Meteoritic Component Research Articles

A likely possible origin of homochirality in amino acids and sugars on prebiotic earth

For life to start on earth and elsewhere, it is critical that the building blocks—amino acids and sugars—be in predominant homochiral form. Over the past century, the origin of terrestrial prebiotic homochirality has been the subject of many speculations. In this Letter I summarize the experimental evidence for ways in which some meteoritic components could have led to the dominance of l amino acids and d sugars on earth, and the most likely way in which the original chiral excesses in the meteorites were formed.

Shallow impact: Isotopic insights into crustal contributions to the Sudbury impact melt sheet

The largest known terrestrial impact melt sheet occurs within the 1850 Ma Sudbury Structure, Ontario. In order to evaluate the relative contributions of different target lithologies to the melt sheet, we have investigated the Pb isotope compositions of feldspar separates from early-formed quartz diorite magmas within Offset Dykes from around the impact structure. The samples define a linear array on plots of age-corrected 206Pb/ 204Pb versus 207Pb/ 204Pb. Samples from Offset Dykes hosted by the Huronian Supergroup (South Range) have a range of 206Pb/ 204Pb 1850 from 15.424 to 17.255 and 207Pb/ 204Pb 1850 from 15.390 to 15.801, whilst those hosted by Archean gneisses of the Superior Province (North Range) cluster around 206Pb/ 204Pb 1850 ≈ 14.8 and 206Pb/ 204Pb 1850 ≈ 15.1. These values can be approximated by binary mixing between the two major groups of target lithologies. A mix of 60–70% of Superior Province gneisses with 30–40% of Huronian metasedimentary material closely matches the Pb isotope compositions of North Range Offset Dyke samples, whereas in the South Range the required Huronian component is up to ca. 80%. These mixing proportions are consistent with Sr, Nd and Os isotope and trace element constraints. A third minor component, either locally-exposed Paleoproterozoic mafic rocks or the lower crust is also required. However, the isotopic, trace element and Ni–Cu–platinum group element characteristics of the melt sheet can be accommodated without the involvement of an average lower crustal or meteoritic component. A major contribution of Huronian supracrustal material, which had a pre-impact thickness of up to 12 km, is required to explain the chemical characteristics of the impact melts, which also have a strong upper crustal affinity (e.g. Eu/Sm = 0.22, Rb/Sr = 0.2–0.35). As such, a shallower level of melting is apparent than that predicted by many previous impact models for the Sudbury event. This can be accommodated by considering approach trajectories for the impactor oblique to the Earth’s surface. In addition, the isotopic and trace element variability identified indicates that the melt sheet was heterogeneous at an early stage, and may not have been completely homogenised during crater formation. Our findings have significant implications for the nature of the Sudbury impact event, the evolution of the melt sheet and the crustal sources of metals contained in Sudbury’s world class Ni–Cu–PGE sulphide ores.

Brownish inclusions and dark streaks in Libyan Desert Glass: Evidence for high-temperature melting of the target rock

Dark streaks and different types of inclusions in Libyan Desert Glass (LDG) collected from the LDG strewn field in Egypt were investigated. Rare transparent spherules enclosed in the glassy matrix are characterized by concentric cracks, irregular internal cracks, intense twinning, and considerable amounts of Ti and Al. Raman spectra show that the spherules are a-cristobalite. Their occurrence together with lechatelierite indicates quick heating of the source rock to at least 1550 � C, followed by rapid quenching leading to crystallization of b-cristobalite, which upon cooling inverted into a-cristobalite. Brownish inclusions are irregularly shaped, elongated objects with smooth contacts to the surrounding glass. They contain small roundish to elliptical droplets, and a few larger angular grains, which compositionally and according to their Raman spectra most closely resemble low-Ca, Al-rich orthopyroxene. Composition and texture of the orthopyroxene suggest that the brownish inclusions formed by incomplete melting of an Al-rich orthopyroxene bearing precursor, e.g., mafic phases present in desert surface sands or also of orthopyroxene- bearing granulite dykes in the LDG target. Experimental data on Ca-poor enstatite also support that the inclusions were heated to about 1550 � C. Analyses of dark streaks in LDG reveal high abundances of Al, Ti, Mn, Cr, Fe, and Ni and a pronounced correlation between the abundances of Cr, Mn, Fe, and Ni. As the Fe ⁄ Ni, Mn ⁄ Ni, and Cr ⁄ Ni ratios are all clearly nonchondritic, the source of this material is most likely terrestrial and the dark streaks studied here represent a different type of schlieren compared to those which contain a meteoritic component. These findings suggest LDG formation during a short high-temperature event. Melting of Al-rich orthopyroxene bearing target material seems to suggest an asteroid impact rather than a near-surface airburst.

