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

在世界海洋盆地的火山热点爆发的熔岩被认为是由浮力上涌的羽流提供的，这些羽流将物质从核心-地幔边界运送到羽流部分融化的浅层地幔。由上涌岩浆柱熔化产生的玄武岩熔体提供了地球深部组成的放射性成因同位素记录。高氦同位素比率（3He/4He）与行星的原始构造块有关，并且在海洋热点熔岩中有证据表明，地幔中存在保存原始（高）3He/4He的域。在整个新生代的大部分时间里，冰岛热点喷发的熔岩记录了最高的陆地地幔3He/4He比率。结合来自其他热点地区的地球化学证据，具有高3He/4He的冰岛熔岩提供了直接证据，表明尽管在地质时期脱气、俯冲带循环和大量地幔物质的对流混合，原始惰性气藏仍然存在于地球的动态地幔中。然而，地幔中3He/4He结构域的起源和长期保存仍然是活跃争论的来源。由于冰岛拥有最高的陆地地幔3He/4He，因此为探索尚不清楚的高3He/4He地幔储层的组成、起源和演化提供了最好的机会。高3He/4He水库有多少年了？它是如何形成的？这项工作将利用氧同位素、传统的放射性成因同位素系统（Sr、Nd、Hf、Pb和Os同位素）以及短寿命182Hf-182W系统的产物来研究冰岛羽流取样的高3He/4He储层的起源。最近的研究表明，冰岛地幔中的3He/4He结构域在地球历史早期（~4.45 Ga之前）形成。此外，在与冰岛早期柱状火山活动有关的62 Ma高3He/4He熔岩中发现了182W/184W异常。寿命较短的182Hf-182W系统，其中182Hf衰变成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，早期冰岛羽流熔岩中182W异常的发现表明，182Hf生命周期（即吸积后50 Ma）形成的储层在地幔中存活了4.5 Ga，并被冰岛羽流采样。从这些发现中产生了几个关键问题，并将作为拟议工作的一部分进行探讨。首先，如果具有182W异常的熔岩与来自早期（62 Ma）冰岛羽流的高3He/4He熔岩有关，那么我们有理由乐观地认为，中新世中期冰岛高3He/4He熔岩中也会发现182W异常，使它们成为182W分析的明确目标。其次，在新火山带的中等高3He/4He熔岩中保存了一个异常的129Xe特征，表明冥古宙地球化学特征在源自现代冰岛火山的地幔中存在。此外，这个保存下来的冥古宙129Xe异常为研究其他冥古宙地球化学特征（包括冥古宙182W异常）是否已经被冰岛的火山活动采样提供了强有力的理由。然而，全球没有一个具有冥古宙129Xe特征的熔岩被用于高精度的182W/184W分析。在同一样品上发现129Xe和182W异常，为研究原始高3He/4He地幔储层的成因和形成提供了重要契机。这项工作的结果将为地幔中3He/4He高储层的长期存在提供基本的新约束，并将阐明冰岛羽流采样的地幔中182W异常的存在。

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.