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

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1 IPGP - Institut de Physique du Globe de Paris 2 THE UNIVERSITY OF CHICAGO - The University of Chicago, Department of the Geophysical Sciences 3 UNIVERSITA DI GENOVA - Universita di Genova Dipartimento per lo studio del Territorio e delle sue Risorse

Abstract : 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.

Keywords : oxygen nitrogen subduction

Author: Pascal Philippot - Vincent Busigny - Marco Scambelluri - Pierre Cartigny -

Source: https://hal.archives-ouvertes.fr/


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