The sources of arc-related lavas are commonly modified by
metasomatic fluids and/or melts believed to be extracted from
parts of the subducting slab (e.g., the basaltic crust and/or overlying
sediments). Evidence from phase equilibria, trace-element
geochemistry, and volatile contents arc-related lavas and melt
inclusions strongly suggest that aqueous fluid derived from
subducted basalt is one of these phases. In contrast, the
properties and even presence of slab-derived melts in the sub-arc
mantle are relatively uncertain. Commonly, a role for slab-derived
melt is implicated where arc lavas are enriched in
sediment-born and fluid-insoluble trace elements such as Th
(e.g., Elliott et al., 1997). However, this evidence is contradicted
by or fails to address other constraints; for instance: Solvi
between water-rich silicate melts and solute-rich aqueous fluids
'close' at T-P conditions within the range of those in the slab and
mantle wedge, suggesting that the dichotomy between 'slab
fluid' and 'slab melt' may be poorly posed (e.g., Bureau and
Keppler, 1999); furthermore, thermal models of subduction
zones suggest that no section of the slab should melt except
under special circumstances or through generally unexpected
physical effects (Peacock, 1991); finally, experimental and
empirical constraints on the expected geochemical properties of
slab fluids and melts are inconsistent with one another in several
respects, adding ambiguity to trace-element arguments for or
against the role of slab melt (e.g., Keppler, 1996; Johnson and
Plank, 1999)
