CAREER: Deciphering the Uranium Isotope Record of Igneous Accessory Phases

锆石和其他辅助矿物，如磷灰石、钛铁矿或斜纹石，构成了地球历史上一些最完整的档案。这些矿物可以使用铀-铅辐射测定法进行精确的测年，它们的耐化学性使它们能够在数十亿年的时间尺度上存活。事实上，碎屑状的锆石是地球第一个500Myr的唯一保存下来的碎片，因此使这一阶段成为了解塑造地球的事件并建立其演化时间表的基石。最近，火成岩及其副相的铀(U)同位素组成的变化引起了人们的极大关注，但这种变化背后的驱动因素仍不清楚。如果更好地理解，高温系统中的晶体间、矿物间和岩石间的铀同位素变化可能成为研究岩浆演化、来源、氧化状态和/或分异过程的有力工具。这一职业奖的目标是(I)确定副相(锆石、钛铁矿、磷灰石、±斜斜晶石)和火成岩熔体中铀同位素分馏的驱动因素，以及(Ii)建立‘读取’火成岩系统铀同位素记录所需的框架。作为该奖项的一部分，PI还将制定一系列适合年龄的教育干预措施，将研究过程带给所有年级的学生(当地和国际)，以培养和加强他们作为未来科学家的认同感，并增加他们在STEM的归属感和保留感。为了确定副相和火成岩熔体中铀同位素分馏的驱动因素，将从微观结构和成分(扫描电子显微镜、化学发光、EBSD、SIMS、EPMA)以及U的价态(XANES)、成键环境(EXAFS)和同位素组成(MC-ICPMS)来测量U的价态(XANES)、成键环境(EXAFS)和同位素组成(MC-ICPMS)。这种方法将有助于解决岩浆环境中铀同位素分馏的驱动因素。在不同fO2条件下进行的探索性锆石生长实验将直接检验锆石结晶和铀氧化态对岩浆铀同位素演化的影响。由此产生的概念将被应用于对一组碎屑状锆石晶体的案例研究，这些碎屑锆石晶体记录了地球上大气氧气条件最戏剧性的变化之一：大氧化事件(GOE)。这最后一项任务将检验这一假设，即地球锆石和岩浆的铀同位素组成的变化与地球水圈和大气的氧化还原演化有关。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。

Zircon and other accessory minerals like apatite, titanite, or baddeleyite constitute some of the most complete archives of Earth’s history. These minerals can be precisely dated using the Uranium-Lead radiometric method, and their chemical resistance allows their survival over billion-year timescales. In fact, detrital zircon are the only preserved fragments remaining from Earth’s first 500 Myr, thus making this phase a cornerstone for understanding the events that shaped the Earth and establishing a timeline of its evolution. Recently, variations in the uranium (U) isotopic composition of igneous rocks and accessory phases have garnered significant attention, but the drivers behind this variability remain unknown. If better understood, inter-crystal, inter-mineral and inter-rock U isotope variations in high-temperature systems could become powerful tools for studying magmatic evolution, source, oxidation state and/or differentiation processes. The goals of this CAREER award are to (i) identify the drivers of U isotope fractionation in accessory phases (zircon, titanite, apatite, ± baddeleyite) and igneous melts, and (ii) build the framework needed to ‘read’ the U isotope record of igneous systems. As part of this award, the PI will also develop a series of age-appropriate educational interventions that will bring the research process to students at all grade levels (locally, and internationally) so as to foster and strengthen their sense of identity as future scientists, and increase their sense of belonging and retention in STEM. To identify the drivers of U isotope fractionation in accessory phases and igneous melts, crystals of zircon, titanite, apatite, and baddeleyite from localities encompassing various redox conditions and covering a range of host-rock compositions and ages, will be characterized microtexturally and compositionally (SEM, CL, EBSD, SIMS, EPMA), and the valence (XANES), bonding environment (EXAFS) and isotopic composition (MC-ICPMS) of U will be measured in the very same grains. This approach will allow resolving the drivers of U isotope fractionation in magmatic settings. Exploratory zircon growth experiments done under variable fO2 conditions will directly test the role that zircon crystallization and U oxidation state have on magma U isotope evolution. The resulting concepts will be applied in a case study of a thoroughly characterized collection of detrital zircon crystals documenting one of the most dramatic changes in atmospheric oxygenation conditions on Earth: the Great Oxidation Event (GOE). This last task will test the hypothesis that shifts in the U isotope composition of terrestrial zircon and magmas are linked to the redox evolution of Earth’s hydrosphere and atmosphereThis 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.