BULK ASSIMILATION, REACTIVE CRYSTALLIZATION AND THE CRYPTIC NATURE OF CRUST-MANTLE MIXING IN PLUTONS: A CONCEPTUAL MODEL

Bulk assimilation of relatively small (e.g. cm to ~1km) fragments of crust - driven and (ultimately) masked by reactions during xenolith melting and pluton crystallization - is an important engine of crust-mantle mixing in plutons. The model can be summarized as follows: 1.) xenoliths containing mica or amphibole undergo dehydration melting reactions when incorporated into a host magma. The products of the reactions are mainly plagioclase, pyroxene, Fe-Ti oxides and melt. 2.) The physical integrity of the xenolith is compromised by partial melting and it begins to disintegrate as it moves through the host magma. Both melt and crystals from the xenolith are mixed into the host magma. 3.) In strongly convective or eruptive systems, mixing may occur on scales down to individual crystals. In less active systems, larger inhomogeneities may persist. 4.) The cryptic character of assimilation is greatly enhanced in hydrous magmas by hydration crystallization (the reverse of dehydration melting) reactions. All anhydrous phases in the system, be they original phenocrysts, xenolith-derived xenocrysts, or crystals having a mixed source will be subject to these reactions. Late-crystallizing amphiboles and micas may incorporate material from several sources, a particularly effective mixing mechanism. Implicit in the model is a reduced energy penalty for bulk assimilation - much of the assimilant remains in solid form. This greatly increases the allowable ratio of assimilation to fractionation compared to melt assimilation models. The model suggests that piecemeal stoping is an important ascent and emplacement mechanism. The model precludes (or obviates) the existence of the long-sought xenolith graveyards at the base of plutons. It has been shown that xenoliths found in plutons reflect only the last stages of assimilation and/or refractory lithologies and are not necessarily relevant to the bulk chemical evolution of the pluton. In short, it's not the xenoliths you see, it’s the xenoliths you don't.