Evidence for liquid immiscibility in the upper mantle

Abstract

Liquid immiscibility, the separation of magmas into two or more immiscible liquid phases, has been shown to occur both experimentally1–3 and in nature4,5, but this phenomenon has received only limited attention in modelling magmatic processes, as known cases of liquid immiscibility are generally restricted to magma compositions encountered during late stages of crystal fractionation, and relatively shallow crustal levels. Here I report the results of a study of trapped quenched liquids in spinel Iherzolite xenoliths, these are fragments of upper mantle material carried to the surface in volcanic rocks. These xenoliths carry quenched coexisting basaltic, carbonatitic and volatile-rich ultramafic liquids, showing the existence of a previously unknown field of three-liquid immiscibility. This type of immiscibility occurs in primitive volatile-rich magma compositions at upper mantle to crustal levels. Furthermore, metasomatic growth of amphibole ± apatite ± phlogopite in the Iherzolites is associated with introduction of carbonatitic and volatile-rich ultramafic liquids, indicating that such liquids may play an important role in mantle metasomatism.