Tuesday, April 22, 2014

Earth hydrosphere #2? What really can be said about the ringwoodite discovery

Earth hydrosphere #1 and a big floating hunk of the cryosphere too. (From http://inter3.kuicr.kyoto-u.ac.jp/album/KH94-4/FH000153.jpg)

Earth has a well known hydrosphere--the oceans that make it up cover two-thirds of the planet. But there may be another hydrosphere deep underground. Recently, a mineral that had only been seen in impact - shocked meteorites, ringwoodite, was discovered in a terrestrial diamond1. Pearson and other workers wrote in the journal Nature that this diamond is thought to have originated in the transition zone between the upper and lower mantle--a zone long known by geologists from a change in earthquake seismic wave behavior as they pass through the earth.

Ringwoodite is a high pressure version or polymorph of olivine, which is a common mineral in basalt. It has the same composition as olivine, but the atoms are arranged differently to be more stable at high pressures found in Earth's mid-mantle. Although only one ringwoodite grain has now been found from the Earth, it is commonly found in meteorites. This is because many asteroids experienced collisions in the 4.5 billion years they've orbited the sun; these collisions produce high shock pressure that allows olivine to recrystallize as ringwoodite. Below is an image showing an incidence in a meteorite where recrystallization from olivine to ringwoodite was incomplete. The box on the left image is blown up on the right image (Ol is olivine; Rgt is ringwoodite.):

Ringwoodite had been predicted to be stable deep within the Earth for decades2, and the presence of water has also been hypothesized, but remained controversial. The authors not only have confirmed a terrestrial ringwoodite, but the finding of water (1.4 to 1.5% with uncertainties "as large as 50%") allow one to make predictions of the hydration of Earth's transition zone. The authors claim that the area local to where the ringwoodite formed (and presumably where the diamond grew around it, preserving it) is about 1% hydrated.

The discovery of a ringwoodite mineral on earth is a significant achievement. The discovery that it hosts water is also very exciting. Furthermore, showing that diamonds and the kimberlite source that brought all the material to the surface must have originated in the transition zone is a very significant result. If other grains are found elsewhere, perhaps it can be shown that the transition zone is pervasively hydrated. It could also show that a volatile rich zone deep in the mantle may be linked to diamond formation and explosive kimberlite eruptions (Water is often linked to vulcanism; one reason is that it lowers the melting temperature of rock). It must be stressed, however, that only one 40 micron diameter ringwoodite grain has been discovered.

Unfortunately, the announcement of this discovery has been accompanied by hyperbole. The University of Alberta, where the first author is from, extrapolates the local area surrounding a tiny 40 micron mineral grain to Earth's entire transition zone: "We show that a special region of the Earth, between 400 and 700km, known as the 'transition zone', is an oasis of water in an otherwise very dry deep interior."3 Sci-News.com states "The first land discovery of ringwoodite confirms the presence of huge water reservoirs beneath the surface of the Earth." Other news outlets, from CBC to BBC to Fox News, have announced similar claims.

There are inklings that the transition zone may be hydrated--for example, seismic waves behave peculiarly in the area that could be explained by pervasive hydration. Many workers (some referenced in the Pearson paper) have given evidence that point to a hydrated transition zone and its role in a deep water cycle. The work by Pearson and his co-authors offers one more link, an exciting link, that the mantle's transition zone may be hydrated at least locally. However, extrapolating interpretations made from a single observation to Earth's entire mantle transition zone is not valid. More samples, preferably from wide-ranging locations, are needed to make such a claim. 

Correction: The amount of water reported was mistakenly first written as 2.5 +/- 50%. The actual number is recorded as "clearly indicative of significant H2O content, and are consistent with a minimum estimate between 1.4 and 1.5 wt% H2O, derived by integrating the spectra in Fig. 2 (see Methods section on FTIR spectroscopy). Although the uncertainty in these estimates may be as large as 50%..." 

1. Pearson et al. Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature. 507, 221-224 (2014)  http://www.nature.com/nature/journal/v507/n7491/full/nature13080.html

2. Ringwood, A. E. & Major, A. The system Mg2SiO4-Fe2SiO4 at high pressures and
temperatures. Phys. Earth Planet. Inter. 3, 89–108 (1970)

3. Hydrous mantle transition zone indicated by ringwoodite included within diamond, University of Alberta Department of Earth and Atmospheric Sciences, http://easweb.eas.ualberta.ca/page/hydrous-mantle-transition 

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