Sample records for hadean upper mantle

  1. A Review of the Isotopic and Trace Element Evidence for Mantle and Crustal Processes in the Hadean and Archean: implications for the Onset of Plate Tectonic Subduction (United States)

    Smart, Katie A.; Tappe, Sebastian; Stern, Richard A.; Webb, Susan J.; Ashwal, Lewis D.


    Plate tectonics plays a vital role in the evolution of our planet. Geochemical analysis of Earth’s oldest continental crust suggests that subduction may have begun episodically about 3.8 to 3.2 billion years ago, during the early Archaean or perhaps more than 3.8 billion years ago, during the Hadean. Yet, mantle rocks record evidence for modern-style plate tectonics beginning only in the late Archaean, about 3 billion years ago. Here we analyse the nitrogen abundance, as well as the nitrogen and carbon isotopic signatures of Archaean placer diamonds from the Kaapvaal craton, South Africa, which formed in the upper mantle 3.1 to 3.5 billion years ago. We find that the diamonds have enriched nitrogen contents and isotopic compositions compared with typical mantle values. This nitrogen geochemical fingerprint could have been caused by contamination of the mantle by nitrogen-rich Archaean sediments. Furthermore, the carbon isotopic signature suggests that the diamonds formed by reduction of an oxidized fluid or melt. Assuming that the Archaean mantle was more reduced than the modern mantle, we argue that the oxidized components were introduced to the mantle by crustal recycling at subduction zones. We conclude, on the basis of evidence from mantle-derived diamonds, that modern-style plate tectonics operated as early as 3.5 billion years ago.

  2. Heat transport in the Hadean mantle: From heat pipes to plates (United States)

    Kankanamge, Duminda G. J.; Moore, William B.


    Plate tectonics is a unique feature of Earth, and it plays a dominant role in transporting Earth's internally generated heat. It also governs the nature, shape, and the motion of the surface of Earth. The initiation of plate tectonics on Earth has been difficult to establish observationally, and modeling of the plate breaking process has not consistently accounted for the nature of the preplate tectonic Earth. We have performed numerical simulations of heat transport in the preplate tectonic Earth to understand the transition to plate tectonic behavior. This period of time is dominated by volcanic heat transport called the heat pipe mode of planetary cooling. These simulations of Earth's mantle include heat transport by melting and melt segregation (volcanism), Newtonian temperature-dependent viscosity, and internal heating. We show that when heat pipes are active, the lithosphere thickens and lithospheric isotherms are kept flat by the solidus. Both of these effects act to suppress plate tectonics. As volcanism wanes, conduction begins to control lithospheric thickness, and large slopes arise at the base of the lithosphere. This produces large lithospheric stress and focuses it on the thinner regions of the lithosphere resulting in plate breaking events.

  3. Issues of oxygen excess in the crust and upper mantle lithosphere (United States)

    Balashov, Y. A.; Martynov, E. V.


    Application of a new geochemical buffer, 'CeB' - Ce+4/Ce+3 for zircons, is promising for oxygen fugacity (FO2) estimation in crust and mantle. Absence of Ce+4 and Eu+2-enriched zircons are typical of the lower lithosphere. Reducing setting dominate in mantle rocks. Subduction adds oxidized substance for lithosphere into deeper mantle (Balashov ea, 2011-2012). The zircons in upper lithosphere are oxidized. Peridotites minerals show increased H2O and OH- preserves to 150-160 km at ΔFMQ -1.4 - -0.1 (Babushkina et al, 2009) comparable with CeB 2.2 - 3.9. Increasing oceanic mass in the geological time controls water efflux and oxidation of upper the lithosphere. Oxygen source in crust and upper mantle is the most important, yet outstanding issues in geochemistry of Earth's upper shells. Oxygen excess in atmosphere correlating with long-term emergence and evolution of Earth's biosphere is an approach reflected in the schemes of cycle- and phase-wise biosphere evolution (Dobretsov et al, 2006; Sorokhtin et al, 2010). The both schemes demonstrate ideas for oxygen evolution of atmosphere, but are not confirmed by geochronology. Applying these outlines an actual picture FO2 evolution. Precambrian granitoids, detrital zircons and upper mantle lithosphere have similar CeB. The initial data include Australian Hadean and Archaean detrital zircons (Peck et al, 2001), CeB: 27.1 -1.96, and Eu+2/Eu+3: 0.015-0.12 (Balashov, Skublov, 2011). Greenland tonalities (3813 Ma) and granodiorite (3638 Ma) (Whitehouse, Kamber, 2002) CeB: 34 - 0.5. In oldest crust rocks dominated zircons with generation under high and heterogeneous FO2. Zircons in younger mantle-crustal rocks of S. American subduction zones (Ballard et al, 2002; Hoskin et al, 2000, etc.) show the same. Upper mantle lithosphere and crust represent continuously interacted with oxygen. If Progressively oxygen increase from Hadean to modern state (Dobretsov ea, 2006; Sorokhtin ea, 2010), contradicts with actual Archaean data. We

  4. The Hadean-Archaean environment. (United States)

    Sleep, Norman H


    A sparse geological record combined with physics and molecular phylogeny constrains the environmental conditions on the early Earth. The Earth began hot after the moon-forming impact and cooled to the point where liquid water was present in approximately 10 million years. Subsequently, a few asteroid impacts may have briefly heated surface environments, leaving only thermophile survivors in kilometer-deep rocks. A warm 500 K, 100 bar CO(2) greenhouse persisted until subducted oceanic crust sequestered CO(2) into the mantle. It is not known whether the Earth's surface lingered in a approximately 70 degrees C thermophile environment well into the Archaean or cooled to clement or freezing conditions in the Hadean. Recently discovered approximately 4.3 Ga rocks near Hudson Bay may have formed during the warm greenhouse. Alkalic rocks in India indicate carbonate subduction by 4.26 Ga. The presence of 3.8 Ga black shales in Greenland indicates that S-based photosynthesis had evolved in the oceans and likely Fe-based photosynthesis and efficient chemical weathering on land. Overall, mantle derived rocks, especially kimberlites and similar CO(2)-rich magmas, preserve evidence of subducted upper oceanic crust, ancient surface environments, and biosignatures of photosynthesis.

  5. Towards a Global Upper Mantle Attenuation Model (United States)

    Karaoglu, Haydar; Romanowicz, Barbara


    Global anelastic tomography is crucial for addressing the nature of heterogeneity in the Earth's interior. The intrinsic attenuation manifests itself through dispersion and amplitude decay. These are contaminated by elastic effects such as (de)focusing and scattering. Therefore, mapping anelasticity accurately requires separation of elastic effects from the anelastic ones. To achieve this, a possible approach is to try and first predict elastic effects through the computation of seismic waveforms in a high resolution 3D elastic model, which can now be achieved accurately using numerical wavefield computations. Building upon the recent construction of such a whole mantle elastic and radially anisotropic shear velocity model (SEMUCB_WM1, French and Romanowicz, 2014), which will be used as starting model, our goal is to develop a higher resolution 3D attenuation model of the upper mantle based on full waveform inversion. As in the development of SEMUCB_WM1, forward modeling will be performed using the spectral element method, while the inverse problem will be treated approximately, using normal mode asymptotics. Both fundamental and overtone time domain long period waveforms (T>60s) will be used from a dataset of over 200 events observed at several hundred stations globally. Here we present preliminary results of synthetic tests, exploring different iterative inversion strategies.

  6. Scales of Heterogeneities in the Continental Crust and Upper Mantle


    M. Tittgemeyer; F. Wenzel; Trond Ryberg; Fuchs, K


    A seismological characterization of crust and upper mantle can refer to large-scale averages of seismic velocities or to fluctuations of elastic parameters. Large is understood here relative to the wavelength used to probe the earth. In this paper we try to characterize crust and upper mantle by the fluctuations in media properties rather than by their average velocities. As such it becomes evident that different scales of heterogeneities prevail in different layers of crust mantle. Although ...

  7. Peridotite xenoliths from the Polynesian Austral and Samoa hotspots: Implications for the destruction of ancient 187Os and 142Nd isotopic domains and the preservation of Hadean 129Xe in the modern convecting mantle (United States)

    Jackson, M. G.; Shirey, S. B.; Hauri, E. H.; Kurz, M. D.; Rizo, H.


    The Re-Os systematics in 13 peridotite xenoliths hosted in young (<0.39 myr) rejuvenated lavas from the Samoan island of Savai'i and 8 peridotite xenoliths from 6 to 10 myr old lavas from the Austral island of Tubuai have been examined to evaluate the history of the oceanic mantle in this region. Modal mineralogy, trace element compositions and 187Os/188Os ratios suggest that these peridotites are not cognate or residual to mantle plumes but rather samples of Pacific oceanic lithosphere created at the ridge. Savai'i and Tubuai islands lie along a flow line in the Pacific plate, and provide two snapshots (separated by over 40 Ma in time) of Pacific mantle that originated in the same region of the East Pacific rise. Tubuai xenoliths exhibit 187Os/188Os from 0.1163 to 0.1304, and Savai'i (Samoa) xenoliths span a smaller range from 0.1173 to 0.1284. The 187Os/188Os ratios measured in Tubuai xenoliths are lower than (and show no overlap with) basalts from Tubuai. The 187Os/188Os of the Savai'i xenoliths overlap the isotopic compositions of lavas from the island of Savai'i, but also extend to lower 187Os/188Os than the lavas. 3He/4He measurements of a subset of the xenoliths range from 2.5 to 6.4 Ra for Tubuai and 10.8 to 12.4 Ra for Savai'i. Like abyssal peridotites and xenoliths from oceanic hotspots that sample the convecting mantle, Os isotopes from the Savai'i and Tubuai xenolith suites are relatively unradiogenic, but do not preserve a record of depleted early-formed (Hadean and Archean) mantle domains expected from earlier cycles of ridge-related depletion, continent extraction, or subcontinental lithospheric mantle erosion. The lack of preservation of early-formed, geochemically-depleted Os-isotopic and 142Nd/144Nd domains in the modern convecting mantle contrasts with the preservation of early-formed (early-Hadean) 129Xe/130Xe isotopic heterogeneities in the convecting mantle. This can be explained if the initial isotopic signatures in Re-Os and Sm-Nd systems

  8. Upper and mid mantle fabric developing during subduction-induced mantle flow (United States)

    Faccenda, Manuele


    Subduction zones are convergent margins where the rigid lithosphere sinks into the Earth's mantle inducing complex 3D flow patterns. Seismic anisotropy generated by strain-induced lattice/crystal preferred orientation (LPO/CPO) of intrinsically anisotropic minerals is commonly used to study flow in the mantle and its relations with plate motions. We computed the seismic anisotropy of the upper and mid mantle due to strain-induced LPO in 3D mechanical models of dynamic subduction by using, respectively, D-Rex and Underworld. Subsequently, FSTRACK was used to compute seismogram synthetics and SKS splitting patterns. Strong anisotropy develops in the upper mantle, while weak or null seismic anisotropy is formed in the upper transition zone/lower mantle and lower transition zone, respectively. The distribution of the fabric in the mantle depends on the distribution and amount of the deformation, and not on the rate at which the slab subducts. The SKS splitting patterns are controlled by the anisotropy in the upper mantle because SKS waves are more sensitive to the anisotropy in the shallowest layers. Horizontally propagating shear waves in the mid mantle originating from local earthquakes are characterized by significant splitting that is mostly due to the fabric in the uppermost lower mantle. We discuss the implications of our results for real subduction settings like Tonga, where a discrete amount of observations have been collected in the past 10 years on the anisotropy in the upper and mid mantle.

  9. European upper mantle tomography: adaptively parameterized models (United States)

    Schäfer, J.; Boschi, L.


    We have devised a new algorithm for upper-mantle surface-wave tomography based on adaptive parameterization: i.e. the size of each parameterization pixel depends on the local density of seismic data coverage. The advantage in using this kind of parameterization is that a high resolution can be achieved in regions with dense data coverage while a lower (and cheaper) resolution is kept in regions with low coverage. This way, parameterization is everywhere optimal, both in terms of its computational cost, and of model resolution. This is especially important for data sets with inhomogenous data coverage, as it is usually the case for global seismic databases. The data set we use has an especially good coverage around Switzerland and over central Europe. We focus on periods from 35s to 150s. The final goal of the project is to determine a new model of seismic velocities for the upper mantle underlying Europe and the Mediterranean Basin, of resolution higher than what is currently found in the literature. Our inversions involve regularization via norm and roughness minimization, and this in turn requires that discrete norm and roughness operators associated with our adaptive grid be precisely defined. The discretization of the roughness damping operator in the case of adaptive parameterizations is not as trivial as it is for the uniform ones; important complications arise from the significant lateral variations in the size of pixels. We chose to first define the roughness operator in a spherical harmonic framework, and subsequently translate it to discrete pixels via a linear transformation. Since the smallest pixels we allow in our parameterization have a size of 0.625 °, the spherical-harmonic roughness operator has to be defined up to harmonic degree 899, corresponding to 810.000 harmonic coefficients. This results in considerable computational costs: we conduct the harmonic-pixel transformations on a small Beowulf cluster. We validate our implementation of adaptive

  10. Thermal and compositional structure of the upper mantle (United States)

    Gilbert, Hersh Joseph

    Constraints for models of the convective, thermal, and mineralogical structure within the mantle depend heavily on seismic observations of the deep, and otherwise inaccessible, Earth. Studies presented within this dissertation focus primarily on the upper mantle discontinuities that bound the transition zone between the upper and lower mantle at the nominal depths of 410 and 660 km. These discontinuities are attributed to phase transitions of the mantle mineral olivine to denser configurations. Additionally, they may demark compositional layers within the mantle. This region figures prominently in the convective style of the planet. I address the questions of whether the 660-km discontinuity in some way inhibits flow from crossing between the upper and lower mantle and, more specifically, if it coincides with a compositional barrier in the mantle. Thermal variations associated with warm-rising and cool-sinking material in the mantle produce observable variations in the depths of the discontinuities. If rising or sinking materials cross the entire extent of the mantle, then the transition zone should respond to its associated thermal perturbations in a correlated manner. If instead, convection were divided between the upper and lower mantle, then thermal perturbations in the transition zone need not be spatially correlated. Observations presented in this dissertation display regions in which both the 410- and 660-km discontinuities possess greater than 20 km of peak-to-peak topography that is not correlated between the two. Studying the upper mantle below the western United States, I find no correlation between the upper mantle and the surface tectonics of the region. The topography on both discontinuities in this region is nearly as pronounced as that found where the cold subducting Tonga slab interacts with the upper mantle, suggesting the presence of a similar thermal anomaly. Additionally, amplitudes of the velocity jumps associated with the discontinuities

  11. The North American upper mantle: density, composition, and evolution (United States)

    Mooney, Walter D.; Kaban, Mikhail K.


    The upper mantle of North America has been well studied using various seismic methods. Here we investigate the density structure of the North American (NA) upper mantle based on the integrative use of the gravity field and seismic data. The basis of our study is the removal of the gravitational effect of the crust to determine the mantle gravity anomalies. The effect of the crust is removed in three steps by subtracting the gravitational contributions of (1) topography and bathymetry, (2) low-density sedimentary accumulations, and (3) the three-dimensional density structure of the crystalline crust as determined by seismic observations. Information regarding sedimentary accumulations, including thickness and density, are taken from published maps and summaries of borehole measurements of densities; the seismic structure of the crust is based on a recent compilation, with layer densities estimated from P-wave velocities. The resultant mantle gravity anomaly map shows a pronounced negative anomaly (−50 to −400 mGal) beneath western North America and the adjacent oceanic region and positive anomalies (+50 to +350 mGal) east of the NA Cordillera. This pattern reflects the well-known division of North America into the stable eastern region and the tectonically active western region. The close correlation of large-scale features of the mantle anomaly map with those of the topographic map indicates that a significant amount of the topographic uplift in western NA is due to buoyancy in the hot upper mantle, a conclusion supported by previous investigations. To separate the contributions of mantle temperature anomalies from mantle compositional anomalies, we apply an additional correction to the mantle anomaly map for the thermal structure of the uppermost mantle. The thermal model is based on the conversion of seismic shear-wave velocities to temperature and is consistent with mantle temperatures that are independently estimated from heat flow and heat production data

  12. Hadean Earth and primordial continents: The cradle of prebiotic life

    Directory of Open Access Journals (Sweden)

    M. Santosh


    Full Text Available The Hadean history of Earth is shrouded in mystery and it is considered that the planet was born dry with no water or atmosphere. The Earth-Moon system had many features in common during the birth stage. Solidification of the dry magma ocean at 4.53 Ga generated primordial continents with komatiite. We speculate that the upper crust was composed of fractionated gabbros and the middle felsic crust by anorthosite at ca. 21 km depth boundary, underlain by meta-anorthosite (grossular + kyanite + quartz down to 50–60 km in depth. The thickness of the mafic KREEP basalt in the lower crust, separating it from the underlying upper mantle is not well-constrained and might have been up to ca. 100–200 km depending on the degree of fractionation and gravitational stability versus surrounding mantle density. The primordial continents must have been composed of the final residue of dry magma ocean and enriched in several critical elements including Ca, Mg, Fe, Mn, P, K, and Cl which were exposed on the surface of the dry Earth. Around 190 million years after the solidification of the magma ocean, “ABEL bombardment” delivered volatiles including H2O, CO2, N2 as well as silicate components through the addition of icy asteroids. This event continued for 200 Myr with subordinate bombardments until 3.9 Ga, preparing the Earth for the prebiotic chemical evolution and as the cradle of first life. Due to vigorous convection arising from high mantle potential temperatures, the primordial continents disintegrated and were dragged down to the deep mantle, marking the onset of Hadean plate tectonics.

  13. Upper mantle viscosity and lithospheric thickness under Iceland

    NARCIS (Netherlands)

    Barnhoorn, A.; Wal, W. van der; Drury, M.R.


    Deglaciation during the Holocene on Iceland caused uplift due to glacial isostatic adjustment. Relatively low estimates for the upper mantle viscosity and lithospheric thickness result in rapid uplift responses to the deglaciation cycles on Iceland. The relatively high temperatures of the upper mant

  14. Seismic anisotropy of upper mantle in Sichuan and adjacent regions

    Institute of Scientific and Technical Information of China (English)

    CHANG LiJun; WANG ChunYong; DING ZhiFeng


    Based on the polarization analysis of teleseismic SKS waveform data recorded at 94 broadband seismic stations in Sichuan and adjacent regions, the SKS fast-wave direction and the delay time between the fast and slow shear waves were determined at each station using the grid searching method of minimum transverse energy and the stacking analysis method, and the image of upper mantle anisotropy was acquired. The fast-wave polarization directions are mainly NW-SE in the study area,NWW-SEE to its northeast and NS to its west. The delay time falls into the interval [0.47 s, 1.68 s]. The spatial variation of the fast-wave directions is similar to the variation of GPS velocity directions. The anisotropic image indicates that the regional tectonic stress field has resulted in deformation and flow of upper mantle material, and made the alignment of upper mantle peridotite lattice parallel to the direction of material deformation. The crust-upper mantle deformation in Sichuan and adjacent regions accords with the mode of vertically coherent deformation. In the eastern Tibetan Plateau, the crustal material was extruded to east or southeast clue to SE traction force of the upper mantle material. The extrusion might be obstructed by a rigid block under the Sichuan Basin and the crust has been deformed. After a long-term accumulation of tectonic strain energy, the accumulative energy suddenly released in Yingxiu town of the Longmenshan region, and Wenchuan Ms8.0 earthquake occurred.

  15. Alpine Lithosphere and Upper Mantle Passive Seismic Monitoring


    Brückl, Ewald; Hausmann, Helmut; Behm, Michael; Lippitsch, Regina; Mitterbauer, Ulrike; Institute of Geodesy and Geophysics Vienna University of Technology (Hrsg.)


    The project ALPASS is a passive seismic monitoring project aiming to reveal upper mantle, lower lithosphere, and asthenosphere beneath the wider Eastern Alpine region, including the Bohemian Massive, the Carpathians, the Pannonian Basin, and the Dinarides. A 3D seismic model which will provide crustal corrections to the seismic travel times has been generated in this area down to the Moho and the uppermost mantle from data of former projects CELEBRATION 2000 and ALP 2002. ALPASS will yield in...

  16. Upper mantle geotherm for eastern China and its geological implications

    Institute of Scientific and Technical Information of China (English)

    徐义刚; 林传勇; 史兰斌; J-C.C.Mercier; J.V.Ross


    The equilibrium temperature and pressure of both spinel and garnet peridotite xenolithsfrom eastern China have been estimated by using different geothermobarometers that are currentlyconsidered to be reliable.Based on these data,the upper mantle geotherm for eastern China has beenconstructed.The obtained geotherm is higher than that for old craton regions,but is similar to that for southeasternAustralia.The most prominent feature of the geotherm is that there is a slight gradient at about 60 kmdepth,and the inflection of the geotherm just passes through the transition line between spinel and garnetfacies.It is likely that the inflection represents the botmdary between the lithosphere and asthenosphere.Thestructures of the upper mantle beneath eastern China have also been discussed by combining the results ofpetrologic,rheological and dynamical studies of the upper mantle.

  17. Compositions of Upper Mantle Fluids Beneath Eastern China:Implications for Mantle Evolution

    Institute of Scientific and Technical Information of China (English)

    ZHANG Mingjie; WANG Xianbin; LIU Gang; ZHANG Tongwei; BO Wenrui


    The composition of gases trapped in olivine, orthopyroxene and clinopyroxene in lherzolite xenoliths collected from different locations in eastern China has been measured by the vacuum stepped-heating mass spectrometry.These xenoliths are hosted in alkali basalts and considered as residues of partial melting of the upper mantle, and may contain evidence of mantle evolution. The results show that various kinds of fluid inclusions in lherzolite xenoliths have been released at distinct times, which could be related to different stages of mantle evolution. In general, primitive fluids of the upper mantle (PFUM) beneath eastern China are dominated by H2, CO2 and CO, and are characterized by high contents of H2 and reduced gases. The compositions of PFUM are highly variable and related to tectonic settings. CO, CO2 and H2 are the main components of the PFUM beneath cratons; the PFUM in the mantle enriched in potassic metasomatism in the northern part of northeastern China has a high content of H2, while CO2 and SO2 are the dominant components of the PFUM in the Su-Lu-Wan (Jiangsu-Shandong-Anhui) region, where recycled crustal fluids were mixed with deeper mantle components. There are several fluids with distinct compositions beneath eastern China, such as primitive fluids of upper mantle (CO, CO2 and H2), partial melting fluids (CO2 and CO) and metasomatic fluids mixed with recycled crustal fluids (CO2, N2, 8O2 and CH4) etc. Fluids of the upper mantle beneath the North China craton are different from that of the South China craton in total gases and chemical compositions: the contents of the reduced gases of the PFUM in the NCC are higher than those in the SCC.

  18. Estimating Upper Mantle Hydration from In Situ Electrical Conductivity (United States)

    Behrens, J.; Constable, S.; Heinson, G.; Everett, M.; Weiss, C.; Key, K.


    The electrical conductivity of 35-40 Ma Pacific plate has been measured in situ; one robust result is the presence of bulk anisotropy in the lithospheric upper mantle. We interpret this anisotropy to be a result of hydrothermal circulation into the upper mantle along spreading-ridge-parallel normal faults: the associated zones of serpentinized peridotite provide the pathways of enhanced electrical conductivity required by the data. Our modeling bounds the range of possible anisotropic ratios, which are then used to estimate the amount of water required to serpentinize the requisite amounts of peridotite. These data sets, however, do not indicate anisotropy in the bulk conductivity of the crust, nor in the asthenospheric mantle. This second point is significant, as recent measurements of sub-continental asthenospheric conductivity have been interpreted to indicate anisotropy aligned with present plate motion, with the diffusion of hydrogen through olivine advanced as an explanation.

  19. Seismic anisotropy of upper mantle in eastern China

    Institute of Scientific and Technical Information of China (English)


    Based on the polarization analysis of teleseismic SKS waveform data recorded at 65 seismic stations which respectively involved in the permanent and temporary broadband seismograph networks deployed in eastern China, the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by use of SC method and the stacking analysis method, and then the image of upper mantle anisotropy in eastern China was acquired. In the study region, from south to north, the fast-wave polarization directions are basically EW in South China, gradually clockwise rotate to NWW-SEE in North China, then to NW-SE in Northeast China. The delay time falls into the interval [0.41 s, 1.52 s]. Anisotropic characteristics in eastern China indicate that the upper mantle anisotropy is possibly caused by both the collision between the Indian and Eurasian Plates and the subduction from the Pacific and Philippine Sea Plates to the Eurasian Plate. The collision between two plates made the crust of western China thickening and uplifting and the material eastwards extruding, and then caused the upper mantle flow eastwards and southeastwards. The subduction of Pacific Plate and Philippine Sea Plate has resulted in the lithosphere and the asthenosphere deformation in eastern China, and made the alignment of upper mantle peridotite lattice parallel to the deformation direction. The fast-wave polarization direction is consistent with the direction of lithosphere extension and the GPS velocity direction, implying that the crust-upper mantle deformation is possibly a vertically coherent deformation.

  20. Seismic anisotropy of upper mantle in eastern China

    Institute of Scientific and Technical Information of China (English)

    CHANG LiJun; WANG ChunYong; Ding ZhiFeng


    Based on the polarization analysis of teleseismic SKS waveform data recorded at 65 seismic stations which respectively involved in the permanent and temporary broadband seismograph networks de-ployed in eastern China,the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by use of SC method and the stacking analysis method,and then the image of upper mantle anisotropy in eastern China was acquired.In the study region,from south to north,the fast-wave polarization directions are basically EW in South China,gradually clock-wise rotate to NWW-SEE in North China,then to NW-SE in Northeast China.The delay time falls into the Interval [0.41 s,1.52 s].Anisotropic characteristics in eastern China indicate that the upper mantle anisotropy is possibly caused by both the collision between the Indian and Eurasian Plates and the subduction from the Pacific and Philippine Sea Plates to the Eurasian Plate.The collision between two plates made the crust of western China thickening and uplifting and the material eastwards extruding,and then caused the upper mantle flow eastwards and southeastwards.The subduction of Pacific Plate and Philippine Sea Plate has resulted in the lithosphere and the asthenosphere deformation in eastern China,and made the alignment of upper mantle peridotite lattice parallel to the deformation direction.The fast-wave polarization direction is consistent with the direction of lithosphere extension and the GPS velocity direction,implying that the crust-upper mantle deformation is possibly a vertically co-herent deformation.

  1. Dihedral angle of carbonatite melts in mantle residue near the upper mantle and transition zone (United States)

    Ghosh, S. K.; Rohrbach, A.; Schmidt, M. W.


    Carbonate melts are thought to be ideal metasomatic agents in the deep upper mantle (Green & Wallace, 1988) and these melts are low in viscosities (10-1-10-3 Pa·s) compared to primitive basalt (101-102 Pa·s), furthermore the ability to form an interconnected grain-edge melt network at low melt fractions (3 GPa (Dasgupta et al. 2006, Ghosh et al., 2009), dissolve a number of geochemically incompatible elements much better than silicate melts (Blundy and Dalton, 2000). Previous studies of carbonate melt dihedral angles in olivine-dominated matrices yielded 25-30oat 1-3 GPa, relatively independent of melt composition (Watson et al., 1990) and temperature (Hunter and McKenzie, 1989). Dihedral angles of carbonate melts in contact with deep mantle silicate phases (e.g. garnet, wadsleyite, and ringwoodite) which constitute more than 70 % of the deep upper mantle and transition zone have not been studied yet. We have performed multi-anvil experiments on carbonate-bearing peridotites with 5.0 wt% CO2 from 13.5 to 20 GPa 1550 oC to investigate the dihedral angle of magnesio-carbonatite melts in equilibrium with garnet, olivine (and its high-pressure polymorphs), and clinoenstatite. The dihedral angle of carbonate melts in the deep upper mantle and transition zone is ~30° for majorite garnet and olivine (and its polymorphs) dominated matrices. It does not change with increasing pressure in the range 13.5-20 GPa. Our results suggest that very low melt fractions of carbonatite melt forming in the deep upper mantle and transition zone are interconnected at melt fractions less than 0.01. Consistent with geophysical observations, this could possibly explain low velocity regions in the deep mantle and transition zone.

  2. Seismic anisotropy of upper mantle in eastern Tibetan Plateau and related crust-mantle coupling pattern

    Institute of Scientific and Technical Information of China (English)

    Paul; SILVER; Lucy; FLESCH


    By using the polarization analysis of teleseismic SKS waveform data recorded at 116 seismic stations which respectively involved in China National Digital Seismograph Network, and Yunnan, Sichuan, Gansu and Qinghai regional digital networks, and portable broadband seismic networks deployed in Sichuan, Yunnan and Tibet, we obtained the SKS fast-wave direction and the delay time between fast and slow waves of each station by use of the stacking analysis method, and finally acquired the fine image of upper mantle anisotropy in the eastern Tibetan Plateau and its adjacent regions. We analyzed the crust-mantle coupling deformation on the basis of combining the GPS observation results and the upper mantle anisotropy distribution in the study area. The Yunnan region out of the plateau has dif-ferent features of crust-mantle deformation from the inside plateau. There exists a lateral transitional zone of crust-mantle coupling in the eastern edge of the Tibetan Plateau, which is located in the region between 26° and 27°N in the west of Sichuan and Yunnan. To the south of transitional zone, the fast-wave direction is gradually turned from S60°―70°E in southwestern Yunnan to near EW in south-eastern Yunnan. To the north of transitional zone in northwestern Yunnan and the south of western Sichuan, the fast-wave direction is nearly NS. From crust to upper mantle, the geophysical parameters (e.g. the crustal thickness, the Bouguer gravity anomaly, and tectonic stress direction) show the feature of lateral variation in the transitional zone, although the fault trend on the ground surface is inconsis-tent with the fast-wave direction. This transitional zone is close by the eastern Himalayan syntaxis, and it may play an important role in the plate boundary dynamics.

  3. Seismic anisotropy of upper mantle in eastern Tibetan Plateau and related crust-mantle coupling pattern

    Institute of Scientific and Technical Information of China (English)

    WANG ChunYong; CHANG LiJun; L(U) ZhiYong; QIN JiaZheng; SU Wei; Paul SILVER; Lucy FLESCH


    By using the polarization analysis of teleseismic SKS waveform data recorded at 116 seismic stations which respectively involved in China National Digital Seismograph Network, and Yunnan, Sichuan,Gansu and Qinghai regional digital networks, and portable broadband seismic networks deployed in Sichuan, Yunnan and Tibet, we obtained the SKS fast-wave direction and the delay time between fast and slow waves of each station by use of the stacking analysis method, and finally acquired the fine image of upper mantle anisotropy in the eastern Tibetan Plateau and its adjacent regions. We analyzed the crust-mantle coupling deformation on the basis of combining the GPS observation results and the upper mantle anisotropy distribution in the study area. The Yunnan region out of the plateau has different features of crust-mantle deformation from the inside plateau. There exists a lateral transitional zone of crust-mantle coupling in the eastern edge of the Tibetan Plateau, which is located in the region between 26° and 27°N in the west of Sichuan and Yunnan. To the south of transitional zone, the fast-wave direction is gradually turned from S60°-70°E in southwestern Yunnan to near EW in southeastern Yunnan. To the north of transitional zone in northwestern Yunnan and the south of western Sichuan, the fast-wave direction is nearly NS. From crust to upper mantle, the geophysical parameters(e.g. the crustal thickness, the Bouguer gravity anomaly, and tectonic stress direction) show the feature of lateral variation in the transitional zone, although the fault trend on the ground surface is inconsistent with the fast-wave direction. This transitional zone is close by the eastern Himalayan syntaxis, and it may play an important role in the plate boundary dynamics.

  4. Magnesium stable isotope composition of Earth's upper mantle (United States)

    Handler, Monica R.; Baker, Joel A.; Schiller, Martin; Bennett, Vickie C.; Yaxley, Gregory M.


    The mantle is Earth's largest reservoir of Mg containing > 99% of Earth's Mg inventory. However, no consensus exists on the stable Mg isotope composition of the Earth's mantle or how variable it is and, in particular, whether the mantle has the same stable Mg isotope composition as chondrite meteorites. We have determined the Mg isotope composition of olivine from 22 mantle peridotites from eastern Australia, west Antarctica, Jordan, Yemen and southwest Greenland by pseudo-high-resolution MC-ICP-MS on Mg purified to > 99%. The samples include fertile lherzolites, depleted harzburgites and dunites, cryptically metasomatised ('dry') peridotites and modally metasomatised apatite ± amphibole-bearing harzburgites and wehrlites. Olivine from these samples of early Archaean through to Permian lithospheric mantle have δ25Mg DSM-3 = - 0.22 to - 0.08‰. These data indicate the bulk upper mantle as represented by peridotite olivine is homogeneous within current analytical uncertainties (external reproducibility ≤ ± 0.07‰ [2 sd]). We find no systematic δ25Mg variations with location, lithospheric age, peridotite fertility, or degree or nature of mantle metasomatism. Although pyroxene may have slightly heavier δ25Mg than coexisting olivine, any fractionation between mantle pyroxene and olivine is also within current analytical uncertainties with a mean Δ25Mg pyr-ol = +0.06 ± 0.10‰ (2 sd; n = 5). Our average mantle olivine δ25Mg DSM-3 = - 0.14 ± 0.07‰ and δ26Mg DSM-3 = - 0.27 ± 0.14‰ (2 sd) are indistinguishable from the average of data previously reported for terrestrial basalts, confirming that basalts have stable Mg isotope compositions representative of the mantle. Olivine from five pallasite meteorites have δ25Mg DSM-3 = - 0.16 to - 0.11‰ that are identical to terrestrial olivine and indistinguishable from the average δ25Mg previously reported for chondrites. These data provide no evidence for measurable heterogeneity in the stable Mg isotope

  5. Superweak asthenosphere in light of upper mantle seismic anisotropy (United States)

    Becker, Thorsten W.


    Earth's upper mantle includes a ˜200 km thick asthenosphere underneath the plates where viscosity and seismic velocities are reduced compared to the background. This zone of weakness matters for plate dynamics and may be required for the generation of plate tectonics itself. However, recent seismological and electromagnetic studies indicate strong heterogeneity in thinner layers underneath the plates which, if related to more extreme, global viscosity reductions, may require a revision of our understanding of mantle convection. Here, I use dynamically consistent mantle flow modeling and the constraints provided by azimuthal seismic anisotropy as well as plate motions to explore the effect of a range of global and local viscosity reductions. The fit between mantle flow model predictions and observations of seismic anisotropy is highly sensitive to radial and lateral viscosity variations. I show that moderate suboceanic viscosity reductions, to ˜0.01-0.1 times the upper mantle viscosity, are preferred by the fit to anisotropy and global plate motions, depending on layer thickness. Lower viscosities degrade the fit to azimuthal anisotropy. Localized patches of viscosity reduction, or layers of subducted asthenosphere, however, have only limited additional effects on anisotropy or plate velocities. This indicates that it is unlikely that regional observations of subplate anomalies are both continuous and indicative of dramatic viscosity reduction. Locally, such weak patches may exist and would be detectable by regional anisotropy analysis, for example. However, large-scale plate dynamics are most likely governed by broad continent-ocean asthenospheric viscosity contrasts rather than a thin, possibly high melt fraction layer.

  6. Upper-Mantle Flow Driven Dynamic Topography in Eastern Anatolia (United States)

    Sengul Uluocak, Ebru; Pysklywec, Russell; Eken, Tuna; Hakan Gogus, Oguz


    Eastern Anatolia is characterized by 2 km plateau uplift -in the last 10 Myrs-, high surface heat flow distribution, shallow Curie-point depth, anomalous gravity field. Seismological observations indicate relatively high Pn and Sn attenuation and significant low seismic velocity anomalies in the region. Moreover, the surface geology is associated predominantly with volcanic rocks in which melt production through mantle upwelling (following lithospheric delamination) has been suggested. It has been long known that the topographic loading in the region cannot be supported by crustal thickness (~45 km) based on the principle of Airy isostasy. Recent global geodynamic studies carried out for evaluating the post-collisional processes imply that there is an explicit dynamic uplift in Eastern Anatolia and its adjacent regions. In this study we investigate the instantaneous dynamic topography driven by 3-D upper-mantle flow in Eastern Anatolia. For this purpose we conducted numerous thermo-mechanical models using a 2-D Arbitrary Lagrangian Eulerian (ALE) finite element method. The available P-wave tomography data extracted along 10 profiles were used to obtain depth-dependent density anomalies in the region. We present resulting dynamic topography maps and estimated 3D mantle flow velocity vectors along these 2-D cross sections for each profile. The residual topography based on crustal thickness and observed topography was calculated and compared with other independent datasets concerning geological deformation and dynamic topography predictions. The results indicate an upper mantle driven dynamic uplift correlated with the under-compensated characteristic in Eastern Anatolia. We discuss our results combined with 3D mantle flow by considering seismic anisotropy studies in the region. Initial results indicate that high dynamic uplift and the localized low Pn velocities in concurrence with Pn anisotropy structures show nearly spatial coherence in Eastern Anatolia.

  7. Olivine crystals align during diffusion creep of Earth's upper mantle. (United States)

    Miyazaki, Tomonori; Sueyoshi, Kenta; Hiraga, Takehiko


    The crystallographic preferred orientation (CPO) of olivine produced during dislocation creep is considered to be the primary cause of elastic anisotropy in Earth's upper mantle and is often used to determine the direction of mantle flow. A fundamental question remains, however, as to whether the alignment of olivine crystals is uniquely produced by dislocation creep. Here we report the development of CPO in iron-free olivine (that is, forsterite) during diffusion creep; the intensity and pattern of CPO depend on temperature and the presence of melt, which control the appearance of crystallographic planes on grain boundaries. Grain boundary sliding on these crystallography-controlled boundaries accommodated by diffusion contributes to grain rotation, resulting in a CPO. We show that strong radial anisotropy is anticipated at temperatures corresponding to depths where melting initiates to depths where strongly anisotropic and low seismic velocities are detected. Conversely, weak anisotropy is anticipated at temperatures corresponding to depths where almost isotropic mantle is found. We propose diffusion creep to be the primary means of mantle flow.

  8. The mantle transition zone and the upper mantle in Central-Eastern Greenland (United States)

    Anja Kraft, Helene; Thybo, Hans; Vinnik, Lev


    We present a receiver function (RF) study of the mantle transition zone (MTZ) and upper mantle in central-eastern Greenland. Our results are based on data from 18 temporary broad-band seismometers and 5 additional stations from the GLISN and GLATIS networks. The stations were operating in the region between Scoresby Sund and Summit (~ 70 ° N) with half of them installed on ice, the other half on bedrock. For our analysis we calculated low frequency PRF and SRF, which use the difference in travel times between converted and not converted phases at discontinuities. We see clear signals from P410s and P660s in most of our PRF and from S410p in the SRF. Their delay times suggest a surprisingly thin MTZ for most parts of the study area with up to 25 km of thinning compared to standard Earth models. The only exception is a small region in the centre of the study area, which shows times close to standard. It is mainly the delay time for P410s, that varies, while P660s is stable throughout our study area. This indicates, that the thinning of the MTZ is mainly due to topography on the 410-discontinuity. We furthermore observe an M-shaped signal for P410s at stations in the western part around Summit. A similar, complicated signal has been observed previously in different settings and is interpreted as a thin low velocity layer between 410 km and 520 km. In addition we jointly inverted the PRF and SRF for upper mantle velocities. These results show velocities slower than IASP91 for the entire study area. Both the low velocities in the upper mantle and the thinning of the MTZ are in contrary to simple models of old continental shields and might indicate a fairly recent heating event.

  9. Tomography images of the Alpine roots and surrounding upper mantle (United States)

    Plomerova, Jaroslava; Babuska, Vladislav


    Teleseismic body-wave tomography represents powerful tool to study regional velocity structure of the upper mantle and to image velocity anomalies, such as subducted lithosphere plates in collisional zones. In this contribution, we recapitulate 3D models of the upper mantle beneath the Alps, which developed at a collision zone of the Eurasian and African plates. Seismic tomography studies indicate a leading role of the rigid mantle lithosphere that functioned as a major stress guide during the plate collisions. Interactions of the European lithosphere with several micro-plates in the south resulted in an arcuate shape of this mountain range on the surface and in a complicated geometry of the Alpine subductions in the mantle. Early models with one bended lithosphere root have been replaced with more advanced models showing two separate lithosphere roots beneath the Western and Eastern Alps (Babuska et al., Tectonophysics 1990; Lippitsch et al., JGR 2003). The standard isotropic velocity tomography, based on pre-AlpArray data (the currently performed passive seismic experiment in the Alps and surroundings) images the south-eastward dipping curved slab of the Eurasian lithosphere in the Western Alps. On the contrary, beneath the Eastern Alps the results indicate a very steep northward dipping root that resulted from the collision of the European plate with the Adriatic microplate. Dando et al. (2011) interpret high-velocity heterogeneities at the bottom of their regional tomographic model as a graveyard of old subducted lithospheres. High density of stations, large amount of rays and dense ray-coverage of the volume studied are not the only essential pre-requisites for reliable tomography results. A compromise between the amount of pre-processed data and the high-quality of the tomography input (travel-time residuals) is of the high importance as well. For the first time, the existence of two separate roots beneath the Alps has been revealed from carefully pre

  10. Sulphide melt evolution in upper mantle to upper crust magmas, Tongling, China

    Directory of Open Access Journals (Sweden)

    Yilun Du


    Full Text Available Sulphide inclusions, which represent melts trapped in the minerals of magmatic rocks and xenoliths, provide important clues to the behaviour of immiscible sulphide liquids during the evolution of magmas and the formation of Ni–Cu–Fe deposits. We describe sulphide inclusions from unique ultramafic clots within mafic xenoliths, from the mafic xenoliths themselves, and from the three silica-rich host plutons in Tongling, China. For the first time, we are able to propose a general framework model for the evolution of sulphide melts during the evolution of mafic to felsic magmas from the upper mantle to the upper crust. The model improves our understanding of the sulphide melt evolution in upper mantle to upper crust magmas, and provides insight into the formation of stratabound skarn-type Fe–Cu polymetallic deposits associated with felsic magmatism, thus promising to play an important role during prospecting for such deposits.

  11. Effect of Upper Mantle Heterogeneities on Lithosphere Stresses and Topography (United States)

    Osei Tutu, A.; Steinberger, B.; Rogozhina, I.; Sobolev, S. V.


    The orientation and magnitude of lithosphere stresses give us knowledge about most of the processes within the Earth that are not easy to observe. It has been established (Steinberger, Schmeling, and Marquart 2001) that large contribution of the forces producing lithosphere stresses have their source origination from the buoyancies of both the upper and lower mantle acting beneath the lithosphere. The contribution of the crustal thickness to the stresses has been estimated to be less than 10% (Steinberger et al. 2001) in most region and increases in areas with high gravitational potential energy like the Himalayas. In most of these studies, the effect of the crust was determined separately by computing the gravitational potential energy from the crust (Ghosh et al. 2013) and applied as correction. (Artyushkov 1973) showed that the inhomogeneous nature of the crust contribute to the stresses observed as against using constant lithosphere thickness in most studies, due to the complexities for implementing a variable lithosphere. We seek extend the approach of Ghosh et al. (2013) by coupling the Crust 1.0 (Laske et al. 2013) to a varaible lithosphere thickness in our numerical method. Using a 3D global lithosphere-asthenosphere model (Popov and Sobolev 2008) with visco-elasto-plastic rheology, coupled at 300 km depth to a mantle modeled with a spectral technique (Hager and O'Connell, 1981), we compute lithosphere stresses and topography. we compare our model with observations; the World Stress Map, Global Strain Rate Map and the observed topgraphy. We use S40RTS seismic tomography below 300 km depth, with radial viscosity distribution (Steinberger et al 2006). To account for all the heterogeneities in the upper mantle (300 km) we used different 3D temperatures models setups. The first model is the thermal lithosphere model (Artemieva and Mooney, 2001) in continental regions and assumes half-space cooling of sea floor with age (Müller et al. 2008) for oceans. For the

  12. Tracking Silica in the Earth's Subduction Zone and Upper Mantle (United States)

    Chen, T.; Wang, X.; Zou, Y.; Gwanmesia, G. D.; Liebermann, R. C.; Li, B.


    The X-discontinuity (~300 km) in the upper mantle has been revealed under some continental or oceanic region by a number of seismic studies, at which depth the P and S wave velocities increase by about 2%. One possible cause for this discontinuity is the coesite-stishovite phase transition. In this study, we conducted ultrasonic interferometry measurements on polycrystalline coesite and stishovite up to 12.6 GPa at ambient temperature and 14GPa 1073K, respectively. While the P wave velocities of coesite continuously increase with pressure, the S wave velocities exhibit a monotonic decrease to the peak pressure of the current experiment followed by a reversible recovery upon release of pressure. As a result, within the pressure range of 8-12 GPa (corresponding to ~250-350 km depths), the velocity contrasts between coesite and stishovite reach as high as ~38% for P wave and 48%-50% for S wave together with impedance contrasts of 71-69% and ~78% for P and S waves, respectively, the highest among all known phase transitions in mantle minerals. With such extreme contrasts, the coesite-stishovite phase transition in the MORB composition with 4-10wt% of SiO2 is sufficient to generate velocity and impedance contrasts comparable to those reported for the X-discontinuity. The current data, together with the seismic X-discontinuity, may provide a geophysical approach to track the ancient subducted oceanic slabs, and place constraints on the amount of silica in the upper mantle.

  13. The upper mantle beneath the Gulf of California from surface wave dispersion. Geologica Ultraiectina (299)

    NARCIS (Netherlands)

    Zhang, X.|info:eu-repo/dai/nl/304835773


    This thesis is a study on upper mantle shear velocity structure beneath the Gulf of California. Surface wave interstation dispersion data were measured in the Gulf of California area and vicinity to obtain a 3-D shear velocity structure of the upper mantle. This work has particular significance for

  14. The upper mantle beneath the Gulf of California from surface wave dispersion. Geologica Ultraiectina (299)

    NARCIS (Netherlands)

    Zhang, X.


    This thesis is a study on upper mantle shear velocity structure beneath the Gulf of California. Surface wave interstation dispersion data were measured in the Gulf of California area and vicinity to obtain a 3-D shear velocity structure of the upper mantle. This work has particular significance for

  15. Upper mantle viscosity and lithospheric thickness under Iceland determined from a microphysical modelling approach of mantle rheology (United States)

    Barnhoorn, A.; van der Wal, W.; Drury, M. R.


    The Vatnajökull glacier, located in the south-east of Iceland is the largest ice cap of Iceland having a mean radius of ~50 km covering an area of ˜8100 km2. The Vatnajökull glacier is situated directly on top of the spreading axis in the eastern volcanic zone (EVZ) of the Icelandic mid-ocean ridge and near the inferred center of the Icelandic hotspot. Due to the vicinity of the glacier to the active tectonic area, the response of the solid earth to melting of the ice cap is strongly controlled by the properties of the hot newly formed upper mantle underneath the mid-ocean ridge. The relatively high temperatures in the mantle during rifting result in relatively low upper mantle viscosities and fast relaxation times in comparison with tectonically inactive glaciated areas such as in. In this study, estimates for lithospheric thickness and upper mantle viscosity under Iceland are produced by a microphysical modelling approach using the theoretical temperature distribution under mid-ocean ridges combined with olivine diffusion and dislocation creep flow laws. Large lateral variations in upper mantle viscosity and especially lithospheric thickness are expected for Iceland perpendicular to the ridge axis due to the large changes in temperatures away from the ridge axis. The lithospheric thickness (27-40 km) and upper mantle viscosity (2 × 1018-1019 Pa s) outcomes for the recent glaciation are consistent with previous reports of viscosity and lithospheric thickness from glacial isostatic adjustment studies. A combination of a 40 km thick elastic lithosphere and an average upper mantle viscosity of 5 × 1018 Pa s would suggest that the upper mantle under Iceland is most likely dry. Also, the results indicate that the presence of a plume under Iceland cannot explain the recent low viscosity values reported for Iceland. Using a larger extent and larger thickness of the Icelandic icecap during the Weichselian glaciation event (˜10,000 BP) this study predicts that during

  16. Density heterogeneity of the North American upper mantle from satellite gravity and a regional crustal model

    DEFF Research Database (Denmark)

    Herceg, Matija; Artemieva, Irina; Thybo, Hans


    and by introducing variations into the crustal structure which corresponds to the uncertainty of its resolution by highquality and low-quality seismic models. We examine the propagation of these uncertainties into determinations of lithospheric mantle density. Given a relatively small range of expected density......We present a regional model for the density structure of the North American upper mantle. The residual mantle gravity anomalies are based on gravity data derived from the GOCE geopotential models with crustal correction to the gravity field being calculated from a regional crustal model. We analyze...... how uncertainties and errors in the crustal model propagate from crustal densities to mantle residual gravity anomalies and the density model of the upper mantle. Uncertainties in the residual upper (lithospheric) mantle gravity anomalies result from several sources: (i) uncertainties in the velocity-density...

  17. Dikes, joints, and faults in the upper mantle (United States)

    Wilshire, H.G.; Kirby, S.H.


    Three different types of macroscopic fractures are recognized in upper-mantle and lower-crustal xenoliths in volcanic rocks from around the world: 1. (1) joints that are tensile fractures not occupied by crystallized magma products 2. (2) dikes that are tensile fractures occupied by mafic magmas crystallized to pyroxenites, gabbros or hydrous-mineral-rich rocks, 3. (3) faults that are unfilled shear fractures with surface markings indicative of shear displacement. In addition to intra-xenolith fractures, xenoliths commonly have polygonal or faceted shapes that represent fractures exploited during incorporation of the xenoliths into the host magma that brought them to the surface. The various types of fractures are considered to have formed in response to the pressures associated with magmatic fluids and to the ambient tectonic stress field. The presence of fracture sets and crosscutting relations indicate that both magma-filled and unfilled fractures can be contemporaneous and that the local stress field can change with time, leading to repeated episodes of fracture. These observations give insight into the nature of deep fracture processes and the importance of fluid-peridotite interactions in the mantle. We suggest that unfilled fractures were opened by volatile fluids exsolved from ascending magmas to the tops of growing dikes. These volatile fluids are important because they are of low viscosity and can rapidly transmit fluid pressure to dike and fault tips and because they lower the energy and tectonic stresses required to extend macroscopic cracks and to allow sliding on pre-existing fractures. Mantle seismicity at depths of 20-65 km beneath active volcanic centers in Hawaii corresponds to the depth interval where CO2-rich fluids are expected to be liberated from ascending basaltic magmas, suggesting that such fluids play an important role in facilitating earthquake instabilities in the presence of tectonic stresses. Other phenomena related to the fractures

  18. Whole-mantle convection with tectonic plates preserves long-term global patterns of upper mantle geochemistry. (United States)

    Barry, T L; Davies, J H; Wolstencroft, M; Millar, I L; Zhao, Z; Jian, P; Safonova, I; Price, M


    The evolution of the planetary interior during plate tectonics is controlled by slow convection within the mantle. Global-scale geochemical differences across the upper mantle are known, but how they are preserved during convection has not been adequately explained. We demonstrate that the geographic patterns of chemical variations around the Earth's mantle endure as a direct result of whole-mantle convection within largely isolated cells defined by subducting plates. New 3D spherical numerical models embedded with the latest geological paleo-tectonic reconstructions and ground-truthed with new Hf-Nd isotope data, suggest that uppermost mantle at one location (e.g. under Indian Ocean) circulates down to the core-mantle boundary (CMB), but returns within ≥100 Myrs via large-scale convection to its approximate starting location. Modelled tracers pool at the CMB but do not disperse ubiquitously around it. Similarly, mantle beneath the Pacific does not spread to surrounding regions of the planet. The models fit global patterns of isotope data and may explain features such as the DUPAL anomaly and long-standing differences between Indian and Pacific Ocean crust. Indeed, the geochemical data suggests this mode of convection could have influenced the evolution of mantle composition since 550 Ma and potentially since the onset of plate tectonics.

  19. Mantle Temperature, Mantle Composition, Mantle Heterogeneity, and the Composition of the Upper Mantle: The View from a Global Synthesis of MORB (United States)

    Langmuir, C. H.; Gale, A.; Dalton, C. A.


    A new comprehensive review of global MORB can address outstanding issues such mantle temperature vs. mantle composition in controlling MORB compositions, the mean composition of ocean ridge basalts, the K/U ratio of the MORB reservoir, and the implications for silicate Earth mass balance of the composition of the upper mantle. We created a global catalogue of ridge segments to assign every sample to a segment. We carried out interlaboratory corrections for major elements, and examined data from each segment to ensure appropriate fractionation correction. We included large unpublished data sets from the Langmuir and Schilling laboratories, assembling the most comprehensive data set for MORB. Data averaged by segment permit calculation of averages that include weighting by segment length and spreading rate. The segment-based approach, comprehensive data set, individualized fractionation correction and interlaboratory corrections distinguish these results from earlier efforts. We also carried out bootstrapping statistical tests for meaningful errors on average compositions. The mean composition of the ocean crust is best determined by a segment length and spreading rate weighted arithmetic mean. As with other recent efforts, notably Su (2002) and also Arevalo and McDonough (2009), the mean composition is substantially more enriched than previous MORB estimates. Average MORB implies a MORB mantle Sm/Nd and Nd isotopic composition similar to the 'non-chondritic primitive mantle' composition based on 142Nd. Then continental crust/MORB mantle mass balance is not possible using a non-chondritic (depleted) bulk silicate earth composition, unless there is a large unsampled depleted reservoir. In contrast to Arevalo and McDonough, who suggested a K/U ratio for MORB of 19,000, we find K/U of 12,340±810, in line with earlier estimates. The discrepancy can be understood from contrasts in methodology, as we determine average K/ average U, while they determine average K/U. To

  20. An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions (United States)

    Warren, J. M.; Shimizu, N.; Sakaguchi, C.; Dick, H. J. B.; Nakamura, E.


    Abyssal peridotites, the depleted solid residues of ocean ridge melting, are the most direct samples available to assess upper oceanic mantle composition. We present detailed isotope and trace element analyses of pyroxene mineral separates from Southwest Indian Ridge abyssal peridotites and pyroxenites in order to constrain the size and length scale of mantle heterogeneity. Our results demonstrate that the mantle can be highly heterogeneous to account adequately for the complexities of ancient and recent melting processes.

  1. Determining upper mantle structures using gravity, seismology, and GIA modelling in Fennoscandia (United States)

    Root, B. C.; van der Wal, W.; Vermeersen, B. L. A.


    The 3D structure of the upper mantle plays a large role in Glacial Isostatic Adjustment (GIA). Finite-element software is able to model this 3D structure, but knowledge of the upper mantle is needed to make these models realistic. Nowadays, global maps are made of the crustal structure and temperature of the upper mantle from seismic observations. Also, satellite gravity missions, such as GOCE and GRACE, determine global gravity fields. Combining these data sets could give new insights in Glacial Isostatic Adjustment and explain some discrepancies seen in currents geological observations with 1D rheology Earth models. We obtain upper mantle models that fit gravity observations. Then, the upper mantle seismic velocities are converted to temperature profiles; that are used to determine the amount of strain according to diffusion and dislocation creep in the upper mantle. The obtained 3D rheology models are used in a finite element GIA model to observe the effect of the 3D structures during GIA. The GIA model results are compared to geological observations of the sea level change, GPS uplift rates, and ongoing gravity change in the area. This study specifically studies the effect of compositional differences in the upper mantle on the modelled remaining uplift and gravity signal. Molecular conversion relations for primitive mantle rock composition, Garnet Lherzolite rock composition, and Archon, iron depleted rock composition are used to compute the temperature and density profiles. The Fennoscandian lithosphere is believed to contain these three types of composition, yet, it is not yet known in what relative amounts and locations. An iterative approach is used to find the best compositional structure to fit the GIA observables in the Fenoscandian upper mantle.

  2. Microstructural evolution and seismic anisotropy of upper mantle rocks in rift zones. Geologica Ultraiectina (300)

    NARCIS (Netherlands)

    Palasse, L.N.


    This thesis investigates field-scale fragments of subcontinental upper mantle rocks from the ancient Mesozoic North Pyrenean rift and Plio-Pleistocene xenoliths from the active Baja California rift, in order to constrain the deformation history of the uppermost mantle. The main focus of the study is

  3. Microstructural evolution and seismic anisotropy of upper mantle rocks in rift zones. Geologica Ultraiectina (300)

    NARCIS (Netherlands)

    Palasse, L.N.


    This thesis investigates field-scale fragments of subcontinental upper mantle rocks from the ancient Mesozoic North Pyrenean rift and Plio-Pleistocene xenoliths from the active Baja California rift, in order to constrain the deformation history of the uppermost mantle. The main focus of the study is

  4. Hadean Thermobarometry Revisited (United States)

    Hopkins, M.; Harrison, T. M.; Manning, C. E.


    Ancient detrital zircons from the Jack Hills region of Western Australia are our principle source of information for the physical and chemical state of Earth’s crust for much of the Hadean Eon (before about 3.85 Ga). Examination of mineral inclusions within these zircons yields information about the magmatic environment in which they formed. We have electron imaged over 1300 pre-4.0 Ga zircons to identify mineral inclusions. We have found 131 which possess inclusions that intersected the polished surfaces. Consistent with our earlier report, the inclusion population is dominated by co-equal abundances of muscovite and quartz (~75%). This observation alone restricts the host melts to have formed at pressure-temperature (P-T) conditions of ~650-800°C and >4 kbars. Individual inclusions (8 muscovites and one hornblende) that were large enough (>5 μm) to be accurately analyzed by electron microprobe were selected for barometric studies. Six out of the eight muscovites contain an average Si per formula unit (normalized to 11 oxygens) of 3.12±0.01, and the sole hornblende inclusion contains an Al content of 2.25 (per 13 cations). Muscovite-phengite and Al-in-hornblende barometers coupled with Ti-in-zircon thermometry for this expanded data set yields a estimate of magmatic P-T conditions of 7±2 kbar and 700±25°C. The two remaining muscovites in our sample suite yield Si contents ~3.4, implying even higher formation pressures at similar temperatures. In all cases, no visible cracks were seen in the vicinity of the inclusions and measured Cr contents of muscovite are low (communication with the fuchsite-rich host metasedimentary rock and support our interpretation that these minerals are primary magmatic inclusions which record crystallization conditions and remained closed chemical systems since crystallization. The newly expanded data set reinforces our previous interpretation that these zircons grew in a crustal environment (<40 km depth) under a geotherm of

  5. Upper Mantle Q and Transmission Studies Using LASA and WWSS Data. (United States)

    complicated than the Heiskanen , Pratt, or Airy mechanisms would suggest and extends well into the upper mantle. A long-range seismic refraction study along the axis of the Rocky Mountains supports a double M-discontinuity. (Author)

  6. Water Distribution in the Continental and Oceanic Upper Mantle (United States)

    Peslier, Anne H.


    Nominally anhydrous minerals such as olivine, pyroxene and garnet can accommodate tens to hundreds of ppm H2O in the form of hydrogen bonded to structural oxygen in lattice defects. Although in seemingly small amounts, this water can significantly alter chemical and physical properties of the minerals and rocks. Water in particular can modify their rheological properties and its distribution in the mantle derives from melting and metasomatic processes and lithology repartition (pyroxenite vs peridotite). These effects will be examined here using Fourier transform infrared spectrometry (FTIR) water analyses on minerals from mantle xenoliths from cratons, plume-influenced cratons and oceanic settings. In particular, our results on xenoliths from three different cratons will be compared. Each craton has a different water distribution and only the mantle root of Kaapvaal has evidence for dry olivine at its base. This challenges the link between olivine water content and survival of Archean cratonic mantle, and questions whether xenoliths are representative of the whole cratonic mantle. We will also present our latest data on Hawaii and Tanzanian craton xenoliths which both suggest the intriguing result that mantle lithosphere is not enriched in water when it interacts with melts from deep mantle upwellings (plumes).

  7. Potassium:rubidium ratio in ultramafic rocks: differentiation history of the upper mantle. (United States)

    Stueber, A M; Murthy, V R


    The increase in K:Rb ratio with decrease in potassium content found in basaltic rocks does not seem to apply to ultramafic rocks. The ratios in a series of alpine ultramafic rocks and ultramafic inclusions in basals and kimberlite pipes are about 200 to 500-significantly lower than those in oceanic tholeiites. This characteristic of ultramafic rocks appears to be consistent with a simplified model in which early differentiation of the primitive mantle led to formation of an upper mantle region enriched in alkali elements and having a low K:Rb ratio. Alpine ultramafic rocks may be residuals from such an upper mantle region.

  8. Seismic velocity variations beneath central Mongolia: Evidence for upper mantle plumes? (United States)

    Zhang, Fengxue; Wu, Qingju; Grand, Stephen P.; Li, Yonghua; Gao, Mengtan; Demberel, Sodnomsambuu; Ulziibat, Munkhuu; Sukhbaatar, Usnikh


    Central Mongolia is marked by wide spread recent volcanism as well as significant topographic relief even though it is far from any plate tectonic boundaries. The cause of the recent magmatism and topography remains uncertain partially because little is known of the underlying mantle seismic structure due to the lack of seismic instrumentation in the region. From August 2011 through August 2013, 69 broadband seismic stations were deployed in central Mongolia. Teleseismic traveltime residuals were measured using waveform correlation and were inverted to image upper mantle P and S velocity variations. Significant lateral variations in seismic velocity are imaged in the deep upper mantle (100 to 800 km depth). Most significant are two continuous slow anomalies from the deep upper mantle to near the surface. One slow feature has been imaged previously and may be a zone of deep upwelling bringing warm mantle to beneath the Hangay Dome resulting in uplift and magmatism including the active Khanuy Gol and Middle Gobi volcanoes. The second, deep low velocity anomaly is seen in the east from 800 to 150 km depth. The anomaly ends beneath the Gobi Desert that is found to have fast shallow mantle indicating a relatively thick lithosphere. We interpret the second deep slow anomaly as a mantle upwelling that is deflected by the thick Gobi Desert lithosphere to surrounding regions such as the Hentay Mountains to the north. The upwellings are a means of feeding warmer than normal asthenospheric mantle over a widely distributed region beneath Mongolia resulting in distributed volcanic activity and uplift. There is no indication that the upwellings are rooted in the deep lower mantle i.e. classic plumes. We speculate the upwellings may be related to deep subduction of the Pacific and Indian plates and are thus plumes anchored in the upper mantle.

  9. Upper Mantle Discontinuity Structure Beneath the Western Atlantic Ocean and Eastern North America from SS Precursors (United States)

    Schmerr, N. C.; Beghein, C.; Kostic, D.; Baldridge, A. M.; West, J. D.; Nittler, L. R.; Bull, A. L.; Montesi, L.; Byrne, P. K.; Hummer, D. R.; Plescia, J. B.; Elkins-Tanton, L. T.; Lekic, V.; Schmidt, B. E.; Elkins, L. J.; Cooper, C. M.; ten Kate, I. L.; Van Hinsbergen, D. J. J.; Parai, R.; Glass, J. B.; Ni, J.; Fuji, N.; McCubbin, F. M.; Michalski, J. R.; Zhao, C.; Arevalo, R. D., Jr.; Koelemeijer, P.; Courtier, A. M.; Dalton, H.; Waszek, L.; Bahamonde, J.; Schmerr, B.; Gilpin, N.; Rosenshein, E.; Mach, K.; Ostrach, L. R.; Caracas, R.; Craddock, R. A.; Moore-Driskell, M. M.; Du Frane, W. L.; Kellogg, L. H.


    Seismic discontinuities within the mantle arise from a wide range of mechanisms, including changes in mineralogy, major element composition, melt content, volatile abundance, anisotropy, or a combination of the above. In particular, the depth and sharpness of upper mantle discontinuities at 410 and 660 km depth are attributed to solid-state phase changes sensitive to both mantle temperature and composition, where regions of thermal heterogeneity produce topography and chemical heterogeneity changes the impedance contrast across the discontinuity. Seismic mapping of this topography and sharpness thus provides constraint on the thermal and compositional state of the mantle. The EarthScope USArray is providing unprecedented access to a wide variety of new regions previously undersampled by the SS precursors. This includes the boundary between the oceanic plate in the western Atlantic Ocean and continental margin of eastern North America. Here we use a seismic array approach to image the depth, sharpness, and topography of the upper mantle discontinuities, as well as other possible upper mantle reflectors beneath this region. This array approach utilizes seismic waves that reflect off the underside of a mantle discontinuity and arrive several hundred seconds prior to the SS seismic phase as precursory energy. In this study, we collected high-quality broadband data SS precursors data from shallow focus (ocean lithosphere to underlying continental lithosphere, as while deeper reflectors are associated with the subduction of the ancient Farallon slab. A comparison of the depth of upper mantle discontinuities to changes in seismic velocity and anisotropy will further quantify the relationship to mantle flow, compositional layering, and phases changes.

  10. Upper mantle convection beneath northwest China and its adjacent region driven by density anomaly

    Institute of Scientific and Technical Information of China (English)

    XU Ping; FU Rong-shan; HUANG Jian-ping; ZHAXian-jie; DAI Zhi-yang


    We assume that the density anomalies, which are transformed from seismic tomography data, are corresponding to temperature distribution in a convective mantle. We take density anomalies as the driving force for mantle convec tion and solve the basic equation with given boundary conditions in a wave-number domain by using the FFT arithmetic. Using the physical model of upper mantle convection and the seismic tomography data supplied by XU et al, we calculated upper mantle convection beneath northwestern China and adjacent region. The flow patterns in the upper mantle show that there are upward and divergent flows in the basin regions, such as Tarim, Qaidam,Junggar and Kazakhstan, where the lithosphere is thin. There are downward and convergent flows in the mountain regions,such as Tianshan, Kunlun and Qilian, where the lithosphere is thick. In addition, because of the divergent flow under the Tarim Basin the upper mantle material in this region is driven southward to the north part of Tibetan Plateau and northward to Tianshan Mountain. Maybe, it is one of the reasons for the recent uplift of the Tianshan Mountain.

  11. Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth's upper mantle (United States)

    Wang, Ze-Zhou; Liu, Sheng-Ao; Liu, Jingao; Huang, Jian; Xiao, Yan; Chu, Zhu-Yin; Zhao, Xin-Miao; Tang, Limei


    compositions of MORB. Therefore, preferential melting of spinel in the peridotites may account for the Zn isotopic difference between spinel peridotites and basalts. By contrast, the absence of Zn isotope fractionation between silicate minerals suggests that Zn isotopes are not significantly fractionated during partial melting of spinel-free garnet-facies mantle. If the studied non-metasomatized peridotites represent the refractory upper mantle, mass balance calculation shows that the depleted MORB mantle (DMM) has a δ66Zn value of +0.20 ± 0.05‰ (2SD), which is lighter than the primitive upper mantle (PUM) estimated in previous studies (+0.28 ± 0.05‰, 2SD, Chen et al., 2013b; +0.30 ± 0.07‰, 2SD, Doucet et al., 2016). This indicates that the Earth's upper mantle has a heterogeneous Zn isotopic composition vertically, which is probably due to shallow mantle melting processes.

  12. Investigations of Eurasian Seismic Sources and Upper Mantle Structure (United States)


    in classical Earth models include the free surface, the Mohorovicic (M) discontinuity, the core-mantle boundary (CMB), and the inner core-outer core...rather to the superposition of first- and higher-order reverberations generated at the Mohorovicic (M) discontinuity. Figure 3.1 depicts the effect of

  13. Shear-wave velocity structure of the crust and upper mantle beneath the Kola Peninsula (United States)

    Dricker, I. G.; Roecker, S. W.; Kosarev, G. L.; Vinnik, L. P.

    We determined the shear-wave velocity structure of the crust and upper mantle beneath the central part of the Kola peninsula from the analysis of P-wave receiver functions and mantle P-SV converted phases recorded at stations Apatity (APA) and Lovozero (LVZ). The times of P-SV converted phases from the 410 and 660 km discontinuities are close to those predicted by the IASP91 model. Phase conversions at the crust-mantle boundary beneath the Baltic shield northeast of LVZ and southwest of APA are consistent with a sharp transition from crust to mantle at a depth of 40 km, while conversions from the intervening Khibina plutonic region are consistent with a gradual transition between depths of 20 and 40 km. We infer that short (∼50 km) wavelength lateral variations in the crust-mantle transition persist in this region, despite the inactivity of the Kola peninsula since Devonian times.

  14. Westward migration of oceanic ridges and related asymmetric upper mantle differentiation (United States)

    Chalot-Prat, Françoise; Doglioni, Carlo; Falloon, Trevor


    Combining geophysical, petrological and structural data on oceanic mantle lithosphere, underlying asthenosphere and oceanic basalts, an alternative oceanic plate spreading model is proposed in the framework of the westward migration of oceanic spreading ridges relative to the underlying asthenosphere. This model suggests that evolution of both the composition and internal structure of oceanic plates and underlying upper mantle strongly depends at all scales on plate kinematics. We show that the asymmetric features of lithospheric plates and underlying upper asthenosphere on both sides of oceanic spreading ridges, as shown by geophysical data (seismic velocities, density, thickness, and plate geometry), reflect somewhat different mantle compositions, themselves related to various mantle differentiation processes (incipient to high partial melting degree, percolation/reaction and refertilization) at different depths (down to 300 km) below and laterally to the ridge axis. The fundamental difference between western and eastern plates is linked to the westward ridge migration inducing continuing mantle refertilization of the western plate by percolation-reaction with ascending melts, whereas the eastern plate preserves a barely refertilized harzburgitic residue. Plate thickness on both sides of the ridge is controlled both by cooling of the asthenospheric residue and by the instability of pargasitic amphibole producing a sharp depression in the mantle solidus as it changes from vapour-undersaturated to vapour-saturated conditions, its intersection with the geotherm at 90 km, and incipient melt production right underneath the lithosphere-asthenosphere boundary (LAB). Thus the intersection of the geotherm with the vapour-saturated lherzolite solidus explains the existence of a low-velocity zone (LVZ). As oceanic lithosphere is moving westward relative to asthenospheric mantle, this partially molten upper asthenosphere facilitates the decoupling between lower asthenosphere

  15. Seismic Evidence for Widespread Serpentinized Forearc Upper Mantle Along the Cascadia Margin (United States)

    Brocher, T. M.; Parsons, T.; Trehu, A. M.; Snelson, C. M.; Fisher, M. A.


    Petrologic models for subducting slabs suggest that metamorphism of such slabs releases water that serpentinizes the overlying forearc mantle wedge. We use controlled-source-seismic and earthquake-tomography data from Cascadia to test the hypothesis that a narrow wedge of low velocity, serpentinized upper mantle extends along the Cascadia forearc from Vancouver Island to the Mendocino triple junction. The primary evidence for this wedge is low upper mantle (Pn) velocities (<7.8 km/s) and/or absent or low-amplitude wide-angle reflections from the top of the upper mantle (PmP) in a narrow belt (about 50 to 70 km wide) beneath the Georgia Strait, Puget Lowland, Willamette Valley, and the northern Great Valley. These results are compatible with a recent teleseismic model from central Oregon showing a weak and even inverted velocity contrast across the forearc Moho. Tomography models from Georgia Strait and the northern Puget Lowland show low upper mantle velocities. The absence of near-vertical and wide-angle PmP reflections from the top of the upper mantle in the SHIPS data from Puget Lowland are consistent with a weak continental Moho contrast there. Similarly, wide-angle reflections from the continental Moho (PmP) along a 1995 SW Washington seismic profile are discontinuous in the forearc. No clear PmP reflections or upper mantle refractions are observed from the North America plate, above the subducting Gorda plate, along Line 6 from the 1993 Mendocino seismic experiment. Local seismic reflectivity within the forearc upper mantle wedge may indicate, among other possibilities, that these low-velocity rocks have undergone extensive shearing. These observations are consistent with the suggestion that this wedge of low-velocity, rheologically-weak, serpentinized rocks facilitates slow slip events on the megathrust by lubricating the megathrust and/or accommodating some of the interplate deformation. The presence of a low-velocity forearc upper mantle wedge in Cascadia

  16. A new density model of the upper mantle of North America (United States)

    Kaban, Mikhail K.; Mooney, Walter D.


    We investigate the density structure of the North America upper mantle based on the integrative analysis of the gravity field and seismic data. The basis of our study is the removal of the gravitational effect of the crust from the observed field to determine the mantle gravity anomalies. We use a new crustal model, which is based on nearly all existing seismic determinations including the most recent. The resultant mantle gravity anomaly map shows a pronounced negative anomaly (-50 to -400 mgal) beneath western NA and the adjacent oceanic region, and positive anomalies (+50 to +350 mgal) east of the NA Cordillera. This pattern reflects the well-known division of NA into the stable eastern region and the tectonically active western region. In the same way we estimate the residual topography, which represents the part of the surface topography not- (or over-) compensated by the crustal structure. We invert these fields jointly with seismic tomography data to image density distribution within the crust and upper mantle. The inversion technique accounts for the fact that the residual gravity and residual topography are controlled by the same factors but in a different way, e.g. depending on depth. In the final stage we separate the effect of mantle temperature variations, which is estimated from seismic tomography models constrained by geothermal modelling. Some features of the composition density distribution, which are invisible in the seismic tomography data, are for the first time detected in the upper mantle. The strongest positive anomaly is co-incident with the Gulf of Mexico, and indicates possibly a high-density eclogite layer that has caused subsidence in the Gulf. Two linear positive anomalies are also seen: one with a NE-SW trend in the eastern USA roughly coincident with the Appalachians, and a second with a NW-SE trend beneath the states of Texas, New Mexico, and Colorado. These anomalies are interpreted as due either to: (1) the presence of remnants of

  17. A tomographic glimpse of the upper mantle source of magmas of the Jemez lineament, New Mexico (United States)

    Spence, W.; Gross, R.S.


    To infer spatial distributions of partial melt in the upper mantle source zones for the Rio Grande rift and the Jemez lineament, the lateral variations of P wave velocity in the upper mantle beneath these features has been investigated. Teleseismic P wave delays recorded at a 22-station network were used to perform a damped least squares, three-dimensional inversion for these lateral variations. Results infer that a large magmatic source zone exists beneath the Jemez lineament but not beneath the Rio Grande rift. This implies that the volcanic potential of the Jemez lineaments continues to greatly exceed that of the Rio Grande rift. The magmatic source zones of the Jemez lineament are modeled as due to clockwise rotation of the Colorado Plateau about a pole in northeastern Colorado. This rotation caused extension of the lithosphere beneath the Jemez lineament, permitting concentration there of partially melted rock in the upper mantle. -from Authors

  18. Upper mantle SH velocity structure beneath Qiangtang Terrane by modeling triplicated phases

    Institute of Scientific and Technical Information of China (English)

    ZHANG RuiQing; WU QingJu; LI YongHua; ZENG RongSheng


    We constrain SH wave velocity structure for the upper mantle beneath western Qiangtang Terrane by comparing regional distance seismic triplicated waveforms with synthetic seismograms, based on an intermediate event (~220 km) recorded by the INDEPTH-Ⅲ seismic array. The ATIP model reveals a low-velocity anomaly with up to -4% variation at the depth of 190--270 km and a relatively small ve-locity gradient above the depth of 410 km in the upper mantle, which is in agreement with previous results. In combination with other geological studies, we suggest that the depth of top asthenosphere is 190 km and no large-scale lithosphere thinning occurs in western Qiangtang Terrane, besides, Qiangtang Terrane has the same kind of upper mantle structure as the stable Eurasia.

  19. Sharpness of upper-mantle discontinuities determined from high-frequency reflections (United States)

    Benz, H.M.; Vidale, J.E.


