Combined analysis of melt and fluid inclusions in recent Cézallier volcanoes, French Massif Central: from mantle melting to magma storage and ascent

There is increasing evidence that the primary magmas at the origin of low-silica alkaline volcanism, such as basanites, are very rich in CO₂ and that they can rise rapidly, directly from the mantle to the Earth’s surface. Such volcanic systems are numerous in intraplate oceanic and continental settings, including the French Massif Central, and some are remarkable for the abundance of large mantle-derived xenoliths. Although they usually represent relatively modest volumes of magma, their eruptions constitute a real and specific volcanic threat because of (1) their high ascent rate, with magmas capable of rising from mantle depths to the surface in less than a day to a few days, (2) the large volumes of CO₂ emitted into the atmosphere at the time of eruption, and (3) the effusion of very fluid lava flows. The recent part of the Cézallier volcanic province, French Massif Central, offers nice examples of such low-silica alkaline volcanoes that erupted less than 200 ka ago.

A study of fluid and melt inclusions has been carried out on three volcanoes from the recent part of the Cézallier volcanic province (Sarran, Mazoires, La Godivelle) in order to characterize the composition of primary magmas and to provide constraints on magma storage and ascent. Mg-rich olivine crystals (forsterite contents in the range of 83-89) were selected for the study of melt inclusions, while CO₂-rich fluid inclusions were analyzed in olivine, pyroxene and amphibole crystals. After in-depth petrographic characterization, the melt inclusions were characterized using a series of analytical techniques, including: X-ray tomography (to characterize the shape and volume of melt inclusions and shrinkage bubbles); electron probe microanalysis (for major elements, Cl, F, S in glasses); Raman spectroscopy (to measure H₂O and CO₂ in glasses and to characterize the CO₂-bearing phases in the shrinkage bubbles of the melt inclusions); and LA-ICP-MS (for trace elements). Microthermometry was used to measure CO₂ densities in fluid inclusions, which were thereafter converted into pressures and into depths.

The glass compositions of the melt inclusions plot into the fields of basanites, basalts and trachy-basalts. The glasses have particularly high CO₂ contents: up to 1.8 wt% dissolved CO₂. These values are minimum values, as CO₂ is also present in the shrinkage bubbles as a fluid phase and as microcrystals of carbonates (Mg-calcite, nahcolite, ferromagnesite) covering the bubble walls. These high CO₂ contents imply that the mantle sources at the origin of these magmas were enriched in carbon. CO₂-rich fluid inclusions in olivine, pyroxene and amphibole crystals are all re-equilibrated and have thus lost their primary densities. At all three volcanoes, the CO₂ density histograms show a major peak at 900 to 1090 kg/m³ (» 750 to 900 MPa), indicating a stage of magma storage at Moho level followed by rapid ascent to the surface. Work is in progress to reconcile the observation of large peridotite xenoliths (at Mazoires) with magma storage at Moho level.