巻号一覧
39 巻, 1 号
選択された号の論文の9件中1～9を表示しています
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Hongtao Liu2005 年 39 巻 1 号 p. 1-20
発行日: 2005年
公開日: 2006/03/15
ジャーナル フリーThe Qimantage volcanic suite (QVS) of late Triassic age, dominated by dacitic to rhyolitic lavas and pyroclastic rocks with minor basaltic andesites, outcrops mainly in the middle segment of Qimantage Mt. region, Kunlun orogenic belt, which extends along the northern margin of Tibetan plateau, western China. Petrological and geochemical studies indicate that the volcanic rocks are of typical calc-alkaline affinity with silica contents ranging from ∼53 to ∼80 wt%, exhibiting a medium- to high-K characteristics. Major element variations define relatively smooth compositional trends, while large ion lithophile (LIL) trace elements correlate positively with increasing SiO2. Chondrite-normalized incompatible trace element spidergram shows conspicuous sparks at Rb and Th, but pronounced Nb and Ti depletion. The chondrite-normalized REE patterns of QVS are generally paralleled to each other, exhibiting highly fractionated LREE/HREE spectra with small to pronounced negative Eu anomalies. Two whole-rock Rb-Sr isochrons yield ages of ca. 209 and ca. 225 Ma, respectively, indicating the volcanic suite was emplaced during the late Triassic. The initial 87Sr/86Sr ratios (0.708-0.709) and initial εNd(t) values (-2.05∼-2.35) imply that the magmas were derived from a slightly enriched mantle source with considerable crustal contamination. Considering its petrological, geochemical and isotopic characteristics in a regional geological background, it is proposed that QVS was generated upon an active continental margin during the late Triassic north-directed subduction event. Both the volcanics and the closely-related coeval granitoid plutons collectively make up the late Triassic arc magmatic belt on the paleogeographically southern margin of the Tarim continent.抄録全体を表示PDF形式でダウンロード (279K) -
Shigeyuki Wakaki, Tsuyoshi Tanaka2005 年 39 巻 1 号 p. 21-28
発行日: 2005年
公開日: 2006/03/15
ジャーナル フリーConventional (multi-)mineral Rb-Sr isochron method requires several mineral species. Not all rock-forming minerals, however, provide the same chronological information. The closure temperature and the extent of weathering differ from mineral to mineral. On the other hand, the whole-rock isochron method has ambiguities on the initial Sr isotope equilibrium. In order to eliminate these complications and to date individual events related to each minerals in a given rock sample accurately, single mineral Rb-Sr isochron method was developed and applied to two different types of Cretaceous volcanic rocks in central Japan, Agigawa welded tuff sheet (Nohi Rhyolite Complex) and quartz porphyry (a dyke intruded into the Toki granite). Precise drilling was carried out on single phenocrystic grains of K-feldspar and chlorite (latter from quartz porphyry only) in order to obtain several aliquots from a single mineral grain. The internal difference of parent/daughter isotope ratios is expected fortuitously. Rb-Sr multi-mineral isochron method failed to yield a precise age from the Agigawa welded tuff sheet, probably because of partial alteration. A single grain K-feldspar age of 72.0 ± 4.1 Ma obtained from the same sample is likely to date annealing of the K-feldspar grain by the intrusion of the Toki granite. In the case of the quartz porphyry dyke, three K-feldspar grains gave the ages of 79.9 ± 6.8 Ma, 71 ± 14 Ma and 82 ± 18 Ma. Combined K-feldspar age for all separates from the three grains is 77.0 ± 4.6 Ma, probably corresponding to the intrusion of the dyke. On the other hand, a combined single mineral isochron age of chlorites (five grains) in the same quartz porphyry dyke is 54.5 ± 4.6 Ma. It seems to date the chlorite formation, which corresponds to the subsequent hydrothermal alteration. Single mineral isochron method proved its effectiveness in the case when Rb-Sr isotope system was partially disturbed between different mineral phases.抄録全体を表示PDF形式でダウンロード (263K) -
Masao Sorai, Takashi Ohsumi2005 年 39 巻 1 号 p. 29-45
発行日: 2005年
公開日: 2006/03/15
ジャーナル フリーFor the assessment of the long-term consequences of the carbon dioxide ocean sequestration, the CO2 injection into the middle depth parts of the ocean was simulated using a geochemical box model of the global carbon cycle. The model consists of 19 reservoir boxes and includes all the essential processes in the global biogeochemical cycles, such as the ocean thermohaline circulation, the solubility pump, the biological pump, the alkalinity pump and the terrestrial ecosystem responses. The present study aims to reveal the effectiveness and consequences of the direct ocean CO2 sequestration in relation to both lowering the atmospheric transient PCO2 peak and reduction in future CO2 uptake potential of the ocean. We should note that the direct ocean injection of CO2 at the present time means the acceleration of the pH lowering in the middle ocean due to the eventual and inevitable increase of CO2 in the atmosphere, if the same amount of CO2 is added into the atmosphere-ocean system. The minimization of impact to the whole marine ecosystem might be attainable by the direct ocean CO2 sequestration through suppressing a decrease in the pH of the surface ocean rich in biota. The geochemical implication of the ocean sequestration is such that the maximum CO2 amount to invade into the ocean, i.e., the oceanic CO2 uptake potential integrated with time until the end of fossil fuel era, is only dependent on the atmospheric PCO2 value in the ultimate steady state, whether or not the CO2 is purposefully injected into the ocean; we gave the total potential capacity of the ocean for the CO2 sequestration is about 1600 GtC in the case of atmospheric steady state value (PCO2) of 550 ppmv.