    AN understanding of the nature of seismic discontinuities in the Earth's upper mantle is important for understanding mantle processes: in particular, the amplitude and sharpness of these discontinuities are critical for assessing models of upper-mantle phase changes and chemical layering. So far, seismic studies aimed at determining the thickness and lateral variability of upper-mantle discontinuities have yielded equivocal results, particularly for the discontinuity at 410km depth1,2. Here we present short-period (0.8-2.0 s) recordings of upper-mantle precursors to the seismic phase P???P??? (PKPPKP) from two South American earthquakes recorded by the ???700-station short-period array in California. Our results show that the 410- and 660-km discontinuities beneath the Indian Ocean are locally simple and sharp, corresponding to transi-tion zones of 4 km or less. These observations pose problems for mineral physics models3-5, which predict a transitional thickness greater than 6 km for the peridotite to ??-spinel phase transition. In contrast to the results of long-period studies6,7, we observe no short-period arrivals from near 520 km depth. ?? 1993 Nature Publishing Group.

  20. Upper-mantle P- and S- wave velocities across the Northern Tornquist Zone from traveltime tomography

    DEFF Research Database (Denmark)

    Hejrani, Babak; Balling, N.; Jacobsen, B. H.


    This study presents P- and S-wave velocity variations for the upper mantle in southern Scandinavia and northern Germany based on teleseismic traveltime tomography. Tectonically, this region includes the entire northern part of the prominent Tornquist Zone which follows along the transition from old...... in basin areas to the southwest and in most of southern Norway. Differences in the VP/VS ratio are believed to be a rather robust indicator of upper-mantle compositional differences. For the depth interval of about 100–300 km, thick, depleted, relatively cold shield lithosphere is indicated in southern...

  1. Three-dimensional shear wave velocity structure in the Atlantic upper mantle (United States)

    James, Esther Kezia Candace

    Oceanic lithosphere constitutes the upper boundary layer of the Earth's convecting mantle. Its structure and evolution provide a vital window on the dynamics of the mantle and important clues to how the motions of Earth's surface plates are coupled to convection in the mantle below. The three-dimensional shear-velocity structure of the upper mantle beneath the Atlantic Ocean is investigated to gain insight into processes that drive formation of oceanic lithosphere. Travel times are measured for approximately 10,000 fundamental-mode Rayleigh waves, in the period range 30-130 seconds, traversing the Atlantic basin. Paths with >30% of their length through continental upper mantle are excluded to maximize sensitivity to the oceanic upper mantle. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two-dimensional parameterization is utilized in order to capture perturbations to age-dependent structure. Phase velocity shows a strong dependence on seafloor age, and removing age-dependent velocity from the 2-D maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. Depth-dependent variations in vertically-polarized shear velocity (Vsv) are determined with two sets of 3-D models: a layered model that requires constant VSV in each depth layer, and a splined model that allows VSV to vary continuously with depth. At shallow depths (˜75 km) the seismic structure shows the expected dependence on seafloor age. At greater depths (˜200 km) high-velocity lithosphere is found only beneath the oldest seafloor; velocity variations beneath younger seafloor may result from temperature or compositional variations within the asthenosphere. The age-dependent phase velocities are used to constrain temperature in the mantle and show that, in contrast to previous results for

  2. Seismic imaging of the upper mantle beneath the northern Central Andean Plateau: Implications for surface topography (United States)

    Ward, K. M.; Zandt, G.; Beck, S. L.; Wagner, L. S.


    Extending over 1,800 km along the active South American Cordilleran margin, the Central Andean Plateau (CAP) as defined by the 3 km elevation contour is second only to the Tibetan Plateau in geographic extent. The uplift history of the 4 km high Plateau remains uncertain with paleoelevation studies along the CAP suggesting a complex, non-uniform uplift history. As part of the Central Andean Uplift and the Geodynamics of High Topography (CAUGHT) project, we use surface waves measured from ambient noise and two-plane wave tomography to image the S-wave velocity structure of the crust and upper mantle to investigate the upper mantle component of plateau uplift. We observe three main features in our S-wave velocity model including (1), a high velocity slab (2), a low velocity anomaly above the slab where the slab changes dip from near horizontal to a normal dip, and (3), a high-velocity feature in the mantle above the slab that extends along the length of the Altiplano from the base of the Moho to a depth of ~120 km with the highest velocities observed under Lake Titicaca. A strong spatial correlation exists between the lateral extent of this high-velocity feature beneath the Altiplano and the lower elevations of the Altiplano basin suggesting a potential relationship. Non-uniqueness in our seismic models preclude uniquely constraining this feature as an uppermost mantle feature bellow the Moho or as a connected eastward dipping feature extending up to 300 km in the mantle as seen in deeper mantle tomography studies. Determining if the high velocity feature represents a small lithospheric root or a delaminating lithospheric root extending ~300 km into the mantle requires more integration of observations, but either interpretation shows a strong geodynamic connection with the uppermost mantle and the current topography of the northern CAP.

  3. Localized seismic deformation in the upper mantle revealed by dense seismic arrays (United States)

    Inbal, Asaf; Ampuero, Jean Paul; Clayton, Robert W.


    Seismicity along continental transform faults is usually confined to the upper half of the crust, but the Newport-Inglewood fault (NIF), a major fault traversing the Los Angeles basin, is seismically active down to the upper mantle. We use seismic array analysis to illuminate the seismogenic root of the NIF beneath Long Beach, California, and identify seismicity in an actively deforming localized zone penetrating the lithospheric mantle. Deep earthquakes, which are spatially correlated with geochemical evidence of a fluid pathway from the mantle, as well as with a sharp vertical offset in the lithosphere-asthenosphere boundary, exhibit narrow size distribution and weak temporal clustering. We attribute these characteristics to a transition from strong to weak interaction regimes in a system of seismic asperities embedded in a ductile fault zone matrix.

  4. Constraints on Shear Velocity in the Cratonic Upper Mantle From Rayleigh Wave Phase Velocity (United States)

    Hirsch, A. C.; Dalton, C. A.


    In recent years, the prevailing notion of Precambrian continental lithosphere as a thick boundary layer (200-300 km), defined by a depleted composition and a steady-state conductively cooled temperature structure, has been challenged by several lines of seismological evidence. One, profiles of shear velocity with depth beneath cratons exhibit lower wave speed at shallow depths and higher wave speed at greater depths than can be explained by temperature alone. These profiles are also characterized by positive or flat velocity gradients with depth and anomalously high attenuation in the uppermost mantle, both of which are difficult to reconcile with the low temperatures and large thermal gradient expected with a thermal boundary layer. Two, body-wave receiver-function studies have detected a mid-lithospheric discontinuity that requires a large and abrupt velocity decrease with depth in cratonic regions that cannot be achieved by thermal gradients alone. Here, we used forward-modeling to identify the suite of shear-velocity profiles that are consistent with phase-velocity observations made for Rayleigh waves that primarily traversed cratons in North America, South America, Africa, and Australia. We considered two approaches; with the first, depth profiles of shear velocity were predicted from thermal models of the cratonic upper mantle that correspond to a range of assumed values of mantle potential temperature, surface heat flow, and radiogenic heat production in the crust and upper mantle. With the second approach, depth profiles of shear velocity were randomly generated. In both cases, Rayleigh wave phase velocity was calculated from the Earth models and compared to the observed values. We show that it is very difficult to match the observations with an Earth model containing a low-velocity zone in the upper mantle; instead, the best-fit models contain a flat or positive velocity gradient with depth. We explore the implications of this result for the thermal and

  5. Measurements of upper mantle shear wave anisotropy from a permanent network in southern Mexico

    NARCIS (Netherlands)

    van Benthem, S.A.C.; Valenzuela, R.W.; Ponce, G.J.


    Upper mantle shear wave anisotropy under stations in southern Mexico was measured using records of SKS phases. Fast polarization directions where the Cocos plate subducts subhorizontally are oriented in the direction of the relative motion between the Cocos and North American plates, and are trench-

  6. Measurements of upper mantle shear wave anisotropy from a permanent network in southern Mexico

    NARCIS (Netherlands)

    van Benthem, S.A.C.; Valenzuela, R.W.; Ponce, G.J.


    Upper mantle shear wave anisotropy under stations in southern Mexico was measured using records of SKS phases. Fast polarization directions where the Cocos plate subducts subhorizontally are oriented in the direction of the relative motion between the Cocos and North American plates, and are

  7. Upper mantle structures beneath the Carpathian-Pannonian region: Implications for the geodynamics of continental collision (United States)

    Ren, Y.; Stuart, G. W.; Houseman, G. A.; Dando, B.; Ionescu, C.; Hegedüs, E.; Radovanović, S.; Shen, Y.; South Carpathian Project Working Group


    The Carpathian-Pannonian system of Eastern and Central Europe represents a unique opportunity to study the interaction between surface tectonic processes involving convergence, extension and convective overturn in the upper mantle. Here, we present high-resolution images of upper mantle structure beneath the region from P-wave finite-frequency teleseismic tomography to help constrain such geodynamical interactions. We have selected earthquakes with magnitude greater than 5.5 in the distance range 30°-95°, which occurred between 2006 and 2011. The data were recorded on 54 temporary stations deployed by the South Carpathian Project (2009-2011), 56 temporary stations deployed by the Carpathian Basins Project (2005-2007), and 131 national network broadband stations. The P-wave relative arrival times are measured in two frequency bands (0.5-2.0 Hz and 0.1-0.5 Hz), and are inverted for Vp perturbation maps in the upper mantle. Our images show a sub-vertical slab of fast material beneath the eastern Alps which extends eastward across the Pannonian basin at depths below ˜300km. The fast material extends down into the mantle transition zone (MTZ), where it spreads out beneath the entire basin. Above ˜300km, the upper mantle below the Pannonian basin is dominated by relatively slow velocities, the largest of which extends down to ˜200km. We suggest that cold mantle lithospheric downwelling occurred below the Pannonian Basin before detaching in the mid-Miocene. In the Vrancea Zone of SE Romania, intermediate-depth (75-180 km) seismicity occurs at the NE end of an upper mantle high velocity structure that extends SW under the Moesian Platform, oblique to the southern edge of the South Carpathians. At greater depths (180-400 km), a sub-circular high velocity anomaly is found directly beneath the seismicity. This sub-vertical high-velocity body is bounded by slow anomalies to the NW and SE, which extend down to the top of the MTZ. No clear evidence of a residual slab is

  8. Upper mantle shear wave velocity structure of the east Anatolian-Caucasus region (United States)

    Skobeltsyn, Gleb Anatolyevich

    The Eastern Anatolian-Caucasus region is a relatively young part of the Alpine- Himalayan orogenic belt and has been formed as the result of the ongoing continental collision of Arabia and Eurasia. In spite of a number of geological studies that have been conducted in this area, there is still no consensus within the geoscience community about the regional tectonic settings and a model for the late Cenozoic tectonic evolution of the Anatolian Plateau. Knowledge of the upper mantle velocity structure in this region can provide the geological community with important constraints that are crucial for developing an understanding of the regional geology and the processes associated with early stages of mountain building. In the present dissertation, I describe two studies of the regional upper mantle S wave velocity structure. In order to derive the absolute velocity structure of the upper mantle, I have applied surface wave tomography to model Rayleigh wave phase velocities as a function of period. Then I inverted the Rayleigh phase velocities to obtain S wave velocities as a function of depth. The resulted high-resolution 3-D S wave velocity model of the regional upper mantle is characterized by a better depth resolution than any preexisting tomographic models. I also conducted an S wave splitting analysis using traditional methods and developed a two-layer grid search algorithm in order to infer the upper mantle anisotropic structure. The results of the S wave splitting analysis for the stations located in Azerbaijan are the first in the region. (Abstract shortened by ProQuest.).

  9. Crustal and upper mantle structure of the Slave craton from P- and S- Receiver Functions (United States)

    Barantseva, Olga; Vinnik, Lev; Artemieva, Irina


    Teleseismic events recorded by POLARIS array in NW Canada (Slave craton) and Yellowknife station were used to calculate a sufficient number of receiver functions for P (PRF) and S (SRF) waves. Velocity (Vp and Vs) and Vp/Vs profiles from the Earth's surface down to 300 km are obtained through the simultaneous inversion of PRF and SRF with teleseismic travel time residuals for the crust and upper mantle. We observe highly heterogeneous structure of the cratonic upper mantle. The Lehman discontinuity (the bottom of the low velocity zone) is found in the western Slave craton, whereas it is not observed in the eastern part of the Slave craton. At stations located in the southern part of the craton, we observe an increase of S-wave velocities (as compared to IASP91 values) at the depths 45-150 km which is typical for depleted cratonic mantle. Low Vp/Vs ratio, obtained for the uppermost mantle (1.65-1.70) can be explained by a high fraction of Opx. A comparison of our results with available xenoliths data shows a good agreement between seismic velocity change at a depth of ca. 160 km and a decrease in mantle depletion at about the same depth.

  10. Upper Mantle Flow Beneath the Subducted Nazca Plate: Slab Contortions and Flattening (Invited) (United States)

    Russo, R. M.


    The form of asthenospheric flow beneath subducted lithospheric slabs can be discerned using splitting of shear waves emanating from earthquakes in the slabs themselves. However, the subducted Nazca plate’s abrupt changes in morphology from a planar slab dipping 30° ENE beneath the central Andes to large areas of flat-lying slab beneath Peru, to the north, and Argentina, to the south, are a potential complication to the sub-slab mantle flow. S waves from earthquakes in the Nazca slab reveal details of the upper mantle flow field below and in the vicinity of the slab. Nazca slab earthquakes large enough to be well recorded (M > 5.4, typically), and deep enough to separate S from pS and sS (30-40 km or more), are suitable for such study, and, for events between 1990 and 2010, recording stations are mostly well-distributed azimuthally about the source event. The S waves were recorded at seismic stations at teleseismic distances from the events, and were corrected for known sub-station seismic anisotropy. Thus, the shear wave splitting engendered during their passage through the asthenospheric upper mantle beneath the slab was isolated, and asthenospheric deformation fabrics resulting from plastic flow beneath the slab mapped in some detail. Shear wave splitting fast directions and upper mantle flow beneath the Nazca plate are most often trench-parallel, consistent with trench-parallel upper mantle flow beneath the slab. Fast splitting polarizations at high angle to the strike of the slab occur in the transition regions from flat to normally dipping slab. Upper mantle flow beneath the slab in these regions appears to be channeled by the slab contortion. Upper mantle flow oceanward of the Nazca slab also appears to change abruptly from trends at a high angle to the Peru-Chile trench to trench-parallel as the top of the Nazca slab attains a depth of around 75 km. Trench-parallel sub-slab flow appears to develop once the asthenosphere beneath the Nazca plate is affected

  11. The Acasta Gneiss - a Hadean cratonic nucleus (United States)

    Sprung, P.; Scherer, E. E.; Maltese, A.; Bast, R.; Bleeker, W.; Mezger, K.


    The known terrestrial rock record lacks undisputed, chemically intact Hadean crust. Direct evidence from this eon has been restricted to zircon grains within younger rocks [1]. The Acasta Gneiss Complex (AGC; NT, CA) has yielded zircon with Hadean domains [e.g., 2,3], but the time at which AGC rocks became closed chemical systems is unclear [4,5]. Determining this `time of last disturbance' (tld) would provide a minimum protolith age, and is crucial for using radiogenic isotope compositions of bulk rocks to trace crust-mantle evolution. Recent studies mostly focused on the `low-strain' eastern AGC [e.g., 6, 7], which records an evolving, early-mid Archean cratonic nucleus [7]. We also studied the `high-strain' banded gneiss in the western AGC, which hosts >4 Ga zircon domains [2,3], too. Our focusing lay on adjoining, lithologically distinct bands [8] of two distinct chemical groups: A) Mafic, chondrite-normalized LaN/YbN ≦20, slightly HFSE- depleted, and B) TTG-like, LaN/YbN up to 145, markedly HFSE-depleted. Six adjacent bands yield a well-defined 4 Ga Sm-Nd isochron with a ɛNd4Ga of +2 and ɛHf4Ga values from +1 to +6. Within-band Sm-Nd and Lu-Hf systematics imply younger mineral re-equilibration [9]. We interpret the 4 Ga Sm-Nd isochron to date the physical juxtaposition of bands in the gneiss unit and to define tld among bands for elements less mobile and diffusive than Sm and Nd. Contrasting Sm-Nd results from the same unit [10] likely are due to sampling at too fine a scale. Digestion of metamict pre-tld zircon likely caused the scatter in Lu-Hf. Both decay systems hint at the existence of a possibly local, strongly depleted Hadean mantle domain. The TTG-like bands are 0.4 Gyr older than similar rocks in the `low-strain' eastern AGC [7]. The AGC was thus an evolved cratonic nucleus already at 4 Ga, possibly with a depleted lithospheric keel. [1] Cavosie et al. (2004) Prec. Res. 135, 251-279 [2] Bowring & Williams (1999) CMP 134, 3-16 [3] Iizuka et al

  12. Upper-mantle P- and S- wave velocities across the Northern Tornquist Zone from traveltime tomography

    DEFF Research Database (Denmark)

    Hejrani, Babak; Balling, N.; Jacobsen, B. H.


    This study presents P- and S-wave velocity variations for the upper mantle in southern Scandinavia and northern Germany based on teleseismic traveltime tomography. Tectonically, this region includes the entire northern part of the prominent Tornquist Zone which follows along the transition from old...... Sweden, contrasting with more fertile, warm mantle asthenosphere beneath most of the basins in Denmark and northern Germany. Both compositional and temperature differences seem to play a significant role in explaining the UMVB between southern Norway and southern Sweden. In addition to the main regional...

  13. A crust and upper mantle model of Eurasia and North Africa for Pn travel time calculation

    Energy Technology Data Exchange (ETDEWEB)

    Myers, S; Begnaud, M; Ballard, S; Pasyanos, M; Phillips, W S; Ramirez, A; Antolik, M; Hutchenson, K; Dwyer, J; Rowe, C; Wagner, G


    We develop a Regional Seismic Travel Time (RSTT) model and methods to account for the first-order effect of the three-dimensional crust and upper mantle on travel times. The model parameterization is a global tessellation of nodes with a velocity profile at each node. Interpolation of the velocity profiles generates a 3-dimensional crust and laterally variable upper mantle velocity. The upper mantle velocity profile at each node is represented as a linear velocity gradient, which enables travel time computation in approximately 1 millisecond. This computational speed allows the model to be used in routine analyses in operational monitoring systems. We refine the model using a tomographic formulation that adjusts the average crustal velocity, mantle velocity at the Moho, and the mantle velocity gradient at each node. While the RSTT model is inherently global and our ultimate goal is to produce a model that provides accurate travel time predictions over the globe, our first RSTT tomography effort covers Eurasia and North Africa, where we have compiled a data set of approximately 600,000 Pn arrivals that provide path coverage over this vast area. Ten percent of the tomography data are randomly selected and set aside for testing purposes. Travel time residual variance for the validation data is reduced by 32%. Based on a geographically distributed set of validation events with epicenter accuracy of 5 km or better, epicenter error using 16 Pn arrivals is reduced by 46% from 17.3 km (ak135 model) to 9.3 km after tomography. Relative to the ak135 model, the median uncertainty ellipse area is reduced by 68% from 3070 km{sup 2} to 994 km{sup 2}, and the number of ellipses with area less than 1000 km{sup 2}, which is the area allowed for onsite inspection under the Comprehensive Nuclear Test Ban Treaty, is increased from 0% to 51%.

  14. Towards the Next Generation Upper-Mantle 3D Anelastic Tomography (United States)

    Karaoglu, H.; Romanowicz, B. A.


    In order to distinguish the thermal and compositional heterogeneities in the mantle, it is crucial to resolve the lateral variations not only in seismic velocities but also in intrinsic attenuation. Indeed, the high sensitivity of intrinsic attenuation to temperature and water content, governed by a form of Arrhenius equation, contrasts with the quasi-linear dependence of velocities on both temperature and major element composition. The major challenge in imaging attenuation lies in separating its effects on seismic waves from the elastic ones. The latter originate from the wave propagation in media with strong lateral elastic gradients causing (de)focusing and scattering. We have previously developed a 3D upper-mantle shear attenuation model based on time domain waveform inversion of long period (T > 60s) fundamental and overtone surface wave data (Gung & Romanowicz, 2004). However, at that time, resolution was limited to very long wavelength structure, because elastic models were still rather smooth, and the effects of focusing could only be estimated approximately, using asymptotic normal mode perturbation theory.With recent progress in constraining global mantle shear velocity from waveform tomography based on the Spectral Element Method (e.g. SEMUCB_WM1, French & Romanowicz, 2014), we are now in a position to develop an improved global 3D model of shear attenuation in the upper mantle. In doing so, we use a similar time domain waveform inversion approach, but (1) start with a higher resolution elastic model with better constraints on lateral elastic gradients and (2) jointly invert, in an iterative fashion, for shear attenuation and elastic parameters. Here, we present the results of synthetic tests that confirm our inversion strategy, as well as preliminary results towards the construction of the next generation upper-mantle anelastic model.

  15. The Bombardment of the Earth During the Hadean and Early Archean Eras (United States)

    Marchi, S.; Bottke, W. F.; Elkins-Tanton, L. T.; Morbidelli, A.; Wuennemann, K.; Kring, D. A.; Bierhaus, M.


    Our knowledge of the Earth during the Hadean and early Archean eons (ca 4.5-3.5 Ga) is very limited, mainly because few rocks older than 3.8 Ga have been found (e.g. Harrison 2009). Hadean-era zircons have allowed us to glean important insights into this era, but their data has led to considerably different evolution models for the evolution of the early Earth; some predict a hellish world dominated by a molten surface with a sporadic steam atmosphere (e.g. Pollack 1997), while others have predicted a tranquil, cool surface with stable oceans (e.g. Wilde et al 2001; Valley et al 2002). To understand whether either model (or both) could be right, we believe it is useful to quantitatively examine the post Moon-forming impact bombardment of the early Earth. Over the last several years, through a combination of observations (e.g., Marchi et al 2012), theoretical models (e.g., Bottke et al 2012), and geochemical constraints from lunar rock (e.g. highly siderophile elements -HSE- abundances delivered to the Moon by impactors; the global number of lunar basins; the record of Archean-era impact spherule beds on Earth; Walker 2009; Neumann et al 2012), we have constructed a calibrated model of the early lunar impactor flux (Morbidelli et al 2012). Our results have now been extrapolated to the Earth, where they can make predictions about its early bombardment. Using a Monte Carlo code to account for the stochastic nature of major impacts, and constraining our results by the estimated HSE abundances of Earth's mantle (that were presumably delivered by impactors; Walker 2009; Bottke et al. 2010), we find the following trends. In the first ~100-200 Myr after the formation of the Moon, which we assume was created ~4.5 Ga, the Earth was almost entirely resurfaced by impacts. This bombardment, which included numerous D > 1000 km diameter impactors, should have vigorously mixed the crust and upper mantle. Between ~4.1-4.3 Ga, the impactor flux steadily decreased; though an uptick

  16. A global horizontal shear velocity model of the upper mantle from multimode Love wave measurements (United States)

    Ho, Tak; Priestley, Keith; Debayle, Eric


    Surface wave studies in the 1960s provided the first indication that the upper mantle was radially anisotropic. Resolving the anisotropic structure is important because it may yield information on deformation and flow patterns in the upper mantle. The existing radially anisotropic models are in poor agreement. Rayleigh waves have been studied extensively and recent models show general agreement. Less work has focused on Love waves and the models that do exist are less well-constrained than are Rayleigh wave models, suggesting it is the Love wave models that are responsible for the poor agreement in the radially anisotropic structure of the upper mantle. We have adapted the waveform inversion procedure of Debayle & Ricard to extract propagation information for the fundamental mode and up to the fifth overtone from Love waveforms in the 50-250 s period range. We have tomographically inverted these results for a mantle horizontal shear wave-speed model (βh(z)) to transition zone depths. We include azimuthal anisotropy (2θ and 4θ terms) in the tomography, but in this paper we discuss only the isotropic βh(z) structure. The data set is significantly larger, almost 500 000 Love waveforms, than previously published Love wave data sets and provides ˜17 000 000 constraints on the upper-mantle βh(z) structure. Sensitivity and resolution tests show that the horizontal resolution of the model is on the order of 800-1000 km to transition zone depths. The high wave-speed roots beneath the oldest parts of the continents appear to extend deeper for βh(z) than for βv(z) as in previous βh(z) models, but the resolution tests indicate that at least parts of these features could be artefacts. The low wave speeds beneath the mid-ocean ridges fade by ˜150 km depth except for the upper mantle beneath the East Pacific Rise which remains slow to ˜250 km depth. The resolution tests suggest that the low wave speeds at deeper depths beneath the East Pacific Rise are not solely due

  17. Negligible sulfur isotope fractionation during partial melting: Evidence from Garrett transform fault basalts, implications for the late-veneer and the hadean matte (United States)

    Labidi, J.; Cartigny, P.


    lower and upper limit for the hadean matte. While the lower bound corresponds to a virtually negligible hadean matte, the upper limit is 3.36 ×1024gS (i.e. ∼10% of the bulk terrestrial S), which remains 5 to 10 times lower than previous estimates. This upper bound nonetheless requires high mantle S content >1000 ppm S before the extraction of the hadean matte. This suggestion would have chronological requirements, requiring any sulfide melt to have formed after the core extraction but before late accretion of the highly siderophile elements.

  18. Rayleigh Wave Phase Velocity in the Upper Mantle Beneath the Indian Ocean (United States)

    Godfrey, K. E.; Dalton, C. A.; Ritsema, J.


    Most of what is currently understood about the seismic properties of oceanic upper mantle is based on either global studies or regional studies of the upper mantle beneath the Pacific Ocean. However, global seismic models and geochemical studies of mid-ocean ridge basalts indicate differences in the properties of the upper mantle beneath the Pacific, Atlantic, and Indian oceans. Though the Indian Ocean is not as well studied seismically, it is host to a number of geologically interesting features including 16,000 km of mid-ocean ridge with a range of spreading rates from 14 mm/yr along the Southwest Indian Ridge to 55-75 mm/yr along the Southeast Indian Ridge. The Indian Ocean also contains multiple volcanic hotspots, the Australian-Antarctic Discordance, and a low geoid anomaly south of India, and it overlies a portion of a large low-shear-velocity province. We are using Rayleigh waves to construct a high-resolution seismic velocity model of the Indian Ocean upper mantle. We utilize a global dataset of phase delays measured at 20 periods, between 37 and 375 seconds; the dataset includes between 700 and 20,000 that traverse our study region exclusively, with a larger number of paths at shorter periods. We explore variations in phase velocity using two separate approaches. One, we allow phase velocity to vary only as a function of seafloor age. Two, we perform a damped least-squares inversion to solve for 2-D phase velocity maps at each period. Preliminary results indicate low velocities along the Southeast Indian Ridge and Central Indian Ridge, but the expected low velocities are less apparent along the slow-spreading Southwest Indian Ridge. We observe a region of fast velocities extending from Antarctica northward between the Kerguelen and Crozet hotspots, and lower than expected velocities beneath the Reunion hotspot. Additionally, we find low velocities associated with a region of extinct seafloor spreading in the Wharton basin.

  19. Volatile-rich Melts in the Earth's Upper Mantle (AGU Kuno Medal) (United States)

    Dasgupta, Rajdeep


    The onset of silicate magma generation in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic fluid with variable water content could be stable ≤250 km beneath mid oceanic ridges [1-3], owing to the small fraction (oxygen fugacity of the mantle in the garnet peridotite field [2, 3], we suggest that on a global scale, carbonated silicate melt generation at ~250-180 km deep redox solidus, with destabilization of metal and majorite in the upwelling mantle, explains oceanic low-velocity zone and electrical conductivity structure of the mantle. In locally oxidized domains (i.e., higher than average Fe3+/Fetotal), deeper carbonated silicate melt may contribute to the X-discontinuity. Furthermore, the new experimental results along with the electrical conductivity of molten carbonated peridotite [8] and that of the oceanic upper mantle [6] suggest that if CO2-rich melt is the only possible agent to explain the high electrical conductivity of the asthenospheric mantle then the mantle at depth is CO2-rich but H2O-poor; higher H2O content in the mantle enhances melting, lowers the CO2 content and likely the conductivity of such melts. Finally, carbonated silicate melts restrict the stability of carbonatite in the Earth's deep oceanic upper mantle and the inventory of carbon, water, and other highly incompatible elements at ridges becomes controlled by flux of the former [7]. Although the stability of carbonatitic melt may be eliminated beneath oceanic ridges at all depths, beneath continents stability of carbonatitic melt is expected. Archean cratonic mantle (geotherms corresponding to surface heat flux of 40-50 mW m-2) crosses the carbonated peridotite solidus, at a depth of ~100-220 km [9]; thus considering the oxygen fugacity profile for cratons [3], carbonatitic melt is expected to be stable at 100-180 km depths, at a narrow

  20. New insight into the Upper Mantle Structure Beneath the Pacific Ocean Using PP and SS Precursors (United States)

    Gurrola, H.; Rogers, K. D.


    The passing of the EarthScope Transportable array has provided a dense data set that enabled beam forming of SS and PP data that resultes in improved frequency content to as much a 1 Hz in the imaging of upper mantle structure. This combined with the application of simultaneous iterative deconvolution has resulted in images to as much as 4 Hz. The processing however results in structure being averaged over regions of 60 to 100 km in radius. This is becomes a powerful new tool to image the upper mantle beneath Oceanic regions where locating stations is expensive and difficult. This presentation will summarize work from a number of regions as to new observations of the upper mantle beneath the Pacific and Arctic Oceans. Images from a region of the Pacific Ocean furthest from hot spots or subduction zones (we will refer to this as the 'reference region'). show considerable layering in the upper mantle. The 410 km discontinuity is always imaged using these tools and appears to be a very sharp boundary. It does usually appear as an isolated positive phase. There appears to be a LAB at ~100 km as expected but there is a strong negative phase at ~ 200 km with a positive phase 15 km deeper. This is best explained as a lens of partial melt as expected for this depth based on the geothermal gradient. If so this should be a low friction point and so we would expect it to accommodate plate motion. Imaging of the Aleutian subduction zone does show the 100 km deep LAB as it descends but this 200 km deep horizon appears as a week descending positive anomaly without the shallower negative pulse. In addition to the 410, 100 and 200 km discontinuities there are a number of paired anomalies, between the 200 and 400 km depths, with a negative pulse 15 to 20 km shallower then the positive pulse. We do not believe these are side lobes or we would see side lobes on the 100 km and 410 km discontinuities. We believe these to be the result of friction induced partial melt along zones of

  1. Contrasting origins of the upper mantle revealed by hafnium and lead isotopes from the Southeast Indian Ridge. (United States)

    Hanan, Barry B; Blichert-Toft, Janne; Pyle, Douglas G; Christie, David M


    The origin of the isotopic signature of Indian mid-ocean ridge basalts has remained enigmatic, because the geochemical composition of these basalts is consistent either with pollution from recycled, ancient altered oceanic crust and sediments, or with ancient continental crust or lithosphere. The radiogenic isotopic signature may therefore be the result of contamination of the upper mantle by plumes containing recycled altered ancient oceanic crust and sediments, detachment and dispersal of continental material into the shallow mantle during rifting and breakup of Gondwana, or contamination of the upper mantle by ancient subduction processes. The identification of a process operating on a scale large enough to affect major portions of the Indian mid-ocean ridge basalt source region has been a long-standing problem. Here we present hafnium and lead isotope data from across the Indian-Pacific mantle boundary at the Australian-Antarctic discordance region of the Southeast Indian Ridge, which demonstrate that the Pacific and Indian upper mantle basalt source domains were each affected by different mechanisms. We infer that the Indian upper-mantle isotope signature in this region is affected mainly by lower continental crust entrained during Gondwana rifting, whereas the isotope signature of the Pacific upper mantle is influenced predominantly by ocean floor subduction-related processes.

  2. Upper Mantle Structure beneath the Chinese Capital Region from Teleseismic Finite-Frequency Seismic Tomography (United States)

    Yang, F.; Huang, J.


    In this study, we applied the finite-frequency seismic tomography(FFST) to teleseismic waveform data to determine 3-D P-wave velocity structure of the upper mantle under the Chinese capital region. The seismic waveform data from more than 300 teleseismic events recorded by the Chinese digital Capital Seismic Network during the period from September 2003 to December 2005 was used in this study. We obtained 18499 high accuracy P-wave relative travel-times by filtering these waveform data on the vertical component into high-, intermediate-, low-frequency bands (1.0-2.0, 0.1-1.0 and 0.05-0.1 hz, respectively) and the multi-channel waveform cross correlation measurement. The 3-D Fréchet sensitivity kernels were calculated by paraxial approximation for each frequency band. We established observation equations with these measured relative travel-times and 3-D Fréchet sensitivity kernels and then determined the 3-D velocity structure by inverting the observation equations. Our results show there are distinct differences of deep velocity structure down to 150 km depth under the four tectonic units of present study region. The Yanshan uplift exhibited the high velocity(high-V) feature. Under the Taihangshan uplift, broad low velocity(low-V) are visible, but it also shows up as small high-V anomalies. A large scale prominent low-V anomaly was revealed in the shallow upper mantle under the North China basin and Bohai bay. In the North China basin the low-V anomaly generally extend from 50 km to 150 km depth, but in the Bohai bay, this low-V anomaly gradually extend down to 200 km depth. The depth of this low-V anomaly is 50-70 km under the North China basin and Bohai bay, which is consistent with the depth of high conductivity layer in the upper mantle determined by the measurement of magnetotelluric sounding and heat flow. This result shows lithosphere thinning in the North China basin and Bohai bay. Most of large earthquakes occurred in the Zhangjiakou-Penglai fault zone

  3. Upper mantle structure of the Pacific and Philippine Sea plates revealed by seafloor seismic array observations (United States)

    Isse, Takehi; Shiobara, Hajime; Suetsugu, Daisuke; Sugioka, Hiroko; Ito, Aki


    Seismic tomography studies have revealed the structure and dynamics of Earth's interior since the 1980s. However, the spatial resolution of the oceanic region is not good enough caused by sparse distribution of the seismic stations. The observations with broadband ocean-bottom seismographs (BBOBSs) since the 2000s enabled us to obtain seismic tomography models with higher spatial resolution. Our Japanese BBOBS group deployed more than 100 BBOBSs in the Pacific Ocean and obtained a high-resolution (300-500 km) three-dimensional shear wave velocity structure in the upper mantle beneath northwestern and south Pacific Ocean by using surface wave tomography technique. In the northwestern Pacific Ocean, where the Pacific plate subducts beneath the Philippine Sea plate, we found that the shear wave structure in the Philippine sea plate is well correlated with the seafloor age in the upper 120 km, three separate slow anomalies in the mantle wedge at depth shallower than 100 km beneath the Izu-Bonin-Mariana arc, which have a close relationship with the three groups of frontal and rear arc volcanoes having distinct Sr, Nd, and Pb isotope ratios, and that the Philippine Sea plate, which is a single plate, shows very large lateral variations in azimuthal and radial anisotropies compared with the Pacific plate. In the South Pacific Ocean, where midplate hotspots are concentrated, we found that the localized slow anomalies are found near hotspots in the upper mantle, estimated thickness of the lithosphere is about 90 km in average and is thinned by ~20 km in the vicinity of hotspots, which may represent thermal erosion due to mantle plumes.

  4. Dynamics and Upper Mantle Structure Beneath the Northwestern Andes: Subduction Segments, Moho Depth, and Possible Relationships to Mantle Flow (United States)

    Monsalve, G.; Yarce, J.; Becker, T. W.; Porritt, R. W.; Cardona, A.; Poveda, E.; Posada, G. A.


    The northwestern South American plate shows a complex tectonic setting whose causes and relationship to mantle structure are still debated. We combine different techniques to elucidate some of the links between slabs and surface deformation in Colombia. Crustal structure beneath the Northern Andes was inferred from receiver functions where we find thicknesses of nearly 60 km beneath the plateau of the Eastern Cordillera and underneath the southern volcanic area of the Central Cordillera. We infer that such crustal thickening resulted from shortening, magmatic addition, and accretion-subduction. Analyses of relative teleseismic travel time delays and estimates of residual surface topography based on our new crustal model suggest that there are at least two subduction segments underneath the area. The Caribbean slab lies at a low angle beneath northernmost Colombia and steepens beneath the Eastern Cordillera. Such steepening is indicated by negative travel time relative residuals in the area of the Bucaramanga Nest, implying a cold anomaly in the upper mantle, and by positive residual topography just off the east of this area, perhaps generated by slab-associated return flow. Results for the western Andes and the Pacific coastal plains are consistent with "normal" subduction of the Nazca plate: travel time relative residuals there are predominantly positive, and the residual topography shows an W-E gradient, going from positive at the Pacific coastline to negative at the Magdalena Valley, which separates the eastern cordillera from the rest of the Colombian Andean system. Azimuthal analysis of relative travel time residuals further suggests the presence of seismically slow materials beneath the central part of the Eastern Cordillera. Azimuthal anisotropy from SKS splitting in that region indicates that seismically fast orientations do not follow plate convergence, different from what we find for the western Colombian Andes and the Caribbean and Pacific coastal plains

  5. Double layering of a thermochemical plume in the upper mantle beneath Hawaii (United States)

    Ballmer, M. D.; Ito, G.; Wolfe, C. J.; Cadio, C.; Solomon, S. C.


    Volcanism far from plate boundaries has traditionally been explained by "classical" plume theory. Classical plumes are typically described as narrow thermal upwellings that rise through the entire mantle to be deflected into a thin (Iceland, are indeed well explained by near-classical thermal plumes. High-resolution seismic velocity images obtained from the PLUME project support the concept of a deep-rooted mantle plume beneath the Hawaiian hotspot. However, in detail these images challenge traditional concepts inasmuch as they indicate a low-velocity body in the upper mantle that is too thick (~400 km) and asymmetric to be interpreted as a classical pancake. Classical plume theory is, moreover, inconsistent with several geochemical characteristics of Hawaiian magmas, which point to a heterogeneous mantle source involving mafic lithologies such as eclogite and not an exclusively thermal (i.e., isochemical) origin¹. To explore the dynamical and melting behavior of plumes containing a substantial fraction (~15%) of eclogite, we performed three-dimensional numerical simulations of thermochemical convection. Relative to ambient-mantle peridotite, eclogite is intrinsically dense. This density contrast is sensitive to phase changes in the upper mantle; the contrast peaks at 410-300 km and lessens at about 250-190 km depth, where eclogite is subsequently removed by melting. For a plume core with an eclogite content >12%, these effects locally increase the density beyond that of the ambient mantle. Therefore, the upwelling column forms a broad and thick pool at depths of 450-300 km (which we term the deep eclogite pool, or DEP). As the DEP is well supported by the deeper stem of the plume and its non-eclogitic outskirts, it inflates to release a shallow thermal plume. This latter plume sustains hotspot volcanism and feeds a hot shallow pancake that compensates the seafloor swell. Our model predictions reconcile a range of characteristics for Hawaiian volcanism. We find

  6. The Upper Mantle Flow Field around South-Africa as Reflected by Isotopic Provinciality (United States)

    Meyzen, C.; Blichert-Toft, J.; Ludden, J.; Humler, E.; Mevel, C.; Albarede, F.


    Isotopic studies of MORB have established the existence of broad isotopic provinces within the underlying asthenosphere, such as in the Indian Ocean (DUPAL). How these features relate to mantle circulation is, however, still unknown. The steepness of the transition between such isotopic provinces will define the geometry of the velocity field in the upper mantle. In this respect, the transition between the Indian and South Atlantic provinces, two domains that are isotopically contrasted, should be readily identifiable over this long ridge segment. Here, we present Hf isotope data for 60 samples dredged along the SWIR between 35° and 69°E. The new Hf isotope data show that the Indian asthenosphere does not spill directly into the South Atlantic upper mantle: the general decreasing southward gradient observed for ^{176}Hf/^{177}Hf down the mid- Atlantic Ridge, and also for Sr isotopes and model Th/U ratios (derived from Pb isotopes), is overprinted by material with radiogenic Sr, unradiogenic Hf and high Th/U. The Indian domain grades into the South Atlantic around Bouvet, while the South Atlantic collides with the Atlantic province around Tristan. We interpret these features to represent fronts between three adjacent isotopic provinces similar to what has been suggested for the Australian-Antarctic Discordance. The common DUPAL signature of MORB and OIB from the Indian province and the geochemistry of Gulf of Aden MORB and the Afar plume suggest that the source of this distinctive mantle component is deep and lies to the north of the province. This is also what the three-dimensional flow field computed by Behn et al. (2004) from shear-wave splitting shows with a major lower mantle upwelling radiating at the base of the asthenosphere under the Afar plume. Lower mantle gushing out from this source flows southward unimpeded along the Indian ridges, whereas it only reaches the South Atlantic ridge after first having been deflected under the deep roots of the South

  7. Variation of the upper mantle velocity structure along the central-south Andes (United States)

    Liang, Xiaofeng; Sandvol, Eric; Shen, Yang; Gao, Haiying


    Variations in the subduction angle of the Nazca plate beneath the South American plate has lead to different modes of deformation and volcanism along the Andean active margin. The volcanic gap between the central and southern Andean volcanic zones is correlated with the Pampean flat-slab subduction zone, where the subducting Nazca slab changes from a 30-degree dipping slab beneath the Puna plateau to a horizontal slab beneath the Sierras Pampeanas, and then to a 30-degree dipping slab beneath the south Andes from north to south. The Pampean flat-slab subduction correlates spatially with the track of the Juan Fernandez Ridge, and is associated with the inboard migration of crustal deformation. A major Pliocene delamination event beneath the southern Puna plateau has previously been inferred from geochemical and geological and preliminary geophysical data. The mechanisms for the transition between dipping- and flat-subduction slab and the mountain building process of the central Andean plateau are key issues to understanding the Andean-type orogenic process. We use a new frequency-time normalization approach with non-linear stacking to extract very-broadband (up to 300 second) empirical Green's functions (EGFs) from continuous seismic records. The long-period EGFs provide the deeper depth-sensitivity needed to constrain the mantle structure. The broadband waveform data are from 393 portable stations of four temporary networks: PUNA, SIEMBRA, CHARGE, RAMP, East Sierras Pampeanas, BANJO/SEDA, REFUCA, ANCORP, and 31 permanent stations accessed from both the IRIS DMC and GFZ GEOFON DMC. A finite difference waveform propagation method is used to generate synthetic seismograms from 3-D velocity model. We use 3-D traveltime sensitivity kernels, and traveltime residuals measurement by waveform cross-correlation to directly invert the upper mantle shear-wave velocity structure. The preliminary model shows strong along-strike velocity variations within in the mantle wedge and

  8. Shear wave velocity, seismic attenuation, and thermal structure of the continental upper mantle (United States)

    Artemieva, I.M.; Billien, M.; Leveque, J.-J.; Mooney, W.D.


    Seismic velocity and attenuation anomalies in the mantle are commonly interpreted in terms of temperature variations on the basis of laboratory studies of elastic and anelastic properties of rocks. In order to evaluate the relative contributions of thermal and non-thermal effects on anomalies of attenuation of seismic shear waves, QS-1, and seismic velocity, VS, we compare global maps of the thermal structure of the continental upper mantle with global QS-1 and Vs maps as determined from Rayleigh waves at periods between 40 and 150 S. We limit the comparison to three continental mantle depths (50, 100 and 150 km), where model resolution is relatively high. The available data set does not indicate that, at a global scale, seismic anomalies in the upper mantle are controlled solely by temperature variations. Continental maps have correlation coefficients of temperatures: most cratonic regions show high VS and QS and low T, while most active regions have seismic and thermal anomalies of the opposite sign. The strongest inverse correlation is found at a depth of 100 km, where the attenuation model is best resolved. Significantly, at this depth, the contours of near-zero QS anomalies approximately correspond to the 1000 ??C isotherm, in agreement with laboratory measurements that show a pronounced increase in seismic attenuation in upper mantle rocks at 1000-1100 ??C. East-west profiles of VS, QS and T where continental data coverage is best (50??N latitude for North America and 60??N latitude for Eurasia) further demonstrate that temperature plays a dominant, but non-unique, role in determining the value of lithospheric VS and QS. At 100 km depth, where the resolution of seismic models is the highest, we compare observed seismic VS and QS with theoretical VST and QST values, respectively, that are calculated solely from temperature anomalies and constrained by experimental data on temperature dependencies of velocity and attenuation. This comparison shows that

  9. Anisotropy in the Pacific Upper Mantle from Inversion of a Combined Global and Regional Dispersion Dataset (United States)

    Eddy, C. L.; Ekstrom, G.; Nettles, M.; Gaherty, J. B.


    Models of seismic anisotropy in oceanic regions have the potential to provide information about the geometry of strain and flow in the mantle, the specific nature of the lithosphere-asthenosphere boundary, and the possible presence of partial melt in the asthenosphere. In order to investigate these and other questions, we are developing a three-dimensional model of the anisotropic velocity structure of the Pacific upper mantle. We use measurements of fundamental-mode dispersion for Rayleigh and Love waves traversing oceanic paths. These observations are drawn from the waveform dataset used to construct the global dispersion model GDM52. To supplement this global dataset, we make additional measurements of surface-wave dispersion on waveform data from the NoMelt experiment, a deployment of broadband ocean-bottom seismometers on ~70 Ma lithosphere between the Clarion and Clipperton fracture zones in the central Pacific. The shorter oceanic paths sampled by this dataset help improve the resolution of the velocity model in the Pacific by providing a regional constraint on the larger plate-scale model. We invert phase-velocity maps from the combined dataset for velocity structure at depth. An oceanic reference model is used to compute the sensitivity kernels. We invert simultaneously for isotropic velocity structure and radial and azimuthal anisotropy beneath the Pacific Basin. The resulting anisotropic velocity model will improve constraints on olivine fabrics and strain geometries in the oceanic upper mantle.

  10. The Influence of Water on Seismic Wave Attenuation in the Upper Mantle (United States)

    David, E. C.; Jackson, I.; Faul, U.; Berry, A.


    Trace amounts of water, present as protons structurally bound in olivine crystal defects, are inferred to significantly enhance the low-strain solid-state viscoelastic relaxation responsible for attenuation and dispersion of seismic waves in the upper mantle. This inferrence is supported by recent observation of water weakening at moderate compressive strains in synthetic, water-undersaturated aggregates (Faul et al., in preparation). In these fine-grained olivine polycrystals of Fo90 composition, doped with 0.02wt% TiO2, "water" is incorporated in the remarkably stable Ti-clinohumite defect. Such synthetic olivine specimens reproduce the infrared spectra of natural mantle olivines (Berry et al., 2005), and present the advantage of being melt-free and of low dislocation density. The water contents in such synthetic polycrystalline olivine aggregates, which can be quantitatively measured by Fourier Transform Infrared Spectroscopy (FTIR), range up to 90 ppm, and are thus representative of water-undersaturated conditions in the upper mantle. We will report here the outcome of torsional-oscillation tests,in which attenuation and shear modulus were measured at seismic frequencies (mHz-Hz) and various temperatures up to 1300C on Pt-encapsulated, Ti-doped olivine specimens, enclosed within a mild-steel jacket.

  11. Anisotropic Structure of the Upper Mantle, Imaged with Surface and S Waveform Tomography (United States)

    Schaeffer, A. J.; Lebedev, S.


    The rapid recent expansion of global and regional seismic networks has paved the way for a new generation of tomographic models, with significantly improved resolution at global and regional scales. We present a new global model of shear velocity and azimuthal anisotropy in the upper mantle, down to the base of the transition zone. The model is constrained by an unprecedentedly large waveform dataset collected from over 2000 stations of GSN and affiliates, USArray, VEBSN, CNSN, PASSCAL experiments, and other networks with data available from IRIS, ORFEUS, and GFZ data centers. Applying the accurate and efficient automated multimode inversion of surface- and S-wave forms to this massive dataset, we generated linear constraints on elastic structure within approximate sensitivity volumes between individual source-receiver pairs, with respect to a 3D reference model. The full waveform inversions resulted in more than one million successful fits (one million seismograms), with structural information extracted from both the fundamental and higher modes. The linear equations were then simultaneously solved for a high-resolution, 3D model of shear velocity and azimuthal anisotropy in the upper mantle. In continental domains, clearly identifiable boundaries between different tectonic features such as basins and relic mountain ranges are readily observable, as well as the signature of deep cratonic roots versus juvenile accretionary margins. Both active and fossil subduction zones are marked by slab signatures deep in the upper mantle and extending through the transition zone. In oceanic regions, largest mid-ocean-ridge anomalies indicative of melting terminate at depths of 100-120 km, with evidence for vertical flow in the upper mantle observed through a combination of VSV, VSH, and azimuthal anisotropy. Spatio-temporal evolution (cooling and thickening) of lithosphere away from the spreading ridges matches the signature expected from geodynamic and thermal modeling. The

  12. Upper Mantle of the Central Part of the Russian Platform by Receiver Function Data. (United States)

    Goev, Andrey; Kosarev, Grigoriy; Sanina, Irina; Riznichenko, Oksana


    The study of the upper mantle of the Russian Platform (RP) with seismic methods remains limited due to the lack of broadband seismic stations. Existing velocity models have been obtained by using the P-wave travel-times from seismic events interpreted as explosions recorded at the NORSAR array in 1974-75 years. Another source of information is deep seismic sounding data from long-range profiles (exceeding 3000 km) such as QUARTZ, RUBIN-1 and GLOBUS and peaceful nuclear explosions (PNE) as sources. However, the data with the maximum distances larger than 1500 km have been acquired on the RP and only in the northern part. Being useful, these velocity models have low spatial resolution. This study analyzes and integrates all the existing RP upper mantle velocity models with the main focus on the central region. We discuss the completeness of the RP area of the LITHO 1.0 model. Based on results of our analysis, we conclude that it is necessary to get up-to-date velocity models of the upper mantle using broadband stations located at the central part of the RP using Vp/Vs ratio data and anisotropy parameters for robust estimation of the mantle boundaries. By applying the joint inversion of receiver-function (RF) data, travel-time residuals and dispersion curves of surface waves we get new models reaching 300 km depth at the locations of broadband seismic stations at the central part of the RP. We used IRIS stations OBN, ARU along with MHV and mobile array NOV. For each station we attempt to determine thickness of the lithosphere and to locate LVL, LAB, Lehman and Hales boundaries as well as the discontinuities in the transition zones at the depth of 410 and 660 km. Also we investigate the necessity of using short-period and broadband RF separately for more robust estimation of the velocity model of the upper mantle. This publication is based on work supported by the Russian Foundation for Basic Research (RFBR), project 15-05-04938 and by the leading scientific school NS

  13. Saudi Arabian seismic-refraction profile: A traveltime interpretation of crustal and upper mantle structure (United States)

    Mooney, W. D.; Gettings, M. E.; Blank, H. R.; Healy, J. H.


    The crustal and upper mantle compressional-wave velocity structure across the southwestern Arabian Shield has been investigated by a 1000-km-long seismic refraction profile. The profile begins in Mesozoic cover rocks near Riyadh on the Arabian Platform, trends southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan, and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, and six shot points were used, including one in the Red Sea. Two-dimensional ray-tracing techniques, used to analyze amplitude-normalized record sections indicate that the Arabian Shield is composed, to first order, of two layers, each about 20 km thick, with average velocities of about 6.3 km/s and 7.0 km/s, respectively. West of the Shield-Red Sea margin, the crust thins to a total thickness of less than 20 km, beyond which the Red Sea shelf and coastal plain are interpreted to be underlain by oceanic crust. A major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition. Elsewhere along the profile, several high-velocity anomalies in the upper crust correlate with mapped gneiss domes, the most prominent of which is the Khamis Mushayt gneiss. Based on their velocities, these domes may constitute areas where lower crustal rocks have been raised some 20 km. Two intracrustal reflectors in the center of the Shield at 13 km depth probably represent the tops of mafic intrusives. The Mohorovičić discontinuity beneath the Shield varies from a depth of 43 km and mantle velocity of 8.2 km/s in the northeast to a depth of 38 km and mantle velocity of 8.0 km/s depth in the southwest near the Shield-Red Sea transition. Two velocity discontinuities occur in the upper mantle, at 59 and 70 km depth. The crustal and upper mantle velocity structure of the Arabian Shield is interpreted

  14. The upper mantle structure of the Tibetan Plateau and its implication for the continent-continent collision

    Institute of Scientific and Technical Information of China (English)

    WU; Qingju; ZENG; Rongsheng; ZHAO; Wenjin


    The upper mantle structures of Himalayas-Tibet have been obtained from the migration of receiver functions of the teleseismic events recorded by INDEPTH-III. The result of migration imaging shows a dipping interface subducting northward from the depth of 100 km to the 410-km discontinuity underneath southern Tibet. It indicates that the lithospheric mantle of the Indian continent had been detached from the crust and deeply subducted to the upper mantle of Eurasia during the Indo-Eurasian collision. This kind of continent-continent collision process is fundamentally different from the oceanic collision.

  15. Small-scale upper mantle flow during the initiation of craton destruction (United States)

    Zhao, Liang; Wang, Kun; Xu, Xiaobing


    The North China Craton (NCC) is an old craton which has experienced multi-episodic tectonism with surrounding plates. Bordered to the north by Xing'an-Mongolian Orogenic Belt, to the south by Qingling-Dabie-Sulu Orogen and to the far-east by (Paleo-) Pacific plate, the NCC has lost the cratonic properties within its eastern part. Evidently, the initiation and mechanism of craton destruction attract tremendous attention and remain hot debated. During the Mesozoic to the Cenozoic, the northeastern part of the NCC has been intensively revoked, along with the transition from NE shortening to NW-SE extension. The subduction of Paleo-Pacific plate becomes the prime suspect due to the same kinematic direction. Here we present a hybrid shear wave splitting measurement to investigate the mantle deformation of the NCC, and intend to constrain geodynamic process during the initiation of craton destruction. The SKS waveform data is recorded from 60 broadband stations with an average spacing of 15 km. We employ the traditional routine method to obtain fast polarization directions (FPDs, Φ) and delay times (δt) for the teleseismic events with epicenter range in 85°-115°. One may often have troubles in delimiting SKS and S wave with regard to the events at distances NCC, the east-end nearly E-W FPD is possibly owing to the fossil anisotropy in the lithosphere during the N-S shortening in Jurassic. The other is located in the Solonker suture zone beneath where the Moho and lithosphere and asthenosphere boundary (LAB) have sharp variation in depth. It suggests that the subduction of Pacific plate apparently reactivates the upper mantle of the north edge of the NCC but has minor effects westwards. The inconsistency in FPDs may result from small-scale mantle flow in the upper mantle, which could be the dominant operating mode of the Pacific subduction during the initiation of cratonic destruction.

  16. Imaging the Mediterranean upper mantle by p- wave travel time tomography

    Directory of Open Access Journals (Sweden)

    A. Morelli


    Full Text Available Travel times of P-waves in the Euro-Mediterranean region show strong and consistent lateral variations, which can be associated to structural heterogeneity in the underlying crust and mantle. We analyze regional and tele- seismic data from the International Seismological Centre data base to construct a three-dimensional velocity model of the upper mantle. We parameterize the model by a 3D grid of nodes -with approximately 50 km spacing -with a linear interpolation law, which constitutes a three-dimensional continuous representation of P-wave velocity. We construct summary travel time residuals between pairs of cells of the Earth's surface, both inside our study area and -with a broader spacing -on the whole globe. We account for lower mantle heterogeneity outside the modeled region by using empirical corrections to teleseismic travel times. The tomo- graphic images show generai agreement with other seismological studies of this area, with apparently higher detail attained in some locations. The signature of past and present lithospheric subduction, connected to Euro- African convergence, is a prominent feature. Active subduction under the Tyrrhenian and Hellenic arcs is clearly imaged as high-velocity bodies spanning the whole upper mantle. A clear variation of the lithospheric structure beneath the Northem and Southern Apennines is observed, with the boundary running in correspon- dence of the Ortona-Roccamonfina tectonic lineament. The western section of the Alps appears to have better developed roots than the eastern, possibly reflecting à difference in past subduction of the Tethyan lithosphere and subsequent continental collision.

  17. Experimental investigation of flow-induced fabrics in rocks at upper-mantle pressures. Application to understanding mantle dynamics and seismic anisotropy

    Energy Technology Data Exchange (ETDEWEB)

    Durham, William B. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)


    The goal of this collaborative research effort between W.B. Durham at the Massachusetts Institute of Technology (MIT) and D.L. Kohlstedt and S. Mei at the University of Minnesota (UMN) was to exploit a newly developed technology for high-pressure, high-temperature deformation experimentation, namely, the deformation DIA (D-DIA), to determine the deformation behavior of a number of important upper mantle rock types including olivine, garnet, enstatite, and periclase. Experiments were carried out under both hydrous and anhydrous conditions and at both lithospheric and asthenospheric stress and temperature conditions. The result was a group of flow laws for Earth’s upper mantle that quantitatively describe the viscosity of mantle rocks from shallow depths (the lithosphere) to great depths (the asthenosphere). These flow laws are fundamental for modeling the geodynamic behavior and heat transport from depth to Earth’s surface.-

  18. Experimental investigation of flow-induced fabrics in rocks at upper-mantle pressures: Application to understanding mantle dynamics and seismic anisotropy

    Energy Technology Data Exchange (ETDEWEB)

    Kohlstedt, David L. [Univ. of Minnesota, Minneapolis, MN (United States)


    The goal of this collaborative research effort between W.B. Durham at the Massachusetts Institute of Technology (MIT) and D.L. Kohlstedt and S. Mei at the University of Minnesota (UMN) was to exploit a newly developed technology for high-pressure, high-temperature deformation experimentation, namely, the deformation DIA (D-DIA) to determine the deformation behavior of a number of important upper mantle rock types including olivine, garnet, enstatite, and periclase. Experiments were carried out under both hydrous and anhydrous conditions and at both lithospheric and asthenospheric stress and temperature conditions. The result was a group of flow laws for Earth’s upper mantle that quantitatively describe the viscosity of mantle rocks from shallow depths (the lithosphere) to great depths (the asthenosphere). These flow laws are fundamental for modeling the geodynamic behavior and heat transport from depth to Earth’s surface.

  19. Upper mantle P-wave velocity structure beneath northern Lake Malawi and the Rungwe Volcanic Province, East Africa (United States)

    Grijalva, A. N.; Kachingwe, M.; Nyblade, A.; Shillington, D. J.; Gaherty, J. B.; Ebinger, C. J.; Accardo, N. J.; O'Donnell, J. P.; Mbogoni, G. J.; Mulibo, G. D.; Ferdinand, R.; Chindandali, P. R. N.; Mphepo, F.


    A recent deployment of 55 broadband seismic stations around the northern Lake Malawi rift as part of the SEGMeNT project have provided a new dataset for imaging crustal and upper mantle structure beneath the Rungwe volcanic center and northern most segment of the Lake Malawi Rift. The goal of our study is to characterize the upper mantle velocity structure and determine to what extent the rifting has been influenced by magmatism. P relative arrival time residuals have been obtained for 115 teleseismic events with magnitudes > 5 in the 30 - 90 degree distance range. They are being tomographically inverted, together with travel time residuals from previous deployments for a 3-D velocity model of the upper mantle. Preliminary results indicate a low wave speed anomaly in the uppermost mantle beneath the Rungwe volcanics. Future results will determine if this anomaly exists under the northern Lake Malawi rift.

  20. Crustal and upper mantle velocity structure of the Salton Trough, southeast California (United States)

    Parsons, T.; McCarthy, J.


    This paper presents data and modelling results from a crustal and upper mantle wide-angle seismic transect across the Salton Trough region in southeast California. The Salton Trough is a unique part of the Basin and Range province where mid-ocean ridge/transform spreading in the Gulf of California has evolved northward into the continent. In 1992, the U.S. Geological Survey (USGS) conducted the final leg of the Pacific to Arizona Crustal Experiment (PACE). Two perpendicular models of the crust and upper mantle were fit to wide-angle reflection and refraction travel times, seismic amplitudes, and Bouguer gravity anomalies. The first profile crossed the Salton Trough from the southwest to the northeast, and the second was a strike line that paralleled the Salton Sea along its western edge. We found thin crust (???21-22 km thick) beneath the axis of the Salton Trough (Imperial Valley) and locally thicker crust (???27 km) beneath the Chocolate Mountains to the northeast. We modelled a slight thinning of the crust further to the northeast beneath the Colorado River (???24 km) and subsequent thickening beneath the metamorphic core complex belt northeast of the Colorado River. There is a deep, apparently young basin (???5-6 km unmetamorphosed sediments) beneath the Imperial Valley and a shallower (???2-3 km) basin beneath the Colorado River. A regional 6.9-km/s layer (between ???15-km depth and the Moho) underlies the Salton Trough as well as the Chocolate Mountains where it pinches out at the Moho. This lower crustal layer is spatially associated with a low-velocity (7.6-7.7 km/s) upper mantle. We found that our crustal model is locally compatible with the previously suggested notion that the crust of the Salton Trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust. However, we observe an apparently magmatically emplaced lower crust to the northeast, outside of the Salton Trough, and propose that this layer in part

  1. Hafnium and iron isotopes in early Archean komatiites record a plume-driven convection cycle in the Hadean Earth (United States)

    Nebel, Oliver; Campbell, Ian H.; Sossi, Paolo A.; Van Kranendonk, Martin J.


    Archean (>2.5 billion years) komatiites are considered expressions of mantle plumes that originate from and thereby sample the lowermost mantle overlying the Earth's core. Some komatiites have reported Hf isotope signatures that require a mantle source with a time-integrated Lu/Hf that is appreciably higher than average modern depleted mantle. The systematic study of the time and locus of parent-daughter fractionation of the mantle sources of these komatiites potentially constrains differentiation processes in the early Earth, and subsequent distribution and storage of early mantle reservoirs. We present radiogenic Hf and stable Fe isotopes for a series of komatiites from the Pilbara craton in Western Australia (aged 3.5 to 2.9 Ga). After careful evaluation of the effects of alteration, we find that pristine samples are characterised by a light Fe isotope mantle source and initial 176Hf/177Hf well above the age-corrected depleted mantle. Taken together these observations require a component of an old, melt-depleted reservoir in their mantle source. The Hf isotope signature of this component appears to be complementary to the first terrestrial crust, as preserved in Hadean (i.e., >4 Ga) detrital zircon cores, suggesting a causal relationship and a Hadean age for this depletion event. We propose that this Early Refractory Reservoir (ERR) is the residue formed by deep melting in hot Hadean mantle plumes, which then accumulated at the base of the first crust. Parts of this primordial lithosphere were destabilised and sank to the core-mantle boundary in cold drips and subsequently returned in hot mantle plumes, whose thermal capacity allows melting of such refractory mantle with its archetype isotope signature. The cycling of this material via cold drips and hot plumes suggests a plume-dominated convection prior to ∼3.9 Ga, which is then replaced by Archean-style plate tectonics.

  2. Sulfide Composition and Melt Stability Field in the Earth's Upper Mantle (United States)

    Zhang, Z.; Hirschmann, M. M.


    In the Earth's upper mantle, sulfur occurs chiefly as (Fe, Ni)xS minerals and melts with near-monosulfide stoichiometries. These could have substantial influence on geochemical and geophysical properties of the Earth's interior. For example, sulfide mineral and melts are the major carriers of chalcophile and platinum group elements (PGEs) and sulfide melts are potentially responsible for mantle geophysical anomalies, as their physical properties (higher density, surface tension, electrical conductivity and lower melting points) differ greatly from those of silicates. Sulfide melts are a potential sink for reduced mantle carbon and perhaps be associated with carbon transport, including diamond precipitation. Sulfides may be molten in large parts of the mantle, but this is determined in part by sulfide composition, which is in turn a product of Fe-Ni exchange with olivine and of the effect of sulfur, oxygen, and carbon fugacities on metal/anion ratios of melts. Melting experiments define the monosulfide (Fe0.35Ni0.12Cu0.01S0.52) solidus from 1-8 GPa at carbon-free and graphite saturated conditions. The resulting carbon-free solidus is below the mantle adiabat to depths of at least 300 km, but does not indicate sulfide melting in continental lithosphere. In contrast, the graphite saturated solidus indicates melting in the lithosphere at 6-7 GPa (~200 km), close to the source conditions typical of diamond formation. To determine the composition of sulfide equilibrated with olivine, we performed experiments on monosulfide-olivine (crushed powders from San Carlos single crystal) under 2 GPa, 1400 ◦C. Our preliminary results suggests that Fe-Ni distribution coefficients KD, defined by (Ni/Fe)sulfide/(Ni/Fe)olivine, have significantly lower values than those determined previously at one atmosphere (Doyle and Naldrett 1987; Fleet and MacRae 1987; Gaetani and Grove 1997). This indicates that sulfide equilibrated with olivine in the mantle is richer in Fe than former

  3. Seismic Structure and Geodynamic Evolution of the Lithosphere and Upper Mantle in the Pannonian - Carpathian Region (United States)

    Houseman, G.; Stuart, G.; Dando, B.; Hetenyi, G.; Lorinczi, P.; Brueckl, E.; Hegedus, E.; Radovanovic, S.; Brisbourne, A.


    The Pannonian Basin is the largest of a group of Miocene-age extensional basins within the arc of the Alpine-Carpathian Mountain Ranges. These basins are extensional in origin, but the surrounding Carpathians result from sustained convergence during and since the period of active extension. A significant part of the mantle lithosphere here has been replaced, as gravitational instability caused an overturn of the upper mantle. The Carpathian Basins Project (CBP) is a major international broadband seismology experiment, supported by geodynamical modelling and designed to improve our understanding of the structure and evolution of the lithosphere and upper mantle beneath the Pannonian and Vienna Basins. Between 2005 and 2007 we deployed 56 portable broadband seismic stations in Austria, Hungary and Serbia, spanning the Vienna Basin and the western part of the Pannonian Basin. Arrival time residuals from teleseismic earthquakes are delayed by about 0.8 sec in the Vienna Basin and early by a similar amount in southwest Hungary. Tomographic inversion of the travel time residuals shows relatively fast P-wave velocities in the upper mantle beneath the western Pannonian Basin and slow P-wave velocities beneath the West Carpathians. Seismic anisotropy (SKS) measurements reveal an intriguing pattern of lithospheric anisotropy: in the north-west the fast direction is generally elongated EW, perpendicular to the shortening direction across the Alps. Across the Vienna Basin the fast direction is NW-SE, perpendicular to the major bounding fault systems. Across the Pannonian Basin the dominant fast direction is EW, but in several locations the vectors are rotated toward NW-SE. The Mid-Hungarian Line, a major strike-slip structure already clearly identified in the gravity field, also is associated with abrupt changes in the azimuth of lithospheric anisotropy. Receiver function analysis of the seismic discontinuity at 670 km shows significant structure on scales of order 100 km, and

  4. Upper mantle discontinuity beneath the SW-Iberia peninsula: A multidisciplinary view. (United States)

    Palomeras, Imma; de Lis Mancilla, Flor; Ayarza, Puy; Afonso, Juan Carlos; Diaz, Jordi; Morales, Jose; Carbonell, Ramon; Topoiberia Working Group


    Evidence for an upper mantle discontinuity located between 60 and 70 km depth have been provided by different seismic data sets acquired in the Southern Iberian peninsula. First indications of such a discontinuity were obtained by the very long offsets seismic refraction shot gathers acquired within the DSS ILIHA project in the early 90's. Clear seismic events recoded by the dense wide-angle seismic reflection shot gathers of the IBERSEIS experiment (2003) provided further constraints on the depth of the discontinuity and first-order estimates of its physical properties beneath the Ossa Morena Zone. Furthermore, the normal incidence Vibroseis deep seismic reflection images of the ALCUDIA transect (2007) extends this structure to the northeast beneath the Central Iberian Zone. This transect images deep laterally discontinuous reflections at upper mantle travel times (19 s) that roughly correspond to depths within the range of 60-70 km. Receiver function studies of the passive seismic recordings acquired by the IBERARRAY (TOPOIBERIA projects) provides additional support for the existence of this upper mantle structure and suggests that this is a relatively large scale regional feature. Two major scenarios need to be addressed when discussing the origin and nature of this deep structure. One is the tectonic scenario in which the structure maybe be related to a major tectonic event such as an old subduction process and therefore represent an ancient slab. A second hypothesis, would relate this feature to a phase change in the mantle. This latter assumption requires this feature ought to be a broader scale boundary which could be identified by different seismic techniques. Reflectivity modeling carried out over the IBERSEIS wide angle reflection data concludes that the observed phase is consistent with an heterogeneous gradient zone located at, approximately, 61-72 km depth. A layered structure with alternating velocities within ranges 8.1 to 8.3 km/s is necessary in

  5. Sm/Nd Evolution of Upper Mantle and Continental Crust:Constraints on Gowth Rates of the Continental Crust

    Institute of Scientific and Technical Information of China (English)



    A new approach to the investigation of the Sm/Nd evolution of the upper mantle directly from the data on lherzolite xenoliths is described in this paper.It is demonstrated that the model age TCHUR of an unmetasomatic iherzolite zenolith ca represent the mean depletion age of its mantle source, thus presenting a correlation trend between fSm/Nd and the mean depletion age of the upper mantle from the data on xenoliths.This correlation trend can also be derived from the data on river suspended loads as well as from granitoids.Based on the correlation trend mentioned above and mean depletion ages of the upper mantle at various geological times, an evolution curve for the mean fSm/Nd value of the upper mantle through geological time has been established.It is suggested that the upwilling of lower mantle material into the upper mantle and the recycling of continental crust material during the Archean were more active ,thus maintaining fairly constantfSm/Nd and εNd values during this time period. Similarly ,an evolution curve for the mean fSm/Nd value of the continental crust through geological time has also been established from the data of continental crust material.In the light of both evolution curves for the upper mantle and continental crust ,a growth curve for the continental crust has been worked out ,suggesting that :(1)about 30%(in volume )of the present crust was present as the continental crust at 3.8 Ga ago ;(2)the growth rate was much lower during the Archean ;and (3)the Proterozoic is another major period of time during which the continental crust wsa built up .

  6. Shear velocity structure of the crust and upper mantle of Madagascar derived from surface wave tomography (United States)

    Pratt, Martin J.; Wysession, Michael E.; Aleqabi, Ghassan; Wiens, Douglas A.; Nyblade, Andrew A.; Shore, Patrick; Rambolamanana, Gérard; Andriampenomanana, Fenitra; Rakotondraibe, Tsiriandrimanana; Tucker, Robert D.; Barruol, Guilhem; Rindraharisaona, Elisa


    The crust and upper mantle of the Madagascar continental fragment remained largely unexplored until a series of recent broadband seismic experiments. An island-wide deployment of broadband seismic instruments has allowed the first study of phase velocity variations, derived from surface waves, across the entire island. Late Cenozoic alkaline intraplate volcanism has occurred in three separate regions of Madagascar (north, central and southwest), with the north and central volcanism active until Madagascar velocity structure. Shallow (upper 10 km) low-shear-velocity regions correlate well with sedimentary basins along the west coast. Upper mantle low-shear-velocity zones that extend to at least 150 km deep underlie the north and central regions of recent alkali magmatism. These anomalies appear distinct at depths <100 km, suggesting that any connection between the zones lies at depths greater than the resolution of surface-wave tomography. An additional low-shear velocity anomaly is also identified at depths 50-150 km beneath the southwest region of intraplate volcanism. We interpret these three low-velocity regions as upwelling asthenosphere beneath the island, producing high-elevation topography and relatively low-volume magmatism.

  7. New constraints on the textural and geochemical evolution of the upper mantle beneath the Styrian basin (United States)

    Aradi, Laszlo; Hidas, Károly; Zanetti, Alberto; János Kovács, István; Patkó, Levente; Szabó, Csaba


    Plio-Pleistocene alkali basaltic volcanism sampled sporadically the upper mantle beneath the Carpathian-Pannonian Region (CPR, e.g. [1]). Lavas and pyroclasts often contain mantle derived xenoliths, and the majority of them have been extensively studied [1], except the westernmost Styrian Basin Volcanic Field (SBVF, Eastern Austria and Slovenia). In the SBVF only a few volcanic centers have been studied in details (e.g. Kapfenstein & Tobaj). Based on these studies, the upper mantle beneath the SBVF is consists of dominantly high temperature, texturally and geochemically homogeneous protogranular spinel lherzolite. New major and trace element data from rock-forming minerals of ultramafic xenoliths, coupled with texture and deformation analysis from 12 volcanic outcrops across the SBVF, suggest that the lithospheric roots of the region are more heterogeneous than described previously. The studied xenoliths are predominantly lherzolite, amphibole is a common phase that replaces pyroxenes and spinels and proves modal metasomatism. Phlogopite coupled with apatite is also present in amphibole-rich samples. The texture of the xenoliths is usually coarse-grained and annealed with low abundance of subgrain boundaries in both olivine and pyroxenes. Olivine crystal preferred orientation (CPO) varies between the three most abundant one: [010]-fiber, orthogonal and [100]-fiber symmetry [2]. The CPO of pyroxenes is usually coherent with coeval deformation with olivine, however the CPO of amphibole is suggesting postkinematic epitaxial overgrowth on the precursor pyroxenes. According to equilibrium temperatures, the studied xenolith suite samples a broader temperature range (850-1100 °C) than the literature data, corresponding to mantle depths between 30 and 60 km, which indicates that the xenolith suite only represents the shallower part of the recent 100 km thick lithospheric mantle beneath the SBVF. The equilibrium temperatures show correlation with the varying CPO symmetries

  8. Seismic anisotropy in the oceanic upper mantle: Evidence from the Bay of Islands Ophiolite Complex

    Energy Technology Data Exchange (ETDEWEB)

    Christensen, N.I.; Salisbury, M.H.


    Olivine fabrics in 17 field-oriented ultramafics and mafics from three widely-spaced traverses in the Bay of Islands Ophiolite Complex, Newfoundland, display a remarkably uniform symmetry in which the olivine a crystallographic axes are aligned subprependicular to the sheeted dikes and the b and c axes lie within the plane of the sheeted dikes. The ultramafics studied consist entirely of tectonites; any olivine formed at the ridge crest by cumulus processes has since been re-oriented by translation gliding and/or syntectonic recrystallization. Deformation has extended from the ultramafics into the overlying gabbro, which suggests that in many oceanic regions the deepest levels of layer 3 consist of gabbroic tectonites. Compressional wave velocities computed from these petrofabrics display 5--6% anisotropy in the plane of the Mohorovicic discontinuity, with V/sub p/ fast parallel to the direction of spreading inferred from dike orientations. Since this pattern is identical to that observed for the oceanic upper mantle, it is concluded that the Bay of Islands Complex is a segment of oceanic crust and upper mantle. Shear wave velocity contours calculated from the same fabrics indicate that the upper mantle is nearly isotropic in terms of the maximum shear wave velocity, V/sub s//sub max/, but that the difference in velocity, s/, between shear waves of orthogonal polarization traveling in the same direction may be sufficiently large parallel to the intersection of the Mohorovicic discontinuity and the sheeted dikes to allow detection of two distinct shear wave arrivals.

  9. Crust and Upper Mantle Velocity Structure of the New Madrid Seismic Zone (United States)

    Nyamwandha, C. A.; Powell, C. A.; Langston, C. A.


