The L3–6 chondritic regolith breccia Northwest Africa (NWA) 869: (I) Petrology,chemistry, oxygen isotopes,and Ar‐Ar age determinations

Abstract– Northwest Africa (NWA) 869 consists of thousands of individual stones with an estimated total weight of about 7 metric tons. It is an L3–6 chondrite and probably represents the largest sample of the rare regolith breccias from the L–chondrite asteroid. It contains unequilibrated and equilibrated chondrite clasts, some of which display shock‐darkening. Impact melt rocks (IMRs), both clast‐free and clast‐poor, are strongly depleted in Fe,Ni metal, and sulfides. An unequilibrated microbreccia, two different light inclusions and two different SiO₂‐bearing objects were found. Although the matrix of this breccia appears partly clastic, it is not a simple mixture of fine‐grained debris formed from the above lithologies, but mainly represents an additional specific lithology of low petrologic type. We speculate that this material stems from a region of the parent body that was only weakly consolidated. One IMR clast and one SiO₂‐bearing object show Δ¹⁷O values similar to bulk NWA 869, suggesting that both are related to the host rock. In contrast, one light inclusion and one IMR clast appear to be unrelated to NWA 869, suggesting that the IMR clast is contaminated with impactor material. ⁴⁰Ar‐³⁹Ar analyses of a type 4 chondrite clast yield a plateau age of 4402 ± 7 Ma, which is interpreted to be the result of impact heating. Other impact events are recorded by an IMR clast at 1790 ± 36 Ma and a shock‐darkened clast at 2216 ± 40 Ma, demonstrating that NWA 869 escaped major reset in the course of the event at approximately 470 Ma that affected many L–chondrites.     

Origin of a metamorphosed lithic clast in CM chondrite Grove Mountains 021536

Abstract– A metamorphosed lithic clast was discovered in the CM chondrite Grove Mountains 021536, which was collected in the Antarctica by the Chinese Antarctic Research Exploration team. The lithic clast is composed mainly of Fe‐rich olivine (Fo₆₂) with minor diopside (Fs_9.7–11.1Wo_48.3–51.6), plagioclase (An_43–46.5), nepheline, merrillite, Al‐rich chromite (21.8 wt% Al₂O₃; 4.43 wt% TiO₂), and pentlandite. Δ¹⁷O values of olivine in the lithic clast vary from ?3.9‰ to ?0.8‰. Mineral compositions and oxygen isotopic compositions of olivine suggest that the lithic clast has an exotic source different from the CM chondrite parent body. The clast could be derived from strong thermal metamorphism of pre‐existing chondrule that has experienced low‐temperature anhydrous alteration. The lithic clast is similar in mineral assemblage and chemistry to a few clasts observed in oxidized CV3 chondrites (Mokoia and Yamato‐86009) and might have been derived from the interior of the primitive CV asteroid. The apparent lack of hydration in the lithic clast indicates that the clast accreted into the CM chondrite after hydration of the CM components.     

The Bencubbin chondrite breccia and its relationship to CR chondrites and the ALH85085 chondrite

