Topaz, a Potential Volatile-Carrier in Cold Subduction Zone: Constraint from Synchrotron X-ray Diffraction and Raman Spectroscopy at High Temperature and High Pressure

The equation of state and stability of topaz at high-pressure/high-temperature conditions have been investigated by in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy in this study. No phase transition occurs on topaz over the experimental pressure-temperature (P-T) range. The pressure-volume data were fitted by the third-order Birch-Murnaghan equation of state (EoS) with the zero-pressure unit-cell volumeV(0)= 343.86 (9) angstrom(3), the zero-pressure bulk modulus K-0= 172 (3) GPa, and its pressure derivativeK'(0)= 1.3 (4), while the obtained K-0= 155 (2) GPa when fixed K'(0)= 4. In the pressure range of 0-24.4 GPa, the vibration modes of in-plane bending OH-groups for topaz show non-linear changes with the increase in pressure, while the other vibration modes show linear changes. Moreover, the temperature-volume data were fitted by Fei's thermal equation with the thermal expansion coefficient alpha(300)= 1.9 (1) x 10(-5)K(-1)at 300 K. Finally, theP-Tstability of topaz was studied by a synchrotron-based single-crystalXRDat simultaneously highP-Tconditions up to similar to 10.9 GPa and 700 K, which shows that topaz may maintain a metastable state at depths above 370 km in the upper mantle along the coldest subducting slab geotherm. Thus, topaz may be a potential volatile-carrier in the cold subduction zone. It can carry hydrogen and fluorine elements into the deep upper mantle and further affect the geochemical behavior of the upper mantle.