GEOCHEMICAL JOURNAL
Online ISSN : 1880-5973
Print ISSN : 0016-7002
ISSN-L : 0016-7002
40 巻 , 6 号
選択された号の論文の9件中1~9を表示しています
  • DAISUKE NAKASHIMA, TOMOKI NAKAMURA
    2006 年 40 巻 6 号 p. 543-555
    発行日: 2006年
    公開日: 2007/06/22
    ジャーナル フリー
    Noble gases in the Allan Hills (ALH) 84206 EH3 chondrite were measured with stepwise heating of a large sample (98.9 mg) and with total extraction of small samples (0.9-2.3 mg). Light noble gases show solar and cosmogenic components. The elemental ratios of solar gases in the samples suggest no significant loss of solar 36Ar. Some of small samples are richer in cosmogenic 21Ne than the large sample. Assuming that the excesses are due to a parent body exposure to cosmic rays, we obtain a parent body exposure age longer than 51 Ma and a meteoroid exposure age shorter than 39 Ma. Heavy noble gases are dominated by elementally fractionated Q-gases and atmospheric noble gases. The presence of fractionated Q-gases combined with no loss of solar 36Ar suggests that solar wind exposure occurred after Q-gas fractionation. Kr isotopic ratios show the presence of neutron-induced 80Kr and 82Kr from Br. The minimum radius of the ALH84206 meteoroid was calculated as 27 cm from the abundances of neutron-induced Kr, assuming that these were produced during exposure of the meteoroid.
  • MOHAMMAD REZA GHORBANI
    2006 年 40 巻 6 号 p. 557-568
    発行日: 2006年
    公開日: 2007/06/22
    ジャーナル フリー
    Basic-intermediate volcanic rocks from the Karaj-Danesfahan area in the Neotethyan magmatic belt of Iran show typical island arc geochemical signatures. The rocks demonstrate calcalkaline affinity with low abundances of “high field strength elements” (HFSE) such as Nb, Ta, Hf and Ti, and highly distinctive spiked trace element patterns at “large ion lithophile elements” (LILE) such as Ba, K, and Sr. Comparing the geochemical characteristics of two sets of the volcanic rocks sampled from two parts of the study area 100 km apart, however, indicate subtle but significant geochemical differences which bear important petrogenetic implications. One set of the volcanic rocks is abnormally enriched in Pb and shows consistently higher abundances of HFSE, particularly Ti, than the other set. These two series are called HTL series (after high titanium and lead) and LTL series (after low titanium and lead), respectively. HTL series volcanic rocks are further from the subduction axis, so they should have had a deeper descending slab contributing to their mantle wedge magmatism. To be able to release Pb-enriched fluids, the subducting slab should not have undergone earlier dehydration. It is because Pb is highly incompatible. It appears that in the early stage, subduction proceeded at a higher rate (cold slab), so slab-dehydration occurred at greater depths triggering HTL series magmatism. Subsequently the subduction rate decreased which in turn raised isotherms and promoted partial melting at shallower depths leading to LTL series magmatism.
  • HONGBING TAN, HAIZHOU MA, HAIZHEN WEI, JIANXIN XU, TINGWEI LI
    2006 年 40 巻 6 号 p. 569-577
    発行日: 2006年
    公開日: 2007/06/22
    ジャーナル フリー
    From the late Cretaceous to the early Tertiary period, the Tethys seawater repeatedly invaded the Shaqa sub-basin and entered the Kuqa sub-basin through the Awati straight. At the same time, as a result of tectonic and paleoenvironmental conditions, many sequences of thick strata of evaporites, especially in its two largest sub-basins of Kuqa in the north and Shaqa in the southwest have been deposited. Therefore, the western Tarim Basin has been considered to be a favorable location to prospect for ancient potash deposits. In order to clearly distinguish between a marine or a continental origin, and also to study the evaporite facies (i.e., late vs. early stage), we analyzed the late stages of evaporites of gypsum and salt minerals and measured chlorine isotope values in halite, sulfur isotopes in gypsum and oxygen isotopes in crystalline water of gypsum. The results showed that the sulfur isotope composition of gypsum can be used to distinguish between a marine and continental origin and chlorine isotopes of salt and oxygen isotopes of crystalline water in gypsum can be used to judge the sedimentary stage. Based on these results, we conclude that the late Cretaceous halite deposited in the Kashi Depression of the southwestern Tarim Basin have a marine origin and represent a later stage of marine evaporite sequence compared to other evaporites in the regions. Hence, the Kashi Depression should be considered as a promising area for finding new sylvinite deposits.
  • YUNG-TAN LEE, JU-CHIN CHEN, JYH-YI SHIH, WEN-SHING JUANG, HUAI-JEN YAN ...
