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

Citation
Jd. Woodhead et al., Hafnium isotope evidence for 'conservative' element mobility during subduction zone processes, EARTH PLAN, 192(3), 2001, pp. 331-346
Citations number
47
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
EARTH AND PLANETARY SCIENCE LETTERS
ISSN journal
0012-821X → ACNP
Volume
192
Issue
3
Year of publication
2001
Pages
331 - 346
Database
ISI
SICI code
0012-821X(20011030)192:3<331:HIEF'E>2.0.ZU;2-7
Abstract
The high field strength elements (HFSE) play a critical role in the interpr etation of chemical variation within subduction-related magmas by providing an assumed mantle-dominated 'baseline' from which enrichments in many othe r stab-derived elements may be gauged. Of the HFSE, hafnium is unique in co mbining the characteristics of HFSE chemistry with it powerful isotopic tra cer and should, in theory, allow the delineation of mantle domains and help constrain the timing of melt depletion processes. A detailed Hf isotope st udy 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 bac k-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 AF C-like processes within the crust are implicated, but also in low-K tholeii tic arcs where hydrous fluids are believed to be the dominant medium of sla b-to-mantle transport. This observation calls into question the concept of 'conservative' or 'immobile' elements and suggests that some transfer of ma terial from the subducting slab into the sub-arc mantle wedge probably occu rs for almost all elements. These conclusions have significant implications for models of are geochemistry. (C) 2001 Elsevier Science B.V. All rights reserved.