Metasomatism and the chemical transformation of rock: Rock-mineral-fluid interaction in terrestrial and extraterrestrial environments

Abstract

The term metasomatism comes from the Greek μετά μετα (meta = change) and σῶμα (soma = body). The term was originally coined by C.F. Naumann in Lehrbuch der Mineralogie, published by Engelman in Leipzig in 1826. The current definition of metasomatism, according to the IUGS Subcommission on the Systematics of Metamorphic Rocks (SSMR), is “a metamorphic process by which the chemical composition of a rock or rock portion is altered in a pervasive manner and which involves the introduction and/or removal of chemical components as a result of the interaction of the rock with aqueous fluids (solutions). During metasomatism the rock remains in a solid state.” Metamorphism comes from the Greek μετα (meta = change) and μορφε (morphe = form) and is defined by the IUGS SSMR as “a process involving changes in the mineral content/composition and/or microstructure of a rock, dominantly in the solid state. This process is mainly due to an adjustment of the rock to physical conditions that differ from those under which the rock originally formed and that also differ from the physical conditions normally occurring at the surface of the Earth and in the zone of diagenesis. The process may coexist with partial melting and may also involve changes in the bulk chemical composition of the rock.” From these two definitions it is obvious that metasomatism and metamorphism are commonly interrelated with each other. However, they can also be mutually exclusive. For example, metamorphism can be isochemical such that the chemical and isotopic characteristics of the protolith sedimentary lithologies are retained. Also, the chemical changes seen during the high-grade transition of metamorphic rocks from amphibolite-to granulite-facies, can be due to the removal of fluid-rich partial melts. In contrast, sediment diagensis, fluid-aided deposition of ore veins or ore deposits, sea floor alteration, and alteration of surrounding rock by hot springs or meteoric fluids are fluid-dominated metasomatic process, which are generally not considered to be metamorphic.