Earth's robe diamonds reveal a water-rich environment

Earth's robe diamonds reveal a water-rich environment

The diamond recently recovered from a mine in Botswana revealed valuable information about the internal processes of our planet, many of which indicate that it formed at a depth of about 660 kilometres below the ground in the transition zone between the top and the bottom of the mantle, in a particularly water-rich environment, and scientists did not expect to find water at that depth.

More than 70 per cent of our planet is covered with water, but the Earth's crust consists of tectonic plates, both oceanic and continental, which move and slide beneath each other, and during these subductive events the water leaks into the Earth's mantle, fuelling a process known as the "deep water cycle", unlike the surface water cycle, which is absorbed in the depths of the Earth by various minerals that eventually rise to the surface as a result of volcanic activity.

Minerals in the upper mantle contain water, but scientists thought it was impossible at great depths.

The vast majority of diamonds come from between 150 and 200 km below the Earth's surface; few are from deeper processes; Tinging Gu, a physicist from the American Geological Institute, and his colleagues have recently studied one of these rare diamonds, the IaB type, from the Karove mine in Botswana; this diamond has a dozen mineral inclusions.

Using non-invasive methods 2SiO4, which is formed from olives at very high temperatures and pressures, Ringwudith usually occurs in the transition zone between the upper and lower mantle, between 410 and 660 km below the surface.

Around this ringwood, researchers also found Ferropericlas.

More importantly, the ringwood, like the other minerals found in the diamond, is hydrogenic, which suggests that the environment in which the diamond was formed was quite wet. "," researchers write in the magazine.

A discovery that sheds light on some seismic and volcanic phenomena

This is not the first time that water has been detected in the mantle transition zone. In 2018, researchers reported that high pressure forms of water called ice-VII were detected in diamonds at depths ranging from 410 to 660 km. But the combination of materials found in this Batswan diamond is indeed unique to researchers.

The data provided in 2018 did not allow an estimate of the amount of water stored at these depths; it was suggested that water enrichment could also be linked to a small localized water pocket, but the Gu study and his colleagues gave more support to the idea of a predominantly aquatic environment. ", they say.

In the transition zone, seismic waves mysteriously change speed. Ringwood holds water better than pheropericlas and enstatitis, so the mineral is likely to release a lot of water when it changes at this border. This potential discharge of water can explain why seismic waves spread differently in this region.

In addition, water captured in minerals is likely to play a role in the viscosity of mantle and internal dynamics, in particular the presence of water can contribute to the creation of magma plumes and influence the explosive force of the volcanic eruption.

The discovery made by Gu and his team suggests that the water in the depths of the Earth can easily exceed the amount of water on the surface. From where does this water come from, it is not yet known. Has it been introduced here as a result of subduction processes or has been present since the Earth's formation? To learn more, researchers will have to analyse the micro-elements present in the inclusions.