Non-nucleosynthetic heterogeneity in non-radiogenic stable Hf isotopes: Implications for early solar system chronology

Nucleosynthetic heterogeneity and secondary neutron capture reactions may have important implications for 176Lu–176Hf chronology and modelling of early planetary evolution. So far, the relevance of these phenomena for the Lu–Hf system has not been explored. We therefore have analyzed the non-radiogenic stable Hf-isotope composition (177Hf, 178Hf, 179Hf, and 180Hf) of meteorites, meteorite components, and terrestrial rock samples to identify nucleosynthetic or neutron capture-induced variations. All analyzed chondrites have uniform 178Hf/177Hf and 180Hf/177Hf values that cannot be resolved from the average terrestrial composition. Thus, there is no evidence for nucleosynthetic heterogeneity in chondrites or Earth and these data support the use of a chondritic reference value for the Hf-isotope composition of the Bulk Silicate Earth. This homogeneity contrasts with nucleosynthetic heterogeneities found in lighter elements and provides evidence for a separate synthesis of light and heavy r-process nuclei.Various mesosiderite samples and one lunar meteorite display coupled 178Hf/177Hf and 180Hf/177Hf anomalies that are associated with neutron capture-induced deviations in 149Sm/154Sm and 150Sm/154Sm. However, the analyzed chondrites and an aubrite show only Sm-isotope anomalies, and these are the result of neutron capture. The Hf-isotope anomalies require substantial capture of epithermal neutrons, whereas Sm anomalies result primarily from thermal neutron capture. The non-radiogenic stable isotope composition of Hf is thus a suitable monitor for epithermal neutron capture reactions. The data reveal distinct neutron energy spectra: mesosiderites are characterized by high epithermal-to-thermal neutron fluence ratios, whereas the remaining samples show low epithermal-to-thermal ratios.Secondary neutron capture may significantly increase the measured 176Hf/177Hf in whole-rock meteorite samples without causing a resolvable shift in 176Lu/177Hf. Thus it could potentially induce scatter in Lu–Hf whole-rock isochrons and produce spurious initial 176Hf/177Hf values. However, the slopes of internal (i.e., mineral) isochrons cannot be increased significantly by secondary neutron capture. This process therefore cannot account for the unrealistically old 176Lu–176Hf ‘ages’ (e.g., ∼4.75Ga) of some meteorites.

