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Organic nitrogen chemistry during low-grade metamorphism |
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| Authors: | Jean-Paul Boudou Arndt Schimmelmann Maria Mastalerz Léon Gengembre |
| Institution: | a Université Pierre et Marie Curie, Paris 6, CNRS-UMR 7618, Case 120, Laboratoire de Biogéochimie et Ecologie des Milieux Continentaux, 4, Place Jussieu, 75252 Paris Cedex 05, France b Department of Geological Sciences, Indiana University, 1001 East Tenth Street, Bloomington, IN 47405-1405, USA c Institut de Physique du Globe de Paris Paris et Université Paris 7, CNRS-UMR 7154, Equipe de physico-chimie des fluides géologiques, 2, Place Jussieu, 75251 Paris Cedex 05, France d Indiana Geological Survey, Indiana University, 611 N. Walnut Grove, Bloomington, IN 47405-2208, USA e Université des Sciences et Technologies de Lille, CNRS UMR 8181, Unité de Catalyse et Chimie du Solide, Bâtiment C3, 59655 Villeneuve d’Ascq Cedex, France |
| Abstract: | Most of the organic nitrogen (Norg) on Earth is disseminated in crustal sediments and rocks in the form of fossil nitrogen-containing organic matter. The chemical speciation of fossil Norg within the overall molecular structure of organic matter changes with time and heating during burial. Progressive thermal evolution of organic matter involves phases of enhanced elimination of Norg and ultimately produces graphite containing only traces of nitrogen. Long-term chemical and thermal instability makes the chemical speciation of Norg a valuable tracer to constrain the history of sub-surface metamorphism and to shed light on the subsurface biogeochemical nitrogen cycle and its participating organic and inorganic nitrogen pools. This study documents the evolutionary path of Norg speciation, transformation and elimination before and during metamorphism and advocates the use of X-ray photoelectron spectroscopy (XPS) to monitor changes in Norg speciation as a diagnostic tool for organic metamorphism. Our multidisciplinary evidence from XPS, stable isotopes, traditional quantitative coal analyses, and other analytical approaches shows that at the metamorphic onset Norg is dominantly present as pyrrolic and pyridinic nitrogen. The relative abundance of nitrogen substituting for carbon in condensed, partially aromatic systems (where N is covalently bonded to three C atoms) increases exponentially with increasing metamorphic grade, at the expense of pyridinic and pyrrolic nitrogen. At the same time, much Norg is eliminated without significant nitrogen isotope fractionation. The apparent absence of Rayleigh-type nitrogen isotopic fractionation suggests that direct thermal loss of nitrogen from an organic matrix does not serve as a major pathway for Norg elimination. Instead, we propose that hot H, O-containing fluids or some of their components gradually penetrate into the carbonaceous matrix and eliminate Norg along a progressing reaction front, without causing nitrogen isotope fractionation in the residual Norg in the unreacted core of the carbonaceous matrix. Before the reaction front can reach the core, an increasing part of core Norg chemically stabilizes in the form of nitrogen atoms substituting for carbon in condensed, partially aromatic systems forming graphite-like structural domains with delocalized π-electron systems (nitrogen atoms substituting for “graphitic” carbon in natural metamorphic organic matter). Thus, this nitrogen species with a conservative isotopic composition is the dominant form of residual nitrogen at higher metamorphic grade. |
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