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

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 degrees C, 0.5-1.0 GPa, and fO(2) 1-1.5 log units above the fayalite-magnetite-quartz (FMQ) buffer. The D-SL/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 D-MSS/SM values for Mn, Zn, As, Mo, Sn, Sb, Pb, and Bi are below 1-3. The D-SL/SM of Au increase with increasing Cu content of the sulfide liquid, but the D-SL/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 D-SL/SM and D-MSS/SM of Mn, Co, Zn, Cd, Sn, and Pb are mainly controlled by the silicate melt FeOtot content ([FeOtot]); the D-SL/SM and D-MSS/SM for Re, Mo, As, Sb, and Bi are mainly controlled by [FeOtot] and fO(2); the DSL=SM and D-MSS/SM for Cu, Ag, and Au are mainly controlled by [FeOtot] and the content of reduced sulfur in the silicate melt; and the D-SL/SM and D-MSS/SM for Se and Te are mainly controlled by fO(2). Using all available D-SL/SM and D-MSS/SM data, a partitioning model was developed for predicting D-SL/SM and D-MSS/SM of chalcophile elements as a multi-function of temperature, pressure, fO(2), and silicate melt and sulfide compositions. Sulfide phase relations suggest that the sulfides precipitating from arc magmas containing >100 mu g/g Cu in the silicate melt occur as Cu-rich sulfide liquid, whereas the sulfides precipitating from arc magmas containing 30-70 mu g/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 fO(2) 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. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The research findings reveal variations in partition coefficients of chalcophile elements in silicate melts under different conditions, influenced by various factors. Furthermore, the results can be used to predict the evolution trends of chalcophile elements in hydrothermal systems.