The volume diffusion of strontium has been measured as a function of temperature, pressure, and crystallographic direction in a synthetic diopside (CaMgSi 2 06), using a Cameca IMS 3f ion microprobe. Temperatures ranged from 1100 to 1300*C for pressures to 20 Kbar. Sr diffusivities were also determined for the c direction of the synthetic crystals by a radiotracer ( 8 5 Sr) and sectioning technique and Rutherford backscattering spectroscopy; and for natural samples by the radiotracer method. The natural sample diffusion coefficients were approximately two orders of magnitude higher than those for the synthetic samples, although the activation energies were similar. The diffusion of samarium in the synthetic crystals was also studied with respect to temperature, pressure, and crystallographic direction using the ion probe. Results for the two cations were comparable (within a factor of 2 at most T's). Values of AVa were negative for strontium diffusion. An empirical relation which adequately described the temperature and pressure dependence of D was calculated for Sr in the c direction of the synthetic samples. The result is:The expression for DSr in the natural samples at one atmosphere is:A single compensation trend for all the data was evident for all values of D and AHa in the synthetic crystals. In the study temperature range at one atmosphere, transport rates for the three crystallographic directions were nearly identical due to compensation with a crossover point at 1111*C and 6 x 10-1 7 cm 2 /sec. Directional compensation was also observed in two other studies Clark and Long, 1971).Comparison of the three analytical techniques yielded excellent agreement in the study range at one atmosphere (D's within a factor of two). The comparison illustrated the fact that technique-generated effects can cause the observed concentration curves to differ from the true diffusion profiles. Treatment for removal of these modifiers was effected through variations in analytical parameters and through empirical data reduction methods. Ion probe depth profiling was proven to be a useful tool for the study of diffusion in crystalline silicates, where extremely slow transport rates limit the applicability of more established techniques such as radiotracer and sectioning.The data were examined in the context of other studies which used similar methods and/or investigated the same variables. Pressure dependence of D in most experiments resulted in positive values of AVa, as opposed to the activation volumes calculated here. The comparisons showed that there are three basic sources of differences between diffusivities for a given phase and element. Sample defect composition, experimental techniques, and analytical methods are all capable of causing variations in the measured D's.A number of models of geologic processes were investigated in light of the present data. Closure temperatures (Tc) were calculated for examples of mineral-mineral age-dating, and major and trace element geothermometry. High values of Tc indicate th...

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