Camel Donga: A Eucrite with High Metal Content

Abstract— The Camel Donga eucrite is unique since it contains about 2% metallic iron. Chemical composition, mineralogy, and noble gas contents of Camel Donga are, however, indistinguishable from other eucrites. The bulk iron content and the composition of the metal show that the Camel Donga metal was formed by reduction of FeO and FeS, probably through loss of S₂ and SO₂. Since coarse- and fine-grained lithologies are affected it is likely that the reduction occurred after brecciation. The inhomogeneous distribution of metal suggests that reduction is limited to areas where sulfides were present. The fact that major parts of the meteorite still contain sulfides, in many cases associated with metal, indicates that the reduction process did not go to completion. A comparatively short heating period is suggested, perhaps in an ejecta blanket.     

The South African polymict eucrite Macibini

Abstract— The polymict eucrite Macibini is a fragmental breccia, predominantly composed of eucritic materials with minor proportions (maximum 2 vol%) of diogenitic material. Hence, it is intermediate between the Yamato‐74159‐type polymict eucrites, which contain negligible amounts of magnesian orthopyroxene, and the howardites. The present study provides mineralogical and bulk compositional data for the meteorite breccia and for six clasts. These clasts include both volcanic and igneous rocks and a variety of impact‐generated rocks. A broad range of degrees of postcrystallization metamorphism affected these materials before the final aggregation of the breccia. Clast A is a fragment of unequilibrated eucrite with subophitic texture. The edges of the zoned pyroxenes in this clast are composed of a host of Fe‐rich augite containing vermicules (blebs) and lamellae composed of a mixture of Fe‐rich olivine and silica. Similar features occur as fragments in lunar breccias and are attributed by some workers to the breakdown of pyroxferroite, an Fe‐rich pyroxenoid. However, textures and compositions of these augite‐olivine‐silica intergrowths in clast A suggest that, in this case, they are the result of decomposition in a series of steps of Fe‐rich subcalcic augite. Among the fragments of impact‐generated material in Macibini is clast 2, an earlier‐formed clastic breccia that was lithified before being broken apart and included in the meteorite breccia. Clast 3 is an impact‐melt breccia that is composed of rock and mineral fragments in a devitrified groundmass. Clast C is also an impact‐melt breccia that has a coarser‐grained, hornfelsic groundmass that resulted from extensive metamorphism after formation.     

Ca‐Al‐Fe‐rich inclusion in the Vigarano CV3 chondrite

Abstract– An anomalous Ca‐Al‐Fe‐rich spherical inclusion (CAFI) was found in the Vigarano CV3 chondrite. The CAFI has an igneous texture and contains large amounts of almost pure and coarse‐grained hercynite grains (approximately 56 vol%) as well as refractory phases such as grossite and perovskite. However, melilite and Mg‐spinel, which are common in ordinary Ca‐Al‐rich inclusions, are very rare (<1 vol%). Another unique characteristic of the CAFI is the presence in its core of dmitryivanovite (CaAl₂O₄), which was formed by shock metamorphism of a low‐pressure form of CaAl₂O₄ that was originally crystallized from a molten droplet. The fine‐grained hercynite and unidentified aluminous phase in the rim of the CAFI may have been produced from grossite during aqueous alteration in the Vigarano parent body.     

CAMEL DONGA METEORITE,A NEW EUCRITE FROM THE NULLARBOR PLAIN,WESTERN AUSTRALIA

Twelve stones, ranging up to 504 g (total weight 2.92 kg), of a very fresh eucrite have been collected from a 1 km² area on the Nullarbor Plain in Western Australia. The location is approximately 75 km N6°E from Nurina Siding on the Trans-Australian Railway; coordinates 30°19′S, 126°37′E. This eucrite consists almost entirely of pyroxene (mean composition Wo 16 Fs 49) and plagioclase (mean composition An 85) in approximate proportions 3: 2, with 2% almost pure Fe metal and accessory amounts of troilite, ilmenite, and an SiO₂ phase. Gabbroic and doleritic clasts are present in a groundmass of comminuted pyroxene and plagioclase. The chemical composition (weight per cent) is: SiO₂ 49.53, TiO₂ 0.74, Al₂O₃ 12.49, Cr₂O₃ 0.33, FeO 16.07, MnO 0.56, MgO 6.31, CaO 10.41, Na₂O 0.49, K₂O 0.04, P₂O₅ < 0.01, H₂O + < 0.1, H₂O — 0.07, Fe metal 2.07, Ni < 0.01, Co < 0.01, FeS 0.19, C 0.03, sum 99.33.     

