Enhanced mantle-to-crust rhenium transfer in undegassed arc magmas

Variations in the 187Os/188Os isotopic signature of mantle and mantle-derived rocks have been thought to provide a powerful chemical tracer of deep Earth structure. Many studies have inferred from such data that a long-lived, high-rhenium component exists in the deep mantle (187Re is the parent isotope decaying to 187Os, with a half-life of approximately 42 billion years), and that this reservoir probably consists of subducted oceanic crust. The interpretation of these isotopic signatures is, however, dependent on accurate estimates of rhenium and osmium concentrations in all of the main geochemical reservoirs, and the crust has generally been considered to be a minor contributor to such global budgets. In contrast, we here present observations of high rhenium concentrations and low Yb/Re ratios in arc-type melt inclusions. These results indicate strong enrichment of rhenium in undegassed arc rocks, and consequently the continental crust, which results in a crustal estimate of 2 p.p.b. rhenium, as compared to previous estimates of 0.4-0.2 p.p.b. (refs 4, 5). Previous determinations of rhenium in arc materials, which were largely measured on subaerially erupted samples, are likely to be in error owing to rhenium loss during degassing. High mantle-to-crust rhenium fluxes, as observed here, require a revaluation of geochemical models based on the 187Re-187Os decay system.     

Release of gold-bearing fluids in convergent margin magmas prompted by magnetite crystallization

A relationship between convergent margin magmas and copper-gold ore mineralization has long been recognized. The nature of the genetic link is controversial, particularly whether the link is due to high-oxygen-fugacity (fO2) melts and fluids released from subducted slabs or to brine exsolution during magmatic evolution. For submarine, subduction-related volcanic glasses from the eastern Manus basin, Papua New Guinea, we here report abrupt decreases in gold and copper abundances, coupled with a switch in the behaviour of titanium and iron from concentration increases to decreases as SiO2 rises. We propose that the abrupt depletion in gold and copper results from concurrent sulphur reduction as a result of fO2 buffering, causing enhanced formation of copper-gold hydrosulphide complexes that become scavenged from crystallizing melts into cogenetic magmatic aqueous fluids. This process is particularly efficient in oxidized arc magmas with substantial sulphate. We infer that subsequent migration and cooling of exsolved aqueous fluids create links between copper-gold mineralization and arc magmatism in the Manus basin, and at convergent margins in general.