Abstracts of Annual Meeting of the Geochemical Society of Japan

We have measured C-14 concentrations of old tree rings with a measuring accuracy of 0.2% to search variation in an ancient cosmic ray intensity fluctuation. From C-14 dating, the calendar ages of the Choukai Jindai cedar in Japan (39°N) were ranged in from 2757 to 2437 calBP with 320 tree rings, and this wood sample is located at a very interesting era because the C-14 concentrations form a peak area as well as Maunder Minimum at which the sunspot numbers were absent. The C-14 concentrations of single-yr tree rings using the Choukai Jindai cedar in 2650 to 2600 yr BP showed a rapid change. We report the structure of the C-14 variation in the 50 years relating an 11-yr solar cycle.

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In order to simulate the formation of interstellar organic compounds, a frozen mixture of methanol, ammonia and water was irradiated with high-energy heavy ions from the HIMAC accelerator. The products were mixtures of complex molecules with large molecular weights, which gave amino acids after acid-hydrolysis. Such complex amino acid precursors could have been delivered to the primitive Earth in the form of cosmic dusts. We are planning to capture cosmic dusts on the Exposure Facility of the Japanese Experimental Module (JEM) of the International Space Station.

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Space dusts are promising candidates for the delivery of extraterrestrial organics to the primitive Earth. It is necessary to capture them out of the terrestrial bisophere. We are planning to capture space dusts on the Exposed Facility of Japanese Experimental Module of International Space Station. We examine the possibility of detection of amino acids in captured space dusts by using ultra-low density silica gel (aerogel). Alpha-aminoisobutyric acid and isovaline, both are non-proteinous amino acids detected in Murchison meteorite, would be target molecules to detect.

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The most representative volcanisms of Paleo-Izu-Bonin-Mariana Ridge were subduction zone-related boninitic magmatisms (49 - 46 Ma). During KT04-28 cruises, adakitic high Mg andesites-dacites (Ar-Ar age: 43.46 ±3.29 Ma, in this study) were firstly recovered from Bonin Ridge forearc seamounts. Adakitic high Mg andesites contain extremely high Sr contents respect to low Y contents, and they show strongly enriched in LREE respect to MREE, and much depleted in HREE, like to typical adakitic rocks; however, the marked positive anomaly in Zr and Hf relative to Sm is much similar to boninites. In adakitic high Mg andesites, we firstly found the presence of the chromian diopside xenocrysts. Based on geochemical modeling calculation, these were chromian diopsides in residual peridotite which had been extracted 10 wt % melt from the original fertile mantle of the Philippine Sea Plate. In the core of chromian diopside xenocrysts, bronzite and pargasite lamellae were crystallized accompanying with Si-rich glass inclusions. By EPMA and SIMS analysis, these Si-rich glass inclusions are dacitic glass inclusions (average: 65.33 wt % SiO2, 0.4 wt % MgO, 2.83 wt % of total alkali) characterized by adakitic-boninitic features (average trace elements compositions: (La/Sm)n = 6.12, (Eu/Tm)n = 9.6, (Hf/Sm)n = 2.4). The co-existing feature of bronzite-pargasite exsolution lamellae and adakitic glass inclusions in chromian diopside xenocrysts is the most direct evidence which showed the chemical reaction between the slab-related adakitic melt and depleted mantle peridotite. These glass inclusions were the primary melts derived from this reaction event. The most important result about petrological studies mentioned above is that the boninites and adakitic high Mg andesites could be produced by the partial meting of the depleted residual mantles which had been reacted with slab melt. According to the slab thermal structure (Iwamori, 2006), the phase diagrams of MORB + H2O and peridotite + H2O system (Maruyama and Okamoto, 2006), the slab melt only can be generated by the partial melting of subducted a young oceanic crust (< 5 Ma). Although now that the tectonic evolution of the Western Pacific during the Eocene is not clear yet, the boninites and adakitic high Mg andesites are the best evidences to demonstrate the existence of an active ridge which was subducting beneath the West Philippine Sea Plate around 50-45 Ma.

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