Carbon and Nitrogen Speciation in N-poor C-O-H-N Fluids at 6.3 GPa and 1100-1400 °C.

Deep carbon and nitrogen cycles played a critical role in the evolution of the Earth. Here we report on successful studying of speciation in C-O-H-N systems with low nitrogen contents at 6.3 GPa and 1100 to 1400 °C. At fO2 near Fe-FeO (IW) equilibrium, the synthesised fluids contain more than thirty species. Among them, CH4, C2H6, C3H8 and C4H10 are main carbon species. All carbon species, except for C1-C4 alkanes and alcohols, occur in negligible amounts in the fluids generated in systems with low H2O, but С15-С18 alkanes are slightly higher and oxygenated hydrocarbons are more diverse at higher temperatures and H2O concentrations. At a higher oxygen fugacity of +2.5 Δlog fO2 (IW), the fluids almost lack methane and contain about 1 rel.% C2-C4 alkanes, as well as fractions of percent of C15-18 alkanes and notable contents of alcohols and carboxylic acids. Methanimine (CH3N) is inferred to be the main nitrogen species in N-poor reduced fluids. Therefore, the behaviour of CH3N may control the nitrogen cycle in N-poor peridotitic mantle. Oxidation of fluids strongly reduces the concentration of CH4 and bulk carbon. However, higher alkanes, alcohols, and carboxylic acids can resist oxidation and should remain stable in mantle hydrous magmas.

Introduction

Abiotic class="Chemical">hydrocarbons (class="Chemical">pan class="Chemical">HCs) and ammonia have been important agents in the Earth’s carbon and nitrogen cycles. Their stability in different tectonic settings in the course of geodynamic evolution could control the habitability of the planet, the amounts of carbon and nitrogen migrating in fluids, the processes of redox melting and freezing, and the formation of diamonds[1-6]. There is a wealth of evidence for the existence of hydrocarbons in the mantle. Namely, Sugisaki and Mimura (ref. 7) studied a collection of 227 samples from fifty localities throughout the world and revealed heavy hydrocarbons (n-alkanes) in mantle-derived rocks, such as peridotites in ophiolite sequences or peridotite xenoliths in alkali basalts, but did not find any in gabbro and granite. HCs in inclusions hosted by diamond and other minerals in mantle xenoliths were reported from the Siberian craton[8-11]. As a recent find, we can cite a thin fluid jacket of CH4 around inclusions of solidified iron-nickel-carbon-sulfur melt in large, exceptional gem (like the Cullinan, Constellation, and Koh-i-Noor) sublithospheric diamonds reported by Smith et al. (ref. 12).

The genesis of class="Chemical">hydrocarbons in the mantle remains unclear. They might be a mixture of class="Chemical">pan class="Chemical">HCs which were recycled or delivered by meteorites and comets to the early (primeval) Earth, or synthesised by the Fischer-Tropsch reaction[7]. The contribution of two former sources may be, however, limited: HCs may oxidise during subduction and can hardly survive the high impact temperatures. The composition of hydrocarbons may vary. For instance, HCs heavier than methane were inferred[13-17] to be energetically favoured at the deep mantle P-T conditions. As for nitrogen in reduced mantle, there is a consensus that most of it resides in ammonia and NH4 + which substitutes for alkali cations in phengite, Cr-bearing clinopyroxene and other silicates[2, 18, 19].

The behaviour of class="Chemical">carbon anclass="Chemical">pan class="Chemical">d nitrogen species in mantle fluids depends on oxygen fugacity (fO2). Estimates for continental lithospheric mantle (CLM) within the Kaapvaal[20], Slave[21] and Siberian[22, 23] cratons show that fO2 generally decreases depthward from +1 Δlog fO2 FMQ (Fayalite-Magnetite-Quartz oxygen buffer) at 100 km (~3.2 GPa) to -4 Δlog fO2 FMQ at a depth of 220 km (~7 GPa). The fO2 values at the CLM base about 200 km below the surface may vary from IW + 1 to IW + 3 log units[1] and the temperatures at this depth may range within 1100–1400 °С[20-23]. According to thermodynamic calculations, lower alkanes (especially methane and ethane) and ammonia are expected to be stable under fO2 conditions of the CLM[3, 24–26], but they almost disappear from fluids at fO2 reaching CW (the maximum H2O content in C-O-H fluids). Moreover, the silicate environment can influence the pH of deep H2O-bearing fluids and thus the carbon and nitrogen speciation[6, 26, 27].

Recent class="Chemical">decaclass="Chemical">pan class="Chemical">des have brought much progress in experimental techniques with fO2 and fH2 buffering used to predict potential compositions of hydrocarbon fluids at mantle pressures and temperatures. Carbon-saturated C-O-H fluids in reduced systems were studied by quenching experiments and chromatography at 2.0–3.5 GPa and 1000–1300 °С[28, 29], as well as at 5.7–6.3 GPa and 1200–1600 °С[30, 31]. Carbon concentrations in strongly reduced fluids synthesised under these Р-Т-fO2 conditions are commonly related as CH4 > C2H6 > C3H8 (other HCs being ≪1%) and agree well with calculations[20, 25, 28, 32]. The concentrations of lower alkanes at pressures from 4 to 7 GPa and typical CLM temperatures[25, 32, 33] vary only little, but the precise contents of other hydrocarbons remain unknown. Among nitrogen species, ammonia was reported[3] to predominate in nitrogen rich N-O-H fluids entrapped in quartz and olivine at 0.2–3.5 GPa, 600–1400 °C and fO2 at the Fe-FeO equilibrium, as well as in ultra-reduced nitrogen-rich C-O-H-N fluids at 5.5–7.8 GPa and 1100–1500 °C, as shown by our recent experiments[34]. In upper mantle aqueous fluids under reducing conditions, nitrogen is expected to occur mainly as ammonium (NH4 +)[26].

In this stuclass="Chemical">dy we aclass="Chemical">pclass="Chemical">ply quenching exclass="Chemical">periments with bufclass="Chemical">pan class="Chemical">fered hydrogen fugacity to study (i) the stability of different carbon and nitrogen species in N-poor C-O-H-N fluids at 6.3 GPa, 1100–1400 °C and fO2 about or slightly below IW corresponding to the conditions at the СLM base (~200 km depth) near the boundary with the asthenosphere, and (ii) the relative stability of HCs and N-bearing compounds at fO2 IW +2.5 log units corresponding to reactions of reduced C-O-H-N fluids with oxidised lithosphere. The estimates[25, 32, 33] that the fluid composition does not depend much on pressure within 4–7 GPa allows us to extrapolate the results onto the upper mantle pressure range.

Results

Effect of cooling rate on behaviour of C-O-H-N fluids

Isobaric class="Chemical">cooling can change the class="Chemical">pan class="Chemical">compositions of fluids as a result of back reactions. In this respect, it is critical for the fluids to cool down at rates sufficient for precluding back reactions and holding the equilibrium composition consistent with the target temperature and run duration. Changes in the species composition of fluids were studied previously in 2.4 GPa experiments in the C-O-H system synthesised at 1000 °C and cooling at rates from 0.3 to 120 deg/s at buffered fO2 [29]. The potential effect was monitored against concentration changes of species involved in the C2H6 + H2 → 2CH4 reaction, and back reactions were shown to stop completely only at relatively rapid cooling of 120 deg/s. The fluids cooling at slower rates contained less hydrogen (2 to 4 mol.%) and, correspondingly, more CH4. The closure temperature for the C-O-H system equilibration was suggested to be <800 °C[29]. Earlier we[31] investigated the effect of cooling rates in the range of 1 to 200 deg/s on the composition of C-O-H fluids synthesised at 6.3 GPa and 1600 °C in 15 hr-long runs. Comparison of fluids generated from the same starting materials but cooled at different rates showed CH4 and C2H6 to increase from 2–3 to 9–10 mole % and from 1 to 3 mole %, respectively, as cooling slowed down from 200 to 27 deg/s. Further deceleration to 1 deg/s did not change the concentration of CH4, but led to C2H6 increase from 3 to 4–5 mol.%. Therefore, synthesis of hydrocarbons most likely was due to back reactions of hydrogen with graphite during slow cooling, as we suggested[31]. Meanwhile, the changes of nitrogen speciation in O-H-N fluids at cooling rates from 70 to 140 deg/s in the forward and reversal experiments of Li and Keppler (ref. 3) were negligible, if any.

