Partial Melting of Oxidized Planetesimals: An Experimental Study to Test the Formation of Oligoclase-rich Achondrites Graves Nunataks 06128 and 06129

Nicole G. Lunning, Kathryn G. Gardner-Vandy, Emma S. Sosa, Timothy J. Mccoy, Emma S. Bullock, Catherine M. Corrigan

Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 11 July 2017


“The meteorites Graves Nunataks (GRA) 06128 and 06129, however, are igneous meteorites dominated by oligoclase feldspar and have a basaltic trachyandesite-like whole rock composition. Formation of the GRA 06128/9 meteorites as primary melts on an oxidized planetesimal has been previously proposed (, , ; ). 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.”