The isotopic composition of magnesium in the inner Solar System

We report high-precision MC-ICPMS measurements of 25Mg/24Mg and 26Mg/24Mg isotope ratios for 14 meteorites, both primitive and differentiated, 5 lunar basalts and breccia samples, 5 terrestrial basalts, 1 dunite and 3 peridotites. We estimate the δ26Mg value of the bulk silicate Earth (BSE) at −0.54±0.04 (2SE) w.r.t. DSM3. Carbonaceous and ordinary chondrites show an average δ26Mg of −0.52±0.04 (2SE), martian meteorites show a narrow range in δ26Mg with an average δ26Mg of −0.57±0.02 (2SE), average δ26Mg of pallasite olivines is −0.54±0.04 (2SE) whereas that of lunar samples, including ilmenite basalts, olivine basalt and one crystalline breccia is −0.51±0.03 (2SE). Our data demonstrate that the overall stable Mg isotopic composition of the inner Solar System (average δ25Mg=−0.273, δ26Mg=−0.535) is homogeneous to within ±0.04‰ for 26Mg/24Mg. This value is indistinguishable from that of the BSE and lunar samples demonstrating that the Mg isotopic composition of the Earth–Moon system is chondritic. Accurate determination of the Mg isotopic composition of the rocky planets is important for Mg isotope studies of surface processes on Earth as well as for using Mg isotope data to understand evaporation and condensation effects in the solar nebula.Consistent with a recent report we find that clinopyroxenes have higher δ26Mg than co-existing olivines and orthopyroxenes in mantle peridotites, although the magnitude of this difference varies (0.08 to 0.2‰). Such small mineral fractionations along with differences between components in primitive meteorites (CAIs, chondrules, and matrix) preclude the resolution of mass-dependent fractionation differences smaller than about 0.05‰ between Solar System objects. We demonstrate that our results do not support the recent claims that: (i) the Earth–Moon system has a unique Mg isotope composition different from chondrites, or (ii) the higher δ26Mg values (up to 0.0) reported for chondrites, achondrites, terrestrial basalts and peridotites. To address the issue of inter-laboratory discrepancies on the Mg isotopic composition of silicate rocks, we have prepared 3 pure Mg standards with wide ranging δ26Mg (−3.75 to −0.69 with respect to DSM3) as well as a large volume of homogenized olivine from the San Carlos peridotite, which will be available to the scientific community, for future comparative studies.

Noble gas study of the Saratov L4 chondrite

Abstract– We have determined the elemental abundances and the isotopic compositions of noble gases in a bulk sample and an HF/HCl residue of the Saratov (L4) chondrite using stepwise heating. The Ar, Kr, and Xe concentrations in the HF/HCl residue are two orders of magnitude higher than those in the bulk sample, while He and Ne concentrations from both are comparable. The residue contains only a portion of the trapped heavy noble gases in Saratov; 40 ± 9% for 36Ar, 58 ± 12% for 84Kr, and 48 ± 10% for 132Xe, respectively. The heavy noble gas elemental pattern in the dissolved fraction is similar to that in the residue but has high release temperatures. Xenon isotopic ratios of the HF/HCl residue indicate that there is no Xe‐HL in Saratov, but Ne isotopic ratios in the HF/HCl residue lie on a straight line connecting the cosmogenic component and a composition between Ne‐Q and Ne‐HL. This implies that the Ne isotopic composition of Q has been changed by incorporating Ne‐HL (Huss et al. 1996) or by being mass fractionated during the thermal metamorphism. However, it is most likely that the Ne‐Q in Saratov is intrinsically different from this component in other meteorites. The evidence of this is a lack of correlation between the isotopic ratio of Ne‐Q and petrologic types of meteorites (Busemann et al. 2000). A neutron capture effect was observed in the Kr isotopes, and this process also affected the 128Xe/132Xe ratio. The 3He and 21Ne exposure ages for the bulk sample are 33 and 35 Ma, respectively.

The tungsten isotopic composition of Eoarchean rocks: Implications for early silicate differentiation and core–mantle interaction on Earth

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

Compositional evolution of the protoplanetary disk: Oxygen isotopes of type-II chondrules from CR2 chondrites

Chondrules are the dominant component of chondritic meteorites and attest to high-temperature transient heating events within the protoplanetary disk. They provide valuable constraint on the disk environments in which they formed and potentially the evolution of primitive planetary materials in the disk. The oxygen isotopic composition of CR2 chondrite type-II chondrules was investigated. Our data show significant variation in the isotopic compositions of the chondrules with no petrographic or geochemical means to identify what chondrule will plot where on a three-isotope diagram. Although we cannot rule out that these chondrules may have come from another chondrite-forming region, we argue in context of type-I chondrules from CR2 chondrites that CR2 type-II chondrules record changes in solid and gas composition during formation due to the vaporization of icy bodies in localized regions of the inner disk.

