Hydrogen content may be the single most important chemical parameter within the mantle, and hence it is very important to quantify its distribution and rates of transport. Volume diffusion measurements for H in mantle minerals indicate extremely fast diffusive equilibration times, unlike that for nearly all other elements. The added component of grain boundary diffusion has the potential to speed equilibration times by an order of magnitude at moderate temperatures (~300-1000°C). However, to date there is no information on effective diffusion rates of water through the upper mantle. These are urgently needed data, as geoscientists are already making calculations based on assumptions about bulk water diffusivity.
In this Small Grant for Exploratory Research, the investigator proposes to do the preliminary experiments to determine the magnitude of grain boundary diffusivity of water through olivine aggregates. The choice of olivine is critical as this is the most abundant upper mantle mineral. Deuterium will be used as a proxy for hydrogen in order to avoid contamination by sample preparation and to improve the analytical background/detection limit. Doubt about the feasibility of these experiments is significant enough to warrant an initial test of the specific aspects of the experimental design, including the quality of olivine aggregates synthesized in the piston-cylinder, the mechanical integrity of aggregates during hydrothermal anneals, and the detection limit of deuterium by SIMS (Secondary Ion Mass Spectrometry) probing in the context of the proposed experiments.
