Large spinel grains in a CM chondrite (Acfer 331): Implications for reconstructions of ancient meteorite fluxes

By dissolving 30–400 kg of marine limestone in HCl and HF acid, our group has previously recovered common relict chromite grains (approximately 63–250 μm) from ordinary chondritic micrometeorites that fell on ancient sea floors, up to 500 Myr old. Here, we evaluate if CM group carbonaceous chondritic material, which makes up an important fraction of the micrometeorite flux today, contains analogous grains that can be searched for in acid residues. We dissolved 8 g of CM2 meteorite Acfer 331 in HF, which yielded a characteristic assemblage of both transparent Mg‐Al‐ and opaque Cr‐spinels >28 μm. We find on average 4.6 and 130 Mg‐Al‐spinel grains per gram in the 63–250 and 28–63 μm size fractions, respectively. These grains are mostly pink or colorless, and often characterized by heterogeneous Cr‐content. Black, opaque Cr‐spinel grains are absent from the >63 μm fraction, but in the 28–63 μm fraction we find approximately 65 such grains per gram meteorite. The individual grains have a characteristic composition, with heterogeneous major element compositions (e.g., 44.4–61.7 wt% Cr₂O₃), but narrow ranges for maximum TiO₂ (0.6–1.6 wt%) and V₂O₃ (0.5–1.0 wt%) concentrations. The content of spinel grains in the 28–63 μm fraction of CM meteorites appears comparable at the order of magnitude level with the content of >63 μm sized chromite grains in fossil L‐chondrites from Ordovician limestone. Our approach of recovering meteoritic spinel from sediment may thus be extended to include CM meteorites, but the smaller size fraction of the acid residues should be searched.     

Reflectance spectroscopy of insoluble organic matter (IOM) and carbonaceous meteorites

Insoluble organic matter (IOM) is the major organic component of chondritic meteorites and may be akin to organic materials from comets and interplanetary dust particles (IDPs). Reflectance spectra of IOM in the range 0.35–25 μm are presented as a tool for interpreting organic chemistry from remote measurements of asteroids, comets, IDPs, and other planetary bodies. Absorptions in the IOM spectra were strongly related to elemental H/C (atom) ratio. The aliphatic 3.4 μm absorption in IOM spectra increased linearly in strength with increasing H/C for H/C > 0.4, but was absent at lower H/C values. When meteorite spectra from the Reflectance Experiment Laboratory (RELAB) spectral catalog (n = 85) were reanalyzed at 3.4 μm, this detection limit (H/C > 0.4) persisted. Aromatic absorption features seen in IOM spectra were not observed in the meteorite spectra due to overlapping absorptions. However, the 3.4 μm aliphatic absorption strength for the bulk meteorites was correlated with both H/C of the meteorite's IOM and bulk C (wt%). Gaussian modeling of the 3 μm region provided an additional estimate of bulk C for the meteorites, along with bulk H (wt%), which is related to phyllosilicate abundance. These relationships lay the foundation for determining organic and phyllosilicate abundances from reflectance spectra. Both the full IOM spectra and the spectral parameters discussed here will aid in the interpretation of data from asteroid missions (e.g., OSIRIS‐REx, Hayabusa2), and may be able to place unknown spectral samples within the context of the meteorite collection.     

Radionuclide activities in recent chondrite falls determined by gamma‐ray spectrometry: Implications for terrestrial age estimates

