Thermal effects of magma degassing


The thermodynamics of diffusive bubble growth is dominated by latent heat of water exsolution (vaporization) and the work of gas expansion (P dV work). A numerical assessment of the effects of these on cooling of bubbly magma (water in albite) indicates that a magma can exsolve volatiles at equilibrium, or with varying degrees of oversaturation, depending on decompression history and degassing kinetics. Heat of water exsolution (vaporization) from magmatic melts has not previously been experimentally or analytically determined, and we have evaluated it from others thermodynamic functions known for the albite-water system. Heat of exsolution is small for pressures over 100 MPa, but can reach 20 kJ/mole at 1–2 MPa (10–20 bar). Oversaturation degassing at 10–20 bar can cause cooling of albite by 8 K/wt.% of exsolved water. The thermal effect of equilibrium degassing depends on the starting pressure of decompression because it follows the solubility curve. For a saturated melt at 100 MPa (4 wt.% water), it can cool a magma at least by 35 K before it disrupts into spray as gas volume fraction exceeds 0.8. Although we do not specifically consider the dynamics of bubble interactions in this study, our results suggest that the dynamic cooling rate could be significant at the vent of an erupting magma column, and under extreme eruption conditions, can lead to glass formation around fast growing bubbles of magmatic foam. This could cause fragmentation into fine ash by brittle failure. Conversely, bubble wall cooling (even before glass formation) can serve to reduce diffusive volatile flux into bubbles, decreasing the overall cooling of the system, and lead to solidification of oversaturated magmas, as occasionally observed in eruption products.


Earth Systems Research Center

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Journal of Volcanology and Geothermal Research



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© 1996 Published by Elsevier B.V.