A new method to quantitatively control oxygen fugacity in externally heated pressure vessel experiments

• Main Authors: A. Alex, Z. Zajacz
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-02-01
• Series: European Journal of Mineralogy
• Online Access: https://ejm.copernicus.org/articles/32/219/2020/ejm-32-219-2020.pdf

<p>Oxygen fugacity (<span class="inline-formula"><i>f</i>O₂</span>) is a fundamental variable affecting phase equilibrium in magmas, and in externally heated pressure vessel experiments it is typically controlled by using redox buffer assemblages. However, these do not allow fine enough resolution; for example, most arc magmas fall between the <span class="inline-formula"><i>f</i>O₂</span> imposed by the neighboring Ni–NiO and Re–<span class="inline-formula">ReO₂</span> buffers and so does the transition of <span class="inline-formula">S²⁻</span> to <span class="inline-formula">S⁶⁺</span> in magmas. Here we propose a new method to quantitatively impose <span class="inline-formula"><i>f</i>O₂</span> in hydrous high-<span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span> experiments in molybdenum hafnium carbide (MHC) pressure vessels by admixing small amounts of hydrogen into the Ar pressure medium. The thermodynamic calculation procedure used to determine the initial amount of hydrogen to be loaded to constrain desired <span class="inline-formula"><i>f</i>O₂</span> values was verified by CoPd alloy redox sensor experiments to be accurate within <span class="inline-formula">±</span>0.3 log units for the pressure (<span class="inline-formula"><i>P</i></span>) – temperature (<span class="inline-formula"><i>T</i></span>) range of 940–2060&thinsp;bar and 800–1100&thinsp;<span class="inline-formula">^∘</span>C. As hydrogen can be slowly lost from the pressure medium due to diffusion through the vessel walls at high <span class="inline-formula"><i>T</i></span>, we also determined the hydrogen permeability of the MHC alloy as a function of <span class="inline-formula"><i>T</i></span>. The such-obtained hydrogen permeability equation for the MHC alloy can be used to determine the rate of <span class="inline-formula"><i>f</i>O₂</span> increase for any MHC pressure vessel configuration. As the rate of <span class="inline-formula"><i>f</i>O₂</span> increase is slow (e.g., 0.36 log units per day in our setup at <span class="inline-formula"><i>T</i>=</span>&thinsp;1000&thinsp;<span class="inline-formula">^∘</span>C), we propose that <span class="inline-formula">H₂</span> addition to the Ar pressure medium is an effective way to accurately impose <span class="inline-formula"><i>f</i>O₂</span> in many types of experiments conducted in MHC vessels allowing experimentation up to <span class="inline-formula"><i>T</i>=</span>&thinsp;1200&thinsp;<span class="inline-formula">^∘</span>C and <span class="inline-formula"><i>P</i>=</span>&thinsp;300&thinsp;MPa.</p>