    Detailed P wave velocity (Vp) and S wave velocity models (Vs) and Vp/Vs ratios for the crust and upper mantle associated with the New Madrid Seismic Zone (NMSZ) are presented. The specific study region spans latitude 34 to 39.5 degrees north and longitude 87 to 93 degrees west and extends to a depth of at least 500 km. The density of data from three networks - The Cooperative New Madrid Seismic Network (CNMSN) operated by CERI, the Earthscope transportable array (TA), and the FlexArray (FA) Northern Embayment Lithospheric Embayment (NELE) project stations - provides us with the opportunity to derive detailed velocity models for this region. We use arrival times from local and regional earthquakes and travel time residuals from teleseismic earthquakes recorded by the three networks from September 2011 to date. The teleseismic body wave arrival times are measured using an Automated and Interactive Measurement of Body Wave Arrival Times (AIMBAT) package (Lou et al., 2012). We perform a joint local and teleseismic inversion (Zhao et al.,1994) to determine the velocity structure. For the local events, the hypocenters are relocated iteratively in the inversion process using an efficient 3-D ray tracing technique. We image a significant low velocity anomaly in the upper mantle with a concentration at about 200 - 300 km depth and it is a consistent feature in both the Vp and Vs tomography results. Checkerboard tests show that the spatial resolution is high in the upper mantle especially for the Vp model. The spatial resolution in the crust is fairly high for most of the study area except at the edges and the southeastern part, which can be attributed to diminished local earthquake activity. We perform synthetic tests to isolate smearing effects and further confirm the features in the tomographic images. Vp/Vs ratios are determined for the portions of the model with highest resolution. Preliminary results indicate that significant Vp/Vs ratio variations are present only at

  10. Improving Earthquake-Explosion Discrimination using Attenuation Models of the Crust and Upper Mantle

    Energy Technology Data Exchange (ETDEWEB)

    Pasyanos, M E; Walter, W R; Matzel, E M; Rodgers, A J; Ford, S R; Gok, R; Sweeney, J J


    In the past year, we have made significant progress on developing and calibrating methodologies to improve earthquake-explosion discrimination using high-frequency regional P/S amplitude ratios. Closely-spaced earthquakes and explosions generally discriminate easily using this method, as demonstrated by recordings of explosions from test sites around the world. In relatively simple geophysical regions such as the continental parts of the Yellow Sea and Korean Peninsula (YSKP) we have successfully used a 1-D Magnitude and Distance Amplitude Correction methodology (1-D MDAC) to extend the regional P/S technique over large areas. However in tectonically complex regions such as the Middle East, or the mixed oceanic-continental paths for the YSKP the lateral variations in amplitudes are not well predicted by 1-D corrections and 1-D MDAC P/S discrimination over broad areas can perform poorly. We have developed a new technique to map 2-D attenuation structure in the crust and upper mantle. We retain the MDAC source model and geometrical spreading formulation and use the amplitudes of the four primary regional phases (Pn, Pg, Sn, Lg), to develop a simultaneous multi-phase approach to determine the P-wave and S-wave attenuation of the lithosphere. The methodology allows solving for attenuation structure in different depth layers. Here we show results for the P and S-wave attenuation in crust and upper mantle layers. When applied to the Middle East, we find variations in the attenuation quality factor Q that are consistent with the complex tectonics of the region. For example, provinces along the tectonically-active Tethys collision zone (e.g. Turkish Plateau, Zagros) have high attenuation in both the crust and upper mantle, while the stable outlying regions like the Indian Shield generally have low attenuation. In the Arabian Shield, however, we find that the low attenuation in this Precambrian crust is underlain by a high-attenuation upper mantle similar to the nearby Red

  11. Upper-mantle seismic discontinuities and the thermal structure of subduction zones (United States)

    Vidale, J.E.; Benz, H.M.


    The precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western United States. The boundary near 410 km depth is locally elevated, that near 660 km depressed. The depths of these boundaries, which mark phase transitions, provide an in situ thermometer in subduction zones: the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth. In addition, a reflector near 210 km depth may mark the bottom of the aesthenosphere.

  12. Upper-Mantle Shear Velocities beneath Southern California Determined from Long-Period Surface Waves


    Polet, J.; Kanamori, H.


    We used long-period surface waves from teleseismic earthquakes recorded by the TERRAscope network to determine phase velocity dispersion of Rayleigh waves up to periods of about 170 sec and of Love waves up to about 150 sec. This enabled us to investigate the upper-mantle velocity structure beneath southern California to a depth of about 250 km. Ten and five earthquakes were used for Rayleigh and Love waves, respectively. The observed surface-wave dispersion shows a clear Love/Rayleigh-wave d...

  13. Geodynamic and metabolic cycles in the Hadean

    Directory of Open Access Journals (Sweden)

    N. T. Arndt


    Full Text Available High-degree melting of hot dry Hadean mantle at ocean ridges and plumes resulted in a crust about 30km thick, overlain in places by extensive and thick mafic volcanic plateaus. Continental crust, by contrast, was relatively thin and mostly submarine. At constructive and destructive plate boundaries, and above the many mantle plumes, acidic hydrothermal springs at ~400°C contributed Fe and other transition elements as well as P and H2 to the deep ocean made acidulous by dissolved CO2 and minor HCl derived from volcanoes. Away from ocean ridges, submarine hydrothermal fluids were cool (≤100°C, alkaline (pH ~10, highly reduced and also H2-rich. Reaction of solvents in this fluid with those in ocean water was catalyzed in a hydrothermal mound, a natural self-restoring flow reactor and fractionation column made up of carbonates and freshly precipitated Fe-Ni sulfide and greenrust pores and bubbles, developed above the alkaline spring. Acetate and the amino acetate glycine were the main products, much of which was eluted to the ocean. Other organic byproducts were retained, concentrated and reacted within the compartments. These compartments comprising the natural hydrothermal reactor consisted partly of greigite (Fe5NiS8. It was from reactions between organic modules confined within these inorganic compartments that the first prokaryotic organism evolved. These acetogenic precursors to the Bacteria diversified and migrated down the mound and into the ocean floor to inaugurate the "deep biosphere". Once there the Bacteria, and the recently differentiated Archaea, were protected from cataclysmic heating events caused by large bolide impacts. Geodynamic forces led to the eventual obduction of the deep biosphere into the photic zone where, initially protected by a thin veneer of sediment, the use of solar energy was mastered and photosynthesis emerged. The further evolution to oxygenic photosynthesis was effected as catalytic [CaMn4+] bearing

  14. Geodynamic and metabolic cycles in the Hadean

    Directory of Open Access Journals (Sweden)

    M. J. Russell


    Full Text Available High-degree melting of hot dry Hadean mantle at ocean ridges and plumes resulted in a crust about 30km thick, overlain in places by extensive and thick mafic volcanic plateaus. Continental crust, by contrast, was relatively thin and mostly submarine. At constructive and destructive plate boundaries, and above the many mantle plumes, acidic hydrothermal springs at ~400°C contributed Fe and other transition elements as well as P and H2 to the deep ocean made acidulous by dissolved CO2 and minor HCl derived from volcanoes. Away from ocean ridges, submarine hydrothermal fluids were cool (≤100°C, alkaline (pH ~10, highly reduced and also H2-rich. Reaction of solvents in this fluid with those in ocean water was catalyzed in a hydrothermal mound, a natural self-restoring flow reactor and fractionation column developed above the alkaline spring. The mound consisted of brucite, Mg-rich clays, ephemeral carbonates, Fe-Ni sulfide and green rust. Acetate and glycine were the main products, some of which were eluted to the ocean. The rest, along with other organic byproducts were retained and concentrated within Fe-Ni sulfide compartments. These compartments, comprising the natural hydrothermal reactor, consisted partly of greigite (Fe5NiS8. It was from reactions between organic modules confined within these inorganic compartments that the first prokaryotic organism evolved. These acetogenic precursors to the bacteria diversified and migrated down the mound and into the ocean floor to inaugurate the 'deep biosphere'. Once there they were protected from cataclysmic heating events caused by large meteoritic impacts. Geodynamic forces led to the eventual obduction of the deep biosphere into the photic zone where, initially protected by a thin veneer of sediment, the use of solar energy was mastered and photosynthesis emerged. The further evolution to oxygenic photosynthesis was effected as catalytic [Mn,Ca]-bearing molecules that otherwise would have been

  15. Can Lower Mantle Slab-like Seismic Anomalies be Explained by Thermal Coupling Between the Upper and Lower Mantles?

    NARCIS (Netherlands)

    Cízková, H. (Hana); Cadek, O.; Berg, A.P. van den; Vlaar, N.J.


    Below subduction zones, high resolution seismic tomographic models resolve fast anomalies that often extend into the deep lower mantle. These anomalies are generally interpreted as slabs penetrating through the 660-km seismic discontinuity, evidence in support of whole-mantle convection. However, th

  16. No evidence for Hadean continental crust within Earth's oldest evolved rock unit (United States)

    Reimink, J. R.; Davies, J. H. F. L.; Chacko, T.; Stern, R. A.; Heaman, L. M.; Sarkar, C.; Schaltegger, U.; Creaser, R. A.; Pearson, D. G.


    Due to the acute scarcity of very ancient rocks, the composition of Earth's embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today's oceanic crust, or included significant amounts of continental crust remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust-mantle system. Here we present new U-Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth--a 4,019.6 +/- 1.8 Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.

  17. The development of slabs in the upper mantle: Insights from numerical and laboratory experiments (United States)

    Becker, Thorsten W.; Faccenna, Caludio; O'Connell, Richard J.; Giardini, Domenico


    We have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab. A two-dimensional finite element code is applied to model Newtonian creep in the numerical experiments. Scaled analog media are used in the laboratory, a sand mixture models the brittle crust, silicone putty simulates creep in the lower crust and mantle lithosphere, and glucose syrup is the asthenosphere analog. Both model approaches show similar results and reproduce first-order observations of the subduction process in nature based on density and viscosity heterogeneities in a Stokes flow model. Subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold. We find that the subduction velocity and angle are time-dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670-km discontinuity. The style of subduction is controlled by the prescribed velocity of convergence, the density contrast between the plates, and the viscosity contrast between the oceanic plate and the mantle. These factors can be combined in the buoyancy number F which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate. Variations in F control the stress in the plates, the speed and the dip of subduction, and the rate of trench retreat, reproducing the contrasting styles of subduction observed in nature. The subduction rate is strongly influenced by the work of bending the lithosphere as it subducts.

  18. Crust and upper mantle electrical conductivity beneath the Yellowstone Hotspot Track (United States)

    Kelbert, A.; Egbert, G. D.


    We have used high-quality electromagnetic data obtained through the EarthScope USArray project to obtain detailed three-dimensional images of electrical resistivity / conductivity in the crust and upper mantle beneath the Snake River Plain/Yellowstone (SRP/Y) volcanic province (Idaho and Wyoming, United States). The lowest resistivities in the area can only plausibly be explained by partial melt and/or fluids, providing valuable new information about the distribution of these phases deep within the Earth beneath the volcanic system. Unexpectedly, in light of the mantle plume models often used to explain Yellowstone volcanism, the electromagnetic data imply that there is no interconnected melt in the lower crust and uppermost mantle directly beneath the modern Yellowstone caldera. Instead, low resistivities consistent with 1-3% melt in the uppermost mantle (depths of 40-80 km) extend at least 200 km southwest of Yellowstone. Shallower areas of reduced resistivity extend upward into the mid-crust around the edges of the seemingly impermeable Snake River Plain province, including beneath Yellowstone. We suggest that the elevated temperatures beneath the active volcanic center have resulted in greater permeability, allowing magma to ascend to shallower depths and pool in the crust. Little melt is entering the system from below at present, perhaps due to intermittency of supply. We describe these results in the context of larger scale electrical resistivity and seismic tomography models of the western US and employ joint interpretation to formulate hypotheses that would explain this unexpected melt distribution beneath the SRP/Y. Our 3-D model is available at

  19. Upper mantle seismic discontinuities in the area of the Indian Ocean Geoid Low (United States)

    Reiss, Anne-Sophie; van Driel, Jac; Heyn, Björn; Thomas, Christine


    We are investigating the upper mantle seismic discontinuities at 410 and 660 km depth beneath the southern tip of India, where the Indian Ocean Geoid Low (IOGL) is located. We use PP and SS waves and their precursors, which reflect off the underside of these seismic discontinuities midway between source and receiver, to map the topography of the two discontinuities bounding the mantle transition zone. Our dataset consists of 9604 events with magnitude 5.8 or higher recorded at 57 different arrays distributed around the Indian Ocean. This results in a dense coverage of reflection points across our area of interest. 599 events out of this dataset show a good PP or SS signal. To enhance the signal-to-noise-ratio for better visibility of the weak precursor signals and to identify out-of-plane arrivals we use array seismology methods. We obtain the depth of the discontinuities by measuring the differential travel time between the main phase and the precursor signal, comparing it with theoretical travel times through the 1D reference Earth model ak135 and correcting the measured travel times for crustal and tomographic features. Most of the signals we observe reflect off the 410 km discontinuity, which is caused by the solid-solid phase transition from olivine to wadsleyite. The 660 km discontinuity, which exists due to the phase transformation from ringwoodite to bridgmanite and magnesiowustite, also causes some visible reflections in our dataset. Besides those two discontinuities we also see few reflections both from shallower and deeper structure. The best quality data show a deepened 410 in the centre of the IOGL as well as an overall elevated 660 km discontinuity. The combined observation of the IOGL, its sensitivity kernel and the behaviour of the mantle transition zone seismic discontinuities can be interpreted as a hot upwelling that currently resides in the mantle transition zone.

  20. Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth's upper mantle. (United States)

    Ohuchi, Tomohiro; Kawazoe, Takaaki; Higo, Yuji; Funakoshi, Ken-Ichi; Suzuki, Akio; Kikegawa, Takumi; Irifune, Tetsuo


    Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth's upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 10(19.6) to 10(20.7) Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size-sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle.

  1. Regionalized temperature variations in the upper 400 km of the Earth's mantle (United States)

    Tralli, David M.; Ita, Joel J.

    Tectonically regionalized variations in the temperature of the upper 400 km of the Earth's mantle are estimated from analysis of global seismic travel-time data cataloged by the International Seismological Centre (ISC). Seismic parameter profiles are determined from estimates of P and S velocities obtained by tau inversion. Summary phase diagrams for the olivine and pyroxene-garnet subsystems are constructed in conjunction with a thermodynamic potential formulation that allows self-consistent determination of density, bulk modulus and adiabats throughout the pressure and temperature regimes of the mantle. Perturbations in estimated seismic parameters are expressed in terms of variations in temperature using the model temperature derivatives of the bulk modulus and density at a given temperature and pressure. Confidence bounds on the velocity estimates are used to place corresponding bounds on the constructed seismic parameters. A simple differential relationship is solved iteratively to obtain a temperature variation for a given variation in seismic parameter. This approach allows the estimation of a range of seismically determined temperature variations by employing a given compositional model. Results indicate that whereas the P and S velocity variations in the upper mantle are consistent with the tectonic regionalization, variations in V p/V s ratios are irregular. This leads to unstable estimates of the seismic parameters and thus estimates of mean temperature anomalies, typically within 600°C of the weighted mean, that are inconsistent with the regionalized seismic data. A comparison of two compositional models is used to show the trade-off with estimated temperature variations. A refined regionalization and analysis of a larger ISC data set are suggested to stabilize the S velocity inversion, reduce statistical uncertainties on the seismic parameters, and thus improve constraints on estimated temperature variations.

  2. Fine scale structure of the eurasian crust and upper mantle from high-frequency waves (United States)

    Jordan, Thomas H.


    The purpose of this project is to provide a better understanding the effects of small-scale inhomogeneities on high-frequency waves excited by nuclear explosions, especially at regional distances. In this project, the PI and students developed and applied methods for extracting phase Uclays and amplitudes from three-component seismograms. These methods delivered new information about impedance discontinuities and velocity gradients in the upper mantle, and large data sets were inverted for radially anisotropic structure. The principle result of this analysis was that the magnitude of the anisotropy observed beneath western Australia and the western Pacific required stochastic variations of isotropic velocities that were petrologically unreasonable, and local anisotropy was inferred. The depth extent of anisotropy is limited to 250 km beneath Australia and 170 km beneath the Pacific. No azimuthal anisotropy was observed in either region, establishing the characteristic outer scale length of the heterogeneous anisotropic structure to be less than 3000 km. This report is split into two sections: the first presents the results for the Australian upper mantle, and has been accepted for publication in Science; the second presents the Pacific results, and has been submitted to Journal of Geophysical Research.

  3. Pressure and temperature evolution of upper mantle under the Rio Grande Rift (United States)

    Kil, Y.; Wendlandt, R. F.


    Spinel peridotite xenoliths associated with the Rio Grande Rift axis (Potrillo and Elephant Butte volcanic fields) and the western rift shoulder (Adam’s Diggings) have been investigated to correlate pre-eruptive pressure and temperature conditions with xenolith deformation textures and rift location. Temperatures of xenolith equilibration at the rift shoulder are 100 250°C cooler for a given pressure than the temperatures at the rift axis. Undeformed xenoliths (protogranular texture) are derived from higher temperature and higher pressure conditions than deformed xenoliths (porphyroclastic and equigranular textures) in the rift axis. Exsolution lamellae in pyroxenes, small decreases in Al contents of orthopyroxenes from core to rim, and small differences in porphyroclastic orthopyroxene compositions versus neoblastic orthopyroxene compositions indicate high temperatures followed by cooling and a larger cooling interval in deformed rocks than in undeformed rocks. These features, along with thermal histories based on calcium zoning in olivine rims, indicate that the upper mantle under Adam’s Diggings and Elephant Butte has undergone cooling from an initial high temperature state followed by a late heating event, and the upper mantle under Potrillo has undergone cooling, reheating, and late heating events.

  4. The electrical conductivity of the upper mantle as estimated from satellite magnetic field data (United States)

    Didwall, E. M.


    The electrical conductivity of the upper mantle is estimated from low-latitude magnetic field variations caused by large fluctuations in the equatorial ring current. The data base is derived from magnetic field magnitude data measured by satellites OGO 2, 4, and 6, which offer better global coverage than land-based observatories. The procedures of analysis consist of: (1) separation of the disturbance field into internal and external parts relative to the surface of the earth, (2) estimation of an electromagnetic response function Q(omega) which relates the internally generated magnetic field variations to the external variations due to the ring current, and (3) interpretation of the estimated response function using theoretical response functions for assumed conductivity profiles. Special consideration is given to possible oceanic and ionospheric effects. Best estimates of the geomagnetic response function Q(omega) for 0.2 to 2.0 cpd indicate an upper mantle conductivity of the order of 0.01 S/m.

  5. The electrical conductivity of the upper mantle as estimated from satellite magnetic field data (United States)

    Didwall, E. M.


    The electrical conductivity of the upper mantle is estimated from low-latitude magnetic field variations caused by large fluctuations in the equatorial ring current. The data base is derived from magnetic field magnitude data measured by satellites OGO 2, 4, and 6, which offer better global coverage than land-based observatories. The procedures of analysis consist of: (1) separation of the disturbance field into internal and external parts relative to the surface of the earth, (2) estimation of an electromagnetic response function Q(omega) which relates the internally generated magnetic field variations to the external variations due to the ring current, and (3) interpretation of the estimated response function using theoretical response functions for assumed conductivity profiles. Special consideration is given to possible oceanic and ionospheric effects. Best estimates of the geomagnetic response function Q(omega) for 0.2 to 2.0 cpd indicate an upper mantle conductivity of the order of 0.01 S/m.

  6. Three-dimensional thermal structure of the Chinese continental crust and upper mantle

    Institute of Scientific and Technical Information of China (English)


    We invert S-wave velocities for the 3D upper-mantle temperatures, in which the position with a temperature crossing the 1300℃ adiabat is corresponding to the top of the seismic low velocity zone. The temperatures down to the depth of 80 km are then calculated by solving steady-state thermal conduction equation with the constraints of the inverted upper-mantle temperatures and the surface temperatures, and then surface heat flows are calculated from the crustal temperatures. The misfit between the calculated and observed surface heat flow is smaller than 20% for most regions. The result shows that, at a depth of 25 km, the crustal temperature of eastern China (500―600℃) is higher than that of western China (<500℃). At a depth of 100 km, temperatures beneath eastern and southeastern China are higher than the adiabatic temperature of 1300℃, while that beneath west China is lower. The Tarim craton and the Sichuan basin show generally low temperature. At a depth of 150 km, temperatures beneath south China, eastern Yangtze craton, North China craton and around the Qiangtang terrane are higher than the adiabatic temperature of 1300℃, but is the lowest beneath the Sichuan basin and the regions near the Indian-Eurasian collision zone. At a depth of 200 km, very low temperature occurs beneath the Qinghai-Tibet Plateau and the south to the Tarim craton.

  7. Upper-mantle velocities below the Scandinavian Mountains from P- and S-wave traveltime tomography (United States)

    Hejrani, Babak; Balling, Niels; Jacobsen, Bo Holm; England, Richard


    The relative traveltime residuals of more than 20 000 arrival times of teleseismic P and S waves measured over a period of more than 10 yr in five separate temporary and two permanent seismic networks covering the Scandinavian (Scandes) Mountains and adjacent areas of the Baltic Shield are inverted to 3-D tomograms of P and S velocities and the VP/VS ratio. Resolution analysis documents that good 3-D resolution is available under the dense network south of 64° latitude (Southern Scandes Mountains), and patchier, but highly useful resolution is available further north, where station coverage is more uneven. A pronounced upper-mantle velocity boundary (UMVB) that transects the study region is defined. It runs from SE Norway (east of the Oslo Graben) across the mountains to the Norwegian coast near Trondheim (around the Møre-Trøndelag Fault Complex), after which it follows closely along the coast further north. Seismic velocities in the depth interval 100-300 km change significantly across the UMVB from low relative VP and even lower relative VS on the western side, to high relative VP and even higher relative VS to the east. This main velocity boundary therefore also separates relatively high VP/VS ratio to the west and relatively low VP/VS to the east. Under the Southern Scandes Mountains (most of southern Norway), we find low relative VP, even lower relative VS and hence high VP/VS ratios. These velocities are indicative of thinner lithosphere, higher temperature and less depletion and/or fluid content in a relatively shallow asthenosphere. At first sight, this might support the idea of a mantle buoyancy source for the high topography. Under the Northern Scandes Mountains, we find the opposite situation: high relative VP, even higher relative VS and hence low VP/VS ratios, consistent with thick, dry, depleted lithosphere, similar to that in most of the Baltic Shield area. This demonstrates significant differences in upper-mantle conditions between the Southern

  8. The electrical conductivity of the upper mantle and lithosphere from the magnetic signal due to ocean tidal flow

    DEFF Research Database (Denmark)

    Schnepf, Neesha Regmi; Kuvshinov, Alexey; Grayver, Alexander

    -based data, satellite data are overall uniform and very high quality. Probing the conductivity of the lithosphere and upper mantle requires EM variations with periods of a few hours. Electric currents generated by oceanic tides are a well-suited source within this period range. Ocean tides interact...... galvanically with Earth’s lithosphere (i.e. by direct coupling of the source currents in the ocean with the underlying substrate), enabling conductivity estimations at shallower depths. Here we present the results of determining a 1-D conductivity-depth profile of oceanic lithosphere and upper mantle using......Oceans cover about seventy percent of the Earth and yet the overwhelming majority of seismological or electromagnetic (EM) observatories are found on continents. This provides a challenge for understanding composition, structure, and dynamics of Earth’s lithosphere and upper mantle in oceanic...

  9. The Oxidation State of Fe in MORB Glasses and the Oxygen Fugacity of the Upper Mantle

    Energy Technology Data Exchange (ETDEWEB)

    E Cottrell; K Kelley


    Micro-analytical determination of Fe{sup 3+}/{Sigma}Fe ratios in mid-ocean ridge basalt (MORB) glasses using micro X-ray absorption near edge structure ({mu}-XANES) spectroscopy reveals a substantially more oxidized upper mantle than determined by previous studies. Here, we show that global MORBs yield average Fe{sup 3+}/{Sigma}Fe ratios of 0.16 {+-} 0.01 (n = 103), which trace back to primary MORB melts equilibrated at the conditions of the quartz-fayalite-magnetite (QFM) buffer. Our results necessitate an upward revision of the Fe{sup 3+}/{Sigma}Fe ratios of MORBs, mantle oxygen fugacity, and the ferric iron content of the mantle relative to previous wet chemical determinations. We show that only 0.01 (absolute, or < 10%) of the difference between Fe{sup 3+}/{Sigma}Fe ratios determined by micro-colorimety and XANES can be attributed to the Moessbauer-based XANES calibration. The difference must instead derive from a bias between micro-colorimetry performed on experimental vs. natural basalts. Co-variations of Fe{sup 3+}/{Sigma}Fe ratios in global MORB with indices of low-pressure fractional crystallization are consistent with Fe{sup 3+} behaving incompatibly in shallow MORB magma chambers. MORB Fe{sup 3+}/{Sigma}Fe ratios do not, however, vary with indices of the extent of mantle melting (e.g., Na{sub 2}O(8)) or water concentration. We offer two hypotheses to explain these observations: The bulk partition coefficient of Fe{sup 3+} may be higher during peridotite melting than previously thought, and may vary with temperature, or redox exchange between sulfide and sulfate species could buffer mantle melting at {approx} QFM. Both explanations, in combination with the measured MORB Fe{sup 3+}/{Sigma}Fe ratios, point to a fertile MORB source with greater than 0.3 wt.% Fe{sub 2}O{sub 3}.

  10. Crustal and upper mantle seismic structure of the Australian Plate, South Island, New Zealand (United States)

    Melhuish, Anne; Holbrook, W. Steven; Davey, Fred; Okaya, David A.; Stern, Tim


    Seismic reflection and refraction data were collected west of New Zealand's South Island parallel to the Pacific-Australian Plate boundary. The obliquely convergent plate boundary is marked at the surface by the Alpine Fault, which juxtaposes continental crust of each plate. The data are used to study the crustal and uppermost mantle structure and provide a link between other seismic transects which cross the plate boundary. Arrival times of wide-angle reflected and refracted events from 13 recording stations are used to construct a 380-km long crustal velocity model. The model shows that, beneath a 2-4-km thick sedimentary veneer, the crust consists of two layers. The upper layer velocities increase from 5.4-5.9 km/s at the top of the layer to 6.3 km/s at the base of the layer. The base of the layer is mainly about 20 km deep but deepens to 25 km at its southern end. The lower layer velocities range from 6.3 to 7.1 km/s, and are commonly around 6.5 km/s at the top of the layer and 6.7 km/s at the base. Beneath the lower layer, the model has velocities of 8.2-8.5 km/s, typical of mantle material. The Mohorovicic discontinuity (Moho) therefore lies at the base of the second layer. It is at a depth of around 30 km but shallows over the south-central third of the profile to about 26 km, possibly associated with a southwest dipping detachment fault. The high, variable sub-Moho velocities of 8.2 km/s to 8.5 km/s are inferred to result from strong upper mantle anisotropy. Multichannel seismic reflection data cover about 220 km of the southern part of the modelled section. Beneath the well-layered Oligocene to recent sedimentary section, the crustal section is broadly divided into two zones, which correspond to the two layers of the velocity model. The upper layer (down to about 7-9 s two-way travel time) has few reflections. The lower layer (down to about 11 s two-way time) contains many strong, subparallel reflections. The base of this reflective zone is the Moho. Bi

  11. Global variations in azimuthal anisotropy of the Earth's upper mantle and crust (United States)

    Schaeffer, A. J.; Lebedev, S.


    Deformation within the Earth's crust and mantle often results in crystallographic preferred orientations that produce measurable seismic anisotropy. Shear wave splitting measurements have the benefit of excellent lateral resolution and are an unambiguous indicator of the presence of seismic anisotropy; however, they suffer from poor depth resolution (integrated measurement from CMB to surface), in addition to being geographically limited (measurements only made at seismometer locations). The analysis of surface wave propagation also provides insight into the azimuthal variations in wave-speed, but with significantly better depth resolution. Thanks to the rapid increase in the number of seismic stations around the world, increasingly accurate, high-resolution 3D models of azimuthal anisotropy can be calculated using surface-wave tomography. We present our new global, azimuthally anisotropic model of the upper mantle and the crust. Compared to its recent predecessor, SL2013sv (Schaeffer and Lebedev, 2013), it is constrained by an even larger waveform fit dataset (>900,000 versus 712,000 vertical-component seismograms, respectively) and was computed using a more precise regularization of anisotropy, tuned to honour the amplitude and orientation of the anisotropic terms uniformly, including near the poles. Automated, multimode waveform inversion was used to extract structural information from surface and S wave forms, yielding resolving power from the crust down to the transition zone. Our unprecedentedly large waveform dataset, with complementary high-resolution regional arrays (including USArray) and global network sub-sets within it, produces improved resolution of global azimuthal anisotropy patterns. The model also reveals smaller scale patterns of 3D anisotropy variations related to regional lithospheric deformation and mantle flow, in particular in densely sampled regions. In oceanic regions, we examine the strength of azimuthal anisotropy, as a function of

  12. Strong lateral variations of S-wave velocity in the upper mantle across the western Alps (United States)

    Lyu, Chao; Pedersen, Helle; Paul, Anne; Zhao, Liang


    Absolute S-wave velocity gives more insight into temperature and mineralogy than relative P-wave velocity variations (ΔV p/ V p) imaged by teleseismic traveltime tomography. Moreover, teleseismic P-wave tomography has poor vertical but good horizontal resolution. By contrast, the inversion of surface waves dispersion data gives absolute S-wave velocity with a good vertical but relatively poor horizontal resolution. However, the horizontal resolution of surface wave imaging can be improved by using closely spaced stations in mini-arrays. In this work, we use Rayleigh wave phase velocity dispersion data to measure absolute S-wave velocities beneath the CIFALPS profile across the French-Italian western Alps. We apply the array processing technique proposed by Pedersen et al. (2003) to derive Rayleigh wave phase dispersion curves between 20 s and 100 s period in 15 mini-arrays along the CIFALPS line. We estimate a 1-D S-wave velocity model at depth 50-150 km beneath each mini-array by inverting the dispersion curves jointly with receiver functions. The joint inversion helps separating the crustal and mantle contributions in the inversion of dispersion curves. Distinct lithospheric structures and marked lateral variations are revealed beneath the study region, correlating well with regional geological and tectonic features. The average S-wave velocity from 50 to 150 km depth beneath the CIFALPS area is ˜4.48km/s, almost the same as in model AK135, indicating a normal upper mantle structure in average. Lateral variations are dominated by relatively low velocities (˜4.4km/s) in the mantle of the European plate, very low velocities (4.0km/s, i.e. approximately 12% lower than AK135) beneath the Dora Maira internal crystalline massif and high velocities (˜ 5.0km/s, i.e. 12% higher than AK135) beneath the Po plain. The lateral variations of S-wave velocity perturbation show the same features as the P wave tomography (Zhao et al., submitted), but with different amplitudes

  13. Shear wave velocity structure of the crust and upper mantle underneath the Tianshan orogenic belt

    Institute of Scientific and Technical Information of China (English)


    From April, 2003 to September, 2004, a passive broadband seismic array consisting of 60 stations was deployed over the Tianshan orogenic belt by State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration. Among them, 51 stations make up an about 500-km-long profile across the Tianshan Mountains from Kuytun to Kuqa. The receiver function profile and S-wave velocity structure of the crust and upper mantle down to 100 km deep are obtained by using the re-ceiver function method (Liu et al. 1996, 2000). The main results can be summarized as follows: (1) A clear mountain root does not exist beneath the Tianshan Mountains, and the crust-mantle boundaries underneath the stations mostly have transitional structures. This implies that the material differentia-tion between the crust and mantle is not yet accomplished and the orogenic process is still going on. (2) The crust beneath the Tianshan Mountains has laterally blocked structures in direction perpendicular to the mountain strike, and the crust-mantle boundary has a clear dislocation structure. Both of them correspond to each other. (3) The offsets of the Moho discontinuity are highly correlated to the tectonic borders on the surface and that corresponding to the frontal southern Tianshan fault reaches to 14 km. This manifests that large vertical divergent movement took place between different blocks. This sup-ports the discontinuous model of the Tianshan orogeny, and the Tarim block subduction is restricted only to the southern side of the South Tianshan. (4) Inside the upper and middle crust of the Tianshan Mountains exist several low-velocity bodies correlated with high seismicity located on the moun-tain-basin jointures on both sides of the mountain and between different blocks, and the low-velocity bodies on the mountain-basin jointures are inclined obviously to the mountain. This implies that the low-velocity bodies may be correlated closely to the thrust and subduction of

  14. Upper Mantle Structure of the transition between Alps and Apennines Revealed by Shear Wave Splitting from the CIFALPS Project (United States)

    Simone, Salimbeni; Liang, Zhao; Marco, Malusà G.; Silvia, Pondrelli; Lucia, Margheriti; Anne, Paul; Xiaobing, Xu; Stefano, Solarino; Stéphane, Guillot; Stéphane, Schwartz; Thierry, Dumont; Coralie, Aubert; Qingchen, Wang; Rixiang, Zhu; Tianyu, Zheng


    Northern Apennines, Alps and surrounding regions are often studied separately. The structure of their upper mantle has been studied repeatedly in the past and some studies reported on the seismic anisotropic properties in the litho-asthenosphere. However, a joint interpretation of the Alps-Apennines transition zone is still lacking, mainly at depth. The China-Italy-France Alps seismic survey (CIFALPS, 2012) provided an improved image of the crust and upper mantle beneath the southwestern Alps and the transition to the Apennines. Here we show the SKS shear wave splitting results obtained from the analysis of teleseismic data recorded by 55 temporary seismic stations along the CIFALPS profile and by some other permanent stations. The strain-induced lattice preferred orientation of olivine minerals within the upper mantle, expressed by the analysis, confirms the NW trending fast polarization directions parallel to the strike of the orogen, in good agreement with the results of previous studies all along the Alpine chain. On the contrary, in the Po Plain, new shear wave splitting measurements show a scattered distribution; the coexistence of both NNE-SSW and E-W directions provides new insights on upper mantle deformation in the complex transition zone between the Alpine and Apenninic subductions. The comparison of this new dataset with recent tomographic studies and geological improvement should compose a more complete picture of the mantle structure and deformation of this puzzling region.

  15. Sensitivity analysis of crustal correction and its error propagation to upper mantle residual gravity and density anomalies

    DEFF Research Database (Denmark)

    Herceg, Matija; Artemieva, Irina; Thybo, Hans


    We investigate the effect of the crustal structure heterogeneity and uncertainty in its determination on stripped gravity field. The analysis is based on interpretation of residual upper mantle gravity anomalies which are calculated by subtracting (stripping) the gravitational effect of the crust...... (including topography) from the observed satellite gravity field data (GOCE Direct release 3). We apply our analysis to Siberia for which a new regional crustal model has recently become available. Uncertainties in the residual upper (lithospheric) mantle gravity anomalies result from several sources: (i...

  16. Crustal and upper mantle seismic structure of the Svalbard Archipelago from the receiver function analysis

    Directory of Open Access Journals (Sweden)

    Wilde−Piórko Monika


    Full Text Available Receiver function provides the signature of sharp seismic discontinuities and the information about the shear wave (S−wave velocity distribution beneath the seismic station. This information is very valuable in areas where any or few reflection and/or refraction studies are available and global and/or regional models give only rough information about the seismic velocities. The data recorded by broadband seismic stations have been analysed to investigate the crustal and upper mantle structure of the Svalbard Archipelago. Svalbard Archipelago is a group of islands located in Arctic, at the north−western part of the Barents Sea continental platform, which is bordered to the west and to the north by passive continental margins. The new procedure of parameterization and selection of receiver functions (RFs has been proposed. The back−azimuthal sections of RF show a strong variation for the HSPB and KBS stations. Significant amplitudes of transversal component of RF (T−RF for the HSPB station indicate a shallow dipping layer towards the southwest. The structure of the crust beneath the SPITS array seems to be less heterogeneous, with very low amplitudes of converted phase comparing to the KBS and HSPB stations. Forward modelling by trial−and−error method shows a division of the crust into 3-4 layers beneath all stations and layering of the uppermost mantle beneath the SPITS array and the HSPB stations. The thickness of the mantle transition zone is larger for western part of archipelago and smaller for eastern part comparing to iasp91 model.

  17. Multi-Observable Thermochemical Tomography of the lithosphere and upper mantle (United States)

    Afonso, J. C.; Yang, Y.; Rawlinson, N.; Jones, A. G.; Fullea, J.; Qashqai, M.


    Current knowledge of the present-day physical state and structure of the lithosphere and upper mantle essentially derives from four independent sources: i) gravity field and thermal modelling, ii) modelling/inversion of different seismic datasets, iii) magnetotelluric studies, and iv) thermobarometric and geochemical data from exhumed mantle samples. Unfortunately, the integration of these different sources of information in modern geophysical studies is still uncommon and significant discrepancies and/or inconsistencies in predictions between these sources are still the rule rather than the exception.In this contribution we will present a thermodynamically-constrained multi-observable probabilistic inversion method capable of jointly inverting i) surface and body wave datasets, gravity anomalies, geoid height, gravity gradients, receiver functions, surface heat flow, magnetotelluric data, and elevation (static and dynamic) in 3D spherical coordinates. Key aspects of the method are: (a) it combines multiple geophysical observables with different sensitivities to deep/shallow, thermal/compositional anomalies into a single thermodynamic-geophysical framework; (b) it works with thermophysical models of the Earth rather than with parameterized structures of physical parameters (e.g. Vs, Vp, density, etc), (c) it uses a general probabilistic (Bayesian) formulation to appraise the data; (d) no initial model is needed; (e) a priori compositional information relies on robust statistical analyses of a large database of natural mantle samples; (f) it provides a natural platform to estimate realistic uncertainties; (g) it handles multiscale parameterizations and complex physical models, and (h) it includes dynamic (convection) effects on surface observables by solving the complete Stokes flow using multi-dimensional decomposition methods. We will present results for both synthetic and real case studies, which serve to highlight the advantages and limitations of this new

  18. Azimuthal seismic anisotropy in the Earth's upper mantle and the thickness of tectonic plates (United States)

    Schaeffer, A. J.; Lebedev, S.; Becker, T. W.


    Azimuthal seismic anisotropy, the dependence of seismic wave speeds on propagation azimuth, is largely due to fabrics within the Earth's crust and mantle, produced by deformation. It thus provides constraints on the distribution and evolution of deformation within the upper mantle. Here, we present a new global, azimuthally anisotropic model of the crust, upper mantle and transition zone. Two versions of this new model are computed: the rough SL2016svAr and the smooth SL2016svA. Both are constrained by a very large data set of waveform fits (˜750 000 vertical component seismogram fits). Automated, multimode waveform inversion was used to extract structural information from surface and S wave forms in broad period ranges (dominantly from 11 to 450 s, with the best global sampling in the 20-350 s range), yielding resolving power from the crust down to the transition zone. In our global tomographic inversion, regularization of anisotropy is implemented to more uniformly recover the amplitude and orientation of anisotropy, including near the poles. Our massive waveform data set, with complementary large global networks and high-density regional array data, produces improved resolution of global azimuthal anisotropy patterns. We show that regional scale variations, related to regional lithospheric deformation and mantle flow, can now be resolved by the global models, in particular in densely sampled regions. For oceanic regions, we compare quantitatively the directions of past and present plate motions and the fast-propagation orientations of anisotropy. By doing so, we infer the depth of the boundary between the rigid, high-viscosity lithosphere (preserving ancient, frozen fabric) and the rheologically weak asthenosphere (characterized by fabric developed recently). The average depth of thus inferred rheological lithosphere-asthenosphere boundary (LAB) beneath the world's oceans is ˜115 km. The LAB depth displays a clear dependence on the age of the oceanic

  19. Experimental study of spinel-garnet phase transition in upper mantle and its significance

    Institute of Scientific and Technical Information of China (English)

    樊祺诚; 刘若新; 谢鸿森; 张月明; 徐平; 林卓然


    Experimental study of spinel-garnet phase transition was carried out using natural mineral and rock specimens from xenolith of mantle rocks in Cenozoic basalt as starting materials. From the result it was found that the condition of spinel lherzolite-garnet lherzolite phase transition ( T = 1100℃ and P = 1.8-2.0 GPa) is consistent with the P-T equilibrium condition of the five-phase assemblage spinel/garnet Iherzolite in eastern China, suggesting that there may exist a spinel-garnet Iherzolite phase transition zone with the thickness of a few km to several ten km at the depth of 55-70 km in the continental upper mantle of eastern China. The depth of phase transition from spinel pyrox-enite to garnet pyroxenite is found to be less than 55 km. Experiment results also show that water promotes metasomatism on one hand but suppresses phase transition on the other. Zoning of mineral composition was also discussed.

  20. 100 Years of Studies of the Crust and Upper Mantle in Croatia (United States)

    Herak, D.; Herak, M.; Tkalcic, H.


    The study of properties of the earth’s crust and upper mantle started in Croatia one century ago, by the seminal paper in which A. Mohorovicic proved the existence of the crust-mantle boundary. Most of Mohorovicic’s work which followed was dedicated to improving travel-time curves of crustal phases. During the 1980-es most of the research—mostly based on interpretation of several deep seismic profiles running across the country—was directed towards determination of the Moho-topography. The interest for the study of elastic properties of the crust was revived in the 1990-es, when several papers appeared which dealt with determination of velocities and attenuation (coda-Q) in the greater circum-Adriatic region. More recently, a large database of Pg-phase arrival times was used to assess the azimuthal anisotropy of the P-wave velocity within the crust in the NW Croatia and in parts of the External Dinarides. It was found that the direction of the fast velocities closely correspond to the direction of predominant current tectonic stress field. Current research based on the data obtained within a large international seismic experiment (ALP2002) lead to new constraints on the crustal structure in northern and western Croatia. Lithospheric structure beneath coastal and continental Croatia is being studied also by broadband teleseismic waveform modeling using receiver functions. The results indicate that the Mohorovicic discontinuity in Dalmatia may lie considerably deeper than presented on recent maps of the Moho topography in Europe.

  1. Upper-mantle velocities below the Scandinavian Mountains from P- and S- wave traveltime tomography

    DEFF Research Database (Denmark)

    Hejrani, Babak; Balling, N.; Jacobsen, B. H.;


    is more uneven. A pronounced upper-mantle velocity boundary (UMVB), transecting the study region is defined. It runs from SE Norway (east of the Oslo Graben) across the mountains to the Norwegian coast near Trondheim (around the Møre-Trøndelag Fault Complex), from where it follows the coast and runs...... between Lofoten and the crest of the Northern Scandes Mountains and stays off the coast further north. Seismic velocities in the depth interval 100-300 km change across the UMVB from low relative VP and even lower relative VS on the western side to high relative VP and even higher relative VS to the east...... these patters is found near the topographic saddle between the Southern and Northern Scandes Mountains. Centred around 64°N, 14°E, a zone of lower S-velocity and hence higher VP/VS ratio is detected in the depth interval between 100 and 300 km. This “Trøndelag-Jämtland Mantle Anomaly” (TJMA) is still...

  2. High-resolution teleseismic tomography of upper-mantle structure using an a priori three-dimensional crustal model (United States)

    Waldhauser, Felix; Lippitsch, Regina; Kissling, Edi; Ansorge, Jörg


    The effect of an a priori known 3-D crustal model in teleseismic tomography of upper-mantle structure is investigated. We developed a 3-D crustal P-wave velocity model for the greater Alpine region, encompassing the central and western Alps and the northern Apennines, to estimate the crustal contribution to teleseismic traveltimes. The model is constructed by comparative use of published information from active and passive seismic surveys. The model components are chosen to represent the present large-scale Alpine crustal structure and for their significant effect on the propagation of seismic wavefields. They are first-order structures such as the crust-mantle boundary, sedimentary basins and the high-velocity Ivrea body. Teleseismic traveltime residuals are calculated for a realistic distribution of azimuths and distances by coupling a finite-difference technique to the IASP91 traveltime tables. Residuals are produced for a synthetic upper-mantle model featuring two slab structures and the 3-D crustal model on top of it. The crustal model produces traveltime residuals in the range between -0.7 and 1.5 s that vary strongly as a function of backazimuth and epicentral distance. We find that the non-linear inversion of the synthetic residuals without correcting for the 3-D crustal structure erroneously maps the crustal anomalies into the upper mantle. Correction of the residuals for crustal structure before inversion properly recovers the synthetic slab structures placed in the upper mantle. We conclude that with the increasing amount of high-quality seismic traveltime data, correction for near-surface structure is essential for increasing resolution in tomographic images of upper-mantle structure.

  3. Can We Probe the Conductivity of the Lithosphere and Upper Mantle Using Satellite Tidal Magnetic Signals? (United States)

    Schnepf, N. R.; Kuvshinov, A.; Sabaka, T.


    A few studies convincingly demonstrated that the magnetic fields induced by the lunar semidiurnal (M2) ocean flow can be identified in satellite observations. This result encourages using M2 satellite magnetic data to constrain subsurface electrical conductivity in oceanic regions. Traditional satellite-based induction studies using signals of magnetospheric origin are mostly sensitive to conducting structures because of the inductive coupling between primary and induced sources. In contrast, galvanic coupling from the oceanic tidal signal allows for studying less conductive, shallower structures. We perform global 3-D electromagnetic numerical simulations to investigate the sensitivity of M2 signals to conductivity distributions at different depths. The results of our sensitivity analysis suggest it will be promising to use M2 oceanic signals detected at satellite altitude for probing lithospheric and upper mantle conductivity. Our simulations also suggest that M2 seafloor electric and magnetic field data may provide complementary details to better constrain lithospheric conductivity.

  4. Crust and upper mantle shear wave structure of Northeast Algeria from Rayleigh wave dispersion analysis (United States)

    Radi, Zohir; Yelles-Chaouche, Abdelkrim; Corchete, Victor; Guettouche, Salim


    We resolve the crust and upper mantle structure beneath Northeast Algeria at depths of 0-400 km, using inversion of fundamental mode Rayleigh wave. Our data set consists of 490 earthquakes recorded between 2007 and 2014 by five permanent broadband seismic stations in the study area. Applying a combination of different filtering technics and inversion method shear wave velocities structure were determined as functions of depth. The resolved changes in Vs at 50 km depth are in perfect agreement with crustal thickness estimates, which reflect the study area's orogenic setting, partly overlying the collision zone between the African and Eurasian plates. The inferred Moho discontinuity depths are close to those estimated for other convergent areas. In addition, there is good agreement between our results and variations in orientations of regional seismic anisotropy. At depths of 80-180 km, negative Vs anomalies at station CBBR suggest the existence of a failed subduction slab.

  5. PALEOMAGNETISM. A Hadean to Paleoarchean geodynamo recorded by single zircon crystals. (United States)

    Tarduno, John A; Cottrell, Rory D; Davis, William J; Nimmo, Francis; Bono, Richard K


    Knowing when the geodynamo started is important for understanding the evolution of the core, the atmosphere, and life on Earth. We report full-vector paleointensity measurements of Archean to Hadean zircons bearing magnetic inclusions from the Jack Hills conglomerate (Western Australia) to reconstruct the early geodynamo history. Data from zircons between 3.3 billion and 4.2 billion years old record magnetic fields varying between 1.0 and 0.12 times recent equatorial field strengths. A Hadean geomagnetic field requires a core-mantle heat flow exceeding the adiabatic value and is suggestive of plate tectonics and/or advective magmatic heat transport. The existence of a terrestrial magnetic field before the Late Heavy Bombardment is supported by terrestrial nitrogen isotopic evidence and implies that early atmospheric evolution on both Earth and Mars was regulated by dynamo behavior.

  6. 3SMAC: an a priori tomographic model of the upper mantle based on geophysical modeling (United States)

    Nataf, Henri-Claude; Ricard, Yanick


    We present an a priori three-dimensional 'tomographic' model of the upper mantle. We construct this model (called 3SMAC — three-dimensional seismological model a priori constrained) in four steps: we compile information on the thickness of 'chemical' layers in the Earth (water, sediments, upper and lower crust, etc); we get a 3D temperature distribution from thermal plate models applied to the oceans and continents; we deduce the mineralogy in the mantle from pressure and temperature and we finally get a three-dimensional model of density, seismic velocities, and attenuation by introducing laboratory measurements of these quantities as a function of pressure and temperature. The model is thus consistent with various geophysical data, such as ocean bathymetry, and surface heat flux. We use this model to compute synthetic travel-times of body waves, and we compare them with observations. A similar exercise is performed for surface waves and normal modes in a companion paper (Ricard et al., 1996, J. Geophys. Res., in press). We find that our model predicts the bulk of the observed travel-time variations. Both the amplitude and general pattern are well recovered. The discrepancies suggest that tomography can provide useful regional information on the thermal state of the continents. In the oceans, the flattening of the sea-floor beond 70 Ma seems difficult to reconcile with the seismic observations. Overall, our 3SMAC model is both a realistic model, which can be used to test various tomographic methods, and a model of the minimum heterogeneities to be expected from geodynamical modeling. Therefore, it should be a useful a priori model to be used in tomographic inversions, in order to retrieve reliable images of heterogeneities in the transition zone, which should, in turn, greatly improve our understanding of geodynamical processes in the deep Earth. 3SMAC and accompanying software can be retrieved by anonymous ftp at

  7. Seismic Velocities and Earthquake Locations in the Central America Upper Mantle: results from the TUCAN Experiment (United States)

    Syracuse, E. M.; Abers, G. A.; Auger, L.; Fischer, K.; Protti, M.; Gonzalez, V.; Strauch, W.; Brewer, J.


    The processes that govern magma generation and extraction at subduction zones are not yet well understood. Velocity tomography and earthquake locations from the TUCAN (Tomography Under Costa Rica and Nicaragua) PASSCAL experiment give insight into the geometry and structure of the Central American subduction zone, which exhibits large variations in downgoing plate roughness and dip, volcano locations, and geochemistry over a short distance along the arc. Approximately 14000 P travel times and 11000 S travel times are used in joint Vp, Vp/Vs and hypocenter inversions. The present-day slab geometry is highlighted by contrasts in dip beneath the two arc sections: a near-vertical slab dip beneath the volcanic front in Nicaragua, similar to that indicated by teleseismic hypocenters and a 30° slab dip beneath central Costa Rica, similar to that indicated by a previous local study. In both regions, the intermediate-depth seismic zone is a single layer as thin as 5 km in some areas and no more than 10 to 20 km thick overall. Tomographic images show that throughout Nicaragua and Costa Rica, the slowest mantle velocities appear directly below the volcanic front, indicating likely zones of mantle melting extending 80 to 120 km depth. This region is much larger beneath Nicaragua than beneath Costa Rica, potentially allowing a greater extent of melting. Within the downgoing plate, a low-velocity region is imaged at depths less than 150 km beneath Nicaragua and in the upper 60 km of the slab beneath Costa Rica. This feature may represent a hydrated layer at the top of the downgoing plate, similar to that seen in waveguide studies. Beneath Nicaragua, we also see evidence for a dipping high-velocity region in the mantle wedge beneath Nicaragua extending from 20 to 100 km trenchward of the arc, consistent with results from receiver function analysis and offshore active source tomography. This high-velocity region may serve as an impediment to mantle flow and fluid migration

  8. Radial anisotropy of the North American upper mantle based on adjoint tomography with USArray (United States)

    Zhu, Hejun; Komatitsch, Dimitri; Tromp, Jeroen


    We use seismic data from USArray to image the upper mantle underneath the United States based on a so-called `adjoint tomography', an iterative full waveform inversion technique. The inversion uses data from 180 regional earthquakes recorded by 4516 seismographic stations, resulting in 586 185 frequency-dependent measurements. Three-component short-period body waves and long-period surface waves are combined to simultaneously constrain deep and shallow structures. The transversely isotropic model US22 is the result of 22 pre-conditioned conjugate-gradient iterations. Approximate Hessian maps and point-spread function tests demonstrate good illumination of the study region and limited trade-offs among different model parameters. We observe a distinct wave-speed contrast between the stable eastern US and the tectonically active western US. This boundary is well correlated with the Rocky Mountain Front. Stable cratonic regions are characterized by fast anomalies down to 250-300 km, reflecting the thickness of the North American lithosphere. Several fast anomalies are observed beneath the North American lithosphere, suggesting the possibility of lithospheric delamination. Slow wave-speed channels are imaged beneath the lithosphere, which might indicate weak asthenosphere. Beneath the mantle transition zone of the central US, an elongated north-south fast anomaly is observed, which might be the ancient subducted Farallon slab. The tectonically active western US is dominated by prominent slow anomalies with magnitudes greater than -6 per cent down to approximately 250 km. No continuous lower to upper mantle upwellings are observed beneath Yellowstone. In addition, our results confirm previously observed differences between oceans and continents in the anisotropic parameter ξ = (βh/βv)2. A slow wave-speed channel with ξ > 1 is imaged beneath the eastern Pacific at depths from 100 to 200 km, reflecting horizontal shear within the asthenosphere. Underneath continental

  9. Silicate melts density, buoyancy relations and the dynamics of magmatic processes in the upper mantle (United States)

    Sanchez-Valle, Carmen; Malfait, Wim J.


    Although silicate melts comprise only a minor volume fraction of the present day Earth, they play a critical role on the Earth's geochemical and geodynamical evolution. Their physical properties, namely the density, are a key control on many magmatic processes, including magma chamber dynamics and volcanic eruptions, melt extraction from residual rocks during partial melting, as well as crystal settling and melt migration. However, the quantitative modeling of these processes has been long limited by the scarcity of data on the density and compressibility of volatile-bearing silicate melts at relevant pressure and temperature conditions. In the last decade, new experimental designs namely combining large volume presses and synchrotron-based techniques have opened the possibility for determining in situ the density of a wide range of dry and volatile-bearing (H2O and CO2) silicate melt compositions at high pressure-high temperature conditions. In this contribution we will illustrate some of these progresses with focus on recent results on the density of dry and hydrous felsic and intermediate melt compositions (rhyolite, phonolite and andesite melts) at crustal and upper mantle conditions (up to 4 GPa and 2000 K). The new data on felsic-intermediate melts has been combined with in situ data on (ultra)mafic systems and ambient pressure dilatometry and sound velocity data to calibrate a continuous, predictive density model for hydrous and CO2-bearing silicate melts with applications to magmatic processes down to the conditions of the mantle transition zone (up to 2773 K and 22 GPa). The calibration dataset consist of more than 370 density measurements on high-pressure and/or water-and CO2-bearing melts and it is formulated in terms of the partial molar properties of the oxide components. The model predicts the density of volatile-bearing liquids to within 42 kg/m3 in the calibration interval and the model extrapolations up to 3000 K and 100 GPa are in good agreement

  10. Seismic structure of the European crust and upper mantle based on adjoint tomography (United States)

    Zhu, Hejun

    We use adjoint tomography to estimate three-dimensional variations in seismic parameters within the crust and upper mantle beneath Europe and the North Atlantic Ocean. Spectral-element and adjoint methods are used to numerically calculate synthetic seismograms and sensitivity kernels in three-dimensional Earth models. Combined with gradient- based optimization algorithms, e.g., preconditioned conjugate-gradient and L-BFGS methods, we iteratively update seismic models of Earth's interior. A three-stage inversion strategy is designed to estimate variations in elastic wavespeeds, anelastic attenuation and radial & azimuthal anisotropy. In stage one, frequency-dependent phase differences between observed and simulated seismograms are used to determine a new radially anisotropic wavespeed model for the European crust and upper mantle, namely EU30. Long-wavelength structures in EU30 compare favorably with previous body- and surface-wave tomographic models. Some hitherto unidentified features naturally emerge from the smooth starting model. In stage two, frequency-dependent amplitude differences combined with remaining phase anomalies are used to simultaneously constrain elastic and anelastic structures. A new anelastic model, named EU50, is constructed in this stage. We observe several notable features, such as enhanced attenuation within the mantle transition zone beneath the North Atlantic Ocean. In the first two stages, long-period surface waves and short-period body waves in three-component seismograms are combined to simultaneously constrain shallow and deep structures. In stage three, frequency-dependent phase and amplitude anomalies of three-component surface waves are used to construct a radially and azimuthally anisotropic model EU60. We find that the direction of the fast axis is closely tied to the tectonic evolution in this region, such as extension along the North Atlantic Ridge, trench retreat in the Mediterranean, and counterclockwise rotation of the

  11. A New Comprehensive Model for Crustal and Upper Mantle Structure of the European Plate (United States)

    Morelli, A.; Danecek, P.; Molinari, I.; Postpischl, L.; Schivardi, R.; Serretti, P.; Tondi, M. R.


    We present a new comprehensive model of crustal and upper mantle structure of the whole European Plate — from the North Atlantic ridge to Urals, and from North Africa to the North Pole — describing seismic speeds (P and S) and density. Our description of crustal structure merges information from previous studies: large-scale compilations, seismic prospection, receiver functions, inversion of surface wave dispersion measurements and Green functions from noise correlation. We use a simple description of crustal structure, with laterally-varying sediment and cristalline layers thickness and seismic parameters. Most original information refers to P-wave speed, from which we derive S speed and density from scaling relations. This a priori crustal model by itself improves the overall fit to observed Bouguer anomaly maps, as derived from GRACE satellite data, over CRUST2.0. The new crustal model is then used as a constraint in the inversion for mantle shear wave speed, based on fitting Love and Rayleigh surface wave dispersion. In the inversion for transversely isotropic mantle structure, we use group speed measurements made on European event-to-station paths, and use a global a priori model (S20RTS) to ensure fair rendition of earth structure at depth and in border areas with little coverage from our data. The new mantle model sensibly improves over global S models in the imaging of shallow asthenospheric (slow) anomalies beneath the Alpine mobile belt, and fast lithospheric signatures under the two main Mediterranean subduction systems (Aegean and Tyrrhenian). We map compressional wave speed inverting ISC travel times (reprocessed by Engdahl et al.) with a non linear inversion scheme making use of finite-difference travel time calculation. The inversion is based on an a priori model obtained by scaling the 3D mantle S-wave speed to P. The new model substantially confirms images of descending lithospheric slabs and back-arc shallow asthenospheric regions, shown in

  12. Upper mantle seismic structure beneath southwest Africa from finite-frequency P- and S-wave tomography

    DEFF Research Database (Denmark)

    Soliman, Mohammad Youssof Ahmad; Yuan, Xiaohui; Tilmann, Frederik


    , probably related to surficial suture zones and the presence of fertile material. A shallower depth extent of the lithospheric plate of ∼100 km was observed beneath the ocean, consistent with plate-cooling models. In addition to tomographic images, the seismic anisotropy measurements within the upper mantle....... Utilizing 3D sensitivity kernels, we invert traveltime residuals to image velocity perturbations in the upper mantle down to 1000 km depth. To test the robustness of our tomographic image we employed various resolution tests which allow us to evaluate the extent of smearing effects and help defining...... the optimum inversion parameters (i.e. damping and smoothness) used during the regularization of inversion process. Resolution assessment procedure includes also a detailed investigation of the effect of the crustal corrections on the final images, which strongly influenced the resolution for the mantle...

  13. Constraining the rheology of the lithosphere and upper mantle with geodynamic inverse modelling (United States)

    Kaus, Boris; Baumann, Tobias


    The rheology of the lithosphere is of key importance for the physics of the lithosphere. Yet, it is probably the most uncertain parameter in geodynamics as experimental rock rheologies have to be extrapolated to geological conditions and as existing geophysical methods such as EET estimations make simplifying assumptions about the structure of the lithosphere. In many geologically interesting regions, such as the Alps, Andes or Himalaya, we actually have a significant amount of data already and as a result the geometry of the lithosphere is quite well constrained. Yet, knowing the geometry is only one part of the story, as we also need to have an accurate knowledge on the rheology and temperature structure of the lithosphere. Here, we discuss a relatively new method that we developed over the last few years, which is called geodynamic inversion. The basic principle of the method is simple: we compile available geophysical data into a realistic geometric model of the lithosphere and incorporate that into a thermo-mechanical numerical model of lithospheric deformation. In order to do so, we have to know the temperature structure, the density and the (nonlinear) rheological parameters for various parts of the lithosphere (upper crust, upper mantle, etc.). Rather than fixing these parameters we assume that they are all uncertain. This is used as a priori information to formulate a Bayesian inverse problem that employs topography, gravity, horizontal and vertical surface velocities to invert for the unknown material parameters and temperature structure. In order to test the general methodology, we first perform a geodynamic inversion of a synthetic forward model of intra-oceanic subduction with known parameters. This requires solving an inverse problem with 14-16 parameters, depending on whether temperature is assumed to be known or not. With the help of a massively parallel direct-search combined with a Markov Chain Monte Carlo method, solving the inverse problem

  14. Global upper-mantle tomography with the automated multimode inversion of surface and S-wave forms

    NARCIS (Netherlands)

    Lebedev, S.; Hilst, R.D. van der


    We apply the Automated Multimode Inversion of surface and S-wave forms to a large global data set, verify the accuracy of the method and assumptions behind it, and compute an Sv-velocity model of the upper mantle (crust–660 km). The model is constrained with ~51 000 seismograms recorded at 368

  15. The crustal and upper mantle structure around the Gulf of California, inferred from surface wave data and receiver functions

    NARCIS (Netherlands)

    de Vos, D.


    This thesis reports on studies that have been carried out to better image the crustal and upper mantle shear velocity structure around the Gulf of California, Mexico. The Gulf of California forms a part of the plate boundary between the Pacific and North-American plates, where transform motion in th

  16. Deep structure of crust and the upper mantle of the Mendeleev Rise on the Arktic­-2012 DSS profile

    DEFF Research Database (Denmark)

    Kashubin, Sergey; Petrov, Oleg; Artemieva, Irina;


    During high­latitude combined geological and geophysical expedition “Arctic­-2012”, deep seismic sounding (DSS) with ocean bottom seismometers were carried out in the Arctic Ocean along the line 740 km long, crossing the Mendeleev Rise at about 77° N. Crustal and upper mantle Vp­velocity and Vp...

  17. Three-dimensional density model of the Upper Mantle in the Middle East : Interaction of diverse tectonic processes

    NARCIS (Netherlands)

    Kaban, Mikhail; El Khrepy, Sami; Al-Arifi, Nassir; Tesauro, Magdala; Stolk, Ward


    We present a three-dimensional density model of the lithosphere and upper mantle for the Middle East and surroundings based on seismic, gravity, and seismic tomography data and analyze the main factors responsible for the density variations. The gravity effect of the crust is calculated and removed

  18. Broadband Seismic Investigations of the Upper Mantle Beneath the Vienna and Pannonian Basins (United States)

    Dando, B. D.; Stuart, G. W.; Houseman, G. A.; Team, C.


    The Carpathian Basins Project (CBP) aims to understand the origin of the Miocene-age extensional basins contained within the compressional arc of the Alpine-Carpathian system. To test competing models for the recent geological evolution of the Carpathian-Pannonian lithosphere and upper mantle, we present a new determination of P-wave velocity structure to depths of 700 km beneath this region. This model is based on inversion of seismic travel-time residuals from 97 broadband seismic stations. We include CBP data from a 15-month deployment of a high resolution network of 46 stations deployed NW-SE across the Vienna and western Pannonian basins through Austria, Hungary and Serbia, together with 10 broadband stations spread across the Pannonian basin and a further 41 permanent broadband stations. We use P-wave arrival times from approximately 341 teleseismic events. The 3-D velocity variation obtained by tomographic inversion of the P-wave travel-time residuals shows an approximately linear belt of fast material of width about 100 km, orientated WNW-ESE beneath the western Pannonian Basin at sub-lithospheric depths. This feature is apparently continuous with structure beneath the Eastern Alps, but becomes more diffuse into the transition zone. Our initial interpretation of these fast velocities is in terms of mantle downwelling related to the early collision of Adria and Europe. We use receiver functions to assess crustal structure variations. We also determine SKS anisotropy; regionally SKS varies systematically in direction, with a delay time of about 1.0s. E-W fast directions above the fast tomographic anomaly change to NW-SE across the Great Hungarian Plane and the Vienna Basin.

  19. Lithosphere structure and upper mantle characteristics below the Bay of Bengal (United States)

    Rao, G. Srinivasa; Radhakrishna, M.; Sreejith, K. M.; Krishna, K. S.; Bull, J. M.


    The oceanic lithosphere in the Bay of Bengal (BOB) formed 80-120 Ma following the breakup of eastern Gondwanaland. Since its formation, it has been affected by the emplacement of two long N-S trending linear aseismic ridges (85°E and Ninetyeast) and by the loading of ca. 20-km of sediments of the Bengal Fan. Here, we present the results of a combined spatial and spectral domain analysis of residual geoid, bathymetry and gravity data constrained by seismic reflection and refraction data. Self-consistent geoid and gravity modelling defined by temperature-dependent mantle densities along a N-S transect in the BOB region revealed that the depth to the lithosphere-asthenosphere boundary (LAB) deepens steeply from 77 km in the south to 127 km in north, with the greater thickness being anomalously thick compared to the lithosphere of similar-age beneath the Pacific Ocean. The Geoid-Topography Ratio (GTR) analysis of the 85°E and Ninetyeast ridges indicate that they are compensated at shallow depths. Effective elastic thickness (Te) estimates obtained through admittance/ coherence analysis as well as the flexural modelling along these ridges led to the conclusions: (i) 85°E Ridge was emplaced in off-ridge environment (Te = 10-15 km); (ii) the higher Te values of ˜25 km over the Afanasy Nikitin Seamount (ANS) reflect the secondary emplacement of the seamount peaks in off-ridge environment, (iii) that the emplacement of the Ninetyeast Ridge north of 2°N occurred in an off-ridge environment as indicated by higher Te values (25-30 km). Furthermore, the admittance analysis of geoid and bathymetry revealed that the admittance signatures at wavelengths >800 km are compensated by processes related to upper mantle convection.

  20. Upper mantle structure beneath the Alpine orogen from high-resolution teleseismic tomography (United States)

    Lippitsch, Regina; Kissling, Edi; Ansorge, JöRg


    To understand the evolution of the Alpine orogen, knowledge of the actual structure of the lithosphere-asthenosphere system is important. We perform high-resolution teleseismic tomography with manually picked P wave arrival times from seismograms recorded in the greater Alpine region. The resulting data set consists of 4199 relative P wave arrivals and 499 absolute P wave arrivals from 76 teleseismic events, corrected for the contribution of the Alpine crust to the travel times. The three-dimensional (3-D) crustal model established from controlled-source seismology data for that purpose represents the large-scale Alpine crustal structure. Absolute P wave arrival times are used to compute an initial reference model for the inversion. Tests with synthetic data document that the combination of nonlinear inversion, high-quality teleseismic data, and usage of an a priori 3-D crustal model allows a reliable resolution of cells at 50 km × 50 km × 30 km. Hence structures as small as two cells can be resolved in the upper mantle. Our tomographic images illuminate the structure of the uppermost mantle to depth of 400 km. Along strike of the Alps, the inversion reveals a high-velocity structure that dips toward the SE beneath the Adriatic microplate in the western and central Alps. In the eastern Alps we observe a northeastward dipping feature, subducting beneath the European plate. We interpret this feature in the western and central Alps as subducted, mainly continental European lower lithosphere. For the east, we propose that parts of the Vardar oceanic basin were subducted toward the NE, forcing continental Adriatic lower lithosphere to subduct northeastward beneath the European plate.

  1. The crust and upper mantle of central East Greenland - implications for continental accretion and rift evolution (United States)

    Schiffer, Christian; Balling, Niels; Ebbing, Jörg; Holm Jacobsen, Bo; Bom Nielsen, Søren


    The geological evolution of the North Atlantic Realm during the past 450 Myr, which has shaped the present-day topographic, crustal and upper mantle features, was dominated by the Caledonian orogeny and the formation of the North Atlantic and associated igneous activity. The distinct high altitude-low relief landscapes that accompany the North Atlantic rifted passive margins are the focus of a discussion of whether they are remnant and modified Caledonian features or, alternatively, recently uplifted peneplains. Teleseismic receiver function analysis of 11 broadband seismometers in the Central Fjord Region in East Greenland indicates the presence of a fossil subduction complex, including a slab of eclogitised mafic crust and an overlying wedge of hydrated mantle peridotite. This model is generally consistent with gravity and topography. It is shown that the entire structure including crustal thickness variations and sub-Moho heterogeneity gives a superior gravity and isostatic topographic fit compared to a model with a homogeneous lithospheric layer (1). The high topography of >1000 m in the western part of the area is supported by the c. 40 km thick crust. The eastern part requires buoyancy from the low velocity/low density mantle wedge. The geometry, velocities and densities are consistent with structures associated with a fossil subduction zone. The spatial relations with Caledonian structures suggest a Caledonian origin. The results indicate that topography is isostatically compensated by density variations within the lithosphere and that significant present-day dynamic topography seems not to be required. Further, this structure is suggested to be geophysically very similar to the Flannan reflector imaged north of Scotland, and that these are the remnants of the same fossil subduction zone, broken apart and separated during the formation of the North Atlantic in the early Cenozoic (2). 1) Schiffer, C., Jacobsen, B.H., Balling, N., Ebbing, J. and Nielsen, S

  2. Seismic tomography reveals the upper-mantle structure beneath the Carpathian-Pannonian system (United States)

    Dando, B. D.; Houseman, G.; Stuart, G. W.; Hegedus, E.; Kovacs, A.; Brueckl, E. P.; Hausmann, H.; Radovanovic, S.


    The Carpathian Basins Project (CBP) aims to understand the formation of the Miocene-age extensional basins contained within the convergent arc of the Alpine-Carpathian system. To test competing models for the recent geological evolution of the Carpathian-Pannonian lithosphere and upper mantle, we present a new tomographic determination of P-wave velocity structure to depths of 700 km beneath this region. This model is based on inversion of seismic travel-time residuals from 97 broadband seismic stations. We include CBP data from a 15-month deployment of a high resolution network of 46 stations deployed NW-SE across the Vienna and western Pannonian basins through Austria, Hungary and Serbia, together with 10 broadband stations spread across the Pannonian basin and a further 41 permanent broadband stations. We use P-wave arrival times from 232 teleseismic events. To avoid contamination of our inversion results from crustal velocity variations, deterministic corrections are applied to our travel-time residuals using crustal velocity models obtained from controlled source experiments and sediment thickness maps. Our 3-D velocity model images the fast velocity structure of the eastern Alps down to ~350 km. Beneath the Pannonian basin the velocity variation at 300 km depth is dominated by a fast region which extends eastward from the Alpine anomaly and reaches down into the mantle transition zone (MTZ). This fast structure is limited on the North side by slow material beneath the North Carpathians. At depths greater than 450 km, below the eastern Pannonian basin, a slow anomaly extends to the base of the model. Beneath the same region Hetenyi et al. (submitted to GRL), used receiver functions from the CBP dataset, to show a localised depression of the 660 km discontinuity of up to ~40 km. We aim to address how the depression of the 660 km discontinuity and its associated density and velocity variations affect our tomographic images. Our results will help to provide

  3. The Mono Arch, eastern Sierra region, California: Dynamic topography associated with upper-mantle upwelling? (United States)

    Jayko, A.S.


    A broad, topographic flexure localized east of and over the central and southern Sierra Nevada, herein named the Mono Arch, apparently represents crustal response to lithospheric and/or upper-mantle processes, probably dominated by mantle upwelling within the continental interior associated Pacific-North American plate-boundary deformation. This zone of flexure is identified through comparison between the topographic characteristics of the active Cascade volcanic arc and backarc regions with the analogous former arc and backarc in the Sierra Nevada and eastern Sierra Nevada. Serial topographic profiles measured normal to the modern Cascade backarc reveal an accordance of topographic lows defined by valley floors with an average minimum elevation of ???1400-1500m for over 175km to the southeast. Although the accordance drops in elevation slightly to the south, the modern Cascade backarc region is remarkably level, and is characterized by relief up to ???750m above this baseline elevation. By contrast, serial topographic profiles over the former arc and backarc transitions of the eastern Sierra region exhibit a regional anticlinal warping defined by accordant valley floors and by a late Miocene-early Pliocene erosion surface and associated deposits. The amplitude of this flexure above regionally flat baseline elevations to the east varies spatially along the length of the former Sierran arc, with a maximum of ???1000m centred over the Bridgeport Basin. The total zone of flexure is approximately 350km long N-S and 100km wide E-W, and extends from Indian Wells Valley in the south to the Sonora Pass region in the north. Previous geophysical, petrologic, and geodetic studies suggest that the Mono Arch overlies a zone of active mantle upwelling. This region also represents a zone crustal weakness formerly exploited by the middle-to-late Miocene arc and is presently the locus of seismic and volcanic activities. This seismic zone, which lies east of the Sierra Nevada block

  4. Crustal and upper mantle structure of central Qiangtang terrane (Tibet Plateau) imaged with magnetotelluric data (United States)

    Zeng, S.; Hu, X.; Li, J.


    Since the Tethys Ocean closed, the ongoing collision between India and Aisa continents has created the Tibet Plateau, which is the most spectacular topographic feature on the surface of the earth. In the last decades, a large number of geological and geophysical studies have been undertaken in the Tibet Plateau, but most of these studies were focused on southern Tibet, where the collision of the Indian tectonic plate with Eurasia was occurred, and southeast Tibet, where lateral extrusion of crustal material may be occurred, absent in the central Tibet. As research continues, it has become clear that a complete understanding of the formation and deformation of the Tibet Plateau requires a study of the entire plateau. The Qiangtang terrane is located in the central Tibet Plateau. In 1993-1994, three profiles of broadband MT data (320 Hz to 2000 s) along N-S trending ranges from 86°E to 91°E were collected by China University of Geoscience in central Qiangtang terrane for the purpose of oil and gas exploration, the previous interpretation was focused on the shallow structures. In this study, we reanalyze the three MT profiles to produce more detailed images of the deep electrical structure of the Qiangtang terrane. Dimensionality analysis and geoelectric strike analysis of these data show that they appear to be two dimensional. 2-D inversion model show that there is a pervasive conductivity layer in the mid- to lower crustal and upper mantle, especially in the north Qiangtang terrane, which was considered to be the result of partial melt. The partial melt fraction is sufficient for crustal flow to occur. The similarity of the inversion models of the three profiles show that there is west-east crustal flow along the Jinsha River suture in central Qiangtang terrane, which seems to be western extension of the crustal flow observed in southeast Tibet by Bai et al. (2010). The inversion results also show difference of the electrical structure between the west and east

  5. Hunting for the Tristan mantle plume - An upper mantle tomography around the volcanic island of Tristan da Cunha (United States)

    Schlömer, Antje; Geissler, Wolfram H.; Jokat, Wilfried; Jegen, Marion


    The active volcanic island Tristan da Cunha, located at the southwestern and youngest end of the Walvis Ridge - Tristan/Gough hotspot track, is believed to be the surface expression of a huge thermal mantle anomaly. While several criteria for the diagnosis of a classical hotspot track are met, the Tristan region also shows some peculiarities. Consequently, it is vigorously debated if the active volcanism in this region is the expression of a deep mantle plume, or if it is caused by shallow plate tectonics and the interaction with the nearby Mid-Atlantic Ridge. Because of a lack of geophysical data in the study area, no model or assumption has been completely confirmed. We present the first amphibian P-wave finite-frequency travel time tomography of the Tristan da Cunha region, based on cross-correlated travel time residuals of teleseismic earthquakes recorded by 24 ocean-bottom seismometers. The data can be used to image a low velocity structure southwest of the island. The feature is cylindrical with a radius of ∼100 km down to a depth of 250 km. We relate this structure to the origin of Tristan da Cunha and name it the Tristan conduit. Below 250 km the low velocity structure ramifies into narrow veins, each with a radius of ∼50 km. Furthermore, we imaged a linkage between young seamounts southeast of Tristan da Cunha and the Tristan conduit.

