Sample records for achondrites

  1. A Classification Table for Achondrites (United States)

    Chennaoui-Aoudjehane, H.; Larouci, N.; Jambon, A.; Mittlefehldt, D. W.


    Classifying chondrites is relatively easy and the criteria are well documented. It is based on mineral compositions, textural characteristics and more recently, magnetic susceptibility. It can be more difficult to classify achondrites, especially those that are very similar to terrestrial igneous rocks, because mineralogical, textural and compositional properties can be quite variable. Achondrites contain essentially olivine, pyroxenes, plagioclases, oxides, sulphides and accessory minerals. Their origin is attributed to differentiated parents bodies: large asteroids (Vesta); planets (Mars); a satellite (the Moon); and numerous asteroids of unknown size. In most cases, achondrites are not eye witnessed falls and some do not have fusion crust. Because of the mineralogical and magnetic susceptibility similarity with terrestrial igneous rocks for some achondrites, it can be difficult for classifiers to confirm their extra-terrestrial origin. We -as classifiers of meteorites- are confronted with this problem with every suspected achondrite we receive for identification. We are developing a "grid" of classification to provide an easier approach for initial classification. We use simple but reproducible criteria based on mineralogical, petrological and geochemical studies. We presented the classes: acapulcoites, lodranites, winonaites and Martian meteorites (shergottite, chassignites, nakhlites). In this work we are completing the classification table by including the groups: angrites, aubrites, brachinites, ureilites, HED (howardites, eucrites, and diogenites), lunar meteorites, pallasites and mesosiderites. Iron meteorites are not presented in this abstract.

  2. Uranium and thorium in achondrites. (United States)

    Morgan, J. W.; Lovering, J. F.


    The abundances of U and Th in 19 achondrites and two pallasite olivines have been measured by radiochemical neutron activation analysis. Brecciated eucrites are enriched relative to chondrites in both elements by factors between 10 and 20, perhaps as a result of a magmatic differentiation process. Two unbrecciated eucrites are far less enriched, possibly due to their origin as igneous cumulates. The diogenites Johnstown and Shalka contain approximately chondritic levels of U and Th, but Ellemeet is 10 times lower. The abundances in three howardites are in good agreement with those expected from major element data for a mixing model with eucrite and diogenite end members. The high O-18 basaltic achondrites Nakhla, Shergotty and Angra dos Reis have a range of U and Th abundances similar to the brecciated eucrites and howardites, but have systematically higher Th/U ratios.

  3. The variation of Zn content in spinel group minerals and daubreelites of primitive achondrites




    We have studied the Zn content of spinel group mineral and daubreelite in primitive achondrites in order to clarify how the content of Zn, a moderately volatile element, reflects their formation process. Primitive achondrites have achondritic texture and chondrite-related chemistry and mineralogy. Because of these characteristics, primitive achondrites are thought to represent the transition from chondrites to achondrites. We have compared the Zn content in spinel group minerals of primitive ...

  4. New Volatile-Rich Clasts from Brecciated Chondrites and Achondrites (United States)

    Patzek, M.; Ebert, S.; Bischoff, A.


    New volatile-rich clasts from brecciated chondrites and achondrites have been analyzed. Some share similarities with CM and some with CI chondrites, but mineralogical and chemical differences still exist.

  5. Hf-W chronometry of primitive achondrites (United States)

    Schulz, T.; Münker, C.; Mezger, K.; Palme, H.


    Metal segregation and silicate melting on asteroids are the most incisive differentiation events in the early evolution of planetary bodies. The timing of these events can be constrained using the short-lived 182Hf- 182W radionuclide system. Here we present new 182Hf- 182W data for major types of primitive achondrites including acapulcoites, winonaites and one lodranite. These meteorites are of particular interest because they show only limited evidence for partial melting of silicates and are therefore intermediate between chondrites and achondrites. For acapulcoites we derived a 182Hf- 182W age of Δ tCAI = 4.1 +1.2/ -1.1 Ma. A model age for winonaite separates calculated from the intercept of the isochron defines an age of Δ tCAI = 4.8 +3.1/ -2.6 Ma (assuming a bulk Hf/W ratio of ˜1.2). Both ages most likely define primary magmatic events on the respective parent bodies, such as melting of metal, although metal stayed in place and did not segregate to form a core. A later thermal event is responsible for resetting of the winonaite isochron, yielding an age of Δ tCAI = 14.3 +2.7/ -2.2 Ma, significantly younger than the model age. Assuming a co-genetic relationship between winonaites and silicates present in IAB iron meteorites (based on oxygen isotope composition) and including data by Schulz et al. (2009), a common parent body chronology can be established. Magmatic activity occurred between ˜1.5 and 5 Ma after CAIs. More than 5 Ma later, intensive thermal metamorphism has redistributed Hf-W. Average cooling rates calculated for the winonaite/IAB parent asteroid range between ˜35 and ˜4 K/Ma, most likely reflecting different burial depths. Cooling rates obtained for acapulcoites were ˜40 K/Ma to ˜720 K and then ˜3 K/Ma to ˜550 K. Accretion and subsequent magmatism on the acapulcoite parent body occurred slightly later if compared to most achondrite parent bodies (e.g., angrites, ureilites and eucrites), in this case supporting the concept of an inverse

  6. Could 433 Eros have a Primitive Achondritic Composition? (United States)

    Burbine, T. H.; McCoy, T. J.; Nittler, L. R.; Bell, J. F., III


    One of the goals of the NEAR (Near Earth Asteroid Rendezvous) mission to 433 Eros is to determine if it has a meteoritic analog. We are currently investigating if primitive achondrites have bulk compositions and spectral properties similar to Eros. Additional information is contained in the original extended abstract.

  7. Assessing the Formation of Ungrouped Achondrite Northwest Africa 8186: Residue, Crystallization Product, or Recrystallized Chondrite? (United States)

    Srinivasan, P.; McCubbin, F. M.; Agee, C. B.


    The recent discoveries of primitive achondrites, metachondrites, and type 7 chondrites challenge the long held idea that all chondrites and achondrites form on separate parent bodies. These meteorites have experienced metamorphic temperatures above petrologic type 6 and have partially melted to various degrees. However, because of their isotopic and compositional similarities to both undifferentiated and differentiated groups, the provenance of these 'type 6+' meteorites remains largely unknown. CK and CV chondrites have recently been linked to a few achondrites due to their strong compositional, mineralogical, and isotopic similarities], suggesting a common origin between these meteorites. Although CVs have generally undergone low degrees of alteration near petrologic type 3, CKs have experienced a wide range of thermal alteration from petrologic type 3 to 6. Thermal evolution models on early accreting bodies predict that an early forming body can partially differentiate due to radiogenic heating, and, as a result, form radial layers of material increasing in thermal grade (types 3 to 6+) from the unmelted chondritic surface towards the differentiated core.Northwest Africa (NWA) 8186 is an ungrouped achondrite that provides compelling evidence for higher degrees of thermal processing and/or melting and differentiation on some CK/CV parent bodies. NWA 8186 plots on the CCAM line on a 3-oxygen isotope diagram directly with CK and CV chondrites and also plots with the CKs in regards to Cr isotopes. This meteorite is dominated by Nickel(II)Oxygen-rich olivine (less than 80%), lacks iron metal, and contains four oxide phases, indicating a high fOxygen (above FMQ) similar to the CKs. Additionally, NWA 8186 does not contain chondrules. We have further investigated the origins of NWA 8186 by examining and comparing the bulk composition of this CK-like achondrite with CK and CV chondrites, allowing us to assess the various scenarios in which NWA 8186 may have formed from


    International Nuclear Information System (INIS)

    The standard planetary formation models assume that primitive materials, such as carbonaceous chondrites, are the precursor materials of evolved planetesimals. Past chronological studies have revealed that planetesimals of several hundred kilometers in size, such as the Howardite-Eucrite-Diogenite (HED) parent body (Vesta) and angrite parent body, began their differentiation as early as ∼3 million years of the solar system formation, and continued for at least several million years. However, the timescale of planetesimal formation in distinct regions of the inner solar system, as well as the isotopic characteristics of the reservoirs from which they evolved, remains unclear. Here we present the first report for the precise 53Mn-53Cr ages of monomict ureilites. Chemically separated phases from one monomict ureilite (NWA 766) yielded the Mn-Cr age of 4564.60 ± 0.67 Ma, identical within error to the oldest age preserved in other achondrites, such as angrites and eucrites. The 54Cr isotopic data for this and seven additional bulk ureilites show homogeneous ε54Cr of ∼-0.9, a value distinct from other achondrites and chondrites. Using the ε54Cr signatures of Earth, Mars, and Vesta (HED), we noticed a linear decrease in the ε54Cr value with the heliocentric distance in the inner region of the solar system. If this trend can be extrapolated into the outer asteroid belt, the ε54Cr signatures of monomict ureilites will place the position of the ureilite parent body at ∼2.8 AU. These observations imply that the differentiation of achondrite parent bodies began nearly simultaneously at ∼4565 Ma in different regions of the inner solar system. The distinct ε54Cr value between ureilite and carbonaceous chondrite also implies that a genetic link commonly proposed between the two is unlikely.

  9. Near Earth Object 2012XJ112 as a source of bright bolides of achondritic nature

    CERN Document Server

    Madiedo, Jose M; Williams, Iwan P; Konovalova, Natalia; Ortiz, Jose L; Castro-Tirado, Alberto; Pastor, Sensi; Reyes, Jose A de los; Cabrera-Caño, Jesus


    We analyze the likely link between the recently discovered Near Earth Object 2012XJ112 and a bright fireball observed over the south of Spain on December 27, 2012. The bolide, with an absolute magnitude of -9 +- 1, was simultaneously imaged during the morning twilight from two meteor stations operated by the SPanish Meteor Network (SPMN). It was also observed by several casual witnesses. The emission spectrum produced during the ablation of the meteoroid in the atmosphere was also recorded. From its analysis the chemical nature of this particle was inferred. Although our orbital association software identified several potential parent bodies for this meteoroid, the analysis of the evolution of the orbital elements performed with the Mercury 6 symplectic integrator supports the idea that NEO 2012XJ112 is the source of this meteoroid. The implications of this potential association are discussed here. In particular, the meteoroid bulk chemistry is consistent with a basaltic achondrite, and this emphasizes the im...

  10. Antarctic primitive achondrites Yamato-74025, -75300, and -75305:Their mineralogy, thermal history and the relevance to winonaite




    Three Antarctic primitive achondrites, Yamato (Y)-74025,-75300,and -75305 were mineralogically and chemically studied. They consist of anhedral to subhedral silicate and opaque minerals. The major constituent minerals are typical of equilibrated ordinary chondrites. However, they do not have any relic of chondrule, and the presence of various accessory minerals, such as K-feldspar, schreibersite, daubreelite, phosphate, Nb-bearing rutile, and magnesiochromite, characterizes these meteorites. ...

  11. Petrography and petrogenesis of some Indian basaltic achondrites derived from the HED parent body: Insights from electron microprobe analyses

    Indian Academy of Sciences (India)

    Rajesh K Srivastava


    Three Indian achondrites, viz., Bholghati howardite, Lohawat howardite and Pipliya Kalan eucrite and two other achondrites, viz., Bé ré ba eucrite and Johnstown diogenite are studied for their petrography and mineral chemistry. All these achondrites are derived from the HED parent body. Both Bholghati and Lohawat howardites are polymict breccias and contain pieces of eucrites and diaogenites (lithic clasts), pyroxene and minor olivine as mineral clasts, and small proportion of ilmenite and pure iron metal. Eucrite clasts are noncumulate basaltic in nature, whereas diogenite clasts are mostly composed of orthopyroxene with minor clinopyroxene and anorthite. Both howardite samples contain orthopyroxene, pigeonite and augite. Notable characteristics observed in Lohawat howardite include crystallization of orthoenstatite first at a high-temperature followed by ferrosilite, pigeonite olivine and augite from a basaltic melt. Piplia Kalan eucrite is noncumulate, unbrecciated and basaltic in nature and display ophitic/sub-ophitic or hypidiomorphic textures. It contains ∼60% pyroxenes (clinoenstatite and pigeonite) and ∼40% plagioclase feldspars (bytownite to anorthite). The observed mineralogy in the Piplia Kalan eucrite suggests its crystallization from a high-temperature basaltic melt crystallized at low pressure. Two other achondrite samples, viz., Bé ré ba eucrite and Johnstown diogenite are also studied. The Bé ré ba eucrite shows cumulate nature which is probably formed by small-degree melts of ilmenitebearing gabbro, whereas the Johnstown diogenite crystallized from a slow cooling of a Ca-poor basaltic melt derived from cumulates formed from the magma ocean, similar to the origin of the noncumulate eucrites.

  12. U-Pb and Al-Mg systematics of the ungrouped achondrite Northwest Africa 7325 (United States)

    Koefoed, Piers; Amelin, Yuri; Yin, Qing-Zhu; Wimpenny, Josh; Sanborn, Matthew E.; Iizuka, Tsuyoshi; Irving, Anthony J.


    Northwest Africa (NWA) 7325 is a unique ungrouped gabbroic achondrite which has characteristics consistent with a possible link to the planet Mercury. In order to understand the origin of this meteorite and the nature of its parent body, we have determined its crystallisation age using the long-lived U-Pb and short-lived Al-Mg chronometers. An internal Pb-Pb isochron defined by six acid leached pyroxene fractions yields an age of 4563.4 ± 2.6 Ma, assuming that the 238U/235U ratio for NWA 7325 is identical to the bulk Earth and Solar System value of 137.794. The Al-Mg isotope analyses of seven fractions (four plagioclase, one pyroxene, one olivine and one whole rock) define a regression line corresponding to 26Al/27Al0 = (3.03 ± 0.14) × 10-7 and an initial δ26Mg∗ of 0.093 ± 0.004‰. When anchored to the D'Orbigny angrite, this initial 26Al/27Al yields an age of 4563.09 ± 0.26 Ma. The Pb-Pb age of 4563.4 ± 2.6 Ma and Al-Mg age of 4563.09 ± 0.26 Ma are in complete agreement, but the low U concentrations of NWA 7325 resulted in a relatively low precision Pb-Pb age. The observed excess in initial δ26Mg∗ can be explained by 27Al/24Mg fractionation and subsequent Mg isotopic evolution after planetary differentiation. Furthermore, the parental magma of NWA 7325 most likely formed within 1.72 Ma after calcium-aluminium rich inclusion (CAI) formation. NWA 7325 formed near simultaneously with quenched angrites and a number of ungrouped achondrites at ∼4563 Ma, suggesting that a multitude of planetary bodies had formed and differentiated by ∼4-5 Myr after CAI formation. This ancient age may be interpreted as an argument against NWA 7325 originating from Mercury, however it does not completely rule it out.

  13. U-Pb systematics of the unique achondrite Ibitira: Precise age determination and petrogenetic implications (United States)

    Iizuka, Tsuyoshi; Amelin, Yuri; Kaltenbach, Angela; Koefoed, Piers; Stirling, Claudine H.


    Ibitira is an unbrecciated, equilibrated vesicular basaltic achondrite that is considered to have originated on a parent body distinct from all other known meteorites. We present the first combined high-precision U and Pb isotopic data for this unique meteorite. The 238U/235U value of 137.777 ± 0.013 determined for the whole rock is comparable to values determined for bulk chondrites and other basaltic achondrites. This value results in corrections of -1.1 Ma for Pb-Pb dates calculated using the previously assumed invariant 238U/235U value of 137.88. Using the determined 238U/235U value, the 7 most radiogenic Pb isotopic analyses for acid-leached pyroxene-rich and whole rock fractions yield an isochron Pb-Pb age of 4556.75 ± 0.57 Ma, in excellent agreement with the results of Mn-Cr chronology which give the ages of 4557.4 ± 2.5 Ma and 4555.9 ± 3.2 Ma using the U-corrected Pb-Pb age of D'Orbigny as a time anchor. Along with the previously proposed thermal history of Ibitira and our closure temperature estimates for Pb diffusion, the Pb-Pb age is interpreted as the timing of the last chemical equilibration and coarse pyroxene exsolution that occurred during high temperature metamorphism. The metamorphism may have been caused by burial of Ibitira lava under successive lava flows and, if so, the Pb-Pb age should post-date the crystallization by a short time interval. The Pb isotopic data for acid leachates suggest partial re-equilibration of Pb between plagioclase and phosphate, perhaps during an impact event at 4.49 Ga, as recorded by K-Ar systematics. The whole rock 238U/204Pb indicates that compared to CI chondrites, Ibitira is less depleted in Pb than in some alkali elements despite a lower condensation temperature of Pb than the alkali elements. The restricted Pb depletion may reflect preferential concentration of metals with high fluid/melt partition coefficients including Pb and Zn as a result of fluid exsolution and migration within the parent magma. We

  14. Determination of primordial and cosmogenic radioactivity in achondritic meteorites by low-level, gamma-ray spectrometry

    International Nuclear Information System (INIS)

    A high-sensitivity, low-background gamma-ray spectrometer containing two 23 cm by 13 cm thallium-activated, sodium iodide detectors was used to measure long-lived primordial and cosmogenic radioactivity in a suite of achondritic meteorites. Potassium, thorium, uranium, and 26Al abundances were established for sixteen brecciated eucrites, two unbrecciated eucrites, a nakhlite, a chassignite, and a unique meteorite from Antarctica by nondestructive counting techniques. In several cases, multiple samples of the same meteorite fall were examined. Concentrations ranged from 79.8 ppM to 1150 ppM for potassium, 55.6 ppb to 663 ppb for thorium, 18.1 ppb to 190 ppb for uranium, and 45.0 dpm/kg to 99.0 dpm/kg for 26Al. In addition, a 137Cs concentration of 264 dpm/kg was observed in the Allan Hills 77005,9 specimen

  15. AR-39Ar-40 dating of basalts and rock breccias from Apollo 17 and the malvern achondrite (United States)

    Kirsten, T.; Horn, P.


    The principles and the potential of the Ar-39/Ar-40 dating technique are illustrated by means of results obtained for 12 Apollo 17 rocks. Emphasis is given to methodical problems and the geological interpretation of lunar rock ages. Often it is ambigious to associate a given lunar breccia with a certain formation, or a formation with a basin. In addition, large-scale events on the Moon have not necessarily reset radiometric clocks completely. One rock fragment has a well-defined plateau age of 4.28 b.y., but the ages of two Apollo 17 breccias define an upper limit for the formation age of the Serenitatis basin at 4.05 b.y. Ages derived from five mare basalts indicate cessation of mare volcanism at Taurus-Littrow approximately 3.78 b.y. ago. Ca/Ar-37 exposure ages show that Camelot Crater was formed by an impact approximately 95 m.y. ago. After a short summary of the lunar timetable as it stands at the end of the Apollo program, we report about Ar-39/Ar-40 and rare gas studies on the Malvern meteorite. This achondrite resembles lunar highland breccias in texture as well as in rare-gas patterns. It was strongly annealed at some time between 3.4 and 3.8 b.y. ago. The results indicate that very similar processes have occurred on the Moon and on achondritic parent bodies at comparable times, leading to impact breccias with strikingly similar features, including the retention of rare-gas isotopes from various sources.

