An experimental study of melt migration in crystal-rich mushes

<p>Increasingly, volcanologists model mature volcanic systems as being fed by stratified magma reservoirs, that is, small lenses of eruptible magma suspended within a larger volume of non-eruptible, crystal-rich mush. Erupted lavas record geochemical evidence for long-term deep storage of distinct magma bodies followed by their ascent and coalescence shortly before eruption. Conditions and flow mechanisms that allow deep-seated magmas to rise quickly in reservoirs despite the high viscosity and low permeability of crystal-rich mushes are a subject of debate. We present results of melt migration experiments conducted in a triaxial, gas-medium apparatus. We prepared multiple crystal-rich mushes by hot pressing crushed borosilicate glass mixed with different amounts of subrounded quartz sand (44-106 μm diameter). Prepared mushes have crystal fractions from 0.59 to 0.83. A single disk of mush is stacked on a disk of soda lime glass, representing the intruding crystal-free magma, then heated to 900°C (above the glass transitions) and pressurized to 100-300 MPa. The bottom and circumference of the mush experience the confining pressure, but the top is at room pressure, resulting in a pore pressure gradient (~33-100 MPa/mm) that could drive the underlying melt into the mush. After several hours samples are cooled, decompressed, cut and imaged to determine the distribution of the soda lime glass that migrated in to the mush while at the experimental conditions. High crystal fraction samples (>0.80; Hi X) have glass filling intergranular space. In low crystal fraction samples (<0.70; Lo X), glass forms finger-like intrusions in the mush, indicating melt migration displaced crystals in the mush. Samples with intermediate crystal fractions (Int X) have both morphologies. Mush crystal fraction significantly influences the amount of melt migration, quantified as the measured area fractions of soda lime glass in mush normalized to the available intergranular space (1 - crystal fraction). Glass fills ~10%, ~60% and ~30% of the intergranular space for Lo X, Int X and Hi X samples, respectively. Glass area fraction is not correlated with the imposed pressure gradient, indicating melt migration is moderated by viscosity contrasts between the melt and mush (melt to mush viscosity ratio: 10<sup>-1.7</sup> to 10<sup>-8.6</sup>). The observed increase in the amount of melt migration with decreasing crystal fraction is coincident with the onset of mush deformation. Applied to natural systems, these results suggest small changes in mush crystal fraction significantly influence the amount of melt migration that occurs and that melt migration in magma reservoirs peaks near the transition from deformable mush to partially-molten rock (i.e., at the rheologically critical melt fraction).</p>