ON THE OXYGEN ISOTOPIC COMPOSITION OF THE SOLAR SYSTEM

The 18O/17O ratio of the Solar System is 5.2 while that of the interstellar medium (ISM) and young stellar objects is ~4. This difference cannot be explained by pollution of the Sun's natal molecular cloud by 18O-rich supernova ejecta because (1) the necessary B-star progenitors live longer than the duration of star formation in molecular clouds; (2) the delivery of ejecta gas is too inefficient and the amount of dust in supernova ejecta is too small compared to the required pollution (2% of total mass or ~20% of oxygen); and (3) the predicted amounts of concomitant short-lived radionuclides (SLRs) conflicts with the abundances of 26Al and 41Ca in the early Solar System. Proposals for the introduction of 18O-rich material must also be consistent with any explanation for the origin of the observed slope-one relationship between 17O/16O and 18O/16O in the high-temperature components of primitive meteorites. The difference in 18O/17O ratios can be explained by enrichment of the ISM by the 17O-rich winds of asymptotic giant branch (AGB) stars, the sequestration of comparatively 18O-rich gas from star-forming regions into long-lived, low-mass stars, and a monotonic decrease in the 18O/17O ratio of interstellar gas. At plausible rates of star formation and gas infall, Galactic chemical evolution does not follow a slope-one line in an three-isotope plot, but instead moves along a steeper trajectory towards an 17O-rich state. Evolution of the ISM and star-forming gas by AGB winds also explains the difference in the carbon isotope ratios of the Solar System and ISM.

A tungsten isotope approach to search for meteoritic components in terrestrial impact rocks

The identification of meteorite impact structures on Earth is based on two main criteria: the presence of shock-metamorphic effects in the crater rock ejecta and/or the confirmation of an extraterrestrial (meteoritic) component in breccias or melt rocks. For the latter, both high elemental abundances of siderophile elements (especially the platinum group elements) with corresponding meteoritic inter-element ratios and the osmium (Os) and chromium (Cr) isotopic signatures characteristic of meteorites have been used successfully. Inspired by earlier suggestions of a meteoritic component in Archean rocks based on tungsten (W) isotope anomalies, here we explore the possible use of 182W, which has been produced by the decay of now extinct 182Hf ( T 1/2 = 8.9 Ma), as a tracer of meteoritic component in terrestrial material. Each group of meteorites has W isotopic compositions that are distinct from each other and from the terrestrial crust. 182W has already been used to try to identify the impactor at the K/T boundary by analyzing the sediments and Ni-rich spinel. In the present study, we broaden the field of investigation by choosing a different approach, namely analyzing a variety of known impact rocks. We measured the W isotope composition in four samples from different impact structures (Gardnos, Norway; Morokweng, South Africa; Vredefort, South Africa; Ries, Germany) as well as in two samples from a distal ejecta layer (K–T boundary samples from Gams, Austria, and Berwind Canyon, USA). All these samples are unambiguously impact-produced and in several of those materials a meteoritic component has unequivocally been identified by other geochemical proxies. In all these samples, the isotopic composition of W is identical with analytical error to that of the Earth's continental crust, and no 182W anomalies are present, even in the samples containing a significant (percent level) meteoritic component. Therefore, we conclude that, in contrast to the Cr or Os isotopes, W isotopes are not suitable for the identification of meteoritic components in terrestrial rocks.