Opaque assemblages in CR2 Graves Nunataks (GRA) 06100 as indicators of shock‐driven hydrothermal alteration in the CR chondrite parent body

We have studied the petrologic characteristics of sulfide‐metal lodes, polymineralic Fe‐Ni nodules, and opaque assemblages in the CR2 chondrite Graves Nunataks (GRA) 06100, one of the most altered CR chondrites. Unlike low petrologic type CR chondrites, alteration of metal appears to have played a central role in the formation of secondary minerals in GRA 06100. Differences in the mineralogy and chemical compositions of materials in GRA 06100 suggest that it experienced higher temperatures than other CR2 chondrites. Mineralogic features indicative of high temperature include: (1) exsolution of Ni‐poor and Ni‐rich metal from nebular kamacite; (2) formation of sulfides, oxides, and phosphates; (3) changes in the Co/Ni ratios; and (4) carbidization of Fe‐Ni metal. The conspicuous absence of pentlandite may indicate that peak temperatures exceeded 600 °C. Opaques appear to have been affected by the action of aqueous fluids that resulted in the formation of abundant oxides, Fe‐rich carbonates, including endmember ankerite, and the sulfide‐silicate‐phosphate scorzalite. We suggest that these materials formed via impact‐driven metamorphism. Mineralogic features indicative of impact metamorphism include (1) the presence of sulfide‐metal lodes; (2) the abundance of polymineralic opaque assemblages with mosaic‐like textures; and (3) the presence of suessite. Initial shock metamorphism probably resulted in replacement of nebular Fe‐Ni metal in chondrules and in matrix by Ni‐rich, Co‐rich Fe metal, Al‐Ti‐Cr‐rich alloys, and Fe sulfides, while subsequent hydrothermal alteration produced accessory oxides, phosphates, and Fe carbonates. An extensive network of sulfide‐metal veins permitted effective exchange of siderophile elements from pre‐existing metal nodules with adjacent chondrules and matrix, resulting in unusually high Fe contents in these objects.     

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.     

A relict‐grain‐bearing porphyritic olivine compound chondrule from LL3.0 Semarkona that experienced limited remelting

Abstract— Chondrule D8n in LL3.0 Semarkona is a porphyritic olivine (PO) chondrule, 1300 times 1900 μm in size, with a complicated thermal history. The oldest recognizable portion of D8n is a moderately high‐FeO, PO chondrule that is modeled as having become enmeshed in a dust ball containing a small, intact, low‐FeO porphyritic chondrule and fine‐grained material consisting of forsterite, kamacite, troilite, and possibly reduced C. The final chondrule melting event may have been a heat pulse that preferentially melted the low‐FeO material and produced a low‐FeO, opaque‐rich, exterior region, 45–140 μm in thickness, around the original chondrule. At one end of the exterior region, a kamacite‐ and troilite‐rich lump 960 μm in length formed. During the final melting event, the coarse, moderately ferroan olivine phenocrysts within the original chondrule appear to have been partly resorbed (These relict phenocrysts have the highest concentrations of FeO, MnO, and Cr₂O₃—7.5, 0.20, and 0.61 wt%, respectively—in D8n.). Narrow olivine overgrowths crystallized around the phenocrysts following final chondrule melting; their compositions seem to reflect mixing between melt derived from the exterior region and the resorbed margins of the phenocrysts. During the melting event, FeO in the relict phenocrysts was reduced, producing numerous small blebs of Ni‐poor metallic Fe along preexisting curvilinear fractures. The reduced olivine flanking the trails of metal blebs has lower FeO than the phenocrysts but virtually identical MnO and Cr₂O₃ contents. Subsequent parent‐body aqueous alteration in the exterior region of the chondrule formed pentlandite and abundant magnetite.     

