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ABSTRACT: Abstract— We report petrography, mineral chemistry, and microdistribution of rare earth elements (REE) in a new lherzolitic shergottite, Grove Mountains (GRV) 99027. The textural relationship and REE patterns of minerals suggest precipitation of cumulus olivine and chromite, followed by equilibrium crystallization of a closed system with a bulk composition of the inferred intercumulus melt. Subsolidus equilibrium temperatures of pyroxenes and olivine range from 1100 to 1210 °C, based on a two-pyroxene thermometry and Ca partitioning between augite and olivine. Oxygen fugacity of the parent magma is 1.5–2.5 (av. 2.0 ± 0.4) log units below the quartz-fayalite-magnetite (QFM) buffer at 960–1360 °C, according to the olivine-orthopyroxene-chromite barometer. The ilmenite-chromite barometer and thermometer show much wider ranges of oxygen fugacity (1.0–7.0 log unit below QFM) and temperature (1130–480 °C), suggesting subsolidus equilibration of the oxides at low temperatures, probably due to deep burial of GRV 99027 on Mars. The low oxygen fugacity and LREE depletion of the parent magma of GRV 99027 suggest low contamination by martian crust.Characteristics of GRV 99027 demonstrate similarity of lherzolitic shergottites, suggesting a high possibility of launch pairing or a homogeneous upper mantle of Mars if they were ejected by individual impact events. However, GRV 99027 probably experienced severe post-shock thermal metamorphism in comparison with other lherzolitic shergottites, based on the re-crystallization of maskelynite, the homogeneity of minerals, and the low subsolidus equilibrium temperatures between chromite and ilmenite.
Meteoritics & Planetary Science. 01/2010; 40(11):1599 - 1619.
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ABSTRACT: To constrain the influence of impact shock on water and hydrogen isotope signatures of Martian meteorite kaersutites, we conducted shock recovery experiments on three terrestrial kaersutite crystals. Homogeneous impact shock to 32 GPa, commensurate with shock levels experienced by Martian meteorite kaersutites, led to increases in kaersutite water contents (ΔH2O=0.25–0.89 wt.%), decreases in Fe3+/ΣFe (4–20%), and enrichments in hydrogen isotope composition (ΔD=+66 to +87‰) relative to pre-shock values. The latter values represent the largest shock-induced hydrogen isotope fractionations measured to date. These observations are explained most completely by a two-step shock process. First, shock-induced devolatilization led to hydrogen isotope enrichment through preferential loss of H relative to D. Second, reaction of the kaersutite with the ambient atmosphere led to increased water contents and reduced Fe. Fe reduction and water addition via the reaction Fe2++OH−↔Fe3++O2−+½H2 explain the Fe3+/ΣFe data and some of the water data. Further water addition mechanisms (irreversible adsorption, shock implantation) are necessary to fully explain the increased water contents. Addition of water from the terrestrial atmosphere, which is isotopically light relative to the experimental kaersutite compositions, means the measured hydrogen isotope enrichments are likely minima. The measured (minimum) levels of hydrogen isotope enrichment are relevant to the hydrogen isotope variability within and among Martian kaersutites, but are minor relative to their absolute δD values. Alternatively, addition of water from the enriched Martian atmosphere could explain both Martian kaersutite hydrogen isotope variability and absolute δD values. However, the low Martian kaersutite water contents leave little room for significant water addition. The importance of the ambient atmosphere to the outcome of the shock experiments makes it difficult to translate our results to Mars given the unknown influence of its more tenuous atmosphere on the processes observed in the experiments. Our results suggest that shock is a feasible mechanism for influencing Martian kaersutite water contents and hydrogen isotope compositions but that its complex signature precludes precise determination of that influence.
Earth and Planetary Science Letters - EARTH PLANET SCI LETT. 01/2008; 266(3):288-302.
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Kevin D McKeegan,
Jerome Aléon,
John Bradley,
Donald Brownlee,
Henner Busemann,
Anna Butterworth,
Marc Chaussidon,
Stewart Fallon,
Christine Floss,
Jamie Gilmour, [......],
Dennis Schlutter,
Christopher J Snead,
Frank J Stadermann,
Rhonda Stroud,
Peter Tsou,
Andrew Westphal,
Edward D Young,
Karen Ziegler,
Laurent Zimmermann,
Ernst Zinner
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ABSTRACT: Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild 2 particle fragments; however, extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Nonterrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single (17)O-enriched circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is (16)O-enriched, like refractory inclusions in meteorites, suggesting that Wild 2 contains material formed at high temperature in the inner solar system and transported to the Kuiper belt before comet accretion.
