Thermal Equation of State of Natural Ti-Bearing Clinohumite

Abstract

The natural occurrence of clinohumite in metabasalts and hydrothermally altered peridotites provides a source of water-rich minerals in subducted slabs, making knowledge of their phase relations and crystal chemistry under high pressure-temperature (P-T) conditions important for understating volatile recycling and geodynamic process in the Earth's mantle. Here we present a synchrotron-based, single-crystal X-ray diffraction study on two natural Ti-bearing clinohumites up to ~28 GPa and 750 K in order to simulate conditions within subducted slabs. No phase transition occurs in clinohumite over this P-T range. Pressure-volume relationships of both compositions at room temperature were fitted to a third-order Birch-Murnaghan equation of state (EoS) with V0 = 650.4(3) Å3, KT0 = 141(4) GPa, and KT0′ = 4.0(6) for Ti-poor clinohumite (0.07 Ti per formula unit, pfu) and V0 = 650.8(3) Å3, KT0 = 144(4) GPa, and KT0′ = 3.6(7) for Ti-rich clinohumite (0.21 Ti pfu). Both clinohumites exhibit anisotropic compression with βb > βc > βa. We also refined P-V-T equation of state parameters using the high-temperature Birch-Murnaghan EoS, yielding (∂KT0/∂T)P = −0.040(10) GPa/K and αT = 5.1(6) × 10−5 K−1 for Ti-poor clinohumite and (∂KT0/∂T)P = −0.045(11) GPa/K and αT = 5.7(6) × 10−5 K−1 for Ti-rich clinohumite. Ti-poor and Ti-rich clinohumites display similar equations of state but are ~20% more incompressible than Mg-pure clinohumite and display ~5% higher bulk sound velocity than olivine at upper mantle conditions. Our results provide constraints for modeling geodynamic process related to the subduction and transport of potentially water-rich slabs in the mantle.