Bulk Meteorite Research Articles

Nepheline and sodalite in the matrix of the Ningqiang carbonaceous chondrite: Implications for formation through parent-body processes

Ningqiang is an ungrouped carbonaceous chondrite that chemically and petrologically resembles CV3 chondrites. The matrix of Ningqiang shows much higher abundances of Na, K, and Al by factors of 4.4, 2.7, and 1.6, respectively, than in CV3 chondrites. Our scanning and transmission electron microscope observations and synchrotron radiation X-ray diffraction measurements reveal that the major proportions of these elements can be attributed to the presence of nepheline and sodalite. Rietveld refinement of X-ray diffraction data shows that the feldspathoids constitute 7.7vol.% of all crystalline phases in the matrix. Nepheline and sodalite occur mostly as discrete, equidimensional grains 2–5μm in diameter that are dispersed homogeneously in the matrix. Most of the grains contain inclusions of Fe-rich olivine and minor Ca pyroxene, magnetite, troilite, and pentlandite.Despite the high abundances of Na, K, and Al in the matrix of Ningqiang, the bulk meteorite abundances of these elements are comparable to those of the CV group (e.g., Rubin et al., 1988). This means that the chondrules, which constitute a major proportion of the volume other than the matrix in Ningqiang, are depleted in Na, K, and Al. In fact, our analyses and observations show that the chondrules in Ningqiang overall contain very small amounts of these elements. Our interpretation of these findings suggests that nepheline and sodalite in the Ningqiang matrix were originally formed by Na-metasomatism of the chondrules and Ca–Al-rich inclusions in the meteorite parent body. Afterward, they were likely disaggregated and scattered into the matrix. However, it is difficult to envisage that the disaggregation and scattering occurred in situ in the present setting of the meteorite. Hence, we suggest that the Ningqiang meteorite underwent these processes before final lithification.

INTERSTELLAR DUST PROPERTIES OF M51 FROMAKARIMID-INFRARED IMAGES

The presence or absence of variations in the mass-independent abundances of Si isotopes in bulk meteorites provides important clues concerning the evolution of the early solar system. No Si isotopic anomalies have been found within the level of analytical precision of 15 ppm in 29Si/28Si across a wide range of inner solar system materials, including terrestrial basalts, chondrites, and achondrites. A possible exception is the angrites, which may exhibit small excesses of 29Si. However, the general absence of anomalies suggests that primitive meteorites and differentiated planetesimals formed in a reservoir that was isotopically homogenous with respect to Si. Furthermore, the lack of resolvable anomalies in the Calcium-Aluminum-rich Inclusion measured here suggests that any nucleosynthetic anomalies in Si isotopes were erased through mixing in the solar nebula prior to the formation of refractory solids. The homogeneity exhibited by Si isotopes may have implications for the distribution of Mg isotopes in the solar nebula. Based on supernova nucleosynthetic yield calculations, the expected magnitude of heavy-isotope overabundance is larger for Si than for Mg, suggesting that any potential Mg heterogeneity, if present, exists below the 15 ppm level.

Insight into the silicate and organic reservoirs of the comet forming region

Cometary interplanetary dust particles (IDPs), collected in Earth’s stratosphere, currently represent the best way to sample outer Solar System primordial dust. The fine-grained (sub-μm) minerals of IDPs show some strong similarities to the textures expected for primary condensates from the solar nebula. In this study we have analysed a set of IDPs for combined bulk carbon, nitrogen and hydrogen isotopes, and high precision oxygen isotopes by NanoSIMS 50L. This study concentrates on combining isotopic analyses, including high precision oxygen, on the silicate and organic components within the same sample on fine-grained primordial materials. Oxygen isotope analyses reveal that some IDPs are more 16O-rich than any bulk meteorite compositions, extending to O isotope compositions in between chondritic- and solar-like values (δ17O=−20‰, δ18O=−20‰). The 16O-rich IDPs display more primitive organic signatures than the chondritic-like 16O-poor IDPs but, rather interestingly, they also have lower presolar grain abundances. The 16O-poor signatures probably indicate an abundant component of chondritic-like material, processed in the inner protoplanetary disk. In order to explain the association of relatively processed material (silicates and organics) with high presolar grain abundances we propose a model whereby the initial dust component is 16O-rich (solar-like) and originally present homogenously throughout the protoplanetary disk. Outward radial transport, potentially associated with aerodynamic sorting, led to an influx of processed silicates from the inner Solar System with chondritic-like isotopic signatures. Comet accretion that occurred late, or at smaller heliocentric distance, included higher abundances of this chondritic-like component and hence had more 16O-poor O isotope compositions. Presolar grains, whose extreme isotopic signatures were destroyed during early homogenisation of solar nebula dust, continue to accrete onto the protoplanetary disk with time such that the later-formed comets, those with a higher proportion of processed silicates within their source, also had time to accumulate a higher abundance of presolar grains. A similar pattern is observed in the organics, with decreasing δD and C/H, from increased exchange with nebula gas, in IDPs that contain more 16O-poor chondritic-like material.

