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A correct interpretation of rare-earth element (REE) patterns requires assessing whether equilibrium between solid and melt was achieved and if there are kinetic controls on the chemical fractionation of REE. An accurate knowledge of REE diffusion rates in pyroxenes is crucial to evaluate the effects of diffusive fractionation during partial melting and to interpret mineral ages in terrestrial and planetary samples. We have measured diffusion coefficients for Eu, Lu and Ce in orthopyroxene (XFe=0.1) using thin film diffusion couples produced by pulsed laser deposition. Diffusion experiments were carried in CO-CO2 vertical gas mixing furnaces in the temperature range of T = 950 – 1100 °C and fO2 = 10-7 to 10-11 Pa. Depth profiles were acquired using time of flight – secondary ion mass spectrometry (TOF-SIMS), which allowed to resolve the concentrations in the lower ppm range of the diffusion profiles. We found faster diffusion rates for Lu, Eu and Ce than previously reported for orthopyroxene and diopside[1,2], but comparable activation energies to the ones obtained in recent studies for Lu in diopside[2] and Nd in orthopyroxene[3]. Our experiments suggest that Lu diffusion is dependent on Lu concentration and fO2, implying a different diffusion mechanism than for Eu and Ce. Additionally, we observed a trend of decreasing diffusivity with increasing ionic radii. The reported data may have important implications for the Lu-Hf systematics. Our results show that if the system is subject to extended periods of high temperatures, the diffusive loss of 176Lu in opx might be more significant than previously estimated, possibly leading to erroneous Lu-Hf ages.

[1] D. J. Cherniak, Y. Liang, Geochim Cosmochim Acta 2007, 71, 1324

[2] E. Bloch et al., Geochim Cosmochim Acta 2017, 204, 32

[3] J. Sano et al., Geochim Cosmochim Acta 2011, 75, 4684