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1

Recycled oceanic crust in the source of 90-40 Ma basalts in North and Northeast China: Evidence, provenance and significance  

Science.gov (United States)

Major, trace element and Sr-Nd-Pb isotopic data of basalts emplaced during 90-40 Ma in the North and Northeast China are compiled in this review, with aims of constraining their petrogenesis, and by inference the evolution of the North China Craton during the late Cretaceous and early Cenozoic. Three major components are identified in magma source, including depleted component I and II, and an enriched component. The depleted component I, which is characterized by relatively low 87Sr/86Sr (1.1) and HIMU-like trace element characteristics, is most likely derived from gabbroic cumulate of the oceanic crust. The depleted component II, which distinguishes itself by its high ?Nd (?8) and moderate 87Sr/86Sr (?0.7038), is probably derived from a sub-lithospheric ambient mantle. The enriched component has low ?Nd (2-3), high 87Sr/86Sr (>0.7065), low 206Pb/204Pb (17), excess Sr, Rb, Ba and a deficiency of Zr and Hf relative to the REE. This component is likely from the basaltic portion of the oceanic crust, which is variably altered by seawater and contains minor sediments. Comparison with experimental melts and trace element modeling suggest that these recycled oceanic components may be in form of garnet pyroxenite/eclogite. These components are young (recycled oceanic crust. Due to different melting temperature of the upper and lower ocean crust and progressive thinning of the lithosphere, the more fertile basaltic crustal component is preferentially sampled during the early stage of volcanism, whereas the more depleted gabbroic lower crust and lithospheric mantle components are preferentially sampled during a late stage. This model is consistent with a protracted destruction process of the lithosphere beneath eastern China. The presence of significant recycled oceanic crust components in the 90-40 Ma basalts highlights the influence of Pacific subduction on the deep processes in the North China Craton, which can be traced back at least to the late Cretaceous. This, along with the conjugation of crustal deformation pattern in this region with the movement of the Pacific plate, makes the Pacific subduction as a potential trigger of the destruction of the North China Craton. Geophysical investigations and morphological analyses indicate that decratonization is largely confined to east of the NSGL, whereas to west of NSGL, in particular the Ordos basin, characteristics typical of a craton are observed (Menzies et al., 2007; Zhu et al., 2011). This spatial pattern of craton destruction, together with NE-NNE-oriented extensional basins, main structural alignments and metamorphic core complexes (Zheng et al., 1978; Ye et al., 1987; Ren et al., 2002; Liu et al., 2006; Zhu G et al., 2012), is consistent with the subduction direction of the Pacific plate. Two main episodes of late Mesozoic magmatism have been identified in the Jurassic and the early Cretaceous. These correspond to the subduction of the Pacific plate underneath the Eurasian content and to subsequent extensions, respectively (Wu et al., 2005, 2006). Global tomography studies indicate that the subducted Pacific oceanic slab has become stagnant within the mantle transition zone and extended subhorizontally westward beneath the East Asian continent (Fukao et al., 1992; Huang and Zhao, 2006; Chen and Ai, 2009; Van der Hilst and Li, 2010). The western end of this stagnant slab does not go beyond the NNE-trending NSGL (Huang and Zh

Xu, Yi-Gang

2014-10-01

2

Potential serpentinization, degassing, and gas hydrate formation at a young (<20 Ma) sedimented ocean crust of the Arctic Ocean ridge system  

Science.gov (United States)

Global assessment of methane must consider the role of mid-ocean ridges. Fluids from serpentinized mantle and gabbro material are noteworthy on ocean ridges, although they are not very well understood. Only a few sedimented ocean ridges exist worldwide, and they may document past and ongoing serpentinization-driven migration of gas-rich fluids. This study is based on two multichannel reflection seismic profiles acquired across a sedimented segment of the ultraslow spreading Knipovich Ridge offshore NW Svalbard. Seismic data allow suggesting a potential link between inferred areas of serpentinization, transfer of carbon from the deep-seated host rocks through the sediments above by diapirism, and methane capture within the gas hydrate stability zone at the eastern flank of the Knipovich Ridge. The origin of sediment remobilization features can be related to intrusions and the degassing process from mantle serpentinization. These disturbances in sediments overlying the oceanic crust can be observed in seismic data and are interpreted as diapirs. In shallower sediments, at the predicted base of the gas hydrate stability zone, the seismic data show a bright spot with all the characteristics of a gas hydrate related bottom-simulating reflector (BSR), such as enhanced reflection amplitude, phase reversal relative to the seabed reflection, and crosscutting of sedimentary strata. The BSR occurs at about 200 ms two-way time within a sequence of marine sediments. Two-dimensional concentration models of methane hydrate using the differential effective medium theory predict saturations of up to 26% of methane hydrate in the pore space of sediments in the gas hydrate reservoir.

Rajan, Anupama; Mienert, Jürgen; Bünz, Stefan; Chand, Shyam

2012-03-01

3

Gas hydrate reservoir and active methane-venting province in sediments on < 20 Ma young oceanic crust in the Fram Strait, offshore NW-Svalbard  

Science.gov (United States)

Seafloor pockmarks are common indicators for vertical fluid flow and frequently associated with methane discharge through the gas-hydrate stability zone (GHSZ). The present-day flux through these degassing systems is presumably at a low level on most rifted continental margins. A pockmark-field on the NW-Svalbard passive margin is located on young ocean crust (methane hydrate and free gas. The model predicts saturations of up to 11% in the hydrate reservoir, which due to the seafloor topography forms a large anticlinal permeability-barrier. Below, in the low-velocity zone (i.e., 1350-1500 m/s), up to 3% of free gas is predicted across the apex of the Vestnesa Ridge and in the immediate vicinity of extensional faults. A conservative estimate indicates that 225 kg/m 2 of pure methane is stored in hydrate and gas in the upper 230 m of the sedimentary column. An elongated pockmark-field, consisting of > 100 individual pockmarks up to 600 m wide, systematically aligns the apex of the Vestnesa Ridge. Active, vigorous degassing from the topography-controlled pressure-valve system was evident from a 750-m-high and ˜ 150-m-wide gas flare observed in the water column during a cruise with R/V Jan Mayen in October 2008. The gas flare documents dynamic degassing through the corresponding chimney, which penetrates the entire GHSZ and into the underlying free gas zone. Cruises in 2006 and 2007 did not detect active gas venting above the pockmark-field. Accordingly, vigorous degassing may operate in an episodic mode, where hydrothermal circulation systems through young ocean crust may play a significant role.

Hustoft, Steinar; Bünz, Stefan; Mienert, Jürgen; Chand, Shyam

2009-06-01

4

Why Does Calcium Carbonate Precipitate in the Ocean Crust?  

Science.gov (United States)

Calcium carbonate (CaCO3) precipitation in extant ocean crust may provide lessons for industrial carbon dioxide (CO2) sequestration to mitigate against future climate change. The formation and alteration of the ocean crust during mid-ocean ridge spreading plays an integral role in the geological carbon (C) cycle. Magma-degassing during mid-ocean ridge volcanism releases large volumes of CO2 to the oceans and atmosphere, whereas the precipitation of CaCO3 minerals in rocks and veins during hydrothermal alteration of the ocean crust is a major mechanism of natural CO2 sequestration. However, the physical and chemical conditions that control the nature, extent and rate of calcium carbonate precipitation in the ocean crust are not well known, and the magnitudes of the carbon fluxes associated with the formation and alteration of the ocean crust remain poorly quantified. Mesozoic ocean crust contains significantly more hydrothermal CaCO3 than young ocean crust, but the past ocean conditions responsible for this enhanced C-uptake by the ocean crust are yet to be determined. The thermal, hydrogeologic, and chemical characteristics of active, low temperature seawater-basalt interaction have been investigated by a series of integrated studies in young ocean crust (<3.6 Ma) on the eastern flank of the Juan de Fuca ridge (JdFR). This has included sampling by scientific ocean drilling of CaCO3 veins, their parent fluids, and the basalts that host the veins. This area provides a unique opportunity to investigate the conditions of in situ hydrothermal CaCO3 precipitation on short geologic timescales. Fluid and vein analyses provide boundary conditions for geochemical modeling using 'Phreeqc' to determine why basement fluids are saturated with respect to CaCO3. We evaluate the controlling effects of key parameters including fluid dissolved inorganic carbon (DIC), Mg and Ca concentrations, alkalinity, temperature, and pH on CaCO3 precipitation. Our results provide insights into how differing past conditions (e.g. ocean temperature and pCO2) affected the ocean crustal C-reservoir, and enable better quantification of the magnitude of this reservoir and how it has varied over geological time. This information is directly applicable to the development of industrial scale CO2 sequestration using methods that mimic but accelerate natural CO2-drawdown processes.

Coggon, R. M.; Teagle, D. A.; Harris, M.; John, C. M.; Smith-Duque, C. E.; Alt, J.

2012-12-01

5

Late Cretaceous (100-89 Ma) magnesian charnockites with adakitic affinities in the Milin area, eastern Gangdese: Partial melting of subducted oceanic crust and implications for crustal growth in southern Tibet  

Science.gov (United States)

Rapid Mesozoic-Early Cenozoic crustal growth in the Gangdese area, southern Tibet, has commonly been attributed to pre-collisional and syn-collisional underplating of mantle-derived magmas. Here, we report on adakitic magnesian charnockites (i.e., hypersthene-bearing diorites and granodiorites) near Milin, in eastern Gangdese, that provide new insights into the crustal growth process of the region. Zircon U-Pb analyses of seven charnockite samples indicate that they were generated in the Late Cretaceous (100-89 Ma). They exhibit variable SiO2 (53.9 to 65.7 wt.%) contents, high Na2O/K2O (1.6 to 14.4) and Sr/Y (27.2 to 138.7) ratios, low Y (6.5 to 18.5 ppm), heavy rare earth element (e.g., Yb = 0.6 to 1.6 ppm) and Th (0.20-2.39 ppm) contents and Th/La (0.02-0.23) ratios, with relatively high Mg# (46 to 56) and MgO (2.0 to 4.5 wt.%) values. They are characterized isotopically by high and slightly variable ?Nd(t) (+ 2.4 to + 4.0) and ?Hf(t) (+ 10.1 to + 15.8) values with relatively low and consistent (87Sr/86Sr)i (0.7042 to 0.7043) ratios. Their pyroxenes have high crystallization temperatures (876 to 949 °C). The Milin charnockites were most probably produced by partial melting of subducted Neo-Tethyan oceanic crust that was followed by adakitic melt-mantle interaction, minor crustal assimilation and fractional crystallization of amphibole + plagioclase. The upwelling asthenosphere, triggered by the roll-back of subducted Neo-Tethyan oceanic lithosphere, provided the heat for slab melting. Therefore, we suggest that, in addition to pre-collisional and syn-collisional underplating of mantle-derived magmas, the recycling of subducted oceanic crust has also played an important role in continental crustal growth in southern Tibet.

Ma, Lin; Wang, Qiang; Wyman, Derek A.; Li, Zheng-Xiang; Jiang, Zi-Qi; Yang, Jin-Hui; Gou, Guo-Ning; Guo, Hai-Feng

2013-08-01

6

Thin oceanic crust and flood basalts: India-Seychelles breakup  

Science.gov (United States)

Recent seismic experiments showed that separation of India from the Seychelles occurred in two phases of rifting. The first brief phase of rifting between India and the Laxmi Ridge formed the Gop Rift, which is characterized by thick oceanic crust and underplating of the adjacent continental margins. The age of the Gop Rift is uncertain, initiation of seafloor spreading being some time between 71 and 66 Ma. This was then followed by rifting and seafloor spreading between the Laxmi Ridge and the Seychelles, the onset of which is well dated by magnetic anomalies at 63.4 Ma and characterized by thin oceanic crust. Both of these rift events occurred within 1000 km of the center of the Deccan flood basalts, which formed at 65 ± 1 Ma. To constrain the age of the Gop Rift and to explore the reasons for the change in crustal structure between the Gop Rift and Seychelles-Laxmi Ridge margins, we employ a geodynamic model of rift evolution in which melt volumes, seismic velocity, and rare earth element (REE) chemistry of the melt are estimated. We explore the consequences of different thermal structures, hydration, and depletion on the melt production during the India-Seychelles breakup to understand the reasons behind the thin oceanic crust observed. Magmatism at the Gop Rift is consistent with a model in which the seafloor spreading began at 71 Ma, ca. 6 Myr prior to the Deccan. The opening occurred above a hot mantle layer (temperature of 200°C, thickness of 50 km) that we interpret as incubated Deccan material, which had spread laterally beneath the lithosphere. This scenario is consistent with observed lower crustal seismic velocities of 7.4 km s-1 and 12 km igneous crustal thickness. The model indicates that when the seafloor spreading migrated to the Seychelles-Laxmi Ridge at 63 Ma, the thermal anomaly was reduced significantly but not sufficient to explain the observed reduction in breakup magmatism. From observations here of 5.2 km oceanic crust, lower crustal seismic velocities of 6.9 km s-1 and a flat REE profile, we infer that breakup occurred in a region of mantle that became depleted by prior extension related to the Gop Rift.

Armitage, J. J.; Collier, J. S.; Minshull, T. A.; Henstock, T. J.

2011-05-01

7

Magnetization of the oceanic crust: TRM or CRM?  

Science.gov (United States)

A model was proposed in which chemical remanent magnetization (CRM) acquired within the first 20 Ma of crustal evolution may account for 80% of the bulk natural remanent magnetization (NRM) of older basalts. The CRM of the crust is acquired as the original thermoremanent magnetization (TRM) is lost through low temperature alteration. The CRM intensity and direction are controlled by the post-emplacement polarity history. This model explains several independent observations concerning the magnetization of the oceanic crust. The model accounts for amplitude and skewness discrepancies observed in both the intermediate wavelength satellite field and the short wavelength sea surface magnetic anomaly pattern. It also explains the decay of magnetization away from the spreading axis, and the enhanced magnetization of the Cretaceous Quiet Zones while predicting other systematic variations with age in the bulk magnetization of the oceanic crust. The model also explains discrepancies in the anomaly skewness parameter observed for anomalies of Cretaceous age. Further studies indicate varying rates of TRM decay in very young crust which depicts the advance of low temperature alteration through the magnetized layer.

Raymond, C. A.; Labrecque, J. L.

1987-01-01

8

Microbial Life of North Pacific Oceanic Crust  

Science.gov (United States)

Information on the microbiology of the deep subsurface is necessary in order to understand the factors controlling the rate and extent of the microbially catalyzed reactions that influence the geophysical properties of these environments. Drilling into 45-Ma oceanic basaltic crust in a deepwater environment during ODP Leg 200 provided a promising opportunity to explore the abundance, diversity and activity of micro-organisms. The combined use of culture-independent molecular phylogenetic analyses and enrichment culture techniques is an advantageous approach in investigating subsurface microbial ecosystems. Enrichment culture methods allow the evaluation of potential activities and functions. Microbiological investigations revealed few aerobic cultivable, in part hitherto unknown, micro-organisms in deep submarine sediments and basaltic lava flows. 16S rDNA sequencing of isolates from sediment revealed the next relatives to be members of the genera Halomonas, Pseudomonas, and Lactobacillus. Within the Pseudomonadaceae the closest relative is Acinetobacter sp., which was isolated from a deep subsurface environment. The next phylogenetical relatives within the Halomonadaceae are bacteria typically isolated from Soda lakes, which are considered as model of early life conditions. Interestingly, not only sediment bacteria could be obtained in pure culture. Aerobic strains could also be successfully isolated from the massive tholeiitic basalt layer at a depth of 76.16 mbsf (46 m below the sediment/basement contact). These particular isolates are gram-positive with low G+C content of DNA, phylogenetically affiliated to the phylum Firmicutes. The closest neighbors are e.g. a marine Bacillus isolated from the Gulf of Mexico and a low G+C gram-positive bacterium, which belongs to the microbial flora in the deepest sea mud of the Mariana Trench, isolated from a depth of 10,897 m. Based on the similarity values, the isolates represent hitherto undescribed species of the deep biosphere. Molecular microbial diversity is currently determined by cloning und comparative 16S rRNA gene analyses. The first results will also be presented. In summary, the low number of isolates, cultivated under aerobic conditions, is in good agreement with the common opinion that most of the bacteria within the deep biosphere are anaerobic. Thus, studies of microbial community structure in solid geological materials are feasible and constitute further evidence that continuing microbiological activity in the challenging exploration of the deep sub-seafloor biosphere environment is absolutely promising.

Schumann, G.; Koos, R.; Manz, W.; Reitner, J.

2003-12-01

9

Helium isotopes in ferromanganese crusts from the central Pacific Ocean  

Science.gov (United States)

Helium isotopes have been measured in samples of two ferromanganese crusts (VA13/2 and CD29-2) from the central Pacific Ocean. With the exception of the deepest part of crust CD29-2 the data can be explained by a mixture of implanted solar- and galactic cosmic ray-produced (GCR) He, in extraterrestrial grains, and radiogenic He in wind-borne continental dust grains. 4He concentrations are invariant and require retention of less than 12% of the in situ He produced since crust formation. Loss has occurred by recoil and diffusion. High 4He in CD29-2 samples older than 42 Ma are correlated with phosphatization and can be explained by retention of up to 12% of the in situ-produced 4He. 3He/4He of VA13/2 samples varies from 18.5 to 1852 Ra due almost entirely to variation in the extraterrestrial He contribution. The highest 3He/4He is comparable to the highest values measured in interplanetary dust particles (IDPs) and micrometeorites (MMs). Helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in-falling terrestrial and extraterrestrial grains of Fe-Mn crusts. The extraterrestrial 3He concentration of the crusts rules out whole, undegassed 4–40 ?m diameter IDPs as the host. Instead it requires that the extraterrestrial He inventory is carried by numerous particles with significantly lower He concentrations, and occasional high concentration GCR-He-bearing particles.

Basu, S.; Stuart, F.M.; Klemm, V.; Korschinek, G.; Knie, K.; Hein, J.R.

2006-01-01

10

Asymmetric Spreading, and the Construction of Oceanic Crust at the Kane Oceanic Core Complex  

Science.gov (United States)

Detachment faulting at the Kane Oceanic Core Complex (OCC) on the Mid-Atlantic Ridge (23° N) has exposed a tectonic window through oceanic crust. Here we present fourteen Pb/U zircon SHRIMP ages of evolved lower crustal gabbro collected by ROV and dredging during R/V Knorr Cruise 180-2 from the Babel, Cain and Abel, and Adam domes of the Kane Oceanic Core complex (up to 46 km off axis), and from ODP Hole 923A, near the present day ridge axis. These data allow us to constrain the construction history of oceanic crust at Kane. Weighted average 206Pb/238U ages range from 2.45 +/-0.06 to 3.70+/-0.16 Ma for samples from the Kane OCC, and 0.86+/-0.14 Ma for Hole 923A. Age-distance plots yield a North American plate-spreading rate of 15.3+/-2.0 cm/yr during formation of the Kane OCC, with a time-averaged rate of 14.3+/-0.95 cm/yr determined from ODP 923A near the ridge axis to the western margin of the OCC, consistent with rates determined from magnetic anomaly data (Williams, 2007). Both the zircon and magnetic data show that the Kane OCC formed during a period of asymmetric spreading with over 60% of the total plate-motion accommodated by detachment faulting, requiring associated ridge migration. The average 206Pb/238U age is consistently ~~250,000 yr older than the magnetic age, implying acquisition of magnetic remanence roughly 4 km off-axis. Ten samples have ages within error of the calculated spreading rate. However, two samples are significantly older, and two significantly younger than those that define the spreading rate trend, implying variability in both depth and location of crustal accretion. One of the older samples (3.7 +/-0.16Ma) comes from the northern Babel Dome, adjacent to the Kane Transform Fault. This single age is 0.6 Ma older than samples from the Cain and Abel domes (15 km) to the south, and may suggest that the gabbros forming Babel Dome crystallized ~~5km deeper in cooler lithosphere adjacent the transform fault. Additional data is being acquired to test this hypothesis. The two anomalously young ages are about 0.5Ma younger than the expected age for their location. Both were collected from the Cain and Abel domes and are in close proximity to samples that give expected ages. We suggest that these young ages reflect the heterogeneous spatial distribution of magmatism within the axial valley. We envisage that the majority of the crust accreted beneath the detachment fault at a depth of ~~6-7 km, consistent with seismicity studies elsewhere, and with high-temperature fault rocks from the Kane OCC. The younger samples may represent later, more shallow magmatism (~~2km depth) that intruded earlier formed crust as it was denuded by the detachment fault. This interpretation is consistent with the observation that both gabbros and basaltic dikes cut recovered fault rocks. Taken together, all data suggest that any given piece of oceanic crust may record up to 0.5Ma of magmatic activity at the Kane OCC.

Cheadle, M.; John, B.; Lusk, M.; Wooden, J.

2008-12-01

11

New Images Suggest Oceanic Crust Generated from Several Magma Sources  

Science.gov (United States)

... Physics Press Release 05-149New Images Suggest Oceanic Crust Generated from Several Magma ... Earth, a process similar to creating an ultrasound image. They published the results of their work ...

12

Structure of the earth: oceanic crust and uppermost mantle.  

Science.gov (United States)

Contents: Rise axis structure: magma chambers. Fracture zone structure: thin oceanic crust. Crustal and uppermost mantle anisotropy. Evolution of the oceanic crust and uppermost mantle. Arctic exploration. Propagation of high frequency Pn/Sn/T phases: reverberation or scattering. Attenuation. Seafloor noise and topographic scattering. Seafloor and subseafloor receivers and sources. Theoretical seismology including the inversion of data. Seismicity. Multichannel and reflection seismology. Seismic refraction studies. Epilogue.

Orcutt, J. A.

13

Forearc oceanic crust in the Izu-Bonin arc - new insights from active-source seismic survey -  

Science.gov (United States)

Petrological studies have suggested that oceanic crust is formed in forearc areas during the initial stage of subduction. However, there is little geophysical evidence for the formation of oceanic crust in those regions. In order to examine crustal formation process associated with a subduction initiation process, we conducted an active-source seismic survey at a forearc region in the Izu-Bonin intra-oceanic arc. The resultant seismic image shows a remarkably thin crust (less than 10 km) at the northern half of the Bonin ridge (at the north of the Chichi-jima) and abrupt thickening the crust (~ 20 km thick) toward the south (at the Haha-jima). Comparison of velocity-depth profiles of the thin forearc crust of the Bonin ridge with those of typical oceanic crusts showed them to be seismologically identical. The observed structural variation also well corresponds to magmatic activities along the forearc. Boninitic magmatism is evident in the area of thin crust and tholeiitic-calcalkaline andesitic volcanism in the area of thick crust. Based on high precision dating studies of those volcanic rocks, we interpreted that the oceanic-type thin crust associated with boninitic volcanism has been created soon after the initiation of subduction (45-48 Ma) and and that the nonoceanic thick crust was created by tholeiitic-calcalkaline andesitic magmatism after the boninitic magmatism was ceased. The above seismological evidences strongly support the idea of forearc oceanic crust (or phiolite) created by forearc spreading in the initial stage of subduction along the intra-oceanic arc.

Kodaira, S.; Noguchi, N.; Takahashi, N.; Ishizuka, O.; Kaneda, Y.

2009-12-01

14

Primitive layered gabbros from fast-spreading lower oceanic crust.  

Science.gov (United States)

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks--in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas--provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt. PMID:24291793

Gillis, Kathryn M; Snow, Jonathan E; Klaus, Adam; Abe, Natsue; Adrião, Alden B; Akizawa, Norikatsu; Ceuleneer, Georges; Cheadle, Michael J; Faak, Kathrin; Falloon, Trevor J; Friedman, Sarah A; Godard, Marguerite; Guerin, Gilles; Harigane, Yumiko; Horst, Andrew J; Hoshide, Takashi; Ildefonse, Benoit; Jean, Marlon M; John, Barbara E; Koepke, Juergen; Machi, Sumiaki; Maeda, Jinichiro; Marks, Naomi E; McCaig, Andrew M; Meyer, Romain; Morris, Antony; Nozaka, Toshio; Python, Marie; Saha, Abhishek; Wintsch, Robert P

2014-01-01

15

Variation of young oceanic crust and upper mantle structure  

International Nuclear Information System (INIS)

Seismic refraction and single-channel reflection data taken along 0.5-, 2.5-, and 4.5-m.y.-age isochrons near the East Pacific Rise during Project ROSE are used to determine if a systematic change in the P velocity-depth function with age can be resolved. Inversion of these data suggests that any change in crustal P velocity structure related to age is smaller than variability in the seismic velocity-depth function along an isochron. The emergence of a 'normal' crust-mantle transition by 4.5-m.y.-age is seen in these data. Crust and crust-mantle transition zone heterogeneity along these isochrons may be related to the along strike variability in processes at the ridge crest. The velocity-depth functions for the threee split profile refractions lines are compared with velocity-depth functions for the Samail ophiolite, which is thought to represent oceanic crust of similar age. The velocity-depth functions for the ROSE data are bounded by different velocity-depth models for the Samail ophiolite; this suggests that those models are not in disagreement but represent the lateral heterogeneity that can be expected in young oceanic crust

16

Anorthositic oceanic crust in the Archean Earth  

Science.gov (United States)

Ultrapure minerals separated from eclogite inclusions in kimberlites were analyzed for Sm, Nd, Sr, and oxygen isotopes and for major and trace elements. Clinopyroxene (cpx) and garnet (gnt) are the only primary mineral phases in these rocks, and mineral phases and their alteration products. The WR sub calc. is the reconstructed bulk composition excluding all the contamination influences. Two groups of eclogites: are distinguished: (1) type A Noritic-anorthositic eclogites; and (2) type B Ti-ferrogabbroic eclogites. The oxygen isotopes are primary mantle-derived features of these rocks and are not caused by posteruption processes, as they were measured on unaltered, clean mineral separates and show a correlation with REE pattern and Sr and Nd isotopes. It is suggested that the variation of the oxygen isotopes are caused by crustal-level fluid-rock interaction at relatively low temperature. It is shown that oxygen isotopes variation in MORB basalts caused by the hydrothermal system are in the same range as the observed oxygen isotope variation in eclogites. A model to explain the new set of data is proposed. It is thought that some of these eclogites might be emplaced into the upper lithosphere or lower crust at the time corresponding to their internal isochron age. The calculated WR composition was used to estimate model ages for these rocks.

Jagoutz, E.; Dawson, J. B.; Hoernes, S.; Spettel, B.; Waenke, H.

1985-01-01

17

Altered oceanic crust as an inorganic record of paleoseawater Sr concentration  

Science.gov (United States)

The minimum Sr content of seawater at 90-95 Ma is determined from the Sr content and Sr isotopic composition of the sheeted dike complex of oceanic crust that was hydrothermally altered at this time on the basis of simple mass and heat balances. When this approach is applied to sheeted dike complexes that were hydrothermally altered by modern seawater a minimum fluid Sr content of sheeted dike complexes in the Cretaceous Troodos and Oman ophiolites gives minimum Sr contents for the ocean at this time of 27 and 38 ?g g-1, respectively. These values are consistent with previous studies of biomineralized carbonate that suggest higher Sr/Ca in seawater at this time and fluid inclusion analyses that suggest higher Ca contents in the ocean at this time. The Ca content of the ocean must have been at the upper end of the range suggested by fluid inclusion analyses (?30 mmol kg-1). The high Ca and Sr content of seawater at 90-95 Ma was, at least partially, due to enhanced submarine hydrothermal circulation which leaches Sr from the basaltic crust because it coincides with a time of low seawater 87Sr/86Sr. Elevated Sr contents at this time may have impacted the evolution and/or biogeochemical role of the Acantharians which secrete a SrSO4 skeleton.

Coogan, Laurence A.

2009-04-01

18

Thrusting in oceanic crust during continental drift offshore Niger Delta, equatorial Africa  

Science.gov (United States)

Two- and three-dimensional (3-D) seismic reflection data acquired over oceanic crust in the deepwater west Niger Delta reveal convincing evidence for compressional tectonics during oceanic crustal spreading. Using the 3-D seismic data set we describe numerous inclined seismic reflections that dissect the entire oceanic crust from the top of the crust to the level of the Moho that are interpreted as thrusts. Thrust propagation results in the development of associated hanging-wall anticlines and footwall synclines. These structures are orthogonal to and clearly postdate normal faults that formed during the accretion of oceanic crust during continental drift and strike at right angles to them. The Charcot Ridge is located 140 km south of these thrusts and is a significantly larger structure. It is a triangular-shaped uplifted region of oceanic crust measuring 80 by 150 km and is located along the NE-SW-oriented Charcot Fracture Zone. Two interpretations are possible for the role of the fracture zone in the development of the Charcot Ridge: (1) A thin-skinned model whereby the oceanic crust west of the fracture zone has been thrust southeastward, with detachment occurring close to the level of the Moho. The ridge forms as a result of translation and folding above a crustal-scale ramp-flat thrust geometry. (2) A thick-skinned model where there is no detachment close to the Moho, with the thrust fault being much steeper, penetrating the crust and probably the mantle lithosphere. In this interpretation the structure formed owing to the compressional reactivation of the fracture zone. Approximate dating of onlapping reflections on either side of the ridge constrains the timing of its formation as between 25 and 120 Ma ago. The Charcot Ridge represents one of the largest thrust structures to be identified in a passive margin setting. Many other compressional folds with the same orientation formed to the northeast in the Benue Trough, probably during the Santonian, as a result of a change in the spreading direction during South Atlantic rifting. We speculate that the same causal mechanism applies for the formation of the Charcot Ridge.

Briggs, Sepribo Eugene; Davies, Richard J.; Cartwright, Joe; Morgan, Richard

2009-02-01

19

Boron contents and isotopic compositions of the hydrothermally altered oceanic crust from the Troodos ophiolite, Cyprus  

Science.gov (United States)

The boron contents and isotopic compositions were determined for the hydrothermally altered oceanic crust through the Troodos ophiolite. The samples were represented by the International Crustal Research Drilling Group (ICRDG) drill-Holes CY1 (479m), CY2A (689m), CY4 (2263m), and selected outcrops along the Akaki river. Hole CY1 was composed upper and lower pillow lava, CY4 constituted sheeted dike complex and gabbro section, and the samples along Akaki river formed from pillow lava to sheeted dike complex. Hole CY2A was composed pillow lava and sheeted dike, drilled near Agrokipia ‘B’ deposit a stockwork type which completely enclosed within the lower pillow lava. The goal of this study is to understand the Boron geochemistry during hydrothermal alteration of the oceanic crust including hydrothermal ore deposit as Agrokipia ‘B’. The average boron contents of each sequence from Troodos ophiolite were pillow lava (63.2ppm), sheeted dike complex (4.5ppm), gabbro section (1.6ppm). But then, those of Oman ophiolite were 7.9ppm, 5.3ppm, 1.7ppm (Yamaoka et al., 2010 submitted). Thus, both of these ophiolites, the vertical profile of boron content decreased with depth, also the boron contents were much richer than fresh-MORB (0.5ppm) (Spivack and Edmond, 1987; Chaussidon and Jambon, 1994). This indicates boron rich of the altered oceanic crust were derived from seawater. And sheeted dike complex and gabbro section were similar value relatively, but pillow lava differed widely. These results may represent the difference of length being submarine, because these ophiolites were generated in deep water of the Tethys sea about 90Ma (Late Cretaceous) (Tilton et al., 1981; Mukasa and Ludden, 1987), and Oman ophiolite was obducted about 70Ma (Lanphere, 1981) but Troodos ophiolite uplifted about 10Ma (Middle Miocene) (Robertson and Woodcock, 1979).

Matsukura, S.; Yamaoka, K.; Ishikawa, T.; Kawahata, H.

2010-12-01

20

Deep structure in rifted crust at the ocean-continent margin in the northwestern Ross Sea  

Science.gov (United States)

The Ross Sea contains several deep sedimentary basins which formed as a result of distributed extension in continental crust during Cenozoic and Cretaceous time. These basins contain sedimentary sequences that are laterally extensive across multiple basins, which in the western Ross Sea represent infill from erosion of the Transantarctic Mountains. The Northern Basin lies in the northwestern Ross Sea, and borders oceanic crust that includes the Adare Trough spreading center, active from 43 to 26 Ma. This area provides an ideal location to study the mechanisms by which strain localized in a spreading center is transferred to adjacent continental crust. Refraction seismic records from 74 sonobuoys with 20 to 30 km of offset were obtained in the Northern and Adare Basins during research cruise NBP0701; they complement the ~2,700 km of multi-channel seismic (MCS) data, by probing the deeper velocity structure of the crust and by providing direct detection of layer velocities. We use standard techniques including linear moveout and conversion of the data into ?-p space (intercept time and slowness) to determine layer depths and velocities; we also construct a finite difference model of each sonobuoy in order to recognize converted phases, confidently tie the refracted arrivals to the reflections from which they originate (which are then tied to the shallower MCS data), and constrain layers’ s-wave velocities. In further support of the hypothesis that volcanic intrusions contributed significantly to the process of extension in the Northern Basin, high crustal velocities do not appreciably deepen when moving from the Adare Basin into the Northern Basin, as would be expected when moving from oceanic to continental crust. We consistently detect high crustal velocities at only a few kilometers depth into the crust, implying that processes such as compaction and erosion of sediment layers and volcanic intrusion have a significant effect on crustal structure.

Selvans, M. M.; Clayton, R. W.; Stock, J. M.; Cande, S. C.; Davey, F. J.

2010-12-01

 
 
 
 
21

Hydrothermal heat flux through aged oceanic crust: where does the heat escape?  

Science.gov (United States)

Recent publications suggest that most of the fluid flow in the upper oceanic crust is channelized through small volumes of rock and vented into the ocean. This implies that at flanks of generally thinly sedimented mid-ocean ridges, focused discharge at the seafloor should be concentrated most likely at outcrops, high-angle normal faults or seamounts. These vents should be associated with a significant heat flow signature. However, only few observations worldwide support this assumption up to now. On our quest for focused fluid exchange between young oceanic crust and the ocean we surveyed a 720 km long and 40-90 km wide off-axis portion of seafloor intersecting the East Pacific Rise near 14°14'S. A wealth of geophysical methods including high-resolution swath mapping bathymetry, single channel seismics, sediment echo sounding, magnetics and heat flow determinations were used. Heat flow data in the tectonic corridor cover crustal ages of 0.3-9.3 Ma. With respect to the conductive plate cooling model the data show the well-known pattern of low values close to the ridge, associated with vigorous hydrothermal circulation of cold seawater through the young upper crust, and a fast recovery to almost lithospheric conductive cooling values at a surprisingly young crustal age of 9.3 Ma. Although the sediment cover is fairly thin, measurements with a 3.6 m violin bow type heat probe were possible almost everywhere within the investigated area. A detailed survey between two large seamounts at 4.5 Ma revealed localized extremely high values of up to 618 mW/m 2 (275% of the expected heat flow) at the foot of the seamount. This is interpreted as a clear indication of focused discharge of hydrothermal fluid. If we, however, relate heat flow normalized by the expected conductive heat loss to the character of igneous basement, heat flow is highest in areas with an almost flat and sedimented basement, and lowest within ˜10-20 km of seamounts and other rough basement relief. We therefore hypothesize that the large number of seamounts covering the ocean floors governs a major amount of convective heat loss of aging oceanic lithosphere.

Villinger, Heinrich; Grevemeyer, Ingo; Kaul, Norbert; Hauschild, Jan; Pfender, Marion

2002-08-01

22

Age dependence and the effect of cracks on the seismic velocities of the upper oceanic crust  

Science.gov (United States)

Seismic velocities in young (e.g., sonobuoy data collected over 0--7 Ma oceanic crust near the East Pacific Rise using the hidden layer method estimates seismic velocities of the upper oceanic crust. The results of sonobuoy analyses indicate that mean top-of-basement velocities and velocity gradients are 2.8 +/- 0.1 km s-1 and 2.7 +/- 0.1 s-1 respectively. Results also suggest that top-of-basement velocities increase at a rate of 0.12 +/- 0.05 km s-1 Ma-1 . A pressure-dependent asperity-deformation model describes the increase in seismic velocities with depth observed from the sonobuoy data. The asperity-deformation model incorporates a velocity variation of the form V( z) = V0 (1 + z/ z0)1/n, where z is depth, V0 is the velocity at the seafloor, and z 0 and n are constants. The asperity-deformation model describes how seismic velocities can increase with pressure simply through the stiffening of cracks without a need for a change in mineral moduli. The observed traveltimes are modeled to within an average root-mean-square misfit of 3.5 ms (less than 0.8 percent).

Cerney, Brian Patrick

23

Cooling history of Atlantis Bank oceanic core complex: Evidence for hydrothermal activity 2.6 Ma off axis  

Science.gov (United States)

We report 26 (U-Th)/He zircon ages from Atlantis Bank, Southwest Indian Ridge, which constrain time scales and rates of lower crustal cooling in ultraslow spreading oceanic crust in this setting. Samples from the detachment fault surface indicate that denuded oceanic crust cooled rapidly (1200°C/Ma, consistent with existing models for the cooling of oceanic crust. (U-Th)/He zircon ages from samples collected along N-S and E-W trending faults scarps record young ages inconsistent with standard cooling models for lower oceanic crust. These samples have a mean (U-Th)/He zircon age 2.6 Ma younger than their corresponding igneous crystallization ages and record cooling through 200°C well outside the rift valley. Similar anomalously young ages are recorded by zircon, sphene, and apatite fission track data from ODP Hole 735B. We interpret these young ages as recording an off-axis thermal/heating event associated with localized high-temperature (>300°C) hydrothermal fluid flow resulting from underplated mafic magmas.

Schwartz, Joshua J.; John, Barbara E.; Cheadle, Michael J.; Reiners, Peter W.; Baines, A. Graham

2009-08-01

24

Microbial Observatories Designed to Assess the Basaltic Ocean Crust Deep Biosphere  

Science.gov (United States)

Although the current census of life indicates a large biomass of life in deep marine subsurface sediments, there is a relative dearth of information regarding microbial life in the hard rock environment of the ocean crust. Considering that the ocean crust comprises the largest hydrologically active environment on Earth, that crustal rocks can be out of redox equilibrium with percolating crustal fluids, and that recent evidence shows that seafloor exposed basalts harbor abundant and highly diverse microbial communities, it is likely that ocean crust basalt supports a significant microbial habitat. Due to current obstacles faced by the scientific community in sampling the oceanic hard rock seafloor, innovative microbial observatories have been deployed at the seafloor and deep in ocean crust to address whether sub-seafloor ocean crust basalts harbors a deep biosphere. These observatories are designed to encourage growth of in situ organisms on specific mineral surfaces for spatially resolved characterization of the microbial processes and populations involved in deep basalt weathering.

Orcutt, B.; Edwards, K.

2008-12-01

25

Boron isotope geochemistry of the oceanic crust from DSDP/ODP Hole 504B  

Energy Technology Data Exchange (ETDEWEB)

Boron contents and boron isotopic compositions were determined for the uppermost 1.3 km section of typical 6.2 Ma oceanic crust from DSDP/ODP Hole 504B, Costa Rica Rift, Galapagos Spreading Center. Both the boron content and the {delta}{sup 11}B value in the oceanic crust are controlled by two types of alteration: (1) low-temperature alteration (0 to 60C; Zones 1 and 2) and (2) high-temperature hydrothermal alteration (200 to 400C; Zones 3 and 4). Basalts subjected to the low-temperature alteration are characterized by their relatively high boron contents (0.69 to 19.3 ppm) and high {delta}{sup 11}B values (+2.2 to +10.6{per thousand}), indicating uptake of boron into secondary phases in equilibrium with seawater or evolved seawater. Hydrothermally altered basalts contain less abundant boron (0.17 to 0.52 ppm) and relatively constant {delta}{sup 11}B values ({minus}0.1 to +1.0{per thousand}). Although basalts from the upper part of these hydrothermal zones (< 1,300 mbsf) show equilibrated boron content and {delta}{sup 11}B value with aqueous fluid, effective leaching of boron from basalt is predominant in the lower part (> 1,300 mbsf). Original boron content and {delta}{sup 11}B value of the Hole 504B MORB were 0.35 ppm and +0.2{per thousand}, respectively. The present data provide fundamental information in understanding of the distribution of boron and boron isotopes in the oceanic crust.

Ishikawa, Tsuyoshi; Nakamura, Eizo (Okayama Univ., Tottori (Japan))

1992-04-01

26

Partial separation of halogens during the subduction of oceanic crust  

Science.gov (United States)

Incompatible elements, such as halogens, have the potential to act as key tracers for volatile transport processes in Earth and planetary systems. The determination of halogen abundances and ratios in different mantle reservoirs gives us the ability to better understand volatile input mechanisms into the Earth's mantle through subduction of oceanic crust. Halogen partition coefficients were experimentally determined between forsterite, orthopyroxene and silicate melt at pressures ranging from 1.0 to 2.3 GPa and temperatures ranging from 1500-1600°C, thus representing partial melting conditions of the Earth's mantle. Combining our data with results of recent studies (Beyer et al. 2012; Dalou et al. 2012) shows that halogen partitioning between forsterite and melt increases by factors of about 1000 (fluorine) and 100 (chlorine) between 1300°C and 1600°C and does not show any pressure dependence. Chlorine partitioning between orthopyroxene and melt increases by a factor of about 1500 for a temperature increase of 100°C (anywhere between 1300°C and 1600°C), but decreases by a factor of about 1500 for a pressure increase of 1.0 GPa (anywhere between 1.0 GPa and 2.5 GPa). At similar P-T conditions, a comparable effect is observed for the fluorine partitioning behaviour, which increases by 500-fold for a temperature increase of 100°C and decreases with increasing pressure. Halogen abundances in mid-ocean ridge basalts (MORB; F=3-15, Cl=0.5-14ppm) and ocean island basalts (OIB; F=35-65, Cl=21-55 ppm) source regions were estimated by combining our experimentally determined partition coefficients with natural halogen concentrations in oceanic basalts (e.g. Ruzié et al. 2012). The estimated chlorine OIB source mantle concentration is in almost perfect agreement with primitive mantle estimates (Palme and O'Neill 2003). If we expect an OIB source mantle slightly depleted in incompatible elements, this suggests that at least small amounts of chlorine are recycled deep into the mantle through subduction of oceanic crust, possibly via marine pore fluids (Sumino et al. 2010). The OIB source region is, however, significantly enriched in fluorine relative to the primitive mantle by a factor of 1.4-3.6, which indicates that significantly larger amounts of fluorine are transported deep into the Earth's mantle through subduction. An explanation for the partial separation of chlorine and fluorine during subduction is that the heavy halogens are more likely to escape from the subducting slab in hydrous fluids at an early subduction stage whereas significant amounts of fluorine are likely to remain in the slab, possibly incorporated in the lattice of hydrous amphibole or mica, or in anhydrous high-pressure phases of eclogite. The MORB source mantle is degassed in fluorine (17-88%) and chlorine (22-99%) relative to primitive mantle estimates. Preliminary data suggest that the bromine partitioning behaviour between forsterite and melt is roughly comparable to the behaviour of fluorine and chlorine. If true, this would imply that the Earth's upper mantle is presumably degassed of all halogens despite the more likely escape of heavy halogens from the slab at an early subduction stage, implying that these halogens are at least partly accumulating in the crust after leaving the slab. Beyer C, Klemme S, Wiedenbeck M, Stracke A, Vollmer C (2012) Earth Planet Sci. Lett. 337-338, pp. 1-9. Dalou C, Koga KT, Shimizu N, Boulon J, Devidal JL (2012) Contrib. Mineral. Petrol. 163, pp. 591-609. Palme H, O'Neill HSTC (2003) Treatise Geochem. 2, pp. 1-38. Ruzié L, Burgess R, Hilton DR, Ballentine CJ (2012) AGU Fall Meeting 2012. V31A-2762 (abstr.). Sumino H, Burgess R, Mizukami T, Wallis SR, Holland G, Ballentine CJ (2010) Earth Planet. Sci. Lett. 294, pp. 163-172.

Joachim, Bastian; Pawley, Alison; Lyon, Ian; Henkel, Torsten; Clay, Patricia L.; Ruzié, Lorraine; Burgess, Ray; Ballentine, Christopher J.

2014-05-01

27

Widespread Occurrence of Zircon in Slow- and Ultraslow Spreading Ocean Crust: A Tool for Studying Ocean Lithospheric Processes  

Science.gov (United States)

The presence of igneous zircon in oceanic gabbro and peridotite provides a new opportunity to constrain absolute ages, and the processes and rates of crustal accretion in oceanic environments. Our recent investigations show zircon to be common in slow and ultraslow spreading oceanic crust including several locations along the Mid-Atlantic Ridge (MAR) and Southwest Indian Ridge (SWIR), and in rock types ranging from trondjhemite dikes to peridotite. Zircon is typically found in felsic intrusions and oxide gabbro, and in many cases may be due to late stage saturation in small pockets of residual melt. We report the morphologic and chemical characteristics of zircon grains collected from >100 rock samples recovered both from the seafloor by manned submersible and ROV, and with depth by ODP/IODP drilling. Grains range from euhedral and faceted to anhedral and fractured, with internal zonation that may be homogeneous, concentric, or patchy, and rarely contain relict cores. Sizes range from 1 mm. Measurements of major, minor, and trace element concentrations and high-resolution Pb/U ages were collected with the SHRIMP-RG. Chondrite-normalized rare earth element (REE) patterns for more than 50 zircon grains are uniform in shape and closely resemble patterns for known terrestrial igneous zircon. This is in contrast to mantle affinity zircon (e.g. kimberlite), which typically show depleted and relatively unfractionated patterns. Observed total REE concentrations range from 330-3765 ppm. Patterns are convex upward and rise sharply towards the HREE, with normalized Sm/La ratios = 16-320 and Lu/Gd ratios = 20-51. Positive Ce and negative Eu anomalies are ubiquitous. Hf abundances range from 5988 to 14,266 ppm. Other elements occurring at minor abundance levels include Y (463-6949 ppm), P (253-2288 ppm), U (7-2827 ppm), and Th (3-7403 ppm). Preliminary Ti concentrations range from 13 to 270 ppm, indicating crystallization temperatures of 765 to 1147°C based on Ti in zircon thermometry. Enrichment of U and Th (>100 ppm) is frequently observed, and allows for the application of isotopic dating techniques on rocks collected near ridge axes. SHRIMP-RG Pb/U age determinations reveal resolution on the order of 2% corresponding to 40,000 years in crust as young as 2 Ma, and errors of <2% for samples up to 13 Ma. Attempts to employ the (U-Th)/He dating method on oceanic zircon have also been successful. With a closure temperature (Tc) of 180±20°C, this technique can be used in conjunction with U/Pb dating (Tc = 900±50°C) to bracket magnetic ages (Tc = 450-580°C). By applying multi-temperature chronometers to zircon collected from boreholes we can measure cooling rates related to emplacement, denudation, and/or tectonic rotation. Finally, the relatively common presence of zircon in oceanic crust precludes the simple assumption that detrital zircons in Archean meta-sedimentary rocks demonstrate the existence of continental crust.

Grimes, C. B.; John, B. E.; Cheadle, M. J.; Schwartz, J. J.

2005-12-01

28

Rock magnetic characterization through an intact sequence of oceanic crust, IODP hole 1256 and comparison with DSDP 504 B  

International Nuclear Information System (INIS)

Complete text of publication follows. One goal of drilling a complete oceanic crust section is to determine the source of marine magnetic anomalies. For crust generated by fast seafloor spreading, is the signal dominated by the upper extrusive layer, do the sheeted dikes play a role, what role do the gabbros play relative to slow spreading centers, and what is the timing of acquisition of the magnetization? To address these questions, we are conducting a comprehensive set of rock magnetic, paleomagnetic measurements and microscopic studies that extend through the intervals drilled on Leg 206 and Expeditions 309 and 312. Recent drilling in the Eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate (i.e. up to 200mm/year full rate) on the Cocos-Pacific plate boundary between 19 and 12 million years ago. Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Continuous downhole variations in magnetic grain size, coercivity, mass-normalized susceptibility, Curie temperatures, and composition have been mapped. Compositionally, we have found that the iron oxides vary from being titanium-rich (TM60) to titanium-poor magnetite as determined semi-quantitatively from Curie temperature analyses. tatively from Curie temperature analyses. Magnetic grain sizes vary from few Single Domain (SD), to the majority of them being Pseudo Single Domain (PSD) and some on the Multi Domain (MD) area of the Day diagram. The low-Ti magnetite or stoichiometric magnetite is present mainly in the lowest part of the section and is associated with higher Curie temperatures (550 A deg C to near 580 A deg C) and higher coercivities than in the extrusive basalts. Skeletal titanomagnetites with varying degrees of alteration is the most common magnetic mineral throughout the section and is often bordered by large iron sulfide grains. Last but not least, absolute paleointensity experiments have been determined on several samples, although the success rate is low as has been found in other studies of oceanic basalts. We also compare our rock magnetic results with the results obtained from DSDP hole 504B to have a better characterization of the rock magnetic properties of the two sampling sites within the Cocos Plate (?15 Ma) and Nazca Plates (?5.9 Ma) respectively.

29

Methane deep in ocean crust could feed chemical-hungry microorganisms  

Science.gov (United States)

This University of Washington news article details the work of oceanographer Deborah Kelley regarding microorganisms found in "layer 3" of the ocean crust. These microbes live deep beneath the sea floor and are thought to survive on methane gas trapped in fractures in the crust.

Hines, Sandra; Information, Uw O.

30

Deformation and rupture of the oceanic crust may control growth of Hawaiian volcanoes  

Science.gov (United States)

Hawaiian volcanoes are formed by the eruption of large quantities of basaltic magma related to hot-spot activity below the Pacific Plate. Despite the apparent simplicity of the parent process - emission of magma onto the oceanic crust - the resulting edifices display some topographic complexity. Certain features, such as rift zones and large flank slides, are common to all Hawaiian volcanoes, indicating similarities in their genesis; however, the underlying mechanism controlling this process remains unknown. Here we use seismological investigations and finite-element mechanical modelling to show that the load exerted by large Hawaiian volcanoes can be sufficient to rupture the oceanic crust. This intense deformation, combined with the accelerated subsidence of the oceanic crust and the weakness of the volcanic edifice/oceanic crust interface, may control the surface morphology of Hawaiian volcanoes, especially the existence of their giant flank instabilities. Further studies are needed to determine whether such processes occur in other active intraplate volcanoes. ??2008 Nature Publishing Group.

Got, J.-L.; Monteiller, V.; Monteux, J.; Hassani, R.; Okubo, P.

2008-01-01

31

Growth and Construction of Oceanic Crust at Atlantis Bank, Southwest Indian Ridge  

Science.gov (United States)

Magmatic zircon is a common accessory mineral in oceanic crustal rocks including gabbro, oxide gabbro, diabase and felsic veins. Its presence in these rocks provides an exceptional opportunity to document crustal growth processes at slow-spreading mid-ocean ridges. We present nineteen Pb/U zircon SHRIMP-RG ion probe ages of lower crustal rocks collected by manned submersible, ROV, dredging and ODP drilling from a 20 x 30 km2 area of Atlantis Bank, Southwest Indian Ridge, which allow us to constrain the growth and construction of oceanic crust. Weighted average 206Pb/238U ages of these samples range from 10.7 to 13.9 Ma, with errors of 0.1-0.6 m.y. (<1 - 4%). At least 75% of these gabbros accreted within error of the predicted sea-surface magnetic age, whereas up to 25% are between 700,000 and 2.5 m.y. older. In one sample, we identified zircon with inherited cores as much as 1.5 m.y. older than their corresponding rims. There is no observable correlation between age and lithology, and the anomalously old samples are not from any specific part of Atlantis Bank; they appear to be randomly distributed amongst the non-anomalous age samples and come from various structural depths. We consider two models to explain the presence of these anomalously old rocks: i) a stochastic intrusion model whereby magma was intruded at different spatial locations within the rift valley as the plates spread apart, resulting in the entrapment of older lower crust by subsequent intrusions; and/or ii) a model in which some gabbroic bodies originally crystallized at depths of ~5-18 km below the base of the crust in a thick, cold, axial lithosphere and were subsequently uplifted along flow-lines and intruded by shallow-level magmas during the creation of Atlantis Bank. In this model, the difference in time between the Pb/U zircon crystallization age and the magnetic age is a proxy for the depth at which zircon crystallized (assuming a constant mantle upwelling rate during the construction of Atlantis Bank over ~1.3 m.y.). We prefer the latter model, although aspects of both models may apply.

Schwartz, J. J.; John, B. E.; Cheadle, M. J.; Miranda, E. A.; Grimes, C. B.; Wooden, J. L.; Dick, H. J.

2005-12-01

32

Continental Growth and Recycling in Convergent Orogens with Large Turbidite Fans on Oceanic Crust  

Directory of Open Access Journals (Sweden)

Full Text Available Convergent plate margins where large turbidite fans with slivers of oceanic basement are accreted to continents represent important sites of continental crustal growth and recycling. Crust accreted in these settings is dominated by an upper layer of recycled crustal and arc detritus (turbidites underlain by a layer of tectonically imbricated upper oceanic crust and/or thinned continental crust. When oceanic crust is converted to lower continental crust it represents a juvenile addition to the continental growth budget. This two-tiered accreted crust is often the same thickness as average continental crustal and is isostatically balanced near sea level. The Paleozoic Lachlan Orogen of eastern Australia is the archetypical example of a tubidite-dominated accretionary orogeny. The Neoproterozoic-Cambrian Damaran Orogen of SW Africa is similar to the Lachlan Orogen except that it was incorporated into Gondwana via a continent-continent collision. The Mesozoic Rangitatan Orogen of New Zealand illustrates the transition of convergent margin from a Lachlan-type to more typical accretionary wedge type orogen. The spatial and temporal variations in deformation, metamorphism, and magmatism across these orogens illustrate how large volumes of turbidite and their relict oceanic basement eventually become stable continental crust. The timing of deformation and metamorphism recorded in these rocks reflects the crustal thickening phase, whereas post-tectonic magmatism constrains the timing of chemical maturation and cratonization. Cratonization of continental crust is fostered because turbidites represent fertile sources for felsic magmatism. Recognition of similar orogens in the Proterozoic and Archean is important for the evaluation of crustal growth models, particularly for those based on detrital zircon age patterns, because crustal growth by accretion of upper oceanic crust or mafic underplating does not readily result in the addition of voluminous zircon-bearing magmas at the time of accretion. This crust only produces significant zircon when and if it partially melts, which may occur long after accretion.

Ben D. Goscombe

2013-07-01

33

IODP Expedition 345: Structural characteristics of fast spread lower ocean crust, implications for growth and cooling of ocean crust  

Science.gov (United States)

IODP Expedition 345 to the Hess Deep Rift sampled ~1 Ma, fast-spread East Pacific Rise gabbroic crust exposed as a dismembered, lower crustal section. Sixteen holes were drilled at Site U1415, centered on a sub-horizontal, 200-m wide E-W-trending bench between 4675 and 4850 mbsl. The bench was formed as a rotational slide within a 1km high slump along the southern wall of the intra-rift ridge. Primitive olivine gabbro and troctolite (Mg# 76-89) were sampled in four discrete, 30 to ? 65 m sized blocks formed by the mass wasting that dominates the southwestern slope of the ridge. Igneous fabric orientations (both layering and foliation) in the blocks vary from sub-vertical to gently dipping, suggesting some of the blocks have rotated at least 90°. Magmatic fabrics including spectacular modal and/or grain size layering are prevalent in >50% of the recovered core. Magmatic foliation in all blocks is defined by plagioclase crystal shape, but may also be defined by olivine and, to a lesser extent, orthopyroxene and clinopyroxene when the crystals have suitable habits. In all cases, this foliation is controlled by both the preferred orientation and shape anisotropy of the crystals. Fabric intensity varies from moderate to strong in the block with simple modal layering, weak to absent in the two blocks of troctolite, and largely absent in the block with heterogeneous textures and/or diffuse banding. Intrinsic to the layering and banding is the common development of dendritic and/or skeletal olivine textures (grain size up to 3 cm). The preservation of these delicate olivine grains showing only limited subgrain formation, and no kinking precludes significant low melt fraction (hydraulic fracturing.

John, B. E.; Ceuleneer, G.; Cheadle, M. J.; Harigane, Y.

2013-12-01

34

Sr-Nd-Pb composition of Mesozoic Pacific oceanic crust (Site 1149 and 801, ODP Leg 185): Implications for alteration of ocean crust and the input into the Izu-Bonin-Mariana subduction system  

Science.gov (United States)

We report Sr, Nd and Pb isotopic compositions of sediments and variably altered igneous rocks from ODP Site 801 (Marianas) and ODP Site 1149 (Izu-Bonin). These Sites provide the most complete drilled ocean crust sections located in front of the Mariana and Izu-Bonin trenches and characterize the unmodified isotopic input into these subduction zones. The subducted ocean crust belongs to the oldest (130-167 Ma) in situ Pacific Ocean crust and thus has end-member character with respect to alteration and sediment load. The lithostratigraphic division of sedimentary units at Site 1149 into clays, cherts, lower clays and carbonates with clay is reflected on isotope correlation diagrams. The Pb isotope data of the sediments show much greater variation than previously reported from this region. Particularly noteworthy are zeolite-bearing clays and clay bearing carbonates from the lower Units that have Pb isotopic compositions identical to the Izu Volcanic Front. The basaltic basement samples display variable 87Sr/86Sr ratios at near constant 143Nd/144Nd ratios, indicating mixing with seawater derived Sr. Most basaltic samples from Site 1149 and 801 exhibit highly variable 206Pb/204Pb (17.88-20.00) at near constant 207Pb/204Pb and 208Pb/204Pb ratios. Three samples from Site 801 display the most extreme 206Pb/204Pb (23.70-26.86) and 207Pb/204Pb (15.73-15.83) ratios ever measured in altered MORB reflecting an increase of 238U/204Pb ratios (?), most likely through addition of seawater derived U. Initial Pb isotopes of most samples overlap with the age corrected field of the Pacific MORB source, thus the increase in ? took place shortly after formation of the crust in most samples. According to our new isotope data the radiogenic end-member of the Izu arc volcanic rocks could either represent Pb from the lower sediment column released from the slab by delayed dewatering or an integrated slab fluid in which 90-95% of the Pb comes from the basaltic crust and 5-10% of the Pb from the sediments. The Pb isotope systematics of the Mariana arc output suggest two component mixing. Both components appear to be input derived with the radiogenic component represented by average Site 801 sediment and the unradiogenic component generated by mixing of ˜80% unaltered crust with ˜20% highly altered crust.

Hauff, Folkmar; Hoernle, Kaj; Schmidt, Angelika

2003-08-01

35

Synthesis of the Geology of Lava and Sheeted Dike Units in Oceanic Crust: Implications for Accretionary Processes  

Science.gov (United States)

The internal structure of lava and sheeted dike units of the upper oceanic crust formed at intermediate to fast spreading rates have been documented in several studies of major escarpments (tectonic windows) and deep crustal drill holes. Geological relationships imply important crustal accretion processes that are not evident from surface geology or geophysical investigations along spreading centers. Available data come from crust of different ages (1-180 Ma) and generated at a range of spreading rates (60->200 mm/yr). Collectively, they reveal a spectrum of structures that may be related to spreading-rate-sensitive parameters, especially magma budget. The following variations occur with increasing spreading rate: 1) decreasing thickness of basaltic lavas with highly variable proportions of sheet flows relative to pillow and lobate forms; 2) increasing thickness of the sheeted dike unit; 3) decreasing variability in the thickness of upper crustal units (lavas plus sheeted dikes); 4) decreasing tilting of inward-dipping lavas and outward-dipping dikes during axial magmatic construction; 5) less pervasive fracturing and increasingly localized faulting and high- temperature hydrothermal alteration. Overall, spreading at faster rates (and presumably higher magma budgets) appears to create a simpler, more uniform upper crust that approaches models based on ophiolite complexes. Geological relationships suggest that sub-axial subsidence (hundreds of meters) accommodates thickening of the lava unit resulting in tectonic rotations of rock units and brittle damage to the underlying crustal units even at superfast spreading rates. Subaxial subsidence in the upper crustal units must be accommodated by mass redistribution in the underlying Axial Magma Chamber and Low-Velocity Zone. The record of processes at these deeper levels may be preserved in the gabbroic rock units of the middle to lower oceanic crust: the next frontier in understanding crustal accretion beneath oceanic spreading centers.

Karson, J.

2008-12-01

36

Transition from oceanic to continental lithosphere subduction in southern Tibet: Evidence from the Late Cretaceous-Early Oligocene (~ 91-30 Ma) intrusive rocks in the Chanang-Zedong area, southern Gangdese  

Science.gov (United States)

Little is known about the detailed processes associated with the transition from oceanic to continental lithosphere subduction in the Gangdese Belt of southern Tibet (GBST). Here, we report zircon U-Pb age, major and trace element and Sr-Nd-Hf isotopic data for Late Cretaceous-Early Oligocene (~ 91-30 Ma) intermediate-acid intrusive rocks in the Chanang-Zedong area immediately north of the Yarlung-Tsangpo suture zone. These rocks represent five magmatic episodes at ~ 91, ~ 77, ~ 62, ~ 48, and ~ 30 Ma, respectively. The 91-48 Ma rocks have slightly lower initial 87Sr/86Sr (0.7037 to 0.7047), and higher ?Nd(t) (+ 1.8 to + 4.3) and ?Hf(t) (+ 3.5 to + 14.7) values in comparison with those (0.7057 to 0.7062, - 3.3 to - 2.5 and + 2.2 to + 6.6) of the ~ 30 Ma intrusive rocks. The ~ 91, ~ 62 and ~ 30 Ma rocks are geochemically similar to slab-derived adakites. The ~ 91 Ma Somka adakitic granodiorites were likely derived by partial melting of the subducting Neo-Tethyan oceanic crust with minor oceanic sediments, and the ~ 91 Ma Somka dioritic rocks with a geochemical affinity of adakitic magnesian andesites likely resulted from interactions between adakitic magmas and overlying mantle wedge peridotite. The ~ 77 Ma Luomu diorites were probably generated by partial melting of juvenile basaltic lower crust. The ~ 62 Ma Naika and Zedong adakitic diorites and granodiorites were likely generated mainly by partial melting of thickened juvenile mafic lower crust but the source region of the Zedong adakitic rocks also contained enriched components corresponding to Indian continental crust. The ~ 48 Ma Lamda granites were possibly generated by melting of a juvenile basaltic crust. The younger (~ 30 Ma) Chongmuda adakitic quartz monzonites and minor granodiorites were most probably derived by partial melting of Early Oligocene northward-subducted Indian lower crust beneath the southern Lhasa Block. Taking into account the regional tectonic and magmatic data, we suggest that the Gangdese Belt of southern Tibet (GBST) underwent a tectonodynamic transition from oceanic subduction to continental subduction between 100 and 30 Ma. It evolved through four stages: 100-65 Ma roll-back of subducted Neo-Tethyan oceanic lithosphere; 65-60 Ma initial collision between Indian and Asian continents; 60-40 Ma breakoff of subducted Neo-Tethyan oceanic lithosphere; and ~ 30 Ma northward subduction of the Indian continent.

Jiang, Zi-Qi; Wang, Qiang; Wyman, Derek A.; Li, Zheng-Xiang; Yang, Jin-Hui; Shi, Xiao-Bing; Ma, Lin; Tang, Gong-Jian; Gou, Guo-Ning; Jia, Xiao-Hui; Guo, Hai-Feng

2014-05-01

37

Trace element chemistry of zircons from oceanic crust: A method for distinguishing detrital zircon provenance  

Science.gov (United States)

We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the field defined by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fingerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid field, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB.

Grimes, C. B.; John, B. E.; Kelemen, P. B.; Mazdab, F. K.; Wooden, J. L.; Cheadle, M. J.; Hanghøj, K.; Schwartz, J. J.

2007-07-01

38

Coring an In Situ Section of Upper Oceanic Crust formed by Superfast Seafloor Spreading: Shipboard Results from Ocean Drilling Program Leg 206  

Science.gov (United States)

Drilling a complete section of oceanic crust has been a long term objective since the inception of scientific ocean drilling. Recovery of in situ oceanic crust is imperative to understand igneous accretion and the complex interplay between magmatic, hydrothermal, and tectonic processes, as well as a means for calibrating remote geophysical observations, particularly seismic and magnetic data. Only by drilling a complete section of upper crust formed far from fracture zones can the processes operating at normal mid-ocean ridges be understood. Leg 206 completed the initial phase of a planned two-leg project to drill a complete in situ section of the upper oceanic crust that will eventually extend through the extrusive lavas and sheeted dikes and into gabbros. Drilling was conducted at ODP Site 1256 (6.736N, 91.934W), which resides on 15-Ma oceanic lithosphere of the Cocos plate that was formed by superfast seafloor spreading (>200 mm/yr) at the East Pacific Rise. Given the observation of an inverse relationship between spreading rate and depth to axial low-velocity zones, it should be possible to reach gabbros on a second leg to this site. Preparatory work at Site 1256 recovered a complete sequence of the 250 m sedimentary overburden and 88.5 m of basement in pilot holes (Holes 1256A, B, and C), before we installed a reentry cone with a 16-inch-diameter casing string extending 20 m into basement. Hole 1256D was then drilled to a total depth of 502 m sub-basement with high recovery (50%). The basement section consists of sheet flows and massive flows with subordinate pillow lavas, hyaloclastites, and rare dikes, capped by a sequence of evolved, massive flows (up to 75 m thick), which apparently flowed several kilometers off axis. The lavas have N-MORB compositions and are only slightly affected by low temperature hydrothermal alteration. Our shipboard results include characterization of the petrology, alteration, structure, physical properties, paleomagnetism, chemistry, heat flow, microbiology, and the digital scanning of basalt whole-rounds for core orientation after integration with the wireline logs. A complete suite of geophysical downhole tools, including the first deployment in a basement hole of the Ultrasonic Borehole Imager, confirmed that the hole is in excellent condition. Hole 1256D is open to its full depth (752 m below seafloor) and primed for future deepening into the sheeted dikes and gabbros early in the next phase of ocean drilling.

Wilson, D. S.; Teagle, D. A. H.; Acton, G. D.; Leg 206 Shipboard Scientific Party

2003-04-01

39

Millennial-scale ocean acidification and late Quaternary decline of cryptic bacterial crusts in tropical reefs.  

