Boron coordination and B/Si ordering controls over equilibrium boron isotope fractionation among minerals, melts, and fluids

Abstract

The high mobility of boron during fluid-rock interaction makes it an effective tracer for the sources of magmatic and metamorphic fluids, as recorded in minerals such as tourmaline and muscovite. Although advances have been made in quantifying the fractionation of boron isotopes among different phases, boron isotope fractionation in complex silicate melts remains poorly understood. Here, we propose appropriate models for the BO3 and BO4 units in silicate melts covering a wide range of chemical compositions and boron coordination structures in silicate magmas, and report the results of a theoretical investigation of boron isotope fractionation among silicate melt, minerals and fluids using a first principles theoretical approach. A comparison of measured and calculated α factors in mineral-melt and fluid-melt systems shows good agreement, suggesting the applicability of a simplified treatment of boron coordination structures in silicate melt. The results of this study show that the proportion of trigonal/tetrahedral coordinated boron and the B/Si ordering in silicate tetrahedral layers control the boron isotope fractionation among different phases, and that the effect of chemical composition is minor (less than 2‰ at 600 K). The temperature-dependent boron isotope fractionations are described as 1000lnαmica-basic fluid = 0.8–2.4 × (1000/T) - 0.8 × (1000/T)2, 1000lnαmica-acidic fluid = 7.0–14.0 × (1000/T) - 1.2 × (1000/T)2 and 1000lnαmica-tur = 1.9–5.4 × (1000/T) -3.4 × (1000/T)2 (T is temperature in Kelvins). At magmatic temperatures, ∆11B values between mineral/fluid and melt also vary with the proportion of the BO4 unit in the melt. This study underpins the applicability of the white mica-tourmaline geothermometers and boron isotopes for fluid source identification, and also offers an explanation of boron isotope fractionation in systems that contain complex silicate melts.