Petrology and bulk chemistry of Yamato‐82094, a new type of carbonaceous chondrite

Carbonaceous chondrites are classified into several groups. However, some are ungrouped. We studied one such ungrouped chondrite, Y‐82094, previously classified as a CO. In this chondrite, chondrules occupy 78 vol%, and the matrix is distinctly poor in abundance (11 vol%), compared with CO and other C chondrites. The average chondrule size is 0.33 mm, different from that in C chondrites. Although these features are similar to those in ordinary chondrites, Y‐82094 contains 3 vol% Ca‐Al‐rich inclusions and 5% amoeboid olivine aggregates (AOAs). Also, the bulk composition resembles that of CO chondrites, except for the volatile elements, which are highly depleted. The oxygen isotopic composition of Y‐82094 is within the range of CO and CV chondrites. Therefore, Y‐82094 is an ungrouped C chondrite, not similar to any other C chondrite previously reported. Thin FeO‐rich rims on AOA olivine and the mode of occurrence of Ni‐rich metal in the chondrules indicate that Y‐82094 is petrologic type 3.2. The extremely low abundance of type II chondrules and high abundance of Fe‐Ni metal in the chondrules suggest reducing condition during chondrule formation. The depletion of volatile elements indicates that the components formed under high‐temperature conditions, and accreted to the parent body of Y‐82094. Our study suggests a wider range of formation conditions than currently recorded by the major C chondrite groups. Additionally, Y‐82094 may represent a new, previously unsampled, asteroidal body.     

Type 1 aqueous alteration in CM carbonaceous chondrites: Implications for the evolution of water‐rich asteroids

The CM carbonaceous chondrite meteorites experienced aqueous alteration in the early solar system. They range from mildly altered type 2 to almost completely hydrated type 1 chondrites, and offer a record of geochemical conditions on water‐rich asteroids. We show that CM1 chondrites contain abundant (84–91 vol%) phyllosilicate, plus olivine (4–8 vol%), magnetite (2–3 vol%), Fe‐sulfide (<5 vol%), and calcite (<2 vol%). The CM1/2 chondrites contain phyllosilicate (71–88 vol%), olivine (4–20 vol%), enstatite (2–6 vol%), magnetite (2–3 vol%), Fe‐sulfides (1–2 vol%), and calcite (~1 vol%). As aqueous alteration progressed, the abundance of Mg‐serpentine and magnetite in the CM chondrites increased. In contrast, calcite abundances in the CM1/2 and CM1 chondrites are often depleted relative to the CM2s. The modal data support the model, whereby metal and Fe‐rich matrix were the first components to be altered on the CM parent body(ies), before further hydration attacked the coarser Mg‐rich silicates found in chondrules and fragments. Based on the absence of tochilinite, we suggest that CM1 chondrites experienced increased alteration due to elevated temperatures (>120 °C), although higher water/rock ratios may also have played a role. The modal data provide constraints for interpreting the composition of asteroids and the mineralogy of samples returned from these bodies. We predict that “CM1‐like” asteroids, as has been proposed for Bennu—target for the OSIRIS‐REx mission—will have a high abundance of Mg‐rich phyllosilicates and Fe‐oxides, but be depleted in calcite.     

Evidence for K‐rich terranes on Vesta from impact spherules

Abstract— The howardite‐eucrite‐diogenite (HED) clan is a group of meteorites that probably originate from the asteroid Vesta. Some of them are complex breccias that contain impact glasses whose compositions mirror that of their source regions. Some K‐rich impact glasses (up to 2 wt% K₂O) suggest that in addition to basalts and ultramafic cumulates, K‐rich rocks are exposed on Vesta's surface. One K‐rich glass (up to 6 wt% K₂O), with a felsic composition, provides the first evidence of highly differentiated K‐rich rocks on a large asteroid. They can be compared to the rare lunar granites and suggest that magmas generated in a large asteroid are more diverse than previously thought.     

The formation and alteration of the Renazzo‐like carbonaceous chondrites III: Toward understanding the genesis of ferromagnesian chondrules

To better understand the formation conditions of ferromagnesian chondrules from the Renazzo‐like carbonaceous (CR) chondrites, a systematic study of 210 chondrules from 15 CR chondrites was conducted. The texture and composition of silicate and opaque minerals from each observed FeO‐rich (type II) chondrule, and a representative number of FeO‐poor (type I) chondrules, were studied to build a substantial and self‐consistent data set. The average abundances and standard deviations of Cr₂O₃ in FeO‐rich olivine phenocrysts are consistent with previous work that the CR chondrites are among the least thermally altered samples from the early solar system. Type II chondrules from the CR chondrites formed under highly variable conditions (e.g., precursor composition, redox conditions, cooling rate), with each chondrule recording a distinct igneous history. The opaque minerals within type II chondrules are consistent with formation during chondrule melting and cooling, starting as S‐ and Ni‐rich liquids at 988–1350 °C, then cooling to form monosulfide solid solution (mss) that crystallized around olivine/pyroxene phenocrysts. During cooling, Fe,Ni‐metal crystallized from the S‐ and Ni‐rich liquid, and upon further cooling mss decomposed into pentlandite and pyrrhotite, with pentlandite exsolving from mss at 400–600 °C. The composition, texture, and inferred formation temperature of pentlandite within chondrules studied here is inconsistent with formation via aqueous alteration. However, some opaque minerals (Fe,Ni‐metal versus magnetite and panethite) present in type II chondrules are a proxy for the degree of whole‐rock aqueous alteration. The texture and composition of sulfide‐bearing opaque minerals in Graves Nunataks 06100 and Grosvenor Mountains 03116 suggest that they are the most thermally altered CR chondrites.     

