High-precision analysis of hydrous rhyolitic glass inclusions in quartz phenocrysts using the electron microprobe and INAA

Abstract

A high-precision electron microprobe (EMP) technique has been developed that is capable of analyzing major, minor, and trace element abundances (Si, Ti, AI, Fe, Mn, Mg, Ca, Na, K, Cl) in hydrous rhyolitic glasses. The technique was developed to characterize the chemical compositions of rhyolitic glass inclusions in phenocrysts that occur in layers of Paleozoic altered volcanic ash. The compositions of these inclusions serve as excellent chemical fingerprints of the altered volcanic ash layers for use in stratigraphic correlation. The precision and reproducibility of the analyses is sufficientnot only to distinguish one altered volcanic ash layer from another on the basis of inclusion compositions, but also to discern differencesin the compositions of different inclusions from the same layer. A high-precision instrumental neutron activation analytical (INAA) technique was also developed that is capable of measuring an additional suite oftrace elements (e.g., Sc, Co, Rb, Cs, Sr, Ba, La, Ce, Sm, Eu, Tb, Yb, Lu, Zr, Hf, Ta) in rhyolitic glass inclusions in quartz phenocrysts with excellent accuracy and precision after correcting for the presence of the host quartz. The abundances of elements measured by the EMP technique and the corrected abundances determined using the INAA technique are identical within analytical uncertainty, thus demonstrating the internal consistency of the results.