  16. 40Ar/39Ar impact ages and time-temperature argon diffusion history of the Bunburra Rockhole anomalous basaltic achondrite (United States)

    Jourdan, Fred; Benedix, Gretchen; Eroglu, Ela.; Bland, Phil. A.; Bouvier, Audrey.


    The Bunburra Rockhole meteorite is a brecciated anomalous basaltic achondrite containing coarse-, medium- and fine-grained lithologies. Petrographic observations constrain the limited shock pressure to between ca. 10 GPa and 20 GPa. In this study, we carried out nine 40Ar/39Ar step-heating experiments on distinct single-grain fragments extracted from the coarse and fine lithologies. We obtained six plateau ages and three mini-plateau ages. These ages fall into two internally concordant populations with mean ages of 3640 ± 21 Ma (n = 7; P = 0.53) and 3544 ± 26 Ma (n = 2; P = 0.54), respectively. Based on these results, additional 40Ar/39Ar data of fusion crust fragments, argon diffusion modelling, and petrographic observations, we conclude that the principal components of the Bunburra Rockhole basaltic achondrite are from a melt rock formed at ∼3.64 Ga by a medium to large impact event. The data imply that this impact generated high enough energy to completely melt the basaltic target rock and reset the Ar systematics, but only partially reset the Pb-Pb age. We also conclude that a complete 40Ar∗ resetting of pyroxene and plagioclase at this time could not have been achieved at solid-state conditions. Comparison with a terrestrial analog (Lonar crater) shows that the time-temperature conditions required to melt basaltic target rocks upon impact are relatively easy to achieve. Ar data also suggest that a second medium-size impact event occurred on a neighbouring part of the same target rock at ∼3.54 Ga. Concordant low-temperature step ages of the nine aliquots suggest that, at ∼3.42 Ga, a third smaller impact excavated parts of the ∼3.64 Ga and ∼3.54 Ga melt rocks and brought the fragments together. The lack of significant impact activity after 3.5 Ga, as recorded by the Bunburra Rockhole suggests that (1) either the meteorite was ejected in a small secondary parent body where it resided untouched by large impacts, or (2) it was covered by a porous heat

  17. 40Ar-39Ar age of the Shergotty achondrite and implications for its post-shock thermal history

    International Nuclear Information System (INIS)

    Analyses of 40Ar-39Ar have been made on a whole rock sample and a maskelynite (feldspar) separate of the shocked Shergotty achondrite. The maskelynite gave a plateau age of 254 +- 10 Myr. The whole rock sample gave a complex release with apparent ages between 240 and 640 Myr. The slightly younger Rb-Sr isochron age of 165 Myr for Shergotty suggests that the maskelynite as well as the whole rock was incompletely degassed. Reasonable Ar diffusion characteristics for Shergotty for shock heating temperatures of 0C indicate D/a2 of 10-11 to 10-13sec-1. The time required to lose 95% of the 40Ar from the plagioclase would be approximately 103 to 104 yr. When this gas diffusion time is introduced into a thermal model of a cooling ejecta blanket of variable thickness, a post-shock cooling time of >= 103 yr and a burial depth of >= 300 m are indicated for Shergotty. These conclusions are not seriously affected by uncertainties in the thermal model. Most likely the shock event occurred approximately 165 Myr ago, but no earlier than 250 Myr ago, when the Shergotty parent object experienced a collision in the asteroid belt. As a result of that collision, feldspar was converted to maskelynite, the K-Ar and Rb-Sr ages were completely or nearly completely reset, and the Shergotty meteorite was heated to 0C and left to cool slowly inside the parent body. (author)

  18. Northwest Africa 6693: A new type of FeO-rich, low-Δ17O, poikilitic cumulate achondrite (United States)

    Warren, Paul H.; Rubin, Alan E.; Isa, Junko; Brittenham, Steve; Ahn, Insu; Choi, Byeon-Gak


    Northwest Africa 6693 is a new type of achondrite, with a unique combination of oxygen-isotopic composition (low Δ17O: -1.08‰; also δ17O = 1.19‰) and FeO-rich, low mg bulk composition. A mode (in vol%) shows 70% pyroxene, 16% olivine and 13% feldspar, along with 0.6% Cr-spinel, and 0.4% NiFe metal (awaruite). Its coarse-poikilitic texture, with pigeonite oikocrysts up to 14 mm, as well as the subchondritic MgO/SiO2 of the rock's bulk composition, indicate origin as an igneous cumulate. The cumulus phases included pigeonite and olivine, and the parent magma was probably also saturated with feldspar, which occurs mainly as anhedral, yet optically continuous, grains intergrown with the pyroxene. The mafic silicates are uniformly ferroan: pigeonite near En57Wo3.2 and olivine near Fo49. The feldspar is uniformly albitic, near Ab92, except for a single tiny grain of Ab57Or43. However, the albite features diverse K/Ca (Or/An) ratios: ranging from consistently ˜0.46 in one end of the oblong NWA 6693 stone, to 5.2 in an olivine-rich enclave that consists mostly of micrographic olivine-feldspar intergrowth. Also, siderophile and incompatible element data show heterogeneity among samples from different regions of this large cumulate. The rock was probably neither an orthocumulate nor an adcumulate, and the proportion of "trapped liquid" probably varied from place to place. After initial crystallization, a shock event caused very minor brecciation, and pervasively mobilized linear-arcuate trails of microinclusions (minute oxides, mostly) and bubbles. A minor proportion of additional melt was formed within, and/or infiltrated into, the rock and formed discrete overgrowth mantles, recognizable based on unusual scarcity of microinclusions, on some pyroxenes. Final cooling, based on mineral-equilibration temperatures, occurred at a moderate rate by intrusive-igneous standards. Olivine, metal, and sulfide phases are all very Ni-rich (e.g., olivine NiO averages 0.77 wt

  19. History of the Pasamonte achondrite: relative susceptibility of the Sm-Nd, Rb-Sr, and U-Pb systems to metamorphic events

    International Nuclear Information System (INIS)

    The Rb-Sr, Sm-Nd, and U-Pb systematics of the eucrite Pasamonte have been studied in order to investigate the relative susceptibility of the different systems to post-crystallization events and to determine the age and history of the meteorite. The Rb-Sr and 238U-206Pb data of mineral separates do not define an isochron but the Sm-Nd data define an internal isochron which corresponds to the formation age of 4.58+-0.12 b.y. (109 years). The 207Pb- 206Pb data of mineral separates define an apparent age of 4.53+-0.03 b.y., however it is concluded that this age, while in agreement with the Sm-Nd age, is not strictly valid since the U-Pb data indicate at least three stages of evolution. The U-Pb data indicate that the parent body of the meteorite experienced brecciation shortly after the formation of the parent body surface (approximately 4.2-4.45 b.y. ago) and a recent disturbance (collision) 6+-30 m.y. ago. The latter age is within the range of cosmic ray exposure ages for achondrites. (Auth.)

  20. A partial melting study of an ordinary (H) chondrite composition with application to the unique achondrite Graves Nunataks 06128 and 06129 (United States)

    Usui, Tomohiro; Jones, John H.; Mittlefehldt, David W.


    Melting experiments of a synthesized, alkali-bearing, H-chondrite composition were conducted at ambient pressure with three distinct oxygen fugacity conditions (IW-1, IW, and IW+2). Oxygen fugacity conditions significantly influence the compositions of partial melts. Partial melts at IW-1 are distinctly enriched in SiO2 relative to those of IW and IW+2 melts. The silica-enriched, reduced (IW-1) melts are characterized by high alkali contents and have silica-oversaturated compositions. In contrast, the silica-depleted, oxidized (≥IW) melts, which are also enriched in alkali contents, have distinctly silica-undersaturated compositions. These experimental results suggest that alkali-rich, felsic, asteroidal crusts as represented by paired achondrites Graves Nunataks 06128 and 06129 should originate from a low-degree, relatively reduced partial melt from a parent body having near-chondritic compositions. Based on recent chronological constraints and numerical considerations as well as our experimental results, we propose that such felsic magmatism should have occurred in a parent body that is smaller in size and commenced accreting later than those highly differentiated asteroids having basaltic crusts and metallic cores.

  1. Carbonaceous Chondrite Clasts in HED Achondrites (United States)

    Zolensky, M. E.; Weisberg, M. K.; Buchanan, P. C.; Mittlefehldt, David W.


    Since carbonaceous chondrite planetesimals are attractive candidates for the progenitors of HED asteroid(s), we have performed a survey of HED meteorites in order to locate and characterize the mineralogy, chemistry, and petrography of the oft-reported carbonaceous chondrite clasts by microprobe, SEM-EDX. and TEM techniques. We examined samples of all HEDs we could lay our gloved hands on, and found carbonaceous chondrite clasts in the howardites Kapoeta, Jodzie, EET 87513, Y 793497, LEW 85441, LEW 87015, and G'Day, the polymict eucrites LEW 97295 and LEW 95300, and the diogenite Ellemeet. We verified previous suggestions that the majority (about 80%) of these clasts are CM2 material, but we discovered that a significant proportion are CR2 (about 20%) and other rare types are present. We conclude that chondritic compounds of mixed CM2 and CR2 materials should be investigated in future geochemical modeling of the origin of the HED asteroid(s).

  2. Petrologic and Oxygen-Isotopic Investigations of Eucritic and Anomalous Mafic Achondrites (United States)

    Mittlefehldt, D. W.; Greenwood, R. C.; Peng, Z. X.; Ross, D. K.; Berger, E. L.; Barrett, T. J.


    The most common asteroidal igneous meteorites are eucrite-type basalts and gabbros rocks composed of ferroan pigeonite and augite, calcic plagioclase, silica, ilmenite, troilite, Ca-phosphate, chromite and Fe-metal. These rocks are thought to have formed on a single asteroid along with howardites and diogenites (HEDs). However, Northwest Africa (NWA) 011 is mineralogically identical to eucrites, but has an O-isotopic composition distinct from them and was derived from a different asteroid. Modern analyses with higher precision have shown that some eucrites have smaller O-isotopic differences that are nevertheless well-resolved from the group mean.

  3. Characterization of crust formation on a parent body of achondrites and the moon by pyroxene crystallography and chemistry (United States)

    Takeda, H.; Miyamoto, M.; Ishii, T.; Reid, A. M.


    Single crystal X-ray diffraction and electron microprobe techniques were used to study lunar crustal pyroxenes in a cataclastic norite, a pyroxene-rich clast, and anorthosite lunar samples, and also in meteorites including diogenites, eucrites, and the Yamoto (1) howardite. The crystallographic and chemical characteristics of pyroxenes in these materials are compared and are discussed in terms of the lower stability limit of pigeonite. A mechanical mixing model of howardite is proposed.

  4. Geochemical identification of impactor for Lonar crater, India

    Digital Repository Service at National Institute of Oceanography (India)

    Mishra, S.; Newsom, H.E.; ShyamPrasad, M.; Geissman, J.W.; Dube, A.; Sengupta, D.

    activation analysis and did not find any significant Ir enrichment. In his study, Lonar glasses were also found to be significantly depleted in Re (~7 times) and Se (~2.5 times) relative to parent basalts, which was attributed to a volatilization effect... of achondrite meteorites. The advantages and disadvantages of the identification of achondrite impactors by Ni, Cr and Fe, are as follows. The achondrites have recently been classified into two groups: PGE-poor achondrites (AI) including howardites, eucrites...

  5. Noble gases in 18 Martian meteorites and angrite Northwest Africa 7812—Exposure ages, trapped gases, and a re-evaluation of the evidence for solar cosmic ray-produced neon in shergottites and other achondrites (United States)

    Wieler, R.; Huber, L.; Busemann, H.; Seiler, S.; Leya, I.; Maden, C.; Masarik, J.; Meier, M. M. M.; Nagao, K.; Trappitsch, R.; Irving, A. J.


    We present noble gas data for 16 shergottites, 2 nakhlites (NWA 5790, NWA 10153), and 1 angrite (NWA 7812). Noble gas exposure ages of the shergottites fall in the 1-6 Ma range found in previous studies. Three depleted olivine-phyric shergottites (Tissint, NWA 6162, NWA 7635) have exposure ages of ~1 Ma, in agreement with published data for similar specimens. The exposure age of NWA 10153 (~12.2 Ma) falls in the range of 9-13 Ma reported for other nakhlites. Our preferred age of ~7.3 Ma for NWA 5790 is lower than this range, and it is possible that NWA 5790 represents a distinct ejection event. A Tissint glass sample contains Xe from the Martian atmosphere. Several samples show a remarkably low (21Ne/22Ne)cos ratio SCR Ne) in addition to the commonly found galactic cosmic ray-produced Ne, implying very low preatmospheric shielding and ablation loss. We revisit this by comparing measured (21Ne/22Ne)cos ratios with predictions by cosmogenic nuclide production models. Indeed, several shergottites, acalpulcoites/lodranites, angrites (including NWA 7812), and the Brachina-like meteorite LEW 88763 likely contain SCR Ne, as previously postulated for many of them. The SCR contribution may influence the calculation of exposure ages. One likely reason that SCR nuclides are predominantly detected in meteorites from rare classes is because they usually are analyzed for cosmogenic nuclides even if they had a very small (preatmospheric) mass and hence low ablation loss.

  6. Large-Scale Melting and Impact Mixing on Early-Formed Asteroids: Evidence from High-Precision Oxygen Isotope Studies

    DEFF Research Database (Denmark)

    Greenwood, Richard; Barrat, J-A; Scott, Edward Robert Dalton;

    into larger-sized bodies [2]. Achondritic meteorites provide a unique source of information about the nature of these early events. However, the fragmentary character of this record makes it difficult to interpret. High-precision oxygen isotope analysis has proved to be an important technique in understanding...... the relationship between the different groups of achondrites [3, 4]. Here we present new oxygen isotope evidence con-cerning the role of large-scale melting and subsequent impact mixing in the evolution of three important achondrite groups: the main-group pallasites, meso-siderites and HEDs....

  7. Micrometeorite flux on Earth during the last ~50,000 years

    Digital Repository Service at National Institute of Oceanography (India)

    ShyamPrasad, M.; Rudraswami, N.G.; Panda, D.K.

    chondritic parent body; a single spherule (0.2% of the population) suggests an achondritic parent body. Unbiased collections spanning large areas temporally and spatially enlarge the inventory of the Earth-crossing meteoroid complex and provide valuable...

  8. Almahata Sitta News: Well-Known Varieties and New Species in the Zoo (United States)

    Bischoff, A.; Ebert, S.; Patzek, M.; Horstmann, M.; Pack, A.; Decker, S.


    Mineralogical characteristics of 18 new samples from the Almahata Sitta strewn field are presented. Among the samples are 5 E chondrites, 12 samples of ureilitic origin (including a new trachyandesite), and an enstatite- and metal-rich achondrite.

  9. The Meteoritical Bulletin, No. 102 (United States)

    Ruzicka, Alex; Grossman, Jeffrey; Bouvier, Audrey; Herd, Christopher D. K.; Agee, Carl B.


    Meteoritical Bulletin 102 contains 3141 meteorites including 12 falls (Boumdeid (2003), Boumdeid (2011), Braunschweig, Chelyabinsk, Dongyang, Draveil, Heyetang, Indian Butte, Katol, Ladkee, Ouadangou, Xining), with 2611 ordinary chondrites, 264 HED achondrites, 124 carbonaceous chondrites, 30 ureilites, 20 Martian meteorites, 16 primitive achondrites, 16 Rumuruti chondrites, 15 mesosiderites, 12 iron meteorites, 10 lunar meteorites, 9 enstatite chondrites, 4 enstatite achondrites, 4 Pallasites, 4 ungrouped achondrites, and 2 angrites, and with 1708 from Antarctica, 956 from Africa, 294 from South America, 126 from Asia, 47 from North America, 6 from Europe (including Russia), and 4 from Oceania. Information about approved meteorites can be obtained from the Meteoritical Bulletin Database (MBD) available on line at">

  10. NWA 1235: A Phlogopite-bearing Enstatite Meteorite (United States)

    Lorenz, C.; Kurat, G.; Brandstätter, F.; Nazarov, M. A.


    NWA 1235 achondrite was formed from an enstatite meteorite source under more oxidized conditions than enstatite meteorites. Its unique features are unusual composition of sulfides, a wide set of microinclusions and the occurrence of fluorphlogopite.

  11. The Meteoritical Bulletin, No. 101 (United States)

    Ruzicka, Alex; Grossman, Jeffrey; Bouvier, Audrey; Herd, Christopher D. K.; Agee, Carl B.


    Meteoritical Bulletin 101 contains 2639 meteorites accepted by the Nomenclature Committee in 2012, including 1 fall (Battle Mountain), with 2308 ordinary chondrites, 156 carbonaceous chondrites, 63 HED achondrites, 17 relict meteorites, 16 Rumuruti chondrites, 15 enstatite chondrites, 15 ureilites, 10 iron meteorites, 9 lunar meteorites, 9 primitive achondrites, 8 ungrouped achondrites, 7 mesosiderites, 4 Martian meteorites, and 2 Pallasites, and with 1812 from Antarctica, 437 from Asia, 301 from Africa, 43 from South America, 21 from Europe (including Russia), 21 from North America, 3 from Oceania, and 1 from unknown. Information about approved meteorites can be obtained from the Meteoritical Bulletin Database (MBD) available on line at">

  12. Rapid timescales for magma ocean crystallization on the howardite-eucrite-diogenite parent body

    DEFF Research Database (Denmark)

    Schiller, M.; Paton, C.; Bizzarro, Martin;


    Asteroid 4 Vesta has long been postulated as the source for the howardite-eucrite-diogenite (HED) achondrite meteorites. Here we show that Al-free diogenite meteorites record variability in the mass-independent abundance of Mg ( Mg*) that is correlated with their mineral chemistry. This suggests ...

  13. Mineralogical Characterization of Baptistina Asteroid Family: Implications for K/T Impactor Source

    CERN Document Server

    Reddy, Vishnu; Lazzaro, Daniela; Michtchenko, Tatiana A; Gaffey, Michael J; Kelley, Michael S; Diniz, Thais Mothé; Candal, Alvaro Alvarez; Moskovitz, Nicholas A; Cloutis, Edward A; Ryan, Erin L; 10.1016/j.icarus.2011.08.027


    Bottke et al. (2007) linked the catastrophic formation of Baptistina Asteroid Family (BAF) to the K/T impact event. This linkage was based on dynamical and compositional evidence, which suggested the impactor had a composition similar to CM2 carbonaceous chondrites. However, our recent study (Reddy et al. 2009) suggests that the composition of (298) Baptistina is similar to LL-type ordinary chondrites rather than CM2 carbonaceous chondrites. This rules out any possibility of it being related to the source of the K/T impactor, if the impactor was of CM-type composition. Mineralogical study of asteroids in the vicinity of BAF has revealed a plethora of compositional types suggesting a complex formation and evolution environment. A detailed compositional analysis of 16 asteroids suggests several distinct surface assemblages including ordinary chondrites (Gaffey SIV subtype), primitive achondrites (Gaffey SIII subtype), basaltic achondrites (Gaffey SVII subtype and V-type), and a carbonaceous chondrite. Based on ...

