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.

Single-crystal elasticity of humite-group minerals by Brillouin scattering

“…than the corresponding OH-bearing phase (Fritzel and Bass, 1997). However, these values are much smaller than the K T0 of 141-144 GPa for two natural (Ti, F)-bearing clinohumite derived from XRD measurements (Qin et al (2017). Further research is needed to provide a comprehensive understanding of the influence of composition on the elasticity of clinohumite.…”

“…This indicates that clinohumite is most incompressible along the b-axis and has the lowest resistance to shear in the (010) crystallographic plane. Previous X-ray diffraction experiments on natural and synthetic clinohumite have also demonstrated the highest degree of linear incompressibility along the b-axis, while the c-axis is most compressible (Qin et al, 2017;Ross and Crichton, 2001). It is known that the stiffness of a crystal is primarily controlled by the gross features of its structure with composition playing a secondary role (Webb and Jackson, 1990).…”

“…The elastic properties of humite-group minerals are critical in determining seismic wave velocities and identifying their potential presence in the Earth's mantle. However, previous studies on the elasticity of clinohumite have shown complex compositional effects (Fritzel and Bass, 1997;Phan, 2008;Qin et al, 2017;Ross and Crichton, 2001). The isothermal bulk modulus of synthetic OH-clinohumite (Mg 9 Si 4 O 16 (OH) 2 ), K T0 , is 119.4(7) GPa with a fixed (∂K T /∂P) T0 = 4.8(2), which is consistent with ultrasonic measurements with the adiabatic bulk modulus, K S0 , of 119(2) GPa (Phan, 2008;Ross and Crichton, 2001).…”

High Pressure Behaviors and a Novel High-Pressure Phase of Cuprous Oxide Cu2O

“…In situ high-pressure single-crystal X-ray diffraction experiments on Beamline 13-BM-C used a monochromatic X-ray beam with a wavelength of 0.4340 Å and focused to a 15 × 15 μm 2 spot. The experimental details were also described previously (Qin et al, 2017;Zhang et al, 2017). To obtain adequate number of diffraction peaks of samples and increase the coverage of the reciprocal space, we collected data at four different detector positions.…”

Crystal chemistry and high-temperature vibrational spectra of humite and norbergite: Fluorine and titanium in humite-group minerals

“…In addition, Ti-rich humite-group minerals are known from meta-basalts and hydrothermally altered peridotites. Therefore, studying the effects of Ti on the physical and chemical properties of these hydrous phases may provide a new understanding of the role of titanium in water recycling in the crust-mantle system (Scambelluri and Rampone 1999;Scambelluri and Philippot 2001;Shen et al 2015;Qin et al 2017).…”