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Journal of Petrology Volume 42 Number 5 Pages 1019-1041 2001
© Oxford University Press 2001

Energy-Constrained Open-System Magmatic Processes II: Application of Energy-Constrained Assimilation–Fractional Crystallization (EC-AFC) Model to Magmatic Systems

WENDY A. BOHRSON,* and FRANK J. SPERA

INSTITUTE FOR CRUSTAL STUDIES AND DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CALIFORNIA, SANTA BARBARA, CA 93106, USA

Evidence for open-system magmatic processes is abundant in igneous rocks from most tectonic settings and with ages spanning most of geologic time. Accurately documenting these processes is critical for understanding magma reservoir dynamics, including the processes that lead to compositional diversity in igneous rocks, and for deciphering the thermochemical evolution of the crust and mantle. Quantitative models describing open-system processes such as assimilation–fractional crystallization (AFC) have provided significant insight into all of these, but, nevertheless, suffer from several serious deficiencies. Foremost among these are the absence of energy conservation and the lack of consideration of country rock partial melting. For a magma body undergoing AFC, a new quantitative model, Energy-Constrained Assimilation Fractional Crystallization (EC-AFC), self-consistently balances energy, species and mass while also tracking compositional variations generated in anatectic melt as country rock undergoes partial melting. EC-AFC represents a significant improvement to existing AFC models for several reasons. First, the inclusion of energy conservation provides a direct and crucial link between thermal parameters and volcanological or geological data. Second, unlike ‘classical’ AFC that models mass and chemical properties only, EC-AFC models mass, chemical and thermal properties of a magma body, thus allowing the energetics of the open-system magma reservoir to be linked to the geochemical evolution. Third, compared with ‘classical’ AFC models, EC-AFC geochemical trends are distinct, exhibiting non-monotonic behaviors that are directly linked to the effects of energy conservation and country rock partial melting. Comparison of EC-AFC trends with data from natural systems indicates that some of the criteria currently used to demonstrate the efficacy of AFC require modification. Finally, comparison of ‘classical’ AFC and EC-AFC results for data from well-documented volcanic centers demonstrates that EC-AFC does a superior job of tracking the compositional trends, provides a plausible physical context for the process of AFC, and allows geologically relevant predictions to be made about particular magmatic systems.

KEY WORDS: assimilation–fractional crystallization; geochemical model; isotope; magma chamber; trace element


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