Metasomatism

Introduction

  • It is the type of alteration which takes place in rocks by the chemical reactions generated by hydrothermal or residual fluids.
  • Metasomatism is defined as metamorphism accompanied by changes in whole-rock composition.
  • Because volatiles are so readily released and mobilized during metamorphism, changes in the volatile content of rocks are generally excluded, and the chemical changes that constitute metasomatism (sensu-stricto) are usually restricted to the redistribution of nonvolatile species.
  • Metasomatism is most dramatically developed in situations where rocks of highly contrasting composition are juxtaposed and elements move easily. Common examples include:
    1. Shallow plutons
    2. Layers, lenses, and pods with contrasting composition in rocks undergoing metamorphism.
    3. Veins where fluids equilibrate with one rock type and then migrate into a contrasting rock type.
  • It is the type of replacement process in rocks by which one or more than one minerals can be displace by the another minerals present in the fluid thus helps in changing the mineralogy of the source rock.
  • In the rock the dissolution and deposition both occurs together.
  • The hydrothermal fluids acts upon the source rocks belong to igneous and metamorphic origin.
  • Skarns and greisens are produced in the metasomatic igneous rocks while hornfels are produced due to contact metamorphism.
  • The mass transfer is take place in metasomatism if taken place in metamorphic environment from the high stress to the lower stress conditions.
  • Metasomatism shows the open system behaviour and differs from the metamorphism which is in-situ in nature.
  • The more complex metasomatism is seen in earth’s mantle.
  • The infiltration of carbonate and silicate melts and volatiles can change the composition of peridotite found in high temperature.
  • Metasomatism leads to metasomatic replacement textures.

The Role of Fluids in Metamorphism

  • Fluids are a critical participant in metamorphic and igneous processes. Aqueous fluids reduce the melting point of rocks and enhance melting. They are also released by rising and cooling magmas where they generate pegmatites and ores. Mixtures of exhaled and meteoric fluids feed hydrothermal systems above plutons and in shallow permeable areas of regional metamorphism.
  • Fluids can dissolve material, transport heat and solutes, precipitate minerals, exchange components as they react with minerals, and catalyze deformation processes by weakening rocks. Contact metamorphic aureoles are largest where fluids are available to transport heat and matter.
  • Release and flow of fluids absorb and transfer large quantities of heat and matter. The absorption and transfer by fluids of heat alone may have profound effects upon the temperature distribution in a contact aureole or even throughout an orogenic belt, and hence upon the geologic evolution of an area.

Agent for metasomatism

Some of the possible agents of metasomatism include:

  • Volatile primary magmas, such as kimberlite and carbonatites that stall at the resistant lithosphere or the solidus ledge where they solidify and released an enriched volatiles phase into the wall rocks.
  • Supercritical volatile fluids from the deep asthenosphere or subduction zones.
  • Silicate melts
  • H2O or CO2 fluid

Metasomatic Processes

Two principal processes of mass transfer in rocks: diffusion and infiltration.

  1. Diffusion is the process by which components move through another medium, either a solid (lattice diffusion) or a stationary fluid.
  • Diffusion is driven by chemical potential (μ) gradients.
  • If the chemical potential of a species is higher in one area than in another, migration of that species from the area of higher μ to that of lower μ will lower the free energy of the whole system.
  • Matter will thus tend to migrate down chemical potential gradients if there is nothing to inhibit such migration.
  • Bimetasomatism refers to the diffusion of components in opposite directions in a reciprocal or exchange fashion.
  1. Infiltration refers to passive mass transfer of solute species carried in a moving fluid medium.
  • Whereas diffusion occurs in response to internal chemical potential gradients, infiltration is driven by external fluid pressure gradients that cause the fluids to move.
  • Fluids in motion permit much more extensive transport of matter than does diffusion.
  • Regional-scale mass transport is not possible via diffusion alone and can only occur with the aid of infiltrating fluids.
  • Permeability plays an important role here.

Types of metasomatism

  • Auto-metasomatism: It is a type of metasomatism that occurs at the top of magmatic bodies during the early postmagmatic stage.
    • Typical autometasomatic processes, for example, are albitisation in granitic plutons and serpentinisation of ultramafic rocks.
  • Contact metasomatism: It is a type of metasomatism that occurs at or near to the contact between a magmatic body and another rock.
    • It may occur at various stages in the magmatic evolution.
    • Endocontact zones develop by replacement of the magmatic rocks.
    • Exocontact zones are formed by replacement of the host rocks.
    • Boundary metasomatism: It is a type of metasomatism that occurs at the contact between two rock types.
    • Bimetasomatism: It is a variety of the contact metasomatism, which causes replacement of both the rocks in contact due to two-way diffusion of different components across the contact.
    • Near-vein metasomatism: It is a type of diffusional metasomatism, which forms symmetrical metasomatic zonation on either side of an infiltrational metasomatic vein (or a vein infilling).
    • Regional metasomatism: It occupies great areas developing in various geological situations. It commonly forms alkaline metasomatic rocks during the magmatic and early postmagmatic stages.

Examples of Metasomatism

  • Chlorite and mica whole rock replacement: The recrystallisation and chemical alteration change the rock mineralogy by chlorite, muscovite and serpentine.
  • Greisen: Metasomatism takes place in granite.
  • Skarns: When the igneous intrusion comes in contact with limestone or marl.
  • Rodingite: It is the metasomatic rock composed of grossular-andradite garnet and calcic pyroxene; vesuvianite, epidote and scapolite.
  • Fenite: It is the carbon dioxide alteration takes place in felsic and mafic rocks.