Chondrite Samples Research Articles

Cosmic‐ray exposure ages of six chondritic Almahata Sitta fragments

AbstractThe Almahata Sitta strewn field is dominated by ureilites, but contains a large fraction of chondritic fragments of various types. We analyzed stable isotopes of He, Ne, Ar, Kr, and Xe, and the cosmogenic radionuclides10Be,26Al, and36Cl in six chondritic Almahata Sitta fragments (EL6 breccia,EL6,EL3‐5,CB,LL4/5, R‐like). The cosmic‐ray exposure (CRE) ages of five of the six samples have an average of 19.2 ± 3.3 Ma, close to the average of 19.5 ± 2.5 Ma for four ureilites. The cosmogenic radionuclide concentrations in the chondrites indicate a preatmospheric size consistent with Almahata Sitta. This corroborates that Almahata Sitta chondrite samples were part of the same asteroid as the ureilites. However,MS‐179 has a lowerCREage of 11.0 ± 1.4 Ma. Further analysis of short‐lived radionuclides in fragmentMS‐179 showed that it fell around the same time, and from an object of similar size as Almahata Sitta, making it almost certain thatMS‐179 is an Almahata Sitta fragment. Instead, its lowCREage could be due to gas loss, chemical heterogeneity that may have led to an erroneous21Ne production‐rate, or, perhaps most likely,MS‐179 could represent the true 4π exposure age of Almahata Sitta (or an upper limit thereof), while all other samples analyzed so far experienced exposure on the parent body of similar lengths. Finally,MS‐179 had an extraordinarily high activity of neutron‐capture36Cl, ~600 dpm kg−1, the highest activity observed in any meteorite to date, related to a high abundance of the Cl‐bearing mineral lawrencite.

Stable strontium isotopic heterogeneity in the solar system from double-spike data

Strontium isotopic anomalies in meteorites are important in assessing nucleosynthetic sources to, and measuring the timing of, early solar system processes. However, conventional use of a constant 88Sr/86Sr value in correcting for instrumental mass fractionation during analysis renders measurements ambiguous and removes information on mass-dependent fractionation variations. From double-spike techniques we obtain data for the four stable strontium isotopes free of this ambiguity, and report measurements from a range of meteoritic, lunar and terrestrial materials. The Earth, Moon, basaltic eucrites and feldspars from angrites (differentiated samples) follow a single mass-dependent fractionation line and have a common nucleosynthetic origin in terms of their strontium isotopes. In contrast, bulk rock CI, CV3, CM and CO chondrite samples serve to define another mass-dependent fractionation line, displaced by 94 ± 28 ppm to heavier 84Sr/86Sr and/or 88Sr/86Sr ratios than that for the differentiated samples. Our Sr-isotopic data are consistent with a primary contrast in early solar system composition between an outer zone of primitive, mostly undifferentiated, materials and an inner zone of (almost entirely) differentiated materials that accumulated to form the terrestrial planets.

Processes of noble gas elemental and isotopic fractionations in plasma-produced organic solids: Cosmochemical implications

The main carrier of primordial heavy noble gases in chondrites is thought to be an organic phase, known as phase Q, whose precise characterization has resisted decades of investigation. The Q noble gas component shows elemental and isotopic fractionation relative to the Solar, in favor of heavy elements and isotopes. These noble gas characteristics were experimentally simulated using a plasma device called the “Nebulotron”. In this study, we synthesized thirteen solid organic samples by electron-dissociation of CO, in which a noble gas mixture was added. The analysis of their heavy noble gas (Ar, Kr and Xe) contents and isotopic compositions reveals enrichment in the heavy noble gas isotopes and elements relative to the light ones. The isotope fractionation is mass-dependent and is consistent with a mn-type law, where n≥1. Based on a plasma model, we propose that the ambipolar diffusion of ions in the ionized CO gas medium is at the origin of the noble gas isotopic fractionation. In addition, the elemental fractionation of experimental and chondritic samples can be accounted for by the Saha law of plasma equilibrium, which does not depend on the respective noble gas masses but rather on their ionization potentials. Our results suggest that the Q noble gases were trapped into growing organic particles starting from solar gases that were fractionated in an ionized medium by ambipolar diffusion and Saha processes. This would imply that both the formation of chondritic organic matter and the trapping of noble gases took place simultaneously in the ionized areas of the protoplanetary disk.