Ferrous silicate spherules with euhedral iron‐nickel metal grains from CH carbonaceous chondrites: Evidence for supercooling and condensation under oxidizing conditions

Abstract— The CH carbonaceous chondrites contain a population of ferrous (Fe/(Fe + Mg) ? 0.1‐0.4) silicate spherules (chondrules), about 15–30 μm in apparent diameter, composed of cryptocrystalline olivinepyroxene normative material, ±SiO₂‐rich glass, and rounded‐to‐euhedral Fe, Ni metal grains. The silicate portions of the spherules are highly depleted in refractory lithophile elements (CaO, Al₂O₃, and TiO₂ <0.04 wt%) and enriched in FeO, MnO, Cr₂O₃, and Na₂O relative to the dominant, volatile‐poor, magnesian chondrules from CH chondrites. The Fe/(Fe + Mg) ratio in the silicate portions of the spherules is positively correlated with Fe concentration in metal grains, which suggests that this correlation is not due to oxidation, reduction, or both of iron (FeO_sil ? Fe_met) during melting of metal‐silicate solid precursors. Rather, we suggest that this is a condensation signature of the precursors formed under oxidizing conditions. Each metal grain is compositionally uniform, but there are significant intergrain compositional variations: about 8–18 wt% Ni, <0.09 wt% Cr, and a sub‐solar Co/Ni ratio. The precursor materials of these spherules were thus characterized by extreme elemental fractionations, which have not been observed in chondritic materials before. Particularly striking is the fractionation of Ni and Co in the rounded‐to‐euhedral metal grains, which has resulted in a Co/Ni ratio significantly below solar. The liquidus temperatures of the euhedral Fe, Ni metal grains are lower than those of the coexisting ferrous silicates, and we infer that the former crystallized in supercooled silicate melts. The metal grains are compositionally metastable; they are not decomposed into taenite and kamacite, which suggests fast postcrystallization cooling at temperatures below 970 K and lack of subsequent prolonged thermal metamorphism at temperatures above 400–500 K.     

Mineralogy of fine‐grained material in the Krymka (LL3.1) chondrite

Abstract— Two dark lithic fragments and matrix of the Krymka LL3.1 chondrite were mineralogically and chemically studied in detail. These objects are characterised by the following chemical and mineralogical characteristics, which distinguish them from the host chondrite Krymka: (1) bulk chemical analyses revealed low totals (systematically lower than 94 wt%) due to high porosity; (2) enrichment in FeO and depletion in S, MgO and SiO₂ due to a high abundance of Fe‐rich silicates and low sulfide abundance; (3) fine‐grained, almost chondrule‐free texture with predominance of a porous, cryptocrystalline groundmass and fine grains; (4) occurrence of a small amount of once‐molten material (microchondrules) enclosed in fine‐grained materials; (5) occurrence of accretionary features, especially unique accretionary spherules; (6) high abundance of small calcium‐ aluminium‐rich inclusions (CAIs) in one of the fine‐grained fragments. It is suggested that the abundance of CAIs in this fragment is one of the highest ever found in an ordinary chondrite. Accretionary, fine‐grained spherules within one of the fragments bear fundamental information about the initial stages of accretion as well as on the evolution of the clast, its incorporation, and history within the bulk rock of Krymka. The differences in porosity, bulk composition, and mineralogy of cores and rims of the fine‐grained spherulitic objects allow us to speculate on the following processes: (1) Low velocity accretion of tiny silicate grains onto the surface of coarse metal or silicate grains in a dusty region of the nebula is the beginning of the formation of accretionary, porous (fluffy) silicate spherules. (2) Within a dusty environment with decreasing silicate/(metal + sulfide) ratio the porous spherules collected abundant metal and sulfide particles together with silicate dust, which formed an accretionary rim. Variations of the silicate/(sulfide + metal) ratio in the dusty nebular environment result in the formation of multi‐layered rims on the surface of the silicate‐rich spherules. (3) Soft accretion and lithification of rimmed, fluffy spherules, fine‐grained, silicate‐rich dust, metal‐sulfide particles, CAIs, silicate‐rich microchondrules, and coarse silicate grains and fragments followed. (4) After low‐temperature processing of the primary, accretionary rock collisional fragmentation occurred, the fragments were subsequently coated by fine‐grained material, which was highly oxidized and depleted in sulfides. (5) In a final stage this accretionary “dusty” rock was incorporated as a fragment within the Krymka host.     

