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

通过传统和非传统稳定同位素追踪岩石圈地幔的古代俯冲作用

• 批准号: 2234385
• 负责人: Jaime Barnes
• 金额: $ 50.58万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2023
• 资助国家: 美国
• 起止时间: 2023-07-01 至 2026-06-30
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2234385&HistoricalAwards=false
• 关键词: Tracing; Ancient; Subduction; Lithospheric; Mantle

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.

地球的外层被分成构造板块，由地壳和最上层地幔组成。这些板块的移动产生了火山、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。它有助于减缓全球碳循环，进而影响气候。板块构造通过板块聚集形成山脉的陆地上和板块分开的海底上的物质风化来影响进入海洋的养分通量。 3至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问题仍然存在很大争议。板块构造的主要后果之一是将表面物质输送到俯冲带的地幔，其中一个板块滑到另一个板块下方并沉回地幔中。追踪地幔中的俯冲物质是限制地球历史上特定时间板块构造是否存在的一种潜在方法。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检查俯冲作用。这项工作将支持本科生和研究生的教育、研究和科学培训。研究生将通过在史密森尼国家自然历史博物馆 (NMNH) 实习，获得博物馆研究/策展/外展方面的职业机会培训。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，是限制太古代和元古代板块构造存在或不存在的一种潜在机制。 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，因为它与对流混合隔离，而且最古老的克拉通根部的形成年龄可以追溯到太古代早期。尽管地壳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，通过来自克拉通（Kaapvaal、Slave、Rae）和非克拉通（纳瓦霍火山）的地幔橄榄岩捕虏体采样，确定 SCLM 中是否存在俯冲成分。 字段）设置。与正常地幔相比，俯冲岩石圈具有明显的 18O 和 44/40Ca 值。渗入地幔楔的板片衍生熔体和流体可以改变其氧和钙同位素组成，根据俯冲成分的性质升高或降低其值。 18O 和 44/40Ca 值以及其他地球化学示踪剂之间的相关性将用于约束板片衍生熔体和流体对地幔化学的起源和影响。此外，放射性同位素系统（例如，Sm-Nd和Lu-Hf）与稳定同位素变化之间的相关性也将用于限制任何已识别的俯冲成分的年龄及其与熔体消耗导致的克拉通岩石圈原始稳定的关系。令人惊讶的是，很少有研究检查地幔捕虏体中耦合的稳定同位素和痕量/放射性元素数据，特别是，先前的研究没有在同一研究中耦合橄榄岩捕虏体的氧和钙同位素比率。确定大陆岩石圈产生和随后改变的机制，以及这两种机制如何随时间变化，从而可以揭示板块构造开始的时间和大陆地壳生产随时间的演变机制。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。