Diffusion of Eu, Ce and Lu in orthopyroxene

Abstract

We have measured diffusion coefficients for Eu, Lu and Ce in single crystal orthopyroxene (opx) using thin film diffusion couples produced by pulsed laser deposition. The diffusion experiments were conducted in CO-CO2 vertical gas mixing furnaces in the temperature range of T = 950–1100 °C and fO2 = 10−7 to 10−11 Pa. The microstructure of a subset of samples was investigated by scanning transmission electron microscopy (STEM) and backscattered secondary electron microscopy (BSE) in lamellae prepared using a focused ion beam – scanning electron microscope (FIB-SEM). The results were used for the interpretation of depth profiles acquired using time of flight – secondary ion mass spectrometry. This analytical approach allowed to identify low-concentration regions of the diffusion profiles, unaffected by artefacts, that could be fit to determine diffusion coefficients of Eu, Lu, and Ce. We found faster diffusion rates for Lu, Eu and Ce than previously reported for orthopyroxene and diopside. The obtained activation energies are comparable to previous studies of Lu diffusion in diopside and Nd diffusion in opx, along the iron-wüstite buffer. Our results suggest that Lu diffusion is seemingly dependent on Lu concentration and fO2, indicating a different diffusion mechanism than for Eu and Ce. Assuming that Eu is mostly in the trivalent state, we observed a trend of decreasing diffusivity with increasing ionic radii. The experimental data defines the following Arrhenius equations for Lu, Ce and Eu: