UC Santa Cruz

Tremolite (Ca2Mg5Si8O22(OH)2), a hydrated calcium-bearing amphibole mineral, is a common constituent of subducting oceanic lithosphere and an important source of water for geological processes in subduction zones. Researchers have proposed that metastably preserved tremolite may figure prominently in intermediate depth earthquakes and conductivity anomalies in the deep Earth, yet the metastability of tremolite has remained largely unstudied. Here, we explore the response of the tremolite structure to high pressures (to 49 GPa) and temperatures (to 540 K) via Raman spectroscopy and single-crystal X-ray diffraction (to 39 GPa). In both studies, tremolite metastably persists to the highest pressures explored with no evidence that the structure undergoes a phase transition, however, plots of the a/b and a/c axial ratios and an FE-fE¬ plot reveal changes in the compressional behavior of tremolite at pressures near 5 and 22 GPa. Concordance between the mode shifts of the hydroxyl stretching vibration in our Raman study and previous infrared spectroscopic results implies a minimal role of Davydov splitting in tremolite and strengthens the conclusion that no approach toward hydrogen-bond symmetrization occurs with increasing pressure in the tremolite structure. The axial compressibilities of each compressional regime of tremolite were determined by fitting of a third-degree Birch-Murnaghan equation of state to our compressional data, and the results demonstrate a strong reduction in the compressional anisotropy of tremolite in high-pressure regimes in contrast to increasingly anisotropic axial compressibilities observed in the high-pressure phases of cummingtonite and grunerite. The persistence of C2/m tremolite throughout the pressure-range of these studies represents the largest known metastability range of any amphibole and implies a correlation between M4 site occupancy and phase stability. However, the decreasing pressure-shifts of the lattice modes in our Raman data above 39 GPa and dramatic stiffening of the axial compressibilities in compression regime III imply an approach toward structural instability/amorphization in tremolite above 40 GPa at ambient temperatures.