Effects of Different Walking Aids on Three-Dimensional Trunk Kinematics of Patients Having Undergone First-Time, Elective Transforaminal Lumbar Interbody Fusion

Sulfide assemblages are commonly found in chondritic meteorites as small inclusions in the matrix or in association with chondrules. These assemblages are widely hypothesized to form through pre-accretionary corrosion of metal by H2S gas or through parent body processes. We report here on two unequilibrated R chondrite samples that contain large, chondrule-sized sulfide nodules in the matrix. Both samples are from Mount Prestrud (PRE) 95404. Chemical maps and spot and broad-beam electron microprobe analyses (EMPA) were used to assess the distribution, stoichiometry, and bulk composition of sulfide nodules and silicate chondrules in the clasts. Oxygen isotope data were collected via secondary ion mass spectrometry (SIMS) to assess the relationship of chondrules to other chondrite groups. Scanning electron microscopy (SEM), focused ion beam (FIB), and transmission electron microscopy (TEM) analyses were used to assess fine-scale features and identify crystal structures in sulfide assemblages. Thermodynamic models were used to assess the temperature, sulfur fugacity (fS2), total pressure, dust-to-gas ratio, and oxygen fugacity (fO2) conditions during sulfide nodule and chondrule formation.The unequilibrated clasts include a mixture of type I and type II chondrules, as well as non-porphyritic chondrules. Chondrule oxygen isotopes overlap with ordinary-chondrite chondrules. Sulfide nodules average 200 µm in diameter, have rounded shapes, and are primarily composed of pyrrhotite, pentlandite, and magnetite. Some are deformed around chondrules in a petrologic relationship similar in appearance to compound chondrules. Both nodules and sulfides in chondrules include phosphate inclusions and Cu-rich lamellae, which suggests a genetic relationship between sulfides in chondrules and in the matrix. Ni/Co ratios for matrix and chondrule sulfides are solar, while Fe and Ni are non-solar and inversely related.We hypothesize that sulfide nodules formed via pre-accretionary melt processes. During chondrule formation, precursors composed of a mixture of silicate and sulfide material were heated to form immiscible melt droplets, which separated and cooled to form Si-rich chondrules and S-rich nodules. Sulfide melt was stabilized by a high total pressure (∼1 atm) in a dust- or ice-enriched environment. Heating of this material contributed to a high fS2 (2 × 10−3 atm at 1138 °C), and high fO2 (IW – 1 to IW – 4), in an environment with peak temperatures between 1539 °C and 1750 °C. Oxygen isotopic compositions in this region were similar to those recorded by the LL-chondrite chondrules.

Formation of unequilibrated R chondrite chondrules and opaque phases

Sulfide assemblages are commonly found in chondritic meteorites as small inclusions in the matrix or in association with chondrules. These assemblages are widely hypothesized to form through pre-accretionary corrosion of metal by H2S gas or through parent body processes. We report here on two unequilibrated R chondrite samples that contain large, chondrule-sized sulfide nodules in the matrix. Both samples are from Mount Prestrud (PRE) 95404. Chemical maps and spot and broad-beam electron microprobe analyses (EMPA) were used to assess the distribution, stoichiometry, and bulk composition of sulfide nodules and silicate chondrules in the clasts. Oxygen isotope data were collected via secondary ion mass spectrometry (SIMS) to assess the relationship of chondrules to other chondrite groups. Scanning electron microscopy (SEM), focused ion beam (FIB), and transmission electron microscopy (TEM) analyses were used to assess fine-scale features and identify crystal structures in sulfide assemblages. Thermodynamic models were used to assess the temperature, sulfur fugacity (fS2), total pressure, dust-to-gas ratio, and oxygen fugacity (fO2) conditions during sulfide nodule and chondrule formation.The unequilibrated clasts include a mixture of type I and type II chondrules, as well as non-porphyritic chondrules. Chondrule oxygen isotopes overlap with ordinary-chondrite chondrules. Sulfide nodules average 200µm in diameter, have rounded shapes, and are primarily composed of pyrrhotite, pentlandite, and magnetite. Some are deformed around chondrules in a petrologic relationship similar in appearance to compound chondrules. Both nodules and sulfides in chondrules include phosphate inclusions and Cu-rich lamellae, which suggests a genetic relationship between sulfides in chondrules and in the matrix. Ni/Co ratios for matrix and chondrule sulfides are solar, while Fe and Ni are non-solar and inversely related.We hypothesize that sulfide nodules formed via pre-accretionary melt processes. During chondrule formation, precursors composed of a mixture of silicate and sulfide material were heated to form immiscible melt droplets, which separated and cooled to form Si-rich chondrules and S-rich nodules. Sulfide melt was stabilized by a high total pressure (∼1atm) in a dust- or ice-enriched environment. Heating of this material contributed to a high fS2 (2×10−3atm at 1138°C), and high fO2 (IW – 1 to IW – 4), in an environment with peak temperatures between 1539°C and 1750°C. Oxygen isotopic compositions in this region were similar to those recorded by the LL-chondrite chondrules.

