从大陆火山岩视角了解深部地幔化学储库的属性

The nature of the deep mantle chemical reservoirs:Perspective from continental intraplate volcanic rocks

洋岛玄武岩(OIB)在同位素组成上的显著变化表明深部地幔在化学上是高度不均一的,存在EM1、EM2和HIMU等地幔端元。现有OIB的地球化学证据支持EM1、EM2和HIMU源区多存在再循环地壳物质,因此对地幔端元属性的精准约束是探讨壳幔物质循环和地球深部过程的关键。基于近年来来自OIB的Mg、Fe、Zn等金属稳定同位素和橄榄石斑晶元素地球化学方面的观察,本文在重新梳理不同地幔端元在成因上的联系和区别的基础上,把EM1和EM2归类为残余型富集储库(residual-type enriched reservoir)、HIMU归为交代型富集储库(metasomatic-type enriched reservoir)。相对于OIB,大陆幔源火山岩的熔融程度普遍要低得多,更有利于保存富集组分的地球化学信息。另外,洋岛玄武岩主要分布在南半球,大陆火山岩主要分布于北半球,后者可以弥补前者在空间上代表性不足的缺陷。因此,大陆火山岩有潜力成为观察深部地幔储库的新视角。

The tremendous isotopic variability in ocean island basalts(OIBs)reflects that the deep mantle is highly heterogeneous and contains several distinct mantle components such as EM1,EM2 and HIMU.Current geochemical evidence indicates that the generation of the EM1,EM2 and HIMU mantle reservoirs could be attributed to the recycling of crustal materials into the mantle.Therefore,a precise and accurate knowledge of the nature of these mantle reservoirs is crucial for exploring the material circulation between Earth s crust and mantle and the deep Earth processes.According to recent observations from stable Mg,Fe,Zn isotopic compositions of OIBs and compositions of olivine phenocrysts,here we re-evaluate the genetic links between these mantle reservoirs and divide them into two groups,the residual-type enriched reservoir(EM1 and EM2)and the metasomatic-type enriched reservoir(HIMU).However,the geochemical signatures of enriched mantle reservoirs recorded by OIBs are usually diluted,as these basalts were formed by melting of high-temperature mantle plumes beneath thin oceanic lithosphere.In comparison,continental intraplate volcanic rocks are often produced by low-degree melting of upwelling mantle beneath thick continental lithosphere,it is thus more likely that they can capture the original geochemical signals of enriched mantle reservoirs.Meanwhile,OIBs are mainly distributed in the Southern Hemisphere,and the latter can compensate for the lack of spatial representation of the former.Therefore,the continental volcanic rock has the potential to be a new window to understand the nature of the chemical reservoirs in the deep mantle.