Assessing the magmatic-hydrothermal history at White Island/Whakaari, New Zealand

<p dir="ltr">Whakaari (White Island) is an active andesite-dacite volcano located on New Zealand’s northernmost continental shelf. It has three unique characteristics: (1) It is one of the most active volcanoes in the world, sitting on the arc front; (2) It erupted a large amount of high-Mg andesites (Mg#: 65–74; SiO₂: 55–58 wt. %) of primary origin in 1976–2000; and (3) It developed an active hydrothermal systems and was believed to be an embryonic porphyry deposit. The production of these magmas was suggested to be involved complex interaction between stored and ascending magmas in a mid-crustal magma chamber that forms part of a larger trans-crustal plumbing system. However, this proposal is inconsistent with the extremely low volatile concentrations in melt inclusions and geophysical observations. Our study re-examines this proposal by using systematic thermodynamic modelling (Rhyolite-MELTS 1.2.0 and Magma Chamber Simulator) and phase equilibrium experiments to simulate liquidus relationships for the 1976–2000 high-Mg andesites. The experimental conditions ranged from 1 atm to 500 MPa at temperatures of 950 to 1200°C with total water concentrations of 0 to10 wt.%. Except for the 500 MPa experiments, ƒO₂ was buffered at 1 or 2 log units above Ni-NiO. The simulations were conducted with broader ranges of P-T-H₂O-fO₂ conditions. </p><p dir="ltr">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 950°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. Furthermore, the effects of mixing and partial re-equilibration between different generations of phenocrysts and melts must also be accounted for. This is needed to explain both the petrography of the natural rocks and differences in the behaviour of FeO_T, Al₂O₃ and TiO₂ during the fractionation of natural Whakaari high-Mg andesitic magmas. </p><p dir="ltr">We are the first to report the co-variation of Cu and Sn isotopic compositions at Whakaari and at another rear-arc volcano, Mt. Taranaki (which lacks hydrothermal activities). Our analysis include lavas and pumices from various eruptions, crater lake water, lake sediments, volcanic ash, and hot springs. Whakaari lavas exhibit significant isotopic fractionation in both Cu (δ⁶⁵Cu = –0.19‰ to 0.59‰) and Sn (δ^122/118Sn = –0.241‰ to 0.361‰), which we interpret through a complex, multi-stage model involving degassing, fluid-associated fractional crystallisation, and sulphide precipitation. The Sn and Cu isotopic compositions in the crater lake and hot spring samples can be explained by a moderate degree of degassing. In contrast, isotopic variation is more limited in Taranaki pumices. Sn isotope variations in these samples likely result from Fe–Ti oxide fractionation, while Cu isotopic compositions can be produced by either a single degassing or fractional crystallisation model. Overall, our findings underscore the significant role of complex magmatic-hydrothermal processes in driving Sn and Cu isotopic fractionation.</p>