C- and N-bearing Species in Reduced Fluids in the Simplified C–O–H–N System and in Natural Pelite at Upper Mantle P–T Conditions

: C- and N-bearing species in reduced fluids weree studied experimentally in C–O–H–N and muscovite–C–O–H–N systems and in natural carbonate-bearing samples at mantle P–T parameters. The experiments reproduced three types of reactions leading to formation of hydrocarbons (HCs) at 3.8–7.8 GPa and 800–1400 ° C and at hydrogen fugacity ( f H 2 ) buffered by the Fe–FeO (IW) + H 2 O or Mo–MoO 2 (MMO) + H 2 O equilibria: (i) Thermal destruction of organic matter during its subduction into the mantle (with an example of docosane), (ii) hydrogenation of graphite upon interaction with H 2 -enriched fluids, and (iii) hydrogenation of carbonates and products of their reduction in metamorphic clayey rocks. The obtained quenched fluids analyzed after the runs by gas chromatography-mass spectrometry (GC–MS) and electronic ionization mass-spectrometry (HR–MS) contain CH 4 and C 2 H 6 as main carbon species. The concentrations of C 2 -C 4 alkanes in the fluids increase as the pressure and temperature increase from 3.8 to 7.8 GPa and from 800 to 1400 ° C, respectively. The fluid equilibrated with the muscovite–garnet–omphacite–kyanite–rutile ± coesite assemblage consists of 50–80 rel.% H 2 O and 15–40 rel.% alkanes (C 1 > C 2 > C 3 > C 4 ). Main N-bearing species are ammonia (NH 3 ) in the C–O–H–N and muscovite–C–O–H–N systems or methanimine (CH 3 N) in the fluid derived from the samples of natural pelitic rocks. Nitrogen comes either from air or melamine (C 3 H 6 N 6 ) in model systems or from NH 4+ in the runs with natural samples. The formula CH 3 N in the quenched fluid of the C–O–H–N system is confirmed by HR– MS. The impossibility of CH 3 N incorporation into K-bearing silicates because of a big CH 3 NH + cation may limit the solubility of N in silicates at low f O 2 and hence may substantially influence the mantle cycle of nitrogen. Thus, subduction of slabs containing carbonates, organic matter, and N-bearing minerals into strongly reduced mantle may induce the formation of fluids enriched in H 2 O, light alkanes, NH 3 , and CH 3 N. The presence of these species must be critical for the deep cycles of carbon, nitrogen, and hydrogen.

natural samples of pelite (Maykop Fm. shale, Russia) and N-bearing mica schist (Polar Ural, Russia) which represented carbonate-bearing slab material subducted to mantle depths. Since methanimine can affect the nitrogen and carbon mantle cycles, it was important to check whether it can exist in hydrocarbon-rich fluids that form during hot subduction. Therefore, special efforts were made to identify methanimine in quenched fluids obtained in the experimental systems at 6.3 GPa and 1000-1200 °C.

