Al-Mg systematics of hibonite-bearing Ca,Al-rich inclusions from Ningqiang

Hibonite-bearing Ca,Al-rich inclusions (CAIs) usually occur in CM and CH chondrites and possess petrographic and isotopic characteristics distinctive from other typical CAIs. Despite their highly refractory nature, most hibonite-bearing CAIs have little or no ^(26)Mg excess (the decay product of ^(26)Al), but do show wide variations of Ca and Ti isotopic anomalies. A few spinel-hibonite spherules preserve evidence of live ^(26)Al with an inferred ^(26)Al/^(27)Al close to the canonical value. The bimodal distribution of ^(26)Al abundances in hibonite-bearing CAIs has inspired several interpretations regarding the origin of short-lived nuclides and the evolution of the solar nebula. Herein we show that hibonite-bearing CAIs from Ningqiang, an ungrouped carbonaceous chondrite, also provide evidence for a bimodal distribution of ^(26)Al. Two hibonite aggregates and two hibonite-pyroxene spherules show no ^(26)Mg excesses, corresponding to inferred ^(26)Al/^(27)Al < 8 × 10^(−6). Two hibonite-melilite spherules are indistinguishable from each other in terms of chemistry and mineralogy but have different Mg isotopic compositions. Hibonite and melilite in one of them display positive ^(26)Mg excesses (up to 25‰) that are correlated with Al/Mg with an inferred ^(26)Al/^(27)Al of (5.5 ± 0.6) × 10^(−5). The other one contains normal Mg isotopes with an inferred ^(26)Al/^(27)Al < 3.4 × 10^(−6). Hibonite in a hibonite-spinel fragment displays large ^(26)Mg excesses (up to 38‰) that correlate with Al/Mg, with an inferred ^(26)Al/^(27)Al of (4.5 ± 0.8) × 10^(−5). Prolonged formation duration and thermal alteration of hibonite-bearing CAIs seem to be inconsistent with petrological and isotopic observations of Ningqiang. Our results support the theory of formation of ^(26)Al-free/poor hibonite-bearing CAIs prior to the injection of ^(26)Al into the solar nebula from a nearby stellar source

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

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_(9.7–11.1)Wo_(48.3–51.6)), plagioclase (An_(43–46.5)), nepheline, merrillite, Al-rich chromite (21.8 wt% Al_2O_3; 4.43 wt% TiO_2), and pentlandite. Δ^(17)O 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

Al-Mg systematics of CAIs, POI, and ferromagnesian chondrules from Ningqiang

We have made aluminum-magnesium isotopic measurements on 4 melilite-bearing calcium-aluminum-rich inclusions (CAIs), 1 plagioclase-olivine inclusion (POI), and 2 ferromagnesian chondrules from the Ningqiang carbonaceous chondrite. All of the CAIs measured contain clear evidence for radiogenic ^(26)Mg^* from the decay of ^(26)Al (τ = 1.05 Ma). Although the low Al/Mg ratios of the melilites introduce large uncertainties, the inferred initial ^(26)Al/^(27)Al ratios for the CAIs are generally consistent with the value of 5 x 10^(−5). There is clear evidence of ^(26)Al^* in one POI and two chondrules, but with considerable uncertainties in the value of (^(26)Al/^(27)Al)0. The (^(26)Al/^(27)Al)_0 ratios for the POI and the chondrules are 0.3–0.6 x 10^(−5), roughly an order of magnitude lower than the canonical value. Ningqiang shows very little evidence of metamorphism as a bulk object and the (^(26)Al/^(27)Al)_0 ratios in its refractory inclusions and chondrules are consistent with those found in other unmetamorphosed chondrites of several different classes. Our observations and those of other workers support the view that ^(26)Al was widely and approximately homogeneously distributed throughout the condensed matter of the solar system. The difference in (^(26)Al/^(27)Al)0 between CAIs and less refractory materials seems reasonably interpreted in terms of a ∼2 million year delay between the formation of CAIs and the onset of formation of less refractory objects. The POI shows clear differences in ^(25)Mg/^(24)Mg between its constituent spinels and olivine, which confirms that they are partially reprocessed material from different sources that were rapidly quenched

Subdivision of petrologic type of unequilibrated ordinary chondrites from Antarctica