  6. Duration of the hydrocarbon fluid formation under thermobaric conditions of the Earth's upper mantle (United States)

    Mukhina, Elena; Kolesnikov, Anton; Kutcherov, Vladimir


    Deep abiogenic formation of hydrocarbons is an inherent part of the Earth's global carbon cycle. It was experimentally confirmed that natural gas could be formed from inorganic carbon and hydrogen containing minerals at pressure and temperature corresponding to the Earth's upper mantle conditions. Reaction between calcite, wustite and water in the large volume device was studied in several works. It was previously proposed that reaction is possible only after 40 minutes of exposure at high pressure and temperature. In this work similar experiment at P = 60 kbar and T = 1200 K were carried out in "Toroid" type chamber with the 5 seconds duration of thermobaric exposure. Gas chromatographic analysis of the reaction products has shown the presence of hydrocarbon mixture comparable to 5 minutes and 6 hours exposure experiments. Based on this fact it is possible to conclude that the reaction of natural gas formation is instant at least at given thermobaric conditions. This experiment will help to better understand the process of deep hydrocarbon generation, particularly its kinetics.

  7. Potential mechanisms of pore-fluid movement from continental lithospheric mantle into upper continental crust

    Institute of Scientific and Technical Information of China (English)

    ZHAO Chong-bin; PENG Sheng-lin; LIU Liang-ming; B.E.HOBBS; A.ORD


    Through integrating the state of the art scientific knowledge in different research fields, some potential mechanisms of large-scale movements of underground pore-fluids such as H2O and CO2 in the continental lithosphere were presented and discussed. The results show that the generation and propagation of porosity waves are important mechanisms to transport mass and heat fluxes from the continental lithospheric mantle into the lower continental crust; the generation and propagation of porosity waves, pore-fluid flow focusing through lower and middle crustal faults, aclvection of pore-fluids through the lower and middle crust, and whole-crustconvection in some particular cases are important mechanisms to transport mass and heat fluxes from the lower into the upper continental crust; heat and mass transport through convective pore-fluid flow is the most effective mechanism of ore body formation and mineralization in hydrothermal systems; due to heat and mass exchange at the interface between the earth surface, hydrosphere and atmosphere, it is very important to consider the hydro-geological effect of the deep earth pore-fluids such as H2O and CO2 on the global warming and climate change in future investigations.

  8. Titanium-hydroxyl defect-controlled rheology of the Earth's upper mantle (United States)

    Faul, Ulrich H.; Cline, Christopher J.; David, Emmanuel C.; Berry, Andrew J.; Jackson, Ian


    Experiments were conducted with hydrous olivine to investigate the defect responsible for the influence of water (hydrogen structurally incorporated as hydroxyl) on the olivine rheology. Solution-gelation derived Fo90 olivine doped with nominally 0.04-0.1 wt.% TiO2 was first hot-pressed and then deformed in platinum capsules at 300 MPa confining pressure and temperatures from 1200- 1350°C. The water content was not buffered so that deformation occurred at water-undersaturated conditions. Due to the enhanced grain growth under hydrous conditions, the samples were at least a factor of three more coarse-grained than their dry counterparts and deformed in powerlaw creep at differential stresses as low as a few tens of MPa. Since all experiments were conducted at the same confining pressure, the essentially linear relationship between strain rate and water content was for the first time determined independently of an activation volume. Infrared spectra are dominated by absorption bands at 3572 and 3525 cm-1. These bands also predominate in infrared spectra of natural olivine, and can only be reproduced experimentally in the presence of titanium. In contrast to the previous interpretation of the hydrous rheology in terms of intrinsic point defects, the experiments show that extrinsic defects (impurities) in natural olivine play the dominant role for water weakening at the water contents expected for most of the upper mantle.

  9. Two-component mantle melting-mixing model for the generation of mid-ocean ridge basalts: Implications for the volatile content of the Pacific upper mantle (United States)

    Shimizu, Kei; Saal, Alberto E.; Myers, Corinne E.; Nagle, Ashley N.; Hauri, Erik H.; Forsyth, Donald W.; Kamenetsky, Vadim S.; Niu, Yaoling


    We report major, trace, and volatile element (CO2, H2O, F, Cl, S) contents and Sr, Nd, and Pb isotopes of mid-ocean ridge basalt (MORB) glasses from the Northern East Pacific Rise (NEPR) off-axis seamounts, the Quebrada-Discovery-GoFar (QDG) transform fault system, and the Macquarie Island. The incompatible trace element (ITE) contents of the samples range from highly depleted (DMORB, Th/La ⩽ 0.035) to enriched (EMORB, Th/La ⩾ 0.07), and the isotopic composition spans the entire range observed in EPR MORB. Our data suggest that at the time of melt generation, the source that generated the EMORB was essentially peridotitic, and that the composition of NMORB might not represent melting of a single upper mantle source (DMM), but rather mixing of melts from a two-component mantle (depleted and enriched DMM or D-DMM and E-DMM, respectively). After filtering the volatile element data for secondary processes (degassing, sulfide saturation, assimilation of seawater-derived component, and fractional crystallization), we use the volatiles to ITE ratios of our samples and a two-component mantle melting-mixing model to estimate the volatile content of the D-DMM (CO2 = 22 ppm, H2O = 59 ppm, F = 8 ppm, Cl = 0.4 ppm, and S = 100 ppm) and the E-DMM (CO2 = 990 ppm, H2O = 660 ppm, F = 31 ppm, Cl = 22 ppm, and S = 165 ppm). Our two-component mantle melting-mixing model reproduces the kernel density estimates (KDE) of Th/La and 143Nd/144Nd ratios for our samples and for EPR axial MORB compiled from the literature. This model suggests that: (1) 78% of the Pacific upper mantle is highly depleted (D-DMM) while 22% is enriched (E-DMM) in volatile and refractory ITE, (2) the melts produced during variable degrees of melting of the E-DMM controls most of the MORB geochemical variation, and (3) a fraction (∼65% to 80%) of the low degree EMORB melts (produced by ∼1.3% melting) may escape melt aggregation by freezing at the base of the oceanic lithosphere, significantly enriching it in

  10. P-wave velocities of main upper mantle minerals at high temperature and high pressure and its geological implication

    Institute of Scientific and Technical Information of China (English)

    宋茂双; 谢鸿森; 郑海飞; 徐有生; 郭捷; 许祖鸣


    At 0 - 5.0GPa and room temperature to 1400℃, the P-wave velocities of olivine, clinopyroxene and orthopyroxene, which are three common minerals from upper mantle, are measured under both simulated oceanic and continental geothermal gradients. The experimental results indicate that the P-wave velocities of these minerals increase with depth under both geothermal gradients. This implicates that pressure is more important than temperature in deep earth in controlling the P-wave velocities of mantle minerals, but the increase of temperature has greater effect on P-wave velocities of main mantle minerals at greater depth than at smaller depth. At low pressure, the measured P-wave velocities of mantle minerals are smaller than their true values due to fracturing, compaction process and recrystallization of mineral powder. The true P-wave velocities of mantle minerals can be obtained at lower pressure by the extrapolation of measured velocities at a high pressure. At higher depth, all these three minerals sho

  11. Evidence for an upper mantle low velocity zone beneath the southern Basin and Range-Colorado Plateau transition zone (United States)

    Benz, H.M.; McCarthy, J.


    A 370-km-long seismic refraction/wide-angle reflection profile recorded during the Pacific to Arizona Crustal Experiment (PACE) detected an upper mantle P-wave low-velocity zone (LVZ) in the depth range 40 to 55 km beneath the Basin and Range in southern Arizona. Interpretation of seismic data places constraints on the sub-crustal lithosphere of the southern Basin and Range Province, which is important in light of the active tectonics of the region and the unknown role of the sub-crustal lithosphere in the development of the western United States. Forward travel time and synthetic seismogram techniques are used to model this shallow upper mantle LVZ. Modeling results show that the LVZ is defined by a 5% velocity decrease relative to a Pn velocity of 7.95 km s−1, suggesting either a ∼3–5% mafic partial melt or high-temperature, sub-solidus peridotite.

  12. The electrical conductivity of the upper mantle and lithosphere from the magnetic signal due to ocean tidal flow

    DEFF Research Database (Denmark)

    Schnepf, Neesha Regmi; Kuvshinov, Alexey; Grayver, Alexander

    Oceans cover about seventy percent of the Earth and yet the overwhelming majority of seismological or electromagnetic (EM) observatories are found on continents. This provides a challenge for understanding composition, structure, and dynamics of Earth’s lithosphere and upper mantle in oceanic...... regions. The recent expansion in magnetic data from low-Earth orbiting satellite missions (Ørsted, CHAMP, SAC-C, and Swarm) has led to a rising interest in probing Earth from space. The largest benefit of using satellite data is much improved spatial coverage. Additionally, and in contrast to ground......-based data, satellite data are overall uniform and very high quality. Probing the conductivity of the lithosphere and upper mantle requires EM variations with periods of a few hours. Electric currents generated by oceanic tides are a well-suited source within this period range. Ocean tides interact...

  13. Rheology of the Deep Upper Mantle and its Implications for the Preservation of the Continental Roots: A Review

    Energy Technology Data Exchange (ETDEWEB)

    Karato, S.


    The longevity of deep continental roots depends critically on the rheological properties of upper mantle minerals under deep upper mantle conditions. Geodynamic studies suggest that the rheological contrast between the deep continental and oceanic upper mantle is a key factor that controls the longevity of the continental roots. Current understanding of rheological properties of deep upper mantle is reviewed to examine how a large enough rheological contrast between the continental and oceanic upper mantle develops that leads to the longevity of the deep continental roots. Based on the microstructures of naturally deformed deep continental rocks as well as on the observations of seismic anisotropy, it is concluded that power-law dislocation creep dominates in most of the deep upper mantle. Deformation by power-law creep is sensitive to water content and therefore the removal of water by partial melting to form depleted continental roots is a likely mechanism to establish a large rheological contrast. The results of experimental studies on the influence of temperature, pressure and water content on plastic flow by power-law dislocation creep are reviewed. The degree of rheological contrast depends critically on the dependence of effective viscosity on water content under 'wet' (water-rich) conditions but it is also sensitive to the effective viscosity under 'dry' (water-free) conditions that depends critically on the influence of pressure on deformation. Based on the analysis of thermodynamics of defects and high-temperature creep, it is shown that a robust estimate of the influence of water and pressure can be made only by the combination of low-pressure (< 0.5 GPa) and high-pressure (> 5 GPa) studies. A wide range of flow laws has been reported, leading to nearly 10 orders of magnitude differences in estimated viscosities under the deep upper mantle conditions. However, based on the examination of several criteria, it is concluded that

  14. "DOBREfraction'99" - Velocity Model of the Crust and Upper Mantle Beneath the Donbas Foldbelt (east Ukraine) (United States)

    Omelchenko, V.; Starostenko, V. I.; Stephenson, R. A.; Guterch, A.; Janik, T.; Grad, M.; Stovba, S. M.; Tolkunov, A.; Thybo, H.; Lang, R.; Lyngsie, S. B.; Keller, G. R.


    The East European Craton (EEC) contains a classic example of the tectonic inversion of a continental rift zone. The Donbas Foldbelt (DF) is the uplifted and deformed part of the up to 20-km thick Dniepr-Donets Basin that formed as the result of rifting of the EEC in the Late Devonian. The DF, especially its southern margin, was uplifted in Early Permian times, in a (trans)tensional tectonic stress regime while folding and reverse faulting mainly occurred later primarily during the Late Cretaceous. A seismic refraction/wide-angle reflection survey was carried out in 1999 to complement existing Deep Seismic Sounding data from the area that, because maximum offsets were generally not greater than about 150 km, did not record significant Pn phase arrivals. The 1999 main survey comprised some 245 recording stations along a line of 360 km length, with 11 in-line shotpoints, extending from the shores of the Azov Sea in the south, across the Azov Massif of the Ukrainian Shield and the DF, ending at the Ukraine-Russia border in the Voronezh Massif of the EEC. Particular scientific targets included the nature of the crust-mantle transition and the geometry of crustal/upper mantle structures related to rifting and subsequent basin inversion. Tomographic inversion as well as ray-trace based velocity modeling has been carried out. The velocity signature of the sedimentary basin itself is well resolved, indicating an asymmetric form (basement surface dipping more gently towards the center of the basin from the north than from the south) and a total thickness of about 20-km, comparable to estimates derived from previous seismic studies and geological interpretations. A thick ( more 10-km), high velocity (more than 6.9 km/s) lower crustal body lies beneath the rift basin itself (DF) but is offset slightly to the north compared to the main basin depocenter. This layer is most likely related to the earlier rifting processes and may represent magmatic underplating. Velocities in the

  15. Seismic Imaging of the crust and upper mantle beneath Afar, Ethiopia (United States)

    Hammond, J. O.; Kendall, J. M.; Stuart, G. W.; Ebinger, C. J.


    In March 2007 41 seismic stations were deployed in north east Ethiopia. These stations recorded until October 2009, whereupon the array was condensed to 13 stations. Here we show estimates of crustal structure derived from receiver functions and upper mantle velocity structure, derived from tomography and shear-wave splitting using the first 2.5 years of data. Bulk crustal structure has been determined by H-k stacking receiver functions. Crustal Thickness varies from ~45km on the rift margins to ~16km beneath the northeastern Afar stations. Estimates of Vp/Vs show normal continental crust values (1.7-1.8) on the rift margins, and very high values (2.0-2.2) in Afar, similar to results for the Main Ethiopian Rift (MER). This supports ideas of high levels of melt in the crust beneath the Ethiopian Rift. Additionally, we use a common conversion point migration technique to obtain high resolution images of crustal structure beneath the region. Both techniques show a linear region of thin crust (~16km) trending north-south, the same trend as the Red Sea rift. SKS-wave splitting results show a general north east-south west fast direction in the MER, systematically rotating to a more north-south fast direction towards the Red Sea. Additionally, stations close to the recent Dabbahu diking episode show sharp lateral changes over small lateral distances (40° over Danakil microplate. Outside of these focused regions the velocities are relatively fast. Below ~250km the anomaly broadens to cover most of the Afar region with only the rift margins remaining fast. At transition zone depths little anomaly is seen beneath Afar, but some low velocities remain present beneath the MER. These studies suggest that in northern Ethiopia the Red Sea rift is dominant. The presence of thin crust beneath northern Afar suggests that the Red Sea rift is creating oceanic like crust in this region. The lack of deep mantle low velocity anomalies beneath Afar suggest that a typical narrow conduit

  16. Preface to the special issue on the crustal and upper mantle structure and geodynamics in Chinese mainland

    Institute of Scientific and Technical Information of China (English)

    Chunyong Wang


    @@ Over the past decade, one important development was the rapid and significant improvement of instru-ments and observation techniques, such as the broad-band seismometers and the seismic-array observations, which permit to widespread collect seismic wave and then clearly demonstrate the crustal and upper mantle structure. The structural and tectonic settings are mainly obtained in the comprehensive analyses of the data from geology, geophysics and seismological phenomena.

  17. Scales of preservation and root causes of heterogeneities in the convecting upper mantle of the Iapetan Ocean (United States)

    O'Driscoll, B.; Walker, R. J.; Day, J. M.; Daly, J. S.


    Ophiolites are particularly valuable resources for assessing the timing, causes and extent of mantle heterogeneity. This is because appropriate examples allow field-based observations to be coupled with geochemical investigations of otherwise inaccessible mantle. The Shetland (UK; ~492 Ma) and Leka (Norway; ~497 Ma) ophiolites each comprise a portion of early-Paleozoic (Iapetan) oceanic lithosphere with well-exposed mantle and lower crustal sections, as well as petrologic Mohorovicic discontinuities. The mantle sections of both ophiolites are highly heterogeneous at the cm-to-m scales, most noticeably manifested by the formation of dunite lenses and layers in dominantly harzburgitic host rock. In the Leka and Shetland rocks, dunite bodies often host podiform chromitite (≥ 60 vol.% Cr-spinel) seams. At both localities, chromitite seams are 0.01-2 m thick and at the outcrop scale may appear complexly deformed, forming stock-work veins and patches. Shetland peridotites are pervasively serpentinized, so that primary silicate minerals (olivine ± orthopyroxene) are rarely preserved. Despite this alteration, comparisons with abyssal peridotites suggest that Shetland peridotites and chromitites generally retain their primary magmatic Re-Os isotope signatures and highly siderophile element (HSE) abundances. Harzburgites preserve evidence for a Mesoproterozoic depletion event, but are dominated by contemporary chondritic, ambient upper mantle compositions (γOs492Ma of -5.3 to +2.6). Dunite bodies have more variable and more radiogenic Os isotope compositions (γOs492Ma of -3.3 to +12.4) than harzburgites; a feature imparted as a consequence of high degrees of melt-rock interaction during supra-subduction zone (SSZ)-type melt extraction. Sub-m scale mantle heterogeneities with respect to Os isotopes are common. One Shetland harzburgite-dunite pair sampled <1 m apart has γOs492Ma values of -5.3 and +1.3, respectively. Individual chromitite seams, separated by hundreds of


    Directory of Open Access Journals (Sweden)

    Alexander V. Pospeev


    Full Text Available The article is aimed at discussion of geological and geophysical aspects of the ‘asthenospheric’ interpretation of the ‘anomalous’ mantle layer that is revealed in the Baikal rift zone by deep seismic sounding (DSS methods. Based on the analysis of the geoelectrical model, estimations of rheological properties, regional geothermal and deep petrological data, it is concluded that the ‘anomalous’ mantle phenomenon should be interpreted within the framework of solid-phase models. It is shown that the actual minimum depth to the top of the asthenosphere is about 60–70 km in the region under study, and temperatures at the surface of the Earth’s mantle varies from 600 to 900 °С. It is most probable that velocities are reduced in the ‘anomalous’ mantle layer due to the presence of hightemperature spinel-pyroxene facies of the mantle rocks.

  19. Transient rheology of the upper mantle beneath central Alaska inferred from the crustal velocity field following the 2002 Denali earthquake (United States)

    Pollitz, F.F.


    The M7.9 2002 Denali earthquake, Alaska, is one of the largest strike-slip earthquakes ever recorded. The postseismic GPS velocity field around the 300-km-long rupture is characterized by very rapid horizontal velocity up to ???300 mm/yr for the first 0.1 years and slower but still elevated horizontal velocity up to ???100 mm/yr for the succeeding 1.5 years. I find that the spatial and temporal pattern of the displacement field may be explained by a transient mantle rheology. Representing the regional upper mantle as a Burghers body, I infer steady state and transient viscosities of ??1 = 2.8 ?? 1018 Pa s and ??2 = 1.0 ?? 1017 Pa s, respectively, corresponding to material relaxation times of 1.3 and 0.05 years. The lower crustal viscosity is poorly constrained by the considered horizontal velocity field, and the quoted mantle viscosities assume a steady state lower crust viscosity that is 7??1. Systematic bias in predicted versus observed velocity vectors with respect to a fixed North America during the first 3-6 months following the earthquake is reduced when all velocity vectors are referred to a fixed site. This suggests that the post-Denali GPS time series for the first 1.63 years are shaped by a combination of a common mode noise source during the first 3-6 months plus viscoelastic relaxation controlled by a transient mantle rheology.

  20. Fine scale heterogeneity in the Earth's upper mantle - observation and interpretation

    DEFF Research Database (Denmark)

    Thybo, Hans


    can be correlated to main plate tectonic features, such as oceanic spreading centres, continental rift zones and subducting slabs. Much seismological mantle research is now concentrated on imaging fine scale heterogeneity, which may be detected and imaged with high-resolution seismic data with dense......High resolution seismic data has over the last decade provided significant evidence for pronounced fine scale heterogeneity in the Earth’s mantle at an unprecedented detail. Seismic tomography developed tremendously during the last 20-30 years. The results show overall structure in the mantle which...... station spacing and at high frequency, e.g. from the Russian Peaceful Nuclear Explosion (PNE) data set and array recordings of waves from natural seismic sources. Mantle body waves indicate pronounced heterogeneity at three depth levels whereas other depth intervals appear transparent, at least...

  1. Crust and upper-mantle seismic anisotropy variations from the coast to inland in central and Southern Mexico (United States)

    Castellanos, Jorge; Pérez-Campos, Xyoli; Valenzuela, Raúl; Husker, Allen; Ferrari, Luca


    Subduction zones are among the most dynamic tectonic environments on Earth. Deformation mechanisms of various scales produce networks of oriented structures and faulting systems that result in a highly anisotropic medium for seismic wave propagation. In this study, we combine shear wave splitting inferred from receiver functions and the results from a previous SKS-wave study to quantify and constrain the vertically averaged shear wave splitting at different depths along the 100-station MesoAmerican Subduction Experiment array. This produces a transect that runs perpendicular to the trench across the flat slab portion of the subduction zone below central and southern Mexico. Strong anisotropy in the continental crust is found below the Trans-Mexican Volcanic Belt (TMVB) and above the source region of slow-slip events. We interpret this as the result of fluid/melt ascent. The upper oceanic crust and the overlying low-velocity zone exhibit highly complex anisotropy, while the oceanic lower crust is relatively homogeneous. Regions of strong oceanic crust anisotropy correlate with previously found low Vp/Vs regions, indicating that the relatively high Vs is an anisotropic effect. Upper-mantle anisotropy in the southern part of the array is in trench-perpendicular direction, consistent with the alignment of type-A olivine and with entrained subslab flow. The fast polarization direction of mantle anisotropy changes to N-S in the north, likely reflecting mantle wedge corner flow perpendicular to the TMVB.

  2. "DOBREfraction'99" - Velocity models of the crust and upper mantle beneath the Donbas Foldbelt (SE Ukraine) (United States)

    Stephenson, R. A.; Dobrefraction'00 Working Group,.


    The Pripyat-Dniepr-Donets basin (PDD) is a Late Devonian rift basin located on the southwestern part of the East-European Craton (EEC). This rift basin strikes in a southeasterly direction and extends from Belarus through Ukraine, where it connects with the Donbas foldbelt and its continuation as the deformed southern margin of the craton (Karpinsky Swell) in southern Russia. The Pripyat and Dniepr-Donets basins are important hydrocarbon provinces. The Donbas foldbelt (DF) is the uplifted and deformed part of the 20-km thick Dniepr-Donets basin. In 1999, an international cooperative deep seismic sounding (DSS) experiment (DOBREfraction'99) was undertaken. This effort involved 11 in-line shotpoints and deployment of some 245 recording stations along a northeast-trending, 360 km long profile extending from the shores of the Azov Sea in the south, across the Azov Massif (Ukrainian Shield), the DF, ending at the Ukraine-Russia border in the Voronezh Massif of the EEC. Particular scientific targets included the nature of the crust-mantle transition and the geometry of crustal/upper mantle structures related to rifting and subsequent basin inversion. Tomographic inversion, as well as, ray-trace based velocity modelling has been carried out. The velocity signature of the sedimentary basin itself is well resolved, indicating an asymmetric form (basement surface dipping more gently towards the center of the basin from the north than from the south) and a total thickness of about 20-km, comparable to estimates derived from previous seismic studies and geological interpretations. A thick (>10-km), high-velocity (>6.9 km/s), lower crustal body lies beneath the rift basin itself. This layer forms a domal structure that is offset slightly to the north compared to the main basin depocenter. A thinner (~5-km) high velocity layer is inferred beneath the southern margin of the Donbas foldbelt and Azov Massif. The former could be related to Permian uplift with the latter being due to

  3. Thermodynamic evolution of lithosphere of the North China craton: Records from lower crust and upper mantle xenoliths from Hannuoba

    Institute of Scientific and Technical Information of China (English)

    LIU Yongsheng; GAO Shan; LIU Xiaoming; CHEN Xiaoming; ZHANG Wenlan; WANG Xuance


    Major element compositions of garnet, clinopyroxene, orthopyroxene andspinel in the garnet-bearing lower crust and upper mantle xenoliths from Hannuoba, North China craton are analyzed by the electron microprobe (EMP). The pressure-temperature estimates reveal the increasing temperature and pressure from core to rim for granulites. In contrast, mantle xenoliths with core temperature > 930℃ recorded a history of decrease in temperature and pressure. However, those with core temperature < 930℃ show a negligible change. The final pressures recorded by these xenoliths cluster at 0.9-1.5 GPa. The presence of high- Na2O cpx in granulite xenoliths suggests that they are products of the transition from granulite to eclogite metamorphism corresponding to the increasing temperature andpressure. Together with previous studies, it is suggested that the P-T changes preserved in the xenoliths are related to lithospheric thickening and subsequentthinning prior to their eruption in the Cenozoic.

  4. Upper mantle anisotropy beneath Peru from SKS splitting: Constraints on flat slab dynamics and interaction with the Nazca Ridge (United States)

    Eakin, Caroline M.; Long, Maureen D.; Wagner, Lara S.; Beck, Susan L.; Tavera, Hernando


    The Peruvian flat slab is by far the largest region of flat subduction in the world today, but aspects of its structure and dynamics remain poorly understood. In particular, questions remain over whether the relatively narrow Nazca Ridge subducting beneath southern Peru provides dynamic support for the flat slab or it is just a passive feature. We investigate the dynamics and interaction of the Nazca Ridge and the flat slab system by studying upper mantle seismic anisotropy across southern Peru. We analyze shear wave splitting of SKS, sSKS, and PKS phases at 49 stations distributed across the area, primarily from the PerU Lithosphere and Slab Experiment (PULSE). We observe distinct spatial variations in anisotropic structure along strike, most notably a sharp transition from coherent splitting in the north to pervasive null (non-split) arrivals in the south, with the transition coinciding with the northern limit of the Nazca Ridge. For both anisotropic domains there is evidence for complex and multi-layered anisotropy. To the north of the ridge our *KS splitting measurements likely reflect trench-normal mantle flow beneath the flat slab. This signal is then modified by shallower anisotropic layers, most likely in the supra-slab mantle, but also potentially from within the slab. To the south the sub-slab mantle is similarly anisotropic, with a trench-oblique fast direction, but widespread nulls appear to reflect dramatic heterogeneity in anisotropic structure above the flat slab. Overall the regional anisotropic structure, and thus the pattern of deformation, appears to be closely tied to the location of the Nazca Ridge, which further suggests that the ridge plays a key role in the mantle dynamics of the Peruvian flat slab system.

  5. Compositional and temperature variations of the Pacific upper mantle since the Cretaceous

    Institute of Scientific and Technical Information of China (English)

    ZHANG Guoliang


    The geological evolution of the Earth during the mid-Cretaceous were shown to be anomalous, e.g., the pause of the geomagnetic field, the global sea level rise, and increased intra-plate volcanic activities, which could be attributed to deep mantle processes. As the anomalous volcanic activities occurred mainly in the Cretaceous Pacific, here we use basalt chemical compositions from the oceanic drilling (DSDP/ODP/IODP) sites to investigate their mantle sources and melting conditions. Based on locations relative to the Pacific plateaus, we classified these sites as oceanic plateau basalts, normal mid-ocean ridge basalts, and near-plateau seafloor basalts. This study shows that those normal mid-ocean ridge basalts formed during mid-Cretaceous are broadly similar in average Na8, La/Sm and Sm/Yb ratios and Sr-Nd isotopic compositions to modern Pacific spreading ridge (the East Pacific Rise). The Ontong Java plateau (125–90 Ma) basalts have distinctly lower Na8 and 143Nd/144Nd, and higher La/Sm and87Sr/86Sr than normal seafloor basalts, whereas those for the near-plateau seafloor basalts are similar to the plateau basalts, indicating influences from the Ontong Java mantle source. The super mantle plume activity that might have formed the Ontong Java plateau influenced the mantle source of the simultaneously formed large areas of seafloor basalts. Based on the chemical data from normal seafloor basalts, I propose that the mantle compositions and melting conditions of the normal mid-ocean ridges during the Cretaceous are similar to the fast spreading East Pacific Rise. Slight variations of mid-Cretaceous normal seafloor basalts in melting conditions could be related to the local mantle source and spreading rate.

  6. Upper- and mid-mantle interaction between the Samoan plume and the Tonga–Kermadec slabs (United States)

    Chang, Sung-Joon; Ferreira, Ana M. G.; Faccenda, Manuele


    Mantle plumes are thought to play a key role in transferring heat from the core–mantle boundary to the lithosphere, where it can significantly influence plate tectonics. On impinging on the lithosphere at spreading ridges or in intra-plate settings, mantle plumes may generate hotspots, large igneous provinces and hence considerable dynamic topography. However, the active role of mantle plumes on subducting slabs remains poorly understood. Here we show that the stagnation at 660 km and fastest trench retreat of the Tonga slab in Southwestern Pacific are consistent with an interaction with the Samoan plume and the Hikurangi plateau. Our findings are based on comparisons between 3D anisotropic tomography images and 3D petrological-thermo-mechanical models, which self-consistently explain several unique features of the Fiji–Tonga region. We identify four possible slip systems of bridgmanite in the lower mantle that reconcile the observed seismic anisotropy beneath the Tonga slab (VSH>VSV) with thermo-mechanical calculations. PMID:26924190

  7. Upper- and mid-mantle interaction between the Samoan plume and the Tonga-Kermadec slabs (United States)

    Chang, Sung-Joon; Ferreira, Ana M. G.; Faccenda, Manuele


    Mantle plumes are thought to play a key role in transferring heat from the core-mantle boundary to the lithosphere, where it can significantly influence plate tectonics. On impinging on the lithosphere at spreading ridges or in intra-plate settings, mantle plumes may generate hotspots, large igneous provinces and hence considerable dynamic topography. However, the active role of mantle plumes on subducting slabs remains poorly understood. Here we show that the stagnation at 660 km and fastest trench retreat of the Tonga slab in Southwestern Pacific are consistent with an interaction with the Samoan plume and the Hikurangi plateau. Our findings are based on comparisons between 3D anisotropic tomography images and 3D petrological-thermo-mechanical models, which self-consistently explain several unique features of the Fiji-Tonga region. We identify four possible slip systems of bridgmanite in the lower mantle that reconcile the observed seismic anisotropy beneath the Tonga slab (VSH>VSV) with thermo-mechanical calculations.

  8. Upper- and mid-mantle interaction between the Samoan plume and the Tonga-Kermadec slabs. (United States)

    Chang, Sung-Joon; Ferreira, Ana M G; Faccenda, Manuele


    Mantle plumes are thought to play a key role in transferring heat from the core-mantle boundary to the lithosphere, where it can significantly influence plate tectonics. On impinging on the lithosphere at spreading ridges or in intra-plate settings, mantle plumes may generate hotspots, large igneous provinces and hence considerable dynamic topography. However, the active role of mantle plumes on subducting slabs remains poorly understood. Here we show that the stagnation at 660 km and fastest trench retreat of the Tonga slab in Southwestern Pacific are consistent with an interaction with the Samoan plume and the Hikurangi plateau. Our findings are based on comparisons between 3D anisotropic tomography images and 3D petrological-thermo-mechanical models, which self-consistently explain several unique features of the Fiji-Tonga region. We identify four possible slip systems of bridgmanite in the lower mantle that reconcile the observed seismic anisotropy beneath the Tonga slab (V(SH)>V(SV)) with thermo-mechanical calculations.

  9. Traces of the crustal units and the upper mantle structure in the southwestern part of the East European Craton

    Directory of Open Access Journals (Sweden)

    I. Janutyte


    Full Text Available The presented study is a part of the passive seismic experiment PASSEQ 2006–2008 which took place around the Trans-European Suture Zone (TESZ from May 2006 to June 2008. The dataset of 4195 manually picked arrivals of teleseismic P waves of 101 earthquakes (EQs recorded in the PASSEQ seismic stations deployed to the east of the TESZ was inverted using the non-linear teleseismic tomography algorithm TELINV. Two 3-D crustal models were used to estimate the crustal travel time (TT corrections. As a result, we obtained a model of P wave velocity variations in the upper mantle beneath the TESZ and the EEC. In the study area beneath the craton we observed 5 to 6.5% higher and beneath the TESZ about 4% lower seismic velocities compared to the IASP91 velocity model. We found the seismic lithosphere-asthenosphere boundary (LAB beneath the TESZ at a depth of about 180 km, while we observed no seismic LAB beneath the EEC. The inversion results obtained with the real and the synthetic datasets indicated a ramp shape of the LAB in the northern TESZ where we observed values of seismic velocities close to those of the craton down to about 150 km. The lithosphere thickness in the EEC increases going from the TESZ to the NE from about 180 km beneath Poland to 300 km or more beneath Lithuania. Moreover, in western Lithuania we possibly found an upper mantle dome. In our results the crustal units are not well resolved. There are no clear indications of the features in the upper mantle which could be related with the crustal units in the study area. On the other hand, at a depth of 120–150 km we possibly found a trace of a boundary of proposed palaeosubduction zone between the East Lithuanian Domain (EL and the West Lithuanian Granulite Domain (WLG. Also, in our results we may have identified two anorogenic granitoid plutons.

  10. Group velocity distribution of Rayleigh waves and crustal and upper mantle velocity structure of the Chinese mainland and its vicinity

    Institute of Scientific and Technical Information of China (English)

    何正勤; 丁志峰; 叶太兰; 孙为国; 张乃铃


    Based on the long period digital surface wave data recorded by 11 CDSN stations and 11 IRIS stations, the dispersion curves of the group velocities of fundamental mode Rayleigh waves along 647 paths, with the periods from 10 s to 92 s, were measured by multi-filter. Their distribution at 25 central periods within the region of 18((54(N, 70(~140(E was inverted by Dimtar-Yanovskaya method. Within the period from 10 s to 15.9 s, the group velocity distribution is laterally inhomogeneous and is closely related to geotectonic units, with two low velocity zones located in the Tarim basin and the East China Sea and its north regions, respectively. From 21 s to 33 s, the framework of tectonic blocks is revealed. From 36.6 s to 40 s, the lithospheric subdivision of the Chinese mainland is obviously uncovered, with distinct boundaries among the South-North seismic belt, the Tibetan plateau, the North China, the South China and the Northeast China. Four cross-sections of group velocity distribution with period along 30(N, 38(N, 90(E and 120(E, are discussed, respectively, which display the basic features of the crust and upper mantle of the Chinese mainland and its neighboring regions. There are distinguished velocity differences among the different tectonic blocks. There are low-velocity-zones (LVZ) in the middle crust of the eastern Tibetan plateau, high velocity featured as stable platform in the Tarim basin and the Yangtze platform, shallow and thick low-velocity-zone in the upper mantle of the North China. The upper mantle LVZ in the East China Sea and the Japan Sea is related to the frictional heat from the subduction of the Philippine slab and the strong extension since the Himalayan orogenic period.

  11. Upper-mantle P- and S-wave velocities below Scandinavia and East Greenland from teleseismic traveltime tomography

    DEFF Research Database (Denmark)

    Hejrani, Babak


    improved resolution when stations follow profiles. The method was tested on the SCANLIPS array across the Scandinavian Peninsula (Paper I). On the data side, I performed a complete reorganization of the in-house MATLAB-based system (Medhus et al., 2012a,b) for handling event extraction, filtering, cross......-correlation and arrival-time picking, involving about 50% rewriting and 50% code extension, now also interfacing with the SAC-system and GMT. Ellipticity correction was built in, following the Kennett and Gudmundsson (1996). With these improved methods, I studied the upper-mantle velocity structure in three regions...

  12. Sensitivity analysis of crustal correction and its error propagation to upper mantle residual gravity and density anomalies

    DEFF Research Database (Denmark)

    Herceg, Matija; Artemieva, Irina; Thybo, Hans


    We investigate the effect of the crustal structure heterogeneity and uncertainty in its determination on stripped gravity field. The analysis is based on interpretation of residual upper mantle gravity anomalies which are calculated by subtracting (stripping) the gravitational effect of the crust...... a relatively small range of expected density variations in the lithospheric mantle, knowledge on the uncertainties associated with incomplete knowledge of density structure of the crust is of utmost importance for further progress in such studies......) uncertainties in the velocity-density conversion and (ii) uncertainties in knowledge of the crustal structure (thickness and average Vp velocities of individual crustal layers, including the sedimentary cover). In this study, we address both sources of possible uncertainties by applying different conversions...... from velocity to density and by introducing variations into the crustal structure which corresponds to the uncertainty of its resolution by high-quality and low-quality seismic models. We examine the propagation of these uncertainties into determinations of lithospheric mantle density. The residual...

  13. Garnet Signatures in Geophysical and Geochemical Observations: Insights into the Thermo-Petrological Structure of Oceanic Upper Mantle (United States)

    Grose, C. J.; Afonso, J. C.


    We have developed new physically comprehensive thermal plate models of the oceanic lithosphere which incorporate temperature- and pressure-dependent heat transport properties and thermal expansivity, melting beneath ridges, hydrothermal circulation near ridge axes, and insulating oceanic crust. These models provide good fits to global databases of seafloor topography and heat flow, and seismic evidence of thermal structure near ridge axes. We couple these thermal plate models with thermodynamic models to predict the petrology of oceanic lithosphere. Geoid height predictions from our models suggest that there is a strong anomaly in geoid slope (over age) above ~25 Ma lithosphere due to the topography of garnet-field mantle. A similar anomaly is also present in geoid data over fracture zones. In addition, we show that a new assessment of a large database of ocean island basalt Sm/Yb systematics indicates that there is an unmistakable step-like increase in Sm/Yb values around 15-20 Ma, indicating the presence of garnet. To explain this feature, we have attempted to couple our thermo-petrological models of oceanic upper mantle with an open system, non-modal, dynamic melting model with diffusion kinetics to investigate trace element partitioning in an ascending mantle column.

  14. Seismic structure of the lithosphere and upper mantle beneath the ocean islands near mid-oceanic ridges (United States)

    Haldar, C.; Kumar, P.; Kumar, M. Ravi


    Deciphering the seismic character of the young lithosphere near mid-oceanic ridges (MORs) is a challenging endeavor. In this study, we determine the seismic structure of the oceanic plate near the MORs using the P-to-S conversions isolated from quality data recorded at five broadband seismological stations situated on ocean islands in their vicinity. Estimates of the crustal and lithospheric thickness values from waveform inversion of the P-receiver function stacks at individual stations reveal that the Moho depth varies between ~ 10 ± 1 km and ~ 20 ± 1 km with the depths of the lithosphere-asthenosphere boundary (LAB) varying between ~ 40 ± 4 and ~ 65 ± 7 km. We found evidence for an additional low-velocity layer below the expected LAB depths at stations on Ascension, São Jorge and Easter islands. The layer probably relates to the presence of a hot spot corresponding to a magma chamber. Further, thinning of the upper mantle transition zone suggests a hotter mantle transition zone due to the possible presence of plumes in the mantle beneath the stations.

  15. Deeper Subduction Zone Melting Explains Enrichment of Upper Mantle and Resolves Dehydration Paradox (United States)

    Dixon, Jacqueline; Bindeman, Ilya; Kingsley, Richard


    We present new volatile and stable isotope data on oceanic basaltic glasses with a range of enriched compositions. Basalt compositions studied here can be modeled by mixing between depleted mantle and various enriched (EM) and prevalent (PREMA) mantle components. We develop a multi-stage metasomatic and melting model for the origin of the enriched components, extending the subduction factory concept to involve melting of different components at different depths, down to the mantle transition zone (660 km), with slab temperature a key variable. EM components are heterogeneous, ranging from wet and heavy (Arctic Ridges) to dry and light (East Pacific Rise), and are derived from the subducted slab at depths of 150 to 250 km by addition of paradox," refering to the following conundrum. The enriched "prevalent mantle" (PREMA) end-member in mid-oceanic ridge and ocean island basalts requires involvement of a mostly dehydrated slab component to explain trace element ratios and radiogenic isotopic compositions, but a fully hydrated slab component to explain stable isotope compositions. In our model, thermal parameters of slabs control the timing and composition of subduction-derived components. This includes deep release of fluids from subcrustal hydrous phases that may rehydrate previously dehydrated slab, resolving the paradox.

  16. Western US intermountain seismicity caused by changes in upper mantle flow. (United States)

    Becker, Thorsten W; Lowry, Anthony R; Faccenna, Claudio; Schmandt, Brandon; Borsa, Adrian; Yu, Chunquan


    Understanding the causes of intraplate earthquakes is challenging, as it requires extending plate tectonic theory to the dynamics of continental deformation. Seismicity in the western United States away from the plate boundary is clustered along a meandering, north-south trending 'intermountain' belt. This zone coincides with a transition from thin, actively deforming to thicker, less tectonically active crust and lithosphere. Although such structural gradients have been invoked to explain seismicity localization, the underlying cause of seismicity remains unclear. Here we show results from improved mantle flow models that reveal a relationship between seismicity and the rate change of 'dynamic topography' (that is, vertical normal stress from mantle flow). The associated predictive skill is greater than that of any of the other forcings we examined. We suggest that active mantle flow is a major contributor to seismogenic intraplate deformation, while gravitational potential energy variations have a minor role. Seismicity localization should occur where convective changes in vertical normal stress are modulated by lithospheric strength heterogeneities. Our results on deformation processes appear consistent with findings from other mobile belts, and imply that mantle flow plays a significant and quantifiable part in shaping topography, tectonics, and seismic hazard within intraplate settings.

  17. Low velocities in the oceanic upper mantle and their relation to plumes: insights from SEM-based waveform tomography (United States)

    Lekic, V.; French, S. W.; Romanowicz, B. A.


    The exchange of heat, mass and momentum between tectonic plates and mantle convection controls lithospheric evolution and hotspot volcanism, and must occur at a range of spatial scales. Yet, the detailed morphology of the associated convection patterns continues to elude geophysicists. Because seismic velocities are affected by temperature, seismic tomography can be used to map the patterns of flow in the Earth's mantle. Here, we present a global-scale long-period full-waveform seismic tomographic model SEMum2 constructed using the Spectral Element Method, which can very accurately model wave propagation through highly complex structures, and account for phenomena such as scattering, (de)focusing, and wavefront healing. Notably, SEMum2 achieves more realistic amplitudes of lateral heterogeneity - particularly low velocities in the upper 250km - than previous generations of global models, while still retrieving the long-wavelength structure present in earlier tomographic models. Cluster analysis of profiles of shear velocity in the SEMum2 oceanic upper mantle, confirms the presence of a well marked shear wave low velocity zone (LVZ) beneath the lithosphere, with a velocity minimum which deepens progressively as a function of age of the plate. The LVZ minimum in SEMum2 reaches values that are lower than in previous tomographic global models and in agreement with local estimates where available. Interestingly, reaching below this "classical" low velocity zone, the model reveals a pattern of alternating lower and higher velocities organized into elongated bands in the direction of absolute plate motion (APM), with a quasi-regular spacing of ~2000 km perpendicular to the APM. This fingerlike structure, most prominent around 200-250 km and extending down to 350-400 km, is most prominent beneath the Pacific plate, but also present under the eastern Antarctic plate, in the south Atlantic and in parts of the Indian Ocean Below this depth, the low velocities appear organized

  18. Tomographic image of crust and upper mantle off the Boso Peninsula using data from an ocean-bottom seismograph array (United States)

    Ito, Aki; Yamamoto, Yojiro; Hino, Ryota; Suetsugu, Daisuke; Sugioka, Hiroko; Nakano, Masaru; Obana, Koichiro; Nakahigashi, Kazuo; Shinohara, Masanao


    We determined the three-dimensional structure of the crust and upper mantle off the Boso Peninsula, Japan, by analyzing seismograms recorded by ocean-bottom seismometers and land stations between 2011 and 2013. We employed seismic tomography to determine the P- and S-wave velocity structures and earthquake locations simultaneously. The tomographic image shows that the mantle parts of the Pacific and the Philippine Sea plates have high-velocity anomalies. The upper boundary of the Philippine Sea plate is delineated as approximately 2-6 km shallower than that previously estimated from land-based data for the area 140.5°E-141.5°E and 35°N-35.5°N. A pronounced low-velocity anomaly in P- and S-waves with low- V p/ V s ratio (1.5-1.6) was observed at depths shallower than 20 km in the overriding North American plate. This anomaly may be caused by the presence of rocks with a low- V p/ V s ratio, such as quartzite, and the water expelled from the subducted Pacific and Philippine Sea plates.[Figure not available: see fulltext.

  19. Evidence for serpentinization of the Ionian upper mantle from simultaneous inversion of P- and S-wave arrival times (United States)

    D'Alessandro, Antonino; Mangano, Giorgio; D'Anna, Giuseppe; Scudero, Salvatore


    Simultaneous inversion of P- and S-wave arrival times, collected during a 3-years Ocean Bottom Seismometer with Hydrophone (OBS/H) monitoring campaign, yields 1D P- and S-wave velocity models for the Ionian lithosphere (Central Mediterranean). The 1D model highlights the presence, in the Ionian upper mantle, of two layers characterized by high seismic P-wave velocity (S1 and S2, 6.3-6.7 and 7.5 km/s, respectively). These two layers, with thicknesses of about 3.3 km and 5 km, respectively, and ranging from ∼8 to ∼16 km in depth, are characterized by low S-wave velocity (S1 = 3.05-3.2 km/s, S2 = 3.85 km/s) and high values of VP/VS (S1 = 2.06-2.09, S2 = 1.95). This is a characteristic feature, often encountered in passive margins and is generally interpreted as partly serpentinized peridotite. The VP, VS and VP/VS values of S1 are consistent with 55-65% of serpentinization of the upper mantle, while the S2 ones are consistent with 15-25% of serpentinization. This research provides a crucial hint about the debated nature of the Ionian crust, suggesting its oceanic structure.

  20. Structural Heterogeneities in Southeast Tibet: Implications for Regional Flow in the Lower Crust and Upper Mantle

    Directory of Open Access Journals (Sweden)

    Zhi Wang


    Full Text Available Our seismic study together with the MT analysis reveal a “R-shape” flow existing in both the lower crust and uppermost mantle, which suggests the crustal deformation along the deep, large sutures (such as the Longmen Shan fault and the Anninghe Fault under the southeastern Tibetan Plateau is maintained by dynamic pressure from the regional flow intermingled with the hot upwelling asthenosphere. The material in the lower crust and uppermost mantle flowing outward from the center of the plateau is buttressed by the old, strong lithosphere that underlies the Sichuan basin, pushing up on the crust above and maintaining steep orogenic belt through dynamic pressure. We therefore consider that the “R-shape” regional flow played a key role in the crustal deformation along the deep suture zones of the Bangong-Nujiang, the Longmen-Shan faults, and other local heavily faulted zones beneath the southeastern Tibetan Plateau.

  1. Seismic structure of the crust and upper mantle in central-eastern Greenland

    DEFF Research Database (Denmark)

    Kraft, Helene Anja

    at the east coast to 50 km in central Greenland. The observed crustal thicknesses indicate that the high topography in eastern Greenland of up to 3700 m cannot be explained by Airy type isostatic equilibrium alone. Major parts of the mantle transition zone below central-eastern Greenland are substantially......Geophysical and geological knowledge of the interior of Greenland is very limited. The lack of knowledge arises mainly due to the logistical challenges related to conducting geophysical fieldwork on the up to 3400 m thick ice sheet, which covers around 80% of the land area. This PhD thesis is based...... on the very first regional passive seismic study in central-Eastern Greenland, focusing on the area between Scoresby Sund and Summit. The study aims to image the structure of subsurface Greenland starting from the crust and down to the mantle transition zone. Furthermore, the thesis links these observations...

  2. Illuminating heterogeneous anisotropic upper mantle: testing a new anisotropic teleseismic body-wave tomography code - part II: Inversion mode (United States)

    Munzarova, Helena; Plomerova, Jaroslava; Kissling, Edi


    Considering only isotropic wave propagation and neglecting anisotropy in teleseismic tomography studies is a simplification obviously incongruous with current understanding of the mantle-lithosphere plate dynamics. Furthermore, in solely isotropic high-resolution tomography results, potentially significant artefacts (i.e., amplitude and/or geometry distortions of 3D velocity heterogeneities) may result from such neglect. Therefore, we have undertaken to develop a code for anisotropic teleseismic tomography (AniTomo), which will allow us to invert the relative P-wave travel time residuals simultaneously for coupled isotropic-anisotropic P-wave velocity models of the upper mantle. To accomplish that, we have modified frequently-used isotropic teleseismic tomography code Telinv (e.g., Weiland et al., JGR, 1995; Lippitsch, JGR, 2003; Karousova et al., GJI, 2013). Apart from isotropic velocity heterogeneities, a weak hexagonal anisotropy is assumed as well to be responsible for the observed P-wave travel-time residuals. Moreover, no limitations to orientation of the symmetry axis are prescribed in the code. We allow a search for anisotropy oriented generally in 3D, which represents a unique approach among recent trials that otherwise incorporate only azimuthal anisotopy into the body-wave tomography. The presented code for retrieving anisotropy in 3D thus enables its direct applications to datasets from tectonically diverse regions. In this contribution, we outline the theoretical background of the AniTomo anisotropic tomography code. We parameterize the mantle lithosphere and asthenosphere with an orthogonal grid of nodes with various values of isotropic velocities, as well as of strength and orientation of anisotropy in 3D, which is defined by azimuth and inclination of either fast or slow symmetry axis of the hexagonal approximation of the media. Careful testing of the new code on synthetics, concentrating on code functionality, strength and weaknesses, is a

  3. Rapid change of atmosphere on the Hadean Earth: Beyond Habitable Trinity on a tightrope (United States)

    Arai, T.; Maruyama, S.


    Surface environment of Hadean Earth is a key to bear life on the Earth. All of previous works assumed that high pCO2 has been decreased to a few bars in the first a few hundreds millions of years (e.g., Zhanle et al., 2011). However, this process is not easy because of material and process barriers as shown below. Four barriers are present. First, the ultra-acidic pH (plate tectonics or pseudo-plate tectonics system. To overcome this barrier, primordial (anorthosite + KREEP) continents must have been above sea-level to increase pH rapidly through hydrological process. Second, major cap rocks on the Hadean oceanic crust must have been komatiite with minor basaltic rocks to precipitate carbonates through water-rock interaction and transport them into mantle through subduction at higher than the intermediate P/T geotherm on the Benioff plane. If not, carbonate minerals are all decarbonated at shallower depths than the Moho plane. Komatiite production depends on mantle potential temperature which must have been rapidly decreased to yield only Fe-enriched MORB by 3.8Ga. Third, the primordial continents composed of anorthosite with subordinate amounts of KREEP basalts must have been annihilated by 4.0Ga to alter pH to be possible to precipitate carbonates by hydrothermal process. The value of pCO2 must have been decreased down to a few bars from c.a. 50 bars at TSI (total surface irradiance) = 75% under the restricted time limit. If failed, the Earth must have been Venus state which is impossible to bear life on the planet. Fourth is the role of tectonic erosion to destroy and transport the primordial continent of anorthosite into deep mantle by subduction. Anorthosite + KREEP was the mother's milk grow life on the Earth, but disappeared by 4.0Ga or even earlier, but alternatively granites were formed and accumulated on the Earth to supply nutrients for life. This is time-dependent process to increase new continents. Fifth is the water content of 3-5km thick, if the

  4. How to draw down CO2 from severe Hadean to habitable Archean? (United States)

    Zhelezinskaia, I.; Ding, S.; Mulyukova, E.; Martirosyan, N.; Johnson, A.; West, J. D.; Kolesnichenko, M.; Saloor, N.; Moucha, R.


    It has been hypothesized that as the magma ocean crystallized in the Hadean, volatiles such as CO2 and H2O were released to the surface culminating with the formation of a liquid ocean by about 4.4 Ga [1] and hot CO2-rich atmosphere [2]. The resulting late Hadean atmospheric pCO2 may have been as high as 100 bars [3] with corresponding surface temperatures ~500 K [4]. Geological evidence suggests that by the early-to-mid Archean, atmospheric pCO2 became less than 1 bar [5]. However, the mechanisms responsible for the great amount of CO2 drawdown in a relatively short period of time remain enigmatic. To identify these possible mechanisms, we have developed a box model during the CIDER 2015 Summer Program that takes into account geological constraints on basalt alteration [6, 7] and possible rate of new oceanic crust formation [8] for the Archean. Our model integrates geodynamic and geochemical approaches of interaction between the Hadean atmosphere, hydrosphere, oceanic crust, and mantle to drawdown CO2. Our primary assumption for the Hadean is the absence of the continental crust and thus continental weathering. Therefore in the model we present, the level of CO2 in the atmosphere is regulated by the formation of oceanic crust (OC), rate of the interaction between the ocean and OC, and carbonate subduction/CO2 degassing. Preliminary results suggest that it would take about 1 billion years for the atmospheric CO2 to decrease to 1 bar if the production of oceanic crust was 10 times more than today and the pH of the ocean was less than 7, making the basalt alteration more efficient. However, there is evidence that some continental crust began to form as early as 4.4 Ga [9] and therefore the role of continental weathering and its rate of CO2 drawdown will need to be further explored. References: [1] Wilde et al. (2001). Nature 409(6817), 175-178. [2] Walker (1985). Origins of Life and Evolution of the Biosphere 16(2), 117-127. [3] Elkins-Tanton (2008). EPSL, 271, 181

  5. Three-dimensional P wave attenuation and velocity upper mantle tomography of the southern Apennines-Calabrian Arc subduction zone (United States)

    Monna, Stephen; Dahm, Torsten


    We propose a 3-D crust-upper mantle seismic attenuation (QP) model of the southern Apennines-Calabrian Arc subduction zone together with a 3-D velocity (VP) model. The QP model is calculated from relative t* using the spectral ratio method and the VP from traveltime data. The final data set used for the inversion of the VP model consists of 2400 traveltime arrivals recorded by 34 short-period stations that are part of the Italian National Seismic Network, and for the QP model, 2178 Pn phases recorded by a subset of 32 stations. Traveltimes and waveforms come from 272 intermediate-depth Calabrian slab events. This 3-D model of attenuation, together with the 3-D velocity model, improves our knowledge of the slab/mantle wedge structure and can be a starting point in determining the physical state of the asthenosphere (i.e., its temperature, the presence of melt and/or fluids) and its relation to volcanism found in the study area. Main features of the QP and VP models show that the mantle wedge/slab, in particular, the area of highest attenuation, is located in a volume underlying the Marsili Basin. The existence and shape of this main low-QP (and low-VP) anomaly points to slab dehydration and fluid/material flow, a process that may explain the strong geochemical affinities between the subduction-related magmas from Stromboli and Vesuvius. Other interesting features in the models are strong lateral variations in QP and VP that are put in relation with known important tectonic structures and volcanic centers in the area.

  6. Os-isotopic Compositions of Peridotite Xenoliths from the Oceanic Mantle: Implications for the Age of Isotopic Domains in the Oceanic Mantle. (United States)

    Jackson, M. G.; Shirey, S. B.; Hauri, E. H.; Kurz, M. D.


    Os-isotopic compositions of abyssal peridotites and peridotite xenoliths from oceanic hotpots that sample the convecting mantle extend to relatively unradiogenic compositions. However, they do not preserve a record of early-formed (Hadean and Archean) depleted mantle domains, either by earlier cycles of ridge-related depletion, continent extraction, or subcontinental lithospheric mantle erosion. The lack of preservation of early-formed (Hadean and Archean) depleted Os-isotopic compositions is consistent with the lack of preservation of Hadean 142Nd/144Nd variability in the modern convecting mantle, but is in stark contrast to the existence of early-formed (early-Hadean), heterogeneous 129Xe/130Xe isotopic anomalies in the modern mantle. Mukhopadhay (Nature, 2012) suggested that the erasure of 142Nd/144Nd, but not 129Xe/130Xe, anomalies from the convecting mantle may be due to the small magnitude of the 142Nd/144Nd anomalies (10% variability observed) because the smaller 142Nd/144Nd anomalies would have been more easily erased than the larger magnitude 129Xe/130Xe anomalies. This model does not work for the Re-Os system because the magnitude of Os-isotopic heterogeneities in the mantle are large, yet early-formed Os-isotopic signatures have been erased. For example, the 187Os/188Os of early-formed depleted mantle at 4.55 Ga was ~0.095, which is >25% lower than the modern mantle. Given that there were substantial amounts of mantle partial melting throughout the Hadean and Archean, it is hard to understand how some refractory residues retaining some vestige of these low, early-formed Os isotopic compositions have not have been preserved in the modern mantle. Therefore, the lack of preservation of early-formed, large magnitude 187Os/188Os excursions in the modern convecting mantle suggests that the preservation of early geochemical heterogeneities was not necessarily a function of the original geochemical anomaly. We explore alternative solutions to the paradox of

  7. Mechanical anisotropy control on strain localization in upper mantle shear zones (United States)

    Herwegh, Marco; Mercolli, Ivan; Linckens, Jolien; Müntener, Othmar


    Mantle rocks at oceanic spreading centers reveal dramatic rheological changes from partially molten to solid-state ductile to brittle deformation with progressive cooling. Using the crustal-scale Wadi al Wasit mantle shear zone (SZ, Semail ophiolite, Oman), we monitor such changes based on quantitative field and microstructural investigations combined with petrological and geochemical analyses. The spatial distribution of magmatic dikes and high strain zones gives important information on the location of magmatic and tectonic activity. In the SZ, dikes derived from primitive melts (websterites) are distributed over the entire SZ but are more abundant in the center; dikes from more evolved, plagioclase saturated melts (gabbronorites) are restricted to the SZ center. Accordingly, harzburgite deformation fabrics show a transition from protomylonite (1100°C), mylonite (900-800°C) to ultramylonite (dikes) and solid-state deformation (protomylonites-mylonites; 1100-900°C), (ii) dominant solid-state deformation in harzburgite mylonites (900-800°C) with some last melt injections (gabbronorites) and ultramylonites (dike-related ridge-parallel deformation controls the onset of the entire strain localization history promoting nucleation sites for different strain weakening processes as a consequence of changing physicochemical conditions.

  8. Volcanism on Mars controlled by early oxidation of the upper mantle. (United States)

    Tuff, J; Wade, J; Wood, B J


    Detailed information about the chemical composition and evolution of Mars has been derived principally from the SNC (shergottite-nakhlite-chassignite) meteorites, which are genetically related igneous rocks of Martian origin. They are chemically and texturally similar to terrestrial basalts and cumulates, except that they have higher concentrations of iron and volatile elements such as phosphorus and chlorine and lower concentrations of nickel and other chalcophile (sulphur-loving) elements. Most Martian meteorites have relatively young crystallization ages (1.4 billion years to 180 million years ago) and are considered to be derived from young, lightly cratered volcanic regions, such as the Tharsis plateau. Surface rocks from the Gusev crater analysed by the Spirit rover are much older (about 3.7 billion years old) and exhibit marked compositional differences from the meteorites. Although also basaltic in composition, the surface rocks are richer in nickel and sulphur and have lower manganese/iron ratios than the meteorites. This has led to doubts that Mars can be described adequately using the 'SNC model'. Here we show, however, that the differences between the compositions of meteorites and surface rocks can be explained by differences in the oxygen fugacity during melting of the same sulphur-rich mantle. This ties the sources of Martian meteorites to those of the surface rocks through an early (>3.7 billion years ago) oxidation of the uppermost mantle that had less influence on the deeper regions, which produce the more recent volcanic rocks.

  9. Chondritic osmium isotopic composition of late Archean convecting upper mantle:Evidence from Zunhua podiform chromitites, Hebei, North China

    Institute of Scientific and Technical Information of China (English)

    XIA Qiongxia; ZHI Xiachen; LI Jianghai; HUANG Xiongnan


    Podiform chromite deposits are a characteristic feature of the mantle sequences of harzburgitic ophiolites. The chromites usually have very low Re and high Os contents, which makes it the most resistant phase remaining from the primary magmatic history of the ultramafic sections of ophiolites. The podiform chromite is one of the robust indicators of initial Os isotopic compositions of the ophiolites where podiform chromites were derived from, which provides strong evidence for the origin and evolution of oceanic lithosphere. The Re and Os contents and the Os isotopic compositions of seven podiform chromitites from Zunhua ophiolitic mélange belt, North China are reported in this study. The Re contents range from 0.019 to 0.128 ng/g, Os from 8.828 to 354.0 ng/g, and the 187Os/188Os ratio from 0.11003 to 0.11145. Three massive chromitites among the sample set have very high Os contents (>300 ng/g), and their 187Os/188Os ratios range from 0.11021 to 0.11030, averaging 0.11026 ± 0.00005 (σ), equivalent to a γOs = -0.12 ± 0.06 at 2.6 Ga, which means that the Os isotopic composition of convecting upper mantle is chondritic in late Archean. It is the Os isotopic composition of podiform chromitites that are derived from the oldest ophiolite in the world till now.

  10. Coexisting contraction-extension consistent with buoyancy of the crust and upper mantle in North-Central Italy

    CERN Document Server

    Aoudia, A; Ismail-Zadeh, A T; Panza, G F; Pontevivo, A


    The juxtaposed contraction and extension observed in the crust of the Italian Apennines and elsewhere has, for a long time, attracted the attention of geoscientists and is a long-standing enigmatic feature. Several models, invoking mainly external forces, have been put forward to explain the close association of these two end-member deformation mechanisms clearly observed by geophysical and geological investigations. These models appeal to interactions along plate margins or at the base of the lithosphere such as back-arc extension or shear tractions from mantle flow or to subduction processes such as slab roll back, retreat or pull and detachment. We present here a revisited crust and upper mantle model that supports delamination processes beneath North-Central Italy and provides a new background for the genesis and age of the recent magmatism in Tuscany. Although external forces must have been important in the building up of the Apennines, we show that internal buoyancy forces solely can explain the coexist...

  11. Crust and upper mantle structure beneath the Pacific Northwest from joint inversions of ambient noise and earthquake data (United States)

    Wagner, Lara S.; Fouch, Matthew J.; James, David E.; Hanson-Hedgecock, Sara


    We perform a joint inversion of phase velocities from both earthquake and ambient noise induced Rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the Pacific Northwest. We focus particularly on the areas affected by mid-Miocene to present volcanic activity. The joint inversion, combined with the high density seismic network of the High Lava Plains seismic experiment and data from the EarthScope Transportable Array, provides outstanding resolution for this area. In Oregon, we find that the pattern of low velocities in the crust and uppermost mantle varies between the High Lava Plains physiographic province and the adjacent northwestern Basin and Range. These patterns may be due to the presence of the Brothers Fault Zone which separates the clockwise rotating northwest Basin and Range from the relatively undeformed areas further north. Further to the east, the Owyhee Plateau, Snake River Plain (SRP) and northeastern Basin and Range are characterized by high crustal velocities, though the depth extent of these fast wave speeds varies by province. Of particular interest is the mid-crustal high velocity sill, previously only identified within the SRP. We show this anomaly extends significantly further south into Utah and Nevada. We suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid-crustal sill structures within the SRP.

  12. The fate of sulfide during decompression melting of peridotite - implications for sulfur inventory of the MORB-source depleted upper mantle (United States)

    Ding, Shuo; Dasgupta, Rajdeep


    Magmatism at mid ocean ridges is one of the main pathways of S outflux from deep Earth to the surface reservoirs and is a critical step in the global sulfur cycle, yet our understanding of the behavior of sulfide during decompression melting of the upper mantle is incomplete. In order to constrain the sulfur budget of the mantle and reconcile the sulfur and chalcophile element budget of mantle partial melts parental to primitive mid-ocean ridge basalts (MORBs), here we developed a model to describe the behavior of sulfide and Cu during decompression melting by combining the pMELTS thermodynamic model and empirical sulfur contents at sulfide concentration (SCSS) models, taking into account the effect of the presence of Ni and Cu in sulfides on SCSS of mantle-derived melts. Calculation of SCSS along melting adiabat at mantle potential temperature of 1380 °C with variable initial S content in the mantle indicates that the complete consumption or partial survival of sulfide in the melting residue depends on initial S content and degree of melting. Primitive MORBs (Mg# > 60) with S and Cu mostly concentrated in 800-1000 ppm and 80-120 ppm are likely mixture of sulfide undersaturated high degree melts and sulfide saturated low degree melts derived from depleted peridotite containing 100-200 ppm S. Model calculations to capture the effects of variable mantle potential temperatures (1280-1420 °C) indicate that for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle owing to the effect of temperature in enhancing sulfide solubility in silicate melt, and higher mantle temperature stabilizing partial melt with higher FeO* and lower SiO2 and Al2O3, all of which generally enhance sulfide solubility. However, sulfide can still be exhausted by ∼ 10- 15% melting with bulk S of 100-150 ppm in the mantle when TP is as low as 1300 °C. We also show that although variation of DCuperidotite/ melt and initial Cu in the

  13. Crust to Upper Mantle Echoes of the Black Sea Opening and Seismotectonic Consequences on the NW Inland (United States)

    Besutiu, L.


    The paper aims at revealing some tectonic and geodynamic imprints of the Black Sea opening mainly inferred from the potential fields analysis. DSS lines and seismic tomography are added in order to strengthen the interpretation and/or deepen the depth of investigation. Crust structure and dynamics (1) The presence of the oceanic crust in the central part of the basin is well reflected in the geomagnetic anomaly pattern. (2) Unlike some previous hypotheses postulating the existence of an unique rifting, up to date filtering techniques pointed out an unexpected pattern of the gravity and geomagnetic anomalies, trending almost perpendicular each-other within eastern and western basin, thus advocating for a distinct opening of the W and E Black Sea. (3) Correlation with the magnetostratigraphic scale revealed a geomagnetic reversal and seems to indicate a later opening of the eastern basin; off-shore seismics confirm the model by showing a slight overthrusting of the E Pontides over W Pontides. (4) It seems that the W Black Sea opening split the Moesian Plate into several slivers by creating/reactivating older faults trending north-westward. (5) Crust expelled by the Black Sea opening accommodated in various circumstances: (i) East Carpathians it met the inclined outer flank of the TTZ and came into an oblique subduction to which specific peculiarities of the South Harghita Mts. volcanism might be associated; (ii) South Carpathians, crustal slivers facing the vertical contact of the Intra-alpine sub-plate could not subduct, but went into a lithosphere buckling to which the lowest gravity low on the Romanian territory, located in front and not beneath the highest mountains in Romania, seems to be related; (iii) within the bending area of East Carpathians, where three tectonic plates met each other, the speed excess provided by the W Black Sea opening created an unstable triple junction. Upper mantle echoes Fingerprints of the Black Sea opening are well reflected in some

  14. The velocity structure of crust and upper mantle in the Wudalianchi volcano area inferred from the receiver function

    Institute of Scientific and Technical Information of China (English)

    贺传松; 王椿镛; 吴建平


    The Wudalianchi volcano is a modern volcano erupted since the Holocene. Its frequent occurrence of the small earthquake is considered to be indicator of active dormancy volcano. The S wave velocity structure is inferred from the receiver function for the crust and upper mantle of the Wudalianchi volcano area. The results show that the low velocity structure of S wave is widely distributed underneath the volcano area and part of the low-velocity-zone located at shallow depth in the Wudalianchi volcano area. The low velocity structure is related to the seismicity. The Moho interface is not clear underneath the volcano area, which may be regard to be an necessary condition for the lava upwelling. Therefore, we infer that the Wudalianchi volcano has the deep structural condition for the volcano activity and may be alive again.

  15. Seismic, petrological and geodynamical constraints on thermal and compositional structure of the upper mantle: global thermochemical models

    DEFF Research Database (Denmark)

    Cammarano, Fabio; Tackley, Paul J.; Boschi, Lapo


    Mapping the thermal and compositional structure of the upper mantle requires a combined interpretation of geophysical and petrological observations. Based on current knowledge of material properties, we interpret available global seismic models for temperature assuming end-member compositional...... lateral compositional variations does not change significantly the thermal interpretation of seismic models, but gives a more realistic density structure. Modelling a petrological lithosphere gives cratonic temperatures at 150 km depth that are only 100 K hotter than those obtained assuming pyrolite......, and thus less buoyant, continental lithosphere is required to explain gravity data. None of the seismic tomography models we analyse is able to reproduce accurately the thermal structure of the oceanic lithosphere. All of them showtheir lowest seismic velocities at~100 km depth beneathmid-oceanic ridges...

  16. The Influence of Water on Seismic Wave Speeds and Attenuation in the Upper Mantle: an update from the Laboratory (United States)

    Cline, C. J., II; David, E. C.; Jackson, I.; Faul, U.; Berry, A.


    A fine-grained synthetic olivine (Fo90) polycrystal, doped with ~0.04 wt. % TiO2, has been prepared with ~70 wt. ppm H2O accommodated in the remarkably stable Ti-clinohumite defect typical of natural olivines from the Earth's generally water-undersaturated upper mantle (Berry et al., 2005). A precision-ground specimen of this material, sleeved in Pt tubing within a mild-steel jacket, was tested in torsional forced oscillation at seismic frequencies (mHz-Hz) and temperatures to 1200 °C, under 200 MPa confining pressure. The shear modulus was observed to decrease systematically with increasing oscillation period and temperature, accompanied by monotonically increasing dissipation, which are characteristic of absorption band or high-temperature-background behaviour. In a previous preliminary report, the new data were compared with the model of Jackson and Faul (Phys. Earth Planet. Interiors, 2010) for a suite of essentially anhydrous Ti-free olivine polycrystals, evaluated at the 25 μm grain size of the hydrous titaniferous olivine specimen, showing that the latter is vastly more dissipative than its anhydrous equivalent (by an order of magnitude at 1200 °C) and correspondingly lower in shear modulus. The results of additional experiments now better constrain the mechanical behaviour of the enclosing Pt sleeve and allow direct comparison with data for an anhydrous titaniferous olivine of comparable grain size. The latest results confirm a very strong influence of water on seismic wave attenuation, even under the water-undersaturated conditions expected to prevail in the Earth's upper mantle.

  17. Seismic structure of the North American lithosphere and upper mantle imaged using Surface and S waveform tomography (United States)

    Schaeffer, A. J.; Lebedev, S.


    The evolution, stability, and dynamics of continental lithosphere remain a central focus of Earth Science research. The continued deployment of the US Array is producing a massive new dataset that samples North America at scales from tectonic units to continent-wide domains and enables resolution of structure and deformation of the lithosphere previously possible only at regional scales. With this resolving power come new challenges relating to efficient management and processing of such large data volumes. In this study, we have assembled a dataset comprising over 3.5 million three-component broadband seismic waveforms from more than 3000 stations. We augment available US Array stations with ~600 additional North American stations of the GSN and affiliates, Canadian National Seismograph Network, regional arrays, past PASSCAL experiments, and other stations from Iceland, Greenland, Central and South America, the Caribbean, and several Mid-Atlantic Islands. We exploit the resolving power of this unprecedentedly large dataset using the Automated Multimode Inversion of surface- and S-wave forms. The waveforms are inverted for path-averaged linear constraints on elastic structure along the source-receiver paths. The linear equations are then simultaneously solved for a high-resolution 3D upper mantle shear velocity model of the continent. We present a model of the North American continent's and the surrounding Ocean's (Pacific, Atlantic, Gulf of Mexico) upper mantle structure down to the 660 km discontinuity. Clearly identifiable boundaries between different tectonic features such as basins and relic mountain ranges are readily observable. For example, a strong correlation between the Hudson Bay geoid anomaly can be identified with an underlying domain of particularily cold cratonic lithosphere. Our model also includes the 3D distribution of azimuthal anisotropy within these structures, which provides new insight into past and present dynamics of the lithosphere and

  18. Strain localization associated with channelized melt migration in upper mantle lithosphere: Insights from the Twin Sisters ultramafic complex, Washington, USA (United States)

    Kruckenberg, Seth C.; Tikoff, Basil; Toy, Virginia G.; Newman, Julie; Young, Laura I.


    We present results of field, microstructural, and textural studies in the Twin Sisters ultramafic complex (Washington State) that document localized deformation associated with the formation of dunite channels in naturally deformed upper mantle. The Twin Sisters complex is a well-exposed, virtually unaltered section of upper mantle lithosphere comprised largely of dunite and harzburgite (in cm- to m-scale primary compositional layers), and variably deformed orthopyroxenite and clinopyroxenite dikes. A series of ˜N-S striking, m-scale dunite bands (typically with porphyroclastic texture) occur throughout the study area and crosscut both the primary compositional layers and older orthopyroxenite dikes. Structural relationships suggest that these dunite bands represent former zones of channelized melt migration (i.e., dunite channels), and that strain localization was associated with melt migration. Early formed orthopyroxenite dikes are either absent within cross-cutting dunite channels, or have been displaced within channels relative to their position in the adjacent host rocks. These pre-existing orthopyroxenite dikes provide strain markers illustrating that displacement was localized primarily along channel margins, which have opposite senses of shear. In all cases where offsets were noted, the center of the channel was moved southward relative to its margins. Material flow and strain was, therefore, partitioned within channels during melt migration, and dunite channels did not accommodate net shear displacement of the adjacent host peridotites. Primary compositional layers adjacent to dunite channels document opposite rotation of olivine [100] crystallographic axes on either side of channel margins, consistent with the kinematic reversal inferred from offset markers at the outcrop scale, suggesting that the formation of dunite channels also induced host rock deformation proximal to channels. Strain localization that was focused at the margin of the bands was

  19. Upper Mantle Composition Beneath the Petit-Spot Area in Northwestern Pacific: Insights From Electrical Conductivity (United States)

    Baba, K.; Ichiki, M.; Abe, N.; Hirano, N.


    The mantle composition beneath the petit-spot area, where is about 500 km offshore from Japan Trench in northwestern Pacific, is discussed through electrical conductivity obtained by seafloor magnetotelluric (MT) survey. The seafloor MT data were collected using ocean bottom electromagnetometers (OBEMs) at four sites with the spacing of 100-150 km, between May and August, 2005. The survey was conducted as a part of the petit-spot multidsciplinary project. The petit-spot is young volcanic activity on very old (~130 Ma) oceanic plate characterized as a clump of small knolls which erupted strong to moderate alkaline basalt. This volcanic field is associated with neither any plate boundaries nor hot spots. To elucidate the magma generation process of this new-type volcanic activity, a collaborative study of various geophysical and geochemical approaches has been carried out. The MT survey aims to constrain the physical state of the lithosphere and asthenosphere where the petit-spot melt is probably generated. The acquired electromagnetic field variation data were analyzed and the MT responses, which is the transfer function between the electric and magnetic fields, were obtained. The effect for the ocean-land distribution and seafloor topography on the MT responses was modeled and stripped. As the result, the corrected responses indicate that the lateral heterogeneity in electrical conductivity is less significant beneath the survey area. One- dimensional inversion study shows that the data require a peak in conductivity (0.05 S/m) at about 200 km depth. The mantle temperature may be calculated from the conductivity using an experimental result for dry olivine (Constable et al., 1992). The resultant temperature is about 1750 °C which is lower than the dry solidus for garnet peridotite. Instead, assuming the temperature as GDH1 model (Stein and Stein, 1992) for 130 Myr old mantle, we calculate water content in olivine using an experimental result by Wang et al. (2006

  20. Seismic structure of the crust and upper mantle in central-eastern Greenland

    DEFF Research Database (Denmark)

    Kraft, Helene Anja

    on the very first regional passive seismic study in central-Eastern Greenland, focusing on the area between Scoresby Sund and Summit. The study aims to image the structure of subsurface Greenland starting from the crust and down to the mantle transition zone. Furthermore, the thesis links these observations...... with topographic features, the recent uplift history of the North Atlantic region, break-up related processes, and the possible track of the Iceland hotspot. The core of this study are P to S- and S to P- receiver functions, which image the difference in arrival time between not-converted and converted phases......Geophysical and geological knowledge of the interior of Greenland is very limited. The lack of knowledge arises mainly due to the logistical challenges related to conducting geophysical fieldwork on the up to 3400 m thick ice sheet, which covers around 80% of the land area. This PhD thesis is based...

