An igneous fragment from cluster IDP L2011#21: An analog for the source of pyrrhotite and taenite in comet 81P/Wild 2 captured in Stardust aerogel

The silica glass extracted from the bulbous parts of Stardust tracks is riddled by electron‐opaque nanograins with compositions that are mostly between pyrrhotite and metallic iron with many fewer nanograins having a Fe‐Ni‐S composition. Pure taenite nanograins are extremely rare, but exist among the terminal particles. Assuming that these Fe‐Ni‐S compositions are due to mixing of pyrrhotite and taenite melt droplets, it is remarkable that the taenite melt grains had discrete Fe/Ni ratios. This paper presents the data from an igneous pyrrhotite/taenite fragment of cluster IDP L2011#21, wherein the taenite compositions have the same discrete Fe/Ni clusters as those inferred for the Stardust nanograins. These Fe/Ni clusters are a subsolidus feature with compositions that are constrained by the Fe‐Ni phase diagram. They formed during cooling of the parent body of this cluster IDP fragment. These specific Fe/Ni ratios, 12.5, 24, 40, and 53 atom% Ni, were preserved in asteroidal taenite that survived radially outward transport to the Kuiper Belt where it accreted into the (future) comet Wild 2 nucleus.     

Microstructure modifications of silicates induced by the collection in aerogel: Experimental approach and comparison with Stardust results

Abstract– Particles from comet 81P/Wild 2 were captured with silica aerogel during the flyby Stardust mission. A significant part of the collection was damaged during the impact at hypervelocity in the aerogel. In this study, we conducted impact experiments into aerogel of olivine and pyroxene powder using a light‐gas gun in similar conditions as that of the comet Wild 2 particles collection. The shot samples were investigated using transmission electron microscopy to characterize their microstructure. Both olivine and pyroxene samples show evidence of thermal alteration due to friction with the aerogel. All the grains have rounded edges after collection, whereas their shape was angular in the initial shot powder set. This is probably associated with mass loss of particles. The rims of the grains are clearly melted and mixed with aerogel. The core of olivine grains is fairly well preserved, but some grains contain dislocations in glide configuration. We interpret these dislocations as generated by the thermal stresses that have emerged due to the high temperature gradients between the core and the rim of the grains. Most of the pyroxene grains have been fully melted. Their high silica concentration reflects a strong impregnation with melted aerogel. The preferential melting of pyroxene compared with olivine is due to a difference in melting temperatures of 300°. This melting point difference probably induces a bias in the measurements of the ratio olivine/pyroxene in the Wild 2 comet. The proportion of pyroxene was probably higher on Wild 2 than expected from the samples collected into aerogel.     

The smallest comet 81P/Wild 2 dust dances around the CI composition

The bulbous Stardust track #80 (C2092,3,80,0,0) is a huge cavity. Allocations C2092,2,80,46,1 nearest the entry hole and C2092,2,80,47,6 about 0.8 mm beneath the entry hole provide evidence of highly chaotic conditions during capture. They are dominated by nonvesicular low‐Mg silica glass instead of highly vesicular glass found deeper into this track which is consistent with the escape of magnesiosilica vapors generated from the smallest comet grains. The survival of delicate (Mg,Al,Ca)‐bearing silica glass structures is unique to the entry hole. Both allocations show a dearth of surviving comet dust except for a small enstatite, a low‐Ca hypersthene grain, and a Ti‐oxide fragment. Finding scattered TiO₂ fragments in the silica glass could support, but not prove, TiO₂ grain fragmentation during hypervelocity capture. The here reported dearth in mineral species is in marked contrast to the wealth of surviving silicate and oxide minerals deeper into the bulb. Both allocations show Fe‐Ni‐S nanograins dispersed throughout the low‐Mg silica glass matrix. It is noted that neither comet Halley nor Wild 2 had a CI bulk composition for the smallest grains. Using the analogs of interplanetary dust particles (IDPs) and cluster IDPs it is argued that a CI chondritic composition requires the mixing of nonchondritic components in the appropriate proportions. So far, the fine‐grained Wild 2 dust is biased toward nonchondritic ferromagnesiosilica materials and lacking contributions of nonchondritic components with Mg‐Fe‐Ni‐S[Si‐O] compositions. To be specific, “Where are the GEMS”? The GEMS look‐alike found in this study suggests that evidence of GEMS in comet Wild 2 may still be found in the Stardust glass.     

