[S-CG51] Hybrid Geochronology from Z to A

[SCG51-05] Development of a micron-resolution zircon U–Pb dating method using LA-ICP-MS

*Shumpei Kudo¹, Sota Niki², Takafumi Hirata², Tetsuo Kawakami¹ (1.Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, 2.Geochemical Research Center, School of Science, The University of Tokyo)

Keywords:Zircon, LA-ICP-MS, geochronology

Internal structures of zircon such as oscillatory zoning and sector zoning contain geological age information during igneous and metamorphic processes [1]. Extracting distinct U–Pb age information from different domains of a single zircon grain is important to reveal petrogenetic histories of the host rocks [e.g. 2]. To take full advantage of zircon preserving chronological information of multiple growth stages corresponding to geological events, in situ isotope analysis of each internal structure of zircon is needed for resolving chronological orders of geological events recorded in zircon [e.g. 3]. However, thickness of growth textures inside typical zircon grains of 100 µm in size is ca. 10 μm or less, which is smaller than the spot sizes of conventional U–Pb isotope analysis. In previous studies, while the spot diameter is large, the drilling depth was on the scale of 100 nm. Therefore, high-spatial-resolution U–Pb dating of thin domains in zircon has been conducted by depth-profiling isotope analysis with a depth resolution of ca. 100 nm using secondary ionisation or a single-shot laser ablation [e.g. 4, 5]. Despite the importance, mixed ages from different domains may be obtained if the growth structures are not parallel to the analytical surface because the drilling is conducted parallel to the outermost layer. Especially when the internal structure of the sample is complex like metamorphic zircon, it becomes difficult to differentiate multiple domains within the ablated volume.
Faced with the above-mentioned problem, an in-house femtosecond laser ablation system with a focused laser beam (ca. 2 µm) is developed for higher-spatial-resolution isotope analysis. In the U–Pb isotope measurement using the focused laser beam, one challenge is that the ion counts of U and Pb isotopes decrease. This is because there is a limitation in achieving deeper excavations due to the shallow depth of field, resulting in the deteriorated accuracy and precision of measurements for U/Pb values. In addition, excessive drilling depth can lead to significant U/Pb fractionation. We firstly examined optimised conditions of the laser to determine the number of laser shots. The analysis spot diameter of zircon approaches 2 μm as the number of shots increased from 1 to 30 shots. On the other hand, the rate of increase in signal intensity decreased when the number of laser shots exceeds 20 although the signal intensity is proportional to the number of shots up to 10 shots. This might be due to the decrease in drilling efficiency derived from the shallow depth of field of the laser. Therefore, we adopted the laser shot counts of 10 that did not result in a decrease in drilling efficiency to conduct U–Pb dating of a zircon reference (Plešovice zircon) using the femtosecond laser ablation system coupled with multiple-collector ICP-MS.
This study compared the results of reference zircon measurements using a high-sensitivity ICP-Q-MS with those obtained using MC-ICP-MS. The data from ICP-Q-MS were 338 ± 39 Ma (2s, n = 20), while the data from MC-ICP-MS were 339.3 ± 8.2 Ma (95% conf., n = 30), both of which are consistent with the literature value of 337 Ma [6]. In the ICP-Q-MS experiment, we obtained data exclusively for three isotopes (²⁹Si ,²⁰⁶Pb, ²³⁸U) and conducted a laser raster scan with 4 μm square to enhance the ion counts. For the analysis of actual geological samples, to use MC-ICP-MS is more favorable in order to achieve higher temporal resolution. In the presentation, we will further evaluate the precision and accuracy of the reference zircon age data obtained using the highly focused beam coupled with multiple-collector ICP-MS, and discuss the potential application of the present technique to metamorphic geochronology.

References
[1] Schaltegger et al.,1999. Contributions to Mineralogy and Petrology, 134, 186-201. [2] Rubatto, 2002. Chemical geology, 184(1-2), 123-138. [3] Möller et al., 2003. Geological Society, London, Special Publications, 220(1), 65-81. [4] Schmitt, A. K., 2003. Geochimica et Cosmochimica Acta, 67(18), 3423-3442. [5] Iwano et al., 2021. Chemical Geology, 559, 119903. [6] Sláma et al., 2008. Chemical Geology, 249(1-2), 1-35.