Linking serpentinite geochemistry with tectonic evolution at the subduction plate-interface: The Voltri Massif case study (Ligurian Western Alps, Italy).

Recent geochemical work shows that subduction-zone serpentinites are repositories for fluid-mobile elements absorbed during interaction with sediment-derived fluids. Unraveling the geochemical fingerprint of these rocks helps to define timing of tectonic accretion of sediments along the subduction interface and the role of serpentinite in element recycling to volcanic arcs. Here we present the trace element and isotopic composition (B-O-H, Sr, Pb) of high-pressure serpentinites from the Voltri Massif (Ligurian Western Alps, Italy), to discuss their role as incompatible element carriers and their contribution to recycling of sediment-derived components in subduction zones. The serpentinites presented here record metamorphic olivine growth during eclogite-facies metamorphism and show unde-formed and mylonitic textures. Field relations show that undeformed rocks are enclosed in deformed ones and that no metasedimentary rocks are present nearby. Undeformed serpentinite has very high delta B-11(SRM951) (from +26% to +30%), low Sr and Pb isotope ratios (Sr-87/Sr-86 = 0.7053-0.7069; Pb-206/Pb-204 = 18.131-18.205) and low As and Sb contents (0.1 and 0.01 mu g/g, respectively). Oxygen and hydrogen isotope compositions are +4.5% and -67%, respectively. In contrast, mylonitic serpentinite shows lower delta B-11 (from +22% to +17%), significant enrichment in radiogenic Sr and Pb isotopes (Sr-87/Sr-86 up to 0.7105; Pb-206/Pb-204 up to 18.725), and enrichment in As and Sb (1.3 and 0.39 mu g/g, respectively). delta O-18 of the mylonitic serpentinites reaches values of +5.9%, whereas delta D is comparable with that of undeformed rocks (approximately -70%). In mylonitic serpentinites, the B and Sr isotopic values and the fluid-mobile element (FME) concentrations are near those for the Voltri metasedimentary rocks (calc- and mica-schists). Pb systematics also reveal influx of a crust-derived component.