中央海嶺の熱水系

概要

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DSDP/ODP Hole 504 B was drilled into 6 Ma crust, about 200 km south of the Costa Rica Rift, penetrating 1.55 km into a section that can be divided into four zones-Zone I: oxic submarine weathering; Zone II: anoxic alteration; Zones III and IV: hydrothermal alteration to greenschist facies. In Zone III there is intense veining of pillow basalts. Cyprus CY2A is located near Agrokipia deposits and cored into 90 Ma crust. The alteration is divided into four zones-Zone I: low-grade alteration; Zone II: silicified hydrothermal alteration; Zone III: propylitic alteration; Zone IV: lower greenschist facies alteration. Both drill cores were located at discharge zones in hydrothermal circulation systems. So the alteration characteristics of both cores were similar. Alteration temperatures of hydrothermally altered rocks were 200-350°C in Hole 504 B and 200-<300°C in CY2A. Boiling didnt occur in both cores. The hydrothermally altered rocks from both cores were formed under low seawater/rock ratio (1-5). The estimated isotopic and/or chemical compositions of hydrothermal solutions are within the range of those of endmember hydrothermal solutions from present seafloor hydrothermal fields. At least 2km3 of magma would be needed for the formation of 6 million tons of sulfide deposit if magma supplies heat energy to make hydrothermal solutions during the cooling from 1200°C to 350°C.
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