GEOCHEMICAL JOURNAL
Online ISSN : 1880-5973
Print ISSN : 0016-7002
ISSN-L : 0016-7002
Volume 56, Issue 1
Displaying 1-4 of 4 articles from this issue
ARTICLE
  • Kenta Asahina, Takeshi Nakajima, Koji U. Takahashi, Miyuki Kobayashi, ...
    Article type: ARTICLE
    2022 Volume 56 Issue 1 Pages 1-15
    Published: 2022
    Released on J-STAGE: February 28, 2022
    Advance online publication: December 08, 2021
    JOURNAL OPEN ACCESS FULL-TEXT HTML

    Paleoceanographic changes in the Akita Basin during the Miocene were investigated using biomarker analysis of source rocks from the Onnagawa and Kusanagi formations in the Yashima area (eastern basin margin) and the Shonai Plain (central basin). During the Nishikurosawa Stage (~13 Ma), the homohopane index indicated a slightly oxidizing depositional environment. At this time, the Japan Sea was an open marginal sea and an oxidizing environment; thus, organic matter preservation in the Akita Basin was low. In the Early Onnagawa Stage (~12 Ma), the Japan Sea became semi-closed because of uplift of the proto-Northeastern Japan Arc. This change is reflected in the pristane/phytane ratio and homohopane index that suggest the Yashima and Shonai Plain areas became reducing, which enhanced organic matter preservation. The depositional environment of the Kusanagi Formation (Shonai Plain) was more reducing than that of the Onnagawa Formation (Yashima area). During the Middle Onnagawa Stage (10 Ma), the homohopane index showed that the Onnagawa Formation (Yashima area) had shifted to an oxidizing depositional environment. Additionally, the C27/C27 + C29 sterane ratio indicated that algae were responsible for the higher overall primary production in the Middle Onnagawa Stage. This high primary productivity was associated with localized coastal upwelling, which resulted in generally high organic matter content in the Middle Onnagawa Formation. In the Shonai Plain area (Kusanagi Formation), bottom water conditions remained anoxic; however, sedimentary organic matter content did not increase because there was no upwelling in the area. During the Late Onnagawa Stage (8 Ma), the seafloor environment in the Shonai Plain area also became oxidizing. As a result of enhanced organic matter decomposition, overall sedimentary organic matter content decreased.

    Our results demonstrate that local variations in primary productivity and bottom water conditions are recorded in the sedimentary record and emphasize the importance of geographic and tectonic settings to the deposition and preservation of organic-rich source rocks. Thus, this study was able to describe spatio-temporal changes over the entire Akita Basin.

ARTICLE
  • Yanpei Dai, Yudi Zhu, Xian Zhu, Lili Wang
    Article type: ARTICLE
    2022 Volume 56 Issue 1 Pages 16-30
    Published: 2022
    Released on J-STAGE: February 28, 2022
    Advance online publication: December 08, 2021
    JOURNAL OPEN ACCESS FULL-TEXT HTML

    The Neoproterozoic western Yangtze block is a vital area for understanding geodynamics of the Rodinia supercontinent. In order to explore temporal and compositional variations of I-type granitoids in this region, we present zircon U-Pb-Hf isotopes and bulk rock geochemical data of the Shimian monzogranite and Kangding tonalite. They yield emplacement ages of 827 ± 5 Ma and 757 ± 3 Ma with εHf(t) values ranging from 6.8 to –1.1 and 9.6 to 3.6, respectively. Typical peraluminous and high-temperature minerals, such as garnet and pyroxene, are scarce. The characteristics of low A/CNK, Ga/Al and FeOT/MgO ratios, as well as low zircon saturation temperatures (770–660°C) can be identified from the granitoids, suggest a typical I-type granite origin. They have variable Y/Nb ratios of 6.18–1.76, Mg# values of 44.3–14.2, and εHf(t) values of 9.6 to –1.1, which can be explained by a heterogeneous source primarily comprising juvenile crust with minor ancient crustal components. Elemental correlations are indicative of fractional crystallization, such as Fe-Ti oxides and plagioclase. In comparison, regional I-type granitoids have geochemical diversities which are bounded by ca. 780 Ma, e.g., contrasting SiO2 contents (average 71.78 wt.% vs. 63.79 wt.%), Y/Nb ratios (average 3.91 vs. 3.49), Rb/Sr ratios (average 1.52 vs. 0.13), Mg# values (average 27.9 vs. 43.6) and εHf(t) values (average 4.84 vs. 7.76). All these evidences suggest a different participation of juvenile crust in their source region before and after ca. 780 Ma. Combined with previous studies, we propose an arc affinity for the Neoproterozoic granitoids in this region, and a slab breakoff on ca. 780 Ma responsible for their compositional variations.

