Short crystal residence times and fast magma ascent prior to effusive eruptions at Mount Ruapehu, New Zealand

Abstract Decoding magma evolution and transport mechanisms requires detailed examination of the groundmass glass and associated mineral phases in the eruptive products. These analyses can help determine crystal residence times and magma ascent rates prior to eruption. Magma storage and ascent rates are particularly relevant when assessing the controls on the type and style of eruptions from andesite volcanoes, which are capable of producing varied eruptive phases from highly explosive to dominantly effusive. Here, we focus on Mount Ruapehu, one of New Zealand’s most active volcanoes and a significant portion of the Tongariro Volcanic Centre, which has a rich geological history of eruptions spanning a broad range of eruptive phases from explosive to effusive. Previous analysis of crystal size distributions of microlites from 10 ka tephra showed that magma ascent occurred over a relatively short period (i.e., ~2 days at ≤0.09 m/s) immediately prior to eruption. Here, we expand this approach to include analysis of massive lava flows emplaced from 50 to <10 ka (Mangawhero and Whakapapa formations) with similar andesitic composition and mineral assemblages dominated by plagioclase and two pyroxenes. We analyze the crystal size distributions of 17 lavas and conduct bulk rock analysis, groundmass laser ablation mass spectrometry analysis, and electron probe microanalysis on plagioclase and pyroxene phenocrysts and microlites. We use the chemical data to calculate the P-T-H2O conditions of crystallization. Then, we combine the derived crystal size distributions with thermobarometry and hygrometry results to determine the residence times and ascent rates of magmas that ultimately fed erupted lavas. Our data yields initial pressures up to 810 MPa, temperatures up to 1160 °C, and water contents up to ~2.7 wt% H2O, with average crystal residence times of ~3 days (maxima of 66 days considering growth rate uncertainty) and ascent rates up to ~0.08 m/s. The similar residence times and ascent rates from effusive and explosive eruptions suggest that similar crystallization paths and undercooling conditions take place irrespective of the eventual eruption style. We propose the eruption style may be modulated by the geometry of the conduit, as observed in other volcanic centers where conduit overpressure and gas accumulation trigger explosive eruptions.