We report results of petrologic and mineralogical studies of 25 unequilibrated ordinary chondrites (19 Chinese Antarctic meteorites) using electron microprobe, scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) techniques. With increasing degree of thermal metamorphism, chemical zoning of olivine was erased; plessite was transformed into intergrowths of coarser-grained kamacite and Ni-rich metal; Crex solved from ferroan olivine, produced needle-like crystals and coarsened into equant chromite grains; and feldspar crystallized during devitrification of glass and recrystallization of matrix. These features can help to identify different metamorphic grades. Based on the scheme by Sears et al ( 1982), we subdivided type 3 Chinese Antarctic ordinary chondrites into petrologic type 3.3 to 4 (3 type 3.3, 3 type 3.4, 1 type 3.5, 2 type 3.6, 2 type 3.7, 5 type 3.8, 3 type 4). This classification scheme is a quite effective way to subdivide Antarctic meteorites. Additionally, we propose to revise the chemical groups of GRV 020032 and GRV 020104 to L and H, respectively

SIMS Analysis of OH and D/H of Apatites from Eucrites

Eucrites, a member of the howardite-eucritediogenite (HED) group, are basaltic meteorites that are believed to come from the asteroid Vesta [e.g., 1]. A recent study of water concentration and hydrogen isotope of eucrite apatite [2] suggested an early accretion of water from a carbonaceous chondrite– like source on Earth, Vesta and other planetary bodies in the inner solar system. In this study, we present results of OH and D/H analyses of apatites from more eucrite samples

Petrogenesis of the Northwest Africa 4898 high-Al mare basalt

Northwest Africa (NWA) 4898 is the only low-Ti, high-Al basaltic lunar meteorite yet recognized. It predominantly consists of pyroxene (53.8 vol%) and plagioclase (38.6 vol%). Pyroxene has a wide range of compositions (En_(12–62)Fs_(25–62)Wo_(11–36)), which display a continuous trend from Mg-rich cores toward Ca-rich mantles and then to Fe-rich rims. Plagioclase has relatively restricted compositions (An_(87–96)Or_(0–1)Ab_(4–13)), and was transformed to maskelynite. The REE zoning of all silicate minerals was not significantly modified by shock metamorphism and weathering. Relatively large (up to 1 mm) olivine phenocrysts have homogenous inner parts with Fo ~74 and sharply decrease to 64 within the thin out rims (~30 μm in width). Four types of inclusions with a variety of textures and modal mineralogy were identified in olivine phenocrysts. The contrasting morphologies of these inclusions and the chemical zoning of olivine phenocrysts suggest NWA 4898 underwent at least two stages of crystallization. The aluminous chromite in NWA 4898 reveals that its high alumina character was inherited from the parental magma, rather than by fractional crystallization. The mineral chemistry and major element compositions of NWA 4898 are different from those of 12038 and Luna 16 basalts, but resemble those of Apollo 14 high-Al basalts. However, the trace element compositions demonstrate that NWA 4898 and Apollo 14 high-Al basalts could not have been derived from the same mantle source. REE compositions of its parental magma indicate that NWA 4898 probably originated from a unique depleted mantle source that has not been sampled yet. Unlike Apollo 14 high-Al basalts, which assimilated KREEPy materials during their formation, NWA 4898 could have formed by closed-system fractional crystallization

An extremely heavy chlorine reservoir in the Moon: Insights from the apatite in lunar meteorites

Chlorine, an extremely hydrophilic volatile element, provides important information regarding the origin of intrinsic volatiles in the Moon. Lunar apatite was found to have a wider spread of δ^(37)Cl (from −1 to +40‰ versus standard mean ocean chloride) than most terrestrial and chondritic ones (0 ± 0.5‰). However, the provenance of the elevated lunar δ^(37)Cl is still enigmatic. Here we report new isotopic data for H and Cl in apatite from three lunar meteorites and discuss possible mechanisms for Cl isotopic fractionation of the Moon. The apatite grain in Dhofar 458 has an average δ^(37)Cl value of +76‰, indicative of an extremely heavy Cl reservoir in the Moon. Volatile loss associated with the Moon-forming Giant Impact and the formation of lunar magma ocean could account for the large Cl isotopic fractionation of the Moon. The observed H_2O contents (220–5200 ppm), δD (−100 to +550‰) and δ^(37)Cl values (+3.8 − +81.1‰) in lunar apatite could be understood if late accretion of hydrous components were added to the Moon after the fractionation of Cl isotopes. The heterogeneous distribution of lunar Cl isotopes is probably resulted from complex lunar formation and differentiation processes

Calcium Isotope Evolution During Differentiation of Vesta and Calcium Isotopic Heterogeneities in the Inner Solar System

