Timing of fluid alteration events by multiscale geochronology of uraninite and (Y,REE,U,Th)–(Nb,Ta,Ti) oxide minerals: An evaluation of the potential to detect hidden fluid events in Earth history

水成流体在地壳地球化学和地球动力学演化中起着重要作用。流体事件的时间主要可以通过直接从这些流体中沉淀出来的矿物来确定，从而结合了可用于同位素测年的可测量数量的放射性元素。然而，流体事件，也可以通过化学和同位素分析的U-和Th-轴承矿物的蚀变域使用微米到纳米分析工具。在这里，我们建议使用激光烧蚀电感耦合等离子体质谱，电子探针，原子探针和透射电子显微镜，通过对晶质铀矿蚀变产物的化学和同位素U-Th-Pb分析，评估流体-岩石相互作用的多尺度U-Pb定年的潜力和局限性，但主要是（Y，REE，U，Th）-（Nb，Ta，Ti）氧化物矿物（例如，异辉锑铅矿、易解石、辉锑铅矿）。来自挪威南部元古宙Evje-Iveland伟晶岩场的三种伟晶岩的钛铁矿、辉石岩/易解石和晶质铀矿及其蚀变产物的首次U-Th-Pb测年结果表明，至少有五种不同的流体-岩石相互作用事件影响了这些伟晶岩，年龄范围可达晚始新世。这些数据进一步表明，迄今为止，并非地质记录中的所有流体事件都得到了确认，特别是当它们与明显的岩浆或构造事件无关时。新的，结合应用微纳米分析技术对进一步的样品从Evje-Iveland伟晶岩领域将使我们能够获得更详细的原子之间的蚀变反应（S）的水溶液和锕系元素轴承，往往是变质矿物和相应的行为U，Th和放射性铅，这是至关重要的正确解释U-Th-Pb年龄从蚀变域。对蚀变的含铀钍矿物和相关蚀变产物进行系统测年，可能为探测地质记录中隐藏的地质事件提供一个难得的机会。

Aqueous fluids play a critical role in the geochemical and geodynamical evolution of the Earth’s crust. The timing of fluid events can principally be determined from minerals that precipitated directly from these fluids and thereby incorporated measurable amounts of radioactive elements that can be used for isotopic dating. Fluid events, however, can also be dated chemically and isotopically by analysis of altered domains of U- and Th-bearing minerals using micro- to- nano-analytical tools. Here, we propose to use laser ablation inductively-coupled plasma mass spectrometry, electron microprobe, atom probe, and transmission electron microscopy to evaluate the potential and limitations of multi-scale U-Pb dating of fluid-rock interactions by chemical and isotopic U-Th-Pb analysis of alteration products of uraninite, but mainly of (Y,REE,U,Th)–(Nb,Ta,Ti) oxide minerals (e.g., euxenite, aeschynite, fergusonite). First U-Th-Pb dating results from titanite, euxenite/aeschynite, and uraninite and its alteration products from three pegmatites from the Proterozoic Evje-Iveland pegmatite field in South Norway indicate that at least five distinct fluid-rock interaction events have affected these pegmatites, ranging in age down to the Late Eocene. This data further suggests that not all fluid events in the geological record have been recognized so far, particularly when they are unrelated to obvious magmatic or tectonic events. The novel, combined application of micro- to nano-analytical techniques on further samples from the Evje-Iveland pegmatite field will enable us to gain a more detailed atomistic understanding of the alteration reaction(s) between aqueous solutions and actinide-bearing, often metamict minerals and the corresponding behaviour of U, Th, and radiogenic Pb, which is crucial to correctly interpret U-Th-Pb ages obtained from altered domains. The systematic dating of altered U-Th-bearing minerals and related alteration products potentially offers an extraordinary opportunity to detect hidden geological events in the geological record.