Assimilation (geology)

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As assimilation or assimilation in the are geology processes referred to in which a magma receives its surrounding rock, either in the form of a chemical reaction or physically by melting. If the chemical composition of the magma is significantly changed by the uptake, one speaks of the formation of a hybrid magma or contamination.

Physico-chemical basics

Magma rising within the earth's crust is usually not in equilibrium with its host rock. The possibility of melting the adjacent rock is limited by the high energy requirements of the melting process. It is therefore only possible to (partially) melt rocks with a liquidus temperature below that of the magma. On the other hand, chemical reactions can also be initiated below this temperature. Assimilation processes therefore usually do not play a major role in petrogenesis .

proof

Assimilation of quartz into basalt
Incomplete assimilation and reaction of quartz in a basaltic magma (thin section, LPL): Round fused quartz is surrounded by glass, at the outer border of which augite is formed by reaction with the basaltic magma
Incomplete assimilation and reaction of quartz in a basaltic magma (thin section, XPL): quartz is white, the edge of the glass remains dark, augite in bright colors.

Assimilation processes are easiest to recognize when they are incomplete and the former host rock is present in the form of clods or xenolites or ocelli in the magmatite formed after the magma has cooled , or when the melted components have not completely mixed with the magma and the rock giving it a streaky appearance. Such a magmatite is also referred to as "immature". On the other hand, if the assimilation is complete, it cannot necessarily be seen macroscopically. It can then only be based on geochemical signatures, e.g. B. special isotope ratios of rare elements in igneous rock that originally come from the adjacent rock can be detected.

Hybrid magmas and corresponding rocks can, however, alternatively be formed by the process of magma mixing .

Economical meaning

According to one theory, the phenomenon of sunburner basalt , which helps determine the economic value of a basalt deposit, is based on an incomplete assimilation of SiO 2 -containing secondary rock by a basaltic magma and the resulting inhomogeneities in the cooled basalt.

Individual evidence

  1. ^ Lexicon of Geosciences. Volume 1. Spectrum Akademischer Verlag, Heidelberg, 2000, p. 135
  2. ↑ a b M. Okrusch, S Matthes: Mineralogie. 8th edition. Springer Verlag, Berlin, 2009, pp. 253-254
  3. ^ Gregor Markl: Minerals and Rocks. 2nd Edition. Spektrum Akademischer Verlag, Heidelberg, 2008, pp. 499–500
  4. ↑ U. Schreiber: New research results on the characteristic patchy weathering phenomenon of basatic rocks (basaltic "sunburn") . In: New Yearbook for Geology and Paleontology, monthly books . No. 6 , 1990, pp. 353-366 .