The Joins FeS2–NiS2 and FeS2–S at 6 GPa

Phase relations along the FeS2–NiS2 and Fe0.36S0.64–S joins were studied at 6 GPa in ceramics capsules using a multianvil Osugi-type press. The FeS2–NiS2 system has an eutectic type of T–X diagram with limited solid solutions. The solubility of NiS2 in pyrite increases with temperature from 1–6 mol % at 450–700°С, to 7–8 mol % at 800–1100°С, and achieves a maximum of 25 mol % at 1250°С. The FeS2 content in vaesite increases from 2–4 mol % at 450–600°С, to 5–20 mol % at 650–900°С, and achieves the maximum, 64 mol %, at 1250°С. The pyrite-vaesite eutectic is situated near 1260°C and has Ni# 42 mol %, where Ni# = Ni/(Ni + Fe) × 100 mol %. At 6 GPa, FeS2 and NiS2 melt congruently near 1380 and 1410°C, respectively. In the range of 1000–1200°C, the solubility of Fe in liquid sulfur does not exceed the detection limit (<0.5 mol %). At 1300°C, liquid sulfur coexisting with pyrite contains 10 mol % Fe. At 1400°C, a continuous transition from a sulfur melt to a sulfide melt is observed. Thus, FeS2 and NiS2 are thermodynamically stable over the entire range of geothermal conditions of subduction zones and the subcontinental lithospheric mantle at a depth of about 180 km and experience congruent melting at temperatures corresponding to the thermal anomalies at the cratonic roots (kinked geotherms) and geothermal conditions of the convective mantle.