Experimental Modeling of Mantle Metasomatic Processes Involving Carbon- and Sulfur-Bearing Agents in an Olivine–Siderite–Sulfur System at Early-Subduction Mantle Conditions

During Precambrian subduction, banded iron formations (BIFs) may have represented an early significant input of carbonates and atmospheric sulfur into the lithospheric mantle, initiated the formation of carbonatite melts and probably triggered mantle metasomatic processes. Experimental modeling of the interaction of BIF-related metasomatic agents with upper mantle rocks, under Archean subduction geotherm, was performed in the simplified olivine–siderite–sulfur system. Experiments were carried out at 6.3 GPa, temperature range of 1150–1450 °C and durations of 20–60 h, using a multi-anvil high-pressure apparatus of a “split-sphere” type. Interaction of Fe,Ni–olivine and siderite with sulfur melt leads to the formation of pyrrhotite + orthopyroxene + ferromagnesite ± magnetite assemblage (1150–1250 °C, 40–60 h), as well as generation of sulfide (1350–1450 °C) and carbonate–silicate melts (1450 °C, 20 h); in all experiments, dissolution of diamond seed crystals is observed. Redox conditions are estimated to approximately correspond to ƒO2 ~ CCO + 0.5 log. units. We demonstrated that during experiments, sulfidation of Fe,Ni–olivine and siderite and redox dissociation of siderite took place. It was shown that indicators of these processes are (1) polycrystalline orthopyroxene + pyrrhotite and pyrrhotite + ferromagnesite aggregates, inheriting shape of olivine and siderite crystals, respectively; (2) orthopyroxene zoned crystals with pyrrhotite inclusions; (3) pyrrhotite with ferromagnesite inclusions and ferromagnesite with magnetite + pyrrhotite inclusions. The obtained results with certain limitations can be used to constrain processes of metasomatic alteration of upper mantle rocks by agents enriched in iron, sulfur, and carbon under relatively oxidizing conditions (ƒO2 ~ CCO + 0.5 log. units).