Radionuclide activities were measured in the low‐background gamma‐ray spectrometry facility GeMSE in eight meteorite falls (Lost City, Tamdakht, Huaxi, Boumdeid, Xining, Kamargaon, Degtevo, and Ouidiyat Sbaa) and two finds (SaU 606 and Mürtschenstock) to evaluate the use of radionuclides for terrestrial age estimates. Results indicate that these meteorites were all derived from small‐ (r < 25 cm) to medium‐sized (r < 65 cm) meteoroids. Short‐lived ⁴⁸V (t_1/2 = 16.0 d) and ⁵¹Cr (t_1/2 = 27.7 d) were only detected in Oudiyat Sbaa (EH), while ⁷Be (t_1/2 = 53.1 d) was also detected in Degtevo (H) and Kamargaon (L), in agreement with reported fall dates. The ²²Na/²⁶Al activity ratio in Huaxi agrees with the previously reported short cosmic‐ray exposure age of this meteorite while ²²Na/²⁶Al in Kamargaon likely records a complex exposure history. Bayesian statistical analysis verifies the detection of very low activities of ⁴⁴Ti (t_1/2 = 60 a) in the relatively large H chondrites (>100 g) Degtevo, Huaxi, Tamdakht, Lost City, and SaU 606. Additionally, large samples from Oudiyat Sbaa (EH) and Kamargaon (L) gave positive detections. For H chondrite target compositions, detected ⁴⁴Ti(Fe+Ni)/²⁶Al averaged 0.055 ± 0.013. Activities of ²²Na and ⁵⁴Mn in SaU 606 show that this meteorite fell between July and September 2012, making SaU 606 the second recent fall from Oman identified using gamma‐ray spectrometry. The upper activity limit of ²²Na in the Mürtschenstock meteorite shows that it fell prior to 1999 and is not related to a bolide observation in 2015. Mürtschenstock shows ¹³⁷Cs ~10× higher than previously determined in Oman meteorites, likely due to Chernobyl fallout.     

Cr isotopes in physically separated components of the Allende CV3 and Murchison CM2 chondrites: Implications for isotopic heterogeneity in the solar nebula and parent body processes

Chromium isotopic data of physically separated components (chondrules, CAIs, variably magnetic size fractions) of the carbonaceous chondrites Allende and Murchison and bulk rock data of Allende, Ivuna, and Orgueil are reported to evaluate the origin of isotopic heterogeneity in these meteorites. Allende components show ε⁵³Cr and ε⁵⁴Cr from ?0.23 ± 0.07 to 0.37 ± 0.05 and from ?0.43 ± 0.08 to 3.7 ± 0.1, respectively. In components of Murchison, ε⁵³Cr and ε⁵⁴Cr vary from ?0.06 ± 0.08 to 0.5 ± 0.1 and from 0.7 ± 0.2 to 1.7 ± 0.1, respectively. The non‐systematic variations of ε⁵³Cr and ⁵⁵Mn/⁵²Cr in the components of Allende and Murchison were likely caused by small‐scale, alteration‐related redistribution of Mn >20 Ma after formation of the solar system. Chondrule fractions show the lowest ⁵⁵Mn/⁵²Cr and ε⁵⁴Cr values of all components, consistent with evaporation of Mn and ε⁵⁴Cr‐rich carrier phases from chondrule precursors. Components other than the chondrules show higher Mn/Cr and ε⁵⁴Cr, suggestive of chemical and isotopic complementarity between chondrules and matrix‐rich fractions. Bulk rock compositions calculated based on weighted compositions of components agree with measured Cr isotope data of bulk rocks, in spite of the Cr isotopic heterogeneity reported by the present and previous studies. This indicates that on a sampling scale comprising several hundred milligrams, these meteorites sampled isotopically and chemically homogeneous nebular reservoirs. The linear correlation of ⁵⁵Mn/⁵²Cr with ε⁵³Cr in bulk rocks likely was caused by variable fractionation of Mn/Cr, subsequent mixing of phases in nebular domains, and radiogenic ingrowth of ⁵³Cr.     

Silica‐rich igneous rims around magnesian chondrules in CR carbonaceous chondrites: Evidence for condensation origin from fractionated nebular gas