Materials
The features of C-and N-bearing species in fluids obtained in synthetic systems were studied using pure natural graphite (99.99% C) pre-dried at 110 °C for at least 30 days and chemical grade synthetic diamond (ACM-20/14, 14-20 μm) pre-annealed in air at 700 °C for 1 h as fluid-generating carbon sources. The starting mixtures also contained at least 99% purity docosane (C22H46), melamine (C3H6N6), distilled water, and silver oxalate (Ag2C2O4) ( Table 1). Air was the only N source in melamine-free samples. Pycnometer testing revealed about 39 vol.% of air in the capsules. Pre-dried graphite contained 700 ppm CO2 and 700 ppm H2O. Distilled water was added to capsules with a microsyringe to ± 0.2 mg accuracy. The capsules were sealed by arc-welding prior to experiments. The capsule assembly details were described in our previous publications [10,19,20,38]. The starting materials used to study C-and N-bearing species in the fluids equilibrated with muscovite (Table 2), natural Maykop Fm. shale (Taman Peninsula, Russia; hereafter referred to as pelite) [39], and N-rich quartz-muscovite-chlorite schist (hereafter referred to as mica schist) (Polar Ural, Russia), as well as melamine or distilled water. The mica schist, with up to 266 ppm NH4 [40], was metamorphosed at 0.2 GPa and 570 to 580 °C. As shown by the thermogravimetric (TG) analysis, it contained up to 0.91 wt.% carbonate CO2 and 2.21 wt.% H2O in total. The pelite sample contained 2.0 wt.% calcite and 1.7 wt.% siderite, according to the quantitative X-ray diffraction (QXRD) data [39] and 1.87 wt.% CO2, 2.05 wt.% of adsorbed H2O, and 3.33 wt.% of hydroxyl according to the TG analysis. The content of nitrogen in the pelite sample was not specially analyzed, but it may be from 424 to 2382 ppm [41,42]. The pelite and mica schist samples exposed to the experimental P-T conditions can be appropriate analogues of metasediments subducted to mantle depths judging by what is known from the literature [6,28,43,44].

Methods
Experiments at 3.8, 5.5, 6.3, and 7.8 GPa were carried out in a split-sphere multi-anvil highpressure apparatus [45]. Pressure was calibrated by recording the change in the resistance of Bi at 2.55 GPa and PbSe at 4.0 and 6.8 GPa at room temperature and by bracketing the quartz-coesite and the graphite-diamond equilibrium at 3.8, 5.5, 6.3, and 7.8 GPa and high temperatures. The second phase transition of Bi occurring at 7.7 GPa at ambient temperature was not used for calibration since the information obtained from other systems is sufficient. Temperature was monitored in each experiment using a PtRh6/PtRh30 thermocouple calibrated at 6.3 GPa and 7.8 GPa using the melting points of Al, Ag, Ni and Pt. Pressure and temperature were measured to an accuracy of ± 0.1 GPa and ± 20 °C [45,46]. The charges were quenched under isobaric conditions at a rate of 200 deg/s. As shown by special studies of the effect of cooling rate on fluid compositions [36,38], cooling from 800-1400 °C to room temperature at 200 deg/s can provide quenching and furnish reliable evidence of the fluid compositions at the applied P-T conditions. The run durations from 2 to 60 hours (Table 1) were chosen because equilibrium fluids rich in hydrocarbons (HC) and N-bearing species in the C-O-H-N experimental system at 6.3 GPa and 1100-1400 °C form in at least two hours [10,38]; one 1400 °C run was as short as 20 min. Long 40-60 h durations were used to bring the fluid-bearing silicate systems close to the equilibrium.
Hydrogen fugacity was buffered by the Fe-FeO (IW) + H2O and Mo-MoO2 (MMO) + H2O equilibria using a modified double-capsule technique, with inner Pt or Au capsules placed inside thick-walled Fe or Mo outer capsules with talc insulation [9]. Our previous experiments at 6.3 GPa and 1150-1400 °C [18,22] showed that fH2 buffering can maintain invariable compositions of HC and C/H ratios in fluids as the run duration increased from 1 min to >40 h with this technique, thus proving its workability at the applied P-T-τ parameters. For more details of the double-capsule technique, see [47]. As we calculated earlier for quenched C-O-H-N fluids [10], their fO2 may generally vary from log fO2 ΔIW − 2.5 to log fO2 ΔIW + 2.5 (ΔIW is the logarithmic difference between experimental fO2 and that imposed by the coexistence of iron and wüstite) depending on the initial H2O contents in the charge and the selected IW or MMO buffers for fH2. Note that the MMO buffer is slightly more oxidized (log fO2 ΔIW + 1) [9]. In the absence of an external fH2 buffer, fO2 in samples was slightly higher than ΔIW + 2 [10].

