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      Drilling into deep-seated hard rocks of the oceanic plate formed at the Mid-Ocean Ridge: results and future perspectives: Deep-seated Hard Rock Drilling

      The Journal of the Geological Society of Japan
      The Geological Society of Japan

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          Age, spreading rates, and spreading asymmetry of the world's ocean crust

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            An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30 degrees N.

            Evidence is growing that hydrothermal venting occurs not only along mid-ocean ridges but also on old regions of the oceanic crust away from spreading centres. Here we report the discovery of an extensive hydrothermal field at 30 degrees N near the eastern intersection of the Mid-Atlantic Ridge and the Atlantis fracture zone. The vent field--named 'Lost City'--is distinctly different from all other known sea-floor hydrothermal fields in that it is located on 1.5-Myr-old crust, nearly 15 km from the spreading axis, and may be driven by the heat of exothermic serpentinization reactions between sea water and mantle rocks. It is located on a dome-like massif and is dominated by steep-sided carbonate chimneys, rather than the sulphide structures typical of 'black smoker' hydrothermal fields. We found that vent fluids are relatively cool (40-75 degrees C) and alkaline (pH 9.0-9.8), supporting dense microbial communities that include anaerobic thermophiles. Because the geological characteristics of the Atlantis massif are similar to numerous areas of old crust along the Mid-Atlantic, Indian and Arctic ridges, these results indicate that a much larger portion of the oceanic crust may support hydrothermal activity and microbial life than previously thought.
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              An ultraslow-spreading class of ocean ridge.

              New investigations of the Southwest Indian and Arctic ridges reveal an ultraslow-spreading class of ocean ridge that is characterized by intermittent volcanism and a lack of transform faults. We find that the mantle beneath such ridges is emplaced continuously to the seafloor over large regions. The differences between ultraslow- and slow-spreading ridges are as great as those between slow- and fast-spreading ridges. The ultraslow-spreading ridges usually form at full spreading rates less than about 12 mm yr(-1), though their characteristics are commonly found at rates up to approximately 20 mm yr(-1). The ultraslow-spreading ridges consist of linked magmatic and amagmatic accretionary ridge segments. The amagmatic segments are a previously unrecognized class of accretionary plate boundary structure and can assume any orientation, with angles relative to the spreading direction ranging from orthogonal to acute. These amagmatic segments sometimes coexist with magmatic ridge segments for millions of years to form stable plate boundaries, or may displace or be displaced by transforms and magmatic ridge segments as spreading rate, mantle thermal structure and ridge geometry change.
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                Author and article information

                Journal
                CHTZA5
                The Journal of the Geological Society of Japan
                Jour. Geol. Soc. Japan
                The Geological Society of Japan
                