The Serreta submarine fissure eruption (December 1998 – April 2001) occurred 10 km west of Terceira Island, Azores, Portugal, in water depths of 300 to 1000 m. Together with dispersed ash, evidence for the eruption was provided by the periodic appearance of “lava balloons” at the sea surface. They remained floating for few minutes and were up to 3 m long. The few successfully collected balloons consist of a large cavity surrounded by a few cm thick crust, within which several different layers have been identified. The original surface of the balloons is golden-coloured, highly porous, glassy, and hypocrystalline. It shows signs of large-scale fluidal deformation, small-scale brittle cracking, and elongated bubbles. In order to better constrain the genesis of these balloons and test models presented by Gaspar et al. (2003), we quantified volatile contents and cooling rates. The concentration of volatiles in the groundmass glass across the crust and in the ash does not vary significantly, with a mean of 0.32 wt.% H2O (sd 0.02) and CO2 below detection. However, melt inclusions, hosted in olivine, pyroxene and feldspar phenocrysts within the crust have very different volatile contents, containing up to 1 wt.% H2O and 1780 ppm CO2.
Cooling rates of glass from the different layers in the crust and the ash were determined using their cptemperature paths, measured by differential scanning calorimetry. During initial heating all samples exhibit a clear trough in cp prior to the glass transition peak, indicating high cooling rates. We discriminated values in the order of 103 K/s for the outermost layer and the ash. The high initial volatile content of the magma, suggested by the inclusions, provides the source of the gases to generate the balloons. The found volatile concentrations will cause gas exsolution and rise long before the magma reaches the eruptive conditions. The cooling rates decrease notably towards the interior of the balloon and are highest for the ash. These results support the proposed model where gas accumulated under a melt skin creates a growing bulge that may finally detached to form the balloons that rose buoyantly through the seawater to the surface.