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 during its ascent may change abruptly as a result of the release of volatiles or through magma mixing or a combination of both. Immediately after this abrupt change of the liquidus temperature occurs a disequilibrium is observed in the melt, and here it is shown that the melt rapidly re-equilibrates to its thermodynamic equilibrium. A quantitative model for cooling and crystallization of a simple two-component model melt is used to investigate such re-equilibration processes. The relaxation time, defined as the time required for the system to re-equilibrate after varying degrees of liquidus temperature perturbations, is found to be ∼3% or significantly less than the time it takes to crystallize ∼40% of the melt, regardless of the amplitude of the perturbation. Associated with the re-equilibration process is an increase in crystal fraction that can easily reach 10% depending on the amplitude of the perturbation. During the compensation of the liquidus temperature perturbation, the temperature of the melt remains nearly constant or may even increase slightly (depending on the latent heat budget, possibly heat of mixing, and heat absorbed during volatile exsolution), which suggests crystallization of the melt without cooling.