Science.gov (United States)

Ocean acidification by atmospheric carbon dioxide has increased almost continuously since the last glacial maximum (LGM), 21,000 years ago. It is expected to impair tropical reef development, but effects on reefs at the present day and in the recent past have proved difficult to evaluate. We present evidence that acidification has already significantly reduced the formation of calcified bacterial crusts in tropical reefs. Unlike major reef builders such as coralline algae and corals that more closely control their calcification, bacterial calcification is very sensitive to ambient changes in carbonate chemistry. Bacterial crusts in reef cavities have declined in thickness over the past 14,000 years with largest reduction occurring 12,000-10,000 years ago. We interpret this as an early effect of deglacial ocean acidification on reef calcification and infer that similar crusts were likely to have been thicker when seawater carbonate saturation was increased during earlier glacial intervals, and thinner during interglacials. These changes in crust thickness could have substantially affected reef development over glacial cycles, as rigid crusts significantly strengthen framework and their reduction would have increased the susceptibility of reefs to biological and physical erosion. Bacterial crust decline reveals previously unrecognized millennial-scale acidification effects on tropical reefs. This directs attention to the role of crusts in reef formation and the ability of bioinduced calcification to reflect changes in seawater chemistry. It also provides a long-term context for assessing anticipated anthropogenic effects. PMID:25040070

Riding, R; Liang, L; Braga, J C

2014-09-01

40

Constant Molybdenum Isotope Composition of Ocean Water and Fe-Mn crusts for the Last 70 Myr  

Science.gov (United States)

In the relatively new field of heavy stable isotope geochemistry, molybdenum (Mo) is one of the very promising elements. Molybdenum is a redox-sensitive trace metal. Isotope fractionation during terrestrial processes such as low-temperature redox transitions, chemical weathering, changes in the composition of the atmosphere, hydrothermal activity and sedimentary cycling is likely. Molybdenum is also an essential element for biological nitrogen fixation. Therefore, biogeochemical Mo isotope fractionation is also probable. The oceans represent an important terrestrial Mo reservoir. Dissolved concentrations in seawater are relatively high (0.01 ppm). The global ocean residence time is corresponding high with 800 kyr. The aim of this study is to characterise the principle present day oceanic Mo reservoirs and their changes with time. Molybdenum isotopic compositions were determined precisely using a Nu instruments MC-ICP-MS. Instrumental and laboratory mass fractionation is separated from natural mass dependent fractionation by addition of a molybdenum double spike prior to chemical separation (Siebert et al., 2001). The external standard reproducibility is at or below 0.1 per mil for the 98Mo/95Mo ratio (2s.d.). We analysed ocean water samples from the Atlantic (n=3, 0m-2400m depth), the Pacific and the Indian Ocean (deep water). These yield a homogeneous Mo isotopic composition as would be expexted from the long residence time of Mo in the oceans. Ocean water has the heaviest Mo isotopic composition measured to date (+2.3 per mil on the 98Mo/95Mo ratio relative to a Johnson Mattey ICP standard solution, lot 602332B). In view of the homogeneous ocean water ratios, we propose the use of present day ocean water as an reference standard (Mean Ocean Molybdenum: MOMO). Significantly lighter compositions from -2.7 to -3.1 per mil on the 98Mo/95Mo ratio relative to MOMO were determined for six Fe-Mn crust surface layers. Pelagic clay (-2.7 per mil) and clastic sediments (-2.3 and -2 per mil) show composition inbetween. A depth profile through a 70Ma old Fe-Mn crust revealed steep gradients for Mo concentrations (300ppm to 400ppm within 2 Myr), precluding post-depositional homogenisation of Mo. In contrast, the Mo isotopic compositions are uniform throughout the entire 70 Myr profile (-3.2 +/- 0.1 per mil relative to 98Mo/95Mo MOMO). We conclude, that a constant Mo isotopic composition must be assumed for ocean water during the last 70 Myr.

Siebert, C.; Nagler, T. F.; von Blankenburg, F.; Kramers, J. D.

2001-12-01

 
 
 
 
41

Compositional variation and genesis of ferromanganese crusts of the Afanasiy—Nikitin Seamount, Equatorial Indian Ocean  

Science.gov (United States)

Eight ferromanganese crusts (Fe-Mn crusts) with igneous and sedimentary substrates collected at different water depths from the Afanasiy-Nikitin Seamount are studied for their bulk major, minor and rare earth element composition. The Mn/Fe ratios < 1.5 indicate the hydrogenetic accretion of the Fe-Mn hydroxides. These Fe-Mn crusts are enriched in Co (up to 0.9%, average ˜ 0.5%) and Ce. The Ce-content is the highest reported so far (up to 3763 ppm, average ˜ 2250 ppm) for global ocean seamount Fe-Mn crusts. In spite of general similarity in the range of major, minor, and strictly trivalent rare earth element composition, the dissimilarity between the present Fe-Mn crusts and the Pacific seamount Fe-Mn crusts in Co and Ce associations with major mineral phases indicates inter-oceanic heterogeneity and region-specific conditions responsible for their enrichment. The decrease in Ce-anomaly (from ˜ 8 to ˜ 1.5) with increasing water depth (from ˜ 1.7 km to ˜ 3.2 km) might suggest that the modern intermediate depth low oxygen layer was shifted and sustained at a deeper depth for a long period in the past.

Rajani, R. P.; Banakar, V. K.; Parthiban, G.; Mudholkar, A. V.; Chodankar, A. R.

2005-02-01

42

The Deep Subsurface Biosphere in Igneous Ocean Crust: Frontier Habitats for Microbiological Exploration  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We discuss ridge flank environments in the ocean crust as habitats for subseafloor microbial life. Oceanic ridge flanks, areas far from the magmatic and tectonic influence of seafloor spreading, comprise one of the largest and least explored microbial habitats on the planet. We describe the nature of selected ridge flank crustal environments, and present a framework for delineating a continuum of conditions and processes that are likely to be important for defining subseafloor microbial "prov...

Edwards, Katrina J.; Fisher, Andrew T.; Wheat, C. Geoffrey

2012-01-01

43

Structure and Geochemistry of the Continental-Oceanic Crust Boundary of the Red Sea and the Rifted Margin of Western Arabia  

Science.gov (United States)

The continental-oceanic crust boundary and an incipient oceanic crust of the Red Sea opening are exposed within the Arabian plate along a narrow zone of the Tihama Asir coastal plain in SW Saudi Arabia. Dike swarms, layered gabbros, granophyres and basalts of the 22 Ma Tihama Asir (TA) continental margin ophiolite represent products of magmatic differentiation formed during the initial stages of rifting between the African and Arabian plates. Nearly 4-km-wide zone of NW-trending sheeted dikes are the first products of mafic magmatism associated with incipient oceanic crust formation following the initial continental breakup. Gabbro intrusions are composed of cpx-ol-gabbro, cpx-gabbro, and norite/troctolite, and are crosscut by fine-grained basaltic dikes. Granophyre bodies intrude the sheeted dike swarms and are locally intrusive into the gabbros. Regional Bouger gravity anomalies suggest that the Miocene mafic crust represented by the TA complex extends westward beneath the coastal plain sedimentary rocks and the main trough of the Red Sea. The TA complex marks an incipient Red Sea oceanic crust that was accreted to the NE side of the newly formed continental rift in the earliest stages of seafloor spreading. Its basaltic to trachyandesitic lavas and dikes straddle the subalkaline-mildly alkaline boundary. Incompatible trace element relationships (e.g. Zr-Ti, Zr-P) indicate two distinct populations. The REE concentrations show an overall enrichment compared to N-MORB; light REEs are enriched over the heavy ones ((La/Yb)n > 1), pointing to an E-MORB influence. Nd-isotope data show ?Nd values ranging from +4 to +8, supporting an E-MORB melt source. The relatively large variations in ?Nd values also suggest various degrees of involvement of continental crust during ascent and emplacement, or by mixing of another mantle source.

Dilek, Y.; Furnes, H.; Schoenberg, R.

2009-12-01

44

Glacial cycles drive variations in the production of oceanic crust  

CERN Document Server

Glacial cycles redistribute water between the oceans and continents causing pressure changes in the upper mantle, with potential consequences for melting of Earth's interior. A numerical model of mid-ocean ridge dynamics that explicitly includes melt transport is used to calculate the melting effects that would be caused by Plio-Pleistocene sea-level variations. Model results interpreted in the context of an analytical approximation predict sea-level induced variations in crustal thickness on the order of hundreds of meters. The specifics of the response depend on rates of sea-level change, mid-ocean ridge spreading rates, and mantle permeability. Spectral analysis of the bathymetry of the Australian-Antarctic ridge shows significant spectral energy near 23, 41, and 100 ky periods, consistent with model results and with the spectral content of Pleistocene sea-level variability. These results support the hypothesis that sea-floor topography records the magmatic response to changes in sea level, reinforcing the...

Crowley, John W; Huybers, Peter; Langmuir, Charles H; Park, Sung-Hyun

2014-01-01

45

Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life  

Science.gov (United States)

We have considered a wide array of scenarios for Europa's chemical evolution in an attempt to explain the presence of ice and hydrated materials on its surface and to understand the physical and chemical nature of any ocean that may lie below. We postulate that, following formation of the jovian system, the europan evolutionary sequence has as its major links: (a) initial carbonaceous chondrite rock, (b) global primordial aqueous differentiation and formation of an impure primordial hydrous crust, (c) brine evolution and intracrustal differentiation, (d) degassing of Europa's mantle and gas venting, (e) hydrothermal processes, and (f) chemical surface alteration. Our models were developed in the context of constraints provided by Galileo imaging, near infrared reflectance spectroscopy, and gravity and magnetometer data. Low-temperature aqueous differentiation from a carbonaceous CI or CM chondrite precursor, without further chemical processing, would result in a crust/ocean enriched in magnesium sulfate and sodium sulfate, consistent with Galileo spectroscopy. Within the bounds of this simple model, a wide range of possible layered structures may result; the final state depends on the details of intracrustal differentiation. Devolatilization of the rocky mantle and hydrothermal brine reactions could have produced very different ocean/crust compositions, e.g., an ocean/crust of sodium carbonate or sulfuric acid, or a crust containing abundant clathrate hydrates. Realistic chemical-physical evolution scenarios differ greatly in detailed predictions, but they generally call for a highly impure and chemically layered crust. Some of these models could lead also to lateral chemical heterogeneities by diapiric upwellings and/or cryovolcanism. We describe some plausible geological consequences of the physical-chemical structures predicted from these scenarios. These predicted consequences and observed aspects of Europa's geology may serve as a basis for further analys is and discrimination among several alternative scenarios. Most chemical pathways could support viable ecosystems based on analogy with the metabolic and physiological versatility of terrestrial microorganisms. ?? 2000 Academic Press.

Kargel, J.S.; Kaye, J.Z.; Head, J. W., III; Marion, G.M.; Sassen, R.; Crowley, J.K.; Ballesteros, O.P.; Grant, S.A.; Hogenboom, D.L.

2000-01-01

46

Deep mantle cycling of oceanic crust: evidence from diamonds and their mineral inclusions.  

Science.gov (United States)

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. PMID:21921159

Walter, M J; Kohn, S C; Araujo, D; Bulanova, G P; Smith, C B; Gaillou, E; Wang, J; Steele, A; Shirey, S B

2011-10-01

47

Timing of Alteration From Uranium-series Geochemistry of Altered Ocean Crust at IODP Site 1256D  

Science.gov (United States)

Hydrothermal alteration at mid-ocean ridges is a crucial component of the global geochemical cycle and influences the chemical and physical properties of the oceanic crust, but the timescales and extent of off-axis alteration processes are poorly constrained. In slow-spreading crust, uranium chemistry has shown the importance of off-axis processes in modifying the composition of the crust. To delimit the timing and extent of off-axis alteration in superfast-spreading crust, we measured 234U- 238U disequilibria and U concentrations in 15 Ma crust drilled at ODP/IODP Site 1256D. Water-rock interaction in the ocean crust imparts elevated (234U)/(238U) to altered crust, and this signature makes 234U- 238U disequilibria sensitive to mineral precipitation or fluid-rock interactions that have occurred within the past ~1.25 Myr. Disequilibria increases with depth, from the volcanic flows through the transition zone into sheeted dikes. In the sheet and massive flows, most samples are close to radioactive equilibrium, but samples in the transition zone have significant 234U excesses (17-25 permil), and all of the samples measured in the sheeted dikes show larger 234U excesses ( >50 permil). The magnitude of disequilibria is independent of the observed textures and alteration mineralogy; complementary fresh and altered whole rock samples have similar disequilibria throughout the section. The concentration of U decreases with depth, and is negatively correlated to the 234U excess. There is no correlation between 87Sr/86Sr and 234U- 238U disequilibria, but rocks with higher 87Sr/86Sr have lower U concentrations. Oxygen isotope data mirror the depth-dependent trends of the 234U excess and U concentration. Because 234U excess is not related to the alteration mineralogy or to 87Sr/86Sr, the fluids imparting 234U disequilibria likely follow different pathways than the high temperature fluids at the ridge axis. The ?18O and the higher U concentrations in the volcanic section are related to oxidizing low-temperature interaction with seawater that results in net addition of U to the crust during on-axis alteration, but 234U-238U analyses suggest either that (1) these processes in the upper section are either no longer active or do not involve recent U exchange or (2) that U interaction persists throughout the section, but that disequilibria in the volcanic section are more subtle due to higher U concentrations that resulted from U addition during on-axis alteration. 234U excesses measured in vein phyllosilicates in the volcanics indicate that water-rock interaction persists in the upper section, but mass balance calculations are necessary to quantify the extent of exchange. 234U- 234U disequilibria may be recording more cryptic water-rock interaction that persists off-axis without strongly affecting enrichment or depletion in other elements.

Flynn, K. S.; Cooper, K. M.; Teagle, D. A.; Banerjee, N.; Smith-Duque, C. E.; Harris, M.; Coggon, R. M.; Cooper, M.

2008-12-01

48

Dating the Growth of Oceanic Crust at a Slow-Spreading Ridge  

Science.gov (United States)

Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas ~25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of ~5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank.

Schwartz, Joshua J.; John, Barbara E.; Cheadle, Michael J.; Miranda, Elena A.; Grimes, Craig B.; Wooden, Joseph L.; Dick, Henry J. B.

2005-10-01

49

Mid-ocean ridge basalt trace element evolution controlled by melt-rock reaction in the lower oceanic crust  

Science.gov (United States)

Mid-ocean ridge basalt (MORB) is the most abundant magma on Earth, and is the main geochemical window into the mantle. When evolution in crustal magma chambers is accounted for, its composition reflects a combination of mantle composition, melting processes and melt migration mechanisms. However, this approach assumes that modification of melts in crustal magma chambers can be corrected for. Recently, it has emerged that the trace element distributions in MORB do not follow fractional crystallization trends, being characterized by a relative over-enrichment of incompatible elements acquired during crustal processing (O'Neill and Jenner, 2012). This implies that it is no longer appropriate to use fractional crystallization models alone to correct for intra-crustal evolution. In order to continue using MORB as messenger from the mantle, it is critical to fully understand the origin of its trace element distributions. O'Neill and Jenner (2012) posit that the trace elements in MORB are the result of repeated replenishment-tapping-fractionation cycles in oceanic magma chambers. Here we explore the alternative hypothesis that the trace elements are instead controlled by melt-rock reaction in the lower oceanic crust. Our hypothesis is based on observations from a suite of lower crustal gabbroic rocks from the East Pacific Rise exposed in Hess Deep (equatorial Pacific Ocean). These gabbros preserve evidence for extensive reactions between ascending melts and a gabbroic framework in the crystal mush that forms the bulk of the lower crust (Lissenberg et al. 2013). In this contribution we compare the trace element distributions generated by melt-rock reaction with those documented in MORB. We treat MORB as mixtures between rapidly transported melts that escape melt-rock reaction and melts that ascend slowly by reactive porous flow, with the trace element enrichment for the latter constrained by the Hess Deep gabbro data. Our results display an excellent correlation with the MORB trace element data, from which we conclude that the trace element distributions in MORB can be explained by melt-rock reaction in the lower oceanic crust. This is fully consistent with geophysical constraints, which show that mid-ocean ridge magma chambers are comprised of crystal mush, rather than pure melt. References Lissenberg, C. J., MacLeod, C. J., Howard, K. A. & Godard, M. Pervasive reactive melt migration though fast-spreading lower oceanic crust (Hess Deep, equatorial Pacific Ocean). Earth and Planetary Science Letters 361, 436-447, doi:10.1016/j.epsl.2012.11.012 (2013). O'Neill, H. S. C. & Jenner, F. E. The global pattern of trace-element distributions in ocean floor basalts. Nature 491, 698-704, doi:10.1038/nature11678 (2012).

Lissenberg, C.; MacLeod, C. J.

2013-12-01

50

Glacial cycles drive variations in the production of oceanic crust  

Science.gov (United States)

Changes in sea level accompanying glacial cycles affect the static pressure within the asthenosphere; these variations could modulate melting rates beneath the mid-ocean ridge system as well as crustal thickness. These effects can be investigated and quantified using models of ridges based on conservation of mass, momentum, energy, and composition for two phases (magma & mantle) and two thermodynamic components (enriched & depleted). The models predict that the sensitivity of crustal thickness to oscillations in sea-level depends on the period of oscillation, the spreading rate of the ridge, and the assumed permeability scale of the melting regime. In contrast to previous studies (Huybers & Langmuir, 2009 and Lund & Asimow, 2011), the new results indicate that effects are larger for ridges with faster spreading rates. They also show that the dominant period of variations in crustal thickness changes with spreading rate and permeability. Sea-level variations with periods in the range of 10 ky - 100 ky can result in significant changes in crustal thickness that are orders of magnitude larger than the sea-level variations that drive them. Accurately modelling this process requires the inclusion of two previously unaccounted for processes: (1) determining the volume of the melting regime that is consistent with the ridge spreading rate and (2) properly treating the transport of melt. These enable us to capture the non-linear dependencies on spreading rate and other model parameters. Spectral analysis of bathymetry at two ridge segments that have a symmetric bathymetric signal and hence are undisturbed by off-axis volcanism or ridge jumps reveals the presence of variability at frequencies associated with precession, obliquity, and the 100 ky glacial/inter-glacial variability. Furthermore, the faster spreading ridge has larger amplitude responses to changes in sea level and shows a proportionately greater response at higher frequencies. These observations reinforce the possible links among climate cycles at the surface, mantle melting at depth and the crustal fabric of the sea floor.

Crowley, J. W.; Katz, R. F.; Langmuir, C. H.; Huybers, P. J.

2013-12-01

51

The oceanic crust in 3D: Paleomagnetic reconstruction in the Troodos ophiolite gabbro  

Science.gov (United States)

The Troodos complex, Cyprus, provides an opportunity to study the structural configuration along a fossil intersection of a spreading axis and a transform fault. We complement studies at Troodos that have reconstructed the brittle deformation of the upper crust by new paleomagnetic data from the gabbro suite. The gabbro suite is exposed at the extinct spreading axis continuing the Solea graben toward the intersection with the fossil Arakapas oceanic transform. This is a unique exposure of deep crustal rocks formed at both an inside-corner and an outside-corner of a ridge-transform intersection. Remanence directions from gabbros (23 sites) were used as indicators for rigid body rotation. The spatial distribution of rotation axes allow recognition of three regions to which deformation is partitioned: 1) a western region (outside corner) that experienced primarily tilt about horizontal axis 2) a central region with minor rotation and, 3) an eastern area (inside corner) where vertical axis rotations are dominant. The absence of significant rotation in the 6 km-wide central domain together with its location between the inside- and the outside corner uncover the root of a fossil axial volcanic zone, a zone sufficiently hot so the upper crust can decouple from the substrate. Clockwise rotation in the gabbro increases from the axial zone eastward, similar to that in the overlying dikes, indicating coupling of the lower crust with the brittle upper oceanic crust. The transition from the decoupled layers of sheeted dikes and gabbro in the axial zone to the dikes-gabbro coupling in the inside corner is in keeping with deepening of the brittle-ductile transition from the dike-gabbro boundary into the lower crust away from the axial zone. Our conclusions are consistent with one of the previous reconstructions in which the Solea spreading axis was orthogonal to the Arakapas transform fault, and with recent studies of the present-day lower oceanic crust. However, the newly inferred surface trace of the Solea spreading axis is further to the east, probably reflecting the tilt of axial upper crust rotated blocks.

Granot, Roi; Abelson, Meir; Ron, Hagai; Agnon, Amotz

2006-11-01

52

Melt flow and hypersolidus deformation in the lower ocean crust: Preliminary observations from IODP Leg 345  

Science.gov (United States)

Models for the construction of the fast-spreading lower ocean crust include the gabbro glacier model (GGM), in which most crystallization occurs within a shallow melt lens and the resulting crystal mush subsides downwards and outwards by crystal sliding. Second, the Sheeted Sill Model (SSM) predicts magmatic injection at many levels in the crust, and requires rapid cooling of the lithosphere. A second set of models seeks to reconcile the relatively unevolved nature of most MORB with the existence of an extensive lower crust with both layering (in the lower crust) and highly evolved gabbros (in the upper plutonic sequence). The mechanisms involved here are melt aggregation during vertical porous flow in the lower crust as opposed to lateral sill injection and in-situ crystallization. Here we report new observations from IODP Expedition 345 to the Hess Deep Rift, where propagation of the Cocos Nazca Ridge (CNR) into young, fast-spreading East Pacific Rise (EPR) crust exposes a dismembered lower crustal section. Drilling in ~4850 m water depth produced 3 holes of 35 to 100 mbsf with ~30% recovery of primitive (Mg# 79-87) plutonic lithologies including troctolite, olivine gabbro, and olivine gabbronorite, showing cumulate textures found in layered mafic intrusions and some ophiolite complexes including: 1. Spectacular modal layering 2. Orthopyroxene very early on the liquidus compared to canonical MORB. 3. Delicate large (2-5 cm) skeletal and hopper structures in olivine. 4. Oikocrystic clinopyroxene enclosing chadacrysts different from the host assemblage. These complex relationships are only hinted at in the existing observations from the ocean floor, and will require significant lab study, however some preliminary inferences can be drawn from the petrographic observations. First, the textures observed in olivine throughout the cores are consistent with rapid crystallization, possibly due to steep thermal gradients in the lower crust. They occur early in the crystallization sequence and survive until crystallization is completed, suggesting that limited hypersolidus deformation occurred during their crystallization. This inference is essentially incompatible with the prediction of substantial hypersolidus flow implicit in the GGM Second, while some authors (Korenaga and Kelemen, 1998) suggest that widespread layering is incompatible with pervasive melt flow models, some of the banding observed is exactly consistent with predictions of such models (Sanfilippo and Dick, 2013). Third, orthopyroxene early on the liquidus cannot be achieved simply by lower crustal processes. High level melt-rock reaction in the mantle combined with dry crystallization in the lower crust may do so, however this requires that some melts crossing the MOHO are not aggregated.

Snow, J. E.; Koepke, J.; Falloon, T.; Abe, N.; Hoshide, T.; Akizawa, N.; Maeda, J.; Jean, M. M.; Cheadle, M. J.

2013-12-01

53

IODP Expedition 345: Primitive Layered Gabbros From Fast-Spreading Lower Oceanic Crust  

Science.gov (United States)

Three-quarters of the ocean crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the seafloor. However, owing to the nearly continuous overlying extrusive upper crust, sampling in situ the lower crust is challenging. Hence, models for understanding the formation of the lower crust are based essentially on geophysical studies and ophiolites. Integrated Ocean Drilling Program (IODP) Expedition 345 recovered the first significant sections of primitive, modally layered gabbroic rocks from the lowermost plutonic crust formed at a fast-spreading ridge, and exposed at the Hess Deep Rift (Gillis et al., Nature, 2014, doi:10.1038/nature12778). Drilling Site U1415 is located along the southern slope of the intrarift ridge. The primary science results were obtained from coring of two ~110 m deep reentry holes and one 35-m-deep single-bit hole, all co-located within an ~100-m-wide area. Olivine gabbro and troctolite are the dominant plutonic rock types recovered, with minor gabbro, clinopyroxene oikocryst-bearing gabbroic rocks, and gabbronorite. All rock types are primitive to moderately evolved, with Mg# 89-76, and exhibit cumulate textures similar to ones found in layered mafic intrusions and some ophiolites. Spectacular modal and grain size layering, prevalent in >50% of the recovered core, confirm a long held paradigm that such rocks are a key constituent of the lowermost ocean crust formed at fast-spreading ridges. Magmatic foliation is largely defined by the shape-preferred orientation of plagioclase. It is moderate to strong in intervals with simple modal layering but weak to absent in troctolitic intervals and typically absent in intervals with heterogeneous textures and/or diffuse banding. Geochemical analysis of these primitive lower plutonics, in combination with previous geochemical data for shallow-level plutonics, sheeted dikes and lavas, provides the best constrained estimate to date of the bulk composition of crust formed at a fast-spreading ridge. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the composition of both the lavas and plutonics. However, the recovered plutonic rocks show unanticipated early crystallization of orthopyroxene, challenging current models of melt extraction from the mantle and mid-ocean ridge basalt differentiation. The core recovered at Site U1415 originated at a stratigraphic level at least 2 km beneath the sheeted dike-plutonic transition, representing intervals of the lower half to one third of the EPR plutonic crust. A more precise depth cannot be assigned as the results of Expedition 345 (e.g., magnetic inclinations) and site survey indicate that the sampled units are tilted, mass-wasted blocks. However, sampling four large blocks of relatively fresh rocks proved facilitated observations of the wide variety and complexity of rock types and textures present in fast spread primitive lowermost crust.

Ildefonse, Benoit; Gillis, Kathryn M.; Snow, Jonathan E.; Klaus, Adam

2014-05-01

54

Osmium isotope variations in the oceans recorded by Fe-Mn crusts  

Science.gov (United States)

This study presents osmium (Os) isotope data for recent growth surfaces of hydrogenetic ferromanganese (Fe-Mn) crusts from the Pacific, Atlantic and Indian Oceans. In general, these data indicate a relatively uniform Os isotopic composition for modern seawater, but suggest that North Atlantic seawater is slightly more radiogenic than that of the Pacific and Indian Oceans. The systematic difference in the Os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high Re/Os ratio in the North Atlantic Ocean, consistent with the distribution of Nd and Pb isotopes. This spatial variation in the Os isotope composition in seawater is consistent with a residence time for Os of between 2 and 60 kyr. Indian Ocean samples show no evidence of a local source of radiogenic Os, which suggests that the present-day riverine input from the Himalaya-Tibet region is not a major source for Os. Recently formed Fe-Mn crusts from the TAG hydrothermal field in the North Atlantic yield an Os isotopic composition close to that of modern seawater, which indicates that, in this area, the input of unradiogenic Os from the hydrothermal alteration of oceanic crust is small. However, some samples from the deep Pacific (???4 km) possess a remarkably unradiogenic Os isotope composition (187Os/186Os ratios as low as 4.3). The compositional control of Os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles, rather than indicating the presence of an unradiogenic deep-water mass. Moreover, this unradiogenic signal appears to be temporary, and local, and has had little apparent effect on the overall evolution of seawater. These results confirm that input of continental material through erosion is the dominant source of Os in seawater, but it is not clear whether global Os variations are due to the input of mantle or meteoritic material, or simply indicate that the continental source itself is not uniform.

Burton, K.W.; Bourdon, B.; Birck, J.-L.; Allegre, C.J.; Hein, J.R.

1999-01-01

55

Building the lower oceanic crust: a view through tectonic windows at the Hess and Pito Deeps  

Science.gov (United States)

Submersible (manned and unmanned) studies of tectonic windows into crust formed at the East Pacific Rise provide insight into the upper ~1000 m of the plutonic complex of the ocean crust at Pito Deep (23° S) and Hess Deep (2° N). Important similarities between the plutonic rocks in these two areas are: (i) the lower abundances of minerals formed during hydrothermal alteration than in the overlying dikes, (ii) the occurrence of steeply dipping magmatic fabrics defined largely by aligned plagioclase crystals, and (iii) that they both provide evidence for some primitive melts crystallising near the top of the plutonic complex. Despite these similarities, the Hess Deep gabbros are, on average, much more evolved (Mg less than 65 versus over 80 at Pito Deep). This is, at least in part, because of much less efficient crystal-melt separation within a crystal mush zone at Hess Deep. The upper crust (lavas and dikes) at Pito Deep is also more primitive and reacted with a greater amount of hydrothermal fluid. Studies of the overlying upper crust using Sr-isotopes constrain the minimum fluid flux through the sheeted dike complex allowing the minimum rate of heat loss through the roof of the axial melt sill to be determined. Based on this, along with thermal models and calculated cooling rates a MINIMUM of ca 30 percent of the mass of the plutonic complex crystallized within the AMC at Hess Deep (more at Pito Deep) and this should be included in any quantitative model of lower crustal processes. The mechanism by which these crystals are transported downwards to form the deeper portions of the lower oceanic crust is not well constrained. However, the sub-vertical alignment of undeformed plagioclase laths in both settings is consistent with some component of magmatic flow. There is currently little direct constraint on where the other less than 70 percent of the mass of the plutonic complex crystallizes. The differences between the crustal sections observed in these two settings may reflect their formation in different parts of segment. Alternatively (or additionally) it may reflect temporal variability in the extent of lateral heat loss from the lower crust on axis allowing crystallization to occur throughout the plutonic complex during formation of the section exposed at Hess Deep but largely within the axial melt sill during formation of the crust exposed at Pito Deep.

Coogan, L. A.

2008-12-01

56

Jurassic (~160 Ma) Lamprophyric Xenoliths from Southern Louisiana Salt Domes: A Unique Perspective on Gulf of Mexico Crust (Invited)  

Science.gov (United States)

No direct information about the age and composition of rift-related igneous activity associated with the Late Jurassic opening of the Gulf of Mexico exists because these rocks are buried beneath several kilometers of sediments. Salt diapirs from southern Louisiana bring to the surface samples of the alkalic igneous rock lamprophyre; these salt domes rise from the base of the sedimentary pile to the surface. Two salt domes (Avery and Weeks) bring up igneous rocks that we studied; a third (Jefferson) brings up igneous rocks but these samples are lost. All three salt diapirs rise from an isolated magnetic high, which may mark the position of an ancient mafic volcano or intrusion in what appears to have evolved as a magma-starved rifted margin. Three lamprophyre samples from Weeks and Avery salt domes were studied, but we cannot tell whether these were lavas or shallow intrusions. The lamprophyres are altered but preserve relict igneous minerals including Cr-rich spinel Cr# ( = 100Cr/Cr+Al) ranging from 36 to 42 rimmed with titanite strongly zoned clinopyroxene (diopside to Ti-augite). Diopside cores have two distinct compositions: one with a high Cr, Si and a second with low Cr, Si. Rims for both core types exhibit titanopyroxene (MgSi2 = TiAl2) and Ca-tschermaks (MgSi = AlviAliv) substitution and are identical to matrix diopside, indicating that rim compositions were in equilibrium with the final melt. Excellent 40Ar/39Ar plateau ages of 158.6±0.2 Ma and 160.1±0.7 Ma for lamprophyric Ti-rich biotite and kaersutite from two different salt domes are interpreted to date when the lamprophyre solidified. Alteration disturbed primary igneous compositions, especially Si, alkali metals, alkaline earths, Pb and H2O, but not high field strength elements (HFSE, e.g., Ti, Zr, Hf, Nb, Y). Elevated abundances of immobile incompatible trace elements are one to two orders of magnitude enriched relative to N-MORB. However, isotopic compositions of Nd and Hf (?Hf ~ +9, ?Nd ~ +7 at 160 Ma) indicate derivation from MORB-like depleted mantle. Trace element and isotopic data, considered together, imply that the lamprophyric magma was derived from low degree of partial melting (~2-3 %) of the mantle. This information supports the idea that crust beneath southern Louisiana formed as a magma-starved rifted margin on the northern flank of the Gulf of Mexico ~160 Ma.

Stern, R. J.; Anthony, E. Y.; Ren, M.; Kimura, J.; Lock, B.; Norton, I. O.

2010-12-01

57

IODP Exp 345: Primitive Layered Gabbros From Fast-Spreading Lower Oceanic Crust  

Science.gov (United States)

Plutonic rocks from the lower ocean crust formed at fast-spreading ridges provide a record of the history of melt transport and crystallization between the mantle and the seafloor. Despite the significance of these rocks, sampling them in situ has proven extremely challenging. This means our models for understanding the formation of the lower crust are based largely on geophysical studies and ophiolites that did not form at typical mid-ocean ridges. Integrated Ocean Drilling Program (IODP) Expedition 345 recovered the first significant recovery of primitive modally layered gabbroic rocks from the lowermost plutonic crust from a fast-spreading ridge exposed at the Hess Deep Rift. Drilling was focused at Site U1415, located along the southern slope of the intrarift ridge. The primary science results were obtained from coring of two ~110 m deep reentry holes and one 35-m-deep single-bit hole, all co-located within an ~100-m-wide area. Olivine gabbro and troctolite are the dominant plutonic rock types recovered, with minor gabbro, clinopyroxene oikocryst-bearing gabbroic lithologies, and gabbronorite. All rock types are primitive (Mg# 76-89) and exhibit cumulate textures similar to ones found in layered mafic intrusions and some ophiolite complexes. Spectacular modal and grain size layering, prevalent in >50% of the recovered core, confirm a long held paradigm that such rocks are a key constituent of the lowermost ocean crust formed at fast-spreading ridges. Magmatic foliation is largely defined by the shape-preferred orientation of plagioclase. It is moderate to strong in intervals with simple modal layering but weak to absent in troctolitic intervals and typically absent in intervals with heterogeneous textures and/or diffuse banding. Geochemical analysis of these primitive lower plutonics, in combination with previous geochemical data for shallow-level plutonics, sheeted dikes and lavas, provides the first robust estimate of the bulk composition of crust formed at a fast-spreading ridge. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the composition of both the lavas and plutonics. However, the recovered plutonic rocks show unanticipated early crystallization of orthopyroxene, challenging current models of melt extraction from the mantle and mid-ocean ridge basalt differentiation. The core recovered at Site U1415 originated at a stratigraphic level at least 2 km beneath the sheeted dike-plutonic transition, representing intervals of the lower half to one third of the EPR plutonic crust. A more precise depth cannot be assigned as the results of Expedition 345 (e.g., magnetic inclinations) and site survey data (Ferrini et al., 2013) indicate that the southern slope of the intrarift ridge formed by mass wasting. However, sampling four large blocks of relatively fresh rocks proved advantageous, as it facilitated observations of the wide variety and complexity of rock types and textures present in fast spread primitive lowermost crust. [Ferrini et al., Mar. Geol., 339, 13-21, 2013

Gillis, K. M.; Snow, J. E.; Klaus, A.

2013-12-01

58

Nickel isotopic compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering effects over the Cenozoic  

Science.gov (United States)

The global variability in nickel (Ni) isotope compositions in ferromanganese crusts is investigated by analysing surface samples of 24 crusts from various ocean basins by MC-ICPMS, using a double-spike for mass bias correction. Ferromanganese crusts have ?Ni60 isotopic compositions that are significantly heavier than any other samples thus far reported (-0.1‰ to 0.3‰), with surface scrapings ranging between 0.9‰ and 2.5‰ (relative to NIST SRM986). There is no well resolved difference between ocean basins, although the data indicate somewhat lighter values in the Atlantic than in the Pacific, nor is there any evidence that the variations are related to biological fractionation, presence of different water masses, or bottom water redox conditions. Preliminary data for laterite samples demonstrate that weathering is accompanied by isotopic fractionation of Ni, which should lead to rivers and seawater being isotopically heavy. This is consistent with the slightly heavier than average isotopic compositions recorded in crusts that are sampled close to continental regions. Furthermore, the isotopic compositions of crusts growing close to a hydrothermal source are clustered around ?1.5‰, suggesting that hydrothermal fluids entering the ocean may have a Ni isotopic composition similar to this value. Based on these data, the heavy Ni isotopic compositions of ferromanganese crusts are likely due to input of isotopically heavy Ni to the ocean from continental weathering and possibly also from hydrothermal fluids. A depth profile through one crust, CD29-2, from the north central Pacific Ocean displays large variations in Ni isotope composition (1.1-2.3‰) through the last 76 Myr. Although there may have been some redistribution of Ni associated with phosphatisation, there is no systematic difference in Ni isotopic composition between deeper, older parts and shallower, younger parts of the crust, which may suggest that oceanic sources and sinks of Ni have largely remained in steady state over the Cenozoic. Additionally, the isotope profile is in agreement with a profile of Mn concentration through the same crust. This implies a link between the Ni isotopic composition recorded in ferromanganese crusts and the release of Ni into the ocean through hydrothermal activity. This supports the conclusions drawn from surface data, that Ni isotope ratios in ferromanganese crusts are largely controlled by the isotopic compositions of the Ni oceanic input sources.

Gall, L.; Williams, H. M.; Siebert, C.; Halliday, A. N.; Herrington, R. J.; Hein, J. R.

2013-08-01

59

The Effect of Hydrothermal Alteration on the Seismic Structure of the Upper Oceanic Crust and the Layer 2A/2B Transition  

Science.gov (United States)

It has long been argued but never demonstrated that alteration "fronts" should be recognizable features of the seismic structure of the oceanic crust. The abrupt transition from crust affected by low-temperature hydrous alteration to crust affected by high-temperature hydrothermal alteration must arise from a step-wise reduction of porosity and permeability that should correspond to a seismic boundary. In Holes 504B and 1256D, the sudden downhole appearance of hydrothermal minerals corresponds to a increase of the velocity gradient that is caused by a change of porosity within the lava-dike transition zone, and models of the downhole variation of permeability computed from apparent porosity logs show a corresponding step-wise change of permeability (by a factor of ~20) that is sufficient to account for the onset of hydrothermal alteration. In principle, the coincidence of the seismic structure with the alteration boundary can be used to interpret the seismic structure of the oceanic crust. In particular, the onset of hydrothermal alteration proves to be a viable candidate for the transition from Layer 2A to Layer 2B, which also occurs within or near the lava-dike transition zone. There is also a systematic decrease of permeability with increasing sonic velocity in both the lavas and the dikes (log(?) ~ a+bv). Remarkably, the extrapolated trend for the lavas is in excellent agreement with in situ permeabilities measured in very young crust, ranging in age from zero to 3.5 Ma. To a good approximation, the permeability of Layer 2A can be estimated from its seismic structure.

Carlson, R. L.

2011-12-01

60

The effect of hydrothermal alteration on the seismic structure of the upper oceanic crust: Evidence from Holes 504B and 1256D  

Science.gov (United States)

It has long been argued but never demonstrated that alteration "fronts" should be recognizable features of the seismic structure of the oceanic crust. The abrupt transition from crust affected by low-temperature hydrous alteration to crust affected by high-temperature hydrothermal alteration must arise from a stepwise reduction of porosity and permeability that should correspond to a seismic boundary. In Holes 504B and 1256D, the sudden downhole appearance of hydrothermal minerals corresponds to a increase of the velocity gradient that is caused by a change of porosity within the lava-dike transition zone, and models of the downhole variation of permeability computed from apparent porosity logs show a corresponding stepwise change of permeability (by a factor of ˜20) that is sufficient to account for the onset of hydrothermal alteration. In principle, the coincidence of the seismic structure with the alteration boundary can be used to interpret the seismic structure of the oceanic crust. In particular, the onset of hydrothermal alteration proves to be a viable candidate for the transition from Layer 2A to Layer 2B, which also occurs within or near the lava-dike transition zone. There is also a systematic decrease of permeability with increasing sonic velocity in both the lavas and the dikes (log(?) ˜ a + bv). Remarkably, the extrapolated trend for the lavas is in excellent agreement with in situ permeabilities measured in very young crust, ranging in age from zero to 3.5 Ma. To a good approximation, the permeability of Layer 2A can be estimated from its seismic structure.

Carlson, R. L.

2011-09-01

 
 
 
 
61

Vertical tectonics at a continental crust-oceanic plateau plate boundary zone: Fission track thermochronology of the Sierra Nevada de Santa Marta, Colombia  

Science.gov (United States)

The topographically prominent Sierra Nevada de Santa Marta forms part of a faulted block of continental crust located along the northern boundary of the South American Plate, hosts the highest elevation in the world (˜5.75 km) whose local base is at sea level, and juxtaposes oceanic plateau rocks of the Caribbean Plate. Quantification of the amount and timing of exhumation constrains interpretations of the history of the plate boundary, and the driving forces of rock uplift along the active margin. The Sierra Nevada Province of the southernmost Sierra Nevada de Santa Marta exhumed at elevated rates (?0.2 Km/My) during 65-58 Ma in response to the collision of the Caribbean Plateau with northwestern South America. A second pulse of exhumation (?0.32 Km/My) during 50-40 Ma was driven by underthrusting of the Caribbean Plate beneath northern South America. Subsequent exhumation at 40-25 Ma (?0.15 Km/My) is recorded proximal to the Santa Marta-Bucaramanga Fault. More northerly regions of the Sierra Nevada Province exhumed rapidly during 26-29 Ma (˜0.7 Km/My). Further northward, the Santa Marta Province exhumed at elevated rates during 30-25 Ma and 25-16 Ma. The highest exhumation rates within the Sierra Nevada de Santa Marta progressed toward the northwest via the propagation of NW verging thrusts. Exhumation is not recorded after ˜16 Ma, which is unexpected given the high elevation and high erosive power of the climate, implying that rock and surface uplift that gave rise to the current topography was very recent (i.e., ?1 Ma?), and there has been insufficient time to expose the fossil apatite partial annealing zone.

Villagómez, Diego; Spikings, Richard; Mora, AndréS.; GuzmáN, Georgina; Ojeda, GermáN.; CortéS, Elizabeth; van der Lelij, Roelant

2011-08-01

62

Insights into magmatic processes and hydrothermal alteration of in situ superfast spreading ocean crust at ODP/IODP site 1256 from a cluster analysis of rock magnetic properties  

Science.gov (United States)

analyze magnetic properties from Ocean Drilling Program (ODP)/Integrated ODP (IODP) Hole 1256D (6°44.1' N, 91°56.1' W) on the Cocos Plate in ˜15.2 Ma oceanic crust generated by superfast seafloor spreading, the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting. Fuzzy c-means cluster analysis and nonlinear mapping are utilized to study down-hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization, the coercive force, the ratio of the remanent coercive force and coercive force, the low-field magnetic susceptibility, and the Curie temperature, to evaluate the effects of magmatic and hydrothermal processes on magnetic properties. A statistically robust five cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas, which differ from two distinct clusters mainly representing the dikes and gabbros. Extensive alteration can obliterate magnetic property differences between lavas, dikes, and gabbros. The imprint of thermochemical alteration on the iron-titanium oxides is only partially related to the porosity of the rocks. Thus, the analysis complements interpretation based on electrofacies analysis. All clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies. Paleointensity determination is difficult because of the propensity of oxyexsolution during laboratory heating and/or the presence of intergrowths. The upper part of the extrusive sequence, the granoblastic dikes, and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization.

Dekkers, Mark J.; Heslop, David; Herrero-Bervera, Emilio; Acton, Gary; Krasa, David

2014-08-01

63

First investigation of the microbiology of the deepest layer of ocean crust.  

Science.gov (United States)

The gabbroic layer comprises the majority of ocean crust. Opportunities to sample this expansive crustal environment are rare because of the technological demands of deep ocean drilling; thus, gabbroic microbial communities have not yet been studied. During the Integrated Ocean Drilling Program Expeditions 304 and 305, igneous rock samples were collected from 0.45-1391.01 meters below seafloor at Hole 1309D, located on the Atlantis Massif (30 °N, 42 °W). Microbial diversity in the rocks was analyzed by denaturing gradient gel electrophoresis and sequencing (Expedition 304), and terminal restriction fragment length polymorphism, cloning and sequencing, and functional gene microarray analysis (Expedition 305). The gabbroic microbial community was relatively depauperate, consisting of a low diversity of proteobacterial lineages closely related to Bacteria from hydrocarbon-dominated environments and to known hydrocarbon degraders, and there was little evidence of Archaea. Functional gene diversity in the gabbroic samples was analyzed with a microarray for metabolic genes ("GeoChip"), producing further evidence of genomic potential for hydrocarbon degradation--genes for aerobic methane and toluene oxidation. Genes coding for anaerobic respirations, such as nitrate reduction, sulfate reduction, and metal reduction, as well as genes for carbon fixation, nitrogen fixation, and ammonium-oxidation, were also present. Our results suggest that the gabbroic layer hosts a microbial community that can degrade hydrocarbons and fix carbon and nitrogen, and has the potential to employ a diversity of non-oxygen electron acceptors. This rare glimpse of the gabbroic ecosystem provides further support for the recent finding of hydrocarbons in deep ocean gabbro from Hole 1309D. It has been hypothesized that these hydrocarbons might originate abiotically from serpentinization reactions that are occurring deep in the Earth's crust, raising the possibility that the lithic microbial community reported here might utilize carbon sources produced independently of the surface biosphere. PMID:21079766

Mason, Olivia U; Nakagawa, Tatsunori; Rosner, Martin; Van Nostrand, Joy D; Zhou, Jizhong; Maruyama, Akihiko; Fisk, Martin R; Giovannoni, Stephen J

2010-01-01

64

Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing.  

Science.gov (United States)

Water and hydrous minerals play a key part in geodynamic processes at subduction zones by weakening the plate boundary, aiding slip and permitting subduction-and indeed plate tectonics-to occur. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization. However, seismological observations bearing on the presence of water within the subducting plate itself are less well documented. Here we use converted teleseismic waves to obtain observations of anomalously high Poisson's ratios within the subducted oceanic crust from the Cascadia continental margin to its intersection with forearc mantle. On the basis of pressure, temperature and compositional considerations, the elevated Poisson's ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values. Combined with observations of a strong negative velocity contrast at the top of the oceanic crust, our results imply that the megathrust is a low-permeability boundary. The transition from a low- to high-permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization. These results may have important implications for our understanding of seismogenesis, subduction zone structure and the mechanism of episodic tremor and slip. PMID:19122639

Audet, Pascal; Bostock, Michael G; Christensen, Nikolas I; Peacock, Simon M

2009-01-01

65

Araxa Group in the type-area: A fragment of Neoproterozoic oceanic crust in the Brasilia Fold Belt  

International Nuclear Information System (INIS)

This study reviews the geological characteristics and puts forward a new evolution model for the Araxa Group in its type-area, the southern segment of the Neo proterozoic Brasilia Belt, Minas Gerais, Brazil. The Araxa Group is confined within a thrust sheet belonging to a syn formal regional fold, the Araxa Syn form, overlying two other thrust sheets made of the Ibia and Canastra Groups. The Araxa Group is described as a tectono stratigraphic terrane in the sense of Howell (1993). It comprises an igneous mafic sequence, with fine and coarse grained amphibolites, associated with pelitic meta sedimentary rocks, and subordinate psanmites. All rocks were metamorphosed to amphibolite facies at ca. 630 Ma ago and were intruded by collisional granites. The amphibolites represent original basaltic and gabbroic rocks, with minor ultramafic (serpentinite/ amphibole-talc schist). The basalts are similar to high Fe O tholeiites, with REE signatures that resemble E-MORB and ?Nd(T) =+ 1.1. The meta sedimentary rocks are interpreted as the result of a marine deep-water sedimentation. They have Sm-Nd model ages of 1,9 Ga, and ?Nd(T) = -10.21. The amphibolites and metasediments could represent a fragment of back-arc oceanic crust. The data presented here differ significantly from the original definition of Barbosa et al. (1970) who describe the Araxa Group as a pelitic/psanmitic sequence and the collisional granites as a basement complex. (author)

66

Salinity changes in the North West Pacific Ocean during the late Pliocene/early Quaternary from 2.73 Ma to 2.52 Ma  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Recent research has increasingly advocated a role for the North Pacific Ocean in modulating global climatic changes over both the last glacial cycle and further back into the geological record. Here a diatom ?18O record is presented from Ocean Drilling Program Site 882 over the Pliocene/Quaternary boundary from 2.73 Ma to 2.52 Ma (MIS G6-MIS 99). Large changes in ?18Odiatom of c. 4‰ from 2.73 Ma onwards are documented to occur on a timeframe broadly coinciding with glacial-int...

Swann, George E. A.

2010-01-01

67

Downdip velocity changes in subducted oceanic crust beneath Northern Japan—insights from guided waves  

Science.gov (United States)

Dispersed P-wave arrivals observed in the subduction zone forearc of Northern Japan suggest that low velocity subducted oceanic crustal waveguide persists to depths of at least 220 km. First arrivals from events at 150-220 km depth show that the velocity contrast of the waveguide reduces with depth. High frequency energy (>2 Hz) is retained and delayed by the low velocity crustal waveguide while the lower frequency energy (WBZ earthquakes at 150-220 km depth are directly compared to synthetic waveforms produced by 2-D and 3-D full waveform finite difference simulations. By comparing both the spectrogram and the velocity spectra of the observed and synthetic waveforms we are able to fully constrain the dispersive waveform, and so directly compare the observed and synthetic waveforms. Using this full waveform modelling approach we are able to tightly constrain the velocity structures that cause the observed guided wave dispersion. Resolution tests using 2-D elastic waveform simulations show that the dispersion can be accounted for by a 6-8 km thick low velocity oceanic crust, with a velocity contrast that varies with depth. The velocities inferred for this variable low velocity oceanic crust can be explained by lawsonite bearing assemblages, and suggest that low velocity minerals may persist to greater depth than previously thought. 2-D simulations are benchmarked to 3-D full waveform simulations and show that the structures inferred by the 2-D approximation produce similar dispersion in 3-D. 2-D viscoelastic simulations show that including elevated attenuation in the mantle wedge can improve the fit of the dispersed waveform. Elevated attenuation in the low velocity layers can however be ruled out.

Garth, Tom; Rietbrock, Andreas

2014-09-01

68

Static and fault-related alteration in the lower ocean crust, IODP Expedition 345, Hess Deep  

Science.gov (United States)

IODP Expedition 345 drilled the first holes in the lower plutonic crust at a fast-spreading ridge, recovering primitive layered gabbros (Gillis et al 2014). Alteration can be subdivided into two series: 1) a largely static pseudomorphic alteration affecting predominantly olivine. This began in the amphibolite facies with minor secondary cinopyroxene and hornblendic amphibole replacing primary pyroxene, and sporadically developed corona textures with tremolite and chlorite replacing olivine and plagioclase respectively, but was predominantly in the greenschist and sub-greenschist facies with talc, serpentine, clay minerals,oxides andsulphides replacing olivine, and prehnite and locally other calcsilicates replacing plagioclase, commonly in micro-vein networks. Albitic plagioclase is sporadically developed, and locally zeolite and carbonate. 2) An overprinting metasomatic alteration under sub-greenschist or perhaps lowermost greenschist conditions(talc and serpentine. Chlorite also ubiquitously occurs as patches replacing plagioclase along grain boundaries, locally associated with carbonate and amphibole needles. Metamorphosed dykes show chilled margins within the cataclasites, and are affected by cataclastic deformation. Faults, dykes and overprinting alteration are all inferred to be related to the westward propagation of Cocos-Nazca spreading that formed Hess Deep. Samples of different alteration and cataclastic domains were cut out of this section chips for isotopic analysis. 87Sr/86Sr ratios of cataclasites and dyke rocks are in the range 0.7037 - 0.7048, indicating alteration by seawater at moderate integrated fluxes. The highest values were in cataclasites overprinted by prehnite. ?18O values range from +1 to + 6 per mil, indicating alteration at temperatures generally >200 °C. Preliminary modelling using Comsol Multiphysics suggests that the temperatures of the overprinting alteration could be achieved in a permeable fault slot cutting through crust 0.5 to 1 m.y. old. Our study reveals a low temperature alteration assemblage dominated by prehnite and chlorite that is not normally associated with the lower oceanic crust. Yet it is likely to be common in any location where faults intersect the Moho off-axis, including transform faults, near axis normal faults at slow spreading ridges, and bending faults at subduction zones, and would be accompanied by serpentinites in upper mantle rocks, as seen at ODP site 895 in Hess Deep. This prehnite + chlorite assemblage may therefore be significant in the release of volatiles in subduction zones. Gillis, K.M., Snow J. E. and Shipboard Science Party (2014) Primitive layered gabbros from fast-spreading lower oceanic crust. Nature, 505,204-207, doi: 10.1038/nature12778

McCaig, Andrew; Faak, Kathrin; Marks, Naomi; Nozaka, Toshio; Python, Marie; Wintsch, Robert; Harigane, Yumiko; Titarenko, Sofya

2014-05-01

69

Depleted Peridotites of Macquarie Island, an Uplifted Section of In-situ Oceanic Crust  

Science.gov (United States)

Macquarie Island, located 1500 km southeast of southernmost Australia, is thought be the sole complete section of ocean crust uplifted in the ocean basin in which it formed. It is an exposure of the Macquarie Ridge complex, which marks the modern Australian-Pacific plate boundary. The oceanic crust of the island formed in the final stages of spreading, ~6 mya, as indicated by Ar-Ar plateau ages of basaltic glass. Geometries of marine faults on the island suggest that it formed near the intersection of a ridge and a transform. At this latitude, the plate boundary evolved from a spreading ridge to a transpressional boundary between ~33 and ~6 mya, thus the rocks of the island record an interesting tectonic history and may provide clues to the mantle process during a major plate motion re-organization. Residual, plagioclase-free mantle peridotite samples were collected along transects through all of the mantle sections on the island, with an average of 100 meter spacing between samples. Orthopyroxenes, clinopyroxenes and chrome spinels were analysed by electron microprobe. Spinel chrome numbers (Cr-nr) ranged from 0.39 to 0.46 (n=23), which corresponds to 15-16% fractional melting applying the empirical melting equation of Hellebrand et al (2001). Their low Ti contents (0.02-0.07) attest to the residual nature of the Macquarie Island peridotites. Cpx is preserved in only 7 samples (alteration, depletion), and occurs mainly as small interstitial grains or as exsolved blebs in opx porphyroclasts. Cpx titanium (0.00 - 0.04 wt% TiO2) and sodium (0.00 - 0.05 wt% Na2O) contents are extremely low, confirming the high depletion and supporting highly efficient melt extraction. Opx porphyroclast cores have very high Mg-nr (0.92 on average). Spreading rates at the time of formation of the Macquarie Island crust have been calculated to be 30mm/yr (full) which is considered "slow". However, the levels of depletion indicated by the spinel Cr-nr and Ti and Na contents of cpx of the Macquarie Island peridotites are more similar to those seen at fast spreading centers or ophiolites. This depletion could be caused by the progressively changing spreading direction disrupts mixing in the mantle, causing repeated melting of the same mantle source or biased sampling in the existing abyssal peridotite database. Further analyses of peridotites and associated basalts will test which model is most likely. Hellebrand et al., (2001) Nature 410, 677-681.

Wertz, K.; Snow, J. E.; Hellebrand, E.; von der Handt, A.; Mosher, S.

2002-12-01

70

Does Fluid-Induced Eclogitization of Subducted Lower Oceanic Crust Produce the Slab Component of Arcs?  

Science.gov (United States)

To investigate fluid-induced transformation processes and associated trace element mobilization, co-genetic gabbros and eclogites of Zambia have been used. The rocks represent relics of subducted lower oceanic crust and gradual stages of the prograde gabbro-to-eclogite transformation are preserved by disequilibrium textures of incomplete reactions. No evidence for prograde blueschist- or amphibolite-facies mineral assemblages was found in the eclogites. Instead, fine-grained intergrowths of eclogite-facies minerals replacing plagioclase indicate the direct eclogitization of gabbroic precursors. Eclogitization occurred at 630-690°C and 2.6-2.8GPa and was accompanied by a channelized fluid flow that produced veins of the peak metamorphic assemblage. Evidences for aqueous fluids with variable salinities, in cases up to brine compositions, were found. Although all of the mafic rocks were subducted, only those gabbros that were infiltrated by fluids were eclogitized. Hence, the eclogites and their veins represent relict fluid pathways through subducted oceanic crust, providing direct evidence of channelized fluid flow within a slab. The gabbros and eclogites have MORB-like trace element patterns and initial Nd and Hf isotope compositions. In some eclogites, however, the LREE have been strongly fractionated from the HFSE and HREE, an effect that cannot be of magmatic origin but must have occurred during metamorphism. Eclogitization was limited by fluid availability, and the fluid flow through the rock is the most likely mechanism for LREE fractionation. Model fluid-rock ratios reveal that the fractionated rocks reacted with an amount of fluid up to 80% of their mass to create the most depleted REE patterns. The lower gabbroic part of the oceanic crust is an unlikely source for such a large volume of fluid and thus we hypothesise that the fluid originated in the underlying serpentinised lithospheric mantle. If, after triggering eclogitization, the resulting LREE-rich, HFSE+HREE-poor slab fluid reaches the zone of partial melting in the mantle wedge, it may contribute significantly to the arc signature. We will evaluate whether the trace element mobilization during fluid-induced eclogitization could be generally responsible for producing the slab component in arc magmas.

Schenk, V.; John, T.

2004-12-01

71

Geochemical relationship between PREMA, FOZO and HIMU: link to chemical heterogeneity of MORB and layered structure of oceanic crust  

Science.gov (United States)

One of important concepts of the geochemistry is a mantle reservoir model, in which isotopic composition of the ocean island basalts (OIBs) are explained by mixing of distinct and isolated reservoirs, those are, depleted MORB mantle (DMM), HIMU (high-?) and EMs (Enriched Mantles). In addition to these reservoirs, importance of reservoirs whose isotopic compositions are intermediate has been pointed out, these are, FOZO (Focal Zone), C (Common component) and PREMA (Prevalent Mantle). Although the existences of these 'intermediate reservoirs' are still in debated, the isotopic compositions of these reservoirs have been used to describe the isotopic distribution of OIBs. Therefore, clarifying the origin of these reservoirs should be significant for the better understanding of cause of mantle heterogeneity. For the evaluation of origin and genetic linkage between these reservoirs, geochemical modeling has been conducted from the perspective of chemical fractionation at mid-ocean ridges and subduction zones. For the modeling, MORB compositions from Mid-Atlantic ridge are compiled for seven trace elements (Rb, Sr, Nd, Sm, Pb, Th and U) and used as representative oceanic crust compositions. Effect of chemical fractionation at a mid-ocean ridge is estimated based on magnesium number and frequency distribution. The results suggest that the chemical fractionation can produce small isotopic variation with moderately depleted isotopic signature that is suitable for PREMA if the age of recycled MORBs is 1.5 Ga. A mixing of dehydrated and dry MORBs can produce isotopic variation from PREMA to FOZO during 1.5 Ga recycling. For the production of typical HIMU (206Pb/204Pb > 21), Pb depletion due to removal of sulfur during subduction and/or U and Th enrichment owing to crystal fractionation at mid-ocean ridge is required. A mixing of dehydrated and dry MORBs can be explained by vertical difference in water content of oceanic crust that might correspond to surface hydrated and deep dry layers, respectively. If this is the case, origin of FOZO could be explained by recycling of commonly exist dehydrated surface layer of oceanic crust. In addition, PREMA may exemplify deep dry layer of oceanic crust. Origin of HIMU can be a totally hydrated evolved oceanic crust that experienced least degassing process to preserve sulfur. Magma evolution at mid-ocean ridges and different degree of dehydration in an oceanic crust beneath subduction zones play an essential role in producing the isotopic variations between PREMA, FOZO and HIMU.

Shimoda, G.; Kogiso, T.

2013-12-01

72

The deep subsurface biosphere in igneous ocean crust: frontier habitats for microbiological exploration  

Directory of Open Access Journals (Sweden)

Full Text Available We discuss ridge flank environments in the ocean crust as habitats for subseafloor microbial life. Oceanic ridge flanks, areas far from the magmatic and tectonic influence of seafloor spreading, comprise one of the largest and least explored microbial habitats on the planet. We describe the nature of the ridge flank crustal environments, and present a framework for delineating a continuum of conditions and processes that are likely to be important for defining subseafloor microbial "provinces." The basis for this framework is three governing sets of conditions that help to determine the nature of subseafloor biomes: crustal age, extent of fluid flow, and thermal state. We present a brief overview of subseafloor conditions, within the context of these three characteristics, for five field sites where microbial studies have been done, are underway, or have been proposed. Technical challenges remain and likely will limit progress in studies of microbial ridge-flank ecosystems, which is why it is vital to select and design future studies so as to leverage as much general understanding as possible. A characterization framework such as presented in this paper, perhaps including alternative or additional physical or chemical characteristics, is essential for achieving the greatest benefit from multidisciplinary microbial investigations of the oceanic ridge flanks.

KatrinaEdwards

2012-02-01

73

Composition of the lunar magma ocean constrained by the conditions for the crust formation  

Science.gov (United States)

The present study aims to constrain the composition of the initial lunar magma ocean (LMO) with fluid dynamic and thermodynamic consideration. A plausible range of the initial LMO composition is investigated by developing an incremental polybaric fractional crystallization model with variable fractionation efficiency to satisfy three conditions for the anorthosite crust formation: (1) the amount of anorthite crystallized from the LMO is abundant enough to form the crust with the observed thickness, (2) the Mg# (=Mg/(Mg + Fe)) of orthopyroxene crystallized with anorthite in the cooling LMO is consistent with that observed in the lunar highland rocks, ferroan anorthosite, and (3) crystallized anorthite separated to float in the turbulent LMO. A plausible range of FeO and Al2O3 contents of the bulk LMO is successfully constrained as a crescent region tight for FeO and loose for Al2O3. The FeO content must be higher than 1.3 times the bulk silicate Earth (BSE) and lower than 1.8 ×BSE unless the Al2O3 content of the Moon is extremely higher than the Earth. These upper and lower limits for FeO are positively correlated with the initial Al2O3 content and fractionation efficiency. The FeO-rich LMO composition may suggest that the circum-Earth disk just after the giant impact of the Earth-Moon system formation was more oxidizing or the impactor was richer in FeO than the Earth’s mantle.

Sakai, R.; Nagahara, H.; Ozawa, K.; Tachibana, S.

2014-02-01

74

Mapping tectonic deformation in the crust and upper mantle beneath Europe and the North Atlantic Ocean.  

Science.gov (United States)

We constructed a three-dimensional azimuthally anisotropic model of Europe and the North Atlantic Ocean based on adjoint seismic tomography. Several features are well correlated with historical tectonic events in this region, such as extension along the North Atlantic Ridge, trench retreat in the Mediterranean, and counterclockwise rotation of the Anatolian Plate. Beneath northeastern Europe, the direction of the fast anisotropic axis follows trends of ancient rift systems older than 350 million years, suggesting "frozen-in" anisotropy related to the formation of the craton. Local anisotropic strength profiles identify the brittle-ductile transitions in lithospheric strength. In continental regions, these profiles also identify the lower crust, characterized by ductile flow. The observed anisotropic fabric is generally consistent with the current surface strain rate measured by geodetic surveys. PMID:23929947

Zhu, Hejun; Tromp, Jeroen

2013-08-23

75

High-Resolution Seismic Constraints on Oceanic Crust and Sediment Subduction  

Science.gov (United States)

Low-velocity layers observed at the top of subducting slabs have long been thought to consist of some or all of the subducted oceanic crust and sediment. However, previous estimates of the thickness of these layers have been poorly constrained giving thicknesses between 2 and 10 km. These results provide only broad constraints on possible subduction factory models and chemical recycling budgets that we seek to improve upon. Here we present a new method for directly determining the thickness of these layers based on observations of S-to-P converted phases from local slab earthquakes. We apply frequency-domain polarisation filtering to isolate mode-converted (P?S and S?P) energy in the seismograms. Two S-to-P converted phases are possible from the upper and lower interfaces of the layer, but they are distinguishable by their relative polarity to the direct P phase. Using differential travel times obtained by measuring these phases, we apply a fitting technique to determine the low-velocity layer thickness. Synthetic tests indicate that thicknesses may be determined to better than 250 m, but the uncertainty depends on event distribution. We present results from two study areas, the Shumagin Islands, Alaska and Guam in the Marianas. These regions were chosen due to the geometry of the slabs, the location of seismic stations and in light of previous work done in each region. In the Shumagins, the thickness of the low-velocity layer is 7 ± 0.25 km, and in the Marianas it is 6 ± 0.5 km. Given sediment input to these trenches, the results in both locations suggest that complete subduction of the oceanic crust and sediment layer occurs. We show an example of how the results may be used to update element recycling budgets through subduction zones.

Horleston, A. C.; Helffrich, G.

2005-12-01

76

Controls on thallium uptake during hydrothermal alteration of the upper ocean crust  

Science.gov (United States)

Hydrothermal circulation is a fundamental component of global biogeochemical cycles. However, the magnitude of the high temperature axial hydrothermal fluid flux remains disputed, and the lower temperature ridge flank fluid flux is difficult to quantify. Thallium (Tl) isotopes behave differently in axial compared to ridge flank systems, with Tl near-quantitatively stripped from the intrusive crust by high temperature hydrothermal reactions, but added to the lavas during low temperature reaction with seawater. This contrasting behavior provides a unique approach to determine the fluid fluxes associated with axial and ridge flank environments. Unfortunately, our understanding of the Tl isotopic mass balance is hindered by poor knowledge of the mineralogical, physical and chemical controls on Tl-uptake by the ocean crust. Here we use analyses of basaltic volcanic upper crust from Integrated Ocean Drilling Program Hole U1301B on the Juan de Fuca Ridge flank, combined with published analyses of dredged seafloor basalts and upper crustal basalts from Holes 504B and 896A, to investigate the controls on Tl-uptake by mid-ocean ridge basalts and evaluate when in the evolution of the ridge flank hydrothermal system Tl-uptake occurs. Seafloor basalts indicate an association between basaltic uptake of Tl from cold seawater and uptake of Cs and Rb, which are known to partition into K-rich phases. Although there is no clear relationship between Tl and K contents of seafloor basalts, the data do not rule out the incorporation of at least some Tl into the same minerals as the alkali elements. In contrast, we find no relationship between the Tl content and either the abundance of secondary phyllosilicate minerals, or the K, Cs or Rb contents in upper crustal basalts. We conclude that the uptake of Tl and alkali elements during hydrothermal alteration of the upper crust involves different processes and/or mineral phases compared to those that govern seafloor weathering. Furthermore, a correlation between the Tl and S concentrations of upper crustal basalts from Holes U1301B, 504B and 896A indicates that Tl is primarily incorporated into secondary sulfides. Given that some of these secondary sulfides formed as a result of microbial sulfate reduction, microbial action is at least indirectly responsible for Tl-uptake. Thallium-enrichment of ridge flank basalts requires a Tl-bearing fluid and physical, chemical and microbial conditions that favor secondary sulfide formation. Uptake of Tl occurs in reducing environments in the background rocks away from fluid flow pathways during early 'open' circulation of oxidizing seawater but more pervasively throughout the system during later 'restricted' circulation of reducing fluids. The Tl-isotope system is therefore a useful tracer of the fluid flux through both the 'open' and 'restricted' ridge flank hydrothermal regimes.

