Sintering of Glass in Hydrous Atmospheres and its Implications for Welding of Volcanic Deposits

Abstract:

Volcanic ash sintering can occur during hot deposition or upon reheating, and recently published models have improved our understanding of viscous sintering timescales at magmatic temperatures. However, in most volcanic environments, water is present either from meteoric or magmatic sources. Water significantly lowers the viscosity of liquids and therefore should alter the onset temperature and timescales of sintering. The diffusion of water in melts and glasses at low (sub-liquidus) temperatures and pressures, and the partitioning between water vapor and dissolved water species are poorly understood. We investigate the impact of a water rich Ar -atmosphere on viscous sintering at temperatures close to the glass transition. Synthetic near-spherical soda-lime silica glass beads with a well-constrained size of about 10-350µm (produced by Spheriglass) were heated in simultaneous thermal analyses of both differential scanning calorimetry and thermogravimetry. Glass transition temperature onset and mass stayed consistent under argon atmosphere during successive heating cycles at a rate of 10 °C.min^-1. Contrastingly, preliminary results show that, when heated, closely packed in a water-argon atmosphere (1) there is a measurable water uptake during timescales as short as 2 hours, and (2) sintering is more efficient and densification takes place at lower temperatures and/or within shorter timescales.

Sintering of volcanic materials reduces both porosity and permeability of volcanic products. The process of sintering is, however, limited by quenching of the material shortly after eruption. External water present during deposition could allow welding of pyroclastic deposits at conditions and timescales otherwise not achievable from the deposited pyroclasts alone.. Viscous sintering in a water-rich atmosphere may enhance resorption and encourage the formation of vesicle-free obsidian.