Analytical Techniques
After quenching and retrieval from the high-pressure cells, the Pt capsules were placed into a crush cell connected on-line to a gas chromatograph before the analytical column. They were heated at 120-130 °C in a stream of carrier gas (99.9999% pure He) for 90 min to clean the surface from adsorbed gases and to evaporate water for further analysis of the released gases. The pre-heating duration depended on the rate of gas desorption and was analyzed on-line. Blank pre-heating runs showed neither speciation nor concentration changes in volatiles in the capsules at 120-130 °C. The capsules that did not sustain heating were excluded from analysis. The gas mixture extracted from capsules by piercing was analyzed by gas chromatography and mass spectrometry using a Thermo Scientific Focus GS/DSQ II Series single quadrupole gas chromatograph-mass analyzer at the V.S. Sobolev Institute of Geology and Mineralogy. The relative concentrations (rel.%) of volatile components in the studied mixture were obtained by normalization: The total area of all chromatographic peaks was normalized to 100%, and the area of an individual component defined its proportion in the mixture. The normalization quality was checked against external standards [10]. The concentration ranges of alkanes during the calibration were the same as in the run products. Analytical uncertainty was below 5% for C1-C4 alkanes and less than 10% for H2O, NH3, and CO2 (determined in the range from 12.5 pptv to 12.5 ppbv and expressed as precision), or even <5% in most cases. For details of the GC-MS method, see [10].
Exact masses of methanimine were determined on a Thermo Fisher Scientific Double Focusing System (DFS) Magnetic Sector high resolution mass-spectrometer at the N. Vorozhtsov Institute of Organic Chemistry (Novosibirsk). The operation conditions were: 70 eV electron ionization and 200 °C ion source temperature. The capsules with mixtures of volatiles were placed into a specially modified inlet for the mass calibration standard, which allowed capsule opening with a needle inside the volume intended for calibration. The inlet system was maintained at 200 °C. The DFS massspectrometer was calibrated with respect to the standard lines of perfluorokerosene (PFK) prior to measurements. The exact masses of methanimine ions were measured relative to the mass lines of known particles (CN + , HCN + ).
In the end of the experiments, the recovered silicate samples were cleaned and mounted in epoxy resin. After the polymerization of resin, the samples were polished under kerosene, without the use of water (final stage 1 μm Al2O3) and examined on a Tescan MIRA 3 LMU scanning electron microscope (SEM). The solid phase compositions were analyzed using the Tescan MIRA 3 LMU scanning electron microscope coupled with an INCA EDS 450 microanalysis system with an Oxford Instruments liquid nitrogen-free Large Area EDS X-Max-80 Silicon Drift Detector. The instruments were operated at an accelerating voltage of 20 keV, a beam current of 1 nA, and a spot size of ~10 nm; the count time for spectra acquisition was 20 s. The EDS spectra were optimized for quantification using the standard XPP procedure built into the INCA Energy 450 software. The TG analyses were accomplished according to the method published by Dementyev et al. [48].

