Chronology of SNC meteorites

Abstract

In this paper a model is presented for the geochemical evolution of Mars which is constrained by the isotope systematics of Pb, Nd, and Sr determined for SNC meteorites (SNCs). The young magmatic crystallization ages (internal or mineral ages) of SNCs may indicate that these meteorites indeed stem from Mars. Internal ages and U-Pb and Pb-Pb systematics strongly suggest that they are the result of two magmatic processes. In addition, shock metamorphism is implied from observed petrographic shock features. For ALHA 77009 a shock-age < 15 Ma is obtained which is within uncertainty identical to the independently determined cosmic ray exposure age. It is therefore plausible that shock and exposure ages are identical for all SNCs. The Rb/Sr data of all common (non-SNC) meteorites form a 4.55 Ga isochron as do the Pb-Pb data (geochron). The SNC data fall close to these two isochrons. The Sr and Pb isotopic compositions in SNCs suggest that they formed in a recent (1.3-0.15 Ga) melting event from reservoirs which had been magmatically differentiated 4.3 ± 0.2 Ga ago. In a concordia diagram (U-Pb evolution plot) the SNC data reflect recent increase of the U/Pb ratio and the same two stage magmatic history as suggested by the other isotopic systems. The oxygen isotopic composition as well as the Nd isotopic systematics strongly suggest that the SNCs stem from one common reservoir which chemically differentiated 4.3 ± 0.2 Ga ago and then formed sub-reservoirs. In contrast to common meteorites, SNCs experienced an early magmatic differentiation where the Sm/Nd, U/Pb and Rb/Sr ratios have been strongly fractionated. In the recent magmatic process (1.3-0.15 Ga ago), in which the SNCs were formed as rocks, Sm/Nd and U/Pb were fractionated, while Rb/Sr remained similar to that of the source from which the magmas originated. During these melting events, mixing of components from different sub-reservoirs might have had occurred. At least three subreservoirs are necessary to explain the isotopic variations observed in SNCs. In contrast to the isotopic evolution of the Earth, Mars conserved remnants of the primary differentiation, a fact, which places important constraints on the tectonic evolution of Mars.