REE and Hf distribution between pyrope and NaCl-bearing water at eclogitic-facies conditions

Due to the interaction of oceanic crust with seawater, NaCl is a main component dissolved in aqueous fluid in subduction zones. The effect of dissolved NaCl (1.64 mol/l) on the partitioning of REE and Hf between garnet (pyrope) and aqueous fluid (doped with approximately 500 ppm of REE and approximately 250 ppm of Hf) has been investigated experimentally at 3 GPa, 850 degrees C (i.e., eclogitic UHP conditions) using the diamond trap technique. Trace elements in both pyrope and diamond aggregate were analyzed using LA-ICP-MS to derive fluid/pyrope partition coefficients (D). Mass balance calculation indicates that lanthanum is present in an accessory phase, tornebohmite. Europium and aluminium concentrations are too high in the aggregate to comply with mass balance and solubility data from the literature, respectively. Transport of these species from the mineral powder to the aggregate is inferred. D (sub MREE) (Gd and Tb) equals 0.39+ or -0.11 and 0.22+ or -0.07, respectively. D (sub HREE) (Dy to Lu) ranges from 0.09+ or -0.02 to 0.02+ or -0.01 with a regular decrease from Dy to Lu. D (sub Hf) is 0.33+ or -0.09. Lanthanum is found to be incompatible (D (sub La) is 2.10+ or -0.95) whereas for the other LREE (from Ce to Sm), D is lower than 1 (D (sub Ce) =0.65+ or -0.15, D (sub Pr) =0.74+ or -0.22, D (sub Nd) =0.71+ or -0.29 and D (sub Sm) =0.61+ or -0.23). Contrary to D (sub HREE) and D (sub MREE) , D (sub LREE) do not fit a near-parabolic dependence on LREE effective ionic radius. The Hf fluid/pyrope partition coefficient (D (sub Hf) =0.33+ or -0.09) indicates that this element favourably partition into garnet. Finally, it is shown that, compared to pure water, the addition of NaCl in water has no significant effect on REE and Hf behaviour.