Extinct radiogenic isotope constraints on mantle dynamics

近年来，在太古宙和新生代岩石中发现了小而广泛的放射性成因182W和142W同位素异常，这些同位素异常是长期灭绝的182Hf和146Sm(半衰期分别为9Myir和103Myir)的衰变产物，从而推动了地幔地球化学的研究。在地球动力学的背景下，这些新的发现表明，在整个地球历史上，Hadean(&gt；4.0Ga)分异过程的遗迹在地幔中幸存下来，需要改变长期接受的范例，即地球地幔在地质时期被板块构造有效地搅动。然而，尽管有这些新的发现，182W和142W同位素异常的确切来源及其在地球历史上的演化仍然不清楚。常见的解释包括早期晚期斑块的不完全叠加、持续的核幔相互作用、岩浆海洋遗迹或早期壳幔分异的遗迹。从晚太古宙到新生代，现有的182W-142记录也有很大的时间差距，组合的182W-142数据集很少。我们提出了一项高精度的182W和142W联合研究，对从太古宙到显生宙的镁铁质岩套进行了精选，以缩小这一时间差距，并对上述对比假设进行了评估。这样的测量处于分析极限，将通过开发多收集器ICPMS的第142次测量的新方案来进一步推动这些测量。这些测量将得到主元素/微量元素、放射性同位素、高度亲铁元素(镁铁质样品)和惰性气体数据(OIB)的补充。这些数据将特别有助于阐明地幔岩石圈、地壳污染和遗失的晚期单板贡献对182W清单的影响。这些补充数据中的许多已经提供给我们的前体或配套项目的样本。我们的综合182W-142测量将涵盖迄今未勘探的关键太古代至早元古代单元，来自不同克拉通以及晚元古代至新生代不同背景的镁铁质岩石，重点关注正在开发各种地幔区域的OIBS和LIPE。这些结果不仅为这些同位素异常的成因提供了新的线索，而且对于从地质时代理解地幔动力学，包括低地幔不均质性的起源和演化，以及羽状岩浆活动的起源也具有普遍的意义。

Over the past years, mantle geochemistry has been spurred by new discoveries of small but widespread isotope anomalies of radiogenic 182W and 142Nd in rocks of Archean and Cenozoic age that are decay products of long extinct 182Hf and 146Sm (half lifes of 9 Myrs and 103 Myrs, respectively). In a geodynamic context, these new findings imply that vestiges of Hadean (> 4.0 Ga) differentiation processes on Earth have survived in the mantle throughout Earth history, requiring a change of the long accepted paradigm that Earth’s mantle has been efficiently stirred through geologic time by plate tectonics. Despite these new findings, however, the exact origins of the 182W and 142Nd isotope anomalies and their evolution through Earth history have remained ambiguous. Popular explanations include incomplete addition of the early late veneer, continuous core-mantle interaction, magma ocean relics or vestiges of early crust-mantle differentiation. There is also a large time gap in the available 182W-142Nd record ranging from the late Archean to the Cenozoic and combined 182W-142Nd datasets are scarce. We propose a combined high precision 182W and 142Nd study on selected mafic rock suites from Archean to Phanerozoic age to close this time gap and to evaluate the contrasting hypotheses above. Such measurements are at the analytical limits, and these will be pushed further by developing a new protocol for 142Nd measurements by Multicollector ICPMS. The measurements will be complemented by major/trace element, radiogenic isotope, highly siderophile element (mafic samples) and noble gas data (OIBs). These data will particularly serve to elucidate the influences of mantle lithosphere, crustal contamination and missing late veneer contributions on the 182W inventory. Many of these complementary data are already available for our samples from precursor or companion projects. Our combined 182W-142Nd measurements will cover hitherto unexplored key Archean to early Proterozoic units from various cratons as well as Late Proterozoic to Cenozoic mafic rocks from various settings, focusing on OIBS and LIPs that are all tapping various mantle domains. The results will not only provide new clues on the origin of these isotope anomalies, but will also have general significance for understanding mantle dynamics through geologic time, including the origin and evolution of lower mantle heterogeneities such as LLSVPs and ULVZs and the origin of plume-derived magmatism.