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

The isotopic composition of lead was determined in different phases from Shergotty, Zagami and EETA 79001. Using phases with low 238U/204Pb, the initial leads of these meteorites could be well defined and are distinct from each other and have high 204Pb/206Pb (from 0.0652 to 0.0739). These leads evolved in different reservoirs over most of solar system history in a low 238U/204Pb approx. equal to 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 approx. 1.8. The U-Th-Pb isotopic relationships involving phases with high 238U/204Pb are quite regular and give ages for three shergottites which indicate an event of U-Th-Pb fractionation at approx. 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. (author)