Thermal expansivity and high-pressure sound velocities of natural topaz and implications for seismic velocities and H2O and fluorine recycling in subduction zones

Abstract

Topaz is an important mineral formed in deeply subducted sediments and might be a major carrier of both H₂O and fluorine into the Earth’s interior. To better understand the seismic velocities and H₂O and fluorine recycling in subduction zones, we determined the thermal expansivity of a natural topaz (Al_1.93(1)Si_1.06(1)O₄(OH)_0.48(3)F_1.52(3), space group pbnm) up to 1073 K using high-temperature powder X-ray diffraction. No phase transition or decomposition was observed within the investigated temperature range. The volume thermal expansion coefficient is 2.24(1) × 10^–5 K⁻¹, and the ratio of the axial thermal expansion coefficients α₀(a):α₀(b):α₀(c) is 1.15:1:1.32 at 300 K. We also investigated its compressional (P) and shear (S) wave velocities up to 13.6 GPa at room temperature using ultrasonic interferometry in a multi-anvil apparatus. The adiabatic bulk modulus (K_s) and shear modulus (G) of topaz and their pressure derivatives are K_S0 = 151(1) GPa, K_S′ = 4.9(1), G₀ = 109.4(10) GPa, and G′ = 1.8(1), respectively, by fitting the velocities and density data to finite strain equations. The density and velocity profiles of the topaz were calculated under the upper mantle P–T conditions. Our results reveal that topaz is prone to subduction which drives H₂O and fluorine to migrate to the deep Earth. Meanwhile, topaz also has unusually high V_P and V_S, and low V_P/V_S ratio relative to common upper mantle phases and the preliminary reference Earth model (PREM, Dziewonski and Anderson, Phys Earth Planet Inter 25:297–356, 1981), which may be diagnostic seismic properties in subducted slabs.