TY - JOUR
T1 - Effect of H2O on upper mantle phase transitions in MgSiO3
T2 - Is the depth of the seismic X-discontinuity an indicator of mantle water content?
AU - Jacobsen, Steven D.
AU - Liu, Zhenxian
AU - Ballaran, Tiziana Boffa
AU - Littlefield, Elizabeth F.
AU - Ehm, Lars
AU - Hemley, Russell J.
N1 - Funding Information:
We thank T. Withers and an anonymous reviewer for constructive feedback on this paper. SDJ acknowledges D. Suetsugu, H. Kawakatsu, and C.R. Bina for the opportunity to visit Japan in 2009 for the Stagnant Slab Project International Symposium on Deep Slab and Mantle Dynamics in Kyoto, and T. Irifune and T. Inoue for discussions at the Geodynamics Research Centre (GRC) at Ehime University in Matsuyama during that trip. SDJ thanks D.J. Frost, H. Keppler, and D. Rubie for hosting visits to Bayerisches Geoinstitut, University of Bayreuth, Germany, where several of the experiments in this paper were preformed, and F. Langenhorst for help with TEM in Bayreuth. J.R. Smyth is acknowledged for extensive discussion about the potentially critical role of water in the mantle. E. Littlefield was supported by the Carnegie Institution of Washington Undergraduate Research Program through NSF-REU grant EAR-0353877 to S.A. Gramsch. Use of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Operation of beamline U2A at the NSLS is supported by COMPRES, the Consortium for Material Properties Research in the Earth Sciences under NSF Cooperative Agreement Grant No. EAR01-35554, and by the U.S. Department of Energy through the Carnegie/DOE Alliance Center (CDAC) contract DE-FC03-03N00144. Funding for this research was provided in part by the U.S. National Science Foundation award EAR-0748707 (CAREER) and by the David and Lucile Packard Foundation to SDJ.
PY - 2010/11
Y1 - 2010/11
N2 - The mantle X-discontinuity, usually assigned to positive seismic velocity reflectors in the 260-330km depth range, has proved difficult to explain in terms of a single mineralogical phase transformation in part because of its depth variability. The coesite to stishovite transition of SiO2 matches deeper X-discontinuity depths but requires 5-10% free silica in the mantle to match observed impedance contrast. The orthoenstatite (OEn) to high-pressure clinoenstatite (HPCen) transformation of MgSiO3 also broadly coincides with depths of the X but requires chemically depleted and orthoenstatite-rich lithology at 300km depth in order to match observed seismic impedance contrast. On the basis of high-pressure infrared spectroscopy, X-ray diffraction, and Raman spectroscopy, we show that 1300ppm variation of H2O content in MgSiO3 can displace the transition of low-pressure clinoenstatite (LPCen) to HPCen by up to 2GPa, similar to previous quench experiments on the OEn to HPCen phase transition, where about 30-45km (1.0-1.5GPa) of deflection could occur per 0.1wt% H2O. If the mantle X-discontinuity results from pyroxene transitions in a depleted harzburgite layer, because of the strong influence of minor amounts of water on the transformation boundary, the depth of the mantle X-discontinuity could serve as a potentially sensitive indicator of water content in the upper mantle.
AB - The mantle X-discontinuity, usually assigned to positive seismic velocity reflectors in the 260-330km depth range, has proved difficult to explain in terms of a single mineralogical phase transformation in part because of its depth variability. The coesite to stishovite transition of SiO2 matches deeper X-discontinuity depths but requires 5-10% free silica in the mantle to match observed impedance contrast. The orthoenstatite (OEn) to high-pressure clinoenstatite (HPCen) transformation of MgSiO3 also broadly coincides with depths of the X but requires chemically depleted and orthoenstatite-rich lithology at 300km depth in order to match observed seismic impedance contrast. On the basis of high-pressure infrared spectroscopy, X-ray diffraction, and Raman spectroscopy, we show that 1300ppm variation of H2O content in MgSiO3 can displace the transition of low-pressure clinoenstatite (LPCen) to HPCen by up to 2GPa, similar to previous quench experiments on the OEn to HPCen phase transition, where about 30-45km (1.0-1.5GPa) of deflection could occur per 0.1wt% H2O. If the mantle X-discontinuity results from pyroxene transitions in a depleted harzburgite layer, because of the strong influence of minor amounts of water on the transformation boundary, the depth of the mantle X-discontinuity could serve as a potentially sensitive indicator of water content in the upper mantle.
KW - Clinoenstatite
KW - Enstatite
KW - MgSiO
KW - Water
KW - X-discontinuity
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U2 - 10.1016/j.pepi.2010.06.015
DO - 10.1016/j.pepi.2010.06.015
M3 - Article
AN - SCOPUS:78649448071
VL - 183
SP - 234
EP - 244
JO - Physics of the Earth and Planetary Interiors
JF - Physics of the Earth and Planetary Interiors
SN - 0031-9201
IS - 1-2
ER -