Fast ascent rate during the 2017–2018 Plinian eruption of Ambae (Aoba) volcano: a petrological investigation
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In September 2017, after more than a hundred years of quiescence, Ambae (Aoba), Vanuatu’s largest volcano, entered a new phase of eruptive activity, triggering the evacuation of the island’s 11,000 inhabitants resulting in the largest volcanic disaster in the country’s history. Three subsequent eruptive phases in November 2017, March 2018, and July 2018 expelled some of the largest tropospheric and stratospheric SO2 clouds observed in the last decade. Here, we investigate the mechanisms and dynamics of this eruption. We use major elements, trace elements, and volatiles in olivine and clinopyroxene hosted melt inclusions, embayments, crystals, and matrix glasses together with clinopyroxene geobarometry and olivine, plagioclase, and clinopyroxene geothermometry to reconstruct the physical and chemical evolution of the magma, as it ascends to the surface. Volatile elements in melt inclusions and geobarometry data suggest that the magma originated from depth of ~ 14 km before residing at shallow (~ 0.5 to 3 km) levels. Magma ascent to the surface was likely facilitated by shallow phreatic eruptions that opened a pathway for magma to ascend. Succeeding eruptive phases are characterised by increasingly primitive compositions with evidence of small amounts of mixing having taken place. Mg–Fe exchange diffusion modelling yields olivine residence times in the magma chamber ranging from a few days to a year prior to eruption. Diffusion modelling of volatiles along embayments (melt channels) from the first two phases of activity and microlite number density suggests rapid magma ascent in the range of 15–270 km/h, 4–75 m/s (decompression rates of 0.1 to ~ 2 MPa/s) corresponding to a short travel time between the top of the shallow reservoir and the surface of less than 2 min.
KeywordsVolatile Melt inclusion Magma ascent Basaltic eruption Geo-speedometer
This research was conducted following the 2017–2018 Ambae volcanic crisis. P.B. field assessment was funded by IRD. Y.M. acknowledges support from the CNRS (projet INSU-TelluS-SYSTER), National Geographic grant number CP-122R-17 (Trail by Fire II–Closing the Ring project) and from IRD. We thank Nordine Bouden and Etienne Deloule of CRPG (France) for their precious guidance during SIMS analysis. We thank Mhammed Benbakkar for ICP-AES analyses, Claire Fonquernie for help with sample preparation, and Jean-Marc Hénot for help with SEM and EBSD imaging. We thank Mike Jollands and anonymous referees for constructive and beneficial comments on the original manuscript.
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