Timescales of texture development in a cooling lava dome

Abstract

Crystal growth and crack development in cooling lava domes are both capable of redistributing and mobilizing water. Cracking and hydration decrease the stability of a dome, which may lead to hazards including partial dome collapse and block and ash flows. By examining the distribution of water around crystals and cracks, we identify and confine temperature and timescales of texture development in glassy rocks of volcanic domes. Four generations of textures have been identified: type a: spherulites, type b: cracks associated with spherulite growth, type c: perlitic cracks, and type d: disparate cracks. High-resolution imaging using Fourier Transform Infrared Spectroscopy (FTIR) performed on samples from the Ngongotaha dome, New Zealand, show an increase in H2O of up to 450% along gradients of around 100μm up to 300μm in length from perlitic cracks, spherulitic cracks and in haloes around spherulites. No gradients in water concentrations across the disparate cracks are present. Water diffusion models show potential timescale–temperature couples that coincide with textural observations and previous studies, and allow us to develop a conceptual model of spherulite growth and cracking in a cooling lava dome. Spherulite growth starts around the glass transition temperature (Tg) when the viscous melt cools to a brittle solid and proceeds with cracking related to volume changes at slightly lower temperatures and shorter timescales (days to weeks) compared to spherulite growth. Perlitic cracking happens at T≪Tg, allowing hydration of a permeable network within weeks to months. Low temperature (≲50°C) cracks could not be hydrated in the time since eruption (≃230ka). Our data show that textures in cooling glass develop during cooling below Tg within days, producing cracks and crystals that create inhomogeneities in the spatial distribution of water. The lengthscales of water diffusion away from spherulites, spherulite cracks, and perlite cracks suggest that most of the rehydration of melt/glass occurs at relatively high temperatures (>400°C). Lack of evidence for water diffusion around other cracks suggests minor low-temperature meteoric water rehydration following emplacement.