Effects of Dynamic Assimilation: Mineral Dissolution Controlled AFC Paths

If N.L. Bowen were alive today, he would probably be conducting experiments aimed at understanding one of the great unresolved challenges of igneous petrology: documenting the time scales of non-equilibrium, magmatic processes. Over the last 20 years thermodynamicists (Ghiorso and Sack, 1995) have developed models that compute liquid-solid equilibrium for a broad range of silicate melt compositions undergoing crystallization processes such as assimilation. Unfortunately, the equilibrium world neglects one variable that is critical to a dynamic world - time. The marriage of equilibrium thermodynamics with time-dependent processes occurs within the field of irreversible thermodynamics - a field petrologists have ventured into via laboratory experiments aimed at documenting the rates at which minerals react with silicate liquids (e.g., Donaldson, 1985). Such experiments, documenting the temporal effects of magmatic reactions, are critical for the future progress of our science. As a first step toward understanding the effects of dynamic assimilation, we developed a simple model for predicting rates of mineral dissolution using the thermodynamic potential affinity. The dissolution rate data used in the model was only slightly modified, for geometric consistency, from the reported raw experimental data. The affinities for the dissolution reactions were calculated using the reported experimental conditions as input to the MELTS software. We developed a linear predictive model based on direct comparison of the modified reaction rates and the calculated affinities. Our model for predicting rates of mineral dissolution, when paired with the MELTS thermodynamic database, predicts the time scales of crystallization and cooling due to dissolution-controlled assimilation. We have applied the model to examine the development of peralkaline magmas at Hoodoo Mountain volcano via AFC processes acting on an AOB parental magma. Given broad age constraints on the formation of the volcano (~80 ky), the time scales of the crystallization process predicted by the model are within geological constraints. Rates of mineral dissolution, compared to rates such as magma transport, rates of crystal growth, and rates of chemical diffusion, dictate that mineral dissolution is an important process in many igneous systems. Models for mineral dissolution may also find application in studies of transport of magma in the mantle, where porosity and melt composition may be controlled by rates of mineral dissolution. Donald CH 1985 Min Mag 54: 67-74 Ghiorso MS, Sack RO 1995 CMP 119: 197-212