Results
The fluid phase was obtained in four runs from charges containing other solid phases besides graphite (see Table 3 for their representative analyses). Recrystallization of muscovite was observed in all runs of muscovite-C-O-H-N systems. The newly formed muscovite gained silica which increased from 3.12 to 3.27 apfu Si upon the temperature and the pressure change from 800 °C to 1000 °C and from 3.8 to 5.5 GPa, respectively, which was associated with growing percentage of the celadionite component (KAl(Mg,Fe 2+ )Si4O10(OH)2) and formation of kyanite. The run with pelite and mica schist produced a muscovite-garnet-omphacite-kyanite-coesite-rutile assemblage ( Figure 1). Muscovite in the assemblage contained 3.52 apfu Si and thus was more strongly phengitized (had a higher percentage of celadionite) than that obtained in the muscovite-C-O-H-N system. Note that the phengite substitution line implies that Si IV and Mg VI substitute, respectively, for Al IV and Al VI [32,33]. Garnet was mainly almandine or pyrope, and a considerable part was grossular; omphacite had a high percentage of jadeite. After the 6.3 GPa and 1000 °C run, mica schist consisted of almost the same muscovite-garnet-omphacite-kyanite-rutile assemblage but without coesite. The newly formed muscovite of this assemblage contained 3.49 apfu Si and was compositionally similar to that in the previous run. Garnet was more often almandine and less often grossular varieties; omphacite had a large jadeite component as well.  According to GC-MS data, the quenched fluids contained light alkanes (Table 4, Figures 2 and 3) which formed by hydrogenation of graphite or diamond in most cases, except for two runs where docosane was used as a source of hydrocarbons (Table 1). Low-N fluids equilibrated in the C-O-H-N system at fH2 buffered by the IW + H2O and MMO + H2O equilibria contained 30 to 70 rel.% alkanes ( Table 4). The hydrogenation reaction stopped at H2O and C 0 formation in a single run with CO2 added to the system, and the run products almost lacked light alkanes. The runs without external H2 buffering produced from 3 to 26 rel.% of alkanes, mainly C1-C4 species, with high contents of CH4 and C2H6 but less than 10 rel.% C3H8 and C4H10 (Figure 2), while C7-C18 were as low as ≤1 rel.%. Other species included notable amounts of aldehydes, ketones and carboxylic acids, the latter especially in non-buffered conditions with CO2 up to 11-43 rel.% (unlike <3.6 rel.% in other runs). Hydrocarbon species appeared for the first time in the quenched fluids formed from the pelite and mica schist samples (Figure 3) at 6.3 GPa, 1000 °C and fH2 buffered by MMO + H2O, at the conditions that simulated subduction of these rocks in a slab to ~200 km depths. Carbon in those charges could only come from carbonates which did not exceed a few wt.% ( Table 2). Dehydration of mica and reduction of carbonates led to water enrichment of the fluid, up to 48-81 rel.% H2O. The fluid contained predominant light HCs, from 15 to 39 rel.%. The fluids synthesized from the charges with natural samples contained lighter alkanes than those obtained in the model C-O-N-H system (Figures 2 and 3). Note that the fluids synthesized in the graphite-bearing muscovite-C-O-H-N system likewise contained mainly light alkanes. As we showed before [18,19], heavier alkanes can form in experimental systems at higher pressures and temperatures, when Pt capsules are used instead of Au ones. Concentrations of heavier alkanes also become lower in fluids with lower H2O contents in the fluid [19]. Thus, the lighter alkane compositions obtained in the runs with the natural samples may be due to (1) a possible catalytic effect of solid matrix silicates (muscovite-garnet-omphacitekyanite-rutile ± coesite; (2) relatively low run temperature (1000 °C); (3) high water content in fluids (Figures 2 and 3), and (4) the use of Au capsules. These factors obviously controlled the composition of alkanes obtained in the muscovite-C-O-H-N system.
According to GC-MS data, N2, NH3 and CH3N are main N-bearing components of quenched fluids (Table 4, Figure 4). Their relative amounts depend on redox conditions, which were varied by either buffering or not the hydrogen fugacity in different runs. In the case of double capsule technique fH2 buffering, the redox conditions in the inner capsule increases with the water content within [47]. The concentration of CO2 measured by GC-MS is an informative parameter reflecting the growth of the fluid oxidation degree. N2 that was captured mainly from the air was a predominant nitrogen specie in the N-poor fluid synthesized in the C-O-H-N system even at CO2 >1 rel.% (Figure 4a), but ammonia predominated at lower CO2 levels (Figure 4b). This trend is consistent with the fluid composition of the muscovite-C-O-H-N system in the presence of an additional N source. Importantly, CH3N (Table 4, Figure 4c) was the main nitrogen specie at low CO2 levels in the fluids that formed in systems with natural rock samples (nitrogen coming mostly from NH4 + of clay minerals and muscovite), as well as in the model muscovite-C-O-H-N system (ambient air N).
