In order to clarify the frequency and distribution of tsunami deposits from the Holocene in the southeast area of the Shizuoka Plain, Shizuoka Prefecture, we reconstructed the Holocene sedimentary environment of this area from two sediment cores (8 m long). The sediments consist mainly of dark blackish blue massive clay, interstratified with a 1-m-thick parallel-laminated clay layer and many thin sand layers with erosional bases. The characteristics of the sedimentary facies and the stratigraphic position of the Kikai-Akahoya tephra (ca. 7,300 cal. years BP) indicate that the sediments represent a lagoon-delta system and one transgressive-regressive cycle during the last 7,300 years. We detected two possible run-up tsunami deposits within the sediments with a probable depositional age of 6,000–3,000 cal. years BP. The upper part of the sediment cores seem to have suffer considerable disturbance by human activities which hinder us from assessing what happened in the last 3000 years.

抄録全体を表示

Machinoyama ultramafic body is the eastern most part of the Yakuno ophiolite in the Maizuru belt. Outcrops of its southeastern margin show various block-in-matrix structures towards the fault boundary to the Ultra-Tanba belt. The blocks consist of serpentinites and serpentinized peridotites and were fragmented into multiple parts, with the spaces between blocks being infilled by scaly serpentinites (i.e. chrysotile). The blocks decreased their sizes and formed phacoidal-shapes towards the fault boundary, whereas serpentinite matrix becomes dominant with intense foliations. These structural developments in Machinoyama body could be related to the fault activity at the boundary with the Ultra-Tanba belt.

抄録全体を表示

Spurrite [Ca₅Si₂O₈(CO₃)], a calc-silicate mineral with a carbonate molecule, is a product of high-temperature contact metamorphism, a typical example of which is found at the Fuka contact aureole, Okayama Prefecture, Japan. We report here carbon and oxygen isotope composition of CO₂ released from the reaction of spurrite with phosphoric acid in the temperature range of 25°C to 100°C. While reacting with the acid carbon isotopes did not fractionate, while oxygen isotope shows considerable isotope fractionation with varying reaction temperature, similar to other carbonate minerals. This suggests that the carbon and oxygen isotope measurements can be carried out using acid extraction method. In the Fuka contact aureole, carbon isotope fractionation of about 3‰ is observed between coexisting spurrite and coarse-grained calcite. If in equilibrium the carbon isotope fractionation between carbonate molecule in spurrite and calcite has the potential application as an isotope geothermometer. However, oxygen isotope distribution between calcite and spurrite could not be critically evaluated because of the uncertainties of source oxygen isotopes values of infiltrating fluids during the skarn formation and the fractionation of it between silicate and carbonate phases.

抄録全体を表示

In this paper, we report the procedure for carbon and oxygen isotopic measurement of small amount of dolomite powder using IR-MS system, MAT-250 at Shizuoka University. Although the dolomite samples had been measured using a batch method, we tried to prepare the carbon dioxide gas of dolomite sample using the existing online carbonate (calcite) sample preparation system. The examined analytical conditions are (1) set reaction vessel temperature 100°C and (2) set reaction time more than 60 minutes. In addition, we describe the operation manual for precise measurement of small amount of carbon dioxide, derived from carbonate minerals reacted with conc. phosphoric acid or carbon dioxide prepared offline and stored in a glass tube. We hope that this paper helps the users to understand of the usage of the mass-spectrometer.

抄録全体を表示