a conjunction of successive geochemical processes during which elements, after a series of migrations, return to the initial state. For the earth’s crust the main geochemical cycle comprises the processes of magmatic differentiation, crystallization with the formation of magmatic rock, postmagmatic rock conversion by endogenic fluids (if it occurs), weathering, water transport of material with chemical differentiation and distribution of substances over facies during precipitation in marine basins, early and late diagenetic processes with formation of sedimentary rock, epigenetic change and metamorphism during subsidences under depositing sediments, and the formation of granite-gneisses and granites (often difficult to differentiate from granites derived from a magmatic melt, particularly if the metamorphosed sedimentary rock has undergone melting) under the action of granitizing fluids.
Geochemical cycles can also be traced for individual chemical elements. These geochemical cycles can be complicated by a biogenic cycle: the element is extracted from the soil or sedimentary rock by plants, the plants are consumed by animals, the plants and animals die, and the element returns to the sedimentary rock, continuing its geochemical cycle. The term “geochemical cycle” was proposed by A. E. Fersman in 1922.
REFERENCEFersman, A. E. Geokhimiia, vol. 2. Leningrad, 1934.
V. V. SHCHERBINA