Martian meteorite chronology and the evolution of the interior of Mars

The Earth, Moon, and Mars are the only three planets for which satellite and ground observations can be confronted with extensive chronological and geochemical observations of rock samples. Here we use new Pb isotope data on shergottites, nakhlites, Chassigny, and the ALH 84001 orthopyroxenite to reassess chronological evidence and discuss its bearing on the evolution of the interior of Mars. We find that the depleted shergottites formed at 4.3 Ga, i.e., similar to 170 Ma after the last equilibration of pyroxene-rich cumulates with the magma ocean, while the younger enriched and intermediate shergottites, as well as ALH 84001, formed similar to 200 Ma later at similar to 4.1 Ga, after the residual part of the mantle had been largely mixed back into these cumulates. We also demonstrate that the Th/U ratios inferred from Pb isotopes require shergottite ages older than similar to 4.0 Ga. We finally reexamine the implications of this new chronological framework for the interpretation of Nd-142 and W-182 anomalies in Martian meteorites. We conclude that shergottites represent samples that derived from a volcanic basement subsequently shattered by the similar to 3.9 Ga Late Heavy Bombardment (LHB). Although the magmatic activity of Mars had strongly declined by the time of the LHB, the new chronology of Martian meteorites proposed here allows the Martian mantle to be convective until today. The 1.3 Ga old emplacement of nakhlites and chassignites corresponds to decompression melting of ilmenite-rich mantle domains and not to renewed global magmatic activity of the planet. With the exception of this event, the history of the interior of Mars therefore somewhat resembles that of the Moon. (C) 2009 Elsevier B.V. All rights reserved.