Abstract
The 2012 Havre eruption evacuated a crystal-poor rhyolite (~3-7 % crystals) producing a volumetrically dominant (~1.4 km3 ) pumice raft, as well as seafloor giant pumice (5-8 %) and lavas (12-14%) at the vent (~0.1 km3), both of which have subtly higher phenocryst contents. For crystal-poor rhyolites like the Havre pumice, it can often remain ambiguous as to whether the few phenocrysts present, in this case, plagioclase, orthopyroxene, clinopyroxene, Fe-Ti oxides +/- quartz, are: a) autocrysts crystallising from the surrounding melt, b) antecrysts being sourced from mush and the magma plumbing system, or c) xenocrysts derived from source materials or chamber walls, or d) possibly a combination of all of the above. In crystal-poor magmas, the few crystals present are strongly relied upon to constrain pre-eruptive conditions such as magmatic temperatures, pressures, water content and fO2. A detailed textural and compositional analysis combined with a range of equilibrium tests and rhyolite-MELTS modelling provide the basis for distinguishing autocrystic vs inherited crystal populations in the Havre eruption. An autocrystic mineral assemblage of andesine plagioclase, enstatite and Fe-Ti oxides constrains the pre-eruptive conditions of the Havre rhyolite magma: magmatic temperatures of 890 ± 27°C, crystallisation pressures at 2-4 kbars, oxygen fugacity of NNO +0.4 and water concentrations (5.6 ± 1.1 wt.%). Inherited phases not in equilibrium with the host melt composition are clinopyroxene, An-rich plagioclase (>An53) and quartz. Rhyolite-MELTs modelling indicates the clinopyroxene and quartz have most likely been sourced from cooler, silicic mush zones in the Havre magmatic system. This study demonstrates that even in crystal-poor rhyolites it cannot be assumed that all crystals are autocrystic and can be used to constrain pre-eruptive magmatic conditions.
Highlights
Crystal CargoIdentifying the origins of crystals in magmatic systems is critical to understanding the inner workings of these systems, in particular, magma petrogenesis and pre-eruptive conditions that place constraints on eruption dynamics
The results presented here are a first-step in understanding the magmatic plumbing system of the Havre volcano, and provide insight into the processes involved in the generation and pre-eruptive storage conditions of a significant volume of crystal-poor rhyolite in an island arc setting
Through our extensive sampling of beach stranded raft pumice, we observe a wider variety of pumice textural types than has been recognized from the Havre summit and seafloor deposits including a subpopulation of white pumice with secondary pink coloration and a ‘brown’ pumice with bread crusted surfaces (Supplementary Material 2)
Summary
Crystal CargoIdentifying the origins of crystals in magmatic systems is critical to understanding the inner workings of these systems, in particular, magma petrogenesis and pre-eruptive conditions that place constraints on eruption dynamics. The Havre 2012 submarine eruption is typical of such rhyolites – it is crystal-poor (generally < 8%) and has a relatively restricted crystal assemblage of plagioclase, two pyroxenes and Fe-Ti oxides (Carey et al, 2018). This assemblage is consistent with the observed mineralogy in other low- to medium-K rhyolites from the Tonga-Kermadec arc (e.g., Wright et al, 2006; Barker et al, 2013). The state of the Havre magmatic system remains unconstrained because the eruptive history and previous eruption products are poorly known due to its remote and deepwater (900 mbsl) location
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