Using Thallium Isotopes to Assess Relative Contributions of Pelagic Sediments and Altered Oceanic Crust to Arc Magmas

使用铊同位素评估远洋沉积物和蚀变洋壳对弧岩浆的相对贡献

• 批准号: 1119373
• 负责人: Sune Nielsen
• 金额: $ 29.67万
• 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: 2011
• 资助国家: 美国
• 起止时间: 2011-07-01 至 2015-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1119373&HistoricalAwards=false
• 关键词: Using; Thallium; Isotopes; Assess; Relative

Intellectual merit: For decades, subduction zones have been studied intensely to understand the physical and chemical mechanisms responsible for the surface volcanism expressed as arcs around the globe. However, there are still large uncertainties about how subducted slab material is discharged into the mantle wedge. For example, despite almost ubiquitous sediment subduction [Plank and Langmuir, 1998], many arc segments appear to have attenuated to negligible sediment signatures [Tera et al., 1986; Morris et al., 1990; Leeman et al., 2005]. Other are inferred to have carried strong chemical signatures from altered ocean crust, even though sediment subduction is clearly taking place [Regelous et al., 1997; Singer et al., 2007]. Are such discrepancies related to the thermal structure of subducting slabs, whereby hotter slabs lose sediment at shallower depths? Or related to sediment accretion in the forearc? Or do we simply not currently have the appropriate means of tracing the different slab fluxes in all arcs? It is proposed to evaluate the utility of novel thallium (Tl) stable isotope systematics in quantifying subduction contributions of pelagic sediments and altered oceanic crust from the Tonga-Kermadec, Aleutian and Central American arcs. The Tl isotope system is unique because the mantle is isotopically homogenous and depleted in Tl, whereas pelagic sediments and altered oceanic crust are enriched in Tl by several orders of magnitude and exhibit highly fractionated Tl isotope compositions. Thallium isotopes appear to be ideally suited to determine slab fluxes to arc lavas. In addition, the three targeted arcs represent different thermal regimes due to the large age range of the subducting oceanic crust. Tl isotopes will be used in conjunction with more conventional trace element and isotope ratios to quantify the role of sediments versus altered oceanic crust and test whether there is a relationship between the age of the subducting oceanic crust and the components incorporated into the arc volcanics. Broader impacts: The results obtained in this study are expected to be of broad interest to all scientists working on subduction zones. Geodynamicists may, for example, be able to incorporate quantitative fluxes into their models on subduction zone melting processes, which could help resolve if melting is driven by fluid fluxing or anhydrous decompression. Quantification of fluid and/or melt fluxes from the slab in individual arcs may also be used to model the thermal state of the slab/mantle interface, where material is discharged to the mantle wedge. This project will provide funds for a WHOI/MIT Joint Program graduate student and a newly appointed WHOI scientist (PI Nielsen). An undergraduate student will also be involved through the WHOI Summer Student Fellowship Program (funded by the NSF REU program), which provides research funds for undergraduate students from across the country. Even though both the chemical separation and mass spectrometric techniques used to measure Tl isotopes are highly advanced, short projects carried out over a 10?12 week period are possible. The PI has previous experience from four undergraduate students in Oxford, UK, who with close supervision learnt the technique and produced publishable results.

学术价值：几十年来，人们对俯冲带进行了深入研究，以了解导致全球范围内以弧形表示的地表火山活动的物理和化学机制。然而，俯冲板片物质如何排放到地幔楔中仍然存在很大的不确定性。 例如，尽管沉积物俯冲几乎无处不在[Plank and Langmuir, 1998]，但许多弧段似乎已经减弱到可以忽略不计的沉积物特征[Tera et al., 1986；莫里斯等人，1990； Leeman 等人，2005]。 尽管沉积物俯冲明显发生，但其他物质被推断带有来自改变的海洋地壳的强烈化学特征[Regelous et al., 1997;辛格等人，2007]。 这种差异是否与俯冲板片的热结构有关，即较热的板片会在较浅的深度失去沉积物？ 还是与弧前沉积物堆积有关？ 或者我们目前是否没有适当的方法来追踪所有弧中的不同板通量？ 建议评估新型铊 (Tl) 稳定同位素系统学在量化汤加-克马德克、阿留申和中美洲弧的远洋沉积物和蚀变洋壳的俯冲贡献方面的效用。 Tl同位素系统是独特的，因为地幔是同位素均质的，并且Tl贫乏，而远洋沉积物和蚀变洋壳的Tl富集了几个数量级，并且表现出高度分馏的Tl同位素组成。铊同位素似乎非常适合确定弧熔岩的板通量。此外，由于俯冲洋壳的年龄范围较大，三个目标弧代表不同的热状态。 Tl同位素将与更传统的微量元素和同位素比率结合使用，以量化沉积物与改变的洋壳的作用，并测试俯冲洋壳的年龄与纳入弧火山岩的成分之间是否存在关系。 更广泛的影响：这项研究获得的结果预计将引起所有研究俯冲带的科学家的广泛兴趣。例如，地球动力学家可能能够将定量通量纳入他们的俯冲带熔化过程模型中，这有助于解决熔化是否是由流体通量或无水减压驱动的问题。 来自各个弧中板片的流体和/或熔体通量的量化也可用于模拟板片/地幔界面的热状态，其中材料被排放到地幔楔。该项目将为 WHOI/MIT 联合项目研究生和新任命的 WHOI 科学家（PI Nielsen）提供资金。一名本科生还将通过 WHOI 暑期学生奖学金计划（由 NSF REU 计划资助）参与其中，该计划为全国各地的本科生提供研究资金。尽管用于测量 Tl 同位素的化学分离和质谱技术都非常先进，但在 10 至 12 周内开展的短期项目也是可能的。该 PI 之前曾从英国牛津的四名本科生那里获得过经验，他们在密切监督下学习了该技术并产生了可发表的结果。