In this stuclass="Chemical">dy we have class="Chemical">performeclass="Chemical">pan class="Chemical">d three 2-hr long runs at 1400 °C, with quenching at 200 deg/s and slow cooling at 1 deg/s (Tables 1–3). The normalised peak areas for particular species in GC-MS spectra showed slight variations in lower alkanes in slowly cooling fluids with similar amounts of water (24–31 rel.%). The CH4/C2H6 ratio was above and below 1 at rapid and slow cooling, respectively. Cooling at 200 times slower rates did not cause notable changes to CH4, C2H6, C3H8 and C4H10 concentrations (Table 2) but led to about ten-fold increase of C15 to C19 alkanes (from ~0.01 to ~0.1) and formation of minor amounts (within ~0.5 rel.%) of olefins, arenes, and oxygenated hydrocarbons: aldehydes, ketones, and carboxylic acids (specifically, 0.2 rel.% acetic acid and higher concentrations of acids with C10. C13 and C15). Note especially that slow cooling resulted in the formation of more methanimine (CH3N), at the same P-T-τ conditions as during quenching (Table 2).

Table 1

Starting compositions (mg). Microscopic amounts of nitrogen in the capsules came from air. 

Run#	Graphite	Docosane (C22H46)	Stearic acid (C18H36O2)
1761_2_3	18.7	1.8	—
1769_2_2	8	0.3	0.3
1315_3_5	26.2	—	3.1
1751_2_2	8.8	—	0.9
1751_2_3	9	0.6	—
1746_2_3	9.4	0.7	—
1746_2_2	8.4	—	0.9
1780_2_2	7.6	—	0.7
1780_2_3	8.1	0.8	—
1780_2_4	8.3	—	1.0
1016_7_2	8.2	—	0.5
1016_7_4	8.8	0.6	—
1753_2_3	18.6	—	1.7
1898_2_1	7.1	—	0.5
1898_2_3	7.8	0.3	0.3
888_7_1	23	1.9	—
889_7_1	23.7	1.4	—
1720_2_2	17.7	—	3.2
1727_2_2	17.8	3.9	—
1019_7_1	8.1	—	0.7
1019_7_3	7.3	0.4	0.3

Table 3

Representative analyses of quenched and slow cooling C-O-H-N fluids (rel.%) obtained in 2-hr runs at 6.3 GPa and 1400 °C.

Run#		1753_2_3	1898_2_3	1898_2_1
Capsule		Pt	Pt	Pt
Starting composition*		Stearic acid	Stearic acid + docosane	Stearic acid
Cooling rate		200 deg/s	1 deg/s	1 deg/s
Water		30.8	24.3	29.7
Alkanes	CH4	37.6	17.0	16.1
	C2H6	20.0	34.5	31.9
	C3H8	8.6	14.3	14.9
	C4H10	2.2	2.1	2.2
	C5H12	0.08	0.1	0.09
	C6-C15	≤0.002	≤0.04	≤0.03
	C15-C19 **	≤0.01	≤0.3	≤0.1
Olefins		—	≤0.06	≤0.02
Arenes		—	≤0.008	≤0.02
Alcohols and ethers		≤0.1	≤0.1	≤0.08
Aldehydes		—	≤0.07	≤0.08
Ketones		≤0.005	≤0.03	≤0.05
Carboxylic acid		—	≤0.5	≤0.05
Furans		≤0.001	≤0.01	≤0.03
Nitrogen species
N2		—	0.05	0.01
CH3N		0.5	>1	>1

*All samples contained graphite. **The quoted concentrations are maximum for a species of the respective group. All GC-MS data are given in Supplementary Table 1.

Table 2

Experimental conditions and concentrations of main species (rel.%) in quenched C-O-H-N fluids synthesised at 6.3 GPa.

Run#	Starting composition*	Capsule	Buffer fH2	Time (h)	T (°C)	H2O	CH4	C2H6	C3H8	C4H10	CH3N/(CH3N + N2)	Calc. fO2
1761_2_3	Docosane	Pt	MMO	0.017	1100	11	10	74	5.2	0.2	1.00	—
1769_2_2	Stearic acid + docosane	Pt	MMO	7	1100	16	35	31	14	0.2	0.22	−10.8
1315_3_5	Stearic acid	Pt	—	7	1200	93	2.9	0.7	0.4	1.0	0.03	−8.5
1751_2_2	Stearic acid	Au	MMO	7	1200	41	37	18	2.7	0.2	0.99	−10.8
1751_2_3	Docosane	Au	MMO	7	1200	9.6	50	31	8.1	0.4	0.99	−15.4
1746_2_3	Docosane	Pt	MMO	2	1300	11	26	38	21	1.7	0.80	−9.0
1746_2_2	Stearic acid	Pt	MMO	2	1300	39	23	28	5.0	1.8	0.98	−10.5
1780_2_2	Docosane	Pt	MMO	7	1300	4.2	27	36	24	5.7	0.95	−9.3
1780_2_3	Stearic acid	Pt	MMO	7	1300	55	13	24	4.9	0.9	Only N2	−8.5
1780_2_4	Stearic acid***	Pt	MMO	7	1300	6.9	5.2	4.2	0.2	0.02	Only N2	—
1016_7_2	Stearic acid	Pt	MMO	0.017	1400	25	11	31	23	3.6	0.93	—
1016_7_4	Docosane	Pt	MMO	0.017	1400	32	11	31	17	4.2	0.93****	—
1753_2_3	Stearic acid	Pt	MMO	2	1400	31	38	20	8.6	2.2	Only CH3N	−10.6
1898_2_1	Stearic acid	Pt	MMO	2	1400**	30	16	32	15	2.2	1.00	−10.5
1898_2_3	Stearic acid + docosane	Pt	MMO	2	1400**	24	17	35	14	2.2	1.00	−10.9
888_7_1	Docosane	Pt	MMO	2	1400**	4.4	23	71	0.8	1.0	1.00	—
889_7_1	Docosane	Pt	MMO	7	1400	0.7	51	38	8.7	0.6	0.69	—
1720_2_2	Stearic acid	Pt	—	7	1400	96	0.06	1.4	0.08	0.8	Only N2	−6.0
1727_2_2	Docosane	Pt	MMO	7	1400	11	22	34	4.9	3.5	0.72	−8.8
1019_7_1	Stearic acid	Pt	MMO	10	1400	52	18	25	2.7	1.5	0.98	−8.1
1019_7_3	Stearic acid + docosane	Pt	MMO	10	1400	26	18	35	14	3.3	0.99	−11.0

Full compositions are given in Supplementary Table 1. MMO is Mo-MoO2 buffer. CH3N/(CH3N + N2) is normalised peak area ratio A(17 + 29 m/z)/(A(17 + 29 m/z) + A(28 m/z)). *All starting compositions include graphite, at 10/1 to fluid generating material. **Fluid cooling at 1 deg/s. ***Very little gas released upon capsule opening. ****NH3 is main nitrogen species.

Starting class="Chemical">comclass="Chemical">positions (mg). Microsclass="Chemical">pan class="Chemical">copic amounts of nitrogen in the capsules came from air.

Experimental class="Chemical">conclass="Chemical">pan class="Chemical">ditions and concentrations of main species (rel.%) in quenched C-O-H-N fluids synthesised at 6.3 GPa.

Full class="Chemical">comclass="Chemical">positions are given in Suclass="Chemical">pclass="Chemical">plementary Table 1. MMO is class="Chemical">pan class="Chemical">Mo-MoO2 buffer. CH3N/(CH3N + N2) is normalised peak area ratio A(17 + 29 m/z)/(A(17 + 29 m/z) + A(28 m/z)). *All starting compositions include graphite, at 10/1 to fluid generating material. **Fluid cooling at 1 deg/s. ***Very little gas released upon capsule opening. ****NH3 is main nitrogen species.

Representative analyses of quencheclass="Chemical">d anclass="Chemical">pan class="Chemical">d slow cooling C-O-H-N fluids (rel.%) obtained in 2-hr runs at 6.3 GPa and 1400 °C.

*All samples class="Chemical">containeclass="Chemical">pan class="Chemical">d graphite. **The quoted concentrations are maximum for a species of the respective group. All GC-MS data are given in Supplementary Table 1.

Therefore, back reactions in slowly class="Chemical">cooling class="Chemical">pan class="Chemical">C-O-H-N fluids have different directions at the pressures 2.4 GPa and 6.3 GPa. Cooling leads mainly to synthesis of CH4 and consumption of alkanes by the reaction C2H6 + H2 = 2CH4 at the lower pressure[29], but CH4 either increases[31] or decreases (this study) slightly at 6.3 GPa, while the ratios of CH4 to C2H6, C3H8 and C4H10 at high and low cooling rates scatter within a reasonable error. However, slow cooling fluids obtained in this study are remarkable by higher concentrations of species that trace the back reactions (some olefins, arenes, aldehydes, ketones, and carboxylic acids). Thus, our and published data, including our previous results for 6.3 GPa and 1600 °C[31], indicate that only cooling from 1100–1400 °C to room temperature at a rate of 200 deg/s can be interpreted as quenching. Note that only quenching can provide reliable molecular compositions of fluids at the P–T conditions of the experiments.

