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[SCG51-01] Distribution of trace elements in ilmenite and mineralogical meaning of U–Pb dates
Keywords:Ilmenite U–Pb dating, Trace elements imaging, Hematite exsolution, LA-ICP-MS, Baddeleyite
Ilmenite (FeTiO3) is one of the iron-titanium oxide and occurs widely in mafic rocks where zircon and other datable minerals are poor. The mineral has a high uranium (U) partition coefficient than lead (Pb) (Klemme et al., 2006; Snape et al., 2022), which causes fractionation during the mineral formation and can be applied for U–Pb dating (Thompson et al., 2021). Ilmenite forms various micro textures, such as exsolution, depending on its major chemical composition, changes in redox state, and thermal history. Determining how elements diffuse in response to these microtextural changes is an important indicator in determining whether ilmenite U–Pb age is interpreted as high-T crystallization, or low-T exsolution formation. The objective of this study is to determine the timing of each event by obtaining trace elements imagings of ilmenite and determining their ages.
In this study, ilmenite is collected from the anorthosite FC1, Duluth complex. Analyses on mineralogy, trace elements distribution, and U–Pb dating of the ilmenite are conducted. The zircon U–Pb age of 1099.0 ± 0.6 Ma is a timing of crystallization (Paces and Miller, 1993). The thermal history, based on the apatite U–Pb age and zircon (U–Th)/He age with different closure temperatures, indicates rapid initial cooling from > 600 ℃ to 200 ℃ during ~60 Myr, and no significant reheating occurred after the initial cooling (Reiners et al., 2004; Iwano et al., 2019; Hartel et al., 2023). Therefore, the sample is suitable for understanding the elemental compatibility in Fe-Ti oxides during crystallization and subsequent exsolution.
Two types of ilmenite in FC1 exist: those with hematite (Fe2O3) exsolution (Hematite exsolution ilmenite) and those without exsolution (No exsolution ilmenite). The presence of exsolution can be attributed to differences in the bulk Fe/Ti ratio in the Fe-Ti oxides, since the textures are formed in immiscible gaps (solvus) during the cooling. The distribution of trace elements in ilmenite differed depending on the presence of exsolution. In the hematite exsolution ilmenite, U is concentrated in hematite and U concentration and 238U/206Pb ratio are high (~0.1 µg g-1 and ~5, respectively), while Zr concentration is low (< 100 µg g-1). On the other hand, in the no exsolution ilmenite, baddeleyite exists in ilmenite and U concentration and 238U/206Pb ratio are low (~0.01 µg g-1 and ~3, respectively), while Zr concentration is high (> 100 µg g-1). The U–Pb ages of each ilmenite are consistent with the hematite exsolution ilmenite: 1088.1 ± 6.5 Ma and the no exsolution ilmenite: 1125.4 ± 45.4 Ma (2σ error), whereas the precision of the no exsolution ilmenite is one order of magnitude lower because U is squeezed into the coexisting baddeleyite. As described above, the distribution of U in Fe-Ti oxides is localized in hematite and baddeleyite, and the U–Pb ages would reflect the timing of hematite exsolution and baddeleyite crystallization. In this presentation, we also report the results of ilmenite in magnetitite of the Bushveld complex.
In this study, ilmenite is collected from the anorthosite FC1, Duluth complex. Analyses on mineralogy, trace elements distribution, and U–Pb dating of the ilmenite are conducted. The zircon U–Pb age of 1099.0 ± 0.6 Ma is a timing of crystallization (Paces and Miller, 1993). The thermal history, based on the apatite U–Pb age and zircon (U–Th)/He age with different closure temperatures, indicates rapid initial cooling from > 600 ℃ to 200 ℃ during ~60 Myr, and no significant reheating occurred after the initial cooling (Reiners et al., 2004; Iwano et al., 2019; Hartel et al., 2023). Therefore, the sample is suitable for understanding the elemental compatibility in Fe-Ti oxides during crystallization and subsequent exsolution.
Two types of ilmenite in FC1 exist: those with hematite (Fe2O3) exsolution (Hematite exsolution ilmenite) and those without exsolution (No exsolution ilmenite). The presence of exsolution can be attributed to differences in the bulk Fe/Ti ratio in the Fe-Ti oxides, since the textures are formed in immiscible gaps (solvus) during the cooling. The distribution of trace elements in ilmenite differed depending on the presence of exsolution. In the hematite exsolution ilmenite, U is concentrated in hematite and U concentration and 238U/206Pb ratio are high (~0.1 µg g-1 and ~5, respectively), while Zr concentration is low (< 100 µg g-1). On the other hand, in the no exsolution ilmenite, baddeleyite exists in ilmenite and U concentration and 238U/206Pb ratio are low (~0.01 µg g-1 and ~3, respectively), while Zr concentration is high (> 100 µg g-1). The U–Pb ages of each ilmenite are consistent with the hematite exsolution ilmenite: 1088.1 ± 6.5 Ma and the no exsolution ilmenite: 1125.4 ± 45.4 Ma (2σ error), whereas the precision of the no exsolution ilmenite is one order of magnitude lower because U is squeezed into the coexisting baddeleyite. As described above, the distribution of U in Fe-Ti oxides is localized in hematite and baddeleyite, and the U–Pb ages would reflect the timing of hematite exsolution and baddeleyite crystallization. In this presentation, we also report the results of ilmenite in magnetitite of the Bushveld complex.