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

智力优点：几十年来，人们对俯冲带进行了深入研究，以了解地球仪周围的火山活动的物理和化学机制。然而，关于俯冲板片物质是如何排入地幔楔的，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年]，但许多弧段似乎已衰减到可忽略不计的沉积物特征[Tera等人，1986; Morris等人，1990; Leeman等人，2005年]。 其他被推断为具有来自改变的海洋地壳的强烈化学特征，即使沉积物俯冲显然正在发生[Regelous等人，1997; 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？ 或者与弧前沉积物的堆积有关？ 或者，我们只是目前没有适当的手段来追踪所有弧中不同的板通量？ 建议评估新的铊（Tl）稳定同位素系统学的效用，在量化俯冲贡献的远洋沉积物和改变大洋地壳从汤加Kermadec，阿留申群岛和中美洲弧。铊同位素系统是独特的，因为地幔是同位素均匀和亏损的铊，而远洋沉积物和蚀变洋壳富集铊的几个数量级，并表现出高度分馏的铊同位素组成。铊同位素似乎是理想的适合于确定板通量弧熔岩。此外，由于俯冲洋壳的年龄范围很大，三个目标弧代表不同的热状态。铊同位素将与更传统的微量元素和同位素比率一起使用，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火山岩成分之间是否存在关系。 更广泛的影响：这项研究所获得的结果预计将引起所有从事俯冲带研究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，这可能有助于解决熔融是由流体通量还是无水减压驱动的问题。 量化的流体和/或熔体通量从板在各个弧也可以用来模拟板/地幔界面的热状态，其中材料被排出到地幔楔。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。一名本科生也将通过WHOI暑期学生奖学金计划（由NSF REU计划资助）参与，该计划为来自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年以上进行的短期项目仍然存在。12周是可能的。PI之前有来自英国牛津大学的四名本科生的经验，他们在密切监督下学习了该技术并产生了可持续的结果。

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.