Experimental studies on diffusion coefficients in pyroxenes.

Diffusion phenomena are commonly observed in pyroxene minerals.In order to analyze such diffusion phenomena on the basis of rate law，the cation diffusion data are indispensable。The reliable experimentl data of diffusion coefficients of cations in pyroxenes，however，are minimal because of some experimental probelms including the sluggishness of the diffusion rates in pyroxenes or the difficulty of synthesizing the active diffusion couples.In the present study the“001”exsolution lamella of pigeonite in augite from Skaergaard gabbro were used as diffusion couples and the osital lamella of pigeonite in augite from.The compositional profile of the natural specimen is almost flat within each phase and changes stepwise across the lamellar interface.However，significant diffusion profiles were observed on the specimens heated at temperatures of 50c interval between1000 and 1200°C for different run durations.These profiles were then analyzed by the simulation model of diffusion，developed by Tanzili and Heckel(1968)，which involves a moving interface in a two-phase region。And we could finally obtain the reliable interdiffusion coefficients of Ca and(Mg，Fe)in augite and pigeonite at temperatures between 1000 and 1200°C.These results reveal that(1)the diffusion coefficient in pigeonite is a order of magnitude greater than that in augite，(2)diffusion rates are slower than ecer thought，(3)there are bendings at ab1100out°C the Arrhenius plot diffusion of the rates are slower than ecer thought，(3)there are bendings at ab1100outAnd about22 and 18kcal/mol，respectively below 1100°C.

Diffusion phenomena are commonly observed in pyroxene minerals. In order to analyze such diffusion phenomena on the basis of rate law, the cation diffusion data are indispensable. The reliable experimentl data of diffusion coefficients of cations in pyroxenes, however, are minimal because of some experimental probelms including the sluggishness of the diffusion rates in pyroxenes or the difficulty of synthesizing the active diffusion couples.In the present study the "001" exsolution lamella of pigeonite in augite from Skaergaard gabbro were used as diffusion couples and the compositional profiles were measured with an analytical electron microscope to overcome the above problems. The compositional profile of the natural specimen is almost flat within each phase and changes stepwise across the lamellar interface. However, significant diffusion profiles were observed on the specimens heated at temperatures of 50 c interval between 1000 and 1200 ゜C for different run durations. These profiles were then analyzed by the simulation model of diffusion, developed by Tanzili and Heckel (1968), which involves a moving interface in a two-phase region. And we could finally obtain the reliable interdiffusion coefficients of Ca and (Mg, Fe) in augite and pigeonite at temperatures between 1000 and 1200゜C.These results reveal that (1) the diffusion coefficient in pigeonite is a order of magnitude greater than that in augite, (2) diffusion rates are slower than ecer thought, (3) there are bendings at about 1100゜C in the Arrhenius plot of diffusion coefficients for both augite and pigeonite, probably due to the intersection of the intrinsic region and the extrinsic one, and (4) the activation energies of diffusion in augite and pigeonite are about 120 and 88kcal/mol, respectively above 1100゜C, and about 22 and 18 kcal/mol, respectively below 1100゜C.