Science 01/2007; 314(5806):1724-8. · 31.20 Impact Factor
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Don Brownlee,
Peter Tsou,
Jérôme Aléon,
Conel M O'd Alexander,
Tohru Araki,
Sasa Bajt,
Giuseppe A Baratta,
Ron Bastien,
Phil Bland,
Pierre Bleuet, [......],
Ian Wright,
Hikaru Yabuta,
Hajime Yano,
Edward D Young,
Richard N Zare,
Thomas Zega,
Karen Ziegler,
Laurent Zimmerman,
Ernst Zinner,
Michael Zolensky
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ABSTRACT: The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.
Science 01/2007; 314(5806):1711-6. · 31.20 Impact Factor
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ABSTRACT: Correlated in situ analyses of the oxygen and magnesium isotopic
compositions of aluminum-rich chondrules from unequilibrated enstatite
chondrites were obtained using an ion microprobe. Among eleven
aluminum-rich chondrules and two plagioclase fragments measured for
26Al-26Mg systematics, only one aluminum-rich chondrule contains excess
26Mg from the in situ decay of 26Al; the inferred initial ratio
(26Al/27Al)o = (6.8 ± 2.4) × 10-6 is consistent with ratios
observed in chondrules from carbonaceous chondrites and unequilibrated
ordinary chondrites.The oxygen isotopic compositions of five
aluminum-rich chondrules and one plagioclase fragment define a line of
slope ˜0.6 ± 0.1 on a three-oxygen-isotope diagram,
overlapping the field defined by ferromagnesian chondrules in enstatite
chondrites but extending to more 16O-rich compositions with a
range in δ18O of about ˜12‰. Based on
their oxygen isotopic compositions, aluminum-rich chondrules in
unequilibrated enstatite chondrites are probably genetically related to
ferromagnesian chondrules and are not simple mixtures of materials from
ferromagnesian chondrules and calcium-aluminum-rich inclusions
(CAIs).
Relative to their counterparts from
unequilibrated ordinary chondrites, aluminum-rich chondrules from
unequilibrated enstatite chondrites show a narrower oxygen isotopic
range and much less resolvable excess 26Mg from the in situ
decay of 26Al, probably resulting from higher degrees of
equilibration and isotopic exchange during post-crystallization
metamorphism. However, the presence of 26Al-bearing
chondrules within the primitive ordinary, carbonaceous, and now
enstatite chondrites suggests that 26Al was at least
approximately homogeneously distributed across the chondrite-forming
region.
Meteoritics & planetary science 12/2005; 41:33-47. · 2.72 Impact Factor
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ABSTRACT: Excesses of sulfur-36 in sodalite, a chlorine-rich mineral, in a calcium- and aluminum-rich inclusion from the Ningqiang carbonaceous chondrite linearly correlate with chorine/sulfur ratios, providing direct evidence for the presence of short-lived chlorine-36 (with a half-life of 0.3 million years) in the early solar system. The best inferred (36Cl/35Cl)o ratios of the sodalite are approximately 5 x 10(-6). Different from other short-lived radionuclides, chlorine-36 was introduced into the inclusion by solid-gas reaction during secondary alteration. The alteration reaction probably took place at least 1.5 million years after the first formation of the inclusion, based on the correlated study of the 26Al-26Mg systems of the relict primary minerals and the alteration assemblages, from which we inferred an initial ratio of (36Cl/35Cl)o > or = 1.6 x 10(-4) at the time when calcium- and aluminum-rich inclusions formed. This discovery supports a supernova origin of short-lived nuclides [Cameron, A. G. W., Hoeflich, P., Myers, P. C. & Clayton, D. D. (1995) Astrophys. J. 447, L53; Wasserburg, G. J., Gallino, R. & Busso, M. (1998) Astrophys. J. 500, L189-L193], but presents a serious challenge for local irradiation models [Shu, F. H., Shang, H., Glassgold, A. E. & Lee, T. (1997) Science 277, 1475-1479; Gounelle, M., Shu, F. H., Shang, H., Glassgold, A. E., Rehm, K. E. & Lee, T. (2001) Astrophys. J. 548, 1051-1070]. Furthermore, the short-lived 36Cl may serve as a unique fine-scale chronometer for volatile-rock interaction in the early solar system because of its close association with aqueous and/or anhydrous alteration processes.
