Arya Udry^a, Nicole G. Lunning^a, Harry Y. McSween JR.^a and Robert J. Bodnar^b

^aPlanetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, TN, USA
^bDepartment of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, VA, USA

Northwest Africa (NWA) 7034 and its paired meteorites NWA 7533 and NWA 7475 are the first recognized martian polymict breccia samples. An unusual, large, subrounded clast in NWA 7034 shows a vitrophyric texture, consisting of skeletal pyroxene and olivine with mesostasis. This lithology has not been observed in the paired meteorites. It crystallized under disequilibrium conditions as indicated by its olivine and pyroxeneK_D^Fe/Mg partitioning values, as well as reversed order of crystallization and mineral compositions relative to those predicted by MELTS. We report the highest bulk Ni value (1020 ppm) measured in any known martian meteorite or martian igneous rock, suggesting an impact melt origin for the vitrophyre. Addition of 5.3-7.7% chondritic material to the target rock would account for the Ni enrichment. The bulk major and trace element abundances of the vitrophyre indicate that the protolith was not the host breccia nor any other martian meteorites. However, the clast is compositionally similar to Humphrey rock in Gusev crater analyzed by the Spirit rover and to a texturally distinct group of clasts in the paired meteorite NWA 7533. Thus, we propose that the target rock was an igneous lithology similar to Gusev basalts, which was subsequently contaminated by a chondritic impactor.

Reference
Udry A, Lunning NG, McSween JR. HY and Bodnar RJ (in press) Petrogenesis of a vitrophyre in the martian meteorite breccia NWA 7034. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.06.026]
Copyright Elsevier

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