  14. Petrology of Anomalous Eucrite QUE 94484 (United States)

    Mittlefehldt, D. W.; Peng, Z. X.


    Most mafic achondrites are broadly "eucritic", being composed of ferroan low-Ca clinopyroxene, high-Ca plagioclase, a silica phase, ilmenite and accessory phases. Their characteristics indicate that eucrite-like basalts formed on asteroids of similar composition under similar petrologic conditions (T, P, fO2). Some eucrite-like basalts have isotopic compositions and petrologic characteristics consistent with formation on different parent asteroids (e.g., Ibitira, NWA 011). Others show small isotopic differences but no distinguishing petrological characteristics (e.g., Caldera, Pasamonte). We have begun a study of anomalous eucrite-like achondrites in an effort to seek resolution to the issues: Did the eucrite parent asteroid fail to homogenize via a magma-ocean stage, thus explaining outliers like Pasamonte? How many parent asteroids are represented by these basalts? Here we present preliminary petrologic information on anomalous basaltic eucrite QUE 94484.

  15. Curation and Allocation of the New Antarctic Nakhlite, MIL03346 (United States)

    McBride, K. M.; Righter, K.; Satterwhite, C. E.; Schwarz, C.; Robinson, P.


    In January 2004, the ANSMET reconnaissance field team (Fig. 1) working in the Miller Range of the Transantarctic Mountains discovered a 715 g achondrite that was instantly recognized as unique. Named MIL03346, initial processing (NASA Johnson Space Center or JSC) and classification (Smithsonian Institution or SI) revealed this achondrite to be a nakhlite (Fig. 2). MIL03346 is the seventh nakhlite recognized in world collections [2], the third nakhlite returned from Antartica, and the first nakhlite in the US Antarctic collection (Table 1). The following is a summary of the steps taken in the processing and allocating of MIL 03346 and some comparisons to some other lunar and martian meteorites processed and allocated at JSC.

  16. 1991 Urey Prize Lecture: Physical evolution in the solar system - Present observations as a key to the past (United States)

    Binzel, Richard P.


    The present evaluation of the use of new observational methods for exploring solar system evolutionary processes gives attention to illustrative cases from the constraining of near-earth asteroid sources and the discovery of main-belt asteroid fragments which indicate Vesta to be a source of basaltic achondrite meteorites. The coupling of observational constraints with numerical models clarifies cratering and collisional evolution for both main-belt and Trojan asteroids.

  17. Geochemical arguments for an Earth-like Moon-forming impactor


    Dauphas, Nicolas; Burkhardt, Christoph; Warren, Paul H.; Fang-Zhen, Teng


    Geochemical evidence suggests that the material accreted by the Earth did not change in nature during Earth's accretion, presumably because the inner protoplanetary disc had uniform isotopic composition similar to enstatite chondrites, aubrites and ungrouped achondrite NWA 5363/5400. Enstatite meteorites and the Earth were derived from the same nebular reservoir but diverged in their chemical evolutions, so no chondrite sample in meteorite collections is representative of the Earth's building...

  18. Oxygen isotopic composition of relict olivine grains in cosmic spherules: Links to chondrules from carbonaceous chondrites

    Digital Repository Service at National Institute of Oceanography (India)

    Rudraswami, N.G.; ShyamPrasad, M.; Nagashima, K.; Jones, R.H.

    evidence of evolved asteroids, similar to chondritic or achondritic parent bodies (e.g., Taylor et al., 2007; Genge et al., 2008; Gounelle et al., 2009; Badjukov et al., 2010; Cordier et al., 2011a, b, 2012; Suavet et al., 2010; Van Ginneken et al., 2012... is much less (Taylor et al., 2000; Yada et al., 2004; Prasad et al., 2013). They are expected to have sampled a wide variety of parent bodies, including those known to us through meteorite studies, and some that have not yet been sampled (Brownlee...

  19. Petrology of Diogenite NWA 5480, A Pristine Olivine-Rich Deformed Harzburgite (United States)

    Peslier, A. H.; Brandon, A. D.; Tarduno, J. A.; Mittlefehldt, D. W.


    Diogenites are achondrites that are part of the HED (howardite, eucrite, diogenite) meteorite group thought to originate from asteroid Vesta. This suite of igneous rocks offers a glimpse of early planetary differentiation and subsequent igneous processes. While eucrites represent asteroidal basaltic crust and howardites the impact brecciated surface, diogenites are samples of the mantle and lower crust. Most of them are orthopyroxene (Opx) dominated cumulates, although harzburgites and rare dunites have also been found. The majority of diogenites are impact breccias. This study describes NWA 5480, a pristine, i.e. hardly altered and minimally shocked, harzburgitic diogenite.

  20. Samarium-neodymium systematics in kimberlites and in the minerals of garnet lherzolite inclusions (United States)

    Basu, A.R.; Tatsumoto, M.


    The initial ratios of neodymium-143 to neodymium-144 in kimberlites ranging in age between 90 ?? 106 to 1300 ?? 106 years from South Africa, India, and the United States are different from the corresponding ratios in the minerals of peridotite inclusions in the kimberlites but are identical to the ratios in the basaltic achondrite Juvinas at the times of emplacement of the respective kimberlite pipes. This correlation between the kimberlites and Juvinas, which represents the bulk chondritic earth in rare-earth elements, strongly indicates that the kimberlite's source in the mantle is chondritic in rare-earth elements and relatively primeval in composition. Copyright ?? 1979 AAAS.

  1. 35 seasons of US antarctic meteorites (1976-2010) a pictorial guide to the collection

    CERN Document Server

    Righter, Kevin; McCoy, Timothy; Harvey, Ralph; Harvey, Ralph


    The US Antarctic meteorite collection exists due to a cooperative program involving the National Science Foundation (NSF), the National Aeronautics and Space Administration (NASA), and the Smithsonian Institution.  Since 1976, meteorites have been collected by a NSF-funded field team, shipped for curation, characterization, distribution, and storage at NASA, and classified and stored for long term at the Smithsonian.  It is the largest collection in the world with many significant samples including lunar, martian, many interesting chondrites and achondrites, and even several unusual one-of-

  2. Early stages of core segregation recorded by Fe isotopes in an asteroidal mantle


    Barrat, Jean-Alix; Rouxel, O; Wang, K; Moynier, F; Yamaguchi, A; Bischoff, A; Langlade, J


    International audience; Ureilites displays  56 Fe values higher than average chondrite. 29-Segregation of Fe-sulfide melts explains the high  56 Fe values in ureilites. 30-Formation of a core can begin at very low degrees of melting through the circulation of a Fe-S melt 31 through a silicate mantle. 32 33 Earth and Planetary Science Letters, in press (11/3/15). 34 2 35 Abstract 36 37 Ureilite meteorites are achondrites that are debris of the mantle of a now disrupted 38 differentiated aste...

  3. Microprobe analyses of rare-earth-element fractionation in meteoritic minerals

    International Nuclear Information System (INIS)

    Two meteorites were analyzed by PIXE with the Los Alamos Nuclear Microprobe. The enstatite achondrite Pena Blanca Spring and the ordinary chondrite St. Severin were chosen as likely candidates for use in 244Pu (t/sub 1/2/ = 82 my) cosmochronology and geochronology. These applications require the meteoritic minerals to have unfractionated actinides and lanthanides relative to cosmic elemental abundance ratios. The PIXE analyses produced evidence of actinide-lanthanide fractionation in Pena Blanca Spring oldhamite (CaS) whereas the St Severin phosphates, whitlockite and chlorapatite, do not exhibit this fractionation

  4. The meteorite collection at Museo di Storia Naturale, Pisa University, Italy (United States)

    Perchiazzi, Natale; D'Orazio, Massimo; Folco, Luigi


    The historical meteorite collection of Museo di Storia Naturale, Pisa University, is presented in this catalog. Dating back to at least 1860, the collection currently (as of June 2003) contains 30 specimens of 26 individual meteorites, representing about 50 kg of extraterrestrial material. The collection includes 2 carbonaceous chondrites, 12 ordinary chondrites, 1 achondrite, 4 stony-iron meteorites, and 7 iron meteorites, including three remarkable specimens: the main mass of Bagnone (48 kg), the fourth largest mass of Quenggouk (717.5 g), and a large (nearly) complete individual of the Siena showerfall (318.8 g).

  5. Petrology of the Baszkowka L5 chondrite: A record of surface-forming processes on the parent body (United States)

    Przylibski, T. A.; Pilski, A. S.; Zagożdżon, P. P.; Kryza, R.


    We review the petrology of Baszkowka, present new microprobe data on mineral constituents, and propose a model for surface properties of the parent body consistent with these data. The low shock index and high porosity of the Baszkowka L5 chondrite mean that considerable primary textural and petrographic detail is preserved, allowing insight into the structure and evolution of the parent body. This meteorite formed in a sedimentary environment resembling that in which pyroclastic rocks are deposited. The origin of the component chondrules, achondritic fragments (mostly olivine and pyroxene aggregates), chondritic-achondritic aggregates, and compound chondrules can be explained by invoking collision of 2 melted or partially melted planetesimals, each covered with a thin crust. This could have happened at an early stage in the evolution of the solar system, between 1 and 2 Myr after its origin. The collision resulted in the formation of a cloud containing products of earlier magmatic crystallization (chondrite and achondrite fragments) from which new chondrules were created. Particle collision in this cloud produced fragmented chondrules, chondritic-achondritic aggregates, and compound chondrules. Within this low-density medium, these particles were accreted on the surface of the larger of the planetesimals involved in the collision. The density of the medium was low enough to prevent grain-size sorting of the components but high enough to prevent the total loss of heat and to enable the welding of fragments on the surface of the body. The rock material was homogenized within the cloud and, in particular, within the zone close to the planetesimal surface. The hot material settled on the surface and became welded as molten or plastic metal, and sulfide components cemented the grains together. The process resembled the formation of welded ignimbrites. Once these processes on the planetesimal surface were completed, no subsequent recrystallization occurred. The high

  6. Lunar and Planetary Science XXXV: Meteorites (United States)


    The session "Meteorites" included the following reports:Description of a New Stony Meteorite Find from Bulloch County, Georgia; Meteorite Ablation Derived from Cosmic Ray Track Data Dhofar 732: A Mg-rich Orthopyroxenitic Achondrite Halogens, Carbon and Sulfur in the Tagish Lake Meteorite: Implications for Classification and Terrestrial Alteration; Electromagnetic Scrape of Meteorites and Probably Columbia Tiles; Pre-Atmospheric Sizes and Orbits of Several Chondrites; Research of Shock-Thermal History of the Enstatite Chondrites by Track, Thermoluminescence and Neutron-Activation (NAA) Methods; Radiation and Shock-thermal History of the Kaidun CR2 Chondrite Glass Inclusions; On the Problem of Search for Super-Heavy Element Traces in the Meteorites: Probability of Their Discovery by Three-Prong Tracks due to Nuclear Spontaneous Fission Trace Element Abundances in Separated Phases of Pesyanoe, Enstatite Achondrite; Evaluation of Cooling Rate Calculated by Diffusional Modification of Chemical Zoning: Different Initial Profiles for Diffusion Calculation; Mineralogical Features and REE Distribution in Ortho- and Clinopyroxenes of the HaH 317 Enstatite Chondrite Dhofar 311, 730 and 731: New Lunar Meteorites from Oman; The Deuterium Content of Individual Murchison Amino Acids; Clues to the Formation of PV1, an Enigmatic Carbon-rich Chondritic Clast from the Plainview H-Chondrite Regolith Breccia ;Numerical Simulations of the Production of Extinct Radionuclides and ProtoCAIs by Magnetic Flaring.

  7. Bunburra Rockhole: Exploring the Geology of a New Differentiated Basaltic Asteroid (United States)

    Benedix, G.K.; Bland, P. A.; Friedrich, J. M.; Mittlefehldt, D.; Sanborn, M. E.; Yin, Q.-Z.; Greenwood, R. C; Franchi, L. A.; Bevan, A. W. R.; Towner, M. C.; Perotta, Grace C.


    Bunburra Rockhole (BR) is the first recovered meteorite of the Desert Fireball Network. It was initially classified as a basaltic eucrite, based on texture, mineralogy, and mineral chemistry but subsequent O isotopic analyses showed that BR's composition lies significantly far away from the HED group of meteorites. This suggested that BR was not a piece of the HED parent body (4 Vesta), but other explanations could also account for the observed oxygen signatures. Possible scenarios include contamination by components from other bodies (chondrites or other achondrites) or that 4 Vesta may not be as equilibrated as hypothesized. After examining multiple pieces with different instruments (CT scans and x-ray maps), no obvious evidence of contamination was found. If BR is not from Vesta, a conundrum exists as no unusual features were found in mineral and bulk trace element chemistry as exist for other anomalous basaltic achondrites such as Ibitira or Asuka 881394. These meteorites have distinct petrological and geochemical characteristics, in addition to their anomalous O isotope compositions, that set them apart from eucrites. Thus, early results provided a somewhat ambiguous picture of BR's petrogenesis and parentage. To clarify the nature of the relationship, if any, between BR and eucrites, we have performed a correlated stable isotope and bulk chemical study of several lithologic fragments.

  8. The initial abundance and distribution of 92Nb in the Solar System

    CERN Document Server

    Iizuka, Tsuyoshi; Akram, Waheed; Amelin, Yuri; Schönbächler, Maria


    Niobium-92 is an extinct proton-rich nuclide, which decays to 92Zr with a half-life of 37 Ma. This radionuclide potentially offers a unique opportunity to determine the timescales of early Solar System processes and the site(s) of nucleosynthesis for p-nuclei, once its initial abundance and distribution in the Solar System are well established. Here we present internal Nb-Zr isochrons for three basaltic achondrites with known U-Pb ages: the angrite NWA 4590, the eucrite Agoult, and the ungrouped achondrite Ibitira. Our results show that the relative Nb-Zr isochron ages of the three meteorites are consistent with the time intervals obtained from the Pb-Pb chronometer for pyroxene and plagioclase, indicating that 92Nb was homogeneously distributed among their source regions. The Nb-Zr and Pb-Pb data for NWA 4590 yield the most reliable and precise reference point for anchoring the Nb-Zr chronometer to the absolute timescale: an initial 92Nb/93Nb ratio of $(1.4 \\pm 0.5) \\times 10^{-5}$ at $4557.93 \\pm 0.36$ Ma, ...

  9. The Mineralogy and Petrology of Anomalous Eucrite Emmaville (United States)

    Barrett, T. J.; Mittlefehldt, D. W.; Ross, D. K.; Greenwood, R. C.; Anand, M.; Franchi, I. A.; Grady, M. M.; Charlier, B. L. A.


    It has long been known that certain basaltic achondrites share similarities with eucrites. These eucrite-like achondrites have distinct isotopic compositions and petrologic characteristics indicative of formation on a separate parent body from the howardite-eucrite-diogenite (HED) clan (e.g., Ibitira, Northwest Africa (NWA) 011). Others show smaller isotopic variations but are otherwise petrologically and compositionally indistinguishable from basaltic eucrites (e.g., Pasamonte, Pecora Escarpment (PCA) 91007). The Emmaville eucrite has a delta O-17 value of -0.137 plus or minus 0.024 per mille (1 sigma), which is substantially different from the eucrite mean of -0.246 plus or minus 0.014 per mille (2 sigma), but similar to those of A-881394 and Bunburra Rockhole (BR). Currently little data exist for Emmaville in terms of petrology or bulk composition. Studying anomalous eucrites allows us to more completely understand the numbers of asteroids represented by eucrite- like basalts and thus constrain the heterogeneity of the HED suite. In this study, we present our preliminary petrological and mineral composition results for Emmaville.

  10. The Foreign Clast Populations of Anomalous Polymict Urelite Almahata Sitta (Asteroid 2008 TC(sub3) and Typical Polymict Ureilites: Implications for Asteroid-Meteorite Connections (United States)

    Goodrich, C. A.; Treiman, A. H.; Zolensky, M.; Kita, N. T.; Defouilloy, C.; Fioretti, A. M.; O'Brien, D. P.; Jenniskens, P.; Shaddad, M. H.


    Almahata Sitta (AhS) is the first meteorite to originate from an asteroid (2008 TC3) that had been studied in space before it hit Earth [1,2]. It is also unique because the fallen fragments comprise a variety of types: approximately 69% ureilites (achondrites) and 31% chondrites [3]. Two models have been proposed for the origin 2008 TC3: 1) an accretionary model [3,4]; or 2) a regolith model [5,6]. Typical polymict ureilites are interpreted to represent regolith, and contain a few % foreign clasts [7,8]. The most common are dark (CC matrix-like) clasts similar to those in many meteoritic breccias [9]. A variety of other chondrites, as well as achondrites (angrites), have also been reported [7,9,10]. We have been working to determine the full diversity of these clasts [10-13] for comparison with AhS. We discuss implications for mixing of materials in the early solar system and the origin of 2008 TC3.

  11. Analysis of Moderately Siderophile Elements in Angrites: Implications for Core Formation of the Angrite Parent Body (United States)

    Righter, K.; Shirai, N.; Irving, A.J.


    Angrites are an enigmatic group of achondrites, that constitute the largest group of basalts not affiliated with the Moon, Mars or Vesta (HEDs). Chemically, angrites are exceptionally refractory element- enriched (e.g., Al, Ca) and volatile element-depleted (e.g., Na and K) achondrites. Highly volatile siderophile and chalcophile elements (Zn, Ge and Se) may be less depleted than alkalis and Ga taken to imply a fractionation of plagiophile elements. Core formation on the angrite parent body (APB) is not well understood due to the dearth of moderately siderophile element (Ga, Ge, Mo, Sb, W) data for angrites, with the exception of Ni and Co [2]. In particular, there are no data for Mo abundances of angrites, while Sb and W abundances are reported for only 3 angrites, and have not always been determined on the same sample. The recent increase in angrite numbers (13) has greatly increased our knowledge of the compositional diversity of the angrite parent body (APB). In this study, we report new Co, Ni, Ga, Mo, Sb and W abundances for angrites by laser ablation inductively coupled plasma mass spectrometry (ICP-MS) in order to place constraints on core formation of the APB.

  12. Cosmochemical and spectroscopic properties of Northwest Africa 7325—A consortium study (United States)

    Weber, I.; Morlok, A.; Bischoff, A.; Hiesinger, H.; Ward, D.; Joy, K. H.; Crowther, S. A.; Jastrzebski, N. D.; Gilmour, J. D.; Clay, P. L.; Wogelius, R. A.; Greenwood, R. C.; Franchi, I. A.; Münker, C.


