Abstract

Average NaSi-CaAl interdiffusion coefficients () have been determined for the plagioclase Huttenlocher interval (~An70 to ~An90) using the method of lamellar homogenization at 1500 MPa, 1050 to 900 °C, ∼1.0 wt% H2O added, and fugacities fixed at the magnetite/hematite buffer. There is a discontinuity in the Arrhenius relation at ∼975 °C that may correlate with the destruction of the compositionally modulated structure in the ∼An70 lamellae. The values are given by  
D˜=4×1016m2/se(103 kJ/mol/RT)(~10501000°C)D˜=11×1016m2/se(371 kJ/mol/RT)(~975900°C).
The values at these conditions are about 3 orders of magnitude larger than previously reported values for the same sample determined by dry annealing experiments in air (Grove et al., 1984). Average values for the peristerite interval (∼An0 to ∼An26) were also determined at the conditions listed above and are given by  
D˜=3×108m2/se(303 kJ/mol/RT)(1050900°C)

Below about 97 5 °C there is little difference in the average values for the Huttenlocher and peristerite intervals, indicating that the effect of bulk composition is minor compared with the effect of H2O or its components dissolved in a plagioclase crystal. Diffusion data for the Huttenlocher interval have been used to estimate cooling rates for chemically zoned plagioclase crystals reported in the Stillwater, Skaergaard, and Kiglapait intrusions. The cooling rates estimated using the dry, 1-atm data compare more favorably with independent estimates ofcooling rates, although the latter are poorly constrained.

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