The geochemical cycle of boron: Constraints from boron isotope partitioning experiments between mica and fluid
The geochemical cycle of boron: Constraints from boron isotope partitioning experiments between mica and fluid
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DOI：
10.1016/j.lithos.2005.02.003
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发表时间：
2005-10
期刊：
影响因子：
3.5
通讯作者：
B. Wunder;A. Meixner;R. Romer;R. Wirth;W. Heinrich
中科院分区：
文献类型：
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作者：
B. Wunder;A. Meixner;R. Romer;R. Wirth;W. Heinrich
在3.0 GPa/500 °C和3.0 GPa/700 °C下，测定了合成硼白云母与流体之间的硼同位素分馏。对于近中性流体，Δ11B（云母-流体）=δ11B（云母）−δ11B（流体）在500 °C时为− 10.9±1.3‰，在700 °C时为− 6.5±0.4‰。这支持了早先的假设，即主要分馏效应是由于从中性流体中的硼的三角形配位到云母、粘土和熔体中的四面体配位硼的变化。这种效应的温度依赖性近似为Δ11B（云母、粘土、熔体-中性流体）=− 10.69·（1000/T [K]）+3.88; R2=0.992，从25 °C的流体-粘土到1000 °C的流体-硅酸盐熔体都有效。在0.4 GPa下，使用强碱性流体的实验产生了明显较低的分馏，在400 °C时Δ 11 B（云母流体）为− 7.4±1.0‰，在500 °C时为− 4.8±1.0‰，表明当大量的硼在基本流体中是四面体配位时，分馏效应降低。野外研究表明，火山弧中的硼浓度和11 B/10 B比值随着离海沟距离的增加而系统地降低，从而反映了俯冲板片的热结构。我们的实验表明，火山弧中的硼同位素特征可能是云母沿着一个明显的P-T范围连续脱水的结果。火山弧中硼同位素特征主要是由于火山弧板片的持续脱水作用和硼通过流体向地幔楔中的迁移。
The fractionation of boron isotopes between synthetic boromuscovite and fluid was experimentally determined at 3.0 GPa/500 °C and 3.0 GPa/700 °C. For near-neutral fluids Δ11B(mica-fluid)=δ11B(mica)−δ11B(fluid)is − 10.9±1.3‰ at 500 °C, and − 6.5±0.4‰ at 700 °C. This supports earlier assumptions that the main fractionation effect is due to the change from trigonal coordination of boron in neutral fluids to tetrahedrally coordinated boron in micas, clays and melts. The T-dependence of this effect is approximated by the equation Δ11B(mica,clay,melt–neutral fluid)=− 10.69·(1000/T [K])+3.88; R2=0.992, valid from 25 °C for fluid–clay up to about 1000 °C for fluid–silicate melt. Experiments at 0.4 GPa that used strongly basic fluids produced significantly lower fractionations with Δ11B(mica–fluid)of − 7.4±1.0‰ at 400 °C, and − 4.8±1.0‰ at 500 °C, showing the reduced fractionation effect when large amounts of boron in basic fluids are tetrahedrally coordinated. Field studies have shown that boron concentrations and11B/10B-ratios in volcanic arcs systematically decrease across the arc with increasing distance from the trench, thus reflecting the thermal structure of the subducting slab. Our experiments show that the boron isotopic signature in volcanic arcs probably results from continuous dehydration of micas along a distinct P–T range. Continuous slab dehydration and boron transport via fluid into the mantle wedge is responsible for the boron isotopic signature in volcanic arcs.