Alchemy in the Earth’s Mantle

The Earth’s “gold kitchen” lies deep beneath the seafloor. Island arcs, whose volcanoes form above subduction zones where one oceanic plate sinks beneath another, are often particularly rich in gold. The reasons for this have long been debated.

A research team led by Dr Christian Timm, a marine geologist at GEOMAR Helmholtz Centre for Ocean Research Kiel, now provides new insights. “Our research shows that hydrous mantle melting beneath island arcs is a key driver of gold enrichment,” says Timm. “In these settings, the mantle behaves like a multi-stage melting system that progressively concentrates gold.”

Clues from Glass on the Seafloor

How do gold and other noble metals behave in submarine subduction zones during mantle melting? To address this question, the researchers analysed 66 glass samples from the seafloor along the oceanic Kermadec island arc and the adjacent Havre Trough north of New Zealand. Such volcanic glasses are formed when lava cools rapidly underwater, which preserves the chemical composition of the original magma.

Particularly informative are so-called primitive glasses. These reflect the original magma composition before it is modified by crystallisation. “When we analysed these samples, we found that their gold concentrations are often several times higher than those of comparable magmas from mid-ocean ridges,” says Timm. “This raised the key question: which processes are responsible for this enrichment?”

The team measured the concentrations of gold with great precision and evaluated it alongside other chalcophile (“sulphur-loving”) elements, including silver, copper, selenium and platinum. These elements behave similarly during melting, providing insights into the conditions within the mantle.

Reading the Mantle’s Chemical Signals

The data show that the mantle beneath the Kermadec island arc undergoes hydrous melting at relatively high temperatures, above the sulphide liquidus. Under these conditions, magmas exhibit silver-to-copper ratios similar to those of the mantle.

At the same time, the researchers observed elevated original gold concentrations of up to six nanograms per gram of rock. The gold-to-copper ratios also significantly exceed those of fertile mantle and primitive mid-ocean ridge basalts.

These observations can only be explained by assuming that the mantle was previously depleted and has since undergone remelting. High-degree, multi-stage melting of a hydrous and oxidised mantle is therefore the primary driver of gold enrichment in these magmas.

Although these gold concentrations are high from a geological perspective, they are not economically viable for mining. Concentrations would need to be around several orders of magnitude higher to be of interest.

When the Mantle melts again and again

“We initially assumed that water released from the subduction zone directly controlled gold enrichment,” says Timm. “However, our data show that water mainly facilitates mantle melting. The key factor for high gold concentrations is the high – and in part repeated – degree of melting.”

The chemical form of gold in the mantle also plays a crucial role. “Gold in the mantle is commonly bound in sulphide minerals,” explains Timm. “At high degrees of melting, these minerals break down, releasing their gold completely into the melt.”

“Our results demonstrate that gold enrichment is not the result of a single melting event, but of multiple stages,” Timm adds. “Only repeated melting allows gold to become strongly concentrated in the magma.”

The First Step in Gold’s Journey

The study makes a significant contribution to our understanding of gold-rich deposits in intra-oceanic island arcs, such as the Kermadec Arc. It shows that hydrous and repeated mantle melting fundamentally controls how much gold is transferred into ascending magmas.

This shifts the focus deeper into the Earth, showing that not only do near-surface processes determine where gold deposits ultimately form, but the chemical evolution of the mantle beneath subduction zones does too.

The findings may also help to explain why hydrothermal sulphide deposits along submarine island arcs are often particularly rich in gold. “The mechanism we identify could contribute to the elevated gold contents observed in hydrothermal systems in subduction zones,” says Timm. “However, this link still needs to be investigated further.”

“We are effectively looking at the first step in the life cycle of gold,” concludes Timm. “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.”

Original Publication:
Timm, C., Portnyagin, M., de Ronde, C. E. J., Hannington, M. D., Garbe-Schönberg, D., Hoernle, K., Brandl, P. A., Layton-Matthews, D., Leybourne, M., Arculus, R. J. (2026): Hydrous mantle melting controls gold in Kermadec arc magmas. Commun Earth Environ 7, 281

https://doi.org/10.1038/s43247-026-03338-w