Intrinsic oxygen fugacity measurements: techniques and results for spinels from upper mantle peridotites and megacryst assemblages

A new technique for the determination of intrinsic oxygen fugacities () of single and polyphase geological samples with solid ZrO₂, oxygen-specific electrolytes is described. Essentially the procedure involves isolating the emf signal from the sample from that unavoidably imposed by the residual atmosphere inside the sample-bearing sensor. By varying the of the residual atmosphere, it is possible to determine a ‘plateau’ value of constant recorded from the sensor which represents a reversed intrinsic measurement for the sample alone, and where the extent of the plateau reflects the innate buffering capability of the sample. A measure of the precision and accuracy of the data obtained is the fact that identical values are obtained whether on a heating or cooling cycle of the sample + compatible atmosphere system.These techniques have been applied to measurements of the intrinsic of spinels from peridotites and megacryst assemblages from Australia, West Germany and the U.S.A. Oxidation states range from ～- 0.25 log₁₀ units more oxidized to 1 log₁₀ unit more reduced than the iron-wüstite (IW) buffer. The overall reduced nature of the spinels and the range of 's obtained are striking features of the data. One implication of the results is that the majority of mantle-derived magmas are initially highly reduced, and the relatively oxidized values observed at surface (～- 4–5 log₁₀ orders more oxidized than IW) reflect late-stage alteration, perhaps by H₂ loss (Sato, 1978).