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

Aleisha Johnson博士被授予NSF EAR博士后奖学金，研究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尼古拉斯·道法教授的指导下进行。稳定的钛同位素最近被提出用来追踪地球上陆壳的演化和板块构造的开始。如果被证明是一种强有力的替代物，钛同位素将填补一个独特的利基市场，因为它们(1)普遍存在于火成岩中，(2)对风化、流体叠加或变质作用的修改不敏感。然而，钛同位素在拉斑玄武岩(羽状)和钙碱性(弧状)岩浆中的表现似乎不同，这使得对古代地质记录的解释变得复杂。约翰逊博士将测量现代弧形火山中的钛同位素，以了解是什么控制了地壳形成过程中钛同位素的分馏，这将使她能够校准替代物，并重新查看地质记录，以了解更多关于地球古代大陆的组成。在担任NSF博士后研究员期间，约翰逊博士将作为导师参与两个组织：第一个是芝加哥大学的FLI网络(第一代，低收入，移民，Ally)，她可以在那里指导与她背景相似的学生，以鼓励私立大学层面的多样性和科学包容性。她还将在国家地球科学教师协会(NAGT)的年度会议上做志愿者，作为研讨会的校友，为学术生涯做准备。现有的地球早期大陆地壳成分的地球化学指标是一个难题。通过提供岩浆分异的时间记录，稳定的钛同位素被用来区分现有的假说。随着分离结晶的进行，较轻的钛同位素优先结合到氧化物中，并最终隔离在岩浆堆积体中。因此，长英质结壳具有重同位素特征，而镁铁质结壳指示最小的分馏作用。最近观察到拉斑玄武岩和钙碱性岩浆不同程度地分馏了钛同位素，这表明还有一些重要的未知变量需要考虑：水含量、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，或许还可以确定钛同位素对其敏感的新过程。将在印度尼西亚林贾尼火山的钙碱性火山岩套中测量钛同位素的矿物熔体分馏系数。配对的铁同位素分析将具体量化氧逸度和磁铁矿结晶的作用。这一分异组合将阐明弧形环境中钛同位素分馏的控制，这将使首次准确地模拟地壳形成过程中的钛同位素分馏。最后，将模拟的钛同位素值与页岩记录进行比较，以更好地识别古代地壳原岩的成分。这个项目得到了地球科学分部地球化学和岩石学项目的资助。这个奖项反映了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，被认为是值得支持的。

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.