Kinetics of mafic magma transfer and destabilization of the deep plumbing system in monogenetic volcanic provinces (Chaîne des Puys, France).

Terrestrial volcanism is the result of a series of magmatic processes governed by the plumbing system architecture and the timescales of melt migration. It has been proposed that large stratovolcanoes are fed by transcrustal magmatic systems with crystals and melts erupted from a range of reservoir depths. At the other end of the spectrum, mafic monogenetic volcanoes are generally thought to be fed directly from the mantle and thus may lack a plumbing system. The presence or absence of a range of magma reservoirs prior to eruption has implications for the timescales of unrest and magma transport to the surface. Here we address these issues by a petrological and mineralogical study of six scoria cones representing the mafic endmember of the Chaîne des Puys (Massif Central, France). This monogenetic intracontinental volcanic province dominates the Clermont-Ferrand agglomeration and its 400,000 inhabitants, with the first eruptions starting about 100 kyr ago and the last only 6.7 kyr ago. Although very active at certain times, the Chaîne des Puys has also been inactive on several occasions over periods of several thousand years.

We investigated the composition and textures of several hundred clinopyroxene and olivine phenocrysts and applied crystal system analyses to propose a range of magmatic environments and also timescales of transfer between them and eruption using diffusion chronometry methods. We found that all scoria cones were fed by two or three magmatic reservoirs and that crystals circulated between them prior to eruption on a range of timescales. We propose that magmas migrate repetitively from the deepest basaltic reservoir (R1; ~25 km depth), partially in the mantle, to slightly more evolved and shallower reservoirs (R2) where they interact with the host magma before eruption onset. Occasionally, magmas from R2 migrate and interact with trachy-basaltic R3 reservoirs (~18 km). The R1 reservoir was involved in both the oldest and most recent eruptions studied (~40 kyr difference), demonstrating its stability over time. The timescale of eruptions triggered by magma transfer from R1 to R2 ranges from about six months to a few years, while the timescale for magma transfer from R2 to R3 ranges from about one week to five months.