Experimental Simulation of Meteorite Ablation during Earth Entry Using a Plasma Wind Tunnel

Abstract Three different types of rocks were tested in a high enthalpy air plasma flow. Two terrestrial rocks, basalt and argillite, and an ordinary chondrite, with a 10 mm diameter cylindrical shape were tested in order to observe decomposition, potential fragmentation, and spectral signature. The goal was to simulate meteoroid ablation to interpret meteor observation and compare these observations with ground based measurements. The test flow with a local mass-specific enthalpy of 70 MJ kg−1 results in a surface heat flux at the meteorite fragment surface of approximately 16 MW m−2. The stagnation pressure is 24 hPa, which corresponds to a flight condition in the upper atmosphere around 80 km assuming an entry velocity of 10 km s−1. Five different diagnostic methods were applied simultaneously to characterize the meteorite fragmentation and destruction in the ground test: short exposure photography, regular video, high-speed imaging with 10 kHz frame rate, thermography, and Echelle emission spectroscopy. This is the first time that comprehensive testing of various meteorite fragments under the same flow condition was conducted. The data sets indeed show typical meteorite ablation behavior. The cylindrically shaped fragments melt and evaporate within about 4 s. The spectral data allow the identification of the material from the spectra which is of particular importance for future spectroscopic meteor observations. For the tested ordinary chondrite sample a comparison to an observed meteor spectra shows good agreement. The present data show that this testing methodology reproduces the ablation phenomena of meteoritic material alongside the corresponding spectral signatures.

Extreme early solar system chemical fractionation recorded by alkali-rich clasts contained in ordinary chondrite breccias

New K–Ca and Rb–Sr isotopic analyses have been performed on alkali-rich igneous rock fragments in the Yamato (Y)-74442 and Bhola LL-chondritic breccias to better understand the extent and timing of alkali enrichments in the early solar system. The Y-74442 fragments yield a K–Ca age of 4.41±0.28 Ga for λ(40K)=0.5543 Ga−1 with an initial 40Ca/44Ca ratio of 47.1618±0.0032. Studying the same fragments with the Rb–Sr isotope system yields an age of 4.420±0.031 Ga for λ(87Rb)=0.01402 Ga−1 with an initial ratio of 87Sr/86Sr=0.7203±0.0044. An igneous rock fragment contained in Bhola shows a similar alkali fractionation pattern to those of Y-74442 fragments but does not plot on the K–Ca or Rb–Sr isochron of the Y-74442 fragments. Calcium isotopic compositions of whole-rock samples of angrite and chondrites are primordial, indistinguishable from mantle-derived terrestrial rocks, and here considered to represent the initial composition of bulk silicate Earth. The initial ε40Ca value determined for the source of the alkali clasts in Y-74442 that is ∼0.5 ε-units higher than the solar system value implies an early alkali enrichment.Multi-isotopic studies on these alkali-rich fragments reveal that the source material of Y-74442 fragments had elemental ratios of K/Ca=0.43±0.18, Rb/Sr=3.45±0.66 and K/Rb∼170, that may have formed from mixtures of an alkali-rich component (possibly an alkali-enriched gaseous reservoir produced by fractionation of early nebular condensates) and chondritic components that were flash-heated during an impact event on the LL-chondrite parent body ∼4.42 Ga ago. Further enrichments of potassium and rubidium relative to calcium and strontium as well as a mutual alkali-fractionation (K/Rb∼50 and heavier alkali-enrichment) would have likely occurred during subsequent cooling and differentiation of this melt. Alkali fragments in Bhola might have undergone similar solid–vapor fractionation processes to those of Y-74442 fragments but appear to have formed via a distinct impact melting event.

