Metasomatism

Metasomatism is a metamorphic process in which a rock or part of a rock, while remaining in a solid state, is pervasively altered by the introduction or removal of chemical components as a result of its interaction with aqueous fluids (solutions). The term is derived from the Greek μετά (meta = beyond, beyond) and σώματος (somatos = body), meaning beyond the body, in the sense of a phenomenon superimposed on a pre-existing rock body. It was introduced by Naumann in 1826.

Metasomatism, metasomatic process, and metasomatosis are synonymous, although some authors use metasomatosis for specific varieties of metasomatism (e.g., Na-metasomatosis, Mg-metasomatosis, etc.). In the metasomatic process the “old” minerals are partially or totally replaced by new minerals of different chemism and in equilibrium with the new chemical and physical conditions in which the rock is found.

This process occurs without volume variations, it is manifested in diagenesis, during magmatic consolidation in the pneumatolitic and hydrothermal phases (autometasomatism), and in allo-chemical metamorphism. In metamorphism, metasomatism adds its effects to the substantial modifications due to physical factors: in dislocation metamorphism it is not important, in thermal metamorphism it favors the contribution and removal of matter and the recrystallization of new minerals, but it is especially significant in general metamorphism in which, by sialic contribution, micaschists and gneisses originate migmatites.

Chemical and physical aspects of metasomatism

The mechanisms of this process have similarities with those of metamorphism, but with a key role of the fluid phase. It becomes the primary cause of the reactions and not only the seat of the reactions. The fluid, in fact, is responsible for the contribution of new chemical species and the dissociation and removal of phases that become unstable in contact with the fluid. It follows that the greater the contribution of fluids, in terms of quantity and time, the greater the transformation of the rock.

These fluids can be of magmatic or metamorphic origin. In the first case, they are volatile-rich fluids or residual fluids; in the second, they are interstitial fluids produced by other metamorphic processes. The effectiveness of metasomatism is related to the reactivity and permeability of the rocks involved. So for high reactivity and low permeability, the metasomatic front will advance slowly, producing a narrow band with very strong transformations; on the contrary, a very wide band with slight transformations will be produced if the rocks are highly permeable and not very reactive.

Metasomatism is distinguished from other endogenous processes in the following respects (Zharikov et al., 1998):

  • By ion-for-ion substitution in minerals through mechanisms in which dissolution of a mineral occurs synchronously with precipitation of a new mineral so that a constant volume is maintained, in accordance with Lindgren’s (1925) rule of constant volume during metasomatism. A good example of metasomatism is the pseudomorphic replacement of a crystal of one mineral by another mineral (or a mixture of other minerals), with preservation of the original shape and volume;
  • by a group of processes that include filling of cavities or fractures, crystallization of magma, and adjacent magma-rock interaction, with preservation of the rock in a solid state during replacement (the volume of the pore-filling solution is negligible compared to the total volume of the rock). The chemical and mineralogical composition of magmatic rocks is uniform through much of the magma body, in contrast to metasomatic rocks that exhibit typical zoning;
  • by isochemical transformation or substantial changes in chemical composition by addition or subtraction of major elements other than H2O and CO2.
  • Changes in the concentration of water and/or carbon dioxide are normal in isochemical metamorphism, so hydration/dehydration or carbonation/decarbonation reactions are not specific to metasomatism and terms such as carbonate metasomatism or hydrometasomatism are to be abandoned. During the metamorphic process only H2O and CO2 are perfectly mobile (in the thermodynamic sense), whereas during metasomatic reactions other components of rocks or minerals may be perfectly mobile. The number of coexisting minerals in the metasomatic zones is normally less than in the replaced rocks, unless the starting rock was monomineralic;
  • by the formation of a regular set of zones. These zones form a characteristic pattern (metasomatic column) through the metasomatic body. The zonal pattern represents the chemical rebalancing between two rocks or between a rock and a filtering fluid (solution). In the case of diffusion metasomatism (see below), changes across zones are transitional while in the case of infiltration metamorphism, changes occur in jumps. The number of metasomatic zones in the column depends on the physicochemical conditions of the interacting media. In the simplest case it can be represented by a single zone. All zones in a metasomatic column are generated and grow simultaneously, increasing in thickness along the direction of mass transport.

A metasomatic column (or pattern of metasomatic zones) is the complete sequence of metasomatic zones that characterize an individual metasomatic facies.

Metasomatic rocks generally have a granofelsic or granoblastic texture. They may be fine- or coarse-grained and sometimes exhibit zoning that may be rhythmic.

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