assimilation

assimilation Assimilation is the process of physical or chemical incorporation of foreign material by a magmatic body that results in a chemical change in the composition of the original liquid. The typical image that this process invokes is that of a magma chamber with bits and pieces of the enclosing country rock or wall rock floating in the magma. If these pieces of wall rock, like ice in a beverage, can melt or, like mud in water, are fine and so widely distributed that they are inseparable, then the resulting liquid will be compositionally different from the original one.

Xenoliths, which are pieces of wall rock, or xenocrysts, which are single mineral grains that originate from xenoliths, can commonly be recognized at outcrops of magmatic rocks. The presence of xenoliths or xenocrysts is generally used as evidence that assimilation could have occurred in the magmatic body. However, the existence of foreign material alone is only circumstantial evidence of assimilation, since their mere presence does not prove conclusively that any of the included material has in fact contaminated the primary magma. Resorption textures, which are features that are suggestive of dissolution of xenoliths or xenocrysts, are more persuasive textural evidence for assimilation. A convincing example of a resorption texture would be minerals such as quartz or alkali feldspar with lobate or embayed grain boundaries in basalt. These minerals could not have crystallized at high temperatures in a magma of basaltic composition, which indicates that they represent foreign material, and their lack of crystal faces or angular broken surfaces strongly suggests resorption or dissolution of the minerals.

In the first half of the twentieth century, the cause of variations in igneous rock compositions was an important topic of debate, and the process of assimilation was one of the explanations that enjoyed many advocates. At that time it was thought that superheated magmas could assimilate large amounts of the enclosing country rock as they moved through the crust. The assimilation of carbonate-rich rocks was proposed as a mechanism of desilicification that could explain the origin of silica-deficient alkaline rock suites, and assimilation of quartz-rich pelitic rocks has been invoked to explain the compositional variation between alkaline and sub-alkaline basalt. The main lines of evidence supporting the modification of the major-element compositions through assimilation were field occurrences of xenoliths with compositions that were deemed appropriate to produce the observed range of magma compositions. As the century progressed, knowledge of the heat contents, latent heats, and melting temperatures of rocks and minerals improved. As a result, it became clear that, even under the most favourable conditions, the volume of assimilated country rock relative to the volume of the host magma was far too small to account for the magnitudes of chemical variations in major elements that are observed. The process of assimilation cannot therefore account for large major-element chemical variations, although chemical modification by remelting may locally alter magma compositions at the borders of large magma bodies.

Despite its limited role in modifying major-element compositions, assimilation can significantly affect the concentrations of radiogenic isotopes and trace elements in magmas. Among common igneous and sedimentary rock types, the concentrations of these elements can vary by several orders of magnitude. As a consequence, the assimilation of only small amounts of material that is highly enriched in a particular trace element can profoundly alter the composition of a magma that is highly depleted in the same element. Other phenomena related to assimilation are contamination, magma mixing, and the AFC process (simultaneous assimilation and fractional crystallization).

J. C. Schumacher