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Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is also known as "alteration".

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

In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact metamorphic aureole adjacent to an intrusive rock mass.

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

In the metamorphic environment, metasomatism is created 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, metasomatism and metamorphism nearly always occur together.

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.

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). 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.

Metasomatic rocks can be extremely varied. Often, metasomatised rocks are pervasively but weakly altered, such that the only evidence of alteration is bleaching, change in colour or change in the crystallinity of micaceous minerals.

In such cases, characterising alteration often requires microscope investigation of the mineral assemblage of the rocks to characterise the minerals, any additional mineral growth, changes in protolith minerals, and so on.

In some cases, geochemical evidence can be found of metasomatic alteration processes. This is usually in the form of mobile, soluble elements such as barium, strontium, rubidium, calcium and some rare earth elements. However, to characterise the alteration properly, it is necessary to compare altered with unaltered samples.

When the process becomes extremely advanced, typical metasomatites can include:

• Chlorite or mica whole-rock replacement in shear zones, resulting in rocks in which the existing mineralogy has been completely recrystallised and replaced by hydrated minerals such as chlorite, muscovite, and serpentine.
• Skarn and skarnoid rock types, typically adjacent to granite intrusions and adjacent to reactive lithologies such as limestone, marl and banded iron formation.
• Greisen deposits within granite margins and cupolas.

Effects of metasomatism in mantle peridotite can be either modal or cryptic. In cryptic metasomatism, mineral compositions are changed, or introduced elements are concentrated on grain boundaries and the peridotite mineralogy appears unchanged. In modal metasomatism, new minerals are formed.

Cryptic metasomatism may be caused as rising or percolating melts interact with surrounding peridotite, and compositions of both melts and peridotite are changed. At high mantle temperatures, solid-state diffusion can also be effective in changing rock compositions over tens of centimeters adjacent to melt conduits: gradients in mineral composition adjacent to pyroxenite dikes may preserve evidence of the process.

Modal metasomatism may result in formation of amphibole and phlogopite, and the presence of these minerals in peridotite xenoliths has been considered strong evidence of metasomatic processes in the mantle. Formation of minerals less common in peridotite, such as dolomite, calcite, ilmenite, rutile, and armalcolite, is also attributed to melt or fluid metasomatism.

Investigation of altered rocks in hydrothermal ore deposits has highlghted several ubiquitous types of alteration assemblages which create distinct groups of metasomatic alteration effects, textures and mineral assemblages.

• Propylitic Alteration is caused by iron and sulfur-bearing hydrothermal fluids, and typically results in epidote-chlorite-pyrite alteration, often with hematite and magnetite facies
• Albite-epidote alteration is caused by silica-bearing fluids rich in sodium and calcium, and typically results in weak albite-silica-epidote
• Potassic Alteration, typical of lode gold deposits, results in production of micaceous, potassic minerals such as biotite in iron-rich rocks, muscovite mica or sericite in felsic rocks, and orthoclase (adularia) alteration, often quite pervasive and producing distinct salmon-pink alteration vein selvages.

Rarer types of hydrothermal fluids may include highly carbonic fluids, resulting in advanced carbonation reactions of the host rock typical of calc-silicates, and silica-hematite fluids resulting in production of jasperoids, manto ore deposits and pervasive zones of silicification, typically in dolomite formations.

• Luth, R. W., Mantle volatiles -- distribution and consequences. In The Mantle and Core (ed. R. W. Carlson) Volume 2 Treatise on Geochemistry (editors H. D. Holland and K. K. Turekian), Elsevier-Pergamon, Oxford, pages 319-361 (2003). ISBN 0-08-043751-6

• Hydrothermal
• Ore genesis
• Greisen
• Skarn
• Hornfels