Reduced methane-bearing fluids as a source for diamond

Diamond formation in the Earth has been extensively discussed in recent years on the basis of geochemical analysis of natural materials, high-pressure experimental studies, or theoretical aspects. Here, we demonstrate experimentally for the first time, the spontaneous crystallization of diamond from CH4-rich fluids at pressure, temperature and redox conditions approximating those of the deeper parts of the cratonic lithospheric mantle (5–7 GPa) without using diamond seed crystals or carbides. In these experiments the fluid phase is nearly pure methane, even though the oxygen fugacity was significantly above metal saturation. We propose several previously unidentified mechanisms that may promote diamond formation under such conditions and which may also have implications for the origin of sublithospheric diamonds. These include the hydroxylation of silicate minerals like olivine and pyroxene, H2 incorporation into these phases and the “etching” of graphite by H2 and CH4 and reprecipitation as diamond. This study also serves as a demonstration of our new high-pressure experimental technique for obtaining reduced fluids, which is not only relevant for diamond synthesis, but also for investigating the metasomatic origins of diamond in the upper mantle, which has further implications for the deep carbon cycle.

carbonate minerals or melts may exist locally at pressures above 5 GPa, either where oxidizing metasomatism has occurred, or potentially within subducting slabs that are isolated from the ambient mantle. On the other hand, the rare occurrence of carbonate inclusions in diamond provides direct evidence for some diamonds having formed through carbonate reduction 17,18 . Under ƒO 2 conditions where pure carbonatite melt would be unstable, metasomatic melts may have a mixed carbonate-silicate character 19 . Spontaneous diamond crystallization through reduction of such a melt was recently demonstrated experimentally by Girnis et al. 20 in a model peridotite-sediment system. Whether or not such processes are generally responsible for diamond formation in the Earth's mantle remains open to debate. Our contribution here addresses an alternative mechanism that is likely to be important for the Earth, as described below.
The formation of diamond from reduced methane-rich fluids is a further possibility that has a number of merits. For example, the ambient ƒO 2 of the lithospheric mantle at depths where diamond becomes stable (i.e. ~150 km) lies well below the stability of CO 2 -rich fluids or carbonatitic melts 15 . In addition, some diamonds exhibiting negatively skewed 13 ∂C signatures 8,10,21 contain CH 4 ± H 2 -bearing fluid inclusions, as detected by Raman spectroscopy 22,23 ). These studies provide direct evidence for the role of CH 4 in the formation of some natural diamonds, including the population of very large "CLIPPIR" diamonds 24 , even so diamond synthesis from strongly reduced fluids has not yet been experimentally observed 25 . There are further reasons to suspect that the mechanism of diamond crystallization through CH 4 oxidation may be more prevalent than previously recognized. Aside from CH 4 having more than double the carbon carrying capacity of carbonates or CO 2 (75 wt.% C in CH 4 versus 27 wt.% in CO 2 and 12 wt.% in CaCO 3 ), the solubility of CH 4 in silicate melts is very low, on the order of 100-500 ppm even under conditions of unit activity of CH 4 26 . Thus, the depression of the peridotite solidus temperature is much less than in the presence of more oxidized H 2 O-CO 2 -rich fluids 27 . As a result, CH 4 (±C 2 H 6 , ±H 2 ) might be the only viable "free" fluid phase stable in the deeper parts of the upper mantle over a large range of temperature and depth. However, some thermodynamic models suggest that the stability of CH 4 -rich fluids requires redox conditions so reducing that the ƒO 2 must lie below that of metal saturation (i.e. below the Ni-precipitation curve, which lies just below the iron-wüstite (IW) oxygen buffer 15,28 ). This could call into question the relevance of such reduced fluids for the formation of diamond in the upper mantle since the Ni-precipitation curve effectively places a lower limit on the feasible ƒO 2 , even if rare moissanite inclusions have been reported 29 . Furthermore, CH 4 may be unstable in the presence of metals as they may react to form carbides (e.g. FeSiC alloy or (Fe, Ni) 3 C) 30 .
