Evolution of copper isotopes in arc systems: Insights from lavas and molten sulfur in Niuatahi volcano, Tonga rear arc

Abstract

Abstract Metal transfer from mantle wedge to primitive arc magmas and subsequent enrichment by magmatic fractionation and volatile exsolution are critical processes for mineralization in arc systems. Copper is one of the most important ore-forming elements whose behavior is sensitive to oxygen fugacity. Copper isotope composition (δ65Cu) may provide valuable insights into Cu transfer and enrichment in hydrous oxidized arc magmas. However, the extent of Cu isotopic variation in arc systems and its link to Cu transfer and enrichment for ore mineralization have been poorly explored. Here we report the Cu isotopes in basalts, dacites and molten sulfur in Niuatahi volcano, Tonga rear arc to address the issue. These samples, as well as associated black smoker chimneys, represent products of magmatic fractionation and degassing of hydrous oxidized arc magmas with ore mineralization. Sulfide-undersaturated differentiation of basalts in the Niuatahi and their high water content and oxygen fugacity suggest complete exhaustion of sulfides in the mantle source during fluxed melting and transfer of nearly all Cu, Ag and other chalcophile metals to the primary magmas. The δ65Cu of Niuatahi basalts thus reflect that of the mantle source. The basalts display δ65Cu of 0.01‰ to 0.17‰ (n = 3; external uncertainty of 0.05‰, 2sd), similar to mid-ocean ridge basalts (MORBs), komatiites and the depleted mantle (0.06 ± 0.20‰, 2sd). These results, together with their Cu contents indistinguishable from MORBs, suggest that oxidized slab components are very likely to have limited influence on the Cu budget and mean δ65Cu of the mantle wedge. The Niuatahi magma became sulfide saturated after magnetite crystallization during magma differentiation from basalt to dacite. Constant Cu/Ag in the basalts and dacites suggests segregation of immiscible sulfide melts instead of crystalline sulfides. The sulfide segregation significantly decreased contents of Cu and other chalcophile metals but hardly changed δ65Cu in dacites (−0.01‰ to 0.35‰, n = 11 with a mean of 0.21 ± 0.24‰, 2sd), implying restricted fractionation of δ65Cu during magnetite fractionation and sulfide melt segregation. Molten sulfurs, which are formed by intensive magmatic degassing of arc lavas and characterized by substantial enrichment of Cu and other metals, show δ65Cu of 0.30‰ to 0.37‰. These values are indistinguishable from those of comagmatic dacites (0.34‰). Although the published δ65Cu of sulfide chimneys in the Niuatahi appears slightly lighter (0.00‰ to 0.29‰ ± 0.18‰, 2sd), the overall limited range of δ65Cu in molten sulfur and sulfide chimneys indicates that discharging magmatic volatiles and hydrothermal venting with significant removal of Cu hardly fractionates δ65Cu. The δ65Cu data from arc lavas, molten sulfur and sulfide chimneys thus reveal limited variations in δ65Cu (within 0.35‰) during fluxed melting, magmatic fractionation, magma degassing and mineralization in arc systems. If these results represent general processes, they imply that the heavier or lighter δ65Cu in other sulfide chimneys and associated deposits should result from the complex hydrothermal processes and/or low-temperature secondary reworking.