Geochemical Processes

processes of change in the chemical composition of rocks and minerals, as well as of melts and solutions from which the rocks were formed. Geochemical processes lead to the migration of chemical elements (removal of some, introduction and concentration of others), changes in their valence states, and so on. They may be subdivided into geologically prehistoric, endogenic, exogenic, and metamorphogenic categories.

The geochemical processes of geological prehistory are associated with the formation of the earth as a celestial body. Endogenic processes start with melting of the magma from the upper mantle and its degasification and differentiation. The nature and degree of magmatic differentiation are determined by the set of a series of physicochemical processes (temperature drop, separation of volatiles, assimilation, crystallization and gravitational differentiation, and so on), as a result of which rocks differing in composition and structure and having different quantitative ratios for the same minerals form from the magma. When basic and ultrabasic magmas cool, primary compounds of iron, magnesium, calcium, chromium, and titanium, as well as platinum and metals of its group, are the first to pass from the melt to the solid phases. The products of the first stage of crystallization are ultrabasic and basic rocks (dunites, peridotites, gabbro, pyroxenites, and so on) and ore minerals associated with them (magnetite, chromite, titanomagnetite, and others), which sometimes form commercial deposits. As a result of the separation of these elements from the magma, it becomes more acid, approaching dioritic magma in its composition. In the course of further crystallization the magma becomes rich in silicon, aluminum, alkali metals, and volatile elements and becomes close to granitic magma in composition. Crystallization of granitic magma gives granites and a residual pegmatitic melt whose solidification gives pegmatitic veins, which are often enriched by rare-element minerals. Interaction between volatiles and rock that has already crystallized results in auto-metamorphic processes. Large amounts of alkali metals in the residual melt give rise to the phenomena of alkaline metasomatosis, often with addition of rare elements, and the conversion of granodiorites and granites to alkaline granites, syenites, and nephelene-syenites. Processes of skarn formation, greisening, propylitization, beresitization, serpentinization, listvenitization, and formation of hydrothermal deposits of copper, lead, silver, zinc, tin, tungsten, gold, and other metals take place with participation of vapors, gases, and hot solutions that have separated from the magma (postmagmatic). The following forms of metasomatosis occur under the influence of solutions of various compositions: alkali, calcium, magnesium-iron-silicate, chlorine-fluorine-boron, and carbonate.

Exogenic geochemical processes embrace all forms of weathering of rocks and the minerals of which they are composed (decomposition, oxidation, hydration, carbonate formation, and so on) that take place in a damp climate with the participation of soil acids or in a dry (arid) climate in an alkaline medium with marked predominance of oxidation reactions. The products of weathering are primarily transported by water flows to oceans, seas, and continental reservoirs (lakes) as mechanical suspensions and true and colloidal solutions. Because of the absorptive capacity of soils and sorption of elements by clays, the compositions of solutions undergo changes; microorganisms play a large part in this process. Chemical differentiation of elements occurs in marine reservoirs: aluminum-bauxite ores are deposited on the shores; ores of iron, manganese, and phosporites are deposited farther out; and limestones and dolomites are deposited even farther out. As a result of processes of coagulation and dehydration, the precipitates thus formed become rock at a stage of early diagenesis, and under the influence of redistribution of substances without addition from without, the formation of concretions takes place at the stage of later diagenesis.

Further chemical change of sedimentary rocks takes place under the influence of the addition of substances from without, as well as of an increase in temperature and pressure when rocks sink to considerable depths.

Metamorphism leads to a deeper transformation of rocks with recrystallization. Various metamorphic rock facies— green slates, epidote-amphibolite, hornblende-gabbro, pyroxene-hornfels, granulitic, and eclogitic—are formed, depending on temperature and pressure. At sufficiently high temperature and pressure, migmatization (the passage of substances into a viscous state preceding fusion) occurs, which closes the cycle of geochemical processes.