Le vendredi 16 janvier 2026, Melvyn BILLON présentera l'examen en vue de l’obtention du grade académique de Docteur en Sciences (Collège de doctorat en Géologie) sous la direction de Jacqueline VANDER AUWERA.
Cette épreuve consistera en la défense publique d’une dissertation intitulée :
« Crystallization kinetics of Plagioclase and application to timescale of crystal mush storage ».
Le Jury sera composé de :
M. F. HATERT (Président), Mmes et MM. B. CHARLIER (Secrétaire), F. FAURE (Université de Lorraine), M. HOLNESS (Cambridge University), O. NAMUR (KULeuven), J. TROCH (RWT Aachen), J. VANDER AUWERA (Promotrice).
Over recent decades, the view of magmatic reservoirs has shifted from large, liquid-rich chambers to complex networks of crystal-rich mushes. This change has increased interest in quantifying magmatic timescales, essential for understanding volcanic processes. Among available methods, textural methods, such as Crystal Size Distributions (CSD) method, combined with experimentally determined growth rates, provide a valuable tool, especially when datable minerals are absent or textures are altered. This thesis evaluates the potential of CSD-based textural approaches by combining 80 controlled crystallization experiments on anhydrous basaltic andesite, with the study of natural samples from three Chilean volcanoes (Osorno, Calbuco and Villarrica).
Experimental results show that nucleation and growth rates peak near the liquidus and decrease during cooling, while crystal textures evolve from euhedral to skeletal and dendritic with increasing undercooling. Thermal pre-treatment strongly affects crystallization, inhibiting nucleation and promoting dendritic growth of large crystals (growth rate evolving from 10-9 to 10-6 cm.s-1).
These growth rates values were then applied to natural CSDs, yielding crystallization times ranging from days to about three years, reflecting different stages of magma evolution: long residence for macrocrysts, intermediate storage for micro-phenocrysts, and final ascent for microlites. These timescales, shorter than those inferred from diffusion or U-series, highlight that each chronometer captures distinct temporal windows and confirm the value of CSD as a complementary tool for understanding magma storage and dynamics in volcanic arcs.
La thèse est intitulée : « Hydrogeomorphological approaches to understanding flood hazards and flood-risk management in northwest Rwanda ».
La thèse est intitulée : « Inferring mechanisms of community assembly from phylogenetic diversity ».
La thèse est intitulée : « Biodiversity under rapid environmental change: trophic interactions and communities’ responses in austral benthic ecosystems ».
