REHOMOGENIZED MELT INCLUSIONS IN NAKHLA AUGITE AND ALHA 77005 OLIVINE: EVIDENCE FOR BASALTIC PARENTAL MAGMAS ON MARS

Previous estimates of parental magma compositions for Martian meteorites have been controversial. Melt inclusions are useful for delineating petrogenetic histories otherwise unavailable from bulk rocks. However, melt inclusions in SNC meteorites contain phases that crystallized after melt entrapment, requiring heating experiments to reconstruct their original trapped compositions. Recent results of heating experiments on melt inclusions in Chassigny olivine and Nakhla augite were interpreted to conflict with the well-accepted igneous origin for these rocks.

Our study involves slow- and fast-rate heating experiments on melt inclusions in Nakhla augite and ALHA77005 olivine. Following rehomogenization of melt inclusions within single minerals, grains were mounted and polished to expose inclusions and major element chemistry was determined by electron microprobe. Nakhla melt inclusions plot near the sub-alkaline/alkaline border on the total-alkali-silica diagram. The most primitive melt inclusion in Nakhla plots in the basalt field and closely matches previous estimates of nakhlite parent magma compositions. Extensive shock and resultant oxidation (darkening) of olivines in ALHA 77005 precluded visual monitoring of melt inclusions during heating. As a result, these melt inclusions were only partially homogenized, with the most primitive inclusion plotting in the basaltic-andesite field. These data trend towards basaltic compositions, but at lower total alkali contents than their Nakhla counterparts.

MELTS crystallization models calculated for the most primitive Nakhla melt inclusion composition produced phase assemblages and compositions consistent with nakhlite assemblages. Perhaps the most significant insight gained from these crystallization models is that olivine and augite co-crystallize from the starting compositions estimated for Nakhla. In the past, disequilibrium between augite and olivine in Nakhla has been attributed to (1) the hypothesis that olivine was xenocrystic or (2) subsolidus re-equilibration of both phases, with the re-equilibration of olivine occurring more rapidly. Our result is consistent with subsolidus re-equilibration. Contrary to previous interpretations, melt inclusions in SNC meteorites do support an igneous origin for these meteorites.