Work

Copper isotope evidence for Cu transfer during oceanic crust subduction: insights from the (ultra)high-pressure metamorphic belt of Southwestern Tianshan, China

Year: 2026

Type: article

Abstract: Constraining the behavior of Cu during subduction is the key to decipher processes leading to the porphyry Cu deposit formation at convergent margins, while also shedding light on S cycling owing to the chalcophile affinity of Cu. This study reported δ 65 Cu values for bulk-rock and chalcopyrite from a metamafic rock from the (ultra-)high pressure metamorphic belt of Southwestern Tianshan, China. The rock consists of abundant glaucophane-dominated breccias (part A) crosscut by a large-scale omphacite-rich vein system (part B) with sharp boundaries. The glaucophane-dominated breccias are interpreted as partially dehydrated metabasalts, while the omphacite-rich vein system reflects the main fluid pathways in the subduction channel. Part A exhibits elevated δ 65 Cu values (0.07‰ to 0.36‰) and low Cu contents (52 ppm on average) compared to MORB (δ 65 Cu = 0.04‰ to 0.14‰; Cu = 90 ppm). Together with heavier δ 65 Cu values of chalcopyrite (−0.17‰ to 0.23‰) in part A compared to magmatic sulfide, these suggest that dehydration preferentially releases isotopically light Cu and leaves residues with heavier Cu isotope compositions. Part B shows complementary light δ 65 Cu values both in bulk rock and chalcopyrite. The negative correlation between S and δ 65 Cu indicates that precipitated sulfides complexed with light Cu isotopes may result in lowered δ 65 Cu values in the rock during fluid-rock interactions. Overall, the data point to the mobilization of isotopically light Cu-bearing fluids during the dehydration of mafic crust in subduction environments. However, the absence of commonly high Cu contents in arc magmas suggests that the released copper may be buffered by sulfides in the sub-arc mantle, resulting in Cu-rich sulfide accumulations.

Source: Chemical Geology

DOI: 10.1016/j.chemgeo.2026.123661