What’s up? Preservation of gravitational direction in the Larkman Nunatak 06299 LL impact melt breccia

Abstract– Larkman Nunatak (LAR) 06299 is a vesicular LL chondrite impact melt breccia that cooled rapidly (0.1–0.3 °C s^?1) during crystallization. Ar‐Ar data from the literature indicate that the impact event that formed this rock occurred approximately 1 Ga ago. About 30 vol% of the meteorite consists of a melt matrix containing faceted and intergrown mafic silicate grains (mainly 4–11 μm size olivine phenocrysts) partially to completely surrounded by 2–20 μm size patches of plagioclase. Suspended in the melt are 30–370 μm size ellipsoidal to spheroidal metal‐sulfide nodules (several hundred per thin section), many connected to 8–600 μm size ellipsoidal to spheroidal vesicles. Most of the metal‐sulfide nodules contain a large oblate metallic Fe‐Ni bleb at one end of the nodule. For approximately 90% of the nodules, the metal blebs are aligned on the same side of the nodules; for approximately 80% of the nodules that are adjacent to vesicles, the vesicles are attached to the opposite end of the nodules from the oblate metal blebs. Most of the oblate metal blebs themselves are flattened in a direction perpendicular to the long axis of the nodule/vesicle. These features result from alignment in the gravitational field on the LL parent asteroid, making LAR 06299 the first known chondrite to indicate gravitational direction. Using reasonable estimates of the cooling rate, viscosity of the metal‐sulfide melt, and asteroid density, as well as the observed sizes of constituent phases in LAR 06299, we obtain a lower limit of approximately 1.5 km for the radius of the LAR 06299 parent body. The body was probably substantially larger.     

Iron oxides in comet 81P/Wild 2

Abstract– We have used synchrotron Fe‐XANES, XRS, microRaman, and SEM‐TEM analyses of Stardust track 41 slice and track 121 terminal area slices to identify Fe oxide (magnetite‐hematite and amorphous oxide), Fe‐Ti oxide, and V‐rich chromite (Fe‐Cr‐V‐Ti‐Mn oxide) grains ranging in size from 200 nm to ～10 μm. They co‐exist with relict FeNi metal. Both Fe‐XANES and microRaman analyses suggest that the FeNi metal and magnetite (Fe₂O₃FeO) also contain some hematite (Fe₂O₃). The FeNi has been partially oxidized (probably during capture), but on the basis of our experimental work with a light‐gas gun and microRaman analyses, we believe that some of the magnetite‐hematite mixtures may have originated on Wild 2. The terminal samples from track 121 also contain traces of sulfide and Mg‐rich silicate minerals. Our results show an unequilibrated mixture of reduced and oxidized Fe‐bearing minerals in the Wild 2 samples in an analogous way to mineral assemblages seen in carbonaceous chondrites and interplanetary dust particles. The samples contain some evidence for terrestrial contamination, for example, occasional Zn‐bearing grains and amorphous Fe oxide in track 121 for which evidence of a cometary origin is lacking.     

