Hydrothermal circulation beneath the spreading axis plays a significant role in the exchange of energy and mass between the solid Earth and the oceans. Deep-seated hydrothermal circulation down to the crust/mantle boundary in the fast-spreading axis has been introduced by a number of studies regarding geological investigations and numerical models. In order to assess a reaction between hydrothermal fluid and host rock around the crust/mantle boundary, we conducted bulk trace element and Sr isotope analyses with a series of in situ investigations for crustal anorthosite, a reaction product between hydrothermal fluid and gabbro in the lowermost crustal section along Wadi Fizh, northern Oman ophiolite. In addition, we conducted titanite U–Pb isotope analyses to evaluate timing of the crustal anorthosite formation in the framework of the evolutional process of the Oman ophiolite. We estimated the formation age of the crustal anorthosite at 97.5 Ma ± 5.0 Ma, overlapping with the timing of the crust formation in the paleo spreading axis. The crustal anorthosite shows high-Th/U ratio (~2.5) and high-initial ⁸⁷Sr/⁸⁶Sr ratio (0.7050) due to seawater-derived hydrothermal fluid ingress into the precursor gabbro. With using analytical technique of micro-excavation at cryo-temperature, we detected Cl from a few micrometer-sized inclusion of aqueous fluid and chromite grains. The solubility of Cr was enhanced by complexation reactions with Cl in the hydrothermal fluid. Regarding reconstructed three-dimensional mass distribution of the inclusion and chromite composition, maximum Cr content of parental fluid was estimated at ~69 000 μg/g. The exceptionally high-Cr content was achieved locally by leaking of fluid and synchronous chromite crystallization during fluid entrapment. Presence of the deep-seated hydrothermal circulation could be assigned to the segment end, where cold seawater penetrates into the lowermost crust and extract heat along widely spaced network-like fluid channel.

中文翻译：

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快速扩张轴下深部热液循环形成的地壳斜长岩：年代学方法、Sr同位素和流体-铬铁矿包裹体研究的限制

扩张轴下方的热液循环在固体地球与海洋之间的能量和质量交换中起着重要作用。许多关于地质调查和数值模型的研究已经引入了深部热液循环到快速扩张轴上的地壳/地幔边界。为了评估地壳/地幔边界周围热液流体与围岩之间的反应，我们进行了大量微量元素和 Sr 同位素分析，并对地壳斜长岩进行了一系列原位调查，地壳斜长岩是最下部热液流体与辉长岩之间的反应产物。阿曼北部蛇绿岩沿 Wadi Fizh 的地壳剖面。此外，我们进行了钛铁矿 U-Pb 同位素分析，以评估在阿曼蛇绿岩演化过程框架内地壳斜长岩形成的时间。我们估计地壳斜长岩的形成年龄为 97.5 Ma±5.0 Ma，与古扩张轴上地壳形成的时间重叠。地壳斜长岩显示出高 Th/U 比 (~2.5) 和高初始⁸⁷锶/ ⁸⁶由于海水衍生的热液进入前体辉长岩，Sr 比 (0.7050)。使用低温微开挖分析技术，从几微米大小的水性流体包裹体和铬铁矿颗粒中检测到Cl。Cr 的溶解度通过与热液中的 Cl 发生络合反应而增强。关于包裹体和铬铁矿成分的重建三维质量分布，母液的最大 Cr 含量估计为 ~69 000 μg/g。异常高的 Cr 含量是通过在流体截留过程中流体泄漏和同步铬铁矿结晶而局部实现的。深部热液循环的存在可以分配到段末端，