FE2-MG INTERDIFFUSION IN ORTHO-PYROXENE - RETRIEVAL FROM THE DATA ON INTRACRYSTALLINE EXCHANGE-REACTION()

Crystal structural data of orthopyroxene suggest that Fe-Mg interdiffu sion should be fastest along the c axis and slowest along the a axis a nd that the overall kinetics of the Fe2+-Mg fractionation, or order-di sorder, between the M1 and M2 sites should be dominated by exchange al ong the c and b directions. Using these crystallographic constraints a nd a formulation of the diffusion process according to absolute reacti on rate theory, we have developed an expression of the average Fe2+-Mg interdiffusion coefficient along the c and b directions of orthopyrox ene, D(Fe-Mg)(c-b), in terms of the rate constant of the disordering p rocess [Fe2+ (M2) + Mg (M1) --> Fe2+ (M1) + Mg (M2)], the equilibrium intracrystalline distribution of Fe2+ and Mg, and the average distance between the neighboring M1 and M2 sites along c and b. D(Fe-Mg)c-b ha s been evaluated from the available experimental data on the order-dis order process as a function of temperature and composition and is defi ned as follows: log D(Fe-Mg)c-b = -5.54 + 2.6X(Fe) - 12530/T. This exp ression is valid at T almost-equal-to 500-800-degrees-C, f02 almost-eq ual-to WI to 0.8 log units above the WI buffer, and X(Fe) almost-equal -to 0.10-0.50 and yields values of D(Fe-Mg)c-b in orthopyroxene that a re at least two orders of magnitude smaller than D(Fe-Mg)c in olivine and about one order of magnitude larger than the isotropic D(Fe-Mg) in garnet at the same X(Fe) and f(o2) conditions. However, because of di ffusion anisotropy in orthopyroxene, the apparent difference between D (Fe-Mg) in orthopyroxene and garnet could seem smaller in terms of com positional zoning in random thin sections of natural rocks. Comparison of the value of the tracer diffusion coefficient of Mg, D(Mg), at X( Fe) = 1, derived from the above interdiffusion data with that determin ed by Cygan et al. (1994) at X(Fe) = 0.1 suggests that D(Mg) increase s with increasing Fe/Mg ratio.