著者

Asako Takamasa Katsuhiko Suzuki Yusuke Fukami Tsuyoshi Iizuka Maria Luisa G. Tejada Wataru Fujisaki Yuji Orihashi Takuya Matsumoto

出版者

GEOCHEMICAL SOCIETY OF JAPAN

雑誌

GEOCHEMICAL JOURNAL (ISSN:00167002)

巻号頁・発行日

vol.54, no.3, pp.117-127, 2020 (Released:2020-06-22)

参考文献数

33

被引用文献数

1 5

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The extinct, relatively short-lived nuclide 182Hf produced 182W as a decay product. Fractionation of Hf-W in the very early Earth led to variations in the 182W/184W ratios of terrestrial rocks; however, because these variations are very small, quantifying 182W/184W ratios requires an extremely precise method. Here, we propose an improved method for highly precise and accurate method for measuring the 182W/184W ratios of terrestrial rocks. Samples were extracted with 4-methyl-2-pentanone and purified by cation and anion exchange chromatography prior to determination of the W isotope ratio by multiple collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) system coupled with a desolvating nebulizer. Sample preparation removed matrix elements (e.g., Hf, Ta, Os, and dimers of Nb and Mo) with masses similar to those of W isotopes, resulting in these elements having a negligible influence on the measured 182W/184W ratios. A W standard solution processed by ion exchange chromatography and/or solvent extraction showed a 183W deficiency, even after mass fractionation correction of the measured isotope data. As reported previously, mass-independent fractionation increases the 182W/184W ratio if the 183W/184W ratio is used to correct for mass fractionation to for better precision in natural samples. However, accurate 182W/184W ratios for a basalt reference material (JB-2) were obtained, even if 183W was used for mass fractionation correction. Our results show that it is also possible to correct for the effects of mass-independent fractionation on the 183W/184W ratio by sample-standard bracketing using a W standard solution subjected to the same preparation procedure used for the samples. A major advantage of the newly developed method is that it requires a smaller amount of sample (0.2–0.3 g; 50–80 ng W for JB-2) compared with that needed for other reported methods (typically 0.7–15 g; 500–1000 ng W). This decrease in sample amount was possible by removing matrix elements from the sample solutions, and cleaning the membrane of the desolvating nebulizer between analyses also contribute to enhancing the W ion beam intensity and to high precision. Analysis of different basalts from the Loihi, Kilauea islands and Ontong Java Plateau with various W isotopic compositions consistent with the previous studies demonstrated the reliability of the method.

著者

Hikaru Sawada Tsuyoshi Iizuka Yukiyasu Tsutsumi Yukio Isozaki

出版者

GEOCHEMICAL SOCIETY OF JAPAN

雑誌

GEOCHEMICAL JOURNAL (ISSN:00167002)

巻号頁・発行日

vol.53, no.3, pp.171-179, 2019 (Released:2019-06-11)

参考文献数

64

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To understand the timing and mode of crustal production and reworking in the Archean, we performed U-Pb and Hf isotopic analyses of detrital zircon grains from the ca. 2.3 Ga Murmac Bay Group in the Rae Craton, central Canada. The zircon U-Pb ages range from 3.9 to 2.3 Ga with a significant gap interval of 3.6–3.3 Ga, indicating that felsic magmatism has semi-continuously within the craton since the early Archean. The combined U-Pb and Hf isotopic data define three distinct Hf isotope-age arrays that share a similar slope equivalent to that of typical 176Lu/177Hf ratio of continental crust, and the slope intersects the mantle evolution curve at 2.9–2.6, 3.3–3.2, and 3.8–3.6 Ga. The secular trends in zircon Hf isotopes illustrate episodic crust formation from depleted mantle during the three periods with subsequent reworking of pre-existing crusts into younger granitoids. Furthermore, these results infer that granitoid crust was rarely reworked for more than 800 million years after its formation. This finding is well explained by assuming that the Archean Rae block has grown outward from the interior by adding new crusts through subduction-related magmatism and/or by secondary accretion of exotic arc crusts. In such a tectonic framework, younger crusts were likely utilized more preferentially in crustal melting during subduction-related magmatism. These observations suggest that plate subduction has operated already in the early Archean, as early as 3.6 Ga Eoarchean.