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The global mid-ocean ridge system is the most extensive magmatic system on our planet and is the site of 75 per cent of Earth's volcanism.sup.1. The vertical extent of mid-ocean-ridge magmatic systems has been considered to be restricted: even at the ultraslow-spreading Gakkel mid-ocean ridge under the Arctic Ocean, where the lithosphere is thickest, crystallization depths of magmas that feed eruptions are thought to be less than nine kilometres.sup.2. These depths were determined using the volatile-element contents of melt inclusions, which are small volumes of magma that become trapped within crystallizing minerals. In studies of basaltic magmatic systems, olivine is the mineral of choice for this approach.sup.2-6. However, pressures derived from olivine-hosted melt inclusions are at odds with pressures derived from basalt major-element barometers.sup.7 and geophysical measurements of lithospheric thickness.sup.8. Here we present a comparative study of olivine- and plagioclase-hosted melt inclusions from the Gakkel mid-ocean ridge. We show that the volatile contents of plagioclase-hosted melt inclusions correspond to much higher crystallization pressures (with a mean value of 270 megapascals) than olivine-hosted melt inclusions (with a mean value of 145 megapascals). The highest recorded pressure that we find equates to a depth 16.4 kilometres below the seafloor. Such higher depths are consistent with both the thickness of the Gakkel mid-ocean ridge lithosphere and with pressures reconstructed from glass compositions. In contrast to previous studies using olivine-hosted melt inclusions, our results demonstrate that mid-ocean-ridge volcanoes may have magmatic roots deep in the lithospheric mantle, at least at ultraslow-spreading ridges. Volatile contents of plagioclase-hosted melt inclusions from volcanoes at the Gakkel mid-ocean ridge suggest that magmatic crystallization extends to depths of 16 kilometres, much deeper than suggested by olivine-hosted melt inclusions., Author(s): Emma N. Bennett [sup.1] , Frances E. Jenner [sup.2] , Marc-Alban Millet [sup.1] , Katharine V. Cashman [sup.3] , C. Johan Lissenberg [sup.1] Author Affiliations: (1) School of Earth [...]

The magmatic systems that give rise to voluminous crystal-poor rhyolite magma bodies can be considered to operate on two contrasting timescales: Those governed by longer-term processes by which a magma acquires its chemical and isotopic characteristics (e.g., fractional crystallisation and assimilation), and those operating at shorter timescales during the physical accumulation of the melt-dominant magma body that finally erupts. We explore the compositional and textural relationships between amphibole and orthopyroxene crystals from the 25.4 ka, 530 [km.sup.33] (magma) Oruanui eruption products (Taupo volcano, New Zealand) to investigate how processes related to the physical assembly of the pre-eruptive magma body are represented in the crystal record. Over 90 % of orthopyroxenes from the volumetrically dominant high-Si[O.sub.2] (>74 wt%) rhyolite pumices record textural evidence for a significant disequilibrium event (partial dissolution ± resorption of cores and interiors) prior to the growth of 40-500 µm thick rim zones. This dissolution/regrowth history of orthopyroxene is recorded in the chemistry of co-crystallising amphiboles as a prominent inflection in the concentrations of Mn and Zn, two elements notably enriched in orthopyroxene relative to amphibole. Textural and chemical features, linked with in situ thermobarometric estimates, indicate that a major decompression event preceded the formation of the melt-dominant body. The decompression event is inferred to represent the extraction of large volumes of melt plus crystals from the Oruanui crystal mush/source zone at pressures of 140-300 MPa (~6-12 km depth). Orthopyroxene underwent partial dissolution during ascent before reestablishing in the melt-dominant magma body at pressures of 90-140 MPa (~3.5-6 km). We model Fe-Mg diffusion across the core-rim boundaries along the crystallographic a or b-axes to constrain the timing of this decompression event, which marked establishment of the melt-dominant magma body. Maximum modelled ages indicate that this event did not begin until ~1,600 years before eruption, consistent with constraints from zircon model-age spectra. Once extraction began, it underwent runaway acceleration with a peak extraction age of ~230 years, followed by an apparent period of stasis of ~60 years prior to eruption. The rapidity of the extraction and accumulation processes implies the involvement of a dynamic driving force which, in the rifted continental arc setting of the Taupo Volcanic Zone, seems likely to be represented by magma-assisted extensional tectonic processes. Keywords Supereruption * Orthopyroxene Fe-Mg diffusion * Oruanui eruption * Crystal mush * Magma chamber, Introduction Deciphering the processes associated with the build-up to large silicic eruptions offers many challenges from petrological and volcanological perspectives. Among the most contentious issues is determining over what timescales [...]