Thermal equation of state of Li-rich schorl up to 15.5 GPa and 673 K: Implications for lithium and boron transport in slab subduction

Abstract

Abstract The thermal equation of state (EoS) of natural schorl has been established at high temperatures up to 673 K and high pressures up to 15.5 GPa using in-situ synchrotron X-ray diffraction combined with a diamond anvil cell. The pressure-volume (P-V) data were fitted to the third-order Birch-Murnaghan EoS with V0 = 1581.45 ± 0.25 Å3, K0 = 111.6 ± 0.9 GPa and K′0 = 4.4 ± 0.2; additionally, when K′0 was fixed at a value of 4, V0 = 1581.04 ± 0.20 Å3 and K0 = 113.6 ± 0.3 GPa. The V0 (1581.45 ± 0.25 Å3) obtained by the third-order Birch-Murnaghan EoS agreed well with the measured V0 (1581.45 ± 0.05 Å3) under ambient conditions; this result confirmed the high accuracy of the experimental data in this study. Furthermore, the axial compression data of the schorl at room temperature were also fitted to a “linearized” third-order Birch-Murnaghan EoS, and the obtained axial moduli for the a- and c-axes were Ka = 621 ± 9 GPa and Kc = 174 ± 2 GPa, respectively. Consequently, the axial compressibilities were βa = 1.61×10-3 GPa-1 and βc = 5.75×10-3 GPa-1 with an anisotropic ratio of βa:βc = 0.28:1.00, indicating axial compression anisotropy. In addition, the compositional effect on the axial compressibilities of tourmalines was also discussed. Fitting our pressure-volume-temperature (P-V-T) data to the high-temperature third-order Birch-Murnaghan EoS provided the following thermal EoS parameters: V0 = 1581.2 ± 0.2 Å3, K0 = 110.5 ± 0.6 GPa, K′0 = 4.6 ± 0.2, (∂KT/∂T)P = -0.012 ± 0.003 GPa K-1 and αV0 = (2.4 ± 0.2) × 10-5 K-1. The obtained thermal EoS parameters in this study were also compared with those of previous studies on other tourmalines. The potential factors influencing the thermal EoS parameters of tourmalines were further discussed.