We employed MC-ICP-MS to measure the mass-dependent Ca isotope compositions of Vesta-related meteorites. Eucrites and diogenites show distinct Ca isotope compositions, which is caused by crystallization of isotopically heavy orthopyroxene. The Ca isotope data support a model where the two lithologies are linked, where the diogenites, mainly composed of orthopyroxene crystallized from an eucritic melt. As normal eucrites are the main Ca reservoir on Vesta, their δ44/40Ca values (per mil 44Ca/40Ca ratios relative to NIST 915a) best represents that of bulk silicate Vesta (0.83 ± 0.04‰). This value is different from those of bulk Earth (0.94 ± 0.05‰) and Mars (1.04 ± 0.07‰), suggesting that there exists notable Ca isotope heterogeneity between inner solar system bodies. The δ44/40Ca difference between chondrules and these planets does not support the pebble accretion model as the main mechanism for planetary growth

Petrography and geochemistry of the enriched basaltic shergottite Northwest Africa 2975

We present a study of the petrology and geochemistry of basaltic shergottite Northwest Africa 2975 (NWA 2975). NWA 2975 is a medium-grained basalt with subophitic to granular texture. Electron microprobe (EMP) analyses show two distinct pyroxene compositional trends and patchy compositional zoning patterns distinct from those observed in other meteorites such as Shergotty or QUE 94201. As no bulk sample was available to us for whole rock measurements, we characterized the fusion crust and its variability by secondary ion mass spectrometer (SIMS) measurements and laser ablation inductively coupled plasma spectroscopy (LA-ICP-MS) analyses as a best-available proxy for the bulk rock composition. The fusion crust major element composition is comparable to the bulk composition of other enriched basaltic shergottites, placing NWA 2975 within that sample group. The CI-normalized REE (rare earth element) patterns are flat and also parallel to those of other enriched basaltic shergottites. Merrillite is the major REE carrier and has a flat REE pattern with slight depletion of Eu, parallel to REE patterns of merrillites from other basaltic shergottites. The oxidation state of NWA 2975 calculated from Fe-Ti oxide pairs is NNO-1.86, close to the QFM buffer. NWA 2975 represents a sample from the oxidized and enriched shergottite group, and our measurements and constraints on its origin are consistent with the hypothesis of two distinct Martian mantle reservoirs: a reduced, LREE-depleted reservoir and an oxidized, LREE-enriched reservoir. Stishovite, possibly seifertite, and dense SiO_2 glass were also identified in the meteorite, allowing us to infer that NWA 2975 experienced a realistic shock pressure of ~30 GPa

Petrogenesis of the Northwest Africa 4734 basaltic lunar meteorite

We report the petrography, mineralogy, trace element abundance geochemistry, and Pb–Pb geochronology of the lunar meteorite Northwest Africa (NWA) 4734 and make a comparison with the LaPaz Icefield (LAP) 02205/02224 low-Ti lunar basaltic meteorites. NWA 4734 is an unbrecciated low-Ti mare basalt composed mainly of subophitic-textured pyroxene (60 vol%) and plagioclase (30%). Pyroxene, plagioclase, and olivine exhibit large compositional variations and intra-grain chemical zoning. Pyroxene and plagioclase in NWA 4734 have rare earth element (REE) concentrations and patterns similar to those of the LAPs. The crystallization age of NWA 4734, determined in situ in baddeleyite, is 3073 ± 15 Ma (2σ), nearly identical to that of the LAPs (3039 ± 12 Ma). NWA 4734 and the LAPs have similar textures, modal abundances, mineral chemistry, and crystallization ages, and are most likely source-crater paired on the Moon. One baddeleyite grain in LAP 02224 displays distinctively older and spatially variable ages, from 3349 ± 62 to 3611 ± 62 Ma (2σ), similar to another baddeleyite grain (3109 ± 29 to 3547 ± 21 Ma) reported by Zhang et al. (2010) for the same meteorite. Raman spectra, cathodoluminescence, and stoichiometric studies of the baddeleyite suggest that the two older grains were not endogenic but were trapped by the parental magma. Equilibrium partition calculation shows that the parental melt from which the NWA 4734 plagioclase crystallized has much lower REE contents than its whole rock, indicating an open system during magma evolution. NWA 4734 could have originated from a parental melt with REE concentrations similar to that of the Apollo 12 olivine basalt. The magma likely assimilated a small amount (∼4 wt%) of KREEP-rich material during its ascent through the lunar crust