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assimilation-fractional crystallization (AFC) An important process in igneous petrology whereby melts with widely differing isotopic and trace elements can be produced. When a primitive magma, such as a basalt, invades crustal rocks, portions of the country rock may become detached and included in the magma as xenoliths. Because of the high temperature and thermal capacity of the basalt, it is capable of melting a proportion of the country rock. In doing so it loses some of its own heat and thus a proportion of the magma crystallizes. The composition of the resulting magma is determined by the relative amounts of magma and country rock initially present; the rates at which assimilation and crystallization proceed; and by the partition coefficients of the various elements between solid and liquid.