EAR-PF: Application of Paired Ti and Fe Isotopes to Understand the Evolution of Earth's Upper Continental Crust

Aleisha约翰逊博士被授予NSF博士后奖学金，以研究Ti（钛）的稳定同位素在大陆地壳形成过程中的行为。她的研究和公共服务将在芝加哥大学进行，由尼古拉斯·多法斯教授监督。稳定钛同位素最近被提出来追踪地球上陆壳的演化和板块构造的开始。如果被证明是一个强大的替代品，Ti同位素将填补一个独特的利基，因为它们（1）普遍存在于火成岩套中，（2）对风化，流体叠加或变质作用的修改不敏感。然而，钛同位素表现出不同的拉斑玄武岩（羽状）和钙碱性（弧状）岩浆，复杂的解释古老的地质记录。约翰逊博士将测量现代弧火山中的钛同位素，以了解在地壳形成过程中是什么控制着钛同位素分馏，这将使她能够校准代用品，并重新访问地质记录，以更多地了解地球古代大陆的组成。在她担任NSF博士后研究员期间，约翰逊博士将在两个组织中担任导师：第一个是芝加哥大学的FLI网络（第一代，低收入，移民，盟友），她可以指导与她自己背景相似的学生，以鼓励私立大学水平的科学多样性和包容性。她还将在全国地球科学教师协会（NAGT）的年度会议上担任志愿者，作为“为学术生涯做准备”研讨会的明矾。地球早期大陆地壳组成的现有地球化学代理人提出了一个难题。稳定的钛同位素被用来区分现有的假说提供了一个时间记录的岩浆分异。随着分离结晶的进展，较轻的钛同位素优先纳入氧化物，并最终在岩浆堆积物隔离。因此，长英质地壳具有重同位素特征，而镁铁质地壳则显示出最小的分馏作用。拉斑玄武岩和钙碱性岩浆在不同程度上对Ti同位素的破碎表明，还有一些重要的未知变量需要考虑：含水量、氧逸度、结晶温度和矿物相。在现代环境中进行适当的校准可以解决这一挑战，并可能确定Ti同位素敏感的新过程。在印度尼西亚林贾尼火山的钙碱性火山岩套中测量了Ti同位素的矿物熔体分馏因子。成对的铁同位素分析将量化的作用，氧逸度和磁铁矿结晶具体。这一分化套件将阐明控制钛同位素分馏弧设置，这将使钛同位素分馏在地壳形成过程中准确建模的第一次。最后，模拟Ti同位素值将与页岩记录进行比较，以更好地确定古地壳原岩的成分。该项目获得了地球科学部地球化学和岩石学项目的资助。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。

Dr. Aleisha Johnson has been awarded an NSF EAR Postdoctoral Fellowship to investigate how the stable isotopes of Ti (Titanium) behave during the formation of continental crust. Her research and public service will take place at the University of Chicago under the supervision of Prof. Nicolas Dauphas. Stable Ti isotopes have been recently proposed to trace the evolution of Earth’s upper continental crust and the onset of plate tectonics. If demonstrated, to be a robust proxy, Ti isotopes would fill a unique niche because they are (1) ubiquitous in igneous rock suites and (2) insensitive to modification by weathering, fluid overprinting, or metamorphism. However, Ti isotopes appear to behave differently in tholeiitic (plume-like) and calc-alkaline (arc-like) magmas, complicating interpretations of the ancient geologic record. Dr. Johnson will measure Ti isotopes in modern arc volcanoes to understand what controls Ti isotope fractionation during the formation of crust, which will allow her to calibrate the proxy and revisit the geologic record to learn more about the composition of Earth’s ancient continents. During her tenure as an NSF Postdoctoral Fellow, Dr. Johnson will become involved as a mentor in two organizations: the first is the FLI network at University of Chicago (First-generation, Low-income, Immigrant, Ally) where she can mentor students similar to her own background to encourage diversity and inclusion in science at the private college level. She will also volunteer at the National Association for Geoscience Teachers (NAGT) annual meetings as an alum of the workshop “Preparing for an Academic Career”.Existing geochemical proxies for the composition of Earth’s early continental crust present a conundrum. Stable Ti isotopes are being used to distinguish between existing hypotheses by providing a temporal record of magmatic differentiation. As fractional crystallization progresses, the lighter isotopes of Ti are preferentially incorporated into oxides and ultimately sequestered in magmatic cumulates. Thus, felsic crust carries an isotopically heavy signature whereas mafic crust indicates minimal fractionation. The recent observation that tholeiitic and calc-alkaline magmas fractionate Ti isotopes to different extents signaled that there are important unknown variables left to consider: water content, oxygen fugacity, crystallization temperature, and mineral phase. Proper calibration in modern settings can address this challenge and perhaps identify new processes which Ti isotopes are sensitive to. Mineral-melt fractionation factors of Ti isotopes will be measured in a calc-alkaline volcanic suite from Rindjani Volcano, Indonesia. Paired Fe isotope analyses will quantify the role of oxygen fugacity and magnetite crystallization specifically. This differentiation suite will elucidate the controls on Ti isotope fractionation in arc settings, which will allow Ti isotope fractionation during crust formation to be modeled accurately for the first time. Finally, modeled Ti isotope values will be compared with the shale record to better identify the compositions of ancient crustal protoliths. This project received funds from the Geochemistry and Petrology program in the Earth Sciences division.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.