硼同位素分馏的实验理论认识和矿床地球化学研究进展

硼是一种中等挥发性元素,具有¹¹B和¹⁰B两个稳定同位素。两个同位素间高达10%的相对质量差使其在地质过程中引起高达-70‰至+75‰的硼同位素变化。硼在自然界主要与氧键合形成三配位(BO₃)和四配位(BO₄)结构,因而¹¹B和¹⁰B间同位素分馏主要受控于三配体(BO₃)和四面体(BO₄)间配分。本文综述了低温和高温地质过程的硼同位素分馏的理论和实验研究进展。在溶液中B(OH)₃和${B(OH)^{-}_{4}}$间硼同位素分馏受pH和热力学p-T条件控制,实验和理论表征获得常温常压条件下的B(OH)₃和$B(OH)^{-}_{4}$间同位素分馏系数(α_3-4)变化范围为1.019 4至1.033 3。低温条件下矿物(如碳酸盐、黏土矿物(蒙脱石和伊利石)、针铁矿、水锰矿、硼酸盐)与溶液间硼同位素分馏行为除了受p-T-pH影响外,矿物表面吸附引起的分馏效应十分显著。在中高温过程(蒙脱石伊利石化、富硼电气石和白云母矿物与热液流体,以及硅酸盐熔体与流体)中硼同位素分馏行为受到硼配位构型、化学成分以及物理化学条件的控制。随着硼同位素分馏机理研究的深入以及越来越完善的地质储库硼同位素端员特征表征,硼同位素地球化学指标可以灵敏示踪成矿物质来源、探究成矿作用与成因模式和重建成矿过程物理化学条件。目前矿床硼同位素地球化学研究的难点在于实现不同赋存相(如流体、矿物和熔体)中硼配位键合结构和硼同位素组成的精细化表征。

Experimental and theoretical understanding of boron isotope fractionation and advances in ore deposit geochemistry study

Boron is a moderately volatile element with two stable isotopes ¹¹B and ¹⁰B. As much as 10% relative mass difference between the two isotopes leads to significant variation in boron isotopic composition from -70‰ to +75‰ in nature. Boron is always bound to oxygen forming tetrahedral (BO₄) and trigonal (BO₃) coordination structures. The isotope fractionation between¹⁰B and ¹¹B is mainly controlled by their partition between the two structures. In this study, we gave a comprehensive review on the advances in equilibrium fractionation of boron isotopes in various processes. In solution, the boron isotope fractionation factor between B(OH)₃ and ${B(OH)^{-}_{4}}$ (α_3-4) is controlled by pH and thermodynamic p-T conditions. At ambient conditions, the α_3-4 values ranged from 1.0194 to 1.0333 by experimental and theoretical approaches. In addition to p-T-pH controls, boron isotope fractionation, caused by mineral surface adsorption between minerals (carbonates, clay minerals (montmorillonite and illite), goethite, hydromanganese, borate, etc.) and solution, is significant at low temperature. In medium and high temperature processes, boron isotope fractionation during illitization of smectite, tourmaline and muscovite minerals and in hydrothermal fluids or silicate melts and fluids are controlled by boron coordination, chemical composition, and physicochemical conditions. With further understanding of boron isotope fractionation mechanisms in individual process and isotopic distribution in various geological reservoirs, boron isotopes may be considered as sensitive indices for tracing ore-forming material sources, exploring ore-forming processes and genesis models, as well as reconstructing physicochemical conditions during ore formation. To better constrain geological concerns using boron isotopes in ore deposit geochemistry, the remaining challenges are to achieve fine characterizations of boron coordination and isotopic compositions in different host phases, such as fluids, minerals and melts.