| 英文摘要: | 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. |