GEOCHEMICAL JOURNAL

High-throughput isotope analysis of sub-nanogram sized lead using MC-ICP-MS with on-line thallium doping technique and desolvating nebulizer system

Kazuya Nagaishi, Ryoichi Nakada, Tsuyoshi Ishikawa

Article type: research-article
2021 Volume 55 Issue 1 Pages 1-9
Published: 2021
Released on J-STAGE: November 22, 2021

DOIhttps://doi.org/10.2343/geochemj.2.0612

JOURNAL FREE ACCESS

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A MC-ICP-MS technique for precise isotope analysis of sub-nanogram sized Pb sample was developed. We found that combined uses of real-time on-line Tl doping, Tl-isotope-based mass discrimination correction, and desolvating nebulizer operated at relatively low temperature give the best analytical results. The 2SD reproducibility for the analyses with 0.4 ng Pb was better than ±0.0032 for 206Pb/204Pb and 207Pb/204Pb, and better than ±0.0069 for 208Pb/204Pb, which were about 2-3 times better than those obtained by standard bracketing technique without Tl normalization. Our method also enables rapid determination of Pb isotope ratios, 10-13 min per sample, which is about 3 times quicker than the bracketing analysis using an auto-sampler.

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Origin of porphyritic texture by magma mixing in the andesitic rocks of Ghansura Rhyolite Dome from the Bathani volcano-sedimentary sequence, Eastern India

Bibhuti Gogoi, Gaurav Hazarika, Hiredya Chauhan

Article type: research-article
2021 Volume 55 Issue 1 Pages 11-25
Published: 2021
Released on J-STAGE: November 22, 2021

DOIhttps://doi.org/10.2343/geochemj.2.0613

JOURNAL FREE ACCESS
Supplementary material

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The Ghansura Rhyolite Dome (GRD) of Bathani volcano-sedimentary sequence in Eastern India provides direct evidence for mafic-felsic magma interaction during its evolution. The interaction led to the formation of different varieties of hybrid rocks that are now preserved at the outcrop scale. One of the hybrid rocks occupying a significant portion of the rhyolite dome is a porphyritic andesite. Here we try to investigate the magmatic processes that led to the formation of the porphyritic andesite in GRD. From field and petrographical observations and mineral chemical data interpretations we infer that the porphyritic andesite crystallized from a hybrid magma that was produced due to the interaction of an invading crystal-rich basaltic magma and melt-dominated portion of the rhyolite magma chamber. When basaltic magma carrying crystals of plagioclase mixed with the rhyolite melt, the plagioclase crystals present in the mafic endmember were incorporated into the resultant hybrid magma. The melt-dominated nature of the hybrid magma allowed the plagioclase crystals to drift and accumulate as synneutic aggregates in the magma. Eventually, the plagioclase crystals in the synneutic aggregates underwent mutual attachment to form large phenocrysts of plagioclase giving the rock its characteristic porphyritic texture. The compositions of plagioclases from the basalts and those occurring as synneutic aggregates in the porphyritic andesite are almost similar. This compositional similarity suggests that the plagioclase crystals joined by synneusis to produce phenocrysts in the porphyritic andesite were probably derived from the basaltic magma.

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Boron determinations of silicate reference rocks by the isotope dilution method in a high-background environment

Shun’ichi Nakai

Article type: research-article
2021 Volume 55 Issue 1 Pages 27-32
Published: 2021
Released on J-STAGE: November 22, 2021

DOIhttps://doi.org/10.2343/geochemj.2.0614

JOURNAL FREE ACCESS

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Boron concentrations in silicate reference rocks were determined using an inductively coupled plasma mass spectrometer (ICP-MS) installed in a laboratory with a high background noise level of boron. The feasibility of three methods of internal standard, standard addition, and isotope dilution were examined. Application of the internal standard analysis is difficult in the absence of an appropriate internal standard element. Repeated analyses of the reference rocks showed that the isotope dilution analysis yielded more reproducible concentrations than the standard addition analysis in the high background environment. Results of boron determinations of 12 standard silicate rocks using the isotope dilution method were mostly consistent with values reported earlier from wet chemical analysis and from prompt gamma ray neutron activation analysis. The isotope dilution method can be recommended for boron determination.

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A record of ²⁴¹Am, ²³⁶U, ²³⁸U, ²³⁹Pu, ²⁴⁰Pu, ¹³⁴Cs and ¹³⁷Cs in surface seawater and ²⁴¹Am in aerosols shortly after the FDNPP incident occurred

Taeko Shinonaga, Katharina Gückel, Masatoshi Yamada, Toshitaka Gamo, P ...

Article type: research-article
2021 Volume 55 Issue 1 Pages 33-38
Published: 2021
Released on J-STAGE: November 22, 2021

DOIhttps://doi.org/10.2343/geochemj.2.0615

JOURNAL FREE ACCESS

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We obtained the rare data on low activity concentration of ²⁴¹Am in the surface seawater collected at about 210 km northeast of Fukushima Daiichi Nuclear Power Plant (FDNPP) on 29 April 2011. The concentration of ²³⁶U, ²³⁸U, ²³⁹Pu, ²⁴⁰Pu, ¹³⁴Cs and ¹³⁷Cs in that seawater was also determined and the atomic ratio ²³⁶U/²³⁸U and ²⁴⁰Pu/²³⁹Pu was calculated. The activity concentration of 137Cs compared to that before the incident and the equal activity ratio of ¹³⁴Cs/¹³⁷Cs clearly indicate that the seawater was contaminated by the radioactive Cs discharged from the FDNPP, while the activity concentration of ²⁴¹Am as well as ²³⁶U, ²³⁹Pu and ²⁴⁰Pu was in the range of the values reported before the incident. ²⁴¹Am in the aerosols collected at about 120 km from the FDNPP, in which FDNPP derived ²³⁶U and Pu isotopes were found, was analyzed. However, no significant larger amount compared to ²³⁶U and ^239+240Pu was found.

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