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Formation of obsidian by resorption of volatiles during slow cooling

Ed Llewellin1, Patrick Sullivan1, Fabian Wadsworth2, Jason Coumans1, Madeleine Humphreys1, Kate Dobson3, Tegan Havard4, Anja Allabar5, Marcus Nowak5, Richard Brooker6, James Gardner7, Thomas Connolly8, Halim Kusumaatmaja9, Ceri Allgood10

  • Affiliations: 1Durham University, UK; 2LMU, Germany; 3University of Strathclyde, UK; 4University of Liverpool, UK; 5University of Tuebingen, Germany; 6University of Bristol, UK; 7University of Texas at Austin, USA; 8Diamond Light Source, UK; 9University of Edinburgh, UK; 10Lancaster University, UK.

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 221

  • Programme No: 3.6.25

  • Theme 3 > Session 6

Abstract

The very glassy nature of obsidian has led to the idea that it is formed through rapid cooling. The near absence of vesicles, however, is problematic, as it requires complete degassing and outgassing of the parent melt. Here we show that dense obsidian can be produced by resorption of bubbles during slow cooling. We performed in-situ experiments in which melt with low H2O content was first heated (lowering solubility) then cooled (raising solubility) during 3D tomographic imaging. Data for the solubility-driven growth and resorption of bubbles were used to validate a numerical model of the process. We use the model to explore the conditions under which rhyolitic melt can resorb bubbles completely during cooling, to form dense obsidian. At atmospheric pressure, melt with low initial gas fraction φ≈0.05 (typical of lava formed by sintering) can form obsidian at relatively fast cooling ≈10-4 K/s, corresponding to cooling from eruption temperature to the glass transition temperature Tg in around a month. Under the same conditions, melt with higher initial gas fraction φ≈0.3 (typical of lava formed by outgassing of a permeable foam) can form obsidian at cooling rate ≈10-5 K/s, corresponding to cooling to Tg in around a year. Modelled resorption timescales are consistent with observations from natural obsidian lavas. Our results indicate that resorption is a viable mechanism for the removal of bubbles during obsidian formation and demonstrates that cooling must be slow enough to allow gas to resorb, challenging the popular idea that obsidian requires 'fast' cooling.