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Pre-eruptive volatile conditions as captured by the apatite record in the crystal mush system of Las Cañadas Caldera, Tenerife

Gemma Brown1, Madeleine C.S. Humphreys1, David J. Colby1, Emma L. Horn2, Victoria C. Smith2, Cees-Jan De Hoog3

  • Affiliations:  1Department of Earth Sciences, Durham University, Durham, UK; 2School of Archaeology, University of Oxford, UK; 3Grant Institute, School of GeoSciences, University of Edinburgh, Edinburgh, UK 

  • Presentation type: Talk

  • Presentation time: Monday 15:00 - 15:15, Room R280

  • Programme No: 1.7.8

  • Theme 1 > Session 7

Abstract

Magma is believed to be stored in the crust as crystal mushes. However, crystal-rich mushes will not be readily mobile and the ways that they are mobilised and erupted is not fully understood. This study assesses whether magmatic volatiles contribute to crystal mush mobilisation, through use of the apatite volatile record. Apatite records changing melt volatile contents by incorporating volatiles (H2O, F, Cl) into its crystal structure, capturing the timing of volatile exsolution in a magma. Using apatite inclusions found in pumice-derived phenocrysts and primocrysts from cumulate nodules interpreted as in-situ mush, we interpret the changing volatile state of the crustal mush column prior to and at the onset of eruption. Pumices and nodules represent a mush ranging in magma composition from basanite to phonolite, with different magma compositions having distinct apatite volatile trends. We use a forward model to fit the compositional range of each apatite population. Our results show that, within a compositionally variable crystal mush, a shallow level, volatile-saturated phonolite reservoir existed. This reservoir was disturbed by an ascending phonotephritic magma initially stored at lower crustal depths in volatile-undersaturated conditions. Volatile saturation occurred in this phonotephrite due to primary boiling and supplied gas to the upper phonolite reservoir. This may have sufficiently destabilised the system to cause eruption. Our work shows that volatile-saturated and undersaturated magmas can exist within one system, and that late-stage volatile saturation of undersaturated parts of the system may be key to mobilisation and eruption of crystal mushes.