Spatio-temporal dynamics of the magmatic plumbing systems, towards an « early-warning clock ». Application to Dominica island (Lesser Antilles) and Kamchatka (Russia)

Volcanic eruptions are among the most devastating events on Earth. In a subduction context, eruptions generally involve differentiated magmas: after long repose times in magma reservoirs, basic magmas resulting from the partial melting of the mantle evolve towards differentiated compositions, thus richer in silica, more viscous and richer in volatile elements. These magmas can give various eruptive styles more or less explosive depending on different parameters such as the composition of the magma, the volume of magma involved, and the behavior of volatile elements during the ascent in the volcanic conduits. 

Estimating the time between changes in magma storage conditions within a magma reservoir and the onset of an eruption is one of the major challenges in volcanology. A direct observation of the magma storage system under the volcanoes is impossible but this timescale can be accessible thanks to the study of crystals brought to the surface by a past volcanic eruption.

Indeed, like tree rings, which can provide information on past events, crystals growing within magma reservoirs feeding volcanoes record changes in magma storage conditions before an eruption. In many large-scale eruptions, it has been shown that magma injections or decompression events can occur in the reservoir and be recorded by the crystals as zonations (i.e. rims of different compositions than the crystal cores). With time, a homogenization of chemical compositions between zonations by diffusion of elements occurs (for example, the diffusion of iron and magnesium for orthopyroxenes or diffusion of titanium in magnetites). During an eruption, the diffusion is frozen by the rapid cooling of the emitted magmas and it is then possible to determine the time between the readjustments in the reservoir and the eruption.

Magmatic reactivation timescales have been studied during this thesis for subduction zone volcanoes generating particularly dangerous arc volcanism, focusing on explosive eruptions from a volcano on the island of Dominica as well as well-monitored recent eruptions from the Kamchatka subduction zone (Russia). For one of the latter examples, the timescales were correlated with the first precursory signals of the eruption (geophysical and geochemical) and will allow to better manage future volcanic crises.

Spatio-temporal models of the dynamics of the magmatic feeding zones at the origin of seven eruptions are thus proposed. This new set of diffusion timescales data on subduction zone volcanoes allows a better understanding of these systems, knowing that diffusion timescales studies on volcanoes of the Lesser Antilles and Kamchatka are few, despite the important volcanic risks for the case of the Lesser Antilles. 

These results on past eruptions of Dominica and Kamchatka with the understanding of reservoir dynamics and correlations between petrological timescales and seismic signals for the 2010 Kizimen eruption are important advances that can be applied for the management of future volcanic crises at subduction zones. If the magma storage area for a given volcano is stable in time, and if the correlation between reservoir readjustments and precursor signals is shown for past eruptions, these timescales will allow, during a reactivation of these volcanoes, to have a useful pre-eruptive warning clock for the protection of the populations.