Metamorphic diogenite Grosvenor Mountains 95555: Mineral chemistry of orthopyroxene and spinel and comparisons to the diogenite suite

Grosvenor Mountains (GRO) 95555 is a relatively newly discovered and unique metamorphic diogenite. It does not show the usual brecciated appearance of other diogenites or wide compositional variability of orthopyroxene or spi nel. Electron microprobe analysis (EMPA) and secondary ion mass spectromete r (SIMS) analysis of orthopyroxene and EMPA of spinel show limited composit ional variability. Compositions for orthopyroxene for Fe/(Fe + Mg) atomic, Al, Zr, Y, and Yb fall in the middle of the compositional ranges of the dio genite suite. Apparently, GRO 95555 formed at sufficient depth or location to escape brecciation by meteorite bombardment and in a thermal regime that led to high-grade metamorphism resulting in homogeneous mineral compositio ns.