Jiddat al Harasis 556: A howardite impact melt breccia with an H chondrite component

Abstract– A petrographic and geochemical study was undertaken to characterize Jiddat al Harasis (JaH) 556, a howardite find from the Sultanate of Oman. JaH 556 is a polymict impact melt breccia containing highly shocked clasts, including mosaicized olivine and recrystallized plagioclase, set in a finely recrystallized vesicular matrix (grain diameter <5–10 μm). Plagioclase (An76–92) and clinopyroxene (En48–62Wo7–15) are associated with orthopyroxene and olivine clasts like in a howardite. JaH 556 oxygen isotope data indicate that it has an anomalous bulk‐rock composition as howardite, resulting from a mixture between HED material and at least one second reservoir characterized by a higher Δ17O. The bulk meteorite has a composition consistent with howardites, but it is enriched in siderophile elements (Ni = 3940 and Co = 159 ppm) arguing for a chondritic material as second reservoir. This is independently confirmed by the occurrence of chondrule relics composed of olivine (Fo56–80), orthopyroxene (En79Wo2), and plagioclase (An61–66). Based on oxygen isotopic signature, siderophile composition, and chondrule core Mg number (Fo80 and En79Wo2), it is proposed that JaH 556 is a howardite containing approximately 20% H chondrite material. This percentage is high compared with that observed petrographically, likely because chondritic material dissolved in the impact melt. This conclusion is supported by the observed reaction of orthopyroxene to olivine, which is consistent with a re‐equilibration in a Si‐undersaturated melt. JaH 556’s unique composition enlarges the spectrum of howardite‐analogs to be expected on the surface of 4 Vesta. Our data demonstrate that oxygen isotopic anomalies can be produced by a mixture of indigenous and impactor materials and must be interpreted with extreme caution within the HED group.

NEUTRON-POOR NICKEL ISOTOPE ANOMALIES IN METEORITES

We present new, mass-independent, Ni isotope data for a range of bulk chondritic meteorites. The data are reported as e 60 Ni58/61, e 62 Ni58/61, and e 64 Ni58/61, or the parts per ten thousand deviations from a terrestrial reference, the NIST SRM 986 standard, of the 58 Ni/ 61 Ni internally normalized 60 Ni/ 61 Ni, 62 Ni/ 61 Ni, and 64 Ni/ 61 Ni ratios. The chondrites show a range of 0.15, 0.29, and 0.84 in e 60 Ni58/61, e 62 Ni58/61, and e 64 Ni58/61 relative to a typical sample precision of 0.03, 0.05, and 0.08 (2 s.e.), respectively. The carbonaceous chondrites show the largest positive anomalies, enstatite chondrites have approximately terrestrial ratios, though only EH match Earth’s composition within uncertainty, and ordinary chondrites show negative anomalies. The meteorite data show a strong positive correlation between e 62 Ni58/61 and e 64 Ni58/61, an extrapolation of which is within the error of the average of previous measurements of calcium-, aluminium-rich inclusions. Moreover, the slope of this bulk meteorite array is 3.003 ± 0.166 which is within the error of that expected for an anomaly solely on 58 Ni. We also determined to high precision (∼10 ppm per AMU) the mass-dependent fractionation of two meteorite samples which span the range of e 62 Ni58/61 and e 64 Ni58/61. These analyses show that “absolute” ratios of 58 Ni/ 61 Ni vary between these two samples whereas those of 62 Ni/ 61 Ni and 64 Ni/ 61 Ni do not. Thus, Ni isotopic differences seem most likely explained by variability in the neutron-poor 58 Ni, and not correlated anomalies in the neutron-rich isotopes, 62 Ni and 64 Ni. This contrasts with previous inferences from mass-independent measurements of Ni and other transition elements which invoked variable contributions of a neutron-rich component. We have examined different nucleosynthetic environments to determine the possible source of the anomalous material responsible for the isotopic variations observed in Ni and other transition elements within bulk samples. We find that the Ni isotopic variability of the solar system cannot be explained by mixing with a component of bulk stellar ejecta from either SN II, Wolf–Rayet or, an asymptotic giant branch source and is unlikely to result from bulk mixing of material from an SN Ia. However, variable admixture of material from the Si/S zone of an SN II can create all the characteristics of Ni isotope variations in solar system materials. Moreover, these characteristics can also be provided by an SN II with a range of masses from 15 to 40 M� , showing that input from SN II is a robust source for Ni isotope variations in the solar system. Correlations of Ni isotope anomalies with O, Cr, and Ti isotope ratios and Pb/Yb in bulk meteorites suggest that the heterogeneous distribution of isotopic anomalies in the early solar system likely resulted from nebular sorting of chemically or physically different materials bearing different amounts of isotopes synthesized proximally to the collapse of the protosolar nebula.

