GEOCHEMICAL JOURNAL
Online ISSN : 1880-5973
Print ISSN : 0016-7002
ISSN-L : 0016-7002
Volume 52 , Issue 1
Showing 1-7 articles out of 7 articles from the selected issue
  • Kohji Marumoto, Akinori Takeuchi, Shoko Imai, Hitoshi Kodamatani, Nori ...
    2018 Volume 52 Issue 1 Pages 1-12
    Published: 2018
    Released: February 02, 2018
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    Re-emission of mercury (Hg), previously deposited in soils and oceans, plays an important role in the global Hg cycle. Mercury evasion from surrounding sea surfaces might be one of the main emission sources of atmospheric Hg in Japan. To obtain information on Hg cycling around the Japanese Islands, dissolved gaseous mercury (DGM) and Hg evasion fluxes from surface seawaters of the Tsushima Strait and East China Sea were investigated using a two-layer gas exchange model. The mean DGM concentrations in the Tsushima Strait are 22 ± 7 pg L–1 in October 2014 and 27 ± 6 pg L–1 in August 2015, suggesting slightly higher values in summer (P < 0.05, t-test). The DGM concentrations in the Kuroshio Current are lower than in the Tsushima Strait and Pacific Ocean. However, the DGM concentrations are supersaturated at all sites, except at the site with the highest atmospheric total gaseous Hg (TGM). This suggests that Hg is emitted from these sea surfaces into the atmosphere. The TGM concentrations are sometimes increased due to the long-range transport from the Asian continent and volcanic activities on the southern Kyushu Island. The DGM concentrations and %DGM of the total Hg in the Tsushima Strait are positively correlated with the solar radiation, indicating that the DGM is partly produced by photochemical formation from Hg(II) in the surface seawater. No positive relationships between the DGM and sunlight were observed in the Kuroshio Current. Given that most of the observations for the Kuroshio Current were made far from the land, at water depths >200 m, the photochemical production of DGM is likely related to the distance from the coast and water depth, more specifically the inflow of terrestrial substances such as Fe and humic substances. The factors controlling the DGM variations differ at various sites, depending on the sea area and seasonal conditions. Based on the literature and results of this study, the Hg emissions of the East China Sea, including the Yellow Sea, are estimated to be 49 ± 17 t yr–1. This value is greater than the annual anthropogenic Hg emission from the Japanese Islands (18–34 t yr–1) but might be low compared with the outflow of Hg from the Asian continent.

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  • Jiang Chen, Zhibo Bing, Zhaoxia Mao, Chunpeng Zhang, Zhongwei Bi, Ze Y ...
    2018 Volume 52 Issue 1 Pages 13-27
    Published: 2018
    Released: February 02, 2018
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    Prospecting target areas generally involves a comprehensive analysis of the geological prospecting criteria or the use of mathematical methods to extract comprehensive prospecting information to compare with the known ore deposits. Geochemical survey data have played an important role for prospecting target areas. However, most target areas are large, their locations are not specific, and the data are mostly based on the main ore-forming elements and ignore the contribution of other geochemical elements. This study rasterizes geochemical data at a scale of 1:200,000 in the Manzhouli area and prepares a geochemical image atlas with a spatial resolution of 1 km using the hyperspectral remote sensing data processing method. The sequential maximum angle convex cone (SMACC) spectral tool is used to identify favorable regions of rock formation, to classify enrichment zones of metallogenic elements in combination with the mineralization intensity of the main ore-forming elements on the surface, and to generate an ore prospecting target area classification map. Our results show that some of the areas known to have ore deposits, including the Wunugetushan, Babayi, and Badaguan copper molybdenum deposits in the Manzhouli area, fall into the potential target area, which demonstrates the accuracy of our proposed method in prospecting target areas. Therefore, this method can be used to accurately identify ore targets based on comprehensive geochemical data.

