Sulfides,oxides and sphene in high-pressure schists from New Caledonia

In the New Caledonia high-pressure schists pyrite, pyrrhotite, chalcopyrite, rutile and sphene are common phases while hematite and ilmenite are rare and magnetite is absent. The parageneses of these minerals were clarified from their occurrence as inclusions in garnet, from phase relations in the Cu-Fe-S and Fe-Ti-O-S systems, and from phase rule considerations for the multi-component system. The sulfur fugacity estimated for pelites and basites containing pyrrhotite, pyrite and rutile increased with increasing metamorphic grade; the oxygen fugacity in these schists was less than 10^–27.6 bars at 400° C, 10 kb and 10^–22.3 bars at 500° C, 11 kb. Among the other components of the metamorphic fluid in pelites, H₂O was major, CH₄, CO₂ and H₂S minor, and H₂, CO, COS and SO₂ rare. The fluid composition altered with advancing metamorphic grade, such that H₂O decreased while CO₂, CH₄ and H₂S increased, and this change was linked to concurrent massive decarbonization in the rock matrices.