Abstract— The outer portions of many type I chondrules (Fa and Fs <5 mol%) in CR chondrites (except Renazzo and Al Rais) consist of silica‐rich igneous rims (SIRs). The host chondrules are often layered and have a porphyritic core surrounded by a coarse‐grained igneous rim rich in low‐Ca pyroxene. The SIRs are sulfide‐free and consist of igneously‐zoned low‐Ca and high‐Ca pyroxenes, glassy mesostasis, Fe, Ni‐metal nodules, and a nearly pure SiO₂ phase. The high‐Ca pyroxenes in these rims are enriched in Cr (up to 3.5 wt% Cr₂O₃) and Mn (up to 4.4 wt% MnO) and depleted in Al and Ti relative to those in the host chondrules, and contain detectable Na (up to 0.2 wt% Na₂O). Mesostases show systematic compositional variations: Si, Na, K, and Mn contents increase, whereas Ca, Mg, Al, and Cr contents decrease from chondrule core, through pyroxene‐rich igneous rim (PIR), and to SIR; FeO content remains nearly constant. Glass melt inclusions in olivine phenocrysts in the chondrule cores have high Ca and Al, and low Si, with Na, K, and Mn contents that are below electron microprobe detection limits. Fe, Ni‐metal grains in SIRs are depleted in Ni and Co relative to those in the host chondrules. The presence of sulfide‐free, SIRs around sulfide‐free type I chondrules in CR chondrites may indicate that these chondrules formed at high (>800 K) ambient nebular temperatures and escaped remelting at lower ambient temperatures. We suggest that these rims formed either by gas‐solid condensation of silica‐normative materials onto chondrule surfaces and subsequent incomplete melting, or by direct SiO_(gas) condensation into chondrule melts. In either case, the condensation occurred from a fractionated, nebular gas enriched in Si, Na, K, Mn, and Cr relative to Mg. The fractionation of these lithophile elements could be due to isolation (in the chondrules) of the higher temperature condensates from reaction with the nebular gas or to evaporation‐recondensation of these elements during chondrule formation. These mechanisms and the observed increase in pyroxene/olivine ratio toward the peripheries of most type I chondrules in CR, CV, and ordinary chondrites may explain the origin of olivine‐rich and pyroxene‐rich chondrules in general.     

Mn‐Cr chronology of five IIIAB iron meteorites

Abstract— Mn‐Cr systematics in phosphates (sarcopside, graftonite, beusite, galileiite, and johnsomervilleite) in IIIAB iron meteorites were investigated by secondary ion mass spectrometry (SIMS). In most cases, excesses in ⁵³Cr are found and δ⁵³Cr is well correlated with Mn/Cr ratios, suggesting that ⁵³Mn was alive at the time of IIIAB iron formation. The inferred Mn‐Cr “ages” are different for different phosphate minerals. This is presumably due to a combined effect of the slow cooling rates of IIIAB iron meteorites and the difference in the diffusion properties of Cr and Mn in the phosphates. The ages of sarcopside are the same for the IIIAB iron meteorites. Johnsomervilleite shows apparent old ages, probably because of a gain of Cr enriched in ⁵³Cr during the closure process. Apparently, old Mn‐Cr ages reported in previous studies can also be explained in a similar way. Therefore, the IIIAB iron meteorites probably experienced identical thermal histories and thus derived from the core of a parent body. Thermal histories of the parent body of IIIAB iron meteorites that satisfy the Mn‐Cr chronology and metallographic cooling rates were constructed by computer simulation. The thermal history at an early stage (<10 Ma after CAI formation) is well determined, though later history may be more model‐dependent. It is suggested that relative timing of various events in the IIIAB parent body may be estimated with the aid of the thermal history. There is a systematic difference in Mn and Cr concentrations in various minerals (phosphates, sulfide, etc.) among the IIIAB iron meteorites, which seems to be mainly controlled by redox conditions.     

Mineralogy and petrography of amoeboid olivine aggregates from the reduced CV3 chondrites Efremovka,Leoville and Vigarano: Products of nebular condensation,accretion and annealing

Abstract— Amoeboid olivine aggregates (AOAs) from the reduced CV chondrites Efremovka, Leoville and Vigarano are irregularly‐shaped objects, up to 5 mm in size, composed of forsteritic olivine (Fa_<10) and a refractory, Ca, Al‐rich component. The AOAs are depleted in moderately volatile elements (Mn, Cr, Na, K), Fe, Ni‐metal and sulfides and contain no low‐Ca pyroxene. The refractory component consists of fine‐grained calcium‐aluminum‐rich inclusions (CAIs) composed of Al‐diopside, anorthite (An₁₀₀), and magnesium‐rich spinel (～1 wt% FeO) or fine‐grained intergrowths of these minerals; secondary nepheline and sodalite are very minor. This indicates that AOAs from the reduced CV chondrites are more pristine than those from the oxidized CV chondrites Allende and Mokoia. Although AOAs from the reduced CV chondrites show evidence for high‐temperature nebular annealing (e.g., forsterite grain boundaries form 120° triple junctions) and possibly a minor degree of melting of Al‐diopside‐anorthite materials, none of the AOAs studied appear to have experienced extensive (>50%) melting. We infer that AOAs are aggregates of high‐temperature nebular condensates, which formed in CAI‐forming regions, and that they were absent from chondrule‐forming regions at the time of chondrule formation. The absence of low‐Ca pyroxene and depletion in moderately volatile elements (Mn, Cr, Na, K) suggest that AOAs were either removed from CAI‐forming regions prior to condensation of these elements and low‐Ca pyroxene or gas‐solid condensation of low‐Ca‐pyroxene was kinetically inhibited.     