A cometary aggregate interplanetary dust particle as an analog for comet Wild 2 grain chemistry preserved in silica‐rich Stardust glass

Abstract— Many of the nanometer‐scale grains from comet 81P/Wild 2 did not survive hypervelocity capture. Instead, they melted and interacted with silica melt derived from the aerogel used by the Stardust mission. Their petrological properties were completely modified, but their bulk chemistry was preserved in the chemical signatures of mostly vesicular Si‐rich glass with its typical Fe‐Ni‐S compound inclusions. Chondritic aggregate IDP L2011A9 that experienced atmospheric pre‐entry thermal modification was selected as an analog to investigate these Wild 2 chemical signatures. The chemical, petrologic, and mineralogical properties of the individual constituents in this aggregate IDP are presented and used to match the chemical signatures of these Wild 2 grains. Mixing of comet material and pure silica, which is used in a diagram that recognizes this mixing behavior, is used to constrain the probable petrologic and minerals that caused the Wild 2 signatures. The Wild 2 nanometer‐scale grain signatures in Si‐rich glass allocations from three different deceleration tracks resembled mixtures of ultrafine‐grained principal components and dense agglomerate‐like material, Mg‐rich silicates (<500 nm) and Fe,Ni‐sulfides (<100 nm), and Si‐rich amorphous material. Dust resembling the mixed matrix of common chondritic aggregate IDPs was present in Jupiter‐family comet Wild 2.     

Chemical identification of comet 81P/Wild 2 dust after interacting with molten silica aerogel

Abstract— Flight aerogel in Stardust allocation C2092,2,80,47,6 contains percent level concentrations of Na, Mg, Al, S, Cl, K, Ca, Cr, Mn, Fe, and Ni that have a distinctive Fe‐ and CI‐normalized distribution pattern, which is similar to this pattern for ppb level chemical impurities in pristine aerogel. The elements in this aerogel background were assimilated in non‐vesicular and vesicular glass with the numerous nanometer Fe‐Ni‐S compound inclusions. After correction for the background values, the chemical data show that this piece of comet Wild 2 dust was probably an aggregate of small (<500 nm) amorphous ferromagnesiosilica grains with many tiny Fe,Ni‐sulfide inclusions plus small Ca‐poor pyroxene grains. This distinctive Fe‐ and CI‐normalized element distribution pattern is found in several Stardust allocations. It appears to be a common feature in glasses of quenched aerogel melts but its exact nature is yet to be established.     

Along‐track compositional and textural variation in extensively melted grains returned from comet 81P/Wild 2 by the Stardust mission: Implications for capture‐melting process

Abstract— Five amorphous (extensively melted) grains from Stardust aerogel capture Track 35 were examined by transmission electron microscopy (TEM); two from the bulb, two from near the bulb‐stylus transition, and one from near the terminal particle. Melted grains consist largely of a texturally and compositionally heterogeneous emulsion of immiscible metal/sulfide beads nanometers to tens of nanometers in diameter in a silica‐rich vesicular glass. Most metal/sulfide beads are spherical, but textures of non‐spherical beads indicate that some solidified as large drops during stretching and breaking while in translational and rotational motion, and others solidified from lenses of immiscible liquid at the silicate‐melt/vesicle (vapor) interface. Melted grains appear to become richer in Fe relative to Mg, and depleted in S relative to Fe and Ni with increasing penetration distance along the aerogel capture track. Fe/S ratios are near unity in grains from the bulb of Track 35, consistent with the dominance of Fe‐monosulfide minerals inferred by previous research on Stardust materials. Near‐stoichiometric Fe/S in melted grains from the bulb suggests that Fe‐sulfides in the bulb were dispersed and melted during formation of the bulb but did not lose S. Along‐track increases in Fe/S in melted grains from the bulb through the bulb‐stylus transition and continuing into the stylus indicate that S initially present as iron monosulfide may have been progressively partially volatilized and lost from the melted grains with greater penetration of the grains deeper into the aerogel during capture‐melting of comet dust. Extensively melted grains from the bulbs of aerogel capture tracks may preserve better primary compositional information with less capture‐related modification than grains from farther along the same capture tracks.     

