PETROGENESIS OF PARENT MAGMAS OF SNC METEORITES

Abstract

Primary magmas for the investigated SNC meteorites have been estimated on the basis of the bulk compositions of "basaltic shergottites" and melt inclusion data for other SNCs. All estimated compositions are similar in terms of major elements. These compositions are high in Mg + Fe and Si mole fractions, and they are similar in this respect to terrestrial boninites.These primary magma compositions could only bave been produced by small degree partial melting of an olivine-rich source, which could be related to the Martian primitive mantle either by previous episodes of the extraction of basaltic melts, which formed a very thick crust, or by formation of olivine-rich cumulates during the consolidation of primordial magma ocean, which originally had the composition of the Martian primitive mantle (similar to terrestrial pyrolite, but with significantly higher Fe/Mg ratio).The estimated contents of heat producing elements in this olivine-rich source are very low, and they are likely to be not sufficient to provide energy for magma genesis, if the whole of the Martian mantle now consists of such very depleted harzburgite.The hypothesis is favored that assumes primitive magma generation in the olivine-rich cumulate pile, originally formed during the consolidation of the primordial global magma ocean. The solidified layer of residual melt, which underlay the olivine-rich cumulate pile, may be enriched in incompatible elements, including K, U, and Th, whose radioactive decay might have provided heat for the initiation of plumes in which primary magmas of SNC meteorites originated at higher levels. This scenario is consistent with trace element and isotope data, which provide evidence that the Martian mantle retains geochemical heterogeneities from the earliest stages of its history resulting from an early isolation of, and subsequent inefficient mixing between, different mantle reservoirs in Mars.