The Affect of Pre and Syn-Ascent Crystallization on Vesiculation Kinetics and Permeability Development in Low Viscosity Magmas

Abstract:

Magma degassing processes are important to understand in order to determine what causes transitions in eruption style. Thus, we conducted high P-T, isothermal decompression experiments to investigate the affects of crystal content on percolation threshold development in low viscosity melts. We use these experiments to examine the mechanical effects phenocrysts and microlites impose on vesiculation during magma ascent and to observe the effects of decompression–induced crystallization on permeability development and magma degassing. We performed experiments using powdered natural K-phonolite (55 wt.% SiO₂; Vesuvius EU2) seeded with andesine crystals to approximate alkalic magma with 20 and 40 vol. % phenocrysts. We performed a second series using a natural basaltic andesite (54 wt.% SiO₂; Westdahl, Alaska) to approximate a mafic magma forming abundant microlites during ascent and degassing. First, we saturated the experiments with water at 100 MPa, holding the feldspar-seeded K-phonolite at 900°C for three hours and the basaltic andesite at 950°C for 24 hours to initiate crystal growth. We decompressed the K-phonolite step-wise at a rate of 0.25 MPa s^-1 and the basaltic andesite continuously at 0.125 MPa s^-1, with ending pressures between 75 and 25 MPa. We measured the permeability of the samples with a bench-top permeameter and calculated the Darcian (k₁) and inertial (k₂) permeabilities using the Forchheimer equation.

In order to understand the effect of crystals on the percolation threshold (Φ_c), we compare our results to crystal-free experiments. K-phonolite seeded with 20 vol.% phenocrysts develops permeability at Φ_c = 44 vol.%. K-phonolite seeded with 40 vol.% phenocrysts are permeable at Φ_c = 53 vol. %. Both thresholds are significantly less than Φ_c determined for crystal-free samples (Φ_c = 75 vol. %). The basaltic andesite developed permeability at Φ_c = 29 vol.%. This finding is significant because our experiments indicate Φ_c > 63 vol.% in crystal-free, vesiculating basaltic andesite melts. Our preliminary results indicate that phenocrysts and microlites significantly lower the percolation threshold of low viscosity melts.