Molybdenum isotope anomalies in meteorites: Constraints on solar nebula evolution and origin of the Earth

The early evolution of the solar nebula involved substantial transport of mass, resulting in mixing and homogenization of isotopically diverse materials that were contributed to the solar system from multiple stellar nucleosynthetic sources. The efficiency of this mixing, as well as its timescale can be quantified by determining nucleosynthetic isotope variations among meteorites and terrestrial planets. Here we present Mo isotopic data for a wide range of samples, including Ca–Al-rich inclusions, chondrites and differentiated meteorites, as well as martian and terrestrial samples. Most meteorites are depleted in s-process Mo relative to the Earth, and only the IAB–IIICD irons, angrites and martian meteorites have terrestrial Mo isotopic compositions. In contrast, most Ca–Al-rich inclusions are enriched in r-process Mo, but one inclusion is characterized by a large s-process deficit. Molybdenum isotopic anomalies in the bulk meteorites correlate with those in Ru exactly as predicted from nucleosynthetic theory, but no obvious correlation is apparent between Mo and Ni anomalies. Therefore, s-process Mo and Ru seem to be hosted in the same carrier, which must be distinct from the carrier responsible for isotopic anomalies in the Fe-group elements (Ni, Cr, Ti). Furthermore, the isotopic heterogeneity in Mo (and other elements) contrasts with the isotopic homogeneity for Hf and Os, indicating that different s-process carriers once existed in the early solar nebula and that only some of these were heterogeneously distributed. The Mo isotopic anomalies of meteorites and their components decrease over time and with increasing size of the parent bodies, providing evidence for a progressive homogenization of the solar nebula. However, the carbonaceous chondrites exhibit larger Mo anomalies than expected for their age, indicating that they received a greater portion of material from the outer solar system (where homogenization was slow) than other meteorite parent bodies and terrestrial planets. Compared to the meteorites, Earth is enriched in s-process Mo and must have accreted from material distinct from the meteorites. Combined Mo and O isotopic data show that the composition of the Earth cannot be reconstructed by any known combination of meteorites, implying that meteorites may be inappropriate proxies for the isotopic composition of the bulk Earth. This is exemplified by the covariation of 92Mo and 142Nd anomalies in chondrites, showing that the 142Nd deficit of chondrites compared to the accessible Earth may not unequivocally be interpreted as a signature of an early differentiation of the Earth. However, further high precision isotopic data are needed to evaluate the role of chondrites in defining the isotopic composition of the Earth.

48 Ca HETEROGENEITY IN DIFFERENTIATED METEORITES

Isotopic heterogeneities of 48Ca have been found in numerous bulk meteorites that are correlated with 50Ti and 54Cr anomalies among differentiated planetary bodies, and the results suggest that a rare subset of neutron-rich Type Ia supernova (nSN Ia) was responsible for contributing these neutron-rich iron-group isotopes into the solar system (SS). The heterogeneity of these isotopes found in differentiated meteorites indicates that the isotopic compositions of the bulk SS are not uniform, and there are significant amounts of nSNe Ia dust incompletely mixed with the rest of SS materials during planetary formation. Combined with the data of now-extinct short-lived nuclide 60Fe, which can be produced more efficiently from an nSN Ia than a Type II supernova ejecta, the observed planetary-scale isotopic heterogeneity probably reflects a late input of stellar dust grains with neutron-rich nuclear statistical equilibrium nuclides into the early SS.

