Geoscience: Diamond from the deep reveals a water-rich environment

A broadly water-saturated environment extends into the Earth’s lower mantle, according to analyses of mineral inclusions trapped inside a rare gem diamond that originates from a depth of 660 km below the Earth’s surface. The findings, published in Nature Geoscience, may improve our understanding of the Earth’s deep water cycle.

Earth is sometimes referred to as a water planet because oceans cover more than 70% of its surface. This ocean water can be transported by hydrated minerals deep into the Earth before it is returned to the surface via volcanic activity — a process known as the deep water cycle. This deep water can affect the explosivity of volcanic eruptions and the nature of seismic activity and plate tectonics. However, sampling and studying the Earth’s deep water cycle has been difficult, as the deepest borehole on Earth reaches just a little over 12 km in depth below the Earth’s surface.

Tingting Gu, Fabrizio Nestola and colleagues studied a gem-quality diamond from the Karowe mine in Botswana that had trapped and protected a sample of the Earth’s lower mantle on its journey from a depth of approximately 660 km to the surface. The authors find evidence for ringwoodite and other hydrous minerals and phases in the diamond, indicating that it formed in a hydrated region of the Earth’s mantle.

The authors conclude that the diamond confirms the presence of mineral-bound water down to, and potentially beyond, 660 km depth in Earth, suggesting broad hydration of this region.