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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

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1945-3027
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Volume 89, Issue 10

Issues

Mg/Si ratios of aqueous fluids coexisting with forsterite and enstatite based on the phase relations in the Mg2SiO4-SiO2-H2O system

Tatsuhiko Kawamoto
  • Corresponding author
  • Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Beppu 874-0903, Japan.
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/ Kyoko N. Matsukage
  • Department of Environmental Sciences, Faculty of Science, Ibaraki University, Mito 310-8512, Japan
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/ Kenji Mibe
  • Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, D.C. 20015, U.S.A.
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/ Maiko Isshiki / Koshi Nishimura
  • Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Beppu 874-0903, Japan
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/ Naoki Ishimatsu
  • Department of Physical Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
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/ Shigeaki Ono
  • Institute for Frontier Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Natsushima, Yokosuka 237-0061, Japan
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Published Online: 2015-03-28 | DOI: https://doi.org/10.2138/am-2004-1010

Abstract

Direct observation of aqueous fluids coexisting with MgSiO3 (enstatite) and/or Mg2SiO4 (forsterite) was performed at 0.5-5.8 GPa and 800-1000 °C with an externally heated diamond-anvil cell and synchrotron X-rays. At 1000 °C in the MgSiO3 -H2O system, forsterite crystallizes below 3 GPa but not above that pressure. At 1000 °C in the Mg2SiO4 -H2O system, forsterite congruently dissolves into the aqueous fluids up to 5 GPa. These observations suggest that the aqueous fluids coexisting with enstatite and forsterite have Mg/Si < 1 below 3 GPa and 1 < Mg/Si < 2 above that pressure.

Comparison with the previous studies reporting Mg/Si ratios of the aqueous fluid coexisting with enstatite and forsterite indicates that the Mg/Si ratios change rapidly from SiO2-rich to MgO-rich at around 3 GPa and 1000 °C. This change can be related to possible structural changes of liquid water under these conditions. The aqueous fluids coexisting with enstatite and forsterite do have Mg/Si ratios similar to those found in the partial melts of H2O-saturated peridotite. Somewhere within the upper mantle, these two fluids unite to form a single regime and cannot be distinguished from each other.

About the article

Received: 2003-07-17

Accepted: 2004-03-23

Published Online: 2015-03-28

Published in Print: 2004-10-01


Citation Information: American Mineralogist, Volume 89, Issue 10, Pages 1433–1437, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2004-1010.

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© 2015 by Walter de Gruyter Berlin/Boston.

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