Two refractory Wild 2 terminal particles from a carrot‐shaped track characterized combining MIR/FIR/Raman microspectroscopy and FE‐SEM/EDS analyses

We present the analyses results of two bulk Terminal Particles, C2112,7,171,0,0 and C2112,9,171,0,0, derived from the Jupiter‐family comet 81P/Wild 2 returned by the Stardust mission. Each particle embedded in a slab of silica aerogel was pressed in a diamond cell. This preparation, as expected, made it difficult to identify the minerals and organic materials present in these particles. This problem was overcome using a combination of three different analytical techniques, viz. FE‐SEM/EDS, IR, and Raman microspectroscopy that allowed identifying the minerals and small amounts of amorphous carbon present in both particles. TP2 and TP3 were dominated by Ca‐free and low‐Ca, Mg‐rich, Mg,Fe‐olivine. The presence of melilite in both particles is supported by IR microspectroscopy, but is not confirmed by Raman microspectroscopy, possibly because the amounts are too small to be detected. TP2 and TP3 show similar silicate mineral compositions, but Ni‐free and low‐Ni, subsulfur (Fe,Ni)S grains are present in TP2 only. TP2 contains indigenous amorphous carbon hot spots; no indigenous carbon was identified in TP3. These nonchondritic particles probably originated in a differentiated body. This work found an unanticipated carbon contamination following the FE‐SEM/EDS analyses. It is suggested that organic materials in the embedding silica aerogel are irradiated during FE‐SEM/EDS analyses creating a carbon gas that develops a strong fluorescence continuum. The combination of the selected analytical techniques can be used to characterize bulk Wild 2 particles without the need of extraction and removal of the encapsulating aerogel. This approach offers a relatively fast sample preparation procedure, but compressing the samples can cause spurious artifacts, viz. silica contamination. Because of the combination of techniques, we account for these artifacts.     

At the interface of silica glass and compressed silica aerogel in Stardust track 10: Comet Wild 2 is not a goldmine

In Stardust tracks C2044,0,38, C2044,0,39, and C2044,0,42 (Brennan et al. 2007 ) and Stardust track 10 (this work) gold is present in excess of its cosmochemical abundance. Ultra‐thin sections of allocation FC6,0,10,0,26 (track 10) show a somewhat wavy, compressed silica aerogel/silica glass interface which challenges exact location identification, i.e., silica glass, compressed silica aerogel, or areas of overlap. In addition to domains of pure silica ranging from SiO₂ to SiO₃ glass, there is MgO‐rich silica glass with a deep metastable composition, MgO = 14 ± 6 wt%, due to assimilation of Wild 2 Mg‐silicate matter in silica melt. This magnesiosilica composition formed when temperatures during hypervelocity capture reached >2000 °C followed by ultrafast quenching of the magnesiosilica melt when it came into contact with compressed aerogel at ~155 °C. The compressed silica aerogel in track 10 has a continuous Au background as result of the melting point depression of gold particles <5 nm that showed liquid‐like behavior. Larger gold particles are scattered found throughout the silica aerogel matrix and in aggregates up to ~50 nm in size. No gold is found in MgO‐rich silica glass. Gold in track 10 is present at the silica aerogel/silica glass interface. In the other tracks gold was likely near‐surface contamination possibly from an autoclave used in processing of these particular aerogel tiles. So far gold contamination is documented in these four different tracks. Whether they are the only tiles with gold present in excess of its cosmochemical abundance or whether more tiles will show excess gold abundances is unknown.     

