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

Abstract

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 analyses 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 lie 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.