    This work is part of a project to build an infrared database in order to link IR data of planetary materials (and therefore possible Mercury material) with remote sensing observations of Mercury, which will probably be obtained by the MERTIS instrument on the forthcoming BepiColombo mission. The unique achondrite Northwest Africa (NWA) 7325, which has previously been suggested to represent the first sample from Mercury, was investigated by optical and electron microscopy, and infrared and Raman spectroscopy. In addition, the oxygen, strontium, xenon, and argon isotopes were measured and the abundance of selected trace elements determined. The meteorite is a cumulate rock with subchondritic abundances of HFSE and REE and elevated Sr contents, which underwent a second heating and partial remelting process. Oxygen isotope measurements show that NWA 7325 plots in the ureilite field, close to the ALM-A trachyandesitic fragment found in the unique Almahata Sitta meteorite breccia. On the other hand, mineralogical investigations of the pyroxenes in NWA 7325 provide evidence for similarities to the lodranites and acapulcoites. Furthermore, the rock is weakly shocked and argon isotope data record ancient (~4.5 Ga) plateau ages that have not been reset. The sample records a cosmogenic exposure age of ~19 Ma. Systematics of Rb-Sr indicate an extreme early volatile depletion of the precursor material, similar to many other achondrite groups. However, despite its compositional similarities to other meteorite groups, our results suggest that this meteorite is unique and unrelated to any other known achondrite group. An origin for NWA 7325 as a sample from the planet Mercury is not supported by the results of our investigation. In particular, the evidence from infrared spectroscopy indicates that a direct relationship between NWA 7325 and the planet Mercury can be ruled out: no acceptable spectral match between laboratory analyses and remote sensing observations from Mercury has

  13. Antarctic Meteorite Newsletter, Volume 31, No. 1 (United States)

    Satterwhite, Cecilia (Editor); Righter, Kevin (Editor)


    This newsletter reports 418 new meteorites from the 2004 and 2006 ANSMET seasons from the Cumulus Hills (CMS), LaPaz Ice Field (LAP), Graves Nunataks (GRA), Grosvenor Mountains (GRO), Larkman Nunatak (LAR), MacAlpine Hills (MAC), Miller Range (MIL), Roberts Massif (RBT), and Scott Glacier (SCO). These new samples include one iron, 1 eucrite, 1 mesosiderite, 6 CK chondrites (2 with pairing), 2 CV3 chondrites, 1 CM1, 7 CM2 (4 with pairing), 3 CR2 (2 with pairing), and one each of a type 3 L and H chondrites. The CK6 chondrites (LAR 06869, 06872, 06873) are unusual in that they have no discernable chondrules, extremely fine-grained texture, and are full of veins. This newsletter represents a break from recent newsletters in which we have announced many unusual and popular samples, including new lunar and martian meteorites, an unusual achondrite (GRA 06128 and 06129 the topic of a special session at this years LPSC).

  14. Mineralogy and Surface Composition of Asteroids

    CERN Document Server

    Reddy, Vishnu; Thomas, Cristina A; Moskovitz, Nicholas A; Burbine, Thomas H


    Methods to constrain the surface mineralogy of asteroids have seen considerable development during the last decade with advancement in laboratory spectral calibrations and validation of our interpretive methodologies by spacecraft rendezvous missions. This has enabled the accurate identification of several meteorite parent bodies in the main asteroid belt and helped constrain the mineral chemistries and abundances in ordinary chondrites and basaltic achondrites. With better quantification of spectral effects due to temperature, phase angle, and grain size, systematic discrepancies due to non-compositional factors can now be virtually eliminated for mafic silicate-bearing asteroids. Interpretation of spectrally featureless asteroids remains a challenge. This paper presents a review of all mineralogical interpretive tools currently in use and outlines procedures for their application.

  15. Search for extinct natural radioactivity of Pb205 via thallium-isotope anomalies in chondrites and lunar soil. (United States)

    Huey, J. M.; Kohman, T. P.


    Thallium and Pb204 contents were determined by stable-isotope-dilution analysis in 16 chondrites, one achondrite, and Apollo 11 and 12 lunar fines. Meteoritic thallium contents vary over a large range, 0.02 to 100 ppb, corresponding to the fact that thallium is a highly fractionated volatile element. Lunar thallium contents are less than 5 ppb. The Tl205/Tl203 ratio was determined in most of the samples, with precision ranging from 0.03% to several percent depending mainly on the amount of thallium present. No variations from the terrestrial ratio were observed. The chondritic isochron slope for Pb205 (13.8-m.y. half-life) is less than or equal to 0.00009 (99% confidence level), corresponding to an interval of at least 60 m.y. and possibly exceeding 120 m.y. between the termination of s-process nucleosynthesis and the lead-thallium fractionations.

  16. Chondrule size and related physical properties: a compilation and evaluation of current data across all meteorite groups

    CERN Document Server

    Friedrich, Jon M; Ebel, Denton S; Biltz, Alison E; Corbett, Bernadette M; Iotzov, Ivan V; Khan, Wajiha S; Wolman, Matthew D


    The examination of the physical properties of chondrules has generally received less emphasis than other properties of meteorites such as their mineralogy, petrology, and chemical and isotopic compositions. Among the various physical properties of chondrules, chondrule size is especially important for the classification of chondrites into chemical groups, since each chemical group possesses a distinct size-frequency distribution of chondrules. Knowledge of the physical properties of chondrules is also vital for the development of astrophysical models for chondrule formation, and for understanding how to utilize asteroidal resources in space exploration. To examine our current knowledge of chondrule sizes, we have compiled and provide commentary on available chondrule dimension literature data. We include all chondrite chemical groups as well as the acapulcoite primitive achondrites, some of which contain relict chondrules. We also compile and review current literature data for other astrophysically-relevant p...

  17. Mercury (Hg) in meteorites: variations in abundance, thermal release profile, mass-dependent and mass-independent isotopic fractionation

    CERN Document Server

    Meier, Matthias M M; Marty, Bernard


    We have measured the concentration, isotopic composition and thermal release profiles of Mercury (Hg) in a suite of meteorites, including both chondrites and achondrites. We find large variations in Hg concentration between different meteorites (ca. 10 ppb to 14'000 ppb), with the highest concentration orders of magnitude above the expected bulk solar system silicates value. From the presence of several different Hg carrier phases in thermal release profiles (150 to 650 {\\deg}C), we argue that these variations are unlikely to be mainly due to terrestrial contamination. The Hg abundance of meteorites shows no correlation with petrographic type, or mass-dependent fractionation of Hg isotopes. Most carbonaceous chondrites show mass-independent enrichments in the odd-numbered isotopes 199Hg and 201Hg. We show that the enrichments are not nucleosynthetic, as we do not find corresponding nucleosynthetic deficits of 196Hg. Instead, they can partially be explained by Hg evaporation and redeposition during heating of ...

  18. Early inner solar system origin for anomalous sulfur isotopes in differentiated protoplanets. (United States)

    Antonelli, Michael A; Kim, Sang-Tae; Peters, Marc; Labidi, Jabrane; Cartigny, Pierre; Walker, Richard J; Lyons, James R; Hoek, Joost; Farquhar, James


    Achondrite meteorites have anomalous enrichments in (33)S, relative to chondrites, which have been attributed to photochemistry in the solar nebula. However, the putative photochemical reactions remain elusive, and predicted accompanying (33)S depletions have not previously been found, which could indicate an erroneous assumption regarding the origins of the (33)S anomalies, or of the bulk solar system S-isotope composition. Here, we report well-resolved anomalous (33)S depletions in IIIF iron meteorites (solar system (solar system S-isotope composition was chondritic (consistent with IAB iron meteorites, Earth, Moon, and Mars). The range of mass-independent sulfur isotope compositions may reflect spatial or temporal changes influenced by photochemical processes. A tentative correlation between S isotopes and Hf-W core segregation ages suggests that the two systems may be influenced by common factors, such as nebular location and volatile content.

  19. Early inner solar system origin for anomalous sulfur isotopes in differentiated protoplanets. (United States)

    Antonelli, Michael A; Kim, Sang-Tae; Peters, Marc; Labidi, Jabrane; Cartigny, Pierre; Walker, Richard J; Lyons, James R; Hoek, Joost; Farquhar, James


    Achondrite meteorites have anomalous enrichments in (33)S, relative to chondrites, which have been attributed to photochemistry in the solar nebula. However, the putative photochemical reactions remain elusive, and predicted accompanying (33)S depletions have not previously been found, which could indicate an erroneous assumption regarding the origins of the (33)S anomalies, or of the bulk solar system S-isotope composition. Here, we report well-resolved anomalous (33)S depletions in IIIF iron meteorites (solar system (solar system S-isotope composition was chondritic (consistent with IAB iron meteorites, Earth, Moon, and Mars). The range of mass-independent sulfur isotope compositions may reflect spatial or temporal changes influenced by photochemical processes. A tentative correlation between S isotopes and Hf-W core segregation ages suggests that the two systems may be influenced by common factors, such as nebular location and volatile content. PMID:25453079

  20. Possible spinel absorption bands in S-asteroid visible reflectance spectra (United States)

    Hiroi, T.; Vilas, F.; Sunshine, J. M.


    Minor absorption bands in the 0.55 to 0.7 micron wavelength range of reflectance spectra of 10 S asteroids have been found and compared with those of spinel-group minerals using the modified Gaussian model. Most of these S asteroids are consistently shown to have two absorption bands around 0.6 and 0.67 micron. Of the spinel-group minerals examined in this study, the 0.6 and 0.67 micron bands are most consistent with those seen in chromite. Recently, the existence of spinels has also been detected from the absorption-band features around 1 and 2 micron of two S-asteroid reflectance spectra, and chromite has been found in a primitive achondrite as its major phase. These new findings suggest a possible common existence of spinel-group minerals in the solar system.

  1. Mercury (Hg) in meteorites: Variations in abundance, thermal release profile, mass-dependent and mass-independent isotopic fractionation (United States)

    Meier, Matthias M. M.; Cloquet, Christophe; Marty, Bernard


    We have measured the concentration, isotopic composition and thermal release profiles of Mercury (Hg) in a suite of meteorites, including both chondrites and achondrites. We find large variations in Hg concentration between different meteorites (ca. 10 ppb to 14,000 ppb), with the highest concentration orders of magnitude above the expected bulk solar system silicates value. From the presence of several different Hg carrier phases in thermal release profiles (150-650 °C), we argue that these variations are unlikely to be mainly due to terrestrial contamination. The Hg abundance of meteorites shows no correlation with petrographic type, or mass-dependent fractionation of Hg isotopes. Most carbonaceous chondrites show mass-independent enrichments in the odd-numbered isotopes 199Hg and 201Hg. We show that the enrichments are not nucleosynthetic, as we do not find corresponding nucleosynthetic deficits of 196Hg. Instead, they can partially be explained by Hg evaporation and redeposition during heating of asteroids from primordial radionuclides and late-stage impact heating. Non-carbonaceous chondrites, most achondrites and the Earth do not show these enrichments in vapor-phase Hg. All meteorites studied here have however isotopically light Hg (δ202Hg = ∼-7 to -1) relative to the Earth's average crustal values, which could suggest that the Earth has lost a significant fraction of its primordial Hg. However, the late accretion of carbonaceous chondritic material on the order of ∼2%, which has been suggested to account for the water, carbon, nitrogen and noble gas inventories of the Earth, can also contribute most or all of the Earth's current Hg budget. In this case, the isotopically heavy Hg of the Earth's crust would have to be the result of isotopic fractionation between surface and deep-Earth reservoirs.

  2. Khatyrka, a new CV3 find from the Koryak Mountains, Eastern Russia (United States)

    MacPherson, Glenn J.; Andronicos, Christopher L.; Bindi, Luca; Distler, Vadim V.; Eddy, Michael P.; Eiler, John M.; Guan, Yunbin; Hollister, Lincoln S.; Kostin, Alexander; Kryachko, Valery; Steinhardt, William M.; Yudovskaya, Marina; Steinhardt, Paul J.


    A new meteorite find, named Khatyrka, was recovered from eastern Siberia as a result of a search for naturally occurring quasicrystals. The meteorite occurs as clastic grains within postglacial clay-rich layers along the banks of a small stream in the Koryak Mountains, Chukotka Autonomous Okrug of far eastern Russia. Some of the grains are clearly chondritic and contain Type IA porphyritic olivine chondrules enclosed in matrices that have the characteristic platy olivine texture, matrix olivine composition, and mineralogy (olivine, pentlandite, nickel-rich iron-nickel metal, nepheline, and calcic pyroxene [diopside-hedenbergite solid solution]) of oxidized-subgroup CV3 chondrites. A few grains are fine-grained spinel-rich calcium-aluminum-rich inclusions with mineral oxygen isotopic compositions again typical of such objects in CV3 chondrites. The chondritic and CAI grains contain small fragments of metallic copper-aluminum-iron alloys that include the quasicrystalline phase icosahedrite. One grain is an achondritic intergrowth of Cu-Al metal alloys and forsteritic olivine ± diopsidic pyroxene, both of which have meteoritic (CV3-like) oxygen isotopic compositions. Finally, some grains consist almost entirely of metallic alloys of aluminum + copper ± iron. The Cu-Al-Fe metal alloys and the alloy-bearing achondrite clast are interpreted to be an accretionary component of what otherwise is a fairly normal CV3 (oxidized) chondrite. This association of CV3 chondritic grains with metallic copper-aluminum alloys makes Khatyrka a unique meteorite, perhaps best described as a complex CV3 (ox) breccia.

  3. Hungaria asteroid family as the source of aubrite meteorites (United States)

    Ćuk, Matija; Gladman, Brett J.; Nesvorný, David


    The Hungaria asteroids are interior to the main asteroid belt, with semimajor axes between 1.8 and 2 AU, low eccentricities and inclinations of 16-35°. Small asteroids in the Hungaria region are dominated by a collisional family associated with (434) Hungaria. The dominant spectral type of the Hungaria group is the E or X-type (Warner et al. [2009]. Icarus, 204, 172-182), mostly due to the E-type composition of Hungaria and its genetic family. It is widely believed the E-type asteroids are related to the aubrite meteorites, also known as enstatite achondrites (Gaffey et al. [1992]. Icarus, 100, 95-109). Here we explore the hypothesis that aubrites originate in the Hungaria family. In order to test this connection, we compare model Cosmic Ray Exposure ages from orbital integrations of model meteoroids with those of aubrites. We show that long CRE ages of aubrites (longest among stony meteorite groups) reflect the delivery route of meteoroids from Hungarias to Earth being different than those from main-belt asteroids. We find that the meteoroids from Hungarias predominantly reach Earth by Yarkovsky-drifting across the orbit of Mars, with no assistance from orbital resonances. We conclude that the CRE ages of aubrites are fully consistent with a dominant source at the inner boundary of the Hungaria family at 1.7 AU. From here, meteoroids reach Earth through the Mars-crossing region, with relatively quick delivery times favored due to collisions (with Hungarias and the inner main-belt objects). We find that, after Vesta, (434) Hungaria is the best candidate for an asteroidal source of an achondrite group.

  4. Proceedings of the 38th Lunar and Planetary Science Conference (United States)


    The sessions in the conference include: Titan, Mars Volcanism, Mars Polar Layered Deposits, Early Solar System Isotopes, SPECIAL SESSION: Mars Reconnaissance Orbiter: New Ways of Studying the Red Planet, Achondrites: Exploring Oxygen Isotopes and Parent-Body Processes, Solar System Formation and Evolution, SPECIAL SESSION: SMART-1, . Impact Cratering: Observations and Experiments, SPECIAL SESSION: Volcanism and Tectonism on Saturnian Satellites, Solar Nebula Composition, Mars Fluvial Geomorphology, Asteroid Observations: Spectra, Mostly, Mars Sediments and Geochemistry: View from the Surface, Mars Tectonics and Crustal Dichotomy, Stardust: Wild-2 Revealed, Impact Cratering from Observations and Interpretations, Mars Sediments and Geochemistry: The Map View, Chondrules and Their Formation, Enceladus, Asteroids and Deep Impact: Structure, Dynamics, and Experiments, Mars Surface Process and Evolution, Martian Meteorites: Nakhlites, Experiments, and the Great Shergottite Age Debate, Stardust: Mainly Mineralogy, Astrobiology, Wind-Surface Interactions on Mars and Earth, Icy Satellite Surfaces, Venus, Lunar Remote Sensing, Space Weathering, and Impact Effects, Interplanetary Dust/Genesis, Mars Cratering: Counts and Catastrophes?, Chondrites: Secondary Processes, Mars Sediments and Geochemistry: Atmosphere, Soils, Brines, and Minerals, Lunar Interior and Differentiation, Mars Magnetics and Atmosphere: Core to Ionosphere, Metal-rich Chondrites, Organics in Chondrites, Lunar Impacts and Meteorites, Presolar/Solar Grains, Topics for Print Only papers are: Outer Planets/Satellites, Early Solar System, Interplanetary Dust, Comets and Kuiper Belt Objects, Asteroids and Meteoroids, Chondrites, Achondrites, Meteorite Related, Mars Reconnaissance Orbiter, Mars, Astrobiology, Planetary Differentiation, Impacts, Mercury, Lunar Samples and Modeling, Venus, Missions and Instruments, Global Warming, Education and Public Outreach, Poster sessions are: Asteroids/Kuiper Belt Objects

  5. Asteroid 4 Vesta: A Fully Differentiated Dwarf Planet (United States)

    Mittlefehldt, David


    One conclusion derived from the study of meteorites is that some of them - most irons, stony irons, some achondrites - hail from asteroids that were heated to the point where metallic cores and basaltic crusts were formed. Telescopic observations show that there remains only one large asteroid with a basaltic crust, 4 Vesta; present day mean radius 263 km. The largest clan of achondrites, the howardite, eucrite and diogenite (HED) meteorites, represent the crust of their parent asteroid. Diogenites are cumulate harzburgites and orthopyroxenites from the lower crust whilst eucrites are cumulate gabbros, diabases and basalts from the upper crust. Howardites are impact-engendered breccias of diogenites and eucrites. A strong case can be made that HEDs are derived from Vesta. The NASA Dawn spacecraft orbited Vesta for 14 months returning data allowing geological, mineralogical, compositional and geophysical interpretations of Vesta's surface and structure. Combined with geochemical and petrological observations of HED meteorites, differentiation models for Vesta can be developed. Proto-Vesta probably consisted of primitive chondritic materials. Compositional evidence, primarily from basaltic eucrites, indicates that Vesta was melted to high degree (>=50%) which facilitated homogenization of the silicate phase and separation of immiscible Fe,Ni metal plus Fe sulphide into a core. Geophysical models based on Dawn data support a core of 110 km radius. The silicate melt vigorously convected and initially followed a path of equilibrium crystallization forming a harzburgitic mantle, possibly overlying a dunitic restite. Once the fraction of crystals was sufficient to cause convective lockup, the remaining melt collected between the mantle and the cool thermal boundary layer. This melt undergoes fractional crystallization to form a dominantly orthopyroxenite (diogenite) lower crust. The initial thermal boundary layer of primitive chondritic material is gradually replaced by a

  6. On the puzzle of space weathering alteration of basaltic asteroids

    CERN Document Server

    Marchi, S; Lazzarin, M; Magrin, S


    The majority of basaltic asteroids are found in the inner main belt, although a few have also been observed in the outer main belt and near-Earth space. These asteroids -referred to as V-types- have surface compositions that resemble that of the 530km sized asteroid Vesta. Besides the compositional similarity, dynamical evidence also links many V-type asteroids to Vesta. Moreover, Vesta is one of the few asteroids to have been identified as source of specific classes of meteorites, the howardite, eucrite, diogenite achondrites (HEDs). Despite the general consensus on the outlined scenario, several questions remain unresolved. In particular, it is not clear if the observed spectral diversity among Vesta, V-types and HEDs is due to space weathering, as is thought to be the case for S-type asteroids. In this paper, SDSS photometry is used to address the question of whether the spectral diversity among candidate V-types and HEDs can be explained by space weathering. We show that visible spectral slopes of V-types...

