Rare-earth element mobility during hydrothermal and metamorphic fluid-rock interaction and the significance of the oxidation state of europium

The Eu3+Eu2+ redox boundary for temperatures up to 600°C and pressures up to 4000 bar has been calculated using the “revised HKF model” for the prediction of the molal Gibbs free energy of formation at elevated pressures and temperatures. The results indicate that the oxygen fugacity (fO2) at which trivalent Eu is reduced to Eu2+ increases with increasing temperature. Thus, in high-temperature regimes Eu can be fractionated from the other REE. If in fluid-rock interaction under mildly acidic conditions the REE pattern of the fluid is controlled by sorption processes, the decreasing ionic radius from La to Lu leads to (LaLu)cn > 1, and, provided Eu occurs as Eu2+, to a positive Eu anomaly (compared to the source-rock for the REE) in the fluid. Under nearly neutral to mildly basic conditions the REE pattern of the same fluid is governed by complexation mechanisms with carbonate, fluoride and hydroxide complexes being the most important REE species. This leads to (LaLu)cn < 1. Because complexation extends the stability of Eu3+ towards lower fO2, reduction to Eu2+ under mildly basic conditions is limited to more reducing environments. Since the REE content of alteration fluids is extremely low, alteration of whole-rock REE patterns during hydrothermal or metamorphic fluid-rock interaction is rather ineffective, unless the water/rock ratio is ⪢ 102–103 or infiltration metasomatism is severe. Apart from these situations, REE systematics should not be affected by hydrothermal or metamorphic fluid-rock interaction.