To investigate the potential conditions under which diamond can form from methane-rich fluids, we have undertaken a series of experiments at pressures and temperatures corresponding to the deeper portions of the cratonic mantle lithosphere under controlled ƒO 2 . A pressure range of 5-7 GPa is of particular interest as this is similar to the range reported for many lithospheric diamonds 21 and where no solid-phase transformation of graphite to diamond is expected (graphite has long been used as a heater for experiments at these pressures without spontaneous transformation). No diamond seed crystals were used to initiate or accelerate diamond growth 2 . Although previous experimental studies have had little to no success in forming diamond at such conditions without diamond seeds 25,31,32 our experiments followed the approach of Matjuschkin et al. 33 , comprising a harzburgitic mineral assemblage of natural olivine and orthopyroxene packed into an olivine capsule along with a coexisting COH-fluid (see Methods). Possible reasons why diamond synthesis in the presence of methane was unsuccessful are briefly discussed in the supplementary information. The ƒO 2 imposed on the sample was measured post-experiment using an Ir-Fe redox sensor 34 . Spectroscopic analysis of the run products provides important insights into the nature of the resulting fluid and mineral phases, including the unequivocal identification of spontaneous diamond formation in our experiments.

Results and discussion
The Ir-Fe redox sensors gave values of 0.2-0.8 log units above the Fe-FeO (IW) oxygen buffer (i.e. ∆logƒO 2 = IW + 0.2 to IW + 0.8, see supp. info Table S2), indicating that our experiments were carried out above FeNi-alloy saturation and at similar to ∆logƒO 2 values reported for some mantle xenoliths originating from ≥150 km depth 19,35,36 . In our experiments, the coexisting fluid phase was effectively trapped in a network of inclusions within the olivine capsule at pressure and temperature (Fig. 1a,b), permitting its composition to be directly probed by Raman spectroscopy (see Methods). While quantitative assessment of the fluid composition was not feasible, spectra reveal fluids composed essentially of CH 4 with minor C 2 H 6 and H 2 (Fig. 2, see also Matjuschkin et al. 33 and Fig. S1 in supp. info.). Although a number of commonly used thermodynamic models for COH-fluids 37,38 , including GFluids 28 , predict a significant H 2 O component (up to 40 mol %) at the P-T-ƒO 2 conditions of our experiments, virtually no H 2 O was detected in the Raman spectra in spite of an extensive search across the samples. The absence of different inclusion populations means that there is no evidence for liquid immiscibility between CH 4 and H 2 O. Our observations imply that CH 4 is much more stable than most models predict and is likely to be a major component of COH fluids at significantly higher ƒO 2 values than generally thought. On the other hand, our results are consistent with the fluid speciation model of Huizenga 39 that predicts ~5 mol % H 2 O at the conditions of our experiments (see supp. info Fig. S1), as such low concentrations might not be detectable in Raman spectra 40 . A finite amount of H 2 O in the fluid phase is not only expected on theoretical grounds (i.e. there must be a finite thermodynamic activity of H 2 O) but is required by the presence of OH as detected in olivine by FTIR spectroscopy (Fig. 3). In our experiments, the amount of OH in olivine increases with increasing ƒO 2 and is related to a concomitant increase in water activity 41 (Fig. 3). While the quantitative assessment of OH concentrations in olivine is beyond the scope of this contribution, the observed incorporation of OH into olivine has important implications for the mechanism of diamond formation as well as the composition of the coexisting fluid in our experiments (see below).
Along with the CH 4 -rich fluid inclusions in olivine, diamond was also observed in many experiments and confirmed by Raman spectroscopy (Figs. 1a-c, 2). The diamonds exhibit a Raman line at 1332 cm −1 , which is the ideal value for well crystallized natural diamond 42 . As no diamond seeds were employed, their presence must be the result of spontaneous nucleation during the experiments. The possibility that the diamond could have been introduced during sample preparation can be ruled out since: i) all diamond-bearing samples were polished with an Al 2 O 3 slurry rather than with diamond paste, and ii) many diamonds including those illustrated in Fig. 1b-e occur well beneath the sample surface. The diamonds are 1-5 µm in size and occur in a variety of textures: type 1) as single-crystals or as polycrystalline inclusions in olivine (Fig. 1d), type 2) within fluid inclusionsin the absence of graphite (Fig. 1b,c,e), type 3) in diamond-rich zones or veins at the interface between olivine-orthopyroxene sample material and the outer Au-capsule (Fig. 1f) or type 4) as concentrations at or near the contact with the inner buffer capsule that supplies H 2 to the sample (Fig. 1g,h). These different types of occurrence emphasize the mobility of CH 4 -fluids, driven in part by unavoidable, but small axial thermal gradients, probably of the order of a few degrees across the capsule.
