Stable isotopic and mineralogical studies of hydrothermal alteration at Arima Spa,Southwest Japan |
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Institution: | 1. Faculty of Education, Kumamoto University, Kurokami 2-40-1, Chuo-ku, Kumamoto 860-8555, Japan;2. Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan;1. Hacettepe University, Department of Geological Engineering, 06800 Ankara, Turkey;2. Indiana Geological and Water Survey, Indiana University, 611 N. Walnut Grove Avenue, Bloomington, IN, USA;3. Institute of Environmental Assessment and Water Research (IDÆA), Consejo Superior de Investigaciones Científicas (CSIC), C/ Jordi Girona 18–26, 08034 Barcelona, Spain;1. ?stanbul University, Department of Geological Engineering, 34320 Avc?lar, ?stanbul, Turkey;2. Hacettepe University, Department of Geological Engineering, 06800 Ankara, Turkey;3. Institute of Environmental Assessment and Water Research (IDÆA), Consejo Superior de Investigaciones Científicas (CSIC), C/Jordi Girona 18-26, 08034 Barcelona, Spain;4. Indiana Geological and Water Survey, Indiana University, 611 N. Walnut Grove Avenue, Bloomington, IN, USA;5. Dokuz Eylül University, Institute of Marine Science and Technology, 35340 ?nciralt?, ?zmir, Turkey |
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Abstract: | The waters of Arima Spa, Southwest Japan, have high salinity (Cl = 54 g/kg) and high isotopic ratios (δD = ? 32, and δ18O = + 10%.), and issue from shallow wells drilled into altered rhyolitic pyroclastic rocks of Cretaceous age.Alteration of the host rocks occurred in two stages. The earlier regional alteration stage is characterized by the presence of 2M- and IM-type muscovite, albite, chlorite, calcite and epidote, whereas muscovite and Fe-chlorite formation at the expense of partly albitized plagioclase and altered biotite or hornblende occurred in the following hydrothermal stage. Pyrite, sphalerite, galena and siderite are present in the central part of the hydrothermal alteration zone. Oxygen and hydrogen isotopic ratios of secondary muscovite show that regional alteration proceeded under the meteoric circulation, and that the hydrothermal fluid for the second stage had chemical and stable isotopic characteristics of non-meteoric origin similar to the present-day Arima brine. The oxygen and to a lesser extent the hydrogen isotopic ratios of the muscovite rapidly decrease with increasing distance from the central zone of hydrothermal alteration. The isotopic variation is best interpreted as reflecting rapidly decreasing fluid/rock ratios with increasing distance of fluid penetration from the narrow hydrothermal alteration zone into the surrounding area.Speciation computation for the present-day brines at Arima Spa indicates that they are saturated with siderite but not with calcite at depth, in good accord with the mineralogical observations. Upon ascent the brines are diluted by HCO3-rich shallow ground water and are saturated with respect to both siderite and calcite. The present-day Arima hydrothermal system is a remnant of the second stage hydrothermal activity. |
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