Crystal-specific insights from rhyolite eruptive deposits into volatile element transfer in the magmatic system below Taupō volcano

Abstract

Knowledge on the chemical pathways of volatiles in large silicic magmatic systems, from the parental mantle-derived mafic magmas to their more evolved silicic derivatives, is central to understanding a range of topics from magmatic processes through to the climate impacts of volcanism, as well as geothermal and ore resources. However, establishing the pre-eruptive volatile contents of magma from silicic eruptive deposits requires capturing the intermediate steps of the magmatic evolution. The concentrations of sulfur, chlorine, and fluorine were investigated in selected deposits from Taupō volcano: (1) silicic groundmass glass and microphenocryst apatite that equilibrated in the shallow melt-dominant magma body; (2) silicic melt and apatite inclusions sourced from deeper in the crystal mush; and (3) olivine-hosted basaltic melt inclusions representing the most primitive magmas. In-situ geochemical analyses were carried out on orthopyroxene and apatite from the Oruanui supereruption, orthopyroxene from post-Oruanui dacites and rhyolites, and high-Mg olivine from Taupō basaltic-andesite. The volatile contents at each stage reveal unique magmatic volatile pathways from the source magmas through to the silicic system, that are primarily a function of intensive parameters (pressure, temperature, oxygen fugacity), crystallising mineral phases, and the presence of an aqueous fluid phase. The relatively high sulfur contents in the basalts drop by ~2000 ppm to near detection limits in dacitic compositions due to pyrrhotite crystallisation during the magnetite crisis. In contrast, chlorine increases from basaltic to dacitic compositions, due to the incompatibility of chlorine in magmatic minerals. Fluorine values are similar between basaltic and dacitic-rhyolitic compositions, primarily controlled by the onset of amphibole/apatite crystallisation. Sulfur and chlorine are both inferred to partition strongly into an aqueous vapour across the whole range of silicic compositions analysed. Volatile pathways within the silicic magmas are faithfully recorded in apatite compositions, which can be a valuable petrological tool for assessing magmatic processes.