Petrology of unique achondrite Queen Alexandra Range 93148: A piece of the pallasite (howardite-eucrite-diogenite?) parent body?

Abstract

Queen Alexandra Range (QUE) 93148 is a small (1.1 g) olivine-rich achondrite (mg 86) that contains variable amounts of orthopyroxene (mg 87) and kamacite (6.7 wt% Ni), with minor augite. Olivine in QUE 93148 contains an unusual suite of inclusions: (1) 5 x 100 μm sized lamellae with a CaO- and Cr2O3-rich (~10 and 22 wt%, respectively) composition that may represent a submicrometer-scale intergrowth of chromite and pyroxene(s); (2) 75 x 500 μm sized lamellar symplectites composed of chromite and two pyroxenes, with minor metal; (3) 15-20 μm sized, irregularly-shaped symplectites composed of chromite and pyroxene(s); (4) 100-150 μm sized, elliptical inclusions composed of chromite, two pyroxenes, metal, troilite, and rare whitlockite. Type 1, 2, and 3 inclusions probably formed by exsolution from the host olivine during slow cooling, whereas type 4 more likely resulted from early entrapment of silicate and metallic melts followed by closed-system oxidation. Queen Alexandra Range 93148 can be distinguished from most other olivine-rich achondrites (ureilites, winonaites, lodranites, acapulcoites, brachinites, Eagle-Station-type pallasites, and pyroxene pallasites), as well as from mesosiderites, by some or all of the following properties: O-isotopic composition, Fe-Mn-Mg relations of olivine, CaO and Cr2O3 contents of olivine, orthopyroxene compositions, molar Cr/(Cr + Al) ratios of chromite, metal composition, texture, and the presence of the inclusions. In terms of many of these properties, it shows an affinity to main-group pallasites. Nevertheless, it cannot be identified as belonging to this group. Meteorite QUE 93148 appears to be a unique achondrite. Possibly it should be considered to be a pyroxene pallasite that is genetically related to main-group pallasites. Alternatively, it may be derived from the mantle of the pallasite (howardite-eucrite-diogenite?) parent body.