Role of boric acids in the formation of pegmatite and hydrothermal minerals: Petrologic consequences of sassolite (H3BO3) discovery in fluid inclusions

Inclusions of boric acid solutions with daughter sassolite crystals were fo und for the first time in a number of minerals from miarolitic pegmatites o f 11 world regions: four localities in central and eastern Transbaikalia, t he middle Urals, the central and southwestern Pamirs, Afghanistan, Pakistan , Nepal, California, Elba and Madagascar islands. Experimental data on the properties of aqueous boric acid solutions were summarized and a method was proposed to calculate the concentration of boric acid from the temperature of sassolite dissolution. The peculiar properties of inclusions with berat e-bearing solutions were described for the first time. They stem from the p olymerization of orthoboric acid and other properties of boron-bearing aque ous solutions. The inclusions contain aqueous or water-carbon dioxide solut ions of boric acid with Na, K, and Ca (sometimes Fe, Mg, Li, and Cs) chlori des, bicarbonates, fluorides, and probably minor amounts of other compounds . Boric acid solutions occurred in a wide range of mineral-forming conditio ns. The homogenization temperatures vary from 450 to 90 degrees C; fluid pr essure, from 1480-40 bar; H3BO3 Concentration, from 26.5-2.4 wt %; and sali nity, from 38.2-1.0 wt % NaCl equiv. The water-carbon dioxide inclusions co ntain 20.2-3.5 wt % CO2. It was found that concentrated boric acid fluids p layed an active role in the mineral-forming processes in zones near and dir ectly in cavities of miarolitic pegmatites independent on the mineral compo sition, association, age, and geologic setting of pegmatites. An inference was substantiated on the crystallization of minerals of tourmaline-bearing assemblages in granitic pegmatites and other rocks of a hydrothermal origin under the influence of concentrated boric acid solutions. First, conclusiv e evidence was obtained on the participation of concentrated boron-bearing fluids in the processes of hydrothermal tourmaline formation. The petrologi c consequences of the discovery of such solution are discussed with applica tion to the understanding of granitic pegmatite genesis. It is suggested th at inclusions of boric acid solutions will be found in all tourmaline-beari ng and some topaz-beryl miarolitic pegmatites of the world, in the USA, Bra zil, Afghanistan, Pakistan, Nepal, Africa (Tanzania, Mozambique, Nigeria, Z imbabwe, and Namibia), Madagascar, and Elba. It is highly probable that sim ilar inclusions will be found in minerals from various boron-bearing hydrot hermal rocks, greisens, hydrothermal associations, and products of volcanic exhalations. The importance of sassolite discovery for boron isotope geoch emistry is emphasized. A method was proposed for the determination of the f actor of boron isotope fractionation between solid phases and mineral-formi ng fluid using water extracts from minerals.