U-series disequilibria provide important constraints on the processes
and time scales of melt production, differentiation, and transport in
subduction settings. Such constraints, which are essential for
understanding the chemical evolution of the continental crust, are
conventionally based on the assumption that the U-series disequilibria
measured in mafic lavas are produced during mantle metasomatism and
melting, and that intracrustal differentiation and assimilation have
limited impacts. Here we show that mantle-derived U-series disequilibria
in mafic lavas erupted at Volcan Llaima, Chile are significantly
diminished by assimilation of plutonic rocks forming Llaima's
subvolcanic basement. This contamination process is extremely subtle in
terms of ``classical'' indicators of crustal assimilation like Sr, Nd
or Pb isotopes because it is a manifestation of assimilative recycling
of the plutonic roots of the arc. This process results in variations in
U-series disequilibria and incompatible trace element ratios that are
significant compared to regional and global variability in arc magmas.
Furthermore, it yields linear correlations between U-series excesses and
incompatible trace element ratios that are generally interpreted as
slab-fluid indicators and chronometers, or tracers of sediment recycling
in subduction zone. Cannibalization of ancestral magmas by ascending
melts warrants careful evaluation when considering the components and
chemical fluxes in subduction zones. Linear arrays defined by activity
ratios of U-series nuclides with different half-lives may be the most
reliable indicators of assimilative recycling of ancestral intrusive
magmas. (C) 2010 Elsevier B.V. All rights reserved.