Hydrogen Effect on the Sound Velocities of Upper Mantle Omphacite

Wade Mans,Przemyslaw Dera,Jin S Zhang,Joseph R Smyth,Gregory J Finkelstein,Ming Hao,Dongzhou Zhang

doi:10.3390/min9110690

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) Å3, and ρ = 3.340(6) g/cm3. We performed Brillouin spectroscopy experiments on four single crystals along a total of 52 different crystallographic directions. The best-fit single-crystal elastic moduli (Cijs), bulk and shear moduli were determined as: C11 = 245(1) GPa, C22 = 210(2) GPa, C33 = 249.6(9) GPa, C44 = 75.7(9) GPa, C55 = 71.2(5) GPa, C66 = 76(1) GPa, C12 = 85(2) GPa, C13 = 70(1) GPa, C23 = 66(2) GPa, C15 = 8.0(6) GPa, C25 = 6(1) GPa, C35 = 34.7(6) GPa, and C46 = 8.7(7) GPa, KS0 = 125(3) GPa, and G0 = 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 C13, C25, C44, and C66.

Highlights

Quantifying the water content in the Earth’s upper mantle through seismic observations requires the knowledge of how structural water content affects the elastic properties for various nominally anhydrous minerals (NAMs) [1,2]
The subducting slabs carry the surface water into the Earth’s interior primarily in the form of structural water; it is important to know how much structural water can be stored in the major mineral phases in the subducting slabs
As a type of Cpx, which can retain the highest amount of water in its structure among all upper mantle NAMs, omphacite is likely both an important water carrier as well as a potential water reservoir in the Earth’s interior [5,6,14,15,16,17]

Summary

Introduction

Quantifying the water content in the Earth’s upper mantle through seismic observations requires the knowledge of how structural water content affects the elastic properties for various nominally anhydrous minerals (NAMs) [1,2]. Previous experimental studies on olivine, another NAM commonly found in the ambient upper mantle, have suggested that the incorporation of hydrogen into the NAMs decreases both the P-wave and S-wave velocities (Vp and Vs ) [19,20,21,22,23]. This softening effect of structural water can potentially affect our understanding of the upper mantle seismic structures and mineralogical composition, as well as the total water budget of the Earth’s interior [6,21,22,23,24,25]. In this study, we performed single-crystal Brillouin spectroscopy experiments on a hydrous omphacite sample with ~710 ppm water in order to study the possible structural water effect on the single-crystal elastic properties of omphacite

Materials and Methods

Discussion

Acoustic

Conclusions