太平洋洋中脊大尺度地幔组成不连续转换带的地球化学制约

The mantle beneath the Pacific mid-ocean ridge can be subdivided into two domains, northern and southern Pacific mantle domains. However, the causes for the formation and separation of the two domains is unclear, because the mid-ocean ridge near the transition zone between the two Pacific mantle domains has been interacting with the Easter mantle plume since 5 Ma that leads to the lack of suitable MORBs samples showing the transitional mantle source nature. The applicant of this project has participated the Integrated Ocean Drilling Program (IODP_329) and obtained two drilled holes of oceanic basalt, U1367 and U1368, which were accreted from the transitional ridge zone before 5Ma and were not influenced by the Easter mantle plume. In this project, we will analyze major and trace elements, Sr-Nd-Pb-Hf-O-He isotopes and age-date the drilled basalt samples. We will investigate the nature of the asthenospheric mantle flow beneath this transitional zone based on reconstruction of the initial position of the drilled oceanic crust and analyses of the mantle source heterogeneity by using the aforementioned geochemical data. We will also investigate the influences of mantle plume activities and recycling of crust and sediment on the formation of the large-scale mantle source heterogeneity based on comparison of isotope compositions between the drilled basalts and oceanic island basalts (OIBs) and the analyses of relationships between these isotope compositions and indicators for mantle melting conditions. As a conclusion, this project will investigate the the causes for the formation and separation of the northern and southern Pacific mantle domains by using data of basalts from IODP Sites U1367 and U1368.

太平洋洋中脊地幔以复活节微板块为界被分为南、北两个地幔域，由于这两个地幔域的转换带自5 Ma以来与复活节地幔柱开始相互作用，缺少能反映转换带本质的现代洋中脊玄武岩样品，这导致对南、北太平洋地幔域形成和分割的原因认识不足。申请人参加综合大洋钻探获得了U1367和U1368站离轴钻孔玄武岩，钻孔玄武岩由该转换带洋中脊>5Ma形成，不受复活节地幔柱活动影响。本项目以U1367和U1368站钻孔玄武岩为研究对象，进行主微量元素、Sr-Nd-Pb-Hf-O-He同位素及Ar-Ar定年分析。基于以上数据，通过恢复钻孔洋壳初始位置和地幔源区不均一性分析，认识转换带性质和软流圈地幔流动性；通过对比钻孔玄武岩与洋岛玄武岩的同位素组成，分析玄武岩同位素组成与地幔熔融指标的耦合关系，揭示地幔柱活动、再循环地壳及沉积物对形成南、北太平洋地幔大尺度不均一性的影响。综上以揭示太平洋洋中脊南、北地幔域形成和分割的原因。

太平洋作为地球上最大的板块，过去研究认为其地幔源区是相对均一的。但是，过去研究主要针对的是北半球太平洋洋中脊玄武岩开展的研究。近年来，南半球太平洋洋中脊玄武岩研究发现，太平洋南北半球之间可能存在差异。对太平洋南北半球之间地幔组成之间的差异和及其机制进行研究，对于认识太平洋地幔不均一性和地幔演化历史具有重要意义。本项目主要围绕半球太平洋地幔组成不均一性和及其产生的原因开展了相关研究。（1）本项目对IODP 329航次获得的U1367和U1368站位钻孔玄武岩开展了主量元素、微量元素和Sr-Nd-Pb-Hf同位素组成分析，发现在南太平洋23°S附近存在一个地幔组成转换带，该转换带反映了南北太平洋不同的演化历史，其中南半球地幔受更多的再循环洋壳组分影响；（2）将不连续转换带与洋中脊水深变化和扩张速率变化对比，发现这个转换带附近也存在水深和扩张速率的突然变化，在恢复板块构造的基础上，我们认为不连续带的空间位置是来自扩张速率对软流圈沿洋脊流动的控制；（3）我们对比了U1365和U1368站玄武岩矿物和化学组成，认为洋壳玄武岩在极薄沉积层下的蚀变持续时间更长，这在洋壳俯冲导致地幔不均一性研究中是需要考虑的；（4）我们对中生代时期以来形成的洋中脊玄武岩与现代太平洋洋中脊玄武岩开展了对比研究，发现中生代时期超级地幔柱活动对太平洋（包括西南太平洋）的洋中脊下地幔组成不均一性有显著影响；（5）本项目分析了IODP 329航次获得的U1365和U1368站位钻孔玄武岩的Mg同位素组成，并与洋岛玄武岩Mg同位素进行对比，发现玄武岩的Mg同位素组成有较大变化，且受到地幔熔融程度的控制，说明Mg同位素组成反映的地幔熔融过程不容忽视。本项目研究的主要意义在于，我们对现代太平洋不同洋脊部位和中生代以来不同地区的太平洋地幔组成有了较为清晰的认识，发现了太平洋在南北半球具有不同的演化历史，并认识到不连续转换带的控制因素，这些对于揭示太平洋地幔演化历史和规律有重要意义。