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Analysis of iron oxidation state across explosive and effusive eruption products to determine the behaviour of degassing and fragmentation at Tūhua and Tambora.

Frankie Haywood1, Richard Brooker1, Ed Llewellin2, Alison Rust1, Geoff Kilgour3, Fabian Wadsworth4, Jess Bassett1

  • Affiliations: 1 School of Earth Sciences, University of Bristol, Bristol, UK; 2Department of Earth Sciences, Durham University, Durham,  UK; 3GNS Science, Wairakei, New Zealand; 4Ludwig-Maximilians-Universität, Germany 

  • Presentation type: Talk

  • Presentation time: Tuesday 14:15 - 14:30, Room S150

  • Programme No: 3.2.5

  • Theme 3 > Session 2

Abstract

Magma involved in volcanic eruptions undergoes large changes in pressure, temperature, and dissolved volatile concentrations. These changes can induce an evolution of iron oxidation state that in turn affects the viscosity of the magmatic liquid and the propensity for crystallisation, both of which are key controls on whether or not magma will fragment to pyroclasts during eruption. Therefore, iron oxidation state changes may be crucially important in eruption physics, as well as potentially acting as a chemical marker of changes that influence eruption style. However, how and why the iron oxidation state evolves during magma ascent, degassing, and eruption, remains poorly constrained. Here, we examine samples from Tūhua, a pantellerite volcano in New Zealand with a history of both explosive and apparently effusive eruptive products enriched in iron. Through x-ray absorption spectroscopy, Mössbauer spectroscopy and electron microprobe analysis, we measure iron valence states across varied eruption styles as well as at the micro-scale in order to un-pick evidence for different syn-eruptive magmatic processes. Initial work has shown similarity in the bulk Fe2+/Fetot state of deposits across Tuhua, despite their apparent difference in eruption and degassing style. These analyses are compared with products from the 1815 Tambora eruption, where changes in iron valence between pre- and syn-eruptive conditions (determined by EPMA analysis of melt inclusions) can be linked with significant sulfur degassing.