Шинэ Зеландын ойролцоох галт уулын шилнээс алтны өндөр агууламж илрүүлэв

Volcanic glass recovered from the Kermadec Arc north of New Zealand contains gold concentrations several times higher than comparable magma from mid-ocean ridges. Chemical measurements show that the enrichment begins when water-rich mantle rock melts repeatedly beneath the chain of submarine volcanoes.

The research examined how gold enrichment starts deep inside Earth, before magma rises toward the seafloor and before hot fluids redistribute metals through volcanic rock. Water encourages the mantle to melt more extensively, while intense melting breaks down sulfur-bearing minerals that normally retain gold.

The study published in Communications Earth & Environment traced this process through the chemistry of seafloor glass. The samples preserved evidence that the mantle beneath the island arc had already melted once and was later melted again, allowing more gold to enter newly formed magma.

Volcanic Glass Preserved the Chemistry of Deep Magma

The researchers analyzed 66 volcanic glass samples collected from the Kermadec island arc and the neighboring Havre Trough in the South Pacific. Both areas lie near a subduction zone, where the Pacific Plate descends beneath the Australian Plate and contributes to volcanic activity above it.

Underwater lava can cool so rapidly that it forms glass before crystals have time to develop. This rapid cooling preserves much of the magma’s original chemical composition, giving scientists a record of conditions below the seafloor. The most useful samples were primitive glasses, which had undergone relatively little chemical change after leaving the mantle.

Researchers measured gold alongside silver, copper, selenium and platinum. These metals are classed as chalcophile elements, a scientific term for elements that tend to associate with sulfur. Their relative concentrations allowed the team to track how sulfur-rich minerals changed as the mantle melted.

The original gold concentrations reached six nanograms per gram of rock. Although that amount is extremely small, it is unusually high for magma derived from the mantle. The glass also had gold-to-copper ratios above those found in fertile mantle and primitive basalt from mid-ocean ridges, according to the GEOMAR research summary.

Water Allows More Mantle Rock to Melt

Subduction zones carry water-bearing minerals into Earth as one tectonic plate sinks beneath another. Water released from the descending plate moves into the overlying mantle and lowers the temperature at which the rock begins to melt. This process is known as hydrous mantle melting.

The team initially examined whether water directly controlled the amount of gold entering the magma. The chemical data instead showed that water mainly increases the extent of mantle melting. A larger proportion of rock can melt, and some mantle material can pass through more than one melting event.

During limited melting, much of the gold remains inside sulfide minerals, compounds in which sulfur is joined with metals. These minerals can survive in partially melted mantle and continue holding gold rather than releasing it into the liquid magma.

Marine geologist Christian Timm described that process directly in the official GEOMAR release: “At high degrees of melting, these minerals break down, releasing their gold completely into the melt.”

Repeated Mantle Melting Releases More Gold

The samples indicate that melting occurred at temperatures above the sulfide liquidus, the point at which the remaining sulfide material becomes fully molten. Once that threshold is crossed, sulfide minerals can no longer retain the same store of gold, silver and related metals.

The chemical patterns also show that the mantle source had been depleted during an earlier melting event before it melted again. This multi-stage melting gradually transferred gold from solid mantle material into magma rather than releasing it through a single episode.

The mantle beneath the Kermadec Arc was both water-rich and oxidized. Combined with a high degree of melting, those conditions reduced the ability of sulfide minerals to hold gold and allowed the metal to move into rising silicate magma.

The glass contained higher gold concentrations than similar magma formed at mid-ocean ridges. The measurements link that difference to repeated melting within a mantle source already altered by an earlier loss of molten material.

Gold Enters Magma Before Reaching the Seafloor

The process identified in the study takes place beneath oceanic island arcs, chains of volcanoes that form above subduction zones. Magma produced there can carry gold upward from the mantle, establishing its initial metal content before it reaches shallower volcanic systems.

Later geological processes can change that composition. As magma cools and crystallizes, water-rich fluids may separate from it and circulate through cracks in the surrounding rock. These hot fluids can dissolve metals, transport them through the crust and deposit them as temperatures and chemical conditions change.

Such activity can produce hydrothermal sulfide deposits beneath the ocean. These deposits form where hot, mineral-rich water interacts with volcanic rock and releases dissolved material. The mantle melting described in the study occurs earlier, supplying gold to the magma that feeds the volcanic system.

The measured rocks are not suitable for commercial extraction. Their gold concentrations remain several orders of magnitude below economically useful levels. The findings describe the deep geological process that moves gold from mantle minerals into magma beneath the Kermadec volcanic system.

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