Sulfide petrology of four nakhlites: Northwest Africa 817, Northwest Africa 998, Nakhla, and Governador Valadares

摘要

The nakhlites contain small proportions of Cu-Fe-Ni sulfide minerals; we have studied these sulfides in Northwest Africa (NWA) 998, Nakhla, Governador Valadares, and NWA 817 with optical microscopy, scanning electron microscope, and electron microprobe. Modal abundances of magmatic sulfides, as estimated by image analysis on thin section, are uniformly low (0.02 to 0.05±0.03vol%), i.e., a factor 5 lower than in shergottites. Sulfides occur within the glassy mesostasis, as composite two-phase Fe-Ti oxide-sulfide grains, intimately associated with interstitial grains or locally enclosed in postcumulus melt inclusions (e.g., Governador Valadares) in olivine. They exhibit a uniform low-Ni monoclinic pyrrhotite composition±chalcopyrite. There is a gradation of sulfide grain sizes and textures across the nakhlites flow(s): droplets in NWA 817; resorbed blebs in Governador Valadares; more massive, true intercumulus blebs in Nakhla and NWA 998. These nakhlites also show evidence for sulfide weathering. Hot desert finds (e.g., NWA 998 and NWA 817) show a few percent fracture-filling iron (oxy) hydroxides of likely terrestrial origin. Original sulfides are 50% altered in our NWA 998 section, with iron (oxy) hydroxides at grain boundaries and as complete pseudomorphs. The compositions of unaltered pyrrhotites are homogeneous, close to that of the monoclinic endmember Fe_7S_8, and are too sulfur-rich to have been in chemical equilibrium with the late magmatic redox state fixed by the fayalite-magnetite-quartz buffer. Therefore, the compositions of the pyrrhotites must have been altered during the later stages of magmatic crystallization, by assimilation of S-rich regolith and hydrothermal circulation.