  7. U-Pb Dating of Zircons and Phosphates in Lunar Meteorites, Acapulcoites and Angrites (United States)

    Zhou, Q.; Zeigler, R. A.; Yin, Q. Z.; Korotev, R. L.; Joliff, B. L.; Amelin, Y.; Marti, K.; Wu, F. Y.; Li, X. H.; Li, Q. L.; Lin, Y. T.; Liu, Y.; Tang, G. Q.


    Zircon U-Pb geochronology has made a great contribution to the timing of magmatism in the early Solar System [1-3]. Ca phosphates are another group of common accessory minerals in meteorites with great potential for U-Pb geochronology. Compared to zircons, the lower closure temperatures of the U-Pb system for apatite and merrillite (the most common phosphates in achondrites) makes them susceptible to resetting during thermal metamorphism. The different closure temperatures of the U-Pb system for zircon and apatite provide us an opportunity to discover the evolutionary history of meteoritic parent bodies, such as the crystallization ages of magmatism, as well as later impact events and thermal metamorphism. We have developed techniques using the Cameca IMS-1280 ion microprobe to date both zircon and phosphate grains in meteorites. Here we report U-Pb dating results for zircons and phosphates from lunar meteorites Dhofar 1442 and SaU 169. To test and verify the reliability of the newly developed phosphate dating technique, two additional meteorites, Acapulco, obtained from Acapulco consortium, and angrite NWA 4590 were also selected for this study as both have precisely known phosphate U-Pb ages by TIMS [4,5]. Both meteorites are from very fast cooled parent bodies with no sign of resetting [4,5], satisfying a necessity for precise dating.

  8. Differentiation of Planetesimals and the Thermal Consequences of Melt Migration

    CERN Document Server

    Moskovitz, Nicholas


    We model the heating of a primordial planetesimal by decay of the short-lived radionuclides Al-26 and Fe-60 to determine (i) the timescale on which melting will occur; (ii) the minimum size of a body that will produce silicate melt and differentiate; (iii) the migration rate of molten material within the interior; and (iv) the thermal consequences of the transport of Al-26 in partial melt. Our models incorporate results from previous studies of planetary differentiation and are constrained by petrologic (i.e. grain size distributions), isotopic (e.g. Pb-Pb and Hf-W ages) and mineralogical properties of differentiated achondrites. We show that formation of a basaltic crust via melt percolation was limited by the formation time of the body, matrix grain size and viscosity of the melt. We show that low viscosity (100 km in size. Differentiation would be mos t likely for planetesimals larger than 20 km in diameter that accreted within ~2.7 Myr of CAI formation.

  9. Nuclear chemistry

    International Nuclear Information System (INIS)

    Topics covered include: mass asymmetry and total kinetic energy release in the spontaneous fission of 262105; calculation of spontaneous fission properties of very heavy nuclei - 98 less than or equal to Z less than or equal to 106 and 150 less than or equal to N less than or equal to 164; energy losses for 84Kr ions in nickel, aluminium and titanium; differences in compound nuclei formed with 40Ar and 84Kr projectiles; measurement of the energy division vs. mass in highly damped reactions; ambiguities in the inference of precompound emission from excitation function analysis; selective laser one-atom detection of neutral prompt fission fragments; laser induced nuclear polarization - application to the study of spontaneous fission isomers; quadrupole and hexadecapole deformations in the actinide nuclei; high-spin states in 164Yb; contrasting behavior of h/sub 9/2/ and i/sub 13/2/ bands in 185Au; multiple band crossings in 164Er; recoil-distance measurement of lifetimes of rotational states in 164Dy, lifetimes of ground-band states in 192Pt and 194Pt and application of the rotation-alignment model; coulomb excitation of vibrational nuclei with heavy ions; surface structure of deformed nuclei; valency contribution to neutron capture in 32S; neutron capture cross section of manganese; search for superheavy elements in natural samples by neutron multiplicity counting; and gamma-ray studies on the geochemistry of achondritic meteorites

  10. Extraterrestrial Amino Acids in Ureilites Including Almahata Sitta (United States)

    Burton, A. S.; Glavin, D. P.; Callahan, M. P.; Dworkin, J. P.


    Ureilites are a class of meteorites that lack chondrules (achondrites) but have relatively high carbon abundances, averaging approx.3 wt %. Using highly sensitive liquid chromatography coupled with UV fluorescence and time-of-flight mass spectrometry (LC-FD/ToF-MS), it was recently determined that there are amino acids in. fragment 94 of the Almahata Sitta ureilite[l]. Based on the presence of amino acids that are rare in the Earth's biosphere, as well as the near-racemic enantiomeric ratios of marry of the more common amino acids, it was concluded that most of the detected amino acids were indigenous to the meteorite. Although the composition of the Almahata Sitta ureilite appears to be unlike other recovered ureilites, the discovery of amino acids in this meteorite raises the question of whether other ureilites rnav also contain amino acids. Herein we present the results of LC-FDlTo.F-MS analyses of: a sand sample from the Almahata Sitta strewn held, Almahata Sitta fragments 425 (an ordinary H5 chondrite) and 427 (ureilite), as well as an Antarctic ureilite (Allan lulls, ALHA 77257).

  11. Studies on Al Kidirate and Kapoeta meteorites

    Energy Technology Data Exchange (ETDEWEB)

    Gismelseed, A.M. [Sudan Atomic Energy Commission, Khartoum (Sudan); Khangi, F. [Sudan Atomic Energy Commission, Khartoum (Sudan); Ibrahim, A. [Sudan Atomic Energy Commission, Khartoum (Sudan); Yousif, A.A. [Coll. of Science, Sultan Qaboos Univ., Alkhoud (Oman); Worthing, M.A. [Coll. of Science, Sultan Qaboos Univ., Alkhoud (Oman); Rais, A. [Coll. of Science, Sultan Qaboos Univ., Alkhoud (Oman); Elzain, M.E. [Coll. of Science, Sultan Qaboos Univ., Alkhoud (Oman); Brooks, C.K. [Coll. of Science, Sultan Qaboos Univ., Alkhoud (Oman); Sutherland, H.H. [Coll. of Science, Sultan Qaboos Univ., Alkhoud (Oman)


    Moessbauer spectroscopy (20-300 K), magnetic susceptibility measurements (77-350 K), scanning electron microscopy and X-ray diffraction experiments have been performed on two meteorite samples: one from an old fall (Kapoeta) and another from a very recent fall (Al Kidirate). The two specimens differ in their mineralogy. Chondrules appear to be absent in Kapoeta and it is probably a pyroxene-plagioclase achondrite with ferrohypersthene as the most abundant mineral. On the other hand, the Al Kidirate meteorite is an ordinary chondrite and the specimen consists of olivine, orthopyroxene, troilite and kamacite. The Moessbauer measurements confirm the above characterization, showing a paramagnetic doublet for the Kapoeta sample and at least two paramagnetic doublets and magnetic sextets for the Al Kidirate specimens. The former were assigned to Fe in pyroxene sites, while the latter was assigned to Fe in pyroxene, olivine, Fe-S and Fe-Ni alloys. The difference in the mineralogy of the two meteorites has also been reflected in the temperature-dependent magnetic susceptibility. The magnetization and the hyperfine interaction parameters will be discussed in relation to the mineralogy. (orig.)

  12. Hibonite, Ca2/Al, Ti/24O38, from the Leoville and Allende chondritic meteorites. (United States)

    Keil, K.; Fuchs, L. H.


    Hibonite was discovered in light-colored, Ca-Al-Ti-rich and Si-Fe-poor, achondritic inclusions of the Leoville and Allende HL-group chondrites. Two varieties of hibonite occur: one emits a bright red-orange luminescence under electron bombardment and has high amounts of Al2O3 (87.7; 87.9) and low amounts of MgO (0.65; 0.8) and TiO2 (0.68; 0.8). The other emits a bright blue luminescence and is low in Al2O3 (78.7; 79.2) and high in MgO (3.3; 3.7) and TiO2 (6.5; 7.9) (in wt. %). The oxide CaO is about the same in both varieties. It is suggested that the change in the color of the visible luminescence results from changes in composition. The origin of hibonite which occurs in complex mineral assemblages together with anorthite, gelhenite, wollastonite, aluminous diopside, andradite, Ca-pyroxene, perovskite, spinel, taenite, chromite, and pentlandite, and in close proximity to nodules containing calcite, whewellite, forsterite and many of the aforementioned phases, is discussed. The proposition that hibonite and associated phases originated by contact metamorphism and metasomatism of calcite-dolomite bearing assemblages cannot, at this time, be completely ruled out.

  13. Petrology and Geochemistry of New Ureilites and Ureilite Genesis (United States)

    Mittlefehldt, David W.; Herrin, J. S.; Downes, H.


    Ureilites are C-bearing, basalt-depleted olivine+pyroxene achondrites from a differentiated asteroid. The group is heterogeneous, exhibiting ranges in O isotopic composition, Fe/Mg, Fe/Mn, pyx/ol, siderophile and lithophile trace element content, and C content and isotopic composition [1]. Some of these characteristics are nebular in origin; others were strongly overprinted by asteroidal igneous processes. The consensus view is that most ureilites are melt-residues, but some are partial cumulates or have interacted with a melt [1,2]. An "unroofing" event occurred while the parent asteroid was hot that froze in mineral core com-positions and resulted in FeO reduction at olivine grain margins. We have studied several new ureilites, but will focus here on two anomalous stones; LAR 04315 and NWA 1241. LAR 04315 is texturally unusual. It contains olivine with angular subdomains, and low-Ca pyroxene riddled with wormy inclusions of metal+troilite, graphite, and possibly other phases, and irregular inclusions of high-Ca pyroxene. Reduction occurred along olivine grain margins and internal fractures, but not along subdomain boundaries. Although texturally odd, LAR 04351 is a typical ureilite in mineral and bulk composition. The olivine is Fo80.8 and falls on the ureilite Fe/Mn-Fe/Mg trend. Its olivine composition falls within the range of the majority of ureilites, and it is typical of these ureilites in bulk rock lithophile and siderophile element contents.

  14. Genesis of Augite-Bearing Ureilites: Evidence From LA-ICP-MS Analyses of Pyroxenes and Olivine (United States)

    Herrin, J. S.; Lee, C-T. A.; Mittlefehldt, D. W.


    Ureilites are ultramafic achondrites composed primarily of coarse-grained low-Ca pyroxene and olivine with interstitial carbonaceous material, but a number of them contain augite [1]. Ureilites are considered to be restites after partial melting of a chondritic precursor, although at least some augite-bearing ureilites may be partially cumulate [1, 2]. In this scenario, the augite is a cumulus phase derived from a melt that infiltrated a restite composed of typical ureilite material (olivine+low-Ca pyroxene) [2]. To test this hypothesis, we examined the major and trace element compositions of silicate minerals in select augite-bearing ureilites with differing mg#. Polished thick sections of the augite-bearing ureilites ALH 84136 , EET 87511, EET 96293, LEW 88201, and META78008 and augite-free typical ureilite EET 90019 were examined by EPMA for major and minor elements and laser ablation ICP-MS (LA-ICP-MS) for trace elements, REE in particular. Although EET 87511 is reported to contain augite, the polished section that we obtained did not.

  15. The Atlas of Vesta Spectral Parameters derived from Dawn/VIR data (United States)

    Frigeri, A.; De Sanctis, M. C.; Ammannito, E.; Tosi, F.; Zambon, F.; Capaccioni, F.; Capria, M. T.; Palomba, E.; Longobardo, A.; Fonte, S.; Giardino, M.; Magni, G.; Jaumann, R.; Raymond, C. A.; Russell, C. T.


    The Dawn mission mapped Vesta from three different orbital heights during Survey orbit (2700 km altitude), HAMO (High Altitude Mapping Orbit, 700 km altitude), and LAMO (Low Altitude Mapping Orbit, 210 km altitude) [1]. From these orbits the Dawn's Visible and Infrared Mapping Spectrometer (VIR) acquired infrared and visible spectra from 0.2 to 5 microns, sampled in 864 channels with a spatial resolution reaching about 150 m/pixel. Studies of the comparison of spectra from remote sensed data and spectra from laboratory allows to synthesize spectral parameters, which can be combined to identify specific physical and compositional states. VIR spectra of Vesta, stored in about 4300 Planetary Data System (PDS) cubes, have been analyzed to derive spectral parameters, each of which is diagnostic of the associated mineralogy on the surface of the asteroid being observed [2]. Maps of spectral parameters show terrain units compositions in their stratigraphic context. Band centers and band depths are among the most important diagnostic parameters of the mineralogy in a spectrum. In most pyroxenes and in the basaltic achondrites there is a strong correlation between the position of BI center and BII center and the associated mineralogy. For example, orthopyroxene bands shift towards longer wavelengths with increasing amounts of iron, whereas clinopyroxene bands shift towards longer wavelengths with increasing calcium content. Band depth is related to scattering effects, thus can be related to the physical state of the material.

  16. Abundance, distribution, and origin of 60Fe in the solar protoplanetary disk

    CERN Document Server

    Tang, Haolan; 10.1016/j.epsl.2012.10.011


    Meteorites contain relict decay products of short-lived radionuclides that were present in the protoplanetary disk when asteroids and planets formed. Several studies reported a high abundance of 60Fe (t1/2=2.62+/-0.04 Myr) in chondrites (60Fe/56Fe~6*10-7), suggesting that planetary materials incorporated fresh products of stellar nucleosynthesis ejected by one or several massive stars that exploded in the vicinity of the newborn Sun. We measured 58Fe/54Fe and 60Ni/58Ni isotope ratios in whole rocks and constituents of differentiated achondrites (ureilites, aubrites, HEDs, and angrites), unequilibrated ordinary chondrites Semarkona (LL3.0) and NWA 5717 (ungrouped petrologic type 3.05), metal-rich carbonaceous chondrite Gujba (CBa), and several other meteorites (CV, EL H, LL chondrites; IIIAB, IVA, IVB iron meteorites). We derive from these measurements a much lower initial 60Fe/56Fe ratio of (11.5+/-2.6)*10-9 and conclude that 60Fe was homogeneously distributed among planetary bodies. This low ratio is consist...

  17. Potassium-bearing Iron-Nickel Sulfides in Nature and High-Pressure Experiments: Geochemical Consequences of Potassium in the Earth's Core (United States)

    Keshav, S.; Corgne, A.; McDonough, W. F.; Fei, Y.


    Introduction: Potassium (K) as a large ion lithophile element has dominantly been concentrated in the Earth s crust and the mantle through differentiation, and in the form of K-40 contributes to the planet s heat budget. However, whether or not K also enters core-forming phases, has been debated for over three decades. Arguments favoring entry of K in the core are based on: (1) K-sulfide (with Fe, Ni, Cu, Na, and Cl; djerfisherite) found in highly reduced enstatite chondrites (or aubrites, enstatite achondrites); (2) demonstration that K, owing to an s-d electronic switch at high-pressure, exhibits transition- element like character, (3) solubility of measurable K in Fe-Ni-S liquids at high pressure, temperature conditions, and (4) models of cooling of the core that seem to require, besides convection, some form of radioactivity, and thus lending support to the experimental work. In this contribution, we assess the effect of sequestering K in the core, as it is perhaps an element that is a key to reconciling geochemistry, paleomagnetism, accretion, and thermal evolution models for the planet.

  18. Hungaria Asteroid Family as the Source of Aubrite Meteorites

    CERN Document Server

    Ćuk, Matija; Nesvorný, David


    The Hungaria asteroids are interior to the main asteroid belt, with semimajor axes between 1.8 and 2 AU, low eccentricities and inclinations of 16-35 degrees. Small asteroids in the Hungaria region are dominated by a collisional family associated with (434) Hungaria. The dominant spectral type of the Hungaria group is the E or X-type (Warner et al, 2009), mostly due to the E-type composition of Hungaria and its genetic family. It is widely believed the E-type asteroids are related to the aubrite meteorites, also known as enstatite achondrites (Gaffey et al, 1992). Here we explore the hypothesis that aubrites originate in the Hungaria family. In order to test this connection, we compare model Cosmic Ray Exposure ages from orbital integrations of model meteoroids with those of aubrites. We show that long CRE ages of aubrites (longest among stony meteorite groups) reflect the delivery route of meteoroids from Hungarias to Earth being different than those from main-belt asteroids. We find that the meteoroids from...

  19. REE Partition Coefficients from Synthetic Diogenite-Like Enstatite and the Implications of Petrogenetic Modeling (United States)

    Schwandt, C. S.; McKay, G. A.


    Determining the petrogenesis of eucrites (basaltic achondrites) and diogenites (orthopyroxenites) and the possible links between the meteorite types was initiated 30 years ago by Mason. Since then, most investigators have worked on this question. A few contrasting theories have emerged, with the important distinction being whether or not there is a direct genetic link between eucrites and diogenites. One theory suggests that diogenites are cumulates resulting from the fractional crystallization of a parent magma with the eucrites crystallizing, from the residual magma after separation from the diogenite cumulates. Another model proposes that diogenites are cumulates formed from partial melts derived from a source region depleted by the prior generation of eucrite melts. It has also been proposed that the diogenites may not be directly linked to the eucrites and that they are cumulates derived from melts that are more orthopyroxene normative than the eucrites. This last theory has recently received more analytical and experimental support. One of the difficulties with petrogenetic modeling is that it requires appropriate partition coefficients for modeling because they are dependent on temperature, pressure, and composition. For this reason, we set out to determine minor- and trace-element partition coefficients for diogenite-like orthopyroxene. We have accomplished this task and now have enstatite/melt partition coefficients for Al, Cr, Ti, La, Ce, Nd, Sm, Eu, Dy, Er, Yb, and La.

  20. Lunar and Meteorite Sample Disk for Educators (United States)

    Foxworth, Suzanne; Luckey, M.; McInturff, B.; Allen, J.; Kascak, A.


    NASA Johnson Space Center (JSC) has the unique responsibility to curate NASA's extraterrestrial samples from past and future missions. Curation includes documentation, preservation, preparation and distribution of samples for research, education and public outreach. Between 1969 and 1972 six Apollo missions brought back 382 kilograms of lunar rocks, core and regolith samples, from the lunar surface. JSC also curates meteorites collected from a US cooperative effort among NASA, the National Science Foundation (NSF) and the Smithsonian Institution that funds expeditions to Antarctica. The meteorites that are collected include rocks from Moon, Mars, and many asteroids including Vesta. The sample disks for educational use include these different samples. Active relevant learning has always been important to teachers and the Lunar and Meteorite Sample Disk Program provides this active style of learning for students and the general public. The Lunar and Meteorite Sample Disks permit students to conduct investigations comparable to actual scientists. The Lunar Sample Disk contains 6 samples; Basalt, Breccia, Highland Regolith, Anorthosite, Mare Regolith and Orange Soil. The Meteorite Sample Disk contains 6 samples; Chondrite L3, Chondrite H5, Carbonaceous Chondrite, Basaltic Achondrite, Iron and Stony-Iron. Teachers are given different activities that adhere to their standards with the disks. During a Sample Disk Certification Workshop, teachers participate in the activities as students gain insight into the history, formation and geologic processes of the moon, asteroids and meteorites.

