SIMS analysis of volatiles in silicate glasses 1. Calibration, matrix effects and comparisons with FTIR

This paper describes microanalysis techniques using secondary ion mass spectrometry (SIMS) to measure the abundances and isotopic compositions of hydrogen, carbon, fluorine, sulfur and chlorine in volcanic glasses. SIMS measurement of total H₂O and total CO₂ abundances compare very well with measurements on the same glasses using vibrational spectroscopy techniques (FTIR). A typical 10-min SIMS measurement for volatile abundances is made on a singly polished specimen, sputtering a crater 15-30 μm in diameter and 2-3 μm deep, utilizing 1-5 × 10^-9 g of sample material. Detection limits are routinely <30 ppm H₂O, <3 ppm CO₂, and <1 ppm F, S and Cl. Measurements of δD, δ¹³C and δ³⁴S in volcanic glasses are currently reproducible and accurate to 2-5%, depending on the concentration of the element. Because of their spatial selectivity, the SIMS methods allow resolution og magmatic volatile signatures from those carried by secondary phases, which can sometimes plague traditional vacuum extraction methods that require large amounts of sample (tens to hundreds of milligrams). Ease of sample preparation, rapid analysis and high sensitivity allow SIMS to be applied to volatile analysis of small samples such as melt inclusions, in which large numbers of individual analyses are often required in order to obtain a representative sample population. Combined abundance and isotopic composition data for volatile elements provide coupled constraints on processes relevant to magma genesis and evolution, including degassing, magma contamination, mixing, and source variability.