RARE-EARTH ELEMENTS IN MINERALS OF THE ALHA77005 SHERGOTTITE AND IMPLICATIONS FOR ITS PARENT MAGMA AND CRYSTALLIZATION HISTORY

Rare earth element (REE) and selected trace element abundances were measured in individual grains of pyroxenes, whitlockite, maskelynite and olivine of the ALHA77005 shergottite. Minor (~ 1%) whitlockite, which displays a negative Eu anomaly indicative of co-crystallization with plagioclase, dominates the bulk-rock REE pattern. Detectable Ce anomalies in ~20% of the pyroxene grains analyzed appear to be the result of weathering in the Antarctic environment and indicate that pyroxene is particularly susceptible to element remobilization. Mineral compositions and textural observations suggest that ALHA77005 is a cumulate with ~50% cumulus material (olivine + chromite). Olivine re-equilibrated with intercumulus liquid on cooling. The intercumulus liquid then crystallized as a closed system to form successively poikilitic low-Ca and high-Ca pyroxene, followed by interstitial pyroxenes, plagioclase, whitlockite (and other minor phases), and reacted with cumulus crystals to form rims of different composition. This crystallization sequence provides a constraint on the shergottite source region composition. The ALHA77005 and Shergotty intercumulus liquids had similar REE compositions; however, as opposed to Shergotty, if ALHA77005 was formed during the last 0.72 Ga, its parent melt was not contaminated by LREE-enriched material. © 1990.