Improved oxygen-buffering techniques for C-O-H fluid-saturated experiments at high pressure

A new oxygen-buffering technique has been developed for experiments in high-pressure, solid media apparatus where saturation with a reduced C-O-H fluid is required. The buffer (abbreviated “WCWO”) consists of a mixture of WC, WO₂ and graphite. Stearic acid (C₁₈H₃₆O₂) is mixed with the buffer components to provide the C-O-H fluid source. The silicate sample is enclosed and separated from the buffer by an inner graphite capsule thereby eliminating iron loss problems. Analysis of quenched fluids by capsule piercing/mass spectrometry demonstrates that WCWO controls oxygen fugacity (ƒO₂) at ∼1 log unit above the iron-wustite (IW) buffer with a reproducibility better than 0.2 logƒO₂ units. Fluid compositions buffered by WCWO can be maintained in sealed noble metal capsules for run times exceeding 50 hours. Fluids are CH₄-H₂O dominated with a wider compositional range (CH₄/(CH₄ + H₂O) ∼0.8 to 0.1 depending on temperature) than is accessible with the more reduced iron-wustite-graphite (IWG) buffer. In liquidus experiments on olivine lamproite, we found the IWG buffer, in contrast to WCWO, to be unreliable in controlling fluid compositions. IWG did, however, perform satisfactorily in the absence of a silicate sample. We show that the IWG technique can be modified so that fluids of H₂O ≫ CH₄ > CO₂ character can be reliably reproduced (termed the “CWI” technique). The WCWO buffer has been employed in melting studies on the system peridotite-C-O-H (Taylor and Green, 1988b) and should have wide application in experiments above 1000°C where control of ƒO₂ and water activity (aH₂O) is required. The CWI technique is based on fluid monitoring and may be used in experiments where high water activity but low ƒO₂ (∼IW+1.5 log units) is desired.