Geochronology of Shergottite Meteorites: Using LA-MC-ICP-MS Analysis to Examine U-Th-Pb Systematics of Baddeleyites and Phosphates

Abstract

I present in-situ analysis of U-Pb systematics in baddeleyite and whitlockite grains from a suite of Martian shergottites. 9 baddeleyite grains (5 from basaltic shergottite NWA 2986 and 4 from olivine-phyric shergottite RBT 04262) were analyzed by LA-MC-ICP-MS. Despite low uranium and radiogenic lead concentrations , maximum ages could be determined for both samples: 187 ± 50 to 1236 ± 430 for NWA 2986 and 100 ± 9 to 526 ± 48 for RBT 04262. The same analytical procedures were used for whitlockites in NWA 2986, ALHA 77005, EETA 79001, NWA 2646 and LAR 06319. As with the baddeleyite analyses, maximum ages were calculated. These ages ranged from 110 ± 1 for LAR 06319 to 561 ± 185 for NWA 2646. These results, which are consistent with previous analyses, mean that the ~ 4 Ga age determined from Pb-Pb analyses cannot time the igneous crystallization of these meteorites.