    2006 年 40 巻 6 号 p. 579-596
    発行日: 2006年
    公開日: 2007/06/22
    ジャーナル フリー
    Cenozoic (Miocene to Pleistocene) basaltic rocks found in Shandong province of northern China include tholeiite, olivine tholeiite and alkali basalt. We present major, trace and rare earth elements data of these basalts and together with Sr-Nd isotopic data in the literatures to discuss the petrogenesis of these basalts. The basalts from Penglai area have higher K, Na and P and incompatible elements, but lower Ca, Mg and compatible elements contents than those from Changle area of Shandong province. Spidergrams indicate that Cenozoic basalts from Shandong province have geochemical characteristics similar to those of ocean island basalts (OIB) with slight positive Nb anomaly. The negative Ba, Rb and K anomalies found in the alkali basalts suggest the presence of residual phlogopite in the mantle source, indicating a metasomatic event occurred before the partial melting. The 143Nd/144Nd vs. 87Sr/86Sr plot suggested that basalts from Shandong province can be produced by MORB and EM-I components mixing. We propose that the EM-I type lithospheric mantle may have been produced by the recent H2O-CO2-fluids metasomatism and the fluids may be derived from dehydration of the subducted slab.
    Based on Shaw's equation, the basalts from eastern and central Shandong province have undergone different degrees of partial melting from the mantle source. Degrees of partial melting and chemical composition of basalts from Shandong province suggest that the lithosphere has thickened progressively since the Miocene. On the basis of Ar-Ar ages of this study and the fractional crystallization model proposed by Brooks and Nielsen (1982), we suggest that basalts from Changle and Penglai areas belong to different magmatic systems which have undergone fractional crystallization and evolved progressively to produce other types of basalts.
  • YUJI SANO, NAOTO TAKAHATA, YUKIYASU TSUTSUMI, TOMOHARU MIYAMOTO
    2006 年 40 巻 6 号 p. 597-608
    発行日: 2006年
    公開日: 2007/06/22
    ジャーナル フリー
    We have developed 238U-206Pb and 207Pb-206Pb dating method of monazite by using a Cameca NanoSIMS NS50 ion microprobe. A ∼4 nA O- primary beam was used to sputter a 5∼7-μm-diameter crater and secondary positive ions were extracted for mass analysis using a Mattauch-Herzog geometry. The multi-collector system was modified to simultaneously detect 140Ce+, 204Pb+, 206Pb+, 238U16O+, and 238U16O2+ ions. A mass resolution of 4100 at 1% peak height was attained with a flat peak top, while the sensitivity of Pb was about 4 cps/nA/ppm. A monazite from North-Central Madagascar with a U-Pb age of 524.9 ± 3.1 Ma (2σ) obtained by thermal ionization mass spectrometry was used as a reference for Pb+/UO+-UO2+/UO+ calibration. Based on the positive correlation, we have determined the 206Pb/238U ratios of samples. 207Pb/206Pb ratios were measured by a magnet scanning with a single collector mode. Then 44 monazite grains extracted from a sedimentary rock in Taiwan were analyzed. Observed ages were compared with the U-Th-Pb chemical ages by electron microprobe. 238U-206Pb ages agree well with those of the chemical ages except for some samples. The discrepancy may be due to an over-estimation of radiogenic Pb by the chemical method. 207Pb-206Pb ages also agree with the chemical ages while there are a few discordant samples. Taking into account the concordant samples, there are three main age groups, 230 Ma, 440 Ma and 1850 Ma of monazites. The age distribution suggests that the provenance of detrital monazites is possibly the North China Craton or the Qinling-Dabie-Sulu zone between the North China and South China blocks.
  • KEISUKE SUZUKI, MAYUMI SHIROHADA
    2006 年 40 巻 6 号 p. 609-623
    発行日: 2006年
    公開日: 2007/06/22
    ジャーナル フリー
    Wet and dry deposits were collected using an automatic fractionation precipitation collector for three and a half years in Matsumoto city, central Japan. Because of the imperfect collection of some inorganic ions, the fraction collected as dry deposit is named “dry dish sample” in this study. The collected samples were analyzed for major cations and anions using ion chromatography. Based on the elemental ratios, Na+, Cl-, and Mg2+ in the precipitation are considered to be of sea salt origin. However, the Cl- found in the dry dish samples was deficient with respect to Na+, possibly resulting from chlorine loss from aerosols, and the Mg2+ content of dry dish samples is thought to be derived from the soil. Abnormally high concentrations of Ca2+ and SO42- were observed in both the precipitation and dry dish samples for a few days after the eruption of Miyakejima volcano. The Na+ and Cl- concentrations in precipitation showed high values in winter, and the yellow sand carried from China in the spring affected the concentrations of Ca2+ and Mg2+ in the precipitation, as well as those in the dry dish sample. High time-resolution sampling of the precipitation disclosed short variations in its composition, which would have otherwise escaped our attention. When continuous precipitation from stratiform clouds occurred due to a warm front, the concentration of chemical constituents in the precipitation was high in the first fraction of precipitation, but was subsequently markedly reduced. On the other hand, when intermittent precipitation was produced from cumuliform clouds due to a cold front, it became more complex: in such cases, there was no predictable pattern for the subsequent concentration fluctuations, i.e., the concentrations of the chemical constituents fluctuated within a day.
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