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

1Nicole G. Lunning, 1,2Kathryn G. Gardner-Vandy, 1,3Emma S. Sosa, 1Timothy J. Mccoya, 4Emma S. Bullock, 1Catherine M. Corrigan
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.07.004]
1Department of Mineral Sciences, Smithsonian Institution, National Museum of Natural History, Washington, DC 20560, USA
2Department of Geosciences, The University of Tulsa, Keplinger Hall L101 The University of Tulsa, 441 South Gary Avenue Tulsa, OK 74104
3Department of Geology and Environmental Geosciences, Lafayette College, 116 Van Wickle Hall, 4 South College Dr. Easton, PA 18042
4Geophysical Laboratory, Carnegie Institution for Science, 5251 Broad Branch Road NW Washington DC 20015
Copyright Elsevier

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 (Day et al., 2009a; Day et al., 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.

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