Argon retentivity of carbonaceous materials: feasibility of kerogen as a carrier phase of Q-noble gases in primitive meteorites

Abstract Extremely large amounts of heavy noble gases are concentrated in phase Q, which seems to be a carbonaceous phase analogous to terrestrial Type III kerogen. Phase Q must have very high noble gas retentivity based on the presence of such extremely large amounts of heavy noble gases in a very minor fraction of the meteorite. To verify that kerogen is a carrier phase of Q-noble gases, X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) using synchrotron radiation were carried on for kerogens (coals) and carbon allotropes that had been bombarded by 3-keV Ar ions, and the Ar retentivities of the two materials were compared. This comparison of the estimated Ar concentrations in the target materials revealed that carbon allotropes (graphite, fullerene, carbon nanotube, and diamond) have a much higher Ar retentivity than kerogens. This unexpected result clearly shows that the terrestrial kerogens tested in our study are not suitable as a carrier phase of Ar and, consequently, that phase Q may not be similar to the terrestrial kerogen tested. If heavy noble gases are really concentrated in carbonaceous components of primitive meteorites, phase Q may have a more ordered structure than terrestrial kerogen based on the fact that the greatest difference between terrestrial kerogen and carbon allotropes is the degree of order of the molecular structure.

Geochemical identification of impactor for Lonar crater, India

Abstract— The only well‐known terrestrial analogue of impact craters in basaltic crusts of the rocky planets is the Lonar crater, India. For the first time, evidence of the impactor that formed the crater has been identified within the impact spherules, which are ?0.3 to 1 mm in size and of different aerodynamic shapes including spheres, teardrops, cylinders, dumbbells and spindles. They were found in ejecta on the rim of the crater. The spherules have high magnetic susceptibility (from 0.31 to 0.02 SI‐mass) and natural remanent magnetization (NRM) intensity. Both NRM and saturation isothermal remanent magnetization (SIRM) intensity are ?2 Am2/Kg. Demagnetization response by the NRM suggests a complicated history of remanence acquisition. The spherules show schlieren structure described by chains of tiny dendritic and octahedral‐shaped magnetite crystals indicating their quenching from liquid droplets. Microprobe analyses show that, relative to the target basalt compositions, the spherules have relatively high average Fe2O3 (by ?1.5 wt%), MgO (?1 wt%), Mn (?200 ppm), Cr (?200 ppm), Co (?50 ppm), Ni (?1000 ppm) and Zn (?70 ppm), and low Na2O (?1 wt%) and P2O5 (?0.2 wt%). Very high Ni contents, up to 14 times the average content of Lonar basalt, require the presence of a meteoritic component in these spherules. We interpret the high Ni, Cr, and Co abundances in these spherules to indicate that the impactor of the Lonar crater was a chondrite, which is present in abundances of 12 to 20 percent by weight in these impact spherules. Relatively high Zn yet low Na2O and P2O5 contents of these spherules indicate exchange of volatiles between the quenching spherule droplets and the impact plume.

Modern U–Pb chronometry of meteorites: Advancing to higher time resolution reveals new problems

In this paper, we evaluate the factors that influence the accuracy of lead (Pb)-isotopic ages of meteorites, and may possibly be responsible for inconsistencies between Pb-isotopic and extinct nuclide timescales of the early Solar System: instrumental mass fractionation and other possible analytical sources of error, presence of more than one component of non-radiogenic Pb, migration of ancient radiogenic Pb by diffusion and other mechanisms, possible heterogeneity of the isotopic composition of uranium (U), uncertainties in the decay constants of uranium isotopes, possible presence of “freshly synthesized” actinides with short half-life (e.g. 234U) in the early Solar System, possible initial disequilibrium in the uranium decay chains, and potential fractionation of radiogenic Pb isotopes and U isotopes caused by alpha-recoil and subsequent laboratory treatment. We review the use of 232Th/ 238U values to assist in making accurate interpretations of the U–Pb ages of meteorite components. We discuss recently published U–Pb dates of calcium–aluminum-rich inclusions (CAIs), and their apparent disagreement with the extinct nuclide dates, in the context of capability and common pitfalls in modern meteorite chronology. Finally, we discuss the requirements of meteorites that are intended to be used as the reference points in building a consistent time scale of the early Solar System, based on the combined use of the U–Pb system and extinct nuclide chronometers.

