著者

オガワ フミオ フクダ ヨウイチ アカマツ ジュンペイ シブヤ カズオ Fumio OGAWA Yoichi FUKUDA Junpei AKAMATSU Kazuo SHIBUYA

雑誌

Proceedings of the NIPR Symposium on Antarctic Geosciences

巻号頁・発行日

vol.4, 1990-09

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Tidal gravity observations were made at Syowa and Asuka Stations, Antarctica, by the 28th Japanese Antarctic Research Expedition (JARE-28). LaCoste & Romberg type G gravity meters were used with electrostatic feedback amplifiers. The tidal output voltage records were sampled at intervals of 30min for both stations. The data with stable drift were selected and calibrated to tidal gravity variation. The number of selected data is 2976 from April 1 to June 1,1987 for Syowa Station and is 8736 from June 2 to November 30,1987 for Asuka Station. The admittance parameters of gravity change for atmospheric pressure and temperature variations are obtained as -0.311 μgal/mbar (1 μgal/mbar=10^<-10>m^2/kg) and 0.003 μgal/℃ (1 μgal/℃=10^<-8>m/s^2℃) for Syowa Station, and -0.264μgal/mbar and 0.029μgal/℃ for Asuka Station, respectively. When we consider appropriate elastic constants of ice, the above admittance parameter of atmospheric pressure change can be explained by both the upward attraction and loading deformation of the ice sheet by air mass for an extent of 30km covering the site. δ-factors which are corrected for oceanic tidal loading effects range from 1.130 (K_1) to 1.250 (M_2) for Syowa Station, and from 1.134 (P_1) to 1.330 (K_2) for Asuka Station, respectively. For diurnal tides, there is no significant difference between the obtained δ-factors at both stations. They are consistent with the theoretical values calculated by WAHR for the 1066A earth model. For semi-diurnal tides, however, the δ-factors at both stations are 1-17% larger than the WAHR's theoretical prediction values. The δ-factors for semi-diurnal tides at Asuka Station on the ice sheet (about 1000m thick) are as a whole larger than those at Syowa Station on the bare rock of East Ongul Island in Lutzow-Holm Bay. We presume that the tidal deformation of the ice sheet may be systematically larger than the ice-free area when we interpret the obtained result, but detailed examination of the earth models including surface ice layers has to be made for accurate interpretation of the tidal gravity observation in the Antarctic region.

著者

シライシ カズユキ カガミ ヒロオ Kazuyuki SHIRAISHI Hiroo KAGAMI

雑誌

Proceedings of the NIPR Symposium on Antarctic Geosciences

巻号頁・発行日

no.3, 1989-09

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We report the preliminary results of whole-rock Rb-Sr and Sm-Nd isochrons dating on granulite-facies metamorphic rocks from Brattnipene, central Sør Rondane Mountains, East Antarctica. Samples : Isotopic ratios of eight samples were measured with a MAT 261 mass spectrometer at the Institute for Study of the Earth's Interior, Okayama University. Four samples are enderbites consisting of Opx, Cpx, Hbl, Bt, Pl and Qtz with or without Grt and Kfs (mineral abbreviations after R. KRETZ : Am. Mineral., 68,277,1983). These rocks were collected within a hundred meters of each other. The other four samples are hornblende gneisses as follows : Bt-Hbl-Pl, Bt-Hbl-Kfs-Pl-Qtz, Cpx-Hbl-Pl-Qtz and Bt-Hbl-Pl-Qtz. Two of them have been slightly altered to form secondary epidote and/or bluish green hornblende. All samples crop out within 500m. The peak metamorphic conditions of the enderbites were estimated from geothermobarometry to be 800℃ and 7-8.5kb (K. SHIRAISHI and S. KOJIMA : Proc. NIPR Symp. Antarct. Geosci., 1,129,1987). The hornblende gneisses were presumably metamorphosed under the same conditions. Rb-Sr data : The ^<87>Rb/^<86>Sr ratios for eight samples are low (0.02-0.15) and no meaningful isochron can be drawn due to the considerable scattering of the data. Four enderbites, however, define an isochron of 1167±127 Ma with initial ratio 0.7040. Sm-Nd data : The ^<143>Nd/^<144>Nd vs. ^<147>Sm/^<144>Nd ratios for all eight samples defined an isochron of 999±164 Ma with an initial ratio of 0.51158. The calculated uncertainty is high because the spread of the ^<147>Sm/^<144>Nd ratios is limited. T_<(DM)> model ages (ε_<ND(0)>=+10) range between 1150 and 1300 Ma.Previous work has shown that the region was affected by a thermal event associated with plutonism about 500Ma ago. This event may be the cause of the scatter in the Rb-Sr data. Nonetheless, four enderbites, which seem to be fresh under the microscope, yield a Rb-Sr isochron whose age is consistent with the Sm-Nd age within the uncertainties. There are two possible explanations for the 1000Ma ages : crust formation or metamorphism. We tentatively interpret the 1000Ma could date granulite-facies regional metamorphism although the Sm-Nd isochron age has been thought to date a crust formation because of the immobility of rare earth elements during metamorphism. Resetting of Sm-Nd whole-rock isochrons may have been possible under the estimated conditions of granulite-facies metamorphism as discussed for the Napier Complex of Enderby Land (M. T. McCULLOCH and L. P. BLACK : Earth Planet. Sci. Lett., 71,46,1984).

