Crustal anorthosite formation by deep‐seated hydrothermal circulation beneath fast‐spreading axis: Constraints from chronological approach, Sr isotope, and fluid–chromite inclusion investigation

Abstract

AbstractHydrothermal 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.