Experimental determination of the diffusion coefficient for calcium in olivine between 900°C and 1500°C

Abstract

Experiments have been carried out to determine the temperature, oxygen fugacity ( fO 2) and compositional dependence of the tracer diffusion coefficient (D) of calcium in olivine. These data constrain the diffusion coefficient over the temperature range 900 to 1500°C for the three principal crystallographic axes. Well constrained linear relationships between the reciprocal of the absolute temperature and log(D) exist at any given oxygen fugacity. There is a strong dependence of the diffusion coefficient on oxygen fugacity with D ∝ fO 2 (1/3). This makes a knowledge of the T- fO 2 path followed by geological samples a prerequisite for modelling Ca diffusion in olivine. The best fitting preexponential factor (Do) and activation energy (E) to the Arrhenius equation log (D) = log [Do exp(−E/RT)] + 0.31Δ log fO 2 for Ca diffusion in olivine at a given oxygen fugacity ( fO 2*) are given by: diffusion along [100]: log [Do (m 2/s)] = −10.78 ± 0.43; E = 193 ± 11 kJ/mol diffusion along [010]: log [Do (m 2/s)] = −10.46 ± 0.37; E = 201 ± 10 kJ/mol diffusion along [001]: log [Do (m 2/s)] = −10.02 ± 0.29; E = 207 ± 8 kJ/mol where Δ log fO 2 = log[ fO 2*] − log[10 −12] with fO 2* in units of bars. There is no measurable compositional dependence of the diffusion coefficient between Fo 83 and Fo 92. Diffusion in Fo 100 has a much higher activation energy than in Fe-bearing olivine and has a weaker fO 2 dependence.