著者

サカイ リョウキ シブヤ カズオ アユカワ マサル Ryoki SAKAI Kazuo SHIBUYA Masaru AYUKAWA

雑誌

Proceedings of the NIPR Symposium on Antarctic Geosciences

巻号頁・発行日

no.4, pp.80-89, 1990-09

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A non-magnetic pillar for observation of geomagnetic field was installed at Seal Rock, East Antarctica in January 1987 by the 28th Japanese Antarctic Research Expedition. A G.S.I. (second-order) precise magnetometer was set on the pillar, and absolute measurements of the geomagnetic field were made 7 times during 19 September-18 December, 1987. The averaged values of the observed declination angle, inclination angle and the total magnetic intensity are -36°19.2' (westward), -63°53.3' (upward) and 43073nT, respectively. These observed values are consistent with the theoretical values calculated by the IGRF (1985) model.

著者

KAWASAKI Toshisuke ISHIKAWA Masahiro MOTOYOSHI Yoichi

出版者

国立極地研究所

雑誌

Proceedings of the NIPR Symposium on Antarctic Geosciences (ISSN:09142029)

巻号頁・発行日

vol.6, pp.47-56, 1993-09

被引用文献数

2

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Diagnostic mineral assemblages involving cordierite, sapphirine, orthopyroxene, sillimanite and garnet are newly reported from Rundvagshetta, Lutzow-Holm Bay, East Antarctica. Cordierite is probably of secondary origin, because it occurs as symplectitic intergrowth with sapphirine at the expense of initial orthopyroxene+sillimanite. In a different domain, vermiculated orthpyroxene+cordierite±sapphirine replaces initial garnet locally. On the basis of the textures, the following reactions are inferred : orthopyroxene+sillimanite+quartz=cordierite, orthopyroxene+sillimanite=cordierite+sapphirine, garnet+quartz=orthopyroxene+cordierite, garnet=orthopyroxene+cordierite+sapphirine. These reactions suggest near isothermal decompression in the model FMAS system, and along with a prograde kyanite from the same locality, the clockwise P-T path of the rocks are confirmed. Moreover, the maximum temperature condition of the Lutzow-Holm Complex is estimated to be as high as 900℃ based on the initial orthopyroxene+sillimanite+garnet assemblage, and this value is apparently higher than those previously estimated. In high-grade metamorphic terranes of East Antarctica, the similar mineral assemblages and reaction textures have been reported from Forefinger Point in Enderby Land and the Rauer Group in Prydz Bay, respectively. However, it is questionable as to whether these terranes can be corrrelatable to Rundvagshetta, because no evidence of prograde P-T paths have been obtained from Forefinger Point and the Rauer Group and there is a lack of relavent geochronological data.