  1. Phlogopite and K-amphibole in the upper mantle: Implication for magma genesis in subduction zones

    Energy Technology Data Exchange (ETDEWEB)

    Sudo, Akira; Tatsumi, Yoshiyuki (Kyoto Univ. (Japan))


    High-pressure phase relations have been examined for phlogopite + diopside with and without enstatite under vapor absent conditions in the pressure range of 5 to 13 GPa and in the temperature range of 1,000 to 1,300C. Phlogopite in these systems can be stable up to 6-7 GPa and decomposes through pressure-dependent reactions to crystallize phases including potassic amphibole. The experimental results suggest that phlogopite, which is one of main hydrous phases in the downdragged hydrated peridotite at the base of mantle wedge, plays an important role in the formation of magmas at the backarc side of a volcanic arc. The existence of potassic amphibole at higher pressure regions may imply the involvement of subduction component in magma generation in the region far away from the trench axis.

  2. Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle (United States)

    van den Bleeken, Greg; Müntener, Othmar; Ulmer, Peter


    Melt-rock reaction in the upper mantle is recorded in a variety of ultramafic rocks and is an important process in modifying melt composition on its way from the source region towards the surface. This experimental study evaluates the compositional variability of tholeiitic basalts upon reaction with depleted peridotite at uppermost-mantle conditions. Infiltration-reaction processes are simulated by employing a three-layered set-up: primitive basaltic powder (`melt layer') is overlain by a `peridotite layer' and a layer of vitreous carbon spheres (`melt trap'). Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0.65 and 0.8 GPa in the temperature range 1,170-1,290°C. In this P-T range, representing conditions encountered in the transition zone (thermal boundary layer) between the asthenosphere and the lithosphere underneath oceanic spreading centres, the melt is subjected to fractionation, and the peridotite is partially melting ( T s ~ 1,260°C). The effect of reaction between melt and peridotite on the melt composition was investigated across each experimental charge. Quenched melts in the peridotite layers display larger compositional variations than melt layer glasses. A difference between glasses in the melt and peridotite layer becomes more important at decreasing temperature through a combination of enrichment in incompatible elements in the melt layer and less efficient diffusive equilibration in the melt phase. At 1,290°C, preferential dissolution of pyroxenes enriches the melt in silica and dilutes it in incompatible elements. Moreover, liquids become increasingly enriched in Cr2O3 at higher temperatures due to the dissolution of spinel. Silica contents of liquids decrease at 1,260°C, whereas incompatible elements start to concentrate in the melt due to increasing levels of crystallization. At the lowest temperatures investigated, increasing alkali contents cause silica to increase

  3. Effects of crystal preferred orientation on upper-mantle flow near plate boundaries: rheologic feedbacks and seismic anisotropy (United States)

    Blackman, D. K.; Boyce, D. E.; Castelnau, O.; Dawson, P. R.; Laske, G.


    Insight into upper-mantle processes can be gained by linking flow-induced mineral alignment to regional deformation and seismic anisotropy patterns. Through a series of linked micro-macro scale numerical experiments, we explore the rheologic effects of crystal preferred orientation (CPO) and evaluate the magnitude of possible impacts on the pattern of flow and associated seismic signals for mantle that includes a cooling, thickening young oceanic lithosphere. The CPO and associated anisotropic rheology, computed by a micromechanical polycrystal model, are coupled with a large scale flow model (Eulerian Finite Element method) via a local viscosity tensor field, which quantifies the stress:strain rate response of a textured polycrystal. CPO is computed along streamlines throughout the model space and the corresponding viscosity tensor field at each element defines the local properties for the next iteration of the flow field. Stable flow and CPO distributions were obtained after several iterations for the two dislocation glide cases tested: linear and nonlinear stress:strain rate polycrystal behaviour. The textured olivine polycrystals are found to have anisotropic viscosity tensors in a significant portion of the model space. This directional dependence in strength impacts the pattern of upper-mantle flow. For background asthenosphere viscosity of ˜1020 Pa s and a rigid lithosphere, the modification of the corner flow pattern is not drastic but the change could have geologic implications. Feedback in the development of CPO occurs, particularly in the region immediately below the base of the lithosphere. Stronger fabric is predicted below the flanks of a spreading centre for fully coupled, power-law polycrystals than was determined using prior linear, intermediate coupling polycrystal models. The predicted SKS splitting is modestly different (˜0.5 s) between the intermediate and fully coupled cases for oceanic plates less than 20 Myr old. The magnitude of azimuthal

  4. Fractional ultrabasic-basic evolution of upper-mantle magmatism: Evidence from xenoliths in kimberlites, inclusions in diamonds and experiments (United States)

    Litvin, Yuriy; Kuzyura, Anastasia


    Ultrabasic peridotites and pyroxenites together with basic eclogites are the upper-mantle in situ rocks among xenoliths in kimberlites. Occasionally their diamond-bearing varieties have revealed within the xenoliths. Therewith the compositions of rock-forming minerals demonstrate features characteristic for primary diamond-included minerals of peridotite and eclogite parageneses (the elevated contents of Cr-component in peridotitic garnets and Na-jadeitic component in eclogitic clinopyroxenes). High-pressure experimental study of melting equilibria on the multicomponent peridotie-pyroxenite system olivine Ol - orthopyroxene Opx - clinopyroxene Cpx - garnet Grt showed that Opx disappeared in the peritectic reaction Opx+L→Cpx (Litvin, 1991). As a result, the invariant peritectic equilibrium Ol+Opx+Cpx+Grt+L of the ultrabasic system was found to transform into the univariant cotectic assemblage Ol+Cpx+Grt+L. Further experimental investigation showed that olivine reacts with jadeitic component (Jd) with formation of garnet at higher 4.5 GPa (Gasparik, Litvin, 1997). Study of melting relations in the multicomponent system Ol - Cpx - Jd permits to discover the peritectic point Ol+Omph+Grt+L (where Omph - omphacitic clinopyroxene) at concentration 3-4 wt.% Jd-component in the system. The reactionary loss of Opx and Ol makes it possible to transform the 4-phase garnet lherzolite ultrabasic association into the bimineral eclogite assemblage. The regime of fractional Ol, Cpx and Grt crystallization must be accompanied by increasing content of jadeitic component in residual melts that causes the complete "garnetization of olivine". In the subsequent evolution, the melts would have to fractionate for basic SiO2-saturated compositions responsible for petrogenesis of eclogite varieties marked with accessory corundum Crn, kyanite Ky and coesite Coe. Both the peritectic mechanisms occur in regime of fractional crystallization. The sequence of the upper-mantle fractional

  5. Microstructural and seismic properties of the upper mantle underneath a rifted continental terrane (Baja California): An example of sub-crustal mechanical asthenosphere?

    NARCIS (Netherlands)

    Palasse, L.N.; Vissers, R.L.M.; Paulssen, H.; Basu, A.R.; Drury, M.R.


    The Gulf of California rift is a young and active plate boundary that links the San Andreas strike-slip fault system in California to the oceanic spreading system of the East Pacific Rise. The xenolith bearing lavas of the San Quintin volcanic area provide lower crust and upper mantle samples from b

  6. Homogenization of carbonate-bearing microinclusions in diamond at P- T parameters of the upper mantle (United States)

    Ragozin, A. L.; Palyanov, Yu. N.; Zedgenizov, D. A.; Kalinin, A. A.; Shatsky, V. S.


    The staged high-pressure annealing of natural cubic diamonds with numerous melt microinclusions from the Internatsional'naya kimberlite pipe was studied experimentally. The results mainly show that the carbonate phases, the daughter phases in partially crystallized microinclusions in diamonds, may undergo phase transformations under the mantle P- T conditions. Most likely, partial melting and further dissolution of dolomite in the carbonate-silicate melt (homogenization of inclusions) occur in inclusions. The experimental data on the staged high-pressure annealing of diamonds with melt microinclusions allow us to estimate the temperature of their homogenization as 1400-1500°C. Thus, cubic diamonds from the Internatsional'naya pipe could have been formed under quite high temperatures corresponding to the lithosphere/asthenosphere boundary. However, it should be noted that the effect of selective capture of inclusions with partial loss of volatiles in relation to the composition of the crystallization medium is not excluded during the growth. This may increase the temperature of their homogenization significantly between 1400 and 1500°C.

  7. Clumped fluoride-hydroxyl defects in forsterite: Implications for the upper-mantle (United States)

    Crépisson, Céline; Blanchard, Marc; Bureau, Hélène; Sanloup, Chrystèle; Withers, Anthony C.; Khodja, Hicham; Surblé, Suzy; Raepsaet, Caroline; Béneut, Keevin; Leroy, Clémence; Giura, Paola; Balan, Etienne


    The mechanism and magnitude of fluorine incorporation in H-bearing forsterite were investigated through a combined experimental and theoretical approach. Forsterite samples were synthesized in a piston cylinder press at 2 and 4 GPa, in hydrous conditions, with or without fluorine. High fluorine solubilities of 1715 and 1308 ppm F were measured by particle induced gamma-ray emission (PIGE) in forsterite samples synthesized at 2 and 4 GPa, respectively. In addition, first-principles calculations based on density functional theory were performed in order to investigate the coupled incorporation mechanisms of fluorine and water in forsterite. Our results demonstrate the close association of fluoride, hydroxyl groups and Si vacancies. Comparison of experimental and theoretical infrared absorption spectra enables assignment of the nine OH stretching bands (3500-3700 cm-1) observed in F-rich synthetic forsterite to clumped fluoride-hydroxyl defects in the forsterite crystal structure. Noteworthily, similar bands were previously recorded on some natural olivine with Mg/(Mg+Fe) molar ratio down to 0.86. Fluorine and water cycles are therefore strongly coupled through the nominally anhydrous minerals and the mantle fluorine budget can be entirely accommodated by these mineral phases.

  8. Upper mantle low-velocity layers beneath the High Lava Plains imaged by scattered-wavefield migration (United States)

    Chen, C.; James, D. E.; Wagner, L. S.


    The High Lava Plains (HLP) in eastern Oregon represents one of the most active intraplate magmatic provinces on Earth. This region's recent tectonic history is dominated by voluminous mid-Miocene outpourings of the Steens and Columbia River flood basalts, followed by a period of bimodal volcanic activities, generating two roughly orthogonal time-progressive rhyolitic hotspot tracks: the northeastern-trending Snake River Plain and the northwestern-trending High Lava Plains. The causes of this complex tectonomagmatic evolution are not well understood, and geophysical constraints have been lacking regarding the detailed crustal and upper mantle structure in this region. From 2006 to 2009, a passive seismic experiment with the deployment of 118 broadband seismic stations was carried out as part of the multidisciplinary High Lava Plains project, which aims to investigate the causes of continental intraplate tectonomagmatism. These stations covered central and eastern Oregon, northern Nevada, and southwestern Idaho, with average spacing of 15-20 km, yielding unprecedented data density in the HLP region. A number of tomographic and receiver function studies has revealed complex structures beneath HLP. These include irregular Moho topography across the HLP, and concentrated low velocity anomalies in the uppermost mantle beneath regions of Holocene volcanism in southeastern Oregon (including areas of the Owyhee Plateau), as well as beneath volcanic centers near Steens Mountain and Newberry volcano. We complement these previous studies by generating high-resolution seismic images from scattered wavefield to detect seismic discontinuities beneath the HLP. We process 80 high-quality teleseismic events with good azimuthal coverage using a 2-D teleseismic migration algorithm based on the Generalized Radon Transform. The resulting migration images indicate the presence of several main features: 1) a prominent and varying Moho topography: the Moho is at ~40 km depth east of the

  9. Oxidation state of the Earth's upper mantle during the last 3800 million years: Implications for the origin of life (United States)

    Delano, J. W.


    A popular, as well as scientifically rigorous, scenario for the origin of life on Earth involves the production of organic molecules by interaction of lightning (or other forms of energy) with a chemically reducing atmosphere in the early history of Earth. Experiments since the 1950's have convincingly demonstrated that the yield of organic molecules is high when the atmosphere contains molecular hydrogen, methane, ammonia, and water vapor. Additional work has also shown that such a highly reducing atmosphere might not, however, have been sufficiently long-lived in the presence of intense solar ultraviolet radiation for life to have formed from it. One way of maintaining such an atmosphere would be to have a continual replenishment of the reduced gases by prolonged volcanic outgassing from a reducing of Earth's interior. The length of time that this replenishment might need to continue is in part constrained by the flux of asteroids onto the Earth's surface containing sufficient energy to destroy most, if not all, life that had developed up to that point in time. If a reducing atmosphere is a key ingredient for the origin of life on Earth, the time of the last environmental sterilization due to large impacts would be an important constraint. In a deep marine setting (e.g., hydrothermal vent), the last global sterilization might have occurred at 4200-4000 Ma. On the Earth's surface, the last global sterilization event might have occurred at 4000-3700 Ma. If these are meaningful constraints, how likely is it that a reducing atmosphere could have survived on the Earth until about 3800 Ma ago? Due to the importance of replenishing this atmosphere with reducing components by volcanic outgassing from the mantle, geochemical information on the history of the mantle's oxidation state would be useful for addressing this question. Geochemical and experimental data discussed in this abstract suggest that extrusive mafic volcanics derived from the upper mantle have had

  10. An Investigation of Crust and Upper Mantle Structure in Western Argentina Utilizing Local Event Receiver Functions (United States)

    Calkins, J. A.; Zandt, G.; Gilbert, H.; Beck, S.


    Images of the crust-mantle boundary and crustal structure obtained using the traditional analysis of teleseismic receiver functions (RFs) exhibit an unusually weak P-S conversion from the Moho in Western Argentina, where the subducting Nazca plate temporarily flattens out beneath the overriding South American plate. In order to better estimate depth to the Moho and search for mid-crustal impedance contrasts, we calculate and stack receiver functions using approximately 45 local earthquakes occurring in the downgoing slab between December of 2000 and February of 2001. The events occurred over a depth range of 76 to 165 km and were all within 128 km horizontal distance of the recording station and thus traveled with ray parameters less than .09 s/km. Radial receiver functions are calculated at two temporary broadband seismic stations located between San Juan and Mendoza, in the region where the Precordillera transitions eastward to the Sierras Pampeanas. Plots of stacked RFs as a function of ray parameter show a strong signal from the Moho at 7 seconds corresponding to a depth near 50 km, as well as conversions from interfaces within the crust at depths of ˜ 20 and 35 km. It should be noted that the narrow time interval between the P and S arrivals, due to the close proximity of events to the stations, precludes the analysis of reverberations within the crust to better constrain crustal Vp/Vs estimates and to refine the depth to interfaces. The observed Moho depth is in good agreement with estimates made using Pn apparent phase velocities along a transect through tectonically similar terrain 200 km to the north. In both cases, areas of relatively low topography are underlain by anomalously thick crust. The discrepancy in the clarity of the Moho Ps between RFs obtained using teleseismic versus local events currently remains unexplained but is an area of ongoing research.

  11. Crust and upper mantle electrical structure of Haiyuan-Liupanshan Thrust Belt and its vicinity revealed by magnetotelluric(MT) detection (United States)

    Han, S.; Liu, G.; Han, J.


    Under the auspices of SinoProbe Project, an array of 91 broad-band magnetotelluric(MT) sites across the southern segment of the Haiyuan-Liupanshan Thrust Belt (HLTB) was occupied to determine the crust and upper mantle structure of the transition zone between the Ordos Block (OB) and the Qilian Orogenic Belt (QOB).An electrical structure model of the crust and the upper mantle was finally obtained after data processing, qualitative analysis and 2D inversion of the observed data.The model revealed the deep structure of the profile.The upper crust of the HLTB is modelled as resistive while the other two tectonic units are modelled as less resistive.The massive high resistive blocks in the upper crust are seen in the HLTB.On the contrast,the lower crust is revealed as conduvtive on the whole.Middle to lower crustal high conductive layers (HCL) are seen both in the QOB and the OB.A strong lower crust conductor is revealed in the HLTB.Electrical structure of the upper mantle is revealed as resistive,respectively.The wedge structure is seen in the uppermost mantle under the Liupanshan Mountain.According to the electrical structure of the profile,the study region can be divided into three tectonic units:the QOB,the Liupan Transition Zone (LTZ,expansion of the HLTB) and the OB.The tectonic deformation for the QB manifest as thrust nappe in the upper crust and shortening strain in the lower crust.The east-dipping conductor in the west of QOB may represent the accumulated weak material in the form of middle crust flow and the HCL of the QB may be the migration channel.The fluctuation of HCL may indicate interior deformation of the OB.The LTZ is quite different from the adjacent tectonic units that fragment structure exists in the upper crust and deep thrust faults cut through the upper crust.The conductor located in the lower crust is interpreted as partial melt zone as a result of the vertical decompression process.Joint interpretation of the electrical structure and the

  12. Anisotropic velocity structure of the crust and upper mantle in the Taiwan region from local travel time tomography (United States)

    Jakovlev, Andrey; Koulakov, Ivan; Wu, Yih-Min


    Taiwan Island located in a contact zone between the Eurasian and Philippine Sea plates, the tectonic processes in this area are mostly controlled by the relative kinematics of these two plates. In the east, the Philippine Sea plate subducts northward under the Eurasian plate along the Ryukyu trench. Off the southern tip of Taiwan, the South China Sea subplate, part of the Eurasian plate, subducts eastward under the Philippine Sea plate underneath the Luzon Island. The Taiwan Island is located at the junction between these two subduction zones. Here we present anisotropic velocity model of the crust and upper mantle in the Taiwan region derived from local travel time tomography. We use more than 300 000 P and more than 150 000 S rays coming from 12910 earthquakes occurred in the Taiwan region and registered by 816 stations of different local Taiwanese seismic networks. The ANITA algorithm, for iterative nonlinear inversion of local earthquake data in orthorhombic anisotropic media with one predefined vertical orientation, was used for the tomographic inversion. This algorithm presumes anisotropy for only P velocity described as horizontally oriented ellipsoid. For S velocity we presume an isotropic model. Results show a good agreement with tectonic structure of the region. Obtained isotropic P and S velocity models show fit to each other. The most prominent features of the models are Philippine Sea plate characterized by increased velocities and decreased velocities observed along the Luzon and Ryukyu arcs. We observe that orientation of the fast velocity axis within the Philippine Sea plate coincides with direction of its displacement. Along the Luzon and Ryukyu arcs orientation of the fast velocities axis coincide with the arcs extension. The results show that upper mantle beneath the eastern Taiwan characterized by decreased velocities and N-S orientation of the fast velocity axis. The western Taiwan characterized by alteration of the relatively small negative

  13. Upper mantle shear wave velocity structure beneath northern Victoria Land, Antarctica: Volcanism and uplift in the northern Transantarctic Mountains (United States)

    Graw, Jordan H.; Adams, Aubreya N.; Hansen, Samantha E.; Wiens, Douglas A.; Hackworth, Lauren; Park, Yongcheol


    The Transantarctic Mountains (TAMs) are the largest non-compressional mountain range on Earth, and while a variety of uplift mechanisms have been proposed, the origin of the TAMs is still a matter of great debate. Most previous seismic investigations of the TAMs have focused on a central portion of the mountain range, near Ross Island, providing little along-strike constraint on the upper mantle structure, which is needed to better assess competing uplift models. Using data recorded by the recently deployed Transantarctic Mountains Northern Network, as well as data from the Transantarctic Mountains Seismic Experiment and from five stations operated by the Korea Polar Research Institute, we investigate the upper mantle structure beneath a previously unexplored portion of the mountain range. Rayleigh wave phase velocities are calculated using a two-plane wave approximation and are inverted for shear wave velocity structure. Our model shows a low velocity zone (LVZ; ∼4.24 km s-1) at ∼160 km depth offshore and adjacent to Mt. Melbourne. This LVZ extends inland and vertically upwards, with more lateral coverage above ∼100 km depth beneath the northern TAMs and Victoria Land. A prominent LVZ (∼4.16-4.24 km s-1) also exists at ∼150 km depth beneath Ross Island, which agrees with previous results in the TAMs near the McMurdo Dry Valleys, and relatively slow velocities (∼4.24-4.32 km s-1) along the Terror Rift connect the low velocity anomalies. We propose that the LVZs reflect rift-related decompression melting and provide thermally buoyant support for the TAMs uplift, consistent with proposed flexural models. We also suggest that heating, and hence uplift, along the mountain front is not uniform and that the shallower LVZ beneath northern Victoria Land provides greater thermal support, leading to higher bedrock topography in the northern TAMs. Young (0-15 Ma) volcanic rocks associated with the Hallett and the Erebus Volcanic Provinces are situated directly

  14. 3-D Teleseismic Tomography of the Crust and Upper Mantle Beneath Northern Tasmania, Australia (United States)

    Rawlinson, N.; Kennett, B. L.; Reading, A. M.


    -felsic compositional variation in the deep crust, possibly caused by a collision-obduction event in the Cambrian; and (4) the pattern of velocity variations within the crust does not have a simple relationship to that in the mantle lithosphere beneath.

  15. Crust and upper mantle heterogeneities in the southwest Pacific from surface wave phase velocity analysis (United States)

    Pillet, R.; Rouland, D.; Roult, G.; Wiens, D. A.


    Direct earthquake-to-station Rayleigh and Love wave data observed on high gain broadband records are analyzed in order to improve the lateral resolution of the uppermost mantle in the southwest Pacific region. We used data of nine permanent Geoscope and Iris stations located in the southern hemisphere and nine other stations as part of two temporary networks, the first one installed in New Caledonia and Vanuatu (hereafter named Cavascope network) by ORSTOM and the EOST from Louis Pasteur University in Strasbourg (France) and the second one installed in the Fiji, Tonga and Niue islands (hereafter named Spase network) by Washington University in St. Louis (USA). In order to collect more significant details on the surficial structures, we included the analysis of short period waves down to 8 s. A multiple frequency filtering technique has been used to recover phase velocities of Rayleigh and Love waves for selected earthquakes with magnitude greater than 5.5 and with known centroid moment tensor (CMT). About 1100 well-distributed seismograms have been processed in the period range 8-100 s and corrections for topography and water depth have been applied to the observed phase velocities. The geographical distribution of phase velocity anomalies have then been computed using the tomographic method developed by Montagner [Montagner, J.P., 1986a. Regional three-dimensional structures using long-period surface waves. Ann. Geophys. 4 (B3), 283-294]. Due to a poor knowledge of dense, well-distributed, crustal thickness values and corresponding velocity models, we did not perform or speculate on the construction of an S-wave 3D velocity model; therefore, we limited this study to the interpretation of the phase velocity distribution. The location of phase velocity anomalies are well determined and the deviations are discussed within the framework of the geological context and compared with other tomographic models. At long periods, from 40 s to 100 s, our results agree well

  16. Upper mantle structure under western Saudi Arabia from Rayleigh wave tomography and the origin of Cenozoic uplift and volcanism on the Arabian Shield

    Energy Technology Data Exchange (ETDEWEB)

    Park, Y; Nyblade, A; Rodgers, A; Al-Amri, A


    The shear velocity structure of the shallow upper mantle beneath the Arabian Shield has been modeled by inverting new Rayleigh wave phase velocity measurements between 45 and 140 s together with previously published Rayleigh wave group velocity measurement between 10 and 45 s. For measuring phase velocities, we applied a modified array method that minimizes the distortion of raypaths by lateral heterogeneity. The new shear velocity model shows a broad low velocity region in the lithospheric mantle across the Shield and a low velocity region at depths {ge} 150 km localized along the Red Sea coast and Makkah-Madinah-Nafud (MMN) volcanic line. The velocity reduction in the upper mantle corresponds to a temperature anomaly of {approx}250-330 K. These finding, in particular the region of continuous low velocities along the Red Sea and MMN volcanic line, do not support interpretations for the origin of the Cenozoic plateau uplift and volcanism on the Shield invoking two separate plumes. When combined with images of the 410 and 660 km discontinuities beneath the southern part of the Arabian Shield, body wave tomographic models, a S-wave polarization analysis, and SKS splitting results, our new model supports an interpretation invoking a thermal upwelling of warm mantle rock originating in the lower mantle under Africa that crosses through the transition zone beneath Ethiopia and moves to the north and northwest under the eastern margin of the Red Sea and the Arabian Shield. In this interpretation, the difference in mean elevation between the Platform and Shield can be attributed to isostatic uplift caused by heating of the lithospheric mantle under the Shield, with significantly higher region along the Red Sea possibly resulting from a combination of lithosphere thinning and dynamic uplift.

  17. Lateral heterogeneity and azimuthal anistropy of the upper mantle: Love and Rayleigh waves 100-250 sec (United States)

    Tanimoto, T.; Anderson, D. L.


    The lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring Rayleigh and Love wave phase velocities in the period range 100-250 sec. Spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1=m=10. Slow regions are evident at the East Pacific rise, northeast Africa, Tibet, Tasman sea, southwestern North America and triple junctions in the Northern Atlantic and Indian oceans. Fast regions occur in Australia, western Pacific and the eastern Atlantic. Details which are not evident in previous studies include two fast regions in the central Pacific and the subduction zone in the Scotia Arc region. Inversion for azimuthal dependence showed (1) little correlation between the fast phase velocity directions and the plate motion vector in plate interiors, but (2) correlation of the fast direction with the perpendicular direction to trenches and ridges. Phase velocity is high when waves propagate perpendicular to these structures. Severe tradeoffs exist between heterogeneity and azimuthal dependence because of the yet unsatisfactory path coverage.

  18. Lateral heterogeneity and azimuthal anisotropy of the upper mantle - Love and Rayleigh waves 100-250 sec (United States)

    Tanimoto, T.; Anderson, D. L.


    The lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring Rayleigh and Love wave phase velocities in the period range 100-250 sec. Spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1=m=10. Slow regions are evident at the East Pacific rise, northeast Africa, Tibet, Tasman sea, southwestern North America and triple junctions in the Northern Atlantic and Indian oceans. Fast regions occur in Australia, western Pacific and the eastern Atlantic. Details which are not evident in previous studies include two fast regions in the central Pacific and the subduction zone in the Scotia Arc region. Inversion for azimuthal dependence showed (1) little correlation between the fast phase velocity directions and the plate motion vector in plate interiors, but (2) correlation of the fast direction with the perpendicular direction to trenches and ridges. Phase velocity is high when waves propagate perpendicular to these structures. Severe tradeoffs exist between heterogeneity and azimuthal dependence because of the yet unsatisfactory path coverage.

  19. Cumulate xenoliths from St. Vincent, Lesser Antilles Island Arc: a window into upper crustal differentiation of mantle-derived basalts (United States)

    Tollan, P. M. E.; Bindeman, I.; Blundy, J. D.


    In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine-gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4-10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89-5.18‰), plagioclase (5.84-6.28‰), clinopyroxene (5.17-5.47‰) and hornblende (5.48-5.61‰) and hydrogen isotope composition of hornblende (δD = -35.5 to -49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth

  20. "DOBREfraction'99"—velocity model of the crust and upper mantle beneath the Donbas Foldbelt (East Ukraine) (United States)

    Grad, M.; Gryn', D.; Guterch, A.; Janik, T.; Keller, R.; Lang, R.; Lyngsie, S. B.; Omelchenko, V.; Starostenko, V. I.; Stephenson, R. A.; Stovba, S. M.; Thybo, H.; Tolkunov, A.; Dobrefraction'99 Working Group


    The Donbas Foldbelt (DF) is the uplifted and deformed part of the up to 20-km-thick Dniepr-Donets Basin (DDB) that formed as the result of rifting of the East European Craton (EEC) in the Late Devonian. Uplift, especially of the southern margin of the basin, occurred in Early Permian times, in a (trans)tensional tectonic stress regime while folding and reverse faulting mainly occurred later—mainly during the Late Cretaceous. A seismic refraction/wide-angle reflection survey was carried out in 1999 (DOBREfraction'99) to complement existing Deep Seismic Sounding (DSS) data from the area that did not record significant Pn phase arrivals because of insufficient maximum offset. DOBREfraction'99 comprised some 245 recording stations along a line of 360 km length with 11 in-line shot points as well as a 100 km away, parallel 190 km long subsidiary fan profile. The main profile runs between the shores of the Azov Sea in the south to the Ukraine-Russia border in the north, across the Azov Massif (Ukrainian Shield), the Foldbelt, and the Voronezh Massif. Particular scientific targets include the nature of the crust-mantle transition and the geometry of crustal-upper mantle structures related to rifting and subsequent basin inversion. Tomographic inversion as well as ray-trace based velocity modelling has been carried out. The velocity signature of the sedimentary basin itself is well resolved, indicating an asymmetric form, with a steeper basement surface in the south than in the north, and a total thickness of about 20 km. A thick (>10 km) high velocity (>6.9 km/s) lower crustal body lies beneath the rift basin itself, offset slightly to the north compared to the main basin depocentre, likely related to the rifting processes. Velocities in the crust below the Ukrainian Shield, south of the Foldbelt, are in general higher than beneath the Voronezh Massif to the north. The Moho displays only slight topography around a depth of 40 km along the profile.

  1. From the Surface Topography to the Upper Mantle Beneath Central-Iberian-Zone. the Alcudia Seismic Experiments. (United States)

    Carbonell, R.; Ehsan, S. A.; Ayarza, P.; Martinez-Poyatos, D. J.; Simancas, J. F.; Azor, A.; Pérez-Estaún, A.


    Normal incidence and wide-angle seismic reflection data acquired in the Central and southern parts of the Iberia Peninsula resolve the internal architecture and constrain the distribution of the physical properties along an almost 350 km long transect that samples the major tectonic domains of the Iberian Massif, including the Central Iberian Zone (CIZ) and the associated sutures. The internal architecture down to almost 70 km depth (~15 s TWTT) is resolved by the normal incidence data set. It images a number of elements that characterize the tectonics of the study area, which is one of the best exposed fragment of the Variscan orogenic Belt. A well marked brittle-to-ductile (B2D) transition separates the crust in two, the upper and mid-lower parts, approximately, 13 km and 18 km thick, respectively. The upper crust appears to be decoupled from the mid-lower crust and responded differently to shortening. The Mohorovicic discontinuity is located at ~10.5 s (TWTT) , it is relatively thick, and highly reflective beneath the CIZ. The wide-angle seismic transect extended the lithospheric section towards the north across the Madrid Basin. This profile provides very strong constraints on the distribution of physical properties (P- and S- wave velocities, Poisson's ratio) of the upper lithosphere as well as a high resolution image of the base of the crust beneath the area. This data is one of the first datasets to present solid evidence of a relatively significant crustal thickening beneath the Madrid Basin. The crustal thickness varies from ~31 km beneath the CIZ to ~35.5 km beneath the Madrid Basin. This data set also reveals two major discontinuity levels, the B2D and the Moho, both represent levels of lithological/rheological variations. The characteristics of the the PmP and SmS seismic phases suggest further details on the internal structure of the Moho. Furthermore, low fold wide-angle P and S wave stacks reveal a marked crust-mantle transition which is most

  2. P and S Wave Velocity Structure of the Crust and Upper Mantle Under China and Surrounding Areas From Body and Surface Wave Tomography (United States)


    1.9 to 1.45 s, after the inversion. [14] High velocities dominate in western China. Beneath several large depressed basins, such as the Tarim...velocity image of Moho discontinuity beneath the Weihe fault depression and its adjacent areas obtained by inversion of travel-time data of Sn waves...the crust and upper mantle. Geo- phys. J. Int. 151, 1–18. Sol, S.J., Meltzer , A., Zurek, B., Zhang, X., Zhang, J., 2004. Insight into the

  3. Crust and upper mantle velocity structure of the northwestern Indian Peninsular Shield from inter-station phase velocities of Rayleigh and Love waves

    Directory of Open Access Journals (Sweden)

    Gaddale Suresh


    Full Text Available We measure the inter-station Rayleigh and Love wave phase velocities across the northwestern Indian Peninsular shield (NW-IP through cross-correlation and invert these velocities to evaluate the underneath crust and upper mantle velocity structure down to 400 km. We consider a cluster of three stations in the northern tip of the Peninsula and another cluster of eight stations in the south. We measure phase velocities along 28 paths for Rayleigh waves and 17 paths for Love waves joining two stations with one from each cluster and using broadband records of earthquakes which lie nearly on the great circle joining the pair of stations. The phase velocities are in the period range of 10 to 275 s for Rayleigh waves and of 10 to 120 s for Love waves. The isotropic model obtained through inversion of the phase velocities indicates 199.1 km thick lithosphere with 3-layered crust of thickness 36.3 km; the top two layers have nearly same velocities and both constitute the upper crust with thickness of 12.6 km. The upper crust is mafic, whereas the lower crust is felsic. In the mantle lid, velocities increase with depth. The velocities of mantle lid beneath NW-IP is lower than those beneath south Indian Peninsula showing the former is hotter than the later perhaps due to large Phanerozoic impact on NW-IP. The significant upper mantle low velocity zone beneath NW-IP indicates high temperature which could be attributed to the past existence of a broad plume head at the west-central part of the Peninsula.

  4. Thermochemical constraints on the thermal state, composition, and mineralogy of the upper mantle of the Moon: Evidence from the seismic models (United States)

    Kuskov, O. L.; Kronrod, V. A.; Kronrod, E. V.


    The thermal state, heat flow, and thermochemical evolution of the Moon are still debatable, and the temperature of lunar interiors is one the most uncertain physical parameters. Transformation of profiles of the velocities of the P and S seismic waves in the lunar mantle obtained by processing the Apollo lunar seismic data into the temperature-depth relationships was performed by the method of thermodynamic modeling in the Na2O-TiO2-CaO-FeO-MgO-Al2O3-SiO2 system. This was the basis for testing of four seismic models in relation to the thermal regime and chemical composition of the mantle in a wide range of the concentrations of CaO (2-5%), Al2O3 (2-6.5%), and FeO (8.5-13%). In contrast to the Earth's mantle, the chemical composition is of key importance for conversion of the velocities in the same seismic model into the temperature effects. The most probable composition of the upper mantle corresponds to olivine-bearing pyroxenite depleted in refrectory oxides (˜2 wt % CaO and Al2O3). Based on the seismic models, constraints on the temperature distribution in the mantle, heat flow, and uranium concentration in the Moon were established. Estimation of the upper limits of the total heat flow resulted in approximately half compared with the Apollo measurements. The results of conversion of the velocities of the P and S seismic waves into the temperature-depth relationships show that, independently on the composition, the positive gradient in the velocities of the P and S waves results in the negative temperature gradient in the mantle, which does not have a physical basis. The velocities of P and S waves should be almost constant or decrease slightly (especially V S ) as a result of the influence of the temperature increasing more rapidly than the pressure for an adequate distribution of temperature in the lunar mantle. The suggested approach to testing of the velocity structure of the lunar mantle based on the methods of thermodynamics and mineral physics provides

  5. Upper mantle temperatures from teleseismic tomography of French Massif Central including effects of composition, mineral reactions, anharmonicity, anelasticity and partial melt (United States)

    Sobolev, Stephan V.; Zeyen, Hermann; Stoll, Gerald; Werling, Friederike; Altherr, Rainer; Fuchs, Karl


    A new technique for interpretation of 3-D seismic tomographic models in terms of temperature, degree of partial melt and rock composition is presented and tested. We consider both anharmonic and anelastic temperature effects on seismic velocities as well as the effects of mineral reactions, composition and partial melt. It is shown that composition effect is small (less than 1% of velocity) if there are no strongly depleted, Mg-rich harzburgites. We calculate anharmonic temperature derivatives of seismic velocities from compositions of mantle xenoliths. The parameters of a non-linear frequency and temperature-dependent model of attenuation have been taken from published laboratory experiments and calibrated using global Q observations in the upper mantle. For every block of the tomographic model we calculate the absolute temperature and melt fraction required to fit the observed Vp perturbation, the average temperature of the tomographic layer being constrained by the observed surface heat flow. With these temperatures we calculate attenuation, density, Vp and Vs from petrophysical modelling, using the average for 80 mantle xenoliths samples from the French Massif Central. The technique is applied to a recently published 3-D teleseismic P wave tomographic model of the upper mantle beneath the French Massif Central. The observed velocity perturbations are probably caused there by variations in temperature. Temperature does not reach the dry solidus temperature (except for a few tomographic blocks), although it comes close to it at the depth of 60-100 km below volcanic areas. At high subsolidus temperatures the contribution of anelasticity to velocity perturbations is at least as important as the combined effect of anharmonicity and mineral reactions. Our model is consistent with the Pn velocities from refraction seismic studies, QS estimations from surface waves, observed gravity, geoid, topography and surface heat flow, as well as with the composition and

  6. Effect of H2O on Upper Mantle Phase Transitions in MgSiO3: is the Seismic X-discontinuity an Indicator of Mantle Water Content

    Energy Technology Data Exchange (ETDEWEB)

    S Jacobsen; Z Liu; T Boffa Ballaran; E Littlefield; L Ehm; R Hemley


    The mantle X-discontinuity, usually assigned to positive seismic velocity reflectors in the 260-330 km depth range, has proved difficult to explain in terms of a single mineralogical phase transformation in part because of its depth variability. The coesite to stishovite transition of SiO{sub 2} matches deeper X-discontinuity depths but requires 5-10% free silica in the mantle to match observed impedance contrast. The orthoenstatite (OEn) to high-pressure clinoenstatite (HPCen) transformation of MgSiO{sub 3} also broadly coincides with depths of the X but requires chemically depleted and orthoenstatite-rich lithology at 300 km depth in order to match observed seismic impedance contrast. On the basis of high-pressure infrared spectroscopy, X-ray diffraction, and Raman spectroscopy, we show that 1300 ppm variation of H{sub 2}O content in MgSiO{sub 3} can displace the transition of low-pressure clinoenstatite (LPCen) to HPCen by up to 2 GPa, similar to previous quench experiments on the OEn to HPCen phase transition, where about 30-45 km (1.0-1.5 GPa) of deflection could occur per 0.1 wt% H{sub 2}O. If the mantle X-discontinuity results from pyroxene transitions in a depleted harzburgite layer, because of the strong influence of minor amounts of water on the transformation boundary, the depth of the mantle X-discontinuity could serve as a potentially sensitive indicator of water content in the uppermantle.

  7. a Global Shear Velocity Model of the Upper Mantle from New Fundamental and Higher Rayleigh Mode Measurements (United States)

    Debayle, E.; Ricard, Y. R.


    We present a global SV-wave tomographic model of the upper mantle, built from a new dataset of fundamental and higher mode Rayleigh waveforms. We use an extension of the automated waveform inversion approach of Debayle (1999) designed to improve the extraction of fundamental and higher mode information from a single surface wave seismogram. The improvement is shown to be significant in the transition zone structure which is constrained by the higher modes. The new approach is fully automated and can be run on a Beowulf computer to process massive surface wave dataset. It has been used to match successfully over 350 000 fundamental and higher mode Rayleigh waveforms, corresponding to about 20 millions of new measurements extracted from the seismograms. For each seismogram, we obtain a path average shear velocity and quality factor model, and a set of fundamental and higher mode dispersion and attenuation curves compatible with the recorded waveform. The set of dispersion curves provides a global database for future finite frequency inversion. Our new 3D SV-wave tomographic model takes into account the effect of azimuthal anisotropy and is constrained with a lateral resolution of several hundred kilometers and a vertical resolution of a few tens of kilometers. In the uppermost 200 km, our model shows a very strong correlation with surface tectonics. The slow velocity signature of mid-oceanic ridges extend down to ~100 km depth while the high velocity signature of cratons vanishes below 200 km depth. At depth greater than 400 km, the pattern of seismic velocities appear relatively homogeneous at large scale, except for high velocity slabs which produce broad high velocity regions within the transition zone. Although resolution is still good, the region between 200 and 400 km is associated with a complex pattern of seismic heterogeneities showing no simple correlation with the shallower or deeper structure.

  8. Constraining the shear zone along the Dead Sea transform fault in the crust and upper mantle using seismic anisotropy (United States)

    Kaviani, Ayoub; Rümpker, Georg; Asch, Günter; Desire Group


    We study seismic anisotropy along the Dead Sea Transform fault (DST) by shear-wave splitting analysis of SKS and SKKS waveforms recorded at a dense network of broad-band and short-period stations of the DESIRE experiment. The DST accommodates the relative motion between Africa and Arabia through a sinistral strike-slip motion. The Dead Sea is a pull-apart basin formed along the DST as a result of stepwise fault-normal displacement. The DESIRE array of stations cover this portion of the DST. We measured the splitting parameters (delay times between the fast and slow components of the shear wave and fast polarization direction) in different period bands. We observed consistent fast polarization directions varying from N14W to N19E at different stations and delay times ranging between 1.0 and 2.5 s. Our preliminary examination reveals that the splitting parameters do not exhibit significant frequency dependence. However, we observe variations in the splitting parameters (mostly delay times) along an E-W profile crossing the DST, with smaller delay times in the middle of the profile, within the surface exposure of the DST shear zone, and with two lobes of relatively large delay times on both sides of the central region. The fast polarization directions along this profile change from a dominant NNW trend in the western side of the DST to a general N-S orientation in the central part and a dominant NNE trend to the east. Waveform modeling is required to infer the lateral and depth variations of the strength and orientation of anisotropy in the crust and upper mantle from these observations. We will also complement our results with the data from the DESERT experiment to provide an overall pattern of seismic anisotropy and structural fabric beneath the DST and surrounding regions from the north of the Dead Sea down to the Red sea.

  9. Investigations of Passive Seismic Body-Wave Interferometry Using Noise Auto-correlations for Crustal and Upper Mantle Structure (United States)

    Oren, C.; Nowack, R. L.


    It is known that the positive lags of the auto-correlation for the seismic transmission response of a layered medium correspond to the reflection seismogram (Claerbout, 1968). In this study, we investigate the use of ambient seismic noise recorded at selected broadband USArray EarthScope Transportable Array (TA) stations to obtain effective reflection seismograms for frequencies up to 1 Hz. The goal is to determine the most suitable parameters used for the processing of ambient seismic noise for the identification of crustal and upper mantle reflections and to minimize unwanted artifacts in the noise correlations. In order to best retrieve the body-wave components of the Green's function beneath a station, a number of processing steps are required. We first remove the instrument response and apply a temporal normalization to remove the effects of the most energetic sources. Next we implement spectral whitening. We test several operators for the spectral whitening where the undulations of the power spectrum are related to the strengths of later arrivals in the auto-correlation. Different filters are then applied to the auto-correlation functions, including Gaussian and zero phase Butterworth filters, in order to reduce the effect of side lobes. Hourly auto-correlations are then stacked for up to one year. On the final stack, Automatic Gain Control (AGC) is applied to equalize the correlation amplitudes in the time domain. The robustness of the resulting ambient noise auto-correlation is first tested on selected TA stations in Nevada, where we are able to identify PmP and SmS arrivals similar to those found by Tibuleac and von Seggern (2012). We then investigate noise auto-correlations applied to selected USArray TA stations in the central US.

  10. Relating the Seismic Character of the Crust and Upper Mantle to Late-Cenozoic Extension in Southwestern N.A. (United States)

    Thurner, S.; Frassetto, A.; Porter, R.; Zandt, G.


    A recent tectonic reconstruction (McQuarrie and Wernicke, 2005) places detailed constraints on the magnitude and scope of late-Cenozoic extension throughout Southwestern North America. This project seeks to better understand the distribution of extension throughout the crust and upper mantle and elucidate the transition from the highly extended Basin and Range to the relatively unextended Colorado Plateau. To this end, we present teleseismic receiver functions generated from 31 broadband seismometers associated with EarthScope's BigFoot Array, TriNet, and PASSCAL stations deployed across Southern California and Arizona. We employ the common-conversion-point stacking method to analyze variations in lithospheric structure. Additionally, in regions with clear converted wave reverberations we analyze the trade-off between crustal thickness and bulk Vp/Vs to improve our view of how crustal thickness and Vp/Vs relate to different tectonic environments and degree of extension. Our preliminary estimates indicate crustal thicknesses of ~25-30 km in eastern California increasing to ~40- 45 km within the southern Colorado Plateau. The transition between thin to thick crust appears to occur over as little as 20 km. Crustal Vp/Vs varies considerably, with Vp/Vs greater than 1.8 near the Transverse Ranges and Colorado Plateau, and less than 1.8 in the southern Basin and Range. We also view a change in the nature of the Moho approaching the Colorado Plateau. Initial calculations indicate the amplitude of the converted wave from the Moho is twice as strong beneath the Mojave and Southern Basin and Range than the Colorado Plateau. Additionally, we observe laminated crust in the western Mojave Desert approaching the Transverse Ranges.

  11. Research of the Conductive Structure of Crust and the Upper Mantle beneath the South-Central Tibetan Plateau

    Institute of Scientific and Technical Information of China (English)

    Ye Gaofeng; Jin Sheng; Wei Wenbo; Martyn Unsworth


    With the super-wide band magnetotelluric sounding data of the Jilong (吉隆)-Cuoqin (措勤) profile (named line 800) which was completed in 2001 and the Dingri (定日)-Cuomai (措迈) profile (named line 900) which was completed in 2004, we obtained the strike direction of each MT station by strike analysis, then traced profiles that were perpendicular to the main strike direction, and finally obtained the resistivity model of each profile by nonlinear conjugate gradients (NLCG) inversion. With these two models, we described the resistivity structure features of the crust and the upper mantle of the center-southern Tibetan plateau and its relationship with Yalung Tsangpo suture: the upper crust of the research area is a resistive layer with resistivity value range of 200-3 000 Ω·m. The depth of its bottom surface is about 15-20 km generally, but the bottom surface of resistive layer is deeper in the middle of these two profiles. At line 900, it is about 30 km deep, and even at line 800, it is about 38 km deep. There is a gradient belt of resistivity at the depth of 15-45 km, and a conductive layer is beneath it with resistivity even less than 5 Ω·m. This conductive layer is composed of individual conductive bodies, and at the south of the Yalung Tsangpo suture, the conductive bodies are smaller with thickness about 10 km and lean to the north slightly. However, at the north of the Yalung Tsangpo suture, the conductive bodies are larger with thickness about 30 km and also lean to the north slightly. Relatively,the conductive bodies of line 900 are thinner than those of line 800, and the depth of the bottom surface of line 900 is also shallower. At last, after analyzing the effect factors to the resistivity of rocks, it was concluded that the very conductive layer was caused by partial melt or connective water in rocks. It suggests that the middle and lower crust of the center-southern Tibetan plateau is very thick, hot,flabby, and waxy.

  12. Compositionally Controlled Volatile Content of Nominally Volatile-Free Minerals in the Continental Upper Mantle of Southern Gondwana (Patagonia & W. Antarctica) (United States)

    Rooks, E. E.; Gibson, S. A.; Leat, P. T.; Petrone, C. M.


    H2O and F contents affect many physical and chemical properties of the upper mantle, including melting temperature and viscosity. These elements are hosted by hydrous and F-rich phases, and by modally abundant, nominally-anhydrous/halogen-free mantle minerals, which can potentially accommodate the entire volatile budget of the upper mantle. We present high-precision SIMS analyses of H2O, and F in mantle xenoliths hosted by recently-erupted (5-10 Ka) alkali basalts from south Patagonia (Pali Aike) and older (c. 25 Ma) alkali basalts from localities along the Antarctic Peninsula. Samples are well characterised peridotites and pyroxenites, from a range of depths in the off-craton lithospheric mantle. Minerals are relatively dry: H2O contents of olivine span 0-49 ppm, orthopyroxene 150-235 ppm and clinopyroxene 100-395 ppm, with highest concentrations found in spinel-garnet lherzolites from Pali Aike. These H2O concentrations fall within the global measured range for off-craton mantle minerals. H2O and F are correlated, and the relative compatibility of F in mantle phases is clinopyroxene>orthopyroxene>olivine. However, elevated F concentrations of 100-210 ppm are found in pyroxenites from two Antarctic localities. This elevated F content is not correlated with high H2O, suggesting that these rocks interacted with a F-rich melt. In clinopyroxenes, F concentration is correlated with Ti, and the ratio of M1Ti to M1Al + M1Cr, suggesting a charge balanced substitution. Consistency between samples (excepting high-F pyroxenites) suggests a constant F-budget, and that concentrations in clinopyroxenes are controlled by mineral chemistry. In orthopyroxene, F correlates with CaO, but no other major or minor elements. Large variability of H2O concentrations within samples is attributed to diffusive loss during ascent. Cl is negligible in all samples, indicating little or no influence of slab fluids from this long-lived subduction zone.

  13. 3-D multi-observable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle. II: General methodology and resolution analysis (United States)

    Afonso, J. C.; Fullea, J.; Yang, Y.; Connolly, J. A. D.; Jones, A. G.


    Here we present a 3-D multi-observable probabilistic inversion method, particularly designed for high-resolution (regional) thermal and compositional mapping of the lithosphere and sub-lithospheric upper mantle that circumvents the problems associated with traditional inversion methods. The key aspects of the method are as follows: (a) it exploits the increasing amount and quality of geophysical datasets; (b) it combines multiple geophysical observables (Rayleigh and Love dispersion curves, body-wave tomography, magnetotelluric, geothermal, petrological, gravity, elevation, and geoid) with different sensitivities to deep/shallow, thermal/compositional anomalies into a single thermodynamic-geophysical framework; (c) it uses a general probabilistic (Bayesian) formulation to appraise the data; (d) no initial model is needed; (e) compositional a priori information relies on robust statistical analyses of a large database of natural mantle samples; and (f) it provides a natural platform to estimate realistic uncertainties. In addition, the modular nature of the method/algorithm allows for incorporating or isolating specific forward operators according to available data. The strengths and limitations of the method are thoroughly explored with synthetic models. It is shown that the a posteriori probability density function (i.e., solution to the inverse problem) satisfactorily captures spatial variations in bulk composition and temperature with high resolution, as well as sharp discontinuities in these fields. Our results indicate that only temperature anomalies of ΔT ⪆150°C and large compositional anomalies of ΔMg# > 3 (or bulk ΔAl2O3 > 1.5) can be expected to be resolved simultaneously when combining high-quality geophysical data. This resolving power is sufficient to explore some long-standing problems regarding the nature and evolution of the lithosphere (e.g., vertical stratification of cratonic mantle, compositional versus temperature signatures in seismic

  14. V_p and V_p/V_s structures in the crust and upper mantle of the Taiwan region, China

    Institute of Scientific and Technical Information of China (English)


    A tomographic study of the Vp and Vp/Vs structures in the crust and upper mantle beneath the Taiwan region of China is conducted by simultaneous inversion of P and S arrival times. Compared with the previous tomographic results, the spherical finite difference technique is suitable for the strong heterogeneous velocity structure, and may improve the accuracy in the travel time and three-dimensional ray tracing calculations. The Vp and Vp/Vs structures derived from joint inversion and the relocated earthquakes can provide better constraints for analyzing the lateral heterogeneity and deep tectonic characters in the crust and upper mantle. Our tomographic results reveal significant relations between the seismic wavespeed structure and the tectonic characters. In the shallow depth, sedimentary basins and orogen show distinct wavespeed anomalies, with low Vp, high Vp/Vs in basins and high Vp, low Vp/Vs in orogen. As the suture zone of Eurasian Plate and Philippine Sea Plate, Longitudinal Valley is characterized by a significant high Vp/Vs anomaly extending to the middle-lower crust and upper mantle, which reflects the impact of rock cracking, partial melting, and the presence of fluids. In the northeast Taiwan, the Vp, Vp/Vs anomalies and relocated earthquakes depict the subducting Philippine Sea Plate under the Eurasian Plate. The high Vp of oceanic plate and the low Vp, high Vp/Vs atop the subducted oceanic plate extend to 80 km depth. Along the east-west profiles, the thickness of crust reaches 60 km at the east of Central Range with eastward dipping trend, which reveals the eastward subduction of the thickened and deformed crust of the Eurasian continental plate.

  15. Geophysical and petrological modelling of the structure and composition of the crust and upper mantle in complex geodynamic settings: The Tyrrhenian Sea and surroundings (United States)

    Panza, G. F.; Peccerillo, A.; Aoudia, A.; Farina, B.


    Information on the physical and chemical properties of the lithosphere-asthenosphere system (LAS) can be obtained by geophysical investigation and by studies of petrology-geochemistry of magmatic rocks and entrained xenoliths. Integration of petrological and geophysical studies is particularly useful in geodynamically complex areas characterised by abundant and compositionally variable young magmatism, such as in the Tyrrhenian Sea and surroundings. A thin crust, less than 10 km, overlying a soft mantle (where partial melting can reach about 10%) is observed for Magnaghi, Vavilov and Marsili, which belong to the Central Tyrrhenian Sea backarc volcanism where subalkaline rocks dominate. Similar characteristics are seen for the uppermost crust of Ischia. A crust about 20 km thick is observed for the majority of the continental volcanoes, including Amiata-Vulsini, Roccamonfina, Phlegraean Fields-Vesuvius, Vulture, Stromboli, Vulcano-Lipari, Etna and Ustica. A thicker crust is present at Albani - about 25 km - and at Cimino-Vico-Sabatini — about 30 km. The structure of the upper mantle, in contrast, shows striking differences among various volcanic provinces. Volcanoes of the Roman region (Vulsini-Sabatini-Alban Hills) sit over an upper mantle characterised by Vs mostly ranging from about 4.2 to 4.4 km/s. At the Alban Hills, however, slightly lower Vs values of about 4.1 km/s are detected between 60 and 120 km of depth. This parallels the similar and rather homogeneous compositional features of the Roman volcanoes, whereas the lower Vs values detected at the Alban Hills may reflect the occurrence of small amounts of melts within the mantle, in agreement with the younger age of this volcano. The axial zone of the Apennines, where ultrapotassic kamafugitic volcanoes are present, has a mantle structure with high-velocity lid ( Vs ˜ 4.5 km/s) occurring at the base of a 40-km-thick crust. Beneath the Campanian volcanoes of Vesuvius and Phlegraean Fields, the mantle

  16. Crustal and Upper Mantle Density Structure Beneath the Qinghai-Tibet Plateau and Surrounding Areas Derived from EGM2008 Geoid Anomalies

    Directory of Open Access Journals (Sweden)

    Honglei Li


    Full Text Available As the most active plateau on the Earth, the Qinghai-Tibet Plateau (TP has a complex crust–mantle structure. Knowledge of the distribution of such a structure provides information for understanding the underlying geodynamic processes. We obtain a three-dimensional model of the density of the crust and the upper mantle beneath the TP and surrounding areas from height anomalies using the Earth Gravitational Model 2008 (EGM2008. We refine the estimated density in the model iteratively using an initial density contrast model. We confirm that the EGM2008 products can be used to constrain the crust–mantle density structures. Our major findings are: (1 At a depth of 300–400 km, high-D(ensity anomalies terminate around the Jinsha River Suture (JRS in the central TP, which suggests that the Indian Plate has reached across the Bangong Nujiang Suture (BNS and almost reaches the JRS. (2 On the eastern TP, low-D(ensity anomalies at a depth of 0–300 km and with high-D anomalies at 400–670 km further verified the current eastward subduction of the Indian Plate. The ongoing subduction process provides force that results in frequent earthquakes and volcanoes. (3 At a depth of 600 km, low-D anomalies inside the TP illustrate the presence of hot weak material beneath it, which contribute to the inward thrusting of external material.

  17. 3-D multiobservable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle. I: a priori petrological information and geophysical observables (United States)

    Afonso, J. C.; Fullea, J.; Griffin, W. L.; Yang, Y.; Jones, A. G.; D. Connolly, J. A.; O'Reilly, S. Y.


    Traditional inversion techniques applied to the problem of characterizing the thermal and compositional structure of the upper mantle are not well suited to deal with the nonlinearity of the problem, the trade-off between temperature and compositional effects on wave velocities, the nonuniqueness of the compositional space, and the dissimilar sensitivities of physical parameters to temperature and composition. Probabilistic inversions, on the other hand, offer a powerful formalism to cope with all these difficulties, while allowing for an adequate treatment of the intrinsic uncertainties associated with both data and physical theories. This paper presents a detailed analysis of the two most important elements controlling the outputs of probabilistic (Bayesian) inversions for temperature and composition of the Earth's mantle, namely the a priori information on model parameters, ρ(m), and the likelihood function, L(m). The former is mainly controlled by our current understanding of lithosphere and mantle composition, while the latter conveys information on the observed data, their uncertainties, and the physical theories used to relate model parameters to observed data. The benefits of combining specific geophysical datasets (Rayleigh and Love dispersion curves, body wave tomography, magnetotelluric, geothermal, petrological, gravity, elevation, and geoid), and their effects on L(m), are demonstrated by analyzing their individual and combined sensitivities to composition and temperature as well as their observational uncertainties. The dependence of bulk density, electrical conductivity, and seismic velocities to major-element composition is systematically explored using Monte Carlo simulations. We show that the dominant source of uncertainty in the identification of compositional anomalies within the lithosphere is the intrinsic nonuniqueness in compositional space. A general strategy for defining ρ(m) is proposed based on statistical analyses of a large database

  18. Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle (United States)

    Lassiter, J. C.; Byerly, B. L.; Snow, J. E.; Hellebrand, E.


    The Lena Trough is a highly oblique, sparsely magmatic, ultra-slow spreading center located at the smallest distance between North America and Eurasia in the Arctic basin. Competing models suggest that it is either floored by oceanic mantle abyssal peridotites (APs) exposed by lithospheric necking, or by subcontinental mantle exposed in a still juvenile rift. To distinguish between these hypotheses, we have examined mineral major and trace element and whole rock Os-isotope variations in Lena Trough peridotites. Lena Trough peridotites are predominantly LREE-depleted, similar to other AP suites, and have 187Os/188Os ranging from ∼0.118 to 0.130 (Ave.=0.1244). This distribution is nearly identical to that of abyssal peridotites globally. Both the REE patterns and the Os-isotope distribution of the Lena Trough peridotites differ starkly from subcontinental mantle xenoliths sampled at Svalbard adjacent to Lena Trough. This suggests that Lena Trough is a site of oceanic spreading, although mid-ocean ridge volcanism as such has not yet begun. Highly refractory APs from several settings have Os- and Hf-isotope compositions indicating ancient (>1 Ga) melt depletion. Some researchers have proposed that at least some APs do not directly sample the convecting upper mantle source of MORB, but instead sample highly melt-depleted residues either entrained in the convecting mantle or present as a buoyant “slag” floating atop the less-depleted MORB-source mantle. However, ocean island peridotite xenoliths and APs reveal an essentially identical, non-Gaussian distribution of Os-isotopes and also span a similar range in Hf-isotopes. The similar mean and distribution of Os-isotopes between APs and ocean island xenoliths indicate that these two sample types derive from the same heterogeneous mantle reservoir. This similarity is inconsistent with the AP “slag hypothesis” due to the significantly greater depth of origin of ocean island xenoliths with respect to APs. Global

  19. Experimentally-determined carbon isotope fractionation in and between methane-bearing melt and fluid to upper mantle temperatures and pressures (United States)

    Mysen, Bjorn


    The behavior of melts and fluids is at the core of understanding formation and evolution of the Earth. To advance our understanding of their role, high-pressure/-temperature experiments were employed to determine melt and fluid structure together with carbon isotope partitioning within and between (CH4 +H2O +H2)-saturated aluminosilicate melts and (CH4 +H2O +H2)-fluids. The samples were characterized with vibrational spectroscopy while at temperatures and pressures from 475° to 850 °C and 92 to 1158 MPa, respectively. The solution equilibrium is 2CH4 +Qn = 2 CH3- +H2O +Q n + 1 where the superscript, n, in the Qn-notation describes silicate species where n denotes the number of bridging oxygen. The solution equilibrium affects the carbon isotope fractionation factor between melt and fluid, αmelt/fluid. Moreover, it is significantly temperature-dependent. The αmelt/fluid < 1 with temperatures less than about 1050 °C, and is greater than 1 at higher temperature. Methane-bearing melts can exist in the upper mantle at fO2 ≤fO2 (MW) (Mysen et al., 2011). Reduced (Csbnd H)-species in present-day upper mantle magma, therefore, are likely. During melting and crystallization in this environment, the δ13C of melts increases with temperature at a rate of ∼ 0.6 ‰ /°C. From the simple-system data presented here, at T ≤ 1050°C, melt in equilibrium with a peridotite-(CH4 +H2O +H2)-bearing mantle source will be isotopically lighter than fluid. At higher temperatures, melts will be isotopically heavier. Degassing at T ≤ 1050°C will shift δ13C of degassed magma to more positive values, whereas degassing at T ≥ 1050°C, will reduce the δ13C of the degassed magma.

  20. New data on seismic wave attenuation in the lithosphere and upper mantle of the northeastern flank of the Baikal rift system (United States)

    Dobrynina, A. A.; Sankov, V. A.; Chechelnitsky, V. V.


    The investigation data on seismic wave attenuation in the lithosphere and upper mantle of the northeastern flank of the Baikal rift system obtained with a seismic coda envelope and sliding window are considered. Eleven local districts were described by one-dimensional attenuation models characterized by alternation of high and low attenuation layers, which are consistent with the results obtained previously by Yu.F. Kopnichev for the southwestern flank of the Baikal rift system [9]. The subcrust of the lithosphere contains a thin layer with high attenuation of seismic waves likely related to higher heterogeneity (fragmentation) and occurrence of fluids. The lithosphere basement depth varies from 100-120 km in the west within the Baikal folded area to 120-140 km in the east within the Siberian Platform. It is concluded that there are two asthenosphere layers. Based on specific features of the lithosphere and upper mantle structure, it can be assumed that they were subject to gradual modification involving fluidization processes and partial melting in the Late Cenozoic extension under the influence of distant tectogenesis sources.

  1. Geophysical study of the crust and upper mantle beneath the central Rio Grande rift and adjacent Great Plains and Colorado Plateau

    Energy Technology Data Exchange (ETDEWEB)

    Ander, M.E.


    As part of the national hot dry rock (HDR) geothermal program conducted by Los Alamos Scientific Laboratory, a regional deep magnetotelluric (MT) survey of Arizona and New Mexico was performed. The main objective of the MT project was to produce a regional geoelectric contour map of the pervasive deep electrical conductor within the crust and/or upper mantle beneath the Colorado Plateau, Basin and Range Province, and Rio Grande rift. Three MT profiles cross the Jemez lineament. Preliminary one-dimensional analysis of the data suggest the lineament is associated with anomalously high electrical conductivity very shallow in the crust. An MT/audiomagnetotelluric (AMT) study of a 161 km/sup 2/ HDR prospect was performed on the Zuni Indian Reservation, New Mexico. Two-dimensional gravity modeling of a 700-km gravity profile at 34/sup 0/30'N latitude was used to study the crust and upper mantle beneath the Rio Grande rift. Several models of each of three consecutive layers were produced using all available geologic and geophysical constraints. Two short-wavelength anomalies along the gravity profile were analyzed using linear optimization techniques.

  2. MORB mantle hosts the missing Eu (Sr, Nb, Ta and Ti) in the continental crust: New perspectives on crustal growth, crust-mantle differentiation and chemical structure of oceanic upper mantle (United States)

    Niu, Yaoling; O'Hara, Michael J.


    We have examined the high quality data of 306 mid-ocean ridge basalt (MORB) glass samples from the East Pacific Rise (EPR), near-EPR seamounts, Pacific Antarctic Ridge (PAR), near-PAR seamounts, Mid-Atlantic Ridge (MAR), and near-MAR seamounts. The data show a correlated variation between Eu/Eu* and Sr/Sr*, and both decrease with decreasing MgO, pointing to the effect of plagioclase crystallization. The observation that samples with MgO > 9.5 wt.% (before plagioclase on the liquidus) show Eu/Eu* > 1 and Sr/Sr* > 1 and that none of the major phases (i.e., olivine, orthopyroxene, clinopyroxene, spinel and garnet) in the sub-ridge mantle melting region can effectively fractionate Eu and Sr from otherwise similarly incompatible elements indicates that the depleted MORB mantle (DMM) possesses excess Sr and Eu, i.e., [Sr/Sr*]DMM > 1 and [Eu/Eu*]DMM > 1. Furthermore, the well-established observation that DNb ≈ DTh, DTa ≈ DU and DTi ≈ DSm during MORB mantle melting, yet primitive MORB melts all have [Nb/Th]PMMORB > 1, [Ta/U]PMMORB > 1 and [Ti/Sm]PMMORB > 1 (where PM indicates primitive mantle normalized), also points to the presence of excess Nb, Ta and Ti in the DMM, i.e., [Nb/Th]PMDMM > 1, [Ta/U]PMDMM > 1 and [Ti/Sm]PMDMM > 1. The excesses of Eu, Sr, Nb, Ta and Ti in the DMM complement the well-known deficiencies of these elements in the bulk continental crust (BCC). These new observations, which support the notion that the DMM and BCC are complementary in terms of the overall abundances of incompatible elements, offer new insights into the crust-mantle differentiation. These observations are best explained by partial melting of amphibolite of MORB protolith during continental collision, which produces andesitic melts with a remarkable compositional (major and trace element abundances as well as key elemental ratios) similarity to the BCC, as revealed by andesites in southern Tibet produced during the India-Asia continental collision. An average amphibolite of MORB

  3. Joint inversion of shear wave travel time residuals and geoid and depth anomalies for long-wavelength variations in upper mantle temperature and composition along the Mid-Atlantic Ridge (United States)

    Sheehan, Anne F.; Solomon, Sean C.


    Measurements were carried out for SS-S differential travel time residuals for nearly 500 paths crossing the northern Mid-Atlantic Ridge, assuming that the residuals are dominated by contributions from the upper mantle near the surface bounce point of the reflected phase SS. Results indicate that the SS-S travel time residuals decrease linearly with square root of age, to an age of 80-100 Ma, in general agreement with the plate cooling model. A joint inversion was formulated of travel time residuals and geoid and bathymetric anomalies for lateral variation in the upper mantle temperature and composition. The preferred inversion solutions were found to have variations in upper mantle temperature along the Mid-Atlantic Ridge of about 100 K. It was calculated that, for a constant bulk composition, such a temperature variation would produce about a 7-km variation in crustal thickness, larger than is generally observed.

  4. Early Differentiation of the Crust-Mantle System: a Hf Isotope Perspective (United States)

    Scherer, E.; Munker, C.; Mezger, K.


    The Lu decay constant recently determined by Scherer et al. 2001 (i.e., 1.865 x 10-11 yr-1) agrees with the results of the two latest physical counting experiments (1.86 x 10-11 yr-1; Dalmasso et al 1992, Nir-El and Lavi 1998), but is ca. 4 percent lower than the decay constants that have been used throughout the Hf isotope literature (e.g., 1.94 x 10-11, Tatsumoto et al., 1981; 1.93 x 10-11 Sguigna et al, 1982). In addition to making Lu-Hf ages older by ca. 4 percent, the revised decay constant also shifts the calculated initial epsilon Hf values of early Archean and Hadean rocks and zircons that are used to constrain crust-mantle differentiation in the early Earth. The initial epsilon Hf values for low-Lu/Hf samples such as zircons and evolved felsic rocks shift downward by 2-4 epsilon units, primarily due to the shift in the position of the CHUR evolution curve rather than that of the samples themselves. Mafic rocks, such as komatiites have higher Lu/Hf ratios that are closer to that of CHUR and therefore their initial epsilon Hf values do not shift as much (up to 1.3 epsilon units lower or 0.4 epsilon units higher). Using the old decay constant, some early Archean rocks (e.g., Amitsoq gneisses; Vervoort et al., 1996, Vervoort and Blichert-Toft, 1999) seemed to have very high initial epsilon Hf values (up to +6), implying that the upper mantle was moderately depleted in the early Archean and that a substantial volume of crust was produced in the Hadean. However, when recalculated with the new decay constant, the data suggest that the mantle was only slightly depleted, requiring less early crust extraction, and allowing a later date for the onset of significant crust production. In contrast, the extremely low recalculated epsilon Hf values of Earth's oldest zircons (Amelin et al., 1999, Amelin et al., 2000) indicate that Earth's first crust formed at or before 4.3 Ga, and that this crust remained intact long enough (>200 million years) to evolve to such low

  5. Discriminating Between Subhorizontal Lower Crustal and Upper Mantle Reflectivity to Delineate Moho Geometry Under the Transylvanian Basin, Romania (United States)

    Fillerup, M. A.; Knapp, J. H.; Knapp, C.


    A thick interval (~16 km) of reflective layered material at the crust/mantle boundary beneath the Transylvanian Basin, Romania, complicates delineation of the Mohorovicic discontinuity based on the deep seismic reflection method. Seismic reflection data from DRACULA I, collected during Project DRACULA (Deep Reflection Acquisition Constraining Unusual Lithospheric Activity), correlated with existing deep (17 s) industry reflection data constrains this regional subhorizontal fabric and may show a genetic relationship to the Vrancea Seismogenic zone. Portions of the DRACULA I line exhibit subhorizontal reflectivity in excess of 25 seconds (~75 km) with no clear reflective break indicating probable Moho location. Other bright reflectors in the southeastern end of DRACULA I at 7-10 seconds TWTT may represent the westward continuation of structural detachments from the Eastern Carpathians. The subhorizontal laterally continuous 10-17 second TWTT crust/mantle reflective interval is the most pervasive feature observed on all the profiles used in this study, and shows continuity of the Transylvanian crust into the Eastern Carpathians. The subhorizontal nature of this crust/mantle reflective interval seems to preclude the existence of a former Miocene-age suture zone to account for Miocene subduction as currently proposed for the geodynamic setting of the Vrancea seismogenic zone. A continuous lower crust without a plate boundary beneath the Transylvanian Basin advocates the less invasive process of active continental lithospheric delamination as a means to explain the Vrancea seismicity. Although still preliminary, results from the DRACULA I and supporting industry profiles, speak strongly against Miocene-age northwestward subduction in the Southeast Carpathians to be responsible for the intermediate-depth seismicity.

  6. Crustal and upper-mantle structure beneath ice-covered regions in Antarctica from S-wave receiver functions and implications for heat flow (United States)

    Ramirez, C.; Nyblade, A.; Hansen, S. E.; Wiens, D. A.; Anandakrishnan, S.; Aster, R. C.; Huerta, A. D.; Shore, P.; Wilson, T.


    S-wave receiver functions (SRFs) are used to investigate crustal and upper-mantle structure beneath several ice-covered areas of Antarctica. Moho S-to-P (Sp) arrivals are observed at ˜6-8 s in SRF stacks for stations in the Gamburtsev Mountains (GAM) and Vostok Highlands (VHIG), ˜5-6 s for stations in the Transantarctic Mountains (TAM) and the Wilkes Basin (WILK), and ˜3-4 s for stations in the West Antarctic Rift System (WARS) and the Marie Byrd Land Dome (MBLD). A grid search is used to model the Moho Sp conversion time with Rayleigh wave phase velocities from 18 to 30 s period to estimate crustal thickness and mean crustal shear wave velocity. The Moho depths obtained are between 43 and 58 km for GAM, 36 and 47 km for VHIG, 39 and 46 km for WILK, 39 and 45 km for TAM, 19 and 29 km for WARS and 20 and 35 km for MBLD. SRF stacks for GAM, VHIG, WILK and TAM show little evidence of Sp arrivals coming from upper-mantle depths. SRF stacks for WARS and MBLD show Sp energy arriving from upper-mantle depths but arrival amplitudes do not rise above bootstrapped uncertainty bounds. The age and thickness of the crust is used as a heat flow proxy through comparison with other similar terrains where heat flow has been measured. Crustal structure in GAM, VHIG and WILK is similar to Precambrian terrains in other continents where heat flow ranges from ˜41 to 58 mW m-2, suggesting that heat flow across those areas of East Antarctica is not elevated. For the WARS, we use the Cretaceous Newfoundland-Iberia rifted margins and the Mesozoic-Tertiary North Sea rift as tectonic analogues. The low-to-moderate heat flow reported for the Newfoundland-Iberia margins (40-65 mW m-2) and North Sea rift (60-85 mW m-2) suggest that heat flow across the WARS also may not be elevated. However, the possibility of high heat flow associated with localized Cenozoic extension or Cenozoic-recent magmatic activity in some parts of the WARS cannot be ruled out.

  7. Sluggish Hadean geodynamics: Evidence from coupled 146,147Sm-142,143Nd systematics in Eoarchean supracrustal rocks of the Inukjuak domain (Québec) (United States)

    Caro, G.; Morino, P.; Mojzsis, S. J.; Cates, N. L.; Bleeker, W.


    The discovery of deficits in 142Nd/144Nd in mafic rocks of the Nuvvuagittuq supracrustal belt (NSB) has triggered a debate about the possible preservation of Hadean (pre-3.85 Ga) crustal remnants in the little-known but areally extensive Innuksuac complex (3.6-3.8 Ga, Inukjuak domain, Northeast Superior Province, Canada). Geochronological investigations in the NSB, however, are hampered by the poor preservation and highly disturbed isotopic record of various mafic (amphibolite) lithologies that host the 142Nd anomalies. Here we present 146Sm-142Nd and 147Sm-143Nd data for rocks of extrusive magmatic and sedimentary protoliths from the Ukaliq supracrustal belt, a newly discovered volcano-sedimentary enclave enclosed in granitoid gneisses of the Inukjuak domain. Our study also includes the first 146Sm-142Nd data for quartz-magnetite rocks (banded iron-formation; BIF) of the NSB and the Eoarchean Isua supracrustal belt (ISB) in southern West Greenland. We show that Ukaliq amphibolites carry variably negative 142Nd anomalies, ranging from 0 to -10 ppm, which are positively correlated with their Sm/Nd ratio. If considered as an isochron relationship, the 146Sm-142Nd array yields an apparent Hadean emplacement age of 4215-76+50 Ma. The negative 142Nd anomalies, however, appear to be mainly restricted to amphibolites with boninitic affinities, likely reflecting inheritance from an enriched mantle source. In contrast, tholeiitic and ultramafic lavas have normal μ142Nd regardless of their Sm/Nd ratio. Furthermore, BIF from Ukaliq and Nuvvuagittuq lack the negative 142Nd anomalies that should have been produced by in situ decay of 146Sm had these sediments been deposited prior to ca. 4.1 Ga. Instead, they exhibit μ142Nd identical to that measured in Isua BIF. Collectively, our results suggest that the 146Sm-142Nd array characterizing mafic lithologies of Ukaliq and Nuvvuagittuq is an inherited signature with doubtful chronological significance. We interpret the volcanic

  8. Cocos Plate Seamounts offshore NW Costa Rica and SW Nicaragua: Implications for large-scale distribution of Galápagos plume material in the upper mantle (United States)

    Herbrich, Antje; Hoernle, Kaj; Werner, Reinhard; Hauff, Folkmar; Bogaard, Paul v. d.; Garbe-Schönberg, Dieter


    The origin of intraplate volcanism not directly part of a hotspot track, such as diffuse seamount provinces, and the extent of mantle plume influence on the upper mantle remain enigmatic. Here we present new 40Ar/39Ar age data and geochemical (major and trace-element and Sr-Nd-Pb isotopic) data from seamounts on the Cocos Plate presently located offshore of NW Costa Rica and SW Nicaragua. The seamounts (~ 7-24 Ma) require mixing of an enriched ocean island basalt composition, similar to that of the Northern Galápagos Domain, with two depleted components. One of the depleted components is similar to East Pacific Rise normal mid-ocean ridge basalt and the other has more depleted incompatible elements, either reflecting secondary melting of MORB or a depleted Galápagos plume component. Seamounts with ages significantly younger than the ocean crust formed in an intraplate setting and can be explained by northward transport of Galápagos plume material along the base of the Cocos Plate up to 900 km away from the hotspot and 250-500 km north of the Galápagos hotspot track. We propose that melting occurs due to decompression as the mantle upwells to shallower depth as it flows northwards, either due to changes in lithospheric thickness or as a result of upwelling at the edge of a viscous plug of accumulated plume material at the base of the lithosphere. The tholeiitic to alkaline basalt compositions of the Cocos Plate Seamounts compared to the more silica under-saturated compositions of Hawaiian rejuvenated and arch (alkali basalts to nephelinites) lavas are likely to reflect the significant difference in age (< 25 vs ~ 90 Ma) and thus thickness of the lithosphere on which the lavas were erupted.

