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（Titanium）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Nicolas Dauphas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大陆地壳的演化和板块构造的开始。如果被证明是一个可靠的代用物，那么钛同位素将填补一个独特的生态位，因为它们(1)普遍存在于火成岩套件中，(2)不受风化、流体套印或变质作用的影响。然而，钛同位素在拉斑岩（羽状）和钙碱性（弧状）岩浆中的表现似乎不同，这使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状火山的钛同位素，以了解在地壳形成过程中是什么控制了钛同位素的分等，这将使她能够校准代用物，并重新审视地质记录，以了解更多关于地球古代大陆组成的信息。在担任美国国家科学基金会博士后期间，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），在那里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。她还将作为“为学术生涯做准备”研讨会的校友，在全国地球科学教师协会（NAGT）年会上担任志愿者。地球早期大陆地壳组成的现有地球化学代用物提出了一个难题。稳定的钛同位素被用来通过提供岩浆分异的时间记录来区分现有的假设。随着分馏结晶的进行，Ti的轻同位素优先融入氧化物，最终被隔离在岩浆堆积中。因此，长英质地壳具有重同位素特征，而基性地壳具有最小分馏特征。最近对拉斑岩和钙碱性岩浆在不同程度上分馏钛同位素的观察表明，还有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，并可能确定钛同位素敏感的新过程。将在印度尼西亚林佳尼火山钙碱性火山套中测量钛同位素的矿物熔融分馏因子。配对铁同位素分析将量化氧逸度和磁铁矿结晶的具体作用。该分异套将阐明弧环境下钛同位素分异的控制因素，这将使地壳形成过程中的钛同位素分异首次得到准确的模拟。最后，将模拟的Ti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组成。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。

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.