Earth’s hidden “gold kitchen” may be operating deep beneath the ocean floor. Island arcs—chains of volcanoes formed where one oceanic tectonic plate sinks beneath another at a subduction zone—are often unusually rich in gold. For decades, scientists have worked to understand why these volcanic regions become so strongly enriched in the precious metal.
New research led by Dr. Christian Timm, a marine geologist at the GEOMAR Helmholtz Centre for Ocean Research Kiel, provides important clues about how gold becomes concentrated beneath submarine island arcs.
“Our research shows that hydrous mantle melting beneath island arcs is a key driver of gold enrichment,” says Timm. “In these environments, the mantle acts as a multistage melting system that progressively concentrates gold.”
Volcanic Glass Preserves Evidence of Ancient Magma
To investigate how gold and other noble metals behave during mantle melting beneath submarine subduction zones, the research team analyzed 66 volcanic glass samples collected from the seafloor along the Kermadec island arc and the neighboring Havre Trough, north of New Zealand.
Volcanic glass forms when underwater lava cools extremely rapidly. This process preserves much of the magma’s original chemical composition, giving scientists a valuable record of conditions deep beneath the seafloor.
The most informative samples were known as primitive volcanic glasses. These rocks retain the chemical signature of the original magma before crystallization altered its composition.
“When we analyzed these samples, we found that their gold concentrations were often several times higher than those in comparable magmas from mid-ocean ridges,” says Timm. “This raised an important question: which geological processes caused this enrichment?”
The researchers measured extremely small amounts of gold and compared the results with other chalcophile, or “sulfur-loving,” elements such as silver, copper, selenium and platinum. Because these elements behave similarly during mantle melting, their chemical relationships can reveal how the mantle evolved.
Chemical Evidence Reveals Repeated Mantle Melting
The results indicate that the mantle beneath the Kermadec island arc melted in the presence of water and at relatively high temperatures above the sulfide liquidus. Under these conditions, sulfide minerals can break down completely. The resulting magma retains silver-to-copper ratios similar to those found in the mantle.
The samples contained original gold concentrations of up to six nanograms per gram of rock. Although this amount is extremely small, it is unusually high for mantle-derived magma. The rocks also showed gold-to-copper ratios significantly higher than those measured in fertile mantle and primitive mid-ocean ridge basalts.
According to the researchers, these chemical signals are best explained by a mantle source that had already been depleted by an earlier melting event and was subsequently melted again.
The primary process responsible for gold enrichment appears to be high-degree, multistage melting within a water-rich and oxidized mantle.
Despite their elevated gold concentrations, the volcanic rocks do not contain enough gold to be economically mined. Commercially valuable deposits would require concentrations several orders of magnitude higher.
Water Helps Trigger Extensive Mantle Melting
The researchers initially suspected that water released from the subducting oceanic plate directly controlled the amount of gold entering the magma. However, the new evidence points to a more complex process.
“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 is the high—and partly repeated—degree of melting.”
Water therefore appears to act primarily as a trigger that helps mantle rock melt more extensively. The stronger and more frequently the mantle melts, the more efficiently gold can be transferred into the magma.
The way gold is stored within the mantle is also critical.
“Gold in the mantle is commonly bound in sulfide minerals,” explains Timm. “At high degrees of melting, these minerals break down and release their gold into the melt.”
During limited melting, gold can remain trapped inside sulfide minerals. When melting becomes intense enough to destroy those minerals, the stored gold is released and incorporated into the rising magma.
“Our results demonstrate that gold enrichment is not caused by a single melting event, but by multiple stages of mantle melting,” Timm adds. “Repeated melting is what allows gold to become strongly concentrated in magma.”
The First Stage of Gold’s Geological Journey
These findings improve scientists’ understanding of gold-rich volcanic systems associated with intra-oceanic island arcs such as the Kermadec Arc. They show that repeated, water-assisted mantle melting plays a major role in determining how much gold magma transports toward the surface.
The research also moves part of the explanation for gold deposits deeper into Earth. Although near-surface processes ultimately determine whether an economically valuable deposit forms, the chemical history of the mantle beneath a subduction zone may establish the conditions for gold enrichment long before magma reaches the seafloor.
The same process may help explain why hydrothermal sulfide deposits along submarine island arcs can contain unusually high gold concentrations. These deposits form when hot, mineral-rich fluids circulate through volcanic regions beneath the ocean.
“The mechanism we identified could contribute to the elevated gold concentrations observed in hydrothermal systems associated with subduction zones,” says Timm. “However, this connection requires further investigation.”
“We are effectively observing the first step in gold’s geological life cycle,” concludes Timm. “The process begins when gold moves from the mantle into a melt that eventually feeds volcanoes. The alchemy starts long before the metal reaches Earth’s surface.”
Source: www.sciencedaily.com