We provide new insights into the geochemistry of serpentinites from the
Alpine orogenic wedge representing a paleo-subduction zone. These
serpentinites are derived from similar oceanic protoliths, but they have
experienced different metamorphic conditions related to three different
structural levels of the paleo-subduction zone ((1) obducted: Chenaillet
ophiolite, (2) accretionary wedge: Queyras Schistes lustres complex and
(3) serpentinite channel: Monviso ophiolite). Metamorphism undergone by
these three units is well defined, increasing eastward from
sub-greenschist to eclogite facies conditions, and allows us to examine
trace element behavior from the oceanic ridge environment to subduction.
Serpentinites first record moderate trace element enrichment due to
seawater interaction resulting in the replacement of olivine and
pyroxene by chrysotile and lizardite below 300 degrees C. In the
sediment-dominated accretionary wedge, serpentinites are strongly
enriched in fluid-mobile-elements (B, Li, As, Sb, and Cs) and act as a
trapping system following the metamorphic gradient (from 300 to 390
degrees C) up to total replacement of the lizardite/chrysotile
assemblage by antigorite. Under higher temperature conditions (T>390
degrees C), no enrichment was observed, and some fluid-mobile elements
were released (B, Li, Cs, and Sr). Moreover, in the serpentinite channel
(T>460 degrees C), most of the fluid-mobile elements are absent due to
the scarcity of metasediments which prevent geochemical exchange between
metasediments and serpentinites. This is also due to the onset of
antigorite breakdown and the release of fluid-mobile elements. Thus, we
emphasize that the geochemistry of Alpine serpentinites is strongly
dependent on (1) the grade of metamorphism and (2) the ability of
metasediments to supply fluid-mobile elements. We conclude that
serpentinites act as a trap-and-release system for fluid-mobile elements
in a subduction context. (C) 2013 Elsevier B.V. All rights reserved.