Coggon, Rosalind M.; Rehkämper, Mark; Atteck, Charlotte; Teagle, Damon A. H.; Alt, Jeffrey C.; Cooper, Matthew J.

2014-11-01

77

Geological storage of CO2 within the oceanic crust by gravitational trapping  

Science.gov (United States)

rise of atmospheric carbon dioxide (CO2) principally due to the burning of fossil fuels is a key driver of anthropogenic climate change. Mitigation strategies include improved efficiency, using renewable energy, and capture and long-term sequestration of CO2. Most sequestration research considers CO2 injection into deep saline aquifers or depleted hydrocarbon reservoirs. Unconventional suggestions include CO2 storage in the porous volcanic lavas of uppermost oceanic crust. Here we test the feasibility of injecting CO2 into deep-sea basalts and identify sites where CO2 should be both physically and gravitationally trapped. We use global databases to estimate pressure and temperature, hence density of CO2 and seawater at the sediment-basement interface. At previously suggested sites on the Juan de Fuca Plate and in the eastern equatorial Pacific Ocean, CO2 is gravitationally unstable. However, we identify five sediment-covered regions where CO2 is denser than seawater, each sufficient for several centuries of anthropogenic CO2 emissions.

Marieni, Chiara; Henstock, Timothy J.; Teagle, Damon A. H.

2013-12-01

78

Oxygen Isotope Composition of the Oceanic Crust in the Oman ophiolite, Wadi Fizh section  

Science.gov (United States)

Oxygen isotope compositions of oceanic crustal rocks of a complete section through the Wadi Fizh area in the Oman ophiolite were determined in order to investigate water-rock interaction at mid-ocean ridges. Bulk rock ?18O values decreased with increasing depth (the mean ?18O values of pillow basalt, sheeted dike and gabbro were 10.9, 7.6 and 4.8 per mil, respectively). Plagiogranites were relatively ?18O-enriched (6.2 to 9.0 per mil), whereas two samples of epidosites showed the lowest ?18O values (1.5 and 2.7 per mil). Average ?18O value (6.1 per mil) of bulk crustal rocks is almost identical to that of MORB basalt (5.7±0.3 per mil). Assuming that the ?18O of hydrothermal solution is 2 per mil and water-rock ratio is 9.4 (calculated from 87Sr/86Sr by the previous study), maximum temperature of alteration in the lower gabbro section is estimated as >500° C, which is consistent with the temperature of alteration estimated from assemblage of secondary minerals. If the reacted solution is more enriched in ?18O, the temperature of alteration might be higher. Although the trend of ?18O depth profile in the Wadi Fizh section is consistent with the previous studies of the other sections (Ibra, Hilti, Shafan and Rajimi) in the Oman ophiolite, the ?18O of the gabbro section in the Wadi Fizh section is highly depleted in comparison with the other sections. It indicates that large volume of high-temperature seawater circulated down to the Moho and the gabbros rarely overprinted by the late stage, low-temperature hydrothermal alteration at off-axis. Thus, the oceanic crust in the Wadi Fizh area is an ideal section to interpret the hydrothermal circulation at fast-spreading ridge system.

Yamaoka, K.; Hiroyasu, Y.; Matsubaya, O.; Ishiyama, D.; Chiba, H.; Kawahata, H.

2008-12-01

79

Petrophysical properties of the root zone of sheeted dikes in the ocean crust: A case study from Hole ODP/IODP 1256D, Eastern Equatorial Pacific  

Science.gov (United States)

ODP (Ocean Drilling Program)/IODP (Integrated Ocean Drilling Program) Site 1256 is located on the Cocos Plate in the Eastern Equatorial Pacific Ocean, in a 15 Ma old oceanic lithosphere formed at the EPR during a period of superfast spreading (> 200 mm/yr). ODP/IODP Hole 1256D reached for the first time the contact between sheeted dikes and underlying gabbros. It consequently offers a unique opportunity to study in situ, in present-day oceanic crust, the root zone of the sheeted dike complex. This root zone is a thin, 100 m thick boundary layer between the magmatic system (i.e., the axial melt lens, ~ 1100 °C), and the overlying high temperature hydrothermal system (? 450 °C). The understanding of interactions within this boundary layer is critical to that of crustal processes along mid-ocean ridges. This work focuses on the petrophysical characterization of the root zone of the sheeted dike complex in order to further constrain the hydrothermal circulation system in the vicinity of the axial melt lens, as recorded in non-granoblastic dikes, granoblastic dikes, and varitextured gabbros. The petrophysical properties were determined from sample measurements in the laboratory and were compared to in situ downhole geophysical probing. The porosity structure is bipolar, depending on lithology, resulting in a layered system. Non-granoblastic dikes are generally altered in the greenschist facies (~>250 °C) with relatively high and interconnected (cementation index m ~ 1.72, electrical tortuosity ? ~ 28.3) porosity (1.5%). In contrast, gabbros are retrogressively metamorphosed in the amphibolite (~>450 °C) and greenschist facies, with lower porosity (1.3%) that involves numerous fissures and cracks, resulting in a more connected medium ( m ~ 1.58, ? ~ 11.8) than non-granoblastic dikes. These cracks are more abundant but also tend to close with increasing depth as indicated in downhole geophysical data. Porosity and alteration, as viewed from surface electrical conductivity, appear to be directly correlated.

Violay, Marie; Pezard, Philippe A.; Ildefonse, Benoît; Belghoul, Akram; Laverne, Christine

2010-10-01

80

Oceanwide imprint of large tectonic and oceanic events on seawater Nd isotope composition in the Indian Ocean from 90 to 40 Ma  

Science.gov (United States)

We have analyzed four sediment cores from the Southern Indian Ocean (ODP sites 757, 758, 1135 and 762) with high carbonate content, in order to reconstruct the neodymium isotopic composition (?Nd) of ancient intermediate South Indian seawater from Late Cretaceous (90 Ma) to Early Eocene (40 Ma). The ?Ndvariations are highly consistent and exhibit reproducible patterns over a very large geographic area, confirming the seawater origin of the signal. Combining geochemical constraints with paleogeographic reconstructions, we highlight the respective roles of (1) large-scale tectonic events, (2) continental weathering from surrounding Precambrian terrains (90-65 Ma), (3) oceanic circulation changes (50-40 Ma) and, possibly, (4) local volcanism of the ultra-fast spreading South East Indian Ridge (SEIR) (60-50 Ma) on the Nd isotopic composition of South Indian seawater. Between 60 Ma and 50 Ma, the regional Nd isotopic variations closely mimic changes in SEIR spreading rate. We suggest that the Nd isotopic composition of seawater could be influenced by Nd of volcanic origin in the vicinity of ultra-fast spreading ridges (>13 cm/yr). The India-Asia collision closed the Equatorial Seaway between Asia and India and drastically changed oceanic circulation patterns in the Indian Ocean: warm and more radiogenic Pacific equatorial seawater was diverted to the South by the East Indian coast. A stronger mixing of this Pacific seawater with South Indian seawater would explain the rapid shift of?Nd from 50 Ma (-11) to 40 Ma (-8).

Le Houedec, Sandrine; Meynadier, Laure; Cogné, Jean-Pascal; AllèGre, Claude J.; Gourlan, Alexandra T.

2012-06-01

 
 
 
 
81

Assimilation of sediments embedded in the oceanic arc crust: myth or reality?  

Science.gov (United States)

Arc magmas are commonly assumed to form by melting of sub-arc mantle that has been variably enriched by a component from the subducted slab. Although most magmas that reach the surface are not primitive, the impact of assimilation of the arc crust is often ignored with the consequence that trace element and isotopic compositions are commonly attributed only to varying contributions from different components present in the mantle. This jeopardises the integrity of mass balance recycling calculations. Here we use Sr and O isotope data in minerals from a suite of volcanic rocks from St Lucia, Lesser Antilles arc, to show that assimilation of oceanic arc basement can be significant. Analysis of 87Sr/86Sr in single plagioclase phenocrysts from four Soufrière Volcanic Complex (SVC; St Lucia) hand samples with similar composition (87Sr/86Sr = 0.7089-0.7091) reveals crystal isotopic heterogeneity among hand samples ranging from 0.7083 to 0.7094 with up to 0.0008 difference within a single hand sample. ?O18 measurements in the SVC crystals show extreme variation beyond the mantle range with +7.5 to +11.1‰ for plagioclase (n=19), +10.6 to +11.8‰ for quartz (n=10), +9.4 to +9.8‰ for amphibole (n=2) and +9 to +9.5‰ for pyroxene (n=3) while older lavas (Pre-Soufriere Volcanic Complex), with less radiogenic whole rock Sr composition (87Sr/86Sr = 0.7041-0.7062) display values closer to mantle range: +6.4 to +7.9‰ for plagioclase (n=4) and +6 to +6.8‰ for pyroxene (n=5). We argue that the 87Sr/86Sr isotope disequilibrium and extreme ?O18 values provide compelling evidence for assimilation of material located within the arc crust. Positive correlations between mineral ?O18 and whole rock 87Sr/86Sr, 143Nd/144Nd and 206,207,208Pb/204Pb shows that assimilation seems to be responsible not only for the isotopic heterogeneity observed in St Lucia but also in the whole Lesser Antilles since St Lucia encompasses almost the whole-arc range of isotopic compositions. This highlights the need for detailed mineral-scale investigation of oceanic arc suites to quantify assimilation that could otherwise lead to misinterpretation of source composition and subduction processes.

Bezard, Rachel; Davidson, Jon P.; Turner, Simon; Macpherson, Colin G.; Lindsay, Jan M.; Boyce, Adrian J.

2014-06-01

82

Molybdenum evidence for expansive sulfidic water masses in ~ 750 Ma oceans  

Science.gov (United States)

The Ediacaran appearance of large animals, including motile bilaterians, is commonly hypothesized to reflect a physiologically enabling increase in atmospheric and oceanic oxygen abundances (pO 2). To date, direct evidence for low oxygen in pre-Ediacaran oceans has focused on chemical signatures in the rock record that reflect conditions in local basins, but this approach is both biased to constrain only shallower basins and statistically limited when we seek to follow the evolution of mean ocean chemical state through time. Because the abundance and isotopic composition of molybdenum (Mo) in organic-rich euxinic sediments can vary in response to changes in global redox conditions, Mo geochemistry provides independent constraints on the global evolution of well-oxygenated environments. Here, we establish a theoretical framework to access global marine Mo cycle in the past from the abundance and isotope composition of ancient seawater. Further, we investigate the ~ 750 Ma Walcott Member of the Chuar Group, Grand Canyon, which accumulated in a rift basin with open connection to the ocean. Iron speciation data from upper Walcott shales indicate that local bottom waters were anoxic and sulfidic, consistent with their high organic content (up to 20 wt.%). Similar facies in Phanerozoic successions contain high concentrations of redox-sensitive metals, but in the Walcott Member, abundances of Mo and U, as well as Mo/TOC (~ 0.5 ppm/wt.%) are low. ? 98Mo values also fall well below modern equivalents (0.99 ± 0.13‰ versus ~ 2.35‰ today). These signatures are consistent with model predictions where sulfidic waters cover ~ 1-4% of the global seafloor, corresponding to a ~ 20-80 fold increase compared to the modern ocean. Therefore, our results suggest globally expansive sulfidic water masses in mid-Neoproterozoic oceans, bridging a nearly 700 million-year gap in previous Mo data. We propose that anoxic and sulfidic (euxinic) conditions governed Mo cycling in the oceans even as ferruginous subsurface waters re-appeared 800-750 Ma, and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic O 2 level, consistent with the hypothesis that pO 2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the Ediacaran Period.

Dahl, Tais W.; Canfield, Donald E.; Rosing, Minik T.; Frei, Robert E.; Gordon, Gwyneth W.; Knoll, Andrew H.; Anbar, Ariel D.

2011-11-01

83

Dike intrusion controls on permeability and hydrothermal circulation of oceanic crust at IODP Hole 1256D  

Science.gov (United States)

We examine the hydrothermal structure of the lava-dike transition zone in oceanic crust of Integrated Ocean Drilling Program (IODP) Hole 1256D using detailed sample measurements of permeability, porosity, metamorphic minerals, and structures. The transition zone consists of basaltic sheet and massive flows, a cataclastic unit, and hyaloclastitic breccias. Structural investigations show that this transition occurs through a larger depth interval than that previously defined, extending 254 m upward from the top of the Sheeted Dike Complex (811.4 to 1065.7 meters below seafloor (mbsf)). Through the transition zone, models predict a general decrease in permeability, based on a corresponding decrease in porosity with depth. Laboratory measurements of physical properties show porosity decreases drastically, as does permeability. Thin sections reveal no open pore space, and all structures are filled: veins (mm- to cm-thick) or sets of parallel veins or vein networks, Riedel-deformation bands, cataclasites (cemented by chalcedony, quartz, calcite, anhydrite, and minor amphibole. The dip angle of planar structures (including cataclasites) show an average downdip increase, thus such sub-vertical structures may represent original cooling fractures and/or might be related to the regional tectonics (i.e., extensional tectonics of the rift zone) or to the local tensional stress field created at the top of the dikes as a direct consequence of dike intrusion. Since this transition zone is located near the boundary between the Low Temperature Alteration Zone (above) and the Hydrothermal Alteration Zone (below), we postulate that fracturing and hydrothermal alteration in the investigated crustal interval likely occurred during dike intrusion. Consequently, the lower part of the lavas (below 811.4 mbsf or 561 meters sub-basement) seems to mark a boundary layer between different stress fields, permeability regimes, and metamorphic imprints.

Gilbert, L. A.; Tartarotti, P.; Fontana, E.; Bona, M. L.; Gross, D.; LaPier, G.; Dempsey, C.

2012-12-01

84

Scattering beneath Western Pacific subduction zones: evidence for oceanic crust in the mid-mantle  

Science.gov (United States)

Small-scale heterogeneities in the mantle can give important insight into the dynamics and composition of the Earth's interior. Here, we analyse seismic energy found as precursors to PP, which is scattered off small-scale heterogeneities related to subduction zones in the upper and mid-mantle. We use data from shallow earthquakes (less than 100 km depth) in the epicentral distance range of 90°-110° and use array methods to study a 100 s window prior to the PP arrival. Our analysis focuses on energy arriving off the great circle path between source and receiver. We select coherent arrivals automatically, based on a semblance weighted beampower spectrum, maximizing the selection of weak amplitude arrivals. Assuming single P-to-P scattering and using the directivity information from array processing, we locate the scattering origin by ray tracing through a 1-D velocity model. Using data from the small-aperture Eielson Array (ILAR) in Alaska, we are able to image structure related to heterogeneities in western Pacific subduction zones. We find evidence for ˜300 small-scale heterogeneities in the region around the present-day Japan, Izu-Bonin, Mariana and West Philippine subduction zones. Most of the detected heterogeneities are located in the crust and upper mantle, but 6 per cent of scatterers are located deeper than 600 km. Scatterers in the transition zone correlate well with edges of fast features in tomographic images and subducted slab contours derived from slab seismicity. We locate deeper scatterers beneath the Izu-Bonin/Mariana subduction zones, which outline a steeply dipping pseudo-planar feature to 1480 km depth, and beneath the ancient (84-144 Ma) Indonesian subduction trench down to 1880 km depth. We image the remnants of subducted crustal material, likely the underside reflection of the subducted Moho. The presence of deep scatterers related to past and present subduction provides evidence that the subducted crust does descend into the lower mantle at least for these steeply dipping subduction zones. Applying the same technique to other source-receiver paths will increase our knowledge of the small-scale structure of the mantle and will provide further constraints on geodynamic models.

Bentham, H. L. M.; Rost, S.

2014-06-01

85

A hydrologic model for the uppermost oceanic crust constrained by temperature estimates from carbonate minerals  

Science.gov (United States)

models of the oceanic crust commonly assume that the uppermost igneous extrusive layer of the aquifer is thermally well mixed, although this assumption has not been rigorously tested. Here, the assumption of a thermally well-mixed aquifer is tested against the geological record using O isotope-derived crystallization temperatures of carbonates in the lavas as a record of the temperatures experienced by the aquifer. It is found that carbonate formation temperatures are higher than can be explained by a model of outcrop-to-outcrop flow in a well-mixed aquifer at four of the seven drilling locations analyzed. A poorly mixed aquifer is developed to further explore the crustal hydrology at these locations. Relative to a well-mixed aquifer, a poorly mixed aquifer can achieve higher average temperatures, develops larger lateral pressure gradients driving flow, and requires a lower permeability to achieve a given lateral fluid flux. O isotope data from most of the carbonate samples analyzed are consistent with temperatures achievable in a poorly mixed aquifer; those samples which are not consistent can be explained by plausible special circumstances (such as formation at a discharge zone, where ascending fluid may warm the uppermost aquifer).

Anderson, B. W.; Gillis, K. M.; Coogan, L. A.

2013-08-01

86

Growth of continental crust and its episodic reworking over >800 Ma: evidence from Hf-Nd isotope data on the Pietersburg block (South Africa)  

Science.gov (United States)

The formation and evolution of the continental crust during the Precambrian, and in particular during the Archaean eon (4.0-2.5 Ga), is still a matter of debate. In particular, it is not yet clear in which tectonic environment the genesis of crust took place and how the large volume of granitoid rocks that form ~70% of the Archaean crust were extracted from the mantle. Many studies highlighted that radiogenic isotope systems, especially Lu-Hf and Sm-Nd, are powerful tools to unravel the respective extent of crustal growth and recycling in Archaean terranes. This work presents coupled Hf and Nd isotope data (analyzed both in situ in accessory minerals and in whole rock samples) of Meso- to Neoarchaean granitoids, applied to unravel the processes of crust formation and evolution of the Pietersburg crustal block in South Africa. This crustal segment, the northermost one of the Archaean Kaapvaal Craton, is separated from older crust (3.65-3.10 Ga) by a large-scale suture zone, and the processes related to amalgamation of both blocks and their subsequent evolution are still unclear. The Pietersburg block is made up of a wide range of Archaean granitoid rocks, including tonalite-trondhjemite-granodiorite (TTG) series, high-K monzogranites as well as (grano)diorites belonging to the so-called "sanukitoid" group [1], all intruded by late Paleoproterozoic alkaline complexes. Age determinations highlighted two stages of granitoid formation: (1) TTG magmatism took place episodically over >400 Ma between 3.34 and 2.89 Ga, with a major pulse at 2.97-2.90 Ga; while (2) all the other (high-K) granitoid types emplaced subsequently between 2.84 and 2.69 Ga before a long magmatic shutdown until the intrusion of alkaline complexes at ~2.00 Ga [2-3]. Isotope systematics reveal that these two stages are related to juvenile crust formation and crust reworking, respectively. Indeed, all Hf-Nd isotope data from TTG gneisses are suprachondritic, pointing to a juvenile origin and precluding any incorporation of older crust from the core of the Kaapvaal craton. In contrast, all data from the younger granitoids, including ~2 Ga-old alkaline complexes, plot along a single, well-defined trend of decreasing ?Hf-Nd towards youngest ages. This trend points to model ages around 2.95-3.05 Ga, which is that of the youngest TTGs, indicating that all ?2.84 Ga-old granitoids formed by episodic reworking, over >800 Ma, of a single crustal reservoir represented by these TTGs. Interestingly enough, this reworking took place "in situ" by both (1) classical intracrustal differentiation (formation of high-K monzogranites) and (2) erosion, sedimentation and recycling of the resulting detrital material in the mantle, producing a hybrid mantle source from which derived the "sanukitoid" magmas and, much later, the alkaline complexes. All these observations are consistent if the Pietersburg block is regarded as an accretionary, Cordilleran-type orogen, formed along the northern margin of the Kaapvaal block by successive amalgamation of small TTG terranes between 3.34 and 2.90 Ga, as a result of episodic subduction dynamics. This gave rise to (1) a juvenile, accretionary crustal terrane and (2) a (isotopically similar) lithospheric mantle source, hybridized with crustal components derived from it. All the younger granitoids thus reult from significant reworking of these two reservoirs, first during ongoing convergence and continental collision between 2.84 and 2.69 Ga; and second, in response to lithosphere heating (intrusion of the Bushveld Complex?) in a within-plate environment at 2.00 Ga. [1] Laurent O. et al., submitted to Lithos [2] Zeh A. et al., 2009. Journal of Petrology 50(5), 933-966 [3] Laurent O. et al., 2013. Precambrian Research 230, 209-226

Laurent, Oscar; Zeh, Armin; Moyen, Jean-François; Doucelance, Régis; Martin, Hervé

2014-05-01

87

Seismic waves guided by untransformed oceanic crust subducting into the mantle: the case of the Kanto district, central Japan  

Science.gov (United States)

A model of the subduction of oceanic crust is established through an analysis of the upper mantle earthquakes occurring beneath the Kanto district, central Japan. A distinct pair of later P and S phases (X p and X s commonly observed at specific stations for these events in a depth range of 40-60 km, and is considered to take place in association with the subducting motion of the Philippine Sea plate. The X p phase is found, from its particle motions, to be a longitudinal seismic wave. A principal component analysis of wavelets including the initial P and the X p phases shows that the latter arrives at the Earth's surface with a slightly shallower angle relative to the former. Close agreement between the relative arrival time ratio of ( Xs - S)/( Xp - P) and the Vp / Vs ratio of crustal or mantle rocks suggests that both X p and X s travel along the same path as longitudinal and tangential waves, respectively. One can clearly observe these later phases at stations located where the upper surface of the Philippine Sea plate is apparently in direct contact with the bottom of the crust of the Honshu island. Apparent velocities of the X p and the X s; phases are comparable to the seismic velocities of the main layer of the oceanic crust or the lower continental crust, which are never expected for an upper mantle earthquake if the velocity structure is laterally homogeneous. Travel-time curves for several earthquakes indicate that these apparent velocities can be directly related to the seismic velocity in the source region. The hypocentral distribution of the events related to the subducting motion of the Philippine Sea plate does not form such a thin seismic zone as observed in southwest Japan but extends over the entire thickness of the Philippine Sea plate. The earthquakes from which the later phases are observed occur only in a shallower portion of this thick and contorted seismic zone. The deeper portion does not generate such events. These characteristics are best explained if these later phases are regarded as the seismic waves guided by the subducted portion of the gabbroic oceanic crust at the top of the Philippine Sea plate. Taking the result of our earlier work into account, we conclude that the subducting oceanic crust remains in a gabbroic phase without transformation to eclogitic rocks at depths down to about 60 km along the entire northern boundary of the Philippine Sea plate.

Hori, S.

1990-05-01

88

Brittle ductile transition in experimentally deformed basalt under oceanic crust conditions  

Science.gov (United States)

The mid-ocean ridge system is the largest continuous volcanic feature on Earth, with significant interactions between tectonic activity, volcanism and sea-water circulation. Iceland is the biggest landmass straddling a mid-ocean ridge. The associated tectonic and volcanic settings resulting from the active rifting provide in this geodynamic context a major heat source for the geothermal exploitation. High-pressure, high-temperature, conventional triaxial compression experiments have been conducted in a Paterson Press to explore the brittle-ductile transition of oceanic crustal rocks under in situ conditions at depth (3-10 Km). The study provides some insights into the prospect of producing geothermal fluids from deep wells drilled into a reservoir at temperatures and pressures of supercritical water (T>400°C). We present a series of 20 axial compression deformation experiments performed on jacketed basalt cores of 10 mm diameter and 20 mm long. The experiments were performed at 100 and 300 MPa, with temperatures ranging from 400°C to 900°C, and pore pressures ranging from 0 to 100 MPa, a constant strain rate of 1 × 10- 5 s- 1 and up to strains of 15%. Two different types of basalts were selected for their simple compositions, low alteration degree and very low porosity (3%). The two samples differed in their percentage of glass, being zero in one case and 15% in the other. For the vitreous sample at a confining pressure of 100 and 300 MPa, our experiments show that deformation takes place by three deformation modes; (1) brittle fracture at 400°C with a maximal strength of 900 MPa, corresponding to failure by localized rupture, (2) strain-hardening at small strains and followed by slipping on a localized fracture plane at a constant strength around 250 MPa at higher strains, for temperatures ranging from 500°C to 700°C, (3) distributed ductile flow at differential stresses from 50 to 100 MPa and temperature from 800 to 900°C. For the non glassy sample, the experiments show (1) at a pressure of 100 MPa between 600°C to 900°C and at pressure of 300 MPa between 600°C and 700°C, the sample fails by localized rupture with a peak strength that depends on temperature and (2) at a pressure of 300 MPa between 800°C and 900°C homogeneous, distributed flow with strengths of 600 MPa and 300 MPa, respectively. Mechanical observations at a constant strain rate of 1 × 10- 5 s- 1 and a confining pressure of 100 MPa and 300 MPa indicate that the rocks are brittle and dilatant up to 700 to 800°C. This indicates that, in the context of the Icelandic geotherm, hydrothermal fluids may circulate, at least briefly, through the oceanic basaltic crust down to 6 to 8 km depth. These results are coherent with the lower limit of the Icelandic seismogenic zone which seems to be associated with a 750 ± 100 °C isothermal surface.

Violay, M.; Gibert, B.; Mainprice, D.; Evans, B.; Pezard, P. A.; Flovenz, O.

2009-04-01

89

Microbial communities in recent and 10 - 28 Ma ocean floor basalt (ODP Leg 187)  

Science.gov (United States)

Previous studies have shown that microbial communities are harboring ocean crust basalt (e.g., Thorseth et al. 1995). The non-hydrothermal regions of ocean ridges are largely unstudied with respect to microbial diversity and physiology. In the present study, the microbial communities resident in samples of recent (PCR -- DGGE) were used to amplify fragments of 16S rRNA genes and to separate individual DNA sequences, corresponding to different species and strains of Bacteria and Archaea in the samples. Relative similarity indices were calculated from DGGE banding patterns using Jaccard's algorithm, and species richness was estimated using Shannon's index. Furthermore, individual DNA bands were excised from the gel and sequenced to evaluate the phylogenetic affiliation of the endolithic microbes. Shannon indices show that the species richness of microbial communities in basalt is higher for seafloor samples (Arctic Ridges) than for subsurface samples (Southeast Indian Ridge). The microbial population in the Arctic Ridge basalt samples affiliates with ten major lineages of the domain Bacteria and 1 major lineage of Archaea. Bacteria in the ODP Leg 187 basalt samples affiliate with six major lineages of the domain Bacteria, whereas no archaeal sequences were retrieved from these samples. Many sequences from both areas appear to be unaffiliated with any previously isolated microbes. The uncultured green nonsulfur bacterium Chloroflexales Arctic 96BD-6, and the three gamma proteobacteria Acinetobacter junii, Pseudoalteromonas sp., and Shewanella frigidimarina affiliate with sequences from both the Arctic Ridges and the Southeast Indian Ridge. The physiological groups of iron and manganese reducers appear to be common in basalt samples, but the major part of the microbial populations can not be assigned to a specific metabolic pathway. There is probably a significant overlap in microbial communities between basalt, sediment, and seawater due to the close contact of the environments. However, part of the microbial population appear to be unique to the basalt environment. Reference: Thorseth, I.H., Torsvik, T., Furnes, H., and Muehlenbachs, K. 1995. Microbes play an important role in the alteration of oceanic crust. Chem. Geol. 126: 137-146.

Lysnes, K.; Steinsbu, B. O.; Einen, J.; Thorseth, I. H.; Pedersen, R. B.; Torsvik, T.

2003-04-01

90

First Paleoproterozoic ophiolite from Gondwana: Geochronologic-geochemical documentation of ancient oceanic crust from Kandra, SE India  

Science.gov (United States)

SHRIMP-RG zircon U-Pb ages confirm the 1.85 Ga age of oceanic crust generated along the SE margin of India. This Paleoproterozoic ophiolite was accreted along a NE-trending suture that juxtaposes the outboard Proterozoic Eastern Ghats Granulite Belt (EGGB) against the inboard Archean Nellore Schist Belt of the Dharwar craton. Collision between the EGGB arc crust and India apparently was highly oblique, involving SW thrusting of oceanic crust and a trailing arc onto the Dharwar craton. Although deformed and dismembered, the original lithologic sequence of the Kandra Ophiolite Complex (KOC) has been largely retained. From SW to NE, the complex consists of layered + isotropic gabbros, sheeted dolerite dikes and amygdaloidal pillow basalts. Ultramafic units are intercalated within the gabbroic rocks, and plagiogranite veins + patches occur within the dolerites. Metacherty layers cap the basalts. The KOC exhibits EMORB geochemistry overprinted by subduction-zone metasomatism. Mafic magmas show high LILE/HFSE, positive Ba and Pb anomalies and negative anomalies for Nb, Zr and Hf in spidergrams — typical of a suprasubduction-zone setting. Its structure and geochemistry suggest that the KOC represents a Chilean-type continental backarc ophiolite. This is the first unequivocal Paleoproterozoic ophiolite reported from India, and probably the first from Gondwana. The 1.85 Ga KOC represents an important Gondwana example in the cascade of arc-continent collisions that assembled the Paleoproterozoic supercontinent Columbia.

Vijaya Kumar, K.; Ernst, W. G.; Leelanandam, C.; Wooden, J. L.; Grove, M. J.

2010-05-01

91

Salinity changes in the North West Pacific Ocean during the late Pliocene/early Quaternary from 2.73 Ma to 2.52 Ma  

Science.gov (United States)

Recent research has increasingly advocated a role for the North Pacific Ocean in modulating global climatic changes over both the last glacial cycle and further back into the geological record. Here a diatom ?18O record is presented from Ocean Drilling Program Site 882 over the Pliocene/Quaternary boundary from 2.73 Ma to 2.52 Ma (MIS G6-MIS 99). Large changes in ?18O diatom of c. 4‰ from 2.73 Ma onwards are documented to occur on a timeframe broadly coinciding with glacial-interglacial cycles. These changes are primarily attributed to large scale inputs of meltwater from glacials surrounding the North Pacific Basin and the Bering Sea. Despite these inputs and associated change in surface water salinity, on the basis of existing opal and U k37 temperature data and new modelled water column densities, no evidence exists to suggests a removal of the halocline stratification or a resumption of the high productivity system similar to that which prevailed prior to 2.73 Ma. The permanence of the halocline suggests that the region played a key role in driving global climatic changes over the early glacial-interglacial cycles that followed the onset of major Northern Hemisphere Glaciation by inhibiting deep water upwelling and ventilation of CO 2 to the atmosphere.

Swann, George E. A.

2010-08-01

92

Platinum group elements and gold in ferromanganese crusts from Afanasiy-Nikitin seamount, equatorial Indian Ocean: Sources and fractionation  

Science.gov (United States)

The major element relationships in ferromanganese (Fe-Mn) crusts from Afanasiy-Nikitin seamount (ANS), eastern equatorial Indian Ocean, appear to be atypical. High positive correlations (r = 0.99) between Mn/Co and Fe/Co ratios, and lack of correlation of those ratios with Co, Ce, and Ce/Co, indicate that the ANS Fe-Mn crusts are distinct from Pacific seamount Fe-Mn crusts, and reflect region-specific chemical characteristics. The platinum group elements (PGE: Ir, Ru, Rh, Pt, and Pd) and Au in ANS Fe-Mn crusts are derived from seawater and are mainly of terrestrial origin, with a minor cosmogenic component. The Ru/Rh (0.5-2) and Pt/Ru ratios (7-28) are closely comparable to ratios in continental basalts, whereas Pd/Ir ratios exhibit values ( 0.75) correlations between water depth and Mn/Co, Fe/Co, Ce/Co, Co, and the PGEs. Fractionation of the PGE-Au from seawater during colloidal precipitation of the major-oxide phases is indicated by well-defined linear positive correlations (r > 0.8) of Co and Ce with Ir, Ru, Rh, and Pt; Au/Co with Mn/Co; and by weak or no correlations of Pd with water depth, Co-normalized major-element ratios, and with the other PGE (r < 0.5). The strong enrichment of Pt (up to 1 ppm) relative to the other PGE and its positive correlations with Ce and Co demonstrate a common link for the high concentrations of all three elements, which likely involves an oxidation reaction on the Mn-oxide and Fe-oxyhydroxide surfaces. The documented fractionation of PGE-Au and their positive association with redox sensitive Co and Ce may have applications in reconstructing past-ocean redox conditions and water masses.

Banakar, V.K.; Hein, J.R.; Rajani, R.P.; Chodankar, A.R.

2007-01-01

93

Platinum group elements and gold in ferromanganese crusts from Afanasiy-Nikitin seamount, equatorial Indian Ocean: Sources and fractionation  

Science.gov (United States)

The major element relationships in ferromanganese (Fe-Mn) crusts from Afanasiy-Nikitin seamount (ANS), eastern equatorial Indian Ocean, appear to be atypical. High positive correlations (r = 0.99) between Mn/Co and Fe/Co ratios, and lack of correlation of those ratios with Co, Ce, and Ce/Co, indicate that the ANS Fe-Mn crusts are distinct from Pacific seamount Fe-Mn crusts, and reflect region-specific chemical characteristics. The platinum group elements (PGE: Ir, Ru, Rh, Pt, and Pd) and Au in ANS Fe-Mn crusts are derived from seawater and are mainly of terrestrial origin, with a minor cosmogenic component. The Ru/Rh (0.5-2) and Pt/Ru ratios (7-28) are closely comparable to ratios in continental basalts, whereas Pd/Ir ratios exhibit values ( 0.75) correlations between water depth and Mn/Co, Fe/Co, Ce/Co, Co, and the PGEs. Fractionation of the PGE-Au from seawater during colloidal precipitation of the major-oxide phases is indicated by well-defined linear positive correlations (r > 0.8) of Co and Ce with Ir, Ru, Rh, and Pt; Au/Co with Mn/Co; and by weak or no correlations of Pd with water depth, Co-normalized major-element ratios, and with the other PGE (r fractionation of PGE-Au and their positive association with redox sensitive Co and Ce may have applications in reconstructing past-ocean redox conditions and water masses.

Banakar, V. K.; Hein, J. R.; Rajani, R. P.; Chodankar, A. R.

2007-01-01

94

Intrasample REE variability in Pacific Ocean Fe-Mn crusts: A potential paleoceanographic indicator  

Energy Technology Data Exchange (ETDEWEB)

Recent studies of the microscale chemistry and morphology of marine Fe-Mn crusts suggest that prevailing oceanographic conditions at the time of formation are recorded in individual layers of the deposits. Using current radiometric crust growth-rate estimates of 1 to 5 mm per million years, thick Fe-Mn oxide accumulations may represent a growth record ranging from 10 to 100 million years. Detailed chemical and mineralogical analyses were performed on 5 mm layers of selected thick (5-10 cm) crusts from our collection. The analyses reveal significant variations in the distribution and abundance of the REE as a function of depth within the crusts. The trivalent REE generally exhibit sympathetic concentration changes in pure Fe-Mn oxides but display a greater degree of fractionation between the light and heavy REE in layers enriched in phosphatic matter. The compositional variability and the extent of REE fractionation can be explained partially by mineralogical control but are also believed to reflect differing depositional conditions and the composition of the seawater from which the deposits formed. Changes in the Ce anomaly throughout the crust correlates with Pt and Ir anomalies observed by other workers and appears to be a very promising indicator of major events. The potential use of Ce as a paleoceanographic tracer stems from its electronic structure, which leads to a preference for the highly insoluble tetravalent CeO{sub 2}, whereas other REE exist principally in the trivalent oxidation state. Because Fe-Mn crusts accrete primarily under oxidizing conditions, variations in the Ce anomaly of crusts may be useful as a paleoredox indicator as previously suggested by other investigators. The REE patterns observed in this study will be discussed in terms of their relation to the major element composition and mineralogy of the crusts.

De Carlo, E.H. (Hawaii Institute of Geophysics, Honolulu (USA))

1990-06-01

95

Pliocene sea surface temperatures of the north atlantic ocean at 3.0 Ma  

Science.gov (United States)

Sea-surface temperature (SST) estimates based on quantitative analysis of planktic foraminifer faunas in North Atlantic deep sea cores suggest that high-frequency, low-amplitude variability related to orbital forcing was superimposed on long-term changes that delineate intervals within the Pliocene that were both warmer and cooler than today. SST estimates from several DSDP and ODP sites, as well as land sections, have been combined into a synoptic view of SST during a Pliocene warm interval centered at about 3.0 Ma. The Pliocene North Atlantic warm interval SST estimates show little evidence for warming in tropical regions whereas mid- to high-latitude areas show moderate to strong warming. SST estimates for the last interglacial (Isotope Stage 5e) show a similar pattern, but warming during the last interglacial was not as pronounced as the Middle Pliocene warming. The regional distribution of SST estimates during these past warm events suggests an increase in ocean circulation. ?? 1991.

Dowsett, H.J.; Poore, R.Z.

1991-01-01

96

Oceanic crust production in the Dinarides during the Senonian: combined U-Pb in situ laser ablation ICP(MC)-MS zircon and mineral separates Ar-Ar dating  

International Nuclear Information System (INIS)

p>Pb/204Pb (15.611 - 15.709) values when compared with recent MORB. Although the Sr and Pb composition could reflect see water contamination, the Nd isotopic composition suggests a subduction-related setting for the generation of these magmas. We used a novel approach in ophiolite research by combining U-Pb and Lu-Hf zircon analyses by in situ laser ablation ICP-(MC)-MS and Ar-Ar dating. Combined Hf (in zircon) and Nd (on whole rock samples) data give the most accurate information about the composition of the source. First data show that zircon from a gabbro have 176Hf/177Hf slightly higher than chondritic values (0.282730 ± 0.000028; 2SD; ?Hf of 0.8 ± 1.0) which is compatible with a subduction-related setting.Our combined Ar- Ar and U-Pb dating show that - at least locally in the Dinarides - oceanic crust formed in the Senonian. The U-Pb zircon ages show that gabbros crystallized at 103.2 ± 1.5 Ma and confirm the amphibole Ar-Ar ages (109 ± 17 Ma). Pillow lavas from another Dinaric locality gave even younger plagioclase Ar-Ar ages (61 ± 14 Ma). Commonly it was thought that oceanic domains in the Dinarides had closed no later than the early Cretaceous. Our data, however clearly, suggest that oceanic crust was produced for a long time and lasted at least till Campanian/Maastrichtian times. It is currently too early to make new geotectonic interpretation but these results give completely new insights in our understanding of geodynamic development of this part of Balkans. (author)

97

Slab-derived water and the petrogenesis of distinct zones of oceanic crust along spreading centers in the Lau back-arc basin  

Science.gov (United States)

Back-arc basin crust formed along the Eastern Lau Spreading Center (ELSC) exhibits dramatic and abrupt changes in magmatic processes and crustal formation with proximity to the nearby Tofua Arc. Systematic variations in seafloor morphology, crustal thickness, seismic properties, and lava composition reflect a decreasing 'subduction influence' with increasing distance from the arc. Results from seismic tomography indicate that the crust that forms near the arc is abnormally thick and compositionally stratified, with a thick low-velocity upper crust and an abnormally high-velocity lower crust. As the ridge moves away from the arc, there is a step-like transition in crustal properties towards crustal velocities and thicknesses more typical of oceanic crust produced at mid-ocean ridges. Likewise, lava compositions exhibit abrupt changes in slab-derived volatiles and trace element enrichments, with silicic, arc-like compositions at the Valu Fa Ridge and southern half of the ELSC, located near the arc, and relatively depleted basalts along the northern ELSC, which is located further from the arc. We attribute the observed changes in the physical and chemical makeup of the crust to excess mantle melting coupled with higher degrees of crustal differentiation near the arc due to higher mantle water contents. We propose a model for the formation of the arc-proximal layered crust whereby water-rich basaltic melts stall and differentiate in the lower crust. High-pressure crystallization concentrates water in the residual melts, decreasing their viscosity and density. Eventually the lighter, more felsic residual melts are extracted from the lower crust, leaving behind a dense, mafic cumulate layer, and go on to produce a silica-rich, porous volcanic layer. We present results of thermodynamic modeling of phase equilibria and develop a petrological model for the formation of this unusual "hydrous" form of oceanic crust.

Eason, Deborah; Dunn, Robert

2014-05-01

98

Continental growth through time by underplating of subducted oceanic crust: evidence from kimberlites in South Africa and SW Pacific  

International Nuclear Information System (INIS)

In the dynamic model of plate tectonics, it is evident that crustal components are returned to the mantle by subduction. Chemical signatures of these subducted components were identified in ocean island volcanics and in island arc volcanics. Indeed, an origin involving a subducted protolith was postulated for certain types of xenoliths in kimberlite, including diamonds. Recent studies of eclogite xenoliths in kimberlite from southern Africa and megacrysts form the Malaitan alnoite, Solomon islands, indicate that lithospheric underplating by subducted oceanic crust has occurred in these two contrasting areas. The results of new eclogite studies from the Bellsbank kimberlite, South Africa, and isotopic data from the Malaitan alnoite megacryst suite. This forms the basis for discerning the role of lithospheric underplating in the growth of cratons and in the evolution of mantle-derived magma

99

Continental growth through time by underplating of subducted oceanic crust: Evidence from kimberlites in South Africa and SW Pacific  

Science.gov (United States)

In the dynamic model of plate tectonics, it is evident that crustal components are returned to the mantle by subduction. Chemical signatures of these subducted components were identified in ocean island volcanics and in island arc volcanics. Indeed, an origin involving a subducted protolith was postulated for certain types of xenoliths in kimberlite, including diamonds. Recent studies of eclogite xenoliths in kimberlite from southern Africa and megacrysts form the Malaitan alnoite, Solomon islands, indicate that lithospheric underplating by subducted oceanic crust has occurred in these two contrasting areas. The results of new eclogite studies from the Bellsbank kimberlite, South Africa, and isotopic data from the Malaitan alnoite megacryst suite. This forms the basis for discerning the role of lithospheric underplating in the growth of cratons and in the evolution of mantle-derived magma.

Taylor, Lawrence A.; Neal, Clive R.

1988-01-01

100

Hydrothermal Alteration of the Ocean Crust: Constraints From O and Sr Isotopic Compositions of Whole Rocks and Secondary Minerals From Macquarie Island  

Science.gov (United States)

Hydrothermal circulation is a fundamental process in the formation and aging of the ocean crust, influencing its structure, physical and chemical properties, and the composition of the oceans and the mantle. The impact of hydrothermal circulation on mid-ocean ridge processes depends on the composition and volume of circulating hydrothermal fluids, and the extent of partitioning between high temperature axial- and low temperature ridge flank- systems, but these processes remain poorly constrained. Macquarie Island, approximately 1500 km south of New Zealand, is a unique sub-aerial exposure of a complete section of ocean crust in the ocean basin in which it formed. The crust formed during a phase of slow spreading along a short segment of mid-ocean ridge approximately 11 Myr ago and was uplifted during recent transpression along the Pacific-Australian plate boundary. Oxygen and Strontium isotopic compositions of whole-rock and secondary minerals from Macquarie Island provide a time-integrated record of fluid-rock exchange and fluid evolution during the hydrothermal cooling of the crust. Sr and O isotope analyses, combined with stratigraphic reconstructions of the island provide the first isotopic profiles through a complete section of normal ocean crust. The oxygen isotope profile is similar to profiles through intact ocean crust and the Troodos and Semail ophiolites, indicating that O-exchange during hydrothermal circulation is controlled by the same processes in different spreading environments. However, the Macquarie crust is on average enriched in 18O relative to fresh MORB (?18O = 7.1 ± 0.5 and 5.9 ± 0.1 ‰, respectively) indicating that Macquarie crust was a net sink for 18O from the oceans. The Macquarie crust also exchanged Sr with the oceans. The lavas are on average enriched in 87Sr (87Sr /86Sr = 0.7028 to 0.7035) relative to fresh MORB glasses (0.7025 to 0.70275) with the 87Sr /86Sr ratio increasing to 0.704 at the lava-dike transition, below which the average ratio decreases with depth through the sheeted dikes and gabbros towards primary magmatic compositions. However, a significant variation in whole-rock 87Sr /86Sr ratios at a given depth in the lower crust (with an average range of 0.0005) indicates that fluids were channelled. High primary Sr contents resulted in more rock-dominated fluid-rock Sr-exchange in the Macquarie Island crust, compared to other crustal sections. Consequently the Sr-isotopic compositions of the upwelling `black smoker' hydrothermal fluids, as recorded by epidote veins, are relatively rock- dominated (0.7031 ± 0.0003). Tracer transport mass balance calculations indicate that a time-integrated fluid flux of 4 ± 1 × 106 kg/m2 is required to produce the observed shift in Sr-isotopic composition. This can be supported by the available mid-ocean ridge magmatic heat and is similar to estimates for sections of intact ocean crust, but a factor of 10 lower than estimates for ophiolites. The Macquarie crust is relatively thin (3 - 4 km), due to its formation on a short segment of a slow spreading ridge. This indicates that hydrothermal circulation removes a greater proportion of the available magmatic heat at slower spreading ridges.

Coggon, R. M.; Teagle, D. A.; Alt, J. C.; Davidson, G. J.

2006-12-01

 
 
 
 
101

Intermediate crust (IC); its construction at continent edges, distinctive epeirogenic behaviour and identification as sedimentary basins within continents: new light on pre-oceanic plate motions  

Science.gov (United States)

Introduction. The plate tectonics paradigm currently posits that the Earth has only two kinds of crust - continental and oceanic - and that the former may be stretched to form sedimentary basins or the latter may be modified by arc or collision until it looks continental. But global analysis of the dynamics of actual plate motions for the past 150 Ma indicates [1 - 3] that continental tectospheres must be immensely thicker and rheologically stiffer than previously thought; almost certainly too thick to be stretched with the forces available. In the extreme case of cratons, these tectospheric keels evidently extend to 600 km or more [2, 3]. This thick-plate behaviour is attributable, not to cooling but to a petrological 'stiffening' effect, associated with a loss of water-weakening of the mineral crystals, which also applies to the hitherto supposedly mobile LVZ below MORs [4, 5]. The corresponding thick-plate version of the mid-ocean ridge (MOR) process [6 - 8], replacing the divergent mantle flow model, has a deep, narrow wall-accreting axial crack which not only provides the seismic anisotropy beneath the flanks but also brings two outstanding additional benefits:- (i) why, at medium to fast spreading rates, MOR axes become straight and orthogonally segmented [6], (ii) not being driven by body forces, it can achieve the sudden jumps of axis, spreading-rate and direction widely present in the ocean-floor record. Furthermore, as we will illustrate, the crack walls push themselves apart at depth by a thermodynamic mechanism, so the plates are not being pulled apart. So the presence of this process at a continental edge would not imply the application of extensional force to the margin. Intermediate Crust (IC). In seeking to resolve the paradox that superficially extensional structures are often seen at margins we will first consider how this MOR process would be affected by the heavy concurrent sedimentation to be expected when splitting a mature continent. I reason that, by blocking the hydrothermal cooling widely seen along MOR axes this must inhibit the freezing-in of diagnostic spreading-type magnetic anomalies and would prolong magmagenesis to give a thicker-than-oceanic mafic crust. I have called this Intermediate Crust (IC) [9, 10], to distinguish it from Mature Continental Crust (MCC). Plate separation will continue to generate IC along the margins for as long/far as the sedimentation input is sufficient to have this effect. Transition to the MOR process will then follow. But if, contrary to the general plate tectonics assumption, based on body forces, plate separation ceases after a limited separation (or perhaps several in differing directions), without proceeding to the oceanic condition, the resulting IC areas will be incorporated within the continent [11]. Where does this lead us? With examples drawn from 40 years' study, I will contend that this is indeed the way the Earth has worked and that it offers potential plate kinematic explanation of the origin of the block-and-sedimentary basin layouts abundantly present in the non-craton areas of continents. I will show that in some cases the intricacy of block outlines and the precision with which they can be fitted together in a kinematically consistent manner rules out that this was purely by chance. The evidently meaningful character of those outlines means that they have been drawn by a narrow-crack separative mechanism which reflects that of our new MOR model. To provide a basis for such Plate Kinematic Analysis (PKA) we now link and compare some features of IC-formation at continental edges and of the crust of sedimentary basins. Characteristics of IC and of sedimentary basin crust (SBC). 1. IC basement, with expected seismic Vp around 6km/s, must look deceptively like that assigned to supposedly stretched MCC. 2. For thermodynamic reasons, the hydrous metamorphic content of deep MCC and of deeply subducted UHP slices of it gives them a big thermal epeirogenic sensitivity which IC lacks. Calculation [8, 9] shows that this type of process yields some 12-30 t

Osmaston, Miles F.

2014-05-01

102

The Igneous Architecture of IODP Hole U1309D: Constructing Oceanic Crust from Multiple Sills  

Science.gov (United States)

Gabbroic rocks comprise a significant portion of the footwall of many oceanic core complexes. The decreasing age of these gabbros from the breakaway to the termination suggests that they are continuously accreted as the bounding detachment fault slips. But, questions remain; how are these large (>1 km diameter) gabbroic bodies constructed and at what scale and frequency is melt added to the system? Here we report a detailed lithologic analysis of IODP Hole U1309D drilled into the Atlantis Massif core complex (30° N, MAR). We present new, high resolution (1m scale) downhole lithological diagrams, compiled from observations of the archive core, refined using other available data including magnetic susceptibility. These data show that the thickness of the individual magmatic units is on the order of 1 to a few 10’s of m. Contacts between many of these units are sharp; many of the units are interpreted to result from small injections of melt as opposed to in-situ fractionation. However, these units are often disaggregated by later intrusive bodies and hence, it is likely that individual gabbro units were initially somewhat thicker. Downhole plots of magmatic fabric dip from both shipboard data and new electron backscatter diffraction (EBSD) data are consistent with the units having initially been intruded as sills, and subsequently rotated by ~40-50°, as constrained by paleomagnetic data (Morris et al., 2009). Using all of the available data, we propose the IODP Hole 1309D gabbro section is a composite body that grew episodically by relatively small (10’s m) repeated sill-like injections of melt. We conclude that the melt lens that formed these gabbros was relatively small at any one time. The EBSD data also provide textural constraints on the model for crustal accretion. Gabbroic samples analyzed so far show a moderate to weak plagioclase and clinopyroxene foliation, similar to those in continental mafic intrusions and are thus interpreted to be magmatic fabrics. J-indices for plagioclase a-axes range between 1.28-1.34, and between 1.26-1.29 for the b-axes. These values are comparable to those for the Dufek Intrusion, Antarctica (1.12-1.39 for plagioclase a-axes and 1.18-1.84 for plagioclase b-axes). There is little evidence for significant deformation, which might be expected if the sills were emplaced at one depth and then underwent gabbro-glacier flow to generate the several km thick gabbro crust. However, bent and kinked grains and deformation twins are commonly found in the coarser grained gabbros. These fabrics are likely produced as later sills intrude recently crystallized gabbros. Grimes et al. (2008) uses U-Pb zircon dating to show that the ~1.5km long borehole records a 200 ky emplacement history. This age range permits an estimate of the length-scale over which the sills were emplaced. Assuming the sills were intruded whilst the detachment fault footwall was moving (18mm/yr), then the present day distribution of the dated igneous units down the core implies that the intrusive activity occurred over a depth of ~3.6km. Therefore, we envision that the gabbros accreted as a series of multiple sills at 6-10km below the seafloor.

Christofferson, C. A.; John, B. E.; Cheadle, M. J.; Swapp, S. M.; Grimes, C. B.

2010-12-01

103

Constraints on the accretion of the lower oceanic crust from plagioclase deformation fabrics in the Oman ophiolite  

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Significant progress has been made in the last two decades in understanding the mechanisms of lower crustal accretion at intermediate-fast spreading mid-ocean ridges. Two dominant competing hypotheses have emerged: The "Gabbro Glacier" model proposes that crystallization occurs primarily in the seismically-imaged axial magma chamber at the base of the sheeted dikes, and the lower crust is built by progressive compaction of these cumulates. By contrast, the "Sheeted Sills" model suggests that the lower crust is accreted in a series of small sills that are emplaced and crystallized in situ. One way to distinguish between these two hypotheses is in the evolution of accumulated strain with depth below the paleo-magma chamber. Cumulates formed in the gabbro glacier setting are expected to experience an exponential increase in strain with depth to the Moho, whereas cumulates formed in situ according to the sheeted sills hypothesis should record the same amount of strain regardless of their position within the stratigraphy. In this study we present plagioclase deformation fabrics in gabbros from a detailed section through the lower crust of the Oman ophiolite. In all samples analyzed, the lattice-preferred orientation (LPO) of plagioclases show maxima in (010) parallel to the pole of compositional banding, with variable girdling in [100] and [001]; and the plagioclase shape-preferred orientation (SPO) in all samples is symmetric relative to the pole of compositional banding, rather than oblique. Our results show no systematic change in the strength (quantified by both M-index and J-index) of the plagioclase LPO with depth to the Moho. In addition, we see no statistical correlation between LPO strength and cooling rate estimates previously determined on the same samples. We also present a model for the evolution of plagioclase LPO with increasing strain in order to quantify the amount of strain experienced by lower crust gabbros.

VanTongeren, J. A.; Hirth, G.; Kelemen, P. B.

2011-12-01

104

Ocean-continent transition and tectonic framework of the oceanic crust at the continental margin off NE Brazil: Results of LEPLAC project  

Science.gov (United States)

In 1992, Brazilian Navy and PETROBRAS carried out a geophysical survey along the continental margin off northeastern Brazil, as part of a governmental plan to delineate the "Legal Continental Shelf" according to the international Law of the Sea. This data set is leading to a better understanding of the crustal transition processes and on the evolution of the oceanic crust over that part of the Brazilian continental margin. On our seismic transects, we show a rifted marginal plateau (Pernambuco Plateau) where crustal extension was controlled by detachment faulting, possibly in a non-volcanic margin setting. Farther north, dealing with the ocean-continent transition nearby a major transform margin, we found a normal passive margin-style transition zone instead of transform-related structures. With the support of multichannel seismic profiles and gravity data derived from GEOSAT altimetry, several well-known oceanic fracture zones and structural lineaments were properly located and correlated. The relationship of these structures with volcanic ridges and extensional, compressive and strike-slip tectonic reactivations suggests that fracture zones at this area behaved either as zones of weakness or as locked transform fault scars. Striking lithospheric flexural deformation is also related to FZs in this region. In the surroundings of the Fernando de Noronha Ridge, lithospheric flexure represents an isostatic response to volcanic loading, while bending across Ascension FZ is likely to have been caused by differential subsidence in crustal segments of contrasting ages. We also correlate some other deformation of the oceanic crust with changes in spreading directions that possibly took place at the Upper Cretaceous.

Gomes, Paulo Otávio; Gomes, Benedito S.; Palma, Jorge J. C.; Jinno, Koji; de Souza, Jairo M.

105

Architecture of the Igneous Lower Crust at Oceanic Core Complexes: constraints from IODP Hole U1309D  

Science.gov (United States)

Slow spreading mid-ocean ridges are a ubiquitous part of the global ridge system, yet remain poorly understood. The crust produced at these ridges is fundamentally different than that produced at fast spreading ridges where the crustal architecture largely conforms to the standard Penrose type crust. At slow spread ridges, a reduced magma supply to the crust allows extensional faulting to play a much more important role in accommodating plate separation than at fast spread ridges. Oceanic core complexes (OCCs), a product of such faulting, denude lower crust to the surface via detachment faulting, and thus provide a means to study the architecture of slow spread lower crust. We report a detailed lithologic analysis of IODP Hole U1309D drilled into the Atlantis Massif OCC (30°N MAR). The abundance of sharp contacts between thin inter-layered gabbroic and ultramafic rocks throughout the core supports crustal construction via small (10-40m thick) injections of magma. Paleomagnetic remanance data allows re-orientation of the observed contacts and igneous fabrics to their original orientation at intrusion, and suggest that most contacts and fabrics were sub-vertical. These data therefore imply that construction of slow spread gabbroic lower crust at OCCs is dominated by dike-like intrusions rather than by sills. Combined U-Pb and (U-Th)/He zircon thermochronometry are used to predict a 3-4km thick zone of accretion that lies 6-7 km below seafloor, at the root of the detachment fault. Existing seismic data estimate the depth of the Moho to be 5km at Atlantis Massif constraining the width of the accretion zone in the footwall of the detachment fault to be 4km. Igneous fabrics from unfaulted gabbroic rocks provide an additional major constraint on the processes occurring within this zone of magmatic accretion. Electron backscatter diffraction (EBSD) was used to characterize the fabrics of slow-spread gabbros from the Atlantis Massif OCC, and other OCCs on the MAR (the 15° 20" N OCC and Kane OCC) and from the Southwest Indian Ridge (Atlantis Bank). Fabrics from the fast spread Oman Ophiolite, and from the Dufek, Stillwater and Rum layered mafic intrusions (LMIs) were analyzed for comparison. Plagioclase fabric strength was quantified using eigenvalues and J-indices, calculated using PFch5 careware (Mainprice, 1990). The E1:E2 ratio for {010} was used as a proxy for foliation strength, and the E1:E2 ratio for as a proxy for lineation strength. Gabbroic rocks from the OCCs have a minimal foliation and a non-existent to very weak lineation. In contrast, gabbros from Oman (a proxy for fast spread crust), have stronger foliations and lineations; gabbros from LMIs show strong foliations and no lineations. The absence of strong fabrics in the OCC gabbros is consistent with their formation as thin, ephemeral dike-like bodies; the presence of a very weak lineation may be related to post intrusion deformation of the solidifying crystal mush during plate separation.

Christofferson, C. A.; Cheadle, M. J.; John, B. E.; Schoolmeesters, N.; Swapp, S. M.; Grimes, C. B.

2011-12-01

106

What happens to the Juan de Fuca plate boundary beneath northern Cascadia? Insight into the methamorphism of the oceanic crust  

Science.gov (United States)

In subduction zones, the plate boundary is the locus where the largest earthquakes occur worldwide. The exact knowledge of the geometry of the plate boundary and the physical properties of the materials brought together in contact are fundamental to understand the earthquake nucleation process and to better asses the seismic hazard in densely populated areas. Due to the metamorphism of the subducted materials and the release of fluids from the subducted plate (e.g. breakout of the serpentinized oceanic upper mantle), the seismic properties of the materials distributed along the plate boundary change with depth (i.e. at different age of subduction). Thus, the clear recognition of the plate boundary at depth using indirect method (e.g. seismic tomography) might be a very complex task. In Northern Cascadia, the position of the plate boundary between the North America (NAM) plate and the Juan de Fuca (JdF) plate is still debated. While along the coast, plate boundary models almost agree, differences in the estimated depth of such interface arise in-land between different models, exactly where large earthquakes enucleate. In this study, we investigate the seismic properties of the subducted JdF crust as it plunges in the upper mantle beneath Northern Cascadia. Harmonic decomposition of a huge Receiver function data-set is used to image both isotropic and anisotropic structures along a trench-normal profile, to better constrain the metamorphism of the crustal materials during the subduction process. The analysis of the seismic anisotropy of the materials is fundamental to better recognize the different components of the subducted plate, where large changes in the seismic velocity of the materials are expected. Our results confirm the 2-layer structure of the JdF crust: basalts over gabbros, and allow us to depict their metamorphism as they plunge into the upper mantle. In the western part of out profile an East-dipping low S-velocity layer is interpreted as fluid-filled basalts, in a region where the uppermost interface of the JdF is sealed. In such region, the JdF upper mantle is strongly anisotropic and the occurrence of intermediate-depth events suggests its progressive de-serpentinization. Further East, fluids coming from the JdF upper mantle trigger the metamorphism of the JdF crust. In the same region, the sealed boundary is finally cracked and fluids hydrate the lower-crust of the NAM plate. Finally, in the eastern portion of our profile, our results clearly depict the final eclogization of the JdF crust and the fluid migration in the NAM upper mantle.

Piana Agostinetti, N.; Miller, M. S.

2012-12-01

107

New constraints on the sources and behavior of neodymium and hafnium in seawater from Pacific Ocean ferromanganese crusts  

Science.gov (United States)

The behavior of dissolved Hf in the marine environment is not well understood due to the lack of direct seawater measurements of Hf isotopes and the limited number of Hf isotope time-series obtained from ferromanganese crusts. In order to place better constraints on input sources and develop further applications, a combined Nd-Hf isotope time-series study of five Pacific ferromanganese crusts was carried out. The samples cover the past 38 Myr and their locations range from sites at the margin of the ocean to remote areas, sites from previously unstudied North and South Pacific areas, and water depths corresponding to deep and bottom waters. For most of the samples a broad coupling of Nd and Hf isotopes is observed. In the Equatorial Pacific ENd and EHf both decrease with water depth. Similarly, ENd and EHf both increase from the South to the North Pacific. These data indicate that the Hf isotopic composition is, in general terms, a suitable tracer for ocean circulation, since inflow and progressive admixture of bottom water is clearly identifiable. The time-series data indicate that inputs and outputs have been balanced throughout much of the late Cenozoic. A simple box model can constrain the relative importance of potential input sources to the North Pacific. Assuming steady state, the model implies significant contributions of radiogenic Nd and Hf from young circum-Pacific arcs and a subordinate role of dust inputs from the Asian continent for the dissolved Nd and Hf budget of the North Pacific. Some changes in ocean circulation that are clearly recognizable in Nd isotopes do not appear to be reflected by Hf isotopic compositions. At two locations within the Pacific Ocean a decoupling of Nd and Hf isotopes is found, indicating limited potential for Hf isotopes as a stand-alone oceanographic tracer and providing evidence of additional local processes that govern the Hf isotopic composition of deep water masses. In the case of the Southwest Pacific there is evidence that decoupling may have been the result of changes in weathering style related to the buildup of Antarctic glaciation. Copyright ?? 2004 Elsevier Ltd.

van de Flierdt, T.; Frank, M.; Lee, D.-C.; Halliday, A.N.; Reynolds, B.C.; Hein, J.R.

2004-01-01

108

Venus trough-and-ridge tessera - Analog to earth oceanic crust formed at spreading centers?  

Science.gov (United States)

The similarity between the morphologies of Venus trough-and-ridge tessera and the earth's ocean floor is discussed. The hypothesis that tessera texture might be related to a crustal fabric produced at spreading centers is examined. It is suggested that the proccesses that produce the ocean floor fabric on earth are good candidates for the origin and production of the trough-and-ridge tessera. To support this hypothesis, the characteristics of the trough-and-ridge terrain in Laima Tessera are described and compared to the seafloor at spreading centers.

Head, James W.

1990-01-01

109

Thermal modelling of a transform-divergent interaction zone, the Demerara Plateau, French Guiana margin: architecture of oceanic and continental crusts  

Science.gov (United States)

The crustal architecture of passive margins is a key to constrain their origin and subsequent evolution, as well as their thermal subsidence. The square shaped continental Demerara Plateau, French Guiana margin, surmounts Central and Equatorial Atlantic oceanic crusts surrounding it. Bounded to the northeast by a WNW-ESE-trending transform fault segment and to both the west and the east by N-S divergent fault segments, the Demerara Plateau is a complex transform-divergent interaction zone. The aim of this study is to refine the crustal architecture of this region as derived from gravity and seismic data, by thermal modelling, and by using surface heat flow data as an additional constraint. Previous studies show that the transform transition domain from continental to oceanic crust occurs across a region of approximately 70-km wide, where the Moho deepens abruptly from 25-27 km beneath the plateau (thinned continental crust), to 11-12 km in the abyssal oceanic domain (3-4 km thick oceanic crust). During the IGUANES cruise (onboard R/V L'Atalante in 2013) 10 surface heat flow measurements crossing the plateau have been carried out. These data are combined with borehole heat flows values around. Measures indicate that surface heat flow values range between 47 and 80 mW/m2 (with an uncertainty on the measurements of ~4mW/m2 on average), and slightly decreases in the continental domain toward the ocean. Preliminary 1D thermal modelling results indicate that these heat flow values are consistent with crustal and sediment thicknesses observed on the Plateau. Along the transform domain, at the transition towards the oceanic crust, heat flow values are lower than model results, if we consider an oceanic crust of more than hundred million years and with a thickness of around 3-4 km. We examine, using a 2D approach, whether this low heat flow could be reasonably accounted for by thermal exchange between oceanic and continental lithospheres.

Grall, Céline; Marcaillou, Boris; Loncke, Lies; Mercier de Lepinay, Marion; Basile, Christophe; Roest, Walter R.; A. M Van Wees, Jan Diederik; A. P. L Cloetingh, Sierd

2014-05-01

110

Lithium isotope as a proxy for water/rock interaction between hydrothermal fluids and oceanic crust at Milos, Greece  

Science.gov (United States)

Hydrothermal activity at Milos in the Aegean island (Greece) is mainly located at rather shallow depth (about 5 m). It is interesting to compare these chemical compositions and the evolution processes of the hydrothermal fluids at deep sea hydrothermal vents in Mid-ocean Ridge (MOR). Lithium (Li) is a highly mobile element and its isotopic composition varies at different geological settings. Therefore, Li and its isotope could be used as an indicator for many geochemical processes. Since 6Li preferential retained in the mineral phase where 7Li is leached into fluid phase during basalt alteration, the Li isotopic fractionation between the rocks and the fluids reflect sensitively the degree of water-rock interaction. In this study, Bio-Rad AG-50W X8 cation exchange resin was used for purifying the hydrothermal fluids to separate Li from other matrix elements. The Li isotopic composition (?7Li) was determined by Multi-collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) with precision better than 0.2‰ (2?, n=20). The Li concentration in the hydrothermal fluids falls between 0.02 to 10.31 mM. The ?7Li values vary from +1.9 to +29.7‰, indicating significant seawater contamination have occurred. These hydrothermal fluids fit well with seawater and brine two end-member binary mixing model. During phase separation, lithium, boron, chlorine, iodine, bromine, sodium and potassium were enriched in the brine phase. On the other hand, aluminum, sulphur and iron were enriched in the vapor phase. There is no significant isotope fractionation between the two phases. The water/rock ratio (W/R) calculated is low (about 1.5 to 1.8) for the Milos fluids, restricted seawater recharge into the oceanic crust. Moreover, the oceanic crust in the region becomes less altered since the W/R is low. The ?7Li value of the hydrothermal fluids can be used as a sensitive tool for studying water-rock interaction.

