GEOCHEMICAL JOURNAL
Online ISSN : 1880-5973
Print ISSN : 0016-7002
ISSN-L : 0016-7002
47 巻 , 1 号
選択された号の論文の8件中1~8を表示しています
  • D. RAMESHWAR RAO, MEGHA M. DAGA, HAKIM RAI
    2013 年 47 巻 1 号 p. 1-19
    発行日: 2013/02/20
    公開日: 2013/11/23
    ジャーナル フリー
    The Shyok valley in the northern Ladakh is a linear NW-SE trending suture zone (Shyok Suture Zone) and separates two distinct terranes, the southern continental margin of Eurasian Plate to north, and Ladakh magmatic arc to south. The crustal growth in the Shyok Suture Zone (SSZ) can be explained by two stage melting process involving the generation of orthogneiss (OG) with mafic enclaves (ME) (pre-collision), and temporal evolution towards increasingly more evolved felsic magmas during reworking processes (post-collision). In this paper, the geochemistry of various ME and host OG that fall within the Shyok-Darbuk corridor of the suture zone are discussed. The field, petrographic and geochemical data suggest that the ME and host OG are genetically related. The ME has formed by low degree of partial melting of a garnet-bearing source or from the partial melting of a basaltic source with a clinopyroxene-garnet eclogite as residue. The LILE and LREE enrichment of ME have been influenced by mantle-magma interaction during the passage of the hydrous primary arc-magma through the overlying lithosphere. The likely interpretation for the genetic linkage of ME and host OG is explained through the mixing of coeval mafic (enclave) and felsic melts, resulting in hybrid magma. Besides, several geochemical lines of evidence suggest that fractional crystallization of the resultant hybrid magma had played a complementary role in the evolution of these rocks.
  • ERIKA YAMAZAKI, SHUN’ICH NAKAI, TETSUYA YOKOYAMA, SHUNSO ISHIHAR ...
    2013 年 47 巻 1 号 p. 21-35
    発行日: 2013/02/20
    公開日: 2013/11/23
    ジャーナル フリー
    An analytical technique is presented for isotope analysis of a Sn ore mineral, cassiterite. Hydroiodic acid was used to successfully decompose the cassiterite samples. An extraction chromatographic material, TRU resin was utilized for the chemical purification of Sn. Although it has been confirmed that this purification process causes isotopic fractionation, the effect can be corrected. The purification method was used to investigate natural isotopic variations in Sn in cassiterite from Asian countries (Japan, China, Thailand, and Malaysia), via multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Consequently, isotopic variations as large as 0.77‰ in 124Sn/120Sn were observed for cassiterite samples from Japan and China. The isotopic fractionations of all the samples were dependent on isotope mass. However, the cause of Sn isotopic variation is yet to be established.
  • JING LI, XING LIANG, MENGGUI JIN, XUMEI MAO
    2013 年 47 巻 1 号 p. 37-50
    発行日: 2013/02/20
    公開日: 2013/11/23
    ジャーナル フリー
    Oxygen-18 (18O), deuterium (D), and chemical patterns were determined in pore water samples extracted from 100-m depth in Caofeidian Harbor, China. The stable isotopic composition indicates that aquitard pore water was of meteoric origin and was basically not influenced by evaporation. Seawater diffusion was identified as the main influence on the isotope signature of pore water, according to the δ18O profile and positive correlation between δ18O and Cl-. The chemistry of aquitard pore water is characterized by high total dissolved solids (TDS) decreasing with depth, ranging from 26.89 to 7.26 g/L. The same trend was observed for Cl-, Na+, and Mg2+, as influenced by seawater. However, the typical ion ratios (i.e., Cl/Br, Sr/Ba) of pore water change significantly in different sedimentary facies along the study profile due to the long-term reaction between pore water and sediment, further supporting the finding that pore water was not replaced by modern water or seawater. Therefore, during long-term aquitard residence, the chemical composition of pore water would be influenced by other processes such as reduction of sulfates and cation exchange. The rare earth elements (REEs) of pore water are mainly affected by water-rock interaction. The shale-normalized REE fractionation patterns display enrichment of heavy REEs (HREE) relative to light REEs (LREE) and even greater fractionation degree in marine sediment pore waters (as larger La/YbNASC). The relative reduction environment in the aquitard and preferential mobilization of Eu2+ in the water-rock interaction lead to the development of positive Eu anomalies (1.13 < (Eu/Eu*)NASC < 1.98) and slightly positive Ce anomalies (-0.13 < (Ce/Ce*)NASC < 0.35).
  • ZHIMIN ZHU, KEJUN HOU, KONGYANG ZHU, HONGQI TAN
    2013 年 47 巻 1 号 p. 51-64
    発行日: 2013/02/20
    公開日: 2013/11/23
    ジャーナル フリー
    The Hekou Group, an outcrop at the west margin of the Yangtze Block, consists of volcanic and sedimentary rocks of sodic lava, sodic pyroclastic rocks, coarse to fine-grained siliciclastic rocks and carbonate rocks. These rocks have endured lower to upper greenschist-facies metamorphism. The Hekou Group represents the basement of the late Mesoproterozoic to early Neoproterozoic successions and hosts many Fe-Cu deposits. We conducted zircon U-Pb and Lu-Hf isotope analyses and whole-rock geochemistry analyses on igneous rocks from the Hekou Group to constrain the formation age of the Hekou Group and to understand the genesis of the Fe-Cu deposits. A zircon 207Pb/206Pb age of 1669 ± 6 Ma was obtained from the tuffaceous schist of the Hekou Group and is considered to represent the deposition age of the group. Most rocks show negative high field strength element (HFSE, e.g., Ta, Nb, P, Zr, Hf and Ti) anomalies, light rare earth element (LREE) enrichment ((La/Yb)n = 12.10-109.01) and positive zircon εHf(t) values (+1.5 to +4.5). The geochemical and isotopic compositions indicate that the Hekou Group was formed in a back-arc basin at 1.7 Ga. Our study also implies that Paleoproterozoic tectonic evolution of the western Yangtze Block is correlated with the assembly and fragmentation of the supercontinent Columbia.
