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Whole Rock Compositions of Aubritic Meteorites: Implications for Their Origin

Introduction: Aubri tes (enstati te achondrites) are heterogeneous assemblages of nearly ironfree enstatite and a variety of rare minerals which formed under highly reducing conditions. Considering the similarities in bulk chemistry of the major and minor mineral phases, and an inferred oxygen fugacity, Watters and Prinz [I] concluded that EL6 chondrites may be the most suitable parent material for aubrites. They further suggested that aubrites formed through the fractional crystallization of such materials. Examining the correlations of several pairs of the elements having great differences in volatility, Wolf et al. [2] also inferred that aubrites may have originated by igneous processes in a differentiated parent body. Okada et al. [3] concluded that the Norton County precursor materials were igneous rocks, mostly plutonic orthopyroxenites, which were formed by extensive melting and igneous differentiation. Brett and Keil [4] and Keil [5] argued that aubrites were produced by melting and fractionation, but not from any known enstatite chondrite group. Recently, Lodder et al. [6] analyzed the whole rock and mineral separates, including oldhamite, from the Pena Blanca Spring aubrite and concluded that the distribution of the REE among the minerals could not be the result of equilibrium fractional crystallization processes. They concluded that a short non-equilibrium melting event, rather than a large degree of equilibrium partial melting, is the k s t explanation for what occurred on the aubrite parent body. In order to contribute to an understanding of the origin of aubrites, we have analyzed, by instrumental neutron activation analysis (INAA), six aubrites (Bishopville, Cumberland Falls, Khor Temilu, Mayo Belwa, Norton Co. and Pena Blanca Spring), which are all observed falls. In addition, we also analyzed three enstatite-rich meteorites (Happy Canyon, Ilafegh 009 and Mt. Egerton). Most of the meteorites in this study were also analyzed by inductively coupled plasma mass spectrometry (ICP-MS) for REE (including Y) and U. Since these meteorites are highly heterogeneous in their mineral abundances and compositions, an effort was made to reduce the bias in sampling small chips of the individual meteorites, so that conclusions as to the petrogenesis of aubrites can be more meaningful. Experiments: Several chips (weighing 200 to 400 mg) were samples from representative samples of each meteorite, and each was broken into fragments in an agate mortar. For aubrites, some fragments were used to prepare polished sections for SEM-EDS study. The residual fragments (100-200 mg) were further pulverized in an agate mortar for chemical analyses by instrumental neutron activation (INAA) and inductively coupled plasma mass spectrometry (ICPMS). For INAA, we followed the basic procedures described by Ebihara and Ozalu [q. A part of each powdered samples (about 40 mg) was used for INAA to determine the bulk chemical composition. REE (lanthanoids and Y) and U were determined by ICP-MS for the samples except Mt. Egerton, Happy Canyon and Ilafegh 009. About 10 mg of each powdered sample was decomposed with HF and HNO3 under high pressure using a teflon bomb. The solution thus prepared was introduced directly into the instrument (modified VG PQ-1). The accuracy of the data was evaluated by analyzing the Allende reference standard. A detailed description of the ICPMS analysis procedures for meteorite samples will appear elsewhere. Results and Discussion: Elemental abundances obtained by INAA are shown in Fig. 1, where the data are normalized to CI values [8]. Na and A1 show large variations in their abundances, over a factor of 70. However, they are highly correlated with one another, indicating that albitic plagioclase is the main host for Na and Al. Mn, Cr and V, which behave as chalcophiles in aubrites, are generally depleted in the aubrites, and their abundances are fairly variable. Siderophiles (Au, Fe, Ni, Co, Ir) also are depleted, and show large variations in their abundances. It is noteworthy that the degree of variation becomes larger from Fe (0.15 0.01 x CI) to Co (0.15 to 0.001 x CI), and possibly further to Ir. However, depletions for the chalcophile elements are relatively small, being within two orders of magnitude. The amount of sulfide loss seems to be somewhat correlated with that for the metal for the aubrites in this study. Khor