Melt inclusions in augite from the nakhlite meteorites: A reassessment of nakhlite parental melt and implications for petrogenesis

摘要

The nakhlites, a subgroup of eight clinopyroxenites thought to come from a single geological unit at the Martian surface, show melt inclusions in augite and olivine. In contrast to olivine-hosted melt inclusions, augite-hosted melt inclusions are not surrounded by fractures, and are thus considered preferential candidates for reconstructing parent liquid compositions. Furthermore, two types of augite-hosted melt inclusion have been defined and characterized in four different nakhlites (Northwest Africa [NWA] 817, Nakhla, Governador Valadares, and NWA 998): Type-I isolated inclusions in augite cores that contain euhedral to subhedral augite, Ti-magnetite, and pigeonite plus silica-rich glass and a gas bubble; Type-II microinclusions that form trails crosscutting host augite crystals. Fast-heating experiments were performed on selected pristine primary augite-hosted melt inclusions from these four samples. Of these, only data from Nakhla were considered robust for reconstruction of a nakhlite parental magma composition (NPM). Based upon careful petrographic selection and consideration of iron-magnesium ratios, our data are used to propose an NPM, which is basaltic (49.1wt% SiO_2), of high Ca/Al (1.95), and K_2O-poor (0.32wt%). Thermodynamic modeling at an oxygen fugacity one log unit below the QFM buffer using the MELTS and PETROLOG programs implies that Mg-rich olivine was not a liquidus phase for this composition. Our analysis is used to suggest that olivine megacrysts found in the nakhlites are unlikely to have coprecipitated with augite, and thus may have been introduced during or subsequent to accumulation in the magma chamber, possibly from more evolved portions of the same chamber.