09:30 〜 09:45
[SMP25-03] 三波川変成帯五良津東部岩体中の高圧結晶質石灰岩に対する柘榴石ウラン–鉛年代測定
キーワード：柘榴石、U–Pb年代測定、三波川変成帯、LA-ICP-MS
Generally, geochronological studies on metamorphic rocks have been dependent on the occurrence of specific minerals (e.g. mica and zircon). For these minerals, the relationships between the timing of mineral growth and metamorphic stages are often unclear [1, 2]. For the accurate reconstruction of the pressure-temperature-time (P-T-t) paths, geochronological data from in-situ dating while differentiating textures inside mineral grains should be linked to P-T conditions determined by co-existing inclusion minerals near the spots measured by in-situ dating methods.
Among metamorphic mineral species, grossular (Grs)-andradite (Adr) series garnet is a potential candidate to define both U–Pb ages and P-T conditions. The U–Pb isotopic system of garnet is robust against thermal events because of the high closure temperature (>800 °C [3]). The P-T conditions of garnet can also be deduced from the co-existing inclusion minerals.
In this study, U–Pb dating of metamorphic Grs garnet was first conducted using a laser ablation-ICP-mass spectrometry. The in-situ spot ages were linked to the P-T conditions deduced from the garnet itself. The garnet sample was collected from the eastern margin of the Eastern Iratsu body in the eclogite-unit of the Sanbagawa metamorphic terrane. Prior to the U–Pb isotopic analysis, the zoning texture inside the garnet sample was observed using backscattered electron images. The Grs grain showed a patchy texture with an almost pure Grs core (Grs98Adr2) and an Adr-rich rim (Grs91Adr8Alm<2). The presence of aragonite in the core indicates formation during the eclogite-facies metamorphism while the rim was formed under the P-T conditions of the juxtaposition of the exhuming eclogite-unit and surrounding schists [4].
For U–Pb isotopic analysis, the 13 spots for the core and 13 spots for the rim were carefully selected. The obtained U–Pb isotopic results formed a regression line on the Tera and Wasserburg Concordia Diagram. The mean square weighted deviation (MSWD) of the regression line was greater than 2 (2.5 for core and 4.1 for rim). Since titanite in this sample recorded ages of an older stage (ca. 200 Ma [5]), the large MSWD can be attributed to the remnant of such old U–Pb isotopic signatures. By rejecting these “old” U–Pb isotopic data, the obtained age of the core was 95.8±8.9 (2σ) with a MSWD = 0.70 and the obtained age of the rim was 95.3±8.3 (2σ) with a MSWD =1.0.
The resulting U–Pb ages of the core were in good agreement with the timing of the eclogite-facies stage, such as 92.6±8.8 Ma of zircon U–Pb ages [6] and 88.8±0.6 Ma of garnet Lu–Hf isochron ages [7]. The core and rim were indistinguishable despite the obvious zoning texture inside the garnet and the difference in metamorphic P-T conditions constrained from co-existing inclusion minerals. The agreement of the U–Pb ages between the core and rim can be explained by two reasons. Firstly, the chemical compositions and the U–Pb isotopic system were decoupled because of coupled dissolution-reprecipitation processes [8]. The rim of Grs was produced through the series of dissolution-reprecipitation reactions among the Grs grains while the U–Pb isotopic system remains unperturbed and partly preserved old (96 Ma) signature. Secondly, the time difference between core and rim formations was too small to detect by present analytical precision. The timing of the rim formation can be earlier than the peak (or cooling) ages of the surrounding schists, i.e. 90-85 Ma [7, 9]. This time period overlapped with the obtained ages in this study within the analytical errors.
References
[1] Itaya et al. 2009, Isl. Arc. [2] Kohn et al., 2015, Am. Min. [3] Mezger et al., 1991, J. Geol. [4] Yoshida et al., 2021, JMPS. [5] Yoshida et al., 2021, JpGU2021. [6] Aoki et al., 2020, Lithos. [7] Wallis et al., 2009, J. Metamorph. Geol. [8] Weinberg et al., 2020, Geochim. Cosmochim. Acta. [9] Dallmeyer and Takasu, 1991, J. Metamorph. Geol.
Among metamorphic mineral species, grossular (Grs)-andradite (Adr) series garnet is a potential candidate to define both U–Pb ages and P-T conditions. The U–Pb isotopic system of garnet is robust against thermal events because of the high closure temperature (>800 °C [3]). The P-T conditions of garnet can also be deduced from the co-existing inclusion minerals.
In this study, U–Pb dating of metamorphic Grs garnet was first conducted using a laser ablation-ICP-mass spectrometry. The in-situ spot ages were linked to the P-T conditions deduced from the garnet itself. The garnet sample was collected from the eastern margin of the Eastern Iratsu body in the eclogite-unit of the Sanbagawa metamorphic terrane. Prior to the U–Pb isotopic analysis, the zoning texture inside the garnet sample was observed using backscattered electron images. The Grs grain showed a patchy texture with an almost pure Grs core (Grs98Adr2) and an Adr-rich rim (Grs91Adr8Alm<2). The presence of aragonite in the core indicates formation during the eclogite-facies metamorphism while the rim was formed under the P-T conditions of the juxtaposition of the exhuming eclogite-unit and surrounding schists [4].
For U–Pb isotopic analysis, the 13 spots for the core and 13 spots for the rim were carefully selected. The obtained U–Pb isotopic results formed a regression line on the Tera and Wasserburg Concordia Diagram. The mean square weighted deviation (MSWD) of the regression line was greater than 2 (2.5 for core and 4.1 for rim). Since titanite in this sample recorded ages of an older stage (ca. 200 Ma [5]), the large MSWD can be attributed to the remnant of such old U–Pb isotopic signatures. By rejecting these “old” U–Pb isotopic data, the obtained age of the core was 95.8±8.9 (2σ) with a MSWD = 0.70 and the obtained age of the rim was 95.3±8.3 (2σ) with a MSWD =1.0.
The resulting U–Pb ages of the core were in good agreement with the timing of the eclogite-facies stage, such as 92.6±8.8 Ma of zircon U–Pb ages [6] and 88.8±0.6 Ma of garnet Lu–Hf isochron ages [7]. The core and rim were indistinguishable despite the obvious zoning texture inside the garnet and the difference in metamorphic P-T conditions constrained from co-existing inclusion minerals. The agreement of the U–Pb ages between the core and rim can be explained by two reasons. Firstly, the chemical compositions and the U–Pb isotopic system were decoupled because of coupled dissolution-reprecipitation processes [8]. The rim of Grs was produced through the series of dissolution-reprecipitation reactions among the Grs grains while the U–Pb isotopic system remains unperturbed and partly preserved old (96 Ma) signature. Secondly, the time difference between core and rim formations was too small to detect by present analytical precision. The timing of the rim formation can be earlier than the peak (or cooling) ages of the surrounding schists, i.e. 90-85 Ma [7, 9]. This time period overlapped with the obtained ages in this study within the analytical errors.
References
[1] Itaya et al. 2009, Isl. Arc. [2] Kohn et al., 2015, Am. Min. [3] Mezger et al., 1991, J. Geol. [4] Yoshida et al., 2021, JMPS. [5] Yoshida et al., 2021, JpGU2021. [6] Aoki et al., 2020, Lithos. [7] Wallis et al., 2009, J. Metamorph. Geol. [8] Weinberg et al., 2020, Geochim. Cosmochim. Acta. [9] Dallmeyer and Takasu, 1991, J. Metamorph. Geol.