THE IGNEOUS CRYSTALLIZATION HISTORY OF AN ANCIENT MARTIAN METEORITE FROM RARE-EARTH ELEMENT MICRODISTRIBUTIONS

Rare earth element (REE) and other selected trace and minor element co ncentrations were measured in individual grains of orthopyroxene, feld spathic glass (of plagioclase composition) and merrillite of the ALH 8 4001 Martian meteorite. Unlike in other Martian meteorites, phosphate is not the main REE carrier in ALH 84001. The REE pattern of ALH 84001 bulk rock is dependent on the modal abundances of three REE-bearing p hases, namely, orthopyroxene, which contains most of the heavy rare ea rth elements (HREEs); feidspathic glass, which dominates the :Eu abund ances; and merrillite, which contains the majority of the light rare e arth elements (LREEs). Variations in the REE abundances previously obs erved in different splits of ALH 84001 can easily be explained in term s of small variations in the modal abundances of these three minerals without the need to invoke extensive redistribution of LREEs. At least some orthopyroxenes (i.e., those away from contacts with feldspathic glass) in ALH 84001 appear to have preserved their original REE zonati on from igneous fractionation. An estimate of the ALH 84001 parent mag ma composition from that of the unaltered orthopyroxene ''core'' (i.e. , zoned orthopyroxene with the lowest REE abundances) indicates that i t is LREE depleted. This implies that the Martian mantle was already p artly depleted within similar to 100 Ma of solar system formation, whi ch is consistent with rapid accretion and differentiation of Mars. Alt hough equilibration and exchange of REEs between phases (in particular , transport of LREEs into the interstitial phases, feldspathic glass a nd merrillite) cannot be ruled out, our data suggest that the LREE enr ichment in melts ''in equilibrium'' with these interstitial phases is most likely the result of late-stage infiltration of the cumulate pile by a LREE-enriched melt.