Lou, U.-Lat; You, Chen-Feng; Wu, Shein-Fu; Chung, Chuan-Hsiung

2014-05-01

111

Generation of felsic melts within fast-spreading oceanic crust: Experimental partial melting of hydrothermally altered sheeted dike  

Science.gov (United States)

In recent oceanic crust and in ophiolites, felsic lithologies are observed. Different processes, like fractional crystallization of MORB and partial melting of mafic rocks are discussed to form these lithologies. Partial melting is expected as a major process in forming felsic lithologies at the base of the sheeted dike complex of fast-spreading ridges, where the axial melt lens is assumed to be located directly beneath the sheeted dikes.It is widely accepted that this melt lens has the potential to trigger partial melting of mafic lithologies at the gabbro/dike transition zone. In this experimental study, the influence of partial melting on the generation of felsic lithologies is examined. Therefore, partial melting experiments at a pressure of 100 MPa were performed. As starting material, a natural basalt from the IODP (Integrated Ocean Drilling Program) drilling at Site 1256 (equatorial East Pacific Rise) was chosen, which is representative for the lower sheeted dike complex. It is characterized as a moderately altered dolerite containing plagioclase (An50-57), clinopyroxene (Mg# 0.55-0.60) and quartz, with chlorite as secondary phase; sulfides and Fe-Ti-oxides are present as accessory minerals. The partial melting experiments were conducted in an H2-controlled IHPV at the Institute of Mineralogy in Hanover, Germany. To investigate the evolution of the partial melts, different experiments were performed at temperatures between 1030°C and 910°C and a constant pressure of 100 MPa. All experiments were water saturated leading to a fO2 corresponding to QFM +1 (QFM = quartz-fayalite-magnetite oxygen buffer). This is slightly more oxidized than MORB crystallization due to the influence of a hydrous fluid which generally increases the oxygen activity. The experimental products were analyzed using electron microprobe for major elements, and a SIMS (CRPG Nancy, France) for trace elements. We present here our first results on phase relations and mineral compositions. The analyzed melts vary in their SiO2 contents between ~50wt% (1030°C) and ~71wt% (910°C) and can be classified as basaltic respectively dacitic. The residual mineral assemblage in the experiments performed at 1030°C contains olivine + plagioclase + clinopyroxene. At lower temperatures, orthopyroxene crystallizes at the expense of olivine, and Fe-Ti-oxides are stabilized. A comparison with corresponding naturally formed lithologies shows similarities both in terms of melt composition and residual phase compositions in those experimental runs performed at lower temperature. First trace element analyses of the experimental melts reveal similar trace element patterns known from naturally formed felsic lithologies recorded in the gabbro/dike transition zone of fast-spread oceanic crust.

Fischer, L. A.; Erdmann, M.; France, L.; Deloule, E.; Koepke, J.

2013-12-01

112

Distributions of REE, Nd, Hf and Pb isotopes in the surfaces of Fe-Mn crusts from across the Pacific Ocean  

Science.gov (United States)

Over the past decade, numerous studies have investigated radiogenic isotopes (i.e. Nd, Pb, Hf, Os) in marine Fe-Mn deposits in an attempt to infer changes in ocean circulation throughout the Cenozoic. The use of radiogenic isotopes as paleoceanographic proxies has been challenged recently by evidence that the isotopic composition of crusts from oceanic domains near major river systems (e.g. Amazon, Congo) or old cratonic areas (i.e. the North Atlantic area) could be influenced significantly by continental inputs. Therefore, for any given crust, it is difficult to deduce the extent to which changes in weathering processes rather than ocean circulation may be responsible for observed isotopic variations. This is partly due to the fact that the oceanic budgets for some of these elements remain poorly constrained. In particular, the influence of both the eastward aeolian transport of Chinese loess and the erosion of the young West-Pacific volcanic belt on the isotopic composition of Pacific water masses has been poorly documented. The deep Pacific Ocean is composed of 4 principal water masses: North Pacific Intermediate Water (NPIW), Antarctic Intermediate Water (AAIW), Pacific Deep Water (PDW) and Antarctic Bottom Water (AABW). AABW, the main source of PDW, flows northward and enters the Central Basin through the Samoan Passage, where it is diverted into two branches: eastward to Line Island Passage and westward through Wake Passage. We will present Nd, Hf and Pb isotopic ratios combined with REE data from the surfaces of 16 Fe-Mn crusts taken at different depths from key areas of the Pacific Ocean. Two crusts were collected from the Izu-Bonin back-arc basin in the western Pacific and, hence, are particularly suitable for monitoring the influence of both continental aeolian and weathering inputs. Two other groups of crusts are from north and south of the equatorial Pacific region. The southern group is situated at the exit of the Samoan Passage, whereas the northern group is located next to the Line Island passage. The two groups represent crusts from a range of water column depths, from 1200 m to 3000 m, sampling the intermediate and deep-water masses. These analyses will allow a preliminary determination of the vertical profile of dissolved Hf isotopes in the Pacific Ocean, coupled with supporting REE, Nd and Pb isotope data.

Chu, N.; Nesbitt, R. W.; German, C. R.; Halbach, P.

2001-12-01

113

Crust and oceans, atmosphere and continents: A new paradigm for the Hadean Earth  

Science.gov (United States)

We report zircon oxygen isotope data (?18Ozircon vs. VSMOW) and reconnaissance Ti-in- zircon concentrations ([Ti]zircon), guided by cathodoluminescence image studies, for detrital zircons up to 4.34 Gyr old from the Narryer Gneiss Complex of Western Australia. Zircon oxygen isotope results bolster the view that some Hadean ({>}3.85 Ga) zircon source melts were enriched in heavy oxygen, a sensitive proxy for contamination by anatectic sediments. Empirical studies show that zircon crystallization temperatures correlate to Ti concentration; our results on a subset of pre-3.8 Ga zircons indicate Hadean zircon core crystallization temperatures are around 680°C in all cases except for one 4.0 Ga grain which yielded temperatures (586-623°C; 3 spots) reconcilable with a sub-solidus origin. Elevated ?18Ozircon values reported here and elsewhere, combined with low minimum-melt crystallization temperature, provide mutually consistent lines of evidence that the Hadean Earth supported an evolved rock cycle which included formation of granitic water-saturated melts, extensive continental crust, hydrosphere- lithosphere interactions and sediment recycling within the first 150 million years of planet formation.

Trail, D.; Mojzsis, S. J.; Harrison, T. M.; Schmitt, A. K.; Watson, E. B.; Young, E. D.

2006-12-01

114

Chemistry of titanite (sphene) in ocean crust: A tool for understanding late-stage igneous and metasomatic processes at mid-ocean ridges  

Science.gov (United States)

Titanite is a common accessory mineral in evolved gabbroic rocks, including Fe-Ti oxide and/or amphibole-bearing gabbro, gabbronorite, olivine-bearing gabbro, troctolite, troctolite microgabbro, quartz diorite, tonalite/trondhjemite veins and their metasomatized equivalents; therefore, it is widespread in oceanic crust. Here we characterize the trace element geochemistry of igneous and metasomatic titanite from in-situ ocean crust formed at varying spreading rates. Understanding titanite chemistry can help constrain conditions of crystallization (melt composition, pressure, temperature, and oxidation state), metasomatic reactions, and the nature of hydrothermal fluids in different ridge settings. Samples come from IODP and ODP cores or dredged samples from one location along the ultraslow-spreading Southwest Indian Ridge (57°E, Hole 735B), three locations on the slow-spreading Mid-Atlantic Ridge (30°N, Hole U1309D; 23°N; and 15°N, Hole 1275D), and one location on the Cocos Plate, formed along the superfast-spreading East Pacific Ridge (Hole 1256D). Textural relations between titanite and adjacent plagioclase, Fe-Ti oxides, and mafic minerals (pyroxene, amphibole, biotite and chlorite) suggest that much oceanic titanite formed under metasomatic conditions. In-situ ion microprobe analysis using the Stanford-USGS SHRIMP-RG of euhedral to anhedral grains has allowed characterization of a large set of trace elements in titanite from these samples. Trace and rare earth element (REE) concentrations range from <10 to 21000 ppm, and show significant variability both between and within samples. Titanite grains from the same sample may have contrasting REE patterns that correlate with Zr concentration, and therefore Zr-in-titanite crystallization temperatures. Zr-in-titanite temperatures range from 548 to 871°C assuming SiO2 and TiO2 activities of 1.0 and 0.7, respectively. Temperature variation within an individual grain ranges up to 161°C, and corresponds to sector zoning in backscattered electron images. The highest average temperatures come from titanite in tonalite/trondhjemite veins, suggesting that these grains likely formed from late-stage fluid-rich melt rather than from later hydrothermal fluids. In contrast, titanite grains from altered olivine-bearing gabbro, troctolite microgabbro, and metagabbro yielded lower Zr-in-titanite temperatures than Ti-in-zircon temperatures from coexisting zircon (i.e. generally below 750°C), implying that these grains formed after zircon crystallization with textural relationships that indicate metasomatic conditions. Thus, metasomatic and late-stage magmatic titanite grains have widely varying REE concentrations that likely relate to temperature, pressure, and/or local compositional effects.

Colwell, L. E.; John, B. E.; Cheadle, M. J.; Wooden, J. L.

2011-12-01

115

Dupal anomaly in existence 115 Ma ago: Evidence from isotopic study of the Kerguelen Plateau (South Indian Ocean)  

Science.gov (United States)

The Kerguelen Plateau (South Indian Ocean), whose oldest age has been dated as early Cretaceous, shows geochemical and isotopic features characteristic of OIB-type magmatism. It is probably related to the early stages of activity of the Kerguelen hot spot which is also responsible for the Ninetyeast Ridge. It shows all evidence of being an oceanic plateau with an impressive volume of magmatism. The Nd-Sr isotopic systematics of the Plateau basalts show a large spread of values comparable to the systematics shown by the basalts from the Kerguelen Islands. However, while the archipelago basalts have Pb isotopic variations almost within analytical errors, the Plateau basalts show large Pb isotopic variations which overlap the whole range observed amongst Indian Ocean ridge basalts. Contamination of a deep, enriched OIB-type plume, i.e., the Kerguelen hot spot with characteristic Dupal signature, by a depleted, MORB-type reservoir can account for both the trace-element and isotopic geochemistry of the Kerguelen Plateau basalts. This indicates the existence of the Dupal anomaly already 115 Ma ago. In addition, evidence for its involvement in the genesis of Indian Ocean basalts occurs throughout time, including the present day. This favors the hypothesis of a deep-seated source for this major geochemical anomaly which is then probably responsible for the special features of the Indian Ocean.

Weis, Dominique; Bassias, Yannis; Gautier, Isabelle; Mennessier, Jean-Paul

1989-08-01

116

75 FR 34929 - Safety Zones: Neptune Deep Water Port, Atlantic Ocean, Boston, MA  

Science.gov (United States)

...1625-AA00 Safety Zones: Neptune Deep Water Port, Atlantic Ocean...for review and comment at the Web site http://www.regulations...rulemaking docket titled: Neptune Deep Water Port, Atlantic Ocean...infrastructure of the Neptune Deep Water Port. Extensive...

2010-06-21

117

Isotopic evidence ( 87Sr/ 86Sr, ? 7Li) for alteration of the oceanic crust at deep-rooted mud volcanoes in the Gulf of Cadiz, NE Atlantic Ocean  

Science.gov (United States)

The chemical and isotopic composition of pore fluids is presented for five deep-rooted mud volcanoes aligned on a transect across the Gulf of Cadiz continental margin at water depths between 350 and 3860 m. Generally decreasing interstitial Li concentrations and 87Sr/ 86Sr ratios with increasing distance from shore are attributed to systematically changing fluid sources across the continental margin. Although highest Li concentrations at the near-shore mud volcanoes coincide with high salinities derived from dissolution of halite and late-stage evaporites, clayey, terrigenous sediments are identified as the ultimate Li source to all pore fluids investigated. Light ? 7Li values, partly close to those of hydrothermal vent fluids (? 7Li: +11.9‰), indicate that Li has been mobilized during high-temperature fluid/sediment or fluid/rock interactions in the deep sub-surface. Intense leaching of terrigenous clay has led to radiogenic 87Sr/ 86Sr ratios (˜0.7106) in pore fluids of the near-shore mud volcanoes. In contrast, non-radiogenic 87Sr/ 86Sr ratios (˜0.7075) at the distal locations are attributed to admixing of a basement-derived fluid component, carrying an isotopic signature from interaction with the basaltic crust. This inference is substantiated by temperature constraints from Li isotope equilibrium calculations suggesting exchange processes at particularly high temperatures (>200 °C) for the least radiogenic pore fluids of the most distal location. Advective pore fluids in the off-shore reaches of the Gulf of Cadiz are influenced by successive exchange processes with both oceanic crust and terrigenous, fine-grained sediments, resulting in a chemical and isotopic signature similar to that of fluids in near-shore ridge flank hydrothermal systems. This suggests that deep-rooted mud volcanoes in the Gulf of Cadiz represent a fluid pathway intermediate between mid-ocean ridge hydrothermal vent and shallow, marginal cold seep. Due to the thicker sediment coverage and slower fluid advection rates, the overall geochemical signature is shifted towards the sediment-diagenetic signal compared to ridge flank hydrothermal environments.

Scholz, Florian; Hensen, Christian; Reitz, Anja; Romer, Rolf L.; Liebetrau, Volker; Meixner, Anette; Weise, Stephan M.; Haeckel, Matthias

2009-09-01

118

Osmium isotope stratigraphy of a marine ferromanganese crust  

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Ferromanganese crusts provide records of long term change in ocean circulation and continental weathering. However, calibrating their age prior to 10 Ma has been entirely based on empirical growth rate models using Co concentrations, which have inherently large uncertainties and fail to detect hiatuses and erosional events. We present a new method for dating these crusts by measuring their osmium (Os) isotope record and matching it to the well-known marine Os isotope evolution of the past 80 Ma. The well-characterised crust CD29-2 from the central Pacific, was believed to define a record of paleooceanographic change from 50 Ma. Previous growth rate estimates based on the Co method are consistent with the new Os isotope stratigraphy but the dating was grossly inaccurate due to long hiatuses that are now detectable. The new chronology shows that it in fact started growing prior to 70 Ma in the late Cretaceous and stopped growing or was eroded between 13.5 and 47 Ma. With this new technique it is now possible to exploit the full potential of the oceanographic and climatic records stored in Fe-Mn crusts. ?? 2005 Elsevier B.V. All rights reserved.

Klemm, V.; Levasseur, S.; Frank, M.; Hein, J.R.; Halliday, A.N.

2005-01-01

119

Linear inversion of a negative gravity anomaly in se Rio Grande cone: a graben on oceanic crust?  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in portuguese Uma anomalia ar-livre com amplitude negativa de 23 mGal em uma região no oceano Atlântico Sul, centrada em 48ºW e 35ºS, foi observada pela primeira vez devido à integração de dados de gravimetria marinha convencionais e dados de gravidade derivados de altimetria por satélite, adquiridos pela missão [...] GEOSAT/ERM. O limite norte desta anomalia coincide com o Lineamento Chuí e o limite sul indica outro lineamento, que é uma extensão da Zona de Fratura Meteoro. A anomalia tem direção NE-SW, sua largura é de 400 km e seu comprimento é de 600 km. Foi utilizada uma metodologia de inversão linear bidimensional, com vínculos relativos e absolutos, para calcular a distribuição de densidades ao longo de três perfis paralelos ao eixo principal da anomalia. O resultado sugere que a espessura de sedimentos na parte mais profunda da bacia é de no mínimo 3,0 km onde a batimetria oceânica é de 4.800 m. Esta feição tectônica, um semi-gráben assimétrico formado entre dois lineamentos, provavelmente situa-se sobre uma crosta oceânica. O volume de sedimentos estimado para esta bacia é de cerca de 50% do volume de sedimentos pós-Mioceno depositados no Rio Grande Cone, onde hidratos de gás foram encontrados. Abstract in english We detect, for the first time, a negative free-air gravity anomaly of 23 mGal amplitude over a region in the South Atlantic Ocean centered at 48ºW and 35ºS. To this end, we used the integration of conventional shipborne gravity data and gravity data derived from GEOSAT/ERM satellite altimetry. The n [...] orth bound of this anomaly coincides with the Chuí Lineament and the south bound indicates another lineament, which is the extension of the Meteor Fracture Zone. The anomaly trend is NE-SW, its width is 400 km and its length is 600 km. Two-dimensional linear inversion with relative and absolute equality constraints was used to calculate the density distribution along three profiles perpendicular to the main axis of the anomaly. The result suggests that the sediment thickness in the deepest part of the basin is at least 3.0 km where the ocean bathymetry is 4,800 m. This tectonic feature, an asymmetric half-graben formed between two lineaments, probably lies over an oceanic crust. The estimated volume of sediments in this basin is approximately 50% of the post-Miocene sediments volume deposited in the Rio Grande Cone where gas-hydrates were found.

Emilson Pereira, Leite; Naomi, Ussami.

120

Fluid flow pathways through the oceanic crust: reaction permeability and isotopic tracing  

Science.gov (United States)

It is generally assumed that the dominant means of creating permeability in ocean floor hydrothermal systems is fracturing, induced either by cooling or by tectonic stress. Here we show textural evidence that metamorphic reactions can create a hierarchy of permeable pathways through gabbroic rocks similar to a fracture hierarchy. Isotopic microsampling shows that just as with fractures, most flow occurs through the larger channelways, and that even at the microscale, flow can be extremely heterogeneous with alteration affecting only certain minerals in the framework, leaving others untouched. Reaction permeability is created in three ways; dissolution creating open porosity, microcracking due to volume increase reactions involving olivine, and expansion of water due to rapid heating in dyke margins, particularly when intruded into brecciated rocks. Our data comes from IODP Hole U1309D, which was drilled to 1400 mbsf in the footwall of the Atlantis Massif detachment fault at the Mid-Atlantic Ridge 30°N. The core is composed of gabbroic rocks interlayered with olivine rich troctolites, with several basalt/diabase sills in the top 130 m. The dominant alteration occurred in the greenschist facies, at depths at least 1 km below seafloor, and decreases in intensity downhole. Whole rock oxygen isotope values range from +5.5 permil to +1.5 permil, indicating variable degrees of interaction with seawater at temperatures generally > 250 °C. Gabbroic rocks and diabases exhibit a range of Sr isotope ratios from MORB values (0.70261) to intermediate ratios (0.70429). Microsampling shows that amphiboles are often more radiogenic than coexisting plagioclase and can sometimes be isotopically altered in the same rock as completely unaltered primary minerals. Large (10 cm) amphibole-filled vugs show values ranging up to 0.708, close to seawater. In some cases however the secondary minerals are virtually unaltered indicating low fluid fluxes in pervasive alteration. SEM textures in broken surfaces reveal extensive evidence for dissolution reactions creating porosity, particularly in diabase where pyroxene is selectively dissolved and the porosity partially filled by actinolite needles. If far-from-equilibrium fluid (such as black smoker fluid) interacts with pyroxene at 300-400 °C, dissolution rates of several microns/day are possible. Fluid volume increase in dyke margins due to heating provides space nearby for dissolved components to precipitate without immediately closing the dissolution porosity, which may be an important part of the process. Amphibole-filled vugs in gabbro are interpreted as the final result of the positive feedback between dissolution and permeability - creating fluid flow tubes analogous to karst in limestone. But in contrast, permeability created by volume increase cracking is self-limiting once the primary phase responsible (olivine) is gone, and hence leads to pervasive olivine replacement but little fluid flux.

McCaig, Andrew; Castelain, Teddy; Klein, Frieder

2013-04-01

 
 
 
 
121

A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge  

Science.gov (United States)

Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle-ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective associations with ductile and cataclastic deformation, and an overall decrease downhole with hydrothermal alteration generally ?5% in the bottom kilometer. Hole 735B provides evidence for a strongly heterogeneous lower ocean crust, and for the inherent interplay of deformation, alteration and igneous processes at slow-spreading ridges. It is strikingly different from gabbros sampled from fast-spreading ridges and at most well-described ophiolite complexes. We attribute this to the remarkable diversity of tectonic environments where crustal accretion occurs in the oceans and to the low probability of a section of old slow-spread crust formed near a major large-offset transform being emplaced on-land compared to sections of young crust from small ocean basins.

Dick, Henry J. B.; Natland, James H.; Alt, Jeffrey C.; Bach, Wolfgang; Bideau, Daniel; Gee, Jeffrey S.; Haggas, Sarah; Hertogen, Jan G. H.; Hirth, Greg; Holm, Paul Martin; Ildefonse, Benoit; Iturrino, Gerardo J.; John, Barbara E.; Kelley, Deborah S.; Kikawa, Eiichi; Kingdon, Andrew; LeRoux, Petrus J.; Maeda, Jinichiro; Meyer, Peter S.; Miller, D. Jay; Naslund, H. Richard; Niu, Yao-Ling; Robinson, Paul T.; Snow, Jonathan; Stephen, Ralph A.; Trimby, Patrick W.; Worm, Horst-Ulrich; Yoshinobu, Aaron

2000-06-01

122

A 5 Ma ocean history of the Bering Sea, observed through diatoms  

Science.gov (United States)

Until now, a paucity of marine records from the arctic and subarctic North Pacific has hindered a complete evaluation of the Pacific's role in abrupt climate events. The Bering Sea is a marginal sea in the North Pacific that has experienced, and is sensitive to, major climatic change. Although only insignificant amounts of North Pacific Intermediate Water (NPIW) forms in the Bering Sea today, the basin not only records, but is potentially involved in causing major climate changes. In 2009, IODP Exp. 323 recovered continuous cores from the Bering Sea extending back into the Pliocene. Diatoms are the dominant microfossil group in this region and reveal major ecological shifts throughout the 5 Ma record. Here we present fossil diatom abundance and assemblage records from core U1340A. The onset of Northern Hemisphere Glaciation is clearly depicted by a shift from warm, nutrient-rich, shade flora species Coscinodiscus marginatus and Lioloma pacificum, to the appearance of sea-ice species at ca. 2.65 Ma. Also at this time, Actinocyclus sp. appears and dominates the assemblage until the present during intergalcial periods. This species is correlated with low nutrient, stratified waters and suggests a reduction in nutrient availability in the subsurface waters. Increasing 15N of bulk organic matter values supports the idea that nutrients were less available and more fully utilized. Neodenticula sp., a species transported by the Alaskan Stream, has a strong presence during interglacial intervals and reflects ice-free open waters at the site. The overall decreasing trend of Neodenticula sp., particularly after 1.0 Ma until the Present, highlights increasing winter sea ice cover (glacial intensification) in the Bering Sea in line with the switch to 100 Ka cycles. A Uk'37-based SST record from this site, indicates about 7 degree C of cooling, in agreement with the idea that the flow of relatively warm Alaskan Stream water decreased since 5 Ma. Laminated sections in core 323-U1340 permit, for the first time, insight into Pliocene diatom seasonal dynamics. High resolution analysis, consisting of diatom counts, SEM, BSEI and X-ray imagery will be presented.

Stroynowski, Z.; Ravelo, C.; Caissie, B.

2011-12-01

123

Mafic granulite xenoliths in the Chilka Lake suite, Eastern Ghats Belt, India: evidence of deep-subduction of residual oceanic crust  

Directory of Open Access Journals (Sweden)

Full Text Available Granulite xenoliths preserve key geochemical and isotopic signatures of their mantle source regions. Mafic granulite and pyroxinite xenoliths within massif-type charnockitic rocks from the Eastern Ghats Belt have recently been reported by us. The mafic granulite xenoliths from the Chilka Lake granulite suite with abundant prograde biotite are geochemically akin to Oceanic Island Basalt (OIB. They can be distinguished from the hornblende-mafic granulite xenoliths with signatures of Arc-derived basalt occurring in the other suites of the Eastern Ghats Belt. These two groups of xenoliths in the Paleoproterozoic Eastern Ghats Province have quite distinct Nd-model ages- 1.9 Ga and 2.5 Ga respectively, which may be interpreted as their crustal residence ages. Strong positive Nb anomalies, indicating subducted oceanic crust in the source, LREE enrichment and strongly fractionated REE pattern are key geochemical signatures attesting to their origin as OIB-type magma. Also low Yb and Sc contents and high (La / YbN ratios can be attributed to melting in the presence of residual garnet and hence at great depths (> 80 km. The variable enrichment in radiogenic 87Sr, between 0.70052 and 0.71092 at 1.9 Ga and less radiogenic 143Nd between ?-1.54 and 7.46 are similar to those of the OIBs compared to MORBs. As OIBs commonly contain some recycled oceanic crust in their sources, we suggest that the residue of the oceanic crust from a previous melting event (~ 2.5 Ga that produced the Arc-derived basalts (protoliths of hornblende-mafic granulite xenoliths could have subducted to great depths and mechanically mixed with the mantle peridotite. A subsequent re-melting event of this mixed source might have occurred at ca. 1.9 Ga as testified by the crustal residence ages of the biotite-mafic granulite xenoliths of the Chilka Lake granulite suite.

S. Bhattacharya

2012-11-01

124

Recycling of nitrogen during subduction of oceanic crust: insights from high-pressure and ultra-high pressure metamorphic rocks  

Science.gov (United States)

During subduction, a significant amount of nitrogen (N) is fixed in the subducting altered oceanic crust (AOC) and, for some margins, the subduction input flux of N in AOC is thought to rival that in sediment. However, the ultimate fate of N during subduction-zone metamorphism remains unclear. N may be released from the AOC and added to arcs, it may be retained in the AOC and incorporated into the mantle, or it may enter fluids along the slab-mantle interface. Moreover, it is not known whether the increase in ?15N accompanying prograde metamorphism of sedimentary rocks also occurs during subduction of mafic igneous rocks and thus what the isotopic contribution of the metabasaltic rocks is when they enter the mantle. In this study, we have analyzed HP/UHP metabasaltic rocks from world-wide localities (Zambia, Italy, Ecuador, China and Spain; range of peak-metamorphic P-T conditions of 14-30 kbar and 500-800°C) for N concentrations and ?15N in an attempt to characterize subduction input flux of N in AOC. Eclogites have variable N concentrations (2 to 20 ppm) and ?15N ranging from -1 to +8. Blueschists contain up to 50 ppm N and overlap in ?15N with the eclogites. In both concentration and ?15N, the HP/UHP metamorphosed mafic rocks are distinct from fresh MORB (N = 1.1 ppm and ?15N = -4 ± 2), but overlap with AOC, consistent with retention of a significant proportion of N during prograde metamorphism. However, trends on diagrams that discriminate between seafloor alteration and metamorphic additions and the concurrent enrichment of N with Ba, Pb, Rb and Cs, together with ?15N values, suggest that some sample sequences (China, Ecuador) were enriched in metasedimentary N by HP/UHP fluid-rock interactions. Others (Italy, Zambia) lack those correlations and appear to more closely reflect the characteristics of the precursor AOC. A sample profile through a prograde blueschist-to-eclogite transformation from the Tianshan (China) reveals that N as well as Rb, Ba and Cs concentrations gradually decrease and N isotopic compositions gradually increase from the blueschist towards the eclogite, suggesting that these elements were leached from the rock and N isotopic fractionation occurs during fluid-induced eclogitization. Our results imply that, if the eclogites represent material that is eventually subducted into the deep mantle, an isotopically heavy N component may be added, which could be of significance for elevated ?15N values in plume-derived magmas (e.g., Society Islands, Hawaii).

Halama, R.; Bebout, G.; John, T.; Schenk, V.

2008-12-01

125

Broadband Marine Magnetotelluric Exploration of the Crust at a Petroleum Prospect and a Mid-Ocean RIdge  

Science.gov (United States)

Broadband marine magnetotelluric (MT) instrumentation developed at Scripps Institution of Oceanography enables resolution of electrical resistivity structure at much shallower depths than previously attainable. While marine seismic reflection surveys have routinely surveyed crustal structure on the continental shelves and mid-ocean ridges, traditional marine MT sensors were only capable of measuring long period fields and so MT experiments were limited to studying mantle structure. The introduction of low-noise sensors allows the broadband MT instrument to now measure the shorter period fields that contain information about crustal resistivity structure. We present two case studies of using the broadband MT system at areas previously surveyed with seismic methods. The joint interpretation of both seismic and MT models for these case studies leads to an improved geological interpretation. At Gemini Prospect in the northern Gulf of Mexico we have collected 42 MT sites in a grid over a three-dimensional (3D) resistive salt structure associated with the petroleum prospect. Depth migrated seismic reflection profiles from a 3D seismic survey at Gemini allow for the verification of two-dimensional (2D) MT inversion models. Combined images of the MT resistivity and seismic reflection profiles show that 2D MT can recover that salt body despite its 3D shape. A steeply dipping and overhanging resistive feature correlates with a previously uninterpreted strong reflection and illustrates how MT can constrain structure in regions where the seismic method performs poorly. A thin and shallow resistive feature shown outside the seismic salt volume may indicate a change in porosity or pore fluids associated with a natural trap in the sediments. At the East Pacific Rise near 9{o50'}N, a pilot survey using the broadband instruments shows sensitivity to structure at shallower depths than previous ridge MT experiments. Two-dimensional inversion of data from 4 MT sites shows a high conductivity zone located in the crust and shallow mantle that is associated with the ridge magmatic system and that agrees well with seismic tomography studies of a nearby section of the ridge. Resistivities beneath the ridge imply a crustal partial melt fraction of about 1-20%, in accordance with the seismic results. Nearly vertical isotherms calculated from the seismic model imply deep hydrothermal circulation acts to convectively cool the flanks of the partial melt region, however, the MT model suggests that this region is probably limited to within a few kilometers of the ridge axis. We augment this study with a preliminary look at data from our experiment this past February, where we collected 69 additional MT sites along two transects at the EPR near 9{o} N.

Key, K. W.; Constable, S. C.

2004-12-01

126

Sulfur geochemistry and microbial sulfate reduction during low-temperature alteration of uplifted lower oceanic crust: Insights from ODP Hole 735B  

Science.gov (United States)

Sulfide petrography plus whole rock contents and isotope ratios of sulfur were measured in a 1.5 km section of oceanic gabbros in order to understand the geochemistry of sulfur cycling during low-temperature seawater alteration of the lower oceanic crust, and to test whether microbial effects may be present. Most samples have low SO4/?S values (? 0.15), have retained igneous globules of pyrrhotite ± chalcopyrite ± pentlandite, and host secondary aggregates of pyrrhotite and pyrite laths in smectite ± iron-oxyhydroxide ± magnetite ± calcite pseudomorphs of olivine and clinopyroxene. Compared to fresh gabbro containing 100–1800 ppm sulfur our data indicate an overall addition of sulfide to the lower crust. Selection of samples altered only at temperatures ? 110 °C constrains microbial sulfate reduction as the only viable mechanism for the observed sulfide addition, which may have been enabled by the production of H2 from oxidation of associated olivine and pyroxene. The wide range in ?34Ssulfide values (? 1.5 to + 16.3‰) and variable additions of sulfide are explained by variable ?sulfate-sulfide under open system pathways, with a possible progression into closed system pathways. Some samples underwent oxidation related to seawater penetration along permeable fault horizons and have lost sulfur, have high SO4/?S (? 0.46) and variable ?34Ssulfide (0.7 to 16.9‰). Negative ?34Ssulfate–?34Ssulfide values for the majority of samples indicate kinetic isotope fractionation during oxidation of sulfide minerals. Depth trends in sulfide–sulfur contents and sulfide mineral assemblages indicate a late-stage downward penetration of seawater into the lower 1 km of Hole 735B. Our results show that under appropriate temperature conditions, a subsurface biosphere can persist in the lower oceanic crust and alter its geochemistry.

Alford, Susan E.; Alt, Jeffrey C.; Shanks, Wayne C., III

2011-01-01

127

antithetic block rotation of the gabbro layer versus the sheeted-dikes complex during oceanic crust accretion: paleomagnetic and petrologic evidence from troodos ophiolite  

Science.gov (United States)

It has been suggested that block rotations in the lower oceanic crust can accommodate extension at slow oceanic spreading environments, and thus may play an important role in oceanic crust tectonics. Yet, there are only few observations supporting the validity of such a mechanism. Here we report paleomagnetic and petrologic evidence for antithetic block rotation of gabrros versus overlying sheeted dikes, which occurred during crust accretion. We study eighteen sites scattered along an exposed transition between gabbro and diabase dikes near the village of Agros, Troodos ophiolite, Cyprus. The magnetization and petrography of the gabbro and the diabase indicate at least two generations of magnetic minerals formations. The first generation are magnetite inclusions in pyroxenes, sub-micrometer to micrometers in size, which were formed during the initial cooling of the rock. This phase occurs in the gabbro as well as in the diabase and carries a primary magnetization associated with rock formation. The second generation of magnetic minerals is associated with hydrothermal activity and carries secondary magnetization. In the gabbro the secondary hydrothermal magnetization is carried by Fe-sulfides filling voids among grains. In the dikes secondary hydrothermal magnetization is carried by maghemite oxy-exsolved from primary titanomagnetite. Paleomagnetic analysis of the magnetizations indicates two stages of block rotations. The first stage includes antithetic block rotations of gabbro and sheeted dikes about sub-horizontal axes. This stage was terminated prior to the acquisition of the secondary hydrothermal magnetization in the gabbro, thus ending close to the ridge. The antithetic block rotation results in a total difference in rotation of ~620 between the gabbro and the sheeted-dikes. A second stage of rotation includes a clockwise rotation of the two sections of ~300 about a sub-vertical axis, most likely as part of regional block rotations associated with the Arakapas transform.

Ron, H.; Shaar, R.; Ebert, Y.; Kessel, R.; Agnon, A.; Abelson, M.

2011-12-01

128

Two stage melt-rock interaction in the lower oceanic crust of the Parece Vela basin (Philippine sea), evidence from the primitive troctolites from the Godzilla Megamullion  

Science.gov (United States)

Godzilla Megamullion is a giant oceanic core complex exposed in an extinct slow- to intermediate-spreading segment of the Parece Vela Basin (Philippine sea) [1; 2]. It exposes lower crust and mantle rocks on the sea-floor, offering a unique opportunity to unravel the architecture and the composition of the lower oceanic lithosphere of an extinct back arc basin. Here we present data on primitive troctolites and associated olivine-gabbros from the breakaway area of the Godzilla Megamullion. On the basis of the olivine/plagioclase volume ratio, the troctolites are subdivided into Ol-troctolites (Ol/Pl >1) and Pl-troctolites (Ol/PlJournal of Petrology, doi:10.1093/petrology/egr029

Sanfilippo, A.; Dick, H. J.; Ohara, Y.

2011-12-01

129

Distribution of various components in a hydrogeneous ferromanganese nodule and an Afanasiy Nikitin Seamount crust from Indian Ocean - A geochemical study using micro-PIXE  

Science.gov (United States)

The present study emphasizes the geochemical features pertaining to the distribution of the major and minor elements in a hydrogenous ferromanganese nodule and a seamount crust originating from the Indian Ocean. The micro-PIXE elemental maps indicate the successive layer formation of Fe and Mn in these deposits. A Ni association with Mn has been further confirmed by observing a Ni-Mn spatial correlation together with their compositional correlation. In addition, the core or the nucleus of the ferromanganese nodule was found to be rich in Fe and not Mn, which strengthens the assumptions made earlier that the nodule formation started with Fe deposition which catalysed the growth of a ferromanganese nodule or crust by successive deposition of Mn and Fe. Irregular patterns of Mn and Fe layers were observed and discussed. Instead of the more often studied Co-Fe association in nodules and crusts, the intra nodule microanalysis revealed a Co-Mn correlation that will be discussed in this paper.

Dutta, R. K.; Sideras-Haddad, E.; Connell, S. H.

2001-07-01

130

Late Quaternary sediment deposition of core MA01 in the Mendeleev Ridge, the western Arctic Ocean: Preliminary results  

Science.gov (United States)

Late Quaternary deep marine sediments in the Arctic Ocean are characterized by brown layers intercalated with yellowish to olive gray layers (Poore et al., 1999; Polyak et al., 2004). Previous studies reported that the brown and gray layers were deposited during interglacial (or interstadial) and glacial (or stadial) periods, respectively. A 5.5-m long gravity core MA01 was obtained from the Mendeleev Ridge in the western Arctic Ocean by R/V Xue Long during scientific cruise CHINARE-V. Age (~450 ka) of core MA01 was tentatively estimated by correlation of brown layers with an adjacent core HLY0503-8JPC (Adler et al., 2009). A total of 22 brown layers characterized by low L* and b*, high Mn concentration, and abundant foraminifera were identified. Corresponding gray layers are characterized by high L* and b*, low Mn concentration, and few foraminiferal tests. Foraminifera abundance peaks are not well correlated to CaCO3 peaks which occurred with the coarse-grained (>0.063 mm) fractions (i.e., IRD) both in brown and gray layers. IRDs are transported presumably by sea ice for the deposition of brown layers and by iceberg for the deposition of gray layers (Polyak et al., 2004). A strong correlation coefficient (r2=0.89) between TOC content and C/N ratio indicates that the major source of organic matter is terrestrial. The good correlations of CaCO3 content to TOC (r2=0.56) and C/N ratio (r2=0.69) imply that IRDs contain detrital CaCO3 which mainly originated from the Canadian Arctic Archipelago. In addition, high kaolinite/chlorite (K/C) ratios mostly correspond to CaCO3 peaks, which suggests that the fine-grained particles in the Mendeleev Ridge are transported from the north coast Alaska and Canada where Mesozoic and Cenozoic strata are widely distributed. Thus, the Beaufort Gyre, the predominant surface current in the western Arctic Ocean, played an important role in the sediment delivery to the Mendeleev Ridge. It is worthy of note that the TOC and CaCO3 peaks are obviously distinct in the upper part of core MA01, whereas these peaks are reduced in the lower part of the core. More study on these contrasting features is in progress. References Adler, R.E., Polyak, L., Ortiz, J.D., Kaufman, D.S., Channell, J.E.T., Xuan, C., Grottoli, A.G., Sellén, E., and Crawford, K.A., 2009. Global and Planetary Change 68(1-2), 18-29. Polyak, L., Curry, W.B., Darby, D.A., Bischof, J., and Cronin, T.M., 2004. Palaeogeography, Palaeoclimatology, Palaeoecology 203, 73-93. Poore, R., Osterman, L., Curry, W., and Phillips, R., 1999. Geology 27, 759-762.

Park, Kwang-Kyu; Kim, Sunghan; Khim, Boo-Keun; Xiao, Wenshen; Wang, Rujian

2014-05-01

131

Araxa Group in the type-area: A fragment of Neoproterozoic oceanic crust in the Brasilia Fold Belt; Grupo Araxa em sua area tipo: um fragmento de crosta oceanica Neoproterozoica na faixa de dobramentos Brasilia  

Energy Technology Data Exchange (ETDEWEB)

This study reviews the geological characteristics and puts forward a new evolution model for the Araxa Group in its type-area, the southern segment of the Neo proterozoic Brasilia Belt, Minas Gerais, Brazil. The Araxa Group is confined within a thrust sheet belonging to a syn formal regional fold, the Araxa Syn form, overlying two other thrust sheets made of the Ibia and Canastra Groups. The Araxa Group is described as a tectono stratigraphic terrane in the sense of Howell (1993). It comprises an igneous mafic sequence, with fine and coarse grained amphibolites, associated with pelitic meta sedimentary rocks, and subordinate psanmites. All rocks were metamorphosed to amphibolite facies at ca. 630 Ma ago and were intruded by collisional granites. The amphibolites represent original basaltic and gabbroic rocks, with minor ultramafic (serpentinite/ amphibole-talc schist). The basalts are similar to high Fe O tholeiites, with REE signatures that resemble E-MORB and {epsilon}{sub Nd(T)} =+ 1.1. The meta sedimentary rocks are interpreted as the result of a marine deep-water sedimentation. They have Sm-Nd model ages of 1,9 Ga, and {epsilon}{sub Nd(T)} = -10.21. The amphibolites and metasediments could represent a fragment of back-arc oceanic crust. The data presented here differ significantly from the original definition of Barbosa et al. (1970) who describe the Araxa Group as a pelitic/psanmitic sequence and the collisional granites as a basement complex. (author)

Seer, Hildor Jose [Centro Federal de Educacao Tecnologica de Araxa, (CEFET), MG (Brazil); Brod, Jose Affonso; Fuck, Reinhardt Adolfo; Pimentel, Marcio Martins; Boaventura, Geraldo Resende; Dardenne, Marcel Auguste [Brasilia Univ., DF (Brazil). Inst. de Geociencias

2001-09-01

132

BATOLITO DE SABANALARGA, PLUTONISMO DE ARCO EN LA ZONA DE SUTURA ENTRE LAS CORTEZAS OCEÁNICA Y CONTINENTAL DE LOS ANDES DEL NORTE / THE SABANALARGA BATHOLITH, ARC PLUTONISM IN THE SUTURE ZONE BETWEEN CONTINENTAL AND OCEANIC CRUST IN THE NORTHERN ANDES  

Scientific Electronic Library Online (English)

Full Text Available SciELO Colombia | Language: Spanish Abstract in spanish El Batolito de Sabanalarga es a un cuerpo alargado de 410 Km2 que se extiende entre las Cordilleras Central y Occidental de Colombia, intruye en el borde occidental la Formación Barroso y las Diabasas de San José de Urama y en el borde oriental las rocas metamórficas del Complejo Cajamarca. El Batol [...] ito de Sabanalarga está formado por al menos dos pulsos magmáticos: uno inicial representado por gabros y dioritas de afinidad subalcalina toleítica y un segundo pulso constituido por cuarzodioritas y tonalitas de afinidad subalcalina de la serie calcoalcalina baja en K. El magmatismo se generó en un ambiente localizado por encima de la zona de subducción, en un arco volcánico plutónico localizado en el borde de sutura entre la corteza continental y la corteza oceánica, afectando ambas cortezas. Hace parte del arco las rocas volcánicas de la Formación Barroso. La edad del Batolito y del arco en general, en concordancia con los datos radiométricos, las relaciones intrusivas y el registro fósil, ocurrió dentro del rango comprendido entre el Cenomaniano-Aptiano superior, localizándose el plutonismo en el rango de edad entre 83 M.a y 102 M.a. Magmatismo como el de la Diorita de Altavista, el Gabro de San Diego y el Batolito Antioqueño, es contemporáneo con el arco que generó el Batolito de Sabanalarga, pudiendo ser parte del mismo evento magmático. Abstract in english The Sabanalarga Batholith is a long shape body reaching 410 Km2, located between the Central and Western cordilleras of Colombia. It intrudes rocks of the Barroso Formation and San Jose de Urama diabases along its western margin and rocks belonging to the Cajamarca complex towards the eastern side o [...] f the pluton. The Sabanalarga batholith is formed by at least two magmatic pulses. The first pulse is represented by gabbros and diorites with tholeiitic sub-alkaline affinity. The second pulse corresponds to cuarzodiorites and tonalites with sub-alkaline to calco-alkalyne low-K affinity. Magmatism is interpreted as being of supra-subduction environment, where the plutonic-volcanic arc is located right into the suture zone bonding and affecting both, continental and oceanic crust. The batholith is part of the volcanic arc of the Barroso Formation. The age of the batholith and related arc, according to available radiometric data, intrusive character and fossil record, occurred between the Cenomanian-Upper Aptian range, constraining the plutonism in the age range between 83 Ma and 102 Ma. The magmatic arc represented by the Sabanalarga batholith is contemporary with the Altavista diorite, San Diego Gabbro and Antioquian batholith and eventually belong to the same magmatic event.

GABRIEL, RODRÍGUEZ G.; MARÍA ISABEL, ARANGO; JOSÉ GILBERTO, BERMÚDEZ.

2012-12-01

133

Application of Sr-isotope and Trace Element Concentrations in Plagioclase to Understanding Melt Evolution in the Lower Crust at Mid-ocean Ridges  

Science.gov (United States)

After extraction from the mantle, melts below mid-ocean ridges must traverse through several kilometers of oceanic crust prior to eruption on the seafloor. During ascent through the lower crust, the chemistry of these melts may be greatly altered by crystal fractionation, reactive transport, and magma mixing. These processes can homogenize and obscure the original signature of the primary mantle melts. Our research presents a new data set utilizing plagioclase phenocrysts in mid-ocean ridge basalts. These phenocrysts preserve an array of geochemical signatures of lower crustal processes, including mixing of primitive melts and reactive transport. Plagioclase is particularly useful for the purpose of investigating crustal processes because it is an early crystallizing phase in the MORB crust and, unlike olivine, has measurable levels of a number of minor and trace elements (e.g. Ti, Sr, Ba, Ce, etc.), all of which have significantly different partitioning behavior and diffusion coefficients. The heterogeneity observed in plagioclase phenocrysts also suggests that these crystals typically grow prior to significant aggregation, allowing them to record processes that are more likely to have been erased in the host lava compositions. Basalts with abundant plagioclase phenocrysts are commonly found at seamounts and ridges with ultra slow to intermediate spreading rates. Major element, trace element and Sr-isotopic data were collected on plagioclase phenocrysts and host glasses from MORBs at ridges with a range of spreading rates and magma supplies including: Southwest Indian Ridge, Southeast Indian Ridge, Blanco Fracture Zone, Juan de Fuca Ridge, Mid-Atlantic Ridge and Axial Seamount. A single lava sample often contains plagioclase phenocrysts with trace element and isotopic characteristics consistent with growth from a range of parental melts. We find these phenocrysts often record the mixing of progressive mantle melts and variable degrees of melt aggregation and diffusive re-equilibration. Despite significant trace element and sometimes isotopic variability, many of the phenocrysts contain relatively constant major element concentrations. Constant anorthite profiles suggest that plagioclase grew in an environment where the major element chemistry was extensively buffered by the wallrock components. Such a process has a much smaller impact on concentration of the incompatible elements (e.g. Ti, Ba), preserving more of the primary magma signature.

Lange, A.; Burleigh, A. W.; Nielsen, R. L.; Tepley, F. J.; Kent, A. J.

2012-12-01

134

Insights on the Formation and Evolution of the Upper Oceanic Crust from Deep Drilling at ODP/IODP Hole 1256D  

Science.gov (United States)

Deep drilling of Hole 1256D on ODP Leg 206 and IODP Expeditions 309/312 provides the first complete section of intact upper oceanic crust down to gabbros. Site 1256 is located on ocean crust of the Cocos Plate that formed at the East Pacific Rise (EPR) 15 million years ago during an episode of superfast rate ocean spreading in excess of 200 mm/yr. Past deep drilling of intact ocean crust has been fraught with difficulties due to the highly fractured nature of oceanic lavas. Site 1256 was specifically chosen because the observed relationship between spreading rate and the depth to axial seismic low velocity zones at modern mid-ocean ridges (thought to be magma chambers), suggests that gabbroic rocks should occur at the shallowest levels in ocean crust formed at the highest spreading rates. In line with pre-drilling predictions, gabbroic rocks were first encountered 1157 m into the basement. Hole 1256D penetrates 754 m of lavas, a 57-m thick transition zone and a thin (346 m) sheeted dike complex. The lower ~60 m of the sheeted dikes are contact metamorphosed to granoblastic textures. After encountering gabbros the hole was deepened a further 100 m before the cessation of drilling operations and the plutonic section comprises two gabbroic sills, 52 and 24 m-thick, intruded into a 24 m screen of granoblastic dikes. The gabbro sills have chilled margins and compositions similar to the overlying lavas and dikes, precluding formation of the cumulate lower oceanic crust from the melt lenses so far penetrated by Hole 1256D. A vertical seismic experiment conducted in Hole 1256D indicates that the bottom of the Hole is still within seismic layer 2 despite gabbroic rocks having been recovered. These data together with 1-D and imaging wire-line logs, have been used to construct a continuous volcano-stratigraphy for Site 1256. Comparison of this data with the recovered cores and the styles of eruption occurring at the modern EPR indicate that ~50% of lava sequences were formed within a few kilometres of the ridge axis, with a further 200 m of lavas that display inflation textures deposited at the base of the axial slope. The great thickness (>75 m) of the ponded lava that makes up the uppermost crust at Site 1256, and an absence of vertical fracturing within those rocks, supports the shipboard interpretation that this unit crystallized a significant distance (~5-10 km) off axis. Geochemical analyses of the Hole 1256D cores have been undertaken to evaluate the melting processes and mantle source heterogeneity under the superfast spreading ancient EPR. Hole 1256D cores have significantly lower incompatible element concentrations than present-day EPR lavas, a signature typically interpreted in terms of greater extents of mantle melting. However, similar crustal thicknesses between Site 1256 (5.5 km) and the EPR (5-7 km) challenge that view. This observation coupled with preliminary radiogenic isotope signatures indicate that the depleted mantle source variation might be caused by upwelling previously depleted Galapagos plume mantle at the paleo-Site 1256. Whole rock and mineral trace element analyses indicate that the gabbros sampled to date cannot be the cumulate rocks remaining after melt extraction. Instead, most of the gabbros represent slowly cooled equivalents of the overlying basaltic rocks. However, some patches of gabbro are magmatically highly evolved, and record late stage melts squeezed out from the crystallizing mush within the melt lens. Secondary mineralogy plus oxygen isotope analyses of vein minerals document the thermal structure of the fossil hydrothermal system penetrated by Hole 1256D, and record a complex history of repeated episodes of magmatism and hydrothermal alteration. The ~800 m volcanic section is partly altered to saponite and celadonite, typical of altered submarine basalts but with little oxidation. There is a stepwise increase in alteration grade across the lava-dike transition and the dikes are variably altered to chlorite and other greenschist minerals (250-350 deg C). The lower 60 m of granoblast

Teagle, D. A. H.

2009-04-01

135

Crustal thickness controlled by plate tectonics : a review of crust-mantle interaction processes illustrated by European examples  

DEFF Research Database (Denmark)

The continental crust on Earth cannot be extracted directly from the mantle, and the primary crust extracted directly from an early magma ocean is not preserved on Earth. We review geophysical and geochemical aspects of global crust–mantle material exchange processes and examine the processes which, on one side, form and transform the continental crust and, on the other side, chemically modify the mantle residue from which the continental crust has been extracted. Major mechanisms that provide crust–mantle material exchange are oceanic and continental subduction, lithosphere delamination, and mafic magmatism. While both subduction and delamination recycle crustal material into the mantle, mafic magmatism transports mantle material upward and participates in growth of new oceanic and continental crusts and significant structural and chemical modification of the latter. We discuss the role of basalt/gabbro–eclogite phase transition in crustal evolution and the links between lithosphere recycling, mafic magmatism, and crustal underplating. We advocate that plate tectonics processes, together with basalt/gabbro–eclogite transition, limit crustal thickness worldwide by providing effective mechanisms of crustal (lithosphere) recycling. The processes of crust–mantle interaction have created very dissimilar crustal styles in Europe, as seen by its seismic structure, crustal thickness, and average seismic velocities in the basement. Our special focus is on processes responsible for the formation of the thin crust of central and western Europe, which was largely formed during the Variscan (430–280 Ma) orogeny but has the present structure of an “extended” crust, similar to that of the Basin and Range province in western USA. Major geophysical characteristics of the Variscan lithosphere are discussed within the frame of possible sequences of crust–mantle material exchange mechanisms during and after main orogenic events in the European Variscides.

Artemieva, Irina M.; Meissner, Rolf

2012-01-01

136

Investigating the link between an iron-60 anomaly in the deep ocean's crust and the origin of the Local Bubble  

International Nuclear Information System (INIS)

Supernova explosions responsible for the creation of the Local Bubble (LB) and its associated HI cavity should have caused geological isotope anomalies via deposition of debris on Earth. The discovery of a highly significant increase of 60Fe (a radionuclide that is exclusively produced in explosive nucleosynthesis) in layers of a deep sea ferromanganese crust corresponding to a time of 2.2 Myr before present, appears very promising in this context. We report on our progress in relating these measurements to the formation of the LB by means of 3D hydrodynamical adaptive mesh refinement simulations of the turbulent interstellar medium in the solar neighborhood. Our calculations are based on a sophisticated selection procedure for the LB's progenitor stars and take advantage of passive scalars for following the chemical mixing process.

137

Large scale obduction of preserved oceanic crust: linking the Lesser Caucasus and NE Anatolian ophiolites and implications for the formation of the Lesser Caucasus-Pontides Arc  

Science.gov (United States)

During the Mesozoic, the Southern margin of the Eurasian continent was involved in the closure of the Paleotethys and opening Neotethys Ocean. Later, from the Jurassic to the Eocene, subductions, obductions, micro-plate accretions, and finally continent-continent collision occurred between Eurasia and Arabia, and resulted in the closure of Neotethys. In the Lesser Caucasus and NE Anatolia three main domains are distinguished from South to North: (1) the South Armenian Block (SAB) and the Tauride-Anatolide Platform (TAP), Gondwanian-derived continental terranes; (2) scattered outcrops of ophiolite bodies, coming up against the Sevan-Akera and Ankara-Erzincan suture zones; and (3) the Eurasian plate, represented by the Eastern Pontides margin and the Somkheto-Karabagh Arc. The slivers of ophiolites are preserved non-metamorphic relics of the now disappeared Northern Neotethys oceanic domain overthrusting onto the continental South Armenian Block (SAB) as well as on the Tauride-Anatolide plateform from the north to the south. It is important to point out that the major part of this oceanic lithosphere disappeared by subduction under the Eurasian Margin to the north. In the Lesser Caucasus, works using geochemical whole-rock analyses, 40Ar/39Ar dating of basalts and gabbro amphiboles and paleontological dating have shown that the obducted oceanic domain originates from a back-arc setting formed throughout Middle Jurassic times. The comprehension of the geodynamic evolution of the Lesser Caucasus supports the presence of two north dipping subduction zones: (1) a subduction under the Eurasian margin and to the south by (2) an intra-oceanic subduction allowing the continental domain to subduct under the oceanic lithosphere, thus leading to ophiolite emplacement. To the West, the NE Anatolian ophiolites have been intensely studied with the aim to characterize the type of oceanic crust which they originated from. Geochemical analyses have shown similar rock types as in Armenia, Mid Ocean Ridge Basalt (MORB) to volcanic arc rocks and Intra-Plate Basalts (IPB). Lithostratigraphic comparisons have shown that the relations between the three units, well identified in the Lesser Caucasus, are similar to those found in NE Anatolia, including the emplacement of stratigraphically conform and discordant deposits. New field data has also shed light on an outcrop of low-grade metamorphic rocks of volcanic origin overthrusted by the ophiolites towards the south on the northern side of the Erzincan basin, along the North Anatolian Fault (NAF). We extend our model for the Lesser Caucasus to NE Anatolia and infer that the missing of the volcanic arc formed above the intra-plate subduction may be explained by its dragging under the obducting ophiolite with scaling by faulting and tectonic erosion. In this large scale model the blueschists of Stepanavan, the garnet amphibolites of Amasia and the metamorphic arc complex of Erzincan correspond to this missing volcanic arc. We propose that the ophiolites of these two zones originate from the same oceanic domain and were emplaced during the same obduction event. This reconstructed ophiolitic nappe represents a preserved non-metamorphic oceanic domain over-thrusting up to 200km of continental domain along more than 500km. Distal outcrops of this exceptional object were preserved from latter collision which was concentrated along the suture zones.

Hassig, Marc; Rolland, Yann; Sosson, Marc; Galoyan, Ghazar; Sahakyan, Lilit; Topuz, Gultelin; Farouk Çelik, Omer; Avagyan, Ara; Muller, Carla

2014-05-01

138

Tectonic implications of post-30 Ma Pacific and North American relative plate motions  

Science.gov (United States)

The Pacific plate moved northwest relative to North America since 42 Ma. The rapid half rate of Pacific-Farallon spreading allowed the ridge to approach the continent at about 29 Ma. Extinct spreading ridges that occur offshore along 65% of the margin document that fragments of the subducted Farallon slab became captured by the Pacific plate and assumed its motion proper to the actual subduction of the spreading ridge. This plate-capture process can be used to explain much of the post-29 Ma Cordilleran North America extension, strike slip, and the inland jump of oceanic spreading in the Gulf of California. Much of the post-29 Ma continental tectonism is the result of the strong traction imposed on the deep part of the continental crust by the gently inclined slab of subducted oceanic lithosphere as it moved to the northwest relative to the overlying continent. -from Authors

Bohannon, R. G.; Parsons, T.

1995-01-01

139

Preliminary Results from IODP Exp. 323 to the Bering Sea: the ocean history for the last 5 Ma  

Science.gov (United States)

The shift from the Late Pliocene to Pleistocene is an important transitional period from global warmth to the initiation of glacial-interglacial cycles on orbital time-scales. Little is understood about the processes responsible for this major shift in the Earth's climate, and until now, a lack of data in critical regions of the Pacific, such as the Bering Sea, has prevented an evaluation of the role of North Pacific processes in global climate change (Takahashi, 1999). The Bering Sea is a marginal sea in the North Pacific that has experienced, and is sensitive to, major climatic change. Here we present preliminary microfossil results from IODP Expedition 323 to the Bering Sea. Diatoms are the dominant microfossil group in this region and reveal major ecological shifts throughout the ~5 Ma record. The onset of Northern Hemisphere Glaciation is clearly depicted by a shift from warm, nutrient-rich, shade flora species Coscinodiscus marginatus and Pyxidicula horridus to the appearance of sea-ice species at ca. 2.5 Ma. A secondary shift is also observed at 1.0-0.9 Ma and coincides with the Middle Pleistocene Transition. The shift to sea-ice dominated assemblages in the northern sector of the Bering Sea, is further intensified at ca. 0.9 Ma and demonstrate a clear response to glacial/interglacial cycles. These observations are further supported by dinoflagellate, radiolarian, foraminifera and geochemical records. The persistence of C. marginatus beyond 2 Ma in the northern latitude sites suggests continued mixing and high nutrient supplies to this zone, and refutes dissolution issues. Increases in intermediate water formation radiolarian species after 1 Ma, together with diatom sea-ice related species suggest the development of NPIW during glacial times.

Stroynowski, Zuzia

2010-05-01

140

Preservation and Recycling of Crust during Accretionary and Collisional Phases of Proterozoic Orogens: A Bumpy Road from Nuna to Rodinia  

Directory of Open Access Journals (Sweden)

Full Text Available Zircon age peaks at 2100–1650 and 1200–1000 Ma correlate with craton collisions in the growth of supercontinents Nuna and Rodinia, respectively, with a time interval between collisions mostly <50 Myr (range 0–250 Myr. Collisional orogens are two types: those with subduction durations <500 Myr and those ?500 Myr. The latter group comprises orogens with long-lived accretionary stages between Nuna and Rodinia assemblies. Neither orogen age nor duration of either subduction or collision correlates with the volume of orogen preserved. Most rocks preserved date to the pre-collisional, subduction (ocean-basin closing stage and not to the collisional stage. The most widely preserved tectonic setting in Proterozoic orogens is the continental arc (10%–90%, mean 60%, with oceanic tectonic settings (oceanic crust, arcs, islands and plateaus, serpentinites, pelagic sediments comprising <20% and mostly <10%. Reworked components comprise 20%–80% (mean 32% and microcratons comprise a minor but poorly known fraction. Nd and Hf isotopic data indicate that Proterozoic orogens contain from 10% to 60% of juvenile crust (mean 36% and 40%–75% reworked crust (mean 64%. Neither the fraction nor the rate of preservation of juvenile crust is related to the collision age nor to the duration of subduction. Regardless of the duration of subduction, the amount of juvenile crust preserved reaches a maximum of about 60%, and 37% of the volume of juvenile continental crust preserved between 2000 and 1000 Ma was produced in the Great Proterozoic Accretionary Orogen (GPAO. Pronounced minima occur in frequency of zircon ages of rocks preserved in the GPAO; with minima at 1600–1500 Ma in Laurentia; 1700–1600 Ma in Amazonia; and 1750–1700 Ma in Baltica. If these minima are due to subduction erosion and delamination as in the Andes in the last 250 Myr; approximately one third of the volume of the Laurentian part of the GPAO could have been recycled into the mantle between 1500 and 1250 Ma. This may have enriched the mantle wedge in incompatible elements and water leading to the production of felsic magmas responsible for the widespread granite-rhyolite province of this age. A rapid decrease in global Nd and in detrital zircon Hf model ages between about 1600 and 1250 Ma could reflect an increase in recycling rate of juvenile crust into the mantle; possibly in response to partial fragmentation of Nuna.

Kent C. Condie

2013-05-01

 
 
 
 
141

Subduction-modified oceanic crust mixed with a depleted mantle reservoir in the sources of the Karoo continental flood basalt province  

Science.gov (United States)

The great majority of continental flood basalts (CFBs) have a marked lithospheric geochemical signature, suggesting derivation from the continental lithosphere, or contamination by it. Here we present new Pb and Os isotopic data and review previously published major element, trace element, mineral chemical, and Sr and Nd isotopic data for geochemically unusual mafic and ultramafic dikes located in the Antarctic segment (Ahlmannryggen, western Dronning Maud Land) of the Karoo CFB province. Some of the dikes show evidence of minor contamination with continental crust, but the least contaminated dikes exhibit depleted mantle - like initial ?Nd (+9) and 187Os/188Os (0.1244-0.1251) at 180 Ma. In contrast, their initial Sr and Pb isotopic compositions (87Sr/86Sr = 0.7035-0.7062, 206Pb/204Pb = 18.2-18.4, 207Pb/204Pb = 15.49-15.52, 208Pb/204Pb = 37.7-37.9 at 180 Ma) are more enriched than expected for depleted mantle, and the major element and mineral chemical evidence indicate contribution from (recycled) pyroxenite sources. Our Sr, Nd, Pb, and Os isotopic and trace element modeling indicate mixed peridotite-pyroxenite sources that contain ˜10-30% of seawater-altered and subduction-modified MORB with a recycling age of less than 1.0 Ga entrained in a depleted Os-rich peridotite matrix. Such a source would explain the unusual combination of elevated initial 87Sr/86Sr and Pb isotopic ratios and relative depletion in LILE, U, Th, Pb and LREE, high initial ?Nd, and low initial 187Os/188Os. Although the sources of the dikes probably did not play a major part in the generation of the Karoo CFBs in general, different kind of recycled source components (e.g., sediment-influenced) would be more difficult to distinguish from lithospheric CFB geochemical signatures. In addition to underlying continental lithosphere, the involvement of recycled sources in causing the apparent lithospheric geochemical affinity of CFBs should thus be carefully assessed in every case.

Heinonen, Jussi S.; Carlson, Richard W.; Riley, Teal R.; Luttinen, Arto V.; Horan, Mary F.

2014-05-01

142

Ocean crust deformation at the North America-South America plate boundary: Results of the 2007 ANTIPLAC marine survey  

Science.gov (United States)

East of the Lesser Antilles active margin, the area of the Barracuda and Tiburon ridges is resulting from of a multidirectional and polyphase tectonic history at the diffuse plate boundary between the North and South American plates. These WNW-ESE trending ridges control the sediment distribution and they are bounded by sedimentary trenches, both ridges and trenches trending parallel to the Mid-Atlantic oceanic fracture zones. A marine survey (called ANTIPLAC) conducted in the beginning of the year (January 2007) has provide new evidences (multibeam and seismic acquisition) of the deformation processes which occurred at this plate boundary. On the seismic lines, a major angular unconformity can be recognized in the whole area of the survey. Interpreting the acquired seismic grid, the lower part of the stratigraphic series can be easily tied to the DSDP/ODP holes of legs 78A, 110, 156, 171A, especially with wells 543 and 672. Thus a Maastrichtian-Pliocene age can be attributed to the geological formations located below the regional unconformity. The very recent geological formations located above the unconformity (attributed to the Late Pliocene-Pleistocene) tend to fill the main depressions of the area and show very heterogeneous thickness. These recent deposits can be more than 3 s(TWT) thick in the Barracuda trough (north of Barracuda ridge). Globally they show clear onlapping characters above the older levels, but in some places these levels show spectacularly evidences of syntectonic deposition. This is notably the case of a narrow WNW-ESE trending fold and fault system trending along the axis of the Barracuda trough. South of Barracuda ridge the recent deposits show also locally spectacular fan geometries characterizing deposition during significant tilting. Also, between Barracuda and Tiburon ridges several fracture zones show evidences of very recent (and probably active) reactivation. This recent deformation is also characterized by recent basin inversion structures. Finally and more generally, the data acquired during the ANTIPLAC survey demonstrate that high deformation occurred at the boundary between the North and South American plates during much more recent times than previously thought, and that notably spectacular compressional structures resulting from the convergence between the two american plates developed recently during Late Pliocene-Quaternary times. The subduction of this structural pattern and its partial incorporation within the Barbados tectonic wedge has widely influenced the deformation processes within the accretionary prism and has also induced segmentation within the overriding Caribbean plate.

Patriat, M.; Benard, F.; Deville, E.; Le Drezen, E.; Loubrieu, B.; Maltese, L.; Roest, W.; Thereau, E.; Umber, M.; Vially, R.

