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American Mineralogist; May 2008; v. 93; no. 5-6; p. 831-843; DOI: 10.2138/am.2008.2653
© 2008 Mineralogical Society of America
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Right arrow Articles by Nishihara, Y.
Right arrow Articles by Karato, S.-i.

Effect of chemical environment on the hydrogen-related defect chemistry in wadsleyite

Yu Nishihara1,2,3,*, Toru Shinmei1,4 and Shun-ichiro Karato1

1 Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, U.S.A.
2 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan
3 Research Center for the Evolving Earth and Planets, Tokyo Institute of Technology, Tokyo 152-8551, Japan
4 Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan

Correspondence: * E-mail: yuu{at}geo.titech.ac.jp

The effect of chemical environment on the hydrogen-related defect chemistry in wadsleyite was investigated using Fourier-transform infrared (FTIR) spectroscopy. Samples were annealed at P = 14–16 GPa and T = 1230–1973 K using Kawai-type multi-anvil apparatus. The effect of oxygen fugacity (fO2) was investigated using three metal-oxide buffers (Mo-MoO2, Ni-NiO, and Re-ReO2). The effect of water fugacity (fH2O) was studied using two different capsule assemblies ("nominally dry" and "dry" assemblies). A range of total OH concentration (COH,Total) of studied wadslyeites varies between <50 H/106Si (<3 wt ppm H2O) and 23 000 H/106Si (1400 wt ppm H2O). The observed FTIR spectra were classified into four different classes, i.e., peaks at 3620 ("3620"), 3480 ("3480"), and 3205 cm–1 ("3205") and the others (Group O), where the Group O includes peaks at 3270, 3330, and 3580 cm–1. The variation in OH concentration corresponding to each peak was analyzed separately. The OH concentrations correspond to "3620," "3480," and "3205" were found to be highly dependent on both fH2O and fO2. Assuming COH,Group O = 2[(2H)Mx] (COH,Group O is OH concentration of Group O), present data were analyzed by using thermodynamic model for concentration of hydrogen-related defects. Based on analytical results, OH concentration of "3620" and "3480" was found to be reasonably explained by q = 1/2 and r = 1/12 (q and r are fH2O and fO2 exponents, respectively), whereas that of "3205" was consistent with q = 1/2 and r = –1/12. These results suggest that "3620" and "3480" correspond to HM' whereas "3205" corresponds to H•, respectively, under the charge neutrality condition of [FeM'] = 2[VM''].

Key Words: Wadsleyite • water • hydrogen-related defect • oxygen fugacity • mantle transition zone






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