Assessment of nebular versus parent body processes on presolar components present in chondrites: Evidence from osmium isotopes

We report Os isotope compositions of acid residues separated from six carbonaceous chondrites that were subjected to varying degrees of aqueous alteration on their parent bodies. The residues from three enstatite chondrites were also investigated in order to evaluate the effects of nebular and parent body processing on the survival of presolar grains in meteorites formed under redox conditions that were markedly reduced, compared to those under which carbonaceous and ordinary chondrites formed. All acid residues from CM and CR chondrites show enrichments in s-process Os isotopes relative to the Solar System average recorded in bulk chondrites. The extent of the anomaly present positively correlates with the degree of aqueous alteration of the host chondrites. This correlation was probably caused by selective destruction/modification of presolar grains carrying r-process-enriched Os during progressive aqueous alteration on the parent bodies. The r-process-enriched component was likely either presolar silicates formed in Type II supernova ejecta, or other unidentified reduced presolar phases such as metal alloys, carbides and silicides. Acid residues from enstatite chondrites have Os isotope anomalies that are much more enriched in the s-process components, relative to the residues from carbonaceous and ordinary chondrites that experienced the same grade of thermal metamorphism. This most likely reflects the selective destruction of s-process-enriched presolar phases that occurred under the more oxidized conditions experienced by carbonaceous and ordinary chondrites, either while components formed in the nebula, or on their parent bodies. To account for the uniform, terrestrial Os isotopic composition in all types of bulk chondrites, it is required that r-process or s-process-enriched Os, released from presolar phases during nebular or parent body processing, was re-incorporated into a new phase(s) which was not lost from the location where the bulk meteorites were derived. However, because parent body processing might have acted differently on other elements (e.g., open system behavior of fluid mobile elements during aqueous alteration), recent findings of isotopic heterogeneities in bulk meteorites should be evaluated not just by invoking nebular heterogeneities, but by also considering the effects of parent body processing.

REE and actinide microdistribution in Sahara 97072 and ALHA77295 EH3 chondrites: A combined cosmochemical and petrologic investigation

We report the results of rare earth elements (REEs) and U–Th inventory of individual minerals (oldhamite, enstatite and niningerite) in two of the most unequilibrated and primitive EH3 known so far, ALHA77295 and Sahara 97072. Under the highly reducing condition that prevailed during the formation of enstatite chondrites, REEs are mainly chalcophile and concentrated in oldhamite. The study is guided by detailed petrographic investigations of the individual minerals in chondrules, complex sulfide-metal clasts and enstatite-dominated matrices. We developed two textural parameters in order to resolve the evolution of oldhamite condensates and their residence in the solar gas prior to their accretion in the individual objects or in matrices and relate these textural features to the measured REE patterns of the individual oldhamite crystals. These textural parameters are the crystal habit of oldhamite grains (idiomorphic or anhedral) and their host assemblages. REE concentrations were measured by SIMS and LA-ICPMS. Oldhamite grains display REE enrichments (10–100 × CI). Four types of REE patterns are encountered in oldhamite in ALHA77295. In general the REE distributions cannot be assigned to a specific oldhamite-bearing assemblage. The most represented REE pattern is characterized by both slight to large positive Eu and Yb anomalies and is enriched in light REEs relative to heavy REEs. This pattern is present in 97% of oldhamite in Sahara 97072, suggesting a different source region in the reduced part of the nebula or different parental EH asteroids for the two EH3 chondrites. Different parental asteroids are also supported by MgS–FeS zoning profiles in niningerite grains adjacent to troilite revealing both normal and reverse zoning trends and different MnS contents. The observed homogeneity of REE distribution in oldhamite grains in Sahara 97072 is not related to the mild metamorphic event identified in this meteorite that caused breakdown of the major K- and Rb-bearing sulfide (djerfisherite). REE concentrations in enstatite range between 0.2 and 8 × CI. Hence, enstatite is an important REE host next to oldhamite. Most patterns are characterized by negative Eu and Yb anomalies. Niningerites are negligible contributors to bulk EH3 REE inventory. Average positive Eu and Yb anomalies observed in most oldhamite are complimentary to the negative ones in enstatite thus explaining the flat patterns of the bulk meteorites. The condensation calculations based on cosmic abundances predict that the first oldhamite condensates should have flat REE patterns with Eu and Yb depletions since Eu and Yb condense at lower temperature than other REE. However, this pattern is seen in enstatite. Our findings are at odds with the predicted negative Eu and Yb anomalies in oldhamite earliest condensates from a closed system in a reduced solar source. Our petrographic, mineral chemistry and REE abundances of oldhamite, enstatite and niningerite discards an origin of oldhamite by impact melting ( Rubin et al., 2009). Our results do not support in first order the scenario of the incorporation of REE in the Earth’s core to explain 142Nd excess in terrestrial samples relative to chondrites because oldhamite is the major REE carrier phase and has super-chondritic Sm/Nd ratios.

