The effect of fO2 on the diffusion of redox-sensitive elements in haplobasaltic melt at 1 GPa and 1300 °C

Abstract

Diffusivities for elements (P, Sc, V, Mn, Co, Zn, Cr, Nb, Mo, La, Ce, Pr, Sm, Eu, Gd, Ta, W) at 1300 °C and 1 GPa in basaltic melts were characterized across a range of oxygen fugacity (fO2) conditions. Experiments were carried out using either a reducing (FMQ-3.1), intermediate (~FMQ-1.2) or oxidizing (FMQ + 6) fO2. For each fO2, three experiments were conducted for durations of 20, 40, and 80 min. For a given time series, changes in diffusivity are typically within 3 standard error at a single fO2. The magnitudes of the elemental diffusivities can be grouped into those of the High-field Strength Elements (HFSE), the Rare Earth Elements (REE), the transition elements, and P. Vanadium and Sc have diffusivities more similar to the REEs and HFSEs respectively, than the other transition elements. The best fits of diffusivities for P also suggest that the diffusivity of this element is more in line with those of the HFSEs. At oxidizing conditions, a fractionation of Nb from Ta with greater magnitude than that at the other oxygen fugacities is seen. Across oxygen fugacities explored here, Eu exhibits unique changes in diffusion. At more reducing conditions, the diffusivity of Eu increases relative to the neighboring REE elements Sm and Gd, with this effect most pronounced at FMQ-3.1 and present in experiments conducted at intermediate fO2 conditions. This demonstrates that an Eu anomaly can be generated by diffusion alone. In oxidizing conditions, because Eu likely is present as mostly Eu3+, the signal vanishes as Eu diffusivity becomes similar to that of other trivalent REEs. There are small systematic changes in element diffusivities for both redox-sensitive and non-redox sensitive elements as fO2 is varied. Averages of the 20, 40, and 80 min diffusivities for all elements done in the intermediate fO2 experiments have the slowest diffusivities of the three oxygen fugacities explored. On average, the diffusivities of the entire contingent of elements studied from the more reducing (FMQ-3.1) conditions are faster than those from the intermediate fO2 by about a factor of 1.5. The elemental diffusivities recovered from the oxidizing experiments are, on average, about ~2 times as fast of those recovered from the intermediate experiments. For elements fit in these experiments, an order of magnitude change in element diffusivities, even for redox-sensitive elements, is never seen over the range of oxygen fugacities explored at 1300 °C. These experiments demonstrate that oxygen fugacity can have an important effect on the diffusivity of certain redox-sensitive elements (e.g. Eu) and that fO2 might play a role in element transport generally.