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 1000lnα_{mica-basic fluid} = 0.8–2.4 × (1000/T) - 0.8 × (1000/T)², 1000lnα_{mica-acidic fluid} = 7.0–14.0 × (1000/T) - 1.2 × (1000/T)² and 1000lnα_mica-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.

正如在电气石和白云母等矿物中所记录的那样，硼在流体-岩石相互作用过程中的高迁移率使其成为岩浆和变质流体来源的有效示踪剂。尽管在量化不同相中硼同位素的分馏方面已取得进展，但对复杂硅酸盐熔体中硼同位素分馏的了解仍然很少。在此，我们为BO ₃和BO ₄提出合适的模型硅酸盐熔体中的单位涵盖了硅酸盐岩浆中广泛的化学成分和硼配位结构，并报告了使用第一原理理论方法对硅酸盐熔体，矿物和流体中硼同位素分馏进行的理论研究的结果。矿物熔体和流体熔体系统中测量和计算的α因子的比较显示出很好的一致性，表明简化处理硅酸盐熔体中硼配位结构的适用性。这项研究的结果表明，三方/四面体配位硼的比例和硅酸盐四面体层中的B / Si有序控制了不同相之间的硼同位素分馏，并且化学成分的影响很小（600时小于2‰ K）。_{云母碱性流体} = 0.8-2.4×（1000 / T） - 0.8×（1000 / T）²，1000lnα_{云母酸性流体} = 7.0-14.0×（1000 / T） - 1.2×（1000 / T）²和1000lnα _mica-tur  = 1.9–5.4×（1000 / T）-3.4×（1000 / T）²（T为开氏温度）。在岩浆温度下，矿物/流体与熔体之间的∆ ¹¹ B值也随熔体中BO ₄单元的比例而变化。这项研究为白色云母-电气石地热仪和硼同位素在流体源识别中的适用性奠定了基础，并为含复杂硅酸盐熔体的系统中硼同位素分馏提供了解释。