Oxygen and nitrogen isotopes as tracers of fluid activities in serpentinites and metasediments during subduction

Summary N and O isotope systematics of a suite of high-pressure (HP) and ultrahigh-pressure (UHP) metasediments of the Schistes Lustrés nappe and metaperidotites of the Erro Tobbio Massif from the Alpine-Appennine system are compared with their unmetamorphosed or hydrothermally-altered equivalent from the same localities and from the South West Indian Ridge (SWIR). The HP and UHP rocks studied represent a sequence of pelagic sediments and altered ultramafic rocks subducted to different depths of down to 90 km along a cold geothermal gradient (8 °C/km). Unmetamorphosed and HP metasediments show the same range in δ¹⁵N values irrespective of their metamorphic grade and bulk nitrogen concentrations. Together with several other geochemical features (K, Rb and Cs contents, δD), this indicates that δ¹⁵N values were unaffected by metamorphism and N was not released during subduction. N isotope analysis of serpentinites coupled with δ¹⁸O systematics suggests the involvement of a mafic (crustal) component during partial deserpentinization of the subducted oceanic mantle at the depth locus of island arc magmatism. This does not imply large-scale fluxes as the metagabbros are spatially associated with the analyzed serpentinites. It rather indicates preservation of presubduction chemical and isotopic heterogeneities on a local scale as documented for the metasediments.