摘要
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 Mg O(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 Mg O 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 ofε_(Nd)(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^(87)Sr/^(86)Sr 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^(87)Sr/^(86)Sr value.
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 Mg O（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 Mg O 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 ofεNd（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.
基金
supported by the State Ocean Administration National Programme on Global Change and Air-Sea Interaction (Grant No. GASI-GEOGE-02)
the Strategic Priority Research Program (B) of Chinese Academy of Sciences (Grant No. XDB18000000)
National Natural Science Foundation of China (Grant No. 41502194)
关键词
凝灰岩
玄武岩
建筑群
火山作用
水下
枕头
NE
海洋
Raohe Complex
Subaqueous volcanism
Jurassic Paleo-Pacific
Ocean Seawater Sr isotope
