The origin of aubrites: Evidence from lithophile trace element abundances and oxygen isotope compositions
Introduction
Among the approximately fifty thousand officially classified meteorites held in institutional collections, enstatite-rich types are rare (about five hundred specimens) and of these, the aubrites are particularly scarce; with only nine falls and a score of finds, including pairings. The mineralogy, geochemistry and isotopic composition of aubrites demonstrate that they are unusual and intriguing achondrites (predominantly pyroxenites) (e.g., Keil, 2010). Most are breccias, containing clasts of coarse-grained pyroxenite that formed in a slowly-cooled environment (plutons), embedded in a clastic matrix composed mainly of enstatite crystal debris. Aubrites are mineralogically diverse, consisting mainly of enstatite, with various proportions of diopside, inverted pigeonite, forsterite, albitic plagioclase, as well as small amounts of troilite and metallic Fe,Ni. In addition, they contain a host of rare accessory sulfides, including oldhamite (CaS), ferroan alabandite [(Mn,Fe)S], daubréelite (FeCr2S4) and caswellsilverite (NaCrS2), all of which formed, under extremely low oxygen fugacities (see Keil, 2010, for a review). As such, aubritic pyroxenites are magmatic rocks that crystallized under the most reducing conditions yet identified (fO2 ≈ IW-5 or below, e.g., Fogel, 1998).
Two of them (Mount Egerton and Larned) are anomalous, with high metal contents. Shallowater, a non-brecciated orthopyroxenite containing large orthoenstatite crystals (up to 4.5 cm), was previously identified as an anomalous aubrite by Keil et al. (1989). It experienced a complex history and almost certainly formed on a separate parent body to that of the main-group aubrites (Keil et al., 1989, Rubin, 2015); an interpretation that is in agreement with Hf-W systematics (Petitat et al., 2008) and Zn isotopes (Moynier et al., 2011). In addition to these aubritic samples, a few other enstatite achondrites are known, such as Itqiy and Northwest Africa 2526. However, these rocks are certainly unrelated to aubrites (e.g., Keil and Bischoff, 2008).
The occurrence of oldhamite and other unusual sulfides indicates that the chemical affinities of many “traditionally” lithophile elements were modified by the prevailing reducing conditions, such that they display a strongly chalcophile behavior. Sulfides, and particularly oldhamite, are major rare earth element (REE) carriers in aubrites (e.g., Floss and Crozaz, 1993, Lodders et al., 1993, Wheelock et al., 1994, Newsom et al., 1996). Thus, REE and possibly other refractory lithophile elements show a unique evolution during the differentiation of the aubrite parent body (bodies) and during the genesis and crystallization of aubritic parental melts.
Contrary to other achondrites, for which the composition of their parental materials remains a matter for discussion, it is almost certain that the aubrites are the early differentiation products of an enstatite chondrite-related precursor. Aubrites and enstatite chondrites clearly formed under similar, extremely reducing formation conditions and in addition display almost identical isotopic compositions for many elements (e.g., O, Ti, Cr, S, Si, Clayton and Mayeda, 1996, Newton et al., 2000, Miura et al., 2007, Trinquier et al., 2007, Zhang et al., 2012, Savage and Moynier, 2013, Defouilloy et al., 2016). This suggests that their parent body (or bodies) accreted from materials which condensed in similar regions of the solar nebula. Both enstatite chondrites and aubrites show many isotopic similarities to the Earth and this has led to the suggestion that the proto-Earth accreted from enstatite-like precursor materials (Javoy et al., 2010).
In this paper, we report the trace element abundances obtained by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for a comprehensive suite of the “common” or main-group aubrites and the two anomalous metal-rich aubrites (Mount Egerton and Larned). Our aim is, first, to evaluate the full range of refractory lithophile element distributions displayed by these meteorites, in order to estimate how sulfides control the budget of these elements. Secondly, we report new high precision oxygen isotope analyses for a representative suite of aubrites and examine whether their parent body (bodies) went through an early phase of large-scale melting (magma ocean stage?).
Section snippets
Samples and analytical methods
We obtained samples from the classical aubrites (Aubres, Bishopville, Bustee, Cumberland Falls, Khor Temiki, LAP 03719, LAR 04316, Mayo Belwa, Norton Co., Peña Blanca Spring, Pesyanoe) and from two anomalous metal-rich finds (Mount Egerton and Larned). Sample sources are listed in Table 1. Powders were prepared using a boron-carbide mortar and pestle. Elemental abundances for most of the samples were determined using a high-resolution ICP-MS spectrometer Thermo Element 2 at Institut
Results
Most aubrites are breccias, often comprising a variety of lithologies, as exemplified by Norton County (Okada et al., 1988). Obtaining a representative sample of a breccia would require crushing and homogenization of tens of grams of sample. Such large amounts of aubrites are just not obtainable for bulk rock analyses. Instead, we have prepared powders (0.5–2 g) from chips of “matrix”, which represent the fine-grained portion of samples, devoid of large clasts (>3–4 mm) of enstatite. This
Evidence for early, large-scale melting on the aubrite parent body
A significant finding of our study is the striking homogeneity of the Δ17O ratios displayed by the aubrite falls. Their Δ17O ratios define a narrow range, from −0.001 to +0.017‰. This homogeneity strongly suggests that these aubrites formed either from primitive materials sharing the same isotopic composition, or alternatively from a single body that was initially heterogeneous and subsequently experienced a high level of O isotope homogenization. The first possibility is unlikely because
Conclusions
- 1.
Two distinct groups of aubrites can be defined using lithophile trace elements abundances: (1) the “normal” or main-group aubrites, and (2) Mount Egerton and Larned. The main-group aubrites display rather flat or slightly light-REE depleted REE patterns with variable Eu and Y anomalies depending on the nature of the accessory phases involved (plagioclase and sulfides). The trace element distributions of their pyroxenes show more diversity, particularly variable light REE depletions, marked
Acknowledgements
Most of the samples analyzed during the course of this study were kindly provided by the National History Museum, London, the Vernadsky Institute, Moscow, the Institute for Meteoritics, Albuquerque, the Muséum National d’Histoire Naturelle de Paris (MNHN), the NASA meteorite working group and Don Stimpson. US Antarctic meteorite samples are recovered by the Antarctic search for Meteorites (ANSMET) program which has been funded by NSF and NASA, and characterized and curated in the department of
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