Diamond was observed in experiments performed at 5, 6 and 7 GPa (Fig. S2 supp. info.). At both 7 and 6 GPa and temperatures from 1050 to 1300 °C diamond crystallized in all experiments, even one that had only a 4 hours duration. The diamond yield appears to increase with the experiment duration, although, it is difficult to quantify this since only a small amount of fluid was initially added (4 wt%, see Methods) and the spatial distribution of diamond is uneven. It is in fact quite remarkable that such a small amount of fluid is capable of producing www.nature.com/scientificreports www.nature.com/scientificreports/ spontaneous diamond precipitation. At 5 GPa only one experiment run at 1250 °C was found to contain diamond (see Supplementary Table S1). The diamond yield was less than observed in experiments at 6 and 7 GPa, which we ascribe to the very close proximity to the graphite-diamond phase boundary 2 .
Although graphite is also present, it formed at the onset of the experiment by the breakdown of the stearic acid, which served as the source of the COH fluid 43 . In most cases, diamond in fluid inclusions is not observed to have any direct textural association with graphite (texture types 1, 2, 3 described above). This means that diamond must have crystallized from the methane-rich fluid itself rather than by solid-state transformation of graphite with diamond precipitating from the CH 4 -fluid as it migrated along cracks in olivine or between the sample and the outer or inner capsule. Where diamond crystallized at or near the surface of the inner buffer capsule (textural type 4), the diamond aggregates developed upon the outer margins of graphite clots (Fig. 1g,h). This spatial relationship suggests an essential role of the fluid phase and the proximity to a source of H 2 in diamond formation. This interpretation is consistent with the observations of Akaishi et al. 31 who proposed a dissolution-reprecipitation mechanism for the crystallization of diamond in their graphite-fluid experiments based upon isotopic labelling of the carbon. Representative Raman spectra of several diamond-bearing fluid inclusionsin olivine. The uppermost spectrum (green) was obtained in non-confocal mode to sample a larger volume of olivine (hence the stronger signal from olivine). In this way, we were able to detect H 2 in the fluid. This also meant that both graphite and diamond were detected, although they were located at different depths within the olivine and not in direct contact with each other (green spectrum only). www.nature.com/scientificreports www.nature.com/scientificreports/ Diamond formation can be considered as an oxidation reaction either directly involving oxygen or involving removal of hydrogen The formation of diamond via equilibrium reaction 1 requires an oxygen source, which could be coupled with the reduction of Fe 3+ to Fe 2+ . As discussed by Stachel and Luth 6 , the Fe 2 O 3 content of upper mantle garnet peridotite is relatively small, limiting the supply of oxygen for such a process. In our experiments, some oxygen could be provided by the natural orthopyroxene in the starting materials that has Fe 3+ /∑Fe = 0.09(2), as determined by Mössbauer spectroscopy (see Methods). Considering the redox conditions of our experiments, reactions 2 and 3 should be more relevant where diamond forms upon removal of H 2 . One way for this to happen is for H to become sequestered in orthopyroxene and olivine via the equilibria proposed by Tollan and Herman 44 for orthopyroxene: The [] in equilibrium (4) denotes a lattice vacancy in orthopyroxene. The formation of OH in olivine during the experiments is documented by the FTIR spectra presented in Fig. 3. Unfortunately, the orthopyroxene grains were too small to analyse spectroscopically, but must also contain OH. Thus, the hydroxylation reactions (4-7) will act to drive reactions (2) and (3) to the right, promoting diamond formation. Such a mechanism should operate in the upper mantle as CH 4 -bearing fluids migrate into "drier" domains, such as those observed in the deeper portions of cratonic roots 46 . These mechanisms require the presence of contrasting mantle domains (i.e. dry vs. fluid-rich).
