Abstracts of Annual Meeting of the Geochemical Society of Japan

For the better understanding of the timescales of various magma processes, I have focused on U-series disequilibria in natural samples. I have developed new analytical methods regarding silicate sample decomposition by acid digestion with very high chemical yield, separation of U, Th, and Ra with high yield and good purity, and isotope analyses of U, Th and Ra using TIMS. These methods were applied to lavas of Miyakejima volcano. Large 238U and 226Ra excesses observed in the Miyakejima lavas are due to the addition of slab-derived fluid to the mantle wedge, and I suggested very rapid ascent time of the slab components in the mantle wedge. I also demonstrated a model that fluid release from the slab and subsequent magma generation occur as episodic events on a several-kyr timescale.

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The stability of amino acids in the ocean sediment are examined by high temperature experiments (100-300oC) in order to understand possible contamination of organic matter in the sub-seafloor hydrothermal system. and to demonstrate the maximum temperature of labile organic matter under the reaction between sediments and hydrothermal solution. Hydrolysable amino acids in the solid and liquid phases were analyzed by the HPLC. The degree of decomposition of amino acids and hexosamines in both sediments and solutions increases with temperature. Both amino acids and hexosamines are completely decomposed in the liquid phase at >250oC and >200oC, respectively, which indicates that these organic matter is not stable in the hydrothermal filed at > 200oC.

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Amino acids were determined in core samples, chimneys, seawaters sampled in deep-sea hydrothermal systems in Suiyo Seamount, in the Izu-Bonin Arc, Pacific Ocean, as a part of the Archaean Park Project. Vertical profiles in amino acids in core samples showed maxima at the deeper part of the core. Proteinous amino acids were predominant, and eneantiomeric (D/L) ratios of the amino acids were quite low. These results strongly suggest the presence of a subjacent microbial oasis in the hydrothermal systems.

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Near-surface deformation related to neotectonics is accommodated by brittle faults.Displacement on the fault planes often results in the development of fault gouge composed ofrock fragments and authigenic illite. Previous studies highlighted the potential for determining theabsolute timing of brittle fault history using isotopic dating techniques (see summary, Zwingmann& Mancktelow 2004). The understanding of fault processes and the timing and extent of clay-richfault gouge formation is important for: (1) hydrocarbon exploration as faults may act as either aconduit or a seal for fluids and/or hydrocarbons; (2) civil engineering in the evaluation ofearthquake hazards and (3) ascertaining the suitability of sites for nuclear waste storage.K-Ar and preliminary 40Ar/39Ar data using micro-encapsulation of authigenic illite fromgouge developed in two different geological settings will be presented. The first study involvesgouge zones from the European Alps where the protolith consists of high grade metamorphic ormagmatic rocks (Zwingmann & Manckeltow, 2004). In these rocks a clear distinction is possiblebetween newly-grown illite, which is the only K-bearing mineral in the fine-grained fractionsseparated from the gouge, and the precursor assemblage. We dated fault movement on youngfaults related to the regional-scale Periadriatic Fault in the European Alps, using neocrystallineillite. The K-Ar ages are consistent both internally and with tightly defined field constraints.Average ages obtained from the < 2 micro m fractions that range from 5.2 to 21.5 Ma (2 sigma) arebracketed by AFTA and ZFTA ages from adjacent localities. This study provides absolute timeconstraints on the youngest, retrograde, neotectonic movements that occurred in the Alpineorogeny.The second study focuses on two fault zones from the northern Sydney Basin, Australia.The faults are hosted by Early Permian siltstones, tuffs and coal seams and foliated gouges,comminution and dilational breccias are developed within the fault zones. Clay minerals ofdetrital or authigenic origin are present within the sedimentary protolith and in the gouges. K-Arages obtained from samples in the gouge and tuffs in the damage zones range from 119 to 172 Marespectively. Illite ages of 196-245 Ma are thought to reflect the time at which diageneticsmectite-illite formed in the host rocks. 119-150 Ma ages from the <0.4 micro m fractions in the tuffsand gouges date the last slip event on the faults and can be related to the rifting of the easternmargin of the Australian continent. The data indicate the time at which diagenesis occurred in thehost rocks and slip movement on the faults and constrain the deformation events associated withthe rifting of the eastern margin of Gondwana during the Early Cretaceous.Zwingmann, H. & Mancktelow, N. 2004 Timing of Alpine fault gouges. EPSL, 223, 415-425.

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