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Electron probe micro-analysis of oxygen in cordierite: potential implications for the analysis of volatiles in minerals

Department of Geology, University of Pretoria, Pretoria, 0002, South Africa, e-mail: martin.rigby{at}up.ac.za

Giles Droop and David Plant

Department of Earth, Atmospheric and Environmental Science, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom, e-mail: giles.droop{at}manchester.ac.uk; david.plant{at}manchester.ac.uk

Peter Gräser

Department of Geology, University of Pretoria, Pretoria, 0002, South Africa, e-mail: peter.graser{at}up.ac.za

The oxygen content of cordierite was analysed in 30, well-characterised samples, using electron probe micro-analysis (EPMA). The major elements were assumed to be stoichiometric and therefore any remaining oxygen from the analysis can be attributed to volatile oxygen, which is defined as the oxygen in H₂O and/or CO₂ within cordierite’s channel structure. The EPMA volatile oxygen contents were compared to volatile oxygen contents obtained from the same samples using secondary-ion mass spectrometry (SIMS) and Fourier-transform infra-red spectroscopy (FTIR). There is a correlation between the data obtained by EPMA and the independent techniques. The data suggests that light-elements, such as oxygen, can be analyzed routinely on the electron microprobe with a degree of accuracy approaching the levels obtained by other, well-established methods. EPMA of oxygen, in addition to the major cations, provides a valuable tool for analyzing the H₂O/OH^– and/or volatile oxygen content of minerals such as amphiboles, micas and cordierite. The assumption implicit in this method, however, relies upon the fact that oxygen is stoichiometric with the major cations. If non-stoichiometry does apply then the method for calculating either H₂O or volatile oxygen is rendered inadequate.