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

在1000万年甚至更长的时间尺度上，地质过程在调节大气中的氧和碳，从而调节地表温度方面发挥着重要作用。碳通过板块俯冲的过程从地球表面转移到地幔中，海洋板块携带富含碳的岩石和沉积物下沉到地幔中。在俯冲的初始阶段，下沉板块中的部分碳被移走，并随着俯冲带火山的岩浆（如环太平洋火山带的火山岩浆）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，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，因为在岩浆上升的过程中，溶解在岩浆中的碳被隔离成一个单独的蒸汽相，以高度可变的速率通过扩散途径穿过地壳进入大气。作为通过俯冲带火山直接测量碳通量的替代方法，地球化学代用物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代理，特别是碳酸盐回收的应用。提出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。该项目包括指导来自地球科学领域代表性不足群体的早期职业研究人员，以及一名一年级硕士研究生。拟议的工作还将在马萨诸塞大学阿默斯特分校和圣路易斯华盛顿大学的早期职业教师之间建立新的合作关系。锶是通过俯冲带进行碳酸盐岩再循环的潜在有力代表，因为相对于地幔，锶在碳酸盐岩中通常以高丰度存在，而且碳酸盐岩具有明显稳定的Sr同位素组成。项目团队将通过在中美洲火山弧（CAVA）建立数据框架，开发火山弧火成岩Sr稳定同位素系统分析和解释的新方法。中美洲火山弧（CAVA）地区存在一层厚的俯冲沉积碳酸盐岩，在以往的研究中对火山岩和气体进行了广泛的测量和表征，并且碳酸盐循环效率的变化具有良好的地球化学证据。通过双尖峰TIMS对Sr稳定同位素和放射性成因同位素比值的综合测量，将提供准确估计从俯冲碳酸盐到火山弧的Sr通量的能力，进而可用于估计碳循环速率。如果成功，这项研究将为全球碳循环提供新的约束条件，建立&amp；#948；俯冲组分及其伴生火山岩的88/86Sr同位素系统，为进一步研究弧地球化学输运提供依据。, # 948;88/86Sr值将与放射性成因Sr同位素相结合，以准确确定弧火山岩的总碳酸盐来源Sr平衡，并确定这些系统是否可用于评估俯冲碳酸盐进入火山弧的再循环效率及其机制。如果&amp；#948；CAVA的88/86Sr体系提供了通过火山弧进行碳回收的基准，然后这些示踪剂可以用来限制世界上其他弧中俯冲碳酸盐的命运。一旦建立了现代稳定的Sr同位素弧系统，这种替代方法也有可能用于评估过去时代俯冲碳酸盐的变异性，为整个地球历史上全球碳循环的变异性提供革命性的见解。建议的工作结果将引起广泛的地球科学团体的兴趣，包括对地表碳储层长期变化感兴趣的低温地球化学和地球历史团体。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。

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.