ALH84001, a cumulate orthopyroxenite member of the martian meteorite clan

Abstract

Abstract— ALH84001, originally classified as a diogenite, is a coarse‐grained, cataclastic, orthopyroxenite meteorite related to the martian (SNC) meteorites. The orthopyroxene is relatively uniform in composition, with a mean composition of Wo 3.3 En 69.4 Fs 27.3. Minor phases are euhedral to subhedral chromite and interstitial maskelynite, An 31.1 Ab 63.2 Or 5.7, with accessory augite, Wo 42.2 En 45.1 Fs 12.7, apatite, pyrite and carbonates, Cc 11.5 Mg 58.0 Sd 29.4 Rd 1.1. The pyroxenes and chromites in ALH84001 are similar in composition to these phases in EETA79001 lithology A megacrysts but are more homogeneous. Maskelynite is similar in composition to feldspars in the nakhlites and Chassigny. Two generations of carbonates are present, early (pre‐shock) strongly zoned carbonates and late (post‐shock) carbonates. The high Ca content of both types of carbonates indicates that they were formed at moderately high temperature, possibly ∼700 °C. ALH84001 has a slightly LREE‐depleted pattern with La 0.67x and Lu 1.85x CI abundances and with a negative Eu anomaly (Eu/Sm 0.56x CI). The uniform pyroxene composition is unusual for martian meteorites, and suggests that ALH84001 cooled more slowly than did the shergottites, nakhlites or Chassigny. The nearly monomineralic composition, coarse‐grain size, homogenous orthopyroxene and chromite compositions, the interstitial maskelynite and apatite, and the REE pattern suggest that ALH84001 is a cumulate orthopyroxenite containing minor trapped, intercumulus material.