Abstract— Bencubbin is an unclassified meteorite breccia which consists mainly of host silicate (～40 vol.%) and host metal (～60%) components. Rare (< 1%) ordinary chondrite clasts and a dark xenolith (formerly called a carbonaceous chondrite clast) are also found. A petrologic study of the host silicates shows that they have textures, modes, mineralogy and bulk compositions that are essentially the same as that of barred olivine (BO) chondrules, and they are considered to be BO chondritic material. Bulk compositions of individual host silicate clasts are identical and differ only in their textures which are a continuum from coarsely barred, to finely barred, to feathery microcrystalline; these result from differing cooling rates. The host silicates differ from average BO chondrules only in being angular clasts rather than fluid droplet-shaped objects, and in being larger in size (up to 1 cm) than most chondrules; but large angular to droplet-shaped chondrules occur in many chondrites. Bencubbin host metallic FeNi clasts have a positive Ni-Co trend, which coincides with that of a calculated equilibrium nebular condensation path. This appears to indicate a chondritic, rather than impact, origin for this component as well. The rare ordinary chondrite clast and dark xenolith also contain FeNi metal with compositions similar to that of the host metal. Two scenarios are offered for the origin of the Bencubbin breccia. One is that the Bencubbin components are chondritic and were produced in the solar nebula. Later brecciation, reaggregation and minor melting of the chondritic material resulted in it becoming a monomict chondritic breccia. The alternative scenario is that the Bencubbin components formed as a result of major impact melting on a chondritic parent body; the silicate fragments were formed from an impact-induced lava flow and are analogous to the spinifex-textured rocks characteristic of terrestrial komatiites. Both scenarios have difficulties, but the petrologic, chemical and isotopic data are more consistent with Bencubbin being a brecciated chondrite. Bencubbin has a number of important chemical and isotopic characteristics in common with the major components in the CR (Renazzo-type) chondrites and the unique ALH85085 chondrite, which suggests that their major components may be related. These include: (1) Mafic silicates that are similarly Mg-rich and formed in similar reducing environments. (2) Similarly low volatiles; TiO₂, Al₂O₃ and Cr₂O₃ contents are also similar. (3) Similar metallic FeNi compositions that sharply differ from those in other chondrites. (4) Remarkable enrichments in ¹⁵N. (5) Similar oxygen isotopic compositions that lie on the same mixing line. Thus, the major components of the Bencubbin breccia are highly similar to those of the ALH85085 and CR chondrites and they may have all formed in the same isotopic reservoir, under similar conditions, in the CR region of the solar nebula.     

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.     

The iodine‐xenon system in clasts and chondrules from ordinary chondrites: Implications for early solar system chronology

Abstract— We have studied the I‐Xe system in chondrules and clasts from ordinary chondrites. Cristobalite‐bearing clasts from Parnallee (LL3.6) closed to Xe loss 1–4 Ma after Bjurböle. Feline (a feldspar‐ and nepheline‐rich clast also from Parnallee) closed at 7.04 ± 0.15 Ma. Two out of three chondrules from Parnallee that yielded well‐defined initial I ratios gave ages identical to Bjurböle's within error. A clast from Barwell (L6) has a well‐defined initial I ratio corresponding to closure 3.62 ± 0.60 Ma before Bjurböle. Partial disturbance and complete obliteration of the I‐Xe system by shock are revealed in clasts from Julesburg (L3.6) and Quenggouk (H4), respectively. Partial disturbance by shock is capable of generating anomalously high initial I ratios. In some cases, these could be misinterpreted, yielding erroneous ages. A macrochondrule from Isoulane‐n‐Amahar contains concentrations of I similar to “ordinary” chondrules but, unlike most ordinary chondrules, contains no radiogenic ¹²⁹Xe. This requires resetting 50 Ma or more later than most chondrules. The earliest chondrule ages in the I‐Xe, Mn‐Cr, and Al‐Mg systems are in reasonable agreement. This, and the frequent lack of evidence for metamorphism capable of resetting the I‐Xe chronometer, leads us to conclude that (at least) the earliest chondrule I‐Xe ages represent formation. If so, chondrule formation took place at a time when sizeable parent bodies were present in the solar system.     

40Ar/39Ar ages of L4, H5, EL6, and feldspathic ureilitic clasts from the Almahata Sitta polymict ureilite (asteroid 2008 TC3)

