Petrological and geochemical tools for unravelling the architecture and dynamic of a magma plumbing system

Abstract

Deciphering the architecture of the plumbing system beneath active volcanoes and the pre-eruptive magma dynamic is of key importance to discuss about the eruptive style and petrological warning signal. Here Dominica in the Lesser Antilles arc is chosen as a key case study to illustrate the scientific approach. The first step is a field study in order to well identify the characteristics of the deposits to well constrain in particular the style, number and age of eruptions, and to well sample them for petrological studies. On Dominica, we proposed a new chronostratigraphy of the explosive eruptions along the island in the last 50 kyrs, based on stratigraphic correlations, lithology, 14C dating and glass chemistry. To reconstruct the magma plumbing architecture, melt inclusions composition entrapped in minerals from the key eruptions were investigated. Their volatile content highlights that two magma ponding zones may be identified: a deepest one located at ~ 4 kbars, giving birth to the large pumiceous eruptions, formely recognized as the ignimbritic eruptions, and a shallowest, at 1.5-2 kbars, one leading to Plinian-type eruptions. These results are similar to those obtained by experimental petrology. Such data allows us to conclude that the magma plumbing system is organized as a transcrustal magma system, with the magma ponding zones linked to the structure of the crust (lower and middle crust), as derived from geophysical studies. Melt inclusions also help us to constrain magma source at depth, and the influence of slab-derived fluid respect to sediment melting in a subduction zone. To go further, the petrological processes occuring within the magma reservoirs may be investigated. Tracking the pre-eruptive history of magma storage and ascent is a key challenge of modern volcanology, in particular to gain insight into the timescale of pre-eruptive processes at active volcanoes. By focusing now on crystal composition, the pre-eruptive magma dynamics may be identified adopting the « Crystal System Analysis » approach, and their timescales estimated using the diffusion chronometry (Fe-Mg interdiffusion in orthopyroxens). On active and monitored volcanoes, the correlation between the petrological clock and the pre-eruptive warnings given by the monitoring network (seismic but not only) is now established on different volcanoes, using various petrological clock. In Dominica, we showed that a mixing processes between different magma batches systematically occures a decade prior the eruption within the deepest reservoir giving birth to the oldest ignimbritic eruptions. For the recent Plinian eruptions, similar mixing process begins ~ 10–30 years prior eruption, with more sustained mixing in the last decade, accelerated in the last 2 years. On active and monitored volcanoes, the correlation between the petrological clock and the pre-eruptive warnings given by the monitoring network (seismic but not only) is now established on different volcanoes, using various petrological clock. Thus, on Dominica, that has no monitored eruption, time constrain may help authorities in volcanic risk mitigation in case of volcanic reactivation. Such integrated study may be conducted on any volcanic target and lead to a more comprehensive understanding on the behaviour of the magma plumbing system, with strong implications in volcanic risks.