A new method for MC-ICPMS measurement of titanium isotopic composition: Identification of correlated isotope anomalies in meteorites

A new protocol is presented for precise measurements of titanium isotopes in natural samples. Titanium was separated via ion-exchange and extraction chromatography in two stages. Tests on Ti standard solutions show that isobaric interferences from Ca, V, and Cr can be adequately corrected, as long as these elements are present at atomic ratios of Ca/Ti < 20, V/Ti < 2, and Cr/Ti < 0.1. Furthermore, Zr2+ and Mo2+ have no influences on Ti+ signals when atomic ratios of Zr/Ti < 0.002 and Mo/Ti < 0.04. Compared with these correction limits, the purified solutions have corresponding ratios several orders of magnitude lower, indicating that the chemical separation technique is effective. This newly developed method has been successfully applied to geostandards and a wide variety of bulk meteorites. Our results are in good agreement with the data from Trinquier et al. (Science, 2009, 324, 374–376)1 and reveal a linear correlation between isotope anomalies of two Ti nuclides in bulk meteorites. The correlation reflects incomplete mixing of the carrier phases for Ti isotope anomalies before bulk meteorite formation.

U–Pb chronology of the Solar System's oldest solids with variable 238U/ 235U

Accurate determination of the absolute ages of the oldest Solar System objects — chondrules and Ca–Al-rich inclusions (CAIs), requires knowledge of their 238U/ 235U ratios. This ratio was assumed to be invariant in all U–Pb dating of meteorites so far, but the recent discovery of U isotope variations in CAIs (Brennecka et al., 2010a) shows that this assumption is invalid. We present the first combined high-precision U and Pb isotopic data for a CAI, and U isotopic data for chondrules and whole rock fractions of the Allende meteorite. The Pb–Pb isochron age of the CAI SJ101 is 4567.18 ± 0.50 Ma, calculated using the measured 238U/ 235U = 137.876 ± 0.043 (2σ), reported relative to 238U/ 235U = 137.837 of the CRM 145 standard. Our best current estimate of the average terrestrial value is: 238U/ 235U = 137.821 ± 0.014.The error in the age includes uncertainties in the Pb–Pb isochron intercept and in the 238U/ 235U ratio. Allende bulk rock and chondrules have 238U/ 235U = 137.747 ± 0.017 (2σ), distinctly lower than the CAI. The difference in the 238U/ 235U ratio of 0.129 ± 0.046 (2σ) between the CAI and chondrules and bulk meteorite increases the 207Pb– 206Pb age difference by ~ 1.4 Ma, and eliminates apparent disagreement between the CAI–chondrule formation time interval determinations with the U–Pb and extinct nuclide ( 26Al– 26Mg and 182Hf– 182W) data. We discuss standardisation of 238U/ 235U measurements for U–Pb geochronology and cosmochronology, elemental and isotopic fractionation induced by intensive acid leaching, ages of CAIs in the context of 238U/ 235U variability, and possible causes of U isotopic variations in CAIs and meteorites.

Heavily fractionated noble gases in an acid residue from the Klein Glacier 98300 EH3 chondrite

Noble gases were measured both in bulk samples (stepped pyrolysis and total extraction) and in a HF/HCl residue (stepped pyrolysis and combustion) from the Klein Glacier (KLE) 98300 EH3 chondrite. Like the bulk meteorite and as seen in previous studies of bulk type 3 E chondrites (“sub-Q”), the acid residue contains elementally fractionated primordial noble gases. As we show here, isotopically these are like those in phase-Q of primitive meteorites, but elementally they are heavily fractionated relative to these. The observed noble gases are different from “normal” Q noble gases also with respect to release patterns, which are similar to those of Ar-rich noble gases in anhydrous carbonaceous chondrites and unequilibrated ordinary chondrites (with also similar isotopic compositions). While we cannot completely rule out a role for parent body processes such as thermal and shock metamorphism (including a later thermal event) in creating the fractionated elemental compositions, parent body processes in general seem not be able to account for the distinct release patterns from those of normal Q noble gases. The fractionated gases may have originated from ion implantation from a nebular plasma as has been suggested for other types of primordial noble gases, including Q, Ar-rich, and ureilite noble gases. With solar starting composition, the corresponding effective electron temperature is about 5000 K. This is lower than inferred for other primordial noble gases (10,000–6000 K). Thus, if ion implantation from a solar composition reservoir was a common process for the acquisition of primordial gas, electron temperatures in the early solar system must have varied spatially or temporally between 10,000 and 5000 K. Neon and xenon isotopic ratios of the residue suggest the presence of presolar silicon carbide and diamond in abundances lower than in the Qingzhen EH3 and Indarch EH4 chondrites. Parent body processes including thermal and shock metamorphism and a late thermal event also cannot be responsible for the low abundances of presolar grains. KLE 98300 may have started out with smaller amounts of presolar grains than Qingzhen and Indarch.

