Graduate Student, School of Environmental Sciences
Thesis Title: Petrological insights into pre- and syn-eruptive degassing at Ambrym volcano, Vanuatu
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Jenni Barclay
Clive Oppenheimer |
About
I have recently completed my thesis (pending viva) on Ambrym volcano, Vanuatu. Ambrym is a basaltic island arc volcano identified in 2005 as a prodigious emitter of SO2, Cl and F. My research is an investigation of shallow magmatic conditions and degassing processes, via petrological study of recent eruptive products. In order to interpret volatile pathways through the system and consider how these might relate to gas emissions and variations in volcanic activity at the surface, I have focused primarily on basaltic scoria from a continuously active central vent.
Typical activity entails low-level degassing from quasi-permanent lava lakes, with strombolian activity of fluctuating intensity. This is punctuated on an approximately decadal timescale by moderate sub-plinian paroxysms, often accompanied by overflow of lava from the intracaldera cones and via flank fissures.
Petrography, major element modelling, mineral chemistry and dissolved volatile contents have been used to constrain degassing behaviour and the nature of the coexisting gas phase. The main focus of this study is basaltic scoria ejected from a central vent in 2005, following a period of heavy SO2 outgassing. Lavas from basaltic intracaldera and flank lava flows have been briefly examined, and a detailed study has been made of a trachyandesitic lava flow erupted in 1986, a geochemically unusual event compared to the majority of larger events in historical time. Examination of the trachyandesite has contributed insights into the occurrence of very occasional silicic products at this predominantly basaltic volcano.
Volatiles (COHS-Cl-F) were examined in melt inclusions, matrix glass, and basaltic experimental glasses (25-190 MPa, 1050-1200 *C, IHPV), in conjunction with detailed petrological study using mineral chemistry/petrography, major element modelling programs (MELTS, Petrolog 2.1) and mineral-melt equilibria for P-T-fO2 and tests of equilibrium (Putirka (2008), France et al. (2010), Roeder & Emslie (1970), etc.).
Experimental and natural samples have been characterised by optical microscopy, SEM, and XRF in the Geotechnical Microanalysis Lab at UEA; by EPMA (major elements, Cl, F, S) at Cambridge Dept of Earth Sciences; and by SIMS (CO2, H2O, Cl, F) at the University of Edinburgh.