With ƒH 2 internally buffered in our experiments, it might be expected that equilibrium 2 and 3 would shift to the left and destabilise diamond 15 . However, this is not supported by the occurrence of euhedral diamonds within CH 4 -rich fluid inclusions (Fig. 1b,c,e) and the crystallization of diamond near the interface with the inner buffer capsule where a H 2 flux is expected (Fig. 1g,h). In fact, H 2 and CH 4 may play an essential role in a more complex process where metastable graphite is dissolved into the fluid in form of CH 4 only to supersaturate and precipitate the more stable diamond. In this way, equilibria 2 and 3 shift to the left in contact with graphite and then shift to the right crystallizing diamond as ƒH 2 is locally lowered. This mechanism is consistent with the observed preferential "etching" of graphite by H 2 and CH 4 compared to diamond 31,47 . The presence of H 2 and CH 4 is also known to stabilize the surface of diamond and promote sp 3 molecular orbital hybridization of carbon, thus promoting diamond growth 48 . Graphite etching is a well-known process in the physics literature 49,50 and can explain the textural occurrence of our type 4 diamond and type 2 diamond-bearing fluid inclusions (Fig. 1b,c,e,g,h). We note that this process can generate diamond at essentially constant temperature, pressure and ƒO 2 , even at sub-solidus conditions. The relevance of such a process in nature can be found in the interaction of subducted graphite 51,52 with CH 4 and H 2 -bearing reduced fluids that may be generated by high-pressure metamorphism of ophiocarbonates (carbonate-bearing ultramafic rocks) in the subducting slab 52,53 . Subducted components and lithologies have frequently been implicated in diamond formation 1,54 . Determining if natural diamond crystallised from reduced fluids is unfortunately problematic in the absence of coexisting fluid inclusions. Even if such inclusions are present, their composition is most likely to have been modified during transport. Significant loss of H 2 and CH 4 from olivine can also occur during sample preparation (heating and vacuum conditions), unlike OH defects in olivine that can be observed by FTIR measurements.
In addition to the afore-mentioned mechanisms, we observe two further processes that are relevant for the mantle environment. In addition to the presence of OH groups (Fig. S2), Matjuschkin et al. 33 report Raman spectra that also indicate incorporation of H 2 into olivine rather than just in fluid inclusions, as was first described by Yang & Keppler 55 . This provides a further mechanism to crystallize diamond by driving both equilibria (2) and (3) to the right. Yang and Keppler 55 report a minimum of 15-40 ppm molecular H 2 residing on interstitial sites of olivine and orthopyroxene (and clinopyroxene) at 2.5 GPa and suggest that 100's of ppm could be incorporated at higher pressures.
Secondly, significant cooling (e.g. from 1200 to 850 °C) will also promote diamond precipitationas the speciation changes and more H 2 O is formed (Fig. S1). This is not only the case for more oxidizing conditions near the "water maximum" as described by Stachel and Luth 6 , but also for CH 4 -rich fluids, as predicted by the speciation model of Huizenga 39 and documented experimentally by Matjuschkin et al. 33 . The amount of diamond precipitation is not only a function of the incremental temperature decrease, but is also related to the final temperature www.nature.com/scientificreports www.nature.com/scientificreports/ and ƒO 2 of the fluid (see Supplementary Data, Fig. S1). Depending on the C-species in the fluid, this process is essentially redox neutral.
Since ƒH 2 and thus ƒO 2 were held constant in our experiments by an internal buffer (see Methods), and pressure and temperature were also kept constant, the observed spontaneous formation of diamond is not related to any significant shift in redox state. Instead, diamond crystallization occurs from very CH 4 -rich fluids by a variety of processes, involving interactions between H 2 and olivine, pyroxene or graphite "etching" in contact with H 2 and CH 4 . Such fluids are stable at pressures and temperatures similar to those expected in the upper mantle at ≥150 km and at realistic ƒO 2 conditions above metal saturation 33 . In addition, their rather weak effect on depressing the peridotite solidus 26 means that CH 4 -rich fluids are likely to exist along a range of geothermal gradients in the deeper lithospheric mantle and in sublithospheric domains without being quantitatively extracted into a melt phase. This is consistent with the detection of CH 4 and H 2 associated with inclusions in the large sublithospheric "CLIPPIR" suite of diamonds 24 . The implication is that CH 4 -rich fluids are not only more prevalent in nature than often thought, but that they may represent a significant source of carbon responsible for diamond formation and that the associated H 2 plays an important role in this process. Thus, this study not only confirms the potential importance of methane in the formation of diamond via several unanticipated mechanisms, but also suggests a high probability for diamond formation at mantle conditions through the involvement of methane-rich fluids. That implies that low density methane-rich fluids play a larger role in a deep carbon cycle as previously appreciated.