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

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“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.”