The small but finite solubility of CO2 in granitic magmas under crustal conditions, together with the common occurrence of CO2 in likely magma source materials, suggests that granitic magmas will often be accompanied by a CO2-H2O fluid phase during their ascent in the crust.

Polybaric and isobaric calculations have been made for model systems with varying total volatile content, initial CO2/H2O ratios, crystallization rates, and closed-system or open-system conditions. The calculations demonstrate that the presence of CO2 in an evolving magma system can result in greatly differing values of H2O activity (and hence H2O content, phase equilibria, and physical properties of the magma). Specifically, if the mass ratio CO2/H2O is ≥0.4 and the initial mass ratio of total volatiles to silicate magma is ≥0.05, then, if little or no loss of the fluid phase occurs during magma evolution, the activity of H2O will remain nearly constant. This is in strong contrast to all other possible cases in which the activity of H2O increases rapidly with decreasing pressure and (or) anhydrous phase crystallization, invariably reaching a value of unity.

It is also demonstrated that if CO2 is present in a fluid phase in the magma source region, then there will be a fluid present throughout the evolutionary history of the magma. The presence of fluid bubbles in the magma should considerably alter many properties of the magma system such as heat transfer, mass transfer, and viscosity.

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