Experimental mixing of hydrous magmas

- We performed the first dynamic mixing experiments between hydrous magmas.
- Mixing textures are isolated crystals, boudins, enclaves, and “diktytaxitic” layers.
- Mixing of hydrous magmas occurs at T = 170 °C lower than between similar but dry magmas
- Mingling and mixing appears feasible at crystal fraction ϕ < 50 vol.%
- Mingling may occur at ϕ > 50 vol.% under higher strain rate conditions (volcanic conduit)

Deformation experiments involving hydrous magmas of different compositions (basalt and haplotonalite) have been performed in a Paterson press at 300 MPa, in the temperature range 600 °C-1020 °C, with water-saturated melts, during 2-4 h. Prior to deformation, the two end-member magmas were annealed at either 950 °C or 1000 °C, yielding magmas with crystal contents in the range 31-53 wt.% and 2 sets of viscosity contrasts. Under the experimental conditions investigated (i.e. moderate shear rates < 10− 3 s− 1), mixing/mingling textures appear at temperatures > 950 °C. In the temperature range 950-985 °C, a few mixing and mingling textures occur, though both end-members essentially retain their physical integrity. It is only at, or above, 1000 °C that a dramatic jump in mingling efficiency happens, corresponding to a crystal fraction of 45 vol.%. Textures include entrainment of mafic crystals into the felsic magma, mafic-felsic banding, enclave formation, and diffusion-induced interface, the latter only over limited distances (< 300 μm) due to the short run durations. In the most strained parcels of interacting magmas, complex mixing/mingling textures were produced, similar to those observed in volcanic and plutonic rocks in arc settings. The experiments show that mixing between hydrous felsic and mafic magmas takes place at around 1000 °C, a temperature which is almost 200 °C lower than mixing under dry conditions. Magma mixing is commonly invoked as a trigger for volcanic eruptions; our experiments suggest that such eruptions can be driven by small (~ 15 °C) temperature fluctuation in the reservoir. Our results also suggest that slow replenishment of a felsic reservoir by mafic inputs will likely result in stratification between end-members rather than in a homogeneous mixture.