Estimation of Fe3+ from electron microprobe analyses: observations on calcic amphibole and chlorite

ABSTRACT Metabasites and metagreywackes from the Pelona and Rand Schists of southern California were analysed using three different electron microprobes. For all three instruments, the estimated Fe³⁺ contents of calcic amphibole, chlorite and epidote are positively correlated. For some samples, there is an additional correlation between high estimated Fe³⁺ and the presence of magnetite. These results imply that microprobe analyses can be used to discern relative differences in Fe³⁺. However, microprobe data and calculations on the sensitivity of the correction procedures to systematic analytical errors indicate that estimated values of Fe³⁺ are not significant in an absolute sense. Thus, estimates of Fe³⁺ are meaningful when comparing samples analysed with a single microprobe, but must be used with caution when comparing analyses obtained on more than one probe.