“Average” interdiffusion of (Fe,Mn)-Mg in natural diopside

Abstract

(Fe,Mn)-Mg interdiffusion has been investigated in single crystals of natural diopside at ambient pressure, high temperatures (900°C to 1240°C) and low oxygen fugacity (pO 2 ≈ 10 −18 atm to 10 −13 atm). Interdiffusion couples were made with polycrystalline ferro-johannsenite thin film (∼ 250 A to ∼ 500 A) as the iron and manganese reservoir (Fe/(Fe + Mn + Mg) ∼ 48 at. %) and with diopside single crystals as the semi-infinite calcium- and magnesium-rich medium. The diopside specimens contain Fe as major impurity (Fe/(Fe + Mg) ≈ 6 at. %). Diffusion profiles of Fe and Mn ranging from ∼ 250 A to ∼ 3500 A were analysed by Rutherford back-scattering spectrometry (RBS). This technique is well adapted to measurements of concentration profiles of heavy elements in silicates, but it can not resolve the signals from elements with nearly same mass (Fe and Mn). Therefore Fe and Mn concentration profiles overlap, but the characteristic shapes of the convoluted diffusion profiles indicate that Fe and Mn diffuse at similar rates. This is also confirmed by secondary-ion mass spectroscopy (SIMS), which allows one to obtain separately Fe and Mn concentration profiles. We obtained an “average” inter-diffusion coefficient characterised by an apparent activation energy of 406 kJ/mol and a pre-exponential factor of 0.955 cm 2 /s. Such a result is surprisingly low for a diffusion process involving Fe and Mg exchanges.