Garnet is a nominally anhydrous mineral that can incorporate several hundreds of ppm H 2 O in the form of OH groups, where H + substitutes for cations in the garnet structure. To understand the effect of such small amounts of H 2 O on the physical and chemical properties of garnet, it is essential to determine where and how the OH groups are incorporated into the mineral structure. We investigated correlations between major and minor element maps acquired with the electron probe micro-analyser and H 2 O maps measured with Fourier transform infrared spectroscopy coupled to a focal plane array detector at the microscale to determine possible coupled substitutions. A set of algorithms was developed to match the maps pixel by pixel. They allow the computation of the garnet structural formula taking the H 2 O content into account and the calculation of correlations between H 2 O and other elements on the basis of 10,000 s of points. This new approach was applied to two hydrous garnet samples both showing H 2 O and chemical zoning. The first sample consists of a grossular-rich garnet from a high-pressure metarodingite ranging from 200 to 900 ppm H 2 O. The second sample contains a Ti-rich andradite garnet ranging from 200 to 8500 ppm H 2 O. For the grossular-rich garnet, a 1:1 correlation between Ti and H has been observed suggesting that H occurs as tetrahedral (2H) 2+ point defect, charge compensated by 2 Ti 4+ on the octahedral site. Based on this, a new hydrous garnet endmember with the formula Ca 3 Ti 2 H 2 Si 2 O 12 is proposed. This 2Ti VI $$\leftrightarrow$$ ↔ (2H) IV exchange mechanism is mainly responsible for the high amounts of TiO 2 (up to 11 wt%) in the investigated Ti-andradite. The incorporation of (2H) 2+ instead of (4H) 4+ on the tetrahedral site has important consequences for the normalisation of the garnet and hence on the determination of Fe 2+ /Fe 3+ based on stoichiometry. In the garnet from the metarodingite, a small-scale zoning in H 2 O contents of less than 100 µm can be resolved, indicating that the Ti–H defect is stable up to eclogite facies conditions and not modified by diffusion even at timescales of millions of years.

中文翻译：

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研究变质石榴石中 Ti-OH 关系的映射方法

石榴石是一种名义上的无水矿物，可以以 OH 基团的形式结合数百 ppm H 2 O，其中 H + 替代石榴石结构中的阳离子。要了解如此少量的 H 2 O 对石榴石物理和化学性质的影响，必须确定 OH 基团在何处以及如何结合到矿物结构中。我们研究了用电子探针微分析仪获得的主要和次要元素图与用傅立叶变换红外光谱仪在微尺度上与焦平面阵列检测器耦合测量的 H 2 O 图之间的相关性，以确定可能的耦合替代。开发了一组算法来逐个像素地匹配地图。它们允许在考虑 H 2 O 含量的情况下计算石榴石结构式，并基于 10,000 秒的点计算 H 2 O 与其他元素之间的相关性。这种新方法适用于两种均显示 H 2 O 和化学分区的含水石榴石样品。第一个样品由来自高压变硬锌矿的富镁石榴石组成，范围为 200 至 900 ppm H 2 O。第二个样品包含范围为 200 至 8500 ppm H 2 O 的富钛红长石石榴石。富石榴石，已观察到 Ti 和 H 之间的 1:1 相关性，表明 H 以四面体 (2H) 2+ 点缺陷的形式出现，电荷由八面体位置上的 2 Ti 4+ 补偿。基于此，提出了一种新的水合石榴石端元，分子式为 Ca 3 Ti 2 H 2 Si 2 O 12。这种 2Ti VI $$\leftrightarrow$$ ↔ (2H) IV 交换机制是研究的 Ti-andradite 中大量 TiO 2 （高达 11 wt%）的主要原因。在四面体位点上引入 (2H) 2+ 而不是 (4H) 4+ 对石榴石的归一化具有重要的影响，因此对基于化学计量的 Fe 2+ /Fe 3+ 的确定具有重要的影响。在来自变黄辉石的石榴石中，可以分辨出 H 2 O 含量小于 100 µm 的小尺度分带，表明 Ti-H 缺陷在榴辉岩相条件下是稳定的，即使在数百万的时间尺度上也不会因扩散而改变年。在四面体位点上引入 (2H) 2+ 而不是 (4H) 4+ 对石榴石的归一化具有重要的影响，因此对基于化学计量的 Fe 2+ /Fe 3+ 的确定具有重要的影响。在来自变黄辉石的石榴石中，可以分辨出 H 2 O 含量小于 100 µm 的小尺度分带，表明 Ti-H 缺陷在榴辉岩相条件下是稳定的，即使在数百万的时间尺度上也不会因扩散而改变年。在四面体位点上引入 (2H) 2+ 而不是 (4H) 4+ 对石榴石的归一化具有重要的影响，因此对基于化学计量的 Fe 2+ /Fe 3+ 的确定具有重要的影响。在来自变黄辉石的石榴石中，可以分辨出 H 2 O 含量小于 100 µm 的小尺度分带，表明 Ti-H 缺陷在榴辉岩相条件下是稳定的，即使在数百万的时间尺度上也不会因扩散而改变年。