Some issues on the role of Ti-H on the physical properties of olivine.

Although there have been many experimental studies on the role of hydrogen (water) on the physical properties of olivine, a few key issues remain unresolved. Among them we focus on the role of Ti in affecting the physical properties when dissolved together with H. In a recent paper, Cline et al. (2018) investigated the role of H on anelastic properties of olivine using samples where H was doped together with Ti and found that seismic attenuation is insensitive to the water content in synthetic olivine. This is in contradiction to an earlier suggestion that is based on an empirical relationship between creep and anelasticity that H enhances anelasticity in olivine.

In order to understand the cause of this puzzling report, we started a series of studies on the role of Ti-H on the physical properties of olivine. A working hypothesis is that the behavior of H in Ti-doped olivine is different from the behavior of H in Ti-poor samples (most of natural samples). During the course of this study, we recognized that when we prepared a sample containing TiO₂ by the sol-gel technique developed by the ANU group, TiO₂ tends to occur as a clump. In most of samples prepared following their procedure, Ti (and H) occurs in small regions (~10 micron size) that occupy a few % of the sample volume. Consequently, we conclude that the bulk physical properties of such an aggregate, say anelasticity or long-term creep, are controlled by Ti (and H) free regions and therefore the conclusion by Cline et al. (2018) that their observations show no sensitivity of anelasticity to hydrogen is not supported by the study.

We also plan to conduct a series of experiments to determine how Ti affects H solubility, and how H in olivine with a variety of H/Ti ratio would affect physical properties such as grain-growth, diffusion (Mg-Fe) and electrical conductivity.

Cline II, C.J., Faul, U. H., David, E. C., Berry, A. J., Jackson, I. Redox-influenced seismic properties of upper-mantle olivine. Nature, 555, 355-358 (2018).