Magmatic differentiation

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The term magmatic differentiation describes the formation of different partial magmas from a so-called trunk magma by changing the chemical composition. This concept comes from a sub-area of geology , endogenous dynamics , which deals with the magmatic processes in the earth's interior. It describes the phenomenon that from a liquid rock melt (magma) with a certain chemical-mineralogical composition (parent magma), a certain igneous rock with the same composition does not just have to crystallize , as would be expected, but that a whole series of different rocks from it with different compositions, i.e. different mineral components, can arise.

Separation by gravity and crystallization

Even in the molten state, immiscible components, such as sulfidic and oxidic phases of a melt, can separate from one another, similar to how oil separates from vinegar (liquid mixture or liquation ). More often, however, is the separation of early crystallized minerals from the remaining partial melt (also called residual ), in which the minerals sink under their own specific weight to the bottom of the magma chamber (segregation of gravity or gravitational differentiation ). Already Charles Darwin had suggested in 1844 that the partial magmas thus created by tectonic can be pressed movements from each other and separated. In the 1920s, Paul Niggli and Hans Cloos developed the idea that the solid and liquid, or immiscible, components of a melt separate, especially when the magma rises to the surface of the earth ( movement differentiation ).

The concept of magmatic differentiation through fractional crystallization was specially developed by Norman Bowen and is based on the fact that rock melts or magmas are multi-component or multi-substance systems , the individual components of which have different melting points . When a magma cools down, the component with the highest melting point crystallizes out first and sinks down in the magma chamber due to its higher density. From the remaining partial melt, the residue , that component with the next higher melting point crystallizes out on further cooling, which in turn drops downwards; thus, the residual melt constantly changes its chemical composition with continuous cooling .

Basaltic melts

Diagram according to Bowen (1928), which illustrates the theoretical development and reaction scheme of the cooling of a silicate , very basic and SiO 2- poor magma. As a result of the crystallization of the mineral phases with a high melting point, the chemical composition of the magma shifts, at least theoretically, towards SiO 2 enrichment.

In basaltic melts, a distinction is made between the continuous series and the discontinuous series .

In the continuous series, the differentiation begins with a calcium-rich plagioclase , the end member of the feldspar solid solution anorthite . The further the magma cools or the more anorthite is deposited, the further the mixed crystal system shifts to the sodium-rich end member albite . At the end of the differentiation only albite is deposited. The discontinuous series describes the differentiation of different minerals according to their melting point. First olivine with the highest melting point, then pyroxene , then amphibole and finally biotite with the lowest melting point is deposited. These two series run parallel to each other and describe the differentiation from a mafic (basic) melt to a rockic (acidic) residue, from which the minerals potassium feldspar , muscovite and quartz finally crystallize.

In general, in the course of magmatic differentiation, the contents of magnesium , iron and calcium in the residual magma decrease and those of silicon , sodium and potassium increase. That is, first of all melting the basaltic mafic crystallize and intermediate minerals and a felsic residual magma is generated that ultimately a granitic solidifies rock.

Example: Bushveld Complex in South Africa

The most prominent example of Differentiationsvorgänge is in South Africa located Bushveld Complex . This basaltic intrusive complex has a rock variation from ultrabasic to granitic composition and is characterized by its very well-developed layer texture , which looks amazingly similar to the stratification of sedimentary rocks . It reflects the various differentiation cycles of the Bushveld. The basic to ultra-basic parts were created by differentiation of a tholeiitic magma, the granites are regarded as melted and contaminated crustal material.

See also

further reading

  • Martin Okrusch, Siegfried Matthes: Mineralogy: An introduction to special mineralogy, petrology and deposit science. 7th revised edition, 2005, 522 pages, ISBN 978-3-540-23812-6

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