Copper isotope evidence for sulfide fractionation and lower crustal foundering in making continental crust

Abstract

The continental crust is strongly depleted in copper compared with its building blocks—primary arc magmas—and this depletion is intrinsically associated with continental crust formation. However, the process by which Cu removal occurs remains enigmatic. Here we show, using Cu isotopes, that subduction-zone processes and mantle melting produce limited fractionation of Cu isotopes in arc magmas, and, instead, the heterogeneous Cu isotopic compositions of lower crustal rocks, which negatively correlate with Cu contents, suggest segregation or accumulation of isotopically light sulfides during intracrustal differentiation of arc magmas. This is supported by the extremely light Cu isotopic compositions of lower crustal mafic cumulates and heavy Cu isotopic compositions of differentiated magmas in thick continental arcs. Intracrustal differentiation of mantle-derived magmas and subsequent foundering of sulfide-rich mafic cumulates preferentially removes isotopically light Cu, leaving a Cu-depleted and isotopically heavy continental crust.

Mantle Oxidation Induced by Recycled Carbonate: Insights From Mg‐Zn‐Fe‐Cu Isotopic Systematics of Intraplate Basalts

“…However, our samples with higher δ 56 Fe and lower δ 65 Cu values have lower SiO 2 contents (Figures 5a and 5b) and are less differentiated (MgO > 5 wt%; Lee et al, 2016;M.-L. Li et al, 2021), which is inconsistent with isotope fractionation induced by differentiation (J. Liu, Rudnick, et al, 2023). When basaltic magmas ascend through the lithospheric mantle, temperature and chemical gradients may lead to kinetic isotope fractionation.…”

“…an S-saturated system and increases δ 65 Cu of the residual melts given that sulfides are isotopically lighter than equilibrium silicate melts (Figure 7a; J. Liu, Rudnick, et al, 2023;Savage et al, 2015;Xia et al, 2019). However, our samples with lower-than-MORB δ 65 Cu values cannot be explained by either sulfide segregation of a sulfide-saturated magma with normal (MORB-like) δ 65 Cu value or partial melting of a normal peridotite source.…”

“…Sulfur in the mantle is chiefly present as an accessory sulfide phase, which may be a monosulfide solid solution, and/or as molten sulfide at reduced conditions (Harvey et al, 2016). Previous experimental studies and natural observations show that sulfides are isotopically lighter than equilibrium silicate melts at magmatic temperatures (Liu, Rudnick, et al, 2023;Savage et al, 2015;Xia et al, 2019). By contrast, distinct Cu isotope fractionation exists during the redox transformation of sulfide into sulfate even at magmatic temperatures, in which sulfate fluids preferentially incorporate isotopically heavy Cu isotopes (Guo et al, 2020;Mathur et al, 2005).…”

Oxidation state of Cu in silicate melts at upper mantle conditions

“…Copper, beyond its critical role as a mineral resource, serves as a sensitive tracer for unraveling deep magmatic processes 1 – 8 . Its geochemical behavior is inextricably linked to the fate of magmatic sulfide, as Cu exhibits pronounced incompatibility within silicate minerals 7 , 9 , 10 .…”

“…This intimate relationship makes Cu a valuable proxy for tracking sulfide evolution, which is highly sensitive to changes in oxygen fugacity (fO 2 ) within the magma reservoir. Furthermore, Cu isotopes can undergo significant fractionation during magmatic differentiation involving sulfides, further amplifying its potential as a geochemical recorder 1 , 4 . Consequently, the combined analysis of Cu content and isotope variations in igneous rocks offers invaluable insights into magma evolution and the fO 2 conditions in the deep Earth.…”