Subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere. A better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial. In this study, major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the Sesia Zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin. Garnet shows a variety of textures requiring dissolution–precipitation processes in presence of fluids. In polycyclic metasediments, garnet preserves a partly resorbed core, related to pre-Alpine high-temperature/low-pressure metamorphism, and one or multiple rim generations, associated with Alpine subduction metamorphism. In eclogites, garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim. In metasediments, pre-Alpine garnet relics show δ 18 O values up to 5.3 ‰ higher than the Alpine rims, while no significant variation is observed among different Alpine garnet generations within each sample. This suggests that an extensive re-equilibration with an externally-derived fluid of distinct lower δ 18 O occurred before, or in correspondence to, the first Alpine garnet growth, while subsequent influxes of fluid had δ 18 O close to equilibrium. The observed shift in garnet δ 18 O is attributed to a possible combination of (1) interaction with sea-water derived fluids during pre-Alpine crustal extension and (2) fluids from dehydration reactions occurring during subduction of previously hydrated rocks, such as the serpentinised lithospheric mantle or hydrated portions of the basement.

中文翻译：

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地壳改造和水化：从高压岩石中石榴石的元素分带和氧同位素的见解（塞西亚区，西阿尔卑斯山）

由于俯冲岩石圈脱水，俯冲带代表了流体循环的最关键环境之一。更好地了解俯冲板片内外的流体流动是揭示流体在埋藏过程中的作用的基础。在这项研究中，主要和微量元素地球化学结合氧同位素被用于研究 Sesia 带的变沉积物和榴辉岩，以重建俯冲大陆边缘内部和外部流体脉冲的影响。石榴石显示出多种质地，需要在流体存在下进行溶解-沉淀过程。在多环变质沉积物中，石榴石保留了部分吸收的核心，与阿尔卑斯山前高温/低压变质作用以及一个或多个边缘世代有关，与高山俯冲变质作用有关。在榴辉岩中，石榴石化学分带表明单环生长，氧同位素从核心到边缘没有变化。在变质沉积物中，前高山石榴石遗迹的 δ 18 O 值比高山边缘高 5.3 ‰，而每个样品中不同高山石榴石世代之间没有观察到显着差异。这表明，在第一次高山石榴石生长之前或与之对应，与外部衍生的 δ 18 O 明显较低的流体发生了广泛的重新平衡，而随后流入的流体使 δ 18 O 接近平衡。