Experimental simulation of magma mixing at high pressure

Abstract : Magma mixing features are observed in many plutonic and volcanic environments. They result from the juxtaposition of two chemically contrasted magmas, usually during the replenishment of a magmatic reservoir, but also syn-eruptively within the conduit. Despite its ubiquity, only a few experimental studies have explored mixing between magmas. Existing data have been mostly acquired at atmospheric pressure and high shear rates (> 10- 1 s- 1), which differ from those accompanying magma mixing in reservoirs. To fill this gap, we performed high pressure mixing experiments at strain rates ranging from 4.10- 4 to 1.10- 3 s- 1. Layers of a synthetic crystal-free haplotonalite and a natural partially-molten basalt were juxtaposed in a Paterson apparatus at 300 MPa, and deformed between 900 and 1200 °C. The experiments shed light on the first stages of magma mixing and illustrate the role and behavior of crystals, either pre-existing or newly grown. Experiments evidence a rheological threshold for mafic material disruption, which sets in abruptly as its melt fraction exceeds 50%, which in the experiments occurs in the narrow temperature interval 1160-1170 °C. Below this threshold, plagioclase crystals in the mafic magma form a rigid touching network and all the deformation is accommodated by the less viscous felsic layer. Above it the crystal network collapses, allowing typical mingling/mixing features to appear altogether, such as enclaves, melt filaments or single xenocrysts isolated into the felsic end-member, coexisting with newly grown phases (plagioclase and pyroxene) whose compositions spread out over considerable ranges. The pre-existing fabric of the mafic magma is only slightly affected by deformation, altogether providing few clues on either the regime or geometry of applied deformation during the magmatic stage.
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Mickaël Laumonier, Bruno Scaillet, Laurent Arbaret, Rémi Champallier. Experimental simulation of magma mixing at high pressure. Lithos, Elsevier, 2014, 196-197, pp.281-300. ⟨10.1016/j.lithos.2014.02.016⟩. ⟨insu-00955227⟩

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