10:45 〜 11:00
[MIS13-07] Advanced geochronological instrument: a sensitive high-resolution ion microprobe (SHRIMP)
With adding multiple data, such as trace-element contents and isotope ratios, to zircon geochronology, discussion about geochemical information of geologic events is becoming increasingly important. A sensitive high-resolution ion microprobe (SHRIMP) is a type of magnetic sector large-geometry secondary ion mass spectrometer (LG-SIMS). SHRIMP has contributed to micro-scale geochemistry by providing highly reliable isotopic data with high mass resolution.
The SHRIMP has the following main features in the analyses of samples: (i) Limited required sample amount for the analysis that permits observation of analyzed spots by SEM and analyses of multiple elements and isotopes, (ii) Suitable for the analyses of elements with high electronegativities, such as O, F, Cl, N, and S. In this presentation, the oxygen isotope analysis is introduced as a representative of the second feature. The Utsubo granitic pluton in the Hida belt is emplaced into the Hida gneiss and has a normal lateral compositional zoning: tonalite, granodiorite, pink coarse-grained granite, and fine-grained granite from its margin to center (Kano, 1990). Zircon grains extracted from tonalite, granodiorite, and coarse-grained granite yielded the U-Pb ages of 192.2 ± 1.4 Ma, 190.0 ± 1.3 Ma, and 188.5 ± 1.4 Ma, respectively. The geochronological results indicate that the time interval from tonalite to coarse-grained granite is 3.7 ± 2.0 Ma. The simple averages of oxygen isotope compositions (δ18OVSMOW) in zircons decrease slightly (by ~0.54‰) for a ~12% increase in SiO2, tonalite to granite. The calculated whole rock δ18O based on the correlation between Δ18O (WR-zircon) and silica content (Δ18O (WR-zircon) = 0.0612(SiO2) - 2.50; Lackey et al., 2008) is 7.81 ‰ (tonalite), 8.13 ‰ (granodiorite), 8.05 ‰ (granite), using the SiO2 contents measured by Arakawa and Shinmura (1995). Closed-system differentiation of mafic magmas typically produces an increase in whole-rock δ18O of ~0.5‰ for each 10% increase in SiO2 (Taylor & Sheppard, 1986). Lackey et al. (2008) reported that the whole-rock δ18O of calc-alkaline systems (the Tuolumne suite) increases by ~ 1.0‰ for each 10% increase in SiO2, while the δ18O of the zircons is relatively constant. In the Utsubo granitic pluton, δ18O of the zircons indicates a decrease from the tonalite to the coarse-grained granite. The data set of oxygen isotope ratio is one of the examples that the high-precision and micro-scale analysis of the oxygen isotopes in zircons allows a new insight.
Moreover, SHRIMP has contributed to the microscale analysis of the oxygen isotope of phosphate, including bioapatite, for estimating the paleoenvironment. In this presentation, we will discuss such contributions and benefits of SHRIMP.
The SHRIMP has the following main features in the analyses of samples: (i) Limited required sample amount for the analysis that permits observation of analyzed spots by SEM and analyses of multiple elements and isotopes, (ii) Suitable for the analyses of elements with high electronegativities, such as O, F, Cl, N, and S. In this presentation, the oxygen isotope analysis is introduced as a representative of the second feature. The Utsubo granitic pluton in the Hida belt is emplaced into the Hida gneiss and has a normal lateral compositional zoning: tonalite, granodiorite, pink coarse-grained granite, and fine-grained granite from its margin to center (Kano, 1990). Zircon grains extracted from tonalite, granodiorite, and coarse-grained granite yielded the U-Pb ages of 192.2 ± 1.4 Ma, 190.0 ± 1.3 Ma, and 188.5 ± 1.4 Ma, respectively. The geochronological results indicate that the time interval from tonalite to coarse-grained granite is 3.7 ± 2.0 Ma. The simple averages of oxygen isotope compositions (δ18OVSMOW) in zircons decrease slightly (by ~0.54‰) for a ~12% increase in SiO2, tonalite to granite. The calculated whole rock δ18O based on the correlation between Δ18O (WR-zircon) and silica content (Δ18O (WR-zircon) = 0.0612(SiO2) - 2.50; Lackey et al., 2008) is 7.81 ‰ (tonalite), 8.13 ‰ (granodiorite), 8.05 ‰ (granite), using the SiO2 contents measured by Arakawa and Shinmura (1995). Closed-system differentiation of mafic magmas typically produces an increase in whole-rock δ18O of ~0.5‰ for each 10% increase in SiO2 (Taylor & Sheppard, 1986). Lackey et al. (2008) reported that the whole-rock δ18O of calc-alkaline systems (the Tuolumne suite) increases by ~ 1.0‰ for each 10% increase in SiO2, while the δ18O of the zircons is relatively constant. In the Utsubo granitic pluton, δ18O of the zircons indicates a decrease from the tonalite to the coarse-grained granite. The data set of oxygen isotope ratio is one of the examples that the high-precision and micro-scale analysis of the oxygen isotopes in zircons allows a new insight.
Moreover, SHRIMP has contributed to the microscale analysis of the oxygen isotope of phosphate, including bioapatite, for estimating the paleoenvironment. In this presentation, we will discuss such contributions and benefits of SHRIMP.