metasomatose

Metasomatose, or metasomatism, is a geological process in which the chemical composition of a rock is substantially altered by hydrothermal or metamorphic fluids that introduce or remove components. This fluid-driven exchange leads to the replacement of original minerals with new mineral assemblages and textures, often changing the rock’s overall identity.

The process involves the circulation of reactive fluids through rock, transporting ions such as silicon, aluminum, calcium, sodium, potassium, magnesium, iron, and volatile species (water, carbon dioxide, chlorine, fluorine). Reactions between the fluids and the host rock produce reaction rims and new minerals, sometimes replacing large portions of the rock. Metasomatism can occur at various temperatures and pressures, ranging from near-surface hydrothermal systems to conditions accompanying metamorphism, and may produce distinct alteration halos around fluid pathways.

Common products and textures of metasomatic alteration include replacement textures, vein fillings, and banded or schlieren patterns. In carbonate rocks, metasomatism near intrusive bodies produces calc-silicate skarns, featuring minerals such as diopside, wollastonite, garnet, and calcite. In granitic or silicic terrains, alkali exchange can lead to albite metasomatism, where original feldspars are replaced by sodium-rich varieties. Metasomatism also plays a crucial role in ore formation, creating replacement and vein deposits enriched in metals such as copper, zinc, lead, and tungsten through fluid-mediated transport and deposition.

Metasomatism is an important mechanism in crustal evolution, in ore geology, and in understanding fluid flow and chemical transport within rocks. It is often distinguished from pure metamorphism by its dominant role of chemical exchange rather than broad recrystallization.