High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite
- Larissa F. Dobrzhinetskayaa,1,
- Richard Wirthb,
- Jingsui Yangc,
- Ian D. Hutcheond,
- Peter K. Weberd and
- Harry W. Green IIa
- aInstitute of Geophysics and Planetary Physics and Department of Earth Sciences, University of California, Riverside, CA 92521;
- bGeoForschungsZentrum, D-14473 Potsdam, Germany;
- cKey Laboratory for Continental Dynamics, Institute of Geology, Beijing 100037, China; and
- dGlenn T. Seaborg Institute, Lawrence Livermore National Laboratory, Livermore, CA 94551
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Edited by Ho-kwang Mao, Carnegie Institution of Washington, Washington, DC, and approved September 25, 2009 (received for review May 21, 2009)
Abstract
The deepest rocks known from within Earth are fragments of normal mantle (≈400 km) and metamorphosed sediments (≈350 km), both found exhumed in continental collision terranes. Here, we report fragments of a highly reduced deep mantle environment from at least 300 km, perhaps very much more, extracted from chromite of a Tibetan ophiolite. The sample consists, in part, of diamond, coesite-after-stishovite, the high-pressure form of TiO2, native iron, high-pressure nitrides with a deep mantle isotopic signature, and associated SiC. This appears to be a natural example of the recently discovered disproportionation of Fe2+ at very high pressure and consequent low oxygen fugacity (fO2) in deep Earth. Encapsulation within chromitite enclosed within upwelling solid mantle rock appears to be the only vehicle capable of transporting these phases and preserving their low-fO2 environment at the very high temperatures of oceanic spreading centers.
Footnotes
- 1To whom correspondence should be addressed. E-mail: larissa{at}ucr.edu
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Author contributions: L.F.D. and J.Y. designed research; L.F.D., R.W., I.D.H., and P.K.W. performed research; L.F.D., R.W., I.D.H., and P.K.W. contributed new reagents/analytic tools; L.F.D., R.W., J.Y., I.D.H., P.K.W., and H.W.G. analyzed data; and L.F.D. and H.W.G. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission.
