Research Topics:

Thesis Subject: Parent magmas of recent volcanism in the Bas-Vivarais (Ardèche, France): Composition, pre-eruptive conditions and ascent rate

Summary: The volcanoes of the Bas-Vivarais (Ardèche) belong to one of the most recent volcanic provinces of the Massif Central. They are characterized by phreatomagmatic eruptions preceded or followed by strombolian activity associated with lava flows. The volcanic products of this province are basanites and alkaline basalts containing numerous crustal and mantle enclaves. Currently, only a few studies have been devoted to these volcanic products and none have focused on their petrology. Thus, the objectives of this thesis are:                                                          1) To characterize the composition, origin and pre-eruptive conditions of the parent magmas of the Bas-Vivarais volcanism. To do this, melt inclusions trapped within olivine phenocrysts of Bas-Vivarais basanites are studied (major, volatile and trace elements). Their study allows to approach the composition of the parental liquid of these lavas while freeing from the crustal and mantle contamination.                                                                                                                                                                                                                                                                              2) To understand the origin of the high CO2 contents of melt inclusions. Indeed, the melt inclusions of the Bas-Vivarais are particularly rich in CO2 (up to 4 wt%). These contents have been quantified using high pressure piston-cylinder homogenization experiments and estimated using 3D tomographies of the inclusion hosted bubble. They are among the highest CO2 contents reported on Earth.                                                                                                                                                                                                                                                                                            3) Characterize the bubbles of melt inclusions and provide constraints on their formation. The bubbles of melt inclusions are particularly rich in microcrystals. They have been characterized by Raman spectrometer and Scanning Optical Microscope coupled to a focused ion beam (SEM-FIB).                                                                                                                                                                                                                                                                                                                                                                                                                                                                4) Estimate the ascent rates of the parent magmas at the origin of the Bas-Vivarais volcanism. It is thought that these magmas, once formed at depth, rise directly to the surface without undergoing differentiation in an intermediate reservoir by having high ascent rates.

Methods:

• Study of melt inclusions trapped in olivines
• Microprobe – SEM – SEM-FIB – FTIR – Raman spectrometer
• Water and CO2 analysis by Raman spectrometry
• 3D tomography of melt inclusion bubbles by Raman spectrometry
• High pressure homogenization experiments with piston-cylinder
• Synthesis of basanitic and CO2 rich reference glasses

Supervisors: Didier Laporte – Federica Schiavi – Nicolas Cluzel

Parrain: Olivier Merle

Financing: Bourse régionale VolcAURA