Hydrogen Effect on the Sound Velocities of Upper Mantle Omphacite

Abstract

Clinopyroxene (Cpx) is commonly believed to be the best structural water (hydrogen) carrier among all major upper mantle nominally anhydrous minerals (NAMs). In this study, we have measured the single-crystal elastic properties of a Cpx, a natural omphacite with~710 ppm water at ambient pressure (P) and temperature (T) conditions. Utilizing the single-crystal X-ray diffraction (XRD) and electron microprobe data, the unit cell parameters and density were determined as a = 9.603(9) Å, b = 8.774(3) Å, c = 5.250(2) Å, β = 106.76(5) o , V = 255.1(4) Å 3 , and = 3.340(6) g/cm 3 . We performed Brillouin spectroscopy experiments on four single crystals along a total of 52 different crystallographic directions. The best-fit single-crystal elastic moduli (C ij s), bulk and shear moduli were determined as: C 11 = 245(1) GPa, C 22 = 210(2) GPa, C 33 = 249.6(9) GPa, C 44 = 75.7(9) GPa, C 55 = 71.2(5) GPa, C 66 = 76(1) GPa, C 12 = 85(2) GPa, C 13 = 70(1) GPa, C 23 = 66(2) GPa, C 15 = 8.0(6) GPa, C 25 = 6(1) GPa, C 35 = 34.7(6) GPa, and C 46 = 8.7(7) GPa, K S0 = 125(3) GPa, and G 0 = 75(2) GPa, respectively. Compared with the anticipated elastic properties of an anhydrous omphacite with the same chemical composition, our results indicate that the incorporation of~710 ppm structural water has no resolvable effect on the aggregate elastic properties of omphacite, although small differences (up to~9 GPa) were observed in C 13 , C 25 , C 44 , and C 66 .

Effect of structural water on the elasticity of orthopyroxene

“…The orientations and unit cell parameters of two samples were determined in the Atlas X-Ray Diffraction Lab at the University of Hawai'i at Manoa and the third one was measured at 13-BM-C, GeoSoilEnviroCARS (GESCARS), Advanced Phonon Source, Argonne National Laboratory. The experimental setup and details can be found in Mans et al (2019).…”

“…However, recent studies showed that high pressure would suppress the effect of structural H 2 O and the hydration-induced reductions of sound velocities vanish at transition zone pressure-temperature conditions (Buchen et al, 2018;Schulze et al, 2018). Moreover, the effects of structural H 2 O on the elastic properties of the Na, Al bearing-clinopyroxene and garnet were found to be negligible at room temperature condition (Fan et al, 2019;Mans et al, 2019). Although the orthorhombic orthopyroxene is the second most abundant mineral in the pyrolitic upper mantle, there has been no report about the effect of structural H 2 O on its single-crystal elastic properties to date.…”