Proceedings of the National Academy of Sciences 03/2005; 102(5):1306-11. · 9.68 Impact Factor
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Science 06/2004; 304(5674):1116-7. · 31.20 Impact Factor
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ABSTRACT: Carbonaceous chondrite condensate olivine grains from two distinct petrographic settings, calcium-aluminum-rich inclusion (CAI) accretionary rims and amoeboid olivine aggregates (AOAs), are oxygen-16 (16O) enriched at the level previously observed inside CAIs. This requires that the gas in the nebular region where these grains condensed was 16O-rich. This contrasts with an 16O-poor gas present during the formation of chondrules, suggesting that CAIs and AOAs formed in a spatially restricted region of the solar nebula containing 16O-rich gas. The 16O-rich gas composition may have resulted either from mass-independent isotopic chemistry or from evaporation of regions with enhanced dust/gas ratios, possibly in an X-wind environment near the young Sun.
Science 03/2002; 295(5557):1051-4. · 31.20 Impact Factor
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Laurie A. Leshin
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ABSTRACT: Oxygen isotopic analyses in approximately 20 micrometer spots in a chemically diverse suite of carbonates from ALH 84001 show highly variable delta(exp 18)O values from +5.4 to +25.3%. The isotopic data are correlated with the major element composition of the carbonate. The earliest forming (Ca-rich) carbonates have the lowest delta(exp 18)O values and the late-forming Mg-rich carbonates have the highest delta(exp 18)O values. Two models that can explain the isotopic variation were investigated. The carbonates could have formed in a water-rich environment at relatively low, but highly variable temperatures. In this open-system case the lower limit to the temperature variation is approximately 125 C, with fluctuations of over 250 C possible within the constraints of the model, depending on fluid composition. Alternatively the data can be explained by a closed-system model in which carbonates precipitated from a limited amount of a CO2-rich fluid. This scenario can reproduce the range of isotopic values observed, even at relatively high temperatures (greater than 500 C). Thus, the oxygen isotopic compositions do not provide unequivocal evidence for formation of the carbonates at low temperature. Neither of these scenarios is consistent with a biological origin of the carbonates and their associated features. Olivine from ALH 84001 occurs as clusters within orthopyroxene adjacent to fractures containing disrupted carbonate globules and feldspathic shock glass. The inclusions are irregular in shape and range in size from approximately 40 micrometers to submicrometer. The olivine exhibits a limited range of chemical composition from Fo(sub 65) to Fo(sub 66). We measured delta(exp 18)O values of the olivine to be +5.1 +/- 1.4%, indistinguishable within uncertainty from the host orthopyroxene. The data suggest that the olivine formed at high temperature (greater than 800 C), and is probably unrelated to carbonate formation. Instead the olivine probably formed by metamorphic reactions involving either dehydration of hydrous silicates or reduction of opx + spinel. If the reactions took place after carbonate formation, they could have caused devolatilization of siderite, producing the magnetite in the ALH carbonate globules. This scenario is also inconsistent with a biological origin for the features in ALH 84001.
03/1999;
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ABSTRACT: We report in situ measurements of oxygen isotopic abundances in individual silicate and oxide minerals from 16 Antarctic micrometeorites (AMMs). The oxygen isotopic compositions of 10 olivine and 11 pyroxene grains are enriched in 16O relative to terrestrial minerals, and on an oxygen three-isotope diagram they plot on the low δ18O side of the 16O mixing line defined by calcium-aluminum-rich inclusions (CAI) from chondritic meteorites. AMM olivine and pyroxene δ18O values range from –9.9‰ to +8.0‰ and δ17O ranges from –11.3‰ to +5.5‰, similar to values measured in individual olivine grains and whole chondrules from carbonaceous chondrites. These data indicate that the mineral grains preserve their pre-terrestrial oxygen isotopic compositions, and provide another link between AMMs and carbonaceous chondrites. However, no clear relationship with one single subgroup of carbonaceous chondrite can be established. Based on their textures, crystal chemistries, and oxygen isotopes, some coarse-grained crystalline AMMs could originate from chondrule fragmentation. Whether the remaining mineral grains were formed by igneous or condensation processes is unclear. No clear correlation is observed between isotopic compositions and mineral compositions of AMM olivine grains, suggesting that the FeO- and 16O-enrichment processes are not coupled in a simple way. Nor are any relatively large 16O enrichments measured in any of the olivine grains, however two Mg-Al spinels and a melilite grain are 16O enriched at the level of δ18O ∼ δ17O ∼ –40‰. The discovery of an 16O-enriched melilite grain in AMMs supports the hypothesis that refractory minerals throughout the solar nebula formed from a relatively uniformly 16O-enriched reservoir. This unique 16O-rich signature of refractory minerals in primitive solar system materials suggests that they either formed from a widespread 16O-rich reservoir in the solar nebula, or that an efficient mechanism (such as bipolar outflows) was acting to spread them from a highly localized 16O-rich region over the early solar nebula.
