Influence of Fe, Cr and Al on hydrogen incorporation in orthopyroxene

Abstract

Three series of enstatite crystals doped with different amounts of Fe, Cr or Al were synthesized under water-saturated conditions at 25 kbar. The OH-incorporation was studied by FTIR-spectroscopy on oriented crystal sections. Strongest absorptions (approximately 65% of total absortion) occurred in all cases when n(gamma) was parallel to the electric field vector E of the incident light, weakest absorptions were observed when n(beta) was parallel to E. In addition to the O-H absortion bands of pure enstatite (at 3070 and 3361 cm(-1), respectively), IR spectra on orthopyroxenes containing trivalent cations reveal additional peaks between 3250 and 3350 cm(-1) as well as above 3400 cm(-1), consistent with the generation of new hydrogen sites and a distortion of the polyhedral chains and cationic effects influencing the local electric field around the H bond. The amount of the incorporated OH correlates well with the concentration of trivalent cations in the crystal, consistent to the exchange mechanisms Fe3+ + H+ double left right arrow 2 Mg2+ and Cr3+ + H+ double left right arrow 2 Mg2+. Comparison to natural samples from the Earth's upper mantle strongly suggests that coupled substitutions involving Al3+ dominate the H+ incorporation.