Original paper

OH-defects in Al- and Cr- doped synthetic enstatites and defect geobarometry on natural orthopyroxenes from the Earth's mantle

Prechtel, Felix; Stalder, Roland

Abstract

Hydrogen incorporation mechanisms in synthetic enstatite (Mg2Si2O6), doped with Al or Cr and Al and Cr, and coexisting with forsterite were studied experimentally at 4-8 GPa at 1150 °C. The IR spectra show two different groups of OH-absorption bands: group 1 (wavenumbers between 3500 and 3730 cm−1) and group 2 (wavenumbers between 2800 and 3500 cm−1). The intensity ratio of group 2 to group 1 bands (e.g., (A3362)/(A3692)) is sensitive to pressure and to the Al/Cr ratio incorporated in enstatite. In addition, for samples doped with Al and Cr, the OH content is not a simple function of Al and Cr, suggesting OH-defect annihilation if both trivalent cations are present. Modelling of OH-incorporation into enstatite suggests that the water storage capacity in mantle orthopyroxene under average conditions of a continental geotherm is higher than in pure enstatite, but much lower than simple substitution mechanisms would suggest. Based on the observed pressure trend in the OH-absorption spectra of the synthetic samples, IR spectra of eleven natural samples were evaluated. Even if synthetic and natural samples do not show the same trend, a pressure trend for the natural samples could be established, and a geobarometer for the OH-characteristics of natural orthoenstatite from the Earth's mantle (coexisting with forsterite) can be formulated asP[GPa] = 63.4 · A(3487−3240) + A(3570−3487) / A(3487−3240) + A(3570−3487) + A(3730−3570) − 48.8,where A(a−b) is the integral absorbance of the component E||nγ + E||nβ of the absorption bands between a cm−1 and b cm−1, respectively.

Keywords

enstatitehigh-pressureir-spectroscopygeobarometryoh point defectwater storage