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

基本信息

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

项目摘要

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.

俯冲带是地球上洋壳和沉积物（俯冲板块）被引入地球深处的位置，导致地表出现突出的弧火山，与大量火山和地震灾害相关。俯冲带也是地球表面和深层之间物质传递的主要场所，它控制着长期气候，并在地球热量收支的演变中发挥着关键作用。众所周知，俯冲板块释放的物质赋予弧火山活动独特的化学特征，地球化学证据表明俯冲沉积物和热液蚀变的洋壳（AOC）在弧熔岩的生成中发挥着重要作用。然而，负责将板坯材料输送到电弧中的确切物理过程是最近争论的主题。从本质上讲，一种模型认为所有俯冲成分都在板片顶部混合，形成“混合”层，该层随后成为弧熔岩的主要源区。第二个模型认为，在俯冲过程的大部分时间里，俯冲板片的总体结构是完整的，并且只有当沉积物变得足够热以熔化并且AOC脱水时，这些成分才会被释放到地幔楔并引起熔化。在这个项目中，研究人员使用一种新型同位素工具来研究这两个模型中哪一个是正确的。该团队已经确定了元素钡 (Ba) 及其同位素作为地球化学示踪剂，可用于研究从俯冲板片，从而区分调用沉积物熔化和洋壳脱水的模型与作为弧熔岩板片材料主要来源的混合岩熔化的模型。从物理上讲，板片物质输运的两个端元模型非常不同，并且对俯冲带的热结构、火山分布和地壳循环的化学预算有不同的影响。然而，这两个模型同样预测了弧熔岩的大部分独特化学和同位素特征。因此，开发能够区分这两种不同模型的科学测试至关重要。在这两个模型中，流体都是板坯材料传输的重要载体。然而，这些流体的最终来源有所不同，一个模型中的流体源自混杂层，而另一种模型中的流体则主要从 AOC 中提取。钡是一种高度流动的移动元素，在弧熔岩中表现出比类似的不相容元素（如镧和钍）富集的特征。这些研究人员认为，Ba 同位素可能在混杂岩、沉积岩和 AOC 流体来源中表现出不同的值。因此，过量 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同位素的证据可以为他们选择研究的俯冲带中的板片物质传输机制提供新的约束。就更广泛的影响而言，该项目将支持研究生将这项工作纳入他们的论文，并对他们进行尖端地球化学工具的培训。这项研究的结果也将引起其他研究俯冲带岩浆作用物理参数的领域的广泛兴趣，例如地震学家和大地电磁学家，他们使用他们的工具来定位俯冲带中的流体流动。该奖项反映了 NSF 的法定使命，并已被通过使用基金会的智力优点和更广泛的影响审查标准进行评估，认为值得支持。

项目成果

期刊论文数量（4）

专著数量（0）

科研奖励数量（0）

会议论文数量（0）

专利数量（0）

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
通讯作者：
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
通讯作者：
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