CAREER: Deciphering the Uranium Isotope Record of Igneous Accessory Phases

锆石和其他辅助矿物，如磷灰石、钛铁矿或斜锆石，构成了地球历史最完整的档案。这些矿物可以使用铀-铅放射性方法精确测定年代，它们的耐化学性使它们能够在数十亿年的时间尺度上生存。事实上，碎屑锆石是地球前5亿年唯一保存下来的碎片，因此使这一阶段成为理解塑造地球的事件并建立其演化时间轴的基石。最近，火成岩和副相的铀（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然未知。如果更好地理解，在高温系统中的晶体间，矿物间和岩石间的U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。该CAREER奖项的目标是（i）确定辅助相（锆石，钛铁矿，磷灰石，斜锆石）和火成岩熔体中U同位素分馏的驱动因素，以及（ii）建立“读取”火成岩系统U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，将研究过程带到所有年级（本地和国际）的学生中，以培养和加强他们作为未来科学家的认同感，并增加他们的归属感和保留感。为了确定副相和火成岩熔体中U同位素分馏的驱动因素，将从微观结构和成分上表征来自不同氧化还原条件并涵盖一系列主岩成分和年龄的地方的锆石、钛铁矿、磷灰石和斜锆石晶体（SEM，CL，EBSD，西姆斯，EPMA），以及U的价态（XANES），成键环境（EXAFS）和同位素组成（MC-ICPMS）将在相同的颗粒中测量。这种方法将允许解决在岩浆环境中的U同位素分馏的驱动程序。在不同的fO 2条件下进行的探索性锆石生长实验将直接测试锆石结晶和U氧化态对岩浆U同位素演化的作用。由此产生的概念将应用于一个案例研究的一个彻底的特点收集碎屑锆石晶体记录在地球上的大气氧化条件的最戏剧性的变化之一：大氧化事件（GOE）。这最后一项任务将测试的假设，即在陆地锆石和岩浆的U同位素组成的变化与地球的水圈和大气的氧化还原演化。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.