Fractional crystallization (petrology)

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Scheme

In the fractional crystallization is a term used in petrology related to the rock formation.

definition

Fractional crystallization is a process that often takes place during what is known as magmatic differentiation . Minerals such as olivine and pyroxene crystallize earlier than others and sink down in the magma and are retained. The crystallization of these minerals changes the composition of the remaining liquid residual magma (the so-called fluid phase).

compatibility

A measure of the incorporation or retention of elements in the fluid or solid phase is incompatibility / compatibility. In order to make statements about the crystallization conditions of magmas, their trace element concentrations are often examined. The so-called mineral separation coefficient and the mass separation coefficient, which indicate the ratio of the concentration in the mineral (or its paragenesis) to the concentration in the liquid, play a decisive role. If this quotient is greater than 1, one speaks of compatible elements, if it is less than 1, it is called incompatible trace elements. Rubidium and zircon are examples of highly incompatible trace elements in many magmas. If one looks at the extent of the crystallization, then via the ratio of the mass of the liquid that remains after the process to the mass at the beginning ('daughter' to 'parent magma'). According to Rayleigh's fractionation equation, the compatible trace elements decrease most rapidly with a large separation coefficient and incompatible elements increase as the differentiation progresses. Different concentrations of incompatible elements, for example in MORB (mid-ocean ridge basalt) and OIB (oceanic island basalt), taking into account the size of the enamel fractions, allow conclusions to be drawn about a similar or different genesis of the basalt magma.

causes

There are several reasons for this process to start:

  1. In gravitational differentiation, the crystals that have already formed, such as olivine and pyroxene, sink to the bottom of the magma chamber due to their greater density and are withdrawn from the melt, so that it can no longer react with the crystals and is depleted in certain chemical substances. The crystals from the early cooling stages accumulate at the bottom of the magma chamber as Kumulat on.
  2. If a tectonic deformation takes place during the crystallization process , the liquid melt can be pressed out of the crystal slurry that has already formed. Crystals gradually form from the cooling magma , which are then separated from it.
  3. In the flow crystallization process, crystals are withdrawn from the melt during its ascent, which have previously crystallized on the cooler walls of the ascent channel.

See also

literature

  • Myron G. Best: Igneous and Metamorphic Petrology . WH Freemann & Company, San Francisco 1982, ISBN 0-7167-1335-7 , pp. 45 ff .
  • Stephen Blake, Tom Argles: Growth and Destruction: Continental Evolution at Subduction Zones , The Open University, Walton Hall, Milton Keynes, 2003 ISBN 978-0-7492-5666-1