地幔Sr-Nd-Pb同位素演化的综合模型与n维同位素空间“地幔面”的地质意义

Synthetic evolution of Sr-Nd-Pb isotopes in the‘mantle plane’—model and interpretation.

在 n 维的 Sr-Nd-Pb 同位素空间中,几乎所有的大洋中脊玄武岩(MORB)和洋岛玄武岩(OIB)都落在一个虚拟平面上,被称为"地幔面(mantle plane)"。"地幔面"描述了大部分玄武岩的同位素地球化学特征,是最重要的、也是最早提出的地幔动力学演化特征之一,但是长期以来关于"地幔面"的内涵和意义并不清楚。本文通过一个综合模型,反演受岩浆作用控制的地幔微量元素(包括各种同位素母体元素)分异、Sr-Nd-Pb同位素演化,并推导出地幔 Sr-Nd-Pb 同位素演化的二元参数方程形式。模型表明,通过部分熔融向地壳输出相对富硅、富碱的物质成分,是地幔长期演化的主要特点,这个过程受到两个参数—部分融融程度(F)和岩浆分离的时间(t)—的控制,即 n 维参数可化为2维,因此在 n 维同位素空间出现"地幔面"的特征。壳源物资循环,能够使局部地幔偏离"地幔面",就地幔总体统计特征而言,地壳混染的比例很低,不同的统计数据显示大约1%～6%的系统偏差,即可能的地壳混染程度;进一步模拟,可能作出更加精确的估算。

In the n-dimension Sr-Nd-Pb isotopic space, almost all the oceanic basalts （ MORB ＋ OIB） over the world fit on a pseudo plane, which was called ＂mantle plane＂. The ＂mantle plane＂ accounts for the characteristics of the majority of the mantle arrays. However, as one of the most important and earliest noticed factors of mantle chemical geodynamic evolution, ＂mantle plane＂ are poorly known even now. Here we present a comprehensive model of Sr-Nd-Pb isotopic evolution in the mantle. In the mantle source of magmas, isotopic rations are mainly controlled by two factors, the fraction of partial melting and the evolution time of the system. This can be described as an equation. As reported that some isotopic data of mantle arrays do not fit on the mantle plane, which means mixing of recycled crustal composition. Statistical results of former researches reveal a deviation of 1% -6% , that infer the fraction of mixed crust.