  • SHEN LIU, RUIZHONG HU, CAIXIA FENG, GUANGYING FENG, YOUQIANG QI, TAO W ...
    2013 年 47 巻 1 号 p. 65-77
    発行日: 2013/02/20
    公開日: 2013/11/23
    ジャーナル フリー
    Associated with several important polymetallic mineral deposits, the origin of the investigated Guilong area granodiorite porphyries (southern Yunnan Province) are presently poorly understood. For this reason we have undertaken a geochronological and geochemical study of these felsic intrusive rocks that includes: U-Pb zircon age-dating, whole-rock geochemical, and Sr-Nd isotopic data, and that are presented and discussed in this paper. Laser ablation inductively coupled plasma mass spectrometry, U-Pb zircon analyses yield an age of 252.5 ± 1.0 Ma for one sample of the felsic rocks. The Guilong area granodiorite porphyries are characterized by variable and high (87Sr/86Sr)i, ranging from 0.7103 to 0.7143, and relatively low εNd(t) values from -13.1 to -11.6. They also exhibit negative Nb, Ta, Ti and Sr anomalies on a primitive mantle-normalized multi-element diagram. Based upon their petrological and geochemical characteristics, we suggest that these rocks derive through the partial melting of crustal rocks at a shallow level. The parental magmas to the Guilong granodiorite porphyries underwent fractional crystallization of K feldspar, plagioclase, and ilmenite or rutile prior to emplacement. Magma generation at around 250 Ma (during Permian times) occurred in response to tectonic interactions between the Yangtze Platform and other crustal blocks, that provided the heat source and elevated geotherms required for partial melting of crust.
Notes
  • YUTAKA KANAI, YOSHIKI SAITO, TORU TAMURA, VAN LAP NGUYEN, THI KIM OANH ...
    2013 年 47 巻 1 号 p. 79-82
    発行日: 2013/02/20
    公開日: 2013/11/23
    ジャーナル フリー
    Deltaic sediments of the Mekong River delta sampled from a tidal beach in Vietnam during the wet season (late October 2011) showed strong 137Cs and 134Cs activities reflecting radionuclides released from the Fukushima Dai-ichi nuclear power plant (FDNPP) after the March 2011 Tohoku-oki earthquake and tsunami, but samples from the same site taken about three months later during the dry season (early February 2012) showed weak activities. This finding indicates that soil from the Mekong drainage basin was deposited along the delta front in the wet season and then removed in the dry season during the winter monsoon. Thus, seasonal changes in the sedimentary environment of sediment supply and accumulation were elucidated by Cs isotopes from the FDNPP accident. This finding may expand the usefulness of radionuclides for obtaining important information about geochemical events.
  • H. KUSUNO, T. FUKUOKA, H. MATSUZAKI
    2013 年 47 巻 1 号 p. 83-88
    発行日: 2013/02/20
    公開日: 2013/11/23
    ジャーナル フリー
    Estimation of 26Al (half-life = 705 kyr) content at the time of meteorite fall is required to calculate the terrestrial age of meteorites using the 26Al content. Previous studies (e.g., Cressy, 1971; Hampel et al., 1980) provided empirical equations for the estimation of 26Al content in meteorites at the time of meteorite fall. However, the equations overestimate the 26Al content of howardites and eucrites, which have a high Al content (5-8%). In this study, we present a new simple empirical equation based on the relationship between 26Al content and the content of two main target elements, Al and Si, in 37 falls meteorites from various classes (one howardite, 10 eucrites, 13 diogenites, 8 chondrites, 3 ureilites, and one angrite). The equation estimates the 26Al content of howardites and eucrites at the time of meteorite fall within a 9% difference of the measured 26Al content. This difference is lower than in any other equations presented in previous literature.
  • SHINGO KATO, TAKAZO SHIBUYA, KENTARO NAKAMURA, KATSUHIKO SUZUKI, V. J. ...
    2013 年 47 巻 1 号 p. 89-92
    発行日: 2013/02/20
    公開日: 2013/11/23
    ジャーナル フリー
    To simulate the discharge zone of hydrothermal systems in the laboratory, we developed a flow-type hydrothermal apparatus that can reproduce water-rock reactions at elevated temperatures and pressures under flow-through conditions and also the mixing of hydrothermal fluids with seawater. Using this apparatus, basaltic rocks were reacted with ultra-pure water at 340°C and 40 MPa for 1271 h under flow-through conditions. An increase in Al, Fe, K, Na, P and Si in the reacted water was observed during the experiment, reproducing the process of dissolution from the basaltic rocks. An increase in pH to 9 during the experiment can be attributed to the dissolution of silicate minerals in the basaltic rocks. That the pH in the experiment is higher than the pH observed in natural hydrothermal fluids at mid-ocean ridges is a result of higher Si concentrations observed in the experiment than those of natural hydrothermal fluids.
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