Fe-Mg interdiffusion in orthopyroxene

Abstract

We have measured Fe-Mg interdiffusion coefficients, D Fe-Mg , parallel to the three main crystallographic axes in two natural orthopyroxene single crystals [approximately En 98 Fs 1 ( X Fs = X Fe = 0.01) and En 91 Fs 9 ] using diffusion couples consisting of a 20–90 nm thick silicate thin film deposited under vacuum on polished and oriented pyroxene single crystals. The thin films were prepared using pulsed laser ablation of polycrystalline olivine pellets (composition: Fo 30 Fa 70 ). Samples were annealed for 4–337 h at 870–1100 °C under atmospheric pressure in a continuous flow of CO + CO 2 to control the oxygen fugacity, f O 2 , between 10 −11 and 10 −7 Pa within the stability field of pyroxene. Film thickness and compositional profiles were measured using Rutherford backscattering spectroscopy (RBS) on reference and annealed samples, and Fe concentration depth profiles were extracted from the RBS spectra and fitted numerically considering a compositional dependence of D Fe-Mg in orthopyroxene. We obtain an Arrhenius relationship for both types of crystals, but only for the more Fe-rich composition a dependence on f O 2 could be clearly identified. For diffusion along [001] in the composition Fs 9 , least-squares regression of the log D Fe-Mg vs. reciprocal temperature yields the following Arrhenius equation: D Fe - Mg [ m 2 / s ] = 1 . 12 × 10 − 6 ( f o 2 [ Pa ] ) 0 . 053 ± 0 . 027 exp [ – 308 ± 23 [ kJ / mol ] / ( R T ) ] . D Fe-Mg in Opx with X Fe = 0.01 obeys a relationship that does not depend on f O 2 : D Fe - Mg [ m 2 / s ] = 1 . 66 × 10 − 4 exp [ – 377 ± 30 [ kJ / mol ] / ( R T ) ] . Diffusion along [001] is faster than diffusion along [100] by a factor of 3.5, while diffusion along [010] is similar to that along [001]. Comparison of D Fe-Mg and rates of order-disorder kinetics indicates that for f O 2 around the IW buffer and lower, diffusion in natural orthopyroxene becomes insensitive to f O 2 , which could be related to a transition in the diffusion mechanism from a transition metal extrinsic (TaMED) domain to a pure extrinsic (PED) domain. This behavior is analogous to that observed for Fe-Mg diffusion in olivine and this complexity precludes the formulation of a closed form expression for the composition and f O 2 dependence of D Fe-Mg in orthopyroxene at present. We were not able to quantitatively constrain the dependence of D Fe-Mg on the X Fs content from the profile shapes, but consideration of the experimentally measured diffusion coefficients along with the data for order-disorder kinetics suggests that the compositional dependence is weaker than previously estimated, at least for orthopyroxene with X Fe T and f O 2 range of available experimental data, Fe-Mg interdiffusion in orthopyroxene is slower than in olivine and aluminous spinel, comparable to garnet, and faster than in clinopyroxene.