2007-12-01

143

The European margin of the Jurassic Tethys in Corsica: dating of Balagne trondhjemites and evidence to support a continental crust beneath the Balagne-Ligurian domain  

International Nuclear Information System (INIS)

Vein trondhjemite in gabbro of the Carnispola Bridge has been dated to 169 ± 3 Ma (U-Pb on zircon). This date indicates that E-MORB-type ophiolites were emplaced in the marginal Balagne part of the Ligurian Jurassic basin about some 10 Ma before the emplacement of N-MORB ophiolites in the most central part of the ocean. In addition, the presence of inherited zircons with Ordovician (431 ± 8 Ma) and Archean (2693 ± 12 Ma) ages reveals that the Balagne ophiolites were emplaced on a thinned continental crust. Finally, the 298 ± 4 Ma age of zircons from eclogitised meta-arkoses in the eclogitic Morteda-Farinole unit ('Schistes lustres' zone) confirms the attribution of these rocks to a paleogeographic area that laid between continent and ocean, along the edge of the Hercynian granite batholith in Corsica. (authors)

144

Geochemistry of the Jurassic Mirdita Ophiolite (Albania) and the MORB to SSZ evolution of a marginal basin oceanic crust  

Science.gov (United States)

The Middle Jurassic Mirdita Ophiolite in northern Albania is part of an ophiolite belt occurring between the Apulian and Pelagonian subcontinents in the Balkan Peninsula. The upper mantle and crustal units of the Mirdita Ophiolite show major changes in thickness, rock types, and chemical compositions from west to east as a result of its complex evolution in a suprasubduction zone (SSZ) environment. The ˜ 3-4-km-thick Western Mirdita Ophiolite (WMO) includes lherzolite-harzburgite, plagioclase-lherzolite, plagioclase-dunite in its upper mantle units and a plutonic complex composed of olivine gabbro, troctolite, ferrogabbro, and gabbro. These peridotites and gabbroic rocks are overlain directly by a ˜ 600-m-thick extrusive sequence containing basaltic pillow lavas and hyaloclastites. Sheeted dikes are rare in the WMO. The ˜ 12-km-thick Eastern Mirdita Ophiolite (EMO) includes tectonized harzburgite and dunite with extensive chromite deposits, as well as ultramafic cumulates including olivine clinopyroxenite, wehrlite, olivine websterite, and dunite forming a transitional Moho with the overlying lower crustal section. The plutonic rocks are made of pyroxenite, gabbronorite, gabbro, amphibole gabbro, diorite, quartz diorite, and plagiogranite. A well-developed sheeted dike complex has mutually intrusive relations with the underlying isotropic gabbros and plagiogranites and feeds into the overlying pillow lavas. Dike compositions change from older basalt to basaltic andesite, andesite, dacite, quartz diorite, to late-stage andesitic and boninitic dikes as constrained by crosscutting relations. The ˜ 1.1-km-thick extrusive sequence comprises basaltic and basaltic andesitic pillow lavas in the lower 700 m, and andesitic, dacitic and rhyodacitic massive sheet flows in the upper 400 m. Rare boninitic dikes and lavas occur as the youngest igneous products within the EMO. The basaltic and basaltic andesitic rocks of the WMO extrusive sequence display MORB affinities with Ti and Zr contents decreasing upsection (TiO 2 = 3.5-0.5%, Zr = 300-50 ppm), while ?Nd( T) (+ 8 to + 6.5) varies little. These magmas were derived from partial melting of fertile MORB-type mantle. Fractional crystallization was important in the evolution of WMO magmas. The low Ti and HREE abundances and Cs and Ba enrichments in the uppermost basaltic andesites may indicate an increased subduction influence in the evolution of the late-stage WMO magmas. Basaltic andesites in the lower 700 m of the EMO volcanic sequence have lower TiO 2 (˜ 0.5%) and Zr (˜ 50 ppm) contents but ?Nd( T) values (+ 7 to + 6.5) are similar to those of the WMO lavas. These rocks show variable enrichment in subduction-enriched incompatible elements (Cs, Ba, Th, U, LREE). The basaltic andesites through dacites and boninites within the upper 400 meters of EMO lavas show low TiO 2 (˜ 0.8-0.3%) and ?Nd( T) (+ 6.5 to + 3.0). The mantle source of these rocks was variably enriched in Th by melts derived from subducted sediments as indicated by the large variations in Ba, K, and Pb contents. EMO boninitic dikes and lavas and some gabbroic intrusions with negative ?Nd ( T) values (- 1.4 and - 4.0, respectively) suggest that these magmas were produced from partial melting of previously depleted, ultra-refractory mantle. The MORB to SSZ transition (from west to east and stratigraphically upwards in the Mirdita Ophiolite and the progression of the ?Nd( T) values from + 8.0 to - 4.0 towards the east resulted from an eastward shift in protoarc-forearc magmatism, keeping pace with slab rollback in this direction. The mantle flow above the retreating slab and in the arc-wedge corner played a major role in the evolution of the melting column, in which melt generation, aggregation/mixing and differentiation occurred at all levels of the sub-arc/forearc mantle. The SSZ Mirdita Ophiolite evolved during the intra-oceanic collapse and closure of the Pindos marginal basin, which had a protracted tectonic history involving seafloor spreading, protoarc rifting, and trench-continent collision.

Dilek, Yildirim; Furnes, Harald; Shallo, Minella

2008-01-01

145

Gondwanaland from 650-500 Ma assembly through 320 Ma merger in Pangea to 185-100 Ma breakup: supercontinental tectonics via stratigraphy and radiometric dating  

Science.gov (United States)

Gondwanaland lasted from the 650-500 Ma (late Neoproterozoic-Cambrian) amalgamation of African and South American terranes to Antarctica-Australia-India through 320 Ma (mid-Carboniferous) merging with Laurussia in Pangea to breakup from 185 to 100 Ma (Jurassic and Early Cretaceous). Gondwanaland straddled the equator at 540 Ma, lay wholly in the Southern Hemisphere by 350 Ma, and then rotated clockwise so that at 250 Ma Australia reached the S pole and Africa the equator. By initial breakup of Pangea at 185 Ma, Gondwanaland had moved northward such that North Africa reached 35°N. The first clear picture of Gondwanaland, in the Cambrian, shows the assembly of continents with later Laurentian, European and Asian terranes along the "northern" margin, and with a trench along the "western" and "southern" margins, reflected by a 10,000-km-long chain of 530-500 Ma granites. The interior was crossed by the Prydz-Leeuwin and Mozambique Orogenic Belts. The shoreline lapped the flanks of uplifts generated during this complex terminal Pan-Gondwanaland (650-500 Ma) deformation, which endowed Gondwanaland with a thick, buoyant crust and lithosphere and a nonmarine siliciclastic facies. During the Ordovician, terranes drifted from Africa as the first of many transfers of material to the "northern" continents. Central Australia was crossed by the sea, and the eastern margin and ocean floor were flooded by grains of quartz (and 600-500 Ma zircon) from Antarctica. Ice centres in North Africa and southern South America/Africa waxed and waned in the latest Ordovician, Early Silurian, latest Devonian, and Early Carboniferous. In the mid-Carboniferous, Laurussia and Gondwanaland merged in the composite called Pangea by definitive right-lateral contact along the Variscan suture, with collisional stress and subsequent uplift felt as far afield as Australia. Ice sheets developed on the tectonic uplands of Gondwanaland south of 30°S. In the Early Permian, the self-induced heat beneath Pangea drove the first stage of differential subsidence of the Gondwanaland platform to intercept sediment from the melting ice, then to accumulate coal measures with Glossopteris, and subsequently Early Triassic redbeds. An orogenic zone along the Panthalassan margin propagated from South America to Australia and was terminally deformed in the mid-Triassic. Coal deposition resumed during Late Triassic relaxation in the second stage of Pangean extension. In the Early Jurassic, the vast ˜200 Ma Central Atlantic magmatic province of tholeiite anticipated the 185 Ma breakup in the Central Atlantic. Another magmatic province was erupted at this time between southern Africa and southeastern Australia. The northeastern Indian Ocean opened from 156 Ma, and the western Indian Ocean from 150 Ma. By the 100 Ma mid-Cretaceous, the Gondwanaland province of Pangea had split into its five constituents, and the Earth had entered the thalassocratic state of dispersed continents. The 650-500 Ma "Pan-Gondwanaland" events (? by mafic underplating) rendered Gondwanaland permanently geocratic. Pangean (320-185 Ma) tectonics, driven by pulses of self-induced heat, promoted widespread subsidence at 300 Ma Early Permian and 230 Ma Late Triassic. Pangea initially broke up at 185 Ma and the five continental pieces of Gondwanaland had broken apart by the 100 Ma mid-Cretaceous. Another long-lasting feature of Gondwanaland was subduction beneath the "southern" margin and export of terranes from the "northern" and "northwestern" margins. Export of terranes was promoted by Gondwanaland-induced heat, and internal breakup by Pangea-induced heat.

Veevers, J. J.

2004-12-01

146

Microstructures and petro-fabrics of lawsonite blueschist in the North Qilian suture zone, NW China: Implications for seismic anisotropy of subducting oceanic crust  

Science.gov (United States)

We conducted a detailed study on the microstructures and petro-fabrics of massive and foliated lawsonite blueschist (LBS) in North Qilian suture zone, NW China. The lattice preferred orientation (LPO) of glaucophane and lawsonite in foliated lawsonite blueschist (LBS) is considered to be dominantly formed by the deformation mechanism of dislocation creep and rigid-body rotation, respectively. The LPO of glaucophane is mainly characterized by the [001] axis aligning parallel to lineation and the [100] axis and (110) pole plunging perpendicular to foliation. In contrast, the LPO of lawsonite features the maximum [010] axis concentrated close to lineation and the [001] axis strongly clustered normal to foliation. The preferred orientation of [010] axis of lawsonite parallel to lineation is supported by a two-dimensional numerical modeling using the finite-volume method (FVM). The mineral LPOs are much stronger in foliated LBS than in massive LBS. In addition, a kinematic vorticity analysis suggests that both pure shear dominant (Wm = 0.18-0.26) and simple shear dominant (Wm = 0.86-0.93) deformation regimes are present in foliated LBS. The [001] axis and (010) pole of glaucophane, and the [100] and [010] axes of lawsonite, tend to distribute in a foliation-parallel girdle in the pure shear dominant samples, but simple shear dominant samples display more lineation-parallel concentrations of a [001] axis of glaucophane and a [010] axis of lawsonite. Because the whole-rock seismic anisotropies in foliated LBS are significantly higher than those in massive LBS and a counteracting effect on seismic anisotropies occurs between glaucophane and lawsonite, the delay time of fast S-wave polarization anisotropy induced by an actual subducting oceanic crust with a high subducting angle (> 45-60°) is expected to range from 0.03 to 0.09 s (lower bound for massive LBS) and from 0.1 to 0.3 s (upper bound for foliated epidote blueschist).

Cao, Yi; Jung, Haemyeong; Song, Shuguang

2014-07-01

147

Interaction between seawater and magma or very hot rock in the deep fast-spreading oceanic crust: Constraints from experimental petrology  

Science.gov (United States)

More and more publications on the dynamics of magmatic system at fast spreading ridges refer to deep interaction of seawater with magma or with just frozen rocks. Prominent models focus on deep hydrothermal circulation at magmatic temperatures [1], seawater involvement into magmatic processes due to ridge tectonics [2], or on stoping/assimilation of hydrothermally altered dikes at the top of the axial melt lens [3]. In order to understand the underlying hydrous MORB magmatism and the corresponding hydrous melting reactions at fast-spreading ocean ridges, we performed different series of crystallization and partial melting experiments in hydrous MORB-type systems at shallow pressures. (1) To understand the magmatic processes ongoing in the top of the axial melt lenses, we present here new phase diagrams for hydrous MORB systems at a pressure of 50 MPa, by extrapolating results of phase equilibria experiments in hydrous systems performed at shallow pressures in primitive to evolved tholeiitic, MORB-type systems. We applied our new phase diagrams to rocks from the sheeted dike/gabbro transition from the EPR crust (IODP Site 1256D) and from the Oman ophiolite, opening interesting perspectives to explain specific aspects of petrogenesis of these rocks. (2) Experimental hydrous partial melting of gabbro results not only in the production of oceanic plagiogranites, but also in the formation of characteristic interstitial residual parageneses like plagioclase with An-rich rims, orthopyroxene, and pargasitic amphibole rimming the primary phases. Such parageneses can be observed in gabbros from IODP Site 1256D and from the Oman ophiolite, documenting that hydrous partial melting proceeded. (3) Experimental partial melting of hydrated dikes results in the formation of plagiogranitic melts and of a typical residual granoblastic mineral paragenesis. Domains with a characteristic texture (microgranular wormy intergrowth of clino- and orthopyroxene, plagioclase, and oxides) can be observed rather often in Gabbros from IODP Site 1256D, implying that partial melting of stoped hydrated sheeted dikes was a major magmatic process which resulted in locally high water activities enabling primary amphibole crystallization. [1] Nicolas, A., and D. Mainprice (2005), Terra Nova, 17, 326-330. [2] Abily, B., G. Ceuleneer, and P. Launeau (2011), Geology, 39, 391-394. [3] Koepke, J., L. France, T. Müller, F. Faure, N. Goetze, W. Dziony, and B. Ildefonse (2011 accepted), Geochem. Geophys. Geosyst.

Koepke, J.; Botcharnikov, R. E.; Berndt, J.; Feig, S.; France, L.

2011-12-01

148

Anoxia and the nitrogen cycle during Cretaceous Oceanic Anoxic Event 1a (~120 Ma): a data-model comparison  

Science.gov (United States)

The Oceanic Anoxic Events (OAEs) of the Mesozoic are among the most dramatic examples of water column oxygen depletion in Earth's history. OAEs were likely characterized by high rates of denitrification and a profoundly different marine nitrogen cycle than that of today. High abundances of 2-methylhopane biomarkers commonly occur in OAE sediments, especially OAE 1a, and are often interpreted to reflect a major increase in the (relative) abundance of N2-fixing cyanobacteria (e.g., Kuypers et al., 2004). However the spatial extent of these postulated cyanobacterial blooms, control of water column anoxia on their occurrence, and impact on the nitrogen cycle is not well constrained and understood. Combining new data with an intermediate-complexity Earth system model (GENIE), we show that modeled reduced oxygenation of the water column is consistent with the majority of available data for OAE 1a. Increased nutrient availability likely caused widespread bottom water anoxia during OAE 1a, similar to OAE 2 (Monteiro et al., 2012). However, the different paleogeography appears to have prevented widespread expansion of euxinic conditions in the photic zone during OAE 1a, consistent with the general absence of isorenieratane in OAE 1a sediments. A compilation of newly generated and previously published 2-methylhopane biomarker data from the Tethys realm (Cismon core and three newly generated records from southern Spain) and Pacific Ocean (Shatsky Rise, DSDP Site 463, and new data from ODP Site 866) shows that relative abundances of 2-methylhopanes and temporal trends herein differ greatly during OAE 1a, even between proximal sites. Although modeled spatial distribution of nitrogen fixation for OAE1a bears similarity with 2-methylhopanes distributions, distinct differences are present. Altogether these results call into question the assumed effects of periods of anoxia on the nitrogen cycle and suggest that the response of the nitrogen cycle to large perturbations of the global carbon cycle and widespread anoxia is more complicated than previously assumed. As oxygen minimum zones are likely to expand in response to global warming (Keeling et al., 2009), these results are important for our understanding of future changes in the global nitrogen cycle. References: - Kuypers, M.M.M., et al., 2004. N2-fixing cyanobacteria supplied nutrient N for Cretaceous oceanic anoxic events. Geology 32 (10), 853-856. - Monteiro, F.M., et al., 2012. Nutrients as the dominant control on the spread of anoxia and euxinia across the Cenomanian-Turonian oceanic anoxic event (OAE2): Model-data comparison. Paleoceanography 27 (4), PA4209. - Keeling, R.F., et al., 2009. Ocean Deoxygenation in a Warming World. Annual Review of Marine Science 2 (1), 199-229.

Naafs, B. D.; Monteiro, F. M.; Froehner, S.; Lowson, C.; Quijano, M.; Castro, J.; Donnadieu, Y.; Schmidt, D. N.; Ridgwell, A. J.; Pancost, R. D.

2013-12-01

149

Absolute plate motion, mantle flow, and volcanism at the boundary between the Pacific and Indian Ocean mantle domains since 90 Ma  

Science.gov (United States)

The Tasman Sea and the southeastern Indian Ocean are situated at the interface between the Indian and Pacific oceans mantle domains. Even though the present location of the boundary between these two reservoirs at the Australian Antarctic Discordance (AAD) is well constrained by geochemical data, its dynamic behaviour is less well understood. Here, we investigate the relationship between volcanism in and south of the Tasman Sea, absolute plate motion, spreading asymmetries and microcontinent formation. We test three hypotheses: since 90 Ma, the upper mantle east of the AAD and west of the Macquarie Triple Junction was attached to 1) the Indian-Atlantic hotspot reference frame, 2) the Pacific hotspot reference frame 3) to the Pacific hotspots from 90 to 40 Ma, and subsequently to the Atlantic-Indian hotspots, due to partial separation of the two mantle domains by subducting slabs. We find that hostspot tracks in this area are best modeled using a Pacific hotspot reference frame, confirming hypothesis (2). If a hotspot is now located close to the Balleny Islands, it could not have produced the Tasmanian seamount chain irrespective of which reference fame is chosen, contrary to previous suggestions. Seafloor spreading asymmetries and microcontinent formation in the Tasman Sea are best accounted for by plume-ridge interactions, rather than the absolute motion of the spreading ridge relative to the mantle. The short-lived nature of most hotspot volcanism in this area, as well as its geochemistry, points to a paleo-superswell in the upper mantle as its source, rather than deep seated plumes, with the exception of the Tasmantid seamounts.

Gaina, Carmen; Dietmar Müller, R.; Cande, Steven C.

150

Southern Ocean Surface and Intermediate Water Temperature from Alkenones and Mg/Ca of Infaunal Foraminifera for the last 1.5 Ma  

Science.gov (United States)

The reconstruction of past surface (SST), intermediate, and deep-water temperatures is critical to our understanding of feedbacks within the ocean-climate system. Intermediate water temperature (IWT) reconstruction is particularly important since intermediate waters, including Antarctic Intermediate Water (AAIW), are proposed to be an important driver in high-low latitude teleconnections, despite limited intermediate-depth records through the Pliocene and Pleistocene. Paleotemperature proxies have caveats, including the 'Carbonate Ion Effect' on the Magnesium to Calcium ratio (Mg/Ca) of benthic foraminifera. However, recent studies demonstrated that the infaunal species, Uvigerina peregrina, co-precipitates Mg independent of secondary effects, affording the use of U.peregrina Mg/Ca as a paleotemperature proxy (Elderfield et al., 2010). We present the first 1.5 Ma record of IWT from Mg/CaU.peregrina coupled with an alkenone- derived UK37' SST record from a sediment core in the Southwest Pacific (DSDP site 593; 1068m water depth), in the core of modern AAIW. Our new data reconstruct interglacial IWTs at ~7°C before and after the Mid-Pleistocene Transition (MPT), whereas values of ~5°C occur in the later Pleistocene. Glacial IWT remained fairly constant (~2°C) throughout the last 1 Ma. These results are in apparent disagreement with the typical idea that glacial-interglacial temperature fluctuations were smaller in the '41-kyr world' before the MPT, than during the '100-kyr world', after the MPT. At proximal ODP site 1123 (3290m water depth; Elderfield et al., 2012), interglacial deepwater temperatures increase by ~1°C after the MPT, with relatively constant glacial deepwater temperatures (~-2°C) over the last 1 Ma. New results from DSDP 593 therefore imply that the mechanisms that drive intermediate and deep water temperatures varied, suggesting that at least one of these watermasses has properties driven by something other than Northern Hemisphere glaciation patterns.

Elmore, Aurora; McClymont, Erin; Elderfield, Harry; Kender, Sev

2014-05-01

151

Cenozoic marine geochemistry of thallium deduced from isotopic studies of ferromanganese crusts and pelagic sediments  

Science.gov (United States)

Cenozoic records of Tl isotope compositions recorded by ferromanganese (Fe-Mn) crusts have been obtained. Such records are of interest because recent growth surfaces of Fe-Mn crusts display a nearly constant Tl isotope fractionation relative to seawater. The time-series data are complemented by results for bulk samples and leachates of various marine sediments. Oxic pelagic sediments and anoxic marine deposits can be distinguished by their Tl isotope compositions. Both pelagic clays and biogenic oozes are typically characterized by ??205Tl greater than +2.5, whereas anoxic sediments have ??205Tl of less than -1.5 (??205Tl is the deviation of the 205Tl/203Tl isotope ratio of a sample from NIST SRM 997 Tl in parts per 104). Leaching experiments indicate that the high ??205Tl values of oxic sediments probably reflect authigenic Fe-Mn oxyhydroxides. Time-resolved Tl isotope compositions were obtained from six Fe-Mn crusts from the Atlantic, Indian, and Pacific oceans and a number of observations indicate that these records were not biased by diagenetic alteration. Over the last 25 Myr, the data do not show isotopic variations that significantly exceed the range of Tl isotope compositions observed for surface layers of Fe-Mn crusts distributed globally (??205 Tl=+12.8??1.2). This indicates that variations in deep-ocean temperature were not recorded by Tl isotopes. The results most likely reflect a constant Tl isotope composition for seawater. The growth layers of three Fe-Mn crusts that are older than 25 Ma show a systematic increase of ??205Tl with decreasing age, from about +6 at 60-50 Ma to about +12 at 25 Ma. These trends are thought to be due to variations in the Tl isotope composition of seawater, which requires that the oceans of the early Cenozoic either had smaller output fluxes or received larger input fluxes of Tl with low ??205Tl. Larger inputs of isotopically light Tl may have been supplied by benthic fluxes from reducing sediments, rivers, and/or volcanic emanations. Alternatively, the Tl isotope trends may reflect the increasing importance of Tl fluxes to altered ocean crust through time. ?? 2004 Elsevier B.V. All rights reserved.

Rehkamper, M.; Frank, M.; Hein, J.R.; Halliday, A.

2004-01-01

152

A geochemical traverse across the North Chilean Andes: Evidence for crust generation from the mantle wedge  

International Nuclear Information System (INIS)

Major and trace element and Sr- and Nd-isotope analyses are presented on 186-0 Ma magmatic rocks along an east-west traverse across North Chile at 220S. ?Sr ranges from -25 to +100 and ?Nd from +6 to -9, but the low ?Nd and high ?Sr values are in rocks generated in the last 15 Ma. It is argued that previous discussions of petrogenesis in North Chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one, but two processes. One results in a progressive shift of ?Sr from -25 to +20 and ?Nd from +6 to -6 in Jurassic to Recent rocks, which is accompanied by increasing Ta/Sm and Sr decreasing Th/Ta. The second is largely confined to the younger rocks and it is characterised by ?Sr increasing up to +100 with increasing SiO2 and decreasing Sr, and it results in relatively shallow trends on an ?Nd-?Sr diagram. The preferred interpretation is that trend 1 is due to the mobilisation of old, late Proterozoic mantle lithosphere as magmatism migrated eastwards, and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust. It follows that the mantle wedge is the principal site of crust generation, and it is argued that <20% of the Sr in the recent northern Chile rocks is derived from the subducted ocean crust. (orig.)

153

Precambrian U-Pb zircon ages in eclogites and garnet pyroxenites from South Brittany (France): An old oceanic crust in the West European Hercynian belt  

International Nuclear Information System (INIS)

U-Pb zircon ages have been determined for tow eclogites from the Vendee and for two garnet pyroxenites from the Baie d'Audierne. In an episodic Pb loss model, the two discordia could give upper intercept ages around 1300-1250 Ma and lower intercepts ages of 436-384 Ma. (orig.)

154

Timescales of Crustal Accretion at a Medium to Fast Spreading Ridge: High Precision U- Pb Zircon Dating of the Intrusive Crust of the Cretaceous Oman Ophiolite  

Science.gov (United States)

The Oman ophiolite is one of the largest subaerial exposures of oceanic lithosphere on Earth. We present new high-precision single grain U-Pb zircon dates from both gabbro and plagiogranite plutons in the ophiolite, which provide new insight into the temporal evolution of oceanic crust. In the Ibra area, at the southern end of the ophiolite, a large and laterally coherent block of oceanic lithosphere exposes a cross section through mantle harzburgites, layered gabbros, massive gabbros, sheeted dikes and rare basaltic flows and pillows. We have dated five upper level massive gabbros and a single plagiogranite from a ~25 km transect perpendicular to the inferred ridge axis. The 206Pb/238U dates range from ~96.2-95.7 Ma and generally confirm that the crust formed at a medium to fast spreading ridge. However, the spatial distribution and spread of dates do not follow the pattern predicted by current models of simple spreading, and suggest that either i) there was significant off-axis magmatism, ii) older plutons crystallized in the mantle and were incorporated into the crust at the ridge axis, or iii) the crust was imbricated by post magmatic strike- slip faulting. Two new dates from the Samail Massif provide insight into the origin of oceanic plagiogranites. A plagioclase- hornblende vein that crosscuts layered gabbros yielded a 206Pb/238U date of ~95.8 Ma, providing a minimum age for the layered gabbros. In contrast, an adjacent plagiogranite pluton generated a range of 206Pb/238U dates of ~95.4-95.2 Ma, indicating that the plagiogranite intrusion post- dates gabbro crystallization by a minimum of ~400,000-600,000 years and likely reflects protracted off- axis modification of the crust.

Rioux, M.; Bowring, S. A.; Kelemen, P. B.

2008-12-01

155

Underplating generated A- and I-type granitoids of the East Junggar from the lower and the upper oceanic crust with mixing of mafic magma: Insights from integrated zircon U-Pb ages, petrography, geochemistry and Nd-Sr-Hf isotopes  

Science.gov (United States)

Whole rock major and trace element, Nd-Sr and zircon Hf isotopic compositions and secondary-ion mass spectrometry zircon U-Pb ages of eleven granitoid intrusions and dioritic rocks from the East Junggar (NW China) were analyzed in this study. The East Junggar granitoids were emplaced during terminal Early to Late Carboniferous (325-301 Ma) following volcanic eruption of the Batamayi Formation. Zircons from the East Junggar granitoids yielded 210 concordant 206Pb/238U ages which are all younger than 334 Ma and exhibit ?Hf(t) values distinctly higher than Devonian arc volcanic-rocks. Seismic P-wave velocities of deep crust of the East Junggar proper resemble those of oceanic crust (OC). These characteristics suggest absence of volcanic rock and volcano-sedimentary rock of Devonian and Early Carboniferous from the source region. The East Junggar granitoids show ?Nd(t) and initial 87Sr/86Sr values substantially overlapping those of the Armantai ophiolite in the area. The Early Paleozoic OC with seamount-like composition as the Zhaheba-Armantai ophiolites remained in the lower crust and formed main source rock of the East Junggar granitoids. Based on petrography and geochemistry, the East Junggar granitoids are classified into peralkaline A-type in the northern subarea, I-type (I1 and I2 subgroups) mainly in the north and A-type in the south of the southern subarea. The perthitic or argillated core and oligoclasic rim with an argillated boundary of feldspar phenocrysts and inclusion of perthites or its overgrowth by matrix plagioclase, in the monzogranites (northern subarea), suggest mixing of peralkaline granitic magma with mafic magma. In the north of the southern subarea, the presence of magmatic microdioritic enclaves (MMEs) in the I1 subgroup granitoids, transfer of plagioclase phenocrysts and hornblendes between host granodiorite and the MME across the boundary and a prominent resorption surface in the plagioclase phenocrysts indicate mixing of crustal magma (I2 subgroup granitoids) with mafic magma. Magma mixing shifted (87Sr/86Sr)i of the I1 subgroup granitoids towards the mantle array. Two generations of hornblende with zonal distribution and similar mineral and geochemical compositions of quartz monzodiorite and hosted MME with unfractionated rare earth elements (REE) suggest extended magma mixing with onset probably at or near source region. These observations imply concurrency of mantle input and the crustal melting and, hence, a causal relationship between underplating/intraplating and the lower OC/upper OC melting. The I-type granitoids experienced plagioclase and hornblende fractionations, whereas fractionated phases of the two groups of A-type granites were alkali feldspar and albite-oligoclase with significant involvement of F--rich fluid. Granodioritic parent magmas of the I2 subgroup granitoids stemmed from the hydrous upper OC. Parent magmas of the two A-type groups possess syenogranitic or quartz syenitic compositions. The peralkaline A-type granites stemmed from the lower OC, whereas the A-type granites from dehydrated upper OC left behind after extensive partial melting and extraction of I-type granitoids. Based on comparison in the ternary system Mg2SiO4-CaAl2SiO6-SiO2, most of the Batamayi volcanic rocks with affinity to ocean-island basalts were derived from asthenospheric upwelling. The gabbro-dioritic rocks with higher light to heavy REE ratios stemmed from metasomatized lithospheric mantle. Both of the above mafic rocks contain subducted slab component.

Liu, Wei; Liu, Xiu-Jin; Liu, Li-Juan

2013-10-01

156

Growth of the continental crust: a planetary-mantle perspective  

International Nuclear Information System (INIS)

The lack of earth rocks older than about 3.8 Ga is frequently interpreted as evidence that the earth formed little or no subduction-resistant continental crust during the first 700 My of its history. Such models obviously imply that the pre-3.8 Ga earth was covered entirely or almost entirely by smoothly subducting oceanic crust. On the other hand, the thermal regime of the early earth probably tended to cause the oceanic crust at this time to be comparatively thin and comparatively mafic. The present earth is covered by about 50 percent oceanic crust, averaging about 7 km in thickness, and 41 percent continental crust, averaging roughly 40 km in thickness. Thus continentless-early-earth models would seem to imply a total mass of crust less than 1/3 that of the present day earth. Possible explanations are examined

157

Crust rheology, slab detachment and topography  

Science.gov (United States)

The collision between continents following the closure of an ocean can lead to the subduction of continental crust. The introduction of buoyant crust within subduction zones triggers the development of extensional stresses in slabs which eventually result in their detachment. The dynamic consequences of slab detachment affects the development of topography, the exhumation of high-pressure rocks and the geodynamic evolution of collision zones. We employ two-dimensional thermo-mechanical modelling in order to study the importance of crustal rheology on the evolution of spontaneous subduction-collision systems and the occurrence of slab detachment. The modelling results indicate that varying the rheological structure of the crust can results in a broad range of collisional evolutions involving slab detachment, delamination (associated to slab rollback), or the combination of both mechanisms. By enhancing mechanical coupling at the Moho, a strong crust leads to the deep subduction of the crust (180 km). These collisions are subjected to slab detachment and subsequent coherent exhumation of the crust accommodated by eduction (inversion of subduction sense) and thrusting. In these conditions, slab detachment promotes the development of a high (> 4.5 km) and narrow (detachment to occur. Further shortening leads to buckling and thickening of the crust resulting in the development of topographic bulging on the lower plate. Collisions involving rheologically layered crust are characterised by a decoupling level at mid-crustal depths. These initial condition favours the delamination of the upper crust as well as the deep subduction of the lower crust. These collisions are thus successively affected by delamination and slab detachment and both processes contribute to the exhumation of the subducted crust. A wide (> 200 km) topographic plateau develops as the results of the buoyant extrusion of the upper crust onto the foreland, this mechanism is further amplified by slab detachment. Our results suggest that the occurrence of both delamination (Apennines) and slab detachment (Himalayas) in orogens may highlight significant differences in their initial rheological structure.

Duretz, T.; Gerya, T. V.

2012-04-01

158

Remnants of the Rheic SSZ Oceanic Lithosphere (320 Ma) Within the Izmir-Ankara-Erzincan Suture Zone in NE Turkey: New Geochemical and Re-Os Isotope Data From the Rehafiye-Erzincan Ophiolite  

Science.gov (United States)

We report on new major-trace-REE and Re-Os isotope compositions and mineral chemistry data from upper mantle peridotites and ultramafic-mafic cumulate rocks in the Rehafiye-Erzincan ophiolite (REO) in NE Turkey, and discuss their siginificance for the tectonic evolution of various oceanic tracts in the eastern Mediterranean region. The REO is part of the Izmir-Ankara-Erzincan Suture Zone (IAESZ) between the Gondwana-derived Tauride-South Armenian ribbon continent to the south and the Rhodope-Pontide micro-continent to the north. It shows bidivergent thrusting along its southern and northern boundaries, resting tectonically on the margins of these continental masses. The IAESZ includes fragments of oceanic lithosphere with WPB, MORB, IAT-Boninite, OIB and LIP affinities that range in age from the Permo-Triassic to the latest Crecateous, although it is commonly interpreted as Neotethyan in origin. The REO consists of upper mantle peridotites including harzburgite with dunite bands/lenses and crosscutting dolerite dikes, ultramafic-mafic cumulate rocks making up a transitional Moho, isotropic gabbro, plagiogranites, and sheeted dikes. Extrusive rocks are missing in the ophiolite sequence but occur as blocks of pillow basalts in an ophiolitic mélange structurally beneath the REO. We have identified two types of upper mantle peridotites, abyssal and SSZ, in the REO. Less depleted, clinopyroxene-rich mantle harzburgites have higher concentrations of Al (1.75-2.12 wt.% Al2O3) and Ca (0.43-1.53 wt.% CaO) and contain spinel phases with Cr# ranging between 33.2 and 37.8. These abyssal peridotites represent a mantle residue of low degrees of partial melting of primitive upper mantle during MOR-type oceanic crust formation. Some peridotite samples, on the other hand, are highly depleted in clinopyroxene and display extremely low contents of Al (0.16-0.89 wt.% Al2O3) and Ca (0.07-0.77 wt.% CaO), characteristic of SSZ peridotites. Spinel phases in these samples have Cr# ranging between 57.0 and 73.5, indicating high degrees of partial melting (in a mantle wedge) of previously depleted peridotites. Both types of peridotites are characterized by low REE contents. Abyssal-type peridotites display a flat pattern from Lu to Tb and negative-flat pattern from Tb to La, although some samples show slight LREE enrichment. SSZ-type, depleted peridotite samples are characterized by nearly similar LREE concentrations, whereas their HREE and especially MREE concentrations are significantly depleted with respect to those of the abyssal peridotites. These rocks show U-shaped REE patterns, reminiscent of forearc peridotites. The PGE content of mantle tectonites in the REO, although low, are significantly higher than those presented by the cumulate gabbros. The 187Os/188Os isotope ratio of these peridotites (0.1195-0.1240) is typical of the depleted mantle and much lower than those of the cumulate gabbros (0.2074-0.5842). Whole-rock samples from the mantle tectonites and cumulate gabbros display a well-defined linear trend in a 187Re/188Os vs. 187Os/188Os isochron diagram. These samples collectively define an isochron with a best-fit age of about 320±16 Ma, suggesting that the melt evolution and crystallization of the SSZ ultramafic-mafic units of the REO may be as old as the late Carboniferous. This new finding of a late Carboniferous SSZ mantle lithosphere between the Gondwana and Eurasia continental masses in NE Turkey indicates: (1) the existence of Rheic Ocean relics farther away in the Tethyan realm of the eastern Mediterranean region than previously considered; (2) a highly heterogeneous character of the IAESZ, containing some lithospheric material that evolved in rift-drift, plume and subduction-accretion tectonic settings of the Rheic, Paleotethyan and Neotethyan Ocean basins. We discuss the geodynamic implications of these inferences.

Uysal, Ibrahim; Dilek, Yildirim; Sarifakioglu, Ender; Meisel, Thomas

2010-05-01

159

Has 7% of Continental Crust been Lost since Pangea Broke Up?  

Science.gov (United States)

After modern plate tectonics began, the net growth or loss of continental crust predominantly involved the mass balance at subduction zones (SZs) between the yin of adding mantle-sourced arc igneous rocks and the subtracting yang of recycling existing crust back into the mantle. Field observations suggest that during Mesozoic and Cenozoic time, a rough long-term balance existed at ocean-margin SZs (e.g., W. N. America, Andes). But a different picture, one of net loss, emerges when additions and losses at collisional or crust-suturing SZs (e.g., India-Tibet) are considered. GAINS AND LOSSES SINCE ~200 Ma Because Mesozoic and Cenozoic convergent margins can be field inspected, the net growth of continental crust after the breakup of Pangea at ~200 Ma can be estimated. Pangea breakup also marked the beginning of the present supercontinent cycle. Newly established (Eocene) ocean-margin SZs (e.g., IBM, Tonga-Kermadec) added juvenile arc crust for at least 10-15 Myr at rates ~10-15 times higher than later and elsewhere at long-established SZs (~30 km3/Myr/km). During the Cenozoic, at colliding SZs (e.g., Alps, India-Tibet, Arabia-Eurasia) tomographic and geological data document losses of subducted continental crust sustained for 15-50 Myr at rates ~15 times that typical from the upper plate at ocean-margin SZs (~70 km3/Myr/km). For additions, we considered that as the Atlantic opened in early Jurassic time, new, prodigiously productive SZs were initiated along the western margin of North and Middle America but not along western South America and the eastern margin of Eurasia. In the Cretaceous, new SZs formed along much of the northern margin of the Tethys, along western Sumatra and southern Java, and at the great arc of the Caribbean. In the early Eocene, in the offshore, a lengthy (~20,000 km) curtain of new, voluminously productive intra-oceanic SZs formed from the Aleutian Islands southward to the Kermadec Islands. For subtractions, we applied subduction losses (~70 km3/Myr/km) previously estimated for Cenozoic ocean-margin SZs. During the past 50 Myr, at Tethyan SZs from Gibraltar to western Australia, we estimate an average rate of lower plate continental crust recycling at ~900 km3/Myr/km. NEGATIVE CRUSTAL GROWTH AND IMPLICATIONS After the breakup of Pangea, the volume of continental crust recycled into the mantle is estimated to be ~500 x 10^6 km3 greater than that created (~400 x 10^6 km3). The net loss is ~7% of Earth's continental crust existing volume (7 x 10^9 km3), an estimate that does not include delamination losses. A large part of SZ losses (~40%) occurred during the past 50 Myr as a consequence of closing the Tethys, a process still underway. It seems likely that a long supercontinent cycle of breakup and dispersal of fragments favors positive crustal growth effected by elevated rates of magmatism at new SZs, whereas a shorter cycle involving rapid reassembly favors negative crustal growth effected by elevated rates of losses at crustal collision zones. Fragmentation of Pangea and the partial reassembly of its parts within 150 Myrs may be representative of a short, crustal destructive supercontinent cycle.

Scholl, D. W.; Stern, R. J.

2012-12-01

160

Combined147,146Sm-143,142Nd constraints on the longevity and residence time of early terrestrial crust  

Science.gov (United States)

silicate differentiation controlled the composition of Earth's oldest crust. Inherited 142Nd anomalies in Archean rocks are vestiges of the mantle-crust differentiation before ca. 4300 Ma. Here we report new whole-rock 147,146Sm-143,142Nd data for the Acasta Gneiss Complex (AGC; Northwest Territories, Canada). Our 147Sm-143Nd data combined with literature data define an age of 3371 ± 141 Ma (2 SD) and yield an initial ?143Nd of -5.6 ± 2.1. These results are at odds with the Acasta zircon U-Pb record, which comprises emplacement ages of 3920-3960 Ma. Ten of our thirteen samples show 142Nd deficits of -9.6 ± 4.8 ppm (2 SD) relative to the modern Earth. The discrepancy between 142Nd anomalies and a mid-Archean 147Sm-143Nd age can be reconciled with Nd isotope reequilibration of the AGC during metamorphic perturbations at ca. 3400 Ma. A model age of ca. 4310 Ma is derived for the early enrichment of the Acasta source. Two compositional end-members can be identified: a felsic component with 142Nd/144Nd identical to the modern Earth and a mafic component with 142Nd/144Nd as low as -14.1 ppm. The ca. 4310 Ma AGC source is ˜200 Myr younger than those estimated for Nuvvuagittuq (northern Québec) and Isua (Itsaq Gneiss Complex, West Greenland). The AGC does not have the same decoupled Nd-Hf isotope systematics as these other two terranes, which have been attributed to the crystallization of an early magma ocean. The Acasta signature rather is ascribed to the formation of Hadean crust that was preserved for several hundred Myr. Its longevity can be linked to 142Nd evolution in the mantle and does not require slow mantle stirring times nor modification of its convective mode.

Roth, Antoine S. G.; Bourdon, Bernard; Mojzsis, Stephen J.; Rudge, John F.; Guitreau, Martin; Blichert-Toft, Janne

2014-06-01

 
 
 
 
161

Evidence for hydrothermal venting in Fe isotope compositions of the deep Pacific Ocean through time  

Science.gov (United States)

Temporal variations in Fe isotope compositions at three locations in the Pacific Ocean over the last 10 Ma are inferred from high-resolution analyses of three hydrogenetic ferromanganese crusts. Iron pathways to the central deep Pacific Ocean appear to have remained constant over the past 10 Ma, reflected by a remarkably constant Fe isotope composition, despite large changes in the Fe delivery rates to the surface ocean via dust. These results suggest that the Fe cycle in the deep ocean is decoupled from that in surface waters. By contrast, one ferromanganese crust from the Izu-Bonin (IB) back-arc/marginal basin of the W. Pacific exhibits large ? 56Fe variations. In that crust, decreases in ? 56Fe values correlate with increases in Mn, Mg, Ni, Cu, Zn, Mo, and V contents, and consistent with periods of intense hydrothermal input and increased growth rates. A second crust located within 100 km of the first IB sample does not record any of these periods of enhanced hydrothermal input. This probably reflects the isolated pathways by which hydrothermally sourced Fe may have migrated in the back arc, highlighting the high degree of provinciality that Fe isotopes may have in the modern (oxic) oceans. Our results demonstrate that despite efficient removal at the source, hydrothermal Fe injected into the deep ocean could account for a significant fraction of the dissolved Fe pool in the deep ocean, and that hydrothermally sourced Fe fluxes to the open ocean may have lower ? 56Fe values than those measured so far in situ at hydrothermal vents. Correlation between ? 56Fe values and elements enriched in hydrothermal fluids may provide a means for distinguishing hydrothermal Fe from other low-? 56Fe sources to the oceans such as dissolved riverine Fe or porewaters in continental shelf sediments.

Chu, N.-C.; Johnson, C. M.; Beard, B. L.; German, C. R.; Nesbitt, R. W.; Frank, M.; Bohn, Marcel; Kubik, P. W.; Usui, A.; Graham, I.

2006-05-01

162

The European margin of the Jurassic Tethys in Corsica: dating of Balagne trondhjemites and evidence to support a continental crust beneath the Balagne-Ligurian domain; La marge europeenne de la Tethys jurassique en Corse: datation de trondhjemites de Balagne et indices de croute continentale sous le domaine Balano-Ligure  

Energy Technology Data Exchange (ETDEWEB)

Vein trondhjemite in gabbro of the Carnispola Bridge has been dated to 169 {+-} 3 Ma (U-Pb on zircon). This date indicates that E-MORB-type ophiolites were emplaced in the marginal Balagne part of the Ligurian Jurassic basin about some 10 Ma before the emplacement of N-MORB ophiolites in the most central part of the ocean. In addition, the presence of inherited zircons with Ordovician (431 {+-} 8 Ma) and Archean (2693 {+-} 12 Ma) ages reveals that the Balagne ophiolites were emplaced on a thinned continental crust. Finally, the 298 {+-} 4 Ma age of zircons from eclogitised meta-arkoses in the eclogitic Morteda-Farinole unit ('Schistes lustres' zone) confirms the attribution of these rocks to a paleogeographic area that laid between continent and ocean, along the edge of the Hercynian granite batholith in Corsica. (authors)

Rossi, Ph.; Cocherie, A.; Lahondere, D. [Bureau de Recherches Geologiques et Minieres (BRGM), 75 - Paris (France); Mark Fanning, C. [Research School of Earth Sciences, ANU, Canberra (Australia)

2002-05-01

163

Investigating the link between an iron-60 anomaly in the deep ocean's crust and the origin of the Local Bubble  

Energy Technology Data Exchange (ETDEWEB)

Supernova explosions responsible for the creation of the Local Bubble (LB) and its associated HI cavity should have caused geological isotope anomalies via deposition of debris on Earth. The discovery of a highly significant increase of {sup 60}Fe (a radionuclide that is exclusively produced in explosive nucleosynthesis) in layers of a deep sea ferromanganese crust corresponding to a time of 2.2 Myr before present, appears very promising in this context. We report on our progress in relating these measurements to the formation of the LB by means of 3D hydrodynamical adaptive mesh refinement simulations of the turbulent interstellar medium in the solar neighborhood. Our calculations are based on a sophisticated selection procedure for the LB's progenitor stars and take advantage of passive scalars for following the chemical mixing process.

Schulreich, Michael; Breitschwerdt, Dieter [Zentrum fuer Astronomie und Astrophysik, TU Berlin, Berlin (Germany)

2013-07-01

164

Crustal magnetization and accretion at the Southwest Indian Ridge near the Atlantis II fracture zone, 0-25 Ma  

Science.gov (United States)

We analyze geophysical data that extend from 0 to 25-Myr-old seafloor on both flanks of the Southwest Indian Ridge (SWIR). Lineated marine magnetic anomalies are consistent and identifiable within the study area, even over seafloor lacking a basaltic upper crust. The full spreading rate of 14 km/Myr has remained nearly constant since at least 20 Ma, but crustal accretion has been highly asymmetric, with half rates of 8.5 and 5.5 km/Myr on the Antarctic and African flanks, respectively. This asymmetry may be unique to a ???400 km wide corridor between large-offset fracture zones of the SWIR. In contrast to the Mid-Atlantic Ridge, crustal magnetization amplitudes correlate directly with seafloor topography along the present-day rift valleys. This pattern appears to be primarily a function of along-axis variations in crustal thickness, rather than magnetic mineralogy. Off-axis, magnetization amplitudes at paleo-segment ends are more positive than at paleo-segment midpoints, suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle. Alteration of the magnetic source layer at paleo-segment midpoints reduces magnetization amplitudes by 70-80% within 20 Myr of accretion. Magnetic and Ocean Drilling Program (ODP) Hole 735B data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust. Thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization. As the crust ages, the lower crust becomes increasingly important in preserving marine magnetic stripes.

Hosford, A.; Tivey, M.; Matsumoto, T.; Dick, H.; Schouten, Hans; Kinoshita, H.

2003-01-01

165

Cooling Rates in the Lower Crust of the Oman Ophiolite: Ca in Olivine, Revisited  

Science.gov (United States)

Debate over the mechanism of accretion of the layered gabbros of the lower oceanic crust has been centered on the gabbro glacier and the sheeted sills models. In the gabbro glacier model it is proposed that the lower oceanic crust (below the sheeted dikes) crystallizes in a single shallow melt lens and is then transported ductilely downward. Conversely, in the sheeted sills model the lower oceanic crust is formed in several melt lenses and crystallizes in situ. The thermal profile of the crust, specifically the roles of hydrothermal circulation and cooling rate, is a key component in distinguishing between these two models. Results from Ca in olivine in this study show no systematic variation of cooling rate with depth in the lower crust of the Khafifah section in the Wadi Tayin massif of the Oman ophiolite. Additionally, very high cooling rates recorded near the base of the crust suggest that hydrothermal circulation plays an important role in the removal of heat throughout the crust. On the basis of these results it can be concluded that the sheeted sills model is a thermally viable mechanism for accretion of oceanic lower crust. Ca in olivine results for the oceanic lower crust are also compared with other magmatic systems such as layered mafic intrusions and arc lower crust to quantify relative cooling rates among these different systems. Perhaps surprisingly, large layered intrusions, such as the Bushveld Complex, are found to cool at approximately the same rate as oceanic lower crust.

Vantongeren, J. A.; Kelemen, P. B.; Hanghoj, K.

2007-12-01

166

Icelandic-type crust.  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Numerous seismic studies, in particular using receiver functions and explosion seismology, have provided a detailed picture of the structure and thickness of the crust beneath the Iceland transverse ridge. We review the results and propose a structural model that is consistent with all the observations. The upper crust is typically 7 ± 1 km thick, heterogeneous and has high velocity gradients. The lower crust is typically 15–30 ± 5 km thick and begins where the velocity gradient decreases...

Foulger, G. R.; Du, Z. J.; Julian, B. R.

2003-01-01

167

Hafnium isotope stratigraphy of ferromanganese crusts  

Science.gov (United States)

A Cenozoic record of hafnium isotopic compositions of central Pacific deep water has been obtained from two ferromanganese crusts. The crusts are separated by more than 3000 kilometers but display similar secular variations. Significant fluctuations in hafnium isotopic composition occurred in the Eocene and Oligocene, possibly related to direct advection from the Indian and Atlantic oceans. Hafnium isotopic compositions have remained approximately uniform for the past 20 million years, probably reflecting increased isolation of the central Pacific. The mechanisms responsible for the increase in 87Sr/86Sr in seawater through the Cenozoic apparently had no effect on central Pacific deep-water hafnium.

Lee, D. -C.; Halliday, A. N.; Hein, J. R.; Burton, K. W.; Christensen, J. N.; Gunther, D.

1999-01-01

168

Ocean Drilling Program  

Science.gov (United States)

This site describes the Ocean Drilling Program (ODP). The ODP conducts basic research into the history of the ocean basins and the overall nature of the crust beneath the ocean floor using the scientific drill ship JOIDES Resolution. There are also links to photographs, core data, and educational material on the site.

Program, Ocean D.; Texas A&M University

169

When did the subduction first initiate in the southern Paleo-Asian Ocean: New constraints from a Cambrian intra-oceanic arc system in West Junggar, NW China  

Science.gov (United States)

We report here a Cambrian southward-subducting intra-oceanic arc system in the southern West Junggar, NW China, where an immature arc occurred initially on SSZ-type ophiolites and finally evolved into a more mature one. The immature arc is dominantly represented by Early-Middle Cambrian (?510 Ma) low-K tholeiitic felsic rocks, whereas the mature arc is characterized by Late Cambrian (?495 Ma) medium- and high-K calc-alkaline felsic and mafic rocks. The SSZ-type ophiolites show remarkable depletion of Nb and Ta and contain high-Cr spinel (Cr# > 0.6), resembling these formed in the forearc. Altogether, they record the initiation of subduction and transformation of crust during early subduction of the Paleo-Asian Ocean in the southern part of the Central Asian Orogenic Belt (CAOB). The subduction initiation might occur in the Early Cambrian (>515 Ma), as constrained by both the SSZ-type ophiolites (516 Ma) and the oldest arc plutons (515-509 Ma) that crosscut the ophiolites. The immature felsic plutons have high SiO2 (>72 wt%) contents and variable MgO (0.42-1.49 wt%) and Mg# values (22-62). Crustal anatexis may be responsible for the genesis of those plutons and thus the transformation from oceanic to continental crust. These results, combined with regional data, convincingly indicate that it is one of the oldest intra-oceanic arc systems in the southern CAOB, which may mark the initial subduction of the Paleo-Asian Ocean in its southern part, much later than those reported in the north. An archipelago-type model is proposed for the evolution of the southern West Junggar and has implications for the development of the southern part of the CAOB.

Ren, Rong; Han, Bao-Fu; Xu, Zhao; Zhou, Yin-Zhang; Liu, Bo; Zhang, Lei; Chen, Jia-Fu; Su, Li; Li, Jiao; Li, Xian-Hua; Li, Qiu-Li

2014-02-01

170

Earth's partial pressure of CO2 over the past 120 Ma; evidence from Ce anomalies in the deep (greater than 600 m) Pacific Ocean, 1  

Science.gov (United States)

It was found that Ce serves as a chemical tracer of paleo-oceanic redox conditions. It was shown that the unoxidized and soluble Ce(3+) in modern seawater exhibits a negative anomaly relative to the other soluble REE(3+). An expression of soluble Ce(3+) in seawater that was approximately 1900X greater than the average observed in Ce in 600-5000 m Pacific seawater was derived. Since Ce(CO3)(+) and Ce(CO3)2(-) complexes greatly exceed the Ce(PO4) complexes in seawater, the formulations of using carbonate complexes were followed and it was found that the calculated Ce and observed concentrations in the deep 600-5000 m Pacific Ocean agree within the uncertainties of the thermodynamic data. As expected, the calculated Ce concentrations are a strong function of pH and found to be lesser functions of CO3(2-) activities.

Liu, Y.-G; Schmitt, R. A.

1993-01-01

171

A unified history of the ocean around southern Africa  

Science.gov (United States)

The movement with respect to Africa of the hotspot marked by present-day Bouvet island is extrapolated backward in time to a position in the Lower Limpopo Valley at the time of the Karoo-Ferrar basalt event (183 Ma). In a tight reconstruction of the Precambrian fragments of Gondwana at this time, the triangular gap that remains between South Africa's Precambrian, that of Dronning Maud Land, Antarctica, and the eastward-extrapolated front of the Cape Fold Belt we fill with the Precambrian fragments of South Patagonia and the Falkland Islands. We postulate that the 183 Ma mantle upwelling produced a triple junction-type fracture marked by the alignments of the Lebombo, the SE margin of the Zimbabwe craton and the giant Botswana dyke swarm (178 Ma) that was rather quickly followed by the expulsion of the South Patagonia terranes from the Gondwana assembly along the alignment of the Falklands-Agulhas Fault Zone (FAFZ) as a transform margin. The space created was filled with igneous material akin to the present day Afar triangle. The magma supply generated not only oceanic crust but also overlying igneous deposits, much probably erupted subaerially. These developed progressively into the Falklands Plateau, the Mozambique Plains, the Mozambique Rise and the Explora Wedge of Antarctica. Not until the early Cretaceous did the growth of normal ocean crust start to exceed the ability of the declining mantle plume to cover the new ocean crust in a confined space with subaerial deposits that substantially thickened otherwise ‘oceanic' crust. When Antarctica and Africa began to separate before about 167 Ma, the future Mozambique Rise moved with Antarctica until, at about 125 Ma, a modest ridge reorganization east of Africa left Madagascar and the Mozambique Rise as part of the Africa Plate. An increasing westerly component to the movement of Antarctica against Africa preceded the initial opening of the South Atlantic and the fusing of the South Patagonia terranes with the bulk of South America. The triple junction jumped from off Cape St Lucia to immediately south of the Mozambique Rise as part of this 125 Ma reorganization and normal ocean crust grew from each of the three ridges emanating from it. The Agulhas Bank represents a reactivation of the mantle plume at about 100 Ma and the Maurice Ewing Bank and other submarine features east of the Falklands Plateau later and smaller ones. The model conforms with limited magnetic anomaly evidence in the oceans and the direction of preserved transforms before Anomaly 34 time (84 Ma). After Anomaly 34, events around the triple junction are well-defined by both magnetic anomalies and preserved transforms. The model may be demonstrated by a geometrically correct animation and offers simple solutions to a number of geological enigmas concerning (a) the Falkland Islands, (b) supposedly ‘continental' plateaus off South Africa, (c) the sudden ending of the Karoo igneous episode well before substantive Gondwana disruption and (d) the exotic nature of the Precambrian rocks of South Patagonia in the context of South American geology. The central role of the Bouvet mantle plume suggests that it has produced a volume of magma comparable with - and a longevity 50 myrs in excess of - that demonstrated by the Kerguelen plume.

Reeves, Colin; Master, Sharad

2010-05-01

172

Late Triassic Batang Group arc volcanic rocks in the northeastern margin of Qiangtang terrane, northern Tibet: partial melting of juvenile crust and implications for Paleo-Tethys ocean subduction  

Science.gov (United States)

The Batang Group (BTG) volcanic rocks in the Zhiduo area, with NW-trending outcrops along the northeastern margin of the Qiangtang terrane (northern Tibet), are mainly composed of volcaniclastic rocks, dacite and rhyolite. Major and trace element, Sr and Nd isotope, zircon U-Pb and Hf isotope data are presented for the BTG dacites. Laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating constrains the timing of volcanic eruption as Late Triassic (221 ± 1 Ma). Major and trace element geochemistry shows that the BTG volcanic rocks are classified as calc-alkaline series. All samples are enriched in large-ion lithophile elements and light rare earth elements with negative-slightly positive Eu anomalies (Eu/Eu* = 0.47-1.15), and depleted in high field strength elements and heavy rare earth elements. In addition, these rocks possess less radiogenic Sr [(87Sr/86Sr) i = 0.7047-0.7078], much radiogenic Nd (?Nd(t) = -4.2 to -1.3) and Hf (?Hf(t) = 4.0-6.6) isotopes, suggesting that they probably originated from partial melting of a crustal source containing a mantle-derived juvenile component. The inferred magma was assimilated by crustal materials during ascending and experienced significant fractional crystallization. By combining previously published and the new data, we propose that the BTG volcanic rocks were genetically related to southwestward subduction of the Ganzi-Litang ocean (a branch of Paleo-Tethys) in the northeastern margin of the Qiangtang terrane. Given the coeval arc-affinity magmatic rocks in the region, we envisage that the Ganzi-Litang ocean may extend from the Zhongdian arc through the Yidun terrane to the Zhiduo area, probably even further northwest to the Tuotuohe area.

Zhao, Shao-Qing; Tan, Jun; Wei, Jun-Hao; Tian, Ning; Zhang, Dao-Han; Liang, Sheng-Nan; Chen, Jia-Jie

2014-09-01

173

Uranium mineralization in sedimentary and metamorphic masses during tectonic evolution of the Earth crust  

International Nuclear Information System (INIS)

Regularities in the location of uranium deposits of different genetic types are considered from the viewpoint of the concept of stepped formation of the Earth crust. Three main stages of the Earth crust evolution and 3 types of the Earth crust structure are singled out: oceanic, transition and continental. Spatial relations, relations in the Earth crust types and confinement to its elements of various uranium deposits are shown

174

The Heat Flow Anomaly across the Ocean Continent Transition: Application to the Gulf of California  

Science.gov (United States)

The effect of the relation between heat flow and age for the ocean crust and the increase in radioactivity on the continents tend to counteract each other creating a decrease in heat flow across the Ocean Continent Transition (OCT) at a young and an increase in heat flow across an older margin. Recent advances in the observational oceanic heat flow field have given us a much better understanding of hydrothermal circulation in the ocean crust. This and two recent papers, that show the expected drop or increase in heat flow at the OCT, encouraged us to take a collective approach and look at all published heat flow profiles across the OCT to investigate if there are any processes common to all or a group of the margins. We examined 16 actual and 2 constructed profiles across the ocean continent transition (OCT) finding 9 where the data justified the examination of the difference between the ocean heat flow and that over the stretched margin (?HF-OCT). For the two margins ultra mafic rock into the thinned continental crust. Currently, a major program of joint research between CICESE, UNAM (Mexico) and SIO (UCSD), UCSC, OSU (USA) is underway to measure the heat flow from shelf to shelf across the axis of three of three-major-deepwater basins (Guaymas, Farallon, Alarcon) of the > 10 Ma Gulf of California Rift System. As a pilot project, we propose to carry out a seismically controlled detailed heat flow measurements across the southeastern side of the Guaymas Basin and a finer resolution survey over a recently imaged "saucer shaped sill" above stretched continental crust. Both surveys will specifically investigate the effect of the intrusion of dykes and sills into the sediments within a recently forming ocean basin on both the continental and oceanic sides of the transition.

Sclater, J. G.; Hasterok, D. P.; Goutorbe, B.; Negrete-Aranda, R.; Contreras, J.; Ferrarri, L.

2012-12-01

175

Zircon xenocrysts from the Kambalda volcanics: Age constraints and direct evidence for older continental crust below the Kambalda-Norseman greenstones  

International Nuclear Information System (INIS)

The Hangingwall Basal at Kambalda, Western Australia, contains zircons that have been shown by ion microprobe analyses to have very high U and Th contents and a wide variety of crystallization ages. Nearly all of these zircons certainly are xenocrysts; a few might relate to intrusive veinlets. The age of the youngest, 2693+-50 Ma (2sigma), shows that the eruptive age of the basalt cannot exceed 2743 Ma. This confirms that the apparent Sm-Nd isochron giving 3200 Ma for Kambalda mafic and ultramafic rocks is a mixing-line between unrelated components enriched and depleted in light rare earth elements. Mixing probably occurred at depth by erosion of 3200-3500 Ma old felsic crust from the walls of HWB conduits. The zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern Yilgarn Block. The latter implies that the Kalgoorlie-Norsemann greenstone sequences were formed in a continental rather than an oceanic environment. (orig.)

176

Microbial alteration of 0-30-Ma seafloor and sub-seafloor basaltic glasses from the Australian Antarctic Discordance  

Science.gov (United States)

Scanning electron microscopic observations of alteration rims in basaltic glasses dredged from 0-2.5-Ma seafloor and drilled from 18-28-Ma ocean crust (Ocean Drilling Program (ODP) Leg 187) in the Australian Antarctic Discordance (AAD) document the presence of endolithic microbes in altered basalt glass. In very young AAD lavas, ˜10-?m-thick alteration rims are developed along intersecting fractures and cracks, and the altered glass contains numerous spherical, rod- and star-shaped, partially fossilised microbial cells, similar to those from the Arctic ridges [1,2]. In 2.5-Ma basalt glasses, altered rims are up to 250 ?m thick, and zeolite (phillipsite) is present within many fractures. Spherical cells occur in porous outer zones of alteration rims and on zeolite crystal surfaces within fractures, indicating that microbial activity persists in the region for at least 2.5 Ma. Mn-rich microbial cell-encrustations associated with zeolite suggest that Mn is used in an energy yielding metabolic process. Combined with recent results from the Arctic ridges the results from this study demonstrate that endolithic microbial growth is a persistent feature of mid-ocean spreading ridges. In glasses from ODP cores, ˜1-mm-thick alteration rims are developed along the widest fractures lined with Mn(Fe)-oxyhydroxides and/or clay and filled by zeolite and calcite. Most common, however, are zeolite-filled, more narrow fractures and cracks. Zeolite-filled fractures with only minor to no alteration indicate several episodes of fracturing followed by relatively fast sealing. There is no age progression in alteration thickness along fractures or other characteristics, suggesting that alteration is essentially completed between 2.5 and 18 Ma. A comparison of alteration in the 2.5-Ma glass with that in the ODP samples indicates that a significant proportion of the glass alteration in the drilled samples developed prior to burial, although one type of diffuse, highly irregular front that is only observed in the ODP samples most likely developed after burial. These diffuse alteration fronts are caused by dissolution and alteration of the glass into minute globules, 0.05-0.2 ?m in diameter, with no associated microbial morphologies. Fossilised, Mn-rich cells do occur within zeolite-filled fractures, possibly indicating that microbial activity continued in the fractures for as long as circulation continued. The apparent non-biological origin of diffuse, irregular alteration fronts in buried AAD glasses indicates that these textural features are not reliable as diagnostic criteria for the existence of a deep biosphere in the volcanic ocean crust.

Thorseth, I. H.; Pedersen, R. B.; Christie, D. M.

2003-10-01

177

Electrical signature of modern and ancient tectonic processes in the crust of the Atlas mountains of Morocco  

Science.gov (United States)

The Atlas Mountains in Morocco are considered as type examples of intracontinental mountain chains, with high topography that contrasts with moderate crustal shortening and thickening. Whereas recent geological studies and geodynamic modelling suggest the existence of dynamic topography to explain this apparent contradiction, there is a lack of modern geophysical data at the crustal scale to corroborate this hypothesis. To address this deficiency, magnetotelluric data were recently acquired that image the electrical resistivity distribution of the crust from the Middle Atlas to the Anti-Atlas, crossing the tabular Moulouya plain and the High Atlas. All tectonic units show different, distinct and unique electrical signatures throughout the crust reflecting the tectonic history of development of each one. In the upper crust, electrical resistivity values and geometries can be associated to sediment sequences in the Moulouya and Anti-Atlas and to crustal scale fault systems in the High Atlas developed likely during Cenozoic times. In the lower crust, the low resistivity anomaly found below the Moulouya plain, together with other geophysical (low velocity anomaly, lack of earthquakes and minimum Bouguer anomaly) and geochemical (Neogene-Quaternary intraplate alkaline volcanic fields) evidences, infer the existence of a small degree of partial melt at the base of the crust. Resistivity values suggest a partial melt fraction of the order of 2-8%. The low resistivity anomaly found below the Anti-Atlas may be associated with a relict subduction of Precambrian oceanic sediments, or to precipitated minerals during the release of fluids from the mantle during the accretion of the Anti-Atlas to the West African Supercontinent during the Panafrican orogeny (ca. 685 Ma).

Ledo, Juanjo; Jones, Alan G.; Siniscalchi, Agata; Campanyà, Joan; Kiyan, Duygu; Romano, Gerardo; Rouai, Mohamed; TopoMed MT Team

2011-04-01

178

U-Pb Dating Reveals Rapid Accretion of Gabbroic Crust at the Ultraslow-Spreading Southwest Indian Ridge  

Science.gov (United States)

Absolute dating of gabbroic rocks at mid-ocean ridges can provide important constraints on the processes of oceanic crustal accretion. We present 206Pb/238U ages of igneous zircon for nine samples from 53- 1430 mbsf in ODP Hole 735B, drilled through 1508 m of gabbroic crust at Atlantis Bank on the Southwest Indian Ridge (SWIR). The Pb/U zircon ages are the same, within error, showing no systematic variation of age with depth. Consequently, individual magmatic series and tectonic blocks recognized within Hole 735B all have the same Pb/U age of 11.97±0.06 Ma (93 analyses). This observation of constant age down hole suggests that >74% (at the 95% confidence level) of Hole 735B accreted in 12.5 km/Myr. This rate of motion is similar to the plate spreading rate at the Southwest Indian Ridge, and so could be accommodated by tectonic rotation and/or denudation of crust during detachment faulting at Atlantis Bank.

Baines, G.; Cheadle, M. J.; John, B. E.; Grimes, C. B.; Wooden, J. L.

2006-12-01

179

Paleo-elevation and subsidence of ˜145Ma Shatsky Rise inferred from CO2 and H2O in fresh volcanic glass  

Science.gov (United States)

Shatsky Rise, a large Mesozoic oceanic plateau in the northwest Pacific, consists of three massifs (Tamu, Ori, and Shirshov) that formed near a mid-ocean-ridge triple junction. Published depth estimates imply that Shatsky Rise has not subsided normally, like typical oceanic lithosphere. We estimated paleo-eruption depths of Shatsky Rise massifs on the basis of dissolved CO2 and H2O in volcanic glass and descriptions of cores recovered from five sites of Integrated Ocean Drilling Program Expedition 324. Initial maximum elevations of Shatsky Rise are estimated to be 2500-3500 m above the surrounding seafloor and the ensuing subsidence of Shatsky Rise is estimated to be 2600-3400 m. We did not observe the anomalously low subsidence that has been reported for both Shatsky Rise and the Ontong Java Plateau. Although we could not resolve whether Shatsky Rise originated from a hot mantle plume or non-plume fusible mantle, uplift and subsidence histories of Shatsky Rise for the both cases are constrained based on the subsidence trend from the center of Tamu Massif (˜2600 m) toward the flank of Ori Massif (˜3400 m). In the case of a hot mantle plume origin, Shatsky Rise may have formed on young (˜5 Ma) pre-existing oceanic crust with a total crustal thickness of ˜20 km. For this scenario, the center of Shatsky Rise is subsequently uplifted by later (prolonged) crustal growth, forming the observed ˜30 km thickness crust. For a non-plume origin, Shatsky Rise may have formed at the spreading ridge center as initially thick crust (˜30 km thickness), with later reduced subsidence caused by the emplacement of a buoyant mass-perhaps a refractory mantle residuum-beneath the center of Shatsky Rise.

