Microlitic crystal-venting episodes at two trachytic lava domes of the Chaîne des Puys (France): Description, experiments and modelling.

Abstract

We document and propose plausible explanations of microlite-rich pyroclastic density currents (PDCs) that ended the main effusive phases of two highly porous trachytic domes: the Puy de Dôme and Grand Sarcoui (Quaternary volcanic field of the Chaîne des Puys, France). At both volcanoes, these PDCs are evidenced by thick, unique pyroclastic deposits that originated from a late summit, open-vent eruption. These deposits are mainly composed of microlitic feldspar laths that are either free or weakly cohesive with some traces of glass. We present HP-HT crystallization experiments to constrain the pre-eruptive conditions. In these experiments, the initial water content of the material is a crucial parameter since the trachytic domes of the Chaîne des Puys exhibit the highest pre-eruptive H2O contents ever reported for alkaline liquids in an intraplate continental context. Our observations and textural and experimental results lead us to propose three models able to explain the occurrence of a microlite suspension at depth after the initial construction of a dome. Model 1 and Model 2 explore the scenario of gas accumulation within the conduit, or at the upper part of the reservoir; Model 3 explores the scenario of gas accumulation below a shallow impermeable plug. Whereas the highly porous nature of the trachyte must have favored gas escape preventing any explosivity of the dome itself during its early growth, the late explosive phase that generated the crystal-rich PDC points to an excess pressure due to drastic porosity reduction of the trachyte above the accumulated mush. We expect that it is the result of the combination of void collapse caused by depressurization and nearly complete outgassing, and pervasive silicification leading to an impermeable carapace. The results of both Models 1 and 2 call for a sector collapse that causes the final pressure drop and conduit evacuation. Since sector collapses at domes are common, the rarity of such microlitic ash deposits could partly be explained by the low viscosity that characterizes these uncommon trachytic melts. Model 3 results suggest that a specific thickness of sealed magma could be the feeding conduit, or alternatively a lateral shallow intrusion of magma of specific dimensions that accumulates gas below an impermeable cap, which can be either roof rocks or cristobalite precipitation within the upper part of the sill. The available data do not allow us to favor any of the models — or a combination thereof if we do not suppose a unique mechanism operating at both volcanoes.