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Experimental textures of mingling and mixing between two chemically contrasted magmas

Abstract : The replenishment by a hot, basic magma is supposed to upset a differentiated magma chamber by magma mixing. The changes in temperature, convection patterns, extent of crystallization, or the vesiculation of the re-heated felsic magma may be all factors that ultimately trigger eruption. However, only little experimental work has been performed so far on mixing/mingling processes, and the existing data base concerns essentially atmospheric pressure. In order to characterize magma mixing, hybridization and related events, we performed experiments juxtaposing two contrasted magma compositions. We have chosen to use a synthetic haplotonalitic composition (SiO2: 68.9%, Al2O3: 19.6%, Na2O: 8.0%, CaO: 3.5%) as the felsic end-member, and natural basalt from Santorini Volcano (Greece). The geometries adopted were (1) a mafic core plug inserted into an outer hollow felsic cylinder and (2) two basaltic disks separated by a felsic one. Then, we simulated a simple shear deformation using a Paterson HP-HT apparatus, at 300 MPa confining pressure, and temperatures from 900 to 1200°C. The temperature range allowed us to get the lowest viscosity contrast at which magma mixing is enhanced. Under these conditions calculated relative viscosities of both felsic and mafic end-members range from Log η = 8.2 to Log η = 1.3 Pa.s, depending on temperature. However, crystal content changes considerably the apparent viscosity. Applied shear strain rates range from γr = 7x10-5 s-1 to γr = 1x10-3 s-1. Finite shear strains (γ) range between 0 (static experiments) and 5.4. SEM images on run products show textures and particular fabrics such as enclave formation, mafic material dislocation, whereas microprobe analyses reveal basalt melting and mixing with the felsic end-member producing andesitic melts. In addition, close to the contact between the two materials, basaltic phenocrysts are replaced by a new generation of crystals, different from plagioclases growing from the haplotonalite only. The comparison with natural textures from e.g. Santorini volcano will constrain the conditions required for magma mixing at this volcano. Experiments under hydrous conditions relevant to the Santorini context are in progress. These experiments will help us to find out the consequences of such a common process partly responsible of explosive eruptions in volcanic arcs.
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Mickaël Laumonier, Laurent Arbaret, Bruno Scaillet, Rémi Champallier, Michel Pichavant, et al.. Experimental textures of mingling and mixing between two chemically contrasted magmas. American Geophysical Union, 2010, United States. ⟨insu-00838449⟩



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