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首页> 外文期刊>The American mineralogist >An experimental approach to quantify the effect of tetrahedral boron in tourmaline on the boron isotope fractionation between tourmaline and fluid
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An experimental approach to quantify the effect of tetrahedral boron in tourmaline on the boron isotope fractionation between tourmaline and fluid

机译：量化硼滨四面硼在硼滨与流体中硼同位素分馏的实验方法

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This study investigates the effect of tetrahedral B (B-[4]) in synthetic tourmaline on the B-isotope fractionation between tourmaline and fluid. This is important for the correct interpretation of B-isotope variations in natural tourmalines containing "excess" B (greater than three atoms per formula unit), which substitutes for Si at tetrahedral sites. Such tourmalines commonly occur in Li, Al-rich pegmatites and have been reported from glaucophane schists that formed at high pressures during subduction. Tourmaline synthesis experiments were performed in a piston-cylinder apparatus in the system SiO2-Al2O3-B2O3-NaCl-H2O at 4 GPa and 700 degrees C using different run durations, starting from quartz-gamma-Al2O3-H3BO3 solid mixtures and NaCl-solutions. We were able to produce "olenitic" tourmaline with excess B between 1.2 and 2.5 B-[4] per formula unit. The B-isotope compositions of the olenitic tourmaline and coexisting fluids were determined by secondary ion mass spectrometry and multi-collector plasma source mass spectrometry to derive isotope fractionation coefficients. The results indicate that for every 10 mol% of total B in tourmaline in tetrahedral coordination, the value of Delta B-11(tur-fluid) is shifted to more negative values by about 1 parts per thousand at 700 degrees C. This is in good agreement with published ab initio calculations and corresponds to an intracrystalline fractionation of B-isotopes between the trigonal B and tetrahedral T sites of tourmaline on the order of 8 +/- 5 parts per thousand, whereby B-10 partitions to the T site.
机译：本研究探讨了合成电气石中四面体B（B-[4]）对电气石与流体之间B同位素分馏的影响。这对于正确解释天然电气石中含有“过量”B（每公式单位大于三个原子）的B同位素变化非常重要，它在四面体位置替代了Si。此类电气石通常出现在富含锂、铝的伟晶岩中，据报道来自俯冲过程中高压形成的蓝辉石片岩。电气石合成实验在活塞-圆筒装置中进行，系统为SiO2-Al2O3-B2O3-NaCl-H2O，在4 GPa和700℃下，从石英-γ-Al2O3-H3BO3固体混合物和NaCl溶液开始，使用不同的运行持续时间。我们能够生产出“油性”电气石，每单位配方奶粉中的过量硼含量在1.2到2.5 B-[4]之间。通过二次离子质谱和多收集器等离子体源质谱测定了油质电气石和共存流体的B同位素组成，得出同位素分馏系数。结果表明，在四面体配位的电气石中，每10 mol%的总硼，Delta B-11（tur流体）的值在700℃时被移动到更负的值，大约为千分之一。这与已发表的从头算计算结果非常一致，对应于电气石的三角B和四面体T位之间的B同位素的晶内分馏，大约为千分之8+/-5，由此B-10被划分到T位。

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