Research led by Dr. Christian Timm of GEOMAR Helmholtz Centre for Ocean Research Kiel reveals that hydrous mantle melting beneath island arcs plays a fundamental role in gold enrichment. Island arcs are chains of volcanoes that form above subduction zones where oceanic plates descend, and these regions have long been recognized as unusually gold-rich. The new findings provide mechanistic explanations for why such enrichment occurs.
The team examined 66 volcanic glass samples collected from the seafloor along the Kermadec island arc and Havre Trough north of New Zealand. Volcanic glass is particularly valuable for scientific study because it forms when underwater lava cools extremely rapidly, preserving the original chemical composition of the magma. Primitive glass samples proved especially informative, as they retain the chemistry of magma before crystallization alters its composition. Analysis revealed gold concentrations several times higher than those found in comparable magmas from mid-ocean ridges. Researchers measured gold alongside other chalcophile elements including silver, copper, selenium, and platinum to determine the processes responsible for this enrichment.
The chemical signatures in the samples indicated that water-rich mantle melted at relatively high temperatures, above the sulfide liquidus, where sulfide minerals break down completely. Primitive samples contained gold concentrations up to six nanograms per gram of rock, along with gold-to-copper ratios substantially higher than those in fertile mantle or primitive mid-ocean ridge basalts. These patterns suggest the mantle source had been depleted by earlier melting events and subsequently melted again, creating the elevated gold concentrations through multi-stage melting.
Water acts primarily as a catalyst enabling extensive melting rather than directly controlling gold transfer into magma. Gold in the mantle typically binds within sulfide minerals and remains trapped during limited melting. When melting becomes intense enough to destroy these minerals, the stored gold releases completely into the rising magma. This repeated, progressive melting process allows gold to concentrate effectively. While the rocks do not contain commercially viable gold concentrations, the research demonstrates that gold enrichment begins in the mantle through repeated, water-assisted melting before magma rises toward the surface.
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