Jean-Paul Boudou
email: boudou@ccr.jussieu.fr

Arndt Schimmelmann

Magali Ader

Maria Mastalerz

Mathieu Sebilo

Léon Gengembre

Organic nitrogen chemistry during low-grade metamorphism

Most of the organic nitrogen (N org ) on Earth is disseminated in crustal sediments and rocks in the form of fossil nitrogen-containing organic matter. The chemical speciation of fossil N org 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 N org and ultimately produces graphite containing only traces of nitrogen. Long-term chemical and thermal instability makes the chemical speciation of N org 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 N org speciation, transformation and elimination before and during metamorphism and advocates the use of X-Ray Photoelectron Spectroscopy (XPS) to monitor changes in N org 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 N org 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 N org 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 N org elimination. Instead, we propose that hot H, O-containing fluids or some of their components gradually penetrate into the carbonaceous matrix and eliminate N org along a progressing reaction front, without causing nitrogen isotope fractionation in the residual N org in the unreacted core of the carbonaceous matrix. Before the reaction front can reach the core, an increasing part of core N org chemically stabilizes in the form of nitrogen atoms substituting for carbon in condensed, partially aromatic systems forming graphite-like structural domains with delocalized πelectron 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.

INTRODUCTION

Earth's geochemical nitrogen cycle links the relatively small pool of organic nitrogen in living biomass with a far more abundant inventory of fossil organic nitrogen and an even larger pool of inorganic nitrogen [START_REF] Berner | Geological nitrogen cycle and atmospheric N 2 over Phanerozoic time[END_REF]and refs. therein), the latter being predominantly in the form of ammonium in minerals. Diagenesis and thermal maturation transform nitrogen-rich biomass into carbonaceous material where, in spite of its lowered abundance, organic nitrogen (N org ) continues to express a diverse and diagnostic geochemical Geochimica et Cosmochimica Acta 72 (2008) Boudou GCA W4836 2 character in terms of its chemical bonding to carbon. Progressive condensation and aromatization of sedimentary organic matter during burial, thermal maturation, and subsequent metamorphism (Fig. 1) produce an insoluble macromolecular carbonaceous matrix. This study investigates changes in N org in anthracite and semi-graphite to constrain the behavior of nitrogen through geological cycles, and to strengthen N org 's utility as a tracer during metamorphism (e.g., [START_REF] Pinti | Nitrogen and argon signatures in 3.8 to 2.8 Ga metasediments: Clues on the chemical state of the Archean ocean and the deep biosphere[END_REF][START_REF] Van Zuilen | Nitrogen and argon isotopic signatures in graphite from the 3.8-Ga-old Isua Supracrustal Belt, Southern West Greenland[END_REF][START_REF] Pitcairn | The behavior of nitrogen and nitrogen isotopes during metamorphism and mineralization: Evidence from the Otago and Alpine Schists, New Zealand[END_REF][START_REF] Ader | Nitrogen isotopic evolution of carbonaceous matter during metamorphism: Methodology and preliminary results[END_REF][START_REF] Jia | Nitrogen isotope fractionations during progressive metamorphism: A case study from the Paleozoic Cooma metasedimentary complex, southeastern Australia[END_REF].

Fig. 1. Correlative overview on kerogen maturation stages, coal ranks (adopting US nomenclature), graphite and transitional phases, selected geochemical parameters, and fluid hydrocarbon generation. N org concentrations and the isotopic range of observed  15 N org data refer to coal samples utilized in the cited studies and are not globally representative. N concentrations in graphite are from [START_REF] Ader | Nitrogen isotopic evolution of carbonaceous matter during metamorphism: Methodology and preliminary results[END_REF] and [START_REF] Van Zuilen | Nitrogen and argon isotopic signatures in graphite from the 3.8-Ga-old Isua Supracrustal Belt, Southern West Greenland[END_REF]. Correlations between temperature and individual parameters are approximate and may vary depending on local temperature and pressure conditions [START_REF] Gretener | Role of temperature and time on organic metamorphism[END_REF][START_REF] Nickelsen | Ambient temperatures during the Alleghany Orogeny[END_REF][START_REF] Paxton | Relationships between Pennsylvanian-age lithic sandstone and mudrock diagenesis and coal rank in the Central Appalachians[END_REF][START_REF] Frey | Very low-grade metamorphism of the Alps -An introduction[END_REF][START_REF] Juster | NH 4 -bearing illite in very low-grade metamorphic rocks associated with coal, northeastern Pennsylvania[END_REF][START_REF] Daniels | Nature and origin of minerals in anthracite from eastern Pennsylvania[END_REF]Daniels and Altaner, 1990;Daniels et al., 1990;[START_REF] Barker | Fluid inclusion and vitrinite reflectance geothermometry compared to heat-flow models of maximum paleotemperature next to dikes, western onshore Gippsland Basin, Australia[END_REF][START_REF] Frey | Low-Grade Metamorphism[END_REF][START_REF] Mullis | Temperature determination between 50 and 270 °C through fluid inclusion microthermometry and vitrinite reflectance values in the external parts of the central Alps[END_REF][START_REF] Hower | Mechanisms of coal metamorphism: Case studies from Paleozoic coalfields[END_REF][START_REF] Petersen | The petroleum generation potential and effective oil window of humic coals related to coal composition and age[END_REF][START_REF] Kwiecińska | Graphite, semi-graphite, natural coke, and natural char classification-ICCP system[END_REF][START_REF] Schimmelmann | Hydrogen isotopic (D/H) composition of organic matter during diagenesis and thermal maturation[END_REF][START_REF] Wang | Comment on "Influence of a basic intrusion on the vitrinite reflectance and chemistry of the Springfield (No. 5) coal, Harrisburg, Illinois[END_REF].

It is difficult to avoid analytical artifacts when analyzing trace amounts of N org in metamorphic carbonaceous matter using methods that rely on solvent extracts or pyrolyzates (e.g., [START_REF] Simoneit | High resolution mass spectrometry of nitrogenous compounds of the Colorado Green River formation oil shale[END_REF][START_REF] Schmitter | Distribution of diaromatic nitrogen bases in crude oils[END_REF][START_REF] Bennett | Fractionation of benzocarbazoles between source rocks and petroleums[END_REF]. Non-destructive X-Ray Photoelectron Spectroscopy (XPS) presently offers the best approach to monitor maturity-related changes in the chemical character of N org . XPS is based on the assessment of orbital ionization potentials by measuring the energy spectrum of electrons ejected from nitrogen atoms following bombardment with monoenergetic rays or particles. The electrostatic interactions between valence and core electrons are expressed as chemical shifts that are dependent on the chemical-structural environment of atoms. In the case of nitrogen,

Temp. o ( C)

Coal rank

the XPS response follows the loss of an energetic electron from the K shell; hence the term N 1s XPS is used. [START_REF] Burchill | Variation of nitrogen content and functionality with rank for some UK bituminous coals[END_REF] used N 1s XPS in a pioneering study to elucidate chemical nitrogen characteristics in three anthracite samples. More recently, natural changes in the chemistry of nitrogen in a sequence from peat to low-volatile bituminous coal have been detected by N 1s XPS [START_REF] Kelemen | Quantification of nitrogen forms in Argonne premium coals[END_REF][START_REF] Kelemen | Thermal transformations of nitrogen and sulfur forms in peat related to coalification[END_REF][START_REF] Straka | X-ray photoelectron spectroscopy of nitrogen and sulfur functionalities in organic substance of coal[END_REF].

Experimental pyrolysis combined with XPS for type I, type II, and type III (coal) kerogens yielded similar observations regardless of the type of kerogen. Our data agree with [START_REF] Kelemen | Nitrogen transformation in coal during pyrolysis[END_REF][START_REF] Kelemen | Thermal chemistry of nitrogen in kerogen and low-rank coal[END_REF] observation of a strong relative increase of quaternary nitrogen with increasing maturity. Following extensive high-temperature pyrolysis of nitrogencontaining amorphous carbon precursors in the laboratory, essentially all edge-located nitrogen atoms (Fig. 2) are eliminated and remaining nitrogen atoms are preferentially located in the interior of the carbon lattice as thermally stable quaternary nitrogen [START_REF] Isaacs | The graphitization of organic compounds. III. Heterocyclic nitrogen derivatives of anthracene and phenanthrene[END_REF][START_REF] Pels | Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis[END_REF][START_REF] Stanczyk | Transformation of nitrogen structures in carbonization of model compounds determined by XPS[END_REF][START_REF] Xiao | Reactions of nitrogen and oxygen surface groups in nanoporous carbons under inert and reducing atmospheres[END_REF]. These trends have not been observed in nature where fluids, temperature, pressure, time and catalysts play important roles in the transformation of carbonaceous matter during metamorphism (e.g., [START_REF] Price | Evidence and characteristics of hydrolytic disproportionation of organic matter during metasomatic processes[END_REF][START_REF] Schwab | Chemical and carbon isotopic evolution of hydrocarbons during prograde metamorphism from 100 °C to 550 °C: Case study in the Liassic black shale formation of Central Swiss Alps[END_REF][START_REF] Guedes | Characterisation of dispersed organic matter from lower Palaeozoic metasedimentary rocks by organic petrography, X-ray diffraction and micro-Raman spectroscopy analyses[END_REF]. It is expected that during metamorphism N org becomes increasingly integrated into sub-nanometric to nanometric sheets of adjoining ring structures (Fig. 2) that occur in isolation or are stacked to form basic structural units [START_REF] Bustin | Mechanisms of graphite formation from kerogen: Experimental 1evidence[END_REF]. The anthracite basic structural unit consists of planar graphitoidal layers with an average C-C bond length of 0.143 nm [START_REF] Bratek | Properties and structure of different rank anthracites[END_REF]. The number of aromatic rings constituting a basic unit increases with rank. The predominant basic structural unit in anthracitic coals has a diameter of 0.75 nm across 3 condensed aromatic rings, whereas the basic unit of meta-anthracite measures 1.2 nm across 5 aromatic rings. During subsequent graphitization, the resulting polyaromatic sheets increase in size and may accommodate nitrogen atoms. 2. Schematic nitrogen-containing molecular structures in condensed ring systems similar to coronene, where nitrogen substitutes for carbon and is either located along the edge and bonded to two carbon atoms (a, b, c), or is bonded to three carbon atoms and is termed N-C 3 nitrogen (d, e, f). (a) Pyrrolic nitrogen; this type of nitrogen is frequently associated with a five-member cycle (e.g., pyrrole) that is not shown here. (b) Pyridonic nitrogen. (c) Pyridinic nitrogen. (d) Cyclazine-type nitrogen common to three fused rings. The oxygen atom in the ring system can be replaced by other heteroatoms (e.g., nitrogen, sulfur), a pyrrolic group, CH 2 , etc. (e) Quaternized cyclazine-type nitrogen or centrally-positioned quaternary nitrogen. (f) Quaternary nitrogen common to two fused rings. (*) Other types of cyclazine structures were presented by [START_REF] Boutique | 5,11,13-tetraazacycl[3.3.3azine: Theoretical (ab initio) and experimental (Xray and ultraviolet photoelectron spectroscopy) studies of the electronic structure[END_REF], [START_REF] Leaver | The synthesis and characterisation of cyclazines and related N-bridged annulenes[END_REF] and [START_REF] Pels | Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis[END_REF].

