Rutile/melt partition coefficients for trace elements and an assessment ofthe influence of rutile on the trace element characteristics of subductionzone magmas

Fractionation of some or all of the high field strength elements (HFSE) Nb, Ta, Zr, Hf, and Ti relative to other trace elements occurs in igneous rock s from convergent margins and in the average continental crust, and is gene rally attributed to a process occurring during subduction. The experimental partitioning of an extensive array of trace elements between rutile/melt p airs is presented which enables the effect of rutile during melting in subd uction zones to be directly assessed. D-Nb and D-Ta are in the range 100-50 0, D-Zr and D-Hf are about 5, whereas all other trace elements analyzed hav e D-rutile/melt less than 0.1. Published D patterns for Nb and Ta between r utile and water-rich fluids are similar to those for melt, whereas the valu es for Zr and Hf are significantly higher. D-Nb and D-Ta values for clinopy roxene and garnet are much lower than for rutile, and cannot cause the frac tionation of HFSE from other elements seen in island area. The presence of rutile in the subducted slab residue during dehydration may be essential in the production of the geochemical signatures of are magmas, whereas that o f the continental crust, including higher Zr/Sm, may be produced by melting of eclogite. Copyright (C) 1999 Elsevier Science Ltd.