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

Highlights

• Oxidized hydrous boninites show low Cu contents and no clear slab addition of Cu.

• Oceanic slabs from initial to mature subduction add little Cu to mantle wedge.

• Slab materials added to the mantle wedge are generally reducing.

• Hydrous slab melt-peridotite reaction is likely vital to produce oxidized arc magma.

• The metasomatized mantle wedge is not enriched in Cu from the oceanic slab.

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 (${\delta }^{65}$Cu) 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 ${\delta }^{65}$Cu, 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 ${\delta }^{65}$Cu, 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 ${\delta }^{65}$Cu 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.

Introduction

Most of the world's economic Cu resources are from porphyry Cu-Au deposits which are mainly sited in magmatic arcs above subduction zones (e.g., Sillitoe, 2010; Sun et al., 2015). Slab subduction has long been recognized to play a key role (e.g., Hedenquist and Lowenstern, 1994; Richards, 2011; Sun et al., 2015). Arc magmas display high oxygen fugacity (fO₂) and high water contents which are often thought to reflect the addition of oxidized fluids and/or melts released from the slab (or water-rich slab melts) to the sub-arc mantle wedge (e.g., Bénard et al., 2018; Debret and Sverjensky, 2017; Evans, 2012; Kelley and Cottrell, 2009; Parkinson and Arculus, 1999). Consequently, as theoretical calculations and experimental constraints indicate, the oxidized slab fluids or melts would be expected to destabilize sulfides and release sulfur and the metals contained within them from the subducted oceanic lithosphere, resulting in noticeable enrichment of the mantle wedge (e.g., Canil and Fellows, 2017; Evans and Tomkins, 2011; Mungall, 2002; Sun et al., 2012, Sun et al., 2017). Metasomatic components in the mantle wedge are preferentially melted at later stages and so, if metal-enriched, should favor the formation of Cu-Au deposits (Griffin et al., 2013; Groves et al., 2019; McInnes et al., 1999; Richards, 2009). However, detailed studies and global compilations of natural samples indicate that the Cu concentration in primitive arc basalts (∼80 ppm) is similar to those of MORBs, implying that Cu is mainly sourced from the mantle wedge with an indiscernibly low contribution from the subducted oceanic lithosphere (e.g., Chiaradia, 2014; Jenner et al., 2010; Lee et al., 2012; Rezeau and Jagoutz, 2020; Wang et al., 2019). Also, metasomatized arc peridotites, even if oxidized, often do not display the expected enrichment of Cu and Au (e.g., McInnes et al., 1999; Parkinson and Pearce, 1998; Savov et al., 2007; Secchiari et al., 2020).

The paradox about Cu contents has been argued to reflect the reduced nature of primitive arc magmas similar to MORBs (e.g., Lee et al., 2012). Magmatic differentiation is probably a key mechanism to increase the oxygen fugacity of evolving magmas in thick continental arcs (Lee and Tang, 2020; Tang et al., 2020). However, high fO₂ is widely observed for metasomatized arc peridotites (Parkinson and Arculus, 1999) and particularly primitive arc magmas of oceanic islands, e.g., sulfate phases and ΔFMQ of +1 to +1.5 (e.g., Bénard et al., 2018; Zelenski et al., 2018). The high fO₂ nature and yet lack of Cu enrichment in primitive arc magmas is apparently contradictory, and the specific cause is enigmatic. It largely reflects the complexity of the deep mantle wedge processes and the lack of sufficient constraints from natural rocks. Recent work suggests that the fO₂ may differ between subduction stages or depths (Cannaò and Malaspina, 2018), and that slab components cannot be as oxidizing as previously thought, and possibly even reducing (Li et al., 2020; Tollan and Hermann, 2019). This further complicates the understanding of redox-sensitive sulfide stability and thus metal release and transfer in subduction zones.

Copper is one of the most important ore-forming metals; combined with the concentration, its isotopes (${\delta }^{65}$Cu) can provide further constraints on metal transfer and enrichment in the metasomatized mantle wedge (Liu et al., 2015; Zheng et al., 2019), because oxidized Cu mainly displays elevated ${\delta }^{65}$Cu with ${\mathrm{\Delta }}^{65}$Cu _{fluid-residue} of 0.5–3‰ (e.g., Fernandez and Borrok, 2009; Mathur and Fantle, 2015). In addition, slab fluids or melts released from variable subduction depths and stages are rich in chlorine (Cl), one of the key transport agents for Cu (Barnes et al., 2018; Kendrick et al., 2020; Rustioni et al., 2019). If the slab fluids or hydrous melts released are rich in Cu, fluid-driven isotopic fractionation would cause Cu complexed by Cl to display heavier ${\delta }^{65}$Cu by 0.1–0.7‰ than the residual phases in the slab (Guo et al., 2020). Any Cu transferred from the slabs to the mantle wedge should thus increase both the Cu content and ${\delta }^{65}$Cu of the wedge, features that are expected in derivative primitive arc magmas as well.

Boninites belong to high-Mg andesites and are widely accepted to form by melting of highly depleted mantle wedge fluxed by hydrous slab melts during the embryonic stage of subduction (Crawford et al., 1989; Pearce and Reagan, 2019). They display water contents (2–5 wt.%) and fO₂ (mainly ΔFMQ: +1 to +2) as high as mature arc lavas, reflecting rapid fluid flux and oxidation of the mantle wedge (e.g., Brounce et al., 2015, Brounce et al., 2021; Umino et al., 2015). Given the incompatible behavior of Cu, previous episodes of high-degree melt extraction would have largely depleted the mantle source of boninites in Cu. Therefore, refractory boninites are more sensitive than primitive arc basalts to show whether subducted slabs are the source of significant quantities of metals. Also, boninites are magmatic products from a large mantle source region, minimizing the effect of local heterogeneity in the mantle wedge.

Here we report the Cu contents and ${\delta }^{65}$Cu of type-locality forearc basalts (FAB, $n=5$), boninites ($n=12$) and high-Mg andesites ($n=8$) from the renowned Izu-Bonin-Mariana (IBM) forearc, as well as fresh MORBs ($n=8$) from the East Pacific rise for comparison. The IBM samples represent a series of magmatic products formed from the early stages of oceanic subduction. When compared to data from mature IBM arc basalts, the results allow us to better understand the role of subducted oceanic slabs in metal recycling through progressive stages of oceanic subduction and the related evolution of redox states. We show that the Pacific oceanic slabs add little Cu to the mantle wedge at all stages from initial to mature subduction, suggesting that the sulfides and Cu are largely retained in the subducting slabs. We also propose a mechanism to explain why primitive arc magmas display no Cu enrichment despite high fO₂.