How to overpressurize a magmatic chamber: the thermodynamic constraints of crystallization-gas exsolution processes
Abstract
The pressure evolution of magmatic chambers is a key question in order to understand the triggering mechanisms and conditions of volcanic eruptions. While this subject is undoubtedly complex, a first answer was provided by the seminal model of Tait et al. (1989), which addresses the pressurization produced by simultaneous crystallization-gas exsolution. Through a set of simple equations, they succeed to tackle this problem by using salient physico-chemical parameters of interest, i.e. magma nature (mafic or felsic), bulk volatile content (H2O and CO2), crystallization variation, initial magma pressure, Henry solubility coefficients, elasticity of the magmatic chamber, melt and crystal densities. Thus, Tait et al. (1989) were able to show that coeval crystallization-gas exsolution is, indeed, an efficient process: a partial crystallization of a few percent in water-saturated magmas is sufficient to generate overpressures exceeding twice the tensile strength of the magmatic chamber. However, in other cases (CO2 or some H2O-CO2 saturated magmas), this model predicts underpressures of the magma chambers, and such a result is counterintuitive and problematic from what is known in volcanology. In fact, to our knowledge, one feature of the theory of Tait et al. has never been clearly stated in the literature: this model applies only in the case of disequilibrium growth of crystals in the magma, as no provision is made to take into account the chemical equilibrium of silicate components between solids and melt. While this simplification hypothesis is certainly valid in fast-evolving magmas (kinetic effects) it may be questioned in the case of slow crystallization at equilibrium with the melt. Thus, we have extended the algorithm of Tait et al. (1989) to deal with this new aspect. The main result is that, in all cases, significant overpressures will be generated in cooling magmas (even CO2-saturated magmas). This can be explained as follows: under the equilibrium assumption, crystals buffer the chemical potentials of silicate components in the melt to lower levels than predicted by the initial model of Tait et al. (1989). As a result, the melt readapts itself by pumping higher amounts of volatiles from the gas and by increasing the pressure. Actually, this effect, quite similar to osmosis, is well known, as it is responsible of the pressure maximum observed along the solidus of any system, involving two markedly different components, like H2O-NaCl or CO2-CH4. Thus, this theoretical study shows the potential control of silicate crystals onto the pressure jump exhibited by volatile-saturated magmas, and opens new exploring ways to study the pre-eruptive phase of magmatic chambers. A calculation software (OS X/Windows platform) has been developed, implementing the new extended version of the model of Tait et al. (1989), and will be of interest to any volcanologist. Tait S., Jaupart C., Vergniolle S., 1989, Earth Planet Sci Let 92: 107-123.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V31D2827C
- Keywords:
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- 3618 MINERALOGY AND PETROLOGY / Magma chamber processes;
- 8411 VOLCANOLOGY / Thermodynamics