High-resolution mass spectrometry was used to measure the exact molecular weight of the 29 Da compound revealed by GC-MS. The measurements were made for quenched fluids obtained in Au capsules after runs 2007_2_3 and 2007_2_4 with model systems at 6.3 GPa and 1200 °C. The calculated mass for CH3N [M] + was 29.0260, and the mass/charge ratio was m/z = 26.0250. The closest mass is atmospheric [ 15 N 14 N] + that has m/z = 29.0025. GC-MS data obtained for gas mixtures in runs with high CH3N contents (pelite and mica schist, Table 4) allowed detecting the mass spectrum more precisely. The mass spectrum we obtained ( Figure 5) shows generally the same relative intensities of signals as the one published by Theule et al. [49], except for the presence of an m/z = 17 signal, which remained cut off in the spectrum of Theule et al. [49], and for the absence of the m/z = 28 signal possibly resulting from N2 pollution mentioned by the authors. Comparison of the HCN spectrum presented by Theule et al. [49] with the reference one from the NIST library (SRD 69) leads to the same inference since the spectrum by Theule et al. [49] has the heightened m/z = 28 signal.

Discussion
The reported experiments provide new knowledge on the composition of C-and N-bearing species of reduced fluids obtained in the C-O-H-N and muscovite-C-O-H-N systems and from natural rock samples. As shown previously [10], fO2 may generally vary from log fO2 ΔIW − 2.5 to log fO2 ΔIW + 2.5 depending on the initial H2O contents in the charge and the selected IW or MMO buffers for fH2. In the absence of external fH2 buffering, fO2 in the samples was slightly above IW + 2.5 log units. The question is whether these redox conditions represent the conditions of slabs and the ambient mantle. Different serpentinization degrees of peridotite, involvement of variable amounts of carbonate in the slab material, and dehydration reactions during subduction can maintain heterogeneous redox conditions within slabs reaching depths of 150-350 km in a range of 6 log units, from log fO2 ΔIW − 1 to log fO2 ΔIW + 5, while the surrounding mantle is generally more reduced [50,51]. Thus, the fluids we analyzed can form and remain stable only in the most reduced mantle and slab regions.

Carbon Species
Carbon species in the quenched fluids from our experiments were mainly light alkanes C1-C4 (Table 4; Figure 2); CO2 reached considerable amounts in two runs with non-buffered fH2. In most of the runs, HCs formed by reactions of H2-CO2-H2O-N2 fluids with carbon of graphite, while volatiles included H2 of the buffer and gases adsorbed on graphite particles of the starting material. The interaction of graphite with H2 discussed in detail in [19,20] started with reduction of the adsorbed CO2, which however stopped in few minutes in the absence of other CO2 sources. No methanation of carbon dioxide can occur at P-T parameters described. Hydrogenation of graphite produces tens of times more HC-bearing fluids than that of diamond. Graphite exfoliation by the fluid leads to significant increase in its surface area and to acceleration of the reaction [19]. In two runs, HCs formed mainly by thermal destruction of docosane at high pressures and temperatures, most likely by the mechanism of thermal cracking [10]. Decomposition of docosane appears to be the main process involving n-alkanes, including n-docosane and alkyl chains of fatty acids. In principle, the initial thermal formation of radical species of higher hydrocarbons in homolysis and rearrangement reactions leads to further β-scission into alkene and alkyl radicals with shorter chains [10]. Ethylene that forms in β-scission of terminal radicals in excess of H2 readily reduces to ethane. The radicals resulting from cracking can recombine or react with alkanes or alkenes to form new HC species and radicals. The speciation of alkanes in quenched fluids (reference to supplementary table) is influenced by the experimental conditions. It was previously shown [10,19,20] that in presence of Pt heavier HCs are formed due to the catalytic homologization [52,53] mainly affecting the CH4/C2H6 ratio. This mechanism may also be responsible for the formation of minor amounts of branched or cyclic alkanes. Capsules of pure gold do not show major catalytic effects in the systems investigated, apparently due to the dissociation of hydrogen on the surface of neutral gold having energy barrier [54,55], in contrast to Pt. Theoretical studies also showed that the adsorption of hydrocarbons, alkyl radicals and atomic H to the neutral Au 0 appears to be disadvantageous [56]. On the other hand, the H2 permeability of Au is substantially lower that of Pt, making the equilibrium to be attained slowly.
