Analyzing nitrogen in natural and synthetic silicate glasses by secondary ion mass spectrometry

Abstract

Volatile releases through volcanic eruptions are one of the major processes contributing to the global nitrogen cycle. Past studies have often estimated the magnitude of this flux using volcanic emission measurements, which are limited to currently active systems and sensitive to atmospheric contamination. Another possible approach is the measurement of nitrogen in melt inclusions, which are parcels of magmatic melt trapped prior to eruption. This methodology requires appropriate analytical parameters for nitrogen analysis in silicate glasses by secondary ion mass spectrometry (SIMS), which have not yet been established. To this end, calibrations for nitrogen were obtained using ion implanted basaltic and rhyolitic glasses. We demonstrate that water content significantly affects the ion yields of 14N+ and 14N16O−, as well as the background intensity of 14N+ and 12C+. Application of implant-derived calibrations to natural samples provide the first reported concentrations of nitrogen in melt inclusions. These measurements were made on samples from the Bishop Tuff in California, the Huckleberry Ridge Tuff of the Yellowstone Volcanic Center, and material from the Okaia and Oruanui eruptions in the Taupo Volcanic Center. In studied material, we often find maximum nitrogen contents of less than 45ppm and that nitrogen concentration varies positively with CO2 concentration, which reflects a partial degassing trend. Using the maximum measured nitrogen contents for each eruption, we find that the Bishop released >3.4×1013, the Huckleberry Ridge released >1.4×1014, the Okaia released >1.0×1011, and the Oruanui released >4.5×1013g of nitrogen. Simple calculations suggest that with concentrations such as these, rhyolitic eruptions may ephemerally increase the nitrogen flux to the atmosphere, but are insignificant compared to the 4×1021g of nitrogen stored in the atmosphere.