Cinétiques syn-éruptives de cristallisation des plagioclases dans les magmas différenciés

Abstract : Magma ascension rate in volcanic conduit plays an important role in determining eruption dynamics of arc volcanoes (effusive or explosive). Investigating the kinetics of processes such as decompression induced crystallization may provide crucial information on the modality of magma ascent. We performed decompressioninduced isobaric crystallization-timed experiments starting with Mt Pelée rhyolitic glass composition and its simplified analogue (haplotonalite) to study plagioclase crystallization (nucleation and growth) kinetics. Our results reveal that a melt depressurized from 200 MPa to a given final pressure from 160 to 25 MPa according to different decompression rates (ΔP/Δt = 1200, 150 and 30 MPa/hr), shows various modalities of crystallization of plagioclases. In particular, the slower the decompression, the higher the numerical density. The crystallization modeling i) provides new constraints on the plagioclase/haplotonalite interfacial energy that controls the nucleation kinetics, by determining shape and roughness parameters ii) reveal that plagioclase crystallization is either controlled by diffusion or crystal/melt interfacial mechanisms, iii) demonstrates that plagioclase diffusion-controlled growth rate is not systematically controlled by the slowest diffusion element, and it closely depends on nucleation. Also, we revealed the importance of melt composition on crystallization kinetics, in particular the inhibitive effect on nucleation of trace arsenic addition in the melt.
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Edith Mollard. Cinétiques syn-éruptives de cristallisation des plagioclases dans les magmas différenciés. Sciences de la Terre. Université d'Orléans, 2011. Français. ⟨NNT : 2011ORLE2066⟩. ⟨tel-00689733v2⟩

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