Efficient mobilization and fractionation of rare-earth elements by aqueous fluids upon slab dehydration

The characteristic REE fractionation pattern in arc magmas compared to MOR-basalts results from the selective mobilization of light rare-earth elements (LREE) by slab-derived mobile components. However, the nature and composition of the slab flux, and the actual mechanisms responsible for the transfer of rare-earth elements (REE) from the slab to the mantle wedge remain unclear. We present experimental data on the solubility of selected REE in ligand-bearing aqueous fluids and a hydrous haplogranitic melt at 2.6 GPa and 600-800 degrees C, spanning the conditions relevant to slab dehydration and melting. The solubilities of REE in aqueous fluids increase more than an order of magnitude with temperature increasing from 600 to 800 degrees C. Addition of ligands such as Cl-, F-, CO32-, SO42- in relatively small concentrations (0.3-1.5 m [mol/kg H2O]) has a pronounced effect further enhancing REE solubilities. Each ligand yields a characteristic REE pattern by preferential dissolution of either the light or the heavy REE. For example, the addition of NaCl to the aqueous fluids yields highly elevated LREE/HREE ratios (La/Yb 17.4 +/- 4.3), whereas the addition of fluoride and sulfate ligands significantly increases the solubility of all REE with moderate LREE/HREE fractionation (La/Yb similar to 4). The addition of Na2CO3 results in preferential increase of HREE solubilities, and yields La/Yb ratio of 1.6 +/- 0.5 by flattening the moderately fractionated REE pattern seen in pure aqueous fluids. The solubilities in hydrous haplogranite melt are moderate in comparison to those observed in aqueous fluids and do not lead to pronounced REE fractionation. Therefore, REE can be effectively mobilized and fractionated by aqueous fluids, compared to felsic hydrous melts. Furthermore, the aqueous fluid chemistry has a major role in determining REE mobilities and fractionation upon slab dehydration in addition to the significant control exerted by temperature. Our results show that chloride-bearing slab-derived aqueous fluids have a significant contribution to the formation of REE-signatures in arc-magmas, especially at lower slab surface temperatures. (C) 2014 Elsevier B.V. All rights reserved.