The Almahata Sitta (AhS) meteorite consists of disaggregated clasts from the impact of the polymict asteroid 2008 TC₃, including ureilitic (70%–80%) and diverse non-ureilitic materials. We determined the ⁴⁰Ar/³⁹Ar release patterns for 16 AhS samples (3–1500 μg) taken from three chondritic clasts, AhS 100 (L4), AhS 25 (H5), and MS-D (EL6), as well as a clast of ureilitic trachyandesite MS-MU-011, also known as ALM-A, which is probably a sample of the crust of the ureilite parent body (UPB). Based on our analyses, best estimates of the ⁴⁰Ar/³⁹Ar ages (Ma) of the chondritic clasts are 4535 ± 10 (L4), 4537–4555 with a younger age preferred (H5), and 4513 ± 17 (EL6). The ages for the L4 and the H5 clasts are older than the most published ⁴⁰Ar/³⁹Ar ages for L4 and H5 meteorites, respectively. The age for the EL6 clast is typical of older EL6 chondrites. These ages indicate times of argon closure ranging up to 50 Ma after the main constituents of the host breccia, that is, the ureilitic components of AhS, reached the >800°C blocking temperatures of pyroxene and olivine thermometers. We suggest that these ages record the times at which the clasts cooled to the Ar closure temperatures on their respective parent bodies. This interpretation is consistent with the recent proposal that the majority of xenolithic materials in polymict ureilites were implanted into regolith 40–60 Ma after calcium–aluminum-rich inclusion and is consistent with the interpretation that 2008 TC₃ was a polymict ureilite. With allowance for its 10-Ma uncertainty, the 4549-Ma ⁴⁰Ar/³⁹Ar age of ALM-A is consistent with closure within a few Ma of the time recorded by its Pb/Pb age either on the UPB or as part of a rapidly cooling fragment. Plots of age versus cumulative ³⁹Ar release for 10 of 15 samples with ≥5 heating steps indicate minor losses of ⁴⁰Ar over the last 4.5 Ga. The other five such samples lost some ⁴⁰Ar at estimated times no earlier than 3800–4500 Ma bp . Clustering of ages in the low-temperature data for these five samples suggests that an impact caused localized heating of the AhS progenitor ~2.7 Ga ago. In agreement with the published work, 10 estimates of cosmic-ray exposure ages based on ³⁸Ar concentrations average 17 ± 5 Ma but may include some early irradiation.     

Mineralogy,petrology and geochemistry of carbonaceous chondritic clasts in the LEW 85300 polymict eucrite

Abstract— We have performed a detailed petrologic and mineralogic study of two chondritic clasts from the polymict eucrite Lewis Cliff (LEW) 85300, and performed chemical analyses by INAA and RNAA on one of these. Petrologically, the clasts are identical and are composed of dispersed aggregates, chondrules and chondrule fragments supported by matrix. The aggregates and chondrules are composed of olivine (Fo_100–45), orthopyroxene (Wo_1–2En_98–60), plus some diopside. The matrix consists of fine-grained olivine (Fo_60–53), and lesser orthopyroxene and augite. Fine-grained saponite is common in the matrix. The bulk major element composition of the matrix is identical in both clasts and similar to that of CM, CO and CV chondrites. The bulk composition of the clast studied by INAA and RNAA shows unusual abundance patterns for lithophile, siderophile and chalcophile elements but is basically chondritic. The INAA/RNAA data preclude assignment of the LEW 85300,15 clast to any commonly accepted group of carbonaceous chondrite. The unusual rare earth element abundance pattern may, in part, be due to terrestrial alteration.     

Fractionated sulfur isotopes in sulfides of the Kaidun meteorite

Abstract— The Kaidun meteorite contains carbonaceous chondrite (CM1) clasts that have been highly altered by reactions with hydrothermal fluids. Pyrrhotite in these clasts occurs as unusual needles wrapped by sheaths of phyllosilicate, and pentlandite forms veins that crosscut aggregates of phyllosilicate and garnet but not pyrrhotite. The isotopic compositions of S (δ³⁴S_CDT) in individual sulfide grains, measured by ion micro-probe, are fractionated compared to troilite in ordinary chondrites. The S in Kaidun sulfides is isotopically light (as much as ?4.2% for pyrrhotite and ?5.7%0 for pentlandite), unlike sulfides in other carbonaceous chondrites, which are enriched in ³⁴S. The unusual S-isotopic composition of these texturally unique sulfides supports the hypothesis that Kaidun CM1 clasts were pervasively altered under extreme thermal conditions, possibly by fluids that had lost isotopically heavy SO₂.     

Bo Xian (LL3.9): Oxygen‐isotopic and mineralogical characterisation of separated chondrules