Geochimica et Cosmochimica Acta.
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ABSTRACT: Ion microprobe oxygen isotopic measurements of a chemically diverse suite of carbonates from Martian meteorite ALH84001 are reported. The δ18O values are highly variable, ranging from +5.4 to +25.3‰, and are correlated with major element compositions of the carbonate. The earliest-forming (Ca-rich) carbonates have the lowest δ18O values and the late-forming (Mg-rich) carbonates have the highest δ18O values. Two models are presented which can explain the isotopic variations. The carbonates could have formed in a water rich environment at relatively low, but highly variable temperatures. In this open-system case the lower limit to the temperature variation is ∼125°C, with fluctuations of over 250°C possible within the constraints of the model. Alternatively, the data can be explained by a closed-system model in which the carbonates precipitated from a limited amount of CO2-rich fluid. This scenario can reproduce the isotopic variations observed at a range of temperatures, including relatively high temperatures (> 500°C). Thus the oxygen isotopic compositions do not provide unequivocal evidence for formation of the carbonates at low temperature. Although more information is needed in order to distinguish between the models, neither of the implied environments is consistent with biological activity. Thus, we suggest that features associated with the carbonates which have been interpreted to be the result of biological activity were most probably formed by inorganic processes.
Geochimica et Cosmochimica Acta.
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ABSTRACT: In situ measurements of 60Fe–60Ni and 53Mn–53Cr isotopic systems with an ion microprobe have been carried out for sulfide assemblages from unequilibrated enstatite chondrites (UECs). Evidence for the initial presence of 60Fe has been observed in nine sulfide inclusions from three UECs: ALHA77295, MAC88136, and Qingzhen. The inferred initial (60Fe/56Fe) [(60Fe/56Fe)0] ratios show a large variation range, from ∼2 × 10−7 to ∼2 × 10−6. The sulfide inclusions with high Fe/Ni ratios yield (60Fe/56Fe)0 ratios of ∼(2–7) × 10−7, similar to most of the (60Fe/56Fe)0 values of troilite and pyroxene observed in unequilibrated ordinary chondrites (UOCs). Inclusions with high inferred (60Fe/56Fe)0 ratios (∼1–2 × 10−6) have low Fe/Ni ratios and the magnitude of the 60Ni excesses is similar in two MAC88136 assemblages in spite of a difference of a factor of two in their Fe/Ni ratios. The inferred high (60Fe/56Fe)0 ratios were probably the result of Fe–Ni re-distribution in the sulfides during later alteration processes.The 53Mn–53Cr system was measured in five of the sulfide assemblages that were examined for their 60Fe–60Ni systematics. The 53Mn–53Cr isochrons yielded variable initial (53Mn/55Mn) [(53Mn/55Mn)0] ratios from ∼(2–7) × 10−7. There is no obvious correlation between the (60Fe/56Fe)0 and (53Mn/55Mn)0 ratios. The variable 53Mn–53Cr isochrons probably also indicate later disturbance to the isotopic systems in these sulfides. Even though no chronological information can be extracted from the 60Fe–60Ni and 53Mn–53Cr systems in these UEC sulfides, our results indicate that 60Fe was present in the enstatite chondrite formation region of the early Solar System.
Geochimica et Cosmochimica Acta.
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ABSTRACT: The oxygen isotopic compositions of seven Al-rich chondrules from four unequilibrated ordinary chondrites were measured in situ using an ion microprobe. On an oxygen three isotope plot, the data are continuous with the ordinary chondrite ferromagnesian chondrule field but extend it to more 16O-enriched values along a mixing line of slope=0.83±0.09, with the lightest value recorded at δ18O=−15.7±1.8‰ and δ17O=−13.5±2.6‰. If Al-rich chondrules were mixtures of ferromagnesian chondrules and CAI material, their bulk chemical compositions would require them to exhibit larger 16O enrichments than we observe. Therefore, Al-rich chondrules are not simple mixtures of these two components. Three chondrules exhibit significant internal isotopic heterogeneity indicative of partial exchange with a gaseous reservoir. Porphyritic Al-rich chondrules are consistently 16O-rich relative to nonporphyritic ones, suggesting that degree of melting is a key factor and pointing to a nebular setting for the isotopic exchange process. Because Al-rich chondrules are closely related to ferromagnesian chondrules, their radiogenic Mg isotopic abundances can plausibly be applied to help constrain the timing or location of chondrule formation.
Earth and Planetary Science Letters.