  9. Crustal and upper-mantle structure beneath the western Atlas Mountains in SW Morocco derived from receiver functions (United States)

    Spieker, Kathrin; Wölbern, Ingo; Thomas, Christine; Harnafi, Mimoun; El Moudnib, Lahcen


    The High Atlas and the Anti Atlas are fold-belts linked to former and still ongoing continent-continent collisions. Despite their high elevation, studies indicate a lack of a deep crustal root (Morocco to analyse teleseismic P- and S-wave receiver functions. Our study yields a crustal thickness ranging from 24 km near the Atlantic coast to 44 km beneath the High Atlas with an average crustal Vp/Vs ratio of 1.77 in the entire region. A crustal thickness of 40 km cannot entirely support the topography in this region. Furthermore, we find the lithosphere-asthenosphere boundary at ˜80 km depth. The lithosphere beneath SW Morocco is thinner than beneath northern Morocco (>150 km). This lithospheric thinning supports the theory of thermal compensation of the mountain ranges. The mantle transition zone thickness amounts to 240 ± 10 km. The transition zone seems to be slightly thinned which might indicate a higher mantle temperature in this region.

  10. On the Chemical Evolution of Upper Mantle of the Early Earth—An Experimental Study on Melting of the Silicate Phase in Jilin Chondrite at High Pressures

    Institute of Scientific and Technical Information of China (English)

    谢鸿森; 方虹; 等


    Relatively old ages of chondrites(normally around 4.5Ga)suggest that their parent bodies did not experience any mely-fractionation under high temperature and high pressure conditions pertaining to the interior of terrestrial plaets.Therefore,it is reasonable to take chondrites as starting materials in the study of the chemical evolution of the early earth.The sillicate phase in the Jilin chondrite (H5)was chosen for this purpose because it possesses a chemical composition similar to that of the primitive mantle.The melting experiment was carried out at 20-30 k bar and has rsulted in a product which contains1-5% melts in addition to solid cryustal phase.The chemical composition of the melt phases and the partitioning of various elements between the coexisting silicate melts are geochemically similar to those of anatectic rocks on the earth.This can thus serve as the basis for discussing the chemical evolution of the early upper mantle.

  11. A Simultaneous Multi-phase Approach to Determine P-wave and S-wave Attenuation of the Crust and Upper Mantle

    Energy Technology Data Exchange (ETDEWEB)

    Pasyanos, M E; Walter, W R; Matzel, E M


    We have generalized the methodology of our regional amplitude tomography from the Lg phase to the four primary regional phases (Pn, Pg, Sn, Lg). Differences in the geometrical spreading, source term, site term, and travel paths are accounted for, while event source parameters such as seismic moment are consistent among phases. In the process, we have developed the first regional attenuation model that uses the amplitudes of four regional phases to determine a comprehensive P-wave and S-wave attenuation model of the crust and upper mantle. When applied to an area encompassing the Middle East, eastern Europe, western Asia, south Asia, and northeast Africa for the 1-2 Hz passband, we find large differences in the attenuation of the lithosphere across the region. The tectonic Tethys collision zone has high attenuation, while stable outlying regions have low attenuation. While crust and mantle Q variations are often consistent, we do find several notable areas where they differ considerably, but are appropriate given the region's tectonic history. Lastly, the relative values of Qp and Qs indicate that scattering Q is likely the dominant source of attenuation in the crust at these frequencies.

  12. A McMC Method for the Inference of Radial and Azimuthal Anisotropy of the Crust and Upper Mantle from Surface-Wave Dispersion Curves (United States)

    Ravenna, Matteo; Lebedev, Sergei


    We develop a Markov Chain Monte Carlo method for joint inversion of Rayleigh- and Love-wave dispersion curves that is able to yield robust radially and azimuthally anisotropic shear velocity profiles, with resolution to depths down to the transition zone. The probabilistic feature of the algorithm is a powerful tool that is able to provide error assessment of the shear velocity models, quantify non-uniqueness and address the issue of data noise estimation by treating it as an unknown parameter in the inversion. In a fixed dimensional Bayesian formulation, we choose to set the number of parameters relatively high, with a more dense parametrization in the uppermost mantle in order to have a good resolution of the Litosphere-Astenosphere Boundary region. We apply the MCMC algorithm to the inversion of surface-wave phase velocities accurately determined in broad period ranges in a few test regions. In the Baikal-Mongolia region we invert Rayleigh- and Love- wave dispersion curves for radially anisotropic structure (Vsv,Vsh) of the crust and upper mantle. In the Tuscany region, where we have phase velocity data with good azimuthal coverage, a different implementation of the algorithm is applied that is able to resolve azimuthal anisotropy; the Rayleigh wave dispersion curves measured at different azimuths have been inverted for the Vsv structure and the depth distribution of the 2-psi azimuthal anisotropy of the region, with good resolution down to asthenospheric depths.

  13. Early mantle dynamics inferred from Nd-142 variations in Archean rocks from southwest Greenland

    DEFF Research Database (Denmark)

    Rizo, Hanika; Boyet, Maud; Blichert-Toft, Janne


    The composition and evolution of the silicate Earth during Hadean/Eoarchean times are widely debated and largely unknown due to the sparse geological record preserved from Earth's infancy. The short-lived Sm-146-Nd-142 chronometer applied to 3.8-3.7 Ga old mantle-derived amphibolites from the Isu...... into the compositional evolution and dynamic workings of Earth's primordial mantle. (C) 2013 Elsevier B.V. All rights reserved.......The composition and evolution of the silicate Earth during Hadean/Eoarchean times are widely debated and largely unknown due to the sparse geological record preserved from Earth's infancy. The short-lived Sm-146-Nd-142 chronometer applied to 3.8-3.7 Ga old mantle-derived amphibolites from the Isua...... of the Greenland samples from a source formed in the Hadean. This mantle source is the oldest yet identified on Earth and therefore provides key information about the nature and evolution of early-differentiated reservoirs. In contrast, modern mantle-derived rocks from around the world do not have Nd-142 anomalies...

  14. Three dimensional shear wave velocity structure of crust and upper mantle in South China Sea and its adjacent regions by surface waveform inversion

    Institute of Scientific and Technical Information of China (English)


    We assembled approximately 328 seismic records. The data set wasfrom 4 digitally recording long-period and broadband stations of CDSN. We carried out the inversion based on the partitioned waveform inversion (PWI). It partitions the large-scale optimization problem into a number of independent small-scale problems. We adopted surface waveform inversion with an equal block (2°′2°) discretization in order to acquire the images of shear velocity structure at different depths (from surface to 430 km) in the crust and upper-mantle. The resolution of all these anomalies has been established with 2check-board2 resolution tests. These results show significant difference in velocity, lithosphere and asthenosphere structure between South China Sea and its adjacent regions.

  15. Evolution of the slab bending radius and the bending dissipation in three-dimensional subduction models with a variable slab to upper mantle viscosity ratio (United States)

    Schellart, W. P.


    Three-dimensional laboratory subduction models are presented investigating the influence of the slab/upper mantle viscosity ratio (η SP/η UM) on the slab bending radius ( RB), with η SP/η UM = 66-1375. Here, RB is non-dimensionalized by dividing it by the upper mantle thickness ( TUM). The results show that RB/ TUM varies with time, reaching a maximum when the subduction velocity is maximum. Furthermore, RB/ TUM increases approximately linearly with increasing η SP/η UM for the investigated viscosity range. The model results show that the slab bending force ( FBe) and the energy dissipation during bending (Ф Be) are small compared to the negative buoyancy force of the slab ( FBu) and the potential energy release during sinking (Ф Bu). Maxima in Ф Be/Ф Bu (≈ FBe/ FBu) are reached in the early stage of subduction when RB/ TUM is minimum and the slab tip is at 220-440 km depth. Maximum Ф Be/Ф Bu increases with increasing η SP/η UM, with Ф Be/Ф Bu(max) = 0.06, 0.11, 0.18 and 0.22 for η SP/η UM = 66, 217, 709 and 1375, respectively. For subduction depths > 220-440 km, Ф Be/Ф Bu = 0.02-0.11 for all viscosity ratios. Assuming that in nature η SP/η UM glucose syrup and silicone oil as modelling materials, highlight the importance of accurate control on temperature during an experiment. New material investigations show that the viscosity of these two materials decreases exponentially with temperature in the range 3-33 °C, their density decreases approximately linearly with temperature, and their coefficient of thermal volumetric expansion is 3.8-4.2 × 10 - 4 C - 1 (glucose syrup) and 9.2 × 10 - 4 C - 1 (silicone oil).

  16. A new view of the He-Ar-CO 2 degassing at mid-ocean ridges: Homogeneous composition of magmas from the upper mantle (United States)

    Paonita, Antonio; Martelli, Mauro


    Deep-sea exploration is rapidly improving our understanding of volatiles geochemistry in mid-ocean-ridge igneous products. It is also placing greater constraints on degassing processes of the Earth's mantle, with the result that degassing models based on vapour-melt equilibrium are no longer able to explain the increasing number of data. In fact, such models force to postulate an upper mantle strongly heterogeneous at any scale, and cannot account for the widespread carbon supersaturation of the recovered igneous products. Here we review the global He-Ar-CO 2 dataset of fluid inclusions in mid-ocean-ridge glasses using the framework of advanced modelling of multicomponent bubble growth in magmas. We display that non-equilibrium fractionations among He, Ar and CO 2, driven by their different diffusivities in silicate melts, are common in most of the natural conditions of magma decompression and their signature strongly depends on pressure of degassing. Due to the comparable Ar and CO 2 diffusivity, magma degassing at low pressure fractionates both the He/Ar and He/CO 2 ratio by a similar extent, while the slower CO 2 diffusion at high pressure causes early kinetic effects on Ar/CO 2 ratio and dramatically changes the degassing path. On this ground, the very different geochemical signatures among suites of data coming from different ridge segments mainly depend on the depth of the magma chamber where the melt was stored. Besides, the variations inside a single suite highlight variable ascent speed and cooling rate of the emplaced lava. The large variations in both the He/CO 2 and Ar/CO 2 ratios at almost constant He/Ar, displayed in glasses coming from the Mid-Atlantic Ridge 24-30°N segment and the Rodriguez Triple Junction, are therefore interpreted as a high-pressure signature. In contrast, the simultaneous increase in both He/CO 2 and He/Ar of the East Pacific Rise, Pito Seamount and South-East Indian Ridge data sets suggests the dominance of low

  17. Single crystal neutron diffraction of hydrous wadsleyite and the reason of sensitivity difference for hydration between upper and lower mantle transition zones (United States)

    Purevjav, N.; Okuchi, T.; Tomioka, N.; Wang, X.; Hoffmann, C.


    Evidences from seismological and mineralogical studies increasingly indicates that water has been transported from the oceans into the Earth's deep mantle, where the mantle transition zone is believed to be the largest reservoir of this transported water. Wadsleyite and ringwoodite are the major constituents and the most important host minerals absorbing this type of water in the transition zone. These minerals are capable of storing the entire mass of the oceans as a hidden reservoirs. In order to understand the effects of such water on the physical properties and chemical evolution of the Earth's interior, it is essential to determine where in the crystal structure the hydration occurs, and which chemical bonds are altered and weakened after hydration. Here we show the result of a neutron single-crystal Laue diffraction study of hydrous wadsleyite. A crystal of homogenously-hydrated wadsleyite involving 1.4 wt. % of H2O was synthesized by our recently-established slow cooling method, which was an effective way to grow high quality large single crystals [1]. By analyzing this crystal using pulsed neutron beam, we demonstrated that the hydrogen atoms exchange only with Mg2+ at the one of the specific octahedron sites (M3) in wadsleyite. We also determined hydrogen's bonding distances and bonding angle. The results unambiguously demonstrated the unique mechanism of hydrogen incorporation into the wadsleyite crystal structure. We previously found that the hydrogen atoms exchanged with both Mg2+ and Si4+ sites simultaneously in the crystal structure of hydrous ringwoodite [2]. Therefore, the current results show that hydration mechanisms are qualitatively different between the upper and the lower transition zones in the wet mantle. The difference is a vital clue towards understanding why these mantle transition zone minerals show different sensitivity for water in their softening behaviors. In addition, we demonstrated that maximum water concentration in wadsleyite is

  18. Rayleigh-wave imaging of upper-mantle shear velocities beneath the Malawi Rift; Preliminary results from the SEGMeNT experiment (United States)

    Accardo, N. J.; Gaherty, J. B.; Shillington, D. J.; Nyblade, A.; Ebinger, C. J.; Mbogoni, G. J.; Chindandali, P. R. N.; Mulibo, G. D.; Ferdinand-Wambura, R.; Kamihanda, G.


    The Malawi Rift (MR) is an immature rift located at the southern tip of the Western branch of the East African Rift System (EARS). Pronounced border faults and tectonic segmentation are seen within the upper crust. Surface volcanism in the region is limited to the Rungwe volcanic province located north of Lake Malawi (Nyasa). However, the distribution of extension and magma at depth in the crust and mantle lithosphere is unknown. As the Western Rift of the EARS is largely magma-poor except for discrete volcanic provinces, the MR presents the ideal location to elucidate the role of magmatism in early-stage rifting and the manifestation of segmentation at depth. This study investigates the shear velocity of the crust and mantle lithosphere beneath the MR to constrain the thermal structure, the amount of total crustal and lithospheric thinning, and the presence and distribution of magmatism beneath the rift. Utilizing 55 stations from the SEGMeNT (Study of Extension and maGmatism in Malawi aNd Tanzania) passive-source seismic experiment operating in Malawi and Tanzania, we employed a multi-channel cross-correlation algorithm to obtain inter-station phase and amplitude information from Rayleigh wave observations between 20 and 80 s period. We retrieve estimates of phase velocity between 9-20 s period from ambient noise cross-correlograms in the frequency domain via Aki's formula. We invert phase velocity measurements to obtain estimates of shear velocity (Vs) between 50-200 km depth. Preliminary results reveal a striking low-velocity zone (LVZ) beneath the Rungwe volcanic province with Vs ~4.2-4.3 km/s in the uppermost mantle. Low velocities extend along the entire strike of Lake Malawi and to the west where a faster velocity lid (~4.5 km/s) is imaged. These preliminary results will be extended by incorporating broadband data from seven "lake"-bottom seismometers (LBS) to be retrieved from Lake Malawi in October of this year. The crust and mantle modeling will be

  19. The not-so-sublime early Earth recorded in Hadean zircons (United States)

    Cavosie, A. J.


    The first few hundred million years following accretion is the least understood eon in the geologic time scale- the Hadean. This poorly defined eon continues to both challenge and fascinate scientists seeking to understand the early Earth, as the most profound planet-wide transition in Earth history occurred during the Hadean: the post-accretion transformation from a meteorite impact dominated, partially molten, steam covered mafic surface on a 'Hot Earth', to a solidified, granitoid-bearing, water covered, life-supporting 'Cool Earth'. Intact rocks from the Hadean have not been identified; other means are thus required to study early Earth processes, such as the appearance, formation, and processing of evolved crust, duration of early impacts and magma oceans, the appearance of liquid water and oceans, and ultimately, stabilization of habitats for life. Hadean detrital zircons found in younger sedimentary rocks in Australia, China, and the USA constitute a mineral record from the early Earth that enables 'ground truth' constraints to be placed on early Earth processes. Hadean zircons are complicated and originate from myriad sources; identification of grains that preserve magmatic composition is critical (as evidenced by growth zoning in CL, concordant U-Pb systematics, trace element abundances and ratios), as many have been modified by secondary processes. Detailed documentation of analyzed material is paramount. A generally consistent understanding of processes on the Hadean Earth is emerging, based on data from well-documented igneous zircons with concordant U-Pb systems: (1) A record of continuous magmatism and rock-forming events starting at 4.4 Ga is recorded in U-Pb ages of Hadean zircons; no periods of magmatic quiescence occur in the Hadean. (2) Coupled Lu/Hf and U/Pb data require formation of evolved crust from extracted Hadean reservoirs by 4.5 to 4.4 Ga. (3) Mineral inclusion suites, low Ti and high Li abundances, trace elements (U-Yb), and elevated

  20. Layered structure in the upper mantle across North America from joint inversion of long and short period seismic data (United States)

    Calò, M.; Bodin, T.; Romanowicz, B.


    We estimate crustal and uppermost mantle shear velocity structure beneath 30 stations in North America by jointly inverting the high frequency scattered wavefield observed in the P wave coda, together with long period surface wave phase and group dispersion data. Several features distinguish our approach from previous such joint inversions. 1) We apply a cross-convolution method, rather than more standard deconvolution approaches used in receiver function studies, and consider both Love and Rayleigh wave dispersion, allowing us to infer profiles of radial anisotropy. 2) We generate probabilistic 1D radially anisotropic depth profiles across the whole uppermost mantle, down to ∼350 km depth. 3) The inverse problem is cast in a trans-dimensional Bayesian formalism, where the number of isotropic and anisotropic layers is treated as unknown, allowing us to obtain models described with the least number of parameters. Results show that the tectonically active region west of the Rocky Mountain Front is marked by a Lithospheric Asthenosphere Boundary and a Lehmann Discontinuity occurring at relatively shallow depths (60-150 km and 100-200 km, respectively), whereas further east, in the stable craton, these discontinuities are deeper (170-200 km and 200-250 km, respectively). In addition, in the stable part of the continent, at least two Mid-Lithospheric Discontinuities are present at intermediate depths, suggesting the existence of strong lithospheric layering, and a mechanism for lithospheric thickening by underplating of additional layers as cratonic age increases. The Moho across the continent as well as mid-crustal discontinuities in the craton are also imaged, in agreement with independent studies.

  1. Upper mantle diapers, lower crustal magmatic underplating, and lithospheric dismemberment of the Great Basin and Colorado Plateau regions, Nevada and Utah; implications from deep MT resistivity surveying (United States)

    Wannamaker, P. E.; Doerner, W. M.; Hasterok, D. P.


    In the rifted Basin and Range province of the southwestern U.S., a common faulting model for extensional basins based e.g. on reflection seismology data shows dominant displacement along master faults roughly coincident with the main topographic scarp. On the other hand, complementary data such as drilling, earthquake focal mechanisms, volcanic occurrences, and trace indicators such as helium isotopes suggest that there are alternative geometries of crustal scale faulting and material transport from the deep crust and upper mantle in this province. Recent magnetotelluric (MT) profiling results reveal families of structures commonly dominated by high-angle conductors interpreted to reflect crustal scale fault zones. Based mainly on cross cutting relationships, these faults appear to be late Cenozoic in age and are of low resistivity due to fluids or alteration (including possible graphitization). In the Ruby Mtns area of north-central Nevada, high angle faults along the margins of the core complex connect from near surface to a regional lower crustal conductor interpreted to contain high-temperature fluids and perhaps melts. Such faults may exemplify the high angle normal faults upon which the major earthquakes of the Great Basin appear to nucleate. A larger-scale transect centered on Dixie Valley shows major conductive crustal-scale structures connecting to conductive lower crust below Dixie Valley, the Black Rock desert in NW Nevada, and in east-central Nevada in the Monitor-Diamond Valley area. In the Great Basin-Colorado Plateau transition of Utah, the main structures revealed are a series of nested low-angle detachment structures underlying the incipient development of several rift grabens. All these major fault zones appear to overlie regions of particularly conductive lower crust interpreted to be caused by recent basaltic underplating. In the GB-CP transition, long period data show two, low-resistivity upper mantle diapirs underlying the concentrated

  2. Crustal and upper mantle S-wave velocity structures across the Taiwan Strait from ambient seismic noise and teleseismic Rayleigh wave analyses (United States)

    Huang, Y.; Yao, H.; Wu, F. T.; Liang, W.; Huang, B.; Lin, C.; Wen, K.


    Although orogeny seems to have stopped in western Taiwan large and small earthquakes do occur in the Taiwan Strait. Limited studies have focused on this region before and were barely within reach for comprehensive projects like TAICRUST and TAIGER for logistical reasons; thus, the overall crustal structures of the Taiwan Strait remain unknown. Time domain empirical Green's function (TDEGF) from ambient seismic noise to determine crustal velocity structure allows us to study an area using station pairs on its periphery. This research aims to resolve 1-D average crustal and upper mantle S-wave velocity (Vs) structures alone paths of several broadband station-pairs across the Taiwan Strait; 5-120 s Rayleigh wave phase velocity dispersion data derived by combining TDEGF and traditional surface wave two-station method (TS). The average Vs structures show significant differences in the upper 15 km as expected. In general, the highest Vs are observed in the coastal area of Mainland China and the lowest Vs appear along the southwest offshore of the Taiwan Island; they differ by about 0.6-1.1 km/s. For different parts of the Strait, the Vs are lower in the middle by about 0.1-0.2 km/s relative to those in the northern and southern parts. The overall crustal thickness is approximately 30 km, much thinner and less variable than under the Taiwan Island.

  3. Feldspar palaeo-isochrons from early Archaean TTGs: Pb-isotope evidence for a high U/Pb terrestrial Hadean crust (United States)

    Kamber, B. S.; Whitehouse, M. J.; Moorbath, S.; Collerson, K. D.


    Feldspar lead-isotope data for 22 early Archaean (3.80-3.82 Ga) tonalitic gneisses from an area south of the Isua greenstone belt (IGB),West Greenland, define a steep linear trend in common Pb-isotope space with an apparent age of 4480+/-77 Ma. Feldspars from interleaved amphibolites yield a similar array corresponding to a date of 4455+/-540 Ma. These regression lines are palaeo-isochrons that formed during feldspar-whole rock Pb-isotope homogenisation a long time (1.8 Ga) after rock formation but confirm the extreme antiquity (3.81 Ga) of the gneissic protoliths [1; this study]. Unlike their whole-rock counterparts, feldspar palaeo-isochrons are immune to rotational effects caused by the vagaries of U/Pb fractionation. Hence, comparison of their intercept with mantle Pb-isotope evolution models yields meaningful information regarding the source history of the magmatic precursors. The locus of intersection between the palaeo-isochrons and terrestrial mantle Pb-isotope evolution lines shows that the gneissic precursors of these 3.81 Ga gneisses were derived from a source with a substantially higher time-integrated U/Pb ratio than the mantle. Similar requirements for a high U/Pb source have been found for IGB BIF [2], IGB carbonate [3], and particularly IGB galenas [4]. Significantly, a single high U/Pb source that separated from the MORB-source mantle at ca. 4.3 Ga with a 238U/204Pb of ca. 10.5 provides a good fit to all these observations. In contrast to many previous models based on Nd and Hf-isotope evidence we propose that this reservoir was not a mantle source but the Hadean basaltic crust which, in the absence of an operating subduction process, encased the early Earth. Differentiation of the early high U/Pb basaltic crust could have occurred in response to gravitational sinking of cold mantle material or meteorite impact, and produced zircon-bearing magmatic rocks. The subchondritic Hf-isotope ratios of ca. 3.8 Ga zircons support this model [5] provided that

  4. Consequences of an Immense Hadean-Archean Heat Flux that Results from Virial Theorem Constraints on the Earth's Initial Axial Spin (United States)

    Hofmeister, A. M.; Criss, R. E.


    Early Earth conditions were largely erased, but the powerful Virial Theorem (VT) constrains Earth's post-accretion state, which largely dictates subsequent thermal and dynamical evolution. Proposals of huge initial inventories of primordial heat are based on Kelvin's disproven theory of starlight. Rather, the VT requires that gravitational potential of the Solar nebula was converted to rotational energy in a conservative, bound accretionary system, which is confirmed by planetary orbit characteristics. In addition, the VT relates axial spin to gravitational self-potential (Ug,self) of each body [2016 Can. J. Phys. p. 380]. From the VT, ½Ug,self binds the body and is unavailable, but spin energy (SE), also equal to ½Ug,self, degrades while gradually evolving heat via friction. The VT likewise restricts primordial heat of core formation, and is consistent with entropy reduction due to ordering and volume restriction [2015 J. Earth Sci., p. 124]. High initial Virial spin is confirmed by (1) data on young stars, (2) independent projections of Earth's initial spin as 2-17 hrs (from fossils and the current rate of spin loss: Lathe 2006), and (3) current SE for all planets defining a power-law trend with Ug,self, which further requires a universal cause for spin loss [2012 Planet. Space Sci. p. 111]. Spin loss is caused by tidal friction and differential rotation of layers. Dissipation is concentrated in the upper layers and especially in the brittle zone, which are much weaker than the highly compressed, essentially hydrostatic interior. With friction, neither mechanical energy nor angular momentum are conserved. Earth's frictional dissipation is immense. Uniform release over time would provide 300-700 TW. This source dominated heat generation for 2 Ga, whereas radiogenic heat dominates today. Exponential spin down suggests 100x more heat production during the Hadean than now, which obliterated early rocks while promoting outgassing and differentiation. Reduction to 10

  5. Constructing a starting 3D shear velocity model with sharp interfaces for SEM-based upper mantle tomography in North America (United States)

    Calo, M.; Bodin, T.; Yuan, H.; Romanowicz, B. A.; Larmat, C. S.; Maceira, M.


    Seismic tomography is currently evolving towards 3D earth models that satisfy full seismic waveforms at increasingly high frequencies. This evolution is possible thanks to the advent of powerful numerical methods such as the Spectral Element Method (SEM) that allow accurate computation of the seismic wavefield in complex media, and the drastic increase of computational resources. However, the production of such models requires handling complex misfit functions with more than one local minimum. Standard linearized inversion methods (such as gradient methods) have two main drawbacks: 1) they produce solution models highly dependent on the starting model; 2) they do not provide a means of estimating true model uncertainties. However, these issues can be addressed with stochastic methods that can sample the space of possible solutions efficiently. Such methods are prohibitively challenging computationally in 3D, but increasingly accessible in 1D. In previous work (Yuan and Romanowicz, 2010; Yuan et al., 2011) we developed a continental scale anisotropic upper mantle model of north America based on a combination of long period seismic waveforms and SKS splitting measurements, showing the pervasive presence of layering of anisotropy in the cratonic lithosphere with significant variations in depth of the mid-lithospheric boundary. The radial anisotropy part of the model has been recently updated using the spectral element method for forward wavefield computations and waveform data from the latest deployments of USarray (Yuan and Romanowicz, 2013). However, the long period waveforms (periods > 40s) themselves only provide a relatively smooth view of the mantle if the starting model is smooth, and the mantle discontinuities necessary for geodynamical interpretation are not imaged. Increasing the frequency of the computations to constrain smaller scale features is possible, but challenging computationally, and at the risk of falling in local minima of the misfit function. In

  6. Crustal and upper mantle 3D shear wave velocity structure of the High Lava Plains, Oregon, determined from ambient noise tomography (United States)

    Hanson-Hedgecock, S.; Wagner, L.; Fouch, M. J.; James, D. E.


    zone, which overlays a prominent low velocity zone. This high velocity zone has been seen in previous studies of the YSRP and has been interpreted as a mid-crustal layer of sill intrusions (Stachnik, et al., 2008). Low velocities underlie the Yellowstone caldera to the base of the model. Further examination of these detailed images will help us to understand the structure of the crust and upper mantle of the High Lava Plains, Oregon and its relationship to surrounding volcanic and tectonic provinces, e.g. the YSRP, CRB, Basin and Range extension, and Cascades.

  7. Evidence for an eolian origin for the silt-enriched soil mantles on the glaciated uplands of eastern Upper Michigan, USA (United States)

    Schaetzl, R.J.; Loope, W.L.


    We provide textural, geochemical, and mineralogical data on a thin, silty deposit that unconformably mantles glaciated uplands in the eastern Upper Peninsula of Michigan. Previous research on this deposit, which we hypothesize to be loess, is nonexistent. The uplands were islands or narrow peninsulas within one or more glacial lakes. We compare the distribution, likely source and nature of the 20-60??cm thick silty mantle by using the loess formation model of Mason et al. [Mason, J.A., Nater, E.A., Zanner, C.W., Bell, J.C., 1999. A new model of topographic effects on the distribution of loess. Geomorphology 28, 223-236], which focuses on the generation of eolian silt by saltating sand across upwind, barren surfaces. Parabolic dunes, with arms open to the NW, are common on former lake floors upwind of the silt-mantled uplands, attesting to the strength and direction of paleowinds. The abrupt termination of the dunes at the footslopes of the uplands, associated with silt deposition on upland soil surfaces in downwind locations, are both consistent with the model of Mason et al. [Mason, J.A., Nater, E.A., Zanner, C.W., Bell, J.C., 1999. A new model of topographic effects on the distribution of loess. Geomorphology 28, 223-236]. Sediments on former lake floors contain abundant strata of fine/medium sand and silt, and thus are likely sources for the silt and dune sand. The cap, dune and lake sediments are similar along many different geochemical axes, whereas the substrate sediment, i.e., the drift below the cap, is unique. Cap sediments, normally containing roughly 30% silt, are enriched in quartz and depleted in Ti and Zr, relative to dune sediment. The dune sediment, a more residual eolian deposit, is enriched in Ti and Zr, relative to the cap, probably due to its greater abundance of heavy minerals. Therefore, we conclude that the silty cap is loess that was deflated from abandoned lake floors after nearby glacial lakes drained, probably contemporaneously with dune

  8. Upper mantle seismic anisotropy beneath the Northern Transantarctic Mountains, Antarctica from PKS, SKS, and SKKS splitting analysis (United States)

    Graw, Jordan H.; Hansen, Samantha E.


    Using data from the new Transantarctic Mountains Northern Network, this study aims to constrain azimuthal anisotropy beneath a previously unexplored portion of the Transantarctic Mountains (TAMs) to assess both past and present deformational processes occurring in this region. Shear-wave splitting parameters have been measured for PKS, SKS, and SKKS phases using the eigenvalue method within the SplitLab software package. Results show two distinct geographic regions of anisotropy within our study area: one behind the TAMs front, with an average fast axis direction of 42 ± 3° and an average delay time of 0.9 ± 0.04 s, and the other within the TAMs near the Ross Sea coastline, with an average fast axis oriented at 51 ± 5° and an average delay time of 1.5 ± 0.08 s. Behind the TAMs front, our results are best explained by a single anisotropic layer that is estimated to be 81-135 km thick, thereby constraining the anisotropic signature within the East Antarctic lithosphere. We interpret the anisotropy behind the TAMs front as relict fabric associated with tectonic episodes occurring early in Antarctica's geologic history. For the coastal stations, our results are best explained by a single anisotropic layer estimated to be 135-225 km thick. This places the anisotropic source within the viscous asthenosphere, which correlates with low seismic velocities along the edge of the West Antarctic Rift System. We interpret the coastal anisotropic signature as resulting from active mantle flow associated with rift-related decompression melting and Cenozoic extension.

  9. Open system models of isotopic evolution in Earth's silicate reservoirs: Implications for crustal growth and mantle heterogeneity (United States)

    Kumari, Seema; Paul, Debajyoti; Stracke, Andreas


    An open system evolutionary model of the Earth, comprising continental crust (CC), upper and lower mantle (UM, LM), and an additional isolated reservoir (IR) has been developed to study the isotopic evolution of the silicate Earth. The model is solved numerically at 1 Myr time steps over 4.55 Gyr of Earth history to reproduce both the present-day concentrations and isotope ratios of key radioactive decay systems (Rb-Sr, Sm-Nd, and U-Th-Pb) in these terrestrial reservoirs. Various crustal growth scenarios - continuous versus episodic and early versus late crustal growth - and their effect on the evolution of Sr-Nd-Pb isotope systematics in the silicate reservoirs have been evaluated. Modeling results where the present-day UM is ∼60% of the total mantle mass and a lower mantle that is non-primitive reproduce the estimated geochemical composition and isotope ratios in Earth's silicate reservoirs. The isotopic evolution of the silicate Earth is strongly affected by the mode of crustal growth; only an exponential crustal growth pattern with crustal growth since the early Archean satisfactorily explains the chemical and isotopic evolution of the crust-mantle system and accounts for the so-called Pb paradoxes. Assuming that the OIB source is located in the deeper mantle, our model could, however, not reproduce its target ɛNd of +4.6 for the UM, which has been estimated from the average isotope ratios of 32 individual ocean island localities. Hence, either mantle plumes sample the LM in a non-representative way, or the simplified model set-up does not capture the full complexity of Earth's lower mantle (Nd isotope) evolution. Compared to the results obtained for a 4.55 Ga Earth, a model assuming a protracted U-Pb evolution of silicate Earth by ca. 100 Myr reproduces a slightly better fit for the Pb isotope ratios in Earth's silicate reservoirs. One notable feature of successful models is the early depletion of incompatible elements (as well as rapid decrease in Th/U) in

  10. Direct Observation of Upper Mantle Shear Flow Since 14.9 Ma in the Central Basin and Range (United States)

    Biasi, G. P.


    Asthenospheric flow in the Basin and Range province of the western United States has been hypothesized in dynamic models of the region. Models have included southwest flow in response to a hotspot-like upwelling in northeast Nevada, rapid northeast flow in response to extension and Pacific-North American margin shear, and slow, top-to-the-west simple shear. Detailed P-wave tomographic imaging in southern Nevada provides a direct measure of mantle flow to >200 km since ~14.9 Ma. A nearly vertical, 1-2.5% high velocity column presently exists immediately beneath the Timber Mountain/Silent Canyon caldera complex (37.2° N, 116.2° W) that is interpreted as the root of the complex. Volcanism reached 500 km3 in eruptive volumes (comparable to Long Valley Caldera) by 14.9 Ma, and a total of over 7,000 km3 by 11.7 Ma, whereupon the system declined drastically in activity. The root is interpreted from tomographic, geochemical, and petrological considerations to correspond to a de-watering feature that conveyed a pulse of volatile rich volcanism and left a dry, depleted, and slightly cooled column behind. As a melt ascension residual of some sort, we infer that it was originally nearly vertical. With that assumption, the root structure comprises a shear flow marker for activity since it formed. The edifice is 0-15 km west of its center at 200 km depth, indicating that there has been little or no top-to-the-west shear and no channelized flow to the east or west since ~14.9 Ma. Approximately 40 km of top-to-the-south flow is observed, consistent with some geological models. Below 200 km the root appears to plunge NE at 45 degrees to the bottom of the model at 400 km. Synthetic modeling indicates that the deep delay structure is not an artifact of the root, but array aperture is too small to eliminate association with even deeper structures. Hydrous, subsolidus conditions to ~150 km are inferred on a NE linear trend southeast of the root structure. In Nevada this trend does

  11. The tungsten isotopic composition of Eoarchean rocks: Implications for early silicate differentiation and core-mantle interaction on Earth (United States)

    Iizuka, Tsuyoshi; Nakai, Shun'ichi; Sahoo, Yu Vin; Takamasa, Asako; Hirata, Takafumi; Maruyama, Shigenori


    We have measured 182W/ 184W for Eoarchean rocks from the Itsaq Gneiss Complex (3.8-3.7 Ga pillow meta-basalts, a meta-tonalite, and meta-sediments) and Acasta Gneiss Complex (4.0-3.6 Ga felsic orthogneisses) to assess possible W isotopic heterogeneity within the silicate Earth and to constrain W isotopic evolution of the mantle. The data reveal that 182W/ 184W values in the Eoarchean samples are uniform within the analytical error and indistinguishable from the modern accessible mantle signature, suggesting that the W isotopic composition of the upper mantle has not changed significantly since the Eoarchean era. The results imply either that chemical communication between the mantle and core has been insignificant in post-Hadean times, or that a lowermost mantle with a distinctive W isotope signature has been isolated from mantle convective cycling. Most terrestrial rock samples have a 0.2 ɛ142Nd/ 144Nd higher than the chondrite average. This requires either the presence of a hidden enriched reservoir formed within the first 30 Ma of the Solar System, or the bulk Earth having a ˜ 5% higher Sm/Nd than the chondrite average. We explored the relevance of the 182Hf- 182W isotope system to the 146Sm- 142Nd isotope system during early silicate differentiation events on Earth. In this context, we demonstrate that the lack of resolvable 182W excesses in the Itsaq rocks, despite 142Nd excesses compared to the modern accessible mantle, is more consistent with the view that the bulk Earth has a non-chondritic Sm/Nd. In the non-chondritic Sm/Nd Earth model, the 182W- 142Nd chronometry constrains the age of the source mantle depletion for the Itsaq samples to more than ˜ 40 Ma after the Solar System origin. Our results cannot confirm the previous report of 182W anomalies in the Eoarchean Itsaq meta-sediments, which were interpreted as reflecting an impact-derived meteoritic component.

  12. Slab-rollback induced upper mantle upwelling near lateral slab edges: A new mechanism for generating intra-plate magmatism in the central Mediterranean (United States)

    Schellart, W. P.


    Most volcanism on Earth is associated with plate boundaries and can thus be explained in a plate tectonic framework. Intra-plate volcanism, however, cannot directly be explained with plate tectonic theory. Intraplate volcanism is frequently explained with the plume model, in which a relatively fixed buoyant plume rises from the lower mantle to the surface and, as the overlying plate moves with respect to the plume source, produces a linear hotspot track along which the age of volcanoes progressively changes. This model has been applied to linear volcanic chains such as the Hawaii-Emperor Ridge in the Pacific and the Walvis Ridge in the Atlantic Ocean. Other intra-plate volcanism that does not occur in linear chains and does not show a preferred age progression in a specific geographical direction is more difficult to explain with the plume model, and might require an alternative explanation. There are several examples of intraplate volcanism on Earth located close to lateral slab edges, suggesting that they might be genetically related to these slab edges. One example of such volcanism is located in Sicily in the Mediterranean, which took place at ~7.0-1.1 Ma on the Iblean plateau and at 0.5 Ma to Present to form Mount Etna. The volcanics are located in close proximity but are laterally offset with respect to the Eolian magmatic arc and the Calabrian subduction zone, where Ionian oceanic lithosphere is subducting west-northwestward below Calabria. The volcanics in Sicily can therefore not be interpreted as arc volcanism. Previous work, primarily based on the geochemistry and petrology of the volcanics, suggests that the volcanism resulted from a plume. The volcanics in Sicily and surrounding seas, however, do not align along a linear chain and show no lateral age progression. Here it is proposed that Mount Etna and the Iblean volcanics are related to decompression melting of upper mantle material that is flowing around the southern Ionian slab edge to accommodate

  13. Excavation and Melting of the Hadean Continental Crust by Late Heavy Bombardment

    CERN Document Server

    Shibaike, Yuhito; Ida, Shigeru


    No Hadean rocks have ever been found on Earth's surface except for zircons---evidence of continental crust, suggesting that Hadean continental crust existed but later disappeared. One hypothesis for the disappearance of the continental crust is excavation/melting by the Late Heavy Bombardment (LHB), a concentration of impacts in the last phase of the Hadean eon. In this paper, we calculate the effects of LHB on Hadean continental crust in order to investigate this hypothesis. Approximating the size-frequency distribution of the impacts by a power-law scaling with an exponent {\\alpha} as a parameter, we have derived semi-analytical expressions for the effects of LHB impacts. We calculated the total excavation/melting volume and area affected by the LHB from two constraints of LHB on the moon, the size of the largest basin during LHB, and the density of craters larger than 20 km. We also investigated the effects of the value of {\\alpha}. Our results show that LHB does not excavate/melt all of Hadean continental...

  14. Distinct Variations in Seismic Velocity Structure of the Crust and Upper Mantle across the Ailao Shan-Red River Shear Zone in Northern Vietnam (United States)

    Hung, S.; Pan, Y.; Huang, B.; Huang, W.; Le, T.; Dinh, V.


    The tectonic evolution of the Aliao Shan-Red River shear zone (RRSZ) that runs from southeast Tibet through North Vietnam to the South China Sea and marks the boundary between the Indochina and South China blocks has been considered closely linked with the northward indention of the strong Indian plate into the Eurasian continent and the consequent uplift of the Tibetan Plateau. A variety of models have been proposed to explain the postcollisional deformation and magmatism of SE Asia and movement along the RRSZ. Since December 2005, Institute of Earth Science, Academia Sinica of Taiwan has deployed a regional broadband array with station spacing of ~50 km in Northern Vietnam for earthquake and seismic structure studies. We collect data from earthquakes with magnitude≥5.5 and epicentral distances of 30-90o between December, 2005 and June, 2008. Using this new dataset, we report 3-D variations of P- and S-wave speeds (δlnVP and δlnVS) and Poisson's ratios via δln(VP/VS) in the crust and upper mantle across the shear zone, obtained with tomographic inversion of P and S relative travel time residuals measured by inter-station cross-correlation of waveforms at both high- and low-frequencies. We employ physically realistic 3-D sensitivity kernels for frequency-dependent traveltime data and data-adaptive, multi-scale parameterization in the inversion. The resulting models reveal noticeable differences across the RRSZ, where the anomalies of distinctly low VS and VP/VS are widely-dispersed in the lower crust and uppermost mantle down to the depth of 100 km to the southwest of the RRSZ. This may indicate that ductile crustal mass has flowed out of Tibet into Indochina accompanying extrusion of relatively hot lithospheric mantle along the RRSZ related to Late Cenozoic volcanism in the region. Though less distinct in the S velocity model, an elongated fast anomaly about 60 km wide that strikes parallel to the RRSZ and subvertically extends to the depth of 60 km clearly

  15. Exploring Geothermal Energy Potential in Ireland through 3-D Geophysical-Petrological Modelling of Surface Heat-Flow and Crustal and Upper-Mantle Structure (United States)

    Fullea, J.; Muller, M. R.; Jones, A. G.


    Little is known of Ireland's deep, low-enthalpy geothermal resources and the potential for space heating and/or electricity generation based on geothermal energy to displace Ireland's significant reliance on carbon-based fuels. IRETHERM ( is a four-and-a-half year, all-island, academic-government-industry collaborative project, initiated in 2011, with the overarching objective of developing a strategic and holistic understanding of Ireland's geothermal energy potential through integrated modelling of new and existing geophysical and geological data. One of the challenges in searching for deep geothermal resources in the relatively unexplored setting of Ireland lies in identifying those areas most likely to support significantly elevated temperatures at depth. Available borehole data, although sparse and clustered around areas of mineral and hydrocarbon interest, suggest a marked regional increase in surface heat-flow across Ireland, from ~40 mW/m2 in the south to >80 mW/m2 in the north. The origins of both the observed regional heat-flow trend and local temperature anomalies have not been investigated and are not currently understood. Although variations in the structure of the crust and lithosphere have been revealed by a number of active-source seismic and teleseismic experiments, their effects on surface heat-flow have not been modelled. Bulk 3-D variation in crustal heat-production across Ireland, which may contribute significantly to the observed regional and local temperature variations, has also not been determined. We investigate the origins of Ireland's regional heat-flow trend and regional and local temperature variations using the software package LitMod. This software combines petrological and geophysical modelling of the lithosphere and sub-lithospheric upper mantle within an internally consistent thermodynamic-geophysical framework, where all relevant properties are functions of temperature, pressure and chemical composition. The major

  16. Upper mantle compositional variations and discontinuity topography imaged beneath Australia from Bayesian inversion of surface-wave phase velocities and thermochemical modeling

    DEFF Research Database (Denmark)

    Khan, A.; Zunino, Andrea; Deschamps, F.


    models of the thermochemical and anisotropic structure of the mantle to 450 km depth. Dispersion data are linked to thermochemical parameters through a thermodynamic formalism for computing mantle mineral phase equilibria and physical properties. The inverse problem is solved using a probabilistic...

  17. Regional difference in small-scale heterogeneities in the crust and upper mantle in Japan derived by the analysis of high-frequency P-wave (United States)

    Takemura, S.; Furumura, T.


    In order to understand distribution properties of small-scale heterogeneities in the crust and upper mantle structure, we analyze three-component seismograms recorded by Hi-net in Japan. We examined relative strength of the P-wave in the transverse (T) component and its change as a function of frequency and propagation distances, which is strongly relating to the strength of seismic wave scattering in the lithosphere. We analyzed 53,220 Hi-net record from 310 shallow (hFDM simulations using stochastic random media. The model covers a zone 204.8 km by 204.8 km by 64.0 km descretized with 0.1 km in horizontal direction and 0.05 km in vertical direction. The small-scale heterogeneity in the lithosphere is constructed by velocity fluctuation from average velocity. The fluctuation is characterized by von Karman-type ACF with the correlation length a, the rms value e and decay order k. We assume average background velocities of P-wave and S-wave are VP = 5.8 km and VS = 3.36 km, respectively. We employ an explosive point source into the model. The FDM simulations were conducted on the Earth Simulator at JAMSTEC. We conducted a number of FDM simulation using different model parameters of stochastic random media for different e (= 0.03, 0.05, 0.07, 0.09) and fixed a and k (a = 5km, k = 0.5). The simulation results confirm EP value increases linearly with increasing e. We also found that larger EP obtained in the back-arc side of Tohoku can be explained by 4% larger e relative to those of other regions.

  18. Widespread mixing and burial of Earth's Hadean crust by asteroid impacts. (United States)

    Marchi, S; Bottke, W F; Elkins-Tanton, L T; Bierhaus, M; Wuennemann, K; Morbidelli, A; Kring, D A


    The history of the Hadean Earth (∼4.0-4.5 billion years ago) is poorly understood because few known rocks are older than ∼3.8 billion years old. The main constraints from this era come from ancient submillimetre zircon grains. Some of these zircons date back to ∼4.4 billion years ago when the Moon, and presumably the Earth, was being pummelled by an enormous flux of extraterrestrial bodies. The magnitude and exact timing of these early terrestrial impacts, and their effects on crustal growth and evolution, are unknown. Here we provide a new bombardment model of the Hadean Earth that has been calibrated using existing lunar and terrestrial data. We find that the surface of the Hadean Earth was widely reprocessed by impacts through mixing and burial by impact-generated melt. This model may explain the age distribution of Hadean zircons and the absence of early terrestrial rocks. Existing oceans would have repeatedly boiled away into steam atmospheres as a result of large collisions as late as about 4 billion years ago.

  19. Evidence for back scattering of near-podal seismic P'P' waves from the 150-220 km zone in Earth's upper mantle

    Energy Technology Data Exchange (ETDEWEB)

    Tkalcic, H; Flanagan, M P; Cormier, V F


    The deepest and most inaccessible parts of Earth's interior--the core and core-mantle boundary regions can be studied from compressional waves that turn in the core and are routinely observed following large earthquakes at epicentral distances between 145{sup o} and 180{sup o} (also called P', PKIKP or PKP waves). P'P' (PKPPKP) are P' waves that travel from a hypocenter through the Earth's core, reflect from the free surface and travel back through the core to a recording station on the surface. P'P' waves are sometimes accompanied by precursors, which were reported first in the 1960s as small-amplitude arrivals on seismograms at epicentral distances of about 50{sup o}-70{sup o}. Most prominent of these observed precursors were explained by P'P' waves generated by earthquakes or explosions that did not reach the Earth's surface but were reflected from the underside of first order velocity discontinuities at 410 and 660 km in the upper mantle mantle. Here we report the discovery of hitherto unobserved near-podal P'P' waves (at epicentral distance less than 10{sup o}) and very prominent precursors preceding the main energy by as much as 55 seconds. We interpret these precursors as a back scattered energy from undocumented structure in the upper mantle, in a zone between 150 and 220 km depth beneath Earth's surface. From these observations, we identify a frequency dependence of Q (attenuation quality factor) in the lithosphere that can be modeled by a flat relaxation spectrum below about 0.05-0.1 Hz and increasing with as the first power of frequency above this value, confirming pioneering work by B. Gutenberg.

  20. Evidence for back scattering of near-podal seismic P'P' waves from the 150-220 km zone in Earth's upper mantle

    Energy Technology Data Exchange (ETDEWEB)

    Tkalcic, H; Flanagan, M P; Cormier, V F


    The deepest and most inaccessible parts of Earth's interior--the core and core-mantle boundary regions can be studied from compressional waves that turn in the core and are routinely observed following large earthquakes at epicentral distances between 145{sup o} and 180{sup o} (also called P', PKIKP or PKP waves). P'P' (PKPPKP) are P' waves that travel from a hypocenter through the Earth's core, reflect from the free surface and travel back through the core to a recording station on the surface. P'P' waves are sometimes accompanied by precursors, which were reported first in the 1960s as small-amplitude arrivals on seismograms at epicentral distances of about 50{sup o}-70{sup o}. Most prominent of these observed precursors were explained by P'P' waves generated by earthquakes or explosions that did not reach the Earth's surface but were reflected from the underside of first order velocity discontinuities at 410 and 660 km in the upper mantle mantle. Here we report the discovery of hitherto unobserved near-podal P'P' waves (at epicentral distance less than 10{sup o}) and very prominent precursors preceding the main energy by as much as 55 seconds. We interpret these precursors as a back scattered energy from undocumented structure in the upper mantle, in a zone between 150 and 220 km depth beneath Earth's surface. From these observations, we identify a frequency dependence of Q (attenuation quality factor) in the lithosphere that can be modeled by a flat relaxation spectrum below about 0.05-0.1 Hz and increasing with as the first power of frequency above this value, confirming pioneering work by B. Gutenberg.

  1. Enrichments of the mantle sources beneath the Southern Volcanic Zone (Andes) by fluids and melts derived from abraded upper continental crust

    DEFF Research Database (Denmark)

    Holm, Paul Martin; Søager, Nina; Dyhr, Charlotte Thorup


    mantle by means of subduction erosion in response to the northward increasingly strong coupling of the converging plates. Both types of enrichment had the same Pb isotope composition in the TSVZ with no significant component derived from the subducting oceanic crust. Pb–Sr–Nd isotopes indicate a major...

  2. Joint inversion of satellite-detected tidal and magnetospheric signals constrains electrical conductivity and water content of the upper mantle and transition zone

    DEFF Research Database (Denmark)

    Grayver, Alexander V.; Munch, F. D.; Kuvshinov, Alexey V.


    We present a new global electrical conductivity model of Earth's mantle. The model was derived by using a novel methodology, which is based on inverting satellite magnetic field measurements from different sources simultaneously. Specifically, we estimated responses of magnetospheric origin and o...

  3. Mantle strength of the San Andreas fault system and the role of mantle-crust feedbacks

    NARCIS (Netherlands)

    Chatzaras, V.; Tikoff, B.; Newman, J.; Withers, A.C.; Drury, M.R.


    In lithospheric-scale strike-slip fault zones, upper crustal strength is well constrained from borehole observations and fault rock deformation experiments, but mantle strength is less well known. Using peridotite xenoliths, we show that the upper mantle below the San Andreas fault system (Californi

  4. TranSCorBe Project: A high-resolution seismic-passive profile to study the variation of the crustal and upper mantle structures under the Betic mountain ranges (United States)

    Morales, José; Martín, Rosa; Stich, Daniel; Heit, Benjamín; Yuan, Xiaohui; Mancilla, Flor; Benito, José; Carrion, Francisco; Serrano, Inmaculada; López-Comino, Jose Angel; Abreu, Rafael; Alguacil, Gerardo; Almendros, Javier; Carmona, Enrique; Ontiveros, Alfonso; García-Quiroga, Daniel; García-Jerez, Antonio


    The goal of this project is to study the crustal and upper mantle structures under the Betic mountain ranges and their variations between the different geological domains. We deployed 50 broadband and short period seismic stations during 18 months following two profiles. We collect teleseismic events to perform a high-resolution P-to-S and S-to-P receiver function analysis. The main profile (TranSCorBe), of 160 km length, starts near the coast in Mazarrón (Murcia) and follows a NW-SE direction, crossing the Cazorla mountain range. It probes, from south to north, the Alboran domain (metamorphic rocks), the External zones (sedimentary rocks) and the Variscan terrains of the Iberian Massif. The spacing between stations is around 3-4 km. This inter-station distance allows us mapping with high accuracy the variations of the crust and upper mantle discontinuities in the Betic Range and their transition to the Iberian Massif. A second profile (HiRe II) with a larger spacing between seismic stations, is a continuation of a previously installed HiRe I profile, a NS profile starting near the Mediterranean coast in Adra (Almería) through Sierra Nevada Mountains. HiRe II profile prolongs HiRe I profile until the Variscan intersecting with TranSCorBe profile near Cazorla.

  5. Finite-difference migration of the field of refracted waves in studies of the deep structure of the Earth's crust and the upper mantle based on the DSS (on the example of the DOBRE profile) (United States)

    Pilipenko, V. N.; Verpakhovskaya, A. O.; Starostenko, V. I.; Pavlenkova, N. I.


    The main results of deep seismic sounding (DSS) are usually presented in the form of high-velocity models of the medium. Some model examples and the international DOBRE profile have shown that the informativeness of the data obtained can be significantly enhanced by the construction of wave images of the Earth’s crust, based on the migration of refracted and wide-angle reflected waves. The Donets Basin Refraction/Reflection Experiment ( DOBRE) profile crosses the Dnieper-Donets paleorift zone in the Donbas region. Along the profile, refracted waves from the basement and the upper mantle and the reflections from the crust basement (the M boundary) are reliably traced. This wave migration has been used to construct a wave image of the structure of the Earth’s crust. As a result, a clear seismic image of the basement surface, whose depth changes along the profile from 0 to 20 km, was obtained. In near-slope parts of the basin, several major faults were identified that had not been identified previously during standard kinematic data processing. It is shown that the crust-upper mantle transition zone is a clearly reflective horizon only within the crystalline massifs; under a depression, it is represented by a lens-shaped highly-heterogeneous area. As shown in the model examples, the images obtained using such a migration accurately reflect the structural features of the medium, in spite of its complicated structure.

  6. Application of normal mode theory to seismic source and structure problems: Seismic investigations of upper mantle lateral heterogeneity. Ph.D. Thesis (United States)

    Okal, E. A.


    The theory of the normal modes of the earth is investigated and used to build synthetic seismograms in order to solve source and structural problems. A study is made of the physical properties of spheroidal modes leading to a rational classification. Two problems addressed are the observability of deep isotropic seismic sources and the investigation of the physical properties of the earth in the neighborhood of the Core-Mantle boundary, using SH waves diffracted at the core's surface. Data sets of seismic body and surface waves are used in a search for possible deep lateral heterogeneities in the mantle. In both cases, it is found that seismic data do not require structural differences between oceans and continents to extend deeper than 250 km. In general, differences between oceans and continents are found to be on the same order of magnitude as the intrinsic lateral heterogeneity in the oceanic plate brought about by the aging of the oceanic lithosphere.

  7. Long-term preservation of early formed mantle heterogeneity by mobile lid convection: Importance of grainsize evolution (United States)

    Foley, Bradford J.; Rizo, Hanika


    The style of tectonics on the Hadean and Archean Earth, particularly whether plate tectonics was in operation or not, is debated. One important, albeit indirect, constraint on early Earth tectonics comes from observations of early-formed geochemical heterogeneities: 142Nd and 182W anomalies recorded in Hadean to Phanerozoic rocks from different localities indicate that chemically heterogeneous reservoirs, formed during the first ∼500 Myrs of Earth's history, survived their remixing into the mantle for over 1 Gyrs. Such a long mixing time is difficult to explain because hotter mantle temperatures, expected for the early Earth, act to lower mantle viscosity and increase convective vigor. Previous studies found that mobile lid convection typically erases heterogeneity within ∼100 Myrs under such conditions, leading to the hypothesis that stagnant lid convection on the early Earth was responsible for the observed long mixing times. However, using two-dimensional Cartesian convection models that include grainsize evolution, we find that mobile lid convection can preserve heterogeneity at high mantle temperature conditions for much longer than previously thought, because higher mantle temperatures lead to larger grainsizes in the lithosphere. These larger grainsizes result in stronger plate boundaries that act to slow down surface and interior convective motions, in competition with the direct effect temperature has on mantle viscosity. Our models indicate that mobile lid convection can preserve heterogeneity for ≈0.4-1 Gyrs at early Earth mantle temperatures when the initial heterogeneity has the same viscosity as the background mantle, and ≈1-4 Gyrs when the heterogeneity is ten times more viscous than the background mantle. Thus, stagnant lid convection is not required to explain long-term survival of early formed geochemical heterogeneities, though these heterogeneities having an elevated viscosity compared to the surrounding mantle may be essential for their

  8. Iron geochemistry of the mantle (United States)

    Humayun, M.; Campbell, T. J.; Brandon, A. D.; Davis, F. A.; Hirschmann, M. M.


    The Fe/Mg ratio is an important constraint on the compositionally controlled density of the mantle. However, this ratio cannot be inferred from erupted lavas from OIB or MORB sources, but must be determined directly from mantle peridotites. Recently, the Fe/Mn ratio of erupted lavas has been used as an indicator of potential Fe variability in the mantle driven by core-mantle interaction, recycled oceanic crust, or even variations in the temperature of mantle melting. The classic compilation of McDonough & Sun (1995) provided the currently accepted Fe/Mn ratio of the upper mantle, 60±10. The uncertainty on this ratio allows for 15-30% variability in mantle iron abundances, which is equivalent to a density variation larger than observed by seismic tomography in the mantle. To better understand the relationship between mantle peridotites and erupted lavas, and to search for real variability in the Fe/Mn ratio of mantle peridotites, we report precise new ICP-MS measurements of the transition element geochemistry of suites of mantle xenoliths that have known Fe/Mg ratios. For 12 Kilbourne Hole xenoliths, we observe a clear correlation between Fe/Mn and MgO (or Fe/Mg) over an Fe/Mn range of 59-72. Extrapolation of this trend to a Primitive Mantle (PM) MgO content of 37.8 yields an Fe/Mn of 59±1 for the PM. Our new analyses of KLB-1 powder and fused glass beads yield an Fe/Mn of 61.4 for both samples, which plots on the Kilbourne Hole Fe/Mn vs. MgO trend. A set of ten xenoliths from San Carlos yield a wide range of Fe/Mn (56-65) not correlated with MgO content. The San Carlos xenoliths may have experienced a metasomatic effect that imprinted variable Fe/Mn. A clinopyroxene-rich lithology from San Carlos yields an Fe/Mn of 38, which plots on an extension of the Kilbourne Hole Fe/Mn vs. MgO trend. These new results, and those from other xenolith localities being measured in our lab, provide new constraints on the compositional variability of the Earth's upper mantle. Mc

  9. CO2 content of andesitic melts at graphite-saturated upper mantle conditions with implications for redox state of oceanic basalt source regions and remobilization of reduced carbon from subducted eclogite (United States)

    Eguchi, James; Dasgupta, Rajdeep


    We have performed experiments to determine the effects of pressure, temperature and oxygen fugacity on the CO2 contents in nominally anhydrous andesitic melts at graphite saturation. The andesite composition was specifically chosen to match a low-degree partial melt composition that is generated from MORB-like eclogite in the convective, oceanic upper mantle. Experiments were performed at 1-3 GPa, 1375-1550 °C, and fO2 of FMQ -3.2 to FMQ -2.3 and the resulting experimental glasses were analyzed for CO2 and H2O contents using FTIR and SIMS. Experimental results were used to develop a thermodynamic model to predict CO2 content of nominally anhydrous andesitic melts at graphite saturation. Fitting of experimental data returned thermodynamic parameters for dissolution of CO2 as molecular CO2: ln( K 0) = -21.79 ± 0.04, Δ V 0 = 32.91 ± 0.65 cm3mol-1, Δ H 0 = 107 ± 21 kJ mol-1, and dissolution of CO2 as CO3 2-: ln (K 0 ) = -21.38 ± 0.08, Δ V 0 = 30.66 ± 1.33 cm3 mol-1, Δ H 0 = 42 ± 37 kJ mol-1, where K 0 is the equilibrium constant at some reference pressure and temperature, Δ V 0 is the volume change of reaction, and Δ H 0 is the enthalpy change of reaction. The thermodynamic model was used along with trace element partition coefficients to calculate the CO2 contents and CO2/Nb ratios resulting from the mixing of a depleted MORB and the partial melt of a graphite-saturated eclogite. Comparison with natural MORB and OIB data suggests that the CO2 contents and CO2/Nb ratios of CO2-enriched oceanic basalts cannot be produced by mixing with partial melts of graphite-saturated eclogite. Instead, they must be produced by melting of a source containing carbonate. This result places a lower bound on the oxygen fugacity for the source region of these CO2-enriched basalts, and suggests that fO2 measurements made on cratonic xenoliths may not be applicable to the convecting upper mantle. CO2-depleted basalts, on the other hand, are consistent with mixing between

  10. Imaging the transition from flat to normal subduction: variations in the structure of the Nazca slab and upper mantle under southern Peru and northwestern Bolivia (United States)

    Scire, Alissa; Zandt, George; Beck, Susan; Long, Maureen; Wagner, Lara; Minaya, Estela; Tavera, Hernando


    Two arrays of broad-band seismic stations were deployed in the north central Andes between 8° and 21°S, the CAUGHT array over the normally subducting slab in northwestern Bolivia and southern Peru, and the PULSE array over the southern part of the Peruvian flat slab where the Nazca Ridge is subducting under South America. We apply finite frequency teleseismic P- and S-wave tomography to data from these arrays to investigate the subducting Nazca plate and the surrounding mantle in this region where the subduction angle changes from flat north of 14°S to normally dipping in the south. We present new constraints on the location and geometry of the Nazca slab under southern Peru and northwestern Bolivia from 95 to 660 km depth. Our tomographic images show that the Peruvian flat slab extends further inland than previously proposed along the projection of the Nazca Ridge. Once the slab re-steepens inboard of the flat slab region, the Nazca slab dips very steeply (˜70°) from about 150 km depth to 410 km depth. Below this the slab thickens and deforms in the mantle transition zone. We tentatively propose a ridge-parallel slab tear along the north edge of the Nazca Ridge between 130 and 350 km depth based on the offset between the slab anomaly north of the ridge and the location of the re-steepened Nazca slab inboard of the flat slab region, although additional work is needed to confirm the existence of this feature. The subslab mantle directly below the inboard projection of the Nazca Ridge is characterized by a prominent low-velocity anomaly. South of the Peruvian flat slab, fast anomalies are imaged in an area confined to the Eastern Cordillera and bounded to the east by well-resolved low-velocity anomalies. These low-velocity anomalies at depths greater than 100 km suggest that thick mantle lithosphere associated with underthrusting of cratonic crust from the east is not present. In northwestern Bolivia a vertically elongated fast anomaly under the Subandean Zone

  11. Rapid South Atlantic spreading changes and coeval vertical motion in surrounding continents: Evidence for temporal changes of pressure-driven upper mantle flow

    DEFF Research Database (Denmark)

    Colli, L.; Stotz, Ingo Leonardo; Bunge, H.P.


    the rapid spreading rate changes, on order 10 million years, require significant decoupling of regional plate motion from the large-scale mantle buoyancy distribution through a mechanically weak asthenosphere. Andean topographic growth in late Miocene can explain the most recent South Atlantic spreading...... velocity reduction, arising from increased plate boundary forcing associated with the newly elevated topography. But this mechanism is unlikely to explain the Late Cretaceous/Tertiary spreading variations, as changes in Andean paleoelevation at the time are small. We propose an unsteady pressure...

  12. Experimental determination of carbon partitioning between upper mantle minerals and silicate melts: initial results and comparison to trace element partitioning (Nb, Rb, Ba, U, Th, K) (United States)

    Rosenthal, A.; Hauri, E. H.; Hirschmann, M. M.; Davis, F. A.; Withers, A. C.; Fogel, M. L.


    Inventories of C in the mantle and magmatic fluxes of C between the mantle and the Earth's outer envelopes are poorly constrained in part owing to challenges in determining undegassed C concentrations of pristine basalts. Saal et al. [1] proposed that the behavior of Nb could be used as a proxy for C, owing to apparently similar behavior of the two elements in Siqueiros Transform MORB, but higher C/Nb ratios in popping rocks [2] call into question the applicability of the C/Nb proxy. Here, we present experimentally determined carbon partition coefficients (D's) between nominally volatile-free mantle minerals (olivine, OL; orthopyroxene, OPX; clinopyroxene, CPX; garnet, GA) and melts at 0.8-3 GPa, and 1250-1500°C. We conducted piston-cylinder experiments using an olivine-tholeiite + 4 wt% CO2, doped with Nb, Rb, U, Th, and 13C to enhance detection limits. To promote growth of crystals big enough for SIMS analyses, experiments were either long (D12C, but a few have the opposite. Continuous exchange of the liquid (initially rich in 13C) with the graphite capsules (rich in 12C) may yield D's with 13C>12C. D's with 12C>13C are likely owing to either low count rates or comparatively high analytical contamination. Concentrations in minerals vary from 0.20-3.46 ppm for C, 25-176 ppm for H2O, and 0.05-1.21 ppm for F, whereas liquids tend to much higher values (C≤0.9 wt%; H2O≤1.5 wt%; F≤34 ppm; P≤0.25 wt%; S≤43 ppm; Cl≤77 ppm). Resulting D's indicate that C is highly incompatible in all major mantle mineral phases, with D's for OL, OPX and CPX of close to 5x10-4, and for GA ~2.2x10-4. D's for H2O (2x10-4 to ~3x10-2) and F (~2.3x10-3 to ~5.8x10-2) are comparable to those found in previous studies. Trace element partition determinations are in progress, but comparison to previous studies indicates that carbon is significantly more incompatible during mantle melting than Nb, U, or Th, and has behavior approximately similar to Ba. We therefore suggest that

  13. Redox conditions for mantle plumes (United States)

    Heister, L. E.; Lesher, C. E.


    The vanadium to scandium ratio (V/Sc) for basalts from mid-ocean ridge (MOR) and arc environments has been proposed as a proxy for fO2 conditions during partial melting (e.g. [1] and [2]). Contrary to barometric measurements of the fO2 of primitive lavas, the V/Sc ratio of the upper mantle at mid-ocean ridges and arcs is similar, leading previous authors to propose that the upper mantle has uniform redox potential and is well-buffered. We have attempted to broaden the applicability of the V/Sc parameter to plume-influenced localities (both oceanic and continental), where mantle heterogeneities associated with recycled sediments, mafic crust, and metasomatized mantle, whether of shallow or deep origin, exist. We find that primitive basalts from the North Atlantic Igneous Province (NAIP), Hawaii (both the Loa and Kea trends), Deccan, Columbia River, and Siberian Traps show a range of V/Sc ratios that are generally higher (average ~9) than those for MOR (average ~ 6.7) or arc (average ~7) lavas. Based on forward polybaric decompression modeling, we attribute these differences to polybaric melting and melt segregation within the garnet stability field rather than the presence of a more oxidized mantle in plume-influenced settings. Like MORB, the V/Sc ratios for plume-influenced basalts can be accounted for by an oxidation state approximately one log unit below the Ni-NiO buffer (NNO-1). Our analysis suggests that source heterogeneities have little, if any, resolvable influence on mantle redox conditions, although they have significant influence on the trace element and isotopic composition of mantle-derived melts. We suggest that variations in the redox of erupted lavas is largely a function of shallow lithospheric processes rather than intrinsic to the mantle source, regardless of tectonic setting. [1] Li and Lee (2004) EPSL, [2] Lee et al. (2005) J. of Petrology

  14. A Tale of Two Earths: Reconciling the Lunar and Terrestrial Hadean Records (United States)

    Boehnke, Patrick

    Studying early Earth history is complicated by the fact that the rock record doesn't extend past 4 Ga and our only record for the Hadean (>4 Ga) comes to us from detrital zircons from the Jack Hills in Western Australia. The Hadean zircon record extends back to almost 4.4 Ga and has revealed that the early Earth may have had liquid water, a felsic crust, plate boundary interactions, and possibly a biosphere. On the other hand, analyses of lunar and meteoritic samples are used to argue for a hellish Hadean Earth where frequent, large impactors repeatedly destroyed the crust. Indeed, these two models stand in direct contradiction. The focus of this thesis is to examine the evidence for these two models and ultimately propose a reconciliation based on a new interpretation of the chronology of the lunar samples used to constrain the impact history into the early Earth-Moon system. In order to improve the understanding of zircon crystallization in igneous settings, we undertook experimental studies of zircon saturation which were analyzed using a novel ion imaging approach by a secondary ion mass spectrometer. This study confirmed the original model for zircon saturation, that it is a function of only temperature, melt composition, and Zr content. Indeed, the primary implication for the early Earth from this work is that zircons are much more likely to crystallize in a felsic rather than mafic magma and therefore simply the existence of Hadean zircons suggests a high likelihood for felsic Hadean magmatism. The majority of the thesis focuses on the interpretation of 40 Ar/39Ar ages of lunar and meteorite samples, specifically with regards to impact histories derived from compilations of such ages. The primary complication with lunar and meteorite 40Ar/ 39Ar ages is that the vast majority show evidence for later disturbances due to diffusive loss of 40Ar. To try and extract meaningful thermal histories from these samples, we undertook investigations of samples from Apollo

  15. Amphibole—Bearing Peridotite Xenoliths from Nushan,Anhui Province:Evidence for Melt Percolation Process in the Upper mantle and Lithospheric uplift

    Institute of Scientific and Technical Information of China (English)

    徐义刚; J.C.MERCIER


    The spinel peridotite xenoliths of Group I in Quaternary basanites from Nushan,Anhui province,can be classified as two suites:a hydrous suite characterized by the ubiquitous occurrence of (Ti-) pargasite and an anhydrous suite.The nineral chemistry reveals that the anhydrous suite and one associated phlogopite-bearing lherzolite are equilibrated under temperature conditions of 1000-1100℃,whereas amplhibole-bearing peridotites display distinct disequilibrum features,indicating partial reequilibration from 1050 to 850℃ and locally down to 750℃. The amplhbole-bearing peridotites were probably the uppermost part of the high temperature anhydrous suite which was modally modifed by fractionating H2O-rich metasomatic agent during regional upwelling.This relatively recent lithospheric uplift event followed an older uplift event recognized from pyroxene unmixing of domains in local equilibrium,as well as the dominant deformation texture in the anhydrous suite.The first thermal disturbance can be linked with the regional extension and widespread basaltic volcanism in Jiangsu-Anhui provinces since Early Tertiary and the formation of the nearby Subei(North Jiangsu) fault-depression basin during the Eocene,while the second event in association with the formation of amphiboles probably indicates the continuation but diminution of upwared mantle flux since Neogene in response to the change in tectonic regime for eastern china.

  16. Experimental, in-situ carbon solution mechanisms and isotope fractionation in and between (C-O-H)-saturated silicate melt and silicate-saturated (C-O-H) fluid to upper mantle temperatures and pressures (United States)

    Mysen, Bjorn


    Our understanding of materials transport processes in the Earth relies on characterizing the behavior of fluid and melt in silicate-(C-O-H) systems at high temperature and pressure. Here, Raman spectroscopy was employed to determine structure of and carbon isotope partitioning between melts and fluids in alkali aluminosilicate-C-O-H systems. The experimental data were recorded in-situ while the samples were at equilibrium in a hydrothermal diamond anvil cell at temperatures and pressures to 825 °C and >1300 MPa, respectively. The carbon solution equilibrium in both (C-O-H)-saturated melt and coexisting, silicate-saturated (C-O-H) fluid is 2CO3 + H2O + 2Qn + 1 = 2HCO3 + 2Qn. In the Qn-notation, the superscript, n, is the number of bridging oxygen in silicate structural units. At least one oxygen in CO3 and HCO3 groups likely is shared with silicate tetrahedra. The structural behavior of volatile components described with this equilibrium governs carbon isotope fractionation factors between melt and fluid. For example, the ΔH equals 3.2 ± 0.7 kJ/mol for the bulk 13C/12C exchange equilibrium between fluid and melt. From these experimental data, it is suggested that at deep crustal and upper mantle temperatures and pressures, the δ13C-differences between coexisting silicate-saturated (C-O-H) fluid and (C-O-H)-saturated silicate melts may change by more than 100‰ as a function of temperature in the range of magmatic processes. Absent information on temperature and pressure, the use of carbon isotopes of mantle-derived magma to derive isotopic composition of magma source regions in the Earth's interior, therefore, should be exercised with care.

  17. Xenoliths From Isla Isabel, Nayarit, Mexico: The Nature of the Upper Mantle Underneath the Western Part of the Mexican Volcanic Belt (United States)

    Housh, T. B.; Aranda-Gomez, J. J.; Luhr, J. F.


    Isla Isabel is located ~65 km NW of San Blas (Nayarit), off the Pacific coast of central Mexico. The island is a Quaternary (Ar/Ar < 0.7 Ma) volcanic complex built atop attenuated continental crust. Isabel lies on the east side of the mouth of the Gulf of California, near the area previously occupied (early Pliocene) by Los Cabos Block. Southeast of Isabel, on the mainland, is the NW-trending Tepic-Zacoalco rift, a major volcano-tectonic structure in the western part of the Mexican Volcanic Belt. On land, the rift is the boundary between the Jalisco and Sierra Madre Occidental blocks, and Isabel lies along its projection. Immediately S of Isabel is the San Blas Trough, a swale that trends NW-SE, co-linear with a gravity lineation parallel to the Tamayo and San Blas fault zones, which are the transform boundaries between the northern Rivera and North American plates. Plio-Quaternary alkaline and calc-alkaline lavas have erupted contemporaneously in the Tepic-Zacoalco rift, but so far no mantle xenoliths have been reported in them. Isabel's rocks are intra-plate type alkaline basalts to trachybasalts, with 5-6%\\ normative Ne. Primary paragenesis in the lavas is: Ol + Pl + Cpx + TMt. Small (< 5 cm) peridotite xenoliths, and xenocrysts derived from them, are ubiquitous in the rocks. Eleven xenoliths were studied comprising 3 dunites, 7 harzburgites (one Pl-bearing), and 1 gabbro. Compared to other Mexican xenolith localities N of the MVB, they are refractory as they are depleted in, or lack, Cpx. Ol crystals in xenoliths are homogenous and their Mg#\\ s range as follows: peridotites (92-88), gabbro (84), and Pl-harzburgite (80). Cores of primary Ol phenocrysts (90.5-86.5) in Isabel's volcanic rocks are significantly higher in Mg#\\ s than in Ol from other Mexican xenolith localities (max. 86) and overlap with the associated peridotite xenoliths. Such overlap has not been reported for other Mexican xenolith localities. Xenolith equilibration temperatures for 5

  18. Analysis of Crust and Upper Mantle Structure in Ningxia and its Adjacent Area%宁夏及邻区地壳上地幔结构特征分析

    Institute of Scientific and Technical Information of China (English)

    谢晓峰; 王伟涛; 崔瑾; 吕俊强; 姚琳


    occurrence of the upper Moho uplift phenomenon in the strong seismic focus,in which super-heated materials likely rise and melt rock as the hot mantle rock ascends in this area.By exami-ning approximately 40 years of research in this area,including the deep seismic sounding profile, high gravity and magnetic anomalies,and inversion of seismic array data,we conclude that the Alashan and Erdos blocks are relatively stable and that the earthquake activity is weak,whereas the earthquake activity of the northeastern margin of the Qingzang-Tibet Plateau and Yinchuan Basin is stronger.The result of the probe of the deep seismic sounding profile shows that the up-per Moho in the Yinchuan Basin is an uplift that resembles a concave bowl from east to west.Geo-thermal data support this theory.A low-velocity body exists in the crust of Yinchuan Basin,which also upholds this theory;many strong earthquakes that occurred in this area also offer direct evi-dence.The northeastern margin of the Qingzang-Tibet Plateau spatially and is the boundary of the active block,exhibiting comprehensive anomalous gradient zones with gravity,aeromagnetism,and crust thickness.Natural and artificial earthquake research reveals significant changes in crust thickness and several low-velocity bodies in the crust.There is obvious underplating in the crust and upper-mantle transition zone.Geothermal data shows the same result.In short,strong earth-quakes that occurred in this area are connected with a deep fault in the crust,a low-velocity body in the middle-lower crust,uplift of the upper mantle,high gravity-magnetic anomalies-and a com-plex crust and upper-mantle transition zone.

