mesostasis

Mesostasis is a term used in igneous petrology to describe the fine-grained, often glassy or microcrystalline portion of an igneous rock that fills the spaces between larger crystals, such as phenocrysts or cumulus grains. It represents the last portion of the original silicate melt to crystallize as the magma evolves and crystals form.

Formation and context: Mesostasis forms during fractional crystallization as earlier-formed crystals remove compatible elements from the melt. The remaining melt becomes enriched in incompatible elements and later crystallizes as a fine-grained interstitial material. In cumulate rocks, mesostasis can be referred to as intercumulus material, occupying the gaps between cumulus crystals.

Texture and composition: The mesostasis is characteristically fine-grained and may be glassy or microcrystalline. Its mineral assemblage can include late-precipitating phases such as alkali feldspar, plagioclase, quartz, and minor minerals, or it may resemble a glassy groundmass. The composition varies with the original magma and cooling history, and the mesostasis often records the composition of the last liquid that crystallized.

Occurrence and significance: Mesostasis is widely recognized in volcanic rocks (where it forms the groundmass between phenocrysts) and in plutonic rocks with cumulate textures. Studying mesostasis helps geologists infer crystallization sequences, cooling rates, and the evolutionary path of the magma. Variations in its chemistry can reveal steps in magma differentiation and the nature of the residual liquid during crystallization.