Abstract

To assess the possible role of residual rutile in the retention of high field strength elements (HFSEs) during dehydration of a subducting slab we have measured rutile/aqueous fluid partition coefficients (Drutfl) for Nb, Ta, Hf, Zr, U and Th at 1–2 GPa and 900–1100°C. Partition coefficients for Nb, Ta, Hf and Zr are all in excess of 100 at 900°C and 1.0 GPa and values become larger with increasing pressure or decreasing temperature. Partition coefficients for U6+ and Th are lower ( ∼ 2 and 2000 times, respectively) than those for the HFSEs, and trends in Drutfl) with ionic radius indicate that cations with a large radius (i.e., > 0.8 Å), including U4+, will have partition coefficients < 1. Results indicate that rutile will therefore selectively deplete coexisting fluids in HFSEs relative to large ion lithophile elements (LILEs). Calculations using these partition coefficients show that only small amounts of residual rutile ( ∼ 0.2wt%) are required to prevent HFSE enrichment of the mantle wedge by fluids derived from either pelagic sediments or the basaltic portion of the subducting slab. In addition, the measured HFSE concentrations of rutiles from eclogite-facies oceanic gabbros from the Rocciavre Massif (Western Alps) indicate that fluids that may have equilibrated with such rocks are strongly depleted in HFSEs and therefore would have no capacity to alter the HFSE content of the subarc mantle. We conclude that fluids derived from slab dehydration can be sufficiently depleted in HFSEs that subsequent enrichment of these elements in the mantle wedge does not occur. Uncertainty still remains, however, with regard to the capacity of such fluids to achieve the necessary enrichments in LILEs inferred for the subarc mantle.