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Magma transfer and degassing budget: Application to the 2009–2010 eruptive crisis of Mt Garet (Vanuatu arc)

Métrich N., A. Bertagnini, E. Garaebiti, S. Vergniolle, P. Bani, A. Beaumais, D.R. Neuville.
Journal of Volcanology and Geothermal Research, doi:10.1016/j.jvolgeores.2015.06.003, In Press, Corrected Proof.


Mt Garet, on Gaua Island, is one of the active volcanoes of the Vanuatu arc. We report here a new dataset on lapilli and lava erupted during Mt Garet unrest in 2009–2010 and on products of the older activity of Gaua composite volcano. The present-day magma of Mt Garet is a trachy-andesite (52 wt.% SiO2) with relatively high Rb/Th (14.6) and Ba/La (41) ratios compared to the Gaua pre- and syn-caldera series, but typical of the central part of Vanuatu arc. Its mineral assemblage is mainly composed of plagioclase (An86–56) and clinopyroxene (Fs5–16) which display significant chemical variations, patchy zones, surface dissolution, and oscillatory zoning that imply episodes of high undercooling and growth rates. The paragenesis is complemented by Fe–Ti oxides and scarce olivine (Fo72–73). The melt inclusions are ubiquitous and their compositions cover a chemical spectrum from basalt to trachy-andesite. Volatile-rich basaltic inclusions (H2O: 2.7 wt.%, S: 0.15 wt.%, and Cl: 0.22 wt.%) are preserved in Mg-rich clinopyroxene whereas the majority of the melt inclusions is volatile poorer with, ≤ 1.0 wt.% of H2O, ≤ 0.05 wt.% of S, and 0.25–0.27 wt.% of Cl. At 1100 °C the measured viscosity of anhydrous magma of Mt Garet is 103.5 Pa s. Adding 0.8 to 2.5 wt.% of H2O decreases the melt viscosity by 0.5 to two orders of magnitude. Combining data on bulk rocks, minerals, and their melt inclusions together with the very first published gas fluxes acquired during the same period of activity, we propose that the high sulfur outgassing in 2009–2010 was produced by the degassing of a basaltic magma batch (~ 0.027 km3) emplaced in a shallow reservoir. This scenario would require temperature and H2O-loss driven resorption/crystallization, magma mixing, and exsolution of an early gas phase rich in H2O, and S.
We suggest here the 2009–2010 activity to be sustained by the existence of thermal convection driven at the bottom of the magma reservoir by cooling, and in which the bubbles are small enough to be stagnant. The most energetic phases are better explained by an additional gas volume, associated to the crystallization of titanomagnetite microcrysts which significantly enhance bubble nucleation. The ultimate step of crystal growth prior to eruption suggests magma ascent within few hours.