Redox-influenced seismic properties of upper-mantle olivine

Abstract

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 content. 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 waves. Structurally bound water (hydroxyl) is believed to affect seismic properties 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.

Lubrication of dislocation glide in forsterite by Mg vacancies: insights from Peierls-Nabarro modeling

“…These vacancies can influence the mechanical properties of olivine, for instance by enhancing seismic attenuation due to grain boundary mechanisms (Cline et al 2018). The solubility of common trivalent elements in olivine is sufficiently high that vacancies created by the substitution reaction (5) can greatly outnumber intrinsic vacancies, such as those associated with Schottky defects (van Orman et al 2009 (Stocker 1978).…”

“…The two mechanisms for creating mobile M-site vacancies may co-exist, particularly in mantle wedges above subduction zones, which are oxidized by fluid transfer from the subducting slab (Kelley and Cottrell 2009). High oxygen fugacity has been shown to enhance seismic attenuation in olivine (Cline et al 2018). The similarly high oxygen fugacities present in the earlier deformation experiments of Faul et al (2016), produced by the platinum sleeve used to wrap the specimen (Faul et al 2017 In this study we use computational simulations to investigate the possibility that cation vacancies may be able to lubricate dislocation glide in forsterite by reducing the Peierls stress  p ., providing a mechanism by which point defects may influence the rheology of this important mantle mineral in the glide-controlled creep regime.…”

Oxidation softens mantle rocks

“…But it was not always so. In the 1960s, biologist Joseph Altman reported that new neurons are generated in the adult brain, specifically in a hippocampal subregion called the dentate gyrus, which is now known to be crucial for memory 2 . Further research languished, however, owing to scepticism about the brain's capacity for such dramatic plasticity.…”

“…Contrary to general thinking, this attenuation seems to be strongly affected by oxidation conditions, rather than by water content. See Letter p.355 2 subjected aggregates of olivine to high temperatures and pressures, and to oscillations that mimic seismic waves known as shear waves. They discovered that the speed and attenuation of the waves were insensitive to the water content of olivine, but strongly dependent on oxidation conditions -findings that could reshape our view of the upper mantle.…”

“…These seismic characteristics are usually associated with low viscosity (a 'softening') in the mantle rock peridotite. On page 355, Cline et al 2 demon strate for the first time that this softening is influenced by oxidation conditions -a result that could have major implications for our understanding of the upper mantle.…”