硼在共存水蒸气-富硼熔体之间分配的实验研究及其地质意义

硼作为一种常用的地球化学指示剂和示踪剂,对研究俯冲带岩石学过程、岩浆-热液分异作用、火山活动以及稀有元素、铜、金的成矿机制具有重要意义。硼具有高水溶性和挥发性,它在气体中的分配、迁移能力和存在形式有助于理解含硼矿物的形成条件、流体化学组成的演化趋势、硼同位素的分馏效应和成矿金属的富集机理。本文在200-350℃、0.19-3.43 MPa条件下实验研究了B2O3-H2O体系中硼在共存水蒸气和富硼熔体（液体）之间的分配,平衡时气相中的B2O3含量为1.06%-32.35%。200℃、250℃、300℃和350℃时硼在气体-熔体之间的表观分配系数分别为0.035、0.042、0.20和0.33,即随温度上升,硼在含水气相中的分配和迁移能力增强。含水的富 B 熔体与硼酸稀溶液体系的气体-液体分配系数变化不大,表明B2O3-H2O±NaCl 体系中硼的气-液分配能力受液体或熔体中硼含量的影响较小,而主要与温度有关。经热力学分析,350℃、0.19-1.74 MPa条件下水蒸气中的气态硼物种可能为H3BO3和HBO2,可以预计随水蒸气压力的升高, H2O的配位数将会增大, H3BO3或其他可能的气态物种H3BO3·H2O会变得更为重要。某些火山活动区可见天然硼酸结壳（升华壳）的形成,灼热和干燥的火山岩石表面有利于硼酸从气相中凝析和沉淀。本实验结果表明,某些富硼酸的火山喷气孔气体的形成可能与地下高温火山岩浆（岩体）中存在因液态不混溶作用或晚期出溶作用产生的含水富硼的残余熔体或流体有关,熔体的去气作用或流体的减压相分离导致含水气相的产生,硼则随之大量分配至含水气体中并喷出地表。

Experimental study on boron distribution between coexisting water vapor and boron-rich melt and its geological implications

As a common geochemical indicator and tracer, boron is of significance for the study of petrologic processes in subduction zone, magmatic-hydrothermal differentiation, volcanic activity and mineralizing mecha-nism of rare elements, copper and gold. Boron is highly water-soluble and volatile, its capacity of distribution and transport and species in vapor phase are helpful for understanding the formation of boron-bearing minerals, evolution trend of chemical composition of fluid, fractionation effect of boron isotope and metallogenic mechanism of ore-forming metals. The distribution of boron between coexisting water vapor and melt （or liquid） in B2O3-H2O system was studied experimentally at temperatures from 200 ℃ to 350℃ and pressures from 0.19 MPa to 3.43 MPa. The equilibrated contents of B2O3 in the vapor phase range from 1.06% to 32.35%. The＆amp;nbsp;apparent distribution coefficients of boron between vapor and melt at 200℃, 250℃, 300℃and 350℃are 0.035, 0.042, 0.20 and 0.33, respectively, show an increasing potential of partition and transport of boron in vapor with temperature. The general variation in distribution coefficients doesn’t seem to be so large between systems of hydrous boron-rich melt and dilute boric acid solution, suggesting the capacity of boron distribution between vapor and liquid in the B2O3-H2O system may be affected slightly by the content of boron in the melt or liquid, but remarkably by the temperature. According to the result of thermodynamic approach, the gaseous species of boron in water vapor at 350℃and 0.19-1.74 MPa are H3BO3 and HBO2 probably. It is predicted that the coordination number of water will increase with the pressure of water vapor, and hence H3BO3 or other possible species such as H3BO3·H2O become more important. Sassolite encrustation （or sublimates） occurs in some volcano areas, the hot and dry surface of volcanic rocks is favorable for the vapor deposition of boric acid from the exhalations. Our experimental results suggest that some fumar