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A previously unknown chemical reaction with water likely creates two mysterious features of Earth’s core. As a result, the core, made mainly of iron, becomes enriched with hydrogen and releases silicon into the overlying Earth’s mantle. This would explain, on the one hand, why the Earth’s outer core is about 10% lighter than expected and, on the other hand, what is happening to the thin layer which, according to seismic data, lies on the Earth’s core surface.
A team led by Sang-Heon Shim of Arizona State University and Yongjae Lee of Yonsei University in Seoul examined the behavior of core and shell components at high pressures and temperatures in a diamond anvil cell. As the group reports in the journal Nature Geoscience, water reacts with the silicon present in the iron alloy of the Earth’s core. This creates silicate, which sticks to the mantle rocks, while the hydrogen remains dissolved in the iron.
Earth’s liquid outer core is made of an iron-nickel alloy, although its exact composition is disputed. Its density is much lower than one would expect from such an alloy. It is therefore assumed that it contains large quantities of lighter elements, but it is not clear which ones and above all how they get there. Part of the answer may lie in a giant conveyor belt stretching from the Earth’s surface to the iron core. In the deep ocean trenches that mark the boundaries between Earth’s plates, water-rich ocean crust rocks descend deep into the Earth’s mantle. They release much of this water into the Earth’s mantle. But some water-containing minerals are so stable that they are transported with the Earth’s old crust to the edge of the Earth’s core, 2,900 kilometers away.
Shim and Lee’s team has now studied what happens to the water there through high-pressure experiments. In a laser-heated diamond anvil cell, in which pressures and temperatures at the core-shell boundary can be reached, water-containing minerals were reacted with an alloy of iron and 9% silicon. Regardless of the exact water source and the respective conditions, silicate always formed on the one hand and hydrogen dissolved in the iron on the other. In the experiment, the iron with dissolved hydrogen also had a significantly lower density than the surrounding alloy, so the reaction could explain why the Earth’s outer core is lighter than expected.
The results of this experiment also suggest that iron, much lighter and richer in hydrogen, is sparsely distributed throughout Earth’s outer core. It is simply too light to be distributed by currents in the metal and instead remains on the surface of the liquid core. This finding agrees surprisingly well with another puzzling observation from the 1990s. As a result, seismic waves that travel through the Earth and are diffracted into the various layers indicate the existence of a thin, stable layer of liquid around the Earth’s outer core.
Until now there was no adequate explanation for this layer, called E’. However, the team’s experiments now indicate that the liquid is the result of a gigantic process that took place over billions of years. Water, along with subducting rocks, travels from the surface to the edge of the Earth’s mantle. There it reacts with the metal in the Earth’s core, producing a hydrogen-rich iron melt that floats on the iron-nickel alloy of the Earth’s core and is visible as a separate layer in the earthquake data.