Experimental sintering of crystal-rich rhyolitic ash at high fluid pressures with implications for degassing of magma

Abstract

We present an experimental investigation of surface tension–driven sintering and associated densification of glassy rhyolitic ash and crystals under shallow volcanic conduit conditions. Rhyolitic glass (< 45 μm) and quartz were run in suites of hydrothermal experiments for 30 min to 9 h. Fluid pressure was isobaric (${P}_{{H}_{2}O}=40 MPa$) for all runs, and temperature was held constant at a value between 675 and 850 °C. Three size populations of quartz were used: 45–76 µm (fine), 90–125 µm (medium), and 250–500 µm (coarse). All samples evolved from loose, cohesion-less particles to a friable, agglutinated framework of glass with an interconnected network of pores of ≥ 15 vol.%. Samples sintered more slowly at cooler temperatures and with finer crystals relative to higher temperatures or coarser crystals. Compared with previous experiments on glass-only samples, all crystal-bearing samples reached a higher final porosity, and those containing fine and medium crystals also sintered more slowly. Permeability was determined via numerical simulation for one sample and was found to be similar to natural samples with equivalent porosity. Our results suggest that solid particles inhibit the sintering process by holding porous networks open. Sintered magma in the shallow conduit that contains crystal (and lithic) particles can thus experience more sustained degassing and outgassing than crystal-free systems.