Preservation of Hadean Geochemical Signatures in the Icelandic High 3He/4He Mantle Domain

在世界大洋盆地的火山热点地区喷发的熔岩被认为是由浮力上升的羽流提供的，这些羽流将物质从核-地幔边界输送到浅地幔，在那里羽流部分融化。上升羽流融化产生的玄武岩熔体提供了地球深部组成的放射性同位素记录。氦同位素比值升高(3He/4He)与行星的原始构造块有关，海洋热点熔岩中有证据表明，地球地幔中保存有原始(高)3He/4He的区域幸存下来。在整个新生代，冰岛热点的熔岩喷发记录了最高的地幔3He/4He比值。与其他热点地区的地球化学证据一起，高3He/4He的冰岛熔岩提供了直接证据，表明原始稀有气藏在地球动力学地幔中持续存在，尽管在地质年代中经历了大量地幔物质的脱气、俯冲带再循环和对流混合。然而，地幔中升高的3He/4He磁区的起源和长期保存仍然是激烈争论的来源。由于冰岛拥有最高的3He/4He地幔，冰岛热点为探索鲜为人知的高3He/4He地幔储集层的组成、起源和演化提供了最好的机会。高3He/4He储集层形成时间有多长？它是如何形成的？这项拟议的工作将利用氧同位素、传统的放射成因同位素系统(锶、钕、氢、铅和铯同位素)和短暂的182Hf-182W系统的产物来研究冰岛烟柱采样的高3He/4He储集层的成因。最近的研究表明，冰岛地幔中升高的3He/4He域形成于地球历史的早期(早于~4.45Ga)。此外，在62 Ma高的3He/4He熔岩中还发现了182W/184W异常，这些熔岩与冰岛早期火山活动有关。短命的182Hf-182W系统，182Hf衰变到182W(t1/2=8.9 Ma)，可以用来探测行星吸积和分化的早期事件。因此，在早期冰岛热柱熔岩中发现的182W异常表明，在182Hf寿命期间(即吸积后50 Ma)形成的储集层在地幔中存活了4.5Ga，并被冰岛热柱采样。从这些调查结果中出现了几个关键问题，并将作为拟议工作的一部分进行探讨。首先，如果具有182W异常的熔岩与来自冰岛早期(62 Ma)的高3He/4He熔岩有关，那么有理由乐观地认为，在中新世中期冰岛高3He/4He熔岩中也将发现182W异常，这将使它们成为182W分析的明确目标。其次，在新火山带中等高的3He/4He熔岩中保存了异常的129Xe特征，这表明在起源现代冰岛火山活动的地幔中保留了Hadean的地球化学特征。此外，这一保存下来的Hadean 129Xe异常为调查其他Hadean地球化学特征，包括异常Hadean 182W，是否在冰岛火山活动中采集了样本提供了强有力的证据。然而，全球所有具有Hadean 129Xe签名的熔岩都不是高精度182W/184W分析的目标。同一样品中129Xe和182W异常的识别，为研究原始高3He/4He地幔储集层的成因和形成提供了重要契机。拟议工作的结果将为高3He/4He储集层在地幔中的长期生存提供新的根本限制，并将揭示冰岛羽流采样的地幔中182W异常的生存情况。

Lavas erupted at volcanic hotspots in the world's ocean basins are thought to be fed by buoyantly upwelling plumes that transport material from the core-mantle boundary to the shallow mantle where the plumes partially melt. The basaltic melts produced by melting upwelling plumes provide a radiogenic isotopic record of the composition of Earth's deep interior. Elevated helium isotope ratios (3He/4He) are associated with the primitive building blocks of planets, and there is evidence in oceanic hotspot lavas for the survival of domains in the Earth's mantle that preserve primitive (high) 3He/4He. Throughout much of the Cenozoic, lavas erupted at the Iceland hotspot record the highest terrestrial mantle 3He/4He ratios. Together with geochemical evidence from other hotspots, Icelandic lavas with high 3He/4He provide direct evidence that primitive noble gas reservoirs have persisted in Earth's dynamic mantle despite degassing, subduction zone recycling, and convective mixing of large volumes of mantle material over geologic time. However, the origin and long-term preservation of elevated 3He/4He domains in the Earth's mantle continues to be the source of active debate. Because it hosts the highest terrestrial mantle 3He/4He, the Iceland hotspot provides the best opportunity to explore the composition, origin and evolution of the poorly understood high 3He/4He mantle reservoir. How old is the high 3He/4He reservoir? How did it form?The proposed work will use oxygen isotopes, traditional radiogenic isotopic systems (Sr, Nd, Hf, Pb, and Os isotopes), and the products of the short-lived 182Hf-182W system to investigate the origin of the high 3He/4He reservoir sampled by the Iceland plume. Recent work indicates that the elevated 3He/4He domain in the Icelandic mantle formed early in Earth's history (prior to ~4.45 Ga). In addition, 182W/184W anomalies have been detected in 62 Ma high 3He/4He lavas associated with early Iceland plume volcanism. The short-lived 182Hf-182W system, where 182Hf decayed to 182W (t1/2 = 8.9 Ma), can be used to probe early events that date planetary accretion and differentiation. Thus, the discovery of 182W anomalies in early Iceland plume lavas indicates that reservoirs formed during the lifetime of 182Hf (i.e., 50 Ma after accretion) have survived 4.5 Ga in Earth's mantle to be sampled by the Iceland plume. Several key questions emerge from these findings, and will be explored as part of the proposed work. First, if lavas with 182W anomalies are associated with high 3He/4He lavas from the early (62 Ma) Icelandic plume, there is reason to be optimistic that 182W anomalies also will be found in mid-Miocene high 3He/4He Icelandic lavas, making them clear targets for 182W analyses. Second, the preservation of an anomalous 129Xe signature in a moderately high 3He/4He lava from the neovolcanic zone indicates that Hadean geochemical signatures have survived in the mantle sourcing modern Icelandic volcanism. Additionally, this preserved Hadean 129Xe anomaly provides a strong justification for investigating whether other Hadean geochemical signatures, including anomalous Hadean 182W, have been sampled by volcanism in Iceland. Yet, none of the lavas with demonstrated Hadean 129Xe signatures globally have been targeted for high-precision 182W/184W analyses. The identification of 129Xe and 182W anomalies on the same sample represents an important opportunity to investigate the origin and formation of the primitive high 3He/4He mantle reservoir. The results of the proposed work will provide fundamental new constraints on the long-term survival of the high 3He/4He reservoir in the Earth's mantle, and will shed light on the survival of 182W anomalies in the mantle sampled by the Iceland plume.