Structure and isotopic composition of carbonate rocks.

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1. Recognition of types of carbonate cement by means of CL-image method: Non-luminescent rim-cement of marine origin and dull to bright luminescent, sparry, drusy cement of freshwater origin have been recognized in such limest ones as; Silurian limestones of Gotland, Jurassic limestones of Nepal-Himalaya and Soma-Fukushima, and Quaternary Ryukyu Limestone of Kikai Island. Roof-pendant type, vadose cements with characteristic zoned patterns have also been recognized for the Silurian Limestone of Gotland, suggesting that the cementation took place under the fluctuating conditions in terms of Eh and Mn concentration. Thus, CL-image method has been proved to be a powerful and useful technique for identification of various cement types. 2. Comparison between luminosity and carbonate chemistry: SEM-EDS analyses on selected samples have revealed that bright-luminescent carbonate contains more Mn than dull to non-luminescent carbonates. Mn-bearing carbonates are considered to have been formed in relatively reduced environments. This implies that bright-luminescent carbonates were formed under oxygen-deficient conditions. 3. Determination of oxygen, carbon, and strontium isotope compositions for micro-samples: Carbon isotopic signatures, mostly within *3 permil, suggest that major phase of carbonate cementation occurred near-surface environments which were open to sea-water and atmosphere. Supposing the near-surface diagenesis, the temperature of precipitaion and/or mixing ratio of sea- water and ground water are estimated from oxygen and strontium isotopic compositions. 4. Chemical and isotoipic analyses would provide critical data concerning diagenetic and sedimentary environments of carbonates only when the micro-textural analyses and the characterization of cement- types had been done. In this sense, CL-image tecnique is very important in carbonate sedimentology.