Fine‐grained material of 81P/Wild 2 in interaction with the Stardust aerogel

Abstract– The deceleration tracks in the Stardust aerogel display a wide range of morphologies, which reveal a large diversity of incoming particles from comet 81P/Wild 2. If the large and dense mineral grains survived the extreme conditions of hypervelocity capture, this was not the case for the fine‐grained material that is found strongly damaged within the aerogel. Due to their low mechanical strength, these assemblages were disaggregated, dispersed, and flash melted in the aerogel in walls of bulbous deceleration tracks. Their petrologic and mineralogical properties are found significantly modified by the flash heating of the capture. Originating from a quenched melt mixture of comet material and aerogel, the representative microstructure consists of silica‐rich glassy clumps containing Fe‐Ni‐S inclusions, vesicles and “dust‐rich” patches, the latter being remnants of individual silicate components of the impacting aggregate. The average composition of these melted particle fragments is close to the chondritic CI composition. They might originate from ultrafine‐grained primitive components comparable to those found in chondritic porous IDPs. Capture effects in aerogel and associated sample biases are discussed in terms of size, chemical and mineralogical properties of the grains. These properties are essential for the grain survival in the extremely hot environment of hypervelocity impact capture in aerogel, and thus for inferring the correct properties of Wild 2 material.     

Shock and annealing in the amphibole‐ and mica‐bearing R chondrites

MIL 11207 (R6) and LAP 04840 (R6) contain hornblende and phlogopite; MIL 07440 (R6) contains accessory titan‐phlogopite and no hornblende. All three meteorites have been shocked: MIL 11207 contains extensive sulfide veins, pyroxene that formed from dehydrated hornblende, and an extensive network of plagioclase glass; MIL 07440 contains chromite‐plagioclase assemblages, chromite veinlets and blebs, pincer‐shaped plagioclase patches, but no sulfide veins; LAP 04840 contains olivine grains with chromite‐bleb‐laden cores and opaque‐free rims, rare grains of pyroxene that formed from dehydrated hornblende, and no sulfide veins. These meteorites appear to have been heated to maximum temperatures of approximately 700–900 °C under conditions of moderately high PH₂O (perhaps 250–500 bars). All three samples underwent postshock annealing. During this process, olivine crystal lattices healed (giving the rocks the appearance of shock‐stage S1), and diffusion of Fe and S from thin sulfide veins to coarse sulfide grains caused the veins to disappear in MIL 07440 and LAP 04840. This latter process apparently also occurred in most S1–S2 ordinary chondrites of high petrologic type. The pressure–temperature conditions responsible for forming the amphibole and mica in these rocks may have been present at depths of a few tens of kilometers (as suggested in the literature). A giant impact or a series of smaller impacts would then have been required to excavate the hornblende‐ and biotite‐bearing rocks and bring them closer to the surface. It was in that latter location where the samples were shocked, deposited in a hot ejecta blanket of low thermal diffusivity, and annealed.     

Northwest Africa 428: Impact‐induced annealing of an L6 chondrite breccia

Abstract— Northwest Africa (NWA) 428 is an L chondrite that was successively thermally metamorphosed to petrologic type‐6, shocked to stage S4–S5, brecciated, and annealed to approximately petrologic type‐4. Its thermal and shock history resembles that of the previously studied LL6 chondrite, Miller Range (MIL) 99301, which formed on a different asteroid. The petrologic type‐6 classification of NWA 428 is based on its highly recrystallized texture, coarse metal (150 ± 150 μm), troilite (100 ± 170 μm), and plagioclase (20–60 μm) grains, and relatively homogeneous olivine (Fa_{24.4 ± 0.6}), low‐Ca pyroxene (Fs_{20.5 ± 0.4}), and plagioclase (Ab_{84.2 ± 0.4}) compositions. The petrographic criteria that indicate shock stage S4–S5 include the presence of chromite veinlets, chromite‐plagioclase assemblages, numerous occurrences of metallic Cu, irregular troilite grains within metallic Fe‐Ni, polycrystalline troilite, duplex plessite, metal and troilite veins, large troilite nodules, and low‐Ca clinopyroxene with polysynthetic twins. If the rock had been shocked before thermal metamorphism, low‐Ca clinopyroxene produced by the shock event would have transformed into orthopyroxene. Post‐shock brecciation is indicated by the presence of recrystallized clasts and highly shocked clasts that form sharp boundaries with the host. Post‐shock annealing is indicated by the sharp optical extinction of the olivine grains; during annealing, the damaged olivine crystal lattices healed. If temperatures exceeded those approximating petrologic type‐4 (?600–700°C) during annealing, the low‐Ca clinopyroxene would have transformed into orthopyroxene. The other shock indicators, likewise, survived the mild annealing. An impact event is the most plausible source of post‐metamorphic, post‐shock annealing because any ²⁶Al that may have been present when the asteroid accreted would have decayed away by the time NWA 428 was annealed. The similar inferred histories of NWA 428 (L6) and MIL 99301 (LL6) indicate that impact heating affected more than 1 ordinary chondrite parent body.     

