Magma of pegmatites from Volhynia: Composition and crystallization parameters determined by magmatic inclusion studies

Recrystallized melt inclusions (relics of the pegmatitic magma) were studied in topaz from the chamber pegmatites of Volhynia, Ukraine. The melt inclusions homogenize into silicate melts, which form glasses upon quenching. The composition of pegmatitic magma was determined by means of an electron and ion microprobe, along with the technique of local infrared spectroscopy. The major-element composition of this magma is close to that of a strongly fractionated ongonitic melt poor in SiO₂ and rich in Al₂O₃ and F. The first direct data on the trace element contents of the homogenized melt inclusions indicate that the pegmatitic magma was rich in Li, Rb, Be, Sn, Nb, Cs, U, and Th, and poor in REE, Zr, Sr, Ba, and Ti. These geochemical features, typical of ongonites, are intrinsic to the pegmatitic magma rather than caused by overprinted processes. The physicochemical parameters of the pegmatitic magma are as follows: liquidus temperature 650-700°C, solidus temperature 530°C, pressure of the water-dominated fluid 2.6-3.0 kbar, magma was composed of melt, crystals, and fluid. The direct determination gave the following contents: 7 wt % H₂O, 157 ppm CO₂, 5.1 wt % F, 0.3 wt % Cl. Judging by the daughter minerals and inclusion textures, the melt inclusions are an approximate model of the chamber pegmatites. The assemblage of the daughter mineral association of the inclusions is essentially the same as the rock-forming and accessory minerals of the pegmatite, a fact indicative of their crystallization from the pegmatitic melt. Grains of the daughter minerals increase toward the contact with the fluid phase, and this provides evidence for the influence of fluid on the origin of the coarse-grained pegmatitic textures. The occurrence of syngenetic melt, crystal, and fluid inclusions suggest the boiling of the pegmatitic magma, from which all main and accessory pegmatite minerals crystallized. The magma was saturated in some minerals of Nb (columbite), Sn (cassiterite), W (wolframite), Be, and U. The partitioning coefficient of Cl between the fluid and melt was 33. The crystallization of pegmatitic magma can be approximated by the model for ongonitic magma, and the increase in Li, Rb, Cs, Sn, Nb, U, Th, H₂O, F, and Cl, and decrease in Ti, Zr, REE, Sr, and Ba contents were caused by fractional differentiation.