Abrupt Change in Magma Liquidus Temperature because of Volatile Loss or Magma Mixing: Effects on Nucleation, Crystal Growth and Thermal History of the Magma

Abstract

The liquidus temperature of magma that reaches shallow levels INTRODUCTION during its ascent may change abruptly as a result of the release of On its way from the source region into the Earth’s volatiles or through magma mixing or a combination of both. crust or even onto the Earth’s surface, magma travelling Immediately after this abrupt change of the liquidus temperature through the mantle and crust evolves, and one of the occurs a disequilibrium is observed in the melt, and here it is shown goals in petrology is to characterize the various processes that the melt rapidly re-equilibrates to its thermodynamic equilibrium. that occur during this passage. Of special interest in the A quantitative model for cooling and crystallization of a simple framework of this study are shallow level processes, such two-component model melt is used to investigate such re-equilibration as magma–crust interaction, magma mixing, deprocesses. The relaxation time, defined as the time required for the volatilization and depressurization, that change the geosystem to re-equilibrate after varying degrees of liquidus temperature chemical signature of the melt, its crystal content, its perturbations, is found to be ~3% or significantly less than the liquidus temperature, its flow behaviour and its erupttime it takes to crystallize ~40% of the melt, regardless of the ability. Whereas crustal contamination of magma can be amplitude of the perturbation. Associated with the re-equilibration traced through, for example, trace element and isotope process is an increase in crystal fraction that can easily reach studies (e.g. Wilson, 1989) and is discussed extensively 10% depending on the amplitude of the perturbation. During the in the other papers of this special volume, cooling, compensation of the liquidus temperature perturbation, the temdepressurization, and possibly magma mixing strongly perature of the melt remains nearly constant or may even increase slightly (depending on the latent heat budget, possibly heat of mixing, affect the thermal history of magma rising in the mantle and heat absorbed during volatile exsolution), which suggests and crust. For example, any pressure decrease in volatilecrystallization of the melt without cooling. saturated systems (not uncommon in natural systems, especially for more evolved compositions) will lead to volatile exsolution and consequent increased liquidus temperature and change in the melting range. One of the major consequences of such behaviour is isothermal crystallization, which was recognized almost 40 years ago by Tuttle & Bowen (1958) (pp. 67 and 68), and the