Geochemical and textural indicators of syn-eruptive degassing during recent rhyolitic eruptions

Silicic volcanoes are capable of producing dangerous explosive eruptions and the effusion of lava and domes. Observations from recent rhyolitic eruptions demonstrated how the transition between these two end-member behaviours is markedly complex. Fractures, commonly preserved as tuffisite veins, are transient permeable pathways that allow for the punctuated but efficient escape of volatiles. They are considered key enablers of degassing but their interactions with deeper, gas-rich reservoirs and influence on melt chemistry are not fully understood. Preserved 210Pb-226Ra isotope disequilibria, volatile trace element heterogeneity, and complex vein and breccia textures in volcanic bombs have been previously associated with gas fluxing within the conduit. In this thesis I utilise both textural and chemical heterogeneity preserved in samples from the first observed rhyolite eruptions, Chaitén (2008) and Cordón Caulle (2011-2012), to assess how degassing processes are preserved from the dm to nm scale. 210Pb-226Ra isotope and trace element analyses were conducted on 'mini-bulk' samples of tuffisite veins and their respective hosts to understand gas flow through fractures, from deeper reservoirs. The lack of 210Pb-226Ra disequilibria preserved in veins constrains the upper limit for the volume of magma that can be degassed by a tuffisite vein during its lifetime. Whilst trace element enrichments and depletions in veins record preferential gas fluxing (e.g. Li, Cu, Pb, Tl, Bi), results are limiting because tuffisites and breccias, which preserve the fracturing process, are comprised of clasts that have each undergone differing degrees of degassing. To further constrain gas fluxing and clasts' individual histories, electron microprobe and laser ablation ICP-MS analyses were undertaken across mm to µm-scale textures in samples from Cordón Caulle. Chemical and textural heterogeneity are strongly linked, depletions of metals (e.g. Zn, Pb, Tl) in veins highlight scavenging by the fluxing volatile phase. Trace element systematics constrain minute-hour fracture lifespans and show how these increase towards the surface and with eruption duration. Trace element analyses are analytically limited to the order of 10s of microns to yield accurate and precise results. But texturally fracturing processes can be investigated to the nm scale. The study of the interfaces between host and vein material show how veins exploit pre-existing bubble networks and can infill them with ash. Rheological weaknesses are exploited to allow cycles of fragmentation, degassing, sintering and healing of melts. Post-fracture melt compaction highlights how even small veins can degas vesicular magmas on the cm-scale.By investigating fracture processes texturally and chemically at differing analytical scales, I constrain (1) the degree to which fractures and permeable gas reservoirs interact; (2) the timescales fractures outgas for; and (3) how heterogeneity is preserved as a function of depth in the conduit and eruption duration. Fractures exploit pre-existing permeable networks at depth, extracting volatiles on a cm-scale, but not enough magma is degassed to induce 210Pb-226Ra isotope disequilibria in tuffisite veins. The channelling of volatiles from reservoirs through fractures in the conduit plug results in metal exchange with transported shards and localised volatile exchange at fracture-host interfaces (constraining fracture lifespans). Outgassing at the near-surface, later in the eruption, occurs for longer resulting in a greater degree of metal and textural heterogeneity. In conclusion, I show fractures play a fundamental spatial and temporal role in degassing of silicic magmas and can directly impact the changing explosive-effusive behaviour observed during eruptions.

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