Basalt, Unveiling Fluid-filled Fractures, Inducing Sediment Intra-void Transport, Ephemerally : Examples from Katla 1918

• Published in: Journal of Volcanology and Geothermal Research
• Main Authors: Owen, Jacqueline, Shea, Thomas, Tuffen, Hugh
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2019
• Subjects: glacier; Iceland
• Online Access: https://eprints.lancs.ac.uk/id/eprint/129702/; https://eprints.lancs.ac.uk/id/eprint/129702/1/Katla_paper_for_submission_editor_comments_without_track_changes.pdf

This article documents textures within basaltic Katla 1918 pyroclasts, where particle-filled fractures and bubbles have been observed. These features are analogous to tuffisite veins; particle-filled fractures which represent the preserved remains of transient degassing pathways in shallow conduits. Such fractures have long been considered restricted to high viscosity silicic melts. However, through BSE images and compositional maps, we have identified similar tuffisite-like features in crystal-poor basalt pyroclasts from the 1918 CE subglacial eruption of Katla, Iceland (K1918). Clast textures record transient mobility of juvenile/lithic particles, melt droplets and gas through magmatic fractures and connected vesicles. Key evidence includes (1) the presence of variably sintered fine-ash particles within variably healed fractures and vesicles (present in >80% of clasts analysed), (2) compositional maps that reveal the presence of foreign particles within preserved and healed permeable pathways, and (3) lower vesicularities immediately surrounding ‘fracture’ walls, suggestive of diffusive volatile loss into a permeable network. The 1918 CE eruption of Katla occurred under a thick glacier, however the ice was quickly breached, owing partly to the explosive nature of the eruption. We propose that the formation and preservation of these transient permeable networks have been facilitated by rapid decompression of a relatively volatile-rich magma, in a confined subglacial environment, with combined magmatic and phreatomagmatic fragmentation, followed by rapid quenching by meltwater. Tuffisite veins in rhyolite demonstrate repeated fracture-healing cycles, which drive incremental release of overpressured gas and help to defuse explosive eruptions. Interestingly, the permeable network at Katla failed to defuse the 1918 CE eruption, which involved a particularly violent subglacial eruptive phase. It is unclear whether this demonstrates an inability of mafic tuffisite-like features to efficiently degas magma (perhaps ...