Subaqueous volcanism in the Paleo-Pacific Ocean based on Jurassic basaltic tuff and pillow basalt in the Raohe Complex, NE China

On-land records of subaqueous explosive volcanic eruptions are rarely reported. To understand this phenomenon and discuss its global significance, we studied the geochronology and geochemistry of basaltic tuff and pillow basalt in the Raohe Complex, NE China. The basaltic tuff consists of well-sorted vitreous, crystal (mostly clinopyroxene), and minor lithic fragments. It is characterized by a high MgO (15.7–15.9%) content and zero Eu anomalies (Eu/Eu*=99–102). The tuff erupted at 172±1 Ma based on SHRIMP zircon U-Pb dating, coeval with the previously reported age of the pillow basalt. The pillow basalt has intermediate MgO content and weakly negative Eu anomalies (Eu/Eu*=90–99). Based on immobile trace element discrimination, the basaltic tuff and pillow basalt belong to alkali basalt displaying an OIB-type trace element pattern, and consistent Nd isotope signatures of eNd(t)=4.4–6.2, indicating an identical mantle source. The pillow basalt has coupled Sr-Nd isotopic values, whereas the basaltic tuff has significantly higher initial 87Sr/86Sr values that are similar to synchronous seawater. This indicates that the elemental exchange between the mantle-derived material and seawater most likely occurred in a subaqueous explosive volcanic eruption, rather than in an effusive eruption. Detailed calculations suggest that the high efficiency of the Sr-isotope exchange between seawater and the mantle-derived material triggered by a subaqueous explosive volcanic eruption is likely one of the main reasons for the rapid decrease of the global seawater 87Sr/86Sr value.