Shimizu, Kenji; Shimizu, Nobumichi; Sano, Takashi; Matsubara, Noritaka; Sager, William

2013-12-01

180

Velocity Structure of the Rifted Crust in the Northwestern Ross Sea, From Seismic Refraction Data  

Science.gov (United States)

Extension in the West Antarctic Rift System produced the Transantarctic Mountains, deep sedimentary basins in the Ross Sea, and the Adare Trough spreading center (43 to 26 Ma). The Adare Basin and Northern Basin are located at the northwesternmost extent of this region of deformation, and are generally assumed to be oceanic and continental crust respectively. Their boundary therefore provides an ideal study area for linking the styles of extension in the two types of crust. We process seismic refraction data collected during research cruise NBP0701 to determine 2D crustal velocity models along four seismic lines at the margin of the Adare and Northern Basins. The 48 closely-spaced sonobuoy records included in this study provide continuous refraction data coverage; three of these lines have reversed sonobuoy records. Finite difference modeling of the individual sonobuoys provides accuracy in our interpreted layer velocities, confidence in tracing refracted arrivals back to their associated reflections in the sonobuoy records, and the ability to match these reflected arrivals with the multi- channel seismic reflection data. Preliminary results from the line trending perpendicular to the margin of the Adare and Northern Basins show no change in crustal velocity structure from one basin to the other, with nearly flat velocity contours along the entire line. An apparent velocity of 8000 m/s is observed along this line in the Northern Basin. A comparable layer velocity is not detected in the sonobuoy record shot in the reverse direction, so this velocity could be due to local basement topography. Alternatively, the high velocity may indicate mantle material, and an unusually thin crust at that location. We model structural layers and associated velocities below the sea floor in order to better understand the physical structure and deformational history of the crust in the northwestern Ross Sea. The velocity horizons determined from this data set provide model constraints for a new type of crust that may be formed during rifting, and should be one end-member resulting from the continental rifting process.

Selvans, M. M.; Stock, J. M.; Clayton, R. W.; Cande, S. C.; Davey, F. J.

2007-12-01

 
 
 
 
181

Late Mesozoic magmatic records in NE Russia and coeval events in Arctic ocean  

Science.gov (United States)

Magmatic records onshore of Arctic Siberia and Alaska may provide constrains on formation of Arctic ocean including Amerasia and Eurasia basins, helps to interpret geophysical data and better understand meaning of spars dredged samples. We used new geochemistry and geochronology (U-Pb and Ar-Ar) data collected in the last decade across North East Russia and partly Alaska to better understand timing and tectonic setting of main magmatic events. It is widely accepted that opening of the Canada basin could have occurred as early as the late Jurassic-earliest Cretaceous (Grantz et al., 1998). Some of the models suggest initial rifting in the early Jurassic and seafloor spreading initiated at 145-142 Ma (Seton et al., 2012). Much older Devonian oceanic crust fragments found in the continental part of Arctic Alaska (Angaucham) as well as in Chukotka (Billings-Provideniya suture zone). Recently we mapped spars outcrops of metaultramafic rocks located within Velitkenay granite-migmatite complex and within Koolen metamorphic dome and Senyavin uplift. Metamorphic growth zircon from ultramafic rocks have U-Pb ages between 360 to 400 Ma. The most important Late Jurassic - Cretaceous collision and subduction-related magmatic provinces in NE Russia related mostly to paleo-Pacific events but some coincide with tectonic episodes in Arctic: (1) 155-145 Ma granitoids of Main Kolyma batholith belt and coeval Uyandino-Yasachnaya volcanic arc (partly coeval with closure of the Anyui Ocean around c. 160-145 Ma); (2) 150-140 Ma volcanics of subduction related Uda-Murgal and Nutesyn margin continental arcs; (3) 130-135 Ma Northern belt granites and oldest granitic complexes in Eastern Chukotka (coeval with beginning of HALIP volcanism); (4) 118-124 Ma Tytylveem continental volcano-plutonic belt in Chukotka coeval with suggested cessation of spreading in the Canada basin; (5) 100-109 Ma extension-related granite-metamorphic core complexes along Arctic cost of Chukotka (coeval with within-plate alkaline basalts of De-Longy archipelago) (6) 106-78 Ma subduction-related Okhotsk-Chukotka volcanic belt (final stage of volcanism in Chukotka at 88-90 Ma coeval with Iceland plume and opening of the Labrador Sea and Baffin Bay between 90 and 55 Ma, which might have affected the Central Arctic region). Small volume alkali basalts eruption happened during 54 to to 37 Ma in the Chersky seismic belt triggered by extension and thinning of the lithosphere combined with adiabatic upwelling of the underlying mantle (coeval with opening of the Eurasia basin at 55-33 Ma).

Akinin, V. V.

2013-12-01

182

The Oligocene-Miocene Pacific-Australia plate boundary, south of New Zealand: Evolution from oceanic spreading to strike-slip faulting  

Science.gov (United States)

Since the Eocene, the Pacific—Australia plate boundary south of New Zealand has evolved from a spreading system into a transform boundary. Swath data acquired in the Southeast Tasman oceanic crust, between the Macquarie Ridge complex and the Resolution Ridge system, show that the spreading fabric changes orientation southwards along the Puysegur Trench, striking successively N60°E, N85°E and N120°E. This reflects the reorganisation of the plate boundary in response to changes in relative plate motion. A comparison of these orientations with the positions of the Pacific-Australia relative poles of rotation enables us to estimate the age of STOC, where there are no identified magnetic anomalies. The youngest age of the oceanic crust is ca. 12 Ma at the south end of the Puysegur Trench. This age is consistent with spreading rates and the amount of crust generated since 31 Ma. Curved fracture zones on either side of the Macquarie Ridge complex suggest a continuous reorientation of transform faults, between 31 Ma and ca. 15 Ma. Small-scale seafloor morphology shows a 13° change of orientation in the L'Atalante Fracture Zone, that indicates incremental, rather than continuous, changes in azimuth of the transform faults. Patterns of fanning ridges indicate that periods of asymmetric spreading accompanied the spreading segment reorientations. Using swath data and plate reconstruction models we infer that between 31 and 12 Ma the plate boundary reorganisation resulted in a continuous increase in the ratio of the cumulative length of transform faults over the cumulative length of spreading segments, along the whole plate boundary. This indicates that, since 14-15 Ma, the plate boundary has become progressively predominantly transcurrent, allowing strike-slip motion to develop along a line of merging transform faults that connected to the intracontinental Alpine Fault.

Lamarche, Geoffroy; Collot, Jean-Yves; Wood, Ray A.; Sosson, Marc; Sutherland, Rupert; Delteil, Jean

1997-04-01

183

Dating Detrital Zircons from Paleozoic Strata on the Northern Margin of the Tarim Craton: Constraints on the Timing of Closure of Paleo-Asian Ocean  

Science.gov (United States)

The Central Asian Orogenic Belt (CAOB) is underwent a long-lived and complicated accretion/collision during Neoproterozoic to Paleozoic times. As the western part of the suture zone of the Paleo-Asian Ocean (PAO) whose closure to have formed the CAOB, the Chinese South Tianshan (STS) belt is considered to have resulted from the closure of the western sector of the PAO, called the STS ocean, between the Tarim craton and the Yili-Central Tianshan block. However, there is a hot debate on the timing of the final closure of the PAO and its subduction polarity. Sedimentary basins at the northwestern margin of the Tarim craton preserve successive Paleozoic deposits, and enable us to determine the timing of the final closure of the STS ocean by tracing the provenance of clastic sediments and especially U-Pb ages of detrital zircons from the Paleozoic strata. Detrital zircons from Late Silurian sandstone gave the youngest age of 417×7 Ma, defining its maximum depositional age. The dating results are dominated by four major populations: 417-470 Ma, 750-1007 Ma, 1740-2100 Ma and 2400-2700 Ma. The later three populations are correlated well with major tectono-magmatic events in the Tarim craton, suggesting that they resulted from the reworking of the old crust of the craton. The age spectrum is consistent with previous dating results of sedimentary rocks in the Central Tianshan (CTS) block, implying the block was part of the Tarim craton prior to ~417 Ma. Five sandstones collected from the Late Devonian, the Early Carboniferous and the middle Carboniferous yielded the youngest zircon U-Pb ages ranging from 413×6 Ma to 442×7 Ma, which are interpreted as their maximum depositional ages. The age spectra of these samples are remarkably similar and show four major age populations at 415-485Ma, 750-1020Ma, 1750-2100Ma and 2400-2680Ma, coinciding with those of the Late Silurian sample. This suggests a long-term recycling of pre-Devonian sediments without addition of juvenile materials, implying a passive continental margin setting. Intense magmatism at 380-340 Ma in the CTS and Yili blocks was not recorded in the Devonian-middle Carboniferous sedimentary rocks at the northwestern margin of the Tarim craton, suggesting that the CTS block was separated from the Tarim craton by the STS Ocean during Devonian to middle Carboniferous times. Zircon grains from two Middle Permian sandstones yielded the youngest age of 264×5 Ma. The two samples preserve detrital zircons with major age populations at 270-330Ma, 410-490 Ma, 720-1100 Ma, 1730-2120 Ma and 2400-2590 Ma. The last four populations are consistent with those of the Silurian-Carboniferous samples, resulting from the recycling of old crust. The population of 270-330 Ma could be related to the Permian mantle plume-related large igneous province in the Tarim craton and the collision-related magmatism and ultra-high pressure metamorphism in the CTS and Yili blocks. Therefore, our data suggest that the final closure of the STS Ocean and the collision between the Tarim craton and CTS-Yili block may have happened in the Late Paleozoic. Acknowledgements: NSFC (41190070, 41190075)

Han, Y.; Zhao, G.

2013-12-01

184

Spreading behaviour of the Pacific-Farallon ridge system since 83 Ma  

Science.gov (United States)

We present improved rotations, complete with uncertainties, for the Pacific-Farallon Ridge (PFR) between geomagnetic chrons 34y (83 Ma) and 10y (28.28 Ma). Despite substantial shortening since ˜55 Ma, this ridge system and its remnants (e.g. the East Pacific Rise) have produced as much as 45 per cent of all oceanic lithosphere created since the Late Cretaceous, but reconstructions face the twin challenges of extensive subduction of Farallon crust-which precludes reconstruction by fitting conjugate magnetic anomaly and fracture zone (FZ) traces-and asymmetric spreading behaviour for at least the past 51 Myr. We have calculated best-fit `half'-angle stage rotations between nine geomagnetic chron boundaries (34y, 33y, 29o, 24.3o, 20o, 18.2o, 17.1y, 13y and 10y) using combined anomaly and FZ data from both the northern and southern Pacific Plate. For rotations younger than chron 24.3o, estimates for spreading asymmetry, derived using anomaly picks from yet-to-be subducted Farallon/Nazca crust in the south Pacific, allow full stage rotations to be calculated. Between 50 and 83 Ma, where no direct constraints on spreading asymmetry are possible, a `best-fit' full stage rotation was calculated based on the net Nazca:Pacific spreading asymmetry (Pacific spreading fraction fPAC = 0.44) over the past 50 Myr, with conservative lower and upper bounds, based on variability in the degree of spreading asymmetry over periods of tightly at 60-75°E, 60-68°N, consistent with the relatively constant trend of the major Pacific FZs. This stability spans at least one episode of Farallon Plate fragmentation caused by subduction of PFR segments beneath the Americas, at 55-48 Ma, which appears to have greatly accelerated divergence on the surviving ridge without significantly affecting the location of the instantaneous rotation pole. Together with quasi-periodic 15-20 Myr variations in the degree of spreading asymmetry that also appear to correlate with changes in spreading rate, this indicates that forces other than slab pull may be a factor in determining Pacific-Farallon Plate motions.

Rowan, Christopher J.; Rowley, David B.

2014-06-01

185

Update on CRUST1.0 - A 1-degree Global Model of Earth's Crust  

Science.gov (United States)

Our new 1-by-1 degree global crustal model, CRUST1.0, was introduced last year and serves as starting model in a comprehensive effort to compile a global model of Earth's crust and lithosphere, LITHO1.0 (Pasyanos et al., 2012). The Moho depth in CRUST1.0 is based on 1-degree averages of a recently updated database of crustal thickness data from active source seismic studies as well as from receiver function studies. In areas where such constraints are still missing, for example in Antarctica, crustal thicknesses are estimated using gravity constraints. The compilation of the new crustal model initially followed the philosophy of the widely used crustal model CRUST2.0 (Bassin et al., 2000; http://igppweb.ucsd.edu/~gabi/crust2.html) to assign elastic properties in the crystalline crust according to basement age or tectonic setting (loosely following an updated map by Artemieva and Mooney (2001; http://www.lithosphere.info). For cells with no local seismic or gravity constraints, statistical averages of crustal properties, including crustal thickness, were extrapolated. However, in places with constraints the depth to basement and mantle are given explicitly and no longer assigned by crustal type. This allows for much smaller errors in both. In each 1-degree cell, boundary depth, compressional and shear velocity as well as density is given for 8 layers: water, ice, 3 sediment layers and upper, middle and lower crystalline crust. Topography, bathymetry and ice cover are taken from ETOPO1. The sediment cover is based on our sediment model (Laske and Masters, 1997; http://igppweb.ucsd.edu/~sediment.html), with some near-coastal updates. In an initial step toward LITHO1.0, the model is then validated against new global surface wave disperison maps and adjusted in areas of extreme misfit. This poster presents the next validation step: compare the new Moho depths with in-situ active source and receiver function results. We also present comparisons with CRUST2.0. CRUST1.0 is available for download. References: Pasyanos, M.E., Masters, G., Laske, G. and Ma, Z., LITHO1.0 - An Updated Crust and Lithospheric Model of the Earth Developed Using Multiple Data Constraints, Abstract T11D-09 presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec, 2012. Artemieva, I.M. and Mooney, W.D., Thermal thickness and evolution of Precambrian lithosphere: A global study, J. Geophys. Res., 106, 16,387-16,414, 2001. Bassin, C., Laske, G. and Masters, G., The Current Limits of Resolution for Surface Wave Tomography in North America, EOS Trans AGU, 81, F897, 2000. Laske, G. and Masters, G., A Global Digital Map of Sediment Thickness, EOS Trans. AGU, 78, F483, 1997. URL: http://igppweb.ucsd.edu/~gabi/crust1.html

Laske, Gabi; Masters, Guy; Ma, Zhitu; Pasyanos, Mike

2013-04-01

186

Continent- Ocean Transition Across the Alarcon Basin, Gulf of California from Seismic Reflection and Refraction Data.  

Science.gov (United States)

A transect of seismic reflection and onshore/offshore refraction data was collected across the Alarcon Basin, Gulf of California in Fall 2002 as part of the MARGINS Rupturing Continental Lithosphere (RCL) initiative. The dataset consists of ~600 km of seismic reflection data together with data from 53 ocean-bottom seismometers and 11 land seismometers along a coincident ~900 km refraction line. This transect crosses the entire conjugate rift system from continent to the thinned and faulted crust of the transition zone, and then to oceanic crust with an active seafloor spreading center. The Alarcon Rise is the southernmost spreading segment in the Gulf of California, separated from the East Pacific Rise by the Tamayo transform fault. Extension in the gulf began about 12 Ma and rifting was initiated at the mouth of the Gulf at the now inactive Magdalena spreading ridge about 5 Ma. Magnetic anomalies reveal that the Alarcon Rise has been spreading at an intermediate rate since 3.6 Ma. The current ridge crest is about 10 km wide, 150 m high and with a small axial valley. The data quality from the ocean-bottom seismometers is excellent, with first arrivals to at least 75 km offset, and past 100 km on many instruments. Land seismometers also produced excellent results- first arrivals are typically observed out to 200 km offset. Pg/Pn crossover distances are around 40 km in the oceanic crust of the Alarcon basin, increase to about 60-75 km in the transition zone and reach a maximum of about 100km for the continental land instruments. The total width of oceanic crust created at the Alarcon Rise, as determined from reflection profiles and initial refraction processing is about 130 km, which agrees with the bathymetric data. The transition zone is characterized by normal faulting- synrift faulting created sedimentary basins, which were later modified by additional normal faulting. The rifted margin appears to be symmetric, with about 180 km of transition zone on either side. However the southern margin is complicated by the fossil Magdalena spreading ridge, which lies about 150 km southeast of the Alarcon rise. We will present MCS results and an initial velocity model across the Alarcon basin.

Sutherland, F. H.; Harding, A. J.; Kent, G. M.; Lizarralde, D.; Holbrook, W. S.; González-Ferná; ndez, A.; Fletcher, J. M.; Umhoefer, P. J.; Axen, G. J.

2003-12-01

187

Subduction of thick crust: the Alaska example (Invited)  

Science.gov (United States)

It is a paradigm of plate tectonics that oceanic lithosphere subducts readily, while lithosphere transporting much thicker continental crust does not. Analyses of plate buoyancy have included a variety of effects, such as eclogitization, crustal compositional stratification, and plate strength, but all lead to the conclusion that crust needs to be thinner than about 15-25 km in order to subduct. A test of this conclusion is underway in southern Alaska, where the Yakutat terrane is being driven by the Pacific plate into the Alaskan margin. Its crust is 15-30 km thick, varying along strike, with a seismic velocity structure resembling an oceanic plateau; thus it spans the predicted limit in thickness of subductable crust. In the eastern thicker part, the terrane appears to be colliding and driving orogenesis in the St. Elias-Chugach ranges, although voluminous volcanism of the Wrangell Volcanic Field may be a consequence of some crust subducting. Farther west, the Yakutat terrane is 15-20 km thick and clearly subducting beneath the Prince William Sound and Kenai Peninsula. It forms the slab subducting beneath the central Alaska Range 400 km inland. The thick crust has been imaged at all depths less than 130 km, through receiver functions, travel-time tomography, and offshore by active-source imaging, with similar structure in most images. Greater than 130 km depth the imaged crust vanishes in seismic images, consistent with predicted depths of eclogitization of weakly hydrated metagabbroic crust, and the lack of a velocity contrast between eclogite and peridotite. Lithosphere including the thick Yakutat crust gives a net buoyancy close to neutral, so its subduction will depend on other factors. The high buoyancy may be responsible for the remarkably shallow dip of the plate at depths less than 50 km, producing one of the widest seismogenic thrust zones on the planet, allowing it to host the great (Mw 9.3) 1964 Gulf of Alaska earthquake. The shallow dip may also aid in driving deformation in the Alaska Range, several hundred km inland. Analogous mountain-building may occur at other sites where oceanic plateaus or rises subduct, for example in southeast Costa Rica, or where leading edges of continents subduct, such as Timor or northern New Guinea.

Abers, G. A.; Kim, Y.; Christensen, D. H.

2013-12-01

188

Radial spreading of viscous-gravity currents with solidifying crust  

Science.gov (United States)

In the present investigation of solidifying-crust effects on the dynamics and surface morphology of radial viscous-gravity currents, polyethylene glycol inflows into the base of a tank holding a cold sucrose solution are used as analogs. As the radial current advanced away from the inlet, its surface solidified and deformed through a combination of folding anf fracturing. When cooling was sufficiently rapid, solid crust formed and caused the spreading rate to increase; progressively colder experiments revealed a sequence of surface morphologies resembling features of cooling lava flows and lava lakes, including multiarmed rift structures with shear offsets and bulbous lobate forms resembling pillow lavas on the ocean floor.

Fink, Jonathan H.; Griffiths, Ross W.

1990-01-01

189

Physics of Neutron Star Crusts  

Directory of Open Access Journals (Sweden)

Full Text Available The physics of neutron star crusts is vast, involving many different research fields, from nuclear and condensed matter physics to general relativity. This review summarizes the progress, which has been achieved over the last few years, in modeling neutron star crusts, both at the microscopic and macroscopic levels. The confrontation of these theoretical models with observations is also briefly discussed.

Chamel Nicolas

2008-12-01

190

Physics of Neutron Star Crusts  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The physics of neutron star crusts is vast, involving many different research fields, from nuclear and condensed matter physics to general relativity. This review summarizes the progress, which has been achieved over the last few years, in modeling neutron star crusts, both at the microscopic and macroscopic levels. The confrontation of these theoretical models with observations is also briefly discussed.

Chamel Nicolas; Haensel Pawel

2008-01-01

191

High-precision TIMS U-Pb dating and SHRIMP trace element analyses of zircons from plutonic crust from ODP Hole 735B, Atlantis Bank, Southwest Indian Ridge  

Science.gov (United States)

Ocean Drilling Program Hole 735B at Atlantis Bank on the Southwest Indian Ridge is the deepest drill hole (1508m) into plutonic oceanic crust. The recovered core provides the opportunity to study both the processes and timescales of lower crustal accretion at a slow-spreading mid-ocean ridge. Major element chemistry suggests that the crust is made up of three 200-1000m thick igneous series (Natland and Dick, 2002). Previous SHRIMP U/Pb dating of zircons from oxide gabbro and felsic/dioritic dikes/veins from the length of the core found no resolvable age differences, suggesting rapid crustal growth; SHRIMP Th-corrected weighted mean 206Pb/238U dates ranged from 11.86 ± 0.20 to 12.13 ± 0.21 Ma (Baines et al., 2009). Here we report combined SHRIMP chemical analyses and high precision TIMS U-Pb geochronology on zircons from a suite of fifteen samples from depths of 26-1430 mbsf. The samples are from each of the three main intrusive series and range from oxide gabbro to diorite and granodiorite dikes/veins. Single grain TIMS 206Pb/238U date uncertainties for most analyses range from ~0.01-0.2 Ma and weighted mean 206Pb/238U date uncertainties range from ~0.004-0.07 Ma, providing precise constraints on the timing and duration of magmatism. Zircon chemistry is variable between samples (John et al., this meeting). Ti and Hf from spot analyses within individual samples range from tight clusters of data to linear trends of decreasing Ti with increasing Hf. Apparent Ti-in-zircon temperature variations within samples range from ~60-230°C, and variations within single grains are as large as 160°C. For zircons with significant chemical zoning, the cores are typically higher in Ti and lower in Hf than the rims. Th-corrected single grain 206Pb/238U dates from individual samples typically overlap within uncertainty, consistent with crystallization of a single batch of magma with no evidence for assimilation of older crust or protracted crystallization, as has been seen in high precision dates from the Mid-Atlantic Ridge and East Pacific Rise (Lissenberg et al., 2009; Rioux et al., 2012). However, two diorite dikes each contain populations of younger zircons with dates of ~11.9 Ma and a single older zircon with a date of ~12.4 Ma, suggesting that these magmas entrained zircons from older but so far unrecognized wall rocks. Resolvable age differences between the most precisely dated rocks suggest that the upper-two magmatic series (0-540 mbsf) under went final crystallization before the lowest series (540-1508 mbsf). Two precisely dated samples from the top two magmatic series have weighted mean 206Pb/238U dates of 12.00 ± 0.02 Ma and 11.96 ± 0.02 Ma. Six precisely dated samples from a range of depths in the deepest magmatic series all have younger weighted mean 206Pb/238U dates of 11.94 ± 0.02 to 11.91 ± 0.01 Ma. The current data do not show resolvable correlations between Th-corrected 206Pb/238U dates and zircon chemistry within individual samples.

Rioux, M. E.; Cheadle, M. J.; John, B. E.; Bowring, S. A.; Wooden, J. L.; Baines, G.

2012-12-01

192

Copper-nickel-rich, amalgamated ferromanganese crust-nodule deposits from Shatsky Rise, NW Pacific  

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A unique set of ferromanganese crusts and nodules collected from Shatsky Rise (SR), NW Pacific, were analyzed for mineralogical and chemical compositions, and dated using Be isotopes and cobalt chronometry. The composition of these midlatitude, deep-water deposits is markedly different from northwest-equatorial Pacific (PCZ) crusts, where most studies have been conducted. Crusts and nodules on SR formed in close proximity and some nodule deposits were cemented and overgrown by crusts, forming amalgamated deposits. The deep-water SR crusts are high in Cu, Li, and Th and low in Co, Te, and Tl concentrations compared to PCZ crusts. Thorium concentrations (ppm) are especially striking with a high of 152 (mean 56), compared to PCZ crusts (mean 11). The deep-water SR crusts show a diagenetic chemical signal, but not a diagenetic mineralogy, which together constrain the redox conditions to early oxic diagenesis. Diagenetic input to crusts is rare, but unequivocal in these deep-water crusts. Copper, Ni, and Li are strongly enriched in SR deep-water deposits, but only in layers older than about 3.4 Ma. Diagenetic reactions in the sediment and dissolution of biogenic calcite in the water column are the likely sources of these metals. The highest concentrations of Li are in crust layers that formed near the calcite compensation depth. The onset of Ni, Cu, and Li enrichment in the middle Miocene and cessation at about 3.4 Ma were accompanied by changes in the deep-water environment, especially composition and flow rates of water masses, and location of the carbonate compensation depth.

Hein, J. R.; Conrad, T. A.; Frank, M.; Christl, M.; Sager, W. W.

2012-01-01

193

Numerical modeling of subduction, accretion, and collision of island arc crust onto continental crust  

Science.gov (United States)

Arc-continent collision occurs when a subducting continent collides into the island arc on the overriding plate or when the island arc is on the subducting plate with an overriding continent. In addition to arc-continent collision, the latter case can lead to full subduction of the arc or accretion parts of arc crust. Accreted terranes composed of arc crustal material are present in the geological record as imbricated slices of crustal rocks or crustal blocks intermingled with accretionary prism rocks. We use numerical experiments to test the importance of different mechanical parameters of the arc crust that allow the arc to subduct, accrete, or collide with an overriding continent. Our experiments specifically focus on the effects of lithospheric buoyancy and detachment layers in the arc crust on subduction geodynamics. We examine the geodynamics of island arc-continent interaction using thermo-mechanical experiments that incorporate a free surface and viscous-plastic rheology. In our numerical experiments, subduction is allowed to freely evolve by including a low viscosity subduction channel. We vary buoyancy and detachment layers for an island arc located on the subducting oceanic plate with an overriding continental plate. The rheological structure of the island arc is represented with upper, middle, and lower crust layers illustrative of the seismic crustal structure of modern arcs. Lithospheric buoyancy is varied by changing the island arc crustal thickness and density. We impose various detachment layers in the arc crust to examine the effect on subduction, accretion, or collision. A basal detachment layer represents the ultramafic crust-mantle transition layer that is posited to founder due to high densities or serve as a weak delamination layer during collision. Detachment layers at other depths represent layers weakened by backarc rifting. Our numerical experiments show that the influence of detachment layers outweighs buoyancy when testing the ability of an island arc to accrete. The depth of the primary detachment layer controls the thickness of arc crust that is underplated to the overriding continent. Numerical experiments without detachment layers show that the island arc lithospheric buoyancy will lead to docking and collision if the downward force of the subducting slab pull is less than the ridge push.

Tetreault, J. L.; Buiter, S.

2011-12-01

194

Crustal thickening prior to 220 Ma in the East Kunlun Orogenic Belt: Insights from the Late Triassic granitoids in the Xiao-Nuomuhong pluton  

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The East Kunlun Orogenic Belt (EKOB) played an important role in plate tectonics, magma generation, and crustal evolution. Late Triassic granodiorites and their mafic micro-granular enclaves (MMEs) from Xiao-Nuomuhong in the EKOB were studied for geochemistry and geochronology to constrain their petrogenesis. Zircon LA-ICP-MS dating indicates that the Xiao-Nuomuhong granodiorites are coeval with their MMEs (?222 Ma). The granodiorites are high-K calc-alkaline rocks that are enriched in Rb, Th, U and LREE, and depleted in Cr, Ni and HFSE, with high Sr/Y ratios (82.2-85.3) and geochemically resemble the lower crust-derived adakites. The MMEs are also high-K calc-alkaline rocks, with high Al2O3 (16.8-18.8 wt.%), low Mg# (30-40), Nb, Zr and Hf, with weak negative Eu anomalies (Eu/Eu# = 0.8-0.9). We suggest the MMEs are mafic magmatic globules that were injected into the felsic host magma. The adakitic rocks from the Xiao-Nuomuhong pluton were generated by partial melting of thickened crust, while the primitive compositions of the MMEs were most likely from the lithospheric mantle beneath the EKOB. The Late Triassic Xiao-Nuomuhong pluton is important evidence that crustal thickening in the EKOB occurred prior to 220 Ma. The pluton is interpreted as the result of mixing between thickened lower crust-derived melts and lithospheric mantle-derived mafic melts and the protracted magmatic response to the break-off of the Paleo-Tethys oceanic slab at ?232 Ma.

Xia, Rui; Wang, Changming; Deng, Jun; Carranza, Emmanuel John M.; Li, Wenliang; Qing, Min

2014-10-01

195

Evaluating Complex Magma Mixing via Polytopic Vector Analysis (PVA in the Papagayo Tuff, Northern Costa Rica: Processes that Form Continental Crust  

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Full Text Available Over the last forty years, research has revealed the importance of magma mixing as a trigger for volcanic eruptions, as well as its role in creating the diversity of magma compositions in arcs. Sensitive isotopic and microchemical techniques can reveal subtle evidence of magma mixing in igneous rocks, but more robust statistical techniques for bulk chemical data can help evaluate complex mixing relationships. Polytopic vector analysis (PVA is a multivariate technique that can be used to evaluate suites of samples that are produced by mixing of two or more magma batches. The Papagayo Tuff of the Miocene-Pleistocene Bagaces Formation in northern Costa Rica is associated with a segment of the Central American Volcanic Arc. While this segment of the arc is located on oceanic plateau, recent (<8 Ma ignimbrites bear the chemical signatures of upper continental crust, marking the transition from oceanic to continental crust. The Papagayo Tuff contains banded pumice fragments consistent with one or more episodes of mixing/mingling to produce a single volcanic deposit. The PVA solution for the sample set is consistent with observations from bulk chemistry, microchemistry and petrographic data from the rocks. However, without PVA, the unequivocal identification of the three end-member solution would not have been possible.

Guillermo E. Alvarado

2013-08-01

196

Structure and Origin of the Crozet Plateau and Conrad Rise, SW Indian Ocean: Insights from Crustal Thickness Mapping Using 3-D Satellite Gravity Inversion  

Science.gov (United States)

We determine Moho depth and crustal thickness for the SW Indian Ocean, using a gravity inversion method which incorporates a lithosphere thermal gravity anomaly correction, in order to investigate the structure and origin of the Crozet Plateau and the Conrad Rise. Data used in the gravity inversion are bathymetry, free-air gravity anomaly and sediment thickness from Smith and Sandwell (2008), Sandwell and Smith (2008) and Laske and Masters (1997) respectively. The gravity inversion method, which is carried out in the 3D spectral domain and predicts Moho depth, incorporates a lithosphere thermal gravity anomaly correction. Lithosphere thermal model re-equilibration (cooling) times, used to calculate the lithosphere thermal gravity anomaly correction, are conditioned by ocean isochron information (Mueller et al. 2008), and continental rifting and breakup ages. The continental lithosphere thinning distribution, used to define the initial thermal model temperature perturbation are derived from the gravity inversion and use no a priori isochron information; as a consequence the gravity inversion method provides a prediction of OCT location which is independent of ocean isochron information. Crustal thickness under the Crozet Plateau, Conrad Rise and Madagascar Ridge are predicted to exceed 25 km. The Crozet Plateau and Conrad Rise are separated by an abandoned sea-floor spreading centre active between 83 and 55 Ma. The Crozet Plateau and the Madagascar Ridge are separated by the currently active South West Indian ocean Ridge (SWIR) which formed at ~50 Ma. Superposition of illuminated satellite gravity data onto crustal thickness maps from gravity inversion provides improved determination of pre-breakup conjugacy and sea-floor spreading trajectory within the SW Indian Ocean. Plate reconstructions of present day crustal thickness shows that at ~ 83 Ma, the Crozet Plateau, Conrad Rise and Madagascar Ridge formed a single entity which was subsequently fragmented and separated by rifting and sea-floor spreading. Comparison of Moho depths determined from gravity inversion and seismic refraction suggests that crustal basement densities for Crozet Plateau, Conrad Rise and Madagascar Ridge may be significantly greater than those for adjacent unequivocal oceanic crust within the Indian Ocean. Average crustal basement densities under Crozet Plateau, Conrad Rise and Madagascar Ridge may be as large as 3000 kg/m3 or more, and suggest that they are underlain by anomalously thick oceanic crust rather than continental fragments, a conclusion consistent with their seismic velocity structure. We propose that the Crozet Plateau, Conrad Rise and Madagascar Ridge, together with the southern portion of the Mascarene and Chagos Plateaus, were part of a single large oceanic plateau prior to 83 Ma, formed as an oceanic large igneous province during the early evolution of the Indian Ocean.

Cross, A.; Kusznir, N. J.; Alvey, A.

2011-12-01

197

Role of crustal stretching on subsidence of the continental crust  

Science.gov (United States)

The principal areas of subsidence of the continental crust are wide intracontinental basins and passive continental margins. In many intracontinental basins crustal extension amounts to only a few percent and is unable to explain their subsidence, as shown for the Timan-Pechora, Pre-Caspian and West Texas basins. The deep Black Sea and South Caspian basins are underlain by highly attenuated continental crust, having velocities in the 7 km/s range, that shows no evidence for significant extension. In passive margins, upper crustal extension ranges between 10 and 40%, whereas the lower crust is attenuated by 2-3 times (e.g., Campos Basin, north Biscay margin). Large discrepancy between upper crustal stretching and lower crustal attenuation is seen in the crustal configuration of many intracontinental grabens (e.g., Baikal and Dniepr-Prypyat basins). Only in highly extended areas, such as the Basin and Range Province and the East China rift system, does crustal stretching account for the observed crustal thinning. The observed discrepancy between upper extension and crustal thinning across intracontinental rifts and passive margins, and particularly in some wide intracontinental basins suggests that crustal stretching alone cannot account for the subsidence of these basins. Phase transformations in the lower crust, probably facilitated by the presence of hydrous fluids, can cause rapid (~ 1 Ma) as well as long-term (200-1500 Ma) upward displacement of the Moho discontinuity, resulting in high amplitude subsidence of the crust. In areas of crustal extension this mechanism can overprint subsidence induced by thermal relaxation of the lithosphere. The case of the Black Sea and South Caspian basins shows that density increase of the lithosphere can also occur independently of crustal stretching.

Artyushkov, E. V.

1992-12-01

198

Crust Formation in Aluminum Cells  

Science.gov (United States)

This paper examines the catalytic effects offlourides on the ???-Al2O3 phase transformation by heat treating commercial alumina samples with 2wt% additions of different flouride compounds. The various additives were ranked according to their effect on transformation temperature. Experiments were conducted to explain the high temperature coherence of crusts. The findings indicate that an alumina network is formed during ??? phase transformation, which reinforces the crust on top of the cryolite bath.

Oedegard, R.; Roenning, S.; Rolseth, S.; Thonstad, J.

1985-11-01

199

Issues of oxygen excess in the crust and upper mantle lithosphere  

Science.gov (United States)

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 ?FMQ -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 believe in correlation of biosphere evolution with cyclic mantle and crustal magma activation (Balashov, Glaznev, 2006) reflecting variation of atmospheric volatiles. This corresponds to abrupt sulphur excess due to volcanogenic activation at the peak of the evolution fatally affected the biosphere state. However, volcanogenic epochs are relatively short-term not to contradict the synthesis of oxygen by the biosphere between them. This should ultimately result in significant oxygen heterogeneity in various rock types. Existence of a wide range of Ce+4/Ce+3 in all the surface systems of the Earth, and upper sequence of the mantle lithosphere is related to constant existence of exactly this heterogeneity. Alongside, various types of geological processes in the crust and mantle should have influence, or even define variation stages in the evolution of the biosphere itself. And, this has already been noted. Another constant oxygen source along the whole interval of the Earth's history should be considered solar wind. The continuous flow of the whole range of elements, which portion in the discharge of H, C, O, and other elements to the atmosphere in a proportion close to the composition of 1 (Anders, Grevesse, 1989), may be regarded as a quite competitive option with other sources of oxygen at the Earth's surface.

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

2012-04-01

200

Evolution of the earth's crust: Evidence from comparative planetology  

Science.gov (United States)

Geochemical data and orbital photography from Apollo, Mariner, and Venera missions were combined with terrestrial geologic evidence to study the problem of why the earth has two contrasting types of crust (oceanic and continental). The following outline of terrestrial crustal evolution is proposed. A global crust of intermediate to acidic composition, high in aluminum, was formed by igneous processes early in the earth's history; portions survive in some shield areas as granitic and anorthositic gneisses. This crust was fractured by major impacts and tectonic processes, followed by basaltic eruptions analogous to the lunar maria and the smooth plains of the north hemisphere of Mars. Seafloor spreading and subduction ensued, during which portions of the early continental crust and sediments derived therefrom were thrust under the remaining continental crust. The process is exemplified today in regions such as the Andes/Peru-Chile trench system. Underplating may have been roughly concentric, and the higher radioactive element content of the underplated sialic material could thus eventually cause concentric zones of regional metamorphism and magmatism.

Lowman, P. D., Jr.

1973-01-01

 
 
 
 
201

Durability of neutron star crust  

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How long do neutron star mountains last? The durability of elastically deformed crust is important for neutron star physics including pulsar glitches, emission of gravitational waves from static mountains, and flares from star quakes. The durability is defined by the strength properties of the Yukawa crystals of ions, which make up the crust. In this paper we extend our previous results [Mon. Not. R. Astron. Soc. 407, L54 (2010)] and accurately describe the dependence of the durability on crust composition (which can be reduced to the dependence on the screening length {lambda} of the Yukawa potential). We perform several molecular dynamics simulations of crust breaking and describe their results with a phenomenological model based on the kinetic theory of strength. We provide an analytical expression for the durability of neutron star crust matter for different densities, temperatures, stresses, and compositions. This expression can also be applied to estimate durability of Yukawa crystals in other systems, such as dusty plasmas in the laboratory (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Chugunov, A.I. [Ioffe Institute, Saint-Petersburg (Russian Federation); Horowitz, C.J. [Nuclear Theory Center and Dept. of Physics, Indiana University, Bloomington (United States)

2012-02-15

202

Ferromanganese crusts as archives of deep water Cd isotope compositions  

Science.gov (United States)

The geochemistry of Cd in seawater has attracted significant attention owing to the nutrient-like properties of this element. Recent culturing studies have demonstrated that Cd is a biologically important trace metal that plays a role in the sequestration of inorganic carbon. This conclusion is supported by recent isotope data for Cd dissolved in seawater and incorporated in cultured phytoplankton. These results show that plankton features isotopically light Cd while Cd-depleted surface waters typically exhibit complimentary heavy Cd isotope compositions. Seawater samples from below 900 m depth display a uniform and intermediate isotope composition of ?114/110Cd = +3.3 ± 0.5. This study investigates whether ferromanganese (Fe-Mn) crusts are robust archives of deep water Cd isotope compositions. To this end, Cd isotope data were obtained for the recent growth surfaces of 15 Fe-Mn crusts from the Atlantic, Pacific, Indian, and Southern oceans and two USGS Fe-Mn reference nodules using double spike multiple collector inductively coupled plasma mass spectrometry. The Fe-Mn crusts yield a mean ?114/110Cd of +3.2 ± 0.4 (2 SE, n = 14). Data for all but one of the samples are identical, within the analytical uncertainty of ±1.1?114/110Cd (2 SD), to the mean deep water Cd isotope value. This indicates that Fe-Mn crusts record seawater Cd isotope compositions without significant isotope fractionation. A single sample from the Southern Ocean exhibits a light Cd isotope composition of ?114/110Cd = 0.2 ± 1.1. The origin of this signature is unclear, but it may reflect variations in deep water Cd isotope compositions related to differences in surface water Cd utilization or long-term changes in seawater ?114/110Cd. The results suggest that time series analyses of Fe-Mn crusts may be utilized to study changes in marine Cd utilization.

Horner, T. J.; SchöNbäChler, M.; RehkäMper, M.; Nielsen, S. G.; Williams, H.; Halliday, A. N.; Xue, Z.; Hein, J. R.

2010-04-01

203

UV - BOSTON MA  

Science.gov (United States)

Brewer 103 is located in Boston MA, measuring ultraviolet solar radiation. Irradiance and column ozone are derived from this data. Ultraviolet solar radiation is measured with a Brewer Mark IV, single-monochrometer, spectrophotometer manufactured by SCI-TEC Instruments, Inc. of S...

204

Nature of crust in the central Red Sea  

Science.gov (United States)

A transition between continental crust in the northern Red Sea and oceanic crust in the southern Red Sea coincides broadly with a southward increase in plate tectonic separation rate and with a decrease in upper mantle seismic velocity. We re-evaluate here the nature of crust in the intervening central Red Sea with the results of legacy seismic refraction experiments and recently released marine gravity anomalies derived from satellite altimeter measurements. In the refraction data, collected east of Thetis Deep, velocities of 6.6-6.9 km s- 1 of a deep refracting layer, which are similar to measured velocities of unaltered gabbro samples, extend outside the deep to 65 km from the axis. The new version of the marine gravity field reveals trends crossing the central Red Sea. Whereas some of them connect with major lineaments in the surrounding African-Arabian shield, those around Thetis Deep die out towards the coastlines. They can be paired across the ridge and lie slightly oblique to plate motions, as is typical of oceanic fracture zones or non-transform discontinuities migrating away from hotspots. Taken together these observations support the view that an oceanic rather than extended continental crust underlies this part of the central Red Sea. The crestal mountains around the median valleys of slow-spreading ridges are typically 500-1000 m lower at spreading discontinuities. Around Thetis Deep, the similar pattern in the gravity field to those of slow-spreading ridges suggests that the crestal mountains may variably block or impede flowage of evaporites towards the spreading centre, whereas the discontinuities may mark areas where flowage is unobstructed. Limited multibeam data collected in transits outside Thetis Deep show oblique fabrics as expected from these predicted movements.

Mitchell, Neil C.; Park, Yongcheol

2014-07-01

205

Tectonic History of the Amerasia Basin, Arctic Ocean  

Science.gov (United States)

Seismic reflection, refraction and potential field data from Amerasia Basin in conjunction with piston cores from Northwind Ridge suggest that the basin was formed by four rotational extensions. The first event stretched and thinned Pangea on a westerly-dipping crustal scale detachment fault system of Sinemurian to no later than Early Hauterivian age that day-lighted on the east along the continental margin of Northwestern Canada. This event rotated Eastern Siberia about 50° anticlockwise from Northwest Canada about a pole in the lower Mackenzie Valley and created transitional crust, which lacks seafloor spreading magnetic anomalies, beneath the marginal areas of the present Amerasia Basin. The second event, 9° or 10° of anticlockwise rotational seafloor spreading, split the earlier-formed transitional crust and emplaced MORB (mid-ocean-ridge basalt) along a northerly trending symmetry axis in the center of the Amerasia Basin. The resultant fan of magnetic anomalies, estimated to be of Late Hauterivian to Late Barremian age (136-125 Ma), is geometrically symmetrical with the first spreading event and likewise converges toward a pole of rotation in the lower Mackenzie Valley. Approximately 45° of clockwise rotation of Chukchi Microplate out of the East Siberian shelf about a pole near 72.5° N, 170° W constitutes the third rotational event, which probably occurred during or shortly following the Late Barremian. This event thrust the northeastern corner of the Chukchi Microplate across the boundary between event 1 and event 2 crusts in the western Canada Basin and created North Chukchi Basin in its wake. North Chukchi Basin is partially filled with post-Barremian to Early Campanian oceanic basalts of the Alpha-Mendeleev Large Igneous Province (125-80 Ma). In the absence of well-defined aeromagnetic anomalies or crustal-scale reflection data we can only speculate, on the basis of morphology, that North Chukchi Basin is a product of localized rotational seafloor spreading The fourth event, mildly rotational Paleocene extension, created basin and range structural morphology and the northerly-trending Northwind Basin in the axial region of the Chukchi Microplate. This extension thinned the continental crust of the microplate beneath the Northwind Basin by about 35 percent and created accommodation space for >2,000 m of water and >4,500 m of clastic sediment within the basin. The pole of this rotation was apparently located on the central Chukchi shelf. Following the four extensional events the southeastern margin of the Amerasia Basin was subject to far-field convergence of Middle Eocene to Quaternary age that appears to have originated at the Pacific Rim and created large thrust-related detachment folds that may be significant for hydrocarbon exploration.

Grantz, A.; Hart, P. E.; Childers, V. A.

2007-12-01

206

Polymetamorphic complexes in the eastern parts of the Balkan Peninsula: 600 Ma of geodynamic evolution  

Science.gov (United States)

Polymetamorphic amphibolite-facies complexes are exposed in the eastern and central parts of the Balkan Peninsula in different Alpine tectonic zones and under different Cadomian to Alpine collisional and exhumation histories and regimes. All complexes consist mostly of biotite and two-mica gneisses and schists, and amphibolites. Strong Cadomian overprint led to intimate mixing (tectonometamorphic amalgamation) of crustal and mantle (and/or oceanic crust)-derived (serpentinized ultramafics, eclogites) products. The pre-Cadomian complex in Central Sredna-gora Mountains evolved through Cadomian collision with c. 617 Ma granites, Hercynian 340 to 250 Ma granitoids, Late Permian exhumation, and Triassic-Jurassic sedimentation followed by Mid-Cretaceous exhumation. P-T conditions never reached amphibolite facies in post-Cadomian times except for some shear zones. The pre-Cadomian amphibolite-facies complex in Sakar Mt. was intruded by c. 500 Ma old granites, deeply eroded in late Permian time, and covered with depositional contact by Triassic terrestrial and marine sediments. Both basement and Triassic cover suffered folding and amphibolite-facies metamorphism (c. 150 Ma BP) followed by exhumation. Included in the Srednogorie Late Cretaceous volcanic arc as crystalline cores, these complexes have been affected by latest Cretaceous exhumation. Amphibolite-facies polymetamorphic cores (Ograzhdenian complex) within the Serbo-Macedonian massif and other units in SW Bulgaria and the adjacent countries were subjected to intense Cadomian (560 - 520 Ma BP) synmetamorphic collision and granite activity. Some units suffered Cadomian collision under greenschist-facies with a Neoproterozoic to Cambrian diabase-phyllitoid complex or have been exhumed and directly covered by Cambrian (followed by Cambrian limestones) or Tremadocian marine sandstones. After Palaeozoic exhumation, their structure was sealed by Permian, Triassic and Jurassic terrestrial and marine sediments, and after intense Mid-Cretaceous folding and thrusting, exhumed again in Palaeogene times. In the Rhodope region, the Mesoproterozoic? to Neoproterozoic supracrustal Rhodopian complex is built up of kyanite-, garnet- and staurolite-bearing biotite and two-mica gneisses and schists, amphibolites, marbles, calcareous schists, quartzo-feldspathic gneisses (derived of possible arkosic or rhyolitic protoliths), and orthoamphibolite, metaperidotite and eclogite rootless bodies. Migmatites and anatexites crop out in the cores of Rhodopian domes. Mid-Cretaceous thrusting in greenschist-facies conditions is documented at the peripheries of the Rhodope massif, and probably contributed to considerable (up to 70 km; now about 50 km) crustal thickening in the interior. The cores underwent very late (Early to Late Palaeogene) exhumation. Eclogitic rootless bodies that witness HP to UHP metamorphic events crop out in all complexes mentioned. Eclogite formation is referred to different mechanisms (burial through subduction of oceanic crust; amalgamation of mantle and crustal slivers in depth; metamorphism of deep norite to troctolite intrusions; local increase of pressure and temperature along minor shear zones) and times (Cadomian, Hercynian, Cimmerian and/or Alpine).

Zagorchev, I.

2007-12-01

207

Dating of the 85°E Ridge (northeastern Indian Ocean) using marine magnetic anomalies  

Science.gov (United States)

The 85°E Ridge extends from the Mahanadi Basin (off northeastern margin of India) through buried hills to the Afanasy Nikitin seamount, is an enigmatic aseismic ridge in the north Indian Ocean. While the north part of the ridge (up to 5°N lat) entirely buried under the Bengal Fan sediments, is associated with negative gravity anomaly, the southern part intermittently rise above the seafloor, is associated with positive gravity anomaly. In contrast to this alternate stripes (variable widths) of positive and negative magnetic signatures are observed along the ridge. Magnetic model studies reveal that oceanic crust in the Bay of Bengal was formed during the long Cretaceous Quiet Period, whereas the ridge was emplaced at a later period during different polarities of Earth's magnetic field. With the available data we have noticed seven positive magnetic anomaly stripes formed during the normal polarity field and six negative magnetic anomaly stripes formed during the reverse polarity field. Positive magnetic anomalies are generated by the relief of the ridge and negative anomalies are largely produced by the polarity contrasts between the ridge and adjacent oceanic crust. Keeping the Paleocene age constraint for the formation of dyke bodies at Afanasy Nikitin seamount and Indian plate moments from the late Cretaceous to early Tertiary in view we opine that the 85°E Ridge was emplaced in intraplate setting by a short-lived hotspot from Mahanadi Basin to Central Indian Basin between 80 and 55 Ma.

Michael, L.; Krishna, K. S.

2008-12-01

208

Phosphorus imbalance in the global ocean?  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The phosphorus budget of the pre-human modern ocean is constrained applying the most recent estimates of the natural riverine, eolian, and ice-rafted input fluxes, the phosphorus burial in marine sediments, and the hydrothermal removal of dissolved phosphate from the deep ocean. This review of current flux estimates indicates that the phosphorus budget of the ocean is unbalanced since the accumulation of phosphorus in marine sediments and altered oceanic crust exceeds the continental input of...

Wallmann, Klaus

2010-01-01

209

An Introduction to Biological Soil Crusts  

Science.gov (United States)

Biological soil crusts are the feature of this USGS Canyonlands Research Station website. The site provides an introduction to biological soil crusts with linked images, divided into the following topics: nomenclature of the crusts- crytogamic, microbiotic, crytobiotic, and microphytic crusts, structure and formation, species composition, ecological functions, response to disturbance, future, and a glossary. In addition, the site provides links to other Canyonlands Research Station webages including an advanced page with a downloadable 90-page report on soil crusts, a gallery of biological soil crust images and figures, references, the Canyon Country Ecosystems Research Site (CCERS), and other related links.

Station, Usgs C.; Usgs

210

Cyanobacteria and the Cryptobiotic Crust  

Science.gov (United States)

Cryptobiotic crusts are featured in this web page, with discussion on cryptobiotic communities, cyanobacteria, and other organisms that comprise this environment. The site provides numerous images of cryptobiotic soil and its inhabiting microorganisms. In addition, this site provides links to a wealth of information on deserts including regions of deserts, organisms found in deserts, and selected deserts in the world.

Deacon, Jim; Edinburgh, University O.

211

42 CFR 495.202 - Identification of qualifying MA organizations, MA-EPs and MA-affiliated eligible hospitals.  

Science.gov (United States)

...Identification of qualifying MA organizations, MA-EPs and MA-affiliated eligible hospitals...Identification of qualifying MA organizations, MA-EPs and MA-affiliated eligible hospitals...seeking reimbursement for qualifying MA EPs and qualifying MA-affiliated...

2010-10-01

212

Continental Crust Recycling at Collision Zones: Insights From Numerical Modeling and Observations  

Science.gov (United States)

In the recent years, an increasing number of geochemical evidences of continental crust (cc) contributions in OIB lavas have been reported worldwide, suggesting that part of the lithospheric continents have sunk and were recycled in the deep mantle. So far, crustal recycling processes have been identified as eclogitization of the lower crust, foundering and subduction of crustal rocks and erosion of continents at ocean-continent and ocean-ocean margins activated by the subducting slab. But what is about continental crust recycling occurring during continental collision? In order to investigate this process, we perform 2D numerical models of subduction under a fix continent of a 1) retreating and 2) advancing oceanic slab attached to a continental passive margin to test this last process. 1) If the slab is retreating, when collision occurs and the continents are poorly coupled, asthenospheric mantle wedges between the continents triggering the retreating and delamination of the converging continental plate. A discrete volume of cc (mainly the lower crust) is then dragged into the deep mantle indicating that this is an efficient process for crustal recycling. Examples of active retreating collisional zone are the Northern Apennines and the Carpatians. 2) If the plates are coupled and convergence does not stop after collision, a large volume of lower cc is transported in the deep mantle. This models is representative of active advancing collisional zone such as the Himalayas where has been calculated that about 2x106 km3 of continental lower crust have been subducted In both cases, the rheologically weak part of the crust (upper crust and sediments, Wet Quartzite flow law) is scraped off and accreted to the accretionary wedge. Results then show that cc recycling occurring after collision is a considerable process that has to be taken into account for both crustal mass balancing and crustal growth models.

Faccenda, M.; Gerya, T. V.; Chakraborty, S.

2008-12-01

213

Lateral Temperature Variations in Upwelling Limbs of the Asthenosphere and its Implications for Thermal Models of the Oceanic Lithosphere  

Science.gov (United States)

Thermal models of the lithosphere proposed to date (the Half-Space Cooling and Plate models) have failed to provide satisfactory accounts of some of the important features in large-scale variations of oceanic heat flow. The systematic difference between model values and observational data have given rise to the so-called "oceanic heat flow paradox", for which no satisfactory solution has been found for over the last forty years. In the present work, we point out that this paradox is a consequence of the model assumption that lateral temperature variations are absent in the sublithospheric mantle. We propose a new thermal model of the oceanic lithosphere that can overcome such inconsistencies. Designated CMI, the new model assumes existence of lateral temperature variations in up-welling limbs of the asthenosphere, similar in character to those commonly observed in tectonothermal processes in the upper crust and in laboratory experiments of thermal plumes. CMI model simulations indicate that the thickness of the young lithosphere increases with distance from the ridge axis, at rates faster than those predicted by Half-Space Cooling and Plate models. As a result, the width of magma injection zone at mid-ocean ridges is relatively narrower in CMI model. Another noteworthy feature of the new model is its ability to provide vastly improved fits for observational heat flow data, in both young (ages less than 55 ma) and old (ages greater than 55 ma) oceanic lithosphere. More importantly, the improved fits to heat flow have been achieved without the need to invoke the ad-hoc hypothesis of large-scale hydrothermal circulation in stable ocean crust. Also, use of CMI model does not lead to artificial discontinuities in the temperature field of the lithosphere, as is the case with GDH reference models. The results of the CMI model provide a better understanding of the global heat flow variations and estimates of global heat loss. In particular, the model is capable of reproducing regional-scale features in the thermal field of the oceanic crust, identified in recent higher degree spherical harmonic representations of global heat flow.

Hamza, V. M.; Cardoso, R. R.

2008-05-01

214

Seasonal Methane Oxidation Potential in Manure Crusts  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Organic crusts on liquid manure storage tanks harbor ammonia- and nitrite-resistant methane oxidizers and may significantly reduce methane emissions. Methane oxidation potential (0.6 mol CH4 m?2 day?1) peaked during fall and winter, after 4 months of crust development. Consequences for methane mitigation potential of crusts are discussed.

Nielsen, Daniel A.; Schramm, Andreas; Nielsen, Lars P.; Revsbech, Niels P.

2013-01-01

215

Nd isotopic evidence from Wopmay Orogen for 2.0-2.4 Ga crust in western North America  

International Nuclear Information System (INIS)

Nd isotopic data from the 1.9 Ga Wopmay Orogen, N.W.T., Canada, strongly suggest the involvement of crust in the range of 2.0-2.4 Ga. Remnants of three compositionally diverse magmatic arcs (1940-1902 Ma, 1890-1880 Ma, and 1875-1840 Ma) are preserved in Wopmay Orogen. Each of the three arcs originated west of the limit of exposed Archean crust. Nineteen samples of plutonic and volcanic rocks from the three arcs yield a narrow range of initial ?Nd from -1.8 to +0.3. There is no apparent correlation between ?Nd and geographic distribution, nor with whole-rock chemistry (SiO2 = 54-75%). Two basalt samples from west of the Archean have initial ?Nd's between +2 and +3. Two 1.87 Ga granites which intrude through Archean crust have much lower initial ?Nd's of -5.9 and -8.6. The data from the three magmatic arcs are best interpreted in terms of a source region which had a very limited range of ?Nd at 1.9 Ga which we interpret as 2.0-2.4 Ga crust, presumably Hottah Terrane basement; Wopmay Fault Zone is therefore the surface manifestation of a suture between 2.0-2.4 Ga Hottah Terrane and the Archean Slave Craton. There is a growing body of data which suggests that much of western North America may be underlain by 2.0-2.4 Ga crust. Geochronologic and isotopic data from other Precambrian cratons also suggest that 2.0-2.4 Ga crust may be much more widespread than previously thought. Models of crustal growth which interpret 1.9-1.7 Ga crust as mixtures of Archean crust and Proterozoic depleted mantle may be overestimating the amount of juvenile material added to the crust in this time period. (orig.)

216

Magnetization of the Lunar Crust  

Science.gov (United States)

Magnetic fields measured by the satellite Lunar Prospector show large scale features resulting from remanently magnetized crust. Vector data synthesized at satellite altitude from a spherical harmonic model of the lunar crustal field, and the radial component of the magnetometer data, have been used to produce spatially continuous global magnetization models for the lunar crust. The magnetization is expressed in terms of localized basis functions, with a magnetization solution selected having the smallest root-mean square magnetization for a given fit to the data, controlled by a damping parameter. Suites of magnetization models for layers with thicknesses between 10 and 50 km are able to reproduce much of the input data, with global misfits of less than 0.5 nT (within the uncertainties of the data), and some surface field estimates. The magnetization distributions show robust magnitudes for a range of model thicknesses and damping parameters, however the magnetization direction is unconstrained. These global models suggest that magnetized sources of the lunar crust can be represented by a 30 km thick magnetized layer. Average magnetization values in magnetized regions are 30-40 mA/m, similar to the measured magnetizations of the Apollo samples and significantly weaker than crustal magnetizations for Mars and the Earth. These are the first global magnetization models for the Moon, providing lower bounds on the magnitude of lunar crustal magnetization in the absence of multiple sample returns, and can be used to predict the crustal contribution to the lunar magnetic field at a particular location.

Carley, R. A.; Whaler, K. A.; Purucker, M. E.; Halekas, J. S.

2012-01-01

217

Metamorphism in the Martian crust  

Science.gov (United States)

Compositions of basaltic and ultramafic rocks analyzed by Mars rovers and occurring as Martian meteorites allow predictions of metamorphic mineral assemblages that would form under various thermophysical conditions. Key minerals identified by remote sensing roughly constrain temperatures and pressures in the Martian crust. We use a traditional metamorphic approach (phase diagrams) to assess low-grade/hydrothermal equilibrium assemblages. Basaltic rocks should produce chlorite + actinolite + albite + silica, accompanied by laumontite, pumpellyite, prehnite, or serpentine/talc. Only prehnite-bearing assemblages have been spectrally identified on Mars, although laumontite and pumpellyite have spectra similar to other uncharacterized zeolites and phyllosilicates. Ultramafic rocks are predicted to produce serpentine, talc, and magnesite, all of which have been detected spectrally on Mars. Mineral assemblages in both basaltic and ultramafic rocks constrain fluid compositions to be H2O-rich and CO2-poor. We confirm the hypothesis that low-grade/hydrothermal metamorphism affected the Noachian crust on Mars, which has been excavated in large craters. We estimate the geothermal gradient (>20 °C km-1) required to produce the observed assemblages. This gradient is higher than that estimated from radiogenic heat-producing elements in the crust, suggesting extra heating by regional hydrothermal activity.

McSween, Harry Y.; Labotka, Theodore C.; Viviano-Beck, Christina E.

2014-07-01

218

Geochemical evolution of Ngorongoro Caldera, Northern Tanzania: Implications for crust magma interaction  

Science.gov (United States)

Ngorongoro Caldera is the largest and best-preserved of nine Plio-Pleistocene volcanoes that make-up the Ngorongoro Volcanic Highlands (NVH) complex situated at the southern bifurcation of Gregory Rift, part of the East African Rift system of northern Tanzania. We report here, major and trace element abundances, Sr-Nd-Pb isotope analyses and 40Ar/ 39Ar laser incremental-heating and total fusion ages on lava and tephra sampled from stratigraphic sections exposed within the Ngorongoro Caldera. Major and trace elements measured on samples collected from the Ngorongoro Caldera wall indicate a stratified magma chamber whose silicic top and basaltic bottom was inverted by sequential eruptions. Samples from the lower part of the exposed Ngorongoro Caldera wall are high in silica, alkalis and HFSE (High Field Strength Elements). The Zr, Nb and Hf concentrations are highly correlated with each other and decrease up-section, indicative of the extent of magma evolution. Modeling of major, trace as well as Sr, Nd and Pb isotope data suggests that assimilation fractional crystallization processes were essential in producing the observed geochemical variations. The Sr and Nd isotope ratios from the Ngorongoro samples are widely dispersed ( 87Sr/ 86Sr = 0.70405 to 0.70801, 143Nd/ 144Nd = 0.512205 to 0.512623) and Pb isotope ratios are consistent with previous studies: 206Pb/ 204Pb = 18.73 to 19.37, 207Pb/ 204Pb = 15.64 to 15.69, 208Pb/ 204Pb = 39.52 to 39.55. Although Sr isotopic ratios are similar to Oceanic Island Basalt (OIB), the more radiogenic samples ( 87Sr/ 86Sr > 0.705) from the lower part of the section suggest crust-magma interaction; this is supported by Ce/Pb ratios (Laser incremental-heating of basalt samples and single grain total fusion of anorthoclase from tephra samples collected from the Ngorongoro Caldera wall section yield 40Ar/ 39Ar ages of 2.25 ± 0.02 Ma to 2.01 ± 0.02 Ma, constraining a duration of volcanism of the order of ~ 240 kyr. These ages suggest correlation of a normal to reverse geomagnetic polarity transition measured in the Ngorongoro Caldera wall section with the ~ 2.14 Ma C2r.1n-1r (normal to reverse) Réunion-Matuyama boundary.

Mollel, Godwin F.; Swisher, Carl C.; Feigenson, Mark D.; Carr, Michael J.

2008-07-01

219

Opening of the Gulf of Mexico and the Nature of the Crust in the Deep Gulf: New Evidence from Seafloor Spreading Magnetic Anomalies  

Science.gov (United States)

The seafloor spreading history in the Gulf of Mexico is poorly constrained due to a lack of recognized seafloor spreading magnetic anomalies, a paucity of deep penetrating seismic data, and absence of drilling to constrain crystalline ocean floor composition and ages. We have identified lineated magnetic anomalies in the eastern Gulf on profiles collected during the Woods Hole R/V Farnella FRNL85-2 cruise that correlate with magnetic chrons M21R to M10. Forward modeling shows that these anomalies formed during creation of weakly magnetized new seafloor in the eastern Gulf between 149-134 Ma at an average half-spreading rate of 3.2 cm/yr. The oldest anomalies are located against stretched continental crust beneath the western Florida shelf on the east and the Yucatan shelf on the west. The youngest anomalies form a juxtaposed conjugate pair that mark the location of an extinct spreading ridge between Yucatan and Florida. Seismic velocities of the crust in the eastern Gulf and the amplitude of the magnetic anomalies are similar to the Iberian and Newfoundland rifted margins, where the early stages of continental breakup were accommodated by exhumation of subcontinental lithosphere rather than creation of new basaltic oceanic crust. We infer that the eastern Gulf of Mexico is underlain by exhumed sub-continental peridotitic mantle intruded by lesser volumes of basaltic igneous rocks generated by decompression melting of the asthenosphere during the late stages of opening of the Gulf. The long wavelength characteristics of the magnetic and gravity fields in the eastern Gulf, as well as the seismic velocity structure of the crust, differ from those in the central and western Gulf, which are more similar to typical magmatic rifted margins. This suggests that the character of the Gulf changes along strike, from a magmatic western portion to an amagmatic eastern portion. Paleogeographic restoration of the lineated magnetic anomaly pattern suggests a 4-phase model for opening of the Gulf. During phase 1 (Early Permian-Late Triassic), Yucatan and associated tectonic blocks that now comprise eastern Mexico were translated eastward from the Pacific realm into positions near the modern western Gulf. During phase 2 (Late Triassic-ca. 160 Ma) Yucatan and the South Florida block were translated southeastward relative to North America, rotating 6.7? counterclockwise about a pole located at 34?N, 74?W. This resulted in ca. 430 km of southeastward extension on the North American coastal plain, 120 km of southward extension on the northern Yucatan shelf, and displacement of the South Florida Block from a pre-rift position on the northwest Florida shelf to its modern position. During phase 3 (ca. 160-149 Ma), Yucatan rotated counterclockwise 46? relative to North America about a pole located at 27.6?N, 84.0?W. Phase 3 may have coincided with seafloor spreading in the central and western Gulf, but predated seafloor spreading in the eastern Gulf. During phase 4 (149-134 Ma), Yucatan moved southwestward relative to North America, rotating counterclockwise 2.2? about a pole located at 17.6?N, 74.2?W and completing opening of the Gulf.

Harry, D. L.; Eskamani, P. K.

2013-12-01

220

Oceanic-type accretion may begin before complete continental break-up  

Science.gov (United States)

Oceanic accretion is thought to be the process of oceanic crust (and lithosphere) edification through adiabatic melting of shallow convecting mantle at oceanic spreading ridges. It is usually considered as a post-breakup diagnostic process following continents rupturing. However, this is not always correct. The structure of volcanic passive margins (representing more than 50% of passive continental margins) outlines that the continental lithosphere is progressively changed into oceanic-type lithosphere during the stage of continental extension. This is clear at least, at crustal level. The continental crust is 'changed' from the earliest stages of extension into a typical -however thicker- oceanic crust with the typical oceanic magmatic layers (from top to bottom: lava flows/tuffs, sheeted dyke complexes, dominantly (sill-like) mafic intrusions in the lower crust). The Q-rich continental crust is highly extended and increases in volume (due to the magma) during the extensional process. At the continent-ocean transition there is, finally, no seismic difference between this highly transformed continental crust and the oceanic crust. Using a large range of data (including deep seismic reflection profiles), we discuss the mantle mechanisms that governs the process of mantle-assisted continental extension. We outline the large similarity between those mantle processes and those acting at purely-oceanic spreading axis and discuss the effects of the inherited continental lithosphere in the pattern of new mafic crust edification.

