HIGH BORON CONCENTRATION IN AN LI-F GRANITIC MELT

Abstract

Boron is a typical lithophile element characterized by a significant accumulation in the Earth's crust (15 ppm) relative to the primitive mantle (0.6 ppm) [1]. In magmatic systems, boron behaves as an incompatible element concentrating in melts and, to an even higher extent, in aqueous fluids. The maximum initial B content is observed in crustal anatectic melts. Their fractional crystallization leads to further boron enrichment of residual magmas. The boron content in the highly differentiated rhyolitic obsidians of Makkuzani (Peru) amounts to 0.62 wt % B2O3 [2]. High concentrations of boron and fluorine lead to a decrease in liquidus and solidus temperatures of silicic melts and an increase in viscosity and water solubility [3, 4]. Recently, high B concentrations were revealed in fluid and melt inclusions in quartz, tourmaline, and beryl from tourmaline-bearing granite pegmatites in different regions of the world [5, 6]. The highest B2O3 concentrations in the melt inclusions (average 5.9 ± 1.0 wt %, maximum 12.5 wt %) were revealed in quartz from pegmatite of the Malkhans pegmatite field [7]. In fluid inclusions, maximal H3BO3 concentrations (23-27 wt %) were revealed in quartz from pegmatites of the southwestern Pamirs and Elba, as well as in tourmalines from pegmatites of Madagascar and the Malkhansk field [5]. Sassolite from these objects is a widespread daughter phase in the fluid inclusions.