Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.v62b1403f&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #V62B-1403
Physics
3640 Igneous Petrology, 3662 Meteorites, 3672 Planetary Mineralogy And Petrology (5410)
Scientific paper
Olivines and pyroxenes in gabbroic rock fragments from the recently found meteorite Northwest Africa 773 (NWA773) show evidence of Fe-enrichment broadly similar to the tholeiitic trend defined for terrestrial rocks. The meteorite consists of two main lithologies: a two-pyroxene olivine gabbro and a fragmental, heterolithic breccia. The olivine gabbro lithology consists of olivine (Fo68), pigeonite (Wo11En65), augite (Wo36En50), and plagioclase (An90), with minor K,Ba-feldspar (Or89Cs04Ab04An02), chromite, Ca-phosphates, ilmenite, troilite and metal. The texture is dominated by cumulate olivine crystals up to 1300 æm across and pyroxenes of slightly smaller grain size. Plagioclase feldspar is interstitial and K,Ba-feldspar is restricted to interstitial sites enriched in incompatible elements. The breccia lithology consists of a variety of clasts, but most of these can be plausibly linked to the olivine gabbro or more Fe-rich differentiates from the same magmatic system. Pyroxenes from the breccia exhibit continuous trends in Mg/(Mg+Fe) and Ti/(Ti+Cr) from values similar to the olivine gabbro (0.79 and 0.18, respectively) to extremely differentiated compositions (0.06 and 1.00, respectively). Olivines are characterized by compositions ranging over Fo71-55 and Fo13-01. The fayalitic olivines occur in clasts with silica, +/- hedenbergitic pyroxene. These variations in mineral composition can be explained as a consequence of extreme Fe-enrichment in a single pluton or related plutonic bodies on the Moon. The presence of silica in NWA 773 is due primarily to Fe-enrichment of residual liquid into ?the forbidden zone? of the pyroxene quadrilateral. The Fe-enrichment trend and cumulate textures suggest that NWA 773 may have originated from a magmatic system broadly similar to terrestrial layered mafic intrusives.
Bunch Ted E.
Fagan Timothy J.
Hicks T. L.
Jeffrey Taylor G.
Keil Klaus
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