LARGE-SCALE OBSIDIAN EMPLACEMENT AT OBSIDIAN CLIFF, YELLOWSTONE (USA)

During its eruptive history, the large volcanic system at Yellowstone has experienced large outpouring of silicic material, both explosively and effusively. Rhyolitic magma at Yellowstone are generally regarded as very hot (850 - 1100°C) and dry in CO₂ and H₂O, but commonly believed to contain high concentrations of halogens (Branney et al. 2008). The key to this enigma may reside in the concentration of these exotic volatiles present in the magmas. Here, we provide a study of magmatic volatiles present in the rhyolitic lava flow of Obsidian Cliff and assess whether the volatiles influenced the eruptive behaviour.

Outcrops at Obsidian Cliff (Yellowstone) expose the interior of a thick rhyolitic lava flow erupted about 180,000 years ago. The rhyolitic lava flow has a thickness of 60m and extends for 6km down Obsidian Creek. Quenching of lava trapped parts of the volatiles species present in the magma, thus providing the opportunity to study the rheological effects of the species on lava flow dynamics. In detail, we investigate the volatile content, cooling rate and glass transition temperature of 15 samples across a 10m vertical section of the exposed lava flow interior.

Bulk rock analysis of the glass, using an electron microprobe, shows the Obsidian Cliff to be a slight peraluminous rhyolite with 78% SiO₂, with trivial variation in major elements. Differential Scanning Calorimetric (DSC) analysis and application of the GRD viscosity model (2008) based on the measured chemical composition, suggest a glass transition interval (Tg) ranging between 723°C and 756°C (at a heating rate of 10K/min, respectively). Tg measured from the DSC data range by 33 °C, but does not show systematic variations along the stratigraphy. The non-systematic variation in Tg of a glass with homogenous composition of major elements may be explained by the different volatiles in the melt phase. Directly Coupled Evolved Gas Analyzing System (DEGAS) data reveal that the obsidian contains H₂O, OH, CO₂, HF, F, SO₂ and Cl, which remain dissolved in the melt up to a temperature of ~1100°C, which is higher than magmatic temperatures inferred in the area. These findings highlight volatile species has an important contributor to local viscosity variation during emplacement of lava flows.