Numerous studies on the structure and chemical composition of artificially produced nitrogen-doped graphitic materials document the occurrence of covalently bonded nitrogen atoms substituting for carbon atoms in condensed, partially aromatic systems. In these systems, only few nitrogen atoms will be linked to another nitrogen atom because the much larger C-N binding energy renders N-N single bonds thermodynamically unstable in the presence of excess carbon [START_REF] Ripalda | An XPS study of carbon nitride synthesized by ion beam nitridation of C 60 fullerene[END_REF]. Instead, most nitrogen atoms are shared by adjoining rings (Fig. 2). When a nitrogen atom substitutes for carbon in a hexagonal structure, it shares the same sp 2 -hybridization as neighboring carbon atoms, and its remaining two electrons participate in the delocalized π-electron network as described, for instance, in graphite (Dos [START_REF] Santos | Nitrogen substitution of carbon in graphite: Structure evolution toward molecular forms[END_REF], fullerene [START_REF] Reuther | Synthesis, properties and chemistry of Aza[60[END_REF], and carbon nanotubes [START_REF] Choi | Distribution and structure of N atoms in multiwalled carbon nanotubes using variable-energy X-ray photoelectron spectroscopy[END_REF]. Three-coordinated N prefers a locally 'buckled', non-planar structure. In principle, this promotes the formation of pentagons [START_REF] Sjöström | Superhard and elastic carbon nitride thin films having fullerenelike microstructure[END_REF], but nitrogen can be accomodated in a hexagonal carbon network as long as the heteroatom concentration remains low [START_REF] Hellgren | Role of nitrogen in the formation of hard and elastic CN x thin films by reactive magnetron sputtering[END_REF].

Our study constrains thermal-evolutionary N org pathways and strengthens our mechanistic understanding of metamorphic transformations of N org leading to inorganic nitrogen species in fluids and residual N org -bearing carbonaceous matter. Although the emphasis of this study is placed on the geochemical fate of N org during low-grade metamorphism up to the semigraphitic stage, it was necessary to also document the N org inventory of some lower-maturity samples in order to properly cover the continuum between pre-metamorphism and metamorphism. Processes at even lower maturity, such as sedimentation of biomass and subsequent diagenesis, are beyond the scope of this paper.

MATERIALS AND METHODS

Sample origin and vitrinite reflectance

Samples from a wide range of ranks from anthracites to semi-graphites were collected from the Western Middle Anthracite Field of Pennsylvania, USA and from north German coal mines in the West European Carboniferous Basin (WECB) (Table 1).

This study utilizes mostly type III kerogens from coals (Table 1). We would have preferred to augment our coal-derived data with more data from kerogens types I and II, but no previously published nitrogen data are available at adequate thermal maturity. The few utilized kerogens of types I and II include: Kerogen type I: A bright shungite from the upper Zaonezhskaya Formation near Lake Onega in the Shunga region of Karelia in Russia derives from one of the richest accumulations of organic material reported from the Proterozoic that generated one of the geologically earliest petroleums, probably from type I kerogen from algal or bacterial biomass (e.g., [START_REF] Mastalerz | Organic and mineral matter in a Precambrian shungite deposit from Karelia, Russia. In Organic Matter and Mineralisation: Thermal Alteration, Hydrocarbon Generation, and Role in Metallogenesis[END_REF]. Green River Shale contains type I kerogen and is the source rock of the primary petroleum system in the Uinta Basin (Utah, USA).

Kerogens type II derive from the New Albany Shale (a ≤140 m thick horizon of Middle Devonian to Early Mississippian organic-rich shales) in the Illinois Basin [START_REF] Werner-Zwanziger | Thermal maturity of type II kerogen from the New Albany Shale assessed by 13 C CP/MAS NMR[END_REF][START_REF] Lis | D/H ratios and hydrogen exchangeability of type-II kerogens with increasing thermal maturity[END_REF] covering a relatively wide range of maturity with vitrinite reflectance R o from 0.29 to 1.5% [START_REF] Hasenmueller | Compilation of gas potential of the New Albany Shale (GRI CD)[END_REF].

Vitrinite reflectance measurements R mean and/or R max of bulk samples were performed according to standard methods [START_REF] Sjöström | Superhard and elastic carbon nitride thin films having fullerenelike microstructure[END_REF][START_REF] Daniels | Nature and origin of minerals in anthracite from eastern Pennsylvania[END_REF][START_REF] Daniels | Nature and origin of minerals in anthracite from eastern Pennsylvania[END_REF]Altaner, 1990, 1993). Due to development of optical anisotropy at higher maturity, R max is the preferred measure at vitrinite reflectance >1.5%. Most samples in this study have high rank and thus justify the consistent use of R max values. An empirical conversion algorithm between R mean and R max (given at the bottom of Table 1) is based on an extensive set of comparative R mean and R max measurements.

Table 1

Sample identification, sample origin, age, maturity, elemental composition (daf = dry, ash-free), stable isotope ratios, and nitrogen XPS data.

(a) Below vitrinite reflectance values of 1.5%, R max values were calculated from R mean values using the following algorithm: R max = -0.00387 . R mean 3 + 0.08533 . R mean 2 + 0.97995 . R mean + 0.03595; r 2 = 0.9971.

(b) Determined on kerogen and expressed on a dry, ash-free basis (daf).

(c) N min is expressed with respect to the dry sample. For low rank samples N min was not determined (n.d.) and, prior to XPS analyses, samples were treated with HF-HCl at room temperature to eliminate ammonium.

(d) Determined on kerogen; data from type III kerogens are from [START_REF] Gerling | New aspects on the origin of nitrogen in natural gas in Northern Germany[END_REF] and Ader et al. (1998a[START_REF] Ader | Nitrogen isotopic evolution of carbonaceous matter during metamorphism: Methodology and preliminary results[END_REF].

(e) XPS was performed either on ammonium-free substrates when R max < 2.5%, or on original dried samples at higher R max .

(f) WMAF = Western Middle Anthracite Field, Pennsylvania, USA.

Grey highlighted rows indicate samples for which XPS analyses were performed on kerogen isolated from ammoniacontaining bulk shale (sample 19J) and on a mineral isolate from a meta-anthracite (sample 16H-LTA); R max , C org , δ 13 C and δ 15 N values for 16H-LTA are those of the parent 16H meta-anthracite (16H was petrographically described by Daniels,92).

Elemental, isotopic and thermal analyses

Demineralization of rocks with acids inevitably causes some organic-geochemical alteration of the resulting kerogen isolate, especially in kerogens of lower rank. Our preparation of kerogens minimized organic-geochemical alteration by choosing standard techniques that are appropriate for various ranks. Each set of kerogen types was treated equivalently in order to maintain reproducibillity and comparability. Relatively low-rank New Albany Shale was processed for the preparation of kerogen using the chemically mild, lowtemperature BF 3 -HF method of [START_REF] Robl | Comparison of the HF-HCl and HF-BF3 maceration techniques and the chemistry of resultant organic concentrates[END_REF], followed by heavy-liquid purification, washing, freeze-drying, and solvent extraction [START_REF] Schimmelmann | D/H isotope ratios of kerogen, oil, and water in hydrous pyrolysis of source rocks containing kerogen types I, II, IIS, and III[END_REF][START_REF] Lis | D/H ratios and hydrogen exchangeability of type-II kerogens with increasing thermal maturity[END_REF]. Higher-rank rocks were demineralized with acids according to [START_REF] Durand | Procedures for kerogen isolation[END_REF] standard method.

Ammonium-silicates from high-rank samples (except the Karelian shungite) were separated from N org and isolated as mineral nitrogen (N min ) by low-temperature plasma ashing of powdered whole rock (Daniels and Altaner, 1990). Ammonium was liberated from the ash, or from the crude rock, via digestion in 1:1 vol/vol mixture of concentrated hydrofluoric and hydrochloric acids in polypropylene bottles overnight at room temperature, followed by the addition of aqueous NaOH, distillation of NH 3 , and absorption of NH 3 in dilute H 2 SO 4 as ammonium sulfate. Ammonium NH 4 + was quantified colorimetrically by detection of indophenol blue at 690 nm using a Spectroquant spectrophotometer Nova 60 (Merck).

Analytical data from different studies are not always directly comparable due to differences among various wet-chemical digestion and combustion techniques (see [START_REF] Holloway | Nitrogen in rock: Occurrences and biogeochemical implications[END_REF]. Total N determination was therefore performed via complete Dumas combustion at high temperature in addition to ASTM D3179 (i.e., Kjeldahl-Gunning analysis) because wet-chemical methods are unable to fully digest N org in metamorphic organic matter.

For most samples, δ 13 C org values of organic carbon and δ 15 N org values of organic nitrogen were determined by sealed tube combustion and off-line isotope measurements using a Finnigan Delta E mass spectrometer, with precisions of ± 0.05‰ and ± 0.15‰, respectively (Ader et al., 1998a) . Quality control of analyzed gases used scanning of m/z 12,15,16,[START_REF] Levine | Tectonic history of coal-bearing sediments in eastern Pennsylvania using coal reflectance anisotropy[END_REF]32 and 40 (Ar). δ 13 C org and δ 15 N org values of type II kerogens were determined on-line using elemental analyzers coupled with Thermo Finnigan isotope ratio mass-spectrometers. δ 13 C and δ 15 N values are reported in ‰ notation relative to Vienna Peedee Belemnite (VPDB) and air nitrogen, respectively. Bulk carbon and nitrogen isotopic compositions of selected original rocks containing no mineral carbon or nitrogen were indistinguishable from values of their kerogens. This serves as evidence that the preparation of kerogen did not isotopically fractionate N org . High temperature and long duration of plasma ashing can change the surface topography and the chemical composition of residual mica [START_REF] Liu | Characterization of oxygen plasmamodified mica surfaces using XPS and AFM[END_REF], possibly causing nitrogen isotopic fractionation due to partial loss of ammonia. However, we found that δ 15 N ammonium values of isolated ammonium were similar to δ 15 N values that were calculated by isotopic mass-balance as the difference between measured N total in original rock and measured N org in kerogen. Assuming that N total = (N org + N ammonium ), these data suggest that no N ammonium was lost during ammonium isolation and N ammonium suffered little, if any isotopic fractionation.

For temperature-programmed pyrolysis mass-spectrometry [START_REF] Boudou | Molecular nitrogen from coal pyrolysis: Kinetic modelling[END_REF][START_REF] Boudou | Nitrogen in aramid-based activated carbon fibers by TPD, XPS and XANES[END_REF], ~5 mg of a powdered sample with a grain size of <80 m was placed in a crucible and heated from 100 to 1450 °C at 30 °C min -1 at atmospheric pressure in He with one volume % Ne at a flow rate of 10 cm 3 min -1 . Product gases were continuously monitored and quantified with a Quadrex 100 Leybold quadrupole mass spectrometer. The N 2 evolution profile was obtained by substracting interfering mass fragments of other gases from the N 2 signal. The system was calibrated using gas mixtures in pure He with 1% Ne. The signal amplitude of each mass was compared to the m/z 20 signal amplitude of Ne. Physically desorbing water from heated samples produced a broad peak, probably due to interactions between polar water molecules and the inner walls of the transfer line between oven and detector. The integrated water peak was used to assess sample moisture content, and hence to correct gas yields with respect to organic carbon or total carbon content on a dry, ash-free basis.

Table 2 Structural assignments of XPS sub-peaks (a) Expressed in eV and calibrated with respect to the maximum principal C 1s XPS sub-peak at 285 eV.

