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Science 31 March 2000:
Vol. 287. no. 5462, pp. 2463 - 2466
DOI: 10.1126/science.287.5462.2463
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Reports

Hot and Dry Deep Crustal Xenoliths from Tibet

Bradley R. Hacker, 1 Edwin Gnos, 2 Lothar Ratschbacher, 3 Marty Grove, 4 Michael McWilliams, 5 Stephen V. Sobolev, 6 Jiang Wan, 7 Wu Zhenhan 7

Anhydrous metasedimentary and mafic xenoliths entrained in 3-million-year-old shoshonitic lavas of the central Tibetan Plateau record a thermal gradient reaching about 800° to 1000°C at a depth of 30 to 50 kilometers; just before extraction, these same xenoliths were heated as much as 200°C. Although these rocks show that the central Tibetan crust is hot enough to cause even dehydration melting of mica, the absence of hydrous minerals, and the match of our calculated P-wave speeds and Poisson's ratios with seismological observations, argue against the presence of widespread crustal melting.

1 Geological Sciences, University of California, Santa Barbara, CA 93106, USA.
2 Mineralogisch-Petrographisches Institut, University of Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland.
3 Institut für Geologie, Technische Universität Bergakademie Freiberg, Bernhard-von-Cottastrasse 2, D-9596 Freiberg/Sachsen, Germany.
4 Department of Earth and Space Sciences, 3806 Geology Building, Los Angeles, CA 90095-1567, USA.
5 Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA.
6 GeoForschungsZentrum Potsdam, Telegrafenberg A17, 14473 Potsdam, Germany.
7 Institute of Geomechanics, Chinese Academy of Geological Sciences, 11 Minzu Xueyuan Nanlu, Beijing, China, 100081.

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