抄録全体を表示PDF形式でダウンロード (278K) -
Xiaodong Liu, Liguang Sun, Xuebin Yin, Renbin Zhu, Zhouqing Xie, Yuhon ...2005 年 39 巻 1 号 p. 47-59
発行日: 2005年
公開日: 2006/03/15
ジャーナル フリーThe elemental geochemical composition of sediments influenced by seal excrements in the Antarctic Fildes Peninsula has been examined in order to establish the source of organic matter, identify potential bio-elements and explore their potential palaeoecological implications. The combination of total organic carbon concentration (TOC), total nitrogen concentration (TN), organic carbon isotope (δ13C) and atomic C/N ratio shows that the organic matter in the sediments with many seal hairs has a marine origin, predominantly derived from seal excrements. Among the determined element/oxide concentrations, S, Se, F, Zn, Hg, FeO and P2O5 were found to be remarkably enriched in the sediments influenced by seal excrements with respect to the sediments with few or without seal hairs, and their concentrations displayed strong and positive correlations with organic matter abundance, indicating that they had the same source and transportation mechanism as the in situ organic material. A comparison of these element/oxide concentrations with the seal hair numbers showed that they had similar distribution patterns with depth, and the correlations were positive and statistically significant. Based upon these results, S, Se, F, Zn, Hg, FeO and P2O5 in the seal excrement sediments were identified as potential bio-elements and their concentrations could potentially be used as inorganic geochemical indicators for tracking seal palaeoecological processes in the Antarctic region.抄録全体を表示PDF形式でダウンロード (384K) -
Xuefen Sheng, Jun Chen, Junfeng Ji, Yunxia Sui2005 年 39 巻 1 号 p. 61-67
発行日: 2005年
公開日: 2006/03/15
ジャーナル フリーElectron Paramagnetic Resonance (EPR) analysis was employed to extract paleoclimatic information from mollusk shells taken from the Luochuan section located on the Loess Plateau of central China. The results show that the EPR spectrum of Mn2+ in mollusk shells is complicated and consists of two sets of six hyperfine lines of Mn2+ that arise from two non-equivalent Mn2+ sites in aragonite. There is no obvious inconsistency of EPR spectral intensities among different species of modern mollusks at Luochuan area; the magnitude of EPR spectra intensities of mollusk shells is correlative with the Mn2+ concentration of the environments. The EPR signals from mollusk shells taken from the loess, which is considered to be formed in the cold and dry glacial environment, are stronger than those from the paleosol deposited under the warm and humid interglacial condition suggesting that the Mn2+ available for mollusks in a cold climate is higher than in a warm climate. We propose that during the glacial period, the mollusks lived in a relatively reductive soil environment with more Mn2+ while during the interglacial period its environment was more oxidizing with less Mn2+. This result provides direct evidence that the loess-paleosol interlayers came into being under alternating redox conditions resulting from fluctuating paleoclimate. The analysis of the EPR spectra of mollusk shells serves as a new tool for retrieving climatic information from the mollusk records.抄録全体を表示PDF形式でダウンロード (357K) -
Tapan Pal, Tanay Dutta Gupta, Partha Pratim Chakraborty, Subhas Chandr ...2005 年 39 巻 1 号 p. 69-82
発行日: 2005年
公開日: 2006/03/15
ジャーナル フリーA thick sequence of bedded tuff alternated with non-volcanogenic turbidites is present in the Archipelago Group of rocks (Mio-Pliocene age) of the Andaman Islands. The tuff occurs in three different facies: a) Facies A, white massive tuff with ill defined bedding contacts; b) Facies B, dominantly green and white tuff with ill defined turbidite Tabc, Tab Bouma sequence; and c) Facies C, well defined ash turbidites of Tabc, Tab, Tabc, Tacde sequence. Dominance of cuspate shards with no pervasive alteration and absence of blocky shards are features of subaerial eruption for the pyroclastic rocks. Absence of glass welding and plastic deformation together with the presence of good sorting and recurrence of ash turbidites indicate that subaerially-erupted ash landed in water and behaved as a cold subaqueous flow. Field features show change of flow character from subaqueous debris flow (for Facies A) to recurrent