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Magma ascent dynamics and its control on compositional diversity of volcanism across tectonic settings

Kai Zhao1, Olivier Bachmann2

  • Affiliations: 1CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences; Nanjing, 210023, China. 2Department of Earth Sciences, ETH Zurich, Clausiusstrasse 25, CH-8092 Zurich, Switzerland.

  • Presentation type: Talk

  • Presentation time: Tuesday 11:00 - 11:15, Room S150

  • Programme No: 3.2.3

  • Theme 3 > Session 2

Abstract

The compositional variability of volcanic rocks in continental settings is generally thought to be related to source melting and/or reservoir processes. However, the modulation effect of ascent dynamics on the composition of erupted magmas remains poorly assessed. Viscosity calculations of data collection for ~90 global volcanoes and numerical modeling suggest that magma eruptibility is influenced by initial magma viscosity at the base of a volcanic conduit rooted in the subvolcanic reservoir, and that ~106.0±0.3 Pa s represents a threshold of eruption below which magmas ascend at rates higher than the critical ascent rates of freezing. Rhyolitic melt viscosity in continental rifts/hotspots is one order of magnitude higher than that in continental arcs (~105.5±0.2 versus ~104.5±0.2 Pa s on average, respectively) due to ~1.5 wt% lower pre-eruptive melt H2O contents in rifts/hotspots at comparable temperatures. The continental rift/hotspot rhyolitic melts thus cannot accommodate large fractions of phenocrysts for eruption, leading to an abundance of crystal-poor (<~30%) rhyolites and a paucity of volcanic rocks with intermediate bulk-rock composition. In contrast, arc rhyolitic melts with lower viscosity can accommodate higher fractions (up to ~50%) of phenocrysts while still remaining below the viscosity threshold, leading to the common appearance of crystal-rich andesites and dacites (with rhyolitic interstitial melts). Therefore, the rheological threshold acts as a filter on the generation of contrasting volcanic rocks in different tectonic settings, which ultimately arises from the different magmatic H2O contents.