Abstract
Recent advances in micro-analysis have revealed that isotopic zoning profiles in igneous crystals reflect progressive changes in liquid composition during open-system magmatic processes. Consequently, the isotope ratio of the integrated (bulk) crystals and that of liquid are inevitably different. This study develops a mass balance model to describe the compositional relationships between the bulk crystals (total suspended crystals), liquid, magma (crystals plus liquid), and crystal rims during open-system magma chamber processes. The model incorporates the effects of concurrent magma recharge (or mixing), assimilation, magma extraction, and fractional crystallization. The mass balance differential equations for suspended crystals and liquid are solved simultaneously. The analytical solution of the equations gives a quantitative account of the evolution paths of trace elements and isotopes within total suspended crystals, liquid, magma, and crystal rims. The isotopic zoning profiles in phenocrysts vary markedly depending on the relative rates of recharge, assimilation, magma extraction, and fractional crystallization. The model can reproduce the 87Sr/86Sr zoning patterns in natural feldspars from different magma systems.
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