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The best proofs for operation of open-system processes remain radiogenic isotope ratios, which are generally not fractionated during closed-system processes such as melting or crystallization. The magmas formed should preserve the isotopic characteristics of their source. In other words, the radiogenic isotope data are totally transparent to mechanisms of magmatic differentiation but are very sensitive to mixing or contamination. This chapter explains mathematical formulae governing binary mixing for a single isotopic ratio (in diagrams such as Sr vs. 87Sr/86Sr or 1/Sr vs. 87Sr/86Sr) and for their pairs (e.g., in 87Sr/86Sr vs. 143Nd/144Nd plots). The following text adds the theoretical treatment of the Assimilation and Fractional Crystallization (AFC) processes and their bearing on isotopic composition of the evolving magmas.
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