This article describes the mode of occurrence, mineral composition, and oxygen and carbon isotope compositions of veins that are ubiquitously observed in the Upper Pliocene Shirahama Formation of the Chikura Group at the southern end of Boso Peninsula, Japan. The veins are characterized by milky to gray white in color, 1 mm to 3cm in thickness, and N 20° W-N40°W in strike, and divided from the textural viewpoints into three types : linear-, twig-, and spider's web-types. Spatial distribution of the veins is restricted within the Shirahama Formation and they do not extend to the Shiramazu Formation conformably overlaying on the Shirahama Formation. The vein minerals consist of, calcite, apophyllite, heulandite (or clinoptilolite), erionite, analcime, natrolite, thomsonite, and/or chabazite. Heulandite and erionite also are common in the matrix of tuffs, sandstones, and conglomerates of the Shirahama Formation. δ
13C
PDB and δ
18O
SMOW values of calcite ranged from -39‰ to -5‰ and +24‰ to +29‰, respectively. In spite of the slightly higher δ
13C values than the published values of carbonates associated with cold seepages, it is inferred from the relatively low δ
13C values that HCO
3- in the vein-forming solution was derived from the oxidation and/or thermal decomposition of methane in sediments. By assuming the two values, 0‰ (a seawater value) and -5‰ (a present, meteoric water value), for the δ
18O of solution equilibrated with calcite and using an oxygen isotope geothermometer, the formation temperatures of veins were calculated to be from 1°C∼48°C. The estimated temperatures of zeolite-bearing veins are significantly higher than those of cold seepage carbonates reported previously. The veins observed in the Shirahama Formation should be mentioned as an example representative of the diversity of cold seepages in their origin, chemistry, and flow mechanism.
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