Melting of sulfide-bearing slab beneath the Western Aleutian Arc: Implications for chalcophile element abundances in slab-derived melts and the origin of continental crust

Abstract Partial melts subducting oceanic crust (slab melts) have been proposed to play an important role in ore-forming processes and in the generation of continental crust, but the scarcity of erupted slab melts hampers direct evaluation of their role. Primitive Mg-rhyodacites with molar Mg/Mg + FeT >0.60 from the Western Aleutians represent nearly unmodified melts from eclogitized mid-ocean-ridge basalt (MORB) of the subducting Pacific plate. Here, we show that the presence of anhydrite phenocrysts and hornblende compositions indicate highly oxidizing conditions (ΔNNO ≥+1) of crystallization. Despite these conditions and the absence of sulfides in the mineral assemblage, the rocks exhibit strongly fractionated chalcophile element ratios (Ag/Cu ~10 × MORB and Bi/Cu ~100 × MORB), suggesting their geochemical signatures were imposed by the presence of monosulfide solid solution (MSS) phase during slab melting. The chalcophile element abundances in the Mg-rhyodacites can be quantitatively explained by partial melting of MORB eclogite at 2 GPa and 900 °C in the presence of MSS. With the exception of Ag and Bi, slab melts have low contents of strongly chalcophile elements, such as Cu, Se, Pt, and Au, and therefore a limited capacity to transport these elements under upper mantle conditions. The distinctively high Ag/Cu and Bi/Cu ratios in the Mg-rhyodacites further suggest that slab-derived magmas may have been significant contributors to the chalcophile element signatures of the continental crust throughout Earth’s history.