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Science 19 January 2007:
Vol. 315. no. 5810, pp. 364 - 368
DOI: 10.1126/science.1135422
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Reports

Water Solubility in Aluminous Orthopyroxene and the Origin of Earth's Asthenosphere

Katrin Mierdel,1 Hans Keppler,1,2* Joseph R. Smyth,2,3 Falko Langenhorst2,4

Plate tectonics is based on the concept of rigid lithosphere plates sliding on a mechanically weak asthenosphere. Many models assume that the weakness of the asthenosphere is related to the presence of small amounts of hydrous melts. However, the mechanism that may cause melting in the asthenosphere is not well understood. We show that the asthenosphere coincides with a zone where the water solubility in mantle minerals has a pronounced minimum. The minimum is due to a sharp decrease of water solubility in aluminous orthopyroxene with depth, whereas the water solubility in olivine continuously increases with pressure. Melting in the asthenosphere may therefore be related not to volatile enrichment but to a minimum in water solubility, which causes excess water to form a hydrous silicate melt.

1 Institut für Geowissenschaften, Universität Tübingen, Wilhelmstr. 56, 72074 Tübingen, Germany.
2 Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany.
3 Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA.
4 Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Burgweg 11, 07749 Jena, Germany.

* To whom correspondence should be addressed. E-mail: hans.keppler{at}uni-bayreuth.de

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