Petrology and mineralogy of the angrite Northwest Africa 1670

Abstract

Northwest Africa (NWA) 1670, contains olivines of up to 5 mm in size representing about 30% of the studied section. With subordinate clinopyroxene and chrome-spinel microphenocrysts (0.2-0.5 mm), they represent a xenocrystic association. Phenocrysts are surrounded by a groundmass, predominantly comprising bundles of plagioclase and clinopyroxene (typically 20 x 200 m crystals). Olivine and kirschsteinite are present in the groundmass in lesser amounts. The olivine xenocrysts (Fo90) are significantly fractured and show mosaicism for their major part, the remaining showing faint undulatory extinction. They are surrounded with a rim of 100-200 micrometers zoned down to Fo80 and overgrown with serrated olivine, Fo80 to Fo60 (about 100 micrometers). Olivine in the groundmass is zoned from Mg# 0.55 to 0.15; its CaO content ranges 2.0 to 8.4%. Subcalcic kirschsteinite is zoned from Mg# 0.13 to 0.03, CaO increasing from 15.8 to 21.3%. Pyroxenes xenocrysts (Mg# = 0.77) are superseded in the groundmass by less magnesian pyroxenes, Mg# 0.61 to 0.17, with an average FeO/ MnO of 98. Their compositions range from En30Fs22Wo27Al-Ts28Ti-Ts2 to En2Fs37Wo22Al-Ts40Ti- Ts1. Anorthite microcrysts (An99-100) are restricted to the groundmass. Accessories are pyrrhotite, kamacite, Ca-phosphate, titanomagnetite, hercynite and Ca-carbonate. The bulk chemical composition confirms that NWA 1670 corresponds to a normal angrite melt that incorporated olivine. High Mg olivine xenocrysts and the associated mineralogy are typical of angrites. We suggest that it is an impact melt with relict phenocrysts. The strong silica undersaturation, the presence of Fo90 olivine xenocrysts and carbonate support their derivation as melilite-like melts in the presence of carbonate.