A compositional tipping point governing the mobilization and eruption style of rhyolitic magma

Abstract

The most viscous volcanic melts and the largest explosive eruptions on our planet consist of calcalkaline rhyolites. These eruptions have the potential to influence global climate. The eruptive products are commonly very crystal-poor and highly degassed, yet the magma is mostly stored as crystal mushes containing small amounts of interstitial melt with elevated water content. It is unclear how magma mushes are mobilized to create large batches of eruptible crystal-free magma. Further, rhyolitic eruptions can switch repeatedly between effusive and explosive eruption styles and this transition is difficult to attribute to the rheological effects of water content or crystallinity. Here we measure the viscosity of a series of melts spanning the compositional range of the Yellowstone volcanic system and find that in a narrow compositional zone, melt viscosity increases by up to two orders of magnitude. These viscosity variations are not predicted by current viscosity models and result from melt structure reorganization, as confirmed by Raman spectroscopy. We identify a critical compositional tipping point, independently documented in the global geochemical record of rhyolites, at which rhyolitic melts fluidize or stiffen and that clearly separates effusive from explosive deposits worldwide. This correlation between melt structure, viscosity and eruptive behaviour holds despite the variable water content and other parameters, such as temperature, that are inherent in natural eruptions. Thermodynamic modelling demonstrates how the observed subtle compositional changes that result in fluidization or stiffening of the melt can be induced by crystal growth from the melt or variation in oxygen fugacity. However, the rheological effects of water and crystal content alone cannot explain the correlation between composition and eruptive style. We conclude that the composition of calcalkaline rhyolites is decisive in determining the mobilization and eruption dynamics of Earth's largest volcanic systems, resulting in a better understanding of how the melt structure controls volcanic processes.

Advances in the rheology of natural multiphase silicate melts: Import for magma transport and lava flow emplacement

“…The current models are applicable within the compositional space that they are based upon, but some compositional regions (e.g. peralkaline compositions) still remain unmapped and the models struggle to repro− duce measured viscosity values [Giordano et al, 2006, Di Genova et al, 2017. Those formulations also put in evidence that the role of water (H 2 O) dissolved in the melt is counterintuitive being opposite to that of net− work modifier cations.…”

High spatial resolution analysis of the iron oxidation state in silicate glasses using the electron probe

“…GRN samples may have suffered oxidation during μXANES (Cottrell et al 2018), therefore their reported Fe oxidation state values are not considered further and the samples are only used to explore the effects of composition on redox changes during EPMA. The high-silica glasses range from peralkaline (FSP in Di Genova et al 2016, andPSG in Di Genova et al 2013) to calcalkaline (Y in Di Genova et al 2017a, andPSB in Riker et al 2015), with both anhydrous (FSP and Y) and hydrous (PSG and PSB) glasses.…”

“…Raman spectroscopy was used to detect the presence of nanolites before and after electron beam irradiation as nanolites alter the Raman spectra of silicate glasses. Magnetite nanolites produce a peak at ~670 cm -1 , which also decreases the intensity of the surrounding silicate peaks (Di Genova et al 2017a, 2017b. Carbon coats were removed prior to analysis.…”

“…An extended grating was used to also collect data on H 2 O content. All samples, except AMS, FSP, PSG, and Y, which have been previously analyzed by Di Genova et al (2017aGenova et al ( , 2017b, were analyzed on non-irradiated areas of glass. Selected glasses that cover a range of compositions (ETNA08, MAS.1.A5, FSP1, FSP9, PSG6, and PSB63) were additionally analyzed following electron beam irradiation.…”

“…Exceptions are AR37 (composition LS) and ETNA(2) (samples ETNA 3, 6, 7, 8, 14, 16, and 30), with a peak at ~670 cm -1 indicating magnetite nanolites. Magnetite nanolites were detected in AMS4 and Y-L using Raman spectroscopy by Di Genova et al (2017aGenova et al ( , 2017b.…”