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，由地壳和最上面的地幔组成。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，可能是地球上生命的关键。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，进而影响气候。板块构造通过陆块聚集造山的陆地上的物质风化，以及板块分散的海底上的物质风化，影响向海洋的营养物质通量。在30亿至25亿年前板块构造的开始或变化与地球大气中氧气的上升之间可能存在联系。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，这一问题一直备受争议。板块构造的主要后果之一是在俯冲带将表面物质输送到地幔，在那里一个板块滑到另一个板块下面，然后沉入地幔。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特定时间板块构造的存在或不存在。该项目将使用一系列地球化学示踪剂来识别地幔中是否存在俯冲物质。该团队将研究通过火山带到地球表面的地幔碎片(称为捕虏体)。天然放射性元素还将用于限制任何已识别的俯冲物质的年龄，这将允许随着时间的推移对俯冲进行检查。这项工作将支持本科生和研究生的教育、研究和科学培养。这位研究生将通过在史密森国家自然历史博物馆(NMNH)的实习，为博物馆的研究/策展/拓展工作机会进行培训。追踪地幔中的俯冲物质，特别是次大陆岩石圈地幔中的俯冲物质，是限制太古代和元古界板块构造存在或不存在的一种潜在机制。与对流地幔相比，SCLM可能提供了更可靠的古代俯冲过程的化学记录，因为它是从对流混合中分离出来的，而且最古老的克拉通根的形成年龄可以追溯到太古宙早期。尽管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。研究小组将使用稳定同位素地球化学(18O，44/40Ca)，结合微量元素和放射性同位素示踪剂，以确定从克拉通(卡帕瓦尔、奴隶、RAE)和非克拉通(纳瓦霍火山场)环境中采集的地幔橄榄岩包体中是否存在俯冲成分。与正常地幔相比，俯冲岩石圈具有明显的18O和44/40Ca值。来自板岩的熔体和渗入地幔楔体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。18O和44/40Ca值之间的相关性以及其他地球化学示踪剂将被用来约束板岩起源的熔体和流体对地幔化学的影响。此外，放射性成因同位素系统(如Sm-ND和Lu-Hf)和稳定的同位素变化之间的相关性也将被用来限制任何已发现的俯冲成分的年龄及其与熔体枯竭对克拉通岩石圈的原始稳定的关系。令人惊讶的是，很少有研究检查地幔包体中稳定同位素和痕量/放射成因元素的耦合数据，特别是以前没有研究在同一研究中耦合橄榄岩包体的氧和钙同位素比率。确定大陆岩石圈形成和随后修改的机制(S)，以及这两者是如何随时间变化的，因此可以揭示板块构造开始的时间和大陆地壳生产随时间演变的机制。这一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智力优势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.