Crystal–Melt Separation and the Development of Isotopic Heterogeneities in Hybrid Magmas

If a magma is a hybrid of two (or more) isotopically distinct end-members, at least one of which is partially crystalline, separation of melt and crystals after hybridization will lead to the development of isotopic heterogeneities in the magma as long as some of the pre-existing crystalline material (antecrysts) retains any of its original isotopic composition. This holds true whether the hybridization event is magma mixing as traditionally construed, bulk assimilation, or melt assimilation. Once a magma-scale isotopic heterogeneity is formed by crystal–melt separation, it is essentially permanent, persisting regardless of subsequent crystallization, mixing, or equilibration events. The magnitude of the isotopic variability resulting from crystal–melt separation can be as large as that resulting from differential contamination, multiple isotopically distinct sources, or in situ isotopic evolution. In one model, a redistribution of one-third of the antecryst cargo yielded a crystal-enriched sample with 87Sr/86Sr of 0·7058, whereas the complementary crystal-poor sample has 87Sr/86Sr of 0·7068. In other models, crystal-rich samples are enriched in radiogenic Sr. Isotopic heterogeneities can be either continuous (controlled by the modal distribution of crystals and melt) or discontinuous (when there is complete separation of crystals and liquid). The first case may be exemplified by some isotopically zoned large-volume rhyolites, formed by the eruptive inversion of a modally zoned magma chamber. In the latter case, the isotopic composition of any (for example) interstitial liquid will be distinct from the isotopic composition of the bulk crystal fraction. The separation of such an interstitial liquid may explain the presence of isotopically distinct late-stage aplites in plutons. Crystal–melt separation provides an additional option for the interpretation of isotopically zoned or heterogeneous magmas. This option is particularly attractive for systems whose chemical variation is otherwise explicable by fractionation-dominated processes. Non-isotopic chemical heterogeneities can also develop in this fashion.