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  • Xiao-Hu Li, Jian-Qiang Wang, Feng-You Chu, Hao Wang, Zheng-Gang Li, Xi ...
    2018 Volume 52 Issue 1 Pages 29-36
    Published: 2018
    Released: February 02, 2018
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    Considerable Cu isotopic fractionation occurs during the formation of mineral deposits and the oxidative weathering processes of modern seafloor hydrothermal sulfides. This report is the first to describe Cu isotopic compositions of Cu-rich and Zn-rich sulfides, and their oxidation products collected from hydrothermal fields along the South Mid-Atlantic Ridge (13–15°S). Results show that the δ65Cu values of these sulfides and their oxidation products are –0.58‰ to +1.36‰, with an average value of 65Cu = +0.30 ± 1.02‰ (n = 17, 2 sd). Cu-rich sulfides have slightly lighter Cu isotope compositions, with an average δ65Cu = +0.37 ± 0.16‰ (n = 5, 2 sd), whereas Zn-rich sulfides are enriched in heavy Cu isotopes, with an average δ65Cu = +0.77 ± 0.59‰ (n = 6, 2 sd). Compared to the Cu-rich and Zn-rich sulfides, their oxidation products are more enriched in light isotopes, with an average δ65Cu = –0.24 ± 0.58‰ (n = 6, 2 sd). The marked enrichment of the oxidation products in light Cu isotopes indicates that Cu isotopic fractionation occurs during the subsequent leaching alteration of seawater, whereas high δ34S values (up to 14.5‰) of oxidation products also reflect large contributions of sulfate reduction S from seawater. The sulfides and oxidation products from the inactive hydrothermal fields reflect that substantial light Cu isotopic compositions can be a light output sink, which might explain the heavy isotopic compositions of the oceans.

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  • Huan Kang, Yue-Long Chen, Da-Peng Li, Chuang Bao, Hua-Zhen Zhang
    2018 Volume 52 Issue 1 Pages 37-57
    Published: 2018
    Released: February 02, 2018
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    In this paper, 219 concordant detrital zircons from the main stem and tributary rivers of the Central Qilian Block (CQB) have been analyzed using excimer laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to determine their U-Pb ages and Lu-Hf isotopic compositions. The detrital zircons from three samples show four major age groups: 246–509 Ma, 899–1176 Ma, 1620–2089 Ma, and 2131–2610 Ma. These age populations indicate that prominent magmatic events occurred at 0.5 Ga, 0.9 Ga, 1.8 Ga, and 2.5 Ga. Archean basement components are rare in these river sands and likely derived from Paleoproterozoic metamorphic complexes. The crustal model ages from Hf isotopic analyses show age peaks at ~2.5 and 1.8 Ga. The crustal accretion of material derived from the depleted mantle of the CQB occurred in two stages at 3.2–1.8 Ga and 1.8–0.6 Ga. The crustal accretion curve based on the Hf model ages indicates that approximately 10% of the present crustal volume of the CQB formed at 3.2 Ga, while ~60% formed at 1.8 Ga. Few crustal components have accreted from the depleted mantle since 0.6 Ga in the CQB. The reworking rate calculation shows that continental crustal accretion also occurred at 2.5 Ga in the CQB; the most intensive crustal reworking occurred at 0.8 Ga. Comparisons of Precambrian continental growth patterns of the African continent and CQB indicate that the zircon Hf isotopic compositions can be used to accurately constrain the growth patterns of continental crust and that a microcontinent can serve as window into the crustal growth of supercontinents during certain periods.

