Water in the Earth’s interior: evidence found inside diamond

Science and Technology


The discovery of a rare mineral called Ringwoodite, buried some 660 kilometres below the Earth’s surface and found formed inside diamonds, confirms the theory that there exist huge reservoirs of water at this depth within our planet. The team of researchers who made the discovery, led by Professor Graham Pearson of the University of Alberta, Canada, were the first scientists to find a terrestrial sample of the mineral. Until 2008, Ringwoodite had only been observed in meteorites, as carrying out fieldwork at such great depths within the Earth is simply not feasible.

The Ringwoodite sample in question was found in the Juina area of Mato Grosso, Brazil, by artisan miners. They found the mineral in shallow rivers, where it was embedded in diamond that had been bought to the Earth’s surface by the most deeply derived of all volcanic rocks, known as kimberlite. At only 60 micrometres across (1 micrometre is equivalent to 0.001 millimetres), the Ringwoodite inclusion in the diamond is invisible to the naked eye. The Brazilian miners collected the diamond in 2008, but it wasn’t until 2009 that the Ringwoodite was found to be present in the mineral. The Ringwoodite was found by John McNeill, a scientist in Professor Pearson’s team, who described the finding as “a piece of luck… as are many scientific discoveries”.

Its fortuitous detection by no means undermines the importance of this discovery, however. Prior to finding the Ringwoodite, it had been theorised that the ultimate origin of all the water in the Earth’s hydrosphere – that is, all the water involved in the water cycle – was deep within the Earth’s mantle. It is notoriously difficult for researchers to experimentally prove theories based around events or objects at such great depths, for obvious practical reasons. Nevertheless, it was hypothetically, and then experimentally, shown that the water-storage capacity of minerals in the transition zone of the mantle (a region between 440 and 660 kilometres beneath the Earth’s surface) is incredibly high. Two such minerals, Ringwoodite and Wadsleyite, can absorb so much water that it may constitute up to 2.5% of their individual masses.  If the transition zone was thus hydrated, then that region of the Earth’s mantle would play a key role in the planet’s magnetism, and also in plate tectonics.

However, it is only with this discovery that the hydration of the transition zone can be conclusively proven, and all of the planet-wide implications that this encompasses can be accepted. 1.5% of the mass of the Ringwoodite found by McNeill in the Brazilian diamond was made up of water.

Professor Pearson commented that: “this sample really provides extremely strong confirmation that there are local wet spots deep in the Earth in this area. That particular zone in the Earth, the transition zone, might have as much water as all the world’s oceans put together”.

During the years of analysis between the initial finding of the hydrated Ringwoodite in the diamond, and the publication of Pearson et al’s conclusions in an article in Nature in March 2014, a combination of X-ray diffraction and Infra-red spectroscopy was used to determine first the presence of water, and then to measure the percentage mass of water in the mineral. In addition to the five years that passed between discovery and publication, there were approximately fifty years between the original, theoretical proposal of a hydrated transition zone, and the experimental evidence provided by the Ringwoodite mineral’s presence in a small river in Brazil.

The findings have helped seismologists, geologists, geophysicists and other scientists to better understand the composition of the Earth’s interior. Until this experimental evidence for the hydration of the transition zone, which lies between the outer and inner Mantle, there has been much controversy within the scientific community as to whether there is any water present in this region at all, or if it is desert-dry.

Professor Pearson concludes by saying: ““One of the reasons the Earth is such a dynamic planet is because of the presence of some water in its interior. Water changes everything about the way a planet works”.


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