Diamonds and water in the deep Earth: a new scenario

AbstractEarth is a water planet, but how much water exists on and in the Earth? Is the water limited to the Earth's surface and limited depths of our planet (molecular water of the hydrosphere), or do deep reservoirs of hydrogen and oxygen really exist as proposed in recent works but not yet proven? Due to the importance of H2O for life and geological processes on the Earth, these questions are among the most significant in all of the Earth sciences. Water must be present in the deep Earth as plate tectonics could not work without water as a major driving force that lowers both viscosity and density of the solid mineral phases of the interior and controls the onset of melting. On subduction, water is returned to the hydrosphere first by dewatering of hydrous phases and second by melting and arc magmatism in and above the subducting slab. The mantle is composed of oxygen minerals, and the extent to which hydrogen is dissolved in them constitutes the true reservoir of the planet's water. Are 'deep water and diamonds' intimately related as indicated in the title of the present article? What is the connection between these two important terrestrial materials? The necessity to review this issue arises from the recent discovery of a strongly hydrous ringwoodite in a Brazilian diamond. As ringwoodite constitutes 60% or more of the lower part of the transition zone, between 525 and 660 km depth, this could correspond to a huge amount of water in this region, comparable or greater in mass to all of Earth's hydrosphere. If the water found in this ringwoodite is representative of the water concentrations of the transition zone, then estimates of Earth's total water reservoir are in need of major revision. This work is an attempt at such a revision.Keywords: diamondwaterEarthreservoirmantle AcknowledgementsMatteo Chinellato is thanked for the picture in Figure 1. We thank Tatiana Mantovani for making Figure 7 and Stefano Castelli for technical photographical support.Additional informationFundingThis work was supported by the European Research Council Starting Grant 2012 [grant agreement number 307,322] to FN [project INDIMEDEA] and by US National Science Foundation grants [EAR 11-13369, EAR 14-16979] to JRS.