•

A pyridinic nitrogen atom substitutes for a carbon atom in an aromatic ring and is bonded to two carbon atoms. None of these two carbon atoms is bonded to atoms other than carbon or hydrogen; see Fig. 2c (e.g., [START_REF] Pels | Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis[END_REF][START_REF] Casanovas | Origin of the large N 1s binding energy in X-ray photoelectron spectra of calcined carbonaceous materials[END_REF] • Ammonium fixed in clay (see Fig. 6d,f). • N-C3 as described for N-Q1 above [START_REF] Casanovas | Origin of the large N 1s binding energy in X-ray photoelectron spectra of calcined carbonaceous materials[END_REF] • Nitrogen oxide [START_REF] Pels | Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis[END_REF] • N-C3 as described for N-Q1 above [START_REF] Casanovas | Origin of the large N 1s binding energy in X-ray photoelectron spectra of calcined carbonaceous materials[END_REF] • A pyrrolic nitrogen atom substitutes for a carbon atom in a non-aromatic ring and is bonded to one hydrogen atom and two carbon atoms. None of these two carbon atoms is bonded to additional atoms other than carbon or hydrogen; see Fig. 2a (e.g., [START_REF] Pels | Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis[END_REF][START_REF] Casanovas | Origin of the large N 1s binding energy in X-ray photoelectron spectra of calcined carbonaceous materials[END_REF].

• Pyrrolic or pyridinic nitrogen in rings bearing oxygen-containing substituents; see Fig. 2b (e.g., pyridone; [START_REF] Pels | Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis[END_REF], or nitrogen in rings that additionally include an oxygen atom substituting for a carbon atom.

• Pyrrolic or pyridinic nitrogen in rings with attached oxygen-containing substituents (e.g. pyridone), or an oxygen atom substituting for a carbon in the ring.

• Amine

• Pyridinic nitrogen associated with an adjacent or nearby located -OH group forming an H-bridge [START_REF] Kelemen | Thermal chemistry of nitrogen in kerogen and low-rank coal[END_REF] • Ammonium salt • N-C3 as above in "Metamorphic nitrogen moieties".

• Quaternary/cyclazine-type nitrogen (named N-C3) where a nitrogen atom substitutes for a carbon atom within a condensed, partially aromatic system. The positively charged or neutral sp 3 -coordinated nitrogen atom is bonded to three carbon atoms. None of these three carbon atoms is bonded to additional atoms other than carbon or hydrogen; see Fig. 2d, e, f [START_REF] Pels | Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis[END_REF][START_REF] Casanovas | Origin of the large N 1s binding energy in X-ray photoelectron spectra of calcined carbonaceous materials[END_REF][START_REF] Gammon | Experimental comparison of N(1s) X-ray photoelectron spectroscopy binding energies of hard and elastic amorphous carbon nitride films with reference organic compounds[END_REF]

X-ray Photoelectron Spectroscopy

During sample preparation for N 1s XPS analysis it is advantageous that N org is less readily oxidized than carbon [START_REF] Babich | Nature of nitrogen specie in coke and their role in NO x formation during FCC catalyst regeneration[END_REF]. In order to minimize surface oxidation artifacts, metamorphic rocks were freshly ground to <80 μm particle size in an automated agate mill. Powdered samples were pressure-mounted onto conducting indium foil. XPS signals were recorded using VG Scientific ESCALAB 220XL or 250 systems equipped with monochromated Al Kα X-ray sources (1486.6 eV, ~1 mm or 650 μm spot size, 200 W) and magnetic immersion lenses which focus electrons emitted from the sample over a cone of up to 45° into the main lens column, thus enhancing sensitivity by several orders of magnitude. Spectra were acquired in constant analyzer energy mode, with pass energies of 150 and 40 eV for survey and narrow regions, respectively. The C 1s level binding energy (285 eV at the maximum of the principal C 1s component) was used to calibrate the binding energy scale. Binding energy values were determined with a precision of ± 0.1 eV. Binding energy values were not subject to charging effects, making it unnecessary to utilize the charge compensation system (i.e., a combination of electron and argon flood guns). After subtraction of background and satellite contributions, N 1s envelopes were curve-fitted with the minimum number of mixed Gaussian-Lorentzian component profiles. The Gaussian-Lorentzian mixing ratio (typically 20% Lorentzian and 80% Gaussian functions), the full width at half maximum, and the positions and intensities of peaks were left unconstrained to result in a best fit.

Chemical-structural assignments of N 1s XPS sub-peaks were guided by known binding energies of nitrogen in model compounds and purified substrates (studies cited in Table 2; NIST XPS database). Table 2 characterizes assignments of N-6, N-5, N-Q1, and N-X XPS peaks to structural N org moieties. Ammonium salt is typically absent or negligible in bituminous and sub-bituminous coals. However, since ammonium salt traces may exhibit an N 1s XPS sub-peak at about -401.4 eV, selected powdered samples were treated with HF/HCl before XPS analysis to reduce the possible contribution from N min when the N-Q1 (N-C 3 ) subpeak was small (HF/HCl-treated samples are identified in Table 1). The N-Q2 XPS peak for fixed ammonium occurs at relatively high binding energy (Table 2).

RESULTS AND DISCUSSION

Elemental, rank, and isotopic characterization

Maturity (rank), elemental composition, carbon and nitrogen stable isotope ratios, and the origin of samples are compiled in Table 1. Vitrinite reflectance R max (%) is one of the best measures of low-metamorphic grade (e.g., [START_REF] Frey | Low-Grade Metamorphism[END_REF]. Our data are plotted both as a function of R max and C org (wt. % on a dry, ash-free basis) because studies on nitrogen during coal/kerogen maturation typically relate changes in nitrogen chemistry to C org content. Type II kerogens show higher N org /C org ratios than type III kerogens at comparable maturities, and the N org /C org ratio of type III kerogen decreases with increasing C org (Fig. 3a) and R max (Fig 3b).

A cross-plot of δ 15 N org versus δ 13 C org values (Fig. 4a) distinguishes shale kerogens from coal kerogens, with mean δ 15 N org values of ~3‰ for NW German coal fields and ~5‰ for North American fields. Although our sample series cover a large range of ranks, δ 13 C org and δ 15 N org values for type II and type III kerogens do not express strong shifts with increasing maturity within each type of kerogen (Fig. 4b), in agreement with earlier δ 13 C org studies [START_REF] Galimov | C 13 /C 12 in kerogen[END_REF][START_REF] Lewan | Stable carbon isotopes of amorphous kerogens from Phanerozoic sedimentary rocks[END_REF].  13 C org and  15 N org values of type I kerogen from bright shungite are in agreement with previously published values [START_REF] Melezhik | Karelian shungite -an indication of 2.0-Ga-old metamorphosed oil-shale and generation of petroleum: geology, lithology and geochemistry[END_REF][START_REF] Mastalerz | Organic and mineral matter in a Precambrian shungite deposit from Karelia, Russia. In Organic Matter and Mineralisation: Thermal Alteration, Hydrocarbon Generation, and Role in Metallogenesis[END_REF][START_REF] Verchovsky | Nitrogen isotopes in shungites[END_REF].  15 N org values of type II kerogens (Fig. 4a) are typical for black shales where  15 N is usually between -3 and +2‰ (e.g., [START_REF] Rau | 15 N/ 14 N variations in Cretaceous Atlantic sedimentary sequences: Implication for past changes in marine nitrogen biogeochemistry[END_REF][START_REF] Dumitrescu | Compositional and isotopic characteristics of organic matter for the early Aptian Oceanic Anoxic Event at Shatsky Rise, ODP Leg 198[END_REF][START_REF] Meyers | Paleoceanographic and paleoclimatic similarities between Mediterranean sapropels and Cretaceous black shales[END_REF]. This study's  15 N org values of type III kerogens fall into the upper range of published  15 N values for coals (~0 to +5‰; Boudou et al., 1984a;[START_REF] Burchill | Variation of nitrogen content and functionality with rank for some UK bituminous coals[END_REF][START_REF] Whiticar | Stable isotope geochemistry of coals, humic kerogens and related natural gases[END_REF]Ader et al., 1998a;[START_REF] Rimmer | Influence of maceral content on δ 13 C and δ 15 N in a Middle Pennsylvanian coal[END_REF].

Deconvolution of the N 1s XPS spectrum: Sub-peaks of inorganic and organic N

Inorganic nitrogen: N 1s XPS of ammonium intercalated in clay

In anthracite and meta-anthracite, ammonium may occur in embedded fine laminae of ammonium-illite in the organic matrix, as documented by scanning electron microscopy images and energy-dispersive X-radiography (SEM-EDX) of meta-anthracite 2C that is typical for the Western Middle Anthracite Field, Pennsylvania [START_REF] Daniels | Nature and origin of minerals in anthracite from eastern Pennsylvania[END_REF]Ader et al., 1998a). The specific N 1s XPS spectral contribution of NH 4 + in bulk coal or rock samples is documented in synoptically shown spectra in Fig. 5c-f. Ammonium is abundant in bulk carbonaceous shale 19J (Fig. 5c,d). Ammonium-rich clay is also evident in the XPS spectra of low-temperature plasma-ashed residue of meta-anthracite 16H-LTA (Fig. 5e,f) where a small amount of Pocahontas #3 coal had been admixed because some un-oxidized carbon is needed for internal calibration of the binding energy scale. Pocahontas #3 coal was chosen because its N 1s XPS spectrum displays a small N-Q1 and weak N-Q2 and N-X sub-peaks (Fig. 6b). The N 1s XPS spectra of bulk shale 19J (Fig. 5d) and 16H-LTA (Fig. 5f) feature a main Lorentzian-Gaussian sub-peak N-Q2 around 402.7 eV reflecting ammonium in illite that had also been identified by X-ray diffraction [START_REF] Daniels | Nature and origin of minerals in anthracite from eastern Pennsylvania[END_REF]. The presented N 1s XPS evidence for an ammonium contribution to sub-peak N-Q2 is corroborated by studies of [START_REF] Buckley | Inorganic nitrogen in Australian semi-anthracites; implications for determining organic nitrogen functionality in bituminous coals by X-ray photoelectron spectroscopy[END_REF][START_REF] Buckley | Heteroatom functionality in a high-sulfur Chinese bituminous coal[END_REF] and [START_REF] Gong | XPS determination of the forms of nitrogen in coal pyrolysis chars[END_REF]. The use of demineralized kerogens in our study eliminated mineral-hosted ammonium and facilitated the deconvolution of N 1s XPS spectra in terms of the exclusive presence of organic nitrogen. (c,d) the same bulk carbonaceous shale Bernice 19J containing ammonium illite; and (e, f) the inorganic residue 16H-LTA from low-temperature plasma ashing of meta-anthracite 16H; the ash was mixed with a

f K 2p 1/2 K 2p 3/2
Residue from low-temperature ashing of ammonium illite-containing meta-anthracite 16H with some Pocahontas #5 coal added

N-X N-Q1 N-Q2 + 4 Illite intercalated NH C 1s K 2p 1/2 K 2p 3/2 K ions in NH illite + + 4
Bulk carbonaceous shale Bernice 19J containing ammonium illite; R 6.30% max C 1s Relative intensity

N-6 N-5 N-Q1 N-Q2 N-X
d b c a e C 1s N-6 N-5 N-Q2 N-X N-Q1 K ions in NH illite + + 4 + 4
Illite intercalated NH C 1s and K 2p XPS N 1s XPS small amount of Pocahontas coal #3 because some carbon is needed for calibration of the binding energy scale.

Fig. 6. N 1s XPS spectra with curve-fitted sub-peaks for four selected samples from low to high maturity (see Table 1 for preparation methods). Some sub-peaks are associated with exemplary chemical structures characterizing nitrogen moieties that are mainly responsible for observed XPS signals.

