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Chemical diffusion profiles in molten CaOAl2O3SiO2have been measured over a large range of compositions at 1500°C and l GPa. The diffusion profiles have been inverted for effective binary diffusion coefficients (EBDCs) and for the chemical diffusion matrix. The EBDCs are shown to depend strongly on both composition and direction of diffusion in composition space. The dependence of EBDCs on direction in composition space, which for the system studied here can be as large as a factor of seven, severely limits the applicability of EBDCs to interdiffusion in any direction other than the one used to derive the EBDCs.

Self diffusion coefficients of calcium (DCa), aluminum (DAl), silicon (DSi), and oxygen (DO) were measured in molten CaOAl 20 3SiO 2at 1500°C and 1 GPa over a range of melt compositions, using the isotope tracer method. For all but one composition, the measured self diffusion coefficients decrease in the order DCa> D Al> D O> D Si, with D Ca∼ 4–19 D Si, D Al∼ 2D Si, and D O∼ 1–2 D Si. The relative uncertainties, based on replicated experiments, are 8% for DCa, 27% for DAl, 28% for DSi, and 18% for DO. Although the self diffusion coefficients of calcium, aluminum, silicon, and oxygen increase with the decrease of melt viscosity, they do not obey the Stokes-Einstein equation or the Eyring equation. The compositional dependence of the self diffusion coefficients of calcium, aluminum, and oxygen were parameterized in terms of melt viscosity (Pa · s) via the power law relation: DCa= (115.141/viscosity 0.389) × 10 −12m 2s −1DA1= (47.302/viscosity 0.800) × 10 −12m 2s −1DO= (33.080/viscosity 0.722) × 10 −12m 2s −1

A series of idealized models for determining the balance of forces in plate tectonics are presented and the question of the driving mechanism is reduced to evaluating several integrals that represent the forces on the bounding surfaces of the lithospheric plates and the total buoyancy of the downgoing slab. The value of the integrals are plotted as a function of depth along the downgoing slab to graphically represent the complete balance of forces, thus showing the relative importance of the different terms and also giving a measure of the internal stresses within the downgoing slab. The models that best explain the observed velocity of plates and the distribution of stresses within the downgoing slabs as inferred from focal mechanisms require the viscosity in the upper mantle to increase with depth by a factor of about 20. The primary balance of forces in these models is between the negative buoyancy of the downgoing slab and the nonhydrostatic pressure acting on its base. The significant role of the nonhydrostatic pressure is the principal new result of these calculations.

A finite amplitude, numerical model of convective motions through a phase boundary is studied. The phase change is assumed to occur over a finite vertical transition zone in the fluid and thus a continuously stratified model is considered. The motions are driven by horizontal temperature gradients on the boundaries or super-critical vertical stratification. In the case of Rayleigh instability, the numerical results reproduce the neutral curve previously obtained in the analytic solution of the linearized problem. When the parameters of the problem are chosen to simulate the olivine-spinel transition in the upper mantle, the following results are obtained: (1) the vertical scale of motion is the entire depth of the fluid; (2) the horizontal scale is not significantly changed from the case of a single phase fluid; (3) the amplitude of the motion is not significantly changed, due to buoyancy changes at the phase boundary being offset by corresponding sources and sinks of latent heat; and (4) the phase boundary varies in depth by as much as 30 km when the vertical velocities are of the order of 10−1 cm yr−1.