The valence and coordination of titanium in ordinary and enstatite chondrites

One way to better understand processes related to chondrite metamorphism is to evaluate changes in chondrite features as a function of petrologic type. Toward this end the valence and coordination of Ti in olivine and pyroxene in suites of ordinary (H, L, and LL) and enstatite (EH and EL) chondrites of types 3 through 6 have been determined with XANES spectroscopy. Trivalent Ti, typically 10–40% of the Ti in the analytical volumes, was found in ordinary chondrites of all types, despite the stability of oxidized iron in the samples. Average valences and the proportions of Ti that are in tetrahedral coordination generally decrease with increasing grade between types 3.0 and 3.5, increase from 3.5 to 4, and then level off. These trends are consistent with previous studies of chondrite oxidation states using other methods, except here the onset of oxidation is observed at a lower type, ∼3.5, than previously indicated (4). These results are also consistent with previous suggestions that oxidation of higher-grade ordinary chondrite samples involved exposure to aqueous fluids from melting of accreted ice. In the enstatite chondrites, typically 20–90% of the Ti is trivalent Ti, so it is reduced compared to Ti in the ordinary chondrites. Valence decreases slightly from petrologic type 3 to 4 and increases from 4 to 6, but no increases in tetrahedral coordination with petrologic type are observed, indicating a redox environment or process distinct from that of ordinary chondrite metamorphism. The presence of Ti4+ in the E chondrites supports previous suggestions that they formed from oxidized precursors that underwent reduction. Unlike ordinary chondrites, enstatite chondrites are thought to have been derived from a body or bodies that did not accrete ice, which could account for their different valence-coordination-petrologic type relationships. The hypothesis, based on observations of unmetamorphosed chondrules and supported by laboratory experiments, that equilibration of Ti valence is sluggish compared to that of Fe could account for the coexistence of reduced Ti and oxidized Fe seen in chondrites of all petrologic types.

Temperature dependent grain growth of forsterite–nickel mixtures: Implications for grain growth in two-phase systems and applications to the H-chondrite parent body

Abstract Grain growth experiments in the system forsterite (Fo) + nickel (Ni) have been performed on two analogue mixtures of ordinary chondrites, with volume % of Fo:Ni (95:5) and (80:20). These two mixtures have been studied at temperatures of 1390 °C and 1340 °C, at an oxygen fugacity ( f O 2 ) three orders of magnitude below the Ni–NiO buffer, for durations between 2 h and 10 days. Microstructures and grain size distributions show that grain growth is normal and that for durations >10 h the Zener relation is verified (i.e., the ratio of Fo and Ni grain size is independent of time). Comparison with results previously obtained at 1440 °C shows a similar grain growth exponent ( n ∼ 5 ) for both phases, consistent with growth of forsterite by grain boundary migration, limited by the growth-rate of nickel. The details of size distribution frequencies and the value of grain-growth exponent indicate that the nickel grains, which pin forsterite grain boundaries, grow by diffusion along one-dimensional paths (i.e., along forsterite triple junctions). The derived activation energies for nickel and forsterite are 235 ± 33 kJ / mol and 400 ± 48 kJ / mol respectively. Within the framework of the Zener relation, this unexpected difference of activation energy is shown to be related to temperature-dependent variations in the ratio of Ni and Fo grain-size that are consistent with observed variations in Fo–Ni–Fo dihedral angle. These data thus indicate that the presence of all phases should be taken into account when considering the activation energy of growth rate of individual phases. As an application, the experimentally derived growth law for metal has been used in conjunction with temperature–time paths taken from models of the thermal history of the H-chondrite parent body to estimate the grain size evolution of metal in H-chondrites. A remarkably self-consistent picture emerges from experimentally derived grain-growth laws, textural data of metal grains in well characterised H-chondrite samples, and geochemically constrained temperature–time paths, providing the potential to use textural data of metal as a window into the thermal history of chondritic samples.

