Abstract
Frequent volcanic activity has occurred in the Okinawa Trough (OT) during the late Quaternary, which attracted much attention to the origin of volcanic rocks. Pumice collected from the seafloor has been extensively investigated, whereas few studies paid attention to the pumice in the sediment. The geochemical compositions of pumice preserved in sediments generally provide insight into past volcanic activity and regional magmatism. Here, we present major and trace element compositions and Sr-Nd-Pb isotope data, together with the established age framework for pumice samples recovered from sediment core S9 in the middle OT (MOT) to investigate their possible formation. Compositionally, the S9 pumice samples are dacite and are characterized by relatively higher Sr (87Sr/86Sr = 0.70480–0.70502) and Pb (206Pb/204Pb = 18.321-18.436, 207Pb/204Pb = 15.622–15.624, and 208Pb/204Pb = 38.52–38.63) and lower Nd (143Nd/144Nd = 0.51272–0.51274) isotope compositions than basalts from the MOT. The geochemical compositions of pumice clasts from different layers of core S9 display no temporal variation trends and vary within narrow ranges. On the basis of the geochemical characteristics of S9 pumice samples, we infer that the parent magma of these samples might generate from hybrid magma through an extensive fractional crystallization process. The Indian Ocean MORB-type mantle was first metasomatized by the subducted Philippine Sea sediments to form the primitive magma; then, followed by assimilation of a small amount of lower crustal component occurred in the lower crust. The long-term magmatism and relatively consistent isotopic compositions indicate that a magma chamber might have existed in the lower crust of the MOT between 11.22 and 12.96 cal. ka BP.
Highlights
The subduction zone, as a crucial interface of material exchange between the Earth’s crust and mantle, is one of the critical areas for plate subduction and consumption as well as for the generation of magmatism [1,2,3]
The normal mid-ocean ridge basalt (N-MORB) normalized incompatible element patterns of S9 pumice samples are shown in Figure 5, which for comparison, includes basalts from the middle OT (MOT) [16,27] and basalts from the northern and middle Ryukyu volcanic front [37]
87Sr/86Sr gradually increases, while 143Nd/144Nd decreases with increasing Th/La ratios among the MOT basalts, andesites, and S9 pumice (Figure 8c,f). These results suggest that the variations in the chemical and isotopic compositions of S9 pumice samples may reflect the process of crustal contamination in the MOT
Summary
The subduction zone, as a crucial interface of material exchange between the Earth’s crust and mantle, is one of the critical areas for plate subduction and consumption as well as for the generation of magmatism [1,2,3]. Extensive volcanic activity has given rise to numerous volcanic rocks, making the OT an ideal area for studying the magmatism associated with back-arc basin initiation [13,14]. Most of the studies suggest that pumice were derived from basaltic magma through fractional crystallization [26,27], while a few studies suggest the partial remelting of andesites [23]. The divergence of these interpretations can be ascribed to the lack of an age framework and the poor recognition of pumice types
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
