Trace element and isotope evolution during concurrent assimilation, fractional crystallization, and liquid immiscibility of a carbonated silicate magma

Liquid immiscibility is an important magmatic process that causes unmixing of magmas into liquids of contrasting compositions. Such magmas may get mod ified by simultaneous wall rock assimilation and fractional crystallization during the liquid immiscibility in a crustal magma chamber. The element an d isotope effects of such a process are likely to be reflected in the final products. To treat these effects and to understand the evolution of the im miscible liquids, a model has been developed modifying the assimilation-fra ctional crystallization (AFC) model of DePaolo (1981). I demonstrate the ap plicability of this model by an example using Sr isotope systematics of sil icate-carbonate melt immiscibility. The initial Sr-87/Sr-86 ratio and Sr co ncentration variation in the silicate rocks of some alkaline-carbonatite co mplexes of Deccan Province are found to be a result of lower crustal contam ination (up to 5%) of the parent carbonated silicate ma,oma, while the Sr-8 7/Sr-86 of the carbonate melt separated out of the parent remained unaffect ed; Though the data on silicate rocks could also be explained by the conven tional AFC model, the processes treated by the model do not include liquid immiscibility, needed for explaining the evolution of the cogenetic carbona tites. it appears from this study that the slightly higher initial Sr-87/Sr -86 (than that of the coexisting carbonatites) shown by the alkaline silica te rocks could be due to crustal contamination of the carbonated silicate p arent magma during concurrent fractional crystallization of silicates and e xsolution of carbonate melt. Though the model has been applied to a very sp ecific case-that of carbonate-silicate melt immiscibility-it can be applied to any case in which both assimilation and immiscibility occur together. C opyright (C) 1998 Elsevier Science Ltd.