UC San Diego

Multiple models exist for forming continental crust (e.g. mantle plumes, relamination, arc magmatism), each with different implications for water budget and distribution. Arc systems produce garnet-pyroxene rich rocks within lower crust and these are generally defined as eclogite (metamorphosed basalt) or arclogite (igneous cumulate). The former should be dehydrated and the latter hydrated. By quantifying water content and trace elements of nominally anhydrous minerals (NAMs) from lower crustal rocks we may assess the mode of their formation. Analyses of Proterozoic, kimberlite-hosted two-pyroxene garnet granulite xenoliths from State Line, CO suggest an igneous origin. Using energy dispersive spectroscopy (EDS) maps and average mineral water contents, we find that whole rocks contain 100 to 650 ppm H2O, with clinopyroxene as the main water-bearing NAM. Analyses of Paleozoic amphibole gabbronorites from the exhumed Famatinia Arc, Argentina indicate secondary mineralization of hydrous phases with amphibole being the primary carrier for water. Additionally, comparison of observed trace element partitioning between NAMs in xenoliths shows that 1) reconstructed bulk rocks can “see through” the host magma contamination, but 2) care must be taken to account for accessory minerals, which may incorporate significant trace elements, in reconstructing bulk compositions. Comparison of xenolith vs. exhumed arc lower crust suggests that the latter may experience retrogression and re-equilibration of NAM water content, resulting in re-partitioning that is not representative of the original hydration state at depth.