high to low concentration turbidity current (for Facies B), and ultimately to low particle concentration turbidity current (for Facies C). All the tuff varieties are vitric to crystovitric in character, and contain broken crystals of quartz, plagioclase, mica, and glass shards of delicate shape (crescent, cuspate, curved) without any welding features. The tuffs do not contain any lithic fragments of volcanic rocks. Petrography as well as XRD studies show that glass alteration is common in Facies A and is least in green tuff of Facies B. Alteration of glass to clinoptilolite and analcime has been linked to burial diagenesis. The similarity of chemical composition of glass shards (mainly dacite) and bulk rocks, however, indicates negligible chemical change during diagenesis. The origin of Andaman tuff in a convergent margin tectonic setting is assumed as the basis of high Zr/Nb and Zr/Y.抄録全体を表示PDF形式でダウンロード (209K) -
Takeshi Hanyu, Shun'ichi Nakai, Riichiro Tatsuta2005 年 39 巻 1 号 p. 83-90
発行日: 2005年
公開日: 2006/03/15
ジャーナル フリー176Hf/177Hf ratios of nine geochemical reference rocks from the Geological Survey of Japan, together with BIR-1 and BCR-2, were determined using multi-collector inductively coupled plasma mass spectrometry. Our data for BIR-1, BCR-2 and JB-1 are in agreement with those previously reported, demonstrating the appropriateness of the chemical procedure and isotopic measurement employed in this study. The reference rocks have a wide range of 176Hf/177Hf covering the field defined by various volcanic rocks, such as mid-ocean ridge basalts, ocean island basalts, and subduction related volcanic rocks. They are therefore suitable as rock standards for Hf isotope measurement of geological samples.抄録全体を表示PDF形式でダウンロード (88K) -
Yoshiro Nishio, Shun'ichi Nakai, Tetsu Kogiso, Hans G. Barsczus2005 年 39 巻 1 号 p. 91-103
発行日: 2005年
公開日: 2006/03/15
ジャーナル フリーTo investigate the origin of the HIMU (high-μ) reservoir in the mantle, we measured Li, Sr, and Nd isotopic compositions of several oceanic island basalts (OIBs) from the Polynesian region. We used a recently developed multiple-collector inductively coupled plasma mass spectrometry method that allows precise and accurate Li isotopic determinations. This study presents the first Li isotopic data on HIMU OIBs. The measured whole-rock δ7Li values (δ7Li = [[7Li/6Li]sample/[7Li/6Li]L-SVEC standard - 1] × 1000) of the Polynesian HIMU OIBs (Mangaia, Tubuai, and Rurutu) range from +5.0‰ to +7.4‰, which are higher than those of fresh normal mid-ocean ridge basalt (N-MORB) glasses (ca. +3‰). The simultaneously measured K/Rb, Ba/Rb, and 87Sr/86Sr ratios indicate that the analyzed HIMU OIBs are free from significant posteruption alteration. These results suggest that the δ7Li value of the Polynesian HIMU source is never lower than those of the N-MORBs. Among the numerous models for the origin of the HIMU source, the most widely accepted model is that it involves subducted (dehydrated) oceanic crust. For this HIMU-origin model, our new Li isotopic results exclude the highly altered portion, that is, the uppermost part of the oceanic crust, because the δ7Li value of subducted highly altered MORB should be extremely low (δ7Li < fresh MORB). For these reasons, we propose that the Polynesian HIMU source is the relatively less-altered oceanic crust underlying the highly altered crust. Whereas Pb, Sr, and Nd isotopic signatures dominantly indicate the involvement of sediments in a source, the Li isotopic signature is more sensitive to the degree of alteration experienced by the basaltic crust and thus can be used to distinguish what part of the crust was trapped in the OIB magma. It therefore provides information complementary to that provided by the radiogenic isotopes.抄録全体を表示PDF形式でダウンロード (142K)
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Masaaki Musashi, Motoyuki Matsuo, Takao Oi, Yasuhiko Fujii2005 年 39 巻 1 号 p. 105-111
発行日: 2005年
公開日: 2006/03/15
ジャーナル フリーIn order to determine the boron isotopic fractionation factor between boron adsorbed onto kaolin clay and boron in solution (S), as a first attempt, a break-through column chromatography at a high pressure (12.0 MPa) was performed. A boric acid standard solution (9.25 mmol/l, NIST SRM 951) was fed into a stainless steel column packed with boron-free kaolin clay with the flow rate of 0.8 ml/h. The S value obtained at 293 K was 1.0023, indicating that the lighter isotope (10B) was preferentially enriched into the clay. The magnitude of the S was apparently smaller than the previously reported values obtained by the batch method with seawater boron at atmospheric pressure. Besides the effect of pH and the influence of coexisting ions that would lead to a larger S value, as an aspect, dependence of pressure on the boron isotopic fractionation was a possible candidate reducing the S value. Assuming an occurrence of the molecular volume isotope effect (MVIE) on boron species, the S value would decrease with increasing pressure.抄録全体を表示PDF形式でダウンロード (61K)
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