Experimental mixing of hydrous magmas

Abstract : Deformation experiments involving hydrous magmas of different compositions (basalt andhaplotonalite) have been performed in a Paterson press at 300 MPa, in the temperature range600°C-1020°C, with water-saturated melts, during 2-4 hours. Prior to deformation the twoend-member magmas were annealed at either 950°C or 1000°C, yielding magmas with crystalcontents in the range 31-53 wt% and 2 sets of viscosity contrasts. Under the experimentalconditions investigated (i.e. moderate shear rates <10-3 s-1), mixing/mingling textures appearat temperatures > 950°C. In the temperature range 950-985°C a few mixing and minglingtextures occur, though both end-members essentially retain their physical integrity. It is onlyat, or above, 1000°C that a dramatic jump in mingling efficiency happens, corresponding to acrystal fraction of 45 vol%. Textures include entrainment of mafic crystals into the felsicmagma, mafic-felsic banding, enclave formation, diffusion-induced interface, the latter only over limited distances (< 300 microns) due to the short run durations. In the most strainedparcels of interacting magmas, complex mixing/mingling textures were produced, similar tothose observed in volcanic and plutonic rocks in arc settings. The experiments show thatmixing between hydrous felsic and mafic magmas takes place at around 1000°C, atemperature which is almost 200°C lower than mixing under dry conditions. Magma mixingis commonly invoked as a trigger for volcanic eruptions; our experiments suggest that sucheruptions can be driven by small (~15°C) temperature fluctuation in the reservoir. Our resultsalso suggest that slow replenishment of a felsic reservoir by mafic inputs will likely result instratification between end-members rather than in a homogeneous mixture.
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Mickaël Laumonier, Bruno Scaillet, Laurent Arbaret, Joan Andújar, Rémi Champallier. Experimental mixing of hydrous magmas. Chemical Geology, Elsevier, 2015, 418, pp.158-170. ⟨10.1016/j.chemgeo.2015.10.031⟩. ⟨insu-01225181⟩

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