Dominant monosulfide solid solution fractionation causes Cu deficit in continental crust

It remains unclear how Earth’s continental crust became deficient in Cu. Lower crustal cumulates from the Gangdese continental arc present key evidence. These cumulates are Cu-rich and exhibit significant δ65Cu variability (−2.09 ‰ to +0.51 ‰), attributed primarily to the fractionation involving Monosulfide Solid Solution (MSS). Combined with the systematic Cu depletion and positive δ65Cu signatures of the Gangdese differentiated arc magmas and the bulk continental crust, we propose that widespread MSS segregation occurs at the base of continental arcs. This process distinguishes continental arcs from island arcs: in the former, higher pressures promote earlier sulfide saturation, and MSS dominated segregation slows the rate of Cu depletion during differentiation. Ultimately, the continuous foundering of these sulfide-rich mafic cumulates drives the continental crust toward a silica-rich and Cu depleted bulk composition.