Volatiles and Fluids in Subduction Zones

Nitrogen geochemistry of subducted oceanic lithosphere

 

Nitrogen (N) isotopes have great potential as a geochemical tracer for crustal and volatile recycling due to the large isotopic differences between the various terrestrial reservoirs mantle, crust and atmosphere. Knowledge about the recycling behavior of N, including the comparison of N input and output fluxes in subduction zones and an assessment of the processes that affect N during subduction, is fundamental to model N cycling and long-term evolution of atmosphere and mantle. Although fresh MORB generally contains little N (mostly less than 2 ppm), N contents in AOC can be considerably higher (up to 18 ppm) due to additions of sedimentary-organic N in pore fluids, and there is a strong possibility that a significant amount of N subducted in oceanic sediments, crust and mantle is not returned via arc volcanism but instead subducted to greater depths.

However, the extent to which HP and ultrahigh-pressure (UHP) metabasaltic rocks retain N has not been investigated to date. This retention beyond subarc depths could potentially greatly impact the global N budget, and the existence of subducted N in the deeper mantle has been proposed based on investigations on diamonds and the study of plume-related magmatism. The major aim of this study is the N isotopic characterization of subducted oceanic lithosphere at high and ultrahigh pressures in order to identify processes that may cause N elemental and isotopic changes during subduction and to determine the N isotopic signature introduced into the mantle by incorporation of eclogitic material.

We are analyzing primarily eclogites and some blueschists, serpentinites and metasediments were investigated from four different localities (Raspas Complex/Ecuador, Lago di Cignana/Italy, Zambezi Belt/Zambia, Cabo Ortegal/Spain). These localities were chosen because they cover a wide range of metamorphic P-T conditions and are petrologically and geochemically well characterized.