Further characterization of carbonaceous materials in Hayabusa‐returned samples to understand their origin

Carbonaceous materials in the sample catcher of the Hayabusa spacecraft were assigned as category 3 particles. We investigated the category 3 particles with a suite of in situ microanalytical methods. Possible contaminants collected from the cleanrooms of the spacecraft assembly and extraterrestrial sample curation center (ESCuC) were also analyzed in the same manner as category 3 particles for comparison. Our data were integrated with those of the preliminary examination team for category 3 particles. Possible origins for the category 3 particles include contamination before and after the operation of the Hayabusa spacecraft.     

Eutectic metal + troilite + Fe‐Mn‐Na phosphate + Al‐free chromite assemblage in shock‐produced chondritic melt of the Yanzhuang chondrite

An assemblage with FeNi metal, troilite, Fe‐Mn‐Na phosphate, and Al‐free chromite was identified in the metal‐troilite eutectic nodules in the shock‐produced chondritic melt of the Yanzhuang H6 meteorite. Electron microprobe and Raman spectroscopic analyses show that a few phosphate globules have the composition of Na‐bearing graftonite (Fe_,Mn,Na)₃(PO₄)₂, whereas most others correspond to Mn‐bearing galileiite Na(Fe,Mn)₄(PO₄)₃ and a possible new phosphate phase of Na₂(Fe,Mn)₁₇(PO₄)₁₂ composition. The Yanzhuang meteorite was shocked to a peak pressure of 50 GPa and a peak temperature of approximately 2000 °C. All minerals were melted after pressure release to form a chondritic melt due to very high postshock heat that brought the chondrite material above its liquidus. The volatile elements P and Na released from whitlockite and plagioclase along with elements Cr and Mn released from chromite are concentrated into the shock‐produced Fe‐Ni‐S‐O melt at high temperatures. During cooling, microcrystalline olivine and pyroxene first crystallized from the chondritic melt, metal‐troilite eutectic intergrowths, and silicate melt glass finally solidified at about 950–1000 °C. On the other hand, P, Mn, and Na in the Fe‐Ni‐S‐O melt combined with Fe and crystallized as Fe‐Mn‐Na phosphates within troilite, while Cr combined with Fe and crystallized as Al‐free chromite also within troilite.     

Minor elements in forsterites of Orgueil (C1), Alais (C1) and two interplanetary dust particles compared to C2-C3-UOC forsterites

Abstract— The rare Mg-rich silicate fraction of the C1 meteorites, Orgueil and Alais, is dominated by minute (< 30 μm) forsterite. Twenty three forsterite grains of these meteorites as well as large forsterites in two chondritic porous interplanetary dust particles (IDPs) are characterized by levels of MnO generally, but not always, higher than found in forsterites of C2, C3 and unequilibrated ordinary chondrites (UOC). Forsterite in Orgueil contains 900 to 6200 ppmw MnO while Alais forsterite has less than 2000 ppmw MnO suggesting that the forsterites in the two meteorites are chemically distinct. Alais forsterite shows lower Cr and Al relative to Orgueil forsterite. The C1 forsterites do not show Fe-poor (FeO < 0.3), refractory-rich (Al, Ca, Ti, V) compositions which are relatively common in the C2-C3-UOC meteorites suggesting that the most primitive forsterite compositions are not present in these C1 meteorites. While minor elements in forsterite can not distinguish unambiguously between C1 and C2-C3-UOC sources, the high Mn levels in some IDP forsterites are similar to some C1 forsterites suggesting a possible relation between the forsterites of these two extraterrestrial samples.     