Kinetics of C-O-H-N fluid equilibration

The efclass="Chemical">fect of kinetics on class="Chemical">pan class="Chemical">concentrations of species that form in the C-O-H-N system at 6.3 GPa and 1400 °С was studied in quenching experiments of different durations from 1 min to 10 hours (Fig. 1 and Table 2). The diversity of alkanes revealed by the GC-MS analysis after capsule opening showed almost no run duration dependence. Lower alkanes, which are the dominant carbon species in the C-O-H-N fluids at 1400 °С, did not change much the CH4/C2H6, CH4/C3H8 and CH4/C4H10 ratios of normalised peak areas at longer durations, irrespective of the starting charge composition (Fig. 2). They were slightly higher in fluids generated in longer runs from samples with docosane or stearic acid and remained almost the same in the case of the docosane + stearic acid mixture: the CH4/C2H6 ratio was about 1 in all cases and CH4/C3H8 varied from 1 to 10. Note that CH4/C4H10 reached or slightly exceeded 100 in fluids obtained in 2- and 7-hr long runs from samples with docosane at low H2O concentrations but remained below 10 in the case of relatively high H2O concentrations with docosane + stearic acid mixture and stearic acid as starting materials.

Figure 1

Diversity of alkanes, alkenes, and oxygenated hydrocarbons revealed by GC-MS analysis in quenched C-O-H-N fluids at 6.3 GPa, 1400 °C, MMO-buffered fH2 and run durations from 0.017 to 10 hours. Symbols on top show starting compositions of charges for fluid generation: d = docosane; d + s = docosane + stearic acid mixture; s = stearic acid. White columns are unquenched samples cooled down at 1 deg/s. Grey and white bands show run groups.

Figure 2

Normalised peak area ratios of CH4/C2H6, CH4/C3H8 and CH4/C4H10 as a function of duration, according to GC-MS analysis of quenched fluids synthesised at 6.3 GPa and 1400 °C from different starting compositions: a = docosane; b = docosane + stearic acid mixture; c = stearic acid.

class="Chemical">Diversity of class="Chemical">pan class="Chemical">alkanes, alkenes, and oxygenated hydrocarbons revealed by GC-MS analysis in quenched C-O-H-N fluids at 6.3 GPa, 1400 °C, MMO-buffered fH2 and run durations from 0.017 to 10 hours. Symbols on top show starting compositions of charges for fluid generation: d = docosane; d + s = docosane + stearic acid mixture; s = stearic acid. White columns are unquenched samples cooled down at 1 deg/s. Grey and white bands show run groups.

Normaliseclass="Chemical">d class="Chemical">peak area ratios of class="Chemical">pan class="Chemical">CH4/C2H6, CH4/C3H8 and CH4/C4H10 as a function of duration, according to GC-MS analysis of quenched fluids synthesised at 6.3 GPa and 1400 °C from different starting compositions: a = docosane; b = docosane + stearic acid mixture; c = stearic acid.

class="Chemical">Olefins, class="Chemical">pan class="Chemical">arenes, alcohols, aldehydes, ketones, and carboxylic acids, as well as N-bearing spices, were more diverse in the shortest 1-min runs. Note that the species diversity of oxygenated hydrocarbons was notably smaller in 2-hr experiments but greater in the longest runs (10 hr), though remained below the level of the 1-min run (Fig. 1; 2-hr runs at slow cooling (white histograms) are shown for comparison). Thus the quenched non-equilibrium C-O-H-N fluids obtained in short runs had strongly variable concentrations and high diversity of olefins, arenes, alcohols and ethers, aldehydes, ketones and carboxylic acids, as well as nitrogen-bearing species. The behaviour of these components can be used as evidence of a non-equilibrium composition of reduced fluids.

At 1100 °C, the class="Chemical">concentrations of main class="Chemical">pan class="Chemical">alkanes and the diversity of oxygenated hydrocarbons in C-O-H-N fluids differed notably in the shortest (1-min) and 7-hour long runs (Tables 2–4). Note that methane was very low while ethane was high (CH4/C2H6 = 0.14) in the shortest run, but their concentrations became similar (CH4/C2H6 = 1.14) and commensurate with those at 1400 °C at the 7 hr duration. The quenched fluids obtained in 7-hr runs at 1100 °C almost lacked oxygenated hydrocarbons, except for some alcohols and ethers and furans. This data shows that the C-O-H-N fluids resulting from thermal decomposition of higher alkanes at 6.3 GPa and 1400 °C can attain equilibrium already in 2-hr runs, while the attainment of equilibrium at 1100 °C most likely requires at least 7-hr durations. Interestingly, alkanes reached equilibrium concentrations in 1 min at 1400 °C, while fluids synthesised in 1-min long runs at 1100 °C were very rich in ethane.

Table 4

Representative analyses of quenched reduced C-O-H-N fluids (rel.%) obtained in runs at 6.3 GPa and 1100–1400 °C.

Run#		1761_2_3**	1751_2_3	1780_2_2	1727_2_2	889_7_1
Capsule		Pt	Au	Pt	Pt	Pt
Temperature (°C)		1100	1200	1300	1400	1400
Duration (hr)		0.017	7	7	7	7
Starting composition: Docosane*
Water		10.4	9.5	4.2	11	0.7
Alkanes	CH4	10.0	49.7	26.9	22.3	51.0
	C2H6	73.8	31.1	36.5	33.9	38.5
	C3H8	5.2	8.1	23.9	4.9	8.7
	C4H10	0.2	1.1	5.7	3.5	0.6
	C5H12	0.01	0.05	0.2	1.5	0.001
	C6-C15	≤0.002	—	—	≤0.6	≤0.01
	C15-C19 ***	≤0.004	—	≤0.4	≤0.2	≤0.01
Olefins		≤0.001	—		—	—
Arenes		≤0.001	—	—	≤0.6	—
Alcohols and ethers		≤0.006	≤0.05	≤0.1	≤0.06	≤0.2
Aldehydes		—	—	—	≤0.3	≤0.001
Ketones		—	—	—	≤0.6	≤0.002
Carboxylic acid		≤0.002	—	—	≤0.2	—
Furans		≤0.001	—	—	—	—
Nitrogen species
N2		0.01	0.02	0.1	0.3	0.02
CH3N		>1	>1	>1	0.7	0.05

*All samples contained graphite; **Data given for comparison. ***The quoted concentrations are maximum for a species of the respective group. All GC-MS data are given in Supplementary Table 1.

Representative analyses of quencheclass="Chemical">d reclass="Chemical">pan class="Chemical">duced C-O-H-N fluids (rel.%) obtained in runs at 6.3 GPa and 1100–1400 °C.

*All samples class="Chemical">containeclass="Chemical">pan class="Chemical">d graphite; **Data given for comparison. ***The quoted concentrations are maximum for a species of the respective group. All GC-MS data are given in Supplementary Table 1.

Speciation of fluids at fO2 near IW

We have class="Chemical">comclass="Chemical">pareclass="Chemical">pan class="Chemical">d the compositions of carbon and nitrogen species in equilibrated C-O-H-N fluids generated by thermal destruction of docosane in 7-hr long runs at 1100–1200 °C and in ≥ 2-hr runs at 1300–1400 °C (Tables 2 and 4; Figs 3 and 4a), at H2O from <1 to 11.2 rel.% (CO2 within 0.006 rel.%; CO not detected). Note that the chosen fH2 buffering technique provided Fe3+ to Fe0 reduction in a 10-hr test run (Supplementary materials, Figs 1–3). At a higher temperature of 1400 °C, the number of detected alkanes increased from 7–11 at 1200–1300 °C to 14 and even reached 29 in one run. The methane-to-ethane ratios (normalised peak areas) varied from slightly above to slightly below 1, while the CH4/C3H8 and CH4/C4H10 ratios were 1 to 30 and ~100, respectively, without any distinct trend (Fig. 4a). Other species were found in minor amounts of <1 rel.%, except for 0.2 rel.% C16-C17 alkanes and 0.4 rel.% C18 in one run at 1300 °C (Table 4, Supplementary Table 1) against ≤0.01 rel.% in all other runs. Aldehydes and ketones likewise were more diverse in higher-temperature runs at 1400 °C (Fig. 3; white histograms are species in Au capsules), while the number of alcohols and ethers remained approximately the same.