  19. On the Earth's paleo-magnetosphere for the late Hadean eon (United States)

    Scherf, Manuel; Khodachenko, Maxim; Blokhina, Marina; Johnstone, Colin; Alexeev, Igor; Belenkaya, Elena; Tarduno, John; Tu, Lin; Lichtenegger, Herbert; Guedel, Manuel; Lammer, Helmut


    Simulations of the terrestrial paleo-magnetosphere for early stages of the solar system are of particular importance for studying the evolution and mass loss of the Earth's atmosphere. Within this presentation, we will present simulations of the paleo-magnetosphere of the Earth for the late Hadean, i.e. for ~4.1 billion years ago. These were performed with an adapted version of the Paraboloid Magnetospheric Model (PMM) of the Skobeltsyn Institute for Nuclear Physics of the Moscow State University, which serves as an ISO standard for the magnetosphere . As an input parameter, the new measurements of the paleomagnetic field strength by Tarduno et al., 2015, are taken. These data from zircons between 3.3 billion and 4.2 billion years old vary between 1.0 and 0.12 of today's equatorial field strength. Available data at ~4.1 billion years ago are among the lowest field strength values. Another input into the adapted PMM is the solar wind pressure, which was derived from a newly developed solar/stellar wind evolution model, which is strongly dependent on the rotation rate of the early Sun.Our simulations of the terrestrial paleo-magnetosphere with the adapted PMM show that for the most extreme case of a fast rotating Sun and a paleomagnetic field strength with 0.12 of today's value, the stand-off distance of the magnetopause rs shrinks significantly down from today's 10-11 RE to 3.43 RE (i.e. 2.43 RE above the Earth's surface, where RE is the Earth's surface). Even for the least extreme case - i.e. the same magnetic field strength as that of today and a slow rotating Sun - rs shrinks down to 8.23 RE. Another outcome of the modelling is that the polar cap was significantly broader ~4.1 billion years ago than today.These results have implications for the early terrestrial atmosphere. Since the EUV flux during the late Hadean eon was significantly higher, the exobase of a nitrogen dominated atmosphere would most probably have reached the magnetopause, leading to enhanced

  20. Multiple Hadean crystallization and reworking events preserved in individual Jack Hills zircon grains (United States)

    Bellucci, Jeremy; Nemchin, Alexander; Whitehouse, Martin; Snape, Joshua


    Five Hadean (>3.9 Ga) aged zircon grains from the Jack Hills metasedimentary belt have been investigated by an improved secondary ion mass spectrometry scanning ion image technique. This technique has the ability to obtain accurate and precise full U-Pb systematics on a scale document the spatial distribution of U, Th and Pb. All five of the grains investigated here have complex cathodoluminescence patterns that correlate to different U, Th, and Pb concentration domains. The age determinations for these different chemical zones indicate multiple reworking events that are preserved in each grain and have affected the primary crystalized zircon on the scale of <10 μm, smaller than traditional ion microprobe spot analyses. These new scanning ion images and age determinations suggest that roughly half, if not all, previous analyses, including those of trace elements and various isotope systems, could have intersected several domains of unfractured zircon, thus making the interpretation of any trace element, Hf, or O isotopic data tenuous. Lastly, all of the grains analyzed here preserve at least two distinguishable 207Pb/206Pb ages. These ages are preserved in core-rim and/or complex internal textural relationships. These secondary events took place during at ca. 4.3, 4.2, 4.1, 4.0, and 3.7 Ga potentially indicating a sequence of magmatic and/or metamorphic events that recycled some volume of early crust during the Hadean and into Paleo- to Mesoarchean several times with an apparent periodicity of ca. 100 Ma.

  1. The Earth's heterogeneous mantle a geophysical, geodynamical, and geochemical perspective

    CERN Document Server

    Khan, Amir


    This book highlights and discusses recent developments that have contributed to an improved understanding of observed mantle heterogeneities and their relation to the thermo-chemical state of Earth's mantle, which ultimately holds the key to unlocking the secrets of the evolution of our planet. This series of topical reviews and original contributions address 4 themes. Theme 1 covers topics in geophysics, including global and regional seismic tomography, electrical conductivity and seismic imaging of mantle discontinuities and heterogeneities in the upper mantle, transition zone and lower mantle. Theme 2 addresses geochemical views of the mantle including lithospheric evolution from analysis of mantle xenoliths, composition of the deep Earth and the effect of water on subduction-zone processes. Theme 3 discusses geodynamical perspectives on the global thermo-chemical structure of the deep mantle. Theme 4 covers application of mineral physics data and phase equilibrium computations to infer the regional-scale ...

  2. Highly siderophile elements were stripped from Earth's mantle by iron sulfide segregation

    CERN Document Server

    Rubie, David C; Jacobson, Seth A; Morbidelli, Alessandro; Palme, Herbert; Vogel, Antje K; Frost, Daniel J


    Highly siderophile elements (HSEs) are strongly depleted in the bulk silicate Earth (BSE) but are present in near-chondritic relative abundances. The conventional explanation is that the HSEs were stripped from the mantle by the segregation of metal during core formation but were added back in near-chondritic proportions by late accretion, after core formation had ceased. Here we show that metal-silicate equilibration and segregation during Earth's core formation actually increased HSE mantle concentrations because HSE partition coefficients are relatively low at the high pressures of core formation within Earth. The pervasive exsolution and segregation of iron sulfide liquid from silicate liquid (the "Hadean matte") stripped magma oceans of HSEs during cooling and crystallization, before late accretion, and resulted in slightly suprachondritic palladium/iridium and ruthenium/iridium ratios.

  3. Highly siderophile elements were stripped from Earth’s mantle by iron sulfide segregation (United States)

    Rubie, David C.; Laurenz, Vera; Jacobson, Seth A.; Morbidelli, Alessandro; Palme, Herbert; Vogel, Antje K.; Frost, Daniel J.


    Highly siderophile elements (HSEs) are strongly depleted in the bulk silicate Earth (BSE) but are present in near-chondritic relative abundances. The conventional explanation is that the HSEs were stripped from the mantle by the segregation of metal during core formation but were added back in near-chondritic proportions by late accretion, after core formation had ceased. Here we show that metal-silicate equilibration and segregation during Earth’s core formation actually increased HSE mantle concentrations because HSE partition coefficients are relatively low at the high pressures of core formation within Earth. The pervasive exsolution and segregation of iron sulfide liquid from silicate liquid (the “Hadean matte”) stripped magma oceans of HSEs during cooling and crystallization, before late accretion, and resulted in slightly suprachondritic palladium/iridium and ruthenium/iridium ratios.

  4. Upper mantle fluids involved in diamond formation and mantle metasomatism (United States)

    Sverjensky, D. A.


    Diamond formation coupled with metasomatic reactions involving the interaction of fluids with silicate host rocks provides important clues about the deep carbon cycle. However, quantitative modeling of diamond formation with silicate rock metasomatism has not been possible. Here the Deep Water (DEW) model [1] was used to generate equilibrium constants for irreversible chemical mass transfer calculations monitoring evolving fluid chemistry during diamond formation and coupled silicate reactions. Conceptual models for diamond formation in two environments were constructed for the purpose of illustrating the role of pH in diamond-forming systems. For cratonic diamonds, fluid at 900°C and 5.0 GPa was derived in equilibrium with a carbonated mafic part of a subducting slab consisting of pure diopside, enstatite, pyrope, phlogopite, magnesite, diamond and pyrite. The fluid was assumed to infiltrate and react at constant T and P with a model metasedimentary eclogite (jadeite, pyrope, kyanite and coesite). Abundant diamond was predicted to precipitate as reactant silicate minerals were destroyed and secondary pyrope, jadeite and kyanite were precipitated, which could represent the solid inclusions in natural diamonds. The final fluid chemistry was extremely enriched in Si and depleted in Ca relative to the initial fluid, consistent with the worldwide fluid inclusion trend from carbonatitic fluid to silicic fluid. The logfO2 changed by only 0.2, whereas pH continuously decreased as reaction with jadeite and kyanite and precipitation of secondary pyrope removed Mg2+ and added H+ to the fluid. Most of the carbon precipitated as diamond was derived from decreasing concentrations of formate and propionate. In the UHPM scenario, fluid at 600°C and 5.0 GPa in carbonated peridotite (forsterite, antigorite, clinochlore, magnesite, and pyrrhotite) in a subducting slab was assumed to infiltrate and react at constant T and P with a different model metasedimentary eclogite (jadeite, diopside, pyrope, almandine, coesite, magnesite and aragonite). Two generations of diamond were produced because of the relative rates of change of pH and fO2. The amount of diamond again correlated closely with the overall decrease in aqueous organic carbon species. [1] Sverjensky, D. A. et al., Geochim. Cosmochim. Acta 129, 125 (2014).

  5. Mantle Volatiles - Distribution and Consequences (United States)

    Luth, R. W.


    O and 10-44% of the CO2 that is subducted is returned to the surface in arc magmatism. He emphasized that the "missing" volatiles may have multiple fates, including incorporation into the mantle wedge, large-scale fluid flow up along the interface between the subducting slab and overlying mantle, and transport into the deeper mantle.Because of the hydrous nature of arc magmatism, a common hypothesis is that there is a hydrous phase that breaks down at subarc conditions to trigger melting in the overlying mantle wedge to produce arc magmas. A key research goal has been to identify this phase, or phases. For example, serpentine in peridotite will break down during subduction to produce olivine+orthopyroxene+fluid or, in cooler slabs, a progression of DHMSs, the last of which may survive into the transition zone.At some point, however, because of the limited thermal or pressure stability of the hydrous phases, water will be liberated from the slab into the surrounding mantle. At this point, the water will either exist as a fluid, a melt - or something intermediate if we are above the second critical end point in the relevant system (Wyllie and Ryabchikov, 2000) - or it may dissolve into nominally anhydrous phases.The understanding of the relevant phase relations for the other volatiles is not as advanced. For carbon, we have a reasonable understanding of its phase stability in the mantle, but there is still no good understanding of the relative importance of carbonates, elemental carbon, and other forms as hosts for carbon in the mantle. In the upper mantle, sulfur resides primarily in sulfides; their behavior during partial melting will play a major role in the geochemical cycling of sulfur as well as of chalcophile elements. The halogens are rare (and rarely studied) in mantle-derived samples; more insight into their behavior is currently coming from the study of mantle-derived magmas.This review will first consider the evidence from mantle-derived magmas pertaining

  6. S-wave crustal and upper mantle's velocity structure in the eastern Tibetan Plateau-Deep environment of lower crustal flow

    Institute of Scientific and Technical Information of China (English)

    WANG ChunYong; Paul SILVER; L(U) Hai; LOU ZhiYong; WU JianPing; CHANG LiJun; DAI ShiGui; YOU HuiChuan; TANG FangTou; ZHU LuPei


    . Flowage of the ductile material in lower crust may be attributable to the variation of the gravitational potential energy in upper crust from higher on the plateau to lower off plateau.

  7. Tectonic evolution and mantle structure of the Caribbean

    NARCIS (Netherlands)

    van Benthem, S.; Govers, R.; Spakman, W.; Wortel, R.


    We investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic P wave velocity structure under the Caribbean region. In the upper mantle, positive seismic anomalies are imaged under the Lesser Antilles and Puerto Ric

  8. Thermoconvective waves in the earth's mantle (United States)

    Birger, B. I.


    The thermoconvective instability of the Earth's mantle is analysed. The mantle is modelled as an infinite horizontal layer with a free upper surface, heated from below. The creep in the mantle is supposed to be transient when strains are small. This transient creep is described by Lomnitz's law modified by Jeffreys (1958a). It is shown that disturbances, in the form of thermoconvective waves with a period of 10 8 - 10 9y and wavelength of the order 10 3 km, can propagate through the mantle without attenuation. These waves induce oscillations of the Earth's surface. The pattern of flows differs greatly from that suggested by plate tectonics. An attempt is made to give a new explanation for the linear magnetic anomalies over oceanic ridges.

  9. How mantle slabs drive plate tectonics. (United States)

    Conrad, Clinton P; Lithgow-Bertelloni, Carolina


    The gravitational pull of subducted slabs is thought to drive the motions of Earth's tectonic plates, but the coupling between slabs and plates is not well established. If a slab is mechanically attached to a subducting plate, it can exert a direct pull on the plate. Alternatively, a detached slab may drive a plate by exciting flow in the mantle that exerts a shear traction on the base of the plate. From the geologic history of subduction, we estimated the relative importance of "pull" versus "suction" for the present-day plates. Observed plate motions are best predicted if slabs in the upper mantle are attached to plates and generate slab pull forces that account for about half of the total driving force on plates. Slabs in the lower mantle are supported by viscous mantle forces and drive plates through slab suction.

  10. A lithospheric cross-section in the eastern Iberian Peninsula and its margins. Modelling the physical properties of the upper mantle; Corte litosferico al Este de la Peninsula Iberica y sus margenes. Modelizacion de las propiedades fisicas del manto superior

    Energy Technology Data Exchange (ETDEWEB)

    Carballo, A.; Fernandez, M.; Jimenez-Munt, I.


    We study the lithosphere structure along a 1700 km transect crossing the Aquitaine basin, Pyrenees, Ebro basin, Catalan Costal Ranges, Valencia Trough, Balearic Promontory, and Sudbalearic basin, and Kabyles, Tell, Atlas, and Saharan Platform in Africa. The objective is to characterize the structure and composition of the lthospheric mantle in the region, using a method (LitMod) that combines elevation, gravity, geoid, surface heat flow, seismic and petrological data. Using this method we can identify along the profile different mantle composition, derived from geochemical analysis and age. This is the main different with the previous model done in the same area, where lithospheric mantle density just varied with temperature. The results obtained with the LitMod methodology show important differences in geometry and depth of the lithosphere-asthenosphere boundary with the previous model. In the present study we had to consider four different lithopheric mantle compositions in order to fit all the observables. (Author) 18 refs.

  11. Mantle wedge dynamics from seismic anisotropy (Invited) (United States)

    Long, M. D.; Wirth, E. A.


    The mantle wedge above subducting slabs plays a critical role in many of the physical processes associated with subduction, including water transport into the upper mantle and the generation and transport of melts. Our understanding of mantle wedge dynamics is incomplete; in particular, the mantle flow field above subducting slabs remains poorly understood. Because seismic anisotropy is a consequence of deformation, observations of anisotropy (such as shear wave splitting and P-to-SH converted waves) can constrain the geometry of the wedge flow field. Additionally, because the presence of water (either in nominally anhydrous minerals or as hydrous phases) can have a large effect on anisotropic structure, a detailed understanding of mantle wedge anisotropy can help to constrain processes related to water cycling in subduction systems. We present a global, synoptic view of anisotropy observations in subduction zone mantle wedges, compiled from a large number of individual studies, with the goal of understanding the first-order controls on wedge anisotropy and flow patterns. This compilation allows us to explicitly test the predictions made by many different conceptual models for wedge anisotropy, as well as to explore the relationships between observed anisotropy parameters and other parameters that describe subduction. We find that no simple model can explain all of the trends observed in the global data set. Mantle wedge flow is likely controlled by a combination of downdip motion of the slab, trench migration, ambient mantle flow, small-scale convection, proximity to slab edges, and slab morphology, with the relative contributions of these in any given subduction system controlled by the subduction kinematics and mantle rheology. There is also a likely contribution from B-type olivine and/or serpentinite fabric in many subduction zones, governed by the local thermal structure and volatile distribution.

  12. Vertical velocity of mantle flow of East Asia and adjacent areas

    Institute of Scientific and Technical Information of China (English)

    CHENG Xianqiong; ZHU Jieshou; CAI Xuelin


    Based on the high-resolution body wave tomo- graphic image and relevant geophysical data, we calculated the form and the vertical and tangential velocities of mantle flow. We obtained the pattern of mantle convection for East Asia and the West Pacific. Some important results and under- standings are gained from the images of the vertical velocity of mantle flow for East Asia and the West Pacific. There is an upwelling plume beneath East Asia and West Pacific, which is the earth's deep origin for the huge rift valley there. We have especially outlined the tectonic features of the South China Sea, which is of the "工" type in the upper mantle shield type in the middle and divergent in the lower; the Siberian clod downwelling dives from the surface to near Core and mantle bounary (CMB), which is convergent in the upper mantle and divergent in the lower mantle; the Tethyan subduction region, centered in the Qinghai-Tibet plateau, is visible from 300 to 2 000 km, which is also convergent in the upper mantle and divergent in the lower mantle. The three regions of mantle convection beneath East Asia and the West Pacific are in accordance with the West Pacific, Ancient Asia and the Tethyan structure regions. The mantle upwelling orig- inates from the core-mantle boundary and mostly occurs in the middle mantle and the lower part of the upper mantle. The velocities of the vertical mantle flow are about 1-4 cm per year and the tangential velocities are 1-10 cm per year. The mantle flow has an effect on controlling the movement of plates and the distributions of ocean ridges, subduction zones and collision zones. The mantle upwelling regions are clearly related with the locations ofhotspots on the earth's surface.

  13. Determining resolvability of mantle plumes with synthetic seismic modeling (United States)

    Maguire, R.; Van Keken, P. E.; Ritsema, J.; Fichtner, A.; Goes, S. D. B.


    Hotspot volcanism in locations such as Hawaii and Iceland is commonly thought to be associated with plumes rising from the deep mantle. In theory these dynamic upwellings should be visible in seismic data due to their reduced seismic velocity and their effect on mantle transition zone thickness. Numerous studies have attempted to image plumes [1,2,3], but their deep mantle origin remains unclear. In addition, a debate continues as to whether lower mantle plumes are visible in the form of body wave travel time delays, or whether such delays will be erased due to wavefront healing. Here we combine geodynamic modeling of mantle plumes with synthetic seismic waveform modeling in order to quantitatively determine under what conditions mantle plumes should be seismically visible. We model compressible plumes with phase changes at 410 km and 670 km, and a viscosity reduction in the upper mantle. These plumes thin from greater than 600 km in diameter in the lower mantle, to 200 - 400 km in the upper mantle. Plume excess potential temperature is 375 K, which maps to seismic velocity reductions of 4 - 12 % in the upper mantle, and 2 - 4 % in the lower mantle. Previous work that was limited to an axisymmetric spherical geometry suggested that these plumes would not be visible in the lower mantle [4]. Here we extend this approach to full 3D spherical wave propagation modeling. Initial results using a simplified cylindrical plume conduit suggest that mantle plumes with a diameter of 1000 km or greater will retain a deep mantle seismic signature. References[1] Wolfe, Cecily J., et al. "Seismic structure of the Iceland mantle plume." Nature 385.6613 (1997): 245-247. [2] Montelli, Raffaella, et al. "Finite-frequency tomography reveals a variety of plumes in the mantle." Science 303.5656 (2004): 338-343. [3] Schmandt, Brandon, et al. "Hot mantle upwelling across the 660 beneath Yellowstone." Earth and Planetary Science Letters 331 (2012): 224-236. [4] Hwang, Yong Keun, et al

  14. Simulation of the Earth's paleo-magnetosphere for the late Hadean eon (United States)

    Scherf, Manuel; Khodachenko, Maxim; Blokhina, Marina; Johnstone, Colin; Alexeev, Igor; Belenkaya, Elena; Tarduno, John; Güdel, Manuel; Lammer, Helmut


    Simulations of the Earth's magnetosphere obstacle, including the shape of the auroral oval and related field lines for early stages of the solar system are of particular importance for studying the evolution and mass loss of the Earth's atmosphere. Within this presentation, we will present simulations of the terrestrial paleo-magnetosphere of the Earth for the late Hadean, i.e. for ˜4.1 billion years ago. These were performed with an adapted version of the Paraboloid Magnetospheric Model (PMM) of the Skobeltsyn Institute for Nuclear Physics of the Moscow State University, which serves as an ISO standard for the Earth's magnetosphere (see e.g. Alexeev et al., 2003). As an input parameter, the new measurements of the paleomagnetic field strength by Tarduno et al., 2015, are taken. These data from zircons between 3.3 billion and 4.2 billion years old vary between 1.0 and 0.12 of today's equatorial field strength. Available data at ˜4.1 billion years ago are among the lowest field strength values. Another input into the adapted PMM is the solar wind pressure, which was derived from a newly developed solar/stellar wind evolution model (Johnston et al., 2015a, b), which is strongly dependent on the rotation rate of the early Sun. Our simulations of the terrestrial paleo-magnetosphere with the adapted PMM show that for the most extreme case of a fast rotating Sun and a paleomagnetic field strength with 0.12 of today's value, the stand-off distance of the magnetopause Rs shrinks down from today's 10 Re to 3.43 Re. Even for a slow rotating Sun Rs would be at only 4.27 Re. Taking the same magnetic field strength as that of today and a slow rotating Sun leads to an Rs of 8.23 Re, which would be the least extreme case for the terrestrial atmosphere. Another outcome of the modelling is that the auroral oval was significantly broader ˜4.1 billion years ago than today. As demonstrated by our calculations a good approach of the relationship between auroral oval size Θpc (

  15. Possible Biosphere-Lithosphere Interactions Preserved in Igneous Zircon and Implications for Hadean Earth. (United States)

    Trail, Dustin; Tailby, Nicholas D; Sochko, Maggie; Ackerson, Michael R


    Granitoids are silicic rocks that make up the majority of the continental crust, but different models arise for the origins of these rocks. One classification scheme defines different granitoid types on the basis of materials involved in the melting/crystallization process. In this end-member case, granitoids may be derived from melting of a preexisting igneous rock, while other granitoids, by contrast, are formed or influenced by melting of buried sedimentary material. In the latter case, assimilated sedimentary material altered by chemical processes occurring at the near surface of Earth-including biological activity-could influence magma chemical properties. Here, we apply a redox-sensitive calibration based on the incorporation of Ce into zircon crystals found in these two rock types, termed sedimentary-type (S-type) and igneous-type (I-type) granitoids. The ∼400 Ma Lachlan Fold Belt rocks of southeastern Australia were chosen for investigation here; these rocks have been a key target used to describe and explore granitoid genesis for close to 50 years. We observe that zircons found in S-type granitoids formed under more reducing conditions than those formed from I-type granitoids from the same terrain. This observation, while reflecting 9 granitoids and 289 analyses of zircons from a region where over 400 different plutons have been identified, is consistent with the incorporation of (reduced) organic matter in the former and highlights one possible manner in which life may modify the composition of igneous minerals. The chemical properties of rocks or igneous minerals may extend the search for ancient biological activity to the earliest period of known igneous activity, which dates back to ∼4.4 billion years ago. If organic matter was incorporated into Hadean sediments that were buried and melted, then these biological remnants could imprint a chemical signature within the subsequent melt and the resulting crystal assemblage, including zircon.

  16. How stratified is mantle convection? (United States)

    Puster, Peter; Jordan, Thomas H.


    We quantify the flow stratification in the Earth's mid-mantle (600-1500 km) in terms of a stratification index for the vertical mass flux, Sƒ (z) = 1 - ƒ(z) / ƒref (z), in which the reference value ƒref(z) approximates the local flux at depth z expected for unstratified convection (Sƒ=0). Although this flux stratification index cannot be directly constrained by observations, we show from a series of two-dimensional convection simulations that its value can be related to a thermal stratification index ST(Z) defined in terms of the radial correlation length of the temperature-perturbation field δT(z, Ω). ST is a good proxy for Sƒ at low stratifications (SƒUniformitarian Principle. The bound obtained here from global tomography is consistent with local seismological evidence for slab flux into the lower mantle; however, the total material flux has to be significantly greater (by a factor of 2-3) than that due to slabs alone. A stratification index, Sƒ≲0.2, is sufficient to exclude many stratified convection models still under active consideration, including most forms of chemical layering between the upper and lower mantle, as well as the more extreme versions of avalanching convection governed by a strong endothermic phase change.

  17. Primary magmas and mantle temperatures through time (United States)

    Ganne, Jérôme; Feng, Xiaojun


    Chemical composition of mafic magmas is a critical indicator of physicochemical conditions, such as pressure, temperature, and fluid availability, accompanying melt production in the mantle and its evolution in the continental or oceanic lithosphere. Recovering this information has fundamental implications in constraining the thermal state of the mantle and the physics of mantle convection throughout the Earth's history. Here a statistical approach is applied to a geochemical database of about 22,000 samples from the mafic magma record. Potential temperatures (Tps) of the mantle derived from this database, assuming melting by adiabatic decompression and a Ti-dependent (Fe2O3/TiO2 = 0.5) or constant redox condition (Fe2+/∑Fe = 0.9 or 0.8) in the magmatic source, are thought to be representative of different thermal "horizons" (or thermal heterogeneities) in the ambient mantle, ranging in depth from a shallow sublithospheric mantle (Tp minima) to a lower thermal boundary layer (Tp maxima). The difference of temperature (ΔTp) observed between Tp maxima and minima did not change significantly with time (˜170°C). Conversely, a progressive but limited cooling of ˜150°C is proposed since ˜2.5 Gyr for the Earth's ambient mantle, which falls in the lower limit proposed by Herzberg et al. [2010] (˜150-250°C hotter than today). Cooling of the ambient mantle after 2.5 Ga is preceded by a high-temperature plateau evolution and a transition from dominant plumes to a plate tectonics geodynamic regime, suggesting that subductions stabilized temperatures in the Archaean mantle that was in warming mode at that time.abstract type="synopsis">Plain Language SummaryThe Earth's upper mantle constitutes a major interface between inner and outer envelops of the planet. We explore at high resolution its thermal state evolution (potential temperature of the ambient mantle, Tp) in depth and time using a multi-dimensional database of mafic lavas chemistry (>22,000 samples formed in

  18. Shear wave splitting and subcontinental mantle deformation (United States)

    Silver, Paul G.; Chan, W. Winston


    We have made measurements of shear wave splitting in the phases SKS and SKKS at 21 broadband stations in North America, South America, Europe, Asia, and Africa. Measurements are made using a retrieval scheme that yields the azimuth of the fast polarization direction ϕ and delay time δt of the split shear wave plus uncertainties. Detectable anisotropy was found at most stations, suggesting that it is a general feature of the subcontinental mantle. Delay times range from 0.65 s to 1.70 s and average about 1 s. Somewhat surprisingly, the largest delay time is found in the 2.7 b.y.-old Western Superior Province of the Canadian Shield. The splitting observations are interpreted in terms of the strain-induced lattice preferred orientation of mantle minerals, especially olivine. We consider three hypotheses concerning the origin of the continental anisotropy: (1) strain associated with absolute plate motion, as in the oceanic upper mantle, (2) crustal stress, and (3) the past and present internal deformation of the subcontinental upper mantle by tectonic episodes. It is found that the last hypothesis is the most successful, namely that the most recent significant episode of internal deformation appears to be the best predictor of ϕ. For stable continental regions, this is interpreted as "fossil" anisotropy, whereas for presently active regions, such as Alaska, the anisotropy reflects present-day tectonic activity. In the stable portion of North America there is a good correlation between delay time and lithospheric thickness; this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant. The acceptance of this hypothesis has several implications for subcontinental mantle deformation. First, it argues for coherent deformation of the continental lithosphere (crust and mantle) during orogenies. This implies that the anisotropic portion of the lithosphere was present since the

  19. Redox-induced lower mantle density contrast and effect on mantle structure and primitive oxygen (United States)

    Gu, Tingting; Li, Mingming; McCammon, Catherine; Lee, Kanani K. M.


    The mantle comprises nearly three-quarters of Earth's volume and through convection connects the deep interior with the lithosphere and atmosphere. The composition of the mantle determines volcanic emissions, which are intimately linked to evolution of the primitive atmosphere. Fundamental questions remain on how and when the proto-Earth mantle became oxidized, and whether redox state is homogeneous or developed large-scale structures. Here we present experiments in which we subjected two synthetic samples of nearly identical composition that are representative of the lower mantle (enstatite chondrite), but synthesized under different oxygen fugacities, to pressures and temperatures up to 90 GPa and 2,400 K. In addition to the mineral bridgmanite, compression of the more reduced material also produced Al2O3 as a separate phase, and the resulting assemblage is about 1 to 1.5% denser than in experiments with the more oxidized material. Our geodynamic simulations suggest that such a density difference can cause a rapid ascent and accumulation of oxidized material in the upper mantle, with descent of the denser reduced material to the core-mantle boundary. We suggest that the resulting heterogeneous redox conditions in Earth's interior can contribute to the large low-shear velocity provinces in the lower mantle and the evolution of atmospheric oxygen.

  20. Diamond growth in mantle fluids (United States)

    Bureau, Hélène; Frost, Daniel J.; Bolfan-Casanova, Nathalie; Leroy, Clémence; Esteve, Imène; Cordier, Patrick


    In the upper mantle, diamonds can potentially grow from various forms of media (solid, gas, fluid) with a range of compositions (e.g. graphite, C-O-H fluids, silicate or carbonate melts). Inclusions trapped in diamonds are one of the few diagnostic tools that can constrain diamond growth conditions in the Earth's mantle. In this study, inclusion-bearing diamonds have been synthesized to understand the growth conditions of natural diamonds in the upper mantle. Diamonds containing syngenetic inclusions were synthesized in multi-anvil presses employing starting mixtures of carbonates, and silicate compositions in the presence of pure water and saline fluids (H2O-NaCl). Experiments were performed at conditions compatible with the Earth's geotherm (7 GPa, 1300-1400 °C). Results show that within the timescale of the experiments (6 to 30 h) diamond growth occurs if water and carbonates are present in the fluid phase. Water promotes faster diamond growth (up to 14 mm/year at 1400 °C, 7 GPa, 10 g/l NaCl), which is favorable to the inclusion trapping process. At 7 GPa, temperature and fluid composition are the main factors controlling diamond growth. In these experiments, diamonds grew in the presence of two fluids: an aqueous fluid and a hydrous silicate melt. The carbon source for diamond growth must be carbonate (CO32) dissolved in the melt or carbon dioxide species in the aqueous fluid (CO2aq). The presence of NaCl affects the growth kinetics but is not a prerequisite for inclusion-bearing diamond formation. The presence of small discrete or isolated volumes of water-rich fluids is necessary to grow inclusion-bearing peridotitic, eclogitic, fibrous, cloudy and coated diamonds, and may also be involved in the growth of ultradeep, ultrahigh-pressure metamorphic diamonds.

  1. A micro-scale investigation of melt production and extraction in the upper mantle based on silicate melt pockets in ultramafic xenoliths from the Bakony-Balaton Highland Volcanic Field (Western Hungary)

    DEFF Research Database (Denmark)

    Bali, Eniko; Zanetti, A.; Szabo, C.;


    Mantle xenoliths in Neogene alkali basalts of the Bakony-Balaton Highland Volcanic Field (Western Hungary) frequently have melt pockets that contain silicate minerals, glass, and often carbonate globules. Textural, geochemical and thermobarometric data indicate that the melt pockets formed at rel...

  2. What can zircon ages from the Jack Hills detrital zircon suite really tell us about Hadean geodynamics? (United States)

    Whitehouse, Martin; Nemchin, Alexander


    As the only direct sample of the Hadean Earth, detrital zircon grains from the Jack Hills, Western Australia, have been the subject of intense investigation over the almost three decades since their discovery. A wide variety of geochemical and isotopic analyses of these grains, as well as their mineral inclusions, have been used variously to support two fundamentally different models for Hadean geodynamics: (i) Some form of (not necessarily modern-style) plate recycling generating felsic (continental-type?) crust at the boundaries [1, 2], or conversely (ii) the persistence of a long-lived, stagnant basaltic lid within which magmatism occurred as a result of internal temperature perturbations and/or impacts [3, 4], a model also generally consistent with a wide range of observations from post-Hadean geochemical reservoirs. Despite the considerable time and resources expended, the majority of these studies uncritically accept the individual U-Pb zircon ages, even though their veracity is key to many of the interpretations [5, 6]. We report here the results of an in-depth evaluation of all published (and new) U-Pb ages from the Jack Hills zircon suite in order to define age populations that can be used with a high degree of confidence in geodynamic interpretations. A notable problem in the interpretation of U-Pb data from ancient zircon grains (including those as young as the Neoarchean) is that disturbance of the systematics even several 100 Ma after crystallization causes data to spread along the concordia curve without becoming discernably discordant within the relatively large error bounds associated with U/Pb ages from in situ dating methods (e.g. SIMS). While 207Pb/206Pb ages are typically more precise, individually they provide no means to detect Pb-loss-induced younging. However, if two or preferably more analyses have been made in the same zircon growth zone, a reasonable evaluation of the possibility of Pb-loss can be made. In the available Jack Hills zircon

  3. 3D models of slow motions in the Earth's crust and upper mantle in the source zones of seismically active regions and their comparison with highly accurate observational data: I. Main relationships (United States)

    Molodenskii, S. M.; Molodenskii, M. S.; Begitova, T. A.


    approach has the following advantages over the method of steepest descent which was used in our previous works: 1. Application of the perturbation method significantly reduces the volume of the computations in the real problems of coseismic and postseismic deformations (by three to four orders of magnitude when the data from a few dozens of observation points are used); 2. In contrast to the method of steepest descent, the suggested method always provides stable results. This means that adding the new satellite data does not alter the previously calculated coefficients in the low-order harmonics of the distributions of the sought parameters in the orthogonalized basis; this only changes the coefficients of the increasingly higher harmonics which determine the smallscale details in the sought distributions. 3. In contrast to the method of steepest descent, the suggested method is not only capable of constructing stable partial solutions of the inverse problem but also estimating the ambiguity of these solutions. The ambiguity is represented in terms of the superposition of the known functions contained in the orthogonal complement and, hence, with the growth of the amount of the analyzed data it is determined by the linear combination of the increasingly higher harmonics. In the second part of the paper, we present the results of the model numerical computations of Green's function for the elastic displacements of the ground surface, which correspond to the case of the arbitrary geometry of the dislocation surface and arbitrary orientation of the dislocation vector for the real model of the radially heterogeneous gravitating Earth with the hydrostatic distribution of the initial stresses. The numerical calculations of the creep function in the upper mantle for the coseismic deformations and the ambiguity of the models of postseismic deformations in the vicinity of the source of the Great Tohoku earthquake (Japan) of March 11, 2011 are illustrated by the examples.

  4. Petrology of periodotites as a tool of insight into mantle processes: a review. Jobu mantle kanrangan no gansekigaku (ikani mantle process wo yomitoruka)

    Energy Technology Data Exchange (ETDEWEB)

    Arai, S. (Kanazawa University, Ishikawa (Japan). Faculty of Science)


    Petrological studies of the upper mantle materials are reviewed mainly in terms of recent topics. Along with on increase in depletion degree (degree of slipped-out magma) of resistate which remains after magma slipped out from the upper mantle materials, a series from lherzolite to Harzburgites is obtained and as an example, the Horoman peridotite complex which intrudes into the Hidaka metamorphic belt (Hokkaido) is shown. In this connection, a theory is introduced where the bedded structure of this peridotite complex was formed by a series of processes accompanied by the cracks generated in the mantle peridotite. Then, the following topics are explained: strongly incompatible trace elements can be enriched in resistate with high depletion degree (REE paradox); and the recent research trend on issues which relates to the interaction between the magma and mantle, such as the formation of dunites, the origin of picrite basalt, and the origin of chromitite are described. 40 refs., 13 figs.

  5. Decreasing µ142Nd Variation in the Archean Convecting Mantle from 4.0 to 2.5 Ga: Heterogeneous Domain Mixing or Crustal Recycling? (United States)

    Brandon, A. D.; Debaille, V.


    The 146Sm-142Nd (t1/2=68 Ma) chronometer can be used to examine silicate differentiation in the first 400 Ma of Earth history. Early fractionation between Sm and Nd is recorded in cratonic Archean rocks in their 142Nd/144Nd ratios that that deviate up to ±20 ppm, or μ142Nd - ppm deviation relative to the present-day convecting mantle at 0. These values likely record early extraction of incompatible trace element (ITE) enriched material with -μ142Nd, either as crust or late stage residual melt from a magma ocean, and resulting in a complimentary ITE depleted residual mantle with +μ142Nd. If this early-formed ITE-enriched material was re-incorporated rapidly back into the convecting mantle, both ITE-enriched and ITE-depleted mantle domains would have been established in the Hadean. Alternatively, if it was early-formed crust that remained stable it could have slowly eroded and progressively remixed into the convecting mantle as subducted sediment during the Archean. Each of these scenarios could potentially explain the decrease in the maximum variation in µ142Nd from ±20 at 4.0 Ga to 0 at 2.5 Ga [1,2,3]. In the scenario where these variations reflect mixing of mantle domains, this implies long mantle mixing times of greater than 1 Ga in the Archean in order to preserve the early-formed heterogeneities. This can be achieved in a stagnant lid tectonic regime in the Archean with sporadic and short subduction cycles [2]. This scenario would also indicate that mixing times in the convecting mantle were much slower than the previously proposed 100 Ma in the Hadean and Archean. In the alternative scenario, sediment with -µ142Nd was progressively mixed into the mantle via subduction in the Archean [3]. This scenario doesn't require slow mantle mixing times or a stagnant-lid regime. It requires crustal resident times of up to 750 Ma to maintain a steady supply of ancient sediment recycling over the Archean. Each of these scenarios evoke very contrasting conditions for

  6. Laboratory-Scale Simulation of Spiral Plumes in the Mantle

    CERN Document Server

    Sharifulin, A N


    On the basis of laboratory simulation a mechanism is established for the formation of the upper mantle convection spiral plumes from a hot point in the presence of a roll-type large-scale convective flow. The observed plume has horizontal sections near the upper limit, which may lead to the formation of chains of volcanic islands.

  7. Multiscale seismic tomography and mantle dynamics (United States)

    Zhao, Dapeng


    Multiscale (local, regional and global) tomographic studies are made to determine the 3-D structure of the Earth, particularly for imaging mantle plumes and subducting slabs. Plume-like slow anomalies are clearly visible under the major hotspot regions in most parts of the mantle, in particular, under Hawaii, Iceland, Kerguelen, South Pacific and Africa (Zhao, 2001, 2004, 2009). The slow anomalies under South Pacific and Africa have lateral extensions of over 1000 km and exist in the entire mantle, representing two superplumes. The Pacific superplume has a larger spatial extent and stronger slow anomalies than that of the Africa superplume. The Hawaiian plume is not part of the Pacific superplume but an independent whole-mantle plume (Zhao, 2004, 2009). The slow anomalies under hotspots usually do not show a straight pillar shape, but exhibit winding images, suggesting that plumes are not fixed in the mantle but can be deflected by the mantle flow. As a consequence, hotspots are not really fixed but can wander on the Earth's surface, as evidenced by the recent paleomagnetic and numeric modeling studies. Wider and more prominent slow anomalies are visible at the core-mantle boundary (CMB) than most of the lower mantle, and there is a good correlation between the distribution of slow anomalies at the CMB and that of hotspots on the surface, suggesting that most of the strong mantle plumes under the hotspots originate from the CMB. However, there are some small-scaled, weak plumes originating from the transition zone or mid mantle depths (Zhao et al., 2006; Zhao, 2009; Lei et al., 2009; Gupta et al., 2009). Clear images of subducting slabs and magma chambers in the upper-mantle wedge beneath active arc volcanoes are obtained, indicating that geodynamic systems associated with arc magmatism and back-arc spreading are related to deep processes, such as convective circulation in the mantle wedge and dehydration reactions of the subducting slab (Zhao et al., 2002, 2007

  8. Zoned mantle convection. (United States)

    Albarède, Francis; Van Der Hilst, Rob D


    We review the present state of our understanding of mantle convection with respect to geochemical and geophysical evidence and we suggest a model for mantle convection and its evolution over the Earth's history that can reconcile this evidence. Whole-mantle convection, even with material segregated within the D" region just above the core-mantle boundary, is incompatible with the budget of argon and helium and with the inventory of heat sources required by the thermal evolution of the Earth. We show that the deep-mantle composition in lithophilic incompatible elements is inconsistent with the storage of old plates of ordinary oceanic lithosphere, i.e. with the concept of a plate graveyard. Isotopic inventories indicate that the deep-mantle composition is not correctly accounted for by continental debris, primitive material or subducted slabs containing normal oceanic crust. Seismological observations have begun to hint at compositional heterogeneity in the bottom 1000 km or so of the mantle, but there is no compelling evidence in support of an interface between deep and shallow mantle at mid-depth. We suggest that in a system of thermochemical convection, lithospheric plates subduct to a depth that depends - in a complicated fashion - on their composition and thermal structure. The thermal structure of the sinking plates is primarily determined by the direction and rate of convergence, the age of the lithosphere at the trench, the sinking rate and the variation of these parameters over time (i.e. plate-tectonic history) and is not the same for all subduction systems. The sinking rate in the mantle is determined by a combination of thermal (negative) and compositional buoyancy and as regards the latter we consider in particular the effect of the loading of plates with basaltic plateaux produced by plume heads. Barren oceanic plates are relatively buoyant and may be recycled preferentially in the shallow mantle. Oceanic plateau-laden plates have a more pronounced

  9. Deep Mantle Cycling of Oceanic Crust: Evidence from Diamonds and Their Mineral Inclusions (United States)

    Walter, M. J.; Kohn, S. C.; Araujo, D.; Bulanova, G. P.; Smith, C. B.; Gaillou, E.; Wang, J.; Steele, A.; Shirey, S. B.


    A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithospheric slabs into the mantle. Seismological studies extend this process to the lower mantle, and geochemical observations indicate return of oceanic crust to the upper mantle in plumes. There has been no direct petrologic evidence, however, of the return of subducted oceanic crustal components from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite, Brazil, which host inclusions with compositions comprising the entire phase assemblage expected to crystallize from basalt under lower-mantle conditions. The inclusion mineralogies require exhumation from the lower to upper mantle. Because the diamond hosts have carbon isotope signatures consistent with surface-derived carbon, we conclude that the deep carbon cycle extends into the lower mantle.

  10. Formation and modification of chromitites in the mantle (United States)

    Arai, Shoji; Miura, Makoto


    Podiform chromitites have long supplied us with unrivaled information on various mantle processes, including the peridotite-magma reaction, deep-seated magmatic evolution, and mantle dynamics. The recent discovery of ultrahigh-pressure (UHP) chromitites not only sheds light on a different aspect of podiform chromitites, but also changes our understanding of the whole picture of podiform chromitite genesis. In addition, new evidence was recently presented for hydrothermal modification/formation chromite/chromitite in the mantle, which is a classical but innovative issue. In this context, we present here an urgently needed comprehensive review of podiform chromitites in the upper mantle. Wall-rock control on podiform chromitite genesis demonstrates that the peridotite-magma reaction at the upper mantle condition is an indispensable process. We may need a large system in the mantle, far larger than the size of outcrops or mining areas, to fulfill the Cr budget requirement for podiform chromitite genesis. The peridotite-magma reaction over a large area may form a melt enriched with Na and other incompatible elements, which mixes with a less evolved magma supplied from the depth to create chromite-oversaturated magma. The incompatible-element-rich magma trapped by the chromite mainly precipitates pargasite and aspidolite (Na analogue of phlogopite), which are stable under upper mantle conditions. Moderately depleted harzburgites, which contain chromite with a moderate Cr# (0.4-0.6) and a small amount of clinopyroxene, are the best reactants for the chromitite-forming reaction, and are the best hosts for podiform chromitites. Arc-type chromitites are dominant in ophiolites, but some are of the mid-ocean ridge type; chromitites may be common beneath the ocean floor, although it has not yet been explored for chromitite. The low-pressure (upper mantle) igneous chromitites were conveyed through mantle convection or subduction down to the mantle transition zone to form

  11. Mantle structure and tectonic history of SE Asia (United States)

    Hall, Robert; Spakman, Wim


    Seismic travel-time tomography of the mantle under SE Asia reveals patterns of subduction-related seismic P-wave velocity anomalies that are of great value in helping to understand the region's tectonic development. We discuss tomography and tectonic interpretations of an area centred on Indonesia and including Malaysia, parts of the Philippines, New Guinea and northern Australia. We begin with an explanation of seismic tomography and causes of velocity anomalies in the mantle, and discuss assessment of model quality for tomographic models created from P-wave travel times. We then introduce the global P-wave velocity anomaly model UU-P07 and the tectonic model used in this paper and give an overview of previous interpretations of mantle structure. The slab-related velocity anomalies we identify in the upper and lower mantle based on the UU-P07 model are interpreted in terms of the tectonic model and illustrated with figures and movies. Finally, we discuss where tomographic and tectonic models for SE Asia converge or diverge, and identify the most important conclusions concerning the history of the region. The tomographic images of the mantle record subduction beneath the SE Asian region to depths of approximately 1600 km. In the upper mantle anomalies mainly record subduction during the last 10 to 25 Ma, depending on the region considered. We interpret a vertical slab tear crossing the entire upper mantle north of west Sumatra where there is a strong lateral kink in slab morphology, slab holes between c.200-400 km below East Java and Sumbawa, and offer a new three-slab explanation for subduction in the North Sulawesi region. There is a different structure in the lower mantle compared to the upper mantle and the deep structure changes from west to east. What was imaged in earlier models as a broad and deep anomaly below SE Asia has a clear internal structure and we argue that many features can be identified as older subduction zones. We identify remnants of slabs

  12. Steady-state creep in the mantle

    Directory of Open Access Journals (Sweden)



    Full Text Available SUMMARY - The creep equations for steady-state flow of olivine at high
    pressure and temperature are compared in an attempt to elucidate the rheological
    behaviour of the mantle. Results are presented in terms of applied deformation
    maps and curves of effective viscosity v depth.
    In the upper mantle, the transition stress between dislocation and diffusion
    creep is between 10 to 102 bar (as orders of magnitude for grain sizes from
    0.01 to 1 cm. The asthenosphere under continents is deeper, and has higher
    viscosity, than under oceans. Predominance of one creep mechanism above the
    others depends on grain size, strain rate, and volume fraction of melt; the
    rheological response can be different for different geodynamic processes.
    In the lower mantle, on the other hand, dislocation creep is predominant
    at all realistic grain sizes and strain rates. If the effective viscosity has to be only
    slightly higher than in the upper mantle, as some interpretations of glacioisostatic
    rebound suggest, then the activation volume cannot be larger than
    11 cm3 mole^1.

  13. Melting of CaO and CaSiO3 at Deep Mantle Condition Using First Principles Simulations (United States)

    Bajgain, S. K.; Ghosh, D. B.; Karki, B. B.


    Accurate prediction of melting temperatures of major mantle minerals at high pressures is important to understand the Hadean Earth as well as to explain the observed seismic anomalies at ultra-low velocity zone (ULVZ). To further investigate the geophysical implications of our recent first principles study of molten CaO and CaSiO­3, we calculated the melting temperatures of the corresponding solid phases by integrating the Clausius-Clapeyron equation. The melting behavior of their high-pressure phases can constrain the lower mantle solidus. Our calculations show melting temperature of 5700 ± 500 kelvins for CaSiO3 and 7800 ± 600 kelvins for CaO at the base of the lower mantle (136 GPa). The bulk sound velocities of CaO and CaSiO3 liquids at the core-mantle boundary are found to be 40 % lower than P-wave seismic velocity and 22 % lower than that of MgSiO3 liquid. With substantial decrease of melting temperature by freezing point depression and iron partitioning, the partial melting of multi-component silicate and its gravitational buoyancy at ULVZ cannot be ruled out.

  14. Seismic anisotropy: tracing plate dynamics in the mantle. (United States)

    Park, Jeffrey; Levin, Vadim


    Elastic anisotropy is present where the speed of a seismic wave depends on its direction. In Earth's mantle, elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation. Earthquakes generate two seismic wave types: compressional (P) and shear (S) waves, whose coupling in anisotropic rocks leads to scattering, birefringence, and waves with hybrid polarizations. This varied behavior is helping geophysicists explore rock textures within Earth's mantle and crust, map present-day upper-mantle convection, and study the formation of lithospheric plates and the accretion of continents in Earth history.

  15. Mantle updrafts and mechanisms of oceanic volcanism (United States)

    Anderson, Don L.; Natland, James H.


    Convection in an isolated planet is characterized by narrow downwellings and broad updrafts-consequences of Archimedes' principle, the cooling required by the second law of thermodynamics, and the effect of compression on material properties. A mature cooling planet with a conductive low-viscosity core develops a thick insulating surface boundary layer with a thermal maximum, a subadiabatic interior, and a cooling highly conductive but thin boundary layer above the core. Parts of the surface layer sink into the interior, displacing older, colder material, which is entrained by spreading ridges. Magma characteristics of intraplate volcanoes are derived from within the upper boundary layer. Upper mantle features revealed by seismic tomography and that are apparently related to surface volcanoes are intrinsically broad and are not due to unresolved narrow jets. Their morphology, aspect ratio, inferred ascent rate, and temperature show that they are passively responding to downward fluxes, as appropriate for a cooling planet that is losing more heat through its surface than is being provided from its core or from radioactive heating. Response to doward flux is the inverse of the heat-pipe/mantle-plume mode of planetary cooling. Shear-driven melt extraction from the surface boundary layer explains volcanic provinces such as Yellowstone, Hawaii, and Samoa. Passive upwellings from deeper in the upper mantle feed ridges and near-ridge hotspots, and others interact with the sheared and metasomatized surface layer. Normal plate tectonic processes are responsible both for plate boundary and intraplate swells and volcanism.

  16. Phase relations in the Fe-Fe3C-Fe3N system at 7.8 GPa and 1350 °C: Implications for carbon and nitrogen hosts in Fe0-saturated upper mantle (United States)

    Sokol, Alexander G.; Kruk, Alexey N.; Seryotkin, Yurii V.; Korablin, Alexander A.; Palyanov, Yury N.


    Phase relations in the Fe-Fe3C-Fe3N system are studied in high-pressure experiments at 7.8 GPa and 1350 °C using a split-sphere multi-anvil apparatus. The starting mixtures consisting of Fe, Fe3C and Fe3N are loaded into ceramic or graphite capsules. Contamination with trace amounts of oxygen leads to the appearance of wüstite in the system retaining oxygen fugacity (fO2) near the iron-wüstite (IW) buffer. The metal melt rich in carbon and nitrogen has a large stability field in the central part of the phase diagram, and this field at 1350 °C is tangent to the Fe-Fe3C side of the Fe-Fe3C-Fe3N triangle at the point of the Fe-Fe3C eutectics. Iron nitride ε-Fe3N (space group P6322 or P63/mmc) contains variable amounts of C and N: up to 2.0-2.5 wt% C and 6.0-7.3 wt% N in equilibrium with a C- and N-rich melt and as little as 1.0 wt% C and 3.2 wt% N in equilibrium with γ- Fe. The limit C and N contents in γ-Fe equilibrated with the C- and N-rich melt is about 1.0 wt%, while the N solubility in cementite (Fe3C) does not exceed 0.5 wt%. The obtained data make basis for the isothermal section of the Fe-Fe3C-Fe3N system. The metal melt phase is inferred to be the main host of carbon and nitrogen in the Fe0-saturated (0.1 wt%) mantle at a depth of ∼250 km. In particular, C- and N-bearing austenite (γ-Fe) and metal melts host carbon and nitrogen in the mantle depleted in volatiles (20 ppm C and 1 ppm N), whereas carbon and nitrogen in the mantle with high concentrations of volatiles (250 ppm C and 100 ppm N) reside in C- and N-rich melts with a minor amount of iron carbide (Fe3C). The presence of nickel and sulphur in metal are expected to inhibit the formation of iron carbide and increases the melt phase stability. Redox freezing of N-rich carbonate melts from subduction slabs in Fe0-saturated mantle may produce iron melts supersaturated with nitrogen and stable ε-Fe3N.

  17. Electrical conductivity of continental lithospheric mantle from integrated geophysical and petrological modeling: Application to the Kaapvaal Craton and Rehoboth Terrane, southern Africa


    Fullea, J.; Muller, M.R.; Jones, A. G.


    The electrical conductivity of mantle minerals is highly sensitive to parameters that characterize the structure and state of the lithosphere and sublithospheric mantle, and mapping its lateral and vertical variations gives insights into formation and deformation processes. We review state-of-the-art conductivity models based on laboratory studies for the most relevant upper mantle minerals and define a bulk conductivity model for the upper mantle that accounts for temperature, pressure, and ...

  18. Accumulation of 'anti-continent' at the base of the mantle and its recycling in mantle plumes (United States)

    Tatsumi, Yoshiyuki; Suzuki, Toshihiro; Ozawa, Haruka; Hirose, Kei; Hanyu, Takeshi; Ohishi, Yasuo


    The continental crust is a unique reservoir of light elements in the solid Earth; it possesses an intermediate composition and is believed to have been created principally along volcanic arcs, which are major sites of terrestrial andesitic magmatism. Mantle-derived arc magmas are, however, generally mafic or basaltic. A simple mechanism to overcome this apparent dilemma and generate andesitic melts in such a setting is through the partial remelting of an initial mafic arc crust by heat supplied from underplating basaltic magmas. An antithesis to the formation of continental crust in this way should be the production of refractory melting residue, here referred to as 'anti-continent'. This anti-continent is likely to detach from arc crust as a result of a density inversion and descend into the upper mantle. High-pressure experiments demonstrate that sinking anti-continent is, in contrast to the subducting oceanic crust, always denser than the surrounding mantle, suggesting that it penetrates through the upper-lower mantle boundary, without stagnation, and accumulates at the base of the mantle to form a 200-400 km thick mass known as the D″ layer. Geochemical modeling provides further evidence that this accumulating anti-continent contributes to a deep-seated hotspot source. Therefore, through complementary processes, Earth creates buoyant continents and dense anti-continents at the top and the base of the mantle, respectively, and has recycled portions of anti-continent in mantle plumes.

  19. Nitrogen speciation in mantle and crustal fluids (United States)

    Li, Yuan; Keppler, Hans


    Seventy-nine experiments have been carried out at 600-1400 °C, 2-35 kbar, and oxygen fugacities ranging from the Fe-FeO to the Re-ReO2 buffer to investigate the nitrogen speciation in mantle and crustal N-H-O fluids. Laser Raman analyses of fluid inclusions trapped in situ in quartz and olivine crystals show that N2 and/or NH3 are the only detectable nitrogen species in the fluids at the conditions of the present study. The results further show that in the fluids of the oxidized shallow upper mantle, nitrogen is mostly present as N2, while in the deep reduced upper mantle, NH3 is the dominant nitrogen species. Nitrogen speciation in subduction zone fluids is also calculated from the experimental data to constrain the efficiency of nitrogen recycling. The data show that a hot, oxidized slab is an efficient barrier for deep nitrogen subduction, while a cold, reduced slab would favor recycling nitrogen into the deep mantle. The nitrogen species in magmatic fluids of mid-ocean ridge basalt and arc magmas are predominantly N2, but a significant fraction of nitrogen can be NH3 at certain conditions. The nitrogen species in fluids released from the solidifying magma ocean and the reduced young mantle may have been mostly NH3. The release of such fluids may have created a reduced atmosphere on the every early Earth, with an elevated concentration of NH3. This may not only resolve the faint young Sun paradox but may also have created favorable conditions for the formation of biomolecules through Miller-Urey type reactions.

  20. Where is mantle's carbon? (United States)

    Oganov, A. R.; Ono, S.; Ma, Y.


    Due to the strongly reducing conditions (the presence of metallic iron was suggested both by experiments [1] and theory [2]), diamond was believed to be the main host of carbon through most of the lower mantle [3]. We showed [4] that cementite Fe3C is another good candidate to be the main host of "reduced" carbon in the mantle, reinforcing an earlier hypothesis [5]. The fate of "oxidised" carbon (in subducted slabs) is of particular importance - if carbonates decompose producing fluid CO2, this would have important implications for the chemistry and rheology of the mantle. Knowledge of crystal structures and phase diagrams of carbonates is crucial here. The high-pressure structures of CaCO3 were predicted [6] and subsequently verified by experiments. For MgCO3, Isshiki et al. [7] found a new phase above 110 GPa, and several attempts were made to solve it [8,9]. Here [4], using an evolutionary algorithm for crystal structure prediction [10], we show that there are two post-magnesite phases at mantle-relevant pressure range, one stable at 82-138 GPa, and the other from 138 GPa to ~160 GPa. Both are based on threefold rings of CO4-tetrahedra and are more favourable than all previously proposed structures. We show that through most of the P-T conditions of the mantle, MgCO3 is the major host of oxidized carbon in the Earth. We predict the possibility of CO2 release at the very bottom of the mantle (in SiO2-rich basaltic part of subducted slabs), which could enhance partial melting of rocks and be related to the geodynamical differences between the Earth and Venus. 1.Frost D.J., Liebske C., Langenhorst F., McCammon C.A., Tronnes R.G., Rubie D.C. (2004). Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle. Nature 428, 409-412. 2.Zhang F., Oganov A.R. (2006). Valence and spin states of iron impurities in mantle-forming silicates. Earth Planet. Sci. Lett. 249, 436-443. 3.Luth R.W. (1999). Carbon and carbonates in the mantle. In: Mantle

  1. Chondritic Xenon in the Earth's mantle: new constrains on a mantle plume below central Europe (United States)

    Caracausi, Antonio; Avice, Guillaume; Bernard, Peter; Furi, Evelin; Marty, Bernard


    data support the notion that the fraction of plutonium-derived Xe in plume sources (oceanic as well as continental) is higher than in the MORB source reservoir. Hence, the MORB - type reservoirs appear to be well distinguished and more degassed than the plume sources (oceanic as well as continental) supporting the heterogeneity of Earth's mantle. Finally this study highlights that xenon isotopes in the Eifel gas have preserved a chemical signature that is characteristic of other mantle plume sources. This is very intriguing because the presence of a mantle plume in this sector of Central Europe was already inferred from geophysical and geochemical studies(Buikin et al., 2005; Goes et al., 1999). Notably, tomographic images show a low-velocity structure down to 2000 km depth, representing deep mantle upwelling under central Europe, that may feed smaller upper-mantle plumes (Eifel volcanic district-Germany). References Buikin A., Trieloff M., HoppJ., Althaus T., Korochantseva E., Schwarz W.H. &Altherr R., (2005), Noble gas isotopessuggestdeepmantleplume source of late Cenozoicmaficalkalinevolcanism in Europe, Earth Planet. Sci. Lett. 230, 143-162. Goes S., Spakman W. &BijwaardH., (1999), A lowermantle source for centraleuropeanvolcanism, Science, 286, 1928-1931.G. Holland, M. Cassidy, C.J. Ballentine, Meteorite Kr in the Earth's mantle suggests a late accretionary source for the atmosphere, Science, 326, 1522-1525, (2009). Marty, B. Neon and xenon isotopes in MORB: implications for the Earth-atmosphere evolution. Earth Planet. Sci. Lett. 94, 45-56 (1989). Mukhopadhyay S., Early differentiation and volatile accretion recorded in deep-mantle neon and xenon
, Nature, 486, 101-106, (2013).

  2. The role of thermodynamics in mantle convection: is mantle-layering intermittent? (United States)

    Stixrude, L. P.; Cagney, N.; Lithgow-Bertelloni, C. R.


    We examine the thermal evolution of the Earth using a 1D model in which mixing length theory is used to characterise the role of thermal convection. Unlike previous work, our model accounts for the complex role of thermodynamics and phase changes through the use of HeFESTo (Stixrude & Lithgow-Bertelloni, Geophys. J. Int. 184, 2011), a comprehensive thermodynamic model that enables self-consistent computation of phase equilibria, physical properties (e.g. density, thermal expansivity etc.) and mantle isentropes. Our model also accounts for the freezing of the inner core, radiogenic heating and Arrhenius rheology, and is validated by comparing our results to observations, including the present-day size of the inner core and the heat flux at the surface.If phase changes and the various thermodynamic effects on mantle properties are neglected, the results are weakly dependent on the initial conditions, as has been observed in several previous studies. However, when these effects are accounted for, the initial temperature profile has a strong influence on the thermal evolution of the mantle, because small changes in the temperature and phase-assemblage can lead to large changes in the local physical properties and the adiabatic gradient.The inclusion of thermodynamic effects leads to some new and interesting insights. We demonstrate that the Clapeyron slope and the thermal gradient at the transition zone both vary significantly with time; this causes the mantle to switch between a layered state, in which convection across the transition zone is weak or negligible, and an un-layered state, in which there is no resistance to mass transfer between the upper and lower mantles.Various plume models describe plumes either rising directly from the CMB to the lithosphere, or stalling at the transition zone before spawning new plumes in the upper mantle. The observance of switching behaviour indicates that both models may be applicable depending on the state of the mantle: plumes

  3. A mantle plume below the Eifel volcanic fields, Germany


    Ritter, Joachim R. R.; Jordan, Michael; Christensen, Ulrich R.; Achauer, Ulrich


    We present seismic images of the upper mantle below the Quaternary Eifel volcanic fields, Germany, determined by teleseismic travel time tomography. The data were measured at a dedicated network of more than 200 stations. Our results show a columnar low P-velocity anomaly in the upper mantle with a lateral contrast of up to 2%. The 100 km wide structure extends to at least 400 km depth and is equivalent to about 150–200 K excess temperature. This clear evidence for a plume below a region of c...

  4. The maximum water storage capacities in nominally anhydrous minerals in the mantle transition zone and lower mantle (United States)

    Inoue, T.; Yurimoto, H.


    Water is the most important volatile component in the Earth, and affects the physicochemical properties of mantle minerals, e.g. density, elastic property, electrical conductivity, thermal conductivity, rheological property, melting temperature, melt composition, element partitioning, etc. So many high pressure experiments have been conducted so far to determine the effect of water on mantle minerals. To clarify the maximum water storage capacity in nominally anhydrous mantle minerals in the mantle transition zone and lower mantle is an important issue to discuss the possibility of the existence of water reservoir in the Earth mantle. So we have been clarifying the maximum water storage capacity in mantle minerals using MA-8 type (KAWAI-type) high pressure apparatus and SIMS (secondary ion mass spectroscopy). Upper mantle mineral, olivine can contain ~0.9 wt% H2O in the condition just above 410 km discontinuity in maximum (e.g. Chen et al., 2002; Smyth et al., 2006). On the other hand, mantle transition zone mineral, wadsleyite and ringwoodite can contain significant amount (about 2-3 wt.%) of H2O (e.g. Inoue et al., 1995, 1998, 2010; Kawamoto et al., 1996; Ohtani et al., 2000). But the lower mantle mineral, perovskite can not contain significant amount of H2O, less than ~0.1 wt% (e.g. Murakami et al., 2002; Inoue et al., 2010). In addition, garnet and stishovite also can not contain significant amount of H2O (e.g. Katayama et al., 2003; Mookherjee and Karato, 2010; Litasov et al., 2007). On the other hand, the water storage capacities of mantle minerals are supposed to be significantly coupled with Al by a substitution with Mg2+, Si4+ or Mg2+ + Si4+, because Al3+ is the trivalent cation, and H+ is the monovalent cation. To clarify the degree of the substitution, the water contents and the chemical compositions of Al-bearing minerals in the mantle transition zone and the lower mantle were also determined in the Al-bearing systems with H2O. We will introduce the

  5. Mapping mantle flow during retreating subduction: Laboratory models analyzed by feature tracking (United States)

    Funiciello, F.; Moroni, M.; Piromallo, C.; Faccenna, C.; Cenedese, A.; Bui, H. A.


    Three-dimensional dynamically consistent laboratory models are carried out to model the large-scale mantle circulation induced by subduction of a laterally migrating slab. A laboratory analogue of a slab-upper mantle system is set up with two linearly viscous layers of silicone putty and glucose syrup in a tank. The circulation pattern is continuously monitored and quantitatively estimated using a feature tracking image analysis technique. The effects of plate width and mantle viscosity/density on mantle circulation are systematically considered. The experiments show that rollback subduction generates a complex three-dimensional time-dependent mantle circulation pattern characterized by the presence of two distinct components: the poloidal and the toroidal circulation. The poloidal component is the answer to the viscous coupling between the slab motion and the mantle, while the toroidal one is produced by lateral slab migration. Spatial and temporal features of mantle circulation are carefully analyzed. These models show that (1) poloidal and toroidal mantle circulation are both active since the beginning of the subduction process, (2) mantle circulation is intermittent, (3) plate width affects the velocity and the dimension of subduction induced mantle circulation area, and (4) mantle flow in subduction zones cannot be correctly described by models assuming a two-dimensional steady state process. We show that the intermittent toroidal component of mantle circulation, missed in those models, plays a crucial role in modifying the geometry and the efficiency of the poloidal component.

  6. Oceanic crust recycling and the formation of lower mantle heterogeneity (United States)

    van Keken, Peter E.; Ritsema, Jeroen; Haugland, Sam; Goes, Saskia; Kaneshima, Satoshi


    The Earth's lower mantle is heterogeneous at multiple scales as demonstrated for example by the degree-2 distribution of LLSVPs seen in global tomography and widespread distribution of small scale heterogeneity as seen in seismic scattering. The origin of this heterogeneity is generally attributed to leftovers from Earth's formation, the recycling of oceanic crust, or a combination thereof. Here we will explore the consequences of long-term oceanic crust extraction and recycling by plate tectonics. We use geodynamical models of mantle convection that simulate plates in an energetically consistent manner. The recycling of oceanic crust over the age of the Earth produces persistent lower mantle heterogeneity while the upper mantle tends to be significantly more homogeneous. We quantitatively compare the predicted heterogeneity to that of the present day Earth by tomographic filtering of the geodynamical models and comparison with S40RTS. We also predict the scattering characteristics from S-P conversions and compare these to global scattering observations. The geophysical comparison shows that lower mantle heterogeneity is likely dominated by long-term oceanic crust recycling. The models also demonstrate reasonable agreement with the geochemically observed spread between HIMU-EM1-DMM in ocean island basalts as well as the long-term gradual depletion of the upper mantle as observed in Lu-Hf systematics.

  7. Importance of Mantle Viscosity in Interseismic Deformation (United States)

    Wang, K.; He, J.; Hu, Y.


    The role of mantle viscosity in subduction earthquake cycles was postulated when the plate tectonics theory had just gained wide acceptance. The process was described using Elsasser's 1-D model for diffusion of stress from the subduction boundary to the plate interior. Main features of interseismic surface deformation predicted by this elegantly simple model were later verified by GPS observations following giant subduction earthquakes. However, and intriguingly, the vast majority of interseismic deformation models developed in the era of space geodesy assume an elastic Earth, incorrectly regarding interseismic deformation as a subdued mirror image of coseismic deformation. The reason is four-fold. (1) The 1-D model and subsequent 2-D viscoelastic models failed to recognize the role of rupture length in the strike direction and could not self-consistently explain deformation following medium and small earthquakes. (2) Based on global mantle viscosity models derived from glacial isostatic adjustment studies, the viscoelastic mantle should indeed behave elastically in earthquake cycles of a few hundred years. (3) The effect of viscous mantle deformation can often be equivalently described by deep fault creep in a purely elastic Earth. (4) The use of an elastic model provides convenience in inverting geodetic data to determine fault locking and creep. Here we use 3D finite element models to show that the main characteristics of surface deformation following subduction earthquakes of all sizes can be explained with a viscoelastic Earth in which the mantle wedge is less viscous than global upper-mantle average of 1020 - 1021 Pa s by one to two orders of magnitude. Following giant earthquakes, such as 1700 Cascadia, 1960 Chile, 1964 Alaska, 2004 Sumatra, and 2011 Japan, upper-plate land deformation undergoes phases of wholesale seaward motion, opposing motion of coastal and inland areas, and wholesale landward motion. The "speed" of the evolution scales inversely with

  8. Sensitivity analysis of crustal correction for calculation of lithospheric mantle density from gravity data

    DEFF Research Database (Denmark)

    Herceg, Matija; Artemieva, Irina; Thybo, Hans


    We investigate how uncertainties in seismic and density structure of the crust propagate to uncertainties in mantle density structure. The analysis is based on interpretation of residual upper-mantle gravity anomalies which are calculated by subtracting (stripping) the gravitational effect of the...

  9. Inference of mantle viscosity for depth resolutions of GIA observations (United States)

    Nakada, Masao; Okuno, Jun'ichi


    Inference of the mantle viscosity from observations for glacial isostatic adjustment (GIA) process has usually been conducted through the analyses based on the simple three-layer viscosity model characterized by lithospheric thickness, upper- and lower-mantle viscosities. Here, we examine the viscosity structures for the simple three-layer viscosity model and also for the two-layer lower-mantle viscosity model defined by viscosities of η670,D (670-D km depth) and ηD,2891 (D-2891 km depth) with D-values of 1191, 1691 and 2191 km. The upper-mantle rheological parameters for the two-layer lower-mantle viscosity model are the same as those for the simple three-layer one. For the simple three-layer viscosity model, rate of change of degree-two zonal harmonics of geopotential due to GIA process (GIA-induced J˙2) of -(6.0-6.5) × 10-11 yr-1 provides two permissible viscosity solutions for the lower mantle, (7-20) × 1021 and (5-9) × 1022 Pa s, and the analyses with observational constraints of the J˙2 and Last Glacial Maximum (LGM) sea levels at Barbados and Bonaparte Gulf indicate (5-9) × 1022 Pa s for the lower mantle. However, the analyses for the J˙2 based on the two-layer lower-mantle viscosity model only require a viscosity layer higher than (5-10) × 1021 Pa s for a depth above the core-mantle boundary (CMB), in which the value of (5-10) × 1021 Pa s corresponds to the solution of (7-20) × 1021 Pa s for the simple three-layer one. Moreover, the analyses with the J˙2 and LGM sea level constraints for the two-layer lower-mantle viscosity model indicate two viscosity solutions: η670,1191 > 3 × 1021 and η1191,2891 ˜ (5-10) × 1022 Pa s, and η670,1691 > 1022 and η1691,2891 ˜ (5-10) × 1022 Pa s. The inferred upper-mantle viscosity for such solutions is (1-4) × 1020 Pa s similar to the estimate for the simple three-layer viscosity model. That is, these analyses require a high viscosity layer of (5-10) × 1022 Pa s at least in the deep mantle, and suggest

  10. Nickel isotopic composition of the mantle (United States)

    Gall, Louise; Williams, Helen M.; Halliday, Alex N.; Kerr, Andrew C.


    This paper presents a detailed high-precision study of Ni isotope variations in mantle peridotites and their minerals, komatiites as well as chondritic and iron meteorites. Ultramafic rocks display a relatively large range in δ60 Ni (permil deviation in 60 Ni /58 Ni relative to the NIST SRM 986 Ni isotope standard) for this environment, from 0.15 ± 0.07‰ to 0.36 ± 0.08‰, with olivine-rich rocks such as dunite and olivine cumulates showing lighter isotope compositions than komatiite, lherzolite and pyroxenite samples. The data for the mineral separates shed light on the origin of these variations. Olivine and orthopyroxene display light δ60 Ni whereas clinopyroxene and garnet are isotopically heavy. This indicates that peridotite whole-rock δ60 Ni may be controlled by variations in modal mineralogy, with the prediction that mantle melts will display variable δ60 Ni values due to variations in residual mantle and cumulate mineralogy. Based on fertile peridotite xenoliths and Phanerozoic komatiite samples it is concluded that the upper mantle has a relatively homogeneous Ni isotope composition, with the best estimate of δ60Nimantle being 0.23 ± 0.06‰ (2 s.d.). Given that >99% of the Ni in the silicate Earth is located in the mantle, this also defines the Ni isotope composition of the Bulk Silicate Earth (BSE). This value is nearly identical to the results obtained for a suite of chondrites and iron meteorites (mean δ60 Ni 0.26 ± 0.12‰ and 0.29 ± 0.10‰, respectively) showing that the BSE is chondritic with respect to its Ni isotope composition, with little to no Ni mass-dependent isotope fractionation resulting from core formation.

  11. Water in the Cratonic Mantle Lithosphere (United States)

    Peslier, A. H.


    The fact that Archean and Proterozoic cratons are underlain by the thickest (>200 km) lithosphere on Earth has always puzzled scientists because the dynamic convection of the surrounding asthenosphere would be expected to delaminate and erode these mantle lithospheric "keels" over time. Although density and temperature of the cratonic lithosphere certainly play a role in its strength and longevity, the role of water has only been recently addressed with data on actual mantle samples. Water in mantle lithologies (primarily peridotites and pyroxenites) is mainly stored in nominally anhydrous minerals (olivine, pyroxene, garnet) where it is incorporated as hydrogen bonded to structural oxygen in lattice defects. The property of hydrolytic weakening of olivine [4] has generated the hypothesis that olivine, the main mineral of the upper mantle, may be dehydrated in cratonic mantle lithospheres, contributing to its strength. This presentation will review the distribution of water concentrations in four cratonic lithospheres. The distribution of water contents in olivine from peridotite xenoliths found in kimberlites is different in each craton (Figure 1). The range of water contents of olivine, pyroxene and garnet at each xenolith location appears linked to local metasomatic events, some of which occurred later then the Archean and Proterozoic when these peridotites initially formed via melting. Although the low olivine water contents ( 6 GPa at the base of the Kaapvaal cratonic lithosphere may contribute to its strength, and prevent its delamination, the wide range of those from Siberian xenoliths is not compatible with providing a high enough viscosity contrast with the asthenophere. The water content in olivine inclusions from Siberian diamonds, on the other hand, have systematically low water contents (water contents. The olivine inclusions, however, may have been protected from metasomatism by their host diamond and record the overall low olivine water content of

  12. Water content in the Martian mantle: A Nakhla perspective (United States)

    Weis, Franz A.; Bellucci, Jeremy J.; Skogby, Henrik; Stalder, Roland; Nemchin, Alexander A.; Whitehouse, Martin J.


    Water contents of the Martian mantle have previously been investigated using Martian meteorites, with several comprehensive studies estimating the water content in the parental melts and mantle source regions of the shergottites and Chassigny. However, no detailed studies have been performed on the Nakhla meteorite. One possible way to determine the water content of a crystallizing melt is to use the water content in nominally anhydrous minerals (NAMs) such as clinopyroxene and olivine. During or after eruption on the surface of a planetary body and during residence in a degassing magma, these minerals may dehydrate. By reversing this process experimentally, original (pre-dehydration) water concentrations can be quantified. In this study, hydrothermal rehydration experiments were performed at 2 kbar and 700 °C on potentially dehydrated Nakhla clinopyroxene crystals. Rehydrated clinopyroxene crystals exhibit water contents of 130 ± 26 (2σ) ppm and are thus similar to values observed in similar phenocrysts from terrestrial basalts. Utilizing clinopyroxene/melt partition coefficients, both the water content of the Nakhla parent melt and mantle source region were estimated. Despite previous assumptions of a relatively dry melt, the basaltic magma crystallizing Nakhla may have had up to 1.42 ± 0.28 (2σ) wt.% H2O. Based on an assumed low degree of partial melting, this estimate can be used to calculate a minimum estimate of the water content for Nakhla's mantle source region of 72 ± 16 ppm. Combining this value with values determined for other SNC mantle sources, by alternative methods, gives an average mantle value of 102 ± 9 (2σ) ppm H2O for the Martian upper mantle throughout geologic time. This value is lower than the bulk water content of Earth's upper mantle (∼250 ppm H2O) but similar to Earth's MORB source (54-330 ppm, average ∼130 ppm H2O).

  13. The coupled 182W-142Nd record of early terrestrial mantle differentiation (United States)

    Puchtel, Igor S.; Blichert-Toft, Janne; Touboul, Mathieu; Horan, Mary F.; Walker, Richard J.


