Abstract
The mineral apatite can incorporate all of the major magmatic volatile species into its structure. Where melt inclusions are not available, magmatic apatite may therefore represent an opportunity to quantify volatile concentrations in the pre-eruptive melt. We analysed apatites and matrix glasses from andesites and dacites erupted from Santiaguito Volcano, Guatemala, between the 1920s and 2002. X-ray mapping shows complex zoning of sulphur in the apatite grains, but typically with sulphur-rich cores and sulphur-poor rims. Apatite microphenocrysts are enriched in F and depleted in Cl relative to inclusions. Matrix glasses are dacite to rhyolite and contain low F but up to 2400ppm Cl. Overall, the data are consistent with progressive depletion of Cl in the most evolved melts due to crystallisation and degassing. In the absence of pristine melt inclusions, we used apatite, together with published partitioning data, to reconstruct the likely volatile contents of the pre-eruptive melt, and hence estimate long-term average gas emissions of SO2, HF and HCl for the ongoing eruption. The data indicate time-averaged SO2 emissions of up to 157 tonnes/day, HCl of 74–1382 tonnes/day and up to 196 tonnes/day HF. Apatite may provide a useful measure of long-term volatile emissions at volcanoes where direct emissions measurements are unavailable, or for comparison with intermittent gas sampling methods. However, significant uncertainty remains regarding volatile distribution coefficients for apatite, and their variations with temperature and pressure.
Highlights
The exsolution of dissolved magmatic volatiles into bubbles during magma ascent and eruption is one of the most important processes affecting the physical properties of any volcanic system
In some magmas, melt inclusions may only be present in phases that are liable to leakage or degassing, or they may be present but too small for analysis, or have undergone devitrification or significant post-‐entrapment modification
The matrix glass compositions exhibit chemical trends that indicate progressive fractionation driven by decompression and degassing; this is consistent with the progressive decrease in H2O content seen in plagioclase-‐
Summary
The exsolution of dissolved magmatic volatiles into bubbles during magma ascent and eruption is one of the most important processes affecting the physical properties of any volcanic system. In the absence of direct measurements of gas emissions, the volatile contents of melt inclusions, trapped in phenocrysts and isolated at depth, are routinely used to infer pre-‐eruptive melt volatile concentrations (e.g. Edmonds et al, 2001; Wallace, 2005; Humphreys et al, 2008, Bouvier et al, 2008) Comparison of these pre-‐eruptive volatile concentrations with those preserved in the matrix glass gives a petrologic estimate of volatiles degassed during volcanic eruptions (Devine et al, 1984; Thordarson et al, 1996). In some magmas, melt inclusions may only be present in phases that are liable to leakage or degassing, or they may be present but too small for analysis, or have undergone devitrification or significant post-‐entrapment modification In such cases, an alternative method for assessment of pre-‐eruptive volatile contents is required. We explore and evaluate the potential use of apatite in place of melt inclusions, to infer pre-‐eruptive concentrations of S, F and
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
