Abstract
At volcanoes where eruptions have not been directly observed, retrospective criteria such as phenocryst growth must be used to determine the magmatic conditions conducive to eruption. Indeed, this can also be the case for some well monitored active volcanoes. New Zealand's andesitic Taranaki volcano is capable of nation-wide ash dispersal and is thus, potentially hazardous. However, since AD1790 it has been anomalously quiescent. To access possible eruption triggers, we examined the textural, geochemical, and isotopic features of plagioclase phenocrysts from its most recent episode of activity ( approximately AD1030-AD1790). In situ 87Sr/86Sr analyses reveal little discordance between the phenocrysts and a uniform source melt for most deposits. However, textural and geochemical features of their final rim zones reveal contrasting conditions just before eruption. In some magmas, plagioclase record late growth in a silicic melt of approximately 0.5-1.0 wt% MgO followed by partial resorption and regrowth in a more mafic melt of >3.5 wt% MgO, indicating magma mixing occurred. Modelling of post-crystallisation Mg diffusion indicates periods of <1 day to a week for these events, consistent with the entry of new magma into the system just before eruption. In contrast, in magmas from intervening eruptions, and from some older pre-1 ka episodes, phenocrysts record uninterrupted growth and increased magma crystallinity prior to eruption. For these eruptions, triggering is better explained by volatile pressurisation induced by crystallisation in a closed system. We infer two coupled modes of eruption operated. The eruptions cause the reorganisation of the system influencing the buoyancy and permeability of the conduit that either favours the ascent and eruption of new magmas or their stagnation and subsequent pressurisation via crystallisation. Both modes resulted in similar eruption styles and magnitudes and likely followed short periods of unrest.