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

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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.