Sr and Nd isotopic systematics in ALHA 77005: Age of shock metamorphism in shergottites and magmatic differentiation on Mars

Abstract

The meteorite ALHA 77005 belongs to the group of SNC meteorites which are thought to be fragments of the planet Mars. New Sr and Nd isotopic data are reported for ALHA 77005 whole rock and petrographically distinct mineral separates. Plagioclase crystallized from a plagioclase shock melt, fractionating the Rb Sr ratio at the time of the shock event. A Rb/Sr age of 15 ± 15 Ma for the shock event is measured using this Rb Sr fractionation. This shock age is, within error, identical to the exposure age of 2.5 Ma. Using augite and pigeonite a Rb Sr crystallization age of 154 ± 6 Ma is measured, which is identical to the crystallization age of the Shergotty mesostasis-plagioclase. Petrographic evidence indicates that the cumulus olivine in ALHA 77005 is derived from a different source than the intercumulus liquid which crystallized the pyroxenes and the plagioclases. Petrographic comparison of all SNC meteorites suggests that most SNC meteorites are orthocumulates in which the cumulus phase comes from a different source than the intercumulus liquid. The interrelation of the different isotopic systems indicates that the SNC meteorites can be explained by mixing three isotopically distinct sources. These three sources were differentiated early in Martian history. In contrast to the Moon, where plagioclase fractionation is the dominant magmatic process, the major magmatic processes on Mars are mafic magmatism and mixing. Comparison of Martian and terrestrial isotopic systematics suggests that Mars accreted from a material chemically similar to C1 and ordinary chondrites, whereas the Earth has lower than chondritic Si Mg and Rb Pb ratios. A similar feature is observed when comparing C2, C3 and CV chondrites with Cl or ordinary chondrites: C2, C3, CV meteorites and the Earth have less Rb for their apparent Pb inventory.