Chemical Separation and Mass Spectrometry of Cr, Fe, Ni, Zn, and Cu in Terrestrial and Extraterrestrial Materials Using Thermal Ionization Mass Spectrometry

A sequential chemical separation technique for Cr, Fe, Ni, Zn, and Cu in terrestrial and extraterrestrial silicate rocks was developed for precise and accurate determination of elemental concentration by the isotope dilution method (ID). The technique uses a combination of cation-anion exchange chromatography and Eichrom nickel specific resin. The method was tested using a variety of matrixes including bulk meteorite (Allende), terrestrial peridotite (JP-1), and basalt (JB-1b). Concentrations of each element was determined by thermal ionization mass spectrometry (TIMS) using W filaments and a Si-B-Al type activator for Cr, Fe, Ni, and Zn and a Re filament and silicic acid-H3PO4 activator for Cu. The method can be used to precisely determine the concentrations of these elements in very small silicate samples, including meteorites, geochemical reference samples, and mineral standards for microprobe analysis. Furthermore, the Cr mass spectrometry procedure developed in this study can be extended to determine the isotopic ratios of 53Cr/52Cr and 54Cr/52Cr with precision of approximately 0.05epsilon and approximately 0.10epsilon (1epsilon = 0.01%), respectively, enabling cosmochemical applications such as high precision Mn-Cr chronology and investigation of nucleosynthetic isotopic anomalies in meteorites.

Highly siderophile element evidence for early solar system processes in components from ordinary chondrites

Separated magnetic and nonmagnetic components from the ordinary chondrites Dhajala (H3.8) and Ochansk (H4) were analyzed for their Re–Os isotopic compositions, as well as for the abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt and Pd. The Re–Os isotopic systematics of these components are used to constrain the timing of HSE fractionations, and assess the level of open-system behavior of these elements in each of the different components. The high precision, isotope dilution mass spectrometric analyses of the HSE are used to constrain the origins of, and possible relations between some of the diverse components present in these chondrites. The relative and absolute abundances of the HSE differ considerably among the components. Metal fractions have Re/Os that are factors of ∼2 (Dhajala) to ∼3 (Ochansk) higher than those of their nonmagnetic fractions. The isotopic data for both meteorites are consistent with the largest Re–Os fractionations occurring between metal and nonmagnetic components early in solar system history, although minor to moderate late stage, open-system behavior, and limited variations in Re/Os preclude a precise determination of the age for that fractionation. Open-system behavior is generally absent to minor in the metal fractions, and highly variable in nonmagnetic fractions. Re/Os ratios of nonmagnetic fractions deviate as much as 40% from a primordial isochron. Although some deviations are large for isochron applications, nearly all are negligible with respect to consideration of fractionation processes controlling the HSE. Metal from both meteorites contains about 90% of the total budget of HSE. Metal in Ochansk has ∼2 to 10 times the abundances of the bulk meteorite, while metal from the matrix of Dhajala has ∼2 to 4 times the abundances of the bulk. Fine metal in both meteorites has higher abundances than coarse metal, as has been previously observed. Nonmagnetic components, consisting of chondrules and matrix from which metal was removed in the laboratory, have highly fractionated HSE, characterized by much lower Re/Os than the bulk meteorites, as well as large relative depletions in Pd. The abundances of Re, Os, Ir, Ru and Pt in the nonmagnetic fractions are 14–120 ng/g, much higher than would be expected if they had equilibrated with the metal phases present (150–16,000 ng/g). Collectively, the data are consistent with the HSE budget in ordinary chondrites being dominated by two HSE-bearing carrier phases with distinct compositions. These phases formed separately, and never subsequently equilibrated. Metal components incorporated a HSE carrier that formed at high through moderate temperatures and relatively high pressures, such that the relatively volatile Pd behaved coherently with the more refractory HSE. Nonmagnetic fractions from both chondrules and matrix have HSE compositions that likely require at least two processes that fractionated the HSE. Depletions in Pd are consistent with the presence of HSE carriers that formed as either highly refractory condensates, or residues of high degrees of metal melting. Depletions in Re may implicate a period of relatively high f O 2 during which a volatile form of Re was separated from the other HSE.