Amino acid compositions in heated carbonaceous chondrites and their compound-specific nitrogen isotopic ratios

A novel method has been developed for compound-specific nitrogen isotope compositions with an achiral column which was previously shown to offer high precision for nitrogen isotopic analysis. We applied the method to determine the amino acid contents and stable nitrogen isotopic compositions of individual amino acids from the thermally metamorphosed (above 500 °C) Antarctic carbonaceous chondrites Ivuna-like (CI)1 (or CI-like) Yamato (Y) 980115 and Ornans-like (CO)3.5 Allan Hills (ALH) A77003 with the use of gas chromatography/combustion/isotope ratio mass spectrometry. ALHA77003 was deprived of amino acids due to its extended thermal alteration history. Amino acids were unambiguously identified in Y-980115, and the δ15N values of selected amino acids (glycine +144.8 ‰; α-alanine +121.2 ‰) are clearly extraterrestrial. Y-980115 has experienced an extended period of aqueous alteration as indicated by the presence of hydrous mineral phases. It has also been exposed to at least one post-hydration short-lived thermal metamorphism. Glycine and alanine were possibly produced shortly after the accretion event of the asteroid parent body during the course of an extensive aqueous alteration event and have abstained from the short-term post-aqueous alteration heating due to the heterogeneity of the parent body composition and porosity. These carbonaceous chondrite samples are good analogs that offer important insights into the target asteroid Ryugu of the Hayabusa-2 mission, which is a C-type asteroid likely composed of heterogeneous materials including hydrated and dehydrated minerals.

Spectral variability on primitive asteroids of the Themis and Beagle families: Space weathering effects or parent body heterogeneity?

Themis is an old and statistically robust asteroid family populating the outer main belt, and resulting from a catastrophic collision that took place 2.5 ± 1.0 Gyr ago. Within the old Themis family a young sub-family, Beagle, formed less than 10 Myr ago, has been identified.We present the results of a spectroscopic survey in the visible and near infrared range of 22 Themis and 8 Beagle families members. The Themis members investigated exhibit a wide range of spectral behaviors, including asteroids with blue/neutral and moderately red spectra, while the younger Beagle family members look spectrally bluer than the Themis ones and they have a much smaller spectral slope variability. Four Themis members, including (24) Themis, have absorption bands centered at 0.68–0.73 μm indicating the presence of aqueously altered minerals.The best meteorite spectral analogues found for both Themis and Beagle families members are carbonaceous chondrites having experienced different degrees of aqueous alteration, prevalently CM2 but also CV3 and CI, and some of them are chondrite samples being unusual or heated. The presence of aqueous altered materials on the asteroids surfaces and the meteorite matches indicate that the parent body of the Themis family experienced mild thermal metamorphism in the past.We extended the spectral analysis including the data available in the literature on Themis and Beagle families members, and we looked for correlations between spectral behavior and physical parameters using the albedo and size values derived from the WISE data. The analysis of this larger sample confirms the spectral diversity within the Themis family and that Beagle members tend to be bluer and to have an higher albedo. The differences between the two families may be partially explained by space weathering processes, which act on these primitive surfaces in a similar way than on S-type asteroids, i.e. producing reddening and darkening. However we see several Themis members having albedos and spectral slopes similar to the young Beagle members. Alternative scenarios are proposed including heterogeneity in the parent body having a compositional gradient with depth, and/or the survival of projectile fragments having a different composition than the parent body.

Precise determination of low abundance isotopes (174Hf, 180W and 190Pt) in terrestrial materials and meteorites using multiple collector ICP-MS equipped with 1012 Ω Faraday amplifiers