Geoffroy, L.; Zalan, P. V.; Viana, A. R.

2011-12-01

 
 
 
 
221

Seawater osmium isotope evidence for a middle Miocene flood basalt event in ferromanganese crust records  

Science.gov (United States)

Three ferromanganese crusts from the northeast, northwest and central Atlantic were re-dated using osmium (Os) isotope stratigraphy and yield ages from middle Miocene to the present. The three Os isotope records do not show evidence for growth hiatuses. The reconstructed Os isotope-based growth rates for the sections older than 10??Ma are higher than those determined previously by the combined beryllium isotope (10Be/9Be) and cobalt (Co) constant-flux methods, which results in a decrease in the maximum age of each crust. This re-dating does not lead to significant changes to the interpretation of previously determined radiogenic isotope neodymium, lead (Nd, Pb) time series because the variability of these isotopes was very small in the records of the three crusts prior to 10??Ma. The Os isotope record of the central Atlantic crust shows a pronounced minimum during the middle Miocene between 15 and 12??Ma, similar to a minimum previously observed in two ferromanganese crusts from the central Pacific. For the other two Atlantic crusts, the Os isotope records and their calibration to the global seawater curve for the middle Miocene are either more uncertain or too short and thus do not allow for a reliable identification of an isotopic minimum. Similar to pronounced minima reported previously for the Cretaceous/Tertiary and Eocene/Oligocene boundaries, possible interpretations for the newly identified middle Miocene Os isotope minimum include changes in weathering intensity and/or a meteorite impact coinciding with the formation of the No??rdlinger Ries Crater. It is suggested that the eruption and weathering of the Columbia River flood basalts provided a significant amount of the unradiogenic Os required to produce the middle Miocene minimum. ?? 2008 Elsevier B.V.

Klemm, V.; Frank, M.; Levasseur, S.; Halliday, A.N.; Hein, J.R.

2008-01-01

222

Presence of ca. 43 Ma highly fractionated and normal calc-alkaline granites first identified in the Gangdese Batholith, southern Tibet  

Science.gov (United States)

The Gangdese Batholith in southern Tibet has long been linked with the subduction of Neo-Tethyan oceanic lithosphere and subsequent India-Asia collision. However, the specific processes of magmatic evolution and petrogenesis of the batholith remain poorly constrained because most existing studies have focused on its geochronological framework. New integrated whole-rock major and trace element, zircon U-Pb age, and zircon Hf isotopic data of granites from Dajia in the western Gangdese Batholith, southern Tibet establish the presence of highly fractionated syenogranite (HFS) and normal calc-alkaline monzogranite (NCM). One syenogranite sample has been dated by LA-ICP-MS zircon U-Pb method to be 43.9 ± 0.3 Ma and three monzogranite samples yielded ages of 42.6 ± 0.3 Ma, 42.7 ± 0.4 Ma, and 43.6 ± 0.3 Ma, representing the late-phase magmatism of the Gangdese Batholith. Six NCM samples display SiO2 of 69-72 wt.%, K2O of 4.9-5.5 wt.%, and Na2O of 3.2-3.8 wt.%, with differentiation index (DI) in the range of 84-93. These rocks are enriched in Rb, Th, U, and LREE and depleted in Ba, Nb, Sr, P, and Ti, with (La/Yb)N = 18.1-26.9 and Eu/Eu* = 0.6-0.83. Eight HFS samples are characterized by high SiO2 (75-78 wt.%) and DI (95-97), and significant negative Eu anomalies (Eu/Eu* = 0.27-0.70), although K2O (4.8-5.3 wt.%) and Na2O (3.2-3.6 wt.%) contents and (La/Yb)N (12.5-27.2) ratios are comparable to those of the NCM samples. These HFS samples display marked concave-upward middle rare earth element (MREE; Gd-Ho) patterns that are not observed in the NCM samples. The NCM and HFS samples have similar zircon Hf isotopic compositions with zircon ?Hf(t) of -5.6 to +6.3 and -1.6 to +4.6, respectively. Such isotopic compositions, together with low heavy REE and Y abundances, indicate that both the NCM and HFS samples were most likely derived from partial melting of the thickened juvenile crust beneath the southern Lhasa subterrane with varying contributions from ancient continental crust material. The absence of fractionation trends between the NCM and HFS samples (e.g., SiO2 vs. Sc and SiO2 vs. Dy/Yb) suggests that the HFS samples with concave-upward MREE patterns can be interpreted as resulting from partial melting of basaltic lower crust leaving an amphibole-rich residuum followed by significant fractional crystallization of feldspar, plagioclase and apatite. The presence of HFS with low heavy REE and Y (6.2-15.3 ppm) abundances identified for the first time in the Gangdese Batholith corroborates that the southern Lhasa crust had already been thickened by ca. 43 Ma. High whole-rock zircon saturation temperatures (815° C-869° C) for the NCM samples suggest high heat supply likely associated with rising asthenospheric flow in response to post-collision breakoff of the Neo-Tethyan oceanic lithosphere.

Wang, Qing; Zhu, Di-Cheng; Cawood, Peter A.; Zhao, Zhi-Dan; Liu, Sheng-Ao; Chung, Sun-Lin; Liu, Dong; Dai, Jin-Gen; Mo, Xuan-Xue

2014-05-01

223

Accretion tectionics of the Japanese islands and evolution of continental crust  

Science.gov (United States)

Revised interpretation of the basement geology of the Japanese island arcs which has emerged in the last 20 years or so, indicates that they are mostly composed of two geological belts: volcanics (greenstone)-granitoid belt (VGB) and turbidite-granitoid belt (TGB). The VGB exhibits thrust-bounded thick sequences of basic volcanics and associated granitoid plutons and was formed by arc-arc collision process. The TGB is composed of turbidite and melange units and was formed by progressive growth of trench accretionary prism and later intrusion of granitoids. Both belts represent a style of crustal growth in a convergent margin. In these orogenic belts, involvement ot many oceanic island arcs and formation of the VGB were a major mechanism of juvenile crust, in addition to the continental crust. The formation of the TGB played a major role in the reworking and recycling of the continental crust which resulted in a long-term secular compositional change of the upper crust.

Taira, Asahiko; Kiyokawa, Shoichi; Aoike, Kan; Saito, Saneatue

1997-10-01

224

Relative contributions of crust and mantle to generation of Campanian high-K calc-alkaline I-type granitoids in a subduction setting, with special reference to the Har?it Pluton, Eastern Turkey  

Science.gov (United States)

We present elemental and Sr-Nd-Pb isotopic data for the magmatic suite (~79 Ma) of the Har?it pluton, from the Eastern Pontides (NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite, tonalite and minor diorite (SiO2 = 59.43-76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO2 = 54.95-56.32 wt%), and exhibits low Mg# (Erzincan oceanic slab beneath the Eurasian block in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr-Nd isotope modeling shows that the generation of these magmas involved ~65-75% of the lower crustal-derived melt and ~25-35% of subcontinental lithospheric mantle. Further, geochemical data and the Ar-Ar plateau age on hornblende, combined with regional studies, imply that the Har?it pluton formed in a subduction setting and that the back-arc extensional period started by least ~79 Ma in the Eastern Pontides.

Karsli, Orhan; Dokuz, Abdurrahman; Uysal, Ibrahim; Aydin, Faruk; Chen, Bin; Kandemir, Raif; Wijbrans, Jan

2010-10-01

225

Zircon Lu-Hf isotopes in high-alumina orthopyroxene megacrysts from the Neoproterozoic Rogaland Anorthosite Province, SW Norway: A window into the Sveconorwegian lower crust  

Science.gov (United States)

The Rogaland Anorthosite Province consists of three massif-type anorthosite bodies and an associated layered Bjerkreim-Sokndal intrusion. The rocks were emplaced between ca. 950 and 920 Ma, following the Sveconorwegian orogeny at ca. 1000 Ma. The anorthosites are commonly thought to have formed by melting of the lower continental crust. They lack zircon or other dateable minerals, so geochronological constraints come from zircon crystals found within opx megacrysts in the rocks. The opx megacrysts contain ca. 7.7 wt.% Al2O3, and are therefore high-alumina opx megacrysts (HAOM). Pressure estimates range from ca. 11 to 13 kbar which implies that the HAOM crystallised in the lower crust and that their zircon cargo has probably been shielded from interaction with the middle and upper crust. If the assumption of a lower-crustal source is correct, the zircons may be viewed as probing the Hf isotopic composition of the lower crust in SW Norway. Zircons were separated from HAOM in the Egersund-Ogna anorthosite and yield a U-Pb age of 948±3 Ma, supporting Andersen and Griffin's (2004) suggestion of onset of magmatism prior to the commonly accepted ca. 930 Ma age for these rocks. Lu-Hf isotopic analyses of the zircons yield a ?Hf948 Ma value of +3.8 ± 3 (2?), which is interpreted to represent the composition of the lower crust in SW Norway. Three crustal components (or, more correctly, their lower-crustal counterparts) were considered as potential source rocks for the anorthosite magma that formed the Egersund-Ogna body: i) ca.1500 Ma Telemarkian magmatic rocks that cover large tracts of S Norway; ii) ca.1260 Ma magmatic rocks that are geographically widespread in south and central Scandinavia, but volumetrically minor; iii) 1050 to 1020 Ma magmatic rocks in the 30-40 km-wide and nearly 200 km-long Sirdal Magmatic Belt in SW Norway. The isotopic data show that 1500 Ma crust cannot be a major component of the lower-crustal source, whereas both 1260 Ma and 1050-1020 Ma crust are possible candidates. Considering the large volumes of 1050-1020 Ma rocks recently identified in SW Norway, a lower-crustal source of this age is likely. This means that Sveconorwegian-age crustal growth in SW Norway was significant, and that the Sveconorwegian orogeny did not merely rework older crust. Andersen, T. and Griffin, W. L., 2004, Lithos, 73, 271-288.

Sauer, Simone; Slagstad, Trond; Andersen, Tom; Kirkland, Christopher L.

2013-04-01

226

Evolution Of The Alpha Ride, The Arctic Ocean, On The Basis Of The Geohistorical Analysis Of The Magnetic Anomalies  

Science.gov (United States)

A new magnetic anomaly map of the Amerasian Basin has been created owing to a joint reprocessing of the Russian and American aeromagnetic data [Glebovsky, Kovacs at all., 2000]. This model produced the base for the magnetic data interpretation on the more qualitative level. As a result three series of seafloor spreading-type magnetic anomalies have been identified within the area of the Alpha Ridge and the adjacent part of the Canada Basin [Gurevich et all, 2003]. Their sources were formed from three spreading centers (SC). Two spreading centers: the western and the eastern, are situated at the axial part of the Alpha Ridge, the third one - the southern, is located on the southern slope of the Alpha Ridge and on the adjacent part of the Canada Basin. The triple junction of these SC had been located in the central part of the recent Alpha Ridge. The geohistorical analysis of these magnetic anomalies is fulfilled using an original computer programs. In consequence of this analysis: the geochronological characteristics are specified; the kinematic characteristics of the oceanic floor movement are determined and the main stages of the area evolution are found. The magnetic anomalies M16r (140 Ma), which signify the position of all three SC, and pair anomalies M20r (146.5 Ma) and M23r (151.5 Ma) are identified enough sure for all three SC and pair anomalies M30r (157.5 Ma) - fore the eastern and the southern SC. Finite and differential Euler poles of the lithospheric plates rotation were calculated for all three SC from best-fit pair magnetic anomalies. All the poles are concentrated around the Nares strait and at the northeastern part of the Ellesmere island. Angle and linear spreading rates were calculated using Euler poles. The calculation has showed that all three SC had low spreading rates. Three stages of the area evolution are found on the basis of the plate tectonic reconstruction for the periods 146.5, 151.5 and 157.5 Ma ago. The first stage, slightly earlier 157.5 Ma ago: the initiation of the oceanic crust formation to the north-west from the recent shelf of the Prince Patrick island. The second stage, from about 157.5 Ma ago: SC had advanced to the north-east; the oceanic crust was forming from one SC. The third stage, from slightly earlier 151.5 Ma ago to 140 Ma ago: the oceanic floor spreading from three SC took place, 140 Ma ago spreading ceased in this area. During the third stage the triple junction of the spreading centers was not stable and changed from type "ridge - ridge - ridge" to type " ridge - ridge - transform". The intraplate volcano-tectonic activity of the oceanic floor that created the Alpha Ridge was t8he fourth stage of the area evolution The kinematic characteristics of the spreading imply of crustal compression in the north of the Greenland and in the north-east of the Ellesmere island and of crustal stretching in the area of the Queen Elizabeth Islands, that agrees with their geological structure. The main stages of the Amerasian Basin evolution correspond by age to unconformities that A. Embry determined in the Mesozoic strata of the Sverdrup Basin [Embry, 1991]. The work has been supported by the Russian Foundation for basic Research (Grant 01-05-65481).

Gurevich, N. I.; Merkouriev, S. A.

2004-12-01

227

Protracted construction of gabbroic crust at a slow spreading ridge: Constraints from 206Pb/238U zircon ages from Atlantis Massif and IODP Hole U1309D (30°N, MAR)  

Science.gov (United States)

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of 24 samples from oceanic crust recovered in Integrated Ocean Drilling Program (IODP) Hole U1309D and from the surface of Atlantis Massif, Mid-Atlantic Ridge (MAR) (30°N) document a protracted history of accretion in the footwall to an oceanic detachment fault. Ages for 18 samples of evolved Fe-Ti oxide gabbro and felsic dikes collected 40-1415 m below seafloor in U1309D yield a weighted mean of 1.20 ± 0.03 Ma (mean square of weighted deviates = 7.1). However, the ages range from 1.08 ± 0.07 Ma and 1.28 ± 0.05 Ma indicating crustal construction occurred over a minimum of 100-200 ka. The zircon ages, along with petrologic observations, indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka. The oldest ages are observed below 600 mbsf, an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault. The data are most consistent with a "multiple sill" model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1.4 km. Zircon ages from broadly spaced samples collected along the southern ridge of Atlantis Massif yield a detachment fault slip rate of 28.7 ± 6.7 mm/a and imply significant asymmetric plate spreading (up to 100% on the North American plate) for at least 200 ka during core complex formation.

Grimes, Craig B.; John, Barbara E.; Cheadle, Michael J.; Wooden, Joseph L.

2008-08-01

228

Protracted construction of gabbroic crust at a slow spreading ridge: Constraints from 206Pb/238U zircon ages from Atlantis Massif and IODP Hole U1309D (30??N, MAR)  

Science.gov (United States)

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of 24 samples from oceanic crust recovered in Integrated Ocean Drilling Program (IODP) Hole U1309D and from the surface of Atlantis Massif, Mid-Atlantic Ridge (MAR) (30??N) document a protracted history of accretion in the footwall to an oceanic detachment fault. Ages for 18 samples of evolved Fe-Ti oxide gabbro and felsic dikes collected 40-1415 m below seafloor in U1309D yield a weighted mean of 1.20 ?? 0.03 Ma (mean square of weighted deviates = 7.1). However, the ages range from 1.08 ?? 0.07 Ma and 1.28 ?? 0.05 Ma indicating crustal construction occurred over a minimum of 100-200 ka. The zircon ages, along with petrologic observations, indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka. The oldest ages are observed below 600 mbsf, an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault. The data are most consistent with a "multiple sill" model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1.4 km. Zircon ages from -broadly spaced samples collected along the southern ridge of Atlantis Massif yield a detachment fault slip rate of 28.7 ?? 6.7 mm/a and imply significant asymmetric plate spreading (up to 100% on the North American plate) for at least 200 ka during core complex formation. Copyright 2008 by the American Geophysical Union.

Grimes, C.B.; John, B.E.; Cheadle, M.J.; Wooden, J.L.

2008-01-01

229

Complex evolution of the lower crust beneath the southeastern North China Craton: the Junan xenoliths and xenocrysts  

Science.gov (United States)

Knowledge of the lower crust beneath the southeastern parts of North China Craton (NCC) is still sparse. The Junan basalts (67 Ma) in the southeastern NCC contain abundant xenoliths of lower crustal granulites, pyroxenites and mantle peridotites. We present integrated in-situ U-Pb ages and Hf isotopes of zircons from the Junan basalts and granulite xenoliths, to investigate accretion and modification processes in the lower crust. The granulite xenoliths define three distinct U-Pb age populations of ca 2.3 Ga, ca 2.0 Ga and 114-126 Ma. The ca 2.3 Ga zircons have widely variable ?Hf(t) and Paleo-Neoarchean model ages (Tcrust = 2.6-4.0 Ga), whereas the ca 2.0 Ga structureless grains give negative ?Hf(t) and Tcrust of 2.7-3.3 Ga. In addition to a few discordant Early Paleoproterozoic xenocrysts, zircons from the basalts are dominantly Early Cretaceous (115-125 Ma), with some Neoproterozoic (550-800 Ma) and Early Paleozoic (437-493 Ma), as well as minor Late Triassic and Late Jurassic grains. These results, combined with previous petrological and geochemical studies and P-T estimates, suggest that the upper part of the Junan lower crust consists of mafic-intermediate granulites, mainly formed at ca 2.3 Ga by crystallization of depleted-mantle-derived magmas that assimilated ancient crust and then fractionated. More significantly, zircon ages and Hf isotopes imply that this lower crust had a complex history of accretion and modification, including initial growth at 3.0-4.0 Ga and 2.5-2.7 Ga, conversion to the dominant granulitic assemblages in the Early Paleoproterozoic (ca 2.3 Ga), modification or metamorphism in the Late Paleoproterozoic (1.8-2.0 Ga) and possibly slight heating in Neoproterozoic time. Episodic thermal events during the Early Paleozoic, Late Triassic and Late Jurassic may also have reworked this Precambrian lower crust. Most of the Early Cretaceous zircons and xenocrysts have uniform ?Hf(t) values similar to those of the nearby coeval magmatic rocks that derived from the enriched lithospheric mantle. This suggests that the Early Cretaceous basaltic underplating, which was contemporary with extensive partial melting of the enriched parts of the NCC lithospheric mantle at the peak of lithospheric thinning, might have substantially modified the Paleoproterozoic granulitic lower crust, and finally gave the diverse cumulate pyroxenites that now make up the deeper lower crust. The discovery of ca 2.1-2.3 Ga lower crust in the southeastern NCC also highlights the heterogeneous nature of the Precambrian lower crust across the eastern NCC.

Tang, Huayun; Zheng, Jianping; Griffin, William L.; O?Reilly, Suzanne Y.; Yu, Chunmei; Pearson, Norman J.; Ping, Xianquan; Xia, Bing; Yang, Huaben

2014-10-01

230

Evidence for a Neoproterozoic ocean in south-central Africa from mid-oceanic-ridge type geochemical signatures and pressure-temperature estimates of Zambian eclogites  

Science.gov (United States)

Precambrian eclogites, metagabbros, and gabbros occur in an ˜200-km-long by 40-km-wide zone in central Zambia. Pressure-temperature (P-T) estimates of kyanite-bearing eclogites (kyanite eclogites) throughout the zone give temperatures of 590 750 °C at minimum pressures of 20 kbar. Phengite-bearing eclogites equilibrated at 720 755 °C and 26 28 kbar and show evidence for a clockwise P-T path. These P-T conditions imply a low geothermal gradient of ˜8 °C/km and a subduction depth of ˜90 km. The eclogites, metagabbros, and gabbros show incompatible element patterns similar to those of recent mid-oceanic-ridge basalts, and thus are interpreted to represent former oceanic crust. The low geothermal gradient indicates a cold subducted oceanic lithosphere, implying long-lived, fast convergence and a relatively large (>1000 km) associated ocean basin. A Sm-Nd isochron defines an age of 595 ± 10 Ma for the eclogite facies metamorphism. These results imply that a Neoproterozoic suture zone exists between the Congo and Kalahari cratons. Suturing occurred during the same orogenic cycle that formed the Zambezi belt and is related to the assembly of Gondwana.

John, Timm; Schenk, Volker; Haase, Karsten; Scherer, Erik; Tembo, Francis

2003-03-01

231

Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples  

Science.gov (United States)

Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the "rolling-hinge" model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46° ± 6° anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011° ± 6°. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases.

Morris, A.; Gee, J. S.; Pressling, N.; John, B. E.; MacLeod, C. J.; Grimes, C. B.; Searle, R. C.

2009-09-01

232

Oceanic crustal structure from seismic measurements  

Science.gov (United States)

The primary source of our knowledge of the structure of oceanic crust is the interpretation of seismic refraction experiments. The first classic compilation of seismic data of Raitt (in The Sea, 1963) subdivided the crust into three distinct layers, which have formed the reference basis for seismic profiles for the last decades. Today we know that the upper igneous crust (layer 2) is a region of strong velocity gradients, while the lower crust (layer 3) is relatively homogeneous, although it does show an increase in velocity with depth. Further, the upper crust has been sub-divided the in Layer 2A, composed of extruded basalts, and Layer 2B, formed by basaltic sheeted dikes. The lower crust, or Layer 3, often called the "oceanic layer", is inferred to be composed of gabbros. As crust ages, sediments accumulate on the igneous basement, creating layer 1. The velocity structure of the oceanic crust formed by seafloor spreading is inherently related to the process of mantle melting. The amount of melt produced by adiabatic decompression of the mantle and the composition of the resultant igneous crust depend on the temperature, composition, and water content of the mantle source. Normal oceanic crust with a thickness of 6-7 km and Mid-Ocean Ridge Basalt (MORB) like composition is the result of decompressional melting of a mantle source composed of dry pyrolite with a mantle temperature of ~1300°C. Thus, crustal formation occurs as passive response to seafloor spreading (i.e., passive upwelling). Higher mantle temperatures or compositional anomalies may cause buoyant upwelling of the mantle (i.e. active upwelling). The combination of active upwelling and higher mantle temperatures, or the presence of a more fertile mantle source, will produce larger amounts of melting and, likely, a thicker crust. Steady state mantle melting models can be used to investigate the relationship between mantle temperature, upwelling, and mantle composition on one hand and lower crustal seismic velocity and crustal thickness on the other hand. Here I use a new compilation of modern seismic refraction data to survey the relationship between crustal thickness and average layer 3 seismic velocity structure to test the hypothesis that mantle temperature is governing the variability of the oceanic crust. Indeed, relating lower crustal seismic velocities to crustal thickness is extremely successful. Thus, the seismic structure of crust generated at the East Pacific Rise is consistent with that expected for a crust generated by passive decompression melting of dry pyrolitic mantle with a potential temperature of 1250-1300°C. However, thinner crust found in some parts of the Indian Ocean indicates along with reduced velocities in the lower crust that crust has been formed at lower mantle temperatures, while thicker crust found near hotspots suggests higher mantle temperatures. Furthermore, focused mantle upwelling along the Mid-Atlantic Ridge suggests that along axis variations of mantle temperature control crustal structure.

Grevemeyer, Ingo

2014-05-01

233

Petrogenesis of Cretaceous adakite-like intrusions of the Gangdese Plutonic Belt, southern Tibet: Implications for mid-ocean ridge subduction and crustal growth  

Science.gov (United States)

We have conducted a whole-rock geochemical, U-Pb zircon geochronological, and in situ zircon Hf-O isotopic compositional study of rocks in southern Tibet from the Langxian igneous suite (including a lamprophyre dyke, mafic enclaves, a granodiorite, and a two-mica granite) and the Nuri igneous suite (a quartz-diorite). U-Pb zircon dating indicates that the timing of crystallization of the mafic enclaves and host granodiorite of the Langxian suite are ca. 105 Ma and 102 Ma, respectively, that the Langxian lamprophyre dyke and the two-mica granite were emplaced at ca. 96 Ma and 80-76 Ma, respectively, and that the Nuri quartz-diorite was emplaced at ca. 95 Ma. With the exception of the lamprophyre dyke and mafic enclaves in the Langxian area, felsic rocks from the Langxian and Nuri igneous suites all show signs of a geochemical affinity with adakite-like rocks. The high Mg-numbers, high abundance of compatible elements, high ?Nd(t) (2.7 and 2.8) and ?18O (8.9 and 9.2‰) values, elevated zircon ?Hf(t) (11.0-17.0) values, and low 87Sr/86Sr(i) ratios (0.7040), collectively indicate that the Nuri adakite-like quartz-diorite was derived from partial melting of the low temperature altered Neo-Tethyan oceanic crust, and that these dioritic magmas subsequently interacted with peridotite as they rose upwards through the overlying mantle wedge. The observation of identical differentiation trends, similar whole-rock Sr-Nd and zircon Hf isotopic compositions, and consistently low (Dy/Yb)N ratios among the Langxian igneous suite rocks, indicates that the adakite-like granodiorite was produced by low-pressure fractional crystallization of precursor magmas now represented by the (relict) mafic enclaves. However, relatively high Al2O3 contents, low MgO, Cr and Ni contents, and low (La/Yb)N and (Dy/Yb)N values indicate that the two-mica granite was derived from partial melting of the southern Tibetan mafic lower crust in the absence of garnet, while isotopic data suggest that at least 70% of the magma source region was juvenile materials. Combined with the presence of HT (high temperature) charnockitic magmatism, HT granulite facies metamorphism, and large volumes of Late Cretaceous batholiths, the oceanic-slab-derived Nuri adakitic rocks indicate a substantial high heat flux in the Gangdese batholith belt during the Late Cretaceous, which may have been related to subduction of a Neo-Tethyan mid-ocean ridge system. According to this model, hot asthenosphere would rise up through the corresponding slab window, and come into direct contact with both the oceanic slab and the base of the overlying plate. This would cause melting of both the oceanic slab and the overlying plate by the addition of heat that was ultimately linked with peak magmatism and the significant growth and chemical differentiation of juvenile crust in southern Tibet during the Late Cretaceous (105-76 Ma). In addition, the petrogenesis of the Langxian adakite-like two-mica granite indicates that the southern Tibetan crust was still of normal thickness prior to the emplacement of these intrusions at ca. 76 Ma. This probably means that large parts of southern Tibet were not very highly elevated prior to the Indian-Asian collision.

Zheng, Yuan-chuan; Hou, Zeng-qian; Gong, Ying-li; Liang, Wei; Sun, Qing-Zhong; Zhang, Song; Fu, Qiang; Huang, Ke-Xian; Li, Qiu-Yun; Li, Wei

2014-03-01

234

The formation of deep basins in High Arctic from metamorphism in continental crust  

Science.gov (United States)

In the East Barents and North Chukchi basins, 16-20 km deep, the crystalline crust is attenuated to 12-18 km (reference profiles 2-AR, 4-AR and 5-AR). P-wave velocities and densities in this layer are characteristic of the oceanic crust. However, the subsidence history in the basins is quite different from that typical of the oceanic crust. In both basins the subsidence continued for several hundred million years and one half of the deposits or more was formed long after the start of the subsidence when cooling of the oceanic plate would be already over. Moreover, the basins are 4-5 km deeper than it could be expected according to the thickness of the crystalline crust above the Moho boundary. In the absence of large free-air gravity anomalies, joint analysis of the gravity and seismic data indicates the existence under the Moho of thick layers of high-density and high-velocity eclogites. As can be seen in high resolution seismic profiles, the intensity of crustal stretching did not exceed 10% in the basins, and their formation can be predominantly attributed to a high-grade metamorphism in the mafic lower part of continental crust. At some episodes, strong increase in the rate of subsidence occurred in the basins. This indicates acceleration of metamorphism catalyzed by infiltration of mantle fluids. A set of the above features, abnormally large depth, long subsidence history with its acceleration at the late stages, and episodes of pronounced acceleration of the subsidence represent characteristic features of some other large hydrocarbon basins, e.g., of the North and South Caspian basins. These features can be used for prospecting new prolific provinces on the Arctic shelf. The Lomonosov ridge, Mendeleev high and the Makarov basin pertain to the same structural type. In the Oligocene they underwent erosion near to sea level with the formation of pronounced unconformity. Then at the end of Oligocene deep-water basins were formed in these regions. Rapid crustal subsidence after a long period of relative stability is atypical of oceanic crust. It can be produced either by intense stretching of continental crust or by a density increase due to metamorphism in this layer. Recent seismic reflection profiles demonstrate only minor stretching of the crystalline basement in the regions. Then metamorphism should be the main cause of formation of deep basins in these regions. This can explain attenuation of crystalline crust and an increase in P-wave velocities in this layer that are typical for many deep basins formed due to intense metamorphism in continental crust.

Artyushkov, Eugene; Belyaev, Igor; Chekhovich, Peter; Petrov, Eugene; Poselov, Viktor

2014-05-01

235

Segmentation of mid-ocean ridges  

Science.gov (United States)

Studies of mid-ocean ridges in the Pacific and Atlantic oceans show that the volcanism that forms the oceanic crust along the spreading-plate boundaries is concentrated at regular intervals related to spreading rate. This observation and a new calculation for a Rayleigh-Taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities. ?? 1985 Nature Publishing Group.

Schouten, H.; Klitgord, K.D.; Whitehead, J.A.

1985-01-01

236

On the origin of granites: a contribution to earth crust research from an isotopic point of view  

International Nuclear Information System (INIS)

Using isotope and element geochemical knowledge the limits of igneous formation are discussed. We offer two explanations which account for extremely different kinds of gineous granite formation: by anatexis of continental crust or by anatexis of subducted (or subfluenced) oceanic crust in the region of the upper mantle. Most of the granites might be mixed forms. Isotope and elemental studies concerning the younger granites of the Erzgebirge (G.D.R.) show a tendency to favour subcrustal formation of their magma source. (author)

237

The Dupal isotopic anomaly in the southern Paleo-Asian Ocean: Nd-Pb isotope evidence from ophiolites in Northwest China  

Science.gov (United States)

It has been suggested that the Dupal isotopic anomaly in the mantle can be traced in the Paleozoic ophiolites from the Neo- and Paleo-Tethyan Ocean (275-350 Ma). The Karamaili ophiolite (KO) and Dalabute ophiolite (DO) in the eastern and western corners, respectively, of the Junggar basin in NW China represent remnants of the relatively older (> 350 Ma) Paleo-Asian Ocean (PAO) crust. Thus, these ophiolites can provide additional constraints on the long-term composition and evolution of the Paleozoic suboceanic mantle. We present new major-trace element and Sr, Nd and high-precision Pb isotope data for the basalts, gabbros and a plagioclase separate from the KO and DO. Our results indicate that the PAO crust indeed has a Dupal-like isotopic signature. In detail, all samples have relatively low ?Nd(t) and high 208Pb/204Pb(t) for given 206Pb/204Pb(t) ratios (i.e., positive ?8/4 values), similar to the Dupal isotopic characteristics of Indian Ocean mid-ocean ridge basalts (MORB). The trace element signature of DO mafic rocks is similar to that of normal- and enriched-MORB whereas that of the KO is transitional between MORB and arc basalt. Therefore, the DO mantle domain reflects the PAO asthenosphere and the KO domain additionally shows the influence of the subduction process. Geochemical modeling using Th/Nd as well as Nd and Pb isotopic ratios indicates that up to 2% subduction component had been added to a depleted Indian MORB-type mantle to produce the bulk of KO rocks. The subduction component in the KO rocks consisted of variable proportions of ? 1% partial melt of unradiogenic sediment similar to modern Izu-Bonin trench sediment and hydrous fluid dehydrated from the subducted altered oceanic crust. The Devonian asthenospheric mantle beneath the southern PAO is isotopically heterogeneous, but lends support to the idea that the Dupal isotopic anomaly existed prior to the opening of the Indian Ocean. Finally, plate tectonic reconstruction indicates that the anomaly was present in the Neo- and Paleo-Tethyan oceans in the southern hemisphere and in the southern part of PAO in the northern hemisphere during the late Paleozoic.

Liu, Xijun; Xu, Jifeng; Castillo, Paterno R.; Xiao, Wenjiao; Shi, Yu; Feng, Zuohai; Guo, Lin

2014-02-01

238

MoMA: SOUNDINGS  

Science.gov (United States)

Soundings is MoMA's first major exhibition of sound art, presenting work by 16 contemporary artists, all born in the 1960s, 70s, and 80s, who work with sound. The artists include: Luke Fowler, Toshiya Tsunoda, Marco Fusinato, Richard Garet, Florian Hecker, Christine Sun Kim, Jacob Kirkegaard, Haroon Mirza, Carsten Nicolai, Camille Norment, Tristan Perich, Susan Philipsz, Sergei Tcherepnin, Hong-Kai Wang, Jana Winderen, and Stephen Vitiello. Probably the best way to view the online exhibition is to skip right to the Artists section (URL listed above), where there is a list of linked artists names. Following the links will lead visitors to images of each artist's works, biographical information, and of course, sound. For example, look at and listen to Susan Philipsz's Study of Strings, 2012, an installation at Kassel Hauptbahnhof. This piece is a reinterpretation of a 1943 composition by Pavel Haas (Czech, 1899âÂÂ1944), who composed the score while imprisoned at the Theresienstadt concentration camp. A performance was filmed in 1944 for a Nazi propaganda film. Philipsz's piece omits the instruments - and musicians - who were executed at the camp.

2013-01-01

239

Discovery and utilization of sorghum genes (Ma5/Ma6)  

Science.gov (United States)

Methods and composition for the production of non-flowering or late flowering sorghum hybrid. For example, in certain aspects methods for use of molecular markers that constitute the Ma5/Ma6 pathway to modulate photoperiod sensitivity are described. The invention allows the production of plants having improved productivity and biomass generation.

Mullet, John E; Rooney, William L; Klein, Patricia E; Morishige, Daryl; Murphy, Rebecca; Brady, Jeff A

2012-11-13

240

Two contemporaneous magma series on Mayotte Island, Comores Archipelago, Indian Ocean  

Science.gov (United States)

The Comores archipelago is comprised of four islands all of which are characterised by alkaline lavas. On Mayotte, two clearly separate magma series can be defined. The origin of such contemporaneous but compositionally different series in several oceanic islands is still a subject of a debate. The mineralogy and geographically locations allow to identify three different lava groups. (1) The north lava group composed of relatively homogeneous alkaline basalts. (2) The north-east group comprised of tephrites to trachy-phonolites series and (3) the south lava group is made up of a nephelinite to phonolite series. Ol+cpx+plag are the main phenocryst phases in the north and north-east lavas, whereas the south lava series have ol+cpx+nepheline as phenocrysts. Composition of clinopyroxene is variable with diopside compositions in the north alkaline basalts and tephrites, and diopside to aegerine compositions in the south nephelinite lavas. A regular enrichment of Na content in plagioclases phenocrysts and groundmass crystals from the lavas of the north group to those from the north-east is observed. In contrast, alkali-feldspars are present in the south lavas. The crystallisation of clinopyroxene instead of plagioclase after olivine fractionation in the north lavas suggests that fractionation occurred at pressure between 0.4 and 0.9 GPa, which is consistent with the presence of Na-rich cpx-cores in all lava groups. The major element composition of lavas from Mayotte allow to define two distinct magma series: a moderately undersaturated and a highly undersaturated series. The moderately undersaturated series is composed of the north alkali basalt and the north-east tephrite lavas, whereas the south nephlinites represent the highly undersaturated series. Compilation of published age determinations and new Ar/Ar datings suggest that the north lavas erupted from 7.7 Ma to 4.4 Ma, followed by the north-east lavas erupted from 4.7 Ma to 1.4 Ma. The south lavas erupted contemporaneously from 7.7 Ma to 2.7 Ma. Basic volcanic activity resumed in the North between 2.9 Ma and 1.2 Ma, and from 2 Ma to 1.5 Ma in the South. Taken together, these preliminary petrological and geochemical results suggest that Mayotte island was constructed by two volcanoes. These volcanoes were active at the same time producing two distinct magma series. Migration of the activity of the northern volcano to the east occurred with emission of increasingly differentiated lavas with time. Depth of crystallisation can be evaluated at more than 15 km for alkaline basalt and tephrite lavas, which corresponds to the mantle-crust interface.

Debeuf, D.; Bachèlery, P.; Sigmarsson, O.

2003-04-01

 
 
 
 
241

Mantle heterogeneity under spreading zones of polar regions of the Atlantic Ocean: sources and formation  

Science.gov (United States)

A number of provinces with prevailing distribution of enriched rift basalts are specified within spreading zones of Indo-Atlantic segment of the World Ocean. The main reason of EMORB-type melts formation is determined by source heterogeneity which is resulted in numerous causes: recycling of old oceanic crust, hotspots within immediate proximity to rift zone, formation of metasomtizated mantle at the early stage of ocean opening which is involved in melting process later on. The spatial distribution of enriched tholeiites within Polar Atlantic is confined by Knipovich, Kolbeinsey and Gakkel ridges. The Knipovich ridge spreading zone formation coincides in time with magmatism appearances in adjacent continental regions. Comparative studying of Neogene and Quaternary magmatism of the Svalbard Island and modern magmatism of the Knipovich ridge reveals pyroxenite mantle participation in the melting process. The main source for Neogene magmas of the Svalbard Island was olivine-free pyroxenite with high 87Sr/86Sr and lower 143Nd/144Nd ratios, which could be a result of interaction of recycled substance of old oceanic crust and low continental crust with mantle peridotite. Due to its preferential fusibility this pyroxenite could be the source for substantial magmas volume under the rigid continental lithosphere that subsequently could have caused its disintegration. With successive rejuvenation of Svaldbard and Knipovich ridge magmatism (from Neogene till nowadays) for its mantle sources there has been traced the decreasing of pyroxenite component share at the expense of increasing of peridotite share accompanied by regular change of Sr and Nd isotope composition of these sources. The old Antarctic continent played a pivot role in the South Ocean formation, geodynamics and magmatism of trap formations and rift zones. The area of Karoo-Maud plume distribution at the early stages (about 180 - 170 Ma) included the southeastern part of Africa and the west of East Antarctic and nowadays it occupies the area of Bouvet hotspot modern location. Development of Karoo-Maud plume caused the formation of considerable mantle heterogeneity and contributed to disintegration of continental blocks within the forming South Ocean. Magmatism of the formed spreading basins of the western Antarctic (Powell and Bransfield) is characterized by greater range of enrichment and evidence to possible melting of pyroxenites which represented the fragments of low parts of continental lithosphere involved into the melting process at mantle asthenospheric upwelling in spreading zones. This component is close by its isotope characteristics to a component revealed within the western edge of Southwest Indian Ridge near the Bouvet triple junction and is represented by a mixture of sources like HIMU and EM-2.

Sushchevskaya, N. M.; Belyatsky, B. V.; Dubinin, E. P.

2012-04-01

242

Protracted construction of gabbroic crust at a slow-spreading ridge: Constraints from SHRIMP Pb/U zircon ages in IODP hole 1309D, Atlantis Massif, MAR (30°N)  

Science.gov (United States)

We present U-Pb zircon ages for 18 samples from lower oceanic crust recovered by IODP Hole 1309D in the footwall to an oceanic detachment fault. These samples are evolved oxide gabbros and felsic dikes from between 40-1415 meters below sea floor (mbsf). Ages range from 1.08±0.07 Ma to 1.28±0.05 Ma with errors as low as 1.6%, and reveal a protracted history of accretion. U-Pb zircon dating was performed using the U.S.G.S.-Stanford SHRIMP-RG. Seven ages from both oxide gabbros and felsic dikes above 570 mbsf give a weighted mean of 1.17±0.02 Ma (MSWD=1.03). Oxide gabbros between 620-1040 mbsf are consistently older and give a weighted mean age of 1.24±0.03 (MSWD=1.4). Two felsic dikes within this interval (867 and 1040 mbsf) give younger ages of 1.14±0.05 Ma. In the deepest section (below ~1040 mbsf) oxide gabbros give varied ages of 1.27±0.05 Ma (1175 mbsf) and 1.14±0.04 (1240 and 1327 mbsf). The deepest sample is a felsic dike intruding gabbro at 1415 mbsf, and has an age of 1.28±0.05 Ma. Abrupt changes in the age of the oxide gabbros (at ~600 and below 1040 mbsf) coincide with petrologic and geochemical variations, and indicate the presence of distinct intrusive bodies, which we interpret as sills. The overall weighted mean age of the crust penetrated by 1309D is 1.20±0.02 Ma (MSWD=7.1). However, the range of zircon crystallization ages indicates that this section of crust was constructed over at least ~100-200kyr. This is a minimum estimate because dated samples intrude more primitive olivine-rich rocks and are cut by later diabase. Shallow paleomagnetic remanence inclinations of -38° to -31.5° from below 180 mbsf in Hole 1309D, along with generally steep magmatic fabrics (~40-60°), imply up to 55° of counter- clockwise rotation associated with detachment faulting. The mean age of the hole, together with additional Pb/U zircon ages determined from dive and dredge samples from the southern wall of Atlantis Massif constrain the slip rate of the detachment fault to be between 12 and 15.8 mm/yr. The higher rate implies asymmetric spreading and is consistent with the gabbros being emplaced at depths of 5-7 km below the ridge axis and transported to the seafloor along a curved detachment fault. A significant result is the observation of increasingly older ages with depth in the hole. This result is not consistent with a model whereby melt intrudes at a constant depth beneath the detachment fault, because such a model predicts younging downwards. The data are however consistent with a multiple sill model whereby sills intrude at random depths.

Grimes, C. B.; John, B. E.; Wooden, J. L.

2007-12-01

243

Protracted construction of gabbroic crust at a slow-spreading ridge: Constraints from SHRIMP Pb/U zircon ages in IODP hole 1309D, Atlantis Massif, MAR (30°N)  

Science.gov (United States)

We present U-Pb zircon ages for 18 samples from lower oceanic crust recovered by IODP Hole 1309D in the footwall to an oceanic detachment fault. These samples are evolved oxide gabbros and felsic dikes from between 40-1415 meters below sea floor (mbsf). Ages range from 1.08±0.07 Ma to 1.28±0.05 Ma with errors as low as 1.6%, and reveal a protracted history of accretion. U-Pb zircon dating was performed using the U.S.G.S.-Stanford SHRIMP-RG. Seven ages from both oxide gabbros and felsic dikes above 570 mbsf give a weighted mean of 1.17±0.02 Ma (MSWD=1.03). Oxide gabbros between 620-1040 mbsf are consistently older and give a weighted mean age of 1.24±0.03 (MSWD=1.4). Two felsic dikes within this interval (867 and 1040 mbsf) give younger ages of 1.14±0.05 Ma. In the deepest section (below ~1040 mbsf) oxide gabbros give varied ages of 1.27±0.05 Ma (1175 mbsf) and 1.14±0.04 (1240 and 1327 mbsf). The deepest sample is a felsic dike intruding gabbro at 1415 mbsf, and has an age of 1.28±0.05 Ma. Abrupt changes in the age of the oxide gabbros (at ~600 and below 1040 mbsf) coincide with petrologic and geochemical variations, and indicate the presence of distinct intrusive bodies, which we interpret as sills. The overall weighted mean age of the crust penetrated by 1309D is 1.20±0.02 Ma (MSWD=7.1). However, the range of zircon crystallization ages indicates that this section of crust was constructed over at least ~100-200kyr. This is a minimum estimate because dated samples intrude more primitive olivine-rich rocks and are cut by later diabase. Shallow paleomagnetic remanence inclinations of -38° to -31.5° from below 180 mbsf in Hole 1309D, along with generally steep magmatic fabrics (~40-60°), imply up to 55° of counter- clockwise rotation associated with detachment faulting. The mean age of the hole, together with additional Pb/U zircon ages determined from dive and dredge samples from the southern wall of Atlantis Massif constrain the slip rate of the detachment fault to be between 12 and 15.8 mm/yr. The higher rate implies asymmetric spreading and is consistent with the gabbros being emplaced at depths of 5-7 km below the ridge axis and transported to the seafloor along a curved detachment fault. A significant result is the observation of increasingly older ages with depth in the hole. This result is not consistent with a model whereby melt intrudes at a constant depth beneath the detachment fault, because such a model predicts younging downwards. The data are however consistent with a multiple sill model whereby sills intrude at random depths.

Grimes, C. B.; John, B. E.; Wooden, J. L.

2004-12-01

244

Early Formation of Terrestrial Crust  

Science.gov (United States)

Early (?4.5 Ga) Formation of Terrestrial Crust T.M. Harrison1, A.K. Schmitt1, M.T. McCulloch2, and O.M. Lovera1 1Department of Earth and Space Sciences and IGPP, UCLA, Los Angeles, CA 90095, USA; 2Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 2601 AUSTRALIA Large deviations in ?repsilonHf(T) from bulk silicate Earth seen in >4 Ga detrital zircons from Jack Hills, Western Australia, have been interpreted as reflecting a major differentiation of the silicate Earth at ca. 4.4 to 4.5 Ga. We have expanded the characterization of 176Hf/177Hf (Hf) in Hadean zircons by acquiring a further 116 laser ablation Lu-Hf measurements on 87 grains with ion microprobe 207Pb/206Pb ages up to 4.36 Ga. Most measurements employed concurrent Lu-Hf and 207Pb/206Pb analyses, permitting assessment of the use of ion microprobe data to characterize the age of the volumetrically larger domain sampled by laser drilling. Our new results confirm and extend the earlier observation of significant negative deviations in ?repsilonHf(T) throughout the Hadean, although no positive ?repsilonHf(T) values were documented in this study. These data yields an essentially uniform spectrum of single-stage model ages between 4.54 and 4.20 Ga for extraction of the zircons' protoliths from a chondritic reservoir. We derived the full error propagation expression for a parameter, ?repsilono, which measures the difference of a sample from solar system initial (Hf) (Hfo), and from this conclude that data plotting close to (Hfo), are statistically meaningful and consistent with silicate differentiation at 4.540±0.006 Ga. ?18O and Ti thermometry for these Hadean zircons show little obvious correlation with initial (Hf), consistent with their derivation through fusion of a broad suite of crustal rock types under near water-saturated conditions. Together with the inclusion assemblage and other isotopic and trace element data obtained from these ancient zircons, our results indicate essentially continuous derivation of crust from the mantle from 4.5 to 4.2 Ga, concurrent with recycling into the mantle and internal crustal re-working. These results represent further evidence that by 4.35 Ga, portions of the crust had taken on continental characteristics.

Harrison, T. M.; Schmitt, A. K.; McCulloch, M. T.; Lovera, O. M.

2007-12-01

245

Isotopic and geochemical constraints on Neoproterozoic crust formation in the Wami River area, eastern Tanzania  

Science.gov (United States)

Neodymium and Sr isotopic data, coupled with major and trace element geochemical data from the Wami River granulite complex of eastern Tanzania, indicate that the granulite protolith was extracted from the mantle during Neoproterozoic times. Crustal growth is interpreted to have occurred through convergent margin magmatism on the eastern flanks of west Gondwana between ˜840 and 700 Ma. Late Archæan crust, now represented by the surrounding amphibolite-facies gneisses, formed the foreland to the Neoproterozoic continental margin. Subsequently, crustal thickening leading to granulite-facie metamorphism of the magmatic arc packages occurred at about 640 Ma. The age and tectonic setting of Neoproterozoic crust generation in the Mozambique Belt is similar to that in the Arabian-Nubian Shield. This suggests that, despite their different metamorphic grades, there is a fundamental geodynamic link between the two sectors of the East African Orogen.

Maboko, M. A. H.

2001-07-01

246

Development of oceanic detachment and asymmetric spreading at the Australian-Antarctic Discordance  

Science.gov (United States)

The largest known oceanic detachment terrains occur in Segment B3 of the Australian-Antarctic Discordance (AAD). Using newly collected bathymetry, magnetic, and gravity data, we show that Segment B3 is divided into two contrasting second-order segments. The western subsegment, B3W, is characterized by well-ordered, ridge parallel abyssal hills and low mantle Bouguer gravity anomalies. The eastern subsegment, B3E, displays rough, chaotic morphology and includes several megamullions characterized by high mantle Bouguer gravity anomaly values. The crust is estimated to be thinner by a maximum of 3 km in southern B3E. The combination of chaotic morphology with thinner crust supports the idea that the megamullions are exposed footwalls of oceanic detachments. Megamullion terrains are characterized by higher magnetization than adjacent terrains, most likely as a result of serpentinization of peridotite exposed at the detachment surfaces. Detachment surfaces constitute up to 70% of the total area of both ridge flanks younger than 2 Ma in B3E, indicating that oceanic detachments have played a major role in its development. Spreading in B3E has been extremely asymmetric, with higher apparent rates associated with the large detachment surfaces, where up to 75% of the total extension occurred. Similar asymmetric spreading on oceanic detachments is also recognized in Segment B4, suggesting that this is the dominant mode of extension associated with cold mantle and low magma supply in this deepest part of the AAD, where it is confined to a mere 100-km-long section of the AAD spreading axis.

Okino, Kyoko; Matsuda, Kohei; Christie, David M.; Nogi, Yoshifumi; Koizumi, Kin-Ichiro

2004-12-01

247

The petrogenesis of oceanic kimberlites and included mantle megacrysts: The Malaitan alnoite  

Science.gov (United States)

The study of unambiguous suboceanic mantle was facilitated by the occurrence of anomalous kimberlite-type intrusives on Malaita in the Solomon Islands. The pseudo-kimberlites were termed alnoites, and are basically mica lamprophyres with melilite in the ground mass. Alnoitic magmas were explosively intruded into the Ontong Java Plateau (OJP) 34 Ma ago. The OJP is a vastly overthickened portion of the Pacific plate which now abuts the Indo-Australian plate. Malaita is considered to be the obducted leading edge of the OJP. Initial diapiric upwelling beneath the OJP produced the proto-alnoite magma. After impingement on the rigid lithosphere, megacrysts fractionation occurred, with augites precipitating first, representing the parental magma. Sea water-altered oceanic crust, which underplated the OJP, was assimilated by the proto-alnoite magma during megacrysts fractionation (AFC).

Neal, Clive R.

1988-01-01

248

Paleozoic to Triassic ocean opening and closure preserved in Central Iran: Constraints from the geochemistry of meta-igneous rocks of the Anarak area  

Science.gov (United States)

The Anarak area belongs to an ophiolitic belt along the northern border of the Central-East Iranian Microcontinent, and is thought to contain fragments of the former Paleotethys and Neotethys oceans. A wide range of meta-igneous rocks from the Late Paleozoic to Triassic Anarak Metamorphic Complex (AMC) and nearby Meraji area have been studied to constrain the origins and modes of emplacement of oceanic remnants in Central Iran. Our samples occur as layers and lenses embedded in extensive sequences of deformed meta-sediments and smaller bodies of serpentinized ultramafic rocks. Petrographical and geochemical data combined with field and satellite observations allow recognition of seven types of meta-igneous rocks preserved from low grade to blueschist facies conditions. Their origins based on relative abundances of immobile trace elements include subduction zone, mid-ocean ridge, ocean intraplate, and continental rift settings. These data and existing geochronological constraints show the AMC formed an accretionary complex formed/exhumed incrementally during the Carboniferous, Permo-triassic and Triassic. Igneous rocks from Meraji formed in the Early Devonian due to opening of the Paleotethys, and belong to a rift sequence extending over 300 km along the edge of the Central-East Iranian Microcontinent. The AMC and nearby rock associations record the evolution of the Paleotethys during a complete Wilson Cycle between ca. 450 and 225 Ma, with implications for: (1) continental rifting; (2) ocean opening; (3) subduction initiation; (4) ocean intraplate and continued mid-ocean volcanism; (5) ridge subduction; and (6) final closure of the ocean during continent-continent collision. Alternate interpretations of the Anarak metabasites are possible, but require radical departures from the widely accepted model for tectonic evolution of the Paleotethys, with the existence of Paleotethyan backarc basin(s) and Permian or earlier collision of continental blocks in Central Iran. In any case, our results show accretionary complexes preserved along suture zones contain an important record of the evolution of oceanic crust from ancient ocean basins.

Buchs, David M.; Bagheri, Sasan; Martin, Laure; Hermann, Joerg; Arculus, Richard

2013-07-01

249

Thermalization time of hot neutron star crust  

Energy Technology Data Exchange (ETDEWEB)

We discuss the thermalization process of the neutron stars crust described by solving the heat transport equation with a microscopic input for the specific heat of baryonic matter. The heat equation is solved with initial conditions specific to a rapid cooling of the core. To calculate the specific heat of inner crust baryonic matter, i.e., nuclear clusters and unbound neutrons, we use the quasiparticle spectrum provided by the Hartree-Fock-Bogoliubov approach at finite temperature. In this framework we analyze the dependence of the crust thermalization on pairing properties and on cluster structure of inner crust matter. It is shown that the pairing correlations reduce the crust thermalization time by a large fraction. The calculations show also that the nuclear clusters have a non-negligible influence on the time evolution of the surface temperature of the neutron star.

Margueron, J [Institut de Physique Nucleaire, IN2P3-CNRS and Universie Paris-Sud, F-91406 Orsay CEDEX (France); Fortin, M [Ecole Normale Superieure, Departement de Physique, 24 rue Lhomond, 75005 Paris (France); Grill, F [Dipartimento di Fisica, Universita degli Studi di Milano, Via Celoria 16, 20133 Milan (Italy); Page, D [Departamento de Astrofisica Teorica, Instituto de AstronomIa, Universidad Nacional Autonoma de Mexico, 04360 Mexico D.F. (Mexico); Sandulescu, N, E-mail: jerome.margueron@ipno.in2p3.fr [National Institute of Physics and Nuclear Engineering, 76900, Bucharest (Romania)

2011-09-16

250

The Siquisique basalts and gabbros, Los Algodones, Venezuela: late Cretaceous oceanic plateau formed within the proto-Caribbean plate?  

Science.gov (United States)

Basalts and gabbros, exposed near Siquisique, Venezuela have previously been interpreted as Jurassic mid-ocean ridge basalts, on the basis of an ammonite found in nearby, but not obviously intercalated, sediments (Bartok, 1985). This, combined with their current tectonic position, well within the continent, and because they accreted before the Cretaceous ‘Great Arc’ of the Caribbean, has led to the Siquisique igneous rocks being widely regarded as Jurassic ‘normal’ mid-ocean ridge basalts and gabbros formed as North and South America rifted apart. We present new geochemical and chronological data which shows that the Siquisique igneous rocks are 95-90Ma and have a chemistry which is more consistent with derivation from a deep mantle plume, than a mid-ocean ridge. It is clear that these basalts represent part of the original ocean floor of the Caribbean, which formed before the tectonic emplacement of the present-day Caribbean from the Pacific. Chemically similar basalts and gabbros at El Copey on Araya Peninsula and Sans Souci in northern Trinidad also accreted to the continental margin of South America before the ‘Great Arc’ of the Caribbean and may well be part of the same intra-Caribbean ‘plume event’. These exposures all indicate that the oceanic crust of the proto-Caribbean, was likely to have consisted (at least in part) of thickened oceanic crust formed by melting of a hot-mantle plume. Although the Siquisique rocks formed at a similar time to the Caribbean-Colombian oceanic plateau they were not derived from the same mantle plume. This supports previous suggestions (Kerr & Tarney, 2005; Snow et al. 2005) that the period around ~90Ma (like that around 120Ma) was marked by a significant upsurge in global plume-related magmatic activity. This activity is likely to have contributed significantly to the major worldwide oceanic anoxia event (OAE2) around the Cenomanian-Turonian boundary (93.4Ma)(Kerr, 1998; Snow et al. 2005). Significantly, this discovery requires a revision of our current understanding of Caribbean plate tectonic evolution. References Bartok, P.E., et al. 1985. The Siquisique Ophiolites, Northern Lara State, Venezuela - a discussion on their Middle Jurassic Ammonites and Tectonic Implications. GSA Bulletin 96, 1050-1055. Kerr, A.C., 1998. Oceanic plateau formation: A cause of mass extinction and black shale deposition around the Cenomanian-Turonian boundary. J Geol Soc London 155, 619-626. Kerr, A.C., Tarney, J., 2005. Tectonic evolution of the Caribbean and northwestern South America: The case for accretion of two Late Cretaceous oceanic plateaus. Geology 33, 269-272. Snow, L.G. et al. 2005. Trace element abundances in the Rock Canyon Anticline, Pueblo, Colorado, marine sedimentary section and their relationship to Caribbean plateau construction and oxygen anoxic event 2. Paleoceanography 20, doi. 10.1029/2004PA001093.

Kerr, A. C.; Neill, I.; Urbani, F.; Spikings, R.; Barry, T.; Tarney, J.

2009-12-01

251

Rb-Sr and Sm-Nd isotope systematics and geochemical studies on metavolcanic rocks from Peddavura greenstone belt: Evidence for presence of Mesoarchean continental crust in easternmost part of Dharwar Craton, India  

Science.gov (United States)

Linear, north-south trending Peddavura greenstone belt occurs in easternmost part of the Dharwar Craton. It consists of pillowed basalts, basaltic andesites, andesites (BBA) and rhyolites interlayered with ferruginous chert that were formed under submarine condition. Rhyolites were divided into type-I and II based on their REE abundances and HREE fractionation. Rb-Sr and Sm-Nd isotope studies were carried out on the rock types to understand the evolution of the Dharwar Craton. Due to source heterogeneity Sm-Nd isotope system has not yielded any precise age. Rb-Sr whole-rock isochron age of 2551 ± 19 (MSWD = 1.16) Ma for BBA group could represent time of seafloor metamorphism after the formation of basaltic rocks. Magmas representing BBA group of samples do not show evidence for crustal contamination while magmas representing type-II rhyolites had undergone variable extents of assimilation of Mesoarchean continental crust (>3.3 Ga) as evident from their initial ? Nd isotope values. Trace element and Nd isotope characteristics of type I rhyolites are consistent with model of generation of their magmas by partial melting of mixed sources consisting of basalt and oceanic sediments with continental crustal components. Thus this study shows evidence for presence of Mesoarchean continental crust in Peddavura area in eastern part of Dharwar Craton.

Rajamanickam, M.; Balakrishnan, S.; Bhutani, R.

2014-06-01

252

Ocean Planet: Ocean Market  

Science.gov (United States)

Unit from Smithsonian multidisciplinary ocean curriculum. Lesson plan focuses on foods, materials and medicines that comes form marine life, how these resources are harvested and processed and the impacts of fisheries. Students identify and classify consumer goods from the ocean and calculate their cost. Unit includes: background essay; teacher instructions; forms for student activity; discussion questions; all online in PDF format. Resources include online version of Smithsonian Ocean Planet exhibition.

253

Sources of continental crust: neodymium isotope evidence from the sierra nevada and peninsular ranges.  

Science.gov (United States)

Granitic rocks from batholiths of the Sierra Nevada and Peninsular Ranges exhibit initial (143)Nd/(144)Nd ratios that vary over a large range and correlate with (87)Sr/(86)Sr ratios. The data suggest that the batholiths represent mixtures of materials derived from (i) chemically depleted mantle identical to the source of island arcs and (ii) old continental crust, probably sediments or metasediments with a provenance age of approximately 1.6 x 10(9) years. These conclusions are consistent with a model for continental growth whereby new crustal additions are repeatedly extracted from the same limited volume of the upper mantle, which has consequently become depleted in elements that are enriched in the crust. There is little evidence that hydrothermally altered, subducted oceanic crust is a primary source of the magmas. PMID:17821189

Depaolo, D J

1980-08-01

254

Biogenic crust dynamics on sand dunes  

Science.gov (United States)

Sand dunes are often covered by vegetation and biogenic crusts. Despite their significant role in dune stabilization, biogenic crusts have rarely been considered in model studies of dune dynamics. Using a simple model, we study the existence and stability ranges of different dune-cover states along gradients of rainfall and wind power. Two ranges of alternative stable states are identified: fixed crusted dunes and fixed vegetated dunes at low wind power; and fixed vegetated dunes and active dunes at high wind power. These results suggest a crossover between two different forms of desertification.

Kinast, Shai; Meron, Ehud; Yizhaq, Hezi; Ashkenazy, Yosef

2013-02-01

255

Biogenic crust dynamics on sand dunes  

CERN Document Server

Sand dunes are often covered by vegetation and biogenic crusts. Despite their significant role in dune stabilization, biogenic crusts have rarely been considered in studies of dune dynamics. Using a simple model, we study the existence and stability ranges of different dune-cover states along gradients of rainfall and wind power. Two ranges of alternative stable states are identified: fixed crusted dunes and fixed vegetated dunes at low wind power, and fixed vegetated dunes and active dunes at high wind power. These results suggest a cross-over between two different forms of desertification.

Kinast, Shai; Yizhaq, Hezi; Ashkenazy, Yosef

2012-01-01

256

Microphytic crusts: 'topsoil' of the desert  

Science.gov (United States)

Deserts throughout the world are the home of microphytic, or cryptogamic, crusts. These crusts are dominated by cyanobacteria, previously called blue-green algae, and also include lichens, mosses, green algae, microfungi and bacteria. They are critical components of desert ecosystems, significantly modifying the surfaces on which they occur. In the cold deserts of the Colorado Plateau (including parts of Utah, Arizona, Colorado, and New Mexico), these crusts are extraordinarily well-developed, and may represent 70-80% of the living ground cover.

Belnap, Jayne

1990-01-01

257

Compositional diversity of ca. 110 Ma magmatism in the northern Lhasa Terrane, Tibet: Implications for the magmatic origin and crustal growth in a continent-continent collision zone  

Science.gov (United States)

The Yanhu area in the northern Lhasa subterrane exposes diverse rock types including basalts, rhyolites, quartz dioritic porphyries, and associated dioritic enclaves. The basalts and rhyolites occur as a bimodal volcanic suite, and the quartz dioritic porphyries intrude into the older basalts as a small apophysis. In this paper, we report for the first time the zircon LA-ICP-MS U-Pb age and Hf isotopic composition data, whole-rock major and trace element composition data, and Sr-Nd isotopic data from the diverse Early Cretaceous magmatic rocks from Yanhu. The three basalt samples that we dated yielded zircon U-Pb ages of 110 ± 0.7 Ma, 108.9 ± 1.1 Ma, and 111.8 ± 3.2 Ma. The zircons from one quartz diorite porphyry yielded an age of 109.7 ± 0.8 Ma, which is coeval with the dioritic enclave (110.4 ± 1.4 Ma). The basalts show a high-K calc-alkaline signature, enriched in Rb, Th, U, and light rare earth elements (REEs) and depleted in Nb, Ta, Ti, Zr, and Hf. These rocks possess varying whole-rock ?Nd(t) (- 0.6 to + 2.1) and zircon ?Hf(t) (+ 0.6 to + 8.9) values. The rhyolite samples are high-K calc-alkaline and are metaluminous to slightly peraluminous. These rocks yielded whole-rock ?Nd(t) of + 0.1 to + 0.9 and zircon ?Hf(t) of + 5.1 to + 12.4. The quartz dioritic porphyries are characterized by high Al2O3 content (15.9-16.1 wt.%), high Sr (466-556 ppm), low Yb (1.36-1.41 ppm), and low Y (13.8 ppm) abundances. Similar geochemical signatures are also present in the dioritic enclaves, revealing that both the quartz dioritic porphyries and the dioritic enclaves have an affinity with adakitic rocks. Moreover, the host rocks and the enclaves display homogeneous ?Nd(t) (+ 3.5 to + 3.6 and + 3.4 to + 4.0, respectively) and zircon ?Hf(t) values (+ 12.4 to + 16.9 and + 11.5 to + 15.7, respectively). Our geochemical data indicate that the magmatic rocks from Yanhu were derived from the partial melting of distinct source regions, i.e., a heterogeneous metasomatized mantle wedge source for basalts, a juvenile crust source for rhyolites, and a thickened mafic lower crust source that mixed with basaltic magmas for the adakitic rocks (including quartz dioritic porphyries and dioritic enclaves). Compared to typical arc basalts, the basalts from Yanhu are relatively enriched with high field strength elements (HFSEs) (e.g., Zr, Nb), resembling those of within-plate basalts elsewhere. In combination with the presence of a coeval bimodal volcanic rock suite, the ca. 110 Ma magmatism in Yanhu is inferred to have occurred in an extensional setting. Our new data, together with recently published data, enable us to correlate the generation of the compositional diversity of the ca. 110 Ma Yanhu magmatic rocks that formed in an extensional setting to the slab break-off during the southward subduction of the Bangong-Nujiang Ocean lithosphere. The presence of basaltic magmatism and coeval silicic magmatic rocks with positive zircon ?Hf(t) indicate that the extensive magmatism at ca. 110 Ma have contributed significantly to the crustal growth of the northern Lhasa subterrane.

Sui, Qing-Lin; Wang, Qing; Zhu, Di-Cheng; Zhao, Zhi-Dan; Chen, Yue; Santosh, M.; Hu, Zhao-Chu; Yuan, Hong-Lin; Mo, Xuan-Xue

2013-05-01

258

The magma ocean as an impediment to lunar plate tectonics  

Science.gov (United States)

The primary impediment to plate tectonics on the moon was probably the great thickness of its crust and particularly its high crust/lithosphere thickness ratio. This in turn can be attributed to the preponderance of low-density feldspar over all other Al-compatible phases in the lunar interior. During the magma ocean epoch, the moon's crust/lithosphere thickness ratio was at the maximum theoretical value, approximately 1, and it remained high for a long time afterwards. A few large regions of thin crust were produced by basin-scale cratering approximately contemporaneous with the demise of the magma ocean. However, these regions probably also tend to have uncommonly thin lithosphere, since they were directly heated and indirectly enriched in K, Th, and U by the same cratering process. Thus, plate tectonics on the moon in the form of systematic lithosphere subduction was impeded by the magma ocean.

Warren, Paul H.

1993-01-01

259

Nagssugtoqidian mobile belt of West Greenland: A cryptic 1850 Ma suture between two Archaean continents - chemical and isotopic evidence  

Energy Technology Data Exchange (ETDEWEB)

New chemical and isotopic data permit the recognition of a cryptic suture zone between two Archean continental masses within the Nagssugtoqidian mobile belt of West Greenland. This discovery has important implications for Precambrian crustal evolution: suture zones may not always be identifiable from geologic field observations, with the consequence that mobile belts in which undetected sutures exist may be mis-identified as ensialic, and thought to require special non-plate tectonic models to account for their development. The Nagssugtoqidian belt consists mainly of Archaean gneisses reworked during the Proterozoic, with metamorphic grade and degree of isotopic disturbance increasing towards the center of the belt. At the centre of the belt the Nagssugtoqidian includes metasediments and calc-alkaline volcanic and plutonic rocks of Proterozoic age, almost always strongly deformed and metamorphosed. From isotopic evidence (Sr/sub i/ ca. 0.703; model ..mu../sub 1/ values ca. 8.0; initial epsilon/sub Nd/ ca. 0) it is clear that the Proterozoic igneous rocks do not include any significant contributions derived from the Archaean crust, and the chemistry of rocks, together with the isotope data, suggests that they were formed at a destructive plate margin. The Proterozoic rocks are found in a narrow zone (up to 30 km wide) between the Archaean gneisses to the north and south of Nordre Stroemfjord, and are interpreted as reflecting the existence of a suture between two Archaean continental blocks. Zircon U-Pb data and other isotope evidence show that subduction started before ca. 1920 Ma ago, and lasted until ca. 1850 Ma when collision occurred, with consequent crustal thickening, high-grade metamorphism and local anatexis. Given the time-span for the operation of subduction, the existence of a wide Nagssugtoqidian ocean can be inferred, even for slow rates of plate motion. (orig./SHOE).

