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

Phase relations in the S-poor portions of the dry condensed Pt-Ir-Fe-S system were determined at 1000° and 1100°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 γ(Ir,Fe,Pt) alloy coexists with γ-(Pt,Fe), Pt₃Fe and PtFe which dissolve respectively at least 5.1, 29.3 and 24.0 at.% Ir at 1100°C (2.2, 23.6 and ⩽17.2 at.% Ir at 1000°C). Gaps between the nearly Ir-free Pt-Fe alloys γ(Pt,Fe), Pt₃Fe s.s., PtFe s.s. and γ(Fe,Pt) were estimated as 20–23 at.%, 40–42 at.% and 54.2–~57 at.% Fe at 1100°C (18–23, 39.5–42.5 and 59–62 at.% Fe at 1000°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_1–xS dissolves up to 5.8 at.% Ir at 1100°C and 3.4 at.% Ir at 1000°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°C. The Fe-Ir thiospinel dissolves up to 2.2 at.% Pt (for the composition Fe_21.6Ir_18.9Pt_2.2S_57.3 at 1100°C). The 25–28 at.% Fe portions of Pt₃Fe with <10 at.% Ir partake in all important sulphide (± melt)-alloy assemblages (with PtS, Ir₂S₃, pyrrhotite, S-rich melt and thiospinel). Therefore, their use in recognizing distinct natural assemblages of genetic importance is of limited value.