5:15 PM - 6:30 PM
[SGL38-P05] Development for in-situ radiogenic 4He analysis in zircon
Keywords:zircon, U-Th-He dating, 4-He, LIMAS
YOSHINARI Koichi1* ; BAJO Ken-ichi1 ; YURIMOTO Hisayoshi1
1 Department of Natural History Sciences, Hokkaido University
Zircon is recently used for U-Th-He dating of various rocks because zircon contains high concentration of U and Th, and radiogenic 4He which is produced by disintegration of U and Th (e.g., Reiners et al., 2004). The closure temperature of U-Th-He dating in zircon is low (~180°C). Recently, a low-temperature thermal history of their samples are revealed by using the U-Th-He dating of zircon, for example, the process of uplift and denudation of mountains (e.g., Sueoka et al., 2011).
There is a question about 4He analysis in current U-Th-He dating. Since a typical range of an α-particle is ~20 µm, the conventional apparatuses for 4He analysis cannot accurately evaluate an escape of radiogenic 4He from minerals byα-decays due to a larger spatial resolution.
LIMAS (Laser Ionization Mass nanoScope) is an analytical system developed for analyzing noble gases in micro region, which shold be a key instrument to solve the issue. In this system, neutral particles sputtered by Ga focused-ion-beam from sample surface are ionized by femtosecond laser for tunneling ionization, and produced ions are separated by the multi-turn time of flight mass spectrometer ‘MULTUM II’ depending on their m/z. LIMAS has a potential to trace a track of the α-particle because a spatial resolution of LIMAS is less than 1 μm in case of analysis for trace amount of 4He (Bajo et al., 2015). LIMAS can measure depth profile, and analyze distribution of the 4He concentration at a given depth. Thus, the depth profiling may tell us the further details of 4He diffusion profile in minerals.
If U-Th-He can be measured by LIMAS, the new U-Th-He dating which should be more reliable than previous measurements will be proposed. As the first step, we carried out quantitative analysis of radiogenic 4He in a zircon standard (zircon91500), that have known data about U and Th concentrations and U-Pb age, by LIMAS.
We measured zircon91500 to detect radiogenic 4He by using LIMAS. Concentrations of U and Th in zircon91500 are 80 and 30 ppm, respectively, of which U-Pb age is 1065 Ma (Wiedenbeck et al., 2004). The concentration of radiogenic 4He was supposed to be 55 ppm based on the U and Th concentrations and U-Pb age.
We measured 28Si ions and 4He+ to estimate concentration of radiogenic 4He. The radiogenic 4He in zircon91500 was calculated to be 30 ± 5 ppm after a blank correction. The blank level of this study was 7 ± 3 ppm, which was derived from residual 4He in the sample chamber of LIMAS.