| ZHANG Guoliang. 2016. Compositional and temperature variations of the Pacific upper mantle since the Cretaceous. Acta Oceanologica Sinica, 35(4):19－25 |
| Compositional and temperature variations of the Pacific upper mantle since the Cretaceous |
| 白垩纪以来太平洋上地幔组成和温度变化 |
| Received:December 03, 2014 Revised:July 02, 2015 |
| DOI：10.1007/s13131-016-0839-4 |
| Key words:mantle temperature mantle source super mantle plume Cretaceous mid-ocean ridge basalt |
| 中文关键词: 地幔温度 地幔源区 超级地幔柱 白垩纪 洋中脊玄武岩 |
| 基金项目:The Strategic Priority Research Program of the Chinese Academy of Sciences under contract No.XDA11030103; the National Natural Science Foundation of China under contract Nos 41376065, 41176043 and 41522602; the project of "AoShan" excellent scholar for Qingdao National Laboratory for Marine Science and Technology. |
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| Abstract: |
| 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 and 143Nd/144Nd, and higher La/Sm and 87Sr/86Sr 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. |
| 中文摘要: |
| 白垩纪中期地球地质过程表现出的一系列异常,如:地磁长期停滞倒转、海平面升高、大量火山活动等,可能与白垩纪大火成岩省的形成和地幔组成及温度演化等深部作用有关。由于白垩纪中期的火山活动异常主要发生在太平洋,本研究通过大洋钻探(DSDP/ODP/IODP)在太平洋获得的玄武岩化学组成研究地幔组成和温度变化。根据距离太平洋Ontong Java洋底高原的距离远近,将这些站位分为洋底高原玄武岩、近洋底高原洋壳玄武岩和正常洋中脊玄武岩。本研究指出:白垩纪时期形成的正常洋中脊玄武岩在平均Na8、La/Sm和Sm/Yb,以及Sr-Nd同位素组成上与现代太平洋扩张中心相近。Ontong Java洋底高原(形成于125-90 Ma)相对洋中脊玄武岩具有明显低的Na8、和143Nd/144Nd比值,以及明显高的La/Sm、87Sr/86Sr比值。然而,这些近洋底高原洋壳玄武岩与洋底高原玄武岩接近,表明Ontong Java洋底高原岩浆活动附近的洋中脊玄武岩具有明显的源区混染。可能来自超级地幔柱的Ontong Java在其活动期间对同一活动时期形成的大面积洋中脊玄武岩源区具有明显影响。基于中生代时期正常洋底玄武岩的化学组成,本研究指出白垩纪时期正常洋中脊地幔组成和熔融条件与现代快速扩张洋中脊段相似。中白垩时期洋中脊玄武岩表现出的熔融条件变化不一定反映地幔温度变化,而更可能与地幔源区和扩张速率有关系。 |
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