The formation of Mg,Fe‐silicates by reactions between amorphous magnesiosilica smoke particles and metallic iron nanograins with implications for comet silicate origins

This thermal annealing experiment at 1000 K for up to 167 h used a physical mixture of vapor phase‐condensed magnesiosilica grains and metallic iron nanograins to test the hypothesis that a mixture of magnesiosilica grains and an Fe‐source would lead to the formation of ferromagnesiosilica grains. This exploratory study found that coagulation and thermal annealing of amorphous magnesiosilica and metallic grains yielded ferromagnesiosilica grains with the Fe/(Fe + Mg) ratios in interplanetary dust particles. Furthermore, decomposition of brucite present in the condensed magnesiosilica grains was the source for water and the cause of different iron oxidation states, and the formation of amorphous Fe³⁺‐ferrosilica, amorphous Fe³⁺‐Mg, Fe‐silicates, and magnesioferrite during thermal annealing. Fayalite and ferrosilite that formed from silica/FeO melts reacted with forsterite and enstatite to form Mg, Fe‐silicates. The presence of iron in different oxidation states in extraterrestrial materials almost certainly requires active asteroid‐like parent bodies. If so, the possible presence of trivalent Fe compounds in comet P/Halley suggests that Halley‐type comets are a mixture of preserved presolar and processed solar nebula dust. The results from this thermal annealing experiment further suggest that the Fe‐silicates detected in the impact‐induced ejecta from comet 9P/Temple 1 might be of secondary origin and related to the impact experiment or to processing in a regolith.     

The most primitive mesosiderite Northwest Africa 1878, subgroup 0

All mesosiderites previously reported were subjected to thermal metamorphism and/or partial melting on the parent body. Therefore, their primordial features have been mostly lost. Here, we report detailed petrological and mineralogical features on a mesosiderite, Northwest Africa (NWA) 1878. This meteorite comprises silicate lithology and aggregates of small spheroidal Fe‐Ni metal grains. Silicate lithology typically shows igneous texture without recrystallization features, and mainly consists of low‐Ca pyroxene and plagioclase. Pyroxenes often show normal zoning. Exsolution lamella of augite is rarely noticed and very thin in width, compared with other mesosiderites. A few magnesian olivine grains are encountered without typical corona texture around them. They are not equilibrated with pyroxene on a large scale. Plagioclase shows a wide compositional range. These results show that NWA 1878 hardly experienced thermal metamorphism, distinguished from mesosiderites of subgroups 1–4. Therefore, we propose that this is classified as subgroup 0 mesosiderite. Nevertheless, NWA 1878 was locally subjected to secondary reactions, such as weak reduction of pyroxene and Fe‐Mg diffusion between olivine and pyroxene, on the parent body.     

Fine‐grained material encased in microtracks of Stardust samples

Dust from comet 81P/Wild 2 was captured at high speed in silica aerogel collectors during the Stardust mission. Studies of deceleration tracks in aerogel showed that a number of cometary particles were poorly cohesive and fragmented during impact. Fragments are now scattered along the walls of impact cavities. Here, we report a transmission electron microscope study of a piece of aerogel extracted from the wall of track 10. We focused on micron‐sized secondary tracks along which fragments of a fine‐grained material are disseminated. Two populations of fragments were identified. The first is made of polycrystalline silicate assemblages (olivine, pyroxene, and spinel) that appear to be chemically related to each other. The second corresponds to silica‐rich glassy clumps characteristic of a mixture of melted cometary material and aerogel. A significant number of fragments have been found with a composition close to chondritic CI for the major elements Fe‐Mg‐S at a submicron scale. These fragments have thus never been chemically differentiated by high‐temperature processes prior to the accretion on the comet, in contrast to terminal particles, which are dominated by larger, denser, and frequently monomineralic components.     