Testing accretion mechanisms of the H chondrite parent body utilizing nucleosynthetic anomalies

Planetary bodies a few hundred kilometers in radii are the precursors to larger planets but it is unclear whether these bodies themselves formed very rapidly or accreted slowly over several millions of years. Ordinary H chondrite meteorites provide an opportunity to investigate the accretion time scale of a small planetary body given that variable degrees of thermal metamorphism present in H chondrites provide a proxy for their stratigraphic depth and, therefore, relative accretion times. We exploit this feature to search for nucleosynthetic isotope variability of ⁵⁴Cr, which is a sensitive tracer of spatial and temporal variations in the protoplanetary disk's solids, between 17 H chondrites covering all petrologic types to obtain clues about the parent body accretionary rate. We find no systematic variability in the mass‐biased corrected abundances of ⁵³Cr or ⁵⁴Cr outside of the analytical uncertainties, suggesting very rapid accretion of the H chondrite parent body consistent with turbulent accretion. By utilizing the μ⁵⁴Cr–planetary mass relationship observed between inner solar system planetary bodies, we calculate that the H chondrite accretion occurred at 1.1 ± 0.4 or 1.8 ± 0.2 Myr after the formation of calcium‐aluminum‐rich inclusions (CAIs), assuming either the initial ²⁶Al/²⁷Al abundance of inner solar system solids determined from angrite meteorites or CAIs from CV chondrites, respectively. Notably, these ages are in agreement with age estimates based on the parent bodies’ thermal evolution when correcting these calculations to the same initial ²⁶Al/²⁷Al abundance, reinforcing the idea of a secular evolution in the isotopic composition of inner disk solids.     

Physical, Chemical, and Mineralogical Properties of Comet 81P/Wild 2 Particles Collected by Stardust

NASA’s Stardust spacecraft collected dust from the coma of Comet 81P/Wild 2 by impact into aerogel capture cells or into Al-foils. The first direct, laboratory measurement of the physical, chemical, and mineralogical properties of cometary dust grains ranging from <10⁻¹⁵ to ∼10⁻⁴ g were made on this dust. Deposition of material along the entry tracks in aerogel and the presence of compound craters in the Al-foils both indicate that many of the Wild 2 particles in the size range sampled by Stardust are weakly bound aggregates of a diverse range of minerals. Mineralogical characterization of fragments extracted from tracks indicates that most tracks were dominated by olivine, low-Ca pyroxene, or Fe-sulfides, although one track was dominated by refractory minerals similar to Ca–Al inclusions in primitive meteorites. Minor mineral phases, including Cu–Fe-sulfide, Fe–Zn-sulfide, carbonate and metal oxides, were found along some tracks. The high degree of variability of the element/Fe ratios for S, Ca, Ti, Cr, Mn, Ni, Cu, Zn, and Ga among the 23 tracks from aerogel capture cells analyzed during Stardust Preliminary Examination is consistent with the mineralogical variability. This indicates Wild 2 particles have widely varying compositions at the largest size analyzed (>10 μm). Because Stardust collected particles from several jets, sampling material from different regions of the interior of Wild 2, these particles are expected to be representative of the non-volatile component of the comet over the size range sampled. Thus, the stream of particles associated with Comet Wild 2 contains individual grains of diverse elemental and mineralogical compositions, some rich in Fe and S, some in Mg, and others in Ca and Al. The mean refractory element abundance pattern in the Wild 2 particles that were examined is consistent with the CI meteorite pattern for Mg, Si, Cr, Fe, and Ni to 35%, and for Ca, Ti and Mn to 60%, but S/Si and Fe/Si both show a statistically significant depletion from the CI values and the moderately volatile elements Cu, Zn, Ga are enriched relative to CI. This elemental abundance pattern is similar to that in anhydrous, porous interplanetary dust particles (IDPs), suggesting that, if Wild 2 dust preserves the original composition of the Solar Nebula, the anhydrous, porous IDPs, not the CI meteorites, may best reflect the Solar Nebula abundances. This might be tested by elemental composition measurements on cometary meteors.     

