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Magma mixing, degassing and late sulfide saturation: Insights into the 1976–2000 eruptive sequence at White Island, New Zealand

Abstract:

This paper reports new petrological and chemical data of ejecta from the 1976–2000 eruptive sequence of White Island, New Zealand. We provide evidence of mixing between shallow stored (0.7–1 km) andesitic to dacitic melts and deeper primitive mafic magma carrying high-Fo olivine, repeatedly injected throughout the sequence. Melt inclusions hosted in plagioclase, pyroxene and olivine were analyzed for major, trace and volatile elements. The low H2O contents in inclusions suggest it exsolves at depth while SO2 degassing is recorded during the rise and crystallization of the mafic magma. The higher solubility of Cl (as HCl and NaCl) delays its exsolution until shallow depth. Convection degassing is suggested to explain the large imbalance between elevated gas emissions and low amount of lava erupted. Frequent inputs of gas-rich, deeply derived magma are essential drivers for the counterflow of viscous degassed melts. Metal contents (Cu, Zn) of the residual mafic magma increases during olivine crystallization. This behavior changes during shallow differentiation of the andesitic magma, with metals leaving the silicate melt. While some metals may be lost to degassing, Cu, Ni, Ag and Au strongly partition into an immiscible sulfide melt. Saturation of such a sulfide melt phase occurs in the shallow environment, likely related to changes in redox state associated with magnetite crystallization and/or SO2 degassing.