The runs with natural samples provided HC generation in strongly reduced conditions at the P-T parameters simulating subduction to depths below 200 km. The experiments actually reproduced metamorphic reactions, including partial dehydration of clay minerals and muscovite, which produced a muscovite-garnet-omphacite-kyanite-rutile ± coesite matrix and a fluid, while the latter provided reduction of carbonates at MMO + H2O-buffered fH2. Reactions of this kind were earlier effectuated in simple model systems consisting of carbonate, FeO and H2O and yielded methane as a main product at high pressures and temperatures [7,12,13,16,17,57]. Methane formation in the reaction of FeO, CaCO3, and H2O in experiments of Scott et al. [13] preferably occurred at temperatures below 1000 °C and pressures in a range of 5-11 GPa, and the yield of methane increased continuously as the temperature was elevated from 700 to ~1200 °C at 4 to 5 GPa in the experiments of Kenney et al. [12]. Unlike those results, the quenched fluids we obtained in the runs with the pelite samples at 6.3 GPa and 1000 °C contained commensurate amounts of methane and ethane and notably smaller amounts of other light alkanes (Figure 3). Our experiments differed by high fH2 which was constrained by oxidation of wüstite at the WM + H2O equilibrium and exceeded that in the experiments with carbonate, FeO and H2O. As we calculated [19], fH2 at the MW + H2O equilibrium in this range of pressures and temperatures was 1.2-2.1 log units below the IW + H2O equilibrium. Since fH2 is the critical parameter governing HCs formation in these reactions [19,57], generation of light alkanes was faster in our case. The pelite sample contained much more carbonates and water than mica schist, and the respective quenched fluid had more water and less light alkanes: 81 rel.% H2O and 15 rel.% alkanes against 48 rel.% H2O and 39 rel.% alkanes in the case of mica schist.
Thus, our experiments reproduced three cases of HC generation under the upper mantle PT conditions: Thermal destruction of docosane; hydrogenation of graphite; hydrogenation of carbonates of pelites and products of their reduction. All reactions occurred at buffered fH2 produced mainly light alkanes, especially CH4 and С2H6 (Table 4; Figure 2). The muscovite-garnet-omphacitekyanite-rutile ± coesite assemblage did not influence much the composition of the generated HCs ( Figure 3).

N-bearing Species
The fluids synthesized in the N-poor C-O-H-N system changed in predominant nitrogen species from N2 to NH3 as they became more reduced and depleted in CO2, whereas the respective change in the fluids obtained using natural samples was from N2 to CH3N (Figure 4c). The limits of NH3 and CH3N redox stability in the analyzed fluids generally coincide ( Figure  4b,c), and their contents increase rapidly as CO2 decreases to <1 rel.%. Taking into account calculations of Stachel and Luth [58], the concentrations of CO2 about 1 rel.% in the C-O-H fluid can be estimated to represent the ∼IW + 1 log unit conditions. The fluids synthesized in our previous study [22] in the lherzolite-C-O-H-N system at 5.5-7.8 GPa and 1150-1350 °C and at fO2 from strongly reduced conditions (IW − 2.5 log. units) to the EMOD equilibrium contained ammonia as predominant nitrogen specie (N2/(NH 3+ N2) = 0.01-0.17) in the presence of metal-saturated lherzolite. However, the N2/(NH3+N2) ratio approached unity in the presence of magnesite-bearing lherzolite, i.e., at more oxidized conditions. In the same way, the reported experiments show the concentration of ammonia decreasing rapidly once CO2 in the fluid from the lherzolite-C-O-H-N system exceeded 1 rel.% (Figure 4b). The concentration of methanimine (CH3N) in the reduced fluid is controlled by the ratio of nitrogen to active C-bearing precursors of CH3N in the system, the content of precursors being substantially higher in the systems containing pelitic rocks. The exact mechanism was not investigated so far. In the presence of melamine in the starting composition, NH3 predominated over CH3N over the whole range of applied fO2, in the case of both metal-and magnesite-saturated lherzolites. The concentrations of NH3 and CH3N remained comparable in N-poor fluids where atmospheric N2 was the only source of nitrogen. Note that most of nitrogen in the fluids obtained in equilibrium with the pelitic mineral assemblage came from NH4 + in clay minerals and micas, and CH3N was the dominant nitrogen species of reduced fluids in that case.
