Chalcophile element partitioning between Cu-rich sulfide phases and silicate melt and implications for the formation of Earth’s continental crust

Abstract

To constrain the behavior of chalcophile (sulfide-loving) elements during arc magmatic differentiation and to understand the formation conditions of Earth’s continental crust, the partition coefficients (D) of Mn, Co, Cu, Zn, As, Se, Mo, Ag, Cd, Sn, Sb, Te, Re, Au, Pb, and Bi between monosulfide-solid-solution (MSS), Cu-rich sulfide liquid (SL; containing 11–45 wt.% Cu), and hydrous silicate melt (SM) of basaltic to dacitic compositions were determined at 1000–1200 °C, 0.5–1.0 GPa, and fO2 1–1.5 log units above the fayalite–magnetite–quartz (FMQ) buffer. The DSL/SM values are 16–160 for Co, 1100–8400 for Cu, 50–220 for Se, 1200–5900 for Ag, 50–1800 for Cd, 700–3300 for Te, 15–510 for Re, 5700–90,000 for Au, 20–440 for Pb, and 140–3300 for Bi. The DSL/SM values for Mn, Zn, As, Mo, Sn, and Sb are below 1–40. The DMSS/SM values are 55–260 for Co, 530–1700 for Cu, 74–110 for Se, 30–110 for Ag, 4–40 for Cd, 15–70 for Te, 200–5900 for Re, and 140–270 for Au. The DMSS/SM values for Mn, Zn, As, Mo, Sn, Sb, Pb, and Bi are below 1–3. The DSL/SM of Au increase with increasing Cu content of the sulfide liquid, but the DSL/SM of the other elements little affected by the Cu concentration in the sulfide liquid. Because of their distinct dissolution mechanisms in the silicate melt, the DSL/SM and DMSS/SM of Mn, Co, Zn, Cd, Sn, and Pb are mainly controlled by the silicate melt FeOtot content ([FeOtot]); the DSL/SM and DMSS/SM for Re, Mo, As, Sb, and Bi are mainly controlled by [FeOtot] and fO2; the DSL/SM and DMSS/SM for Cu, Ag, and Au are mainly controlled by [FeOtot] and the content of reduced sulfur in the silicate melt; and the DSL/SM and DMSS/SM for Se and Te are mainly controlled by fO2. Using all available DSL/SM and DMSS/SM data, a partitioning model was developed for predicting DSL/SM and DMSS/SM of chalcophile elements as a multi-function of temperature, pressure, fO2, and silicate melt and sulfide compositions. Sulfide phase relations suggest that the sulfides precipitating from arc magmas containing >100 µg/g Cu in the silicate melt occur as Cu-rich sulfide liquid, whereas the sulfides precipitating from arc magmas containing 30–70 µg/g Cu in the silicate melt occur as mixed MSS and Cu-rich sulfide liquid. Modeling the Cu evolution trends of global arc magmas illustrates that the precipitating sulfides are dominantly MSS in continental arcs with a crustal thickness of >30 km, with the proportion of sulfide liquid being less than 20%; whereas, in island arcs with a crustal thickness of <20 km, the proportion of sulfide liquid may reach up to 90%. Applying the model to predict the evolution trends of Ag, As, Sn, Sb, Se, Mo, Re, Mo, Au, Pb, and Bi in global arc magmas under various fO2 conditions, we find that when no more than 10% of the precipitating sulfides are sulfide liquid, the chalcophile element patterns of oxidized magmas (0–1 log unit above FMQ) in continental arcs match that of Earth’s bulk continental crust, which implies that Earth’s continental crust formed mainly in oxidized continental arcs.