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

The main obstacle to understanding of the geological role of reduced, CH ₄ -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 (fO ₂ ) 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 CaF ₂ pressure medium to minimise H ₂ -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–MoO ₂ and Fe–FeO buffer conditions. IrFe alloy sensors demonstrate successful achievement and maintenance of reduced fluid environment at ∆logfO ₂ ≈ 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 CH ₄ , along with small amounts of higher hydrocarbons like C ₂ H ₆ . No water was detected, but H ₂ 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 H ₂ O contents at these fO ₂ conditions. It is also apparent that fluids with significant CH ₄ contents are likely to be stable under the conditions recorded by some mantle samples.