We present a novel method for the measurement of lithium isotopes in garnet utilising glass reference materials and secondary ion mass spectrometry (SIMS). Measured lithium isotopic compositions of natural garnets are heterogeneous, making them unreliable reference materials for in situ determination. However, SIMS lithium isotope measurements of glasses derived from these natural garnets are isotopically identical to their parent garnets and more homogeneous, demonstrating that they can be used as reliable reference materials. To characterise the composition dependence of instrumental mass fractionation (IMF), oxide and silicate powders were used to synthesise custom-made glass reference materials (CGRMs) with garnet-equivalent compositions. Results for six CGRMs measured by SIMS show a significant linear relationship between IMF and FeO and MnO contents. Corrections for this compositional IMF result in changes of up to 12‰ within the compositional range explored. Uncertainty in IMF-corrected SIMS analyses with a 20 μm spot is in the range of 2.5 to 4.5‰, depending on the garnet composition and reference materials used. The method for in situ lithium isotope measurement in garnet by SIMS presented here is highly adaptable, valid across a range of Al-rich garnet compositions, and yields spatial resolution and precision necessary to address a range of geological applications.

中文翻译：

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石榴石中锂同位素的二次离子质谱测量方法：玻璃参考材料的效用

我们提出了一种利用玻璃参考材料和二次离子质谱法 (SIMS) 测量石榴石中锂同位素的新方法。测得的天然石榴石的锂同位素组成是异质的，使其成为不可靠的原位参考材料决心。然而，从这些天然石榴石中提取的玻璃的 SIMS 锂同位素测量值与它们的母石榴石在同位素上相同，并且更均匀，表明它们可以用作可靠的参考材料。为了表征仪器质量分馏 (IMF) 的成分依赖性，使用氧化物和硅酸盐粉末合成具有石榴石等效成分的定制玻璃参考材料 (CGRM)。SIMS 测量的六个 CGRM 的结果显示 IMF 与 FeO 和 MnO 含量之间存在显着的线性关系。对该成分 IMF 的修正导致在探索的成分范围内发生高达 12‰的变化。使用 20 μm 光斑进行 IMF 校正的 SIMS 分析的不确定度在 2.5 到 4.5‰ 的范围内，具体取决于所使用的石榴石成分和参考材料。此处介绍的 SIMS 对石榴石中的原位锂同位素测量具有很强的适应性，适用于一系列富含铝的石榴石成分，并产生解决一系列地质应用所需的空间分辨率和精度。