J. Ganguly et V. Tazzoli, FE2-MG INTERDIFFUSION IN ORTHO-PYROXENE - RETRIEVAL FROM THE DATA ON INTRACRYSTALLINE EXCHANGE-REACTION(), The American mineralogist, 79(9-10), 1994, pp. 930-937
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.