Figure 3

Diversity of alkanes, alkenes, and oxygenated hydrocarbons revealed by GC-MS analysis in quenched C-O-H-N fluids at 6.3 GPa, 1100–1400 °C, MMO-buffered fH2 and run durations ≥ 2 hours. White columns are samples obtained in Au capsules. Letter symbols are same as in Fig. 2.

Figure 4

Normalised peak area ratios of CH4/C2H6, CH4/C3H8 and CH4/C4H10 as a function of temperature, according to GC-MS analysis of quenched fluids synthesised at 6.3 GPa from different starting compositions: a = docosane; b = docosane + stearic acid mixture and stearic acid.

class="Chemical">Diversity of class="Chemical">pan class="Chemical">alkanes, alkenes, and oxygenated hydrocarbons revealed by GC-MS analysis in quenched C-O-H-N fluids at 6.3 GPa, 1100–1400 °C, MMO-buffered fH2 and run durations ≥ 2 hours. White columns are samples obtained in Au capsules. Letter symbols are same as in Fig. 2.

Normaliseclass="Chemical">d class="Chemical">peak area ratios of class="Chemical">pan class="Chemical">CH4/C2H6, CH4/C3H8 and CH4/C4H10 as a function of temperature, according to GC-MS analysis of quenched fluids synthesised at 6.3 GPa from different starting compositions: a = docosane; b = docosane + stearic acid mixture and stearic acid.

As for class="Chemical">nitrogen sclass="Chemical">peciation, the exclass="Chemical">perimental results (Tables 2, 4, Suclass="Chemical">pclass="Chemical">plementary Table 1) showeclass="Chemical">pan class="Chemical">d methanimine (CH3N, CAS # 2053-29-4) to be the predominant nitrogen host in N-poor C-O-H-N fluids. However, N2 was persistent, though minor (0.1 to 0.01 rel.%), even in ultra-reduced C-O-H-N fluids resulting from decomposition of docosane, with the CH3N/N2 > 2 ratio in all cases. Ammonia appeared in a single 1-min run at 1400 °С, with its concentration notably exceeding both N2 and CH3N. Other nitrogen species were detectable in trace amounts (Fig. 3 and Supplementary Table 1).

Speciation of fluids at fO2 slightly above IW

The normaliseclass="Chemical">d class="Chemical">peak areas of class="Chemical">pan class="Chemical">water in the chromatograms of quenched equilibrium fluids generated by thermal decomposition of stearic acid or the stearic acid + docosane mixture varied from 16 to 55 rel.%. The concentration of CO2 reached 0.9 rel.% at 1100 °С but decreased to 0.01–0.1 rel.% at 1300–1400 °С. No CO was detectable at higher water contents, as in the case of more reduced fluids. C1-C4 alkanes in C-O-H-N fluids showed more prominent trends with increasing water contents (Figs 2–5). As the run temperature increased to 1400 °С instead of 1200 °С, the CH4/C2H6 ratio of normalised peak areas decreased only from slightly above 1 to slightly below 1, as at low H2O, but the CH4/C3H8 and CH4/C4H10 ratios became ten times smaller: from 10 to 1 and from 100 to 10, respectively. Note that CH4/C4H10 at 1400 °С were much lower at high than at low water contents.

Figure 5

Diversity of alkanes, alkenes, and oxygenated hydrocarbons revealed by GC-MS analysis in quenched C-O-H-N fluids as a function of normalised areas of H2O at 6.3 GPa, 1400 °C, run duration ≥2 hours in MMO-buffered fH2 and in unbuffered experiments. Letter symbols are same as in Fig. 2.

class="Chemical">Diversity of class="Chemical">pan class="Chemical">alkanes, alkenes, and oxygenated hydrocarbons revealed by GC-MS analysis in quenched C-O-H-N fluids as a function of normalised areas of H2O at 6.3 GPa, 1400 °C, run duration ≥2 hours in MMO-buffered fH2 and in unbuffered experiments. Letter symbols are same as in Fig. 2.

Synthesis in class="Chemical">Pt caclass="Chemical">psules at 1100  anclass="Chemical">pan class="Chemical">d 1300 °С gave fluids with C16-C19 alkanes from 0.2 to 0.9 rel.% and to 0.2 rel.%, respectively (Table 5). The respective alkane contents in fluids from Au capsules at 1200 °С were no higher than 0.06 rel.% and were still lower in Pt capsules at 1400 °С. Oxygenated hydrocarbons in the fluids were diverse in both Au and Pt capsules (Fig. 3). The contents of the detected species did not show marked variation trends. Among oxygenated hydrocarbons, alcohols and carboxylic acids had the largest contents. Note that a water-rich quenched fluid synthesised at 1300 °С in a 7-hr long run contained 0.1 rel.% methanol, ethanol, and benzoic acid (C7H6O2).

Table 5

Representative analyses of quenched H2O-rich C-O-H-N fluids (rel.%) obtained in runs at 6.3 GPa and 1100–1400 °C.

Run#		1769_2_2	1751_2_2	1780_2_3	1019_7_3	1019_7_1
Capsule		Pt	Au	Pt	Pt	Pt
Temperature (°C)		1100	1200	1300	1400	1400
Duration (hr)		7	7	7	10	10
Starting composition*		Stearic acid + docosane	Stearic acid	Stearic acid	Stearic acid + docosane	Stearic acid
Water		16.4	40.6	54.6	26.5	51.9
Alkanes	CH4	35.2	37.2	13.1	17.9	17.9
	C2H6	30.8	18.2	24.0	35.3	24.6
	C3H8	13.6	2.7	4.9	13.9	2.7
	C4H10	0.2	0.9	0.9	3.3	1.5
	C5H12	0.02	0.05	0.03	0.2	0.1
	C6-C15	≤0.04	≤0.001	≤0.1	≤0.04	≤0.02
	C15–C19 **	≤0.9	≤0.06	≤0.2	≤0.02	≤0.01
Olefins	—	—	—	≤0.001	≤0.01	≤0.01
Arenes	—	—	—	≤0.003	≤0.001	≤0.001
Alcohols and ethers	≤0.2	≤0.002	≤0.8	≤0.01	≤0.02	≤0.02
Aldehydes	—	≤0.05	—	≤0.003	≤0.02	≤0.02
Ketones	—	≤0.01	—	≤0.007	≤0.01	≤0.01
Carboxylic acid	—	<0.001	≤0.1	≤0.09	≤0.02	≤0.02
Furans	≤0.006	≤0.001	≤0.003	≤0.001	≤0.01	≤0.01
Nitrogen species
N2	0.4	0.02	0.08	0.01	0.01	0.01
CH3N	0.1	>1	—	>1	0.9	0.9

*All samples contained graphite. **The quoted concentrations are maximum for a species of the respective group. All GC-MS data are given in Supplementary Table 1.

Representative analyses of quencheclass="Chemical">d class="Chemical">pan class="Chemical">H2O-rich C-O-H-N fluids (rel.%) obtained in runs at 6.3 GPa and 1100–1400 °C.

*All samples class="Chemical">containeclass="Chemical">pan class="Chemical">d graphite. **The quoted concentrations are maximum for a species of the respective group. All GC-MS data are given in Supplementary Table 1.

class="Chemical">Methanimine (class="Chemical">pan class="Chemical">CH3N) was the main nitrogen species of C-O-H-N fluids at high-H2O as well (Table 2, Fig. 6 and Supplementary Table 1), but no ammonia was detected in any run. Persistent N2, from 0.02 to 0.4 rel.%, was observed in the fluids generated by decomposition of stearic acid or the stearic acid + docosane mixture, as in the case of reduced fluids. The normalised peak area ratios corresponding to CH3N/N2 in quenched fluids were A(17 + 29 m/z)/A(28 m/z) ≫ 1 in most of analysed spectra. Methanimine predominated in the fluids from both Au and Pt capsules. It was found in small amounts only in four runs with Pt capsules (out of those, fH2 was not buffered in two runs and little fluid was released upon capsule opening in one run). Note that the concentration of CH3N increased at slow cooling (Fig. 6a,b).

Figure 6

Fragments of chromatograms of quenched fluids extracted from Pt capsules after runs 1019_7_3 (a) and 1898_2_3 (b). 1 = Total ion current (TIC) chromatograms; 2 = Reconstructed ion chromatograms (RIC) m/z 28 that characterise traces of molecular nitrogen, ethane, propane and butane; 3 = RIC m/z 16: methane; 4 = RIC m/z 17 + 29: methanimine; 5 = RIC m/z 44: traces of carbon dioxide, propane and butane; 6 = RIC m/z 30: ethane; 7 = RIC m/z 18: water.

