The role of sulfides in the chalcophile and siderophile element budget of the subducted oceanic crust

Abstract

Abstract Subduction zones are the site of long-term chemical cycling between Earth’s surface and interior. During subduction, dehydration and melting may partition mobile elements into the overlying mantle wedge and arc system. Volcanic arcs also host much of the world’s base (e.g., Cu, Sn, Sb, and Mo) and precious (e.g., Au, Ag) metal deposits. However, slab contributions to the chalcophile and siderophile element (CSE) budget of arc magmas and their associated ore deposits remain controversial. Although previous studies have identified the mobility of some CSE in slab fluids, few studies have examined the contribution from sulfides to the CSE budget of the slab. Here we report in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of Cr, Co, Ni, Cu, Zn, Ga, Ge, As, Mo, Ag, Cd, In, Sn, Sb, Te, Tl, Pb, and Bi in sulfides and silicates in thirteen rocks from six exhumed high-pressure (HP) terranes worldwide. Combined with data from the literature, we demonstrate that sulfides host nearly the entire Cu, As, Ag, Cd, and Te budget of subducted rocks, whereas Co, Ni, Zn, Ga, Ge, Mo, Sn, and Tl are dominantly hosted in silicate and oxide phases. Prograde sulfide-breakdown during subduction is expected to liberate Cu, As, Ag, Cd, and Te into the overlying arc-mantle wedge system, whereas Co, Ni, Zn, Ga, and Pb may be partially lost during lawsonite, epidote, and amphibole dehydration reactions. In contrast, Ge, Tl, Sn, and Mo are largely retained in the eclogitic slab residue. We also demonstrate that CSE zoning in pyrite may be used to track high pressure (HP) metasomatism. Oscillatory Co and Ni zoning in metasomatic pyrite is linked to transitions between fluid and rock buffered regimes, consistent with seismic- and dehydration-induced cyclic fluctuations in fluid pore pressure and flux. Thus, minor and trace CSEs in HP rocks may also be a useful tool to constrain slab fluid migration.