Phase relations in the metal-rich portions of the phase system Pt-Ir-Fe-S at 1000 degrees C and 1100 degrees C

Phase relations in the S-poor portions of the dry condensed Pt-Ir-Fe-S system were determined at 1000 degrees and 1100 degrees C with a particular emphasis on delineation of the solid solubility fields of the Pt-Ir-Fe alloys. At both temperatures, a broad field of gamma (Ir,Fe,Pt) alloy coexists with gamma -(Pt,Fe), Pt (sub 3) Fe and PtFe which dissolve respectively at least 5.1, 29.3 and 24.0 at.% Ir at 1100 degrees C (2.2, 23.6 and < or =17.2 at.% Ir at 1000 degrees C). Gaps between the nearly Ir-free Pt-Fe alloys gamma (Pt,Fe), Pt (sub 3) Fe s.s., PtFe s.s. and gamma (Fe,Pt) were estimated as 20-23 at.%, 40-42 at.% and 54.2- approximately 57 at.% Fe at 1100 degrees C (18-23, 39.5-42.5 and 59-62 at.% Fe at 1000 degrees C). The first gap agrees with data from natural phases by Cabri et al. (1996). The Fe-rich sulphide melt dissolves only traces of Pt and Ir; Fe (sub 1-x) S dissolves up to 5.8 at.% Ir at 1100 degrees C and 3.4 at.% Ir at 1000 degrees C. These solubilities and the compositions of coexisting alloys (before low-temperature exsolution) may serve as geothermometers. The S-rich melt dissolves up to 8.2 at.% Pt and 2.5 at.% Ir (measured on a solidified melt) at 1100 degrees C. The Fe-Ir thiospinel dissolves up to 2.2 at.% Pt (for the composition Fe (sub 21.6) Ir (sub 18.9) Pt (sub 2.2) S (sub 57.3) at 1100 degrees C). The 25-28 at.% Fe portions of Pt (sub 3) Fe with <10 at.% Ir partake in all important sulphide (+ or - melt)-alloy assemblages (with PtS, Ir (sub 2) S (sub 3) , pyrrhotite, S-rich melt and thiospinel). Therefore, their use in recognizing distinct natural assemblages of genetic importance is of limited value.