Zone Fusion

the hypothetical process of melting and degassing of the matter in the earth’s mantle; a process analogous to the mechanism of zone melting, which leads to the formation of the earth’s envelopes (the lithosphere, hydro-sphere, and atmosphere).

The hypothesis of zone fusion was proposed by the Soviet scientist A. P. Vinogradov in 1955 to explain the shell-like structure of the planet and the regularities of the distribution of chemical elements in the earth’s crust. The Soviet scientists V. A. Magnitskii (1964) and A. N. Tikhonov and co-workers (1969) have given a physicomathematical substantiation of the process. According to the hypothesis, in the early

stage of the earth’s evolution individual fused magmatic chambers, whose subsequent chemical differentiation according to the laws of zone melting leads to the division of the original matter into high-melting and low-melting phases, formed at various depths in the mantle (whose composition is close to that of stony meteorites), primarily in the astheno-sphere, as a result of warming by the heat generated by radioactive elements. The low-melting phase moves upward, toward the surface of the earth, according to the principle of zone melting. The physical cause of the upward movement of the molten matter is the convective instability of the radially extending molten masses in the planet’s gravitational field. This instability leads to the development of convective fluxes in the liquid. A consequence of these motions is intensified heat transfer within the fused chamber from the bottom up-ward, which leads to the relative cooling of the liquid metal and its crystallization in the lower parts of the chamber and to relative heating and melting of the roof rock in the upper parts. The upward movement of the molten matter according to the principle of zone melting is accompanied by a change in the composition of the liquid metal and by its enrichment with elements and compounds that lower the melting point of the system (the “low-melting” components, including “volatile” components). The remaining solid phase, which may not pass through a stage of complete melting, is enriched with “refractory” elements and compounds that raise its melting point. Thus, the uplift of the liquid metal leads to chemical differentiation of the matter in the mantle and the evacuation to the earth’s surface of the substances that are concentrated in the earth’s crust, hydrosphere, and atmosphere. According to the physicochemical laws governing the crystallization of silicate systems, the liquid matter that is evacuated from the bowels of the earth and that forms the crust is relatively rich in silicon, aluminum, potassium, sodium, calcium, uranium, thorium, strontium, barium, rubidium, and many other (lithophilic) elements. The remaining (“refractory”) matter of the mantle is made up primarily of magnesium and iron silicates, as well as compounds of nickel, chromium, and some other elements. The geochemical laws governing the distribution of chemical elements in the rock of the earth’s crust (granite and basalt), in dunites and peridotites (which make up the differentiated mantle), and in the silicate phase of stony meteorites (chondrites) correspond to the distribution of the elements in the process of zone fusion of the original undifferentiated mantle of chondritic composition.