Intuitively, one might think that gold is an incredibly rare metal on Earth. However, this is far from the case. What specialists were unaware of until now is that it does not remain as confined to the depths of our planet as previously thought: very large quantities of this famous yellow metal even seep into the upper geological layers. A fascinating discovery for geologists, for whom it represents a great source of wealth... purely scientifically.
Many researchers have already attempted to estimate the quantity of gold present on our planet – and the figures are dizzying. A 2006 study conducted by Professor Bernard Wood, which is still a benchmark today, concluded, for example, that the Earth contains more than 16 billion tonnes of the precious metal! Unfortunately for miners, it is completely impossible to extract the vast majority of this resource. Indeed, more than 99.999% of this mass is confined in the Earth's core, approximately 3,000 kilometres deep, held back by several physical and chemical factors that prevent it from escaping. At least, that was the general consensus until the publication of a new study by German researchers.
The Earth's core, a veritable prison...
To understand the ins and outs of this work, we need to take a short detour to look at the history of our good old blue planet. Around 4.5 billion years ago, the young Earth, still largely made of molten material at the time, underwent what is called planetary differentiation. The various elements that compose it were divided into several layers: the densest elements, such as iron and nickel, migrated toward the center, leaving behind silicate materials that today constitute the crust and mantle.
During this great migration, iron brought with it other elements such as gold, called siderophiles (“iron-loving” in Greek), and therefore more likely to associate with iron than with silicates. Since the end of this great differentiation, specialists traditionally consider that all this material is sequestered by a geological boundary, the Gutenberg discontinuity, which physically and chemically isolates the core from the mantle. According to this traditional model, gold and other siderophile elements remain mostly trapped in the core indefinitely, with a few exceptions. These materials are therefore unable to escape and rise again thanks to mantle plumes, these upwellings of hot rocks from the deep mantle.
This is where Nils Messling's team from the Department of Geochemistry comes in and isotope geology from the University of Göttingen in Germany. The group was studying various rocks in Hawaii, an incredibly important archipelago in the history of Earth science. It is located on a volcanic hotspot and was directly formed by material brought up from the bowels of the Earth via the mantle plumes mentioned above. This is an incredible geological archive that can teach us a lot about the planet's history and internal dynamics... and that's exactly what happened during this expedition.
...not as hermetic as we thought
Messling's team has developed a new technique that allows them to very precisely measure the isotopic composition of a rock, that is, the distribution of different variants (called isotopes) of a given chemical element in a sample. This is a fantastic resource for geologists, because this isotopic composition can reveal valuable information about the origin and evolution of the rock in question - a kind of geological signature, in short.
In Hawaii, they used this new tool, capable of detecting subtle differences in this isotopic composition, to analyze the origin of ruthenium in local rocks. It is a highly siderophile element: like gold, almost all of the ruthenium originally present on Earth migrated to the core. The few traces that remain in the current crust come from other sources, such as meteorites. However, to their great surprise, the isotopic distribution corresponded precisely to that expected at the interface between the core and the mantle!
A remarkable observation, and for good reason: it strongly suggests that this geological boundary is not as impermeable as previously thought. Against all odds, it finally appears that elements like ruthenium and gold can still make their way from the core to the surface.
A promising new avenue of research
"As soon as the first results arrived, we realized we had found a gold mine," Nils Messling said in a press release. "Our findings not only show that the Earth's core is not as isolated as previously thought, but also that enormous volumes of superheated mantle material – several hundred million billion tons of rock – originate at the core-mantle boundary and rise to the Earth's surface to form oceanic islands like Hawaii," says his colleague Matthias Willbold. This does not mean that there are gigantic unknown gold reserves just waiting to be exploited. On the other hand, these results could force geologists to reconsider an entire chapter of Earth's history, and will undoubtedly fuel other work that could also lead to fascinating new discoveries.
"Was the processes we observe today also at work in the past? This remains to be proven. But our findings open up an entirely new perspective on the evolution of our planet's internal dynamics," concludes Willbold.
The text of the study is available here.
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