Abstract— We report the results of a mineralogical and O‐isotopic study of 362 chondrules disaggregated from the Bo Xian chondrite. The range of mineral compositions (Fa = 0.8–31.2%, mean = 23.5%, mode = 27–28%) are consistent with a reclassification of this meteorite from LL4 to LL3.9. Chondrule diameters range from 0.20 to 3.40 mm (mean = 0.74 mm) in the disaggregated population. A lower mean diameter (0.64 mm) calculated from thin‐section measurements partly reflects the high proportion of chondrule fragments. The chondrule size distribution, which is approximately log‐normal, is consistent with size‐sorting mechanisms. This sorting could be linked to the fragmentation of many chondrules on the parent body. However, in detail, the variation in diameter of different chondrule types and a hiatus in the size distribution at 0.6 mm indicate that there may have been complex controls perhaps partly being determined by the chondrule formation mechanism. Seven percent of the sectioned chondrules (102) contain chemically fractionated mineral assemblages: cristobalite‐bearing and Al‐rich. This significant degree of chemical heterogeneity probably resulted from both igneous and volatility controls. Oxygen‐isotopic compositions were determined on mineral separates and 16 of the sectioned chondrules. Three separate isotopic exchange events have been identified. The dominant one is a low‐temperature hydrous gas‐solid exchange event between ¹⁶O‐rich solid and ¹⁶O‐poor gas reservoirs that lay along a slope 1.0 line on three‐isotope plots. Partial equilibration with the gas by feldspar and cristobalite, which exchanged more rapidly than olivine or pyroxene, led to formation of a slope 0.77 mixing line for Bo Xian and other LL chondrites. Mineralogy is the dominant control on the extent of this exchange; no relationship between isotopic composition and chondrule texture or size was identified. The feldspar separate and cristobalite‐rich chondrules have the most ¹⁶O‐poor compositions. Subsequently, thermal metamorphism in the parent body led to partial isotopic equilibration between the different mineral phases. A third exchange event, predating the other two events, is probably shown by one of the Al‐rich chondrules. This has an ¹⁶O‐rich composition, lying below the terrestrial fractionation line. Another Al‐rich chondrule has a normal ordinary chondrite isotopic composition. It is not clear whether the isotopic fractionation recorded in some Al‐rich chondrules can be achieved by the dominant gas‐solid exchange. Instead, the precursor O to the mineral phases may have become ¹⁶O‐rich during an earlier phase of mass‐independent fractionation.     

A clast of Bali‐like oxidized CV material in the reduced CV chondrite breccia Vigarano

Abstract— The CV (Vigarano‐type) chondrites are a petrologically diverse group of meteorites that are divided into the reduced and the Bali‐like and Allende‐like oxidized subgroups largely based on secondary mineralogy (Weisberg et al., 1997; Krot et al., 1998b). Some chondrules and calcium‐aluminum‐rich inclusions (CAIs) in the reduced CV chondrite Vigarano show alteration features similar to those in Allende: metal is oxidized to magnetite; low‐Ca pyroxene, forsterite, and magnetite are rimmed and veined by ferrous olivine (Fs_40–50); and plagioclase mesostases and melilite are replaced by nepheline and sodalite (Sylvester et al., 1993; Kimura and Ikeda, 1996, 1997, 1998). Our petrographic observations indicate that Vigarano also contains individual chondrules, chondrule fragments, and lithic clasts of the Bali‐like oxidized CV materials. The largest lithic clast (about 1 times 2 cm in size) is composed of opaque matrix, type‐I chondrules (400–2000 μm in apparent diameter) surrounded by coarse‐grained and fine‐grained rims, and rare CAIs. The matrix‐chondrule ratio is about 1.1. Opaque nodules in chondrules in the clast consist of Cr‐poor and Cr‐rich magnetite, Ni‐ and Co‐rich metal, Ni‐poor and Ni‐rich sulfide; low‐Ni metal nodules occur only inside chondrule phenocrysts. Chromium‐poor magnetite is preferentially replaced by fayalite. Chondrule mesostases are replaced by phyllosilicates; low‐Ca pyroxene and olivine phenocrysts appear to be unaltered. Matrix in the clast consists of very fine‐grained (<1 μm) ferrous olivine, anhedral fayalite grains (Fa_80–100), rounded objects of porous Ca‐Fe‐rich pyroxenes (Fs_10–50Wo₅₀), Ni‐poor sulfide, Ni‐ and Co‐rich metal, and phyllosilicates; magnetite is rare. On the basis of the presence of the Bali‐like lithified chondritic clast—in addition to individual chondrules and CAIs of both Bali‐like and Allende‐like materials—in the reduced CV chondrite Vigarano, we infer that (1) all three types of materials were mixed during regolith gardening on the CV asteroidal body, and (2) the reduced and oxidized CV materials may have originated from a single, heterogeneously altered asteroid.     

