Does pre-ascent magma storage influence eruption style? An evaluation from textures and geochemistry of phenocrysts at Taranaki volcano

Understanding the explosivity of future volcanic eruptions is a cornerstone of hazard evaluation, and magma storage days to years leading up to eruption could provide timely information for mitigation strategies. To explore the influence of pre-ascent conditions on eruption style we compared the growth histories of plagioclase and amphiboles phenocrysts from four prominent, Holocene-aged, explosive erupted deposits with six effusive eruptions at Taranaki volcano, New Zealand. The explosively erupted andesite-dacite magmas display a range of bulk compositions and crystallinities that overlap with those erupted effusively. Regardless of their eruption style, each phenocryst population displays a high degree inter-crystal textural, geochemical and Sr-isotope discordance, including in their final rim growth. This can be explained by the periodic intrusive disruption of a redeveloping crystal mush that lacked a unidirectional assimilation-fractional-crystallisation path in the centuries to millennia between eruptions. However, amphiboles from most of the effusive eruptions are distinguished from those erupted explosively by recording late-stage halogen (and likely other volatiles) accumulation in the magma and subsequent decomposition during slow ascent. Also more prevalent in the effusive eruptions are plagioclase textures that record late-stage mingling with a mafic melt. These features support conceptual models that suggest magma compressibility due to volatile accumulation allows more efficient magma mingling during recharge and thus heating. The resulting melt viscosity reduction and enhanced outgassing of the magma then promotes effusive eruption. Testing this hypothesis via mineralogical determination of intensive parameters would need to overcome the high degree of disequilibrium that occurred at all stages of the magma evolution. • Mineral texture and chemistry are compared for Taranaki explosive & effusive deposits. • Effusively erupted amphiboles display reaction rims & F enrichment gradients. • Effusively erupted plagioclases more commonly display calcic/mafic rims. • Significant exsolved volatiles and extensive magma mixing preceded effusive eruption.