Crystallisation history of rhyolite magmas at Tarawera volcano, New Zealand, from in-situ 238U-230Th disequilibrium dating and trace element geochemistry of zircon

Abstract

Tarawera volcano formed during four intra-caldera rhyolite eruptive episodes in the Okataina Volcanic Centre (OVC), New Zealand, at 21.9, 17.7, 13.7, and 0.7 ka that were all triggered by basalt intrusion. Most of the episodes involved at least two distinct magma types that were erupted from the same conduit. New SIMS 238U-230Th disequilibrium dating and trace element analyses (U, Th, Y, Hf, Ce, Ti) of individual zircon crystals provide insight to the timescales of silicic magma system processes. Zircon geochronology reveals protracted yet disparate crystallisation histories in all of the erupted magmas. Rim analyses on unpolished crystal faces were performed to minimise the overlap of the ion beam onto different age zones. This documents the last stage of crystal growth. Zircon crystallisation frequently stopped long before eruption (1->100 ka). Interior analyses (>200 ka in some crystals) suggest a minimum onset of crystallisation long before the formation of the volcano. Continuous age depth profiling from the crystal rims inward reveals both uninterrupted and intermittent crystallisation over protracted periods of up to ~114 ka. Hiatuses in growth of up to ~40 ka in some crystals are asynchronous between different zircons, even from the same lava block, and imply contemporaneous crystal growth and stagnation in different parts of the system. In addition, age-correlated compositional depth profiles indicate contemporaneous but contrasting zircon compositional histories and Ti-in-zircon temperatures (~650-850 °C). All of these features require heterogeneous crystallisation conditions as well as significant crystal transport and mixing prior to eruption. Transport and mixing could be the result of buoyancy instabilities and overturn caused by mafic intrusions. Spatially heterogeneous freeze-thaw cycles could explain the co-existence of high (~75-95 %) and low degrees of crystallinity in the mush inferred from fractional crystallisation modelling of U, Th, and Th/U. The continuity of zircon growth throughout the eruptions from Tarawera and even the ~45 ka OVC caldera-forming event suggest a decoupling between the crystallisation state of the magma system and eruption triggers such as tectonism and mafic intrusion. This highlights the need to distinguish between the longevity of the magmatic system and the time for accumulation of eruptible magma when investigating the timescales of rhyolite magma residence in the crust.