Deep mantle cycle of chalcophile metals and sulfur in subducted oceanic crust

Understanding the subduction-driven recycling of economically important chalcophile metals (e.g., Cu, Au, Ag, and platinum-group elements, PGEs) and volatiles (e.g., sulfur) is critical for assessing models of ore formation. However, whether the sulfide remains stable in recycled altered oceanic crust (AOC) in the mantle is unclear, and the influence of AOC-derived metasomatic agents on the extraction efficiency of chalcophile elements from the deep mantle remains poorly understood. Here we report whole-rock Cu-Ag-Au-Re-PGEs of Cenozoic alkaline basalts at Hannuoba, eastern China, which were derived from the interaction of carbonated eclogite (recycled AOC)-derived melts with lithospheric peridotites. Volatile-rich primitive melts from carbonated eclogite are poor in strongly chalcophile metals (e.g., ∑PGE < 1 ng/g, Au < 1 ng/g, Cu < 30 μg/g). Remarkably, primitive melts show significant fractionation of elements with variable chalcophile affinities, such as lower Cu/Ag ratios than those in primitive mantle and MORBs. These features reflect the sequestration of these metals in residual mono-sulfide solid solution (MSS) during low-degree melting of the AOC in the deep mantle. Given the retention of sulfides in the AOC, partial melts derived from it are rich in sulfur but poor in strongly chalcophile metals. The Hannuoba alkaline melts scavenged some strongly chalcophile metals from peridotites (especially PGEs) during their ascent. However, this did not necessarily lead to the formation of chalcophile metal-enriched melts (e.g., Au < 1 ng/g and Cu < 60 μg/g). These processes appear to be common in eastern China, as evidenced by the low abundances of Cu and Au found in many primitive intraplate alkaline basalts. We propose that strongly chalcophile elements in subducted AOC are efficiently transported into, and stored in, the deep mantle, with a small proportion returning to the exosphere through volcanism. Moreover, the chalcophile metal-poor and volatile-rich melts of subducted AOC could also explain the addition of abundant sulfur (secondary sulfides), but limited chalcophile metals to the metasomatized mantle.