Pre-eruptive volatile content and degassing history of an evolving peralkaline volcano

Abstract

The volatile contents of melt inclusions trapped within phenocrysts of quartz and feldspar in peralkaline rhyolites from Mayor Island, New Zealand, have been studied using Fourier transform infrared spectroscopy and ion microprobe analyses. The glass inclusions analyzed span the ~ 130 ka subaerial eruptive history of the island and come from volcanic deposits representing a wide range of eruptive styles (explosive vs. effusive, magmatic vs. phreatomagmatic, low vs. high discharge rates). The water content of all inclusions analysed is uniformly high at ~ 4.4 wt.% H 2O, whereas CO 2 contents are below the limits of detection (~ 50 ppm). Chlorine in melt inclusions ranges from 2070 to 5200 ppm, while coexisting matrix glasses have generally lower Cl concentrations of 1700–4200 ppm; the apparent bulk distribution coefficient describing Cl partitioning between melt and vapor phase ([Cl] fluid/[Cl] melt) during degassing is 5–15. Fluorine appears to be less affected by eruptive degassing than is Cl, and melt inclusion and matrix glass F concentrations show significant overlap (1400–2520 ppm F in inclusions, 1550–2890 ppm F in matrix glass). The observed invariance of melt inclusion water content with sample age contrasts with ion microprobe data on incompatible trace elements (e.g., Zr, Nb), which suggest ~ 35% fractionation between the oldest and youngest samples. This, along with the Cl partitioning behaviour, suggests that at least the upper erupting portion of the Mayor Island magma chamber was water saturated throughout the volcano's 130 ka eruptive history. Furthermore, the large range of eruptive styles observed on Mayor Island are not due to differences in water concentration of the erupting magma but instead must reflect differences in rates of magma ascent and supply (which control the efficiency of non-explosive degassing).