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首页> 外文期刊>Contributions to Mineralogy and Petrology >Boron-isotope fractionation between tourmaline and fluid: an experimental re-investigation
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Boron-isotope fractionation between tourmaline and fluid: an experimental re-investigation

机译：电气石和液体之间的硼同位素分馏：实验性重新研究

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The fractionation of boron isotopes between synthetic dravitic tourmaline and fluid was determined by hydrothermal experiments between 400 and 700°C at 200 MPa and at 500°C, 500 MPa. Tourmaline was crystallized from an oxide mix in presence of water that contained boron in excess. In one series of experiments, [B]fluid/[B]tour was 9 after the run; in another series it was 0.1. All experiments produced tourmaline as the sole boron-bearing solid, along with traces of quartz and talc. Powder XRD and Rietveld refinements revealed no significant amounts of tetrahedrally coordinated boron in tourmaline. 11B always preferentially fractionated into the fluid. For experiments where [B]fluid/[B]tour was 9, a consistent temperature-dependent boron isotope fractionation curve resulted, approximated by Δ11B(tour–fluid) = −4.20 · [1,000/T (K)] + 3.52; R 2 = 0.77, and valid from 400 to 700°C. No pressure dependence was observed. The fractionation (−2.7 ± 0.5‰ at 400°C; and −0.8 ± 0.5‰ at 700°C) is much lower than that previously presented by Palmer et al. (1992). Experiments where [B]fluid/[B]tour was 0.1 showed a significant larger apparent fractionation of up to −4.7‰. In one of these runs, the isotopic composition of handpicked tourmaline crystals of different size varied by 1.3‰. This is interpreted as resulting from fractional crystallization of boron isotopes during tourmaline growth due to the small boron reservoir of the fluid relative to tourmaline, thus indicating larger fractionation than observed at equilibrium. The effect is eliminated or minimized in experiments with very high boron excess in the fluid. We therefore suggest that values given by the above relation represent the true equilibrium fractionations.
机译：通过在200 MPa和500℃，500 MPa下在400和700°C之间的水热实验确定合成的德拉威电气石和流体之间的硼同位素分馏。在含有过量硼的水中，电气石从氧化物混合物中结晶出来。在一系列实验中，运行后[B] fluid / [B] tour 为9；在另一个系列中，该值为0.1。所有实验均产生电气石作为唯一的含硼固体，以及微量的石英和滑石。粉末XRD和Rietveld精炼显示，电气石中没有大量的四面体配位硼。 11 B总是优先分馏到液体中。对于[B] fluid / [B] tour 为9的实验，得出了一致的随温度变化的硼同位素分馏曲线，近似为Δ 11 B （游水） = −4.20·[1,000 / T（K）] + 3.52; R 2 = 0.77，在400至700°C下有效。没有观察到压力依赖性。分馏率（400°C下为-2.7±0.5‰； 700°C下为-0.8±0.5‰）远低于Palmer等人先前提出的分馏率。 （1992）。 [B] fluid / [B] tour 值为0.1的实验显示，明显的更大的表观馏分高达-4.7‰。在这些实验之一中，不同大小的精选电气石晶体的同位素组成相差1.3‰。这被解释为是由于电气石相对于电气石的硼储量小，在电气石生长过程中硼同位素的分步结晶所致，因此表明分馏比在平衡状态下观察到的要大。在流体中硼含量非常高的实验中，这种影响被消除或最小化。因此，我们建议上述关系式给出的值代表真实的平衡分数。

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