Spade: An H chondrite impact‐melt breccia that experienced post‐shock annealing

Abstract— The low modal abundances of relict chondrules (1.8 vol%) and of coarse (i.e., ≥200 μm‐size) isolated mafic silicate grains (1.8 vol%) in Spade relative to mean H6 chondrites (11.4 and 9.8 vol%, respectively) show Spade to be a rock that has experienced a significant degree of melting. Various petrographic features (e.g., chromite‐plagioclase assemblages, chromite veinlets, silicate darkening) indicate that melting was caused by shock. Plagioclase was melted during the shock event and flowed so that it partially to completely surrounded nearby mafic silicate grains. During crystallization, plagioclase developed igneous zoning. Low‐Ca pyroxene that crystallized from the melt (or equilibrated with the melt at high temperatures) acquired relatively high amounts of CaO. Metallic Fe‐Ni cooled rapidly below the Fe‐Ni solvus and transformed into martensite. Subsequent reheating of the rock caused transformation of martensite into abundant duplex plessite. Ambiguities exist in the shock stage assignment of Spade. The extensive silicate darkening, the occurrence of chromite‐plagioclase assemblages, and the impact‐melted characteristics of Spade are consistent with shock stage S6. Low shock (stage S2) is indicated by the undulose extinction and lack of planar fractures in olivine. This suggests that Spade reached a maximum prior shock level equivalent to stage S6 and then experienced post‐shock annealing (probably to stage S1). These events were followed by a less intense impact that produced the undulose extinction in the olivine, characteristic of shock stage S2. Annealing could have occurred if Spade were emplaced near impact melts beneath the crater floor or deposited in close proximity to hot debris within an ejecta blanket. Spade firmly establishes the case for post‐shock annealing. This may have been a common process on OC asteroids.     

The Fukang pallasite: Characterization and implications for the history of the Main‐group parent body

We report the results of a study of the Fukang pallasite that includes measurements of bulk composition, mineral chemistry, mineral structure, and petrology. Fukang is a Main‐group pallasite that consists of semiangular olivine grains (Fo 86.3) embedded in an Fe‐Ni matrix with 9–10 wt% Ni and low‐Ir (45 ppb). Olivine grains sometimes occur in large clusters up to 11 cm across. The Fe‐Ni phase is primarily kamacite with accessory taenite and plessite. Minor phases include schreibersite, chromite, merrillite, troilite, and low‐Ca pyroxene. We describe a variety of silicate inclusions enclosed in olivine that contain phases rarely or not previously reported in Main‐group pallasites, including clinopyroxene (augite), tridymite, K‐rich felsic glass, and an unknown Ca‐Cr silicate. Pressure constraints determined from tridymite (<0.4 GPa), two‐pyroxene barometry (0.39 ± 0.07 GPa), and geophysical calculations that assume pallasite formation at the core–mantle boundary (CMB), provide an upper estimate on the size of the Main‐group parent body from which Fukang originated. We conclude that Fukang originated at the CMB of a large differentiated planetesimal 400–680 km in radius.     

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.     

Analysis of ordinary chondrites using powder X‐ray diffraction: 2. Applications to ordinary chondrite parent‐body processes

