SIMS analysis of volatiles in silicate glasses

Abstract

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 H_2O and total CO_2 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 H_2O, <3 ppm CO_2, and <1 ppm F, S and Cl. Measurements of δD, δ^(13)C and δ^(34)S 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 of 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.

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