Quantifying Surface and Bulk Proton Transport in Olivine
Creators: name: Samaya Manthri Patabendi Gedara, Sarath
Publisher: name: Macquarie University
Year: 2022
Subjects: Other education not elsewhere classified

description: Water <strong>in </strong>nominally anhydrous minerals (NAMs) significantly affects the geochemical and geophysical conditions of the interior of the Earth. NAMs retain water as hydroxyl groups or free fluid inclusions, providing a possible mechanism to transfer water from the surface of the Earth into the deep mantle. The water content of the mantle has been estimated <strong>in</strong> different ways, such as studying samples at the surface of the Earth (e.g. volcanic emission and xenoliths), mineral physics experiments and electromagnetic geophysical observations (e.g. magnetotelluric). Electromagnetic observations can, <strong>in </strong>principle, achieve a reliable estimate of water content in the mantle and geospatial distribution due to its extreme sensitivity to the presence of water in rocks. This requires an understanding of the relationship between water in NAMs and the electromagnetic signal, which has led to many investigations of the effect of water on rocks and minerals' electrical conductivity. Proton assisted conductivity in mantle minerals has been studied using impedance spectroscopy and tracer diffusion techniques. These studies revealed that water content dramatically enhances the electrical conductivity of NAMs by orders of magnitude. Despite agreement on the overall effect of water on conductivities of minerals, the actual proton diffusion values determined by the conductivity and diffusion methods do not agree. One reason for this could be that the two methods measure different things: surface versus bulk diffusion. This thesis quantifies water transport and its effects on proton conductivities in olivine for both surface and bulk diffusion. In order to achieve this, I used quasi-elastic neutron scattering (QENS) to measure bulk diffusion and neutron imaging to measure surface diffusion in olivine. The heterogeneities of distribution of water in minerals can also have a significant effect on the proton conductivity. Therefore, I also studied the nanoscale heterogeneities of water incorporation and distribution using photo-induced force microscopy. The data from these studies were also used to model the possible water diffusion mechanism in olivine and the effects of water diffusion on proton conductivity. The bulk proton self-diffusion coefficients derived from QENS for both olivine and forsterite were found to be almost identical (8.2 x 10-10 m2s-1 at 973 Kand 2.9 x 10-9 m2s-1 at 1193 K, respectively). Activation energies of 0.78 (± 0.04) eV for olivine and 0.78 (± 0.13) eV for forsterite were determined from this data which are similar to those determined using impedance spectroscopy. Macroscopic effective diffusion coefficients of water in forsterite, which are a weighted average of grain boundary and in-grain diffusion, are several orders of magnitude higher than previously reported for water diffusion in single crystals at corresponding pressures and temperatures. The self-diffusion data, which were measured in this work, overlay with the impedance spectroscopy data. The macroscopic effective diffusion coefficients are also relatively close to the impedance spectroscopy data. The activation energy of the effective diffusion is 0.74 (± 0.02) eV. Nanoscale heterogeneities may control the water inter-diffusivity of olivine grains and the total diffusion mechanism in polycrystalline olivine. This work demonstrates that the governing mechanism for proton diffusion in olivine below 1273 K is hopping between silicon-vacancy sites.