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Can experimental oxygen fugacity be controlled?

More precise data is required to determine whether a novel experimental technique designed to control experimental fO2 is effective under H2O-undersaturated conditions.

Piston cylinder experiments typically employ noble metals as sample containers due to their low reactivity and high melting temperature. Many years of experimental research has demonstrated that choice of capsule metal often involves a payoff between melting temperature and the ability to control important compositional parameters (e.g., loss of Fe and H2O).

Oxygen fugacity (ƒO2) is intrinsically linked to these variables and is a key property of an experiment because it controls the valence of multivalent elements. In turn, this alters phase relations and mineral compositions, and affects speciation of other volatiles elements such as sulphur.

Earlier work by Jakobsson (2012) presented a novel experimental technique for controlling ƒO2 by physically separating a redox buffer from an Au-Pd alloy inner capsule with a hydrogen-permeable barrier. This technique relies on hydrogen fugacity being equal in both in the outer and inner capsule as fixed by the solid buffer; however, the original study failed to take into account the H2O-undersaturated nature of the experimental melts, which actually act to reduce the ƒO2 imposed on the inner capsule.

Jakobsson 12 capsule setup

Diagram of the Jakobsson (2012) capsule setup

This amendment acknowledges this oversight, and corrects the measured ƒO2 in the original study for H2O-undersaturation. The authors conclude that whilst this sample assembly is capable of controlling fO2 in H2O-saturated runs, more precise analysis of other parameters (such as the activity of Fe and H2O in the melt) are needed to assess whether the same holds true for H2O-undersaturated variants.

Jakobsson, S., Blundy, J., & Moore, G. (2014). Oxygen fugacity control in piston-cylinder experiments: a re-evaluation. Contributions to Mineralogy and Petrology, 167(6), 1-4. http://dx.doi.org/10.1007/s00410-014-1007-5

SCIENTIFIC ABSTRACT

Jakobsson (Contrib Miner Petrol 164(3):397–407, 2012) investigated a double capsule assembly for use in piston-cylinder experiments that would allow hydrous, high-temperature, and high-pressure experiments to be conducted under controlled oxygen fugacity conditions. Using a platinum outer capsule containing a metal oxide oxygen buffer (Ni–NiO or Co–CoO) and H2O, with an inner gold–palladium capsule containing hydrous melt, this study was able to compare the oxygen fugacity imposed by the outer capsule oxygen buffer with an oxygen fugacity estimated by the AuPdFe ternary system calibrated by Barr and Grove (Contrib Miner Petrol 160(5):631–643, 2010). H2O loss or gain, as well as iron loss to the capsule walls and carbon contamination, is often observed in piston-cylinder experiments and often go unexplained. Only a few have attempted to actually quantify various aspects of these changes (Brooker et al. in Am Miner 83(9–10):985–994, 1998; Truckenbrodt and Johannes in Am Miner 84:1333–1335, 1999). It was one of the goals of Jakobsson (Contrib Miner Petrol 164(3):397–407, 2012) to address these issues by using and testing the AuPdFe solution model of Barr and Grove (Contrib Miner Petrol 160(5):631–643, 2010), as well as to constrain the oxygen fugacity of the inner capsule. The oxygen fugacities of the analyzed melts were assumed to be equal to those of the solid Ni–NiO and Co–CoO buffers, which is incorrect since the melts are all undersaturated in H2O and the oxygen fugacities should therefore be lower than that of the buffer by 2 log aH2O.

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