Continuous water supply from the subducted pacific plate to the Eastern Asian big mantle wedge: New insights from the water content of late Cretaceous OIB-like basalts

Abstract

A large number of seismic topographical studies have shown the westward subducted Pacific plate in the mantle transition zone beneath the eastern Asian, forming the so-called Big Mantle Wedge (BMW). Several recent studies also suggest that such BMW has been initiated from the early Cretaceous. Some previous investigations on the water contents from Early Cretaceous and Cenozoic basalts from the BWM region have demonstrated that their sources were extensively hydrated, consistent with the numerical simulation. However, there are two questions about the water supply from the subducted Pacific slab to the eastern Asian BMW remaining unanswered: 1) was the upper mantle beneath the eastern Asian continuously hydrated from the early Cretaceous and late Cenozoic, and if yes, 2) could the subducted Pacific itself carry a considerable amount of water to the upper mantle of Eastern Asian? In order to answer these questions, we have determined the water contents of a suite of basalts from north and eastern North China Craton (Zhanglaogongtun basalts in Yixian (ZLGT), 106Ma; Pishikou basalts in Qingdao (QDPS), 78-86Ma), based on the water contents of clinopyroxene phenocrysts in the basalts. The results show that the estimated water contents of the primary magmas of the late Cretaceous ZLGT and QDPS basalts are 2.18 and 2.87 wt%, respectively, falling in the water content range of back-arc basin basalts and island-arc basalts while being significantly higher than the levels in mid-ocean ridge basalts and ocean island basalts, and the calculated minimum water contents of the mantle sources are approximately 2800 ppm and 1200 ppm, respectively. In combination with the previous studies, our results suggest that the subducted Pacific slab may have continuously supplied water to the BMW during the Early Cretaceous to late Cenozoic period, with high water-carriage efficiency.