RUTILE-AQUEOUS FLUID PARTITIONING OF NB, TA, HF, ZR, U AND TH - IMPLICATIONS FOR HIGH-FIELD STRENGTH ELEMENT DEPLETIONS IN ISLAND-ARC BASALTS

To assess the possible role of residual rutile in the retention of hig h field strength elements (HFSEs) during dehydration of a subducting s lab we have measured rutile/aqueous fluid partition coefficients (D-ru t/fl) for Nb, Ta, Kf, Zr, U and Th at 1-2 GPa and 900-1100 degrees C. Partition coefficients for Nb, Ta, Hf and Zr are all in excess of 100 at 900 degrees C and 1.0 GPa and values become larger with increasing pressure or decreasing temperature. Partition coefficients for U6+ and Th are lower(similar to 2 and 2000 times, respectively) than those fo r the HFSEs, and trends in D-rut/fl with ionic radius indicate that ca tions with a large radius (i.e., > 0.8 Angstrom), including U4+, will have partition coefficients < 1. Results indicate that rutile will the refore selectively deplete coexisting fluids in HFSEs relative to larg e ion lithophile elements (LILEs). Calculations using these partition coefficients show that only small amounts of residual rutile (similar to 0.2wt%) are required to prevent HFSE enrichment of the mantle wedge by fluids derived from either pelagic sediments or the basaltic porti on of the subducting slab. In addition, the measured HFSE concentratio ns 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 hav e no capacity to alter the HFSE content of the subarc mantle. We concl ude that fluids derived from slab dehydration can be sufficiently depl eted in HFSEs that subsequent enrichment of these elements in the mant le wedge does not occur. Uncertainty still remains, however, with rega rd to the capacity of such fluids to achieve the necessary enrichments in LILEs inferred for the subarc mantle.