EAGER: Collaborative Research: Development and application of Sr stable isotopes as a novel tracer of carbonate through subduction

地质作用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，其时间尺度为一千万年或更长时间。碳通过板块俯冲过程从地球表面转移到地幔，在板块俯冲过程中，携带富碳岩石和沉积物的海洋板块下沉到地幔中。在俯冲的最初阶段，下沉板块中的一小部分碳被移走，并在俯冲带火山的岩浆中运回地球表面，例如太平洋火环火山的岩浆。因此，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，有必要确定有多少俯冲的碳迅速返回地球表面，有多少碳被输送到地幔深处。然而，碳循环的速度很难直接测量，因为在岩浆上升过程中，溶解在岩浆中的碳被隔离成单独的气相，以高度不同的速度穿过地壳进入大气，并通过扩散路径。作为直接测量通过俯冲带火山的碳通量的替代方法，可以使用地球化学指标来估计最初存在于岩浆中的碳量。这项研究将纳入分析技术的最新进展，这些技术将使用锶同位素系统作为替代，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区化学循环的理解，同时促进教学、培训和学习。该项目包括对来自地球科学中代表性不足的群体的早期职业研究人员以及一名一年级硕士学生的指导。这项拟议的工作还将在美国马萨诸塞州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。锶是通过俯冲带进行碳酸盐循环的潜在强大代理人，因为它通常存在于碳酸盐中相对于地幔的高丰度，并且因为碳酸盐具有独特的稳定的锶同位素组成。该项目组将通过在中美洲火山弧(CAVA)建立一个数据框架来开发新的程序，以分析和解释来自火山弧的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，火山岩和气体已被广泛测量和表征，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，并且有很好的地球化学证据表明碳酸盐循环效率不同。通过双峰TIMS对锶稳定同位素和放射性同位素比值的联合测量，将提供准确估计从俯冲碳酸盐到火山弧的锶通量的能力，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制约因素，建立俯冲成分及其相关火山岩之间的同位素系统学，并为进一步研究弧形地球化学运移提供基础。88/86Sr值将与放射性成因的锶同位素相结合，以准确地确定弧形火山岩中碳酸盐来源的锶总量，以及这些系统是否可用于评估俯冲碳酸盐再循环成火山弧的效率和机制。如果CAVA的88/86sr系统提供了通过火山弧进行碳循环的基准，那么这些示踪剂就可以用来限制世界其他火山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，这一指标也有可能被用来评估过去时代俯冲碳酸盐的变异性，提供对地球历史上全球碳循环变异性的变化性洞察。拟议工作的结果将引起广泛的地球科学界的兴趣，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社区。这一奖项反映了NSF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。

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.