Aluminum Enrichment in Silicate Melts by Fractional Crystallization: Some Mineralogic and Petrographic Constraints

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Abstract
The degree of aluminum saturation of an igneous rock may be described by its Aluminum Saturation Index (ASI) defined as the molar ratio Al2O3(CaO + K2O + Na2O). One suggested origin for mildly peraluminous granites (ASI between 1 and about 1.1) is by fractional crystallization of subaluminous (ASI < 1) magmas; hornblende, having ASI < 0.5, could be a major driving force in such a fractionation process. The efficacy of the process depends not only on precipitation of hornblende and its effective removal from the reacting system, but on the composition and nature of other coprecipitating phases, weighted by their modal abundances in the reactive system. Precipitation of feldspar (ASI = 1), for instance, would retard or even prevent aluminum enrichment in the melt if the ASI of melt is < 1, but would enhance such evolution if the ASI of the melt is > 1. Discussion of the efficacy of any mineral must be made in the context of the total reacting system.

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