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

Abstract

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.