The solar system abundances of phosphorus and titanium and the nebular volatility of phosphorus

Abstract— The bulk chemical composition of Orgueil and 25 other carbonaceous chondrites was determined by x‐ray fluorescence analysis. The sample sizes of the analyzed meteorites were in all cases 120 mg. The abundances of P and Ti in Orgueil and Ivuna were precisely determined by the standard addition method. The new P CI abundance is 926 ± 65 ppm. Excluding the low P of Ivuna and one Orgueil sample with unusual chemistry gives a CI P content of 930 ± 23 ppm. A CI abundance of 926 ppm corresponds to a P/Si wt ratio of 8.66 times 10^?3 (atomic ratio 7.85 times 10^?3). For Ti a CI content of 458 ± 18 ppm and a Ti/Si wtratio of 4.28 times 10^?3 (atomic ratio 2.51 times 10^?3) were found. A Si content of 10.69% was obtained for average CI. The new P CI abundance is 20 to 30% below earlier estimates, while the Ti CI abundance is in agreement with earlier determinations. From the results of the analyses of bulk carbonaceous chondrites it is concluded: (1) Refractory element/Mg ratios increase from CI through CM and C3O to C3V, but ratios among Al, Ca and Ti are constant, except for low Ca/Al ratios in the reduced subgroup of C3V. (2) The Si/Mg ratios are constant in all groups of carbonaceous chondrites. (3) There is a volatility related depletion of Cr and Fe, but the Cr/Fe ratios are constant. (4) The sequence of volatility related depletions of the moderately volatile elements P, Au, As, Mn, and Zn follows condensation temperatures (except for As), if in condensation calculations non‐ideal solid solution in the host phase is considered.     

Investigation of cutting methods for small samples of Hayabusa and future sample return missions

We report the investigation of cutting methods for Hayabusa samples. The purpose of our study is to explore the possibility of applying multiple analyses to a single particle effectively. We investigated the cutting performance of a blade dicing saw, laser, focused ion beam (FIB), and physical breaking by microindenter. Cutting performance was examined by estimating the aspect ratio of the cut slit, i.e., depth over width of the slit. We also investigated the possible contamination and sample damage by cutting. The result of the investigation shows that we can cut the samples from <50 μm to 500 μm using those methods with aspect ratios from 10 to 20, although they would introduce some contamination or damage to the samples. Our investigations also provide an important basis for the analysis of samples obtained by future sample return missions.     

Origin of the metamorphosed clasts in the CV3 carbonaceous chondrite breccias of Graves Nunataks 06101, Vigarano,Roberts Massif 04143, and Yamato‐86009

We observed metamorphosed clasts in the CV3 chondrite breccias Graves Nunataks 06101, Vigarano, Roberts Massif 04143, and Yamato‐86009. These clasts are coarse‐grained polymineralic rocks composed of Ca‐bearing ferroan olivine (Fa_24–40, up to 0.6 wt% CaO), diopside (Fs_7–12Wo_44–50), plagioclase (An_52–75), Cr‐spinel (Cr/[Cr + Al] = 0.4, Fe/[Fe + Mg] = 0.7), sulfide and rare grains of Fe‐Ni metal, phosphate, and Ca‐poor pyroxene (Fs₂₄Wo₄). Most clasts have triple junctions between silicate grains. The rare earth element (REE) abundances are high in diopside (REE ~3.80–13.83 × CI) and plagioclase (Eu ~12.31–14.67 × CI) but are low in olivine (REE ~0.01–1.44 × CI) and spinel (REE ~0.25–0.49 × CI). These REE abundances are different from those of metamorphosed chondrites, primitive achondrites, and achondrites, suggesting that the clasts are not fragments of these meteorites. Similar mineralogical characteristics of the clasts with those in the Mokoia and Yamato‐86009 breccias (Jogo et al. 2012 ) suggest that the clasts observed in this study would also form inside the CV3 chondrite parent body. Thermal modeling suggests that in order to reach the metamorphosed temperatures of the clasts of >800 °C, the clast parent body should have accreted by ~2.5–2.6 Ma after CAIs formation. The consistency of the accretion age of the clast parent body and the CV3 chondrule formation age suggests that the clasts and CV3 chondrites could be originated from the same parent body with a peak temperature of 800–1100 °C. If the body has a peak temperature of >1100 °C, the accretion age of the body becomes older than the CV3 chondrule formation age and multiple CV3 parent bodies are likely.     

Refractory forsterite in primitive meteorites: Condensates from the solar nebula?

