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Volume 24 (1990)
- Issue 6 Pages 349-
- Issue 5 Pages 309-
- Issue 4 Pages 193-
- Issue 3 Pages 123-
- Issue 2 Pages 47-
- Issue 1 Pages 1-
Volume 24, Issue 2
Displaying 1-5 of 5 articles from this issue
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Toshio Nakamura, Tsunemasa Shiki, Nobuyuki Nakai1990 Volume 24 Issue 2 Pages 47-56
Published: 1990
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSRadiocarbon (14C) ages have been measured for four organic-carbon fractions, i.e., humic acid and solid organic materials of plant residues, and humic acid and humin of sandy-mud component, separated from a turbidite sediment, with a Tandetron accelerator mass spectrometer at Radioisotope Center, Nagoya University. The KT7819-24 piston-cored sediment, 275 cm in length, was collected at Suruga Trough, off Suruga Bay. The sediment materials from the lowermost turbidite bed of the cored sediment were used for the 14C analysis. Different organic fractions from the turbidite sediment showed relevant 14C ages. The non-hydrolyzable humic acid of the plant materials was the youngest, with a 14C age of 270±80 y BP. The solid substances of the plant fragments showed a much older age of 1550±130 y BP. The non-hydrolyzable humic acid from the sandy mud was dated to be 2270±90 y BP, and the humin component was to be the oldest, with a 14C age of 9400±170 y BP. Concerning the chemical and physical character of the sedimentary organics in the turbidites, and their adsorption and mixing mechanism, it is inferred that the turbidite bed was produced shortly after 1550±130 y BP, which is the age of the solid substances of the plant residues.View full abstractDownload PDF (2704K) -
Kazuhiro Suzuki, Mamoru Adachi, Koshi Yamamoto1990 Volume 24 Issue 2 Pages 57-74
Published: 1990
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSAbundances of rare earth elements (REEs) in the whole rock and coexisting accessory minerals from a sample of the Cretaceous Naegi granite from central Japan, were determined with isotope dilution and electron microprobe methods. The granite contains the following amounts of accessory minerals: 853.8 mg fluorite, 215.2 mg apatite, 166.5 mg ilmenite, 145.5 mg zircon, 78.1 mg monazite, 10.5 mg fergusonite, 9.5 mg allanite, 3.6 mg rutile, 1.7 mg columbite and 1.4 mg thorite per 1 kg. REEs in the granite reside mainly in the accessory minerals, but the contribution of the accessories to the whole-rock REE abundance becomes less important toward the light REE end; the sum of REEs in the accessories accounts for only 53±6% of the whole-rock La, 59±6% Ce, 62±8% Nd, 66±10% Sm and 74±11% Gd-Yb. The unaccountable REEs in the rock are stored at the grain-boundaries of constituent minerals. The chondrite-normalized REE pattern for the grain-boundary (whole-rock minus sum of accessories) of the granite is light REE-enriched and consists of two linear segments with turning point at Gd. Since grain-boundaries are considered to melt first at partial melting, the early-formed melt will retain the REE characteristics of grain-boundaries. Some granitoids, which possess REE patterns consisting of two linear segments with turning point at around Gd and are considered to be formed by partial melting of crustal rocks, may have originated from source rocks with a grain-boundary REE distribution like that of the Naegi granite.View full abstractDownload PDF (3765K) -
Hideo Ishizuka, Yoshihisa Kawanobe, Hitoshi Sakai1990 Volume 24 Issue 2 Pages 75-92
Published: 1990
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSVolcanic rocks dredged from the Okinawa Trough, an active back-arc basin behind the Ryukyu island arc (IA)-trench system associated with a subduction of the Philippine Sea plate, encompass a wide variety of petrology and geochemistry, ranging from basalts to rhyolites through andesites. The basalts, consisting of highly vesicular pillow lavas, are moderately porphyritic with phenocrysts of olivine, plagioclase and minor clinopyroxene. Their glass and bulk rock compositions, especially their large-ion lithophile element (LILE) abundances, are transitional between the Ryukyu IA basalts (LILE-enriched) and normal-type