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

学术价值：几十年来，人们对俯冲带进行了深入的研究，以了解全球以弧形表示的地表火山活动的物理和化学机制。然而，关于俯冲板块物质如何排放到地幔楔形中，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和Langmuir，1998]，但许多弧段似乎已经衰减到可以忽略的沉积物特征[Tera等人，1986年；Morris等人，1990年；Leeman等人，2005年]。另一些则被推断为携带了来自蚀变洋壳的强烈化学特征，尽管沉积物俯冲显然正在发生[Regelous等人，1997年；Singer等人，2007年]。这种差异是否与俯冲板块的热结构有关，即较热的板块在较浅的深度失去沉积物？或与弧前沉积有关？或者我们只是目前没有适当的方法来追踪所有弧区中不同的板坯通量？建议评估新的铊(Tl)稳定同位素系统学在量化汤加-克尔马德克弧、阿留申弧和中美洲弧的远洋沉积物和蚀变洋壳俯冲贡献方面的作用。Tl同位素体系是独一无二的，因为地幔在同位素上是均一的，并且亏损Tl，而远洋沉积物和蚀变洋壳则富含几个数量级的Tl，并显示出高度分馏的Tl同位素组成。铊同位素似乎非常适合用来确定板坯到弧状熔岩的流量。此外，由于俯冲洋壳的年龄范围很大，这三个目标弧线代表了不同的热体制。Tl同位素将与更传统的微量元素和同位素比率结合使用，以量化沉积物与蚀变洋壳的作用，并测试俯冲洋壳的年龄与弧形火山岩中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关于俯冲带熔融过程的模型中，这可能有助于解决熔融是由流体熔化还是由无水减压驱动的问题。对各个弧区中板材的流体和/或熔体流量的量化也可用于模拟板材/地幔界面的热状态，其中材料被排入地幔楔体。该项目将为世界卫生组织/麻省理工学院联合项目的一名研究生和一名新任命的世界卫生组织科学家(皮尼尔森)提供资金。一名本科生也将参与WHOI暑期学生奖学金计划(由NSF REU计划资助)，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学技术都非常先进，但在10-12周内进行短期项目是可能的。PI之前有来自英国牛津的四名本科生的经验，他们在密切的监督下学习了这项技术，并产生了可发表的结果。

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.