Abstract— All groups of chondritic meteorites contain discrete grains of forsteritic olivine with FeO contents below 1 wt% and high concentrations of refractory elements such as Ca, Al, and Ti. Ten such grains (52 to 754 μg) with minor amounts of adhering matrix were separated from the Allende meteorite. After bulk chemical analysis by instrumental neutron activation analysis (INAA), some samples were analyzed with an electron microprobe and some with an ion microprobe. Matrix that accreted to the forsterite grains has a well‐defined unique composition, different from average Allende matrix in having higher Cr and lower Ni and Co contents, which implies limited mixing of Allende matrix. All samples have approximately chondritic relative abundances of refractory elements Ca, Al, Sc, and rare‐earth elements (REE), although some of these elements, such as Al, do not quantitatively reside in forsterite; whereas others (e.g., Ca) are intrinsic to forsterite. The chondritic refractory element ratios in bulk samples, the generally high abundance level of refractory elements, and the presence of Ca‐Al‐Ti‐rich glass inclusions suggest a genetic relationship of refractory condensates with forsteritic olivine. The Ca‐Al‐Ti‐rich glasses may have acted as nuclei for forsterite condensation. Arguments are presented that exclude an origin of refractory forsterite by crystallization from melts with compositions characteristic of Allende chondrules: (a) All forsterite grains have CaO contents between 0.5 and 0.7 wt% with no apparent zoning, requiring voluminous parental melts with 18 to 20 wt% CaO, far above the average CaO content of Allende chondrules. Similar arguments apply to Al contents. (b) The low FeO content of refractory forsterite of 0.2‐0.4 wt% imposes an upper limit of ～1 wt% of FeO on the parental melt, too low for ordinary and carbonaceous chondrule melts, (c) The Mn contents of refractory forsterites are between 30 to 40 ppm. This is at least one order of magnitude below the Mn content of chondrule olivines in all classes of meteorites. The observed Mn contents of refractory forsterite are much too low for equilibrium between olivine and melts of chondrule composition, (d) As shown earlier, refractory forsterites have O‐isotopic compositions different from chondrules (Weinbruch et al., 1993a). Refractory olivines in carbonaceous chondrites are found in matrix and in chondrules. The compositional similarity of both types was taken to indicate that all refractory forsterites formed inside chondrules (e.g., Jones, 1992). As refractory forsterite cannot have formed by crystallization from chondrule melts, we conclude that refractory forsterite from chondrules are relic grains that survived chondrule melting and probably formed in the same way as refractory forsterite enclosed in matrix. We favor an origin of refractory forsterite by condensation from an oxidized nebular gas.     

A gamma‐ray spectroscopy survey of Omani meteorites

The gamma‐ray activities of 33 meteorite samples (30 ordinary chondrites, 1 Mars meteorite, 1 iron, 1 howardite) collected during Omani‐Swiss meteorite search campaigns 2001–2008 were nondestructively measured using an ultralow background gamma‐ray detector. The results provide several types of information: Potassium and thorium concentrations were found to range within typical values for the meteorite types. Similar mean ²⁶Al activities in groups of ordinary chondrites with (1) weathering degrees W0‐1 and low ¹⁴C terrestrial age and (2) weathering degree W3‐4 and high ¹⁴C terrestrial age are mostly consistent with activities observed in recent falls. The older group shows no significant depletion in ²⁶Al. Among the least weathered samples, one meteorite (SaU 424) was found to contain detectable ²²Na identifying it as a recent fall close to the year 2000. Based on an estimate of the surface area searched, the corresponding fall rate is ~120 events/10⁶ km²*a, consistent with other estimations. Twelve samples from the large JaH 091 strewn field (total mass ~4.5 t) show significant variations of ²⁶Al activities, including the highest values measured, consistent with a meteoroid radius of ~115 cm. Activities of ²³⁸U daughter elements demonstrate terrestrial contamination with ²²⁶Ra and possible loss of ²²²Rn. Recent contamination with small amounts of ¹³⁷Cs is ubiquitous. We conclude that gamma‐ray spectroscopy of a selection of meteorites with low degrees of weathering is particularly useful to detect recent falls among meteorites collected in hot deserts.     