Identification of the projectile component in the impact structures Rochechouart, France and Sääksjärvi, Finland: Implications for the impactor population for the earth

A set of 11 impact melt rock samples from the Rochechouart impact structure, France and nine impact melt rock samples from Sääksjärvi impact structure, Finland were studied for their major and trace element compositions, including the abundances of the platinum group elements. The main goal of this study was to identify the projectile type(s) responsible for the formation of these two impact structures. The results confirmed previous studies that suggested extraterrestrial contamination in both the Rochechouart and Sääksjärvi impact melt rocks. The projectile types found for Rochechouart and Sääksjärvi are quite similar, and compatible with the composition of non-magmatic iron meteorites (IA and IIIC). This interpretation is based on: identical platinum group element patterns as well as peculiar Ni/Cr, Ni/Ir and Cr/Ir ratios, which can be explained by mixing of the different components of non-magmatic iron meteorites. This result indicates that, besides ordinary chondrites, non-magmatic iron may be among the most common material impacting the Earth, as they also represent the majority of the projectiles for craters smaller that 1.5 km. The abundance of non-magmatic irons as projectiles as well as their composition (olivine, pyroxene and iron) supports the assumption that a fraction of the S-type asteroids could by related to this type of material.

Effect of pressure, temperature, and oxygen fugacity on the metal-silicate partitioning of Te, Se, and S: Implications for earth differentiation

We have measured liquid Fe metal–liquid silicate partitioning ( D i) of tellurium, selenium, and sulfur over a range of pressure, temperature, and oxygen fugacity (1–19 GPa, 2023–2693 K, fO 2 −0.4 to −5.5 log units relative to the iron-wüstite buffer) to better assess the role of metallic melts in fractionating these elements during mantle melting and early Earth evolution. We find that metal-silicate partitioning of all three elements decreases with falling FeO activity in the silicate melt, and that the addition of 5–10 wt% S in the metal phase results in a 3-fold enhancement of both D Te and D Se. In general, Te, Se, and S all become more siderophile with increasing pressure, and less siderophile with increasing temperature, in agreement with previous work. In all sulfur-bearing experiments, D Te is greater than D Se or D S, with the latter two being similar over a range of P and T. Parameterized results are used to estimate metal-silicate partitioning at the base of a magma ocean which deepens as accretion progresses, with the equilibration temperature fixed at the peridotite liquidus. We show that during accretion, Te behaves like a highly siderophile element, with expected core/mantle partitioning of >10 5, in contrast to the observed core/mantle ratio of ∼100. Less extreme differences are observed for Se and S, which yielded core/mantle partitioning 100- to 10 times higher, respectively, than the observed value. Addition of ∼0.5 wt% of a meteorite component (H, EH or EL ordinary chondrite) is sufficient to raise mantle abundances to their current level and erase the original interelement fractionation of metal-silicate equilibrium.

Osmium isotopic evidence against an impact at the Frasnian-Famennian boundary

Two sections across the Frasnian-Famennian boundary were analyzed for Re and Os concentrations and ¹⁸⁷Os/¹⁸⁸Os ratios to evaluate evidence for a meteoritic input coincident with this boundary and its associated mass extinction. These sections are from a siltstone and shale sequence at Irish Gulf in New York, US and a calcareous shale and ferromanganese oxide sequence at La Serre in France. The Irish Gulf section, with an initial ¹⁸⁷Os/¹⁸⁸Os of about 0.49, does not show the characteristic meteoritic Os imprint with an ¹⁸⁷Os/¹⁸⁸Os of about 0.13. The integrity of the section with regard to Re and Os retention is indicated by the construction of an isochron with an age of 388 ± 41 Ma, consistent with independently determined ages for the Frasnian-Famennian boundary. Although the La Serre section, with Os concentrations as high as 33 ppb and Re concentrations ranging from 1.4 to 7.4 ppb, might be expected to show excellent evidence for a meteoritic contribution, the highly radiogenic isotopic composition (¹⁸⁷Os/¹⁸⁸Os 2.42-3.61) instead suggests recent massive Re loss or addition of radiogenic Os. These possibilities prevent the reconstruction of an initial ¹⁸⁷Os/¹⁸⁸Os value for the boundary for this site. Assuming reasonable Re concentrations prior to loss, the Os isotopic composition is inconsistent with a large meteoritic component. This inconsistency emphasizes that platinum group element (PGE) concentrations are not reliable indicators of extraterrestrial addition.

