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

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 degrees 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., (A(3362))/(A(3692))) 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 as P[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 parallel to n(gamma) + E parallel to n(beta) of the absorption bands between a cm(-1) and b cm(-1), respectively.