Geochemistry of Sc in the magmatic process: Experimental evidence

Data on Sc distribution between aluminosilicate melt and fluoride phases (cryolite in the Na-K parts of the system and fluoride melt in its Li-bearing parts) were experimentally obtained for the Si-Al-Na-K-O-H-F system at 800, 750, and 700°C and $ P_{H_2 O} $ = 1000 bar, at the saturation of the aluminosilicate melt with respect to H₂O and F. The Sc partition coefficients between aluminosilicate melt and fluoride phases vary from 0.005 to 1.6 depending on phase relations in the system, which are, in turn, controlled by its bulk composition and experimental conditions. At 800°C, the Sc partition coefficients between aluminosilicate melt and fluoride phases are lower than 1 in all of the examined parts of the system; i.e., Sc is preferably distributed into the fluoride phase regardless of whether it is crystals of (Na, K-Na, or K)-cryolite or alkali-aluminofluoride melt. This is the fundamental difference of Sc behavior from those of REE, and most other elements, under the same conditions. A temperature decrease to 700°C and the transition from quartz-normative to nepheline-normative melts, as well as an increase in their agpaitic coefficient, leads to an increase in the Sc partition coefficients to values greater than 1. This tendency is related to an increase in the degree of depolymerization of the aluminosilicate melt. Sc is demonstrated to be characterized by strong affinity to F and can substitute up to half of Al atoms in the cryolite structure, and Sc is even more significantly incorporated into Li-bearing aluminofluoride melts. The high chemical affinity of Sc to F and the ability of the former to produce fluoride complexes in residual magmatic melts is one of the main reasons for Sc enrichment in pegmatites and high-temperature postmagmatic metasomatic rocks: greisens, skarns, and albitized granites. The data obtained on Sc behavior in the sequence of zones of reaction column developing during the interaction of dolerite with granitic melt do not validate the idea that Sc is selectively extracted during the migmatization and assimilation of host rocks.