A new method for quantitative analysis of total water contents and estimating molecular water and hydroxyl contents in rhyolitic glasses by SIMS

Abstract

We have succeeded in developing a new analytical method to estimate the concentration of hydrous chemical species (molecular water; H₂O_m and hydroxyl groups; OH) and total water (H₂O_total) in rhyolitic glass using secondary ion mass spectrometry (SIMS). This method makes it possible to estimate the H₂O_m contents, OH contents, and H₂O_total contents at the micrometer scale (<10 μm) in silicate glasses such as melt inclusions by SIMS. With a ¹⁶O^– primary ion beam, a linear correlation between ¹H⁺/³⁰Si⁺ ratios and H₂O_total contents is observed. On the other hand, there is a linear correlation between ¹H^–/³⁰Si^– ratios and OH contents when using a ¹³³Cs⁺ primary ion beam. These results suggest that the ¹H⁺/³⁰Si⁺ ratios reflect the H₂O_total contents, while the ¹H^–/³⁰Si^– ratios mainly reflect the OH contents. Therefore, the selective estimation of the H₂O_total or OH contents can be achieved through the selection of the primary ion beam (¹⁶O^– and ¹³³Cs⁺). This new method will aid in the estimation of the pressure and temperature of the magma reservoir in the Earth’s interior depending on the solubilities of H₂O_m, OH, and H₂O_total in silicate melts, and in the understanding of water transport in the silicate glasses and melts.