Primitive mantle magmas recorded as silicate melt inclusions in igneous minerals

Abstract

This paper reviews work on primitive silicate melt inclusions in basalt phenocrysts and evaluates its significance. Primary melt inclusions are small quantities of silicate melt included in minerals during coarsening, growth or recrystallisation of the crystal structure. Because they contain liquids formed in thermodynamic equilibrium with their host minerals, primary melt inclusions produced at different stages of evolution of the melts will record the liquid line of descent of magmatic systems. Moreover, assuming they become closed to the surrounding systems since the time of entrapment, primary melt inclusions in early-formed crystals may isolate pristine samples of high-pressure mantle-derived melts that were trapped prior to mixing at shallower levels. They can thus be used to obtain information on the processes that create magmas and the nature of their mantle source regions. The first part of the review focuses on the general characteristics of silicate melt inclusions. Several aspects of the evolution of silicate melt inclusions after formation are investigated, in particular (1) the effect of elastic deformation of the host phase on the evolution of pressure inside an inclusion; (2) the changes that may occur in the inclusions during cooling, including the separation of immiscible fluid phases and crystallisation of new minerals; and (3) the efficiency of the melt inclusion isolation from the influence of the external system. The second part documents inferences derived from the composition of primitive melt inclusions in basalt phenocrysts, on mantle source compositions, melting and melts transportation. In order to discuss these results, two examples are detailed. First, the trace element compositions of primitive melt inclusions in early-formed phenocrysts in mid-ocean-ridge basalts are consistent with those expected for small melt fractions of peridotite, progressively depleted by fractional melting with a small threshold porosity and followed by high-level mixing. Secondly, the compositional changes in primitive melt inclusions in basalts from Mount Etna volcano (Eastern Sicily, Italy) reflects a progressive transition from a predominantly mantle-plume source to an island-arc magmatic source.