Abstract
Melt and gas transfer processes are essential to the formation and growth of the Earth’s crust and for sustaining volcanic activity. These processes also play a major role in magma fractionation at shallow depths (< 10 km) where magmas stall rheologically and solidify. In this scenario, the conditions of melt and gas mobilisation during progressive cooling of crystal mushes down to their solidus remain poorly understood. We present experimental data (at 1.1 kbar) showing how a combination of temperature and crystal content control the ability of melt and gas to escape from cooling and solidifying hydrous silicic magmas with initial crystal fractions of 0.6, 0.7, and 0.8, and for temperature snapshots of 850, 800, and 750 °C. Microstructural observations and chemical data show that the amount of extracted melt increases by 70% from 850 to 750 °C and by 40% from 0.6 to 0.8 in crystal fraction at 750 °C, due to the formation of interconnected crystal frameworks, gas expansion in constricted pore space, and filter pressing during cooling. As a result, our experiments suggest that melt and gas extraction from cooling mushes increases in proximity to their solidus and can operate efficiently at crystal fractions between 0.6 and 0.93. These observations shed light on maximum estimates of the segregation of gas-rich, crystal-poor magmas (0.02 m/year at 850 °C to 9 m/year at 750 °C) to form felsic dykes or eruptible systems feeding volcanoes.
Highlights
IntroductionThese factors impede any gravitational segregetation of crystals from melt, yet there is clear field evidence for such segregation in natural rocks (Holness, 2018)
We present the first experimental study to show melt segregation driven by gas filter pressing from crystal mushes at conditions relevant to the Earth’s shallow crust
Through the use of conventional high-temperature and high-pressure experiments combined with high-spatial resolution techniques and RhyoliteMELTS simulations, we have illustrated the physical conditions that allow gas-driven filter pressing to be an efficient process for melt extraction from near-solidus systems
Summary
These factors impede any gravitational segregetation of crystals from melt, yet there is clear field evidence for such segregation in natural rocks (Holness, 2018). Instead, these near-solidus systems are capable of releasing high-silica melt (formed during crystallization at low-pressure; Gualda and Ghiorso, 2013) through porous flow (Olsen et al, 2004) to form crystal-poor, evolved magmas of rhyolitic composition that are capable of eruption at the surface (Hildreth, 1981, 2004; Marsh, 1981; Bachmann and Bergantz, 2004; Hildreth and Wilson, 2007; Dufek and Bachmann, 2010; Waters and Lange, 2017). The competition between the rate of heat loss and the rate of latent heat release due to crystallization during progressive cooling and solidification of magmas exerts an important control on the extraction of interstitial melts (Huber et al, 2009; Caricchi and Blundy, 2015; Lee et al, 2015)
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