ARTICLE
  • Junji Yamamoto, Hidemi Ishibashi, Yuuki Hagiwara, Lena Yokokura, Kiyoa ...
    Article type: ARTICLE
    2022 Volume 56 Issue 1 Pages 31-39
    Published: 2022
    Released on J-STAGE: February 28, 2022
    Advance online publication: December 08, 2021
    JOURNAL OPEN ACCESS FULL-TEXT HTML
    Supplementary material

    In the Horoman peridotite complex, the peridotite containing olivine-filled channels is known to exist. To examine the distribution pattern of the channel olivine, we performed non-destructive microanalyses of Raman spectra at 51 points of the olivine with 5 mm intervals along the channel. Compared with the Raman spectra of a reference euhedral olivine, for which the crystallographic orientation is known, there is no abrupt change in the crystallographic orientation in the 250 mm line analysis region of the channel. Moreover, Mg/Fe ratios of the channel olivine show gradual change over the entire measured area. If the channel olivine is an aggregate of olivine grains crystallized from magma infiltrating into the channel, then there should be olivine grains with both uneven crystallographic orientation and a homogeneous Mg/Fe ratio. Therefore, the olivine in the channel is regarded as a single crystal with slight growth zoning. The peridotite with the channel had been a part of mantle that was uplifted by the collision of plates. Therefore, the channel is a trace of magma migrating in the mantle. Magma migration in mantle drives the material–thermal circulation system connecting the Earth’s interior and surface. Furthermore, the size of the mineral in mantle reflects its stress field. Therefore, the magma channel involving such a large olivine is a unique specimen that reveals the particular characteristics of magmatism occurring in the Earth’s interior.

ARTICLE
  • Yoshiaki Endo, Sebastian O. Danielache, Moeko Ogawa, Yuichiro Ueno
    Article type: ARTICLE
    2022 Volume 56 Issue 1 Pages 40-56
    Published: 2022
    Released on J-STAGE: February 28, 2022
    Advance online publication: December 08, 2021
    JOURNAL OPEN ACCESS FULL-TEXT HTML
    Supplementary material

    The sulfur isotope fractionation that occurs during SO2 photolysis is key to explaining the isotope signatures stored in ancient sedimentary rocks and understanding the atmospheric compositions of the early Earth and early Mars. Here, we report the photoabsorption cross-sections of 32SO2, 33SO2, 34SO2, and 36SO2 measured from 206 to 220 nm at 296 K. The wavelength resolution was set to 1 cm–1, 25 times higher than that of previous SO2 isotopologue absorption spectra measurements. The precision of ~10% is in agreement with previously reported SO2 absorption spectra. In comparison with previously reported high-resolution spectra of natural abundance, SO2 measurements demonstrate smaller cross-sectional magnitudes at absorption peaks and an offset wavelength by ~0.016 nm. Using the newly recorded isotopologue spectra, we calculated the sulfur isotope fractionation for self-shielding during SO2 photolysis. The calculated 34S fractionation (34ε) roughly reproduces the observed relationship between 34ε and the SO2 column density in previous photolysis experiments. Thus, the cross-section is useful for predicting 34S/32S isotope fractionation in an optically thick SO2 atmosphere. In contrast, for mass-independent fractionation (MIF-S, i.e., non-zero Δ33S), the measured spectra predicted a weakly negative Δ33S/δ34S slope of about –0.1. The small Δ33S/δ34S slope is consistent with the slopes of SO2 photolysis experiments under high-pressure atmospheres (i.e., the pressure broadened absorption line width will be comparable to the spectral resolution). Therefore, MIF-S during photolysis experiments was linked to spectroscopic measurements for the first time. We conclude that reasonable precision and high-resolution spectroscopic measurements are key to explaining the origin of MIF-S at column densities below 1018 cm–2. However, MIF-S production in chamber experiments or atmospheric conditions may require understanding pressure or temperature effects, such as linewidth broadening on the UV-absorption spectra, and how these effects manifest themselves on isotopologues.

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