Petrology of a nonindigenous microgranitic clast in polymict ureilite EET 87720: Evidence for formation of evolved melt on an unknown parent body

EET 87720 is a polymict ureilite breccia known to contain numerous nonindigenous fragments. We have discovered a microgranitic clast in an interior chip of Elephant Moraine (EET) 87720. The clast consists of a granophyre‐like intergrowth of a pure SiO₂ phase (tridymite) and albite, mantling a zoned oligoclase phenocryst. In the intergrowth, the tridymite occurs as thin elongate vermicular blebs within larger albite crystals. The granophyre‐like intergrowth and the oligoclase phenocryst share a common margin, suggesting that the clast was originally part of a larger fragment. An estimate of its bulk composition is equivalent to that of granite (77 wt% SiO₂). Patches of high‐Si K‐bearing glass occur interstitially within the clast; they have high concentrations of SO₃ (11–12 wt%) and contain Cl (0.6 wt%), suggesting that the clast formed on a volatile‐rich parent body perhaps resembling early Mars. The mean oxygen isotope composition of the feldspar and tridymite in the clast is very different from the oxygen isotope compositions of ureilites, and is similar to those of silicate inclusions in IIE and IVA irons. Thus, the clast is not indigenous to the ureilite parent body, but it provides evidence for the formation of evolved melts on an unknown parent body in the early solar system.     

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.     

THE ATLANTA ENSTATITE CHONDRITE BRECCIA

Atlanta is the fifth known brecciated enstatite chondrite. It contains a centimeter-sized troilite-rich clast, similar to those that occur in Blithfield. All of these clasts probably formed in the solar nebula under high pS₂/pO₂ conditions in a gas of non-cosmic composition. The absence of ordinary or carbonaceous chondrite clasts in any of the enstatite chondrite breccias and absence of enstatite chondrite clasts or materials formed at high pS₂/pO₂ ratios in ordinary and carbonaceous chondrite breccias support the model that enstatite chondrites were formed at a location distant from those of the other chondritic groups.     

Petrology and oxygen isotopic compositions of clasts in HED polymict breccia NWA 5232

Northwest Africa (NWA) 5232, an 18.5 kg polymict eucrite, comprises eucritic and exogenic CM carbonaceous chondrite clasts within a clastic matrix. Basaltic clasts are the most abundant eucritic clast type and show a range of textures and grain size, from subophitic to granoblastic. Other eucritic clast types present include cumulate (high‐En pyroxene), pyroxene‐lath, olivine rich with symplectite intergrowths as a break‐down product of a quickly cooled Fe‐rich metastable pyroxferroite, and breccia (fragments of a previously consolidated breccia) clasts. A variable cooling rate and degree of thermal metamorphism, followed by a complex brecciation history, can be inferred for the clasts based on clast rounding, crystallization (and recrystallization) textures, pyroxene major and minor element compositions, and pyroxene exsolution. The range in δ¹⁸O of clasts and matrix of NWA 5232 reflects its origin as a breccia of mixed clasts dominated by eucritic lithologies. The oxygen isotopic compositions of the carbonaceous chondrite clasts identify them as belonging to CM group and indicate that these clasts experienced a low degree of aqueous alteration while part of their parent body. The complex evolutionary history of NWA 5232 implies that large‐scale impact excavation and mixing was an active process on the surface of the HED parent body, likely 4 Vesta.     

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.     

Mineralogy,petrology, and oxygen isotopic compositions of chondritic and achondritic lithologies in the anomalous CB carbonaceous chondrites Sierra Gorda 013 and Fountain Hills

