Hafnium isotope evidence for 'conservative' element mobility during subduction zone processes

The high field strength elements (HFSE) play a critical role in the interpretation of chemical variation within subduction-related magmas by providing an assumed mantle-dominated 'baseline' from which enrichments in many other stab-derived elements may be gauged. Of the HFSE, hafnium is unique in combining the characteristics of HFSE chemistry with it powerful isotopic tracer and should, in theory, allow the delineation of mantle domains and help constrain the timing of melt depletion processes. A detailed Hf isotope study of oceanic are lavas and paired are/backarc settings has been conducted. Here we show, contrary to expectations, that the Hf isotopic compositions of arc lavas are always displaced significantly from their co-existing back-are spreading centres which can be considered to sample the local mantle. This is true not only of those arcs in which direct sediment melting or AFC-like processes within the crust are implicated, but also in low-K tholeiitic arcs where hydrous fluids are believed to be the dominant medium of slab-to-mantle transport. This observation calls into question the concept of 'conservative' or 'immobile' elements and suggests that some transfer of material from the subducting slab into the sub-arc mantle wedge probably occurs for almost all elements. These conclusions have significant implications for models of are geochemistry. (C) 2001 Elsevier Science B.V. All rights reserved.