Analytical protocols for measurements of isotope ratios involving low-abundance isotopes 174Hf (~0.16%), 180W (~0.12%), and 190Pt (~0.014%) using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) at ca. tenfold improved precision compared to previous studies are presented. The improvements in precision result in part from the use of Faraday amplifiers equipped with 1012Ω resistors for collection of the isotope beams of interest, as well as for the isotope beams used to monitor isobaric interferences. For ion currents >5×10−13 A (>50mV on an amplifier with 1011Ω resistor), uncertainties (2s.d.) of better than 100ppm are typically achieved for isotope ratios involving 174Hf, 180W and 190Pt. Terrestrial isotope abundances (atomic percent ±2s.d.) of 174Hf (0.16106±0.00006; n=15); 180W (0.11910±0.00009; n=10); and 190Pt (0.01286±0.00005; n=9) were determined on reference solutions throughout multiple analytical sessions over ~3years. The results, however, rely on the accuracy of the reference value chosen for the isotope ratios employed for mass bias correction and, to a lesser extent, on the accuracy of the exponential mass fractionation law used for mass bias correction. For the precise determination of isotope ratios involving 174Hf, 180W and 190Pt in natural materials, efficient chromatographic separation schemes for Hf and W from silicate-rich matrices, and for Pt from iron meteorite matrices, are presented that provide effective separation of the element of interest from interfering elements. First results for Pt suggest that IAB, IIAB and IIIAB iron meteorites exhibit terrestrial 190Pt abundances within uncertainty. Three chondrite samples exhibit indistinguishable 174Hf from terrestrial Hf. The isotope abundance of 180W in meteorites is more variable due to radiogenic 180W that was produced by 184Os-decay.

Ru isotope heterogeneity in the solar protoplanetary disk

Nucleosynthetic isotope anomalies in bulk chondrites and differentiated meteorites reflect variable proportions of isotopically diverse presolar components in bulk planetary bodies, but the origin of these heterogeneities is not well understood. Here, the Ru isotope composition of a comprehensive suite of iron meteorites and bulk samples of ordinary, enstatite and carbonaceous chondrites, as well as acid leachates and an insoluble residue of the Allende chondrite are examined using newly developed multi-collector inductively coupled plasma mass spectrometry techniques. Except for IAB iron meteorites and enstatite chondrites, all investigated meteorites show well-resolved Ru isotope anomalies. Of these, within-group Ru isotopic variations observed for samples from a given chemical group of iron meteorites reflect secondary neutron capture induced Ru isotope shifts during prolonged cosmic ray-exposure. After correction of these cosmogenic effects using Pt isotopes as a neutron-dose monitor, the remaining Ru isotope anomalies are nucleosynthetic in nature and are consistent with a deficit in s-process Ru in iron meteorite parent bodies. Similarly, Ru isotope anomalies in bulk ordinary and carbonaceous chondrites also reflect a deficiency in s-process Ru. The sequential dissolution of Allende reveals the presence of an HF-soluble s-process carrier, which is either an unidentified presolar phase or a component that incorporated s-process Ru liberated from SiC grains during nebular or parent body processes. We show that varying proportions of the s-process carrier identified in Allende resulted in the correlated Ru isotope anomalies observed for bulk meteorites, and that all meteorites (except possibly IAB irons and enstatite chondrites) are depleted in this s-process component relative to Ru from the Earth’s mantle. Bulk meteorites exhibit correlated Ru and Mo isotope anomalies, reflecting variable deficits of a common s-process component, but some iron meteorites and carbonaceous chondrites appear to deviate from this correlation. This may reflect unaccounted cosmic effects on Mo isotopes in iron meteorites, sample heterogeneities in carbonaceous chondrites or nebular and parent body processes acting differently on presolar Mo and Ru components.The identification of s-deficits in Ru isotopes in almost all iron meteorites and chondrites investigated so far implies that meteorites do not seem to represent the material delivered to the Earth’s mantle as a late veneer after cessation of core formation. However, additional analyses of a more comprehensive set of chondrites are necessary to firmly arrive at this conclusion.

Raman spectroscopic thermometry of carbonaceous material in chondrites: four–band fitting analysis and expansion of lower temperature limit

Raman spectra of carbonaceous materials were observed for 20 chondrite samples (one R, four CV, four CO, two CM, one CR, one CB, one C–ung, three LL, one L, and two H chondrites). The observed specimens were chips (fifteen samples) and thin sections (five samples). The obtained spectra were decomposed into four components: GL, D1, D3, and D4 bands at 1600, 1350, 1510, and 1245 cm−1, respectively. The full width at half maximum of the D1 band (ΓD1) decreased with increasing peak metamorphic temperature (PMT) of the parent body, as published previously. By increasing the number of band components to four, the applicable range of the Raman spectroscopic thermometer using ΓD1 extended its lower limit to 20–30 °C. The upper limit of the present Raman spectroscopic thermometer is 540 °C, which is derived from the bottoming out of ΓD1. This thermometer was applied to chondrites whose PMT are unknown. The obtained temperature values were consistent with the order of petrological type.

