Pyrrhotite–silicate melt partitioning of rhenium and the deep rhenium cycle in subduction zone

The Re-Os isotopic system serves as an important tracer of recycled crust in Earth’s deep mantle because of the large Re/Os ratios and time-integrated enrichment of radiogenic Os in Earth’s crust. However, the Re distribution in Earth’s known reservoirs is mass imbalanced, and the behavior of Re during subduction remains little understood. We performed laboratory experiments to determine the partition coefficients of Re between pyrrhotite and silicate melt (DRepo/sm) at 950–1080 °C, 1–3 GPa, and oxygen fugacities (in log units relative to the fayalite-magnetite-quartz [FMQ] buffer) of FMQ –1.3 to FMQ +2. The obtained DRepo/sm values are 200–25,000, which increase with decreasing oxygen fugacity and the total iron content (FeOtot) of silicate melt but decrease with increasing temperature or decreasing pressure. Applying DRepo/sm to constrain the behavior of Re during slab melting demonstrates that slab melts contribute minimal Re to the sub-arc mantle, with most Re dissolved in sulfides subducted into Earth’s deep mantle. Deep storage of recycled oceanic basalts and sediments can explain the mass imbalance of Re in Earth’s primitive mantle, depleted mantle, and crust.

Supplemental Material: Pyrrhotite–silicate melt partitioning of rhenium and the deep rhenium cycle in subduction zones

Description of experimental and analytical methods, Figures S1–S7, and Tables S1–S6.

Supplemental Material: Pyrrhotite–silicate melt partitioning of rhenium and the deep rhenium cycle in subduction zones

Description of experimental and analytical methods, Figures S1–S7, and Tables S1–S6.

The sulfur solubility minimum and maximum in silicate melt

The behaviour of sulfur in magmas is complex because it dissolves as both sulfide (S2-) and sulfate (S6+) in silicate melt. An interesting aspect in the behaviour of sulfur is the solubility minima (SSmin) and maxima (SSmax) with varying oxygen fugacity (fO2). We use a simple ternary model (silicate–S2–O2) to explore the varying fO2 paths where these phenomena occur. Both SSmin and SSmax occur when S2- and S6+ are present in the silicate melt in similar quantities due to the differing solubility mechanism of these species. At constant T, a minimum in dissolved total S content (wmST) in vapour-saturated silicate melt occurs along paths of increasing fO2 and either constant fS2 or P; for paths on which wmST is held constant with increasing fO2, the SSmin is expressed as a maximum in P. However, the SSmin is not encountered during closed-system depressurisation in the simple system we modelled. The SSmax occurs when the silicate melt is multiply-saturated with vapour, sulfide melt, and anhydrite. The SSmin and SSmax influence processes throughout the magmatic system, such as mantle melting, magma mixing and degassing, and SO2 emissions; and calculations of the pressures of vapour-saturation, fO2, and SO2 emissions using melt inclusions.