Cr and Al diffusion in chromite spinel: experimental determination and its implication for diffusion creep

Diffusion coefficients of Cr and Al in chromite spinel have been determined at pressures ranging from 3 to 7 GPa and temperatures ranging from 1,400 to 1,700 degrees C by using the diffusion couple of natural single crystals of MgAl(2)O(4) spinel and chromite. The interdiffusion coefficient of Cr-Al as a function of Cr# (=Cr/(Cr + Al)) was determined as D(Cr-Al) = D(0) exp {-(Q' + PV*)/RT}, where D(0) = exp{(10.3 +/- 0.08) x Cr#(0.54 +/- 0.02)} + (1170 +/- 31.2) cm(2)/s, Q' = 520 +/- 81 kJ/mol at 3 GPa, and V* = 1.36 +/- 0.25 cm(3)/mol at 1,600 degrees C, which is applicable up to Cr# = 0.8. The estimation of the self-diffusion coefficients of Cr and Al from Cr-Al interdiffusion shows that the diffusivity of Cr is more than one order of magnitude smaller than that of Al. These results are in agreement with patterns of multipolar Cr-Al zoning observed in natural chromite spinel samples deformed by diffusion creep.