Garnet is one of the most robust and ubiquitous minerals that record element zoning during crustal metamorphism. In addition to major elements, zoning in trace elements can provide a wealth of information to document the changing conditions of garnet growth and modification. However, mapping trace elements at low concentrations, over large areas and with high resolution has remained a major challenge. We present a comprehensive investigation of the TE distribution in garnet from three Alpine samples that underwent a complex evolution at different metamorphic conditions. The TE distribution in garnet grains is mapped in 2D in thin section with a novel approach using laser ablation inductively coupled plasma time of flight mass spectrometry (LA-ICP-TOFMS) to achieve a lateral resolution of 5 µm and limits of detection for the heavy rare earth elements (REE) down to 0.2 µg/g. Comparison with major element zoning measured by electron probe microanalysis and trace elements measured by conventional LA-ICPMS spot analysis testifies to the accuracy of the measurements. Garnet in an amphibolite-facies metapelite from Campolungo, Central Alps, that recorded metamorphism to 600 °C preserves Y + REE trace element zoning that closely matches that of Ca. In this sample, there is no notable diffusive modification for trace elements. Y + REE zoning is dominated by Rayleigh fractionation in the core and by the sporadic breakdown of accessory phases producing annuli in the rim of the garnet. A granulite-facies garnet from Malenco, Eastern Central Alps, formed during subsolidus heating, followed by peritectic melting reactions up to temperatures of 800–850 °C. Major and trace element zoning are decoupled indicating diffusional resetting of major elements, whereas trace elements still largely document the growth history. Enrichment of trace elements in the garnet mantle may be related to the consumption of biotite (V, Cr) and the dissolution of zircon (Zr) and monazite (Y + REE) in the melt. Diffusion of Y + HREE at the core–mantle boundary occurred over a length scale of ~ 200 µm. Garnet in an eclogite from the Sesia Zone, Western Alps ( P ~ 2 GPa, T ~ 600 °C), displays pronounced fluid-related veinlets, visible in FeO, MgO and MnO, which cross-cut the primary growth zoning. Surprisingly, complex Y + REE and Cr zoning is not affected by the veinlets, indicating that they did not form by a crack-seal mechanism but are rather related to a selective replacement process. The trace element maps provide a detailed insight into the growth and modification of garnet and thus allow assessment of equilibrium versus disequilibrium processes, and assist in determination of P – T conditions, garnet dating, diffusion modelling as well as documenting fluid-induced modifications.

中文翻译：

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通过 LA-ICP-TOFMS 高分辨率微量元素映射识别变质石榴石的生长机制

石榴石是在地壳变质过程中记录元素分带的最坚固和无处不在的矿物之一。除了主要元素外，微量元素的分区可以提供丰富的信息来记录石榴石生长和改性的变化条件。然而，以低浓度、大面积和高分辨率绘制微量元素仍然是一项重大挑战。我们对来自三个高山样品的石榴石 TE 分布进行了全面调查，这些样品在不同变质条件下经历了复杂的演变。使用激光烧蚀电感耦合等离子体飞行时间质谱 (LA-ICP-TOFMS) 以实现 5 µm 的横向分辨率和重离子检测限的新方法，在薄截面中以 2D 方式绘制石榴石晶粒中的 TE 分布。稀土元素 (REE) 低至 0.2 µg/g。与通过电子探针微量分析测量的主要元素分区和通过传统 LA-ICPMS 点分析测量的痕量元素进行比较，证明了测量的准确性。来自中阿尔卑斯山 Campolungo 的角闪岩相变质岩中的石榴石记录了 600 °C 的变质作用，保留了与 Ca 密切匹配的 Y + REE 微量元素分带。在该样品中，微量元素没有显着的扩散改性。Y + REE 分带主要由核心中的瑞利分馏和在石榴石边缘产生环带的副相的零星分解支配。来自中阿尔卑斯山脉东部马伦科的麻粒岩相石榴石，在亚固相线加热过程中形成，随后在高达 800-850 °C 的温度下发生包晶熔融反应。主要元素和微量元素分区是分离的，表明主要元素的扩散重置，而微量元素仍然在很大程度上记录了生长历史。石榴石地幔中微量元素的富集可能与黑云母（V、Cr）的消耗以及熔体中锆石（Zr）和独居石（Y+REE）的溶解有关。Y + HREE 在核-地幔边界的扩散发生在~200 µm 的长度尺度上。来自西阿尔卑斯山 Sesia 带榴辉岩中的石榴石 ( P ~ 2 GPa, T ~ 600 °C), 显示明显的流体相关细脉，在 FeO、MgO 和 MnO 中可见，它们横切了主要生长分区。令人惊讶的是，复杂的 Y + REE 和 Cr 分带不受细脉的影响，表明它们不是由裂缝密封机制形成的，而是与选择性置换过程有关。微量元素图可以详细了解石榴石的生长和变化，从而可以评估平衡与不平衡过程，并有助于确定 P-T 条件、石榴石测年、扩散建模以及记录流体引起的变化。表明它们不是由裂缝密封机制形成的，而是与选择性替换过程有关。微量元素图可以详细了解石榴石的生长和变化，从而可以评估平衡与不平衡过程，并有助于确定 P-T 条件、石榴石测年、扩散建模以及记录流体引起的变化。表明它们不是由裂缝密封机制形成的，而是与选择性替换过程有关。微量元素图可以详细了解石榴石的生长和变化，从而可以评估平衡与不平衡过程，并有助于确定 P-T 条件、石榴石测年、扩散建模以及记录流体引起的变化。