Copper recycling and redox evolution through progressive stages of oceanic subduction: Insights from the Izu-Bonin-Mariana forearc

Abstract

Addition of subducted materials from the slab to the mantle wedge is often thought to elevate the oxygen fugacity of arc magmas and also to fertilize the mantle wedge in metals for subduction-related Cu-Au deposits. However, it remains controversial if slab-driven metal addition is effective and whether it occurs at all stages of subduction. The Izu-Bonin-Mariana (IBM) forearc preserves a full record of arc development from the initiation of subduction of the Pacific plate to mature arc volcanism. Here, we study the ore-forming and redox-sensitive metal Cu and its isotopes (δ65Cu) in the type-locality forearc basalts (FABs), boninites and high-Mg andesites from the IBM forearc, as well as MORBs from the East Pacific rise. Overall, the FABs display variably high Cu contents and MORB-like δ65Cu, consistent with limited Cu isotopic fractionation during partial melting of typical MORB mantle sources. Beginning with the boninites, the magma products from the IBM record strong slab signals, high water contents and high fO2 (ΔFMQ > +1). However, the oxidized, hydrous boninites and subsequent high-Mg andesites display low Cu contents, and mantle-like Cu/Sc ratios and δ65Cu, with no clear indication of the addition of Cu from the slab. The boninites show noticeably lower TiO2, Yb and Cu contents than the FABs, consistent with refractory mantle sources. Combined with available data from boninites and arc basalts worldwide, our results lead to a general conclusion that subducted oceanic slabs from initial to mature subduction contribute little Cu to the mantle wedge. This is further supported by the compiled Cu contents of arc peridotites. Adding Cu-poor, water-rich slab melts to the mantle wedge at any stage of Pacific oceanic subduction causes limited release of Cu, which remains trapped predominantly by reduced sulfides in the subducting slab. This likely reflects the overall reducing nature of slab materials added to the mantle wedge. However, we propose that the subsequent reaction of such reducing, hydrous slab melts with peridotites and flux melting produce oxidized primitive arc magmas deep in the mantle wedge. The reactive process promotes sulfide dissolution and metal release from the mantle wedge itself to oxidized arc magmas with high sulfur solubility, which explains the inheritance of mantle-like δ65Cu and the sulfide-undersaturated early-stage evolution in boninites and arc basalts. This study elucidates the role of subducted oceanic slabs in metal transfer and redox evolution and implies no significant Cu enrichment in the metasomatized mantle sources of magmatic arcs.