Tracing Ancient Subduction in the Lithospheric Mantle via Traditional and Non-Traditional Stable Isotopes

地球的外层被分解成构造板块，由地壳和上地幔组成。这些板块四处移动，产生火山、地震和山脉。板块构造，即这些板块的运动，是地球独有的，可能是地球生命的关键。板块构造可能在维持行星宜居性方面发挥着重要作用。它有助于缓和全球碳循环，从而影响气候。板块构造通过风化作用影响海洋营养物质的流动，这些风化作用既发生在板块聚集形成山脉的陆地上，也发生在板块分散的海底。在30亿到25亿年前板块构造的开始或变化与地球大气中氧气的增加之间可能存在联系。尽管板块构造对地球过程很重要，但板块构造何时开始以及开始是突然的、渐进的还是间歇性的，这些问题一直争论不休。板块构造的一个主要后果是，在俯冲带，一个板块滑到另一个板块下面，并沉入地幔，从而将表面物质输送到地幔。追踪地幔中的俯冲物质是一种潜在的方法，可以限制地球历史上某一特定时期板块构造的存在与否。该项目将使用一系列地球化学示踪剂来确定地幔中是否存在俯冲物质。该小组将研究地幔的碎片（称为捕虏体），这些碎片是通过火山带到地球表面的。天然放射性元素也将用于确定任何已确定的俯冲物质的年龄，这将允许对随时间的俯冲进行检查。这项工作将支持本科生和研究生的教育、研究和科学训练。研究生将通过在史密森国家自然历史博物馆（NMNH）实习，为博物馆的研究/策展/推广工作提供就业机会。追踪地幔中的俯冲物质，特别是次大陆岩石圈地幔（SCLM）中的俯冲物质，是限制太古宙和元古代板块构造存在与否的一种潜在机制。SCLM可能比对流地幔提供更可靠的古代俯冲过程的化学记录，因为它与对流混合分离，最古老的克拉通根的形成年龄可以追溯到太古代早期。虽然地壳再循环本身并不能证明板块构造的存在，但它是板块构造的必然结果。该研究团队将利用稳定同位素地球化学（胸径18O、胸径44/40Ca），结合微量元素和放射性成因同位素示踪剂，从克拉通（Kaapvaal、Slave、Rae）和非克拉通（纳瓦霍火山场）的地幔橄榄岩捕虏体中，确定scm中俯冲成分的存在（或不存在）。与正常地幔相比，俯冲岩石圈具有明显的<s:1> 18O和<s:1> 44/40Ca值。板块衍生的熔体和渗透地幔楔的流体可以改变其氧和钙同位素组成，根据俯冲成分的性质提高或降低这些值。将利用该模型与其他地球化学示踪剂之间的相关性来约束板块衍生熔体和流体的成因及其对地幔化学的影响。此外，放射性成因同位素系统（如Sm-Nd和Lu-Hf）与稳定同位素变化之间的相关性也将用于约束任何已确定的俯冲成分的年龄及其与熔体衰竭引起的克拉通岩石圈原始稳定的关系。令人惊讶的是，很少有研究对地幔捕虏体中稳定同位素和微量/放射性元素的耦合数据进行研究，特别是在同一研究中，没有研究对橄榄岩捕虏体的氧和钙同位素比值进行耦合研究。确定大陆岩石圈产生和随后变化的机制，以及这两者如何随时间变化，从而可以阐明板块构造开始的时间和大陆地壳产生的演化机制。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The outer layer of the Earth is broken into tectonic plates, consisting of the crust and uppermost mantle. Those plates move around producing volcanoes, earthquakes, and mountains. Plate tectonics, the movement of these plates, is unique to Earth and may be key to life on Earth. Plate tectonics likely plays an important role in maintaining planetary habitability. It helps to moderate the global carbon cycle which in turn affects climate. Plate tectonics affects nutrient fluxes to the oceans through weathering of material both on land where plates come together to build mountains, and on the seafloor where plates spread apart. There may be a link between the start of or change in plate tectonics between 3 and 2.5 billion years ago and the rise of oxygen in the Earth’s atmosphere. Despite the importance of plate tectonics to Earth processes, the question of when plate tectonics started and whether the start was sudden, gradual, or intermittent, are highly debated. One of the major consequences of plate tectonics is the delivery of surface material to the mantle at subduction zones, where one plate slides under another plate and sinks back into the mantle. Tracing subducted material in the mantle is one potential way to constrain the presence or absence of plate tectonics at a given time in Earth’s history. This project will use a series of geochemical tracers to identify the presence (or absence) of subducted material in the Earth’s mantle. The team will study pieces of the mantle (called xenoliths) which are brought to the Earth’s surface via volcanoes. Natural radioactive elements will also be used to constrain the ages of any identified subducted materials, which will allow examination of subduction over time. This work will support the education, research, and scientific training of both undergraduate and graduate students. The graduate student will train for career opportunities in research/curation/outreach at museums via an internship at the Smithsonian National Museum of Natural History (NMNH). Tracking subducted material in the mantle, and in particular in the sub-continental lithospheric mantle (SCLM), is one potential mechanism for constraining the presence or absence of plate tectonics in Archean and Proterozoic times. The SCLM may provide a more robust chemical record of ancient subduction processes than the convecting mantle because it is isolated from convective mixing, and the oldest cratonic roots have formation ages that extend back into the early Archean. Although crustal recycling in and of itself does not prove the existence of plate tectonics, it is a necessary consequence of plate tectonics. The research team will use stable isotope geochemistry (18O, 44/40Ca), in combination with trace element and radiogenic isotope tracers, to identify the presence (or absence) of subducted components in the SCLM as sampled by mantle peridotite xenoliths from cratonic (Kaapvaal, Slave, Rae) and non-cratonic (Navajo Volcanic Field) settings. Subducting lithosphere has distinct 18O and 44/40Ca values compared to normal mantle. Slab-derived melts and fluids that infiltrate the mantle wedge can alter its oxygen and calcium isotope composition, raising or lowering the values depending on the nature of the subducted components. Correlations between 18O and 44/40Ca values and other geochemical tracers will be used to constrain the origin and effects of slab-derived melts and fluids on mantle chemistry. In addition, correlations between radiogenic isotope systems (e.g., Sm-Nd and Lu-Hf) and stable isotope variations will also be used to constrain the age of any identified subduction components and their relationship to the original stabilization of cratonic lithosphere by melt depletion. Surprisingly, very few studies have examined coupled stable isotope and trace/radiogenic element data in mantle xenoliths and, in particular, no prior study has coupled oxygen and calcium isotope ratios of peridotite xenoliths in the same study. Determining the mechanism(s) by which continental lithosphere is generated and subsequently modified, and how both of these have varied through time, can thus shed light onto both the timing of plate tectonics onset and on the evolving mechanisms of continental crust production through time.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.