中大西洋洋脊33-35ºN玄武岩成分多样性成因：Sr-Nd-Pb-Hf同位素制约

Mantle temperature variation, plate spreading rate and mantle composition have been considered to be the fundamental variables that control the compositions of mid-ocean ridge basalts (MORB). However, there is debate over which variable exerts the major control and whether it is the same under different ridges. Moreover, it remains unclear whether the Hf-Nd isotopic coupling/decoupling exists on a global or local scale and its origin. Along the portion of the Mid-Atlantic Ridge (MAR) between the Oceanographer (35ºN) and Hayes (33ºN) transforms, the mantle potential temperature and the plate spreading rate are the same, but the compositions of the basalts vary drastically (almost covering the global MORB ranges, Niu et al., 2001). Therefore, it is an ideal place to study the relationship between mantle composition and MORB geochemical variations from Sr-Nd-Pb-Hf isotopic perspectives, and to examine MORB Hf-Nd isotopic correlation at a specific portion of ridge. We employ a Sr-Nd-Pb-Hf approach to unravel the along-ridge isotopic variations and their relationships with major elements and abundances and ratios of incompatible elements. These results allow us to further evaluate the mantle compositional control on MORB geochemical variations, the origin of enriched mantle component and Hf-Nd correlation from Sr-Nd-Pb-Hf perspectives. This has bearing on understanding the origin of MORB and mantle geodynamics.

洋中脊玄武岩（MORB）成分变化主要受三个因素控制：地幔潜能温度、地幔组成和洋脊扩张速率。但哪种因素起主导作用，主控因素在不同洋脊是否一样，存在争议。另外，对全球MORB Hf-Nd同位素耦合/解耦的空间尺度和成因认识也不清楚。中大西洋洋脊33-35ºN之间OH-1和OH-3洋脊下地幔潜能温度、洋脊扩张速率相同（Niu等，2001），但玄武岩微量元素成分变化几乎涵盖全球MORB变化范围。因此是从Sr-Nd-Pb-Hf同位素角度研究地幔组成与MORB成分变化联系的理想场所，也是从区域上研究Hf-Nd同位素耦合/解耦的理想场所。本项目拟对OH-1和OH-3 MORB样品进行密集的Sr-Nd-Pb-Hf同位素分析，揭示其沿着洋脊的变化及其与主量元素和不相容元素比值的关系，进一步探讨地幔组成与MORB组分的联系、富集地幔组分成因和Hf-Nd相关性，对于深入理解MORB成因和地幔动力学具有重要意义。

富集型洋中脊玄武岩(E-MORB)成分通常被认为是洋脊与地幔柱相互作用的结果, 而远离地幔柱的E-MORB不能用洋脊-地幔柱相互作用来解释，其成因存在争议。其源区主要有再循环洋壳的石榴石辉石岩或者榴辉岩、亏损地幔中混入少量的陆源或者深海沉积物和交代富集的大洋岩石圈地幔。本项目对深海钻探计划(DSDP)Leg 82航次559和561钻孔的玄武岩样品，进行了主量和微量元素及Sr-Nd-Pb同位素分析。在球粒陨石标准化稀土元素和原始地幔标准化多元素图解上，559钻孔玄武岩样品显示轻稀土元素(LREE)和不相容元素(如Nb, Ta, Th)富集，与E-MORB相似。561钻孔玄武岩样品分为两组：一组为亏损不相容元素，与正常的(N)-MORB相似；另一组富集不相容元素，与典型的E-MORB相似。在MgO=8wt.%时，559钻孔E-MORB样品具有高的TiO2和Fe2O3T含量, 低的CaO含量和CaO/Al2O3比值。而且在相同的Zr含量时，561钻孔E-MORB样品具有较高的La、Ce、Nb和Th等元素含量，表明它们具有不同的母岩浆组成。559钻孔玄武岩样品87Sr/86Sri (0.70292~0.70341)、143Nd/144Ndi (0.51296~0.51299)和208Pb/204Pbi (38.944~39.371)同位素比值与FOZO (focal zone)地幔组分相似，可能是FOZO地幔组分部分熔融的产物。FOZO地幔组分被认为是洋中脊玄武岩地幔源区普遍存在的一种组分，可能起源于俯冲交代的大洋岩石圈地幔。而561钻孔E-MORB样品具有比559钻孔样品更为富集的初始Sr和Pb同位素组成，在87Sr/86Sri-143Nd/144Ndi图解上，561钻孔E-MORB样品落在FOZO地幔组分的外部，其源区并非是FOZO地幔组分。561钻孔E-MORB样品和中大西洋KP2-3洋脊段（受Azores地幔柱影响最大的洋脊）MORB样品以及Azores列岛中São Miguel洋岛玄武岩的Sr-Nd-Pb同位素组组成相似，表明561钻孔E-MORB样品可能是Azores地幔柱组分部分熔融的结果。