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Submitted by Patrick Allard, November 2015
Improved quantification of the global emissions of deep carbon through volcanism is one key objective of the DCO’s DECADE (Deep Earth Degassing) initiative supported by the Reservoirs & Fluxes Directorate. One approach is by measuring deep carbon degassing in very active but yet undocumented remote volcanic regions. The Vanuatu island arc, in southwest Pacific, is one of these. In a just issued JVGR publication*, an international team led by DECADE bod members Patrick Allard (IPGP, France), Alessandro Aiuppa (Palermo University, Italy) and Hiroshi Shinohara (JSG, Japan) demonstrates that Ambrym basaltic volcano, in central Vanuatu arc, ranks among the top-three persistent emitters of volcanic gas yet identified at the global scale.
Ambrym volcano consists of a huge (500 km3) submarine shield-pile rising 1,800 m above the ocean floor, that is topped by a 12-km wide and young (2 ka) Hawaiian-type caldera. Two major cones in this caldera, Benbow and Marum, host recurrent basaltic lava lakes whose degassing sustains voluminous gas plumes (Figure 1). A very high SO2 emission rate from Ambrym was detected recently (Bani et al., 2009, 2012). Despite challenging accessibility (Figure 2), the international team could determine for the first time the overall degassing budget of major gases, trace elements and short-lived radioactive daughters of 222radon from Ambrym, combining both ground-based and airborne field measurements and lab analyses with various techniques. Results reveal prodigious emission rates of magma-derived volatiles from this basaltic arc volcano, ranging from 5 to 10% of current global volcanic flux estimates of H2O, CO2, HCl, Cu, Cd, Au, Cs, Tl, 210Po and 210Pb, between 10% and 17% for SO2, HF, HBr, Hg, and over 30% for Ag, Se and Sn (Figure 3). The CO2 flux, in particular, averages ≈10 Gg per day. Such emission rates characterize Ambrym as one of the three most powerful gas emitters amongst persistently degassing subaerial volcanoes on Earth, together with Nyiragongo (East-African rift) and Etna (Sicily). Moreover, simultaneous melt inclusion study shows that Ambrym basalt is not anomalously enriched in volatiles. Instead, voluminous gas release from the volcano is shown to result from a very high basalt supply/degassing rate (25 m3 s-1), from a 0.5 km3 large and ∼4-km deep magma reservoir that may be entirely renewed in about 240 days. Hence, current global estimates of volcanic carbon and other volatile emissions will have to take account of the previously unknown volatile contribution of Ambrym and be revised accordingly. New investigations of anologous volcanoes in other poorly documented regions of the southwest Pacific but also of Indonesia might reveal larger volcanic carbon emission rates than currently expected.
This study was supported by the French National Agency for Research (ANR contracts 06-CATT-002, ‘Arc Vanuatu’, and 06-CATT-012-01, ‘VOLGASPEC’), Palermo University (European Research Council, GA305377, Pi: A.A.), INGV-Pisa and the Geological Survey of Japan. This work was realized in straight collaboration with our Vanuatu scientific partner GEOHAZARD (Port Vila).
* Allard P., Aiuppa A., Bani P., Métrich N., Bertagnini A., Gauthier P-J., Shinohara H., Sawyer G., Parello F., Bagnato E., Pelletier B., Garaebiti E. (2015) Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc. J. Volcanol. Geotherm. Res. 304, 378–402, http://dx.doi.org/10.1016/j.jvolgeores.2015.08.022.
Bani, P., Oppenheimer, C., Tsanev, V.I., Carn, S.A., Cronin, S.J., Crimp, R., Charley, D., Lardy, M., Robert, T.R., 2009b. Surge in sulphur and halogen degassing from Ambrym volcano, Vanuatu. Bull. Volcanol. 71, 10, 1159-1168, doi:10.1007/s00445-009-0293-7.
Bani, P., Oppenheimer, C., Allard, P., Shinohara, H., Lardy, M., Garaebiti, E., 2012. First estimate of volcanic SO2 budget for Vanuatu island arc. J. Volcanol. Geotherm. Res. 211-212, 36-46, doi: 10.1016/j.jvolgeores.2011.10.005.
Figure 1 – Two volcanoes in one: volcanic plumes from Benbow (right) and Marum (left) active cones of Ambrym (source: NASA, 2013/08/09).
Figure 2 – Eastern inner slopes of Ambrym’s Benbow cone (1160 m asl, ~300 m deep) and location of its active lava lake.
Figure 3 – Ambrym volatile fluxes as percentages of global fluxes from subaerial volcanism (GVF).