LAUSR

Abstract Clinohumite is a potentially abundant silicate mineral with high water concentration (2~3 wt% H2O) that is generated from dehydration of serpentine-group minerals in subduction zones. Previous studies show that fluorine substitution (OH- = F-) can stabilize clinohumite to significantly higher temperature in subduction zones, although temperatures within the slabs are thought to be well within the stability field of both F-bearing and OH-clinohumite. We collected in-situ high-temperature Raman and Fourier transform infrared (FTIR) spectra for both the synthetic [Mg9Si4O16(OH)2] and natural [Mg7.84Fe0.58Mn0.01Ti0.25(SiO4)4O0.5(OH)1.30F0.20] clinohumite samples up to 1243 K. Three OH bands above 3450 cm–1 are detected for both the natural and synthetic samples with negative temperature dependence, due to neighboring H-H repulsion in the crystal structure. Additional OH peaks are detected for the natural sample below 3450 cm–1 with positive temperature dependence, which could be explained by non-polar F- substitution in the OH site. Hence, F- substitution significantly changes the high-temperature behavior of hydrogen bonds in the humite-group minerals. On the other hand, we evaluated the mode Grüneisen parameters (γiP, γiT), as well as the intrinsic anharmonic parameters (ai) for clinohumite, chondrodite, and phase A, the dense hydrous magnesium silicate (DHMS) phases along the brucite–forsterite join. The estimated averaged anharmonic parameters (ai_avg) for these DHMS phases are systematically smaller than those of olivine. To model the thermodynamic properties of minerals (such as heat capacity) at the high-temperature conditions of the mantle, the DeBye model, which simply approximates the lattice vibrations as harmonic oscillators, is commonly used. In contrast to forsterite, such quasi-harmonic approximations are valid for clinohumite at subduction zone temperatures, as the anharmonic contribution is no more than 2% when extrapolated to 2000 K. Hence, the classic DeBye model can reasonably simulate the thermodynamic properties of these DHMS phases in subduction zones.