Sensitive high resolution ion microprobe – stable isotope (SHRIMP-SI) analysis of water in silicate glasses and nominally anhydrous reference minerals

Abstract

Low-level water measurements of geological materials are fundamental in understanding the volatile inventories of the Earth from the mantle to crustal reservoirs. Here we describe the development of microanalytical techniques using the new SHRIMP SI ion microprobe to measure the abundances of OH⁻ (as a proxy for water) in volcanic glass and nominally anhydrous minerals (NAMs). Samples were first analysed at the Carnegie Institute of Washington (CIW) on their Cameca ims-6f with calibrations based on previous FTIR and manometry analyses. SHRIMP SI is a large geometry ion microprobe and is currently mainly used for O and S isotope analyses. The analytical protocol used here incorporates multiple collection of ¹⁶O⁻ and ¹⁶O¹H⁻ allowing rapid measurements. A single calibration line incorporating all glasses and NAMs for the SHRIMP SI allows calibration of ¹⁶O¹H⁻/¹⁶O⁻ to H₂O over a wide range in concentration (50 to 15 000 ppm H₂O). This calibration line has around a 10% uncertainty, which appears to be limited only by sample heterogeneity. The current background for SHRIMP analysis is between 20–40 ppm but this is expected to improve with improved pumping on the source chamber. A current limitation to water analysis of NAM samples, by any technique, is having a range of standard materials to enable OH⁻ calibration to absolute H₂O concentrations. Data are presented for 7 NAM samples (2 olivines, 2 orthopyroxenes and 3 clinopyroxenes) that appear to be promising as potential standards for international laboratory H₂O measurements. These NAM samples have been analysed and characterised here by SHRIMP SI, FTIR, EMP and the Cameca ims-6f ion microprobe at CIW. Four of these samples have previously been measured by manometry to determine absolute H₂O concentrations.