Re-Os isotopic evidence for early differentiation of the Martian mantle

Variations in the short-lived systems of Hf-182-W-182 and Sm-146-Nd-142 in the SNC meteorites indicate an early isolation of, and subsequent inefficient mixing between, mantle reservoirs in Mars. Correlations of epsilon (W) and epsilon (142Nd) with initial gamma (Os) are consistent with the Re-Os isotopic systematics of these meteorites being set during the earliest differentiation history of Mars. Contamination by a juvenile Martian crust may have affected Zagami Os isotopic systematics but successful contamination models combining Nd and Os systematics, are inconsistent with such a process affecting the isotopic compositions of the shergottite lherzolites (EETA 77005, LEW 88516, Y 793605). At least two long-lived mantle reservoirs, and possibly three, are required to explain the observed systematics. One mantle reservoir (NC Group), represented by Nakhla and Chassigny, has a projected present day gamma (Os) of -5.4 +/- 2.6. Another mantle reservoir represented by the shergottite lherzolites and possibly Zagami, has a present day gamma (Os) of about +4. This represents a 2 to 3% enrichment in Re/Os relative to the primitive mantle estimated for the Earth (+1.6). A third mantle reservoir may be represented by DaG 476, having a nearly chondritic gamma (Os) coupled with very high epsilon (143Nd) of around +40. The isotopic systematics of these reservoirs may be linked to development of cumulate crystal piles in a Martian magma ocean and variable amounts of late stage intercumulus melt (following Borg et al., 1997). In this model, fractional crystallization of olivine and Possibly other phases with slightly subchondritic Re/Os, from a solidifying magma ocean, resulted in a lower Re/Os ratio in the NC Group source cumulates, and a resultant low gamma (Os). Later cumulates or evolved melts crystallized with higher Re/Os ratios to produce the shergottite mantle reservoir(s), and hence, consequent higher gamma (Os). Crystallization of the Martian magma ocean followed earliest core formation, as indicated by the correlation of epsilon (W) with epsilon (142Nd) and initial gamma (Os). Copyright (C) 2000 Elsevier Science Ltd.