diagram of a subduction zone
Cross-section of a subduction zone with the layered downgoing subducted slab, which includes a thin sediment layer (brown/grey), the igneous oceanic crust (dark lilac), and the sulfur-bearing serpentinized top of the lithospheric mantle (green) with the water-rich serpentine minerals lizardite (Liz), chrysotile (Ctl) and antigorite (Atg). Blue arrows indicate the release of hydrous fluids from slab to mantle after antigorite breaks down which infiltrate the source region of arc magmas in the mantle (red). The state of magma oxidation is expressed in units relative to the QFM (quartz-fayalite-magnetite) buffer, which are systematically higher in arc magmas together with higher ratios of Cu and S isotopes, denoted as δ65Cu and δ34S, respectively. In contrast, the QFM, δ65Cu and δ34S are lower in mid-oceanic ridge basalts (MORB) and back-arc magmas which form in mantle domains not affected by oxidizing slab fluids. Credit: Chen et al. [2022], Figure 9
Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Journal of Geophysical Research: Solid Earth

Arc magmas are ‘oxidized’ which means they have more oxygen available than other magmas to participate in chemical reactions. How arc magmas become oxidized deeply affects how and how much of elements sensitive to oxidation – including the climate-active elements carbon and sulfur – are transferred by arc magmatism from the Earth’s mantle to the surface. A key problem is whether arc magmas oxide only after melt formation en route to the Earth’s surface, or whether the mantle source of arc magmas is oxidized prior to melting by fluids from the subducted slab. Isotopically heavy sulfur isotopes with high δ34S values in arc magmas have previously been linked to the mantle source oxidation of arc magmas by means of oxidizing sulfate-bearing fluids from the lithospheric serpentinite layer of the slab.

Chen et al. [2022] now posit that such fluids should also mobilize the sulfur-loving element copper (Cu) and enrich the mantle beneath arcs with isotopically heavy copper isotopes which have high δ65Cu values. As test of this hypothesis, the authors present new δ65Cu data from three cold oceanic subduction zones, where the slab fluid signals are best revealed. Here, the arc magmas indeed have collectively higher δ65Cu values – and in one case also higher δ34S – relative to magmas produced from other mantle domains that are unaffected by slab fluids. Thus, Cu isotopes may emerge as a new tracer of slab-related arc magma oxidation, which has implications not only for the subduction fluxes but also for the evolution of the deeper mantle beyond.

Citation: Chen, Z., Chen, J., Tamehe, L. S., Zhang, Y., Zeng, Z., Xia, X., et al. (2022). Heavy copper isotopes in arc-related lavas from cold subduction zones uncover a sub-arc mantle metasomatized by serpentinite-derived sulfate-rich fluids. Journal of Geophysical Research: Solid Earth, 127, e2022JB024910. https://doi.org/10.1029/2022JB024910

—Susanne Straub, Associate Editor, Journal of Geophysical Research: Solid Earth

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