Insights into the dynamics of mafic magmatic-hydromagmatic eruptions from volatile degassing behaviour: The Hverfjall Fires, Iceland

Abstract

The style and intensity of hydromagmatic activity is governed by a complex interplay between the relative volumes of magma and water that interact, their relative viscosities, the depth of subsurface explosions, the substrate properties, and the vent geometry. Fundamental questions remain, however, regarding the role of magmatic vesiculation in determining the dynamics of magma-water interaction (MWI). Petrological reconstructions of magmatic degassing histories are commonly employed to interpret the pre-and syn-eruptive conditions during 'dry' magmatic eruptions, but the application of similar techniques to hydromagmatic activity has not yet been fully explored. In this study, we integrate glass volatile measurements (S, Cl, H2O and CO2) with field observations and microtextural measurements to examine the relationship between degassing and eruptive style during the Hverfjall Fires fissure eruption, Iceland. Here, coeval fissure vents produced both 'dry' magmatic (Jarðbaðshólar scoria cone complex) and variably wet hydromagmatic (Hverfjall tuff ring) activity, generating physically distinct pyroclastic deposits with contrasting volatile signatures. Matrix glass volatile concentrations in hydromagmatic ash (883 ± 172 [1σ] ppm S; 0.45 ± 0.03 [1σ] wt% H2O; ≤20 ppm CO2) are consistently elevated relative to magmatic ash and scoria lapilli (418 ± 93 [1σ] ppm S; 0.12 ± 0.48 [1σ] wt% H2O; CO2 below detection) and overlap with the range for co-erupted 2 phenocryst-hosted melt inclusions (1522 ± 127 [1σ] ppm S; 165 ± 27 [1σ] ppm Cl). Measurements of hydromagmatic glasses indicate that the magma has degassed between 17 and 70% of its initial sulfur prior to premature quenching at variably elevated confining pressures. By comparing volatile saturation pressures for both magmatic and hydromagmatic glasses, and how these vary through the eruptive stratigraphy, we place constraints on the conditions of MWI. Crucially, our data demonstrate that the magma was already vesiculating when it encountered groundwater at depths of 100-200 m, and that the external water supply was sufficient to maintain MWI throughout the eruption with no significant vertical or lateral migration of the fragmentation surface. We propose that development of an invent watersediment slurry provides a mechanism through which the elevated confining pressures of ~1.6-2.6 MPa (or up to 6 MPa accounting for uncertainty in CO2 below analytical detection) could be maintained and buffered throughout the eruption, whilst enabling vertical mixing and ejection of fragmented juvenile and lithic material from a range of depths. Importantly, these results demonstrate that the volatile contents of hydromagmatic deposits provide valuable records of (1) the environment of MWI (e.g., groundwater versus surface water, vertical migration of the fragmentation level) and (2) the state of the magma at the time of fragmentation and quenching. We further suggest that the volatile content of tephra glasses provides a reliable alternative (or additional) indicator of a hydromagmati...

Properties of dust source material and volcanic ash in Iceland

“…Shape characteristics can also be used to assist in determining the source of the material, especially with volcanic ash. The ash from each volcanic eruption has distinct characteristics depending on the environment it was formed in Liu et al (2018), Schmith et al (2018), Shoji et al (2018).…”

Evidence of explosive hydromagmatic eruptions during the emplacement of the North Atlantic Igneous Province

“…Elevated volatile concentrations in glass-above that expected in equilibrium with atmospheric pressures-indicate quenching before the melt is fully degassed. This would be most likely attributable to quenching under elevated pressures, such as during subaqueous or subglacial magma-water interactions [Dixon et al 2002;Liu et al 2018;Mastin et al 2004;Métrich et al 1991;Óladóttir et al 2007]. Although most quantitative assessments of degassing histories have focussed on CO 2 and H 2 O exsolution [e.g.…”

“…Rapid quenching arrests the degassing process, leading to elevated residual volatile contents compared to magmatic tephra degassed to atmospheric pressure [Mastin et al 2004;Németh and Kósik 2020;Óladóttir et al 2007]. Therefore, one method that can potentially identify hydromagmatic eruptions is measuring the volatile contents of quenched glass, particularly late-stage degassing species such as sulfur and chlorine [Liu et al 2018;Mastin et al 2004;Óladóttir et al 2007;Thórdarson et al 2003]. Recent studies of basaltic eruptions in Iceland have used this approach to identify changes in glacial thickness above Katla volcano [Óladóttir et al 2007] and to differentiate between coeval magmatic and hydromagmatic eruptions along a single fissure [Liu et al 2017;Liu et al 2018].…”

“…Therefore, one method that can potentially identify hydromagmatic eruptions is measuring the volatile contents of quenched glass, particularly late-stage degassing species such as sulfur and chlorine [Liu et al 2018;Mastin et al 2004;Óladóttir et al 2007;Thórdarson et al 2003]. Recent studies of basaltic eruptions in Iceland have used this approach to identify changes in glacial thickness above Katla volcano [Óladóttir et al 2007] and to differentiate between coeval magmatic and hydromagmatic eruptions along a single fissure [Liu et al 2017;Liu et al 2018]. Coupled with volcanic glass morphology, Liu et al [2018] demonstrated that glass sulfur concentrations were elevated in the hydromagmatic products due to quenching at pressures greater than atmospheric (1 atm).…”

“…Recent studies of basaltic eruptions in Iceland have used this approach to identify changes in glacial thickness above Katla volcano [Óladóttir et al 2007] and to differentiate between coeval magmatic and hydromagmatic eruptions along a single fissure [Liu et al 2017;Liu et al 2018]. Coupled with volcanic glass morphology, Liu et al [2018] demonstrated that glass sulfur concentrations were elevated in the hydromagmatic products due to quenching at pressures greater than atmospheric (1 atm). Although the Fur Formation ashes are considerably older (56.0-54.6 Ma) than these Holocene tephras, they are remarkably well-preserved.…”