Deep beneath the waves north of New Zealand, Earth appears to run a natural gold kitchen. New research has revealed that repeated melting of water-rich mantle rock under submarine volcanoes is the hidden process that first liberates gold from the planet’s interior and sends it on its long journey towards the surface.
Scientists led by Dr Christian Timm of the GEOMAR Helmholtz Centre for Ocean Research Kiel examined 66 samples of volcanic glass recovered from the seafloor along the Kermadec island arc and the neighbouring Havre Trough. The glasses form when underwater lava cools almost instantly, locking in the chemical signature of the original magma before crystals can alter it. The most pristine “primitive” samples proved especially telling.
Gold concentrations in these magmas were several times higher than those found in comparable melts from mid-ocean ridges. Some glasses contained up to six nanograms of gold per gram of rock — tiny by everyday standards, yet unusually elevated for material that has only just left the mantle. Ratios of gold to copper and patterns among other sulphur-loving elements pointed to a clear mechanism.
Beneath island arcs, where one oceanic plate dives beneath another, water is driven out of the descending slab. That water does not simply ferry gold into the melt.
Instead it acts as a flux, allowing the mantle to melt more extensively and repeatedly. At sufficiently high temperatures the sulphur-rich minerals that normally hold gold captive break down completely. Only then is the metal released in full into the rising magma.
Mr Timm explained: “Our research shows that hydrous mantle melting beneath island arcs is a key driver of gold enrichment.
“In these settings, the mantle behaves like a multi-stage melting system that progressively concentrates gold.” Water, he added, mainly facilitates the melting; the critical factor is the high — and in part repeated — degree of melting itself.
The findings help explain why submarine island arcs are so often unusually rich in gold. The same process may also contribute to the elevated gold contents of hydrothermal sulphide deposits that form when hot fluids circulate through seafloor volcanic systems, although that link still requires further study.
Importantly, the concentrations measured remain far too low for commercial mining. Economically viable deposits demand enrichment by several further orders of magnitude, processes that occur much closer to the surface. What the new work demonstrates is the crucial first step: the transfer of gold from the mantle into the melt that eventually feeds volcanoes.
Dr Timm concluded: “We are effectively looking at the first step in the life cycle of gold. It begins with the transfer of gold from the mantle into a melt that eventually forms volcanoes. The alchemy starts long before the metal reaches the surface.”
By peering into volcanic glass quenched on the ocean floor, researchers have caught Earth’s deep gold kitchen in the act — a slow, watery, multi-stage recipe written in the chemistry of the mantle itself.
