H2O-Controlled Eruptive Filtering on the Bimodality of Continental Volcanism Across Tectonic Settings

Abstract The contemporary basaltic and rhyolitic volcanism with a dearth of intermediate compositions creates a compositional gap (‘Daly gap’), which is known to be pronounced in continental rifts/hotspots but absent in continental arcs. The intrinsic mechanism(s) producing the Daly gap remain poorly understood. Similar bimodal spectra recorded in melt inclusions from continental rifts/hotspots and arcs suggest that similar processes modulate the bulk composition of erupted magmas. A compiled data set of melt inclusion composition, phenocryst fraction, pre-eruptive temperature and melt H2O content enables precise calculations of melt viscosities and bulk magma viscosities for >100 eruptions of global volcanoes in various tectonic settings. Analyses of the data set suggest lower viscosities of rhyolitic melts in arcs than in rifts/hotspots (~104.5 ± 0.2 versus ~105.5 ± 0.2 Pa·s, respectively), at comparable temperatures, because of ~1.5 wt % higher melt H2O contents in arcs. Combining numerical constraints on ascent dynamics and data set observations, our study defines an eruption threshold of ~106.0 ± 0.3 Pa·s at the base of a volcanic conduit rooted in the subvolcanic reservoir. The lower viscosities of rhyolitic melts in arcs facilitate stirring/re-homogenization of upper crustal reservoirs upon recharge and accommodation of more phenocrysts in erupted products, leading to the common appearance of crystal-rich (30–50%) andesites and dacites (with rhyolitic interstitial melts) with mixing signatures in magmatic provinces of continental arcs. In contrast, the lower mixing efficiency and eruptibility of the more viscous magmas present in continental rifts/hotspots lead to preferential eruption of crystal-poor (<~30%) rhyolites, enhancing compositional bimodality in the erupted records. This H2O-controlled eruptive filter modulates the presence versus absence of the Daly gap and is key to the distinct differentiation patterns across tectonic settings.