Metasomatism

Metasomatism (from the Greek μετά metá "change" and σῶμα sôma "body") is the chemical alteration of a rock by hydrothermal and other fluids.[1] It is traditionally defined as metamorphism which involves a change in the chemical composition, excluding volatile components.[2] It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur simultaneously and the rock remains solid.

Synonyms of the word metasomatism are metasomatosis[3] and metasomatic process. The word metasomatose can be used as a name for specific varieties of metasomatism (for example Mg-metasomatose and Na-metasomatose).[4]

Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source.

Metasomatic albite + hornblende + tourmaline alteration of metamorphosed granite, Stone Mountain, Atlanta

In the igneous environment, metasomatism produces skarns, greisen, and may affect hornfels in the contact metamorphic aureole adjacent to an intrusive rock mass. In the metamorphic environment, metasomatism is driven by mass transfer from a volume of metamorphic rock at higher stress and temperature into a zone with lower stress and temperature, with metamorphic hydrothermal solutions acting as a solvent. This can be envisaged as the metamorphic rocks within the deep crust losing fluids and dissolved mineral components as hydrous minerals break down, with this fluid percolating up into the shallow levels of the crust to chemically change and alter these rocks.

This mechanism implies that metasomatism is open system behaviour, which is different from classical metamorphism which is the in-situ mineralogical change of a rock without appreciable change in the chemistry of the rock. Because metamorphism usually requires water in order to facilitate metamorphic reactions, metamorphism nearly always occurs with metasomatism.

Further, because metasomatism is a mass transfer process, it is not restricted to the rocks which are changed by addition of chemical elements and minerals or hydrous compounds. In all cases, to produce a metasomatic rock some other rock is also metasomatised, if only by dehydration reactions with minimal chemical change. This is best illustrated by gold ore deposits which are the product of focused concentration of fluids derived from many cubic kilometres of dehydrated crust into thin, often highly metasomatised and altered shear zones and lodes. The source region is often largely chemically unaffected compared to the highly hydrated, altered shear zones, but both must have undergone complementary metasomatism.

Metasomatized dike in serpentinite Nelson New Zealand

Metasomatism is more complicated in the Earth's mantle, because the composition of peridotite at high temperatures can be changed by infiltration of carbonate and silicate melts and by carbon dioxide-rich and water-rich fluids, as discussed by Luth (2003).[5] Metasomatism is thought to be particularly important in changing the composition of mantle peridotite below island arcs as water is driven out of ocean lithosphere during subduction. Metasomatism has also been considered critical for enriching source regions of some silica-undersaturated magmas. Carbonatite melts are often considered to have been responsible for enrichment of mantle peridotite in incompatible elements.

Metasomatism can be similar to other endogenic processes and is separated by 4 main features.[6] The first of these is the ion-by-ion replacement in minerals, this can happen from the precipitation of new minerals at the same time as the dissolution of existing minerals.[6] The second feature used to identify metasomatism is that it is from the preservation of rocks in its solid state during replacement.[6] The third distinctive feature is from isochemical metamorphism, or the addition or subtraction of major elements other than water (H2O) and carbon dioxide (CO2).[6] The last feature is the distinct zones of metasomatism. These are formed from magmatism and metamorphism and form a characteristic pattern of a metasomatic column.[6]

  1. ^ Harlov, D.E.; Austrheim, H. (2013). Metasomatism and the Chemical Transformation of Rock: Rock-Mineral-Fluid Interaction in Terrestrial and Extraterrestrial Environments. Berlin: Springer. doi:10.1007/978-3-642-28394-9_1. ISBN 978-3-642-28393-2.
  2. ^ Putnis, A.; Austrheim, H. (2010-12-23). "Fluid‐Induced Processes: Metasomatism and Metamorphism". Frontiers in Geofluids: 254–269. doi:10.1002/9781444394900.ch18.
  3. ^ "metasomatosis". Merriam-Webster.com Dictionary. Merriam-Webster. Retrieved 10 April 2023.
  4. ^ Zharikov V.A.; Pertsev N.N.; Rusinov V.L.; Callegari E.; Fettes D.J. "9. Metasomatism and metasomatic rocks" (PDF). Recommendations by the IUGS Subcommission on the Systematics of Metamorphic Rocks: Web version 01.02.07. British Geological Survey.
  5. ^ Luth, R. W. (2003). Mantle volatiles - distribution and consequences in The Mantle and Core (Volume 2 Treatise on Geochemistry ed.). Elsevier-Pergamon, Oxford. pp. 319–361. ISBN 0-08-043751-6.
  6. ^ a b c d e Zharikov V.A.; Pertsev N.N.; Rusinov V.L.; Callegari E.; Fettes D.J. "9. Metasomatism and metasomatic rocks" (PDF). Recommendations by the IUGS Subcommission on the Systematics of Metamorphic Rocks: Web version 01.02.07. British Geological Survey.