Sulfide-bearing cumulates in deep continental arcs: The missing copper reservoir

Abstract

Resolving the geochemical discrepancies between the bulk continental crust and its building blocks, basaltic arc magmas, can provide insights into the processes by which the continental crust is formed. One of the discrepancies is that the bulk continental crust is depleted in Cu and has a lower Cu/Ag ratio (∼500) than basaltic arc magmas (∼3500). How arc magmas become Cu-depleted and where the missing Cu resides remain unclear. Here, we report Cu and Ag concentrations in Arizona garnet-pyroxenite xenoliths, which represent cumulates formed in a deep continental arc. Many of these cumulates are Cu-enriched, with Cu concentrations as high as ∼1000 ppm. Furthermore, for the first time, we show that these cumulates have higher Cu/Ag ratios (up to ∼10,000) than basaltic arc magmas, complementary to the low Cu/Ag in felsic arc magmas and the continental crust (≤500). In these cumulates, Cu and Ag concentrations are initially low, but begin to rise with progressive differentiation due to sulfide saturation when cumulate Mg# (atomic ratio of Mg/(Mg+Fe)) decreases to ∼0.7. The onset of sulfide saturation in the cumulates is accompanied by a Cu decrease in continental arc magmas. Fractional crystallization modelling shows that the early sulfide saturation during the differentiation of continental arc magmas was facilitated principally by cooling and iron depletion in the melt. Our findings suggest that most of the Cu extracted from the mantle in continental arc settings is sequestered in sulfides in deep arc cumulates during magma differentiation. The sulfide-bearing cumulates represent the missing reservoir with both high Cu concentrations and high Cu/Ag ratios. The low Cu abundance and low Cu/Ag ratio in the average continental crust requires the formation of sulfide-bearing cumulate layer at arc roots followed by its delamination into the mantle. Moreover, we confirm that sulfide saturation in continental arc magmas occurs earlier (at higher Mg#) than in island arc settings, resulting in lower Cu concentrations in the felsic upper crust of continental arcs. Because the bulk continental crust is strongly depleted in Cu, these observations together point to syn-magmatic crustal thickening as a key process in making continents.