EAGER: Collaborative Research: Development and Application of Sr Stable Isotopes as a Novel Tracer of Carbonate Through Subduction

EAGER：合作研究：Sr稳定同位素的开发和应用作为俯冲碳酸盐的新型示踪剂

• 批准号: 1939189
• 负责人: Rita Parai
• 金额: $ 3.85万
• 依托单位: Washington University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-09-01 至 2023-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1939189&HistoricalAwards=false
• 关键词: EAGER; Collaborative; Research; Development; Application

Geological processes play an important role in regulating atmospheric oxygen and carbon, and thus surface temperatures, on timescales of ten million years and greater. Carbon is transferred from the surface of the Earth into the mantle by the process of plate subduction, where oceanic plates carrying carbon-rich rocks and sediments sink into the mantle. During the initial stages of subduction, some fraction of the carbon in the sinking plate is removed and transported back to the Earth's surface in the magmas of subduction-zone volcanoes, such as those of the Pacific Ring of Fire. To account for the mass balance of the carbon cycle, and to fully understand variations in Earth's climate on geological time scales, it is therefore necessary to determine how much subducting carbon is rapidly returned back to Earth's surface, and how much is instead transported deep into the mantle. Rates of carbon recycling are difficult to measure directly, however, because carbon dissolved in magmas is sequestered into a separate vapor phase during magmatic ascent, traversing through the crust and into the atmosphere at highly variable rates, and via diffuse pathways. As an alternative to direct measurements of carbon fluxes through subduction zone volcanoes, geochemical proxies can be used to estimate the amount of carbon that was initially present in the magmas. This study will incorporate recent advances in analytical techniques that will use the strontium isotope system as a proxy, with applications to carbonate recycling in particular. The proposed work aims to advance understanding of chemical cycling at subduction zones while promoting teaching, training, and learning. This project involves mentoring of early-career researchers from under-represented groups in the Earth sciences, as well as a first-year Master's student. The proposed work also will build new collaborative relationships among early career faculty members of UMass Amherst and Washington University in St. Louis.Strontium is a potentially powerful proxy of carbonate recycling though subduction zones because it is typically present at high abundances in carbonates relative to the mantle, and because carbonates have distinct stable Sr isotope compositions. The project team will develop new procedures for the analysis and interpretation of the Sr stable isotope system among igneous rocks from volcanic arcs by building a data framework in the Central American Volcanic Arc (CAVA), where there is a thick layer of subducting sedimentary carbonate, the volcanic rocks and gases have been extensively measured and characterized by previous studies, and there is good geochemical evidence for variable carbonate recycling efficiency. Combined measurements of Sr stable and radiogenic isotope ratios via double-spike TIMS will provide the ability to accurately estimate the Sr flux from the subducting carbonate to the volcanic arc, which can in turn be used to estimate the rates of carbon recycling. If successful, this study will provide novel constraints on the global carbon cycle, establish δ88/86Sr isotope systematics among subducting components and their associated volcanics, and provide the basis for further studies of arc geochemical transport. δ88/86Sr values will be combined with radiogenic Sr isotopes to accurately determine the total carbonate-derived Sr budget of the arc volcanics and whether these systems can be used to assess the efficiency of, and the mechanisms that enable, the recycling of subducting carbonate into volcanic arcs. If the δ88/86Sr system at CAVA provides a benchmark on carbon recycling through volcanic arcs, then these tracers can be used to constrain the fate of subducted carbonate in other arcs around the world. Once modern-day stable Sr isotope arc systematics are established, this proxy can also potentially be used to assess the variability of subducting carbonate in past eras, providing transformative insight into the variability of the global carbon cycle throughout Earth's history. The results of the proposed work will be of interest to the wide geoscience community, including low-temperature geochemistry and Earth history communities interested in long-term variations in surface carbon reservoirs.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.

地质过程在调节大气中的氧和碳以及因此在一千万年甚至更长的时间尺度上的地表温度方面发挥着重要作用。碳通过板块俯冲过程从地球表面转移到地幔中，其中携带富含碳的岩石和沉积物的海洋板块沉入地幔中。在俯冲的初始阶段，下沉板块中的一部分碳被去除，并在俯冲带火山（例如太平洋火环火山）的岩浆中被运回地球表面。为了解释碳循环的质量平衡，并充分了解地球气候在地质时间尺度上的变化，因此有必要确定有多少俯冲碳快速返回地球表面，以及有多少碳被输送到深处地幔。然而，碳回收率很难直接测量，因为溶解在岩浆中的碳在岩浆上升过程中被隔离到单独的气相中，以高度可变的速率穿过地壳并通过扩散路径进入大气。作为直接测量通过俯冲带火山的碳通量的替代方法，地球化学代理可用于估计岩浆中最初存在的碳量。这项研究将结合分析技术的最新进展，使用锶同位素系统作为代理，特别是在碳酸盐回收方面的应用。拟议的工作旨在增进对俯冲带化学循环的了解，同时促进教学、培训和学习。该项目涉及对来自地球科学领域代表性不足群体的早期职业研究人员以及一年级硕士生的指导。拟议的工作还将在麻省大学阿默斯特分校和圣路易斯华盛顿大学的早期职业教员之间建立新的合作关系。锶是俯冲带碳酸盐回收的潜在强大代表，因为相对于碳酸盐，它通常以高丰度存在。地幔，并且因为碳酸盐具有独特的稳定的锶同位素组成。该项目团队将通过在中美洲火山弧（CAVA）建立数据框架来开发新的程序，用于分析和解释火山弧火成岩中的锶稳定同位素系统，那里有厚厚的俯冲沉积碳酸盐层，以前的研究已经对火山岩和​​气体进行了广泛的测量和表征，并且有良好的地球化学证据表明碳酸盐回收效率的变化。通过双尖峰 TIMS 对 Sr 稳定同位素比和放射性同位素比进行综合测量，将能够准确估计从俯冲碳酸盐到火山弧的 Sr 通量，进而可用于估计碳回收率。如果成功，这项研究将为全球碳循环提供新的约束，建立俯冲成分及其相关火山岩之间的88/86Sr同位素系统学，并为弧地球化学输运的进一步研究提供基础。 δ88/86Sr 值将与放射性 Sr 同位素相结合，以准确确定弧火山岩中碳酸盐衍生的 Sr 预算总量，以及这些系统是否可用于评估回收的效率和机制将碳酸盐俯冲到火山弧中。如果 CAVA 的 88/86Sr 系统提供了火山弧碳回收的基准，那么这些示踪剂可用于限制世界其他弧中俯冲碳酸盐的命运。一旦建立了现代稳定的锶同位素弧系统学，该代理也可用于评估过去时代俯冲碳酸盐的变化性，从而为整个地球历史上全球碳循环的变化性提供变革性的见解。拟议工作的结果将引起广泛的地球科学界的兴趣，包括对表面碳库长期变化感兴趣的低温地球化学和地球历史界。该奖项反映了 NSF 的法定使命，并被认为值得通过以下方式获得支持：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。