Fragments of chromatograms of quencheclass="Chemical">d fluiclass="Chemical">pan class="Chemical">ds extracted from Pt capsules after runs 1019_7_3 (a) and 1898_2_3 (b). 1 = Total ion current (TIC) chromatograms; 2 = Reconstructed ion chromatograms (RIC) m/z 28 that characterise traces of molecular nitrogen, ethane, propane and butane; 3 = RIC m/z 16: methane; 4 = RIC m/z 17 + 29: methanimine; 5 = RIC m/z 44: traces of carbon dioxide, propane and butane; 6 = RIC m/z 30: ethane; 7 = RIC m/z 18: water.

Composition of fluids near CW

class="Chemical">Oxygen fugacity of class="Chemical">pan class="Chemical">C-O-H-N fluids could reach the water maximum (CW) in two unbuffered runs at 1200 and 1400 °С (Table 6) because of hydrogen leakage[30, 31]. The loss of hydrogen from the fluid led to its water enrichment and oxidation of the hydrocarbons originally produced by thermal decomposition of the starting materials. Contrary to our expectations, HCs demonstrated different degrees of stability to oxidation. The concentrations of methane and ethane decreased dramatically from 20–30% to a few percent and even to fractions of percent (Table 6) while the C3, C4 and C5 alkanes remained almost invariable. Some higher alkanes, especially, C16, were present in notable amounts: 0.04 rel.% at 1200 °С to 0.2 rel.% at 1400 °С. In the lower-temperature runs phtalates were observed as the main species among oxygenated hydrocarbons, possibly as a result of capsule contamination. The detected species were markedly more diverse in higher-temperature runs, where acetic acid was predominant. The diversity of HC species in fluids with 1 to 96 rel.% H2O (normalised peak areas) exhibited no distinct trends (Fig. 5). The fluids near CW contained N2 as the predominant nitrogen species and trace amounts of nitriles and azines (Supplementary Table 1).

Table 6

Representative analyses of quenched H2O-rich C-O-H-N fluids (rel.%) obtained in unbuffered runs at 6.3 GPa.

Run#		1315_3_5	1720_2_2
Capsule		Pt	Pt
Temperature (°C)		1200	1400
Duration (hr)		7	7
Starting composition*		Stearic acid	Stearic acid
Water		93.5	96.3
Alkanes	CH4	2.9	0.06
	C2H6	0.7	1.4
	C3H8	0.4	0.08
	C4H10	1.0	0.8
	C5H12	0.02	0.1
	C6-C14 **	—	≤0.08
	C15-C19	≤0.04	≤0.2
Olefins		—	—
Arenes		—	≤0.008
Alcohols and ethers		≤0.7	≤0.01
Aldehydes		≤0.003	≤0.01
Ketones		—	≤0.01
Carboxylic acid		≤0.008	<0.07
Furans		≤0.006	≤0.01
Nitrogen species
N2		0.5	0.01
CH3N		—	—

*All samples contained graphite. **The quoted concentrations are maximum for a species of the respective group. All GC-MS data are given in Supplementary Table.

Representative analyses of quencheclass="Chemical">d class="Chemical">pan class="Chemical">H2O-rich C-O-H-N fluids (rel.%) obtained in unbuffered runs at 6.3 GPa.

*All samples class="Chemical">containeclass="Chemical">pan class="Chemical">d graphite. **The quoted concentrations are maximum for a species of the respective group. All GC-MS data are given in Supplementary Table.

Calculated fO2 and carbon contents

class="Chemical">Oxygen fugacity (class="Chemical">pan class="Chemical">fO2) is the key parameter of fluid systems against which to compare the compositions of fluids synthesised in quenching experiments with data on natural mantle fluids. fO2 in the experimental samples was calculated by Gibbs energy minimisation at 6.3 GPa and 1100–1400 °C knowing the components of the obtained fluids. However, chromatography–mass spectrometry revealed more than 30 carbon and nitrogen species in the fluids, and the calculations required some assumptions. Primarily, the presence of nitrogen species found in very low concentrations was neglected, i.e., the C-O-H-N system was reduced to the C-O-H one. The contribution of alkanes higher than C5, as well as oxygenated hydrocarbons and CO2, all below 1 rel.%, into the fO2 of the system was likewise assumed negligible. With these assumptions, the system composition was simplified to H2O, CH4, C2H6, C3H8, and C4H10, and this limited number of main species was used in CG-MS calibrations (see Supplementary information). The normalised peak areas of particular components were converted to mole % only for a part of quenching experiments where equilibrium concentrations of the main components were obtained. They were further used to estimate C contents in the fluids as a function of temperature at constant fO2 and as a function of fO2 at a constant temperature of 1400 °C (Table 7 and Fig. 7). The results generally record markedly lower carbon contents at lower temperatures and higher fO2.

Table 7

Mole ratios of CH4, C2H6, C3H8 and C4H10, carbon content (mole %), and calculated fO2 in fluid phase, according to GC-MS calibration (see text for explanation).

Run#	Capsule	Buffer fH2	T, (°C)	Calc. fO2	CH4/C2H6	CH4/C3H8	CH4/C4H10	C, mole %
1769_2_2	Pt	MMO	1100	−10.8	20	5.1	10.8	3.0
1315_3_5	Pt	—	1200	−8.5	227	1384	839	0.01
1751_2_2	Au	MMO	1200	−10.8	2012	14912	47973	8.7
1751_2_3	Pt	MMO	1200	−15.4	2774	18853	59694	19.9
1746_2_3	Pt	MMO	1300	−9.0	22	109	237	6.3
1746_2_2	Pt	MMO	1300	−10.5	2063	36898	85957	14.1
1780_2_2	Pt	MMO	1300	−9.3	6.9	8.7	17	8.9
1780_2_3	Pt	MMO	1300	−8.5	5.7	2.8	5.8	4.9
1753_2_3	Pt	MMO	1400	−10.6	433	2412	4733	18.3
1898_2_1	Pt	MMO	1400	−10.5	224	1677	4297	18.2
1898_2_3	Pt	MMO	1400	−10.9	244	1181	4804	18.8
1720_2_2	Pt	—	1400	−6.0	5.7	6.7	11.1	0.01
1727_2_2	Pt	MMO	1400	−8.8	26	12	24.98	12.6
1019_7_1	Pt	MMO	1400	−8.1	176	2016	3471	7.3
1019_7_3	Pt	MMO	1400	−11.0	274	1711	4797	18.9

Figure 7

Composition of quenched fluids converted using GC-MS calibration with respect to main species: CH4, C2H6, C3H8, C4H10 and H2O. a: mole ratios CH4/C2H6, CH4/C3H8 and CH4/C4H10 in strongly reduced fluids at 6.3 GPa, 1100–1400 °C and constant fO2; b: same at 6.3 GPa, 1400 °C in a range of fO2; c: amount of carbon in fluids as a function of temperature; d: amount of carbon in fluids as a function of fO2.

Mole ratios of class="Chemical">CH4, class="Chemical">pan class="Chemical">C2H6, C3H8 and C4H10, carbon content (mole %), and calculated fO2 in fluid phase, according to GC-MS calibration (see text for explanation).

class="Chemical">Comclass="Chemical">position of quencheclass="Chemical">pan class="Chemical">d fluids converted using GC-MS calibration with respect to main species: CH4, C2H6, C3H8, C4H10 and H2O. a: mole ratios CH4/C2H6, CH4/C3H8 and CH4/C4H10 in strongly reduced fluids at 6.3 GPa, 1100–1400 °C and constant fO2; b: same at 6.3 GPa, 1400 °C in a range of fO2; c: amount of carbon in fluids as a function of temperature; d: amount of carbon in fluids as a function of fO2.

Discussion

Processes of fluid equilibration

Acclass="Chemical">corclass="Chemical">pan class="Chemical">ding to the reported results, the fluids experimentally generated at 6.3 GPa and 1100–1400 °C attain equilibrium with respect to all C-O-H-N components for the time from 2 to 7 hours. The greatest diversity of carbon species observed in 1-min runs indicates that thermal decomposition of docosane and stearic acid produces numerous non-equilibrium components of the system which disappear in longer runs. GC-MS analysis reveals light alkanes as main run products, with predominant methane and ethane (Table 2). The changes the HCs experience at the experimental conditions can either lengthen or shorten the hydrocarbon chains. For higher hydrocarbons, decomposition most likely occurs as thermal cracking[35] and 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 from higher hydrocarbons in homolysis and rearrangement reactions leads to further β-scission into alkene and alkyl radicals with shorter chains (Fig. 8a). Ethene 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 alkanes and new radicals.