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

Geochemical evidence suggests that the material accreted by the Earth did not change in nature during Earth's accretion, presumably because the inner protoplanetary disc had uniform isotopic composition similar to enstatite chondrites, aubrites and ungrouped achondrite NWA 5363/5400. Enstatite meteorites and the Earth were derived from the same nebular reservoir but diverged in their chemical evolutions, so no chondrite sample in meteorite collections is representative of the Earth's building blocks. The similarity in isotopic composition (Δ(17)O, ε(50)Ti and ε(54)Cr) between lunar and terrestrial rocks is explained by the fact that the Moon-forming impactor came from the same region of the disc as other Earth-forming embryos, and therefore was similar in isotopic composition to the Earth. The heavy δ(30)Si values of the silicate Earth and the Moon relative to known chondrites may be due to fractionation in the solar nebula/protoplanetary disc rather than partitioning of silicon in Earth's core. An inversion method is presented to calculate the Hf/W ratios and ε(182)W values of the proto-Earth and impactor mantles for a given Moon-forming impact scenario. The similarity in tungsten isotopic composition between lunar and terrestrial rocks is a coincidence that can be explained in a canonical giant impact scenario if an early formed embryo (two-stage model age of 10-20 Myr) collided with the proto-Earth formed over a more protracted accretion history (two-stage model age of 30-40 Myr).

Homogeneous distribution of Fe isotopes in the early solar nebula

AbstractTo examine the iron (Fe) isotopic heterogeneities of CI and ordinary chondrites, we have analyzed several large chips (approximately 1 g) from three CI chondrites and three ordinary chondrites (LL5, L5, and H5). The Fe isotope compositions of five different samples of Orgueil, one from Ivuna and one from Alais (CI chondrites), are highly homogeneous. This new dataset provides a δ56Fe average of 0.02 ± 0.04‰ (2SE, n = 7), which represents the best available value for the Fe isotopic composition of CI chondrites and probably the best estimate of the bulk solar system. We conclude that the homogeneity of CI chondrites reflects the initial Fe isotopic homogeneity of the well‐mixed solar nebula. In contrast, larger (up to 0.26‰ in δ56Fe) isotopic variations have been found between separate approximately 1 g pieces of the same ordinary chondrite sample. The Fe isotope heterogeneities in ordinary chondrites appear to be controlled by the abundances of chondritic components, specifically chondrules, whose Fe isotope compositions have been fractionated by evaporation and recondensation during multiple heating events.

Variations in quadrupole splitting of the 57 Fe in the M1 and M2 sites of meteoritic olivines with different origin

A comparative study of meteoritic olivine in bulk samples of Farmington L5 and Tsarev L5 ordinary chondrites and extracted from Omolon and Seymchan the main-group pallasites was performed using Mossbauer spectroscopy with a high velocity resolution. Mossbauer spectra for each specimen were measured at 295 and 90 K. Mossbauer spectral components related to the 57Fe in crystallographically non-equivalent sites M1 and M2 in olivines were determined and their Mossbauer hyperfine parameters were evaluated. It was found small variations of quadrupole splitting for the 57Fe in both the M1 and M2 sites of olivines in bulk ordinary chondrites and olivines extracted from pallasites.

A noble gas and cosmogenic radionuclide analysis of two ordinary chondrites from Almahata Sitta

Abstract– We present the results of a noble gas (He, Ne, Ar) and cosmogenic radionuclide (10Be, 26Al, 36Cl) analysis of two chondritic fragments (#A100, L4 and #25, H5) found in the Almahata Sitta strewn field in Sudan. We confirm their earlier attribution to the same fall as the ureilites dominating the strewn field, based on the following findings: (1) both chondrite samples indicate a preatmospheric radius of approximately 300 g cm−2, consistent with the preatmospheric size of asteroid 2008 TC3 that produced the Almahata Sitta strewn field; (2) both have, within error, a 21Ne/26Al‐based cosmic ray exposure age of approximately 20 Ma, identical to the reported ages of Almahata Sitta ureilites; (3) both exhibit hints of ureilitic Ar in the trapped component. We discuss a possible earlier irradiation phase for the two fragments of approximately 10–20 Ma, visible only in cosmogenic 38Ar. We also discuss the approximately 3.8 Ga (4He) and approximately 4.6 Ga (40Ar) gas retention ages, measured in both chondritic fragments. These imply that the two chondrite fragments were incorporated into the ureilite host early in solar system evolution, and that the parent asteroid from which 2008 TC3 is derived has not experienced a large break‐up event in the last 3.8 Ga.