Sub-peak N-Q1: Organic N-C 3

The N 1s XPS sub-peak N-Q1 with a mean binding energy of 401.6 eV and mean full width at half-maximum (FWHM) = 1.6 eV is assigned to N-C 3 where each N atom is bonded covalently to three C atoms (Figs. 2, 6 and 7). The higher binding energy of N-C 3 relative to two-coordinated nitrogen (i.e., each N atom bonded to two carbon atoms, as in pyridine and pyrrole rings; Table 2) is partly attributed to electron charge transfer from C to N [START_REF] Boutique | 5,11,13-tetraazacycl[3.3.3azine: Theoretical (ab initio) and experimental (Xray and ultraviolet photoelectron spectroscopy) studies of the electronic structure[END_REF]. The predominant association of N-Q1 with N-C 3 [START_REF] Pels | Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis[END_REF] in metamorphic samples is indicated by a strong increase in the relative abundance of N-Q1 with rank increase (Figs. 7e, f, 8c, d,) after hydroxyl and carboxyl groups have long been eliminated [START_REF] Blom | Chemical structure and properties of coal. XVIII. Oxygen groups in coal and related products[END_REF][START_REF] Van Krevelen | Coal[END_REF]. The resulting patterns over maturity depend on the proxy for rank; the C org content is appropriate for samples with lower maturity, whereas the use of R max is preferred throughout metamorphism to a semi-graphitic stage. Each organic nitrogen moiety is quantified in terms of the area of its diagnostic XPS sub-peak N-6, N-5, or N-Q1 relative to the overall XPS N 1s signal area.

The organic carbon content C org is expressed as weight % on a dry, ash-free basis (daf). Vitrinite reflectance is given as R max (%). Solid lines are drawn to guide the eye. Dashed gray lines indicate hypothetical trends for type II kerogen, assuming that the evolutionary paths of type II and III kerogens converge during metamorphism. Panels (b), (d) and (f) also offer exemplary chemical structures characterizing nitrogen moieties that are mainly responsible for observed XPS signals.

Trace amounts of quaternary nitrogen in the pre-metamorphic stage reflect a form of pyridinic nitrogen associated with adjacent or nearby hydroxyl or carboxyl groups that are protonated via formation of H-bridges [START_REF] Kelemen | Thermal chemistry of nitrogen in kerogen and low-rank coal[END_REF] see structural example in Table 2). Quaternary nitrogen XPS peaks in some materials may also reflect the presence of ammonium salts [START_REF] Gong | Identification of inorganic nitrogen in an Australian bituminous coal using X-ray photoelectron spectroscopy (XPS) and timeof-flight secondary ion mass spectrometry (TOFSIMS)[END_REF]; NIST XPS database) and protonated amines (e.g., [START_REF] Clark | An experimental and theoretical investigation of the core level spectra of a series of amino acids, dipeptides and polypeptides[END_REF][START_REF] Vidyadhar | Mechanisms of amine-feldspar interaction in the absence and presence of alcohols studied by spectroscopic methods[END_REF], although this has limited relevance for coals and kerogens where these species are essentially absent. . Temperature-programmed pyrolysis of coal at temperatures up to 1450 °C produces nitrogen (N 2 ), methane (CH 4 ), and hydrogen (H 2 ). Data from a coal reference series [START_REF] Boudou | Molecular nitrogen from coal pyrolysis: Kinetic modelling[END_REF] exhibit distinct patterns of gas yields that are expressed in mg of CH 4 , N 2 and H 2 per 100 mg of total organic carbon content in the analyzed sample. Thermal maturity is expressed as vitrinite reflectance R max (%).

Fig. 9. The concentration of organic nitrogen N org moieties (in weight % on a dry, ash-free basis, daf) in a series of type III kerogens is plotted relative to (a) organic carbon content (C org , wt. % daf) and (b) vitrinite reflectance R max (%). The resulting patterns over maturity depend on the proxy for rank; the C org content is appropriate for samples with lower maturity, whereas the use of R max is preferred throughout metamorphism to a semi-graphitic stage. Pyridinic, pyrrolic, and N-C 3 were quantified via their diagnostic XPS sub-peaks N-6, N-5, and N-Q1, respectively, relative to the overall XPS N 1s signal and the known total nitrogen content (determined via elemental analysis). Panels (c) and (d) express N-Q1-related data in more detail and associate distinct peaks with specific N org moieties relating to increasing thermal maturity. Solid lines are drawn to guide the eye. The dashed line in panel (c) represents a theoretical slow increase in N org that is covalently bonded within condensed, partially aromatic systems (chemical-structural explanations offered in Table 2). In contrast to N 1s XPS sub-peaks N-6, N-5 and N-Q1, the chemical-structural assignment of the weak high-binding energy sub-peaks >402 eV still remain unresolved (Fig. 6). Candidates for generating the N-Q2 sub-peak at 402.7 eV (FWHM = 1.5 eV) and the N-X sub-peak at 403.6 eV (FWHM = 1.5 eV) are (i) pyridinic nitrogen atoms bonded to oxygen atoms, (ii) nitrogen substituting for carbon in condensed, partially aromatic systems (a higher nitrogen binding energy would arise both from charge transfer and hybridization; [START_REF] Casanovas | Origin of the large N 1s binding energy in X-ray photoelectron spectra of calcined carbonaceous materials[END_REF], and (iii) ammonium fixed in clays (Fig. 5; [START_REF] Buckley | Inorganic nitrogen in Australian semi-anthracites; implications for determining organic nitrogen functionality in bituminous coals by X-ray photoelectron spectroscopy[END_REF][START_REF] Buckley | Heteroatom functionality in a high-sulfur Chinese bituminous coal[END_REF][START_REF] Gong | XPS determination of the forms of nitrogen in coal pyrolysis chars[END_REF] where the outermost sheet is composed of oxygen anions inducing a particular local interlayer environment, in contrast to simple ammonium salts [START_REF] Yu | The preparation and characterization of cetyltrimethylammonium intercalated muscovite[END_REF][START_REF] He | A X-ray photoelectron spectroscopy study of HDTMAB distribution within organoclays[END_REF]. Kerogen does not contain any silicate minerals that can host ammonium, therefore the N 1s XPS N-Q2 and N-X sub-peaks of kerogen from the meta-anthracitecontaining shale 19J cannot be caused by clay-bound NH 4 + (Fig. 5b). The corresponding potassium K 2p XPS spectrum confirms the absence of illite (Fig. 5a). The N-X sub-peak in Fig. 5b may relate to organic R=N-O-R where nitrogen is linked to carbon (R) and to one oxygen atom (e.g., [START_REF] Gong | XPS determination of the forms of nitrogen in coal pyrolysis chars[END_REF][START_REF] Xiao | Reactions of nitrogen and oxygen surface groups in nanoporous carbons under inert and reducing atmospheres[END_REF].

Although anthracites contain a network of closed micropores [START_REF] Fryer | The micropore structure of disordered carbons determined by high resolution electron microscopy[END_REF][START_REF] Mahajan | Physical characterization of coal[END_REF][START_REF] Daulan | Influence of anthracite pretreatment in the preparation of activated carbons[END_REF][START_REF] Radlinski | Application of SAXS and SANS in evaluation of porosity, pore size distribution and surface area of coal[END_REF]) that may originally contain some nitrogen gas, [START_REF] Boudou | Molecular nitrogen from coal pyrolysis: Kinetic modelling[END_REF] showed that elemental nitrogen is removed from micropores by crushing and evacuation. The mean relative peak areas of N-Q2 and N-X are ~9% to 10% and ~6% to 7% of the total N 1s XPS area, respectively. The presence of N-Q2 and N-X sub-peaks in kerogen from carbonaceous shale 19J (Fig. 5b) documents that these sub-peaks can largely originate from nitrogen species other than NH 4 + in minerals. In agreement with the presence of significant sulfur S 2p XPS peaks relating to sulfate, relatively low-rank New Albany Shale type II kerogens exhibit N-Q2 and N-X sub-peaks that could result from oxidation during preparation and storage rather than from a significant abundance in N-C 3 .

Sub-peak N-6: Contribution from pyridinic nitrogen

Sub-peak N-6 corresponds to nitrogen expressing a mean binding energy of 398.8 eV with a FWHM = 1.6 eV. It is associated with pyridinic nitrogen (Fig. 6), as well as with a few other nitrogen species, possibly including ammonia NH 3 . Good accuracy of the binding energy calibration of our N 1s spectra is indicated by the gap Δ|N 1s -C 1s| between the binding energies of pyridinic nitrogen and carbon when the C 1s signal is calibrated to 285 eV at the maximum of the C 1s envelope. Pyridinic nitrogen atoms are bonded only to two C atoms in an aromatic π-electron system. The observed 113.8 eV offset Δ|N 1s -C 1s| is in agreement with reported values for pyridinic model compounds, such as 3,5,11,13tetraazacycl-[3,3,3]-azine or pyrolysates of acridine, 9-aminoacridine, 9-cyanoanthracene, 2aminoanthracene, poly(vinylpyrrolidone), and phenazine (reviewed by [START_REF] Lahaye | Chemical transformation during the carbonisation in air and the pyrolysis under argon of a vinylpyridine-divinylbenzene copolymer by X-ray photoelectron spectroscopy[END_REF]. Temperature-programmed pyrolysis of anthracites demonstrated the absence of NH 3 in closed micropores, although traces of N 2 were found in meta-anthracite [START_REF] Boudou | Molecular nitrogen from coal pyrolysis: Kinetic modelling[END_REF]. During the pre-metamorphic stage, the relative importance of the N-6 sub-peak area (as a percentage of the total N 1s XPS peak area) increases up to a rank corresponding to ~85 wt. % carbon (Fig. 7a). During subsequent anthracitization-graphitization, the relative abundance of N-6 nitrogen strongly decreases (Fig. 7b). This metamorphic decrease of N-6 parallels the decline of the atomic H/C ratio [START_REF] Van Krevelen | Coal[END_REF] and lowered yields of molecular hydrogen during temperature-programmed pyrolysis (Fig. 8).

Sub-peak N-5: Pyrrolic and pyridinic rings with or without oxygen-containing substituents

N 1s XPS sub-peak N-5 corresponds to nitrogen expressing a mean binding energy of 400.6 eV with FWHM = 1.7 eV (Fig. 6). Its intensity hardly varies during the premetamorphic stage until C org ~ 90 wt. %, followed by a strong decrease above R max 2% during anthracitization-graphitization (Figs. 9c,8a). The N-5 signal has been associated with three types of organic nitrogen. (i) Pyrrolic N atoms are bonded to one H atom and two C atoms in a non-aromatic or partially aromatic ring, with or without oxygen-containing substituents. (ii) Aromatic pyridinic rings contain N atoms in addition to oxygen-containing substituents (e.g., hydrated pyridinic rings, pyridone and pyridine carboxylic acids; defined by [START_REF] Mitra-Kirtley | Nitrogen chemistry of kerogens and bitumens from X-ray absorption near-edge structure spectroscopy[END_REF][START_REF] Zhu | Determination of the fate of nitrogen functionality in carbonaceous materials during pyrolysis and combustion using X-ray absorption near edge structure spectroscopy[END_REF][START_REF] Vairavamurthy | Organic nitrogen in geomacromolecules: Insights on speciation and transformation with K-edge XANES spectroscopy[END_REF][START_REF] Schnadt | Structural study of adsorption of isonicotinic acid and related molecules on rutile TiO 2 (110) II: XPS[END_REF]. Oxygencontaining substituents, such as hydroxyl groups, may provide chemically active sites for ring opening and subsequent elimination of N org [START_REF] Katritzky | Aqueous high-temperature chemistry of carbo-and heterocycles. 30[END_REF][START_REF] Siskin | A review of the reactivity of organic compounds with oxygen-containing functionality in superheated water[END_REF]. (iii) Amine and amide moieties may be present in less mature coals and kerogens [START_REF] Kelemen | Thermal transformations of nitrogen and sulfur forms in peat related to coalification[END_REF]. Within the accuracy of XPS measurements, pyridone nitrogen cannot be distinguished from pyrrolic nitrogen [START_REF] Zhu | Determination of the fate of nitrogen functionality in carbonaceous materials during pyrolysis and combustion using X-ray absorption near edge structure spectroscopy[END_REF]. Similar to pyridinic nitrogen, these reactive nitrogen species are primarily located along the edge of condensed and partially aromatic systems.