Figure 8

Main chemical processes during formation of equilibrium fluids. (a) initial thermal formation of radical species from higher hydrocarbons in homolysis, rearrangement,  their further rearrangement and β-scission reactions; (b) thermal decarboxylation of carboxylic acids; (c) processes involving carboxyl acids; (d) hydrogen reduction of carbonyl compounds.

Main chemical processes class="Chemical">during formation of equilibrium fluiclass="Chemical">pan class="Chemical">ds. (a) initial thermal formation of radical species from higher hydrocarbons in homolysis, rearrangement,  their further rearrangement and β-scission reactions; (b) thermal decarboxylation of carboxylic acids; (c) processes involving carboxyl acids; (d) hydrogen reduction of carbonyl compounds.

Thermal class="Chemical">decarboxylation (Fig. 8b) of class="Chemical">pan class="Chemical">carboxylic acids (such as stearic acid) produces alkanes shorter for one carbon atom (e.g., relatively high amounts of heptadecane (Supplementary Table 1) in run #1016_7_2) which decompose in a regular way through dehydrogenation and β-scission (Fig. 8a). Catalytic reduction of fatty acids in excess of hydrogen yields primary alcohols. Further dehydration of alcohols to alkenes may be followed by catalytic hydrogenation on the Pt surface producing a mixture of alkanes and terminal alkenes (Fig. 8c).

However, raclass="Chemical">dical reactions of intermeclass="Chemical">pan class="Chemical">diates are also possible at each step of reduction, and varieties of shortened primary alcohols were observed in the experiments. The reactions can be catalysed by Pt, judging by the presence of methanol and ethanol in the fluids from Pt capsules and their absence in the case of Au capsules in 1200 °C runs # 1315_3_5, 1751_2_2 and 1751_2_3 (Supplementary Table 1).

The fluiclass="Chemical">ds generateclass="Chemical">pan class="Chemical">d in Au and Pt capsules differ in carbon speciation irrespective of run duration and temperature: methane is one of major fluid components (along with water) in both cases but the methane/ethane ratios of normalised peak areas are, respectively, CH4/C2H6 > 1 and <0.8 in the Au and Pt capsules (Table 2). This difference may be due to catalytic processes that involve Pt, which may facilitate homolytic dissociation of methane to methyl radicals on the surface at temperatures over 1000 °C[36]. The process accelerates with heating, and the methane/ethane ratio may decrease at higher temperatures. Indeed, the CH4/C2H6, CH4/C3H8, and CH4/C4H10 mole ratios we obtained by calibration were the lowest at 1400 °C and at constant fO2 (Fig. 7a). The 7-hr run at 1100 °C gave a spike, possibly, because the system failed to attain equilibrium. Further recombination of the formed radicals produces a mixture of lower HCs: ethane, propane and n-butane. The reactivity of ethane is restricted to catalytic dehydrogenation on the Pt surface with formation of ethene, but the excess of hydrogen (fH2 buffered at MMO) impedes this process by shifting the equilibrium toward the reagents. For this reason, ethane does not contribute to the formation of methyl/ethyl radicals and, hence, methane and higher HCs, and tends to accumulate. Nevertheless, the catalytic fission of n-butane produces two ethyl radicals that recombine and react with alkenes with further formation of higher HCs.

The fluiclass="Chemical">ds generateclass="Chemical">pan class="Chemical">d in both Au and Pt capsules contained various oxygen-bearing compounds, mainly alcohols, ethers, aldehydes, ketones and furans (Tables 4, 5, Supplementary Table 1 and Fig. 3). However, the contents of carbonyl compounds (aldehydes and ketones) were lower in less reduced fluids from Pt samples (Table 5), at comparable T-τ conditions, possibly, because of catalytic hydrogen reduction (Fig. 8d) on the Pt surface and subsequent dehydration of alcohols. The run duration dependence of carbon speciation patterns in Pt capsules (Fig. 1) shows that aldehydes and ketones are the least diverse in the case of 2-hr runs but more diverse in fluids generated in 10-hr runs, possibly, because the catalytical activity of Pt decreases with time.

Carbon speciation

Main class="Chemical">carbon sclass="Chemical">pecies in the synthesiseclass="Chemical">pan class="Chemical">d equilibrium fluids are CH4, C2H6, C3H8 and C4H10, and their concentrations (according to GC-MS analysis) agree well with the calculated relations of CH4 > C2H6 > C3H8 > C4H10 [20, 25, 28, 32, 33] almost in all cases (Fig. 7а,b and Table 7). The CH4/C2H6, CH4/C3H8, and CH4/C4H10 ratios are lower in the case of both higher temperature (1400 °С) and fO2 (2.5 log units above IW) at 6.3 GPa (Fig. 7а,b). Taking into account the H2O variations, this dependence is the reason why temperature and fO2 control the amount of carbon the hydrocarbon fluids can carry (Fig. 7c,d). Previous calculations by Huizenga et al. (ref. 25) for 5.0 GPa and 1227 °C gave a similar fO2 dependence of carbon contents in fluids synthesised in the C-O-H system. This similarity indicates that thermodynamic calculations provide a faithful idea of C behaviour in fluids subject to oxidation at CLM pressures. On the other hand, as we have demonstrated experimentally, the amount of carbon in the fluid phase can be notably lower at a lower temperature (1100 °C) at a constant oxygen fugacity of −log(fO2) = 11.

The equilibrium class="Chemical">comclass="Chemical">positions of the GC-MS analyseclass="Chemical">pan class="Chemical">d C-O-H-N fluids count more than 30 carbon and nitrogen species and contain up to 1 rel.% С15-С18 alkanes, alcohols, aldehydes, ketones, carboxyl acids, and furans (Tables 4–6 and Figs 1, 3 and 5). In some cases, the contents of species in the obtained fluid vary largely even in runs of the same temperature and starting composition. This variation may result from the presence of moisture that adsorbs, in smaller or larger amounts, on graphite flakes during the capsule assembly. The reduced fluids at fO2 near IW generated by thermal decomposition of docosane are relatively rich only in alcohols, and their composition apparently corresponds to fO2 at which diamonds crystallise from metal melts (ref. 12). At higher H2O, the concentrations of С15-С18 alkanes increase slightly, while the diversity remains basically the same in alkanes and increases notably in oxygenated hydrocarbons (especially, ketones and carboxylic acids). The effect of lower temperature (1200 °C instead of 1400 °C) is to reduce the diversity of alkanes and oxygenated hydrocarbons in more reduced fluids. As H2O exceeds 90 mole % and fO2 reaches 2.5 log units above IW, methane in the fluid almost zeroes down, while higher alkanes do not change much (Tables 4–6 and Fig. 5). For instance, the fluids with fO2 near CW contain 1 rel.% C2H6, fractions of percent C3H8 and C4H10, and C15–19 alkanes, as well as quite large amounts of oxygenated hydrocarbons, especially alcohols and carboxylic acids.

The efclass="Chemical">fect of class="Chemical">pan class="Chemical">Pt and Au capsules on the composition of equilibrium C-O-H-N fluids is seen in the results of 7-hr long runs at 1100–1300 °C (Table 2 and Fig. 3) where CH4/C2H6 in the fluids from Au capsules are higher than with Pt capsules (Table 2) which become involved in catalytic processes (see above). Fluids generated in the two types of capsules are similar in diversity of alkanes (Fig. 4), while H2O-rich fluids from Pt capsules have low or absent aldehydes and ketones, possibly, due to catalytic hydrogen reduction at the Pt surface, and subsequent dehydration of the formed alcohols. The composition similarity of the quenched fluids synthesised in Pt and Au capsules indicates that the Pt effect consists in faster attainment of equilibrium in the system and must be the smallest in 10-hr runs.

All class="Chemical">detecteclass="Chemical">pan class="Chemical">d carbon species, except alkanes, turn out to be very sensitive to cooling rates: they are much more diverse at slow (1 deg/s) cooling (Fig. 1). Therefore, back reactions upon cooling lead primarily to synthesis of oxygenated hydrocarbons.

