Diamonds Play the Water Dowser
(appeared on 19th March 2014)

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Diamonds from deep in the earth suggest there are vast reserves of water down there, says S.Ananthanarayanan.

In Jules Verne’s Journey to the Centre of the Earth (1864), Prof Hardwigg and his group descend into the entrails of the earth through an extinct volcano in Iceland, and they discover a subterranean sea, over which they make a raft voyage and meet monstrous creatures.

Even if it is not this sea of water which has been found, D. G. Pearson, F. E. Brenker, F. Nestola, J. McNeill, L. Nasdala, M. T. Hutchison, S. Matveev, K. Mather, G. Silversmit, S. Schmitz, B. Vekemans and L. Vincze, from Canada, USA, Germany, Austria, Italy, Belgium and UK, report in the journal, Nature, that a mineral found between 400 metres to 670 km below the earth’s surface may be rich in water, and hold as much as the water in all the surface oceans!

The structure of the earth, going down from the surface, is that after the outer crust, of 8-40 km, there is the largest, semi-solid region, 2,900 km deep, known as the mantle. Below this is the molten and hence liquid, outer core, and further down is the inner core, which is solid, because of the high pressure. The mantle itself, which is composed of silicate rock, richer in magnesium and iron that the crust, has two zones, the outer, rigid mantle and the inner mantle, which is stiffer, again due to high pressure.

What kind of material there is at great depths underground is known mainly from the way seismic waves, which are disturbances caused by violent events, are reflected by the surfaces of different material that the waves encounter. The reflected waves, detected at the surface, tens or hundreds of km away, have revealed a structure of changing strata as we go down, and also a transition zone, from 400 deep and till 670 kms deep, between the upper and lower mantle. The material of the mantle is mostly a combination of magnesium, iron and silica, or common sand, called olivine. The structure of olivine keeps changing with depth, as the pressure increases, and the geologist Alfred E Ringwood first proposed that the changes detected by seismic waves were a result of transitions in the forms, particularly a form known as ‘spinel’, where there is a change in the positions of magnesium and iron atoms, that olivine assumes. While we cannot find actual samples of olivine from great depths, what kind of olivine there should be at different depths can be worked out from laboratory studies. When the proposed ‘spinel’ form was found present in meteorites that had experienced the high shock pressures during collisions in space, this was verification and the form of olivine was named ringwoodite

Ringwood’s group, in fact had found that a high pressure form of olivine, synthesized by them in the laboratory, had a structure and distribution of charges that could allow water to be trapped. Substantial water content, in fact was later realized in the laboratory in this high pressure form, which is called wadstleyite and, surprisingly, also to the extent of 1% in ringwoodite. To find water in ringwoodite was surprising, because water, or H2O, exists as H+ and OH-, and the category to which the ringwoodite structure belongs does not admit the OH- group in its composition. But it was later found that the ringwoodite structure contains charged areas that allow the OH group to find a place and water can exist in ringwoodite, as a reverse solution, of molecules of a liquid being fitted into the structure of solid crystals.

There was hence the possibility of the transition zone, below the shallow mantle, being rich in water, which would influence the movement of the softer material below the earth’s crust and also the way the rigid plates that form the crust and upper mantle may shift or flow. But despite the possibility of water being established, proof that water exists has not been discovered. The usual methods to assess the nature of the deep earth, like studies of electrical conductivity, gave conflicting results and the question has stayed controversial.

Limited samples from the mantle, derived from deeper volcanic pipes, show that the upper mantle is predominantly olivine, and diamonds, which arise at great depths, have been studied for traces of wadsleyite or ringwoodite, which were suspected to be the forms that change into olivine when they rise to lower pressures. But samples from the transition zone appear to undergo changes that defaces original, possibly ringwoodite forms. The international group featured in Nature worked on focused on diamonds from the Juina district of Mato Grosso, Brazil, a well-known site for deep diamonds originating in the transition zone and lower mantle, using ultra-sensitive techniques like high energy X Ray based tomography and Raman scattering.

The X Rays reveal the atom to atom level structure of mantle material brought up with diamonds and Raman spectra show the signature of low energy inter-atom interactions. The Nature group reports that the X Ray data shows the clear pattern the belongs to ringwoodite. The Raman spectra are not inconsistent with the X Ray findings and furthermore, indicates the presence of significant OH, or water content. Further analysis of the data yields a measure of the water content and the depth of formation of the crystal. The fact that the upper mantle has little water indicates that the water in the ringwoodite must have come locally from the transition zone.

The presence of water in the transition zone is also consistent with other data, like electromagnetic effects and the transmission of seismic waves and it supports the idea that fluid activity, notably of water, played a role in the formation of ultra-deep diamonds. The estimate of water content is 1%. Considering the huge size of the transition zone, this 1% is comparable with the water content of the surface oceans. The great depths, which are storehouses of industrially important metals are hence seen hold huge water too. But the significance is for the structure of the earth, not as a source of water, as the water content is highly spread out.

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