The new experimental results confirm the common trend of NH3 stability in fluids of the model systems at 3.8 to 7.8 GPa and 800 to 1400 °C, in the upper mantle redox conditions. Therefore, considerable amounts of ammonia may be present in a fluid stable either in a relatively cold reduced lithosphere or in a metal-saturated asthenosphere. This study has provided the first evidence that methanimine (CH3N) may be an essential N-bearing specie in the reduced fluid obtained from natural pelite samples in the P-T conditions of 6.3 GPa and 1000 °C corresponding to a hot slab subducted to 200 km. The ammonia concentration controls the solubility of nitrogen in K-bearing silicates since NH4 + can partially replace K + [3,30]. On the other hand, the basicity of CH3N and the big size of methanimine cation CH3NH + prevent the replacement of K + in silicates. Thus, the stability of CH3N in reduced fluids may substantially influence the mantle cycle of nitrogen because methanimine cannot incorporate into silicates (like NH4 + → K + substitutions), which limits the solubility of N in silicates at low fO2.

Conclusions
Our experiments at the upper mantle P-T conditions reproduced three potentially important ways of HC generation: (i) Thermal desctruction of docosane; (ii) hydrogenation of graphite; (iii) hydrogenation of pelitic carbonates or products of their reduction. The aforementioned processes conducted under fH2 buffering produced mainly light alkanes, especially methane and ethane revealed by the GC-MS analysis of the quenched fluids. The alkanes became heavier upon pressure and temperature increase from 3.8 to 7.8 GPa and from 800 to 1400 °C. The quenched fluids obtained at unbuffered fH2 contained >10 rel.% CO2, lesser amounts of alkanes but more O-bearing organic compounds. The fluids produced by partial dehydration of hydrous minerals or by hydrogenation of carbonates (or their reduction products) in equilibrium with the muscovite-garnet-omphacitekyanite-rutile ± coesite assemblage consisted of 50-80 rel.% H2O and 15-40 rel.% alkanes (C1 > C2 > C3 > C4). Therefore, subduction of H2O-and carbonate-bearing clayey sediments into strongly reduced mantle below 200 km, where fO2 is as low as the IW buffer, can be expected to induce generation of light alkanes. The HCs generated in slabs may form a hydrocarbon link in the deep cycles of carbon and hydrogen.
The dominant nitrogen specie in the quenched fluids synthesized in the simplified C-O-H-N system, where nitrogen comes from air or melamine, changes from N2 to NH3 as the system becomes more reduced. The respective change in the fluids obtained from natural samples, with NH4 + as the carbon source, is from N2 to mainly CH3N. The concentration of CH3N in the reduced fluid increases at higher ratio of C-bearing precursors to bulk nitrogen in the system. The presence of CH3N in the quenched fluid derived from the C-O-H-N system has been checked by high resolution massspectrometry. The CH3NH + ion, being much bigger than NH4 + , cannot substitute for K + in K-bearing silicates. Therefore, the stability of CH3N in reduced N-poor fluids may limit the solubility of N in silicates at low fO2 and thus influence considerably the mantle cycle of nitrogen.