Nitrogen speciation

class="Chemical">Nitrogen in the stuclass="Chemical">pan class="Chemical">died C-O-H-N fluids is mainly air N2 entrapped by the capsule assembly, which reacts with hydrogen, hydrocarbons, and graphite at the experiment P-T conditions to form methanimine (Table 2, Figs 6 and 8): CH3N with the formula . The fluids also contain trace amounts of other nitrogen species, such as some nitriles, mainly at 1400 °C and high H2O contents. CH3N/N2 ratios estimated by GC-MS (Fig. 9) show that CH3N is the main nitrogen species in almost all equilibrium reduced fluids; CH3N/N2 < 1 only at 1100 °C and fO2 about −11 log units. However, CH3N disappears at 1400 °C when fO2 becomes four orders of magnitude higher and approaches CW. The temperature and redox dependence of CH3N stability (with regard to the slope of buffers in the T-fO2 diagram of Fig. 10) indicates that the formation of CH3N is possible at low fO2.

Figure 9

Normalised peak area ratios of CH3N and N2 that characterise their role as nitrogen species in C-O-H-N fluids in a large range of T-fO2 parameters at 6.3 GPa. (a) 1100–1400 °C and constant fO2; (b) redox interval from strongly to moderately reduced conditions.

Figure 10

Concentrations of carbon (mole %) in reduced C-O-H-N fluids at 6.3 GPa, according to GC-MS analysis. Red circles are GC-MS-measured values for experimental compositions; blue dash line is the carbon isopleth. Light-green field shows T-fO2 range for cratonic lithospheric mantle at a depth of ~200 km (see text for explanation). IW and CW curves are after refs 1, 24 and 56.

Normaliseclass="Chemical">d class="Chemical">peak area ratios of class="Chemical">pan class="Chemical">CH3N and N2 that characterise their role as nitrogen species in C-O-H-N fluids in a large range of T-fO2 parameters at 6.3 GPa. (a) 1100–1400 °C and constant fO2; (b) redox interval from strongly to moderately reduced conditions.

class="Chemical">Concentrations of class="Chemical">pan class="Chemical">carbon (mole %) in reduced C-O-H-N fluids at 6.3 GPa, according to GC-MS analysis. Red circles are GC-MS-measured values for experimental compositions; blue dash line is the carbon isopleth. Light-green field shows T-fO2 range for cratonic lithospheric mantle at a depth of ~200 km (see text for explanation). IW and CW curves are after refs 1, 24 and 56.

Thus, class="Chemical">CH3N class="Chemical">pan class="Chemical">discovered in the C-O-H-N system at 6.3 GPa, 1100–1400 °C, and a fluid C/N ratio of ∼20 may be an important component in reduced mantle containing minor amounts of nitrogen. It may predominate as a nitrogen host already at fO2 near and below IW. A more detailed study is obviously required to constrain the stability of CH3N at the mantle P-T-fO2 conditions, with a special focus on nitrogen sources in the reactions that produce this species. Note that NH3 and N2 are stable in the N-rich C-O-H-N system generated by decomposition of melamine, as well as its mixture with stearic acid or docosane at C/N ratio in the fluid from 0.4 to 4.6, at similar P-T-fO2 [34], while the redox stability of CH3N is comparable with that of ammonia[3, 34]. To sum up, the behaviour of CH3N may control the deep nitrogen cycle in N-poor reduced peridotitic mantle. Specifically, silicate phases capable of dissolving ammonia in the presence of NH3-bearing fluids[2, 19, 37] hardly can host nitrogen in equilibrium with CH3N-bearing mantle fluids.

Carbon and nitrogen transport across redox and thermal barriers in the mantle

The most important class="Chemical">carbon anclass="Chemical">pan class="Chemical">d nitrogen species of C-O-H-N fluids in the upper mantle revealed in this study of N-poor fluids, as well as in previous results for N-rich fluids[34], are CH4 and CH3N in the depleted domains with ∼20 ppm C and ∼1 ppm N[38, 39], and CH4 and NH3 in the enriched domains containing ∼250 ppm C and ∼100 ppm N[38, 39] (Fig. 11). Note that this inference is based on experiments with a simplified C-O-H-N system limited to four components, whereas the speciation in the natural mantle fluids is much more complex. Namely, N-rich fluids derived from a subducting slab may be quite rich in chlorides[40]. The appearance of the Cl− ion in the fluid may be coupled to NH4 + forming a stable ligand. Furthermore, the real fluid phase in the mantle occurs in the interstitial space of silicate rocks. As it was shown previously[6], the pH values of the eclogitic fluids are strongly alkaline, which supports the model of ionic C-bearing species. At the same time, fluids in equilibrium with mantle peridotite minerals generally contain species in the molecular form[6], which is consistent with the conventional fluid models.

Figure 11

Schematic of the upper mantle volatile cycle and main carbon and nitrogen species of C-O-H-N fluids in depleted and enriched domains[38, 39]. Black arrows show paths and estimates of the relative magnitudes of carbon, nitrogen and hydrogen fluxes and blue arrow is mantle upwelling. Right panel: potential mantle fO2 as a function of depth. The Fe0-FeO and Fe3+/Fe2+ (equilibria involving the exchange of Fe3+ and Fe2+ between silicate minerals) curves are according to refs 1 and 56. Oxygen fugacity of CW (maximum H2O content in C-O-H fluids), EMOD/G (equilibria enstatite + magnesite = olivine + graphite/diamond) and MORB (mid-ocean ridge basalts) are according to refs 1 and 24. ‘Redox melting’ process at depths of 120–150 km is according to refs 1 and 24.

Schematic of the upper mantle volatile cycle anclass="Chemical">d main class="Chemical">pan class="Chemical">carbon and nitrogen species of C-O-H-N fluids in depleted and enriched domains[38, 39]. Black arrows show paths and estimates of the relative magnitudes of carbon, nitrogen and hydrogen fluxes and blue arrow is mantle upwelling. Right panel: potential mantle fO2 as a function of depth. The Fe0-FeO and Fe3+/Fe2+ (equilibria involving the exchange of Fe3+ and Fe2+ between silicate minerals) curves are according to refs 1 and 56. Oxygen fugacity of CW (maximum H2O content in C-O-H fluids), EMOD/G (equilibria enstatite + magnesite = olivine + graphite/diamond) and MORB (mid-ocean ridge basalts) are according to refs 1 and 24. ‘Redox melting’ process at depths of 120–150 km is according to refs 1 and 24.

The ascent of N-bearing reclass="Chemical">duceclass="Chemical">pan class="Chemical">d fluids is an important element of the carbon and nitrogen cycles in the mantle[2, 5]. On their way to shallow mantle, these fluids can cross redox and/or temperature barriers, such as multiple redox fronts[24] in the lithosphere or the boundary between hotter asthenosphere and colder lithosphere[41]. Our experiments and published calculation results[20, 25, 28, 32, 33] have implications for the behaviour of fluids crossing such redox and thermal barriers. Carbon concentrations in reduced fluids within the T-fO2 range of CLM at a depth of ∼200 km vary as plotted in Fig. 10. The reduced fluids stable at ∼IW lose almost all carbon (about 18 mole % С, Figs 7e and 10) upon isothermal oxidation at 1400 °С to fO2 about CW. For the fluids with methane as a predominant carbon species, the experimentally obtained and calculated[25] for 5.0 GPa and 1227 °C amounts of carbon released during oxidation are in good agreement. The novelty of the experimental results is that HCs show different degrees of stability to oxidation, and this difference can affect the carbon speciation in the fluid. Namely, oxidation can be expected to decrease the concentrations of methane and ethane (much more strongly for the former) but to cause almost no effect on C3, C4 and C5 alkanes (Table 2 and Fig. 7b), as well as on some C15-C19 alkanes which remain within fractions of percent (Table 6). Note also that H2O-rich fluids at 1400 °С contain diverse oxygenated hydrocarbons (Fig. 5), with predominant acetic acid. As for nitrogen species, we infer CH3N to become less important and the role of N2 to grow as oxidation progresses (Fig. 9b).

The question whether any class="Chemical">hydrocarbons can survive transclass="Chemical">port across the mantle reclass="Chemical">pan class="Chemical">dox fronts is essential for understanding the deep carbon cycle. Neither the stability of higher alkanes at fO2 near CW nor the oxidation patterns of hydrocarbon fluids have been studied experimentally at mantle P-T conditions. In our previous quenching experiments with anthracene (C14H10), performed without external fH2 buffering[31], oxidation of the C-O-H system produced an aqueous fluid with trace amounts of CH4 (0.5 to 0.7 mole%) and C2H6 (≤0.1 mol.%). According to the data of this study, hydrous silicate magmas generated within multiple redox fronts can entrap minor amounts of some higher alkanes and oxygenated hydrocarbons of mantle origin and carry them further to shallow mantle.

