Primitive Helium Is Sourced From Seismically Slow Regions in the Lowermost Mantle
Primitive Helium Is Sourced From Seismically Slow Regions in the Lowermost Mantle
复制标题
原始氦源自最下地幔的地震缓慢区域
DOI：
10.1029/2019gc008437
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Romanowicz, B.
中科院分区：
文献类型：
--
作者：
Williams, C. D.;Mukhopadhyay, S.;Rudolph, M. L.;Romanowicz, B.
地球科学的一个主要目标是了解地球表面熔岩的地球化学特征与地球内部特定区域的位置和形成历史之间的关系。例如，一些最有力的原始物质保存的证据来自海洋岛屿玄武岩的低4he /3He比率，但原始氦储层的位置仍然未知。在这里，我们结合了全地幔地震层析成像，地幔流动模拟，以及海洋岛屿玄武岩中4he /3He比率的新测量和现有测量的全球汇编，以限制原始4he /3He物质的来源位置。我们的地球动力学模拟预测，现今地幔柱的地表表现将从它们的下地幔源位置横向偏移。当考虑到这种横向偏移时，海洋玄武岩的最小4he /3He比率与地震缓慢区之间存在很强的关系，这些区域通常位于两个大的低横波速度省（llsvp）内。相反，最大208pb */206Pb*比值与最下地幔地震慢区之间没有显著关系。这些结果表明，原始物质在地理上仅限于llsvp，而回收物质更广泛地分布于下地幔。llsvp的原始性质表明这些区域并不完全由回收板组成，而互补的氙和钨同位素异常要求llsvp的原始部分在地球吸积期间形成，在月球形成的巨大撞击中幸存下来，并在随后的45亿年的地幔对流中保持相对未混合。
A major goal in Earth Science has been to understand how geochemical characteristics of lavas at the Earth's surface relate to the location and formation history of specific regions in the Earth's interior. For example, some of the strongest evidence for the preservation of primitive material comes from low4He/3He ratios in ocean island basalts, but the location of the primitive helium reservoir(s) remains unknown. Here we combine whole‐mantle seismic tomography, simulations of mantle flow, and a global compilation of new and existing measurements of the4He/3He ratios in ocean island basalts to constrain the source location of primitive4He/3He material. Our geodynamic simulations predict the present‐day surface expression of plumes to be laterally offset from their lower mantle source locations. When this lateral offset is accounted for, a strong relationship emerges between minimum4He/3He ratios in oceanic basalts and seismically slow regions, which are generally located within the two large low shear‐wave velocity provinces (LLSVPs). Conversely, no significant relationship is observed between maximum208Pb*/206Pb*ratios and seismically slow regions in the lowermost mantle. These results indicate that primitive materials are geographically restricted to LLSVPs, while recycled materials are more broadly distributed across the lower mantle. The primitive nature of the LLSVPs indicates these regions are not composed entirely of recycled slabs, while complementary xenon and tungsten isotopic anomalies require the primitive portion of the LLSVPs to have formed during Earth's accretion, survived the Moon‐forming giant impact, and remained relatively unmixed during the subsequent 4.5 billion years of mantle convection.
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