First evidence for infiltration metasomatism in a martian meteorite, ALH 84001

Details First evidence for infiltration metasomatism in a martian meteorite, ALH 84001 First evidence for infiltration metasomatism in a martian meteorite, ALH 84001

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.545w&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 545

Other

7

Infiltration, Mars (Planet), Meteorites, Meteoritic Composition, Abundance, Ion Probes, Mineralogy, Rare Earth Elements, Trace Elements

Scientific paper

ALH 84001, originally classified as a diogenite, was recently recognized by Mittlefehldt as a new member of the clan of martian meteorites. It is a coarse-grained orthopyroxenite with same O isotopic composition as the nakhlites. Most of this meteorite consists of orthopyroxene grains; it also contains maskelynite, chromite, and accessory minerals including apatite, augite, pyrite, and Mg-Ca-Mn-Fe carbonates. With the ion microprobe, we measured the concentrations of Rare Earth Elements (REEs) and other selected minor and trace elements in individual grains of orthopyroxene, maskelynite, and apatite. Although in all SNCs phosphate is the mineral with the highest REE concentrations, it is not the major REE carrier in ALH 84001. Using the most appropriate partition coefficients for these minerals in SNCs, we estimated the compositions of the metals that may have been in equilibrium with the 'average' orthopyroxene, the apatite, and the maskelynite. The orthopyroxene equilibrium melt is slightly LREE depleted, whereas the apatite and maskelynite equilibrium melts have higher REE concentrations and are strikingly LREE enriched. We tried unsuccessfully to derive the apatite and maskelynite melts from the orthopyroxene melt by fractional crystallization. We therefore suggest that an infiltrating fluid enriched in LREE is responsible for the formation of the apatite and maskelynite that occur as interstitial grains in ALH 84001. The similarity of REE patterns for the parent melts of ALH 84001, Shergotty, and Zagami seems to indicate that the new SNC meteorite is more closely related to these two shergottites than to any of the other meteorites thought to have come from Mars.

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