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  • Kenta Yoshida, Rustam Orozbaev, Takao Hirajima, Akira Miyake, Akira Ts ...
    2018 Volume 52 Issue 1 Pages 59-67
    Published: 2018
    Released: February 02, 2018
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    Direct chemical analysis of a single fluid inclusion was conducted by micro-excavation at cryo-temperatures. A scanning electron microscope (SEM) equipped with a focused ion beam (FIB), an energy dispersive X-ray spectrometer (EDS), and a cold stage, were used to analyze the chemical composition of the sample fluid inclusion, which was trapped in the ultrahigh-pressure (UHP) talc-garnet-chloritoid schist obtained from the Makbal metamorphic complex, Kyrgyz. Conventional techniques such as microthermometry and cryo-temperature and room-temperature Raman spectroscopy enabled the detection of NaCl and CaCl2 as solute species in the fluid inclusions, as well as high salinity of 20.5 mass% CaCl2 and 1.7 mass% NaCl with the assumption of a NaCl-CaCl2-H2O ternary system. However, additional chemical analysis using the present cryo-FIB-SEM-EDS system further revealed the presence of K as a solute element, which is hardly identified by conventional techniques. Petrographic examination of the fluid inclusions indicated that they had been trapped during the exhumation stage of the UHP talc-garnet-chloritoid schist, possibly originating from the decompression breakdown of lawsonite. The data acquired from the present study challenges previous reports of the existence of simple chemical system of aqueous fluids in high-pressure and UHP metamorphic terrane in eastern Asia. This is imperative because misidentification of solute species introduces errors into salinity estimation, resulting in inaccuracy propagation in quantitative analytical processes such as LA-ICP-MS. A detailed fluid inclusion petrography would thus require accurate quantitative analysis involving FIB-based sample-preparation and SEM-EDS analysis.

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  • Morihisa Hamada, Jun-Ichi Kimura, Qing Chang, Takeshi Hanyu, Takayuki ...
    2018 Volume 52 Issue 1 Pages 69-74
    Published: 2018
    Released: February 02, 2018
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    We report a high-precision 206Pb-based lead isotope composition of olivine-hosted melt inclusions in basaltic rocks from Rarotonga Island, Polynesia, in the southern Pacific, using femtosecond laser ablation (FsLA)-multiple Faraday collector-inductively coupled plasma-mass spectrometry (MFC-ICP-MS). This improved method enables high-precision analysis of Pb isotopes from low-Pb (≤10 ppm) melt inclusions with a crater size of ~30 μm in diameter and depth. The small crater size allows for further analysis of major and trace elements from the same melt inclusions using FsLA-sector field (SF)-ICP-MS. Using Pb isotope ratios of two olivine-hosted melt inclusions, we suggest that the mantle source beneath Rarotonga Island is heterogeneous. Such identification becomes possible owing to high-precision in situ analysis of Pb isotopes in melt inclusions and application of statistical approaches such as Independent Component Analysis to the analytical data.

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  • Tsuyoshi Tanaka, Seung-Gu Lee, Taehoon Kim, Seunghee Han, Hyo Min Lee, ...
    2018 Volume 52 Issue 1 Pages 75-79
    Published: 2018
    Released: February 02, 2018
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    The concentrations of fourteen rare earth elements (REEs) in GSJ/AIST geochemical reference materials JCp-1 (coral) and JCt-1 (giant clam) were identified by isotope dilution inductively-coupled plasma-mass spectrometry, using a quadrupole mass filter (ID-ICP-QMS). Large samples (2~4.1 g) were used to eliminate inhomogeneities in the sample powders. The sample powders were spiked with enriched isotopes and decomposed using HCl. The REEs were separated from the matrix elements by Fe(OH)3-coprecipitation and purified by cation-exchange chromatography. The REEs were further divided into light REE, middle REE, and heavy REE fractions using cation-exchange chromatography, eluted with 2-hydroxyisobutyric acid (HIBA). The abundances for ten poly-isotopic REEs were obtained using both the isotope dilution method and the conventional peak height comparison method, using a calibration curve. The recovery rates for the ten REEs were then calculated by comparing the values obtained by these two methods. The obtained recovery rates were then interpolated for mono-isotopic REEs, and their concentrations were corrected from their relative peak heights. The obtained values were determined twice, using different spike/sample ratios, and agreed within +/–2%, with the exception of La, Ce, and Ho. The ID gave precise abundances for the fourteen different REEs rapidly and easily.

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