Raman spectroscopic constraints on compression and metastability of the amphibole tremolite at high pressures and temperatures

Abstract

Amphiboles, containing up to 2 wt% water in the form of hydroxyl units, contribute significantly to water transport into the deep lithosphere and upper mantle in subduction zones, where dehydration reactions make the water available for metasomatic processes and flux melting of the overlying mantle wedge. Here, we investigate the high-pressure and temperature behavior of the calcic end-member amphibole Ca2Mg5Si8O22(OH)2-tremolite. Tremolite is characterized by Raman spectroscopy at pressures to ~ 49 GPa at room temperature, and at temperatures up to 540 K at ambient pressure. The behavior of the hydroxyl stretching vibration, concordant with previous infrared spectroscopic results, implies that the role of Davydov splitting in this amphibole is small, and the monotonically increasing mode shift of the hydroxyl peak under pressure indicates no approach toward hydrogen-bond symmetrization. As with a range of other hydroxyl-bearing minerals, the hydroxyl librations have slightly negative pressure-induced mode shifts. Intensity trends of Raman peaks that involve Ca–O displacements suggest changes in the calcium cation bonding environment above 10 GPa. The peak shifts of 18 modes are used to determine their isothermal and isobaric mode-Gruneisen parameters (γiT and γiP). The intrinsic mode anharmonicities, calculated from the Gruneisen parameters, indicate largely quasi-harmonic internal mode vibrations while the lower frequency vibrations associated with cations display significant anharmonicity. The general topology of the tremolite structure remains metastable to ~ 50 GPa, and our work provides constraints on the metastability of amphiboles in subduction zones and the upper mantle, as well as the intrinsic crystallographic stability of the monoclinic amphibole structure.