华北克拉通东部新生代碱性玄武岩Mg-Ca-Sr同位素组成：制约深部循环碳酸盐组成和玄武岩成因

Carbonates in marine sediments and the oceanic lithosphere can be recycled into the mantle at convergent margins, which has an important influence on the physical and chemical properties of the mantle as well as the global carbon cycle. A combined study of Mg、Ca and Sr isotopes can provide a tight constraint on the composition of deeply recycled carbonates and the petrogenesis of basalts. A preliminary study shows that some Cenozoic alkaline basalts from eastern North China Craton, have Mg isotopic compositions (δ26Mg = -0.36 ~ -0.47‰) much lighter than that of the normal mantle (δ26Mg = -0.25 ± 0.07‰). This light Mg isotopic composition can be attributed to interaction of their mantle source with low δ26Mg carbonate melts derived from the subducted west Pacific plate. Consequently, we intend to carry out a combined study of Mg-Ca-Sr-Nd-Pb isotopes as well as major and trace elements of widely-distributed Cenozoic alkaline basalts from eastern North China Craton, trying to play constraints on (1) the composition of deeply recycled carbonates in the mantle beneath eastern North China Craton; (2) the petrogenesis and mantle characteristics of Cenozoic basalts from eastern North China Craton; and (3) the links between the subducted West Pacific Plate and the petrogenesis of Cenozoic basalts.

沉积物和大洋岩石圈中的碳酸盐能够通过板块俯冲进入地幔，从而影响地幔的物理和化学性质以及全球碳循环。综合Mg、Ca、Sr同位素研究，能够有效制约深部循环碳酸盐的组成和玄武岩的成因。申请人前期研究发现，华北克拉通东部部分新生代碱性玄武岩的Mg同位素组成（δ26Mg = -0.36 ~ -0.47‰）显著轻于“地幔值”（δ26Mg = -0.25 ± 0.07‰），暗示其地幔源区曾经受到来自西太平洋板块的低δ26Mg碳酸盐熔体的交代作用。据此，本项目拟调查华北克拉通东部大面积出露的新生代碱性玄武岩的Mg-Ca同位素组成，结合Sr-Nd-Pb同位素和主微量元素，制约（1）再循环进入华北克拉通东部上地幔的碳酸盐的组成；（2）华北克拉通东部新生代玄武岩的成因和源区性质；以及（3）西太平洋板块俯冲与新生代玄武岩成因的联系。

本项目分析了华北、华南和东北等地的玄武岩的主量和微量元素以及Sr-Nd-Mg-Ca-Fe-Zn同位素。结果显示，中国东部晚中生代-新生代玄武岩从硅不饱和的霞石岩变化到硅饱和的拉斑玄武岩，具有洋中脊玄武岩型微量元素分配模式和亏损到富集的Sr-Nd同位素组成；它们的δ26Mg值变化范围为−0.60‰到−0.30‰，δ44/40Ca为0.59‰到0.94‰，δ66Zn为0.30‰到0.63‰，δ56Fe为0.10‰到0.30‰。综合分析显示，（1）中国东部晚中生代-新生代玄武岩高度变化的化学组成是古太平洋板块俯冲导致的不同壳源组分（即沉积碳酸盐、硅质沉积物和碳酸盐化榴辉岩）加入到地幔的结果；（2）深部再循环碳酸盐主要为富镁-富锌的菱镁矿和白云石；（3）中国东部上地幔具有比正在地幔高的氧逸度；（4）Zn同位素是一种新的示踪深部碳循环的地球化学指标。研究成果制约了中国东部晚中生代-新生代玄武岩的成因、地幔源区的性质和动力学背景，并揭示中国东部深部地幔是一个巨大的再循环碳库。除了上述成果，负责人取得的成果还包括（1）厘定蚀变洋壳的铜锌同位素组成，揭示地幔交代和岩浆演化等重要地质过程中铜锌同位素分馏机理，为铜锌同位素示踪地幔演化提供重要的理论依据；（2）证实俯冲洋壳释放的熔流体对弧岩浆岩铁-镁-锌同位素体系的影响非常有限，提出弧岩浆岩轻铁-重镁同位素异常是蛇纹岩流体交代地幔楔的结果，为研究俯冲带壳-幔相互作用提供新的示踪工具。在本项目的支持下，负责人已发表论文9篇，包括7篇Nature Index论文。