Journal of Conference Abstracts

Nature and Extent of Hydrothermal System-Magma Interactions During the 1995-96 Eruption of Soufriere Hills Volcano, Montserrat: Implications for Eruptive Style?

Benoit Villemant¹ (villema@ccr.jussieu.fr), Jean-Christophe Komorowski¹ (komorow@ipgp.jussieu.fr), Georges Boudon¹ (boudon@ipgp.jussieu.fr), Philippe Ildefonse¹ (ildefons@lmcp.jussieu.fr) & Gilbert Hammouya² (obsgua@outremer.com)

¹ Institut de Physique du Globe de Paris, 4, Place Jussieu, 75252 Paris Cedex 05, France.

² Observatoire Volcanologique de la Soufriere de Guadeloupe, Le Houelmont, 97113 Gourbeyre, Guadeloupe, FWI.

Interactions at the hydrothermal system-magma interface can modify the physico-chemical characteristics of the magma and the host-rock influencing eruptive style (Mt. Pelee, Villemant et al., 1996).

Castle Peak dome, 1995-96 domes, and 1995 phreatic tephra are chemically similar except for highly mobile elements (Au, As, Br, Mo, W, Ag). Tephra show systematic As, Br, Sb, Mo, W enrichment. Castle Peak dome and tephra have extreme Au contents. Thus tephra represent mixtures of dome magma and material strongly enriched in mobile and volatile elements. Hydrothermal material from Tar River Soufriere show a quasi-systematic strong depletion in all measured elements except highly incompatible Ta, Hf, to a lesser degree Th and U, which are little or unmodified. The enriched component of tephra is clearly distinct from the signature of hydrothermalized material and was likely inherited from deep hydrothermal fluids.

Castle Peak phreatic tephra is strongly hydrothermally altered (vein-filling pyrite, cristobalite, vuggy silica, sulfates). Porosity of such initially dense material is almost nil. Explosions clearly involved the permanent acid-sulfate hydrothermal system. Leachate, XRD, binocular/SEM analysis of distal tephra (Guadeloupe) from the September 17 1996 dome explosion reveal it's origin from the gas-rich interior and areas affected by acid-sulfate fluids at the Castle Peak dome/active conduit interface.

Silicification has drastically reduced the porosity of Castle Peak dome through which magma has extruded. How and to which extent can ascending viscous magma degass to form thick-carapaced domes? A locally sealed environment (host-rock/conduit interface, dense carapace) could maintain overpressures sufficient for rapid vesiculation of the dome's interior following major collapse or injection of gas-rich magma (Sept. 17 vesicular/reticulitic tephra).

Correlation of ongoing U-Th disequilibrium studies on dome rocks, inclusions and hydrothermalized rocks with microtextural data might reveal the extent and nature of interactions with the hydrothermal system providing constraints on their influence on eruptive style.