Using Barium Isotopes to Investigate the Origin of Fluids in Subduction Zones

使用钡同位素研究俯冲带流体的起源

• 批准号: 1829546
• 负责人: Sune Nielsen
• 金额: $ 55.07万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-07-01 至 2022-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1829546&HistoricalAwards=false
• 关键词: Using; Barium; Isotopes; Investigate; Origin

Subduction zones are locations on Earth where oceanic crust and sediments (the subducted slab) are introduced back into the deep Earth resulting in the prominent arc volcanoes at the surface that are associated with substantial volcanic and earthquake hazards. Subduction zones are also the main places of mass transfer between the surface and deep Earth, which controls long-term climate and plays a critical role in the evolution of Earth's heat budget. It is well-known that material released from the subducted slab imparts distinct chemical signatures to arc volcanism and geochemical evidence suggests that both subducted sediment and hydrothermally altered oceanic crust (AOC) play significant roles in arc lava generation. However, the exact physical processes responsible for transporting slab material into the arc is the subject of significant recent debate. In essence, one model poses that all the subducted components are mixed at the top of the slab, forming a 'melange' layer, which subsequently is the main source region for arc lavas. The second model invokes that the general structure of the subducted slab is intact through a large portion of the subduction process and only when sediments become hot enough to melt and AOC dehydrates are these components released to the mantle wedge and induce melting. In this project, researchers use a novel isotopic tool to investigate which of these two models is correct.This team has identified the element barium (Ba) and its isotopes as a geochemical tracer that can be used to investigate the origin of material released from the subducting slab and, thereby, distinguish between models that invoke sediment melting and ocean crust dehydration versus melange melting as the primary source of slab material in arc lavas. Physically, the two end-member models of slab material transport are very different and have different consequences for the thermal structure, distribution of volcanoes, and chemical budgets of crustal recycling in subduction zones. However, both models equally predict most of the unique chemical and isotopic characteristica of arc lavas. It is, therefore, critical to develop scientific tests that are capable of distinguishing the two different models. In both models, fluids are important vectors of slab material transport. However, the ultimate source of these fluids are different in that fluids are sourced from melange layers in one model and primarily extracted from AOC in the other. Barium is a highly fluid mobile element that displays characteristic enrichment over similarly incompatible elements like lanthanum and thorium in arc lavas. These researchers argue that Ba isotopes likely displays different values in melange, sedimentary and AOC sources of fluids. The isotope composition of the excess Ba is, therefore, likely to constrain the ultimate source of the fluids that carry the Ba. They hypothesize that evidence based on Ba isotopes can provide new constraints on the slab material transport mechanism in the subduction zones they have selected for study. In terms of Broader Impacts, this project will support graduate students who will incorporate this work into their theses, and will train them in cutting-edge geochemical tools. The results of this research will also be of broad interest to other fields that investigate the physical parameters that govern subduction zone magmatism, like seismologists and magnetotelluricists who use their tools to locate fluid flow in subduction zones.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.

俯冲带是地球上的位置，将海洋壳和沉积物（俯冲平板）引入深层地球，从而导致表面上突出的弧火山与大量的火山和地震危害相关。俯冲带也是地表与深地球之间传质的主要场所，它控制着长期的气候，并在地球热预算的进化中起着至关重要的作用。众所周知，从俯冲板上释放的材料赋予了弧火山的不同化学特征，地球化学证据表明，俯冲的沉积物和水热改变的海洋壳（AOC）在弧熔岩产生中起着重要作用。但是，负责将平板材料运输到ARC的确切物理过程是最近重大争论的主题。从本质上讲，一个模型认为所有俯冲成分都在平板顶部混合，形成了“混合物”层，后来是弧熔岩的主要源区域。第二个模型表明，俯冲板的一般结构通过俯冲过程的很大一部分完好无损，只有当沉积物变得足够热以熔化并且AOC脱水时，这些成分才释放到地幔楔并诱导熔化。 In this project, researchers use a novel isotopic tool to investigate which of these two models is correct.This team has identified the element barium (Ba) and its isotopes as a geochemical tracer that can be used to investigate the origin of material released from the subducting slab and, thereby, distinguish between models that invoke sediment melting and ocean crust dehydration versus melange melting as the primary source of slab material in arc lavas.从物理上讲，平板材料运输的两个末端模型截然不同，对俯冲带中的地壳回收的热结构，火山的分布和化学预算产生了不同的影响。但是，这两个模型同样预测了弧熔岩的大多数独特化学和同位素特征。因此，开发能够区分两个不同模型的科学测试至关重要。在这两种模型中，流体都是平板材料传输的重要载体。但是，这些流体的最终来源是不同的，因为流体来自一种模型中的混杂层，主要是从另一个模型中提取的。钡是一种高流动性的移动元件，它表现出与类似不相容的元素（如弧形熔岩中的Lanthanum和Thorium）相似的特征富集。这些研究人员认为，BA同位素可能在流体的混合物，沉积和AOC来源中显示出不同的值。因此，多余BA的同位素组成可能会限制携带BA的流体的最终来源。他们假设基于BA同位素的证据可以在他们选择进行研究的俯冲区中对平板材料传输机制的新约束。在更广泛的影响方面，该项目将支持将这项工作纳入本文的研究生，并将培训他们的尖端地球化学工具。这项研究的结果也将引起其他领域的广泛关注，这些领域研究了俯冲带岩浆的物理参数，例如地震学家和磁性智能主义者，他们使用其工具在俯冲区中定位流体流动。这奖反映了NSF的法定任务，并通过使用基金会的智力效果和宽阔的范围进行评估，并通过评估值得评估。

项目成果

Barium isotope systematics of subduction zones

• DOI: 10.1016/j.gca.2020.02.006
• 发表时间: 2020-04
• 期刊: Geochimica et Cosmochimica Acta
• 影响因子: 5
• 作者: S. Nielsen;Yunchao Shu;M. Auro;G. Yogodzinski;R. Shinjo;T. Plank;S. Kay;T. Horner

Thallium Isotope Fractionation During Magma Degassing: Evidence From Experiments and Kamchatka Arc Lavas

• DOI: 10.1029/2020gc009608
• 发表时间: 2021-04
• 期刊: Geochemistry
• 影响因子: 3.7
• 作者: S. Nielsen;Yunchao Shu;Bernard J. Wood;J. Blusztajn;M. Auro;C. Ashley Norris;G. Wörner

Barium Isotopes: Drivers, Dependencies, and Distributions through Space and Time

钡同位素：空间和时间的驱动因素、依赖性和分布

• DOI: 10.1017/9781108865845
• 发表时间: 2021
• 期刊: Elements in geochemical tracers in earth system science
• 作者: Horner, Tristan J;Crockford, Peter W
• 通讯作者: Crockford, Peter W