Tiny diamond impurity reveals water riches of deep Earth Mineral never before found on Earth points to a vast reservoir in the mantle. Nature 12 March 2014 doi:10.1038/nature.2014.14862 nature/news/tiny-diamond-impurity-reveals-water-riches-of-deep-earth-1.14862 A microscopic crystal of a mineral never before seen in a terrestrial rock holds clues to the presence of vast quantities of water deep in Earth’s mantle, scientists report in a paper published today in Nature (1). The discovery came from a diamond weighing less than one-tenth of a gram, found in Brazil. Further studies of the sample could help to answer the long-standing question of the origin of the planets water. Most diamonds form at depths of about 150 to 200 kilometres, but ultradeep diamonds come from a region of the mantle known as the transition zone, 410 to 660 kilometres below the surface, says Graham Pearson, a mantle geochemist at the University of Alberta in Edmonton and the lead author of the study. Impurities in ultradeep diamonds can be used as probes to study the regions in which the stones formed — and in particular to understand what minerals are present at those depths. Certain minerals have crystal structures that can form only at high pressures or temperatures, or both, and many rearrange themselves into different structures when the pressure is taken off or the temperature goes down. Thus, when the churning of the mantle brings rock towards the surface, some of the minerals that formed at great depths can no longer be found. But if the minerals are trapped inside diamonds, they stay compressed in their original forms. “These high-pressure diamonds give you a window into the deep Earth,” says Pearson. He and his team studied one such diamond, which weighed 0.09 grams and came from the Juína district in Brazil. As they examined impurities in it using a light-scattering method known as Raman spectroscopy, they came across something unusual: a grain 40 micrometres across that turned out to be ringwoodite — a high-pressure form of olivine, a mineral that makes up much of the upper mantle. Ringwoodite had previously been found only in meteorites or synthesized in the lab. Rock star Mineralogical theory and seismic findings had long suggested that ringwoodite is a major component of the transition zone, and the finding backs that up. “It confirms that our ideas of how the mantle is constructed are correct,” says Hans Keppler, a geophysicist at the University of Bayreuth in Germany, who wrote about the find in an accompanying News & Views (2). Unlike better-studied forms of olivine, ringwoodite can hold a substantial amount of water. The sample therefore had the potential to help resolve a long-standing controversy over just how much water the transition zone contains. Using infrared spectroscopy, Pearson’s team found that its tiny fleck of ringwoodite contained about 1% water by weight. “That may not sound like much,” Pearson says, “but when you realize how much ringwoodite there is, the transition zone could hold as much water as all the Earth’s oceans put together.” But the water content of a single crystal is not necessarily representative of the entire zone, says Norm Sleep, a geophysicist at Stanford University in California. Diamonds are produced by an unusual type of volcanism that is normally associated with water-rich rock, he says. He compares the situation to that of someone panning for gold and finding a large nugget: “It would be unwise to assume that all the gravel in the stream is gold nuggets.” Pearson agrees. Remote-sensing studies of the mantle have produced conflicting results, suggesting that the water content of the transition zone may be “spotty”, he says. “Our sample appears to come from one of the wet spots.” Where it all began There are two theories as to where the mantles water came from. One is that it was ocean water that was carried deep underground when sea-floor rocks were subducted by plate tectonics. The other is that deeper layers of the Earth still contain water that was part of the materials that formed the Earth. If the water has been there since Earth formed, its ratio of deuterium to normal hydrogen could be different from that found in sea water today—and closer to the composition of the Earth’s primordial water. If so, that ratio could provide clues as to whether the water came from asteroids or from comets, says Humberto Campins, an asteroid researcher at the University of Central Florida in Orlando. Pearson sees a value to checking the isotope ratio, but so far his group has been unwilling to do such destructive tests on the only known piece of mantle ringwoodite. “We have to think really carefully on what we do next on this sample because it’s very small: 40 micrometres,” he says. “That means you can only think of doing one or two additional analyses.” - Diamond Suggests Presence of Water Deep Within Earth news.sciencemag.org/chemistry/2014/03/scienceshot-diamond-suggests-presence-water-deep-within-earth Imperfections can reduce a diamonds value to a jeweler, but they may render it priceless to a geologist. Take the tiny speck in the diamond above; too small to be visible to the naked eye, it could help settle a long-standing debate about the amount of water in Earths mantle. Down to about 400 km below the surface, the mantle is mainly a mineral called olivine, which does not absorb water. However, below this, the immense heat and pressure cause the olivine to adopt different chemical structures, one of which is called ringwoodite, which laboratory tests have shown can contain up to 2.5% water. The chemical structure of the diamond above, unearthed by magma pushing its way to the surface in the Juina district of Brazil, shows that it was formed more than 400 km deep. Under a microscope, the researchers spotted a 40 micrometer crystal trapped inside the diamond called an inclusion. Spectroscopic analysis showed this to be ringwoodite. Further analysis detailed online today in Nature shows the ringwoodite contains hydrogen-oxygen bonds, which suggests the crystal lattice contains at least 1.4% water. The place where the diamond was produced may not be typical of the entire lower mantle, but if it is then there could be a lot of water down there. This would be important, as changes in the temperature in the mantle could cause it to expel highly pressurized steam, which could lead to volcanic eruptions. References Editors summary in Nature It is not clear just how much water resides within the solid Earth, and where it is to be found, with many indirect measurements yielding conflicting results. Here Graham Pearson and co-authors present evidence from a diamond inclusion from Juína, Brazil, for the first known terrestrial occurrence of ringwoodite — a high-pressure polymorph of olivine first identified in meteorites and thought to be a major constituent of the Earths mantle transition zone. The water-rich nature of this inclusion provides direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent. 1. Keppler, H. Nature 507, 174–175 (2014). Geology: Earths deep water reservoir Nature 507, 174–175 (13 March 2014) doi:10.1038/507174a nature/nature/journal/v507/n7491/full/507174a.html News & Views A tiny sample of a mineral included in a diamond confirms predictions from high-pressure laboratory experiments that a water reservoir comparable in size to all the oceans combined is hidden deep in Earths mantle. See Letter p.221 (2) 2. Pearson, D. G. et al. Nature 507, 221–224 (2014). Hydrous mantle transition zone indicated by ringwoodite included within diamond Nature 507, 221–224 (13 March 2014) doi:10.1038/nature13080 nature/nature/journal/v507/n7491/full/nature13080.html Abstract The ultimate origin of water in the Earth’s hydrosphere is in the deep Earth—the mantle. Theory and experiments have shown that although the water storage capacity of olivine-dominated shallow mantle is limited, the Earth’s transition zone, at depths between 410 and 660kilometres, could be a major repository for water, owing to the ability of the higher-pressure polymorphs of olivine—wadsleyite and ringwoodite—to host enough water to comprise up to around 2.5 per cent of their weight. A hydrous transition zone may have a key role in terrestrial magmatism and plate tectonics, yet despite experimental demonstration of the water-bearing capacity of these phases, geophysical probes such as electrical conductivity have provided conflicting results, and the issue of whether the transition zone contains abundant water remains highly controversial. Here we report X-ray diffraction, Raman and infrared spectroscopic data that provide, to our knowledge, the first evidence for the terrestrial occurrence of any higher-pressure polymorph of olivine: we find ringwoodite included in a diamond from Juína, Brazil. The water-rich nature of this inclusion, indicated by infrared absorption, along with the preservation of the ringwoodite, is direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent. The finding also indicates that some kimberlites must have their primary sources in this deep mantle region.
Posted on: Thu, 13 Mar 2014 19:01:05 +0000
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