Evaluating links between primordial mantle domains and deep mantle structures using 182W and 3He/4He

许多地幔热柱衍生的大洋岛玄武岩包含多个地幔成分，代表再循环的、环境的和原始的同位素域。由板内火山采集的这些组分的混合物，定义了每个羽流特有的同位素混合阵列。其中一个成分FOZO(“震源区”)是不寻常的，因为它在许多地幔热柱中都是常见的。FOZO组分的特点是铅、钕、锶和氦同位素的组成范围特定且相对较窄，大多数羽流的同位素组成重叠。值得注意的是，FOZO类型的储集层保留了原始的氦同位素组成，与地幔的其他部分相比，这些组成没有被随后的过程显著改变。尽管FOZO的锶-铅-钕组成定义自首次确定以来经历了几次修订，但原始氦同位素仍然是这一储集层类型的重要定义特征。最近，原始氦比与深部地幔中地震可探测的结构以及羽流系统中的参数，如物质浮力和温度有关，这表明原始氦升高的储集层在地震、热和化学上与“典型”的下地幔物质截然不同。这项研究的发现将提高我们对早期地球的地球化学演化的理解。与地球物理学家和地球动力学家合作，这项研究将提供对原始地幔区域的大小、形状和发展的限制，这些区域经常导致深源火山活动。此外，这个项目将为PI提供两年的支持，并扩大她在同位素地球化学方面的专业知识，包括短暂的同位素系统。最近，在羽状火山作用中发现了钨同位素的变化，这种变化是由一种已灭绝的Hf同位素的衰变引起的。在岩柱中，在含有原始氦的样品中发现了异常的钨，这意味着FOZO型熔体可能存在第二原始同位素约束。初步发现，携带异常钨和氦的FOZO类物质与核幔边界附近探测到的超低速带有关。在羽流中发现的其他被认为是对周围地幔区域进行采样的成分没有明显的氦和钨异常，但需要更多的数据来充分测试地震结构和同位素组成之间的联系。为了测试这一设想，将对缺乏任何可回收地壳材料的证据的羽流进行氦和钨同位素分析。路易斯维尔、胡安·费尔南德斯、鲍伊、科布和卡罗琳羽状物由于缺乏可回收组分和可变的氦同位素组成而被选中。然后，新的数据将与地震解析的下地幔结构的空间相关性(或缺乏空间相关性)联系起来，以确定FOZO型熔体、周围的下地幔成分和深地幔结构之间的关系。这一裁决反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Many mantle plume-derived ocean island basalts entrain multiple mantle components, representing recycled, ambient, and primordial isotopic domains. The mixtures of these components, sampled by intraplate volcanoes, define isotopic mixing arrays unique to each plume. One of these components, FOZO (“Focal Zone”), is unusual in that it is common to many mantle plumes. The FOZO component is characterized by a specific and relatively narrow compositional range in lead, neodymium, strontium, and helium isotopes, where isotopic compositions of most plumes overlap. Notably, FOZO-type reservoirs retain primordial helium isotopic compositions that have not been as significantly modified by subsequent processes compared to other parts of the mantle. While the strontium-lead-neodymium compositional definition of FOZO has undergone several revisions since it was first identified, primordial helium isotopes have remained an important defining characteristic of this reservoir type. Primordial helium ratios have recently been linked to seismically-detectable structures in the deep mantle as well as parameters in plume systems such as material buoyancy and temperature, suggesting that reservoirs with elevated primordial helium are seismically, thermally, and chemically distinct from “typical” lower mantle material. The findings from this study will improve our understanding of the geochemical evolution of the early Earth. In collaboration with geophysicists and geodynamicists, this study will provide constraints on the size, shape, and development of primordial mantle domains that frequently contribute to deeply-sourced volcanism. In addition, this project will provide two years of support for the PI and broaden her expertise in isotope geochemistry to include short-lived isotope systematics.Recently, variations in tungsten isotopes, driven by the decay of an extinct isotope of hafnium, were discovered in plume-derived volcanism. In plumes, anomalous tungsten is found in samples with primordial helium, implying that a second primordial isotopic constraint may exist for FOZO-type melts. Tentatively, FOZO-type material carrying anomalous tungsten and helium have been linked to ultra-low velocity zones detected near the core-mantle boundary. Other compositions thought to sample ambient mantle domains found in plumes lack distinct helium and tungsten anomalies, but additional data are needed to fully test the links between seismic structures and isotopic composition. To test this scenario, plumes that lack evidence of any recycled crustal material, a possible contaminant, will be analyzed for helium and tungsten isotopes. The Louisville, Juan Fernandez, Bowie, Cobb, and Caroline plumes have been selected for their lack of recycled componentry and variable helium isotopic compositions. The new data will then be linked to spatial correlations (or lack thereof) with seismically-resolved lower mantle structures to determine the relationship of FOZO-type melts, ambient lower mantle compositions, and the structure of the deep mantle.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.