Insights into the 1976–2000 eruption episode of Whakaari/White Island, New Zealand: an eruption fuelled by repeated mafic recharge

Whakaari is New Zealand’s most active volcano, and its last major magmatic eruption occurred during a protracted episode between 1976 and 2000. This eruption episode involved a sustained phase of heightened unrest, punctuated by regular, and small-volume, phreatic, magmatic and phreatomagmatic eruptions. We examine the scoria sample suite erupted throughout this period to develop a model for the storage conditions and the underlying cause of this long-lived activity. From our analysis of the major element composition of minerals, groundmass glass and phenocryst-hosted melt inclusions, along with the volatile content of melt inclusions, Whakaari magmas are relatively hot and dry arc andesites when compared globally. Based on a combination of plagioclase hygrometry, volatile contents of phenocryst-hosted melt inclusions and thermodynamic modelling, we conceptualize a distributed magmatic system that is dominated by andesite. We find that the 1976–2000 eruption episode was largely driven by volatile poor andesite-dacite magma that was periodically intruded by a relatively primitive mafic magma throughout this period. This andesite-dacite crustal magma reservoir (to at least 9 km) is capped by a shallow storage reservoir at ~1- to 2-km depth, where magmas stalled before erupting. Mafic magma/s periodically intruded the andesite-dacite reservoir providing heat, melt, volatiles, and mafic minerals, including olivine, into the system. Finally, we show that the volatile budget during this small-volume eruption episode released significant S and Cl into the atmosphere, similar in scale to large basaltic eruptions.