On the Stability of Sulfides, Oxides, and Native Metals in Serpentinite

Abstract

Topologic relations indicate that the lowest relative oxygen fugacity attained in magnetite-bearing serpentinites occurs in the presence of the assemblage serpentine-olivine-brucite. This is consistent with the observation that Ni-Fe alloys are preferentially associated with this assemblage, both in prograde metaserpentinites, where the stable serpentine mineral is antigorite, and in retrograde serpentinites, where either antigorite, lizardite, or chrysotile may be present. The presence of the Ni-Fe alloy indicates that the assemblage serpentine-olivine-brucite-magnetite in prograde metaserpen-tinites equilibrated with an oxygen fugacity four to five log units below the FMQ buffer whereas in retrograde environments it equilibrated at oxygen fugacities as low as six or seven log units below FMQ. In carbonate-bearing metaperidotites the oxygen fugacity is buffered by the assemblage Fe-Mg carbonate-Fe oxide and this buffering allows the oxygen fugacity to attain values above those of the HM buffer for rocks with relatively high-Xco2> such as those containing the assemblage talc-1- magnesite. A considerable gradient in fOl may therefore be present across a serpentinite body from a partially serpentinized core to a carbonatized margin. This gradient is reflected in the composition of the Fe and Ni sulfides. Sulfur-rich sulfides, such as millerite or vaesite and pyrite, occur in carbonate-rich