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Compositional and temperature variations of the Pacific upper mantle since the cretaceous

Acta oceanologica Sinica 2016年第4期35卷 19-25页

作者： ZHANG Guoliang Key Laboratory of Marine Geology and Environment Institute of Oceanology Chinese Academy of Sciences Laboratory for Marine Geology Qingdao National Laboratory for Marine Science and Technology

The geological evolution of the Earth during the mid-cretaceous were shown to be anomalous, e.g., the pause of the geomagnetic field, the global sea level rise, and increased intra-plate volcanic activities, which could be attributed to deep mantle processes. As the anomalous volcanic activities occurred mainly in the cretaceous Pacific, here we use basalt chemical compositions from the oceanic drilling（DSDP/ODP/IODP） sites to investigate their mantle sources and melting conditions. Based on locations relative to the Pacific plateaus, we classified these sites as oceanic plateau basalts, normal mid-ocean ridge basalts, and near-plateau seafloor basalts. This study shows that those normal mid-ocean ridge basalts formed during mid-cretaceous are broadly similar in average Na8, La/Sm and Sm/Yb ratios and Sr-Nd isotopic compositions to modern Pacific spreading ridge（the East Pacific Rise）. The Ontong Java plateau（125–90 Ma） basalts have distinctly lower Na8 and143Nd/144 Nd, and higher La/Sm and 87Sr/86 Sr than normal seafloor basalts, whereas those for the near-plateau seafloor basalts are similar to the plateau basalts, indicating influences from the Ontong Java mantle source. The super mantle plume activity that might have formed the Ontong Java plateau influenced the mantle source of the simultaneously formed large areas of seafloor basalts. Based on the chemical data from normal seafloor basalts, I propose that the mantle compositions and melting conditions of the normal mid-ocean ridges during the cretaceous are similar to the fast spreading East Pacific Rise. Slight variations of mid-cretaceous normal seafloor basalts in melting conditions could be related to the local mantle source and spreading rate.

关键词： mantle temperature mantle source super mantle plume cretaceous mid-ocean ridge basalt

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