Abstract: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 CH 4-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 experim… Show more
“…Notably, methane-rich fluids have recently been shown to generate pure carbon in the form of diamond under very high pressure (5 to 7 GA) ( 51 ). This process occurs by the removal of hydrogen from methane:…”
Elemental carbon exists in different structural forms including graphite, diamond, fullerenes, and amorphous carbon. In nature, these materials are produced through abiotic chemical processes under high temperature and pressure but are considered generally inaccessible to biochemical synthesis or breakdown. Here, we identified and characterized elemental carbon isolated from consortia of anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria (SRB), which together carry out the anaerobic oxidation of methane (AOM). Two different AOM consortia, ANME-1a/HotSeep-1 and ANME-2a/c/Seep-SRB, produce a black material with similar characteristics to disordered graphite and amorphous carbon. Stable isotope probing studies revealed that the carbon is microbially generated during AOM. In addition, we found that select methanogens also produce amorphous carbon with similar characteristics to the carbon from AOM consortia. Biogenic amorphous carbon may serve as a conductive element to facilitate electron transfer, or redox active functional groups associated with the carbon could act as electron donors and acceptors.
RESULTS
Initial inspection of black matter present in AOM culturesBoth the AOM50 and the AOM20 cultures are dominated by large microbial consortia, formed either by ANME-1a and HotSeep-1 partner bacteria or by ANME-2a/c and Seep-SRB partner bacteria, respectively. The consortia exhibited amber-like color, which is due
“…Notably, methane-rich fluids have recently been shown to generate pure carbon in the form of diamond under very high pressure (5 to 7 GA) ( 51 ). This process occurs by the removal of hydrogen from methane:…”
Elemental carbon exists in different structural forms including graphite, diamond, fullerenes, and amorphous carbon. In nature, these materials are produced through abiotic chemical processes under high temperature and pressure but are considered generally inaccessible to biochemical synthesis or breakdown. Here, we identified and characterized elemental carbon isolated from consortia of anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria (SRB), which together carry out the anaerobic oxidation of methane (AOM). Two different AOM consortia, ANME-1a/HotSeep-1 and ANME-2a/c/Seep-SRB, produce a black material with similar characteristics to disordered graphite and amorphous carbon. Stable isotope probing studies revealed that the carbon is microbially generated during AOM. In addition, we found that select methanogens also produce amorphous carbon with similar characteristics to the carbon from AOM consortia. Biogenic amorphous carbon may serve as a conductive element to facilitate electron transfer, or redox active functional groups associated with the carbon could act as electron donors and acceptors.
RESULTS
Initial inspection of black matter present in AOM culturesBoth the AOM50 and the AOM20 cultures are dominated by large microbial consortia, formed either by ANME-1a and HotSeep-1 partner bacteria or by ANME-2a/c and Seep-SRB partner bacteria, respectively. The consortia exhibited amber-like color, which is due
“…Observations of CH 4 in diamonds are rare (Smit et al 2016) and growth from CH 4 , or CH 4 -H 2 O low-density fluids was suggested based on modeling of isotopic fractionation (e.g., Thomassot et al 2007;Smit et al 2019). Experimental studies found that "in oxidized fluids (CO 2 ; CO 2 -H 2 O; H 2 O), the intensity of spontaneous nucleation, the rate of diamond growth on seed crystals, and the degree of graphite-diamond transformation are considerably higher than those in reduced fluids (H 2 O-CH 4 ; CH 4 -H 2 )" (Palyanov et al 2015), but a recent study does suggest nucleation and growth of diamond in the harzburgite+CH 4 system (Matjuschkin et al 2020). Smith et al (2016) reported the finding of iron-nickel-carbon-sulfur inclusion in lower mantle diamonds, but metallic iron inclusions are known from shallower diamonds as well (Bulanova et al 1998), and this mode of growth may be important and efficient where metallic melts exist (e.g., below 250 km, Frost and McCammon 2008).…”
Section: High-density Fluids and Diamond Formationmentioning
Giardini (1974, 1975) reported the composition of gases released by crushing diamonds under vacuum. Gas signals of transparent or light-yellow diamonds were similar to the system blank levels (0.2-2 × 10 −5 cm 3 STP, for an empty crushing chamber at 0 °C and 1 bar). However, the crushing of cubic and translucent diamonds at 200 °C yielded gas volumes that were an order of magnitude larger. The main gases released were H 2 O and CO 2 . Gas released by graphitization of diamonds contained mostly CO with minor H 2 and the quantities were >10 −2 cm 3 STP (Kaiser and Bond 1959;Melton and Giardini 1976), but the H 2 may be attributed to a reaction of water during graphitization (Fesq et al. 1975).Early attempts to determine the nature of the material trapped in the microinclusions by X-ray diffraction and X-ray fluorescence were not conclusive (Lonsdale and Milledge 1965;Seal 1966Seal , 1970Harris 1968). Prinz et al. (1975) found "fluffy, filamentous material" in two diamonds from the Democratic Republic of the Congo (DRC), but did not describe the diamonds. The composition obtained by electron probe microanalysis (EPMA) of this material was SiO 2 67 wt%; TiO 2 2.5 wt%; Al 2 O 3 14-17 wt%; FeO 2-3 wt%; MgO 1 wt%; Na 2 O 0.2-2.5 wt% and K 2 O 8.5-10 wt%. As mentioned below, this composition agrees roughly with that of later studies of microinclusion-bearing fibrous diamonds. Fesq et al. (1975) used instrumental neutron activation analysis (INAA) to investigate the nature of impurities in diamonds. They examined groups of diamonds (~10-20 crystals in each sample) with mineral inclusions as well as diamonds with no visible inclusions. They concluded that in addition to mineral inclusions, the diamonds also carried picritic (high-Mg basaltic) melt associated with an H 2 O + CO 2 rich component or phase that carried incompatible trace elements. The presence of a sulfide-rich phase was inferred from the correlation between Fe, Ni, Cu and Co. These impurities were found in diamonds with and without visible inclusions.INAA of coated diamonds was attempted by Kodochigov and co-workers (according to Orlov 1977) and Glazunov et al. (1967). Bibby (1979) used INAA to determine the traceelement composition of a coated diamond. He found low levels in the core, but the coat carried up to 50 ppm Fe and a few ppm of Na, K, Ba and Ce. Based on the good intercorrelation of K, Na, Ba, and the REE and their poor correlation with Sc, Cr, and Mn, he suggested the presence of carbonate microinclusions.
“…Diamond growth is characterised by temperature-dependent induction times, certainly for nucleation of new diamond crystals, and in some cases even for growth on pre-existing seed diamonds. More recent studies such as Matjuschkin et al (2020) have demonstrated the ability of these fluids to grow diamonds at lower temperatures (e.g., that are more realistic for lithospheric mantle geotherms). In this study, extensive efforts were devoted to improving experimental design to minimize changes in fluid composition over the course of the experiments.…”
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