The use of Auger spectroscopy for the in situ elemental characterization of sub‐micrometer presolar grains

Abstract— Presolar grains are small samples of stardust that can be found at low abundances in some of the most unaltered types of extraterrestrial materials. While earlier laboratory studies of stardust mainly focused on grain types that can be extracted from bulk meteorites by acid dissolution techniques, such as silicon carbide and graphite, recent analyses of presolar silicates rely on isotope imaging searches for locating these grains in situ. Since presolar silicates are generally less than a micrometer in diameter and represent at best only a few hundred ppm of their host materials (e.g., primitive meteorites or interplanetary dust particles), locating and studying these particles can be analytically challenging. Recently, we began using scanning Auger spectroscopy for the in situ elemental characterization of presolar silicate grains as a complement to NanoSIMS isotopic studies for obtaining spatially matched compositional data. Auger spectroscopy is a well‐established analytical technique for elemental characterizations in the material sciences, but has not been widely used in geological applications. We discuss the application of this technique to sub‐micrometer sized silicate grains and address practical issues such as sample preparation, measurement settings, spatial resolution, data processing, and elemental quantification.

Helium loss from Martian meteorites mainly induced by shock metamorphism: Evidence from new data and a literature compilation

Abstract— Noble gas data from Martian meteorites have provided key constraints about their origin and evolution, and their parent body. These meteorites have witnessed varying shock metamorphic overprinting (at least 5 to 14 GPa for the nakhlites and up to 45–55 GPa (e.g., the lherzolitic shergottite Allan Hills [ALH] A77005), solar heating, cosmic‐ray exposure, and weathering both on Mars and Earth. Influences on the helium budgets of Martian meteorites were evaluated by using a new data set and literature data. Concentrations of 3He, 4He, U, and Th are measured and shock pressures for same sample aliquots of 13 Martian meteorites were determined to asses a possible relationship between shock pressure and helium concentration. Partitioning of 4He into cosmogenic and radiogenic components was performed using the lowest 4He/3He ratio we measured on mineral separates (4He/3He = 4.1, pyroxene of ALHA77005). Our study revealed significant losses of radiogenic 4He. Systematics of cosmogenic 3He and neon led to the conclusion that solar radiation heating during transfer from Mars to Earth and terrestrial weathering can be ruled out as major causes of the observed losses of radiogenic helium in bulk meteorites. For bulk rock we observed a correlation of shock pressure and radiogenic 4He loss, ranging between ˜20% for Chassigny and other moderately shocked Martian meteorites up to total loss for meteorites shocked above 40 GPa. A steep increase of loss occurs around 30 GPa, the pressure at which plagioclase transforms to maskelynite. This correlation suggests significant 4He loss induced by shock metamorphism. Noble gas loss in rocks is seen as diffusion due to (1) the temperature increase during shock loading (shock temperature) and (2) the remaining waste heat after adiabatic unloading (post shock temperature). Modeling of 4He diffusion in the main U, Th carrier phase apatite showed that post‐shock temperatures of ˜300 °C are necessary to explain observed losses. This temperature corresponds to the post‐shock temperature calculated for bulk rocks shocked at about 40 GPa. From our investigation, data survey, and modeling, we conclude that the shock event during launch of the meteorites is the principal cause for 4He loss.

Study of iron meteorite Sikhote–Alin and extracted iron–nickel phosphides using Mössbauer spectroscopy with high velocity resolution

Comparative study of schreibersite and rhabdite extracted from Sikhote–Alin (IIAB) iron meteorite using Mossbauer spectroscopy, X-ray diffraction and magnetic measurements was carried out. Mossbauer spectra of bulk meteorite sample and extracted phosphides were recorded in 4,096 channels and then presented in 1,024 channels. Differences of the unit cell structure, the Fe, Ni and Co content, the room temperature Mossbauer spectra and Curie temperature for rhabdite and schreibersite were found.

