IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A2_2G-O8

Room B2Date/Time: July 23 11:00-11:15

The 2011-2012 submarine eruption of volatile-rich magma at El Hierro, Canary Islands
Marc-Antoine Longpré1 , John Stix1 , Nobumichi Shimizu2 , Andreas Klügel3
1

McGill University, Canada, 2 Woods Hole Oceanographic Institution, USA, 3 University of Bremen,
Germany
E-mail: m-a.longpre@mcgill.ca

From October 2011 to March 2012 a submarine eruption took place 2 km south of El Hierro, the youngest andwesternmost of the Canary Islands. The eruption produced buoyant, gas-filled “lava balloons” that sporadically rosefrom 100-300 m depth below sea level to the ocean surface and were sampled. Here we report ion microprobe
measurements of the volatile element concentrations (CO2, H2O, S, F, Cl) of basanite matrix glass and basanite
to phonotephrite melt inclusions trapped in olivine and Fe-Ti oxide phenocrysts collected from the glassy lava
samples. The results reveal the volatile-rich nature and degassing history of the 2011-2012 magma. Whereas
matrix glasses contain 70-710 ppm CO2, 0.4-0.9 wt.% H2O, 410-1030 ppm S, 1540-1790 ppm F and 850-1010
ppm Cl, melt inclusions show much greater ranges extending to high concentrations. Carbon dioxide concentrations
in melt inclusions exhibit extreme variations from <50 to 13700 ppm, with 50% of values above 2000 ppm CO2.
Most melt inclusions have dissolved water contents in the range 0.5-1.5 wt.%, but values of up to 3 wt.% areobserved. Sulphur ranges from 320 to 5580 ppm and is positively correlated with H2O. Fluorine and chlorine varyfrom 860 to 2110 ppm and 610 to 1370 ppm, respectively. These volatile concentrations are among the highest
ever measured for ocean island magmas. The high CO2 and H2O contents require high entrapment pressures(>200 MPa) for most melt inclusions, in agreement with fluid inclusion and clinopyroxene-melt barometry as well aswith syn-eruptive seismicity. This suggests that the volatile-rich nature of the El Hierro magma was inherited fromthe mantle source. Intense shallow level degassing probably led to powerful submarine Strombolian explosions atEl Hierro, as indicated by strong bubbling at the ocean surface. First-order calculations, using an eruptive volume of329 million m3 (converted to 160-260 million m3 DRE assuming 20-50% porosity) determined from high-resolution
bathymetric surveys (Rivera et al., 2013) and assuming 1 wt.% CO2 and 5000 ppm S total degassing, indicate that
the El Hierro eruption could have released roughly 4-6 Tg CO2 and 2-3 Tg S to the hydrosphere and atmosphere.
However, the formation of abundant sulphide globules associated with magnetite and clinopyroxene phenocrysts
may have provided a sink for much of the sulphur. A positive correlation between S6+ /ΣS, as measured by the peakshift method, and S concentration in melt inclusions suggests that a reduction in the oxidation state of the magma
during sulphur degassing, possibly aided by extensive magnetite crystallisation, could have caused saturation witha sulphide liquid. Our findings thus have far-reaching implications for the dynamics of the 2011-2012 El Hierro
eruption, the volatile-rich nature and degassing of Canarian and other mafic alkaline magmas, and the nature ofthe mantle source beneath the Canary Islands.

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