The recycling of crystals into new magma batches can result from magma recharge events, the mobilization of, and entrainment from, crystal mush zones, remelting of recently formed subsolidus materials, or unmelted remains (restite) of partially melted source materials (e.g., Bachmann and Bergantz, 2004, 2008a, b Hildreth and Wilson, 2007 Bryan et al., 2008 Deering et al., 2011 Deering and Bachmann, 2010 Huber et al., 2012 Cooper and Kent, 2014 Lubbers et al., 2020). The past 20 years has seen an evolution in understanding of the origin of crystals from the generally held assumption that all phenocrysts were autocrysts nucleated and grown from the erupted magma to being a potentially complex mixture of autocrysts, xenocrysts (e.g., quartz, K-feldspars in basalts, and ancient zircon in granites and rhyolites) and antecrysts that record related, but earlier phases of magmatism ( Charlier et al., 2005 Gill et al., 2006 Charlier et al., 2007 Davidson et al., 2007 Hildreth and Wilson, 2007 Jerram and Martin, 2008 Bryan et al., 2008). Identifying 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 then place constraints on eruption dynamics. 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. Rhyolite-MELTs modeling indicates the clinopyroxene and quartz have most likely been sourced from cooler, silicic mush zones in the Havre magmatic system. Inherited phases not in equilibrium with the host melt composition are clinopyroxene, An-rich plagioclase (> An 53) and quartz.
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☌, crystallization pressures at 2–4 kbars, oxygen fugacity of NNO + 0.4 and water concentrations (5.6 ± 1.1 wt.%). A detailed textural and compositional analysis combined with a range of equilibrium tests and rhyolite-MELTS modeling provide the basis for distinguishing autocrystic vs inherited crystal populations in the Havre eruption.
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 fO 2.
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 crystallizing 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. The 2012 Havre eruption evacuated a crystal-poor rhyolite (∼3–7% crystals) producing a volumetrically dominant (∼1.4 km 3) pumice raft, as well as seafloor giant pumice (5–8%) and lavas (12–14%) at the vent (∼0.1 km 3), both of which have subtly higher phenocryst contents.
2Central Analytical Research Facility, Institute for Future Environments, Queensland University of Technology, Brisbane, QLD, Australia.1School of Earth and Atmospheric Sciences, Queensland University of Technology, Brisbane, QLD, Australia.