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Solve the gold puzzle?  – Why did heavy metals such as gold and platinum not sink into the ground?

Solve the gold puzzle? – Why did heavy metals such as gold and platinum not sink into the ground?

October 13, 2023 Reading time: 5 minutes.

An inexplicable wealth of minerals: In fact, there should be no gold, platinum or other heavy metals in the Earth’s crust and mantle – they have long since sunk into the Earth’s core. Researchers may now have discovered why this doesn’t happen. According to this, a combination of early impacts, semi-molten rocks, and mantle convection ensured that precious metals reached the Earth, but they did not reach the Earth’s core. Instead, some of it falls into the Earth’s deep mantle, and some of it has returned to the surface.

Gold, platinum and the like are not only precious metals and precious raw materials, but they also pose a number of mysteries. It is only partially clear how these elements were formed in the universe and how they reached Earth. But even more puzzling is why gold, platinum and other iron-loving metals are present in the Earth’s mantle and crust at all. Because of its strong association with iron and its weight, it must have sunk into the iron core of our planet in the early days of the Earth’s formation.

Did early planetary debris impacts bring gold to Earth? © Southwest Research Institute

Early influences as gold suppliers?

But it is also clear that metals such as gold, platinum, rubidium or palladium are still present in the Earth’s mantle and crust. but why? A possible explanation is that these minerals reached the Earth at a later time – after the formation of the Earth’s layers was complete and the Earth’s core had already formed. The remains of large planets and asteroids that struck the young Earth about 3.8 to 3.6 billion years ago are potential suppliers of this shipment of gold. Their metallic cores could contain enough gold and carbon dioxide to enrich Earth’s deposits.

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But the problem is that even with these effects, most of the heavy metals would have sunk relatively quickly and permanently into the Earth’s core. The large planetary debris core will penetrate so deeply upon impact that it will merge directly with Earth’s core. Even with smaller impacts, this will only be a matter of time. According to simulations, their minerals will also sink through the Earth’s mantle to the core over several tens of thousands of years.

Semi-melted layer as a gold trap

“This raises the question of how the Earth gets its precious metals,” says Simone Marchi of the Southwest Research Institute in Colorado. “Even during late accretion, it seems impossible to enrich the land cover with mineral components.” Or is it? Marchi, along with her colleague Jun Kurinaga of Yale University, studied the conditions under which Gold and Co. might be able to avoid sinking into the ground.

In their study, the team used geophysical models to reconstruct processes associated with a large asteroid impact. This showed that if the impacting body did not penetrate directly into the Earth’s core, a local magma ocean would form at the impact site, under which a thin, semi-molten transition layer lay. This plays a crucial role as a temporary trap for metals in the impact object, the researchers explain.

Reconstruction of the three-step process: During impact, the metal from the impactor collects in a semi-molten transition layer (A). This mineral-rich rock then sinks and hardens (B). Mantle convection moves some of these mineral-rich rocks upward (c) © Southwest Research Institute

What happened after effects

Scenario: After impact, gold, platinum and other iron-loving metals sink rapidly to the bottom of the magma ocean and then to the partially molten mantle. There, its sinking initially slows and minerals accumulate in the transition layer. “As a result, this region beneath the local magma ocean can reach a metal concentration of one to ten percent by weight,” Marchi and Kurinaga say.

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However, this transition zone does not remain permanently preserved either: due to its high mineral content, it becomes gravitationally unstable and now also begins to sink into the depths – along with minerals trapped in the semi-melted silicate rock. But the material does not melt as it sinks, but actually solidifies, as simulations have shown. This is possible because the temperature rises more slowly than the pressure. “Adiabatic pressure holds mineral components in mantle rocks,” the researchers explain.

Return transport with mantle convection

Highlights: Because the minerals are trapped in mantle rocks, they cannot cross the boundary between the core and mantle and disappear into the Earth’s core. Instead, they are under the influence of slow mantle convection along with the rest of the rock. Over billions of years, the hotter parts of the mantle gradually rise and the cooler parts of the mantle sink. While purely mineral relics would be too heavy to be carried by this current, this is not the case for metal-rich mantle rocks from the former transition zone, the team determined.

“This three-stage system, combined with long-term mixing through mantle convection, allows the preservation of iron-loving minerals in the Earth’s mantle,” Korinaga explains. This could solve the mystery of “impossible” gold deposits in the Earth’s mantle and crust.

An explanation for the mantle anomaly as well?

It is also interesting that the processes reconstructed in the simulation could solve another geological mystery. Seismic data show that there are several large anomalies in the lower mantle, just above the core-mantle boundary, which are areas that are denser and more mineral-rich than the rest of the mantle. It is still unclear where these mineral enrichments come from. Possible causes include leaks in the Earth’s core, as well as early impacts or other processes from the planet’s earliest days.

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If the scenario proposed by Marchi and Kurinaga is correct, these deep mantle anomalies could be remnants of metal-rich transition zones formed by collisions that sank to the base of the mantle. “Our study therefore also provides a concrete and quantitative mechanism for the primitive origin of large low-shear-velocity provinces in the lower mantle,” say the researchers. (Proceedings of the National Academy of Sciences, 2023; Droy: 10.1073/pnas.2309181120)

Source: Southwest Research Institute, Yale University

October 13, 2023 – Nadia Podbrigar