mid-ocean ridge basalts (N-MORBs, LILE-depleted). The rhyolites are moderately porphyritic, including phenocrysts of plagioclase and orthopyroxene with or without clinopyroxene and hornblende, and have the close geochemical affinity with the Ryukyu IA rhyolites. It follows that the rhyolites were derived from an IA-type source material similar to that for the Ryukyu IA volcanics, but the basalts were derived from a less LILE-enriched source material approaching that for N-MORBs. Available K-Ar ages indicate the rhyolitic volcanism prior to the basaltic one. It is, therefore, most likely that the magma source region beneath the Okinawa Trough has changed with time from an IA type to a N-MORB type during the back-arc rifting. On the other hand, the andesites are highly porphyritic with phenocrysts of plagioclase, orthopyroxene and clinopyroxene, and have the K-Ar age similar to that of the basalts. Most importantly, the andesites exhibit many lines of textural and compositional evidence for mixing between basaltic melt, and rhyolitic partial melt of pre-existing IA lower crust that was presumably heated by the former melt. This again suggests the structural transition of the magma source region beneath the Okinawa Trough, which may be most characteristic of the initial stage of back-arc basin volcanism in general.View full abstractDownload PDF (4821K) -
Yong-Joo Jwa, Takashi Nakajima, Shigeru Uchiumi, Ken Shibata1990 Volume 24 Issue 2 Pages 93-103
Published: 1990
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSK-Ar and Sr isotopic age determinations were made for Mesozoic granitic rocks intruding into Precambrian gneisses and schists in the Inje-Hongcheon district, South Korea. They include hornblende-biotite granodiorite (K-Ar biotite ages of 154 to 165 Ma), porphyritic biotite granite (149 to 157 Ma) and two-mica granite (149 to 155 Ma). The hornblende-biotite granodiorite gives K-Ar hornblende ages of 168 to 180 Ma and a Rb-Sr whole-rock isochron age of 212.0±26.6 Ma with an initial 87Sr/86Sr ratio of 0.71198±0.00046. These granitic rocks have been derived from crustal melt during late Triassic to early Jurassic time. The cooling rate of the hornblende-biotite granodiorite was slow in early stage and became greater as it approached shallow depths.View full abstractDownload PDF (2582K) -
Yoji Seki1990 Volume 24 Issue 2 Pages 105-121
Published: 1990
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSThe gas and other solute concentrations have been estimated for the aquifer fluid of the Oku-aizu geothermal system prior to discharge-induced boiling. The model takes into consideration the excess enthalpy (i.e. two-phase) reservoir conditions which developed at production depths following depressurization. The model consists of two end-member types of boiling. One is a “high flow-low temperature drop type” and the other is a “low flow-high temperature drop type”. The first is a process which could be present in an aquifer with high permeability, and is characterized by a large total discharge, accompanied by a small temperature and pressure drop around the well. The fractionation of gases into vapor phase in the downhole feed zone may be smaller than predicted for equilibrium conditions, due to single step steam separation under dynamic conditions. The second type is a process which could occur in an aquifer with low permeability, and is characterized by a low flow rate and a high temperature and pressure drop. The net gas fractionation into vapor phase in the feed zone is very large, resulting in almost all the gases fractionating into the vapor phase, due to multi-step steam separation. Based on this model, the concentration of gases and other solutes are estimated for the reservoir liquid of the Oku-aizu geothermal system. Calculated ranges of gas concentrations in the reservoir liquid (prior to flashing) are 0.3 to 1.0 wt% for CO2 and 150 to 250 mg/kg for H2S. The estimates of gas concentrations in the original (pre-boiled) fluid are necessary to calculate mineral-fluid equilibria and to estimate such development-related factors as the potential for scaling.View full abstractDownload PDF (3713K)
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