Magma Evolution and Storage Conditions in an Andesite-Dacite Volcanic System, Whakaari (White Island), New Zealand

Whakaari (White Island) is an active andesite-dacite volcano located on New Zealand’s northernmost continental shelf. During a series of Strombolian eruptions in 1976–2000 it produced high-Mg andesite magmas (Mg#: 65–74; SiO2: 55–58 wt. %). It has already been suggested that the production of these magmas involved complex interaction between stored and ascending magmas in a mid-crustal magma chamber that forms part of a larger trans-crustal plumbing system. Here we re-examine this proposal by employing Rhyolite-MELTS 1.2.0 and Magma Chamber Simulator to simulate liquidus relationships for one of the 1976–2000 high-Mg andesites from Whakaari (P41600). It was found that production of the main phenocryst assemblage (olivine + Cr-spinel + orthopyroxene + clinopyroxene + plagioclase + magnetite), mineral compositions, and liquid line of descent (as determined from matrix glasses) requires 30–60% fractional crystallisation at comparatively low pressures (< 100 MPa) and melt-H2O concentrations (< 2 wt. %) with moderate fO2 (from Ni-NiO to one log unit above Ni-NiO) and temperatures of 1140°C to 1000°C. At least 0.5 wt.% water is required to stabilise olivine at 60 MPa although original magmatic water concentrations may have been significantly higher. The early loss of magmatic water is a predictable consequence of both the low pressures of fractionation and the partitioning of H2O into a CO2-rich vapour phase. Polybaric crystallisation does not improve the simulations when compared to the isobaric model. Considering these observations, it is unlikely that evolution of the Whakaari high-Mg andesites involved significant fractionation within the mid- to lower-crust. However, there are aspects of magma evolution (particularly for FeOT and TiO2) that are not well modelled by any of our simulations. This may be because our model could not accurately simulate the effects of mixing between magmatic products or interaction between evolved melts and earlier crystallized mineral phases.