Jd. Woodhead et al., Hafnium isotope evidence for 'conservative' element mobility during subduction zone processes, EARTH PLAN, 192(3), 2001, pp. 331-346
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.