The CB (Bencubbin-like) metal-rich carbonaceous chondrites are subdivided into the CB_a and CB_b subgroups. The CB_a chondrites are composed predominantly of ~cm-sized skeletal olivine chondrules and unzoned Fe,Ni-metal ± troilite nodules. The CB_b chondrites are finer grained than the CB_as and consist of chemically zoned and unzoned Fe,Ni-metal grains, Fe,Ni-metal ± troilite nodules, cryptocrystalline and skeletal olivine chondrules, and rare refractory inclusions. Both subgroups contain exceptionally rare porphyritic chondrules and no interchondrule fine-grained matrix, and are interpreted as the products of a gas–melt impact plume formed by a high-velocity collision between differentiated planetesimals about 4562 Ma. The anomalous metal-rich carbonaceous chondrites, Fountain Hills and Sierra Gorda 013 (SG 013), have bulk oxygen isotopic compositions similar to those of other CBs but contain coarse-grained igneous clasts/porphyritic chondrule-like objects composed of olivine, low-Ca-pyroxene, and minor plagioclase and high-Ca pyroxene as well as barred olivine and skeletal olivine chondrules. Cryptocrystalline chondrules, zoned Fe,Ni-metal grains, and interchondrule fine-grained matrix are absent. In SG 013, Fe,Ni-metal (~80 vol%) occurs as several mm-sized nodules; magnesiochromite (Mg-chromite) is accessory; daubréelite and schreibersite are minor; troilite is absent. In Fountain Hills, Fe,Ni-metal (~25 vol%) is dispersed between chondrules and silicate clasts; chromite and sulfides are absent. In addition to a dominant chondritic lithology, SG 013 contains a chondrule-free lithology composed of Fe,Ni-metal nodules (~25 vol%), coarse-grained olivine and low-Ca pyroxene, interstitial high-Ca pyroxene and anorthitic plagioclase, and Mg-chromite. Here, we report on oxygen isotopic compositions of olivine, low-Ca pyroxene, and ±Mg-chromite in Fountain Hills and both lithologies of SG 013 measured in situ using an ion microprobe. Oxygen isotope compositions of olivine, low-Ca pyroxene, and Mg-chromite in these meteorites are similar to those of magnesian non-porphyritic chondrules in CB_a and CB_b chondrites: on a three-isotope oxygen diagram (δ¹⁷O vs. δ¹⁸O), they plot close to a slope-1 (primitive chondrule mineral) line and have a very narrow range of Δ¹⁷O (=δ¹⁷O–0.52 × δ¹⁸O) values, −2.5 ± 0.9‰ (avr ± 2SD). No isotopically distinct relict grains have been identified in porphyritic chondrule-like objects. We suggest that magnesian non-porphyritic (barred olivine, skeletal olivine, cryptocrystalline) chondrules in the CB_as, CB_bs, and porphyritic chondrule-like objects in SG 013 and Fountain Hills formed in different zones of the CB impact plume characterized by variable pressure, temperature, cooling rates, and redox conditions. The achondritic lithology in SG 013 represents fragments of one of the colliding bodies and therefore one of the CB chondrule precursors. Fountain Hills was subsequently modified by impact melting; Fe,Ni-metal and sulfides were partially lost during this process.     

NWA 10214—An LL3 chondrite breccia with an assortment of metamorphosed,shocked, and unique chondrite clasts

NWA 10214 is an LL3‐6 breccia containing ~8 vol% clasts including LL5, LL6, and shocked‐darkened LL fragments as well as matrix‐rich Clast 6 (a new kind of chondrite). This clast is a dark‐colored, subrounded, 6.1 × 7.0 mm inclusion, consisting of 60 vol% fine‐grained matrix, 32 vol% coarse silicate grains, and 8 vol% coarse opaque grains. The large chondrules and chondrule fragments are mainly Type IB; one small chondrule is Type IIA. Also present are one 450 × 600 μm spinel‐pyroxene‐olivine CAI and one 85 × 110 μm AOI. Clast 6 possesses a unique set of properties. (1) It resembles carbonaceous chondrites in having relatively abundant matrix, CAIs, and AOIs; the clast's matrix composition is close to that in CV3 Vigarano. (2) It resembles type‐3 OC in its olivine and low‐Ca pyroxene compositional distributions, and in the Fe/Mn ratio of ferroan olivine grains. Its mean chondrule size is within 1σ of that of H chondrites. The O‐isotopic compositions of the chondrules are in the ordinary‐ and R‐chondrite ranges. (3) It resembles type‐3 enstatite chondrites in the minor element concentrations in low‐Ca pyroxene grains and in having a high low‐Ca pyroxene/olivine ratio in chondrules. Clast 6 is a new variety of type‐3 OC, somewhat more reduced than H chondrites or chondritic clasts in the Netschaevo IIE iron; the clast formed in a nebular region where aerodynamic radial drift processes deposited a high abundance of matrix material and CAIs. A chunk of this chondrite was ejected from its parent asteroid and later impacted the LL body at low relative velocity.     