Mass-independent fractionation of oxygen isotopes in the mesostasis of a chondrule from the Semarkona LL3.0 ordinary chondrite

A large chondrule from Semarkona, the most primitive ordinary chondrite known, has been discovered to contain a record of mass transport during its formation. In most respects, it is a normal Type I, group A1, low-FeO chondrule that was produced by reduction and mass-loss during the unidentified flash-heating event that produced the chondrules, the most abundant structural component in primitive meteorites. We have previously measured elemental abundances and abundance profiles in this chondrule. We here report oxygen isotope ratio abundances and ratio abundance profiles. We have found that the mesostasis is zoned in oxygen isotope ratio, with the center of the chondrule containing isotopically heavier oxygen than the outer regions, the outer regions being volatile rich from the diffusion of volatiles into the chondrule during cooling. The δ 17O values range from −2.0‰ to 9.9‰, while δ 18O range from −1.9‰ to 9.6‰. More importantly, a plot of δ 17O against δ 18O has a slope of 1.1 ± 0.2 (1 σ) and 0.88 ± 0.10 (1 σ) when measured by two independent methods. Co-variation of δ 17O with δ 18O that does not follow mass-dependent fractionation has often been seen in primitive solar system materials and is usually ascribed to the mixing of different oxygen reservoirs. We argue that petrographic and compositional data indicate that this chondrule was completely melted at the time of its formation so that relic grains could not have survived. Furthermore, there is petrographic and compositional evidence that there was no aqueous alteration of this chondrule subsequent to its formation. Although it is possible to formulate a series of exchanges between the chondrule and external 16O-rich and 16O-poor reservoirs that may explain the detailed oxygen isotope systematics of this chondrule, such a sequence of events looks very contrived. We therefore hypothesize that reduction, devolatilization, and crystallization of the chondrule melt may have produced 16O-rich olivines and 16O-poor mesostasis plotting on a slope-one line as part of the chondrule-forming process in an analogous fashion to known chemical mass-independent isotopic fractionation mechanisms. During cooling, volatiles and oxygen near the terrestrial line in oxygen isotope composition produced the outer zone of volatile rich and 16O-rich mesostasis. The chondrule therefore not only retains a record of considerable mass transport accompanying formation, but also may indicate that the isotopes of oxygen underwent mass-independent fractionation during the process.

Electrochemical studies of iron meteorites: phosphorus redox chemistry on the early Earth

AbstractThe mineral schreibersite, (Fe,Ni)3P, a ubiquitous component of iron meteorites, is known to undergo anoxic hydrolytic modification to afford a range of phosphorus oxyacids. H-phosphonic acid (H3PO3) is the principal hydrolytic product under hydrothermal conditions, as confirmed here by 31P-NMR spectroscopic studies on shavings of the Seymchan pallasite (Magadan, Russia, 1967), but in the presence of photochemical irradiation a more reduced derivative, H-phosphinic (H3PO2) acid, dominates. The significance of such lower oxidation state oxyacids of phosphorus to prebiotic chemistry upon the early Earth lies with the facts that such forms of phosphorus are considerably more soluble and chemically reactive than orthophosphate, the commonly found form of phosphorus on Earth, thus allowing nature a mechanism to circumvent the so-called Phosphate Problem.This paper describes the Galvanic corrosion of Fe3P, a hydrolytic modification pathway for schreibersite, leading again to H-phosphinic acid as the key P-containing product. We envisage this pathway to be highly significant within a meteoritic context as iron meteorites are polymetallic composites in which dissimilar metals, with different electrochemical potentials, are connected by an electrically conducting matrix. In the presence of a suitable electrolyte medium, i.e., salt water, galvanic corrosion can take place. In addition to model electrochemical studies, we also report the first application of the Kelvin technique to map surface potentials of a meteorite sample that allows the electrochemical differentiation of schreibersite inclusions within an Fe:Ni matrix. Such experiments, coupled with thermodynamic calculations, may allow us to better understand the chemical redox behaviour of meteoritic components with early Earth environments.