Three-dimensional observation and morphological analysis of organic nanoglobules in a carbonaceous chondrite using X-ray micro-tomography

Organic nanoglobules are submicrometer spherical, often hollow organic grains ubiquitously distributed throughout primitive solar materials, such as carbonaceous chondrites. Until now, organic nanoglobules have been examined by TEM only after sectioning by ultramicrotomy so it has not been possible to determine whether fluids or mineral grains occur in the hollow cores. H2O-rich fluids might be present in hollows of the nanoglobules if they originate from dust particles composed of organic materials and ice prior to or in an early stage of the solar system formation or fluids incorporated into nanoglobules during aqueous alteration on the asteroidal parent body. In order to determine whether or not any fluids or mineral grains are present in the nanoglobules, a carbonaceous chondrite sample (Tagish Lake C2 meteorite) was observed non-destructively using synchrotron radiation-based X-ray CT (computed tomography), and then microtomed sections were observed using a transmission electron microscope (TEM). We observed three-dimensional shapes of thirty-eight organic nanoglobules in the meteorite sample. Their size and shape distributions are consistent with a hypothesis that nanoglobules originate from icy dust particles. Nanoglobule candidates observed in CT images were confirmed by the TEM images. However, the presence or absence of fluid could not be judged because CT images of nanoglobules are affected by X-ray refraction. Simulation of CT images by considering X-ray refraction shows that the presence or absence of water in nanoglobules cannot be distinguished with CT images alone. However the outer shapes of nanoglobules can be determined quantitatively and nanoglobules containing silicate cores can be easily identified. The thirty-eight nanoglobules we examined did not have silicate cores.

Variations of Li and Mg isotope ratios in bulk chondrites and mantle xenoliths

We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic) meteorites. The accuracy of our new Mg isotope ratio measurement protocol is substantiated by a combination of standard addition experiments, the absence of mass independent effects in terrestrial samples and our obtaining identical values for rock standards using two different separation chemistries and three different mass-spectrometric introduction systems. Carbonaceous, ordinary and enstatite chondrites have irresolvable mean stable Mg isotopic compositions (δ 25Mg = −0.14 ± 0.06; δ 26Mg = −0.27 ± 0.12‰, 2SD), but our enstatite chondrite samples have lighter δ 7Li (by up to ∼3‰) than our mean carbonaceous and ordinary chondrites (3.0 ± 1.5‰, 2SD), possibly as a result of spallation in the early solar system. Measurements of equilibrated, fertile peridotites give mean values of δ 7Li = 3.5 ± 0.5‰, δ 25Mg = −0.10 ± 0.03‰ and δ 26Mg = −0.21 ± 0.07‰. We believe these values provide a useful estimate of the primitive mantle and they are within error of our average of bulk carbonaceous and ordinary chondrites. A fuller range of fresh, terrestrial, ultramafic samples, covering a variety of geological histories, show a broad positive correlation between bulk δ 7Li and δ 26Mg, which vary from −3.7‰ to +14.5‰, and −0.36‰ to + 0.06‰, respectively. Values of δ 7Li and δ 26Mg lower than our estimate of primitive mantle are strongly linked to kinetic isotope fractionation, occurring during transport of the mantle xenoliths. We suggest Mg and Li diffusion into the xenoliths is coupled to H loss from nominally anhydrous minerals following degassing. Diffusion models suggest that the co-variation of Mg and Li isotopes requires comparable diffusivities of Li and Mg in olivine. The isotopically lightest samples require ∼5–10 years of diffusive ingress, which we interpret as a time since volatile loss in the host magma. Xenoliths erupted in pyroclastic flows appear to have retained their mantle isotope ratios, likely as a result of little prior degassing in these explosive events. High δ 7Li, coupled with high [Li], in rapidly cooled arc peridotites may indicate that these samples represent fragments of mantle wedge that has been metasomatised by heavy, slab-derived fluids. If such material is typically stirred back into the convecting mantle, it may account for the heavy δ 7Li seen in some oceanic basalts.