Organic nitrogen loss during low-grade metamorphism

The scope and goals of this study of metamorphic transformation of kerogen and carbonaceous matter justify the reduced emphasis on kerogens of type I (i.e., lipid-rich organic matter from aquatic organisms) and type II, because with increasing thermal maturity the geochemical properties of different types of kerogens approach each other and eventually become indistinguishable. For example, at high rank with C org >97 wt. % and an atomic H/C ratio <0.4, the composition of carbonaceous organic matter is narrowly predicted by van Krevelen's (1961) H/C and O/C diagrams, regardless of prior kerogen maturation paths of types I, II, or III [START_REF] Tissot | Influence of nature and diagenesis of organic matter in formation of petroleum[END_REF][START_REF] Durand | Elemental analysis of kerogens (C, H, O, N, S, Fe)[END_REF]. This principle is evident in Fig. 3 where shungite type I kerogen is very similar to type III kerogens from coals of comparable rank.

At high rank, the carbonaceous matter includes insoluble kerogen and small amounts of extractable bitumen. Kerogen becomes increasingly aromatic whereby smaller aromatic ring structures condense to larger polyaromatic clusters that eventually mimic nanographitic sheets and eventually approach a well-ordered graphitic structure [START_REF] Oberlin | Carbonization and graphitization[END_REF][START_REF] Beyssac | Graphitization in a high pressure low-temperature metamorphic gradient: A Raman microspectroscopy and HRTEM study[END_REF].

The thermal evolution of atomic N org /C org ratios of type III kerogen can be divided into three stages when viewed as a function of the C org concentration (Fig. 3a), or of vitrinite reflectance R max (Fig. 3b). First, a relative enrichment of N org in kerogen occurs due to preferential loss of carbon, hydrogen and oxygen during diagenesis and maturation into the early oil window corresponding to C org below ~85 wt. % daf (Figs. 1,3a). Subsequent loss of N org from kerogen results in decreasing N org /C org ratios from the end of the oil window to the onset of the dry gas window of thermogenic methane generation (corresponding to R max ~2%;Figs. 1, 3b;[START_REF] Boudou | Molecular nitrogen from coal pyrolysis: Kinetic modelling[END_REF]. Finally, the observation of very low N org contents in semi-graphite and graphite [START_REF] Ader | Nitrogen isotopic evolution of carbonaceous matter during metamorphism: Methodology and preliminary results[END_REF] suggests extensive loss of N org during semigraphitization and graphitization. Our N org /C org data support an earlier conceptual evolutionary path for N org in type III kerogen where an early phase of relative N org accumulation is followed by a phase of metamorphic N org loss (Boudou et al., 1984a, b;[START_REF] Boudou | Molecular nitrogen from coal pyrolysis: Kinetic modelling[END_REF]. Our findings are also corroborated by recent data from German Creek and Moranbah Coal measures of the Bowen Basin in Australia where total nitrogen concentrations start a significant decrease at R max ~2% [START_REF] Ward | Variations in coal maceral chemistry with rank advance in the German Creek and Moranbah Coal Measures of the Bowen Basin, Australia, using electron microprobe techniques[END_REF].

Mechanism for organic nitrogen loss without isotope fractionation

In spite of a significant loss of N org (Fig. 3), the natural maturation path of type III kerogens does not produce large  15 N org variation or trends over a wide range of vitrinite reflectance values (Fig. 4b). Limited or absent  15 N org isotope fractionation with thermal maturation has been observed in previous studies of type III kerogen (Boudou et al., 1984a, b;[START_REF] Rigby | The isotopic composition of nitrogen in Australian coals and oil shales[END_REF][START_REF] Whiticar | Stable isotope geochemistry of coals, humic kerogens and related natural gases[END_REF]Ader et al., 1998a[START_REF] Ader | Nitrogen isotopic evolution of carbonaceous matter during metamorphism: Methodology and preliminary results[END_REF]. The limited  15 N org data for type I and II kerogens determined in this study are insufficient to reach the same conclusions as for type III kerogen (Table 1). Fig. 10. Proposed idealized transformations of organic nitrogen during low-temperature maturation and metamorphism: Hot H, O-containing fluids or some of their components gradually penetrate toward the core of a dense and ultra-microporous carbonaceous matrix. The resulting reactions between fluid components and organic nitrogen lead to the elimination of organic nitrogen along a reaction front, generating a nitrogendepleted peripheral reacted zone (black). Liberated ammonia and some elemental nitrogen diffuse out of the carbonaceous matrix. Before the reaction front can reach deeply embedded 'core' nitrogen in the interior of the carbonaceous matrix (grey part of rectangles), an increasing fraction of core nitrogen chemically stabilizes as single nitrogen atoms substituting for carbon in condensed and partially aromatic systems (i.e., N-C 3 ). With ongoing thermal chemical evolution, the chemical speciation of the remaining bulk organic nitrogen changes distinctly. Pie charts indicate average relative abundances of pyridinic N, pyrrolic N, and N-C 3 nitrogen substituting for carbon inside the condensed and partially aromatic systems for late catagenesis, late anthracitization, and graphitization. Temperature ranges are adopted from estimates for Pennsylvania's anthracite fields and other ones (see Fig. 1; [START_REF] Hower | Mechanisms of coal metamorphism: Case studies from Paleozoic coalfields[END_REF]and refs. therein). Several reasons may account for the conservative character of δ 15 N org during thermal evolution. (i) Some nitrogen-containing heterocycles express remarkable thermal stability, especially structures containing N-C 3 . (ii) All isotope fractionations tend to decrease with increasing temperature. However, even high pyrolysis temperatures in the laboratory are unable to eliminate isotopic fractionation during thermal disproportionation of N org from coal (e.g., [START_REF] Stiehl | Isotopenvariationen des Stickstoffs humoser und bituminöser natürlicher organischer Substanzen[END_REF]. (iii) Nitrogen isotope exchange between N org in source rocks and NH 4 + in aqueous fluids has been observed in hydrous pyrolysis experiments [START_REF] Schimmelmann | Chitin: 'Forgotten' source of nitrogen -From modern chitin to thermally mature kerogen: Lessons from nitrogen isotope ratios[END_REF] and buffers δ 15 N org values if the available pool of NH 4 + is large relative to that of N org . Formation fluids contain abundant and mobile aqueous NH 4 + (e.g., [START_REF] Manning | Distribution and mineralogical controls on ammonium in deep groundwaters[END_REF] until metamorphism reaches stability fields where ammonium ions and ammonia are no longer stable in the free fluid phase (e.g., [START_REF] Eugster | Ammonium silicate stability relations[END_REF][START_REF] Andersen | N 2 and CO 2 in deep crustal fluids: evidence from the Caledonides of Norway[END_REF]. The effect of isotopic buffering of δ 15 N org would extend into the temperature range of metamorphism if NH 3 or NH 4 + can penetrate into ultramicroporous carbonaceous grains. Regardless of the contributing factors, the observed small variance of δ 15 N org before and during metamorphism (Fig. 4b) in the presence of a large and progressive loss of N org (Fig. 3) calls for essentially non-fractionating mechanisms of N org elimination.

Late Catagenesis

Increasing metamorphic grade

Simple thermal degradation would isotropically affect all sites of a carbonaceous solid phase simultaneously and would likely entail Rayleigh-type isotope fractionation in the residual N org , as observed during experimental coal pyrolysis (e.g., [START_REF] Stiehl | Isotopenvariationen des Stickstoffs humoser und bituminöser natürlicher organischer Substanzen[END_REF] or oil formation (e.g., [START_REF] Oldenburg | Nitrogen isotope systematics of petroleum fractions of differing polarity -Neutral versus basic compounds Org[END_REF]. The mechanism of our proposed nonfractionating loss of N org during metamorphism is depicted in Fig. 10. In brief, the overall isotope ratio of N org in the carbonaceous matrix remains constant when N org is eliminated along a reaction front that proceeds through the carbonaceous matrix. Progress along the reaction front in the absence of overall isotope fractionation requires (i) diffusive transport of hot H, O-containing fluids or some of their components towards the interior of the carbonaceous matrix, (ii) a relatively fast and essentially complete conversion of N org to inorganic nitrogen species along the reaction front, and (iii) diffusive removal of reaction products NH 4 + and N 2 out of the reacted zone. In other words, essentially all N org is located in the unreacted core of the carbonaceous matrix and its δ 15 N org value is relatively constant and equal to bulk δ 15 N org . The following sections will discuss details about the stabilization of N org as N-C 3 in the core of the carbonaceous matrix, and the role of hot fluids in the chemical transformation of N org .

Increase of N-C 3 substituting for carbon in condensed, partially aromatic systems, at the expense of edge-located organic nitrogen

This study provides the first direct evidence for organic nitrogen atoms substituting for "graphitic" carbon in natural metamorphic organic matter. With increasing thermal maturity, chemically less stable N org functional groups and N-containing heterocycles are eliminated or transformed to more stable moieties. These chemical changes with increasing rank are reflected by changes in the relative areas of N 1s XPS sub-peaks (Fig. 7). The XPS signals from distinct N org moieties can also be expressed in terms of weight percent of participating nitrogen as a function of rank (Fig. 9).

Our data indicate that, at the metamorphic onset at C org ~90 wt. % (daf) and R max 2%, N org is mainly located along the outer edge of condensed, partially aromatic systems in pyrrolic and pyridinic functional groups (corresponding to N-6 and N-5 sub-peaks; Fig. 7a-d). With increasing metamorphic grade, the percentage of N-C 3 strongly increases (N-Q1 sub-peak; Fig. 7e,f; Fig. 9c,d). [START_REF] Burchill | Variation of nitrogen content and functionality with rank for some UK bituminous coals[END_REF]. (a) Re-drafted data with original trend lines from [START_REF] Burchill | Variation of nitrogen content and functionality with rank for some UK bituminous coals[END_REF] original Figure 3, where N 1s XPS-quantified relative abundances of total, pyridinic, and "pyrrolic" nitrogen are plotted against the carbon content on a 'dry, mineral matter free' (dmmf) basis. The same XPS data for "pyrrolic" and pyridinic nitrogen are expressed in terms of relative abundances (% of the total XPS signal) along one common Y-axis (b) and using two separately scaled Y-axes for visualization of "mirror trends" (c), with newly assigned trend lines. (d) This study's new data on coals with increasing organic carbon content (i.e., with increasing thermal maturity, R max in %) indicate that Burchill and Welch's (1989) so-called "pyrrolic" nitrogen actually represents a combination of the XPS subpeaks N-5 (pyrrolic, pyridonic, and other nitrogen species) and N-Q1 (N-C 3 ). The trend line of the (N-5 + N-Q1) summary peak area approximates the mirror image of the trend line for pyridinic nitrogen N-6. Graph (d) is directly comparable to [START_REF] Burchill | Variation of nitrogen content and functionality with rank for some UK bituminous coals[END_REF] data shown in (c) because 'carbon weight % on a dry, mineral matter free basis' closely approximates the total organic carbon content on a dry, ash free (daf) basis in coals. Lines are drawn to guide the eye.