The differentiation of eucrites: The role of in situ crystallization

Abstract— We report on major and trace element analyses of 17 eucrites, including three cumulate eucrites (Binda, Moore County, and Serra de Magé), determined by, respectively, inductively‐coupled plasma atomic emission spectrometry and inductively‐coupled plasma mass spectrometry. The results obtained for Binda and Moore County are consistent with the model of Treiman (1997) for the formation of cumulate eucrites, which holds that these meteorites were produced from a eucritic melt. Our sample of Serra de Magé contains unusually large amounts of pyroxene and probably an accessory phase rich in heavy rare earth elements and is therefore not representative of this eucrite as known from literature data. Our results for the noncumulate eucrites Bereba, Bouvante, Cachari, Caldera, Camel Donga, Ibitira, Jonzac, Juvinas, Lakangaon, Millbillillie, Padvarninkai, Pasamonte, Sioux County, and Stannern are in good agreement with literature data. The observed decoupling between major and trace elements for noncumulate eucrites can be explained by in situ crystallization during the differentiation of an asteroidal magma ocean. This model can further account for both the Nuevo Laredo and the Stannern trends but has as a consequence that none of the analyzed eucrites represents a primary melt.     

The Isheyevo meteorite: Mineralogy,petrology, bulk chemistry,oxygen, nitrogen,carbon isotopic compositions,and 40Ar‐39Ar ages

Abstract— Isheyevo is a metal‐rich carbonaceous chondrite that contains several lithologies with different abundances of Fe,Ni metal (7–90 vol%). The metal‐rich lithologies with 50–60 vol% of Fe,Ni metal are dominant. The metal‐rich and metal‐poor lithologies are most similar to the CB_b and CH carbonaceous chondrites, respectively, providing a potential link between these chondrite groups. All lithologies experienced shock metamorphism of shock stage S4. All consist of similar components—Fe,Ni metal, chondrules, refractory inclusions (Ca, Al‐rich inclusions [CAIs] and amoeboid olivine aggregates [AOAs]), and heavily hydrated lithic clasts—but show differences in their modal abundances, chondrule sizes, and proportions of porphyritic versus non‐porphyritic chondrules. Bulk chemical and oxygen isotopic compositions are in the range of CH and CB chondrites. Bulk nitrogen isotopic composition is highly enriched in ¹⁵N (δ¹⁵N = 1122‰). The magnetic fraction is very similar to the bulk sample in terms of both nitrogen release pattern and isotopic profile; the non‐magnetic fraction contains significantly less heavy N. Carbon released at high temperatures shows a relatively heavy isotope signature. Similarly to CB_b chondrites, ～20% of Fe,Ni‐metal grains in Isheyevo are chemically zoned. Similarly to CH chondrites, some metal grains are Ni‐rich (>20 wt% Ni). In contrast to CB_b and CH chondrites, most metal grains are thermally decomposed into Ni‐rich and Ni‐poor phases. Similar to CH chondrites, chondrules have porphyritic and non‐porphyritic textures and ferromagnesian (type I and II), silica‐rich, and aluminum‐rich bulk compositions. Some of the layered ferromagnesian chondrules are surrounded by ferrous olivine or phyllosilicate rims. Phyllosilicates in chondrule rims are compositionally distinct from those in the hydrated lithic clasts. Similarly to CH chondrites, CAIs are dominated by the hibonite‐, grossite‐, and melilite‐rich types; AOAs are very rare. We infer that Isheyevo is a complex mixture of materials formed by different processes and under different physico‐chemical conditions. Chondrules and refractory inclusions of two populations, metal grains, and heavily hydrated clasts accreted together into the Isheyevo parent asteroid in a region of the protoplanetary disk depleted in fine‐grained dust. Such a scenario is consistent with the presence of solar wind—implanted noble gases in Isheyevo and with its comparatively old K‐Ar age. We cannot exclude that the K‐Ar system was affected by a later collisional event. The cosmic‐ray exposure (CRE) age of Isheyevo determined by cosmogenic ³⁸Ar is ～34 Ma, similar to that of the Bencubbin (CB_a) meteorite.     

Differentiation processes in FeO‐rich asteroids revealed by the achondrite Lewis Cliff 88763

