Elsevier

Geochimica et Cosmochimica Acta

Volume 192, 1 November 2016, Pages 29-48
Geochimica et Cosmochimica Acta

The origin of aubrites: Evidence from lithophile trace element abundances and oxygen isotope compositions

https://doi.org/10.1016/j.gca.2016.07.025Get rights and content

Abstract

We report the abundances of a selected set of “lithophile” trace elements (including lanthanides, actinides and high field strength elements) and high-precision oxygen isotope analyses of a comprehensive suite of aubrites. Two distinct groups of aubrites can be distinguished: (a) the main-group aubrites display flat or light-REE depleted REE patterns with variable Eu and Y anomalies; their pyroxenes are light-REE depleted and show marked negative Eu anomalies; (b) the Mount Egerton enstatites and the silicate fraction from Larned display distinctive light-REE enrichments, and high Th/Sm ratios; Mount Egerton pyroxenes have much less pronounced negative Eu anomalies than pyroxenes from the main-group aubrites.

Leaching experiments were undertaken to investigate the contribution of sulfides to the whole rock budget of the main-group aubrites. Sulfides contain in most cases at least 50% of the REEs and of the actinides. Among the elements we have analyzed, those displaying the strongest lithophile behaviors are Rb, Ba, Sr and Sc.

The homogeneity of the Δ17O values obtained for main-group aubrite falls [Δ17O = +0.009 ± 0.010‰ (2σ)] suggests that they originated from a single parent body whose differentiation involved an early phase of large-scale melting that may have led to the development of a magma ocean. This interpretation is at first glance in agreement with the limited variability of the shapes of the REE patterns of these aubrites. However, the trace element concentrations of their phases cannot be used to discuss this hypothesis, because their igneous trace-element signatures have been modified by subsolidus exchange. Finally, despite similar O isotopic compositions, the marked light-REE enrichments displayed by Mount Egerton and Larned suggest that they are unrelated to the main-group aubrites and probably originated from a distinct parent body.

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

References (79)

  • C. Floss et al.

    Heterogeneous REE patterns in oldhamite from the aubrites: their nature and origin

    Geochim. Cosmochim. Acta

    (1993)
  • C. Floss et al.

    Rare earth elements and the petrogenesis of aubrites

    Geochim. Cosmochim. Acta

    (1990)
  • A. Ghosh et al.

    A thermal model for the differentiation of asteroid 4 Vesta based on radiogenic heating

    Icarus

    (1998)
  • R.C. Greenwood et al.

    The oxygen isotope composition of diogenites: evidence for early global melting on a single, compositionally diverse, HED parent body

    Earth Planet. Sci. Lett.

    (2014)
  • R.C. Greenwood et al.

    Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection

    Geochim. Cosmochim. Acta

    (2015)
  • L.J. Hallis et al.

    The oxygen isotope composition, petrology and geochemistry of mare basalts: evidence for large scale compositional variation in the lunar mantle

    Geochim. Cosmochim. Acta

    (2010)
  • M. Javoy et al.

    The chemical composition of the Earth: enstatite chondrite models

    Earth Planet. Sci. Lett.

    (2010)
  • K. Keil

    Enstatite achondrite meteorites (aubrites) and the histories of their asteroidal parent bodies

    Chem. Erde

    (2010)
  • K. Keil et al.

    The Shallowater aubrite: evidence for origin by planetesimal impacts

    Geochim. Cosmochim. Acta

    (1989)
  • M.E. Lipschutz et al.

    Cumberland Falls chondritic inclusions: III. Consortium study of relationship to inclusions in Allan Hills 78113 aubrite

    Geochim. Cosmochim Acta

    (1988)
  • S. Lorenzetti et al.

    History and origin of aubrites

    Geochim. Cosmochim. Acta

    (2003)
  • M.F. Miller

    Isotopic fractionation and the quantification of 17O anomalies in the oxygen three isotopes system: an appraisal and geochemical significance

    Geochim. Cosmochim. Acta

    (2002)
  • Y.N. Miura et al.

    Noble gas and oxygen isotope studies of aubrites: a clue to origin and histories

    Geochim. Cosmochim. Acta

    (2007)
  • C.W. Neal et al.

    Cumberland falls chondritic inclusions: mineralogy/petrology of a forsterite chondrite suite

    Geochim. Cosmochim. Acta

    (1981)
  • W.C. Phinney et al.

    Partition coefficients for calcic plagioclase: implications for Archean anorthosites

    Geochim. Cosmochim. Acta

    (1990)
  • A. Pun et al.

    Subsolidus REE partitioning between pyroxene and plagioclase in cumulate eucrites: an ion microprobe investigations

    Geochim. Cosmochim. Acta

    (1997)
  • A.E. Rubin

    Impact features of enstatite-rich meteorites

    Chem. Erde

    (2015)
  • P.S. Savage et al.

    Silicon isotopic variation in enstatite meteorites: Clues to their origin and Earth-forming material

    Earth Planet. Sci. Lett.

    (2013)
  • E.R.D. Scott et al.

    Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

    Geochim. Cosmochim. Acta

    (2009)
  • M.J. Spicuzza et al.

    Oxygen isotope constraints on the origin and differentiation of the Moon

    Earth Planet. Sci. Lett.

    (2007)
  • N.A. Starkey et al.

    Triple oxygen isotopic composition of high-3He/4He mantle

    Geochim. Cosmochim. Acta

    (2016)
  • A.H. Treiman

    The perils of partition: difficulties in retrieving magma compositions from chemically equilibrated basaltic meteorites

    Geochim. Cosmochim. Acta

    (1996)
  • D. van Acken et al.

    Siderophile trace elements in metals and sulfides in enstatite achondrites record planetary differentiation in an enstatite chondritic parent body

    Geochim. Cosmochim. Acta

    (2012)
  • R.M. Verkouteren et al.

    Cumberland Falls chondritic inclusions-II. Trace element contents of forsterite chondrites and meteorites of similar redox state

    Geochim. Cosmochim. Acta

    (1983)
  • M.M. Wheelock et al.

    REE geochemistry of oldhamite-dominated clasts from the Norton County aubrite: igneous origin of oldhamite

    Geochim. Cosmochim. Acta

    (1994)
  • L. Wilson et al.

    Volcanic activity on differentiated asteroids: a review and analysis

    Chem. Erde Geochem.

    (2012)
  • R. Wolf et al.

    Aubrites and diogenites: trace element clues to their origin

    Geochim. Cosmochim. Acta

    (1983)
  • J.A. Barrat

    Determination of the parental magmas of HED cumulates: The effects of interstitial melts

    Meteori. Planet. Sci.

    (2004)
  • J.A. Barrat et al.

    Geochemistry of the martian meteorite ALH84001, revisited

    Meteorit. Planet. Sci.

    (2010)
  • Cited by (0)

    View full text