Quantifying the water content of Earth’s mantle transition zone (between the upper and lower mantle) has proved extremely challenging. The water content of the mantle potentially affects many of its properties, including rheology, electrical conductivity, melting point, and preservation of chemical heterogeneity, and may have a profound effect on plate tectonics. However, until now indirect estimates of water in the transition zone have yielded conflicting results. In a paper published in the 13 March 2014 edition of Nature , Graham Pearson (University of Alberta) and colleagues, three of whom are project leaders of the Diamonds and Mantle Geochemistry of Carbon (DMGC) initiative of DCO’s Reservoirs and Fluxes Community, finally provide concrete data on the subject, thanks to a unique and fortuitous find.
Listen to Graham Pearson on the Nature Podcast (12 March 2014).
The authors describe a series of analyses performed on a microscopic ringwoodite inclusion (~40μm in its longest dimension) found in a so-called “ultradeep” diamond from the Juína district of Mato Grosso, Brazil. Ultradeep diamonds formed within the transition zone, and therefore any included minerals must have formed at the same depth as the surrounding diamond, and either at the same time or before the diamond grew around it.
“It’s so small, this inclusion, it’s extremely difficult to find, never mind work on,” Pearson said, adding, “so it was a bit of a piece of luck, this discovery, as are many scientific discoveries.”
The tiny, greenish inclusion allowed Pearson and colleagues to directly measure the water content of a mineral from Earth’s mantle transition zone. Through a combination of X-ray diffraction, Raman spectroscopy, and infrared spectroscopy the team showed that their small sample contained ~1.4% water by weight. From this, they calculated that the bulk water content of the mantle is ~1% by weight, a figure that agrees with previous estimates generated from electromagnetic remote sensing.
“This sample really provides extremely strong confirmation that there are local wet spots deep in the Earth in this area,” said Pearson. “That particular zone in the Earth, the transition zone, might have as much water as all the world’s oceans put together.”
The study is a great example of a modern international collaboration with some of the top leaders from various fields, including the University of Alberta, Geoscience Institute at Goethe University, University of Padova, Durham University, University of Vienna, Trigon GeoServices and Ghent University. Indeed, DCO is acknowledged as playing a key role in the initiation of this research partnership.
“This is clearly one of the most important measurements ever made of this kind. It demonstrates that a wet mantle transition is possible and that pristine concentrations can be preserved and measured through a diamond host”, said Steve Shirey (Carnegie Institution for Science, USA), co-leader with Pearson and others of the DMGC effort. “It’s a stunning example of how and why natural diamonds are valuable to studies of Earth’s deep mantle.”
Image credits: University of Alberta.