Cathodoluminescence microscopy and spectroscopy of forsterite from Kaba meteorite: An application to the study of hydrothermal alteration of parent body

Highly forsteritic olivine (Fo: 99.2–99.7) in the Kaba meteorite emits bright cathodoluminescence (CL). CL spectra of red luminescent forsterite grains have two broad emission bands at approximately 630 nm (impurity center of divalent Mn ions) in the red region and above 700 nm (trivalent Cr ions) in the red–IR region. The cores of the grains show CL blue luminescence giving a characteristic broad band emission at 400 nm, also associated with minor red emissions related to Mn and Cr ions. CL color variation of Kaba forsterite is attributed to structural defects. Electron probe microanalyzer (EPMA) analysis shows concentrations of Ca, Al, and Ti in the center of the forsterite grain. The migration of diffusible ions of Mn, Cr, and Fe to the rim of the Kaba meteoritic forsterite was controlled by the hydrothermal alteration at relatively low temperature (estimated at about 250 °C), while Ca and Al ions might still lie in the core. A very unusual phase of FeO (wüstite) was also observed, which may be a terrestrial alteration product of FeNi‐metal.     

The potential for metal contamination during Apollo lunar sample curation

Curation and preparation of samples for chemical analysis can occasionally lead to significant contamination. This issue is of concern in the study of lunar samples, especially those from the Apollo sample collection, where available masses are finite. Here we present compositional data for stainless steels that have commonly been used in the processing of Apollo lunar samples at NASA Johnson Space Center, including a chisel and a vessel typically used to transfer Apollo samples to principal investigators. The Type 304 stainless steels are Cr-rich, with high concentrations of Mn (4000–18,000 μg g⁻¹), Cu (1000–22,900 μg g⁻¹), Mo (1030–1120 μg g⁻¹), and W (72–193 μg g⁻¹). They have elevated highly siderophile element (HSE) concentrations (up to 92 ng g⁻¹ Os), ¹⁸⁷Os/¹⁸⁸Os ranging from 0.1310 to 0.1336, and negligible lithophile element abundances. We find that, while metal contamination is possible, significant (≫0.01% by mass) addition of stainless steel is required to strongly affect the composition of the HSE, W, Mo, Cr, or Cu for most Apollo lunar samples. Nonetheless, careful appraisal on a case-by-case basis should take place to ensure contamination introduced through sample processing during curation is at acceptably low levels. A survey of lunar mare basalts and crustal rocks indicates that metal contamination plays a negligible role in the compositional variability of the HSE and W compositions preserved in these samples. Further work to constrain contamination for other properties of Apollo samples is required (e.g., organics, microbes, water, noble gases, and magnetics), but the effect of metal contamination can be well-constrained for the Apollo lunar collection.     

Atom‐probe tomography and transmission electron microscopy of the kamacite–taenite interface in the fast‐cooled Bristol IVA iron meteorite

We report the first combined atom‐probe tomography (APT) and transmission electron microscopy (TEM) study of a kamacite–tetrataenite (K–T) interface region within an iron meteorite, Bristol (IVA). Ten APT nanotips were prepared from the K–T interface with focused ion beam scanning electron microscopy (FIB‐SEM) and then studied using TEM followed by APT. Near the K‐T interface, we found 3.8 ± 0.5 wt% Ni in kamacite and 53.4 ± 0.5 wt% Ni in tetrataenite. High‐Ni precipitate regions of the cloudy zone (CZ) have 50.4 ± 0.8 wt% Ni. A region near the CZ and martensite interface has <10 nm sized Ni‐rich precipitates with 38.4 ± 0.7 wt% Ni present within a low‐Ni matrix having 25.5 ± 0.6 wt% Ni. We found that Cu is predominantly concentrated in tetrataenite, whereas Co, P, and Cr are concentrated in kamacite. Phosphorus is preferentially concentrated along the K‐T interface. This study is the first precise measurement of the phase composition at high spatial resolution and in 3‐D of the K‐T interface region in a IVA iron meteorite and furthers our knowledge of the phase composition changes in a fast‐cooled iron meteorite below 400 °C. We demonstrate that APT in conjunction with TEM is a useful approach to study the major, minor, and trace elemental composition of nanoscale features within fast‐cooled iron meteorites.