Olivine‐dominated (70–80 modal %) achondrite meteorite Lewis Cliff (LEW) 88763 originated from metamorphism and limited partial melting of a FeO‐rich parent body. The meteorite experienced some alteration on Earth, evident from subchondritic Re/Os, and redistribution of rhenium within the sample. LEW 88763 is texturally similar to winonaites, has a Δ¹⁷O value of ?1.19 ± 0.10‰, and low bulk‐rock Mg/(Mg+Fe) (0.39), similar to the FeO‐rich cumulate achondrite Northwest Africa (NWA) 6693. The similar bulk‐rock major‐, minor‐, and trace‐element abundances of LEW 88763, relative to some carbonaceous chondrites, including ratios of Pd/Os, Pt/Os, Ir/Os, and ¹⁸⁷Os/¹⁸⁸Os (0.1262), implies a FeO‐ and volatile‐rich precursor composition. Lack of fractionation of the rare earth elements, but a factor of approximately two lower highly siderophile element abundances in LEW 88763, compared with chondrites, implies limited loss of Fe‐Ni‐S melts during metamorphism and anatexis. These results support the generation of high Fe/Mg, sulfide, and/or metal‐rich partial melts from FeO‐rich parent bodies during partial melting. In detail, however, LEW 88763 cannot be a parent composition to any other meteorite sample, due to highly limited silicate melt loss (0 to <<5%). As such, LEW 88763 represents the least‐modified FeO‐rich achondrite source composition recognized to date and is distinct from all other meteorites. LEW 88763 should be reclassified as an anomalous achondrite that experienced limited Fe,Ni‐FeS melt loss. Lewis Cliff 88763, combined with a growing collection of FeO‐rich meteorites, such as brachinites, brachinite‐like achondrites, the Graves Nunataks (GRA) 06128/9 meteorites, NWA 6693, and Tafassasset, has important implications for understanding the initiation of planetary differentiation. Specifically, regardless of precursor compositions, partial melting and differentiation processes appear to be similar on asteroidal bodies spanning a range of initial oxidation states and volatile contents.     

Zoning patterns of Fe and V in spinel from a type B Ca‐Al‐rich inclusion: Constraints on subsolidus thermal history

Abstract We obtained two‐dimensional concentration maps for the minor elements Fe and V in 21 spinel crystals in the Allende type B1 inclusion TS‐34 with a 4–5 μm resolution. Locally high concentrations of Fe occur along at least one edge of the spinels and decrease toward the center of the grains. Enrichment in V can also occur along edges or at corners. In general, there is no overall correlation of the Fe and V distributions, but in local regions of two grains, the V and Fe distributions are correlated, strongly suggesting a local source for both elements. In these two grains, opaque assemblages are present that appear to locally control the V distributions. This, coupled with previous work, suggests that prior to alteration, TS‐34 contained V‐rich metal. Oxidation of this metal during alteration can account for the edge/corner V enrichments, but provide only minor FeO contributions, explaining the overall lack of correlation between Fe and V. Most of the FeO appears to have been externally introduced along spinel boundaries during alteration. These alteration phases served as sources for diffusion of FeO into spinel. FeO distributions in spinel lead to a mean attenuation length of ?8 μm and, using literature diffusion coefficients in isothermal and exponential cooling approximations for peak temperatures in the range 600–700°C, this leads to a time scale for calcium‐aluminum‐rich inclusion (CAI) alteration in the range of decades to centuries.     

Petrology and geochemistry of feldspathic impact‐melt breccia Abar al' Uj 012, the first lunar meteorite from Saudi Arabia

Abar al' Uj (AaU) 012 is a clast‐rich, vesicular impact‐melt (IM) breccia, composed of lithic and mineral clasts set in a very fine‐grained and well‐crystallized matrix. It is a typical feldspathic lunar meteorite, most likely originating from the lunar farside. Bulk composition (31.0 wt% Al₂O₃, 3.85 wt% FeO) is close to the mean of feldspathic lunar meteorites and Apollo FAN‐suite rocks. The low concentration of incompatible trace elements (0.39 ppm Th, 0.13 ppm U) reflects the absence of a significant KREEP component. Plagioclase is highly anorthitic with a mean of An_96.9Ab_3.0Or_0.1. Bulk rock Mg# is 63 and molar FeO/MnO is 76. The terrestrial age of the meteorite is 33.4 ± 5.2 kyr. AaU 012 contains a ~1.4 × 1.5 mm² exotic clast different from the lithic clast population which is dominated by clasts of anorthosite breccias. Bulk composition and presence of relatively large vesicles indicate that the clast was most probably formed by an impact into a precursor having nonmare igneous origin most likely related to the rare alkali‐suite rocks. The IM clast is mainly composed of clinopyroxenes, contains a significant amount of cristobalite (9.0 vol%), and has a microcrystalline mesostasis. Although the clast shows similarities in texture and modal mineral abundances with some Apollo pigeonite basalts, it has lower FeO and higher SiO₂ than any mare basalt. It also has higher FeO and lower Al₂O₃ than rocks from the FAN‐ or Mg‐suite. Its lower Mg# (59) compared to Mg‐suite rocks also excludes a relationship with these types of lunar material.