Some Theoretical Aspects of Contamination in Acid Magmas

Contamination in acid magmas is intimately connected with the volatiles which they contain. The chief function of these volatiles is to form a medium of low viscosity between the xenolith (or margin) and the magma through which material can diffuse with comparative freedom. This diffusion of substance to and from a xenolith constitutes reciprocal reaction. Its object is to stabilise the mineral assemblage of the xenolith so that its mineral phases are in equilibrium with the contaminated magma. The material gained by reciprocal reaction is incorporated in phases which crystallise from the contaminated magma. But material is also gained by the magma through other methods which may be loosely described as mechanical and which disintegrate the xenolith so that their constituent minerals are set free in the magma. So far, three such methods have been noted.