Redox-influenced seismic properties of uppermantle olivine

• Published in: Nature (London) Vol. 555; no. 7696; pp. 355 - 358D
• Main Authors: Cline, C J, Faul, U H, David, E C, Berry, A J, Jackson, I
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group 15-03-2018
• Subjects: Asthenosphere; Bound water; Earth; Earth mantle; Experiments; Fugacity; Gasoline; Grain boundaries; Grain boundary sliding; Lithosphere; Moisture content; Olivine; Organic chemistry; Oxygen; Physical properties; Point defects; Rheology; Seismic activity; Seismic properties; Seismic waves; Sleeving; Subduction (geology); Temperature; Upper mantle; Viscoelasticity; Water content; Wave attenuation; Wave propagation
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature25764

Lateral variations of seismic wave speeds and attenuation (dissipation of strain energy) in the Earth's upper mantle have the potential to map key characteristics such as temperature, major-element composition, melt fraction and water content1-3. The inversion of these data into meaningful representations of physical properties requires a robust understanding of the micromechanical processes that affect the propagation of seismic waves2,3. Structurally bound water (hydroxyl) is believed to affect seismic properties2,3 but this has yet to be experimentally quantified. Here we present a comprehensive low-frequency forced-oscillation assessment of the seismic properties of olivine as a function of water content within the under-saturated regime that is relevant to the Earth's interior. Our results demonstrate that wave speeds and attenuation are in fact strikingly insensitive to water content. Rather, the redox conditions imposed by the choice of metal sleeving, and the associated defect chemistry, appear to have a substantial influence on the seismic properties. These findings suggest that elevated water contents are not responsible for low-velocity or high-attenuation structures in the upper mantle. Instead, the high attenuation observed in hydrous and oxidized regions of the upper mantle (such as above subduction zones) may reflect the prevailing oxygen fugacity. In addition, these data provide no support for the hypothesis whereby a sharp lithosphere-asthenosphere boundary is explained by enhanced grain boundary sliding in the presence of water.