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

Methane-bearing fluids in the upper mantle: an experimental approach

The main obstacle to understanding of the geological role of reduced, CH4-bearing fluids is the absence of a reliable experimental technique applicable to solid-media high-pressure apparatuses, allowing their observation and direct characterisation under laboratory conditions. In this study, we describe the main pitfalls of earlier designs and technical aspects related to achievement of strongly reduced oxygen fugacity (fO2) conditions (i.e., Fe–FeO, IW) and maintenance of a constant fluid equilibrium during an experiment. We describe a new triple-capsule design made of an Au outer capsule with an Au-inner capsule containing a metal/metal oxide oxygen buffer and water, as well as an inner olivine container filled with a harzburgitic sample material and Ir powder that serves as a redox sensor. The bottom of the outer capsule is covered with a solid fluid source (e.g., stearic acid). The outer capsule is surrounded by a polycrystalline CaF2 pressure medium to minimise H2-loss from the assembly. Application of this design is limited to temperatures below the melting temperature of Au, which is pressure dependent. Metals other than Au can lead to fluid disequilibrium triggered by a dehydrogenation and carbonation of the methane. Test experiments were carried out at 5 GPa, temperatures < 1300 °C, at Mo–MoO2 and Fe–FeO buffer conditions. IrFe alloy sensors demonstrate successful achievement and maintenance of reduced fluid environment at ∆logfO2 ≈ IW + 0.5. The fluid phase was trapped in numerous inclusions within the olivine sample container. Raman spectra reveal that the fluid consists mainly of CH4, along with small amounts of higher hydrocarbons like C2H6. No water was detected, but H2 was found to be present in fluid and incorporated into the olivine structure. Our results are inconsistent with published fluid speciation models that predict significant H2O contents at these fO2 conditions. It is also apparent that fluids with significant CH4 contents are likely to be stable under the conditions recorded by some mantle samples.The Deutsche Forschungsgemeinschaft is gratefully acknowledged for funding the project WO652/26-1

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