Boron coordination in haplogranite glasses

Abstract. The coordination of boron in silicate melts has been extensively studied in synthetic industrial glasses but rarely in natural volcanic glasses or their synthetic analogues. Because coordination is a controlling factor in the boron isotope exchange between melts and coexisting phases, it is important to close this knowledge gap. We synthesized a set of boron-rich (2 wt % and 5 wt % B2O3) haplogranite glasses with water content from 0 wt % to 7 wt % and a variable alumina–alkali ratio, expressed by the aluminum saturation index (ASI), the molar ratio of Al2O3 / (CaO + Na2O + K2O). Boron coordination was determined by 11B MAS-NMR analyses and is expressed as R(IVB), the ratio of tetrahedral (BO4) to tetrahedral and trigonal (BO3) groups. There is a first-order dependency of boron coordination on the ASI ratio within the studied range of 0.8 to 1.7. All glasses with ASI > 1.1 showed nearly exclusive trigonal boron regardless of water and boron concentration. The maximum value of R(IVB) was 7 %. Glasses with lower ASI values showed a steady increase in R(IVB) up to 90 % in a sample with ASI = 0.8. High water contents may favor formation of BO4 groups as suggested by other glass studies, but there are masking effects related to the quench rates that make this trend inconclusive. Our results concur with the few existing NMR studies of natural glasses that boron is dominantly in trigonal coordination in peraluminous melts. The trigonal coordination of boron as B(OH)3 in neutral to acidic aqueous fluids means that there should be little if any fractionation of boron isotopes between a granitic melt and exsolved fluid if the granite is peraluminous. For granites with ASI <1.1, the ratio R(IVB) and thus the B-isotope fractionation are expected to strongly increase. We present a predictive model based on ab initio fractionation factors that links Δ11Bmelt–fluid with ASI in the melt, which suggests a fractionation of −4 ‰ to −7 ‰ at 730 and 530 °C, respectively, for a granite with ASI = 0.8.