An Experimental Calibration of the Fractionation of Boron Isotopes among Granitic Melt, Aqueous Fluid, and Tourmaline

了解地球是如何在时间上演化的是地球科学的核心。为此，总是需要分析工具或方法，以阐明这些变化过程以及这些过程如何对自然环境起作用。轻元素稳定同位素的相对丰度往往在共存阶段(即不同的矿物、气体、岩浆、生物群)之间通过一些有机和无机过程进行分馏。随着地球及其生物圈的演化，这些产生稳定同位素的自然相互作用不断发生。因此，稳定的同位素分布在阐明贯穿地球化学史的一些过程中发挥了非常大的作用。氢、碳和氧的稳定同位素体系得到了广泛的研究、校准和利用。最近的研究表明，10B和11B的硼同位素系统具有潜在的革命性意义。硼集中在海水、蚀变的洋壳、海洋沉积物、硅酸盐岩浆及其衍生的水溶液中。研究人员希望，10B和11B在矿物、熔体和水溶液中的分馏可能会对大陆地壳的化学演化产生重要的见解。到目前为止，实验室对天然样品中的硼同位素的分析未能提供其化学行为的可理解和一致的图景。一项工作指出，在共存的水溶液、花岗岩液体和电气石(地球上硼的主要矿物学载体)之间，硼同位素组成很少或没有分馏，而其他分析数据表明，即使在一个地质事件中，电气石中的硼同位素分馏范围很大。因此，这项研究的目标是完成一项实验计划，在高温高压下，对硼同位素在花岗岩熔体、水溶液和电气石之间的分配进行内部一致的校准。三个系列的实验将测量10B和11B在花岗岩熔体和水溶液之间的分馏。多种实验方法的使用，以及同位素扩散混合方向的反转，将为实现平衡分布提供可靠的测试。还将通过合成实验测量电气石和熔体之间的硼同位素分馏。这些后面的实验将检验一种假设，即电气石同位素组成的大范围源于远离矿物熔体平衡的状态下参与电气石结晶的硼同位素的扩散控制分馏。

Understanding how the Earth evolves in time is central to the geosciences. Toward that end, there is always a need for analytical tools or methods that may shed light on those processes of change and how these processes operate on the natural environment. The relative abundance of stable isotopes of light elements tend to be fractionated between coexisting phases (i.e., different minerals, gas, magma, biota) by a number of organic and inorganic processes. These natural interactions that fractionate stable isotopes are occurring constantly as the earth and its biosphere evolve. For that reason, stable isotope distributions have played a very large role in elucidating some of the processes operating throughout the earth's chemical history. The stable isotope systematics of H, C, and O are widely studied, calibrated, and utilized. Recent studies have heralded the boron isotopic system, 10B and 11B, as potentially revolutionary. Boron is concentrated in ocean waters, altered oceanic crust, in marine sediments, and in silicate magmas and aqueous fluids that are derived from them. There is a hope among researchers that the fractionation of 10B and 11B among minerals, melts, and aqueous solutions may yield important insights into the chemical evolution of the continental crust. So far, the laboratory analyses of boron isotopes in natural samples have failed to provide an understandable and consistent picture of their chemical behavior. One body of work cites little or no fractionation of boron isotopic composition among coexisting aqueous solution, granitic liquid, and tourmaline, the principal mineralogical carrier of boron in the earth, while other analytical data point to very large ranges in the fractionation of boron isotopes in tourmaline, even within a single geologic occurrence. The goal of this research, therefore, is to complete an experimental program that yields an internally consistent calibration of the partitioning of the boron isotopes among granitic melt, aqueous solution, and tourmaline at elevated pressures and temperatures.Three series of experiments will measure the fractionation of 10B and 11B between granitic melt and aqueous solution. The use of multiple experimental methods, and reversals in the direction of diffusive mixing of the isotopes, will provide robust tests of the attainment of equilibrium distributions. The fractionation of boron isotopes between tourmaline and melt will also be measured through synthesis experiments. These latter experiments will test a hypothesis that the wide range in the isotopic composition of tourmaline stems from diffusion-controlled fractionation of the isotopes of boron attending the crystallization of tourmaline at states far from the mineral-melt equilibrium.