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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

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Raman spectroscopy offers new insights into the CO2 contents of magmas

A new calibration for micro-Raman spectroscopy paves the way for easy and accurate quantification of CO2 dissolved in volcanic glasses

CO2 is an important volcanic volatile. It is commonly the second most abundant dissolved gaseous species in a molten rock (after H2O) and it can have a dramatic effect on the phase relations and rheology of degassing magmas. The release of CO2 dissolved in magmas is also a vital part of the global carbon cycle. Thus, there has been considerable experimental effort dedicated to measuring CO2 solubility in silicate melts.

Raman laser

Laser path of the micro-Raman spectrometer in the School of Earth Sciences at the University of Bristol

Raman spectroscopy is a non-destructive spectroscopic technique that harnesses the scattering of light to provide information about the molecular structure of sample, e.g., CO2 content. Micro-Raman has advantages over other comparable techniques because it can analyse <10 μm spot sizes and it requires relatively minimal sample preparation; however, the analysis requires a compositionally dependent calibration.

To this end, Morizet and co-authors present a new calibration for the quantification of CO2 in geologically relevant glass compositions by micro-Raman. The study collected micro-Raman CO2 data for an extensive database of synthetic and natural samples, whose CO2 content had previously been quantified by bulk analysis, and found a relationship between the spectral features in the high-frequency region of aluminosilicate glasses and the spectral peak associated with dissolved carbonate. This new calibration is found to be accurate to better than ±0.4 wt% CO2.

Morizet Y, Brooker RA, Iacono-Marziano G, & Kjarsgaard BA (2013) Quantification of dissolved CO2 in silicate glasses using micro-Raman spectroscopy. American Mineralogist, 98(10), http://dx.doi.org/10.2138/am.2013.4516

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