Reduced methane-bearing fluids as a source for diamond

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 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, H 2 incorporation into these phases and the "etching" of graphite by H 2 and cH 4 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.

Biogenic formation of amorphous carbon by anaerobic methanotrophs and select methanogens

“…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:…”

Fluid Inclusions in Fibrous Diamonds

“…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).…”

Experimental Petrology Applied to Natural Diamond Growth

“…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.…”