Formation ages and evolution of Shergotty and its parent planet from U-Th-Pb systematics

Abstract

The isotopic composition of lead was determined in different phases from Shergotty, Zagami and EETA 79001. Using phases with low 238U204Pb, the initial leads of these meteorites could be well defined and are distinct from each other and have high 204Pb206Pb (from 0.0652 to 0.0739). These leads evolved in different reservoirs over most of solar system history in a low 238U204Pb ≈ 5 environment. It follows that the parent planet of the shergottites has a high 204Pb concentration relative to U and must also be high in other volatiles. If the parent planet of the shergottites is Mars, then this planet must be volatile-rich compared to the Earth by a factor of ~ 1.8. This implies that Mars lost its original atmosphere and that the present atmosphere was derived from a previously outgassed Martian upper mantle.The U-Th-Pb isotopic relationships involving phases with high 238U204Pb are quite regular and give ages for three shergottites which indicate an event of U-Th-Pb fractionation at ~200 m.y. The initial leads at that time were highly evolved relative to the primordial lead. The absence of substantial additions of radiogenic lead (above the initial lead) in high U phases precludes an older age for the U-Th-Pb fractionation. However, details of the data arrays are complex. The young age is in general agreement with some of the ages obtained by other methods, but precise concordance between the different methods is not established. It is suggested that the shock event that produced maskelynite may possibly be associated with the late-stage breakup at ~2.5 m.y. and not necessarily related to the ~ 180 m.y. record. The major event of chronometer resetting is taken to be ~ 180 m.y. This event does not appear to be an endogenic igneous event but an almost complete equilibration during a major impact on more ancient igneous rocks which were contained in the target. Problems of the polymetamorphic history of meteorites appear as complicated as those of polymetamorphic terrestrial terranes.