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

Abstract

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 93 148 contains an unusual suite of inclusions: (1) 5 x 100 p m sized lamellae with a CaO-and Cr203-rich (-10 and 22 wt%, respectively) composition that may represent a submicrometer-scale intergrowth of chromite and pyroxene(s); (2) 75 x 500 p m sized lamellar symplectites composed of chromite and two pyroxenes, with minor metal; (3) 15-20 p m sized, irregularly-shaped symplectites composed of chromite and pyroxene(s); (4) 100-150 p m 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 93 148 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: 0-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 93 148 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-eucrite4iogenite?) parent body.

Queen Alexandra Range 93148: A new type of pyroxene pallasite?

“…Incompatible trace element (e.g., Ti, Zr) distributions in orthopyroxene do indicate large amounts of melting and are consistent with the ultramafic assemblage observed in this meteorite. Trace element abundances in olivine are consistent with suggestions that QUE 93148 may be related to the main group pallasites (Goodrich and Righter, 2000), although there are some inconsistencies. Its trace element distributions are most like those of the pyroxene pallasites, suggesting that it may have formed in a similar manner.…”

“…3) are broadly similar to those observed in orthopyroxene. Manganese abundances are lower than those observed in acapulcoite/ lodranite olivines, consistent with the low Mn/Mg ratios determined by Goodrich and Righter (2000), and Cr concentrations are higher. Zirconium concentrations in QUE 93148 olivine are also lower than those seen in olivine from the acapulcoite/lodranite meteorites, but Ti abundances fall between the ranges observed in the two groups; Y abundances are similar to those ofacapulcoite/lodranite olivine.…”

“…Several small metal-troilitechromite blebs are enclosed within the olivine, as is a symplectic intergrowth similar to the ones described by Goodrich and Righter (2000). QUE 93148,3 contains a large rounded olivine grain adjacent to a large grain ofFe,Ni metal, as well as several smaller olivine and orthopyroxene grains.…”

“…Queen Alexandra Range (QUE) 93148 is an olivine-rich achondrite with lesser amounts of orthopyroxene, metal, troilite and chromite, that was originally classified as a lodranite (Mason, 1995). However, Goodrich and Righter (2000) noted that its oxygen isotopic composition (measured by Mayeda and Clayton) precludes a genetic relationship to the acapu1coites and lodranites. They furthermore observed that it has a lower Mn/Mg ratio than any major group of primitive or evolved achondrites and suggested that QUE 93148 may be a piece of the deep mantle of the HED parent body.…”

“…Although QUE 93148 was originally classified as a lodranite, a variety of data including oxygen isotopic compositions (Goodrich and Righter, 2000), preclude a genetic relationship with the acapulcoites/lodranites. Incompatible trace element (e.g., Ti, Zr) distributions in orthopyroxene do indicate large amounts of melting and are consistent with the ultramafic assemblage observed in this meteorite.…”