As moclass="Chemical">deling by Stachel anclass="Chemical">pan class="Chemical">d Luth (ref. 5) demonstrates, less than 50 ppm fluid are required to completely reset the redox state of depleted cratonic peridotite to that of the fluid. Taking into account the strongly reduced chemistry of most peridotites at the depths of diamond stability[20-23], Stachel and Luth (ref. 5) conclude that redox fronts can be unstable to interaction with hydrocarbon fluids and that the last fluids to interact with the deep CLM are generally reducing. The fluids which penetrate into reduced but colder CLM domains and cool down from 1400 °С and fO2 ∼IW to 1100 °С at fO2 slightly above IW are inferred to loose carbon (decreasing from 18 to 3 mole %) but gain water (Figs 7a,c and 10). The cooled fluids contain more ethane, propane, and butane but much less diverse oxygenated hydrocarbons, mainly methanol and ethanol (Fig. 3 and Table 5, Supplementary Table 1). As for nitrogen speciation, N2 is expected to be predominant instead of CH3N (CH3N/N2 < 1) (Fig. 9a) in the cooled peridotitic fluid, which contains mainly molecular forms of species.

Diamond formation

class="Chemical">Carbon-bearing fluiclass="Chemical">pan class="Chemical">ds in CLM can act as both carbon carriers and diamond crystallisation environment[5, 42–45]. The reaction of hydrocarbons with O2 at multiple redox fronts[24] stimulates the activity of H2O and causes drastic reduction of total carbon content in the fluids, as well as rapid drop in the solidus temperature of the ambient rocks. The redox melting is considered to be an important process in the cratonic mantle lithosphere[24]. As a result of oxidation, carbon of methane and other hydrocarbons is inferred to release in the form of C0 and to become consumed for saturation with the aqueous fluid. Experimental data show that this C0 could be involved in diamond crystallisation. The chemistry of mineral inclusions in diamond[46, 47] and stable isotope data of carbon and nitrogen[48] indicate distinct possibility of diamond precipitation from CH4. Higher alkanes and some oxygenated hydrocarbons have been shown experimentally to be stable against oxidation of hydrocarbon fluids till the water maximum (Table 6). Therefore, they should be present in the diamond crystallisation environment, along with water, and can be entrapped as inclusions. Recent studies[8-11] confirm the presence of higher HCs in inclusions from diamond and its syngenetic mantle-derived minerals.

The available experimental class="Chemical">data on crystallisation of class="Chemical">pan class="Chemical">diamond from the fluid phase[49-53] indicate the existence of important, possibly, kinetic barriers impeding its spontaneous nucleation and further growth in the mantle fluid. Diamond has never been synthesised from strongly reduced fluids in the field of its thermodynamic stability, even in the presence of metastable graphite[31, 52]. Note that none of our experiments, from 1 minute to 10 hours long, led to spontaneous diamond nucleation, even near CW. As we showed earlier[31], diamond can nucleate and grow at run duration at least 40 hours, at 1600 °C and at relatively high fO2 near CW, in H2O-rich fluids. At lower temperatures, diamond nucleation begins even in oxidised carbonatitic fluids after a much longer induction period[44, 51, 54]. Thus, only oxidation of fluids within redox fronts at temperatures approaching 1400 °С can maintain effective diamond formation. Fluids cooling in strongly reduced mantle can release excess carbon as metastable graphite. The fate of this graphite within CLM may be different: it may either remain in the metastable state for an infinitely long time or convert to diamond upon interaction with oxidised alkaline metasomatic fluids[44, 51, 54]. This mechanism is similar to the model suggested by Jablon and Navon (ref. 55) as universal for most diamonds from CLM.

Conclusions

Experiments at 6.3 class="Gene">GPa show that fluiclass="Chemical">pan class="Chemical">ds generated by thermal decomposition of docosane and stearic acid can attain equilibrium for 2 and 7 hours at 1400 and 1100 °C, respectively. The shortest 1-min runs lead to the formation of numerous non-equilibrium components of the system, especially oxygenated hydrocarbons which disappear in longer runs. The processes leading to equilibrium are mainly radical and include thermal formation of radical species from higher hydrocarbons and carboxylic acids in homolysis, rearrangement reactions, and further β-scission into alkenes and alkyl radicals. Carboxylic acids additionally undergo thermal decarboxylation. Equilibrium fluids contain CH4, C2H6, C3H8 and C4H10 as main carbon species, which is consistent with previous experimental and theoretical results[25, 28, 29, 31, 32]. It has been demonstrated for the first time that equilibrium N-poor C-O-H-N fluids can contain more than 30 carbon and nitrogen species. Besides the main species, they include С15-С19 alkanes, alcohols, aldehydes, ketones, carboxylic acids, and furans.

The class="Chemical">carbon anclass="Chemical">pan class="Chemical">d nitrogen speciation in the equilibrium fluids depends on temperature and redox conditions. The CH4/C2H6, CH4/C3H8, and CH4/C4H10 ratios and C concentrations decrease both under isobaric cooling from 1400 to 1200 °С at constant fO2 and under oxygen fugacity increase from IW-2.5 to IW + 2.5 log units at 1400 °С. As the temperature and water content increase, the concentrations of С15-С18 alkanes increase slightly while oxygenated hydrocarbons become more diverse. In reduced fluids, only alcohols can reach notable amounts. The fluids with fO2 IW + 2.5 log units, almost lack methane and contain about 1 rel.% of C2H6, C3H8 and C4H10, as well as C15–19 alkanes, and relatively high oxygenated hydrocarbons, especially alcohols and carboxylic acids. The material of capsules causes a catalytic effect: CH4/C2H6 ratios are slightly higher in quenched fluids from Au capsules than in those from Pt capsules. Methanimine (CH3N) is the main nitrogen species in the studied fluids, but it loses importance (CH3N/N2 < 1) at a lower temperature of 1100 °C at constant fO2 in the system IW-2.5 log units; CH3N almost disappears while N2 becomes the predominant species as fO2 decreases to IW + 2.5 log units at 1400 °C.

The behaviour of the class="Chemical">CH3N sclass="Chemical">pecies can strongly class="Chemical">pan class="Chemical">control the mantle nitrogen cycle, especially in N-poor fluids equilibrated with peridotite. Specifically, silicate phases capable of dissolving ammonia in the presence of NH3-bearing fluids hardly can be nitrogen hosts in equilibrium with CH3N-bearing fluids. Oxidation of peridotitic fluids with small N contents upon interaction with multiple redox fronts is inferred to decrease strongly the concentrations of methane and methanimine and slightly reduce the amount of ethane, but it causes significant changes neither to C3, C4 and C5 alkanes nor to C15-C19 alkanes and oxygenated hyrocarbons. As a result, hydrous magma can capture species stable to oxidation, as well as N2, which can be involved in diamond formation and carried to shallow mantle.

Materials and Methods

Starting mixtures for generation of class="Chemical">carbon- anclass="Chemical">pan class="Chemical">d nitrogen-bearing fluids consisted of chemical-grade docosane (C22H46) and stearic acid (С18Н36O2), and 99.9999% pure natural graphite (Table 1). The pre-dried graphite contained 700 ppm CO2 and 700 ppm H2O determined by chromatography of gases extracted during graphite annealing at 600 °С in a U-shaped quartz cell. Graphite with docosane and/or stearic acid, at a weight ratio of ~10/1, were placed into Pt or Au capsules (Supplementary Fig 1). The 10/1 ratio of graphite to fluid-generating material provided an amount of C-O-H-N fluid sufficient for GC/MS analysis but did not produce overpressure in the capsule after the runs and during drying before gas analysis. Microscopic amounts of nitrogen in the capsules came from air. The samples had different H2O concentrations changed by varying the aliquots of stearic acid. Capsules containing starting material for the generation of a C-O-H-N fluid were arc-welded using a Lampert Werktechnik GmbH PUK-4U impulse micro welding device. Experiments were carried out in a high-pressure split-sphere multi-anvil apparatus. Fluid was generated at buffered fH2 in 6.3 GPa runs in a cell with a large low-gradient zone and was analyzed using a Thermo Scientific DSQ II Series Dual Stage Quadrupole GC/MS [34]. Analytical uncertainty for H2O, NH3, and CO2, expressed as precision, was less than 10% and in most cases less than 5% (determined in the range from 12.5 pptv to 12.5 ppbv). The efficiency of the chosen fH2 buffering technique has been proven in special tests (Supplementary Figs 1–3).

Supplementary information

Relatepan class="Chemical">d Manuscriclass="Chemical">pan class="Chemical">pt File