Oxygen and nitrogen isotopes as tracers of fluid activities in serpentinites and metasediments during subduction
Résumé
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-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 d15N values independently of their metamorphic grade and bulk nitrogen concentrations. Together with several other geochemical features (K, Rb and Cs contents, dD), this indicates that d15N values were unaffected by metamorphism and N was not released during subduction. N isotope analysis of serpentinites coupled with d18O systematics envisages 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, rather preservation of presubduction chemical and isotopic heterogeneities on a local scale as documented for the metasediments.
Domaines
Océan, Atmosphère
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