Chemical and Isotopic Constraints for the Martian Crust

Introduction: The Martian meteorite whole rock Rb-Sr isotopic compositions and the initial Pb data from plagioclase separates suggest strongly that the crust of their parent body originated from an ancient planetary global differentiation process about 4.5 Ga ago. The absence of plate tectonic activity on early Mars excludes a crustal recycling and preserves the isotopic systems derived from the early crustal differentiation. Together with geophysical and geochemical data from spacecraft missions, the Martian meteorites provide constraints on the nature of the crust. Based on element correlations observed in the Martian meteorites and the measured K content of the Martian surface we propose an estimate of the Rb-Sr systematics of the crust. Isotopic reservoirs of the crust: All whole rock data of the Rb-Sr isotopes cluster in 3 groups close to the meteoritic isochron of 4.55 Ga. Borg et al [1, 2] interpreted the Rb-Sr whole rock isochron as a mixing line of an ancient depleted mantle and an old evolved crust. We favored an early crust-mantle differentiation forming 3 distinct isotopic reservoirs, which remained isolated since 4.3 ± 0.2 Ga. [3, 4]. The representatives of the isotopically enriched crustal reservoir are the basaltic shergottites: Shergotty, Zagami and Los Angeles. They have relatively high abundances of radiogenic Sr, which might originate from a planetary crust enriched in incompatible elements. The Pb systematic on the SNCs reveals also that U was enriched in the crust and depleted in the mantle ~ 4.5 Ga. The conformity of the U-Pb and Rb-Sr isotopic systematics reflects similar magmatic fractionation behaviour of Rb and U during the evolution of curst and mantle.