Kalsbeek, F.; Pidgeon, R.T.; Taylor, P.N.

1987-10-01

260

Nagssugtoqidian mobile belt of West Greenland: A cryptic 1850 Ma suture between two Archaean continents - chemical and isotopic evidence  

International Nuclear Information System (INIS)

New chemical and isotopic data permit the recognition of a cryptic suture zone between two Archean continental masses within the Nagssugtoqidian mobile belt of West Greenland. This discovery has important implications for Precambrian crustal evolution: suture zones may not always be identifiable from geologic field observations, with the consequence that mobile belts in which undetected sutures exist may be mis-identified as ensialic, and thought to require special non-plate tectonic models to account for their development. The Nagssugtoqidian belt consists mainly of Archaean gneisses reworked during the Proterozoic, with metamorphic grade and degree of isotopic disturbance increasing towards the center of the belt. At the centre of the belt the Nagssugtoqidian includes metasediments and calc-alkaline volcanic and plutonic rocks of Proterozoic age, almost always strongly deformed and metamorphosed. From isotopic evidence (Sri ca. 0.703; model ?1 values ca. 8.0; initial ?Nd ca. 0) it is clear that the Proterozoic igneous rocks do not include any significant contributions derived from the Archaean crust, and the chemistry of rocks, together with the isotope data, suggests that they were formed at a destructive plate margin. The Proterozoic rocks are found in a narrow zone (up to 30 km wide) between the Archaean gneisses to the north and south of Nordre Stroemfjord, and are interpreted as reflecting the existence of a suture between two Archaean continental blocks. Zircon U-Pb data and other isotope evidence show that subduction started before ca. 1920 Ma ago, and lasted until ca. 1850 Ma when collision occurred, with consequent crustal thickening, high-grade metamorphism and local anatexis. Given the time-span for the operation of subduction, the existence of a wide Nagssugtoqidian ocean can be inferred, even for slow rates of plate motion. (orig./SHOE)

 
 
 
 
261

Do Fault-Controlled Hydrothermal Systems Control the Thermal Evolution of the Lower Crust at Fast-Spreading Ridges?  

Science.gov (United States)

Some geophysical data (e.g. seismic, compliance, bathymetry) suggest that the lower oceanic crust loses heat far more rapidly in the near-axis region that is predicted by conductive cooling models. Hydrothermal circulation through the lower oceanic crust provides one potential mechanism to cool the lower crust in the near axis region. However, there is scant evidence in samples of the lower oceanic crust for extensive pervasive fluid flow occurring over a wide temperature range. This suggests that any large-volume fluid flow must be channelised, most likely in fault-zones. Observations in the Oman ophiolite demonstrate that faults in the plutonic complex can form at high-temperatures. The Sr-isotopic composition of rocks within these fault zones are as elevated as those of the sheeted dike complex (~0.7045) requiring large fluid fluxes (Coogan et al., 2006). Thermal modeling suggests that fluid flow in faults in the lower crust could lead to much more heterogeneous cooling than if the permeability is constant. If fluid flow in the lower crust is focused in faults, with heat conducted through the wall-rock to the fluid in a fault, then the separation of faults and how far from the axis they form are critical parameters in controlling the impact of fluid flow on the axial thermal structure. The best estimate of the separation of lower crustal faults in the Oman ophiolite is approximately 1km but in the Troodos ophiolite, where this is better constrained by seismic data, it is approximately 7 km (Mackenzie et al., 2006). We currently have little data to support the existence of deeply penetrating faults in modern lower oceanic crust close to the ridge axis. Future off-axis micro-seismic surveys, and geological observations of lower crust exposed at tectonic windows, provide perhaps our best opportunities to better understand if such faults exist and, if so, what role they play in controlling the hydrology of the lower oceanic crust. References: Coogan et al, 2006, Am. J. Sci. v306, p389; Mackenzie et al, 2006, Geophys. J. Int. v167, p1385.

Coogan, L. A.

2007-12-01

262

The breaking strain of neutron star crust  

Energy Technology Data Exchange (ETDEWEB)

Mountains on rapidly rotating neutron stars efficiently radiate gravitational waves. The maximum possible size of these mountains depends on the breaking strain of neutron star crust. With multimillion ion molecular dynamics simulations of Coulomb solids representing the crust, we show that the breaking strain of pure single crystals is very large and that impurities, defects, and grain boundaries only modestly reduce the breaking strain to around 0.1. Due to the collective behavior of the ions during failure found in our simulations, the neutron star crust is likely very strong and can support mountains large enough so that their gTavitational wave radiation could limit the spin periods of some stars and might be detectable in large scale interferometers. Furthermore, our microscopic modeling of neutron star crust material can help analyze mechanisms relevant in Magnetar Giant and Micro Flares.

Kadau, Kai [Los Alamos National Laboratory; Horowitz, C J [INDIANA UNIV

2009-01-01

263

Comparison of Oceanic and Continental Ultramafic Hosted Hydrothermal Sulfide Deposits under Slow-spreading Mid-ocean Ridge Setting  

Science.gov (United States)

Recently, slow-spreading mid-ocean ridges have attracted lots of researchers, especially in the MAR (Mid-Atlantic Ridge) and the Indian Ocean Ridge. People have found many hydrothermal vents or hydrothermal sulfide deposits around MAR, such as TAG, Rainbow and Lost City. The slow-spreading ridges are characterized by variations in magmatic, tectonic, and alteration processes along ridge segments. Because of the difficulties of the seafloor exploration, we need an example on the continent for compare with the hydrothermal sulfide deposits on mid-ocean ridge. So we found De'erni Cu (Co) ore deposit on the north of Tibet. De'erni Cu (Co) ore deposit is a typical VHMS developing in the north of Tibet, China. The ore body is hosted by the ultramafic rocks of the A'nyemagen ophiolite suite, which is the symbol of the residual crust of Paleo-Tethys Ocean. Through the detailed geological analysis to De'erni Cu (Co) ore deposit, lots of reminded geological records of submarine hydrothermal system, including: 1) thin-layer exhalative rock covering on the ore body; 2) the colloform structure, raspberry-like structure and breccia structure reserved in the porous-type ores; 3) the main mineral composition; 4) the calcite and felsic cement in the synchronization with the pyrite clast; 4) the ore zonality similar to the TAG hydrothermal sulfide deposit. According to the TiO2 content in the MORB basalts, the approximate half-spreading rate is 1.1-2.5cm/a, of the Paleo-Tethys Ocean represented by the De'erni ophiolite. Comparing to the mineralization processes of present mid-ocean hydrothermal sulfide deposits, we insist that De'erni Cu (Co) deposit has experienced three stages: submarine exhalation stage, cooling deposition and subduction emplacement. And the OCC (Oceanic Core Complex) may be the host setting of the submarine hydrothermal exhalation stage. Compared to other similar sulfide deposits on the continent in the world, De'erni Cu (Co) sulfide deposit has a younger age (340Ma Carboniferous) and a more completed ore deposit structure. De'erni Cu (Co) ore deposit is a typical case of ultramafic hosted hydrothermal sulfide deposits.

Li, Honglin; Li, Jianghai; Zhang, Huatian

2013-04-01

264

Implications of regional gravity for state of stress in the earth's crust and upper mantle.  

Science.gov (United States)

Topography is maintained by stress differences within the earth. Depending on the distribution of the stress we classify the support as either local or regional compensation. In general, the stresses implied in a regional compensation scheme are an order of magnitude larger than those corresponding to local isostasy. Gravity anomalies, a measure of the earth's departure from hydrostatic equilibrium, can be used to distinguish between the two compensation mechanisms and thus to estimate the magnitude of deviatoric stress in the crust and upper mantle. Topography created at an ocean ridge crest or in a major contiental orogenic zone appears to be locally compensated. Such features were formed on weak crust incapable of maintaining stress differences much greater than the stress from the applied load. Oceanic volcanoes formed on an already cooled, thickened lithosphere are regionally supported with elastic stresses. -Author

McNutt, M.

1980-01-01

265

Receiver function analysis of the crust and upper mantle in Fennoscandia - isostatic implications  

DEFF Research Database (Denmark)

The mountains across southern Norway and other margins of the North Atlantic Ocean appear conspicuously high in the absence of recent convergent tectonics. We investigate this phenomenon with receiver functions calculated for seismometers deployed across southern Fennoscandia. These are used to constrain the structure and seismic properties of the lithosphere and primarily to measure the thickness and infer the bulk composition of the crust. Such parameters are key to understanding crustal isostasy and assessing its role, or lack thereof, in supporting the observed elevations. Our study focuses on the southern Scandes mountain range that has an average elevation >1.0 km above mean sea level. The crust-mantle boundary (Moho) is ubiquitously imaged, and we occasionally observe structures that may represent the base of the continental lithosphere or other thermal, chemical, or viscous boundaries in the upper mantle. The Moho resides at similar to 25-30 km depth below mean sea level in southeastern coastal Norwayand parts of Denmark, similar to 35-45 km across the southern Scandes, and similar to 50-60 km near the Norwegian-Swedish border. That section of thickest crust coincides with much of the Transscandinavian Igneous Belt and often exhibits a diffuse conversion at the Moho, which probably results from the presence of a high wave speed, mafic lower crust across inner Fennoscandia. A zone of thinned crust (

Frassetto, Andrew; Thybo, Hans

2013-01-01

266

Growth of the Afanasy Nikitin Seamount, Central Indian Ocean - the product of short-lived hotspots  

Science.gov (United States)

The Afanasy Nikitin seamount (ANS) is a major structural feature in the Central Indian Basin. An understanding of its evolution using multibeam bathymetry, magnetic and seismic reflection data provides new insights on growth of the seamount through time, emplacement of the 85°E Ridge in the Bay of Bengal and deformation of the lithosphere in the equatorial Indian Ocean. The seafloor morphology and internal structure of the ANS show that the seamount consists of extensive plateaus extending from 2°15' to 5°30'S in water depths of 3000-4500 m, numerous elevated features (seamount highs) pierce through the northern part of the seamount plateau reaching up to 1600 m water depth and faulted blocks up to 1.0 s TWT throw in the southern part of the seamount plateau. Model studies of magnetic profiles suggest that the main plateau of the seamount was emplaced during the normal magnetisation period between the formation of seafloor spreading anomalies 33-32n.2 (79-73 Ma) and that the seamount high was formed in a reverse magnetisation period later than the main seamount plateau formation. Integrated geophysical results clearly demonstrate that the ANS was constructed in two phases, initially coeval with the formation of the oceanic crust during 79-73 Ma, and later at about 55 Ma in an intraplate setting. Based on present geophysical results and published plate reconstruction results of the Indian Ocean from Late Cretaceous to Early Cenozoic, we believe that the Conrad hotspot has emplaced the main plateau of the ANS and Conrad Rise during the period 79-73 Ma in an on-ridge setting, after which the hotspot has continued its activity in Antarctica plate leaving the main plateau of the ANS as an isolated feature on the Indian plate. Subsequently another hotspot that formed the 85°E Ridge and buried hills in the Bay of Bengal has rebuilt the existing main plateau of the ANS at around 55 Ma and eventually the hotspot became defunct in the vicinity of the ANS.

Krishna, K. S.; Scrutton, R. A.; Bull, J. M.; Shankar, S. Jai; Banakar, V. K.

2009-04-01

267

Phantom Archean crust in Mangaia hotspot lavas and the meaning of heterogeneous mantle  

Science.gov (United States)

Lavas from Mangaia in the Cook-Austral island chain, Polynesia, define an HIMU (or high ?, where ?=U238/Pb204) global isotopic end-member among ocean island basalts (OIB) with the highest 206,207,208Pb/204Pb. This geochemical signature is interpreted to reflect a recycled oceanic crust component in the mantle source. Mass independently fractionated (MIF) sulfur isotopes indicate that Mangaia lavas sampled recycled Archean material that was once at the Earth's surface, likely hydrothermally-modified oceanic crust. Recent models have proposed that crust that is subducted and then returned to the surface in a mantle plume is expected to transform to pyroxenite/eclogite during transit through the mantle. Here we examine this hypothesis for Mangaia using high-precision electron microprobe analysis on olivine phenocrysts. Contrary to expectations of a crustal component and, hence pyroxenite, results show a mixed peridotite and pyroxenite source, with peridotite dominating. If the isotopic compositions were inherited from subduction of recycled oceanic crust, our work shows that this source has phantom-like properties in that it can have its lithological identity destroyed while its isotope ratios are preserved. This may occur by partial melting of the pyroxenite and injection of its silicic melts into the surrounding mantle peridotite, yielding a refertilized peridotite. Evidence from one sample reveals that not all pyroxenite in the melting region was destroyed. Identification of source lithology using olivine phenocryst chemistry can be further compromised by magma chamber fractional crystallization, recharge, and mixing. We conclude that the commonly used terms mantle “heterogeneities” and “streaks” are ambiguous, and distinction should be made of its lithological and isotopic properties.

Herzberg, C.; Cabral, R. A.; Jackson, M. G.; Vidito, C.; Day, J. M. D.; Hauri, E. H.

2014-06-01

268

Spectral expressions for modelling the gravitational field of the Earth’s crust density structure:  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We derive expressions for computing the gravitational field (potential and its radial derivative) generated by an arbitrary homogeneous or laterally varying density contrast layer with a variable depth and thickness based on methods for a spherical harmonic analysis and synthesis of gravity field. The newly derived expressions are utilised in the gravimetric forward modelling of major known density structures within the Earth’s crust (excluding the ocean density contrast) beneath the geoid ...

Tenzer, R.; Novak, P.; Hamayun; Vajda, P.

2011-01-01

269

Growth of the Afanasy Nikitin seamount and its relationship with the 85°E Ridge, northeastern Indian Ocean  

Science.gov (United States)

The Afanasy Nikitin seamount (ANS) is a major structural feature (400 km-long and 150 km-wide) in the Central Indian Basin, situated at the southern end of the so-called 85°E Ridge. Combined analyses of new multibeam bathymetric, seismic reflection and geochronological data together with previously described magnetic data provide new insights into the growth of the ANS through time, and its relationship with the 85°E Ridge. The ANS comprises a main plateau, rising 1200 m above the surrounding ocean floor (4800 m), and secondary elevated seamount highs, two of which (lie at 1600 and 2050 m water depths) have the morphology of a guyot, suggesting that they were formed above or close to sea-level. An unbroken sequence of spreading anomalies 34 through 32n.1 identified over the ANS reveal that the main plateau of the ANS was formed at 80-73 Ma, at around the same time as that of the underlying oceanic crust. The 40Ar/39Ar dates for two basalt samples dredged from the seamount highs are consistent, within error, at 67 Ma. These results, together with published results of late Cretaceous to early Cenozoic Indian Ocean plate reconstructions, indicate that the Conrad Rise hotspot emplaced both the main plateau of the ANS and Conrad Rise (including the Marion Dufresne, Ob and Lena seamounts) at 80-73 Ma, close to the India-Antarctica Ridge system. Subsequently, the seamount highs were formed by late-stage volcanism c. 6-13 Myr after the main constructional phase of the seamount plateau. Flexural analysis indicates that the main plateau and seamount highs of the ANS are consistent with Airy-type isostatic compensation, which suggest emplacement of the entire seamount in a near spreading-center setting. This is contrary to the flexural compensation of the 85°E Ridge further north, which is interpreted as being emplaced in an intraplate setting, i.e., 25-35 Myr later than the underlying oceanic crust. Therefore, we suggest that the ANS and the 85°E Ridge appear to be unrelated as they were formed by different mantle sources, and that the proximity of the southern end of the 85°E Ridge to the ANS is coincidental.

Krishna, K. S.; Bull, J. M.; Ishizuka, O.; Scrutton, R. A.; Jaishankar, S.; Banakar, V. K.

2014-02-01

270

Is Macquarie Island a Section of Slow-spread Crust?  

Science.gov (United States)

Peridotite melting at mid-ocean ridges is believed to be affected by spreading rate, with faster spreading rates leading to higher degrees of melting. Macquarie Island, located 1500 km southeast of southernmost Australia, is thought be the sole complete section of ocean crust uplifted in the ocean basin in which it formed; moreover it has been proposed that it formed during slow spreading (30mm/yr full rate, calculated from geophysical studies). The oceanic crust of the island formed in the final stages of spreading, ˜6 mya, as indicated by Ar-Ar plateau ages of basaltic glass. At this latitude, the plate boundary evolved from a spreading ridge to a transpressional boundary between ˜33 and ˜6 mya, thus the rocks of the island record an interesting tectonic history and may provide clues to the mantle processes during a major plate motion re-organization, and slow spreading. Residual, plag-free peridotites were collected along transects through all of the mantle sections on the island, with an average of 100 meter spacing between samples. Spinel chrome numbers (Cr#) ranged from 0.39 to 0.46 (n=23), which corresponds to 15-16% fractional melting [1]. Their low Ti contents (0.02-0.07) attest to the residual nature of the Macquarie Island peridotites. Cpx is preserved in only 7 samples (alteration, depletion), and occurs mainly as small interstitial grains or as exsolved blebs in opx porphyroclasts. Cpx titanium (0.00-0.04 wt% TiO_2) and sodium (0.00-0.05 wt% Na_2O) contents are extremely low, confirming the high depletion and supporting highly efficient melt extraction. Opx porphyroclast cores have very high Mg# (0.92 on average). Trace element analysis (SIMS) revealed a strong depletion of HREE, and enrichment of LREE, as well as a positive Sr anomaly in some samples. Exsolved and matrix grains have identical compositions. The levels of depletion indicated by the spinel Cr# and HREE contents of cpx of the Macquarie Island peridotites are more similar to those seen at fast spreading centers or ophiolites, not at most slow spreading centers. This depletion could be caused by the progressively changing spreading direction disrupting mixing in the mantle, causing repeated melting of the same mantle source. Alternately, this depletion may be caused by melting enhanced by the presence of fluids, possibly introduced by some limited (temporal?) subduction in the region. This could also explain the LREE- and fluid mobile element enrichment. Further analyses of associated gabbros and basalts will test which model is most likely. [1] Hellebrand et al., (2001) Nature 410, 677-681.

Wertz, K.; Hellebrand, E.; Snow, J. E.; von der Handt, A.; Mosher, S.

2003-04-01

271

Tectonic and Magmatic Modification of Ocean Crustal Bulk Physical Properties: A Global Perspective  

Science.gov (United States)

We compiled crustal physical property data such as porosity, density, and velocity from several decades of ocean drilling to compare normal ocean crust with crust influenced by fracture zones, hotspots, hydrothermal alteration, and other magmatic or tectonic influences. Cores and holes from IODP Expeditions and ODP and DSDP Legs that have penetrated 100 m or more into basement have been divided into the following geologic settings: a) normal crust, b) tectonic windows, c) off-axis seamounts, d) mid-ocean ridge seamounts e) off-axis plateaus and f) areas of arc volcanism. This study aims not only to provide a more global perspective of the ocean crust with respect to factors such as geologic setting, age, and rock type from many seafloor holes globally that have yet to be compared, but also to focus more specifically on how normal crustal properties compare with those of crust which has interacted with other tectonic and magmatic zones. Preliminary analyses of the different geologic settings with respect to their average physical properties without regard for differences in rock type, depth, age, or composition show some interesting trends. Although the average crustal velocity is highest in normal oceanic crust, we find no significant difference between normal crust and any other setting. All the settings exhibit variability in the upper 200 m of crust, but below 200 m the velocity of normal oceanic crust never drops below 4 km/s. The average porosity of normal crust is similar to that of tectonic windows, but much lower than the average of any of the other geologic settings. We find that the average bulk density is again highest in normal oceanic crust and similar to the average of tectonic windows, but normal crust is not significantly different from the other settings. Average grain density is also highest in normal crust. The present data suggest a comprehensive model of both normal crust and crust that has been influenced by tectonic and magmatic processes. Using physical properties and the identifying the controlling factors such as hydrothermal alteration, mechanical collapse, and faulting, we can synthesize the global datasets provided by ocean drilling during the last four decades.

Kuenzel, N. A.; Gilbert, L. A.

2007-12-01

272

Intracratonic (non-Oceanic) deep subduction and exhumation, East Greenland Caledonides  

Science.gov (United States)

The Caledonian orogeny in the North Atlantic region is typically portrayed as Ordovician to Silurian destruction of the Iapetus Ocean ending with continent-continent collision, and partial subduction of Baltica, below the overriding Laurentian plate. Geometries of geologic cross-sections from both sides of the orogen for the late Silurian and early Devonian show tectonic wedges squeezed out between a basal thrust(s) and an overlaying extensional detachments at a higher structural level. High pressures (HP) or ultra HP (UHP)-rocks occur on both sides of the orogen. In Scandinavia they occur in the tail of the subducted continental crust, generally occurring below thrusts and exotic nappes. In contrast, the HP and molten syn-orogenic rocks in the Greenland Caledonides occurs in thrust sheets structurally above low grade Laurentian platform sediments margin and below one or more extensional detachments also carrying low grade Laurentian shelf sediments. The tectonic scenarios of both side of the orogen are here compared by combining new U-Pb TIMS and 40Ar/39Ar -data from NE Greenland with published structural, stratigraphic, metamorphic and geochronologic data from both sides of the orogen. In the Scandinavian Caledonides Late Silurian to Early Devonian HP metamorphism, occur in the continental margin subducted below a stack of collapsed nappes. The lower nappes comprise Baltic rocks and the structurally higher nappes contain exotic rocks probably including remnants of both the Iapetus Ocean, and exotic continental rocks inferred to have been parts of Laurentia. Some of these upper nappes show evidence of early Caledonian magmatism and deformation related to Taconic/Grampian (~470Ma) events typical for the Laurentian margin. The driving force for Baltic continental subduction is proposed to be the pull of oceanic crust (Iapetus), and subsequent continental eclogitization. A different tectonic scenario appears to exist for the East Greenland Caledonides. The inferred intracratonic HP metamorphism of the East Greenland Caledonides, requires models not involving a nearby oceanic subduction zone. Clues to the intracratonic subduction mechanism may be found in the extremely thick pre-Caledonian basin deposits occurring above the detachment and high grade nappes. These basins formed above a 500 m.y. low geoid probably reflecting a thinned crust, and updomed dense lithospheric mantle. By the onset of Caledonian thrusting below these thick deposits, the underlying dense lithosphere is freed to sink, into a major intracontinental subduction zone. Shortly after, the high grade nappes are exhumed by combined tectonic extrusion and emplacement towards west above the low grade the foreland, and crustal scale N-S extensional asymmetric boudinage, where the Devonian Late orogenic basins formed above exhumed rocks in the thinned boudin necks.

Hartz, E. H.; Corfu, F.; Andresen, A.; Andersen, T. B.; Bowring, S. A.; Hodges, K. V.

2003-04-01

273

Investigation of thallium fluxes from subaerial volcanism-Implications for the present and past mass balance of thallium in the oceans  

Science.gov (United States)

A suite of 34 volcanic gas condensates and particulates from Kilauea (Hawaii), Mt. Etna and Vulcano (Italy), Mt. Merapi (Indonesia), White Island and Mt. Nguaruhoe (New Zealand) were analysed for both Tl isotope compositions and Tl/Pb ratios. When considered together with published Tl-Pb abundance data, the measurements provide globally representative best estimates of Tl/Pb = 0.46 ?? 0.25 and ??205Tl = -1.7 ?? 2.0 for the emissions of subaerial volcanism to the atmosphere and oceans (??205Tl is the deviation of the 205Tl/203Tl isotope ratio from NIST SRM 997 isotope standard in parts per 10,000). Compared to igneous rocks of the crust and mantle, volcanic gases were found to have (i) Tl/Pb ratios that are typically about an order of magnitude higher, and (ii) significantly more variable Tl isotope compositions but a mean ??205Tl value that is indistinguishable from estimates for the Earth's mantle and continental crust. The first observation can be explained by the more volatile nature of Tl compared to Pb during the production of volcanic gases, whilst the second reflects the contrasting and approximately balanced isotope fractionation effects that are generated by partial evaporation of Tl during magma degassing and partial Tl condensation as a result of the cooling and differentiation of volcanic gases. Mass balance calculations, based on results from this and other recent Tl isotope studies, were carried out to investigate whether temporal changes in the volcanic Tl fluxes could be responsible for the dramatic shift in the ??205Tl value of the oceans at ???55 Ma, which has been inferred from Tl isotope time series data for ferromanganese crusts. The calculations demonstrate that even large changes in the marine Tl input fluxes from volcanism and other sources are unable to significantly alter the Tl isotope composition of the oceans. Based on modelling, it is shown that the large inferred change in the ??205Tl value of seawater is best explained if the oceans of the early Cenozoic featured significantly larger Tl output fluxes to oxic pelagic sediments, whilst the sink fluxes to altered ocean crust remained approximately constant. ?? 2009 Elsevier Ltd.

Baker, R. G. A.; Rehkamper, M.; Hinkley, T. K.; Nielsen, S. G.; Toutain, J. P.

2009-01-01

274

42 CFR 495.204 - Incentive payments to qualifying MA organizations for MA-EPs and MA-affiliated eligible hospitals.  

Science.gov (United States)

...payments to qualifying MA organizations for MA-EPs and MA-affiliated eligible hospitals...payments to qualifying MA organizations for MA-EPs and MA-affiliated eligible hospitals...incentive payment for its qualifying MA-EPs and its qualifying MA-eligible...

2010-10-01

275

BaMa / Raivo Juurak  

Index Scriptorium Estoniae

Eesti ülikoolide üleminekust 3+2 süsteemile. Lühend BaMa on tulnud kasutusele seoses Euroopa ülikoolide õppekavade reformimisega ning tähistab õppekava, kus esimese astme läbimise järel omandatakse bakalaureuse- ja teise järel magistrikraad. Õppekavade tüüpidest Eesti ja Euroopa Liidu kõrgkoolides ning Bologna deklaratsioonist

Juurak, Raivo, 1949-

2002-01-01

276

Dredged bedrock samples from the Amerasia Basin, Arctic Ocean  

Science.gov (United States)

Between 2008-2012, as part of the U.S. Extended Continental Shelf project in the Amerasia Basin, Arctic Ocean, 17 dredges were successfully collected sampling the first rock outcrops in the Chukchi Borderland and surrounding regions for the purpose of describing the geologic nature of the bathymetric features in this area. Multiple lines of evidence indicate that the specimens were collected from submarine rock exposures and were not samples of ice rafted debris, common in the ice covered waters of the Arctic Ocean. Using the USCGC Healy, each dredge was collected along very steep slopes (>35 degrees) measured with high resolution multibeam swath bathymety data. Each haul yielded samples of similar lithologies and identical metamorphic grade with manganese crusts on the surfaces exposed to seawater and fresh surfaces where the rocks were broken from outcrop. High tension pulls on the dredge line also indicated sampling of bedrock exposures. Dredged samples from a normal fault scarp in the central Chukchi Borderland consisted of Silurian (c. 430 Ma) orthogneisses that intruded older (c. 487-500 Ma) gabbros and luecogranties that were all metamorphosed to amphibolite grade (Brumley et al., 2011). Samples from the northern Northwind Ridge consisted of metasediments (greenschist facies) interpreted to have been deposited in a proximal arc setting with detrital zircon U-Pb age peaks at 434, 980 Ma with lesser peaks between 500-600, 1100-2000 Ma, and rare 2800 Ma grains (Brumley et al, 2010). Other dredges in the region of the Northwind Ridge yielded deformed and metamorphosed calcareous sandstones and low-grade phyllites (O'Brien et al., 2013). Taken together these rocks indicate a relationship to the Pearya Terrane of northern Ellesmere Island and S.W. Svalbard that were thought to represent a Cambro-Ordovician volcanic arc terrane that was involved in Caledonian orogenesis (Brumley et al., 2011). These findings constrain plate tectonic reconstruction models and bring into question long held ideas that the Chukchi Borderland was made up of an undeformed platformal sequence that was part of the Laurentian passive margin from Proterozoic through Jurassic time (e.g. Grantz et al., 1998). Dredges collected along fault scarps that border the edges of the Nautilus Basin yielded undeformed but highly altered volcaniclastic rocks that were deposited in a shallow water setting and contain primary potassium feldspar phenocrysts that are not associated with mafic magmas. Also in this region, several dredges contained undeformed Late Cretaceous (112, 88-80 Ma) basalts (Andronikov et al., 2008; Mukasa et al., 2009) interpreted to have been derived from a continental lithospheric source similar to continental flood basalts from other regions (Mukasa et al., 2009). These dredged rock samples not only have implications for the Extended Continental Shelf projects of Arctic nations, but add greatly to the body of geologic information about the history of the Arctic Ocean and provide the first ground truth as to the nature of the bathymetric features within the Amerasia Basin.

Brumley, K. J.; Mukasa, S. B.; O'Brien, T. M.; Mayer, L. A.; Chayes, D. N.

2013-12-01

277

Diversity in early crustal evolution: 4100 Ma zircons in the Cathaysia Block of southern China.  

Science.gov (United States)

Zircons are crucial to understanding the first 500?Myr of crustal evolution of Earth. Very few zircons of this age (>4050?Ma) have been found other than from a ~300?km diameter domain of the Yilgarn Craton, Western Australia. Here we report SIMS U-Pb and O isotope ratios and trace element analyses for two ~4100?Ma detrital zircons from a Paleozoic quartzite at the Longquan area of the Cathaysia Block. One zircon ((207)Pb/(206)Pb age of 4127 ± 4?Ma) shows normal oscillatory zonation and constant oxygen isotope ratios (?(18)O = 5.8 to 6.0‰). The other zircon grain has a ~4100?Ma magmatic core surrounded by a ~4070?Ma metamorphic mantle. The magmatic core has elevated ?(18)O (7.2 ± 0.2‰), high titanium concentration (53 ± 3.4?ppm) and a positive cerium anomaly, yielding anomalously high calculated oxygen fugacity (FMQ + 5) and a high crystallization temperature (910°C). These results are unique among Hadean zircons and suggest a granitoid source generated from dry remelting of partly oxidizing supracrustal sediments altered by surface waters. The ~4100?Ma dry melting and subsequent ~4070?Ma metamorphism provide new evidence for the diversity of the Earth's earliest crust. PMID:24888297

Xing, Guang-Fu; Wang, Xiao-Lei; Wan, Yusheng; Chen, Zhi-Hong; Jiang, Yang; Kitajima, Kouki; Ushikubo, Takayuki; Gopon, Phillip

2014-01-01

278

Thermal models of dyke intrusion during development of continent-ocean transition  

Science.gov (United States)

A consensus has emerged in recent years from a variety of geoscientific disciplines that extension during continental rifting is achieved only partly by plate stretching: dyke intrusion also plays an important role. Magma intrusion can accommodate extension at lower yield stresses than are required to extend thick, strong, unmodified continental lithosphere mechanically, thereby aiding the breakup process. Dyke intrusion is also expected to heat and thereby weaken the plate, but the spatial extent of heating and the effect of different rates of magmatic extension on the timescales over which heating occurs are poorly understood. To address this issue, a numerical solution to the heat-flow equation is developed here to quantify the thermal effects of dyke intrusion on the continental crust during rifting. The thermal models are benchmarked against a priori constraints on crustal structure and dyke intrusion episodes in Ethiopia. Finite difference models demonstrate that magmatic extension rate exerts a first-order control on the crustal thermal structure. Once dyke intrusion supersedes faulting and stretching as the principal extensional mechanism the crust will heat and weaken rapidly (less than 1 Ma). In the Main Ethiopian Rift (MER), the majority of present-day extension is focused on ?20 km-wide Quaternary-Recent axial magmatic segments that are mostly seismogenic to mid-crustal depths and show P-wave seismic velocities characteristic of heavily intruded continental crust. When reviewed in light of our models, these observations require that no more than half of the MER's extension since ?2 Ma has been achieved by dyke intrusion. Magmatic heating and weakening of the crust would have rendered it aseismic if dyke intrusion accounted for the entire 6 mm/yr extension rate. In the older, faster extending (16 mm/yr) Red Sea rift (RSR) in Afar, dyke intrusion is expected to have had a more dramatic impact on crustal rheology. Accordingly, effective elastic plate thickness and Moho depth in the Danakil region of northernmost Afar are markedly reduced and seismicity is shallower than in the MER. Thermally driven variations in crustal rheology over time in response to dyke intrusion thus play an important role in the development of continent-ocean transition.

Daniels, K. A.; Bastow, I. D.; Keir, D.; Sparks, R. S. J.; Menand, T.

2014-01-01

279

Superfluid Properties of Neutron Star Crust  

CERN Document Server

Superfluid properties of the inner crust matter of neutron stars, formed by nuclear clusters immersed in a dilute neutron gas, are analysed in a self- consistent HFB approach. The calculations are performed with two pairing forces, fixed so as to obtain in infinite nuclear matter the pairing gaps provided by the Gogny force or by induced interactions. It is shown that the nuclear clusters can either suppress or enhance the pairing correlations inside the inner crust matter, depending on the density of the surrounding neutrons. The profile of the pairing field in the inner crust is rather similar for both pairing forces, but the values of the pairing gaps are drastically reduced for the force which simulates the polarisation effects in infinite neutron matter.

Sandulescu, N; Liotta, R J; Giai, Nguyen Van

2004-01-01

280

The composition of the continental crust  

Science.gov (United States)

A new calculation of the crustal composition is based on the proportions of upper crust (UC) to felsic lower crust (FLC) to mafic lower crust (MLC) of about 1:0.6:0.4. These proportions are derived from a 3000 km long refraction seismic profile through western Europe (EGT) comprising 60% old shield and 40% younger fold belt area with about 40 km average Moho depth. A granodioritic bulk composition of the UC in major elements and thirty-two minor and trace elements was calculated from the Canadian Shield data (Shaw et al., 1967, 1976). The computed abundance of thirty-three additional trace elements in the UC is based on the following proportions of major rock units derived from mapping: 14% sedimentary rocks, 25% granites, 20% granodiorites, 5% tonalites, 6% gabbros, and 30% gneisses and mica schists. The composition of FLC and MLC in major and thirty-six minor and trace elements is calculated from data on felsic granulite terrains and mafic xenoliths, respectively, compiled by Rudnick and Presper (1990). More than thirty additional trace element abundances in FLC and MLC were computed or estimated from literature data. The bulk continental crust has a tonalitic and not a dioritic composition with distinctly higher concentrations of incompatible elements including the heat producing isotopes in our calculation. A dioritic bulk crust was suggested by Taylor and McLennan (1985). The amount of tonalite in the crust requires partial melting of mafic rocks with about 100 km thickness (compared with about 7 km in the present MLC) and water supply from dehydrated slabs and mafic intrusions. At the relatively low temperatures of old crustal segments MLC was partly converted into eclogite which could be recycled into the upper mantle under favourable tectonic conditions. The chemical fractionation of UC against FLC + MLC was caused by granitoidal partial melts and by mantle degassing which has controlled weathering and accumulation of volatile compounds close to the Earth's surface.

Wedepohl, K. Hans

 
 
 
 
281

Early magma ocean and core formation on Vesta  

Science.gov (United States)

The Dawn mission confirms predictions that the asteroid 4 Vesta is differentiated in an iron rich core, a silicate mantle and a basaltic crust, supports its identification as the parent body of the HEDs and provides revised values of e.g. the mass, the bulk density and the dimensions of the asteroid 4 Vesta. Although no distinct volcanic regions have been identified, resurfacing by igneous processes distinguishes Vesta from asteroids like Ceres with its primitive surface, or Lutetia, which retained its primordial surface composition (and may still be partially differentiated[1]). Vesta's core radius is estimated to be 107-113 km[2] (derived from the mass concentration towards the centre). We performed numerical calculations of the thermo-chemical evolution of Vesta adopting the new data obtained by the Dawn mission (mass, bulk density, radius). We have expanded the thermo-chemical evolution model of [3], which includes accretion, compaction, melting, associated changes of the material properties, advective heat transport and differentiation by porous flow, by considering convection and thus effective cooling in a magma ocean to analyse its formation and evolution on Vesta. For melt fractions below the rheologically critical melt fraction (RCMF) of ?50% the heat transport by melt segregation is modeled assuming melt flow in porous media and by supplementing the energy balance equation with additional advection terms. Above the RCMF the effective thermal conductivity keff is computed from the convective heat flux in the soft turbulence regime[4]. The parameter keff mimics the vigorous convection and heat flux of the magma ocean with a low viscosity. It amounts to O(106) W m-1K-1 and substitutes the thermal conductivity in the energy balance equation. We consider both instantaneous and continuous accretion (assuming late runaway material accumulation). In particular, we compare the evolution scenarios arising from the instantaneous accretion of Vesta at different formation times t0 (relative to the formation of the CAIs) with those for which the accretion durations ta is between 0.5 and 2.0 Ma. According to our results core formation is possible for formation times of up to 2.5 Ma after the CAIs. An important process for the formation and evolution of a magma ocean is the partitioning of 26Al and its relocation with the silicate melt. Previous models[5] suggest the formation of an internal magma ocean throughout the whole mantle beneath a solid crust. Thereby, the partitioning of 26Al is neglected. In contrast to that, if partitioning of 26Al into the melt is considered we obtain an about 1 km thick superficial magma ocean due to the enrichment of the radioactive nuclides in the liquid phase and redistribution towards the surface with the rising melt (for t0

Neumann, Wladimir; Breuer, Doris; Spohn, Tilman

2013-04-01

282

The crust role at Paramillos Altos intrusive belt: Sr and Pb isotope evidence  

International Nuclear Information System (INIS)

Paramillos Altos Intrusive Belt (PAIB) (Ostera, 1996) is located in the thick skinned folded-thrust belt of Malargue, southwestern Mendoza, Argentina. Geochemical, geochronologic and isotopic studies were carried out in it (Ostera 1996, 1997, Ostera et al. 1999; Ostera et al. 2000) and these previous papers suggested a minor involvement of the crust in the genesis of the PAIB. According with Ostera et al. (2000) it is composed by stocks, laccoliths, dykes and sills which range in composition from diorites to granodiorites, and from andesites to rhyolites, and divided in five Members, which range in age from Middle Miocene to Early Miocene: a- Calle del Yeso Dyke Complex (CYDC), with sills and dykes of andesitic composition (age: 20±2 Ma). b- Puchenque-Atravesadas Intrusive Complex (PAIC), composed by dykes and stocks ranging from diorites to granodiorites (age: 12.5±1 Ma). c- Arroyo Serrucho Stock (SAS), an epizonal and zoned stock, with four facies, with K/Ar and Ar/Ar dates of 10±1 and 9.5±0.5 Ma. d- Portezuelo de los Cerros Bayos (PCB), that includes porphyritic rocks of rhyolitic composition, of 7.5±0.5 Ma. e- Cerro Bayo Vitrophyres (CBV), with andesitic sills and dykes (age: 4.8±0.2 Ma). We present in this paper new Sr and Pb isotopes data that constrain the evolution of the PAIB (au)

283

Shallow origin for South Atlantic Dupal Anomaly from lower continental crust: Geochemical evidence from the Mid-Atlantic Ridge at 26°S  

Science.gov (United States)

We measured trace element concentrations and Pb isotope compositions of fresh volcanic glass samples from the Mid-Atlantic Ridge at 26°S, and from nearby off-axis seamounts. The samples have previously been studied for major elements and Sr-Nd-He isotopes. All samples are depleted MORB, and include some of the most incompatible trace element depleted lavas yet reported from the Atlantic. The seamount lavas are more depleted in highly incompatible elements than the axial lavas, but have high Sr, Pb and Eu concentrations, relative to REE of similar incompatibility. The lavas with the highest Sr/Nd, Pb/Ce and Eu/Eu* have the highest 3He/ 4He (up to 11.0 R/RA) ratios and the lowest incompatible trace element concentrations. They also have the highest 87Sr/ 86Sr (up to 0.7036) and 208Pb/ 204Pb for a given 206Pb/ 204Pb ratio, which are characteristics of lavas from the Dupal Anomaly in the South Atlantic, and of many EM-1 type intraplate lavas generally. Our data place constraints on the origin of the Dupal Anomaly. The enrichments in Sr, Pb and Eu, together with the low Ca/Al ratios of the seamount lavas indicate that their mantle source consists of material that at one time contained plagioclase, and must therefore have resided at crustal pressures. We argue that the trace element and isotopic compositions of the seamount lavas are best explained by derivation from a mantle source contaminated with lower continental crust, which was introduced into the upper mantle during continental rifting and breakup in the South Atlantic. Our results support previous suggestions that the Dupal Anomaly in the South Atlantic has a relatively recent, shallow origin in lower continental crust and continental lithospheric mantle, rather than in recycled material supplied from the deeper mantle by plumes. Plate reconstructions place the Parana-Etendeka flood basalt province over the central part of the Dupal Anomaly at the time of rifting of South America and Africa at 134 Ma. The flood basalts which have undergone the least crustal-level contamination also have extreme Dupal compositions. We speculate that delamination of dense lower continental crust during continental rifting causes flood basalt magmatism, whilst variably polluting the upper oceanic mantle with continental material.

Regelous, Marcel; Niu, Yaoling; Abouchami, Wafa; Castillo, Pat R.

2009-09-01

284

Callisto Cutaway with Ocean  

Science.gov (United States)

This artist's concept, a cutaway view of Jupiter's moon Callisto, is based on recent data from NASA's Galileo spacecraft which indicates a salty ocean may lie beneath Callisto's icy crust.These findings come as a surprise, since scientists previously believed that Callisto was relatively inactive. If Callisto has an ocean, that would make it more like another Jovian moon, Europa, which has yielded numerous hints of a subsurface ocean. Despite the tantalizing suggestion that there is an ocean layer on Callisto, the possibility that there is life in the ocean remains remote.Callisto's cratered surface lies at the top of an ice layer, (depicted here as a whitish band), which is estimated to be about 200 kilometers (124 miles) thick. Immediately beneath the ice, the thinner blue band represents the possible ocean, whose depth must exceed 10 kilometers (6 miles), according to scientists studying data from Galileo's magnetometer. The mottled interior is composed of rock and ice.Galileo's magnetometer, which studies magnetic fields around Jupiter and its moons, revealed that Callisto's magnetic field is variable. This may be caused by varying electrical currents flowing near Callisto's surface, in response to changes in the background magnetic field as Jupiter rotates. By studying the data, scientists have determined that the most likely place for the currents to flow would be a layer of melted ice with a high salt content.These findings were based on information gathered during Galileo's flybys of Callisto in November 1996, and June and September of 1997. JPL manages the Galileo mission for NASA's Office of Space Science, Washington, DC. This artist's concept and other images and data received from Galileo are posted on the World Wide Web on the Galileo mission home page at http://galileo.jpl.nasa.gov . Background information and educational context for the images can be found at http://www.jpl.nasa.gov/galileo/sepo

1998-01-01

285

A relatively reduced Hadean continental crust  

Science.gov (United States)

Among the physical and chemical parameters used to characterize the Earth, oxidation state, as reflected by its prevailing oxygen fugacity (fO2), is a particularly important one. It controls many physicochemical properties and geological processes of the Earth's different reservoirs, and affects the partitioning of elements between coexisting phases and the speciation of degassed volatiles in melts. In the past decades, numerous studies have been conducted to document the evolution of mantle and atmospheric oxidation state with time and in particular the possible transition from an early reduced state to the present oxidized conditions. So far, it has been established that the oxidation state of the uppermost mantle is within ±2 log units of the quartz-fayalite-magnetite (QFM) buffer, probably back to ~4.4 billion years ago (Ga) based on trace-elements studies of mantle-derived komatiites, kimberlites, basalts, volcanics and zircons, and that the O2 levels of atmosphere were initially low and rose markedly ~2.3 Ga known as the Great Oxidation Event (GOE), progressively reaching its present oxidation state of ~10 log units above QFM. In contrast, the secular evolution of oxidation state of the continental crust, an important boundary separating the underlying upper mantle from the surrounding atmosphere and buffering the exchanges and interactions between the Earth's interior and exterior, has rarely been addressed, although the presence of evolved crustal materials on the Earth can be traced back to ~4.4 Ga, e.g. by detrital zircons. Zircon is a common accessory mineral in nature, occurring in a wide variety of igneous, sedimentary and metamorphic rocks, and is almost ubiquitous in crustal rocks. The physical and chemical durability of zircons makes them widely used in geochemical studies in terms of trace-elements, isotopes, ages and melt/mineral inclusions; in particular, zircons are persistent under most crustal conditions and can survive many secondary processes such as metamorphism, weathering and erosion. Thus, zircons in granites of shallow crust may record the chemical/isotopic composition of the deep crust that is otherwise inaccessible, and offer robust records of the magmatic and crust-forming events preserved in the continental crust. In fact, due to the absence of suitable rock records (in particular for periods older than ~4.0 Ga), studies in recent years concerning the nature, composition, growth and evolution of the continental crust, and especially the Hadean crust, have heavily relied on inherited/detrital zircons. Natural igneous zircons incorporate rare-earth elements (REE) and other trace elements in their structure at concentrations controlled by the temperature, pressure, fO2 and composition of their crystallization environment. Petrological observations and recent experiments have shown that the concentration of Ce relative to other REE in igneous zircons can be used to constrain the fO2 during their growth. By combining available trace-elements data of igneous zircons of crustal origin, we show that the Hadean continental crust was significantly more reduced than its modern counterpart and experienced progressive oxidation till ~3.6 billions years ago. We suggest that the increase in the oxidation state of the Hadean continental crust is related to the progressive decline in the intensity of meteorite impacts during the late veneer. Impacts of carbon- and hydrogen-rich materials during the formation of Hadean granitic crust must have favoured strongly reduced magmatism. The conjunction of cold, wet and reduced granitic magmatism during the Hadean implies the degassing of methane and water. When impacts ended, magma produced by normal decompression melting of the mantle imparted more oxidizing conditions to erupted lavas and the related crust.

Yang, Xiaozhi; Gaillard, Fabrice; Scaillet, Bruno

2014-05-01

286

Central Andean Giant Ore Deposits: Links to Forearc Subduction Erosion, Shallowing Subduction and Thickening Crust  

Science.gov (United States)

An outstanding question on the Central Andean margin is the relationship between tectonic processes like ebbing arc volcanism, shallowing of the subducting slab and crustal thickening, and the origin of giant porphyry and epithermal Cu, Au and Ag deposits. Another potentially important factor in forming these major mineral deposits is forearc subduction erosion, which is postulated to have removed up to ~250 km of Central Andean forearc crust since the Jurassic. Geochemical and geophysical studies provide insights into possible links. Evidence for partial melts of removed and subducted forearc crust reaching the arc magma source and thus the magmas that host the ore deposits comes from the chemistry of late Neogene volcanic rocks on both the northern and southern margin of the Chilean-Pampean flat-slab (28°-33°S), where the frontal arc was displaced ~50 km into the foreland between ~10 and 3 Ma. This chemical evidence consists of transient ultra-steep REE patterns, elevated Mg, Cr and Ni contents and steps in isotopic ratios that are particularly notable in the glassy adakitic 8-3 Ma (Pircas Negras) andesites on the northern flat-slab margin at 27°-28°S. Well constrained reconstructions of the margin near 26-28°S that assume a sustained 300 km wide arc-trench gap and ~50 km of forearc removal suggest an accelerated average forearc subduction erosion rate over 150 km3/my/km between 8 and 3 Ma. Noting that the late Miocene arc is now at least ~ 260 km from the trench from 26°S to 34°S and that the active arc extrapolates through the amagmatic flat-slab region (28°-33°S) at 300 km from the trench, accelerated forearc removal could be inferred from ~34°S to 26°S at ~10 to 3 Ma. Geophysical evidence for forearc crust entering the mantle wedge as the flatslab shallowed could come from low Vp/Vs seismic ratios in the mantle wedge under the flatslab, which Wagner et al. (2010) attribute to orthopyroxene. Formation of this orthopyroxene could be explained by forearc crust reacting with the mantle wedge. Thus, the slab shallowing, crustal thickening and forearc subduction erosion in the flatslab region, which began at ca 20-18 Ma and accelerated after 11-10 Ma could have set the stage for the formation of the Los Pelambres, Rio Blanco and El Teniente giant Cu porphyries between ~ 11-4 Ma. The backarc 8-6 Ma Bajo de la Alumbrera Cu-Au district near 27°S, also formed east of the migrating volcanic arc on the northern flatslab margin at this time. This deposit is notable for now being above a high Qp mantle seismic anomaly overlying the slab, which is at a depth of ~150 km. Elsewhere, Ag-Zn mineralization in the ~14-12 Ma Potosi district near 19.5°S in the Altiplano backarc, which has been suggested to have occurred in the early stages of steepening of a shallow slab, would potentially predate flushing of eroded forearc material from an expanding mantle wedge. In the same vein, a lack of known big Cu-Au-Ag deposits associated with the late Neogene giant plateau ignimbrite complexes, considered to be fomed over steepening subduction zones characterized by low Vp and Vs and high Qp tomographic seismic anomalies, could also partially reflect loss of forearc subducted components from an expanding wedge.

Kay, S. M.; Mpodozis, C.

2013-05-01

287

Seismic stratigraphy and sediment thickness of the Nansen Basin, Arctic Ocean  

Science.gov (United States)

A Norwegian expedition to the western Nansen Basin, Yermak Plateau and the Hinlopen margin in 2001 acquired about 1100 km of 2-D multichannel seismic profiles and 50 wide-angle sonobuoy record sections. Analysis of these data establishes a regional seismic stratigraphic framework for the western Nansen Basin integrating previously published stratigraphic schemes. P-wave velocities and sediment thickness were derived within 7-8 per cent uncertainty from 2-D seismic ray tracing models of each sonobuoy section. Sediment thickness reaches 2 km in the studied area and increases towards the depocentre of the giant Franz-Victoria fan on the Barents-Kara continental margin. High-relief oceanic crystalline crust with 3.7 km s-1 average near-top velocity is infilled by four seismic sediment units with typical velocities 2.4, 2.2, 2.0 and 1.8 km s-1. A prominent regional seismic horizon between units 2 and 3 is tentatively correlated by basement onlap and sedimentation rates to a Miocene (~10 Ma) palaeoceanographic event, possibly the opening of the Fram Strait. The youngest unit is correlated to prograding sequences on the margin and to the onset of major slope failure caused by intensified glacio-fluvial drainage and ice sheet erosion during Northern Hemisphere glaciations (2.6-0.01 Ma).

Engen, Øyvind; Gjengedal, Jakob Andreas; Faleide, Jan Inge; Kristoffersen, Yngve; Eldholm, Olav

2009-03-01

288

Fractionation of the geochemical twins Zr-Hf and Nb-Ta during scavenging from seawater by hydrogenetic ferromanganese crusts  

Science.gov (United States)

In contrast to igneous systems, the geochemical twins Zr and Hf are decoupled from each other in seawater, and specific Zr/Hf ratios appear to be characteristic of individual marine water masses. Hydrogenetic marine ferromanganese (Fe-Mn) crusts which accumulate trace metals from seawater may be an archive of Zr/Hf ratios that reveal changes in oceanic paleocirculation over millions of years. To verify whether Fe-Mn crusts truly reflect the Zr-Hf distribution in seawater, we studied these particle-reactive elements together with Nb and Ta (another geochemical twin pair) in bulk Fe-Mn crusts and their surface layers from different locations in the Atlantic and Pacific oceans. Zirconium (400-1000 mg kg-1), Hf (5-18 mg kg-1), Nb (42-83 mg kg-1) and Ta (0.5-1.5 mg kg-1) are significantly enriched in Fe-Mn crusts relative to the average continental crust, and their Zr/Hf and Nb/Ta ratios are super-chondritic (57-87 and 35-96, respectively), whereas the continental crust shows ratios close to those of chondrites. We emphasize that neither bulk Fe-Mn crusts nor their surface layers match the Zr/Hf or Nb/Ta ratios of modern deep seawater, but are lower and higher, respectively. The presence of aluminosilicate detritus cannot explain the different Zr/Hf ratios of crusts and ambient seawater, as potential detritus has much lower Zr and Hf concentrations. Consequently, these geochemical twins must be fractionated during their removal from seawater and their incorporation into Fe-Mn (oxyhydr)oxides. Hafnium is preferentially scavenged as shown by Zr/Hf ratios of crust surface layers (75-100) that are always below those of modern deep seawater (150-300). The decoupled behavior of geochemical twins during sorption, which is also observed for Nb-Ta, can be related to differences in the electron structures of these elements. Iron-normalized concentrations of Zr, Hf, Nb, and Ta increase with increasing size of the positive Ce anomaly (known to increase with decreasing growth rate), which is accompanied by decreasing Zr/Hf and Nb/Ta ratios. Concentrations and ratios of Zr-Hf and Nb-Ta in the crusts are controlled by the Fe/Mn ratio, the growth rate of a crust, and by the composition of ambient seawater. On a basin-wide scale, the variation of Zr/Hf ratios in Fe-Mn crusts, although different from those of ambient seawater, generally reflects the variation of the regional seawater. Compared to Fe-Mn crusts from the Central Pacific, crusts from the NE Atlantic display lower Zr/Hf and Nb/Ta ratios. This likely reflects the lower Zr/Hf ratios in Atlantic deep water compared to Pacific deep water. Although the Zr/Hf ratio of Fe-Mn crusts broadly reflects major changes in seawater circulation, the Zr/Hf signatures of individual Fe-Mn crust layers cannot be used as a paleoceanographic proxy because they are continuously modified by ongoing sorption. Sorption of Zr and Hf onto Fe and/or Mn (oxyhydr)oxides that occur either as discrete particles or as particle coatings, appear to be an efficient way to fractionate Zr and Hf and may significantly contribute to the continuous fractionation of dissolved Zr and Hf observed in seawater.

Schmidt, Katja; Bau, Michael; Hein, James R.; Koschinsky, Andrea

2014-09-01

289

Neutron Star Crust in Strong Magnetic Fields  

Science.gov (United States)

We discuss the effects of strong magnetic fields through Landau quantization of electrons on the structure and stability of nuclei in neutron star crust. In strong magnetic fields, this leads to the enhancement of the electron number density with respect to the zero field case. We obtain the sequence of equilibrium nuclei of the outer crust in the presence of strong magnetic fields adopting most recent versions of the experimental and theoretical nuclear mass tables. For B ~ 1016G, it is found that some new nuclei appear in the sequence and some nuclei disappear from the sequence compared with the zero field case. Further we investigate the stability of nuclei in the inner crust in the presence of strong magnetic fields using the Thomas-Fermi model. The coexistence of two phases of nuclear matter - liquid and gas, is considered in this case. The proton number density is significantly enhanced in strong magnetic fields B ~ 1017G through the charge neutrality. We find nuclei with larger mass number in the presence of strong magnetic fields than those of the zero field. These results might have important implications for the transport properties of the crust in magnetars.

Nandi, Rana; Bandyopadhyay, Debades

2011-09-01

290

Actual timing of neodymium isotopic variations recorded by Fe-Mn crusts in the western North Atlantic  

Science.gov (United States)

Hydrogenetic ferromanganese (Fe-Mn) crusts from the western North Atlantic record variations in the Nd and Pb isotopic composition of Cenozoic deep water preserved during their growth. The timing and cause of the most striking change have been the subject of debate. Some have proposed that the shift took place after 4 Ma in response to the closure of the Panama gateway. Others have argued that the major change in isotope composition occurred as early as 8 Ma. This study presents high-resolution Nd isotope records for crusts previously dated using 10Be/9Be chronology. These data confirm that the shifts in Nd occurred after 4 Ma, consistent with a likely relationship with the closure of the Central American Isthmus and intensification of Northern Hemisphere Glaciation, and in accordance with changes seen in other physical and chemical records. These results illustrate the need for both a robust chronological framework and high-resolution records before a reliable paleoceanographic interpretation can be made of the variations recorded by Fe-Mn crusts.

Burton, K.W.; Lee, D.-C.; Christensen, J.N.; Halliday, A.N.; Hein, J.R.

1999-01-01

291

Comparative Study on the Electrical Properties of the Oceanic Mantle Beneath the Northwest Pacific Ocean  

Science.gov (United States)

We have been conducting long-term seafloor electromagnetic (EM) observations at two sites in the northwest Pacific since 2001. The older site was established at the deep seafloor (~5600m) on the northwest Pacific basin (Site NWP), while the new one was installed on the west Philippine basin (Site WPB) in 2006 at the slightly deeper (~5700m) seafloor. The ages of the oceanic basins at those sites are approximately 129 Ma for Site NWP (Shipboard Scientific Party of ODP Leg 191, 2000) and 49 Ma for Site WPB (Salisbury et al., 2006), respectively. The EM instruments deployed at those sites are seafloor EM stations (SFEMS; Toh et al., 2004 and 2006) and capable of measuring vector EM fields at the seafloor for as long as one year or more with other physical quantities such as the instruments' attitude, orientation and temperature. One of the objectives of the seafloor long-term EM observations by SFEMSs is to make a comparative study of the oceanic mantle with and without influence of the so-called 'stagnant slabs' in terms of their electrical conductivity. It is anticipated that the mantle transition zone under the influence of the stagnant slab has a higher electrical conductivity because the transition zone there could be wetter than that in the absence of the stagnant slab. In this context, the mantle transition zone beneath Site WPB can be said to have influence by the stagnant slab, while that beneath Site NWP does not. It, therefore, is basically possible to estimate how much water is present in each transition zone by comparison of the electrical conductivity profiles of the two. The one-dimensional electrical profile beneath Site NWP has been derived so far using the magnetotelluric (MT) and geomagnetic depth sounding (GDS) methods with significant jumps in the electrical property at 410 and 660km discontinuities. The jumps are approximately factors of 10 and 2, respectively (Ichiki et al., 2009). Here we show a profile beneath Site WPB using both MT and GDS responses as well. It, however, should be also noted here that the penetration depth beneath Site WPB is significantly smaller than that beneath Site NWP because the solar activity has been very low since 2006. References Ichiki, M., K. Baba, H. Toh and K. Fuji-ta, An overview of electrical conductivity structures of the crust and upper mantle beneath the northwestern Pacific, the Japanese Islands, and continental East Asia, Gondwana Research, 16, 545?562, doi:10.1016/j.gr.2009.04.007, 2009. Salisbury MH et al (2006) 2. Leg 195 Synthesis: Site 1201?A geological and geophysical section in the West Philippine Basin from the 660-km discontinuity to the mudline. Proc. Ocean Drilling Program, Scientific Reports 195:27. Shipboard Scientific Party of ODP Leg 191 (2000) Northwest Pacific seismic observatory and hammer drill tests, Proc. Ocean Drilling Program, Initial Reports 191. Toh, H., Y. Hamano and M. Ichiki, Long-term seafloor geomagnetic station in the northwest Pacific: A possible candidate for a seafloor geomagnetic observatory, Earth Planets Space, 58, 697-705, 2006. Toh, H., Y. Hamano, M. Ichiki and H. Utada, Geomagnetic observatory operates at the seafloor in the Northwest Pacific Ocean, Eos, Trans. Am. Geophys. Union, 85, 467/473, DOI: 10.1029/2004EO450003, 2004.

Toh, H.

2013-12-01

292

Time-Averaged Rate of Detachment Faulting at Atlantis Bank, Southwest Indian Ridge: Evidence for Highly Asymmetric Spreading Rates During the Formation of Oceanic Core-Complexes  

Science.gov (United States)

Determining the rates of oceanic detachment faulting and how they compare to rates of seafloor spreading is of fundamental importance for constraining how these features form and their relationship to tectonic processes, including magmatic accretion, in slow-spreading environments. We use Pb/U ages of igneous zircon from lower crustal gabbros and sea-surface magnetic data from Atlantis Bank to determine half-spreading rates during detachment faulting. We suggest that these rates, which record crustal accretion and cooling of lower crust in the Antarctic plate, also reflect the time-averaged rate of motion on the detachment fault. During the time Atlantis Bank formed, from 11.2-12.5Ma, both techniques yield a half-spreading rate of 14±1km/Ma. The magnetically determined full-spreading rate has been 14km/Ma since 20Ma and magnetic data from neighboring segments show that this full spreading rate did not increase during the period of detachment faulting. Our data also show that the period of highly asymmetric spreading was confined to the segment containing Atlantis Bank, and so was associated with ridge migration and closure of the non-transform discontinuity to the East. Therefore, detachment faulting at Atlantis Bank occurred during a period of highly asymmetric spreading; a result consistent with sea-surface magnetic data from the FUJI dome at 63° 45'E on the SWIR. Our data require a re-evaluation of existing models that predict structural asymmetry but not asymmetric plate spreading rates during the formation of oceanic core-complexes. The half-spreading rate during detachment faulting suggests that for ~1.3Ma the detachment fault acted as the primary plate boundary and that the axial valley was (within resolution) stationary with respect to the hanging wall (African Plate). If volcanism continued within the axial valley, a thick layer of extrusive material would have formed. This has not been identified. A combination of hydrothermal cooling and strain localization on the detachment fault might lead to limited diking and volcanism within the hanging wall, thus restricting crustal accretion to the lower crust and producing a thick plutonic layer in the Antarctic Plate, a prediction consistent with seismic velocities and crustal thickness observed beneath Atlantis Bank.

Baines, A. G.; Schwartz, J. J.; Cheadle, M. J.; John, B. E.

2005-12-01

293

Guided wave observations and evidence for the low-velocity subducting crust beneath Hokkaido, northern Japan  

Science.gov (United States)

At the western side of the Hidaka Mountain range in Hokkaido, we identify a clear later phase in seismograms for earthquakes occurring at the uppermost part of the Pacific slab beneath the eastern Hokkaido. The later phase is observed after P-wave arrivals and has a larger amplitude than the P wave. In this study, we investigate the origin of the later phase from seismic wave observations and two-dimensional numerical modeling of wave fields and interpret it as a guided P wave propagating in the low-velocity subducting crust of the Pacific plate. In addition, the results of our numerical modeling suggest that the low-velocity subducting crust is in contact with a low-velocity material beneath the Hidaka Mountain range. Based on our interpretation for the later phase, we estimate P-wave velocity in the subducting crust beneath the eastern part of Hokkaido by using the differences in the later phase travel times and obtain velocities of 6.8 to 7.5 km/s at depths of 50 to 80 km. The obtained P-wave velocity is lower than the expected value based on fully hydrated mid-ocean ridge basalt (MORB) materials, suggesting that hydrous minerals are hosted in the subducting crust and aqueous fluids may co-exist down to depths of at least 80 km.

Shiina, Takahiro; Nakajima, Junichi; Toyokuni, Genti; Matsuzawa, Toru

2014-12-01

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Adakite-TTG connection and fate of Mesoarchaean basaltic crust of Holenarsipur Nucleus, Dharwar Craton, India  

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Holenarsipur Nucleus (HN) is one of the oldest part (3.5-3.1 Ga) of the continental crust of India. It is made up of mafic-ultramafic-komatitic (3.35 Ga) greenstone belts, migmatitic TTG (tonalite-trondhjemite-granodiorite 3.3 Ga) and diapiric trondhjemites (3.1 Ga). New major, trace and REE data on these rocks presented here provide constraints on adakite-TTG connection model and petrogenesis of these rocks of HN, in addition to recycling of the Early-Mesoarchaean basaltic crust and evolution of Mesoarchaean continental crust. Although of two different ages, the chemical characteristics of the migmatitic TTGs and diapiric trondhjemites of the HN resemble with the Archaean and Cenozoic high silica adakites (HSA), and the adakites from two Neoarchaean greenstone belts of Dharwar Craton, they also display similarities with the Early-Mesoarchaean TTGs from different cratons. Based on the compositional variations observed in TTG and diapiric trondhjemites of HN, we propose that plume fed, basaltic and/or basaltic komatiitic oceanic crust/slab flatly (shallow angle) subducted and melted under high Archaean geothermal regime to produce TTGs. Identification of HSA in 2.7 Ga old greenstone belt subduction complexes that are compositionally similar to TTG, suggests the possibility of continuation of this process into Neoarchaean and supports a genetic connection between TTG and adakites.

Naqvi, S. M.; Mohan, M. Ram; Prathap, J. G. Rana; Sarma, D. Srinivasa

2009-08-01

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Postcollisional mafic igneous rocks record crust-mantle interaction during continental deep subduction  

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Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins. PMID:24301173

Zhao, Zi-Fu; Dai, Li-Qun; Zheng, Yong-Fei

2013-01-01

296

Post-rift km-scale uplift of passive continental margins can be caused by compressive stresses within continental crust  

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Many passive continental margins are flanked by a mountain range up to more than 2 km high (Elevated Passive Continental Margins; EPCMs), e.g. Norway, east and west Greenland, East Brazil, eastern Australia and other margins elsewhere, that have been uplifted long after continental break-up. Explanations for these uplifted margins have been ad hoc, but there has hitherto been no explanation that accounts for their presence at both volcanic and non-volcanic margins and in both polar and tropical climatic environments. A continent breaks up by extension and thinning of the continental crust. Thinning varies from small amounts in the proximal rift to perhaps a factor of 5 or more adjacent to oceanic crust. Continental crust > ca. 25 km thick contains two weak layers, one between strong upper (quartz-rich) and lower (dioritic) crust and the other between strong lower crust and strong mantle. Continental crust buckle-folds with a wavelength of 200-400 km and an amplitude of ca. 0.5 km. Under moderate compression, material in the crust's weak layers starts to flow towards the rift from under the adjacent continent. The lack of weak layers under the thinned, distal rift basin means, however, that flow cannot continue towards the ocean. Mid- and lower crustal material therefore accumulates under the proximal rift, thickening the crust there and lifting it by isostatic response to the thickening. Material flows into the rift until the crust under it is once more as thick as it was prior to extension, but no thicker. This thickened layer extends gradually further and further below t