  1. Thulium anomalies and rare earth element patterns in meteorites and Earth: Nebular fractionation and the nugget effect

    CERN Document Server

    Dauphas, N


    This study reports the bulk rare earth element (REEs, La-Lu) compositions of 41 chondrites, including 32 falls and 9 finds from carbonaceous (CI, CM, CO and CV), enstatite (EH and EL) and ordinary (H, L and LL) groups, as well as 2 enstatite achondrites (aubrite). The CI-chondrite-normalized REE patterns and Eu anomalies in ordinary and enstatite chondrites show more scatter in more metamorphosed than in unequilibrated chondrites. This is due to parent-body redistribution of the REEs in various carrier phases during metamorphism. The dispersion in REE patterns of equilibrated ordinary chondrites is explained by the nugget effect associated with concentration of REEs in minor phosphate grains. Terrestrial rocks and samples from ordinary and enstatite chondrites display negative Tm anomalies of ~-4.5 % relative to ca chondrites. In contrast, CM, CO and CV (except Allende) show no significant Tm anomalies. Allende CV chondrite shows large excess Tm (~+10 %). These anomalies are similar to those found in group II...

  2. Disk-resolved photometry of Vesta and Lutetia and comparison with other asteroids (United States)

    Longobardo, Andrea; Palomba, Ernesto; Ciarniello, Mauro; Tosi, Federico; De Sanctis, Maria Cristina; Capaccioni, Fabrizio; Zambon, Francesca; Ammannito, Eleonora; Filacchione, Gianrico; Raymond, Carol A.


    . The phase functions found in bright material units on Vesta are similar to those found for Steins (E-type in the Tholen taxonomy, Xe-type in the Bus one), suggesting a photometric analogy between achondritic surfaces. The latter are brighter and with a flatter phase function with respect to chondritic surfaces: we argued that this behavior is driven by optical properties of asteroid surfaces (e.g. albedo, role of multiple scattering) rather than by physical ones (e.g. grain size, roughness). Dark material units on Vesta show an intermediate behavior between achondrites and the C-type Mathilde, confirming once again that these regions are characterized by mixtures of HED and carbonaceous chondrites. While a clear anti-correlation is observed between reflectance and steepness of phase function for V, S and C asteroids, Lutetia shows an anomalous photometric behavior, presenting both a low reflectance and a flat phase curve, and hence cannot be grouped with other spectral classes here considered. This behavior is similar to some X-type asteroids ground-observed at low phase angles and is consistent with a chondritic composition of its surface.

  3. Late Chondritic Additions and Planet and Planetesimal Growth: Evaluation of Physical and Chemical Mechanisms (United States)

    Righter, Kevin


    Studies of terrestrial peridotite and martian and achondritic meteorites have led to the conclusion that addition of chondritic material to growing planets or planetesimals, after core formation, occurred on Earth, Mars, asteroid 4 Vesta, and the parent body of the angritic meteorites [1-4]. One study even proposed that this was a common process in the final stages of growth [5]. These conclusions are based almost entirely on the highly siderophile elements (HSE; Re, Au, Pt, Pd, Rh, Ru, Ir, Os). The HSE are a group of eight elements that have been used to argue for late accretion of chondritic material to the Earth after core formation was complete (e.g., [6]). This idea was originally proposed because the D(metal/silicate) values for the HSE are so high, yet their concentration in the mantle is too high to be consistent with such high Ds. The HSE also are present in chondritic relative abundances and hence require similar Ds if this is the result of core-mantle equilibration. Since the work of [6] there has been a realization that core formation at high PT conditions can explain the abundances of many siderophile elements in the mantle (e.g., [7]), but such detailed high PT partitioning data are lacking for many of the HSE to evaluate whether such ideas are viable for all four bodies. Consideration of other chemical parameters reveals larger problems that are difficult to overcome, but must be addressed in any scenario which calls on the addition of chondritic material to a reduced mantle. Yet these problems are rarely discussed or emphasized, making the late chondritic (or late veneer) addition hypothesis suspect.

  4. Probing Core Processes in the Earth and Small Bodies Using Paleomagnetism (United States)

    Fu, R. R.; Weiss, B. P.; Lima, E. A.; Glenn, D. R.; Kehayias, P.; Walsworth, R. L.


    Convective motion in the cores of differentiated metal-silicate bodies may sustain a global dynamo magnetic field. Progressive crystallization in a dynamo-generating core is expected to play a central role in determining the observable properties of the hosted magnetic field. Importantly, the release of light elements and latent heat during core crystallization is a key source of entropy for sustaining core convection. Therefore, the persistence and intensity of a dynamo magnetic field depend directly on the extent and style of core crystallization. We present and discuss paleomagnetic data from the Earth and asteroid-sized bodies to characterize internally generated magnetic fields during the early histories of these objects. In the case of the Earth, recent and ongoing paleomagnetic experiments of zircons from the Jack Hills of Australia can potentially constrain the existence and intensity of the geodynamo before 3.5 Ga. If robust, such measurements hold strong implications for the energy budget of the Earth's early core and the dynamics of the early mantle. We will discuss both recently published and preliminary results and assess carefully the challenges and uncertainties of paleomagnetic experimentation on ancient zircon samples. In the case of small bodies, several classes of meteorites record ancient magnetic fields likely produced by core dynamos on their parent bodies. Data from the CV carbonaceous chondrites and pallasites indicate that dynamos in planetesimal-sized bodies persisted for a broad range of timescales between ~10 My and >100 My. Meanwhile, measurements of the angrite group of achondrites show that their earliest-forming members crystallized in an almost non-magnetic environment, suggesting a delayed onset of the planetesimal dynamo until several My after initial differentiation. We will discuss the possible causes for this observed diversity of small body dynamo properties, including the role of core crystallization and the distribution of

  5. On the Nature of the Impactor That Formed the Shackleton Crater on the Moon (United States)

    Pugacheva, Svetlana G.; Feoktistova, Ekaterina A.; Shevchenko, Vladislav V.


    The present paper attempts to assess the characteristics of the impactor that formed the Shackleton crater, located at the south pole of the Moon. The crater's morphometric parameters were analyzed based on the data of the Lunar Orbiter Laser Altimeter aboard the Lunar Reconnaissance Orbiter. Conclusions were drawn regarding the possible range of the impact angle and the parameters of the transient crater, such as depth and volume. The thickness of ejecta deposits on the transient crater rim and the volume of these deposits at a certain distance from the crater rim were assessed. These assessments enabled determining the type and characteristics of impactors (velocity, density, size, and impact angle) that could have formed the Shackleton crater. It was shown that the Shackleton crater could have been formed by an impact of a low-velocity (3 km/s) comets with diameter 4-4.5 km, chondrite or achondrite with a diameter of 2 km at a 45°-50° angle, whose velocity did not exceed 6 km/s, as well as stony-iron or iron-nickel impactors with a 1-2 km diameter for stony-iron asteroids and 1-1.5 km for iron-nickel asteroids. The impact velocity of stony-iron impactors, according to the authors' calculations, can reach 12 km/s. The impact velocities of iron-nickel asteroids range from 6 to 9 km/s. The impactor's substance mass that could have remained in the crater after it was formed was assessed.

  6. Experimental Constraints on a Vesta Magma Ocean (United States)

    Hoff, C.; Jones, J. H.; Le, L.


    A magma ocean model was devised to relate eucrites (basalts) and diogenites (orthopyroxenites), which are found mixed together as clasts in a suite of polymict breccias known as howardites. The intimate association of eucritic and diogenitic clasts in howardites argues strongly that these three classes of achondritic meteorites all originated from the same planetoid. Reflectance spectral evidence (including that from the DAWN mission) has long suggested that Vesta is indeed the Eucrite Parent Body. Specifically, the magma ocean model was generated as follows: (i) the bulk Vesta composition was taken to be 0.3 CV chondrite + 0.7 L chondrite but using only 10% of the Na2O from this mixture; (ii) this composition is allowed to crystallize at 500 bar until approx. 80% of the system is solid olivine + low-Ca pyroxene; (iii) the remaining 20% liquid crystallizes at one bar from 1250C to 1110C, a temperature slightly above the eucrite solidus. All crystallization calculations were performed using MELTS. In this model, diogenites are produced by cocrystallization of olivine and pyroxene in the >1250C temperature regime, with Main Group eucrite liquids being generated in the 1300-1250C temperature interval. Low-Ca pyroxene reappears at 1210C in the one-bar calculations and fractionates the residual liquid to produce evolved eucrite compositions (Stannern Trend). We have attempted to experimentally reproduce the ocean. In the MELTS calculation, the change from 500 bar to one bar results in a shift of the olivine:low-Ca pyroxene boundary so that the 1250C liquid is now in the olivine field and, consequently, olivine should be the first-crystallizing phase, followed by low-Ca pyroxene at 1210C, and plagioclase at 1170C. Because at one bar the olivine:low-Ca pyroxene boundary is a peritectic, fractional crystallization of the 1210C liquid proceeds with only pyroxene crystallization until plagioclase appears. Thus, the predictions of the MELTS calculation are clear and

  7. Eucrite Impact Melt NWA 5218 - Evidence for a Large Crater on Vesta (United States)

    Wittmann, Axel; Hiroi, Takahiro; Ross, Daniel K.; Herrin, Jason S.; Rumble, Douglas, III; Kring, David A.


    Northwest Africa (NWA) 5218 is a 76 g achondrite that is classified as a eucrite [1]. However, an initial classification [2] describes it as a "eucrite shock-melt breccia...(in which) large, partially melted cumulate basalt clasts are set in a shock melt flow...". We explore the petrology of this clast-bearing impact melt rock (Fig. 1), which could be a characteristic lithology at large impact craters on asteroid Vesta [3]. Methods: Optical microscopy, scanning electronmicroscopy, and Raman spectroscopy were used on a thin section (Fig. 1) for petrographic characterization. The impact melt composition was determined by 20 m diameter defocused-beam analyses with a Cameca SX-100 electron microprobe. The data from 97 spots were corrected for mineral density effects [4]. Constituent mineral phases were analyzed with a focusedbeam. Bidirectonal visible and near-infrared (VNIR) and biconical FT-IR reflectance spectra were measured on the surface of a sample slab on its central melt area and on an eucrite clast, and from 125-500 m and rock. The thin section captures a central, subophitic-textured melt that contains 1 cm to tens of m-size subangular to rounded, variably-shocked eucrite clasts. Clasts >100 m are coarse-grained with equigranular 1 mm size plagioclase, quartz, and clinopyroxene (Fig. 1). Single crystals of chromite, ilmenite, zircon, Ca-Mg phosphate, Fe-metal, and troilite are embedded in the melt. Polymineralic clasts are mostly compositionally similar to the above mentioned larger clasts but scarce granulitic fragments are observed as well.

  8. The stable Cr isotopic compositions of chondrites and silicate planetary reservoirs (United States)

    Schoenberg, Ronny; Merdian, Alexandra; Holmden, Chris; Kleinhanns, Ilka C.; Haßler, Kathrin; Wille, Martin; Reitter, Elmar


    The depletion of chromium in Earth's mantle (∼2700 ppm) in comparison to chondrites (∼4400 ppm) indicates significant incorporation of chromium into the core during our planet's metal-silicate differentiation, assuming that there was no significant escape of the moderately volatile element chromium during the accretionary phase of Earth. Stable Cr isotope compositions - expressed as the ‰-difference in 53Cr/52Cr from the terrestrial reference material SRM979 (δ53/52CrSRM979 values) - of planetary silicate reservoirs might thus yield information about the conditions of planetary metal segregation processes when compared to chondrites. The stable Cr isotopic compositions of 7 carbonaceous chondrites, 11 ordinary chondrites, 5 HED achondrites and 2 martian meteorites determined by a double spike MC-ICP-MS method are within uncertainties indistinguishable from each other and from the previously determined δ53/52CrSRM979 value of -0.124 ± 0.101‰ for the igneous silicate Earth. Extensive quality tests support the accuracy of the stable Cr isotope determinations of various meteorites and terrestrial silicates reported here. The uniformity in stable Cr isotope compositions of samples from planetary silicate mantles and undifferentiated meteorites indicates that metal-silicate differentiation of Earth, Mars and the HED parent body did not cause measurable stable Cr isotope fractionation between these two reservoirs. Our results also imply that the accretionary disc, at least in the inner solar system, was homogeneous in its stable Cr isotopic composition and that potential volatility loss of chromium during accretion of the terrestrial planets was not accompanied by measurable stable isotopic fractionation. Small but reproducible variations in δ53/52CrSRM979 values of terrestrial magmatic rocks point to natural stable Cr isotope variations within Earth's silicate reservoirs. Further and more detailed studies are required to investigate whether silicate

  9. Internal state of Lutetia as a function of the macroporosity (United States)

    Neumann, W.; Breuer, D.; Spohn, T.


    like Lutetia can be a parent body of undifferentiated (chondritic), partially differentiated (primitive achondritic), and differentiated (achondritic) meteorites. Thus, enstatite chondritic, iron, and primitive achondritic meteorites can, in principle, originate from Lutetia, because it may be partially differentiated and was clearly disturbed by several major impacts. Furthermore, the timing of the occurrence of thermal convection in the metallic core suggests the possibility of an internal dynamo on Lutetia. This could explain the remanent magnetization found in undifferentiated chondritic meteorites.

  10. 40 Years of Processing Pieces of Space (United States)

    Satterwhite, C. E.; Funk, R. C.; Righter, K.; Harrington, R. H.


    This year marks the 40th year anniversary for the Antarctic Search for Meteorite (ANSMET) program. In 1976, the ANSMET program led the first expedition to Antarctica. The ANSMET program is a US-led field-based science project that recovers meteorite samples from Antarctica. Once a year from late November to late January, a field team consisting of 8 to 12 people, spends 6-8 weeks camping on the ice and collecting meteorites. Since 1976, more than 22,000 meteorite samples have been recovered. These meteorites come from asteroids, planets and other bodies of the solar system. Once collected, the Antarctic meteorites are shipped to NASA/Johnson Space Center (JSC) Houston, TX. in a refrigerated truck and are kept frozen to minimize oxidation until they are ready for initial processing. In Antarctica each meteorite is given a field tag which consists of numbers, once in the lab, this is replaced by an official tag, consisting of the Antarctic field location and year collected. The types and numbers of meteorites that have been classified include 849 carbonaceous chondrites, 135 enstatites, 512 achondrites, 64 stony, 115 irons, 48 others (27 R chondrites, 7 ungrouped), 6,161 H chondrites, 7,668 L chondrites, and 4,589 LL chondrites. Although 80-85 percent of the collected meteorites fall in the ordinary chondrite group, the other approximately 15 percent represent rare types of achondrites and carbonaceous chondrites. These rare meteorites include 25 lunar meteorites, 15 Martian meteorites, scores of various types of carbonaceous chondrites, and unique achondrites. The Antarctic meteorites that have been collected are processed in the Meteorite Processing Lab at JSC in Houston, TX. Initial processing of the meteorites begins with thawing/drying the meteorites in a nitrogen glove box for 24 to 48 hours. The meteorites are then photographed, measured, weighed and a description of the interior and exterior of each meteorite is written. The meteorite is broken and a

  11. The Acapulco Parent Planetesimal: An Early Collisional History in the Solar System (United States)

    Marti, K.; Kim, Y.


    The Acapulco, Mexico, meteorite was recovered in 1976 from a crater of approx. 30 cm diameter. An old crystallization age of 4.60 (error 0.03) Ga (Prinzhofer et al., 1992) establishes that its parent object is one of the oldest known planetesimals in the solar system, although not in a pristine form. Other dating systems indicated somewhat younger ages and isotopic variabilities in several elements documented a complex early history. The younger ages date the closure times in secondary minerals. The initial parent object was in a partially molten state when isotopically distinct foreign matter invaded the chondritic parent and some of the isotopic signatures survived. Nitrogen in the primitive achondrite Acapulco was found to have distinct isotopic signatures for the metal and silicate phases and also in different morphologies of graphites (El Goresy, 1995, 2005). The delayed collisional event probably disrupted the parent object, as Acapulco cooled very rapidly. Nitrogen in the injected metal and graphite did not isotopically exchange with the host silicates. We observed nitrogen isotopic signatures of several separated mineral phases which cover a range of delta 15N values from -150 permil to +13 permil. The lightest nitrogen signatures observed in metal separates are comparable to those in some morphologies of Acapulco graphites. The heavy N signatures observed in several silicate minerals are consistent with each other, while nitrogen in chromite is distinctly light (delta 15N of -80 permil), intermediate between those of metal and silicates. The incipient rapid cooling history is well documented down to approx. 120° C, as recorded by U/Th-4He ages in phosphates (Min et al., 2003). The history of the Acapulco parent object was uneventful after its early evolution in an environment where no perturbation by collisions occurred, until the meteorite's recent (6.0 Ma ago) injection into an earth-crossing orbit. References: El Goresy, A., Zinner, E., and Marti, K

  12. Origin, Internal Structure and Evolution of 4 Vesta (United States)

    Zuber, Maria T.; McSween, Harry Y.; Binzel, Richard P.; Elkins-Tanton, Linda T.; Konopliv, Alexander S.; Pieters, Carle M.; Smith, David E.


