A Two Gigayear History of Germanium Outgassing from Shergottites

Germanium (Ge) and Zn enrichment in martian sedimentary rocks has been reported from rocks at Gale Crater, showing concentrations of Ge from tens to hundreds ppm [1]. The Ge concentrations in martian meteorites are significantly lower (0.5-2.5 ppm) [2]. Our recent studies [3-4] have revealed that Ge is lost from shergottites due to volatility. Recent experimental studies confirm that Ge and Zn are both significantly volatile under magmatic conditions [5-7]. Further, Ge is moderately incompatible during magmatic differentiation [8] so Ge contents in olivines or pyroxenes increase during igneous fractionation in nakhlites and chassignites [4]. Shergottites for which Ge abundances had been determined included rocks with ages of 150-600 Ma, while the enrichments reported from Gale Crater rocks likely occurred over 3 Ga ago. The recent discovery of two unpaired ancient (2.4 Ga) depleted shergottites, NWA 7635 [9] and NWA 8159 [10], afforded the prospect of obtaining an extended history of martian volcanic outgassing. Both of the ancient shergottites are depleted in incompatible elements and share a similar GCR exposure age to younger depleted shergottites implying derivation from a single, long-lived (>2 Ga) volcanic center [9].