A determination of the viscosity of partially molten peridotite at melt fractions up to the RCMF and the effect of incompatible elements in olivines on the rates of cation diffusion

In this dissertation, I report on experiments to measure the viscosity of partially molten mantle rocks and experiments to measure the effect of mantle rock grain boundary chemistry on the diffusional transport of cations.; In chapter 2, I determine the influence of a large melt fraction, 0.15 ≤ &phis; ≤ 0.30, on the rheological behavior of partially molten rocks, after performing a series of creep experiments on synthetic peridotites in the diffusion and dislocation creep regimes. The creep data reveal a sharp drop in rock viscosity as &phis; is increased from &phis; = 0.25 to 0.30, indicative of a rheologically critical melt fraction (RCMF). Over the range 0 ≤ &phis; ≤ 0.25, the flow behavior of peridotite is well described by 3&d2; ∝ exp(alpha&phis;) with alpha = 21 for diffusion creep and alpha = 32 for dislocation-accommodated GBS creep.; In chapter 3, I extend the work in Chapter 2 to determine the relationship between alpha and melt composition. I (with Justin Hustoft) performed a series of creep experiments on samples of olivine + Fe-S and olivine + Au and combine the results with previously published measurements for olivine + MORB. Our results reveal an abrupt change in the value of alpha, from alpha = 21 for silicate melts to alpha = 6 for metallic melts. The low solubility of olivine in the metallic melts combined with stress enhancement at grain contacts may be responsible for change in alpha.; In chapter 4, I performed a series of interdiffusion experiments between pure and Ca or Sc-doped polycrystalline Mg2SiO4 and Co2SiO4. Doping the olivines with Ca, retards grain boundary diffusion, resulting in a lower value of D eff that is still enhanced a factor of ∼10 relative to lattice diffusion. Doping the olivines with Sc, has the effect of increasing both the lattice and grain boundary diffusivity, such that Deff is a factor of ∼3 greater than undoped samples. Therefore, relative to the host cation, a segregant with either higher valence state or larger ionic radius can significantly affect the mobility of the divalent cations, which will effect the mechanical and electrical properties of olivine in the earth.