Partial melting of oxidized planetesimals: An experimental study to test the formation of oligoclase-rich achondrites Graves Nunataks 06128 and 06129

Abstract

The meteorites Graves Nunataks (GRA) 06128 and 06129 are igneous meteorites dominated by oligoclase feldspar and have basaltic trachyandesite-like whole rock compositions. Formation of the GRA 06128/9 meteorites as primary melts on an oxidized planetesimal has been previously proposed (Day et al. 2009a, 2012a; Gardner-Vandy et al., 2013; Wang et al., 2014). We show experimentally that anhydrous partial melting of an oxidized R chondrite at IW to IW+1 between 1120–1140°C produces melts of GRA 06128/9-like compositions: intermediate SiO2 and FeO concentrations that are enriched in volatile sodium. From a process perspective, GRA 06128/9-like magmas are complementary to partial melt residues such as olivine-rich brachinite and FeO-rich brachinite-like meteorites. Magmas of GRA 06128/9’s composition can be generated under equilibrium conditions, as demonstrated by MELTS modeling, but only at temperatures ∼1140°C. At lower degrees of partial melting liquids formed under equilibrium and non-equilibrium conditions follow distinct compositional pathways to reach GRA 06128/9-like melts. For lower degrees of melting, the non-equilibrium trend more closely resembles GRA 06128/9’s composition. Phase abundance modeling indicates that GRA 06128/9-composition magmas form by 14–22% silicate melting of an oxidized R-chondrite. We conclude that GRA 06128/9-composition magmas can be generated at ∼1140°C from partial melting of an oxidized chondritic precursor under both non-equilibrium and equilibrium conditions.