Metasomatism

Metasomatism (from the Greek μετά metá "change" and σῶμα sôma "body") is the chemical alteration of a rock by hydrothermal and other fluids.

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

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.

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.

[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.

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

Formation of minerals less common in peridotite, such as dolomite, calcite, ilmenite, rutile, and armalcolite, is also attributed to melt or fluid metasomatism.

[11] Chemical weathering strongly effects the levels and contents of the metasomatic liquid and the major element geochemistry and mineralogy of siliciclastic sediments.