SODIUM-22 FROM SUPERNOVAE: A METEORITE CONNECTION

There are several meteoritic noble gas components whose isotopic abundances are distinctly different from the solar abundance. One of these components, Ne-E(L), is a 22Ne-rich component carried by presolar graphite. Neon-22 is dominantly from the radiogenic decay of 22Na (T 1/2 = 2.6 a), but it also contains 22Ne produced by 14N(α, γ)18F(e +ν)18O(α, γ)22Ne. The proportions of 22Ne from the two different origins vary with graphite density. Many low-density graphite grains (1.65-1.72 g cm–3) exhibit 18O and 28Si excesses, signatures of supernova origin. We revisited noble gas data on bulk (=aggregates) and single-grain analyses of presolar graphite to better understand the origin of Ne-E(L). We conclude that essentially all 22Ne in low-density graphite grains originated from 22Na produced in the O/Ne zone in supernovae. The initial presence of 22Na in these grains indicates that grains formed within a few years after the supernova explosion. The absence of implanted 22Ne can be explained if the grains and the gas in the ejecta became decoupled when they encountered the reverse shock and sputtering eroded the grains in the outer H-rich zone and in the interstellar medium.

The Dhala structure, Bundelkhand craton, Central India—Eroded remnant of a large Paleoproterozoic impact structure

Abstract— The newly discovered Dhala structure, Madhya Pradesh State, India, is the eroded remnant of an impact structure with an estimated present‐day apparent diameter of about 11 km. It is located in the northwestern part of the Archean Bundelkhand craton. The pre‐impact country rocks are predominantly granitoids of ˜2.5 Ga age, with minor 2.0–2.15 Ga mafic intrusive rocks, and they are overlain by post‐impact sediments of the presumably >1.7 Ga Vindhyan Supergroup. Thus, the age for this impact event is currently bracketed by these two sequences. The Dhala structure is asymmetrically disposed with respect to a central elevated area (CEA) of Vindhyan sediments. The CEA is surrounded by two prominent morphological rings comprising pre‐Vindhyan arenaceous‐argillaceous and partially rudaceous metasediments and monomict granitoid breccia, respectively. There are also scattered outcrops of impact melt breccia exposed towards the inner edge of the monomict breccia zone, occurring over a nearly 6 km long trend and with a maximum outcrop width of ˜170 m. Many lithic and mineral clasts within the melt breccia exhibit diagnostic shock metamorphic features, including multiple sets of planar deformation features (PDFs) in quartz and feldspar, ballen‐textured quartz, occurrences of coesite, and feldspar with checkerboard texture. In addition, various thermal alteration textures have been found in clasts of initially superheated impact melt. The impact melt breccia also contains numerous fragments composed of partially devitrified impact melt that is mixed with unshocked as well as shock deformed quartz and feldspar clasts. The chemical compositions of the impact melt rock and the regionally occurring granitoids are similar. The Ir contents of various impact melt breccia samples are close to the detection limit (1–1.5 ppb) and do not provide evidence for the presence of a meteoritic component in the melt breccia. The presence of diagnostic shock features in mineral and lithic clasts in impact melt breccia confirm Dhala as an impact structure. At 11 km, Dhala is the largest impact structure currently known in the region between the Mediterranean and southeast Asia.