The Origin of Ni-bearing Sulfides in CI Carbonaceous Chondrites

Experimental studies of sulfide formation in H2-H2S gas show that sulfurization of an FeNi alloy does not change the bulk Fe/Ni ratio of the metal. Instead, the Fe/Ni ratio of the resulting sulfides is equal to that of the metal. This is in contrast to thermodynamic equilibrium calculations which predict that sulfide formation in the solar nebula produced Ni-free troilite (FeS) and enriched the remaining metal in Ni. Our data show that the Ni content of the sulfide layer increases with distance from the metal. The outer edge of the sulfide layer is enriched in Ni and is composed of both the monosulfide solid solution and pentlandite. These results suggest that mss and pentlandite formed concurrently in the solar nebula. The experimental sulfides are compositionally and morphologically similar to sulfides in the Alais CI chondrite, suggesting that these meteoritic sulfides are solar nebula condensates.