    New Sm-Nd, Lu-Hf, Hf-W, and Re-Os isotope data, in combination with highly siderophile element (HSE, including Re, Os, Ir, Ru, Pt, and Pd) and W abundances, are reported for the 3.55 Ga Schapenburg komatiites, South Africa. The Schapenburg komatiites define a Re-Os isochron with an age of 3550 ± 87 Ma and initial γ187Os = +3.7 ± 0.2 (2SD). The absolute HSE abundances in the mantle source of the Schapenburg komatiite system are estimated to be only 29 ± 5% of those in the present-day bulk silicate Earth (BSE). The komatiites were derived from mantle enriched in the decay products of the long-lived 147Sm and 176Lu nuclides (initial ɛ143Nd = +2.4 ± 0.1, ɛ176Hf = +5.7 ± 0.3, 2SD). By contrast, the komatiites are depleted, relative to the modern mantle, in 142Nd and 182W (μ182W = -8.4 ± 4.5, μ142Nd = -4.9 ± 2.8, 2SD). These results constitute the first observation in terrestrial rocks of coupled depletions in 142Nd and 182W. Such isotopic depletions require derivation of the komatiites from a mantle domain that formed within the first ˜30 Ma of Solar System history and was initially geochemically enriched in highly incompatible trace elements as a result of crystal-liquid fractionation in an early magma ocean. This mantle domain further must have experienced subsequent melt depletion, after 182Hf had gone extinct, to account for the observed initial excesses in 143Nd and 176Hf. The survival of early-formed 182W and 142Nd anomalies in the mantle until at least 3.55 Ga indicates that the products of early planetary differentiation survived both later planetary accretion and convective mantle mixing during the Hadean. This work moreover renders unlikely that variable late accretion, by itself, can account for all of the observed W isotope variations in Archean rocks.

  14. The mantle transition zone beneath the Afar Depression and adjacent regions: implications for mantle plumes and hydration (United States)

    Reed, C. A.; Gao, S. S.; Liu, K. H.; Yu, Y.


    The Afar Depression and its adjacent areas are underlain by an upper mantle marked by some of the world's largest negative velocity anomalies, which are frequently attributed to the thermal influences of a lower-mantle plume. In spite of numerous studies, however, the existence of a plume beneath the area remains enigmatic, partially due to inadequate quantities of broad-band seismic data and the limited vertical resolution at the mantle transition zone (MTZ) depth of the techniques employed by previous investigations. In this study, we use an unprecedented quantity (over 14 500) of P-to-S receiver functions (RFs) recorded by 139 stations from 12 networks to image the 410 and 660 km discontinuities and map the spatial variation of the thickness of the MTZ. Non-linear stacking of the RFs under a 1-D velocity model shows robust P-to-S conversions from both discontinuities, and their apparent depths indicate the presence of an upper-mantle low-velocity zone beneath the entire study area. The Afar Depression and the northern Main Ethiopian Rift are characterized by an apparent 40-60 km depression of both MTZ discontinuities and a normal MTZ thickness. The simplest and most probable interpretation of these observations is that the apparent depressions are solely caused by velocity perturbations in the upper mantle and not by deeper processes causing temperature or hydration anomalies within the MTZ. Thickening of the MTZ on the order of 15 km beneath the southern Arabian Plate, southern Red Sea and western Gulf of Aden, which comprise the southward extension of the Afro-Arabian Dome, could reflect long-term hydration of the MTZ. A 20 km thinning of the MTZ beneath the western Ethiopian Plateau is observed and interpreted as evidence for a possible mantle plume stem originating from the lower mantle.

  15. Conflicting Geophysical and Geochemical Indicators of Mantle Temperature Beneath Tibet (United States)

    Klemperer, S. L.


    In Tibet a small number of earthquakes occurs at 75-100 km depth, spanning the Moho, reaching >350 km and >550 km north of the Himalayan front in south-eastern Tibet and western Tibet respectively. 'Earthquake thermometry' implies these deep earthquakes occur in anhydrous lower lithosphere, either anorthitic or ultramafic, at 0.1RA (~1% mantle fluid) are conventionally taken to imply an unequivocal mantle component. Time-averaged upward flow rates calculated from measured 3He/4He ratios (R) and [4He] range from ~1-15 cm/a, implying transport times of 0.5-7 Ma through a 70-km thick crust. Discussion of 3He in Tibet in the western literature has been dominated by a single paper (Hoke et al., EPSL, 2000) that reported modest mantle helium (0.110% mantle fluids are reported 120 km and 150 km south of the northern limit of deep earthquakes in southeastern and western Tibet respectively. These hot springs apparently sampled mantle with T>800°C south of the locations where earthquake thermometry implies Moho temperatures India, Nepal and Pakistan, even though the 800°C isotherm is substantially shallower there than beneath southern Tibet? More plausibly the mantle helium is derived from an Asian mantle wedge above the region of deep earthquakes, in which case underthrusting Indian lithosphere is not intact, but breaks into an upper layer forming the lower crust of the Tibetan Plateau, and a lower seismogenic layer that is subducted more deeply into the mantle. Based on the geothermal springs, an Asian mantle wedge extended south of the Indus Tsangpo suture in SE Tibet and to the Karakoram fault in W Tibet until the latest Miocene, or even more recently.

  16. The Elephants' Graveyard: Constraints from Mantle Plumes on the Fate of Subducted Slabs and Implications for the Style of Mantle Convection (United States)

    Lassiter, J. C.


    The style of mantle convection (e.g., layered- vs. whole-mantle convection) is one of the most hotly contested questions in the Geological Sciences. Geochemical arguments for and against mantle layering have largely focused on mass-balance evidence for the existence of "hidden" geochemical reservoirs. However, the size and location of such reservoirs are largely unconstrained, and most geochemical arguments for mantle layering are consistent with a depleted mantle comprising most of the mantle mass and a comparatively small volume of enriched, hidden material either within D" or within seismically anomalous "piles" beneath southern Africa and the South Pacific. The mass flux associated with subduction of oceanic lithosphere is large and plate subduction is an efficient driver of convective mixing in the mantle. Therefore, the depth to which oceanic lithosphere descends into the mantle is effectively the depth of the upper mantle in any layered mantle model. Numerous geochemical studies provide convincing evidence that many mantle plumes contain material which at one point resided close to the Earth's surface (e.g., recycled oceanic crust ± sediments, possibly subduction-modified mantle wedge material). Fluid dynamic models further reveal that only the central cores of mantle plumes are involved in melt generation. The presence of recycled material in the sources of many ocean island basalts therefore cannot be explained by entrainment of this material during plume ascent, but requires that recycled material resides within or immediately above the thermo-chemical boundary layer(s) that generates mantle plumes. More recent Os- isotope studies of mantle xenoliths from OIB settings reveal the presence not only of recycled crust in mantle plumes, but also ancient melt-depleted harzburgite interpreted to represent ancient recycled oceanic lithosphere [1]. Thus, there is increasing evidence that subducted slabs accumulate in the boundary layer(s) that provide the source

  17. A perovskitic lower mantle inferred from high-pressure, high-temperature sound velocity data. (United States)

    Murakami, Motohiko; Ohishi, Yasuo; Hirao, Naohisa; Hirose, Kei


    The determination of the chemical composition of Earth's lower mantle is a long-standing challenge in earth science. Accurate knowledge of sound velocities in the lower-mantle minerals under relevant high-pressure, high-temperature conditions is essential in constraining the mineralogy and chemical composition using seismological observations, but previous acoustic measurements were limited to a range of low pressures and temperatures. Here we determine the shear-wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle using Brillouin scattering spectroscopy. The mineralogical model that provides the best fit to a global seismic velocity profile indicates that perovskite constitutes more than 93 per cent by volume of the lower mantle, which is a much higher proportion than that predicted by the conventional peridotitic mantle model. It suggests that the lower mantle is enriched in silicon relative to the upper mantle, which is consistent with the chondritic Earth model. Such chemical stratification implies layered-mantle convection with limited mass transport between the upper and the lower mantle.

  18. Origin and Distribution of Water Contents in Continental and Oceanic Lithospheric Mantle (United States)

    Peslier, Anne H.


    The water content distribution of the upper mantle will be reviewed as based on the peridotite record. The amount of water in cratonic xenoliths appears controlled by metasomatism while that of the oceanic mantle retains in part the signature of melting events. In both cases, the water distribution is heterogeneous both with depth and laterally, depending on localized water re-enrichments next to melt/fluid channels. The consequence of the water distribution on the rheology of the upper mantle and the location of the lithosphere-asthenosphere boundary will also be discussed.

  19. Mantle Xenoliths of Cerro Mercedes, Costa Rica, Central America (United States)

    Lindsay, F. N.; Carr, M. J.; Herzberg, C. T.; Feigenson, M. D.


    Mantle peridotite occurs as xenoliths in lavas and bombs at Cerro Mercedes, a Plio-Quaternary potassic alkaline basalt volcano approximately 70 km behind the volcanic front of northern Costa Rica (Tournon and Alvarado, 1997). Mineral exploration led to the first discovery of abundant mantle xenoliths in Central America (Vargas and Alfaro, 1992). The compositions of 71 xenoliths recovered in January 2003 include dunite, harzburgite, lherzolite and olivine websterite. Twenty xenoliths have a diameter of at least 3 cm. The nodules are abundant in basalt outcrops and the rare bombs. In spite of substantial soil development in a rain forest environment, both xenoliths and host lava remain well preserved. Olivine, pyroxenes and spinel are common, plagioclase is present and garnet appears to be absent. There is no obvious shearing or deformation and several pyroxenes are as much as 1 cm in diameter. The mineralogy suggests a relatively shallow upper mantle source, within either the lithosphere or possibly the uppermost asthenosphere. Cerro Mercedes, at latitude 10° 58' N and longitude 82° 21' W, lies along the Rio San Juan, which is locally the border between Nicaragua and Costa Rica, Central America. This location approximately coincides with a boundary between dominantly depleted mantle to the northwest and OIB or Galapagos-like mantle to the southeast. We will use mineralogical data to better define the likely depths and oxidation states of representative nodules and isotopic data to define the type of mantle source.

  20. Platinum-group elements for the mantle peridotites in the Dazhuka ophiolite, Tibet, China

    Institute of Scientific and Technical Information of China (English)


    The total PGE amounts of mantle peridotites in the Dazhuka ophiolite, Tibet, are 28.37-50.67 ng/g, slightly higher than those of mantle peridotites in the primitive mantle, and typical ophiolites in the world, and the Alps-type mantle peridotites. The PGE distribution patterns in the Dazhuka mantle peridotites are also different from those of the mantle peridotites of partial melting relict origin. The Dazhuka mantle peridotites have relatively high total PGE amounts and are enriched in Pt, Pd, and Ru. Their PGE distribution patterns belong to the positively inclined- or swallow-type patterns. The PGE distribution patterns in the mantle peridotites of partial melting relict origin belong to the negative-slope patterns or flat patterns. This reflects the unique features of the upper mantle in this region. Relative enrichment in Pt and Pd, as well as in the incompatible ele-ments Cu, Au, Cs, Rb, Ba, Th, U and LREE, indicates that the partial melting-derived relict mantle peridotites in the Dazhuka ophiolite had experienced intensive permeating and mixing processes of the melt and fluid both containing abundant incompatible elements.

  1. Occurrence of Tourmaline in Metasedimentary Rocks of the Isua Supracrustal Belt, Greenland: Implications for Ribose Stabilization in Hadean Marine Sediments (United States)

    Mishima, Shinpei; Ohtomo, Yoko; Kakegawa, Takeshi


    possible during the Hadean, providing a stabilization agent for ribose.

  2. Occurrence of Tourmaline in Metasedimentary Rocks of the Isua Supracrustal Belt, Greenland: Implications for Ribose Stabilization in Hadean Marine Sediments. (United States)

    Mishima, Shinpei; Ohtomo, Yoko; Kakegawa, Takeshi


    possible during the Hadean, providing a stabilization agent for ribose.

  3. Origin of geochemical mantle components: Role of subduction filter (United States)

    Kimura, Jun-Ichi; Gill, James B.; Skora, Susanne; van Keken, Peter E.; Kawabata, Hiroshi


    We quantitatively explore element redistribution at subduction zones using numerical mass balance models to evaluate the roles of the subduction zone filter in the Earth's geochemical cycle. Our models of slab residues after arc magma genesis differ from previous ones by being internally consistent with geodynamic models of modern arcs that successfully explain arc magma genesis and include element fluxes from the dehydration/melting of each underlying slab component. We assume that the mantle potential temperature (Tp) was 1400-1650°C at 3.5-1.7 Ga and gradually decreased to 1300-1350°C today. Hot subduction zones with Tp ˜1650°C have a thermal structure like modern SW Japan where high-Mg andesite is formed which is chemically like continental crust. After 2.5-1.7 Gyr of storage in the mantle, the residual igneous oceanic crust from hot subduction zones can evolve isotopically to the HIMU mantle component, the residual base of the mantle wedge to EMI, the residual sediment becomes an essential part of EMII, and the residual top of the mantle wedge can become the subcontinental lithosphere component. The Common or Focal Zone component is a stable mixture of the first three residues occasionally mixed with early depleted mantle. Slab residues that recycled earlier (˜2.5 Ga) form the DUPAL anomaly in the southern hemisphere, whereas residues of more recent recycling (˜1.7 Ga) underlie the northern hemisphere. These ages correspond to major continental crust forming events. The east-west heterogeneity of the depleted upper mantle involves subcontinental mantle except in the Pacific.

  4. Volatile cycling and the thermal evolution of planetary mantle (United States)

    Sandu, Constantin

    The thermal histories of terrestrial planets are investigated using two parameterized mantle convection models for either Earth like planets and planets with no active plate tectonics. Using parameterized models of mantle convection, we performed computer simulations of planetary cooling and volatile cycling. The models estimate the amount of volatile in mantle reservoir, and calculate the outgassing and regassing rates. A linear model of volatile concentration-dependent is assumed for the activation energy of the solid-state creep in the mantle. The kinematic viscosity of the mantle is thus dynamically affected by the activation energy through a variable concentration in volatile. Mantle temperature and heat flux is calculated using a model derived from classic thermal boundary layer theory of a single layered mantle with temperature dependent viscosity. The rate of volatile exchanged between mantle and surface is calculated by balancing the amount of volatiles degassed in the atmosphere by volcanic and spreading related processes and the amount of volatiles recycled back in the mantle by the subduction process. In the cases that lack plate tectonics, the degassing efficiency is dramatically reduced and the regassing process is absent. The degassing effect is dependent on average spreading rate of tectonic plates and on the amount of volatile in the melt extract in the transition zone between mantle and upper boundary laver. The regassing effect is dependent on the subduction rate and on the amount of volatile present on a hydrated layer on top of the subducting slab. The degassing and regassing parameters are all related to the intensity of the convection in the mantle and to the surface temperature of the planet, and they are regulated by the amount of volatiles in reservoir. Comparative study with the previous models display significant differences and improve the versatility of the model. The optimum efficiency factors found are in the range of 0.01--0.06 for

  5. Mantle hydrocarbons: abiotic or biotic? (United States)

    Sugisaki, R; Mimura, K


    Analyses of 227 rocks from fifty localities throughout the world showed that mantle derived rocks such as tectonized peridotites in ophiolite sequences (tectonites) arid peridotite xenoliths in alkali basalts contain heavier hydrocarbons (n-alkanes), whereas igneous rocks produced by magmas such as gabbro arid granite lack them. The occurrence of hydrocarbons indicates that they were not derived either from laboratory contamination or from held contamination; these compounds found in the mantle-derived rocks are called here "mantle hydrocarbons." The existence of hydrocarbons correlates with petrogenesis. For example, peridotite cumulates produced by magmatic differentiation lack hydrocarbons whereas peridotite xenoliths derived from the mantle contain them. Gas chromatographic-mass spectrometric records of the mantle hydrocarbons resemble those of aliphatics in meteorites and in petroleum. Features of the hydrocarbons are that (a) the mantle hydrocarbons reside mainly along grain boundaries and in fluid inclusions of minerals; (b) heavier isoprenoids such as pristane and phytane are present; and (c) delta 13C of the mantle hydrocarbons is uniform (about -27%). Possible origins for the mantle hydrocarbons are as follows. (1) They were in organically synthesized by Fischer-Tropsch type reaction in the mantle. (2) They were delivered by meteorites and comets to the early Earth. (3) They were recycled by subduction. The mantle hydrocarbons in the cases of (1) and (2) are abiogenic and those in (3) are mainly biogenic. It appears that hydrocarbons may survive high pressures and temperatures in the mantle, but they are decomposed into lighter hydrocarbon gases such as CH4 at lower pressures when magmas intrude into the crust; consequently, peridotite cumulates do not contain heavier hydrocarbons but possess hydrocarbon gases up to C4H10.

  6. Electrical Structure and Fault Features of Crust and Upper Mantle beneath the Western Margin of the Qinghai-Tibet Plateau: Evidence from the Magnetotelluric Survey along Zhada-Quanshui Lake Profile

    Institute of Scientific and Technical Information of China (English)

    Jin Sheng; Ye Gaofeng; Wei Wenbo; Deng Ming; Jing Jian'en


    The magnetotelluric (MT) survey along the Zhada (札达)-Quanshui (泉水) Lake profile on the western margin of the Qinghai (青海)-Tibet plateau shows that the study area is divided into three tectonic provinces by the Yalung Tsangpo and Bangong (班公)-Nujiang (怒江) sutures. From south to north these are the Himalayan terrane, Gangdise terrane, and Qiangtang (羌塘) terrane. For the study area, there are widespread high-conductivity layers in the mid and lower crust, the top layers of which fluctuate intensively. The high-conductivity layer within the Gangdise terrane is deeper than those within the Qiangtang terrane and the Himalaya terrane, and the deepest high-conductivity layer is to the south of the Bangong-Nujiang suture. The top surface of the high-conductivity layer in the south of the Bangong-Nujiang suture is about 20 km lower than that in the north of it. The high-conductivity layer within the Gangdise terrane dips toward north and there are two high-conductivity layers within the crust of the southern Qiangtang terrane. In the upper crust along the profile, there are groups of lateral electrical gradient zones or distortion zones of different scales and occurrence indicating the distribution of faults and sutures along the profile. According to the electrical structure, the structural characteristics and space distribution of the Yalung Tsangpo suture,Bangong-Nujiang suture, and the major faults of Longmucuo (龙木错) and Geerzangbu are inferred.

  7. Tracing Mantle Plumes: Quantifying their Morphology and Behavior from Seismic Tomography (United States)

    O'Farrell, K. A.; Eakin, C. M.; Jones, T. D.; Garcia, E.; Robson, A.; Mittal, T.; Lithgow-Bertelloni, C. R.; Jackson, M. G.; Lekic, V.; Rudolph, M. L.


    Hotspot volcanism provides a direct link between the deep mantle and the surface, but the location, depth and source of the mantle plumes that feed hotspots are highly controversial. In order to address this issue it is important to understand the journey along which plumes have travelled through the mantle. The general behavior of plumes in the mantle also has the potential to tell us about the vigor of mantle convection, net rotation of the mantle, the role of thermal versus chemical anomalies, and important bulk physical properties of the mantle such as the viscosity profile. To address these questions we developed an algorithm to trace plume-like features in shear-wave (Vs) seismic tomographic models based on picking local minima in velocity and searching for continuous features with depth. We apply this method to several of the latest tomographic models and can recover 30 or more continuous plume conduits that are >750 km long. Around half of these can be associated with a known hotspot at the surface. We study the morphology of these plume chains and find that the largest lateral deflections occur near the base of the lower mantle and in the upper mantle. We analyze the preferred orientation of the plume deflections and their gradient to infer large scale mantle flow patterns and the depth of viscosity contrasts in the mantle respectively. We also retrieve Vs profiles for our traced plumes and compare with velocity profiles predicted for different mantle adiabat temperatures. We use this to constrain the thermal anomaly associated with these plumes. This thermal anomaly is then converted to a density anomaly and an upwelling velocity is derived. We compare this to buoyancy fluxes calculated at the surface and use this in conjunction with our measured plume tilts/deflections to estimate the strength of the "mantle wind".

  8. Intraplate volcanism and mantle dynamics in East Asia: Big mantle wedge (BMW) model (Invited) (United States)

    Zhao, D.


    In the East Asia continent there are many Cenozoic volcanoes, but only a few are still active now, such as the Changbai, Wudalianchi, and Tengchong volcanoes which have erupted several times in the past 1000 years. Although many studies have been made by using various approaches, the origin of the intraplate volcanoes in East Asia is still not very clear. Recently we used regional and global seismic tomography to determine high-resolution 3-D mantle structure under Western Pacific to East Asia (Zhao, 2004; Huang and Zhao, 2006; Zhao et al., 2009). Our results show prominent low-velocity anomalies from the surface down to 410 km depth beneath the intraplate volcanoes and a broad high-velocity anomaly in the mantle transition zone under East Asia. Focal-mechanism solutions of deep earthquakes indicate that the subducting Pacific slab under the Japan Sea and the East Asia margin is subject to compressive stress regime. These results suggest that the Pacific slab meets strong resistance at the 660-km discontinuity and so it becomes stagnant in the mantle transition zone under East Asia. The Philippine Sea slab has also subducted down to the mantle transition zone under western Japan and the Ryukyu back-arc region. The western edge of the stagnant slab is generally parallel with the Japan trench and the Ryukyu trench and roughly coincides with a prominent surface topography and gravity boundary in East China, which is located approximately 1800 km west of the trenches. The upper mantle under East Asia has formed a big mantle wedge (BMW) above the stagnant slab. The BMW exhibits low seismic-velocity and high electrical-conductivity, which is hot and wet because of the deep dehydration reactions of the stagnant slab and the convective circulation process in the BMW. These processes lead to the upwelling of hot and wet asthenospheric materials and thinning and fracturing of the continental lithosphere, leading to the formation of the active intraplate volcanoes in East

  9. Abiotic ammonium formation in the presence of Ni-Fe metals and alloys and its implications for the Hadean nitrogen cycle

    Directory of Open Access Journals (Sweden)

    Strongin Daniel R


    Full Text Available Abstract Experiments with dinitrogen-, nitrite-, nitrate-containing solutions were conducted without headspace in Ti reactors (200°C, borosilicate septum bottles (70°C and HDPE tubes (22°C in the presence of Fe and Ni metal, awaruite (Ni80Fe20 and tetrataenite (Ni50Fe50. In general, metals used in this investigation were more reactive than alloys toward all investigated nitrogen species. Nitrite and nitrate were converted to ammonium more rapidly than dinitrogen, and the reduction process had a strong temperature dependence. We concluded from our experimental observations that Hadean submarine hydrothermal systems could have supplied significant quantities of ammonium for reactions that are generally associated with prebiotic synthesis, especially in localized environments. Several natural meteorites (octahedrites were found to contain up to 22 ppm Ntot. While the oxidation state of N in the octahedrites was not determined, XPS analysis of metals and alloys used in the study shows that N is likely present as nitride (N3-. This observation may have implications toward the Hadean environment, since, terrestrial (e.g., oceanic ammonium production may have been supplemented by reduced nitrogen delivered by metal-rich meteorites. This notion is based on the fact that nitrogen dissolves into metallic melts.

  10. Ca isotope fingerprints of early crust-mantle evolution (United States)

    Kreissig, K.; Elliott, T.


    isotope ratios in some mafic samples reflect anomalous K/Ca ratios as a result of intense K-metasomatism ˜3.6 Ga. Thus Ca isotope measurements are not a robust tracer of crustal growth in the presence of intense tectono-metamorphic processes. Coupled with other isotope data, however, the degree of overprint can be estimated and the 40Ca/ 44Ca ratio of a little disturbed sample hints at a small contribution of Hadean protocrust in the coastal part of the Godthåbsfjord area (Southwest Greenland). In the majority of Zimbabwe TTG samples, unradiogenic initial Ca isotope ratios point to very little prior crustal history and minor subsequent disturbance. We thus infer that the modest initial ɛ Nd ˜0.8 of the Zimbabwean samples is representative of the depleted mantle at ˜3.6 Ga. Furthermore, Ca isotope systematics provide little support for a "steady state" model of crustal growth.

  11. An olivine-free mantle source of Hawaiian shield basalts. (United States)

    Sobolev, Alexander V; Hofmann, Albrecht W; Sobolev, Stephan V; Nikogosian, Igor K


    More than 50 per cent of the Earth's upper mantle consists of olivine and it is generally thought that mantle-derived melts are generated in equilibrium with this mineral. Here, however, we show that the unusually high nickel and silicon contents of most parental Hawaiian magmas are inconsistent with a deep olivine-bearing source, because this mineral together with pyroxene buffers both nickel and silicon at lower levels. This can be resolved if the olivine of the mantle peridotite is consumed by reaction with melts derived from recycled oceanic crust, to form a secondary pyroxenitic source. Our modelling shows that more than half of Hawaiian magmas formed during the past 1 Myr came from this source. In addition, we estimate that the proportion of recycled (oceanic) crust varies from 30 per cent near the plume centre to insignificant levels at the plume edge. These results are also consistent with volcano volumes, magma volume flux and seismological observations.

  12. Upper Endoscopy

    Medline Plus

    Full Text Available ... Clinical Topics / Procedures F - Z / Upper Endoscopy (EGD) Upper Endoscopy (EGD) The Latest Practice Guidelines Technology Reviews ... the Safety of Your Endoscopic Procedure Brochure Understanding Upper Endoscopy Brochure Make the Best Choice for Your ...

  13. Seismic Constraints on the Mantle Viscosity Structure beneath Antarctica (United States)

    Wiens, Douglas; Heeszel, David; Aster, Richard; Nyblade, Andrew; Wilson, Terry


    Lateral variations in upper mantle viscosity structure can have first order effects on glacial isostatic adjustment. These variations are expected to be particularly large for the Antarctic continent because of the stark geological contrast between ancient cratonic and recent tectonically active terrains in East and West Antarctica, respectively. A large misfit between observed and predicted GPS rates for West Antarctica probably results in part from the use of a laterally uniform viscosity structure. Although not linked by a simple relationship, mantle seismic velocities can provide important constraints on mantle viscosity structure, as they are both largely controlled by temperature and water content. Recent higher resolution seismic models for the Antarctic mantle, derived from data acquired by new seismic stations deployed in the AGAP/GAMSEIS and ANET/POLENET projects, offer the opportunity to use the seismic velocity structure to place new constraints on the viscosity of the Antarctic upper mantle. We use an Antarctic shear wave velocity model derived from array analysis of Rayleigh wave phase velocities [Heeszel et al, in prep] and examine a variety of methodologies for relating seismic, thermal and rheological parameters to compute a suite of viscosity models for the Antarctic mantle. A wide variety of viscosity structures can be derived using various assumptions, but they share several robust common elements. There is a viscosity contrast of at least two orders of magnitude between East and West Antarctica at depths of 80-250 km, reflecting the boundary between cold cratonic lithosphere in East Antarctica and warm upper mantle in West Antarctica. The region beneath the Ellsworth-Whitmore Mtns and extending to the Pensacola Mtns. shows intermediate viscosity between the extremes of East and West Antarctica. There are also significant variations between different parts of West Antarctica, with the lowest viscosity occurring beneath the Marie Byrd Land (MBL

  14. The Acasta Gneisses revisited: Evidence for an early depleted mantle (United States)

    Scherer, E. E.; Sprung, P.; Bleeker, W.; Mezger, K.


    The oldest known mineral samples crystallized on the Earth are the up to 4.4 Ga zircon grains from the Jack Hills, Australia [e.g., 1,2]. Zircon, which is datable by U-Pb, contains ca. 1 wt% Hf, and has very low Lu/Hf, is well suited to recording the initial 176Hf/177Hf of its parent magma. It has therefore been widely used to track Earth’s crust-mantle differentiation over time and to estimate the relative amounts of juvenile and recycled components that contributed to Archean and Hadean crust. [e.g., 3,4,5,6]. Zircon studies may be subject to sampling bias, however: Juvenile mafic magmas are likely to stem from depleted sources, but are less likely to crystallize zircon. Processes such as host-rock metamorphism, remelting, weathering of the host rock, and sedimentary transport of grains may have further biased the zircon population. Metamict grains or those with high aspect ratios are likely to be destroyed by these processes, potentially biasing the zircon Hf record toward enriched compositions such that the degree of mantle depletion remains poorly defined before 4 Ga. In addition, incorrect age assignments to Hf analyses result in spurious initial ɛHf values. Here, we attempt to overcome these issues by investigating the bulk rock Lu-Hf and Sm-Nd systematics of some of the oldest rocks on Earth, the Acasta Gneisses (Northwest Territories, Canada). Earlier studies showed that zircon grains in these gneisses tend to come from enriched sources [e.g, 3,7,8] and are thus of little use for directly tracking the degree of mantle depletion. Furthermore, the gneisses themselves have been multiply metamorphosed and are often affected by mixing: The banded gneisses in particular comprise several magmatic precursor rocks of different age that have been repeatedly folded into each other. This promted questions of whether zircon ages should be used in the calculation of bulk rock initial epsilon Nd, and whether linear trends on Sm-Nd isochron represented meaningful

  15. Radial profiles of temperature and viscosity in the Earth's mantle inferred from the geoid and lateral seismic structure

    NARCIS (Netherlands)

    Cadek, O.; Berg, A.P. van den


    In the framework of dynamical modelling of the geoid, we have estimated basic features of the radial profile of temperature in the mantle. The applied parameterization of the geotherm directly characterizes thermal boundary layers and values of the thermal gradient in the upper and lower mantle.

  16. Reconciling laboratory and observational models of mantle rheology in geodynamic modelling (United States)

    King, Scott D.


    Experimental and geophysical observations constraining mantle rheology are reviewed with an emphasis on their impact on mantle geodynamic modelling. For olivine, the most studied and best-constrained mantle mineral, the tradeoffs associated with the uncertainties in the activation energy, activation volume, grain-size and water content allow the construction of upper mantle rheology models ranging from nearly uniform with depth to linearly increasing from the base of the lithosphere to the top of the transition zone. Radial rheology models derived from geophysical observations allow for either a weak upper mantle or a weak transition zone. Experimental constraints show that wadsleyite and ringwoodite are stronger than olivine at the top of the transition zone; however the uncertainty in the concentration of water in the transition zone precludes ruling out a weak transition zone. Both observational and experimental constraints allow for strong or weak slabs and the most promising constraints on slab rheology may come from comparing inferred slab geometry from seismic tomography with systematic studies of slab morphology from dynamic models. Experimental constraints on perovskite and ferropericlase strength are consistent with general feature of rheology models derived from geophysical observations and suggest that the increase in viscosity through the top of the upper mantle could be due to the increase in the strength of ferropericlase from 20-65 GPa. The decrease in viscosity in the bottom half of the lower mantle could be the result of approaching the melting temperature of perovskite. Both lines of research are consistent with a high-viscosity lithosphere, a low viscosity either in the upper mantle or transition zone, and high viscosity in the lower mantle, increasing through the upper half of the lower mantle and decreasing in the bottom half of the lower mantle, with a low viscosity above the core. Significant regions of the mantle, including high

  17. Trench migration, net rotation and slab mantle coupling (United States)

    Funiciello, F.; Faccenna, C.; Heuret, A.; Lallemand, S.; Di Giuseppe, E.; Becker, T. W.


    Laboratory models have been conducted to improve our understanding of the role that the resistance of the slab to bending and its coupling to the ambient mantle play in subduction dynamics over geological time scales. Our models are set up with a viscous plate of silicone (lithosphere) subducting under negative buoyancy in a viscous layer of glucose syrup (mantle). For our study, the lithosphere/upper mantle viscosity contrast has been systematically varied, from ~ 10 to ~ 10 5 in order to explore the parameter space between weak and strong slab dynamics. We found that subduction is characterized by a retreating mode for viscosity ratios > 10 4, by the coexistence of a retreating mode and an advancing mode for viscosity ratios between ~ 10 4 and ~ 10 2, and quasi-stationary, Rayleigh-Taylor like behaviour for ratios < 10 2. By combining our experimental results and kinematic data from current subduction zones in four reference frames which differ in the amount of net rotation, we infer that a lithosphere/upper mantle viscosity contrast of 150-500 is necessary to obtain realistic trench/subducting plate velocity ratios as well as the variability of subduction styles observed in nature.

  18. Nd-isotopes in selected mantle-derived rocks and minerals and their implications for mantle evolution (United States)

    Basu, A.R.; Tatsumoto, M.


    The Sm-Nd systematics in a variety of mantle-derived samples including kimberlites, alnoite, carbonatite, pyroxene and amphibole inclusions in alkali basalts and xenolithic eclogites, granulites and a pyroxene megacryst in kimberlites are reported. The additional data on kimberlites strengthen our earlier conclusion that kimberlites are derived from a relatively undifferentiated chondritic mantle source. This conclusion is based on the observation that the e{open}Nd values of most of the kimberlites are near zero. In contrast with the kimberlites, their garnet lherzolite inclusions show both time-averaged Nd enrichment and depletion with respect to Sm. Separated clinopyroxenes in eclogite xenoliths from the Roberts Victor kimberlite pipe show both positive and negative e{open}Nd values suggesting different genetic history. A whole rock lower crustal scapolite granulite xenolith from the Matsoku kimberlite pipe shows a negative e{open}Nd value of -4.2, possibly representative of the base of the crust in Lesotho. It appears that all inclusions, mafic and ultramafic, in kimberlites are unrelated to their kimberlite host. The above data and additional Sm-Nd data on xenoliths in alkali basalts, alpine peridotite and alnoite-carbonatites are used to construct a model for the upper 200 km of the earth's mantle - both oceanic and continental. The essential feature of this model is the increasing degree of fertility of the mantle with depth. The kimberlite's source at depths below 200 km in the subcontinental mantle is the most primitive in this model, and this primitive layer is also extended to the suboceanic mantle. However, it is clear from the Nd-isotopic data in the xenoliths of the continental kimberlites that above 200 km the continental mantle is distinctly different from their suboceanic counterpart. ?? 1980 Springer-Verlag.

  19. The upper to uppermost Cenomanian oceanic anoxic event: a review and an interpretation involving a seawater stratification by the CO{sub 2} of mantle origin; L`evenement oceanique anoxique du Cenomanien superieur-terminal: une revue et une interpretation mettant en jeu une stratification des eaux marines par le CO{sub 2} mantellique

    Energy Technology Data Exchange (ETDEWEB)

    Busson, G.; Cornee, A. [Laboratoire de Geologie du Museum, 75 - Paris (France)


    Oil exploration data have revealed the exceptional richness of the middle Cretaceous in source rocks worldwide. Oceanic drillings have shown the existence of oceanic anoxic events (OAE) well defined in time. This study analyzes the OAE 2 event dated from the upper Cenomanian-lower Turonian. This event has been recognized in numerous sites from the northern, central and southern Atlantic and punctually in the Pacific and Indian oceans. It occurs in both numerous deep oceanic sites and orogenic zones, and stable platforms covered by epeiric seas. It coincides with a sea level rise which is one of the most sudden and highest in Phanerozoic times and it stands out as a remarkable episode of massive faunal extinction which led to the deposition of organic matter of marine planktonic dominant nature. The first part of the study recalls the previous interpretations of this event (oceanic stratification, euxinic conditions, spreading of an oxygen minimum zone, greenhouse climate effect, sluggish atmospheric and oceanic circulations, high planktonic production, great oceanic overturns, marginal or general upwellings, marine transgressions on epeiric areas etc..). The second part gives the basis of the new hypothesis: connection between separated seas due to the transgression, retreat of evaporite facies, high sea-floor spreading rates, intense volcanic activity and high mantle outgassing with huge CO{sub 2} influxes. The last part describes the proposed interpretation: CO{sub 2} accumulation in deep and intermediate waters and sea overflows on marginal and continental areas which led to a rise of the CO{sub 2}-rich hypolimnion. (J.S.) 236 refs.

  20. Simple scaling relations in geodynamics:the role of pressure in mantle convection and plume formation

    Institute of Scientific and Technical Information of China (English)

    Don L. Anderson


    Scaling relations are important in extrapolating laboratory experiments to the Earth's mantle. In planetary interiors, compression becomes an important parameter and it is useful to explore scalings that involve volume. I use simple volume scaling relations that allow one to extrapolate laboratory experiments and upper mantle behavior, in a thermodynamically self-consistent way, to predict lower mantle behavior. The relations are similar to the quasi- harmonic approximation. Slabs and plates have characteristic dimensions of hundreds of kilometers and time constants of 100 million years, but the volume scalings predict order of magnitude higher values in the deep mantle. The scaling relations imply that the deep mantle is a sluggish system with ancient features. They imply irreversible chemical stratification and do not favor the plume hypothesis.

  1. Experiments on melt-rock reaction in the shallow mantle wedge (United States)

    Mitchell, Alexandra L.; Grove, Timothy L.


    This experimental study simulates the interaction of hotter, deeper hydrous mantle melts with shallower, cooler depleted mantle, a process that is expected to occur in the upper part of the mantle wedge. Hydrous reaction experiments ( 6 wt% H2O in the melt) were conducted on three different ratios of a 1.6 GPa mantle melt and an overlying 1.2 GPa harzburgite from 1060 to 1260 °C. Reaction coefficients were calculated for each experiment to determine the effect of temperature and starting bulk composition on final melt compositions and crystallizing assemblages. The experiments used to construct the melt-wall rock model closely approached equilibrium and experienced phase equilibria, melt compositions, and reaction coefficients provide a framework for understanding how melt-wall rock reaction occurs in the natural system during melt ascent in the mantle wedge.

  2. The dynamics of plate tectonics and mantle flow: from local to global scales. (United States)

    Stadler, Georg; Gurnis, Michael; Burstedde, Carsten; Wilcox, Lucas C; Alisic, Laura; Ghattas, Omar


    Plate tectonics is regulated by driving and resisting forces concentrated at plate boundaries, but observationally constrained high-resolution models of global mantle flow remain a computational challenge. We capitalized on advances in adaptive mesh refinement algorithms on parallel computers to simulate global mantle flow by incorporating plate motions, with individual plate margins resolved down to a scale of 1 kilometer. Back-arc extension and slab rollback are emergent consequences of slab descent in the upper mantle. Cold thermal anomalies within the lower mantle couple into oceanic plates through narrow high-viscosity slabs, altering the velocity of oceanic plates. Viscous dissipation within the bending lithosphere at trenches amounts to approximately 5 to 20% of the total dissipation through the entire lithosphere and mantle.

  3. Postcollisional mantle-derived magmatism, underplating and implications for basement of the Junggar Basin

    Institute of Scientific and Technical Information of China (English)

    韩宝福; 何国琦; 王式洸


    The late Paleozoic postcollisional granitoids, mafic-ultramafic complexes, and volcanic rocks are extensively distributed around the Junggar Basin; they are generally characterized by positive εNd(t) values, implying that the magmas were mantle-derived and contaminated with crustal materials to some extents. The emplacement of mantle-derived magmas and their differentiates in the upper crust is the expression of deep geological processes at shallow level, while much more mantle-derived magmas were underplated in the lower crust and the region near the crust-mantle boundary, being component part of basement of the Junggar Basin. The postcollisional mafic-ultramafic complexes would not be generated by re-melting of residual oceanic crust, which was considered as the basement of the Junggar Basin, unless very high degrees of partial melting occurred. Even if old continental crust had been present before collision, it would have been strongly modified by the mantle-derived magma underplating. This inter

  4. Geoelectromagnetic investigation of the earth’s crust and mantle

    CERN Document Server

    Rokityansky, Igor I


    Electrical conductivity is a parameter which characterizes composition and physical state of the Earth's interior. Studies of the state equations of solids at high temperature and pressure indicate that there is a close relation be­ tween the electrical conductivity of rocks and temperature. Therefore, measurements of deep conductivity can provide knowledge of the present state and temperature of the Earth's crust and upper mantle matter. Infor­ mation about the temperature of the Earth's interior in the remote past is derived from heat flow data. Experimental investigation of water-containing rocks has revealed a pronounced increase of electrical conductivity in the temperature range D from 500 to 700 DC which may be attributed to the beginning of fractional melting. Hence, anomalies of electrical conductivity may be helpful in identitying zones of melting and dehydration. The studies of these zones are perspective in the scientific research of the mobile areas of the Earth's crust and upper mantle where t...

  5. Subducted slabs and the geoid: Constraints on mantle rheology and flow (United States)

    Hager, B. H.


    The total geoid anomaly which is the result of a given density contrast in a convecting viscous earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries, as well as by the density contrast itself is discussed. If the internal density contrasts can be estimated, the depth and variation of viscosity with depth of the convecting system can be constrained. The observed long wavelength geoid is highly correlated with that predicted by a density model for seismically active subducted slabs. The amplitude of the correlation is explained if the density contrasts associated with subduction extend into the lower mantle or if subducted slabs exceeding 350 km in thickness are piled up over horizontal distances of thousands of km at the base of the upper mantle. Mantle wide convection in a mantle that has a viscosity increasing with depth provides the explanation of the long-wavelength geoid anomalies over subduction zones.

  6. Mantle Structure Beneath Central South America (United States)

    Vandecar, J. C.; Silver, P. G.; James, D. E.; Assumpcao, M.; Schimmel, M.; Zandt, G.


    Making use of 60 digital broadband seismic stations that have operated across central South America in recent years, we have undertaken an inversion for the upper- and uppermost lower-mantle P- and S-wave velocity structures beneath the region. We have combined data from four portable PASSCAL-type experiments as well as the 3 GTSN permanent stations (LPAZ, BDFB and CPUP) and 1 Geoscope station (SPB) located in the region. The portable data were deployed at various times between 1992 and 1999 and include: 28 sites from the Brazilian Lithosphere Seismic Project (BLSP: Carnegie Institution of Washington and Universidade de Sao Paulo), 16 sites from the Broadband ANdean JOint experiment (BANJO: Carnegie Institution of Washington and University of Arizona), 8 sites from the Seismic Exploration of the Deep Altiplano project (SEDA: Lawrence Livermore National Laboratory) and 4 sites from the University of Brasilia. The P- and S-wave relative delay times are independently obtained via a multi-channel cross correlation of band-passed waveforms for each teleseismic event. These data are then inverted using an iterative, robust, non-linear scheme which parameterizes the 3-D velocity variations as splines under tension constrained at over 120,000 nodes across South America between latitudes of 15 and 30 degrees South. Amongst other features, we robustly image the high-velocity subducting Nazca plate penetrating into the lower mantle and the high-velocity root of the ~3.2 Gyr old Sao Francisco Craton extending to depths of 200-300 km. We will discuss the consistency between our tomographic models and predictions of dynamic mantle models based on plate tectonic reconstructions of subduction.

  7. Trans-Pacific whole mantle structure (United States)

    Liu, Lijun; Tan, Ying; Sun, Daoyuan; Chen, Min; Helmberger, Don


    Recent reports on modeling USArray data reveal mostly vertical microplates with little resemblance to preliminary reference Earth model (PREM). Such complexity at plate boundaries makes it difficult to form reliable images of ocean basins using global paths. Here, we report on modeling stacked seismograms obtained from the first broadband array (TriNet) situated on the edge of the Pacific Plate, southern California, with no major subduction zone blocking its view. Extended records, including multi-S and ScS waves up to four bounces from 18 Tonga-Fiji deep events (140 to 620 km) are analyzed to check the validity of existing models and derive the whole mantle shear velocity structure along this corridor. Synthetics generated from 3-D tomographic models do not fit the upper mantle triplication data or the mantle reverberations associated with the ScS multiples as well as the 1-D model PAC06. We construct a hybrid model (HPAC), which remains one dimensional down to 800 km (PAC06). The lower portion of HPAC is essentially the tomography model S20RTS with velocity variation inflated by a factor of 2 for the lowermost 600 km. Thus, the mid-Pacific large low shear velocity province (LLSVP) has a lower shear velocity of about 2% relative to PREM and extends into the midmantle, similar to that beneath South Africa. Moreover, rapid changes in the differential (ScS-S) and (ScS2-S) times as a function of distance suggest ultra low velocity zones near the eastern edge and under the LLSVP, again similar to that found beneath Africa.

  8. Pyroxenes as tracers of mantle water variations (United States)

    Warren, Jessica M.; Hauri, Erik H.


    The concentration and distribution of volatiles in the Earth's mantle influence properties such as melting temperature, conductivity, and viscosity. To constrain upper mantle water content, concentrations of H2O, P, and F were measured in olivine, orthopyroxene, and clinopyroxene in mantle peridotites by secondary ion mass spectrometry. Analyzed peridotites are xenoliths (Pali Aike, Spitsbergen, Samoa), orogenic peridotites (Josephine Peridotite), and abyssal peridotites (Gakkel Ridge, Southwest Indian Ridge, Tonga Trench). The comparison of fresh and altered peridotites demonstrates that low to moderate levels of alteration do not affect H2O concentrations, in agreement with mineral diffusion data. Olivines have diffusively lost water during emplacement, as demonstrated by disequilibrium between olivine and coexisting pyroxenes. In contrast, clinopyroxene and orthopyroxene preserve their high-temperature water contents, and their partitioning agrees with published experiments and other xenoliths. Hence, olivine water concentrations can be determined from pyroxene concentrations using mineral-mineral partition coefficients. Clinopyroxenes have 60-670 ppm H2O, while orthopyroxenes have 10-300 ppm, which gives calculated olivine concentrations of 8-34 ppm. The highest olivine water concentration translates to an effective viscosity of 6 × 1019 Pa s at 1250°C and ~15 km depth, compared to a dry effective viscosity of 2.5 × 1021 Pa s. Bulk rock water concentrations, calculated using mineral modes, are 20-220 ppm and correlate with peridotite indices of melt depletion. However, trace element melt modeling indicates that peridotites have too much water relative to their rare earth element concentrations, which may be explained by late-stage melt addition, during which only hydrogen diffuses fast enough for reequilibration.

  9. Compositional Evolution of the Mantle (United States)

    Bennett, V. C.


    The mantle is the Earth's largest chemical reservoir comprising 82% of its total volume and 65% of its mass. The mantle constitutes almost all of the silicate Earth, extending from the base of the crust (which comprises only 0.6% of the silicate mass) to the top of the metallic core at 2,900 km depth. The chemical compositions of direct mantle samples such as abyssal peridotites (Chapter 2.04) and peridotite xenoliths (Chapter 2.05), and of indirect probes of the mantle such as basalts from mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) (Chapter 2.03), and some types of primitive granites, tell us about the compositional state of the modern mantle, with ever increasingly detailed information providing strong evidence for chemical complexity and heterogeneity at all scales (Chapter 2.03). This chemical heterogeneity must reflect the complex physical interplay of a number of distinct long-lived geochemical reservoirs that are identified primarily by their radiogenic isotopic compositions.Many of the chapters in this volume provide detailed images of the current chemical and physical state of the Earth's mantle, whereas other contributions examine the starting composition for the Earth (Chapter 2.01). This chapter attempts to link these two areas by tracking the composition of the mantle through time. The first part of this chapter is a summary of the empirical evidence for secular change in the chemical composition of the mantle from the formation of the Earth at 4.56 Ga throughto the present day. The emphasis is on results from the long-lived radiogenic isotopic systems, in particular 147Sm-143Nd, 176Lu-176Hf, 87Rb-87Sr, and 187Re-187Os systems as these isotopic data provide some of the best constraints on the composition of the mantle in the first half of Earth history, and the timing and extent of chemical differentiation that has affected the mantle over geologic time. Selected trace element data and the "short-lived" 146Sm-142Nd isotopic systems

  10. Electrical conductivity of (Mg,Fe)SiO3 Perovskite and a Perovskite-dominated assemblage at lower mantle conditions (United States)

    Li, Xiaoyuan; Jeanloz, Raymond


    Electrical conductivity measurements of Perovskite and a Perovskite-dominated assemblage synthesized from pyroxene and olivine demonstrate that these high-pressure phases are insulating to pressures of 82 GPa and temperatures of 4500 K. Assuming an anhydrous upper mantle composition, the result provides an upper bound of 0.01 S/m for the electrical conductivity of the lower mantle between depths of 700 and 1900 km. This is 2 to 4 orders of magnitude lower than previous estimates of lower-mantle conductivity derived from studies of geomagnetic secular variations.

  11. Mapping small-scale mantle heterogeneities using seismic arrays (United States)

    Bentham, H. L.; Rost, S.


    In recent years array seismology has been used extensively to detect and locate the small scale (~10 km) structure of the Earth. In the mantle, small scale structure likely represents chemical heterogeneity and is essential in our understanding of mechanical mixing processes within mantle convection. As subducted crust is chemically distinct from the background mantle, imaging the remains of the crust provides a tracer for convectional flow. Evidence for heterogeneities has been found in the lower mantle in previous seismology studies but the arrivals associated with such heterogeneities are difficult to detect in the seismic data as they are typically low amplitude and are often masked by a multitude of larger amplitude arrivals. In this study we find global and regional seismic heterogeneities in the mantle by processing teleseismic earthquake data through array seismology methods. We find global patterns of heterogeneity using a stacking approach. To locate regional heterogeneities, we target the "quiet" window prior to the PP arrival for earthquakes with epicentral distances of 90-110°. Within this time window, we enhance the weak coherent energy that arrives off great circle path by calculating the observed directivity (slowness and backazimuth) and using a semblance weighted beampower measure. We use the directivity and travel times of suitable precursors to back-trace the energy to the origin of P-to-P reflections, using a 1D raytracer. Most of the P-to-P reflections that we observe have reflection origins in the upper/mid mantle. Beneath the western Pacific subduction zones, such reflections show a good correlation with subduction zone contours that are derived from subduction zone seismicity, and correlate well with tomography gradients of 0.01-0.5% per degree, interpreted as the edge of the slab. Deep mantle reflections (>600 km) are also observed to depths of ~1900 km. The locations of these heterogeneities are combined with previous seismological

  12. Upper mantle and crustal structure of the East Greenland Caledonides

    DEFF Research Database (Denmark)

    Schiffer, Christian; Balling, N.; Jacobsen, B. H.;

    of the North Atlantic passive margins, including the gravitational collapse, extension, rifting and a possible influence by volcanism related to the Iceland hot spot. The landscape and topography were finally shaped by extensive erosion, finding its peak in the quaternary glaciations. Seismological data were...

  13. Crustal and Upper Mantle Velocity and Q of Mainland China (United States)


    corded at two SRO stations located in Mashad , Iran (11AIO) and Taipei, Taiwan (TATO). SRO stations operate at high gain, thus lowering the detection...the relative amplitude of surface wave energy arrivals over the path Yunnan- Mashad for both vertical and radial Rayleigh waves is shown in figures 4...Along the path Sichuan- Mashad , five events each with two (vertical and radial) Rayleigh components are used to obtain the observed dispersion data

  14. Upper mantle structure from body-wave coda and amplitudes

    NARCIS (Netherlands)

    Neele, F.P.


    The last decade has seen a large increase in the amount of high-quality data from a growing number of digitally operating seismic stations. Both short- and long-period data are reported on a regular basis to central data centres. As global seismology is hampered by the unequal distribution of statio

  15. Investigations of Eurasian Seismic Sources and Upper Mantle Structure. (United States)


    possibility is Ringwood’s (19821 " megalith " hypothesis, illustrated in Figure 3.3b, where the buckling of former oceanic crust and the harzburgite...instability owing to its compositional stratification [from Oxburgh and Parmentier, 1977, Figure 4]. Panel (b) depicts Ringwood’s [1982, Figure 9] " megalith

  16. Correlation between mobile continents and elevated temperatures in the subcontinental mantle (United States)

    Jain, Charitra; Rozel, Antoine; Tackley, Paul


    Rolf et al. (EPSL, 2012) and Coltice et al. (Science, 2012) have previously shown that continents exert a first order influence on Earth's mantle flow by affecting convective wavelength and surface heat flow. With stationary continents, Heron and Lowman (JGR, 2014) highlighted the decreasing role of continental insulation on subcontinental temperatures with higher Rayleigh number (Ra). However, the question whether there exists a correlation between mobile continents and elevated temperatures in the subcontinental mantle or not remains to be answered. By systematically varying parameters like core-mantle boundary (CMB) temperature, continental size, and mantle heating modes (basal and internal); we model thermo-chemical mantle convection with 2D spherical annulus geometry (Hernlund and Tackley, PEPI 2008) using StagYY (Tackley, PEPI 2008). Starting with a simple incompressible model having mobile continents, we observe this correlation. Furthermore, this correlation still holds when the model complexity is gradually increased by introducing internal heating, compressibility, and melting. In general, downwellings reduce the mantle temperature away from the continents, thereby resulting in correlation between mobile continents and elevated temperatures in the subcontinental mantle. For incompressible models (Boussinesq approximation), correlation exists and the dominant degree of convection varies with the continental distribution. When internal heating is switched on, correlation is observed but it is reduced as there are less cold regions in the mantle. Even for compressible models with melting, big continents are able to focus the heat underneath them. The dominant degree of convection changes with continental breakup. Additionally, correlation is observed to be higher in the upper mantle (300 - 1000 km) compared to the lower mantle (1000 - 2890 km). At present, mobile continents in StagYY are simplified into a compositionally distinct field drifting at the top of

  17. Role of the mantle exhumation channel in the formation of ultramafic seafloor (United States)

    Muntener, O.; Lavier, L. L.


    Mantle peridotites from ocean-continent transition zones (OCT's) illustrate the distribution of the scale of upper mantle heterogeneity in extensional systems that evolve from rifting to (ultra-) slow seafloor spreading. We summarize research on mantle processes of the conjugate Iberia-Newfoundland rift and from the Alps that show that the basement of OCT's consists of 3 mantle domains. Thermally undisturbed, cold subcontinental mantle formed the ocean floor next to thinned continental crust. This 'subcontinental domain' is separated by ductile shear zones from an infiltrated (e.g. hot) domain dominated by plagioclase peridotite. The footwall of these mantle shear zones display complex refertilization processes and high-temperature deformation. These rocks are highly heterogeneous and are juxtaposed with depleted lherzolites and dunites (extraction domain). Upwelling of partial melts that enter the conductive lithospheric mantle inevitably leads to freezing of the melt and to the formation of a chemical and rheological barrier, which we term mantle exhumation channel. We will discuss an example that displays km-scale refertilization with active deformation (and melt focusing?) on top, and the formation of a domain that represents focused melt extraction at the bottom. We show that an actively deforming refertilization front in Alpine plagioclase peridotites and in the Iberia Newfoundland rift moved ahead of a melting front. Melt lubricated shear zones (or melt bands) focus melt flow. Continuous uplift leads to crystallization and deformation will prevail in the subsolidus state. Final exposure of infiltrated peridotites on the seafloor is accommodated by faults in which hydrous phases crystallize (chlorite, serpentine, talc). We combine petrologic data and numerical models to illustrate that these processes play a key role in the rejuvenation and erosion of the lithospheric mantle and ultimately form ultramafic seafloor in ocean-continent transitions and ultra

  18. Mapping the mass distribution of Earth's mantle using satellite-derived gravity gradients (United States)

    Panet, Isabelle; Pajot-Métivier, Gwendoline; Greff-Lefftz, Marianne; Métivier, Laurent; Diament, Michel; Mandea, Mioara


    The dynamics of Earth's mantle are not well known. Deciphering mantle flow patterns requires an understanding of the global distribution of mantle density. Seismic tomography has been used to derive mantle density distributions, but converting seismic velocities into densities is not straightforward. Here we show that data from the GOCE (Gravity field and steady-state Ocean Circulation Explorer) mission can be used to probe our planet's deep mass structure. We construct global anomaly maps of the Earth's gravitational gradients at satellite altitude and use a sensitivity analysis to show that these gravitational gradients image the geometry of mantle mass down to mid-mantle depths. Our maps highlight north-south-elongated gravity gradient anomalies over Asia and America that follow a belt of ancient subduction boundaries, as well as gravity gradient anomalies over the central Pacific Ocean and south of Africa that coincide with the locations of deep mantle plumes. We interpret these anomalies as sinking tectonic plates and convective instabilities between 1,000 and 2,500km depth, consistent with seismic tomography results. Along the former Tethyan Margin, our data also identify an east-west-oriented mass anomaly likely in the upper mantle. We suggest that by combining gravity gradients with seismic and geodynamic data, an integrated dynamic model for Earth can be achieved.

  19. The Fe-rich clay microsystems in basalt-komatiite lavas: importance of Fe-smectites for pre-biotic molecule catalysis during the Hadean eon. (United States)

    Meunier, Alain; Petit, Sabine; Cockell, Charles S; El Albani, Abderrazzak; Beaufort, Daniel


    During the Hadean to early Archean period (4.5-3.5 Ga), the surface of the Earth's crust was predominantly composed of basalt and komatiite lavas. The conditions imposed by the chemical composition of these rocks favoured the crystallization of Fe-Mg clays rather than that of Al-rich ones (montmorillonite). Fe-Mg clays were formed inside chemical microsystems through sea weathering or hydrothermal alteration, and for the most part, through post-magmatic processes. Indeed, at the end of the cooling stage, Fe-Mg clays precipitated directly from the residual liquid which concentrated in the voids remaining in the crystal framework of the mafic-ultramafic lavas. Nontronite-celadonite and chlorite-saponite covered all the solid surfaces (crystals, glass) and are associated with tiny pyroxene and apatite crystals forming the so-called "mesostasis". The mesostasis was scattered in the lava body as micro-settings tens of micrometres wide. Thus, every square metre of basalt or komatiite rocks was punctuated by myriads of clay-rich patches, each of them potentially behaving as a single chemical reactor which could concentrate the organics diluted in the ocean water. Considering the high catalytic potentiality of clays, and particularly those of the Fe-rich ones (electron exchangers), it is probable that large parts of the surface of the young Earth participated in the synthesis of prebiotic molecules during the Hadean to early Archean period through innumerable clay-rich micro-settings in the massive parts and the altered surfaces of komatiite and basaltic lavas. This leads us to suggest that Fe,Mg-clays should be preferred to Al-rich ones (montmorillonite) to conduct experiments for the synthesis and the polymerisation of prebiotic molecules.

  20. Reactions between komatiite and CO2-rich seawater at 250 and 350 °C, 500 bars: implications for hydrogen generation in the Hadean seafloor hydrothermal system (United States)

    Ueda, Hisahiro; Shibuya, Takazo; Sawaki, Yusuke; Saitoh, Masafumi; Takai, Ken; Maruyama, Shigenori


    To understand the chemical nature of hydrothermal fluids in the komatiite-hosted seafloor hydrothermal system in the Hadean, we conducted two hydrothermal serpentinization experiments involving synthetic komatiite and a CO2-rich acidic NaCl fluid at 250 and 350 °C, 500 bars. During the experiments, the komatiites were strongly carbonated to yield iron-rich dolomite (3-9 wt.% FeO) at 250 °C and calcite (<0.8 wt.% FeO) at 350 °C, respectively. The carbonation of komatiites suppressed H2 generation in the fluids. The steady-state H2 concentrations in the fluid were approximately 0.024 and 2.9 mmol/kg at 250 and 350 °C, respectively. This correlation between the Fe content in carbonate mineral and the H2 concentration in the fluid suggests that the incorporation of ferrous iron into the carbonate mineral probably limited magnetite formation and consequent generation of hydrogen during the serpentinization of komatiites. The H2 concentration of the fluid at 350 °C corresponds to that of modern H2-rich seafloor hydrothermal systems, such as the Kairei hydrothermal field, where hydrogenotrophic methanogens dominate in the prosperous microbial ecosystem. Accordingly, the high-temperature serpentinization of komatiite would provide the H2-rich hydrothermal environments that were necessary for the emergence and early evolution of life in the Hadean ocean. In contrast, H2-rich fluids may not have been generated by serpentinization at temperatures below 250 °C because carbonate minerals become more stable with decreasing temperature in the komatiite-H2O-CO2 system.

  1. Upper Endoscopy

    Medline Plus

    Full Text Available ... Procedure Brochure Understanding Upper Endoscopy Brochure Make the Best Choice for Your Endoscopic Procedure Brochure Members-only ... Procedure Brochure Understanding Upper Endoscopy Brochure Make the Best Choice for Your Endoscopic Procedure Brochure View more ...

  2. Upper Endoscopy

    Medline Plus

    Full Text Available ... Endoscopic Submucosal Dissection (ESD) Endoscopic Ultrasound (EUS) Procedures F - Z GI Bleeding Manometry Photodynamic Therapy (PDT) Polypectomy ... Gastrointestinal Glossary of Terms Home / Clinical Topics / Procedures F - Z / Upper Endoscopy (EGD) Upper Endoscopy (EGD) The ...

  3. Mantle fluids in the Karakoram fault: Helium isotope evidence (United States)

    Klemperer, Simon L.; Kennedy, B. Mack; Sastry, Siva R.; Makovsky, Yizhaq; Harinarayana, T.; Leech, Mary L.


    The Karakoram fault (KKF) is the 1000 km-long strike-slip fault separating the western Himalaya from the Tibetan Plateau. From geologic and geodetic data, the KKF is argued either to be a lithospheric-scale fault with hundreds of km of offset at several cm/a, or to be almost inactive with cumulative offset of only a few tens of kilometers and to be just the upper-crustal localization of distributed deformation at depth. Here we show 3He/4He ratios in geothermal springs along a 500-km segment of the KKF are 3-100 times the normal ratio in continental crust, providing unequivocal evidence that a component of these hydrologic systems is derived from tectonically active mantle. Mantle enrichment is absent along the Indus-Yarlung suture zone (ISZ) just 35 km southwest of the KKF, suggesting that the mantle fluids flow only within the KKF. Within the last few Ma, the KKF must have accessed tectonically active Tibetan mantle northeast of the "mantle suture" which we therefore locate vertically beneath the KKF, very close to the surface trace of the ISZ. Hence, in southwestern Tibet, Indian crust may not now be underthrusting substantially north of the ISZ, even though Miocene underthrusting may have placed Indian crust north of the ISZ in the lower half of the Tibetan Plateau crust. This is in significant contrast to central and eastern Tibet where underthrust Indian material not only forms the lower half of the Tibetan crust but is also currently underthrusting for ∼200 km north of the ISZ. Our new constraint on KKF penetration to the mantle allows an improved description of the continuously evolving Karakoram fault, as a tectonically significant yet perhaps geologically ephemeral lithospheric structure.

  4. Mantle flow and dynamic topography associated with slab window opening (United States)

    Guillaume, Benjamin; Moroni, Monica; Funiciello, Francesca; Martinod, Joseph; Faccenna, Claudio


    A slab window is defined as an 'hole' in the subducting lithosphere. In the classical view, slab windows develop where a spreading ridge intersects a subduction zone. The main consequences of this phenomenon are the modifications of the physical, chemical and thermal conditions in the backarc mantle that in turn affect the tectonic and magmatic evolution of the overriding plate. In this work, we perform dynamically self-consistent mantle-scale laboratory models, to evaluate how the opening of a window in the subducting panel influences the geometry and the kinematics of the slab, the mantle circulation pattern and, finally, the overriding plate dynamic topography. The adopted setup consists in a two-layer linearly viscous system simulating the roll-back of a fixed subducting plate (simulated using silicone putty) into the upper mantle (simulated using glucose syrup). Our experimental setting is also characterized by a constant-width rectangular window located at the center of a laterally confined slab, modeling the case of the interaction of a trench-parallel spreading ridge with a wide subduction zone. We find that the geometry and the kinematics of the slab are only minorly affected by the opening of a slab window. On the contrary, slab induced mantle circulation, quantified using Feature Tracking image analysis technique, is strongly modified and produces a peculiar non-isostatic topographic signal on the overriding plate. Assuming that our modeling results can be representative of the natural behavior of subduction zones, we compare them to the Patagonian subduction zone finding that anomalous backarc volcanism that developed since middle Miocene could result from the lateral flowage of subslab mantle, and that part of the Patagonian uplift could be dynamically supported.

  5. Upper Endoscopy

    Medline Plus

    Full Text Available ... Upper Endoscopy (EGD) Upper Endoscopy (EGD) The Latest Practice Guidelines Technology Reviews Articles Videos Events & Products Ensuring the Safety of Your Endoscopic Procedure Brochure Understanding Upper Endoscopy Brochure Make the Best Choice for Your Endoscopic Procedure Brochure Members-only ...

  6. Toward a coherent model for the melting behavior of the deep Earth's mantle (United States)

    Andrault, D.; Bolfan-Casanova, N.; Bouhifd, M. A.; Boujibar, A.; Garbarino, G.; Manthilake, G.; Mezouar, M.; Monteux, J.; Parisiades, P.; Pesce, G.


    Knowledge of melting properties is critical to predict the nature and the fate of melts produced in the deep mantle. Early in the Earth's history, melting properties controlled the magma ocean crystallization, which potentially induced chemical segregation in distinct reservoirs. Today, partial melting most probably occurs in the lowermost mantle as well as at mid upper-mantle depths, which control important aspects of mantle dynamics, including some types of volcanism. Unfortunately, despite major experimental and theoretical efforts, major controversies remain about several aspects of mantle melting. For example, the liquidus of the mantle was reported (for peridotitic or chondritic-type composition) with a temperature difference of ∼1000 K at high mantle depths. Also, the Fe partitioning coefficient (DFeBg/melt) between bridgmanite (Bg, the major lower mantle mineral) and a melt was reported between ∼0.1 and ∼0.5, for a mantle depth of ∼2000 km. Until now, these uncertainties had prevented the construction of a coherent picture of the melting behavior of the deep mantle. In this article, we perform a critical review of previous works and develop a coherent, semi-quantitative, model. We first address the melting curve of Bg with the help of original experimental measurements, which yields a constraint on the volume change upon melting (ΔVm). Secondly, we apply a basic thermodynamical approach to discuss the melting behavior of mineralogical assemblages made of fractions of Bg, CaSiO3-perovskite and (Mg,Fe)O-ferropericlase. Our analysis yields quantitative constraints on the SiO2-content in the pseudo-eutectic melt and the degree of partial melting (F) as a function of pressure, temperature and mantle composition; For examples, we find that F could be more than 40% at the solidus temperature, except if the presence of volatile elements induces incipient melting. We then discuss the melt buoyancy in a partial molten lower mantle as a function of pressure

  7. Tracking Crust-Mantle Recycling through Superdeep Diamonds and their Mineral Inclusions (United States)

    Walter, Michael; Bulanova, Galina; Smith, Chris; Thomson, Andrew; Kohn, Simon; Burnham, Antony


    Sublithospheric, or 'superdeep' diamonds, originate in the deep upper mantle, transition zone, and at least as deep as the shallow lower mantle. When diamonds crystallize in the mantle from fluids or melts they occasionally entrap coexisting mineral phases. Because of their great physical resiliency, diamonds can potentially preserve information over long distance- and time-scales, revealing important information about the petrologic, tectonic and geodynamic environment in which the diamonds grew and were transported. Superdeep diamonds and their inclusions have proven especially powerful for probing processes related to subduction of slabs into the deep mantle [1-3]. In contrast to lithospheric diamonds that are effectively frozen-in geodynamically, mineral inclusions in superdeep diamonds often record hundreds of kilometers of uplift in the convecting mantle from their original depth of origin [3-5]. The phase equilibria of unmixing of original deep mantle phases such as Ca- and Mg-perovskite, NAL-phase, CF-phase, CAS-phase, and majorite provide a means to establish amounts of uplift. The few available age constraints indicate superdeep diamond growth from the Proterozoic to the Cretaceous, and further dating can potentially lead to constraining mantle upwelling rates [4]. Here we will provide several examples showing how superdeep diamonds and their inclusions record processes of subduction and slab foundering, and ultimately recycling of slab material from the transition zone and lower mantle into the shallow upper mantle. 1. Harte, B., Mineralogical Magazine, 2010. 74: p. 189-215. 2. Tappert, R., et al., Geology, 2005. 33: p. 565-568. 3. Walter, M.J., et al., Science, 2011. 333: p. 54-57. 4. Bulanova, G.P., et al., Contributions to Mineralogy and Petrology, 2010. 160: p. 489-510. 5. Harte, B. and N. Cayzer, Physics and Chemistry of Minerals, 2007.

  8. Dynamics of Pre-3 Ga Crust-Mantle Evolution (United States)

    Patchett, P. J.; Chase, C. G.; Vervoort, J. D.


    During 3.0 to 2.7 Ga, the Earth's crust underwent a non-uniformitarian change from a pre-3.0 Ga environment where long-term preservation of cratons was rare and difficult, to post-2.7 Ga conditions where cratons were established and new continental crust generation took place largely at craton margins. Many models view the Earth's surface during pre-3 Ga time as broadly equivalent to the post 2.7 Ga regime. Any such uniformitarian or gradual evolution cannot explain the conundrum that only a tiny amount of pre-3 Ga crust is preserved today coupled with the fact that very little pre-3 Ga crust was incorporated into the large amount of new craton that came into existence during 3.0-2.7 Ga. If large volumes of pre-3 Ga continental crust existed, it disappeared either just prior to 3 Ga, or during 3.0-2.7 Ga. To explain sudden appearance of surviving but dominantly juvenile continental crust in a model where continents were large prior to 3 Ga, it would be necessary either that pre-3 Ga continent was recycled into the mantle at sites systematically different from those where new 3.0-2.7 Ga crust was made, or that widespread continent destruction preceded the 3.0-2.7 Ga crustal genesis. From expected mantle overturn in response to the heat budget, it is likely that most pre-3 Ga crust was both more mafic and shorter-lived than after 3 Ga. Although Nd and Hf ratios for pre-3 Ga rocks are uncertain due to polymetamorphism, it appears that depleted upper mantle was widespread by 2.7 Ga, even pre-3 Ga. Depletion may have been largely achieved by formation, subduction and storage of mafic crust for periods of 200-500 m.y. The rapid change to large surviving continents during 3.0-2.7 Ga was due to declining mantle overturn, and particularly to development of the ability to maintain subduction in one zone of the earth's surface for the time needed to allow evolution to felsic igneous rock compositions. In as much as storage of subducted slabs is probably occurring today, and

  9. Bases of the Mantle-Carbonatite Conception of Diamond Genesis (United States)

    Litvin, Yuriy; Spivak, Anna; Kuzyura, Anastasia


    In the mantle-carbonatite conception of diamond genesis, the results of physic-chemical experiments are coordinated with the data of analytic mineralogy of primary inclusions in natural diamonds. Generalization of the solutions of principal genetic problems constitutes the bases of the conception. The solutions are following: (1) it is grounded that diamond-parental melts of the upper mantle have peridotite/eclogite - carbonatite - carbon compositions, of the transition zone - (wadsleite↔ringwoodite) - majorite - stishovite - carbonatite - carbon compositions, and of the lower mantle - periclase/wustite - bridgmanite - Ca-perovskite -stishovite - carbonatite - carbon compositions; (2) a construction of generalized diagrams for the diamond-parental media, which reveal changeable compositions of the growth melts of diamonds and associated phases, their genetic relations to the mantle substance, and classification connections of the primary inclusions in natural diamonds; (3) experimental equilibrium phase diagrams of syngenesis of diamonds and primary inclusions, which characterize the nucleation and growth conditions of diamonds and a capture of paragenetic and xenogenetic minerals by the growing diamonds; (4) a determination of the phase diagrams of diamonds and inclusions syngenesis under the regime of fractional crystallization, which discover the regularities of ultrabasic-basic evolution and paragenesis transitions in the diamond-forming systems of the upper and lower mantle. The evidence of the physic-chemically united mode of diamond genesis at the mantle depths with different mineralogy is obtained. References. Litvin Yu.A. (2007). High-pressure mineralogy of diamond genesis. In: Advances in High-Pressure Mineralogy (edited by Eiji Ohtani), Geological Society of America Special paper 421, 83-103. Litvin Yu.A. (2012). Experimental study of physic-chemical conditions of natural diamond formation on an example of the eclogite

  10. Widespread tungsten isotope anomalies and W mobility in crustal and mantle rocks of the Eoarchean Saglek Block, northern Labrador, Canada: Implications for early Earth processes and W recycling (United States)

    Liu, Jingao; Touboul, Mathieu; Ishikawa, Akira; Walker, Richard J.; Graham Pearson, D.


    Well-resolved 182W isotope anomalies, relative to the present mantle, in Hadean-Archean terrestrial rocks have been interpreted to reflect the effects of variable late accretion and early mantle differentiation processes. To further explore these early Earth processes, we have carried out W concentration and isotopic measurements of Eoarchean ultramafic rocks, including lithospheric mantle rocks, meta-komatiites, a layered ultramafic body and associated crustal gneisses and amphibolites from the Uivak gneiss terrane of the Saglek Block, northern Labrador, Canada. These analyses are augmented by in situ W concentration measurements of individual phases in order to examine the major hosts of W in these rocks. Although the W budget in some rocks can be largely explained by a combination of their major phases, W in other rocks is hosted mainly in secondary grain-boundary assemblages, as well as in cryptic, unidentified W-bearing 'nugget' minerals. Whole rock W concentrations in the ultramafic rocks show unexpected enrichments relative, to elements with similar incompatibilities. By contrast, W concentrations are low in the Uivak gneisses. These data, along with the in situ W concentration data, suggest metamorphic transport/re-distribution of W from the regional felsic rocks, the Uivak gneiss precursors, to the spatially associated ultramafic rocks. All but one sample from the lithologically varied Eoarchean Saglek suite is characterized by generally uniform ∼ + 11 ppm enrichments in 182W relative to Earth's modern mantle. Modeling shows that the W isotopic enrichments in the ultramafic rocks were primarily inherited from the surrounding 182W-rich felsic precursor rocks, and that the W isotopic composition of the original ultramafic rocks cannot be determined. The observed W isotopic composition of mafic to ultramafic rocks in intimate contact with ancient crust should be viewed with caution in order to plate constraints on the early Hf-W isotopic evolution of the

  11. Dehydrogenation of goethite in Earth’s deep lower mantle

    Energy Technology Data Exchange (ETDEWEB)

    Hu, Qingyang; Kim, Duck Young; Liu, Jin; Meng, Yue; Yang, Liuxiang; Zhang, Dongzhou; Mao, Wendy L.; Mao, Ho-kwang


    The cycling of hydrogen influences the structure, composition, and stratification of Earth’s interior. Our recent discovery of pyrite-structured iron peroxide (designated as the P phase) and the formation of the P phase from dehydrogenation of goethite FeO2H implies the separation of the oxygen and hydrogen cycles in the deep lower mantle beneath 1,800 km. Here we further characterize the residual hydrogen, x, in the P-phase FeO2Hx. Using a combination of theoretical simulations and high-pressure–temperature experiments, we calibrated the x dependence of molar volume of the P phase. Within the current range of experimental conditions, we observed a compositional range of P phase of 0.39 < x < 0.81, corresponding to 19–61% dehydrogenation. Increasing temperature and heating time will help release hydrogen and lower x, suggesting that dehydrogenation could be approaching completion at the high-temperature conditions of the lower mantle over extended geological time. Our observations indicate a fundamental change in the mode of hydrogen release from dehydration in the upper mantle to dehydrogenation in the deep lower mantle, thus differentiating the deep hydrogen and hydrous cycles.

  12. Rogue mantle helium and neon. (United States)

    Albarède, Francis


    The canonical model of helium isotope geochemistry describes the lower mantle as undegassed, but this view conflicts with evidence of recycled material in the source of ocean island basalts. Because mantle helium is efficiently extracted by magmatic activity, it cannot remain in fertile mantle rocks for long periods of time. Here, I suggest that helium with high 3He/4He ratios, as well as neon rich in the solar component, diffused early in Earth's history from low-melting-point primordial material into residual refractory "reservoir" rocks, such as dunites. The difference in 3He/4He ratios of ocean-island and mid-ocean ridge basalts and the preservation of solar neon are ascribed to the reservoir rocks being stretched and tapped to different extents during melting.

  13. Experimental Constraints on Mantle Heterogeneity and Mantle-Melt Equilibration Depths along the Volcanic Front of the Trans-Mexican Volcanic Belt (United States)

    Weaver, S.; Wallace, P. J.; Johnston, A.