The strong increase of N-C 3 during natural low-grade metamorphism from anthracite to semi-graphite occurs at the expense of pyridinic and pyrrolic/pyridonic nitrogen (Fig. 7). Our observations need to be reconciled with [START_REF] Burchill | Variation of nitrogen content and functionality with rank for some UK bituminous coals[END_REF] pioneering N 1s XPS study of coals from the British Isles where a figure (re-drawn here as Fig. 11a) depicts contrasting trends of XPS sub-peak abundances of only two species, namely pyridinic and "pyrrolic" nitrogen. The authors combined pyrrolic and N-C 3 sub-peaks into a so-called "pyrrolic" peak and arrived at the perception that "pyrrolic" nitrogen reached its absolute maximum at ~84 wt. % C, whereas pyridinic nitrogen continued to increase in relative abundance into higher maturity until ~90 wt. % C. They concluded that "pyrrolic" nitrogen was predominant, although the fraction of pyridinic nitrogen increased with rank and seemed to be more stable during later stages of coalification. However, by normalizing [START_REF] Burchill | Variation of nitrogen content and functionality with rank for some UK bituminous coals[END_REF] "pyrrolic" and pyridinic XPS areas to the total N 1s XPS area (Fig. 11b), a Ushaped "pyrrolic" nitrogen curve becomes apparent with increasing concentrations along anthracitization, in contrast to the pattern of decreasing values for pyrrolic (N-5) alone (Fig. 7c). Our new data plotted in the same fashion (Fig. 11d) express a similar pattern as in Fig. 7c and thus indicate that Burchill and Welch's (1989) so-called "pyrrolic" nitrogen actually represents a combination of the XPS sub-peaks N-5 and N-Q1 (N-C 3 ). The trend line of the (N-5 + N-Q1) summary peak area approximates the mirror image of the trend line for pyridinic nitrogen N-6. We conclude that separate integrations of [START_REF] Burchill | Variation of nitrogen content and functionality with rank for some UK bituminous coals[END_REF] N-5 and N-Q1 sub-peaks bring their data into agreement with ours suggesting that similar trends as those described in this study can be found independently in other sample series. [START_REF] Kelemen | Quantification of nitrogen forms in Argonne premium coals[END_REF][START_REF] Kelemen | Thermal transformations of nitrogen and sulfur forms in peat related to coalification[END_REF] have shown that the pre-metamorphic increase in the relative abundance of pyridinic nitrogen may be due to transformations of amide, amine and protonated pyridinic structures (i.e., quaternary nitrogen) during peat, lignite and subsequent bituminous coal stages. Overall, the presented evidence points toward universal trends in N org speciation with increasing rank along a continuous introduction of N atoms into condensed ring systems as N-C 3 , at the expense of edge-located N.

Assuming that the formation of N-C 3 is controlled kinetically, one can expect that this nitrogen species accumulates slowly rather than appearing suddenly as a result of a "maturation jump" at the onset of anthracitization. We propose a continuous pre-metamorphic increase in N-C 3 (N-Q1) that is expressed as a dashed line in Fig. 9c for low-temperature maturation. Our hypothesis is supported by the fact that even in modern and immature sediments, small amounts of naturally occurring condensed aromatic ring structures can be found that have been linked to wildfires, anthropogenic contamination, and catalytic/enzymatic processes (e.g., [START_REF] Delfourne | Structure revision of the marine pentacyclic aromatic alkaloid: Cystodamine[END_REF][START_REF] Silliman | A hypothesis for the origin of perylene based on its low abundance in sediments of Green Bay[END_REF][START_REF] Finkelstein | Can biomass burning produce a globally significant carbon-isotope excursion in the sedimentary record? Earth Planet[END_REF][START_REF] Hockaday | The transformation and mobility of charcoal in a fire-impacted watershed[END_REF]. It is likely that some of these condensed ring systems contain covalently bonded nitrogen. Nitrogen-containing and partially aromatic compounds may also form spontaneously in humic compounds at low temperature [START_REF] Thorn | Ammonia fixation by humic substances: a nitrogen-15 and carbon-13 NMR study[END_REF][START_REF] Kraus | Carbon and nitrogen dynamics in a forest soil amended with purified tannins from different plant species[END_REF][START_REF] Hsu | New evidence for covalent coupling of peptides to humic acids based on 2D NMR spectroscopy: A means for preservation[END_REF] or may have a catalytic origin [START_REF] Jokic | Integration of the polyphenol and Maillard reactions into a unified abiotic pathway for humification in nature: the role of δ-MnO 2[END_REF]. Several studies report the occurrence of nitrogen-containing aromatic systems in brown coals [START_REF] Imuta | Isolation of adamantane from coal extract[END_REF][START_REF] Chaffee | Polycyclic aromatic hydrocarbons in Australian coals. I. Angularly fused pentacyclic tri-and tetraaromatic components of Victorian brown coal[END_REF][START_REF] Chaffee | Polycyclic aromatic hydrocarbons in Australian coals. II. Novel tetracyclic components from Victorian brown coal[END_REF][START_REF] Murata | Structural analysis of coal through RICO reaction: Detailed analysis of heavy fractions[END_REF][START_REF] Kashimura | Evidence of polycondensed aromatic rings in a Victorian brown coal[END_REF]. We expect that these nitrogen species were produced by aromatization of inherited or neoformed condensed polycyclic compounds (e.g., terpenoids, steroids, hopanoids, alkaloids; [START_REF] Hayatsu | Thermal catalytic transformation of pentacyclic triterpenoids: Alteration of geochemical fossils during coalification[END_REF] and could secondarily react with organic nitrogen and/or ammonia, nitrogen oxides and nitrates.

Aromatization of isolated polycyclic structures and condensation of aromatic units result in growth of small aromatic clusters throughout catagenesis, as suggested by modeling (e.g., [START_REF] Béhar | Chemical modelling of kerogens[END_REF] and demonstrated by high resolution transmission electron microscopy [START_REF] Neidhardt | Correlated high resolution transmission electron microscopy and X-ray photoelectron spectroscopy studies of structured CNx (0< x <0.25) thin solid films[END_REF]. The increase in the abundance of N-C 3 (N-Q1) during early metamorphism in relative terms (Fig. 7e,f) and in absolute terms (Fig. 9c,d) can be explained by a combination of (i) selective preservation of thermally stable 'core' nitrogen inside aromatic carbon lattices, and (ii) neoformation of N-C 3 in condensed ring systems while edge-located pyrrolic and pyridinic nitrogen would become progressively sparse due to structural rearrangement and chemical elimination. The steep metamorphic decline of pyridinic nitrogen (N-6; Fig. 7a,b) could be due to the tendency of aromatic rings to condense and form small polyaromatic clusters. In contrast, the comparatively more delayed metamorphic decrease of pyrrolic/pyridone nitrogen (N-5; Fig. 7c,d) may be explained by (i) a conversion of pyridone to pyrrole [START_REF] Brent | Pyrolysis of 2-pyrone, coumarin, and 2pyridone[END_REF][START_REF] Schmiers | Change of chemical bonding of nitrogen of polymeric N-heterocylcic compunds during pyrolysis[END_REF], and (ii) by the difficulty of expanding pyrrole-type rings to pyridine-type rings as suitable building blocks for accreting graphitic structures. Few nitrogen concentration data have been published for natural graphite [START_REF] Van Zuilen | Nitrogen and argon isotopic signatures in graphite from the 3.8-Ga-old Isua Supracrustal Belt, Southern West Greenland[END_REF]. [START_REF] Ader | Nitrogen isotopic evolution of carbonaceous matter during metamorphism: Methodology and preliminary results[END_REF] assessed the loss of nitrogen from around thousand ppm in semi-graphites to few ppm in disordered-graphites. We therefore propose that the N-C 3 content increases to reach an intermittent maximum during anthracitization, followed by a final decline towards well-ordered graphite with negligible concentrations of heteroelements associated with crystal lattice defects (Fig. 9c,d).

The role of H, O-containing hot fluids during transformation of organic nitrogen

Experimental dry, open-system programmed pyrolysis of model substances shows that pyridinic, pyrrolic and N-C 3 -type nitrogen species are remarkably stable at high temperature (e.g., [START_REF] Stanczyk | Elimination of nitrogen from coal in pyrolysis and hydropyrolysis -a study of coal and model chars[END_REF][START_REF] Schmiers | Change of chemical bonding of nitrogen of polymeric N-heterocylcic compunds during pyrolysis[END_REF][START_REF] Xiao | Reactions of nitrogen and oxygen surface groups in nanoporous carbons under inert and reducing atmospheres[END_REF][START_REF] Boudou | Nitrogen in aramid-based activated carbon fibers by TPD, XPS and XANES[END_REF]. Modelling with Optkin or 2D Petromod software, for example, justifies the early pyrolytic elimination of only a minor amount of anthracitic N org , even under high heat flow of up to 80 W/cm 2 from the Carboniferous to present time [START_REF] Littke | Molecular nitrogen in natural gas accumulations: Generation from sedimentary organic matter at high temperatures[END_REF], in reality most N org is not eliminated throughout low grade metamorphism [START_REF] Boudou | Molecular nitrogen from coal pyrolysis: Kinetic modelling[END_REF]. In the laboratory under dry conditions, most Norg can only be decomposed at unnaturally high temperatures [START_REF] Everlien | High-temperature programmed pyrolysis of Paleozoic source rocks from Northern Germany and adjacent areas and its thermodynamic constraints[END_REF]. XPS data of solid pyrolysis residues indicate that programmed pyrolysis in the laboratory can reproduce some trends in naturally metamorphosed samples (Fig. 7), for example a decrease in the relative abundance of pyrrolic nitrogen and an increase in the relative abundance of quaternary N-C 3 nitrogen, especially when kerogen type III is used as a starting material [START_REF] Kelemen | Nitrogen transformation in coal during pyrolysis[END_REF][START_REF] Kelemen | Thermal chemistry of nitrogen in kerogen and low-rank coal[END_REF]. Important differences remain between artificial and natural paths of chemical evolution:

(i) Atomic N/C ratios of pyrolysis residues from coals in the laboratory are systematically larger than those found in naturally heated coals, in agreement with kinetic modelling predictions of thermal N org stability in anthracite in the temperature range of natural anthracitization (e.g., [START_REF] Boudou | Molecular nitrogen from coal pyrolysis: Kinetic modelling[END_REF].

(ii) During gradual heating in the laboratory, pyridinic nitrogen experiences only a slight decrease at higher temperatures. In contrast, natural chemical evolution of N org causes significant losses of both pyrrolic and pyridinic nitrogen simultaneously with an exponential relative increase in C-N 3 .

(iii) Laboratory thermal decomposition of organic matter isotopically disproportionates N org and causes 15 N-enrichment in the residue (e.g., [START_REF] Stiehl | Isotopenvariationen des Stickstoffs humoser und bituminöser natürlicher organischer Substanzen[END_REF]. This study provides evidence that natural chemical evolution of N org in type III kerogen causes no significant isotope fractionation in remaining N org (Fig. 4).

