Uranium-lead age of baddeleyite in shergottite Roberts Massif 04261: Implications for magmatic activity on Mars

摘要

The issue of the duration of Martian magmatic activity is one of the controversial debates recently: whether the young radiometric ages of～180 Ma for shergottites,a class of Martian meteorites, reflect the timing of crystallization from a magma or rather record later events related to shock metamorphism. In order to address the timescale over which Mars was geologically active, we have undertaken U-Pb isotopic studies on baddeleyite in the Roberts Massif (RBT) 04261 shergottite. Baddeleyite in RBT 04261 is usually associated with ilmenite and shows monoclinic structure, suggesting that the baddeleyite could have formed by crystallization from a residual liquid and not by shock metamorphism. In situ U-Th-Pb isotopic analyses of baddeleyite yield a young ~(238)U_~(206)Pb age of～200 Ma. Since the U-Pb system of baddeleyite is considered to be more resistant to resetting during reheating events, and olivine in RBT 04261 preserves an igneous calcium zoning, the young age of baddeleyite could be interpreted as a crystallization age of RBT 04261. The present results imply that Martian magma was still forming only 200 Ma ago, and that Mars had been geologically active until the recent past.