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

地质过程在调节大气中的氧和碳，从而在一千万年或更长的时间尺度上调节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。在俯冲的初始阶段，下沉板块中的一部分碳被移除，并通过俯冲带火山的岩浆被运送回地球表面，例如太平洋火环的火山。为了解释碳循环的质量平衡，并充分了解地质时间尺度上地球气候的变化，因此有必要确定有多少俯冲碳迅速返回地球表面，以及有多少被输送到地幔深处。然而，碳循环的速率很难直接测量，因为溶解在岩浆中的碳在岩浆上升过程中被隔离成单独的气相，以高度可变的速率穿过地壳进入大气，并通过扩散途径。作为直接测量俯冲带火山碳通量的替代方法，地球化学代用品可用于估计最初存在于岩浆中的碳量。这项研究将结合分析技术的最新进展，将使用锶同位素系统作为替代，特别是碳酸盐回收的应用。拟议的工作旨在促进对俯冲带化学循环的理解，同时促进教学，培训和学习。该项目涉及指导来自地球科学代表性不足群体的早期职业研究人员以及一年级硕士生。拟议的工作还将建立新的合作关系，早期职业教师之间的马萨诸塞大学阿默斯特分校和华盛顿大学圣路易斯分校。锶是一个潜在的强有力的代理碳酸盐循环虽然俯冲带，因为它通常是在高丰度的碳酸盐相对于地幔，因为碳酸盐有独特的稳定锶同位素组成。项目小组将制定新的程序，分析和解释火山弧火成岩中的锶稳定同位素系统，在中美洲火山弧建立一个数据框架，那里有一层厚的俯冲沉积碳酸盐，火山岩和气体已在以前的研究中得到广泛测量和表征，碳酸盐再循环效率的变化有很好的地球化学证据。通过双尖峰TIMS的Sr稳定和放射性同位素比值的组合测量将提供准确估计从俯冲碳酸盐岩到火山弧的Sr通量的能力，这反过来又可以用于估计碳循环的速率。如果成功，这项研究将为全球碳循环提供新的限制，建立δ 88/86 Sr同位素系统学研究，为进一步研究弧地球化学迁移提供了依据。&九四八; 88/86 Sr值将与放射成因Sr同位素相结合，以准确地确定弧火山岩的碳酸盐来源的Sr总预算，以及这些系统是否可用于评估俯冲碳酸盐再循环到火山弧中的效率和机制。如果δ CAVA的88/86 Sr体系为火山弧的碳循环提供了一个基准，这些示踪剂可用于控制世界其他弧中俯冲碳酸盐的命运。一旦现代稳定的Sr同位素弧系统学建立，这个代理也可以潜在地用于评估过去时代的俯冲碳酸盐的变化，提供变革性的洞察全球碳循环的变化在整个地球的历史。拟议工作的结果将引起广大地球科学界的兴趣，包括对地表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。

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.