A key distinction between natural and artificial paths of N org chemical evolution is the availability of hot water during maturation. Laboratory pyrolysis experiments at high temperatures are typically performed in the absence of water, for example in a flow of inert gas at ambient pressure (e.g., [START_REF] Littke | Molecular nitrogen in natural gas accumulations: Generation from sedimentary organic matter at high temperatures[END_REF][START_REF] Boudou | Molecular nitrogen from coal pyrolysis: Kinetic modelling[END_REF][START_REF] Kelemen | Nitrogen transformation in coal during pyrolysis[END_REF][START_REF] Kelemen | Thermal chemistry of nitrogen in kerogen and low-rank coal[END_REF]. In contrast, deep sedimentary environments provide ubiquitous and abundant hot H, O-containing formation fluids at considerable pressure. In the Pennsylvanian anthracite field, hydrothermal convecting fluids induced by Alleghanian uplift significantly increased [START_REF] Daniels | Nature and origin of minerals in anthracite from eastern Pennsylvania[END_REF][START_REF] Harrison | Stratigraphic control of hot fluids on anthracitization, Lackawanna synclinorium, Pennsylvania[END_REF]. On local to regional scales, igneous intrusions can induce hydrothermal activity in porous host rocks with important repercussions on organic maturation below 725°C [START_REF] Galushkin | Thermal effects of igneous intrusions on maturity of organic matter: A possible mechanism of intrusion[END_REF].

Therefore, we propose that naturally occurring loss of N org is not simply due to thermal elimination reactions within organic matter (as is the case during dry heating in the laboratory), but that nature primarily relies on chemical reactions between organic matter and hot H, O-containing fluids causing the gradual conversion of N org to inorganic nitrogen species. The anthracite fields of Pennsylvania have been studied as a model for coal metamorphism [START_REF] Hower | Mechanisms of coal metamorphism: Case studies from Paleozoic coalfields[END_REF]. Their paleotemperatures range from nearly 200 °C in semi-anthracites [START_REF] Nickelsen | Ambient temperatures during the Alleghany Orogeny[END_REF][START_REF] Levine | Tectonic history of coal-bearing sediments in eastern Pennsylvania using coal reflectance anisotropy[END_REF] to 260-275 °C for higher rank coals [START_REF] Juster | NH 4 -bearing illite in very low-grade metamorphic rocks associated with coal, northeastern Pennsylvania[END_REF]Daniels and Altaner, 1990;Daniels et al., 1990). Our samples from this region were exposed to, and reacted with hot fluids during anthracitization.

At relatively low temperatures of 225 to 365 °C, hydrous pyrolysis experiments of an oil shale across the oil window to incipient gas generation [START_REF] Lewan | Evaluation of petroleum generation by hydrous pyrolysis[END_REF] released significant amounts of aqueous NH 4 + by decomposition of N org [START_REF] Barth | The distribution of nitrogen between bitumen, water and residue in hydrous pyrolysis of extracted Messel oil shale[END_REF]. Ammonia formation was found to be slow at relatively low temperatures during hydrous pyrolysis of sedimentary organic matter, but accelerated above a temperature threshold corresponding to natural anthracitization [START_REF] You | Hydrothermal alteration of hemi-pelagic sediments: experimental evaluation of geochemical processes in shallow subduction zones[END_REF]. [START_REF] Everlien | Hydrous pyrolysis of high-maturity Paleozoic coals and black shales from Central Europe and adjacent areas -Thermodynamic considerations[END_REF] and [START_REF] Gerling | New aspects on the origin of nitrogen in natural gas in Northern Germany[END_REF] perfomed hydrous pyrolysis experiments at 370 °C (just below the 374 °C supercritical temperature of water at 22.1 MPa) on Westphalian coals with different maturities to simulate the genesis of molecular nitrogen in the North German Basin. They observed larger N 2 yields and higher δ 15 N N2 values with increasing coal rank, in agreement with δ 15 N N2 values from elemental nitrogen in natural NW German gases [START_REF] Boigk | Isotopenphysikalische Untersuchungen zur Herkunft und Migration des Stickstoffs nordwestdeutscher Erdgase aus Oberkarbon und Rotliegend[END_REF]. Our estimate of the coals' N org content and Gerling et al.'s (1997) N 2 yields suggest that ~0.5% of the initial N org from coal with R max ~1% (C daf 85 wt. %) had been converted to N 2 . Only 5% of the initial N org from more mature coal with R max ~ 5.5% (C daf 95 wt. %) had been converted to N 2 . These experimental low N 2 yields during artificial heating of coals cannot explain the strong loss of N org during natural anthracitization. Instead, they indirectly support other experimental evidence for ammonia being the main inorganic nitrogen product from hydrous pyrolysis of kerogen and model compounds (Ader et al., 1998a(Ader et al., , b, 2000)), as well as of other nitrogen heterocyles [START_REF] Houser | Reactivity of some organic compounds with supercritical water[END_REF][START_REF] Houser | The reactivity of tetrahydroquinoline, benzylamine and bibenzyl with supercritical water[END_REF][START_REF] Katritzky | Aqueous high-temperature chemistry. 22. Nitrogen-containing heterocycles in supercritical water at 460 °C[END_REF][START_REF] Katritzky | Aqueous high-temperature chemistry of carbo-and heterocycles. 30[END_REF][START_REF] Ogunsola | Decomposition of isoquinoline and quinoline by supercritical water[END_REF][START_REF] Yuan | Catalytic denitrogenation of hydrocarbons through partial oxidation in supercritical water[END_REF].

Artificial heating experiments at 460-600 ºC and a pressure of 0.2 GPa for 7 days in sealed gold cells used mixtures of (i) Pennsylvanian semi-anthracite and kaolinite (acting as water source and scavenger for ammonia) and Pennsylvanian semi-anthracite and (ii) model semi-anthracite-kaolinite such as carbazole-kaolinite mixtures (Ader et al., 1998a(Ader et al., , b, 2000)). These experiments produced ammonium-illite containing residual solids with δ 15 N org and δ 15 N NH4 values that are similar to those observed in the organic and mineral matrix of natural meta-anthracites. The experiments also demonstrated that heating in the presence of water under pressure produced strong N org losses and the formation of ammonia (ammonium-illite) of the same order of magnitude as observed during natural anthracitization. The observations are in agreement with earlier reports on the genesis of NH 4 + -bearing authigenic minerals in the Pennsylvanian anthracite field [START_REF] Juster | NH 4 -bearing illite in very low-grade metamorphic rocks associated with coal, northeastern Pennsylvania[END_REF][START_REF] Daniels | Nature and origin of minerals in anthracite from eastern Pennsylvania[END_REF]Altaner, 1990, 1993) and show that metamorphic fluids involved in NH 4 + -clay authigenesis can only be derived from an internal source, e.g. from alteration of the anthracite-clay assemblage [START_REF] Daniels | Nature and origin of minerals in anthracite from eastern Pennsylvania[END_REF]Altaner, 1990, 1993).

Auxiliary information on the reactivity of N org moieties is available from experimental studies on water-carbon reaction mechanisms at ~800 °C and low pressure during carbon activation and gasification. In these conditions, water vapor appears to be more efficient in the elimination of N org than other reagents, such as carbon dioxide [START_REF] Boudou | Nitrogen in aramid-based activated carbon fibers by TPD, XPS and XANES[END_REF]. Water vapor's high efficiency to eliminate N org is more pronounced when the carbon matrix still offers a relatively high abundance of accessible and reactive nitrogen sites along the edges of small polyaromatic clusters [START_REF] Rouzaud | Coke microtexture: a key for coke reactivity[END_REF] at an early stage of anthracitization-graphitization. According to Kapteijn et al.'s model (1999), steam gasification chemically erodes reactive carbon structures/cycles that shield interior N-C 3 . The resulting chemical exposure of formerly 'encapsulated' nitrogen atoms along exterior edges subsequently causes the elimination of N org without discrimination among different nitrogen structural moieties.

Energy released from decay of naturally occurring radioactive isotopes in Earth's crust dissociates water molecules into energetic radicals that can form other reactive species, e.g. hydrogen and hydroxyl radicals, H 2 , H 2 O 2 , hydrous electrons (e aq -), and H - [START_REF] Hall | Radiolytic H 2 in continental crust: Nuclear power for deep subsurface microbial communities[END_REF]. Hydrogen radicals can also be produced by reaction of water with hydrocarbons and nitrogencontaining heterocycles (e.g., [START_REF] Townsend | Solvent effects during reactions in supercritical water[END_REF][START_REF] Moriya | Conversion of polyethylene to oil using supercritical water and donation of hydrogen in supercritical water[END_REF][START_REF] Yuan | Catalytic denitrogenation of hydrocarbons through partial oxidation in supercritical water[END_REF] and with fluids (e.g., via gas shift reaction; [START_REF] Sato | Water gas shift reaction kinetics under noncatalytic conditions in supercritical water[END_REF]. Aided by high temperatures, some of these highly reactive species may provide sufficiently large activation energies for abstracting structurally accessible organic nitrogen atoms from ultramicroporous carbonaceous matter during metamorphism. Alternatively, radiation from radioactivity can directly provide abundant activation energy. In the process of reaction with hydrogencontaining free radicals and water, N org would be converted to NH 3 and the small polyaromatic clusters would be locally oxidized and re-arranged. The impact of long-term, natural radiolysis on nitrogen compounds is demonstrated by the fact that NH 4 + in minerals is partly converted to elemental N 2 [START_REF] Grishina | Organic inclusions in salt. Part 3. Oil and gas inclusions in Cambrian evaporite deposit from East Siberia. A contribution to the understanding of nitrogen generation in evaporites[END_REF].

Proposed model of progressive metamorphism of organic nitrogen

Porosity in anthracite is significantly reduced as a result of nanometer-scale tectonic deformation [START_REF] Ju | Relationship between nano-scale deformation of coal structure and metamorphic-deformed environments[END_REF], perhaps due to pressure-enhanced alignment and stacking of small polyaromatic clusters [START_REF] Bustin | Mechanisms of graphite formation from kerogen: Experimental 1evidence[END_REF]. Ultramicropores with a diameter of <0.4 nm (i.e., d002 interlayer spacing of the crystalline carbon structures) are already dominant in anthracites [START_REF] Bratek | Properties and structure of different rank anthracites[END_REF] and gain importance with increasing rank and growth of graphitic-like structures [START_REF] Lu | Quantitative X-ray diffraction analysis and its application to various coals[END_REF]. The porous texture of metamorphic carbonaceous matter from kerogens other than type III can be inferred from experimental carbonization of algal precursor materials which form large polyaromatic clusters followed by planar graphite during high-pressure breaking of pore walls [START_REF] Oberlin | Carbonization and graphitization[END_REF].

The flow of heated water likely occurs through cleats (i.e., small fractures in coal) and interconnected open pores. On a smaller scale, an ultramicoporous matrix of mostly closed pores essentially consists of interconnected small polyaromatic clusters where heteroatoms are predominantly located along edges. Although individual water molecules measure only ~0.28 nm in diameter and can be adsorbed between graphite layers (e.g., [START_REF] Ruuska | Ab initio model study on a water molecule between graphite layers[END_REF], water does not easily penetrate the interlayer spacing of crystalline carbon structures into <0.4 nm diameter ultramicropores in high-rank coal, possibly due to individual water molecules' inability to maintain liquidity [START_REF] Prinz | Development of the micro-and ultramicroporous structure of coals with rank as deduced from the accessibility to water[END_REF]. At the same time, the mobility of water within the carbonaceous ultramicoporous matrix is reduced by increasing surface hydrophobicity resulting from heteroatom removal and aromatization, although hydration of the exterior carbon surface of carbonaceous grains is facilitated by oxidation (e.g., [START_REF] Lerf | Hydration behavior and dynamics of water molecules in graphite oxide[END_REF], and the reduced polarity and viscosity of water at elevated temperatures facilitate the access of water to the carbonaceous matrix [START_REF] Molina-Sabio | Development of porosity in a char during reaction with steam or supercritical water[END_REF]. The solvent properties of liquid water at high temperature are similar to those of polar organic solvents at room temperature, thus favoring ionic versus free radical reactions with organic compounds. Supercritical water is especially reactive as a catalyst and reactant in high-temperature organic geochemistry [START_REF] Siskin | A review of the reactivity of organic compounds with oxygen-containing functionality in superheated water[END_REF]. We hypothesize that, in spite of rising temperature with increasing metamorphic grade, the rate of elimination of N org would decrease exponentially, unless tectonic deformation and fracturing of anthracites provide new conduits for circulation and access of fluids [START_REF] Cao | Deformation metamorphism of bituminous and anthracite coals from China[END_REF].

