The igneous crystallization history of an ancient Martian meteorite from rare earth element microdistributions

Details The igneous crystallization history of an ancient Martian meteorite from rare earth element microdistributions The igneous crystallization history of an ancient Martian meteorite from rare earth element microdistributions

Jul 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998m%26ps...33..685w&link_type=abstract

Meteoritics & Planetary Science, vol. 33, no. 4, pp. 685-692.

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Scientific paper

Rare earth element (REE) and other selected trace and minor element concentrations were measured in individual grains of orthopyroxene, feldspathic glass (of plagioclase composition) and whitlockite of the ALH84001 martian meteorite. Unlike in other martian meteorites, phosphate is not the main REE carrier in ALH84001. The REE pattern of ALH84001 bulk rock is dependent on the modal abundances of three REE-bearing phases, i.e., orthopyroxene (which contains most of the HREEs), feldspathic glass (which dominates the Eu abundances) and whitlockite (which contains the majority of the LREEs). Variations in the REE abundances previously observed in different splits of ALH84001 can easily be explained in terms 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 ALH84001 appear to have preserved their original REE zonation from igneous fractionation. Estimate of the ALH84001 parent magma composition from that of the unaltered orthopyroxene `core' (i.e., zoned orthopyroxene with the lowest REE abundances) indicates that it is LREE-depleted. This implies that the martian mantle was already partly depleted within ~100 Ma of solar system formation, consistent with rapid accretion and differentiation of Mars. Although equilibration and exchange of REEs between phases (in particular, transport of LREEs into the interstitial phases, feldspathic glass and whitlockite) cannot be ruled out, our data suggest that the LREE-enrichment 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.

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