It's all about water

ScienceDaily has an intriguing little article today called Keeping the Earth's Plates Oiled. I haven't had time to dive into it, but I'll put a few blurbs from it here.

Björn Winker, a mineralogist at the Johann Wolfgang Goethe University in Frankfurt, Germany, believes that the key to the asthenosphere is water. "We have to have water in the asthenosphere to get it plastically deforming," he explains. This water is no longer in its liquid state, but is bound to oxygen in crystal structures to form hydroxyl (OH-) groups instead.

Winkler, and another researcher Refson's study attempts to address the nature of the water through experimentation.

Unfortunately we can't get samples from the asthenosphere ... Winkler finds samples of these candidate minerals on the Earth's surface and...subjects them to the pressures and temperatures estimated for the asthenosphere.

What they found was that...

...clinochlore was found to be good at holding onto water, but showed some interesting changes in its structure at around 8GPa. "The nature of the hydrogen bonds start to change and the layers within the structure slide," explains Refson.

The article then moves on to a separate, but related study.

These kind of results have been invaluable for Hans Keppler, a geologist at the University of Bayreuth in Germany. He has been trying to work out why the asthenosphere exists. Previous theories have suggested that this 'wet' and slippery layer exists because minerals leave their water behind them when they melt and turn into magma. "This explains why the asthenosphere appears beneath oceans, but it doesn't explain why we have an asthenosphere beneath the continents," says Keppler. Lava continually bubbles up at mid-ocean ridges, but continental plates don't have an equivalent spring of constant magma. It also fails to explain why there is a lower boundary to the asthenosphere.

What Keppler found was that...

...water solubility in olivine continuously increases with temperature and pressure, whereas in aluminium-saturated enstatite the solubility reaches a distinct minimum at asthenosphere temperatures and pressures. "It means that the mantle minerals cannot contain all the water and the excess water forms a hydrous silicate melt"

I'm not a mineralogist/petrologist by training, so I honestly can't comment on the details of these studies (I welcome comments from my geoblogger colleagues). But, reading it made me take a step back and think about the question posed at the beginning of the article regarding why the plate tectonics operating on Earth seems so unique. Water is key to subduction, and subduction seems to be what is unique. Other planetary bodies have different forms of volcanism, but not subduction (as far as we know).

These ideas are nothing new (see reference below, for example), but it should be very interesting as we explore other planets in the future. We will be able to put Earth's plate tectonics into a broader context, which will certainly help us understand it better.

Like I said, I'm used to studying modern and ancient Earth surface processes, so I encourage any comments about other papers or ideas out there.

Ragenauer-Lieb, K., Yuen, D. and Branlund, J., The initiation of subduction: criticalilty by addition of water? Science, B. 294, p. 578-580, (October 2001)

image above from here

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