Nitrogen isotopes in ophiolitic metagabbros: A re-evaluation of modern nitrogen fluxes in subduction zones and implication for the early Earth atmosphere | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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  Nitrogen isotopes in ophiolitic metagabbros: A re-evaluation of modern nitrogen fluxes in subduction zones and implication for the early Earth atmosphere

Publication Type:

Journal Article

Source:

Geochimica Et Cosmochimica Acta, Volume 75, Issue 23 (2011)

ISBN:

0016-7037

Accession Number:

WOS:000297214400009

Keywords:

UMR 7154 ; Géobiosphère actuelle et primitive

Abstract:

Nitrogen contents and isotope compositions together with major and trace element concentrations were determined in a sequence of metagabbros from the western Alps (Europe) in order to constrain the evolution and behavior of N during hydrothermal alteration on the seafloor and progressive dehydration during subduction in a cold slab environment (8 °C/km). The rocks investigated include: (i) low-strain metagabbros that equilibrated under greenschist to amphibolite facies (Chenaillet Massif), blueschist facies (Queyras region) and eclogite facies (Monviso massif) conditions and (ii) highly-strained mylonites and associated eclogitic veins from the Monviso Massif. In all samples, nitrogen (2.6–55 ppm) occurs as bound ammonium (View the MathML source) substituting for K or Na–Ca in minerals. Cu concentrations show a large variation, from 73.2 to 6.4 ppm, and are used as an index of hydrothermal alteration on the seafloor because of Cu fluid-mobility at relatively high temperature (>300 °C). In low-strain metagabbros, δ15N values of +0.8‰ to +8.1‰ are negatively correlated with Cu concentrations. Eclogitic mylonites and veins display Cu concentrations lower than 11 ppm and show a δ15N–Cu relationship that does not match the δ15N–Cu correlation found in low-strain rocks. This δ15N–Cu correlation preserved in low-strain rocks is best interpreted by leaching of Cu–N compounds, possibly of the form Cu(NH3)22+, during hydrothermal alteration. Recognition that the different types of low-strain metagabbros show the same δ15N–Cu correlation indicates that fluid release during subduction zone metamorphism did not modify the original N and Cu contents of the parent hydrothermally-altered metagabbros. In contrast, the low Cu content present in eclogitic veins and mylonites implies that ductile deformation and veining were accompanied either by a loss of copper or that externally-derived nitrogen was added to the system.