Our proposed concept of progressive N org transformation and loss with increasing rank explains why N org is strongly depleted during metamorphism while its isotopic composition remains essentially unchanged (Fig. 10). N org is not subject to purely thermal isotropic elimination which would produce a Rayleigh-type isotopic fractionation. Instead, N org is removed from the periphery of carbonacous particles along a progressing reaction front while the unreacted core retains N org and its δ 15 N org value. The reacted peripheral zone of the carbonaceous matrix loses essentially all N org [START_REF] Ader | Nitrogen isotopic evolution of carbonaceous matter during metamorphism: Methodology and preliminary results[END_REF], therefore the core's N org and its δ 15 N org represent bulk nitrogen of the carbonaceous matrix.

Changes in the overall chemical speciation of N org in anthracite and graphite simultaneously occur in two fundamental ways. First, hot H, O-containing fluids or some of their components penetrate into the dense, poorly permeable, ultramicroporous carbonaceous matrix and eliminate N org in the form of NH 3 and N 2 along the reaction front. Hot fluids are the likely source of energetic chemical reactants (e.g., H 2, hydrogen and hydroxyl radicals) for N org elimination. The supply of fluids to coal or other rocks containing organic material may derive from external formation fluids and from internal sources, e.g. from phyllosilicates, or via elimination of organic fuctional groups that generate new H 2 O. The slow penetration of H, O-containing fluids or some of their components into an increasingly hydrophobic and ultramicroporous carbonaceous matrix, and the outward diffusion of products (NH 3 , CO 2 , N 2 , etc.) limit the progress of the reaction front. Second, before the reaction front can reach N org in the unreacted core of the carbonaceous matrix, core N org is subject to purely thermal transformation, whereby this 'encapsulated' N org stabilizes in the form of N-C 3 nitrogen substituting for carbon in graphite-like structural domains with delocalized π-electron systems. Thus, the residual N org in the unreacted core is preserved without significant isotope fractionation.

CONCLUSIONS

Multidisciplinary evidence from X-Ray Photoelectron Spectroscopy, nitrogen stable isotopes, traditional quantitative coal analyses, and other analytical approaches provide insight into the mechanisms and chemical consequences of low-grade metamorphism of organic nitrogen.

At the onset of metamorphism with a vitrinite reflectance R max ~2%, organic nitrogen is dominantly present as pyrrolic and pyridinic nitrogen.

The relative abundance of isolated organic nitrogen N-C 3 atoms that are bonded covalently to three carbon atoms in condensed, partially aromatic ring systems increases during metamorphism. The occurrence of N-C 3 (i.e., nitrogen atoms substituting for "graphitic" carbon in natural metamorphic organic matter) in natural metamorphic carbonaceous matter is documented here for the first time.

There is no evidence for systematic 15 N-enrichment with increasing rank in naturally matured samples. The apparent absence of Rayleigh-type nitrogen isotopic fractionation suggests that direct thermal loss of nitrogen from an organic matrix in an isotropic fashion does not serve as a major pathway for organic nitrogen elimination. Pyridinic, pyrrolic and N-C 3 nitrogen in anthracite and semi-graphite is relatively stable during dry experimental heating in the laboratory, but can be eliminated at much lower temperature during wet experimental heating in the presence of sub-or supercritical water. This suggests that the chemical elimination of organic nitrogen during metamorphism is facilitated by hot H, O-containing fluids.

We propose that hot H, O-containing fluids or some of their components gradually penetrate into the dense, ultramicroporous carbonaceous matrix and eliminate organic nitrogen along a progressing reaction front. Organic nitrogen is essentially eliminated from the reacted periphery of the carbonaceous matrix. Organic nitrogen in the unreacted core represents the bulk organic nitrogen of the entire carbonaceous matrix and preserves a conservative nitrogen isotope ratio, although the overall nitrogen concentration in the carbonaceous matrix decreases. Before the reaction front can reach the center of the carbonaceous matrix, an increasing part of core organic nitrogen chemically stabilizes as N-C 3 . Thus, this nitrogen species becomes the dominant form of organic nitrogen at higher metamorphic grade.

  Fig.2. Schematic nitrogen-containing molecular structures in condensed ring systems similar to coronene, where nitrogen substitutes for carbon and is either located along the edge and bonded to two carbon atoms (a, b, c), or is bonded to three carbon atoms and is termed N-C 3 nitrogen (d, e, f). (a) Pyrrolic nitrogen; this type of nitrogen is frequently associated with a five-member cycle (e.g., pyrrole) that is not shown here. (b) Pyridonic nitrogen. (c) Pyridinic nitrogen. (d) Cyclazine-type nitrogen common to three fused rings. The oxygen atom in the ring system can be replaced by other heteroatoms (e.g., nitrogen, sulfur), a pyrrolic group, CH 2 , etc. (e) Quaternized cyclazine-type nitrogen or centrally-positioned quaternary nitrogen. (f) Quaternary nitrogen common to two fused rings. (*) Other types of cyclazine structures were presented by[START_REF] Boutique | 5,11,13-tetraazacycl[3.3.3azine: Theoretical (ab initio) and experimental (Xray and ultraviolet photoelectron spectroscopy) studies of the electronic structure[END_REF],[START_REF] Leaver | The synthesis and characterisation of cyclazines and related N-bridged annulenes[END_REF] and[START_REF] Pels | Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis[END_REF].

Fig. 3 .

 3 Fig.3. Organic atomic N org /C org ratios of kerogens plotted against (a) the organic carbon content in bulk rocks/coals (in weight % on a dry, ash-free basis, daf), and (b) rank expressed by vitrinite reflectance R max (%). Gray dashed lines represent expected trends for type II kerogens. Solid lines are drawn to guide the eye.

Fig. 4 .

 4 Fig. 4. (a) Stable isotopic cross-plot of  15 N org and  13 C org values of kerogens from shales and coals, indicating distinct clustering of sample sets used in this study. (b)  15 N org values of kerogens from North American and NW German coal series do not express strong dependence on rank (i.e., vitrinite reflectance, R max %). Dashed lines are drawn to guide the eye.

Fig. 5 :

 5 Fig. 5: N 1s XPS and C 1s XPS spectra with curve-fitted sub-peaks for: (a, b) kerogen isolated from carbonaceous shale Bernice 19J, a carbon-poor shale that was sampled close to an anthracite bed;(c, d) the same bulk carbonaceous shale Bernice 19J containing ammonium illite; and (e, f) the inorganic residue 16H-LTA from low-temperature plasma ashing of meta-anthracite 16H; the ash was mixed with a

Fig. 7 .

 7 Fig. 7. Dependence of pyridinic (a, b),pyrrolic (c, d), and quaternary/cyclazine-type nitrogen (e, f) moieties on organic carbon content and vitrinite reflectance R max (%). The resulting patterns over maturity depend on the proxy for rank; the C org content is appropriate for samples with lower maturity, whereas the use of R max is preferred throughout metamorphism to a semi-graphitic stage. Each organic nitrogen moiety is quantified in terms of the area of its diagnostic XPS sub-peak N-6, N-5, or N-Q1 relative to the overall XPS N 1s signal area. The organic carbon content C org is expressed as weight % on a dry, ash-free basis (daf). Vitrinite reflectance is given as R max (%). Solid lines are drawn to guide the eye. Dashed gray lines indicate hypothetical trends for type II kerogen, assuming that the evolutionary paths of type II and III kerogens converge during metamorphism. Panels (b), (d) and (f) also offer exemplary chemical structures characterizing nitrogen moieties that are mainly responsible for observed XPS signals.

  Fig.8. Temperature-programmed pyrolysis of coal at temperatures up to 1450 °C produces nitrogen (N 2 ), methane (CH 4 ), and hydrogen (H 2 ). Data from a coal reference series[START_REF] Boudou | Molecular nitrogen from coal pyrolysis: Kinetic modelling[END_REF] exhibit distinct patterns of gas yields that are expressed in mg of CH 4 , N 2 and H 2 per 100 mg of total organic carbon content in the analyzed sample. Thermal maturity is expressed as vitrinite reflectance R max (%).

  Sub-peaks N-Q2 and N-X: Uncertain N org assignments

  carbonaceous matrix that has not yet been reached by hydrogen and oxygen-containing hot fluid or some of its components diffusing inward

Fig. 11 .

 11 Fig. 11. Comparison of this study's nitrogen XPS data with data from[START_REF] Burchill | Variation of nitrogen content and functionality with rank for some UK bituminous coals[END_REF]. (a) Re-drafted data with original trend lines from[START_REF] Burchill | Variation of nitrogen content and functionality with rank for some UK bituminous coals[END_REF] original Figure3, where N 1s XPS-quantified relative abundances of total, pyridinic, and "pyrrolic" nitrogen are plotted against the carbon content on a 'dry, mineral matter free' (dmmf) basis. The same XPS data for "pyrrolic" and pyridinic nitrogen are expressed in terms of relative abundances (% of the total XPS signal) along one common Y-axis (b) and using two separately scaled Y-axes for visualization of "mirror trends" (c), with newly assigned trend lines. (d) This study's new data on coals with increasing organic carbon content (i.e., with increasing thermal maturity, R max in %) indicate that[START_REF] Burchill | Variation of nitrogen content and functionality with rank for some UK bituminous coals[END_REF] so-called "pyrrolic" nitrogen actually represents a combination of the XPS subpeaks N-5 (pyrrolic, pyridonic, and other nitrogen species) and N-Q1 (N-C 3 ). The trend line of the (N-5 + N-Q1) summary peak area approximates the mirror image of the trend line for pyridinic nitrogen N-6. Graph (d) is directly comparable to[START_REF] Burchill | Variation of nitrogen content and functionality with rank for some UK bituminous coals[END_REF] data shown in (c) because 'carbon weight % on a dry, mineral matter free basis' closely approximates the total organic carbon content on a dry, ash free (daf) basis in coals. Lines are drawn to guide the eye.

Acknowledgments

We are indebted to many expert colleagues for sharing their valuable samples and offering inspiration. Eric Daniels, ChevronTexaco Energy Research & Technology Company, Richmond, California (http://www.clays.org/AskTheClayScientist/Eric%20Daniels.html), and Jeffrey Levine, Consultant Geologist, Dallas (http://www.levineonline.com/jeffrey/index.asp), provided Pennsylvanian anthracites. We gratefully acknowledge Angelika Vieth's (Geologischer Dienst Nordrhein-Westfalen, Krefeld, Germany, http://www.gd.nrw.de/home.php) contribution of samples from the Bramscher Massiv, and Peter Gerling's (Bundesanstalt für Geowissenschaften und Rohstoffe Geozentrum Hannover, http://www.bgr.bund.de/) coal samples from Northern Germany. We thank Mohamed Chehimi and Carol Bilem for technical assistance with some XPS analyses at ITODYS, University of Paris VII (France). Our manuscript benefited greatly from the constructive criticism of Jay Brandes, JoAnn Holloway, Iain Pitcairn, and an anonymous reviewer. This work was supported by a grant from Chevron Petroleum Technology Company to J. P. B., and by U.S. Department of Energy, Basic Energy Research Grant number DEFG02-00ER15032 to A. S. and M. M.