Unexpected mineral hints at large amounts of water in Earth’s mantle

Nature : Some diamonds are formed far deeper than others—anywhere from 410 km to 660 km below Earth’s surface in the mantle. Impurities trapped in the diamonds can provide details, such as the temperature, pressure, and presence of different molecules, about the area in which the diamonds formed. Graham Pearson of the University of Alberta and his colleagues have found inside one of those ultradeep diamonds the first sample of a mineral called ringwoodite that previously had only been found in meteorites or created in labs. Ringwoodite is a form of olivine that is formed under very high pressures, and an analysis of the 40-µm crystal found that it was 1% water by weight. While not significant itself, an extrapolation suggests that if that concentration of water is common, the amount of water trapped in the mantle could equal the amount in Earth’s oceans. Pearson cautions that other studies of the mantle suggest that the estimate is on the high side and that it is likely that water in the mantle is not evenly distributed.