Superliquidus differentiation of fluid-bearing magmatic melts under reducing conditions as a possible mechanism of formation of layered massifs: Experimental investigations

Previous experimental work (Bezmen, 1992; Bezmen and Elevich, 1998) demonst rated that, at certain critical thermodynamic parameters (temperature, pres sure, and fluid phase composition), melts become unstable and show cryptic or contrast layering. Layering in ultrabasic melts, separation from silicat e melts of ore liquids enriched in chromite, ilmenite, and apatite were obt ained under superliquidus conditions at H-O-C-S fluid pressure, The proport ions of gases in the fluid phase were specified to provide the closest appr oach to the compositions of natural fluids. At constant thermodynamic param eters and the absence of temperature gradient, fluid-bearing melts exhibit liquid-state layering, which develops gravitationally on a macromolecular s cale. An increase in the duration of experiments results in a stronger cont rast and appearance of layers with new compositions. The transmission elect ron microscopic investigation of quench glasses revealed ellipsoid-shaped i nclusions with a crystalline structure and diffuse outlines, 6 nm (60 Angst rom) and more in size. It is supposed that these are cores of clusters, whi ch occur in strongly depolymerized fluid melts. The formation of clusters i s a consequence of the fluctuation quasi-crystalline structure of magmatic melts. According to modern data obtained in situ (Cohen and Knight, 1990), clusters are a transitional state of matter between liquid and crystal. The cluster is composed of an ordered core and a shell consisting of ligands ( Tredoux et al., 1995). The latter provide cluster stability in time. Atoms in the ligand shell are more mobile and the structure as a whole is a pseud ocrystalline core with a liquid-like surface. High-pressure experiments dem onstrated that the presence of a fluid phase similar in composition to natu ral magmatic fluid provides necessary conditions for the gravitational move ment of clusters and their aggregates. The liquid-state cluster differentia tion of melts allows us to explain a number of issues in the evolution of d ifferentiated complexes including the nature of cryptic layering, rhythmic structure of layered sections, selective concentration of ore components by melts, development of fine-grained and homogranular textures, concentratio n of dense minerals in the upper portions of massif sections, formation of monomineral rocks and massive ores, and others.