Abstract– We evaluate the chemical and physical conditions of metamorphism in ordinary chondrite parent bodies using X‐ray diffraction (XRD)‐measured modal mineral abundances and geochemical analyses of 48 type 4–6 ordinary chondrites. Several observations indicate that oxidation may have occurred during progressive metamorphism of equilibrated chondrites, including systematic changes with petrologic type in XRD‐derived olivine and low‐Ca pyroxene abundances, increasing ratios of MgO/(MgO+FeO) in olivine and pyroxene, mean Ni/Fe and Co/Fe ratios in bulk metal with increasing metamorphic grade, and linear Fe addition trends in molar Fe/Mn and Fe/Mg plots. An aqueous fluid, likely incorporated as hydrous silicates and distributed homogeneously throughout the parent body, was responsible for oxidation. Based on mass balance calculations, a minimum of 0.3–0.4 wt% H₂O reacted with metal to produce oxidized Fe. Prior to oxidation the parent body underwent a period of reduction, as evidenced by the unequilibrated chondrites. Unlike olivine and pyroxene, average plagioclase abundances do not show any systematic changes with increasing petrologic type. Based on this observation and a comparison of modal and normative plagioclase abundances, we suggest that plagioclase completely crystallized from glass by type 4 temperature conditions in the H and L chondrites and by type 5 in the LL chondrites. Because the validity of using the plagioclase thermometer to determine peak temperatures rests on the assumption that plagioclase continued to crystallize through type 6 conditions, we suggest that temperatures calculated using pyroxene goethermometry provide more accurate estimates of the peak temperatures reached in ordinary chondrite parent bodies.     

Heavily metamorphosed clasts from the CV chondrite breccias Mokoia and Yamato‐86009

Abstract– Metamorphosed clasts in the CV carbonaceous chondrite breccias Mokoia and Yamato‐86009 (Y‐86009) are coarse‐grained, granular, polymineralic rocks composed of Ca‐bearing (up to 0.6 wt% CaO) ferroan olivine (Fa_34–39), ferroan Al‐diopside (Fs_9–13Wo_47–50, approximately 2–7 wt% Al₂O₃), plagioclase (An_37–84Ab_63–17), Cr‐spinel (Cr/(Cr + Al) = 0.19–0.45, Fe/(Fe + Mg) = 0.60–0.79), nepheline, pyrrhotite, pentlandite, Ca‐phosphate, and rare grains of Ni‐rich taenite; low‐Ca pyroxene is absent. Most clasts have triple junctions between silicate grains, indicative of prolonged thermal annealing. Based on the olivine‐spinel and pyroxene thermometry, the estimated metamorphic temperature recorded by the clasts is approximately 1100 K. Few clasts experienced thermal metamorphism to a lower degree and preserved chondrule‐like textures. The Mokoia and Y‐86009 clasts are mineralogically unique and different from metamorphosed chondrites of known groups (H, L, LL, R, EH, EL, CO, CK) and primitive achondrites (acapulcoites, brachinites, lodranites). On a three‐isotope oxygen diagram, compositions of olivine in the clasts plot along carbonaceous chondrite anhydrous mineral line and the Allende mass‐fractionation line, and overlap with those of the CV chondrule olivines; the Δ¹⁷O values of the clasts range from about ?4.3‰ to ?3.0‰. We suggest that the clasts represent fragments of the CV‐like material that experienced metasomatic alteration, high‐temperature metamorphism, and possibly melting in the interior of the CV parent asteroid. The lack of low‐Ca pyroxene in the clasts could be due to its replacement by ferroan olivine during iron‐alkali metasomatic alteration or by high‐Ca ferroan pyroxene during melting under oxidizing conditions.     

A TEM study of four particles extracted from the Stardust track 80

Abstract— Four particles extracted from track 80 at different penetration depths have been studied by analytical transmission electron microscopy (ATEM). Regardless of their positions within the track, the samples present a comparable microstructure made of a silica rich glassy matrix embedding a large number of small Fe‐Ni‐S inclusions and vesicles. This microstructure is typical of strongly thermally modified particles that were heated and melted during the hypervelocity impact into the aerogel. X‐ray intensity maps show that the particles were made of Mg‐rich silicates (typically 200 nm in diameter) cemented by a fine‐grained matrix enriched in iron sulfide. Bulk compositions of the four particles suggest that the captured dust particle was an aggregate of grains with various iron sulfide fraction and that no extending chemical mixing in the bulb occurred during the deceleration. The bulk S/Fe ratios of the four samples are close to CI and far from the chondritic meteorites from the asteroidal belt, suggesting that the studied particles are compatible with chondritic‐porous interplanetary dust particles or with material coming from a large heliocentric distance for escaping the S depletion.