Korra Korrabes: A new,large H3 chondrite breccia from Namibia

Abstract— A new, large, ordinary chondrite has been recovered from near the strewn field of Gibeon iron meteorites in Namibia, and is designated Korra Korrabes, after the farm property on which the specimens were found in 1996–2000. A total of ～140 kg of related specimens were recovered, including a large stone of 22 kg, and hundreds of smaller objects between 2 g and several kilograms. Cut surfaces indicate that Korra Korrabes is a breccia, containing 10–20% of light grey‐brown clasts up to 3 cm across in a uniform, darker grey‐brown host that contains abundant round chondrules, and irregular grains of Fe‐Ni metal and troilite up to 1 cm across. The vast majority of the stone is unshocked, although some clasts show mild shock features (stage S2), and one chondrule fragment is moderately shocked (stage S3). Weathering grade varies between W1 and W2. Microprobe analyses indicate variable compositions of olivine (Fa_13.8–27.2, n = 152, percent mean deviation = 7.82%) and low‐Ca pyroxene (multiply twinned clinobronzite, Fs_8.4–27.8, n = 68). There is excellent preservation of magmatic textures and mineralogy within many chondrules, including normally zoned olivine (Fa_13.8–18.9) and low‐Ca pyroxene (Fs_0.2–20.9) phenocrysts, and abundant glass, some of whose compositions are unusually alkaline (Na₂O + K₂O = 13.6–16.3 wt%) and Ca‐deficient (CaO = 0‐0.75 wt%), seemingly out of magmatic equilibrium with associated clinoenstatite or high‐Al calcic clinopyroxene crystals. Textural and mineralogical features indicate that Korra Korrabes is an H3 chondrite breccia, which represents the largest and least equilibrated stony meteorite yet recovered from Namibia; it is now one of the four largest unequilibrated ordinary chondrites worldwide.     

Solar and galactic cosmic‐ray records of the Fermo (H) chondrite regolith breccia

Abstract— ‐We demonstrate the presence of solar flare as well as neutron capture effects in the isotopic composition of rare gases in the Fermo regolith breccia acquired on its parent body based on the measurements of tracks, rare gases and radionuclides. The track density along a 3.2 cm long core decreases by a factor of about 6 and by more than a factor of 13 within the meteorite, indicating small (2–9 cm) and asymmetrical ablation. Rare gases show a large trapped component; the isotopic ratios, particularly ²⁰Ne/²²Ne ? 11 and ²⁰Ne/³⁶Ar = 10 are indicative of a solar component. The galactic cosmic‐ray exposure age is determined to be 8.8 Ma. Activities of a dozen radionuclides ranging in half‐life from 16 day ⁴⁸V to 0.73 Ma ²⁶Al are consistent with their expected production rates. Track, rare gas and radionuclide data show that the meteoroid was a small body (≤ 120 kg) and had a simple, one‐stage exposure history to cosmic rays in the interplanetary space. However, ⁸²Kr and ¹²⁸Xe show an excess due to neutron irradiation on the parent body of the meteorite. The presence of solar gases and the neutron capture effects indicate several stages of irradiation on the parent asteroid. The chemical composition of Fermo confirms that it belongs to the H group of ordinary chondrites with lithic clasts having varying compositions. δ¹⁵N is found to be 8.3 ± 1.2%0, close to the typical values observed in H chondrites.     

Rb-Sr ages of clasts from within Boulder 1, Station 2, Apollo 17

Rb, Sr and⁸⁷Sr/⁸⁶Sr have been determined for fragments of matrix and clasts from three of the hand-specimens of Boulder 1, 72275, 72255, and 72215. Total-rock and certain plagioclase samples from a crushed norite clast (Civet Cat) define an age of 4.17±0.05AE (2σ) for the pre-Serenitatis igneous differentiation of the norite. Pyroxene and other mineral separates were affected by a later event at about 3.9±0.1AE. An unshocked clast of pigeonite basalt has a well-fitted mineral isochron of 4.01±0.04AE. Samples of the competent breccia matrix comparatively rich in small clasts of highly radiogenic microgranite define a mixing line equivalent to 4.03±0.03AE, which denotes the age of the microgranite. Other samples of the matrix dominated by small anorthosite clasts define a 4.4AE mixing-line and demonstrate that Sr isotope equilibration between plagioclase and matrix did not occur during the high-temperature event that indurated the matrix.