Oxygen fugacities directly measured in magmatic gases

An electrochemical device was used to measure the fugacity of oxygen (f_O2) in holes drilled through the crust of Makaopuhi lava lake, Kilauea Volcano, Hawaii. Results obtained within 6 months of the lake formation show that log f_O2 normally varies linearly with the reciprocal of the absolute temperature, and that chemical changes occurring in the cooling tholeiitic basalt are reflected in the f_O2 values measured in the holes.