Extinct radiogenic isotope constraints on mantle dynamics

在过去的几年里，地幔地球化学一直受到新发现的小，但广泛的同位素异常的放射性成因的182 W和142 Nd的岩石中的太古代和新生代的衰变产物的长期灭绝的182 Hf和146 Sm（半衰期分别为9百万年和103百万年）。在地球动力学背景下，这些新发现意味着地球上冥古宙（> 4.0 Ga）分异过程的遗迹在整个地球历史中一直存在于地幔中，需要改变长期接受的范式，即地球地幔在地质时期被板块构造有效地搅动。然而，尽管有这些新发现，182 W和142 Nd同位素异常的确切起源及其在地球历史中的演变仍然是模糊的。流行的解释包括早期晚期单板的不完全添加，连续的核幔相互作用，岩浆海洋遗迹或早期壳幔分异的遗迹。从晚太古代到新生代，现有的182 W-142 Nd记录也存在很大的时间间隔，组合的182 W-142 Nd数据集很少。我们提出了一个高精度的182 W和142 Nd相结合的研究选定的基性岩套从太古代到中生代的年龄，以关闭这个时间差距，并评估上述对比的假设。这种测量是在分析极限，这些将进一步推动通过开发一个新的协议，142 Nd测量多集电极ICPMS。这些测量将辅之以主要/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。这些数据将特别有助于阐明地幔岩石圈，地壳污染和失踪的晚单板贡献的182 W清单的影响。这些补充数据中的许多已经可以用于我们的前体或伴随项目的样本。我们的182 W-142 Nd测量组合将涵盖迄今为止尚未探索的关键太古代到早元古代单元，从各种地幔，以及晚元古代到新生代基性岩，从各种设置，重点是OIBS和LIPs，都是利用各种地幔域。研究结果不仅为这些同位素异常的成因提供了新的线索，而且对认识地质时代的地幔动力学，包括下地幔低层异常物质（LLSVP）和超低层异常带（ULVZ）的起源和演化，以及地幔柱岩浆活动的起源等具有普遍意义。

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.