Evolution of the Fluoride–Calcium Melt Composition According to Experimental Data and Fluorite Formation in Rhyolites

Abstract

The paper reports the results of melting experiments on the fluorite-enriched rhyolites of the Nyalga Basin (Central Mongolia). The fluoride–calcium (F–Ca) melt was obtained in a wide range of P-T parameters (1250–750°С, 5.5–1 kbar). Fluoride–silicate liquid immiscibility was observed at F > 2.5 wt % and CaO > 5.3 wt % in the initial system. An increase of temperature and pressure was accompanied by a significant increase in the concentrations of REE, Y, Sr, P, Th, U, Nb, Co, Cu, Sn, Sb, and Mo in the F–Ca melt. The peculiarity of the DREE distribution coefficients between F–Ca and silicate melts can lead to the formation of M-type tetrad effects for the first, third, and fourth tetrads in the chondrite-normalized REE patterns of silicate melt. The F–Ca melt has existed up to subsolidus temperatures of the rhyolitic melt. None of the models of magmatic crystallization of fluorite in haplogranitic melts or subsolidus postmagmatic and hydrothermal fluoritization can explain the origin of fluorite-enriched rhyolites. It is suggested that these rocks were formed from magma containing the emulsion of rhyolitic and F–Ca melts. The fluoride–silicate liquid immiscibility resulted in the redistribution of trace elements (REE, Y, Sr, P, Zr, Hf, Ta, Nb, Sc, Li, Be, and Rb) between melts. The formation of rock matrix was accompanied by degassing of the rhyolitic melt. An effective viscosity of the magma (melts emulsion with fluid bubbles) is comparable with the viscosity of a liquid. The F–Ca melt was quenched into F–Ca phase consisting of submicron fluorite particles, while solidification of the rhyolitic melt and silicate glass devitrification resulted in the formation of quartz–sanidine symplectites. The F–Ca phase has the elevated contents of O, Sr, LREE, Y, Si, sometimes Sc, P, and Al. The isomorphous substitution of O2– → F– in the fluorite structure led to the formation of aggregates of oxygen-vacancy centers, which are responsible for the laser-induced luminescence of the F–Ca phase in the rhyolite matrix. The wide variations of the REE, Y, Sr, Th, Nb, Ta, Zr, and Hf contents in the F–Ca phase are related to its recrystallization under the effect of fluid, which was released during degassing of rhyolitic melt. The submicron-sized fluorite particles in the F–Ca phase during the interaction with fluid were gradually liberated from impurities (besides Sr) and transformed into larger crystalline segregations. It is suggested that the oxygenated F–Ca melt exists in a metastable supercooled state under oxidizing conditions during eruption of rhyolitic magma. This is inconsistent with previously obtained experimental data on crystallization of fluorite from CaF2- and H2O-saturated haplogranitic melts at < 950°С and 1–2 kbar. Using rhyolites as example, it is shown that fluorite and associated ore mineralization (monazite-group minerals, cerianite) were derived from F–Ca melt with elevated REE and Y contents. In many igneous rocks and magmatogenic ores, fluorite could be a product of transformation of the F–Ca melt.