Study of metal grains extracted from chondrite Tsarev L5 using Mössbauer spectroscopy with high velocity resolution

Study of metal grains extracted from ordinary chondrite Tsarev L5 using Mossbauer spectroscopy with high velocity resolution was made for the first time. Three magnetic and one paramagnetic components were revealed in Mossbauer spectrum of extracted metal. These components were related to correspondent phases of Fe–Ni–Co alloy while only two magnetic components related to metal were revealed in the spectrum of bulk meteorite sample. The results obtained were in agreement with metallographic study of metal grains in meteorite Tsarev L5 which demonstrated the presence of four Fe(Ni, Co) phases.

Osmium isotope evidence for uniform distribution of s- and r-process components in the early solar system

We have precisely measured Os isotopic ratios in bulk samples of five carbonaceous, two enstatite and two ordinary chondrites, as well as the acid-resistant residues of three carbonaceous chondrites. All bulk meteorite samples have uniform 186Os/ 188Os, 188Os/ 189Os and 190Os/ 189Os ratios, when decomposed by an alkaline fusion total digestion technique. These ratios are also identical to estimates for Os in the bulk silicate Earth. Despite Os isotopic homogeneity at the bulk meteorite scale, acid insoluble residues of three carbonaceous chondrites are enriched in 186Os, 188Os and 190Os, isotopes with major contributions from stellar s-process nucleosynthesis. Conversely, these isotopes are depleted in acid soluble portions of the same meteorites. The complementary enriched and depleted fractions indicate the presence of at least two types of Os-rich components in these meteorites, one enriched in Os isotopes produced by s-process nucleosynthesis, the other enriched in isotopes produced by the r-process. Presolar silicon carbide is the most probable host for the s-process-enriched Os present in the acid insoluble residues. Because the enriched and depleted components present in these meteorites are combined in proportions resulting in a uniform chondritic/terrestrial composition, it requires that disparate components were thoroughly mixed within the solar nebula at the time of the initiation of planetesimal accretion. This conclusion contrasts with evidence from the isotopic compositions of some other elements (e.g., Sm, Nd, Ru, Mo) that suggests heterogeneous distribution of matter with disparate nucleosynthetic sources within the nebula.

On the mean oxygen isotope composition of the Solar System

Since the first discovery of extraordinary oxygen isotope compositions in carbonaceous meteorites by Clayton et al. [Clayton, R.N., Grossman, L., Mayeda, T.K., 1973. Science 182, 485–488], numerous studies have been done to explain the unusual mass-independent isotope fractionation, but the problem is still unresolved to this day. Clayton's latest interpretation [Clayton, R.N., 2002. Nature 415, 860–861] sheds new light on the problem, and possible hypotheses now seem to be fairly well defined. A key issue is to resolve whether the oxygen isotopes in the Solar System represented by the Sun (solar oxygen) are the same as oxygen isotopes in planetary objects such as bulk meteorites, Mars, Earth, and Moon, or whether the solar oxygen is more similar to the lightest oxygen isotopes observed in CAIs (Calcium Aluminum-rich Inclusions) in primitive meteorites. Here, we examined the problem using oxygen isotope analytical data of about 400 bulk meteorite samples of various classes or types (data compiled by K. Lodders). We used in our discussion exclusively the parameter ΔO17, a direct measure of the degree of mass-independent isotope fractionation of oxygen isotopes. When ΔO17 is arranged according to a characteristic size of their host planetary object, it shows a systematic trend: (1) ΔO17 values scatter around zero; (2) the scatter from the mean (ΔO17=0) decreases with increasing representative size of the respective host planetary object. This systematic trend is easily understood on the basis of a hierarchical scenario of planetary formation, that is, larger planetary objects have formed by progressive accretion of planetesimals by random sampling over a wide spectrum of proto-solar materials. If this progressive random sampling of planetesimals were the essential process of planetary formation, the isotopic composition of planetary oxygen should approach that of the solar oxygen. To test this random sampling hypothesis, we applied a multiscale, multistep bootstrap statistical method [Shimodaira, H., 2004. Ann. Statist. 32, 2616–2641] to the meteorite oxygen isotope data, and deduced a σ–N relation, where σ is the standard deviation of ΔO17, and N is the representative size of a host planetary object. If we assign 200 and 500 km as a representative sizes of the chondrite and achondrite parent bodies, the observed σ of ΔO17 agree well with the values predicted by the σ–N relation. A common mean value of ΔO17=0 for all planetary objects also agrees with the progressive random sampling process. Therefore, we conclude that the solar oxygen is the same as planetary oxygen, but differs from CAI oxygen. The conclusion implies that a massive enrichment in 17O and 18O resulting from CO self-shielding, a current influential interpretation of CAI-O, did not occur.