    Asteroid 4 Vesta is the only preserved intact example of a large, differentiated protoplanet like those believed to be the building blocks of terrestrial planet accretion. Vesta accreted rapidly from the solar nebula in the inner asteroid belt and likely melted due to heat released due to the decay of 26Al. Analyses of meteorites from the howardite-eucrite-diogenite (HED) suite, which have been both spectroscopically and dynamically linked to Vesta, lead to a model of the asteroid with a basaltic crust that overlies a depleted peridotitic mantle and an iron core. Vesta’s crust may become more mafic with depth and might have been intruded by plutons arising from mantle melting. Constraints on the asteroid’s moments of inertia from the long-wavelength gravity field, pole position and rotation, informed by bulk composition estimates, allow tradeoffs between mantle density and core size; cores of up to half the planetary radius can be consistent with plausible mantle compositions. The asteroid’s present surface is expected to consist of widespread volcanic terrain, modified extensively by impacts that exposed the underlying crust or possibly the mantle. Hemispheric heterogeneity has been observed by poorly resolved imaging of the surface that suggests the possibility of a physiographic dichotomy as occurs on other terrestrial planets. Vesta might have had an early magma ocean but details of the early thermal structure are far from clear owing to model uncertainties and paradoxical observations from the HEDs. Petrological analysis of the eucrites coupled with thermal evolution modeling recognizes two possible mechanisms of silicate-metal differentiation leading to the formation of the basaltic achondrites: equilibrium partial melting or crystallization of residual liquid from the cooling magma ocean. A firmer understanding the plethora of complex physical and chemical processes that contribute to melting and crystallization will ultimately be required to

  13. Meteorite Source Regions as Revealed by the Near-Earth Object Population (United States)

    Binzel, Richard P.; DeMeo, Francesca E.; Burt, Brian J.; Polishook, David; Burbine, Thomas H.; Bus, Schelte J.; Tokunaga, Alan; Birlan, Mirel


    Spectroscopic and taxonomic information is now available for 1000 near-Earth objects, having been obtained through both targeted surveys (e.g. [1], [2], [3]) or resulting from all-sky surveys (e.g. [4]). We determine their taxonomic types in the Bus-DeMeo system [5] [6] and subsequently examine meteorite correlations based on spectral analysis (e.g. [7],[8]). We correlate our spectral findings with the source region probabilities calculated using the methods of Bottke et al. [9]. In terms of taxonomy, very clear sources are indicated: Q-, Sq-, and S-types most strongly associated with ordinary chondrite meteorites show clear source signatures through the inner main-belt. V-types are relatively equally balanced between nu6 and 3:1 resonance sources, consistent with the orbital dispersion of the Vesta family. B- and C-types show distinct source region preferences for the outer belt and for Jupiter family comets. A Jupiter family comet source predominates for the D-type near-Earth objects, implying these "asteroidal" bodies may be extinct or dormant comets [10]. Similarly, near-Earth objects falling in the spectrally featureless "X-type" category also show a strong outer belt and Jupiter family comet source region preference. Finally the Xe-class near-Earth objects, which most closely match the spectral properties of enstatite achondrite (aubrite) meteorites seen in the Hungaria region[11], show a source region preference consistent with a Hungaria origin by entering near-Earth space through the Mars crossing and nu6 resonance pathways. This work supported by the National Science Foundation Grant 0907766 and NASA Grant NNX10AG27G.[1] Lazzarin, M. et al. (2004), Mem. S. A. It. Suppl. 5, 21. [2] Thomas, C. A. et al. (2014), Icarus 228, 217. [3] Tokunaga, A. et al. (2006) BAAS 38, 59.07. [4] Hasselmann, P. H., Carvano, J. M., Lazzaro, D. (2011) NASA PDS, EAR-A-I0035-5-SDSSTAX-V1.0. [5] Bus, S.J., Binzel, R.P. (2002). Icarus 158, 146. [6] DeMeo, F.E. et al. (2009), Icarus

  14. Classification and radiative-transfer modeling of meteorite spectra (United States)

    Pentikäinen, H.; Penttilä, A.; Peltoniemi, J.; Muinonen, K.


    The interpretation of asteroid spectra is closely tied to surface structure and composition. Asteroid surfaces are usually assumed to be covered with a regolith, which is a mixture of mineral grains ranging from micrometers to centimeters in size. The inverse problem of deducing the characteristics of the grains from the scattering of light (e.g., using photometric and polarimetric observations) is difficult. Meteorite spectroscopy can be a valuable alternative source of information considering that unweathered meteoritic ''falls'' are almost pristine samples of their parent bodies. Reflectance spectra of 18 different meteorite samples were measured with the Finnish Geodetic Institute Field Goniospectrometer (FIGIFIGO) covering a wavelength range of 450--2250 nm [1,2]. The measurements expand the database of reflectance spectra obtained by Paton et al. [3] and Gaffey [4]. Principal Component Analysis (PCA) performed on the spectra indicates a separation of the undifferentiated ordinary chondrites and the differentiated achondrites. The principal components also suggest a discrimination between the spectra of ordinary chondrites with petrologic grades 5 and 6. The distinction is not present when the data are supplemented with the spectra from the two other data sets obtained with differing measuring techniques. To further investigate the different classifications, the PCA is implemented with selected spectral features contrary to the previous analyses, which encompassed the complete spectra. Single-scattering albedos for meteoritic fundamental scatterers were derived with a Monte Carlo radiative-transfer model [1]. In the derivation, realistic scattering phase functions were utilized. The functions were obtained by fitting triple Henyey-Greenstein functions to the measured scattering phase functions of olivine powder for two different size distributions [5,6]. The simulated reflectances for different scattering phase functions were matched to the measured meteorite

  15. Re-analysis of previous laboratory phase curves: 2. Connections between opposition effect morphology and spectral features of stony meteorites (United States)

    Déau, Estelle; Spilker, Linda J.; Flandes, Alberto


    We investigate connections between the opposition phase curves and the spectra from ultraviolet to near infrared wavelengths of stony meteorites. We use two datasets: the reflectance dataset of Capaccioni et al. ([1990] Icarus, 83, 325), which consists of optical phase curves (from 2° to 45°) of 17 stony meteorites (three carbonaceous chondrites, 11 ordinary chondrites, and three achondrites), and the spectral dataset from the RELAB database consisting of near-ultraviolet to near-infrared spectra of the same meteorites. We re-analyzed the first dataset and fit it with two morphological models to derive the amplitude A, the angular width HWHM of the surge and the slope S of the linear part. Our re-analysis confirms that stony meteorites have a non-monotonic behavior of the surge amplitude with albedo, which is also observed in planetary surfaces (Déau et al. [2013] Icarus, 226, 1465), laboratory samples (Nelson et al. [2004] Proc. Lunar Sci. Conf., 35, p. 1089) and asteroids (Belskaya and Shevchenko [2000] Icarus, 147, 94). We find a very strong correlation between the opposition effect morphological parameters and the slope of the spectra between 0.75 μm and 0.95 μm. In particular, we found that meteorites with a positive amplitude-albedo correlation have a positive spectral slope between 0.75 μm and 0.95 μm, while meteorites with a negative amplitude-albedo correlation have a negative spectral slope between 0.75 μm and 0.95 μm. We have ruled out the role of the meteorite samples' macro-properties (grain size, porosity and macroscopic roughness) in the correlations found because these properties were constant during the preparation of the samples. If this hypothesis is correct, this implies that other properties like the composition or the micro-properties (grain inclusions, grain shape or microscopic roughness) could have a preponderant role in the non-monotonic behavior of the surge morphology with albedo at small and moderate phase angles. Further

  16. Heliocentric zoning of the asteroid belt by aluminum-26 heating (United States)

    Grimm, R. E.; Mcsween, H. Y., Jr.


    Variations in petrology among meteorites attest to a strong heating event early in solar system history, but the heat source has remained unresolved. Aluminum-26 has been considered the most likely high-energy, short-lived radionuclide (half-life 0.72 million years) since the discovery of its decay product - excess Mg-26 - in Allende CAI's. Furthermore, observation of relict Mg-26 in an achondritic clast and in feldspars within ordinary chondrites (3,4) provided strong evidence for live Al-26 in meteorite parent bodies and not just in refractory nebular condensates. The inferred amount of Al-26 is consistent with constraints on the thermal evolution of both ordinary and carbonaceous chondrite parent objects up to a few hundred kilometers in diameter. Meteorites can constrain the early thermal evolution of their parent body locations, provided that a link can be established between asteroid spectrophotometric signature and meteorite class. Asteroid compositions are heliocentrically distributed: objects thought to have experienced high metamorphic or even melting temperatures are located closer to the sun, whereas apparently unaltered or mildly heated asteroids are located farther away. Heliocentric zoning could be the result of Al-26 heating if the initial amount of the radionuclide incorporated into planetesimals was controlled by accretion time, which in turn varies with semimajor axis. Analytic expressions for planetary accretion may be integrated to given the time, tau, required for a planetesimal to grow to a specified radius: tau varies as a(sup n), where n = 1.5 to 3 depending on the assumptions about variations in the surface density of the planetesimal swarm. Numerical simulations of planetesimal accretion at fixed semimajor axis demonstrate that variations in accretion time among small planetesimals can be strongly nonlinear depending on the initial conditions and model assumptions. The general relationship with semimajor axis remains valid because it

  17. Oxygen and Magnesium Isotopic Compositions of Asteroidal Materials Returned from Itokawa by the Hayabusa Mission (United States)

    Yurimoto, H; Abe, M.; Ebihara, M.; Fujimura, A.; Hashizume, K.; Ireland, T. R.; Itoh, S.; Kawaguchi, K.; Kitajima, F.; Mukai, T.; Nagao, K.; Nakamura, T.; Naraoka, H.; Noguchi, T.; Okazaki, R.; Sakamoto, N.; Seto, Y.; Tsuchiyama, A.; Uesugi, M.; Yada, T.; Yoshikawa, M.; Zolensky, M.


    The Hayabusa spacecraft made two touchdowns on the surface of Asteroid 25143 Itokawa on November 20th and 26th, 2005. The Asteroid 25143 Itokawa is classified as an S-type asteroid and inferred to consist of materials similar to ordinary chondrites or primitive achondrites [1]. Near-infrared spectroscopy by the Hayabusa spacecraft proposed that the surface of this body has an olivine-rich mineral assemblage potentially similar to that of LL5 or LL6 chondrites with different degrees of space weathering [2]. The spacecraft made the reentry into the Earth s atmosphere on June 12th, 2010 and the sample capsule was successfully recovered in Australia on June 13th, 2010. Although the sample collection processes on the Itokawa surface had not been made by the designed operations, more than 1,500 grains were identified as rocky particles in the sample curation facility of JAXA, and most of them were judged to be of extraterrestrial origin, and definitely from Asteroid Itokawa on November 17th, 2010 [3]. Although their sizes are mostly less than 10 microns, some larger grains of about 100 microns or larger were also included. The mineral assembly is olivine, pyroxene, plagioclase, iron sulfide and iron metal. The mean mineral compositions are consistent with the results of near-infrared spectroscopy from Hayabusa spacecraft [2], but the variations suggest that the petrologic type may be smaller than the spectroscopic results. Several tens of grains of relatively large sizes among the 1,500 grains will be selected by the Hayabusa sample curation team for preliminary examination [4]. Each grain will be subjected to one set of preliminary examinations, i.e., micro-tomography, XRD, XRF, TEM, SEM, EPMA and SIMS in this sequence. The preliminary examination will start from the last week of January 2011. Therefore, samples for isotope analyses in this study will start from the last week of February 2011. By the time of the LPSC meeting we will have measured the oxygen and

  18. Accretional Impact Melt From the L-Chondrite Parent Body (United States)

    Wittmann, A.; Weirich, J. R.; Swindle, T. D.; Rumble, D.; Kring, D. A.


    MIL 05029, a unique achondritic Antarctic meteorite with L-chondritic affinity, has a medium-grained, well equilibrated texture of large poikilitic low-Ca pyroxenes that overgrew smaller, euhedral olivines. Plagioclase filled interstitial spaces and has an abundance that is twice that typical for L-chondrites, while Fe-Ni metal and troilite are strongly depleted in that respect. No relic clasts or shock features were found in the thin section analyzed. However, based on its chemical affinity to L-chondrites, MIL 05029 was classified as an impact melt. This is confirmed by its olivine and low-Ca pyroxene compositions, the Co content in Fe-Ni metal, and its oxygen isotopic composition that lies very close to that of L-chondrites. An igneous origin of MIL 05029 cannot be ruled out but would have to be reconciled with thermochronometric constraints for the formation of the ordinary chondrite parent bodies. These studies infer delayed accretion of the parent asteroids of the ordinary chondrites and, thus, insufficient heating from short-lived radiogenic isotopes to produce endogenic magmatism. Metallographic cooling rates of ˜2-22 °C/Ma in the temperature range between ˜700-400°C were determined on five zoned metal particles of MIL 05029. Thermal modeling showed that such cooling rates relate to metamorphic conditions at depths of 5-12 km on the L-chondrite parent body. For an impact to deposit material at this depth, scaling relationships for an impact event on the 100-200 km diameter parent asteroid require a 15 to 60 km diameter simple crater that produced a basal melt pool, in which MIL 05029 crystallized. Further constraints for the formation conditions of MIL 05029 were derived from three whole-rock samples that gave well-defined Ar-Ar plateau ages of 4.53±0.02 Ga. This age indicates the time at which MIL 05029 cooled below ˜180°C, the Ar-closure temperature of plagioclase. Considering its slow metallographic cooling, the impact event that formed MIL 05029

  19. What do Meteorite Falls Tell Us about the Strength of Asteroid Boulders (United States)

    Britt, Daniel; Demasi, Michael; Kring, David


    One possible source of data on the strength of a boulder on an asteroid's surface is the meteorite collection and the observations of meteorite falls. Since highly fractured boulders should breakup in the atmosphere and arrive as meteorite showers, the relative ratio of boulders to showers can provide insight into boulder strength. Since about 85-95% of the mass of a meteoroid is lost during atmospheric entry, we have chosen to investigate only those falls with a final recovered mass of at least 10 kg. This corresponds to a minimum pre-atmospheric mass of 100-200 kg and roughly 25 centimeter minimum diameter. Using the Catalogue of Meteorites and the Meteoritical Bulletins we compiled a list of observed meteorite falls with a total recovered mass greater than or equal to 10 kg. We found a total of 269 meteorites that met these criteria, of which 263 entries reported or estimated the number of fragments associated with their falls. The overall percentage of observed showers was found to be around 34%. The ratio of "boulders" to showers was determined to be around 1.94:1. Comparing the percentage of showers within the meteorite types shows a trend in strength with irons (showers only 4.3%) very rarely exhibit reported showers, stony-irons (25%), ordinary chondrites (28.5%), achondrites (35.7%), and carbonaceous chondrites (70%) are dominantly showers.The meteorite fall data primarily sample the "boulder" population of meteoroids roughly 0.25 meters to a few meters in their pre-atmospheric diameter because of the 85-95% atmospheric loss. The relative rarity of showers seems to indicate that most meteoroids that survive to produce meteorites in this size range are fairly strong and coherent. Not surprisingly, irons and stony-irons are the strongest class which is consistent with the overwhelmingly high production of Earth's smallest impact craters by iron meteorites. Carbonaceous chondrites are by far the weakest and most fracture-prone meteorite class with 70% of the

  20. Unraveling the chemical space of terrestrial and meteoritic organic matter (United States)

    Schmitt-Kopplin, Philippe; Harir, Mourad; Hertkorn, Norbert; Kanawati, Basem; Ruf, Alexander; Quirico, Eric; Bonal, Lydie; Beck, Pierre; Gabelica, Zelimir


    )High resolution analytical approaches will be presented in their application to unravel the chemical nature and organic signatures in bio-geosystems and especially in selected chondritic (organic and ordinary) and achondritic meteorites. We will focus on thermal effects in CM types of materials and describe the effect of shock events on the changes in chemodiversity and the formation of unique novel organic compounds using high magnetic field ultrahigh resolution mass spectrometry (12 Tesla ion cyclotron resonance Fourier transform mass spectrometry - ICR-FT/MS) and nuclear magnetic resonance spectroscopy (Cryo 800MHz NMR).

  1. Vesta and Ceres by the light of Dawn (United States)

    Russell, Christopher T.


    Ceres and Vesta are the most massive bodies in the main asteroid belt. They both appear to be intact protoplanets whose growth may have been drastically altered by the concomitant formation of Jupiter.. These two bodies have witnessed 4.6 Ga of solar system history, much, but not all, of which has been recorded in their surfaces. Dawn’s objective is to interview these two witnesses to learn as much as possible about the early epoch. These bodies are protoplanets, our best archetypes of the early building blocks of the terrestrial planets. In particular, siderophile elements in the Earth’s core were probably first segregated in Vesta-like bodies, and its water was likely first condensed in Ceres-like bodies.Many of the basaltic achondrites originated from a common parent body. Dawn verified that Vesta was consistent with that parent body. hence strengthening geochemical inferences from these samples on the formation and evolution of the solar system and supporting hypotheses for their delivery from Vesta to Earth. Ceres has not revealed itself with a meteoritic record. While the surface is scarred with craters, it is probable that the ejecta from the crater-forming event created little competent material from the icy crust and any such ejected projectiles that reached Earth might have disintegrated upon entry into the Earth’s atmosphere.Ceres’ surface differs greatly from Vesta’s. Plastic or fluidized mass wasting is apparent, as are many irregularly shaped craters, including many polygonal crater forms. There are many central-pit craters possibly caused by volatilization of the crust in the center of the impact. There are also many central-peak craters, which were made by rebound or pingo-like formation processes. Bright deposits dot the landscape, which are possibly salt-rich, suggesting fluvial activity beneath the crust. Observations of the brightest spots on Ceres could suggest sublimation from the surface of the bright area, which may be water vapor

  2. Variable microstructural response of baddeleyite to shock metamorphism in young basaltic shergottite NWA 5298 and improved U-Pb dating of Solar System events (United States)

    Darling, James R.; Moser, Desmond E.; Barker, Ivan R.; Tait, Kim T.; Chamberlain, Kevin R.; Schmitt, Axel K.; Hyde, Brendt C.


    The accurate dating of igneous and impact events is vital for the understanding of Solar System evolution, but has been hampered by limited knowledge of how shock metamorphism affects mineral and whole-rock isotopic systems used for geochronology. Baddeleyite (monoclinic ZrO2) is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust U-Pb isotopic systematics, but there is little understanding of shock-effects on this phase. Here we present new nano-structural measurements of baddeleyite grains in a thin-section of the highly-shocked basaltic shergottite Northwest Africa (NWA) 5298, using high-resolution electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) techniques, to investigate shock-effects and their linkage with U-Pb isotopic disturbance that has previously been documented by in-situ U-Pb isotopic analyses. The shock-altered state of originally igneous baddeleyite grains is highly variable across the thin-section and often within single grains. Analyzed grains range from those that preserve primary (magmatic) twinning and trace-element zonation (baddeleyite shock Group 1), to quasi-amorphous ZrO2 (Group 2) and to recrystallized micro-granular domains of baddeleyite (Group 3). These groups correlate closely with measured U-Pb isotope compositions. Primary igneous features in Group 1 baddeleyites (n = 5) are retained in high shock impedance grain environments, and an average of these grains yields a revised late-Amazonian magmatic crystallization age of 175 ± 30 Ma for this shergottite. The youngest U-Pb dates occur from Group 3 recrystallized nano- to micro-granular baddeleyite grains, indicating that it is post-shock heating and new mineral growth that drives much of the isotopic disturbance, rather than just shock deformation and phase transitions. Our data demonstrate that a systematic multi-stage microstructural evolution in

  3. Proceedings of the 40th Lunar and Planetary Science Conference (United States)


    ; Seek Out and Explore: Upcoming and Future Missions; Mars: Early History and Impact Processes; Mars Analogs II: Chemical and Spectral; Achondrites and their Parent Bodies; and Planning for Future Exploration of the Moon The poster sessions were: Lunar Missions: Results from Kaguya, Chang'e-1, and Chandrayaan-1; LRO and LCROSS; Geophysical Analysis of the Lunar Surface and Interior; Remote Observation and Geologic Mapping of the Lunar Surface; Lunar Spectroscopy; Venus Geology, Geophysics, Mapping, and Sampling; Planetary Differentiation; Bunburra and Buzzard Coulee: Recent Meteorite Falls; Meteorites: Terrestrial History; CAIs and Chondrules: Records of Early Solar System Processes; Volatile and Organic Compounds in Chondrites; Crashing Chondrites: Impact, Shock, and Melting; Ureilite Studies; Petrology and Mineralogy of the SNC Meteorites; Martian Meteorites; Phoenix Landing Site: Perchlorate and Other Tasty Treats; Mars Polar Atmospheres and Climate Modeling; Mars Polar Investigations; Mars Near-Surface Ice; Mars: A Volatile-Rich Planet; Mars: Geochemistry and Alteration Processes; Martian Phyllosilicates: Identification, Formation, and Alteration; Astrobiology; Instrument Concepts, Systems, and Probes for Investigating Rocks and Regolith; Seeing is Believing: UV, VIS, IR, X- and Gamma-Ray Camera and Spectrometer Instruments; Up Close and Personal: In Situ Analysis with Laser-Induced Breakdown Spectroscopy and Mass Spectrometry; Jupiter and Inscrutable Io; Tantalizing Titan; Enigmatic Enceladus and Intriguing Iapetus; Icy Satellites: Cryptic Craters; Icy Satellites: Gelid Geology/Geophysics; Icy Satellites: Cool Chemistry and Spectacular Spectroscopy; Asteroids and Comets; Comet Wild 2: Mineralogy and More; Hypervelocity Impacts: Stardust Models, LDEF, and ISPE; Presolar Grains; Early Nebular Processes: Models and Isotopes; Solar Wind and Genesis: Measurements and Interpretation; Education and Public Outreach; Mercury; Pursuing Lunar Exploration; Sources and Eruptionf

  4. A Mid-Infrared Emission and Reflectance Library of Meteorites and Fine Particulate Phases (United States)

    Osterloo, M. M.; Hamilton, V. E.


    We have acquired middle infrared (MIR) emission and reflectance spectra of fine particulate minerals and solid and fine particulate meteorite samples under ambient conditions. This dataset is intended to help expand our ability to interpret the MIR spectra of asteroids, possible extinct comets, and other rocky bodies, which contain a record of processes that are key to understanding the formation of our solar system 4.6 G.y. ago. Non-destructive MIR spectra of all samples in this study were collected in the spectroscopy laboratory at Southwest Research Institute [Hamilton and Lucey, 2005, LPSC XXXVI]. We measured the MIR spectra of fine particulate meteorite samples with the objective of providing the planetary community a dataset that can be used for qualitative comparison to asteroid spectra. The current library [Salisbury et al., 1991, Icarus] that is used for comparison to asteroid emissivity spectra represents particle size fractions of size fraction to be analogous to low inertia asteroid regoliths [e.g., Barucci et al. 2002, Icarus; Emery et al. 2006, Icarus]. Therefore, our MIR spectral library includes a series of meteorite powders having smaller size fractions that may be more analogous to the regoliths of large, low inertia asteroids and will be beneficial for qualitative comparisons. The Smithsonian Institute's analyzed Meteorite Powered Collections (USNM 7073) provided particulate meteorite samples. At this time we have focused on chondritic and a few achondrite samples with particle size fractions samples chosen represent many of the samples that were originally measured by Salisbury et al. [1991, Icarus], providing continuity with the existing larger particle size spectral dataset. Furthermore, this work is the first to obtain spectral data of meteorites at wavelengths greater than 13.5 μm, which contain diagnostic features in silicates, carbonates, and oxides and is important for comparison to longer wavelength space-based observations

  5. Modelling the thermal evolution and differentiation of the parent body of acapulcoites and lodranites (United States)

    Neumann, Wladimir; Breuer, Doris; Spohn, Tilman; Henke, Stephan; Gail, Hans-Peter; Schwarz, Winfried; Trieloff, Mario; Hopp, Jens


    The acapulcoites and lodranites are rare groups of achondritic meteorites. Several characteristics such as unique oxygen isotope composition and similar cosmic ray exposure ages indicate that these meteorites originate from a common parent body (Weigel et al. 1999). By contrast to both undifferentiated and differentiated meteorites, acapulcoites and lodranites are especially interesting because they experienced melting that was, however, not complete (McCoy et al. 2006). Thus, unravelling their origin contributes directly to the understanding of the initial differentiation stage of planetary objects in the Solar system. The information preserved in the structure and composition of meteorites can be recovered by modelling the evolution of their parent bodies and comparing the results with the laboratory investigations. Model calculations for the thermal evolution of the parent body of the Acapulco and Lodran-like meteorite clan were performed using two numerical models. Both models (from [3] and [4], termed (a) and (b), respectively) solve a 1D heat conduction equation in spherical symmetry considering heating by short- and long-lived radioactive isotopes, temperature- and porosity-dependent parameters, compaction of initially porous material, and melting. The calculations with (a) were compared to the maximum metamorphic temperatures and thermo-chronological data available for acapulcoites and lodranites. Applying a genetic algorithm, an optimised set of parameters of a common parent body was determined, which fits to the data for the cooling histories of these meteorites. The optimum fit corresponds to a body with the radius of 270 km and a formation time of 1.66 Ma after the CAIs. Using the model by (b) that considers differentiation by porous flow and magmatic heat transport, the differentiation of the optimum fit body was calculated. The resulting structure consists of a metallic core, a silicate mantle, a partially differentiated layer, an undifferentiated

  6. Effect of parent body evolution on equilibrium and kinetic isotope fractionation: a combined Ni and Fe isotope study of iron and stony-iron meteorites (United States)

    Chernonozhkin, Stepan M.; Goderis, Steven; Costas-Rodríguez, Marta; Claeys, Philippe; Vanhaecke, Frank


    Various iron and stony-iron meteorites have been characterized for their Ni and Fe isotopic compositions using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) after sample digestion and chromatographic separation of the target elements in an attempt to further constrain the planetary differentiation processes that shifted these isotope ratios and to shed light on the formational history and evolution of selected achondrite parent body asteroids. Emphasis was placed on spatially resolved isotopic analysis of iron meteorites, known to be inhomogeneous at the μm to mm scale, and on the isotopic characterization of adjacent metal and silicate phases in main group pallasites (PMG), mesosiderites, and the IIE and IAB complex silicate-bearing iron meteorites. In a 3-isotope plot of 60/58Ni versus62/58Ni, the slope of the best-fitting straight line through the laterally resolved Ni isotope ratio data for iron meteorites reveals kinetically controlled isotope fractionation (βexper = 1.981 ± 0.039, 1 SD), predominantly resulting from sub-solidus diffusion (with the fractionation exponent β connecting the isotope fractionation factors, as α62/58 =α60/58β). The observed relation between δ56/54Fe and Ir concentration in the metal fractions of PMGs and in IIIAB iron meteorites indicates a dependence of the bulk Fe isotopic composition on the fractional crystallization of an asteroidal metal core. No such fractional crystallization trends were found for the corresponding Ni isotope ratios or for other iron meteorite groups, such as the IIABs. In the case of the IIE and IAB silicate-bearing iron meteorites, the Fe and Ni isotopic signatures potentially reflect the influence of impact processes, as the degree of diffusion-controlled Ni isotope fractionation is closer to that of Fe compared to what is observed for magmatic iron meteorite types. Between the metal and olivine counterparts of pallasites, the Fe and Ni isotopic compositions show clearly

  7. Accretion timescales and style of asteroidal differentiation in an 26Al-poor protoplanetary disk (United States)

    Larsen, K. K.; Schiller, M.; Bizzarro, M.


    The decay of radioactive 26Al to 26Mg (half-life of 730,000 years) is postulated to have been the main energy source promoting asteroidal melting and differentiation in the nascent solar system. High-resolution chronological information provided by the 26Al-26Mg decay system is, therefore, intrinsically linked to the thermal evolution of early-formed planetesimals. In this paper, we explore the timing and style of asteroidal differentiation by combining high-precision Mg isotope measurements of meteorites with thermal evolution models for planetesimals. In detail, we report Mg isotope data for a suite of olivine-rich [Al/Mg ∼ 0] achondritic meteorites, as well as a few chondrites. Main Group, pyroxene and the Zinder pallasites as well as the lodranite all record deficits in the mass-independent component of μ26Mg (μ26Mg∗) relative to chondrites and Earth. This isotope signal is expected for the retarded ingrowth of radiogenic 26Mg∗ in olivine-rich residues produced through partial silicate melting during 26Al decay and consistent with their marginally heavy Mg isotope composition relative to ordinary chondrites, which may reflect the early extraction of isotopically light partial melts from the source rock. We propose that their parent planetesimals started forming within ∼250,000 years of solar system formation from a hot (>∼500 K) inner protoplanetary disk region characterized by a reduced initial (26Al/27Al)0 abundance (∼1-2 × 10-5) relative to the (26Al/27Al)0 value in CAIs of 5.25 × 10-5. This effectively reduced the total heat production and allowed for the preservation of solid residues produced through progressive silicate melting with depth within the planetesimals. These 'non-carbonaceous' planetesimals acquired their mass throughout an extended period (>3 Myr) of continuous accretion, thereby generating onion-shell structures of incompletely differentiated zones, consisting of olivine-rich residues, overlaid by metachondrites and

  8. Mineralogical and Geochemical Analysis of Howardite DaG 779: understanding geological evolution of asteroid (4) Vesta (United States)

    Marcel Müller, Christian; Mengel, Kurt; Singh Thangjam, Guneshwar; Weckwerth, Gerd


    The HED meteorites, a clan of stony achondrites, are believed to originate from asteroid (4) Vesta (e.g. Mittlefehldt et al. (2015)). Recent evolution models (e.g. Toplis et al. (2013)) and observations from Dawn spacecraft data (e.g., Prettyman et al. (2013)) indicate that diogenites form the lower crust and uppermost mantle of (4) Vesta. Deep seated material excavated by large impacts such as the Rheasilvia- and Veneneiaforming event should be present in howardites. We analysed a slice of howardite DaG 779 which had been recovered from the Libyan Desert in 1999 and was briefly described by Grossmann (2000). The data presented here include electron microprobe, bulk-rock XRD and XRF as well as trace element analysis by ICP-MS and INA. The petrographic results confirm earlier observations that DaG 779 is polymict and mainly contains diogenite and eucrite clasts. Mass balance calculations using bulk-rock and microprobe major element data reveal a modal mineralogy of 77% orthopyroxene, 8% plagioclase, 7% clinopyroxene and 2% spinels, the rest being olivine, SiO2-phases, sulphides, and native Fe(Ni). When compared with the element compilation recently reported by Mittlefehldt (2015) the 39 trace element analysed here (including REE and PGE) confirm that this howardite is clearly dominated by diogenite. Beside the modal petrographic information, a number of more detailed observations obtained from microprobe investigations reveal fresh and recrystallized glasses, troilite-orthopyroxene symplectites from a mixed silicate-sulphide melt giving rise to graphic intergrowths as well as vermicular and reticular FeS in highly disrupted clasts. While the origin of the FeS in these clasts is not clear yet, its particular shape and distribution indicates that this mineral has been (partially) molten and recrystallized from a sulphide melt. The silicate minerals around these FeS occurrences are recrystallized but there is no indication for a partial silicate melt. Further

  9. Lunar and Meteorite Sample Education Disk Program - Space Rocks for Classrooms, Museums, Science Centers, and Libraries (United States)

    Allen, Jaclyn; Luckey, M.; McInturff, B.; Huynh, P.; Tobola, K.; Loftin, L.


    violent impact history of the Moon. The disks also include two regolith soils and orange glass from a pyroclastic deposit. Each Meteorite Disk contains two ordinary chondrites, one carbonaceous chondrite, one iron, one stony iron, and one achondrite. These samples will help educators share the early history of the solar system with students and the public. Educators may borrow either lunar or meteorite disks and the accompanying education materials through the Johnson Space Center Curatorial Office. In trainings provided by the NASA Aerospace Education Services Program specialists, educators certified to borrow the disk learn about education resources, the proper use of the samples, and the special security for care and shipping of the disks. The Lunar and Meteorite Sample Education Disk Program will take NASA exploration to more people. Getting Space Rocks out to the public and inspiring the public about new space exploration is the focus of the NASA disk loan program.

  10. Isotopic constraints on the age and early differentiation of the Earth. (United States)

    McCulloch, M T


    The Earth's age and early differentiation history are re-evaluated using updated isotopic constraints. From the most primitive terrestrial Pb isotopic compositions found at Isua Greenland, and the Pilbara of Western Australia, combined with precise geochronology of these localities, an age 4.49 +/- 0.02 Ga is obtained. This is interpreted as the mean age of core formation as U/Pb is fractionated due to sequestering of Pb into the Earth's core. The long-lived Rb-Sr isotopic system provides constraints on the time interval for the accretion of the Earth as Rb underwent significant depletion by volatile loss during accretion of the Earth or its precursor planetesimals. A primitive measured 87Sr/86Sr initial ratio of 0.700502 +/- 10 has been obtained for an early Archean (3.46 Ga) barite from the Pilbara Block of Western Australia. Using conservative models for the evolution of Rb/Sr in the early Archean mantle allows an estimate to be placed on the Earth's initial Sr ratio at approximately 4.50 Ga, of 0.69940 +/- 10. This is significantly higher than that measured for the Moon (0.69900 +/- 2) or in the achondrite, Angra dos Reis (0.69894 +/- 2) and for a Rb/Sr ratio of approximately 1/2 of chondrites corresponds to a mean age for accretion of the Earth of 4.48 + /- 0.04 Ga. The now extinct 146Sm-142Nd (T1/2(146)=103 l0(6)yrs) combined with the long-lived 147Sm-143Nd isotopic systematics can also be used to provide limits on the time of early differentiation of the Earth. High precision analyses of the oldest (3.8-3.9 Ga) Archean gneisses from Greenland (Amitsoq and Akilia gneisses), and Canada (Acasta gneiss) do not show measurable (> +/- l0ppm) variations of 142Nd, in contrast to the 33 ppm 142Nd excess reported for an Archean sample. The general lack of 142Nd variations, combined with the presence of highly positive epsilon 143 values (+4.0) at 3.9 Ga, indicates that the record of large-scale Sm/Nd fractionation events was not preserved in the early-Earth from 4

  11. Evolution of asteroid (4) Vesta in the light of Dawn (United States)

    Thangjam, Guneshwar; Mengel, Kurt; Nathues, Andreas; Schmidt, Kai H.; Hoffmann, Martin


    Asteroid (4) Vesta has been visited by the NASA Dawn spacecraft in 2011/12. The combination of compositional/elemental information from the three onboard instruments with mineralogical information from the howardite-eucrite-diogenite (HED) clan of stony achondrites has shed new light on the surface lithologic heterogeneity and the early evolution. Although petrologic/chemical models have tried to unravel the evolutionary processes, inconsistencies exist for some chemical major element/phase [e.g., 1, 2]. A revised evolutionary model is presented here [3]. The three oxygen isotope signature of HEDs and, thus, of proto-Vesta is best met by a mixture of 80% ordinary plus 20 % CV chondrites. Assuming a 27Al-triggered magma ocean within the first MA after accretion and taking into account the reliable major element data of the silicate fraction of the chondritic mixture results a crystallization sequence that differs from the earlier models [1, 2, 3]. The crystallized phase obtained by 'MELTS' software [4] starts with olivine and continues with minor olivine plus orthopyroxene until the liquid reaches a Kd value (partition coefficient) of 0.31 where the fractionated melt is in equilibrium with the residual liquid [5]. The abundance of minerals and rocks formed in this model are converted in volume proportions assuming a spherical shape of early Vesta (262 km radius) with a core (FeNi, FeNiS) radius of 110 km [6]. Two scenarios are considered to describe the early bulk silicate Vesta. First, the early-crystallized olivine accumulated at the base of the silicate shell is accounted for a dunitic lower mantle having a thickness of 46 km while the later crystallized phases form an orthopyroxenitic upper mantle and a crust of thickness 84 and 22 km, respectively. Second, an olivine-rich lower mantle that gradually changes to orthopyroxene-rich upper mantle is expected having an overall shell thickness of 137 km, with a 15 km thick crust. An important result is that the deep

  12. Can Halogen Enrichment in Reduced Enstatite Chondrites Provide Clues to Volatile Accretion in the Early Earth? (United States)

    Clay, P. L.; Burgess, R.; Busemann, H.; Ruzié, L.; Joachim, B.; Ballentine, C.


    chondrites measured and have high molar I/Cl (~10-3) and Br/Cl (~10-3) ratios. For comparison, the ordinary chondrites have highly variable halogen concentrations and very low molar I/Cl (~10-6) and Br/Cl (~10-4) ratios. Halogen concentrations in the ECs are up to ~ 8 times higher for Cl, up to ~ 40 times higher for Br and up to ~ 50 times higher for I, when compared to estimates of halogen concentrations in the primitive mantle [2]. Potential halogen carrier phases in the ECs include Cl-rich feldspathic glass in chondrules, enstatite and/or the halogen-bearing sulfide djerfisherite. Accretion of halogen-rich, reduced material such as that observed here with the ECs could support models for heterogeneous accretion. Ongoing analyses of the primitive enstatite achondrites will shed additional light on these issues. [1] Wänke, H. Dreibus, G., Jagoutz E., Archaean Geochemistry, A. Kröner, G. N. Hanson, A. M. Goodwin, Eds. (Springer, Berlin, 1984), pp. 1-24. [2] Newsom, H.E., 1995. Global Earth Physics, A Handbook of Physical Constants, AGU Reference Shelf, vol. 1. American Geophysical Union, Washington.

  13. Chromium valences in ureilite olivine and implications for ureilite petrogenesis (United States)

    Goodrich, C. A.; Sutton, S. R.; Wirick, S.; Jercinovic, M. J.


    Ureilites are a group of ultramafic achondrites commonly thought to be residues of partial melting on a carbon-rich asteroid. They show a large variation in FeO content (olivine Fo values ranging from ∼74 to 95) that cannot be due to igneous fractionation and suggests instead variation in oxidation state. The presence of chromite in only a few of the most ferroan (Fo 75-76) samples appears to support such a model. MicroXANES analyses were used in this study to determine the valence states of Cr (previously unknown) in olivine cores of 11 main group ureilites. The goal of this work was to use a method that is independent of Fo to determine the oxidation conditions under which ureilites formed, in order to evaluate whether the ureilite FeO-variation is correlated with oxidation state, and whether it is nebular or planetary in origin. Two of the analyzed samples, LEW 88774 (Fo 74.2) and NWA 766 (Fo 76.7) contain primary chromite; two others, LAP 03587 (Fo 74.4) and CMS 04048 (Fo 76.2) contain sub-micrometer-sized exsolutions of chromite + Ca-rich pyroxene in olivine; and one, EET 96328 (Fo 85.2) contains an unusual chromite grain of uncertain origin. No chromite has been observed in the remaining six samples (Fo 77.4-92.3). Chromium in olivine in all eleven samples was found to be dominated by the divalent species, with valences ranging from 2.10 ± 0.02 (1σ) to 2.46 ± 0.04. The non-chromite-bearing ureilites have the most reduced Cr, with a weighted mean valence of 2.12 ± 0.01, i.e., Cr2+/Cr3+ = 7.33. All low-Fo chromite-bearing ureilites have more oxidized Cr, with valences ranging from 2.22 ± 0.03 to 2.46 ± 0.04. EET 96328, whose chromite grain we interpret as a late-crystallizing phase, yielded a reduced Cr valence of 2.15 ± 0.07, similar to the non-chromite-bearing samples. Based on the measured Cr valences, magmatic (1200-1300 °C) oxygen fugacities (fO2) of the non-chromite-bearing samples were estimated to be in the range IW-1.9 to IW-2.8 (assuming