The timing and mechanisms of sulfur release by Icelandic flood lava eruptions. Holuhraun 2014–15 CE and Laki 1783–84 CE a case study

Abstract

Flood lavas (FL), or high magnitude (1–100 km3) basaltic fissure eruptions have a recurrence interval of 250–500 years in Iceland. These events can produce atmospheric volcanic pollution at tropospheric–stratospheric levels via their sulfur (S) emissions. Current knowledge of the modulating role of shallow conduit processes on the vent activity, as well as the mechanisms and timing of peak S release into the atmosphere, is limited.

This project has two key aims: first, to evaluate the influence of shallow conduit processes on eruption style and dynamics during peak intensities of explosive phases; second, to produce a novel chalcophile stable isotope proxy for the speciation of released S and the redox state of the system upon eruption. Two selected case studies for this project are: (i) the recent low intensity and magnitude end-member, 2014–15 CE Holuhraun, and (ii) a high intensity and magnitude end-member, 1783–84 CE Laki which is well-documented in detailed contemporary accounts.

Micro-textural analysis of the ‘frozen’ outer rinds of pyroclasts from both eruptions, which record the state of the magma prior to fragmentation, identified relative shifts in vesicle number density associated with changes in eruptive intensity. Whole pyroclast textural mapping and in-situ geochemical analysis constrain the modification of the interior of the clasts via post-fragmentation expansion. This process enhanced the contrasts between discrete pre-existing melt domains which mingled prior to eruption. These domains record different shallow conduit histories, in particular contrasting ascent rates, and therefore preserve evidence of equilibrium and disequilibrium vesiculation within the same clast. Zinc and copper stable isotope compositions of lava and tephra from distinct phases of eruptive activity were utilised to fingerprint the mechanisms of S loss. Changes in the efficiency of volatile loss of S can be linked to known changes in vent dynamics, as well as changes in S loss associated with both the evolving transport system, redox conditions at the vent and in the flow field.

The textural and geochemical findings from these two eruptions will help further our understanding of shallow conduit, eruptive and emplacement processes during the many ill-constrained basaltic fissure eruptions of different intensities and magnitudes worldwide.