Magma mixture

In geology , magma mixing is the term used to describe processes in which a differentiated magma is changed in its composition by the addition of a differently composed magma into the magma chamber .

Mechanisms

A distinction is made between two different processes:

• the “magma mingling” , in which one type of magma can be distinguished from the other in the cooled rock; and
• the "Magma mixing" , which is a unified in a complete homogenization of both Magmentypen and forming hybrid magma leads.

Similar to assimilation , the energy balance also plays a role in magma mixing: two types of magma can basically be completely mixed with one another; But the situation can change if one magma is significantly hotter than the other, because “colder” magmas often show reduced miscibility due to increased viscosity.

Appearances in the terrain

Composite dike near Streitishvarf in East Iceland: rounded inclusions of basalt in rhyolite

The effect is often noticeable when a differentiated magma in a magma chamber has already had the opportunity to cool down and a burst of fresh, hot magma flows into the chamber. It supplies heat to the cooled magma, which reduces its viscosity, and is itself cooled. In such cases, inclusions of the magma with the higher liquidus temperature can be found in the cooled rock in the magma with the lower, whereby these inclusions show through their rounded outlines that the mixing took place in the liquid state. The inclusion-forming magma may also show signs of thermal deterrence at the edges of the inclusions. Examples of such processes can be found in the "composite dikes" in the east of Iceland , where there are numerous dike-shaped deposits of rhyolite in which numerous large and small basalt "drops" are enclosed.

If the liquidus temperatures of the two magmas are sufficiently far apart, one can already be in a solid state, while the other is still sufficiently thin to penetrate into fine crevices that break open in the already solidified magma. The result is a so-called "net-vein complex", in which angular fragments of the previously solidified magma are traversed by a network of ducts of the later solidified.

If the two magmas are completely hybridized, signs of this are most likely to be found in the fact that crystals precipitated from the melt before the mixture, which were in equilibrium with their original magma, are now in an imbalance with the hybrid magma and are either reabsorbed or continue to grow with a different composition (zoned crystals). This explains the formation of the Rapakiwi structure in the so-called granites, where early precipitated potash feldspars were overgrown by plagioclase .

Individual evidence

1. ↑ M. Okrusch, S Matthes: Mineralogie. 8th edition. Springer Verlag, Berlin 2009, p. 250
2. ↑ Ron H. Vernon: A Practical Guide to Rock Microstructure. Cambridge University Press, Cambridge 2004, pp. 126-134
3. ↑ Ron H. Vernon: A Practical Guide to Rock Microstructure. Cambridge University Press, Cambridge 2004, p. 484
4. ↑ Ari Trausti Gudmundsson: Living Earth. Facets of the geology of Iceland. Mál og Menning, Reykjavik 2007, pp. 50-52
5. ^ David Shelley: Igneous and Metamorphic Rocks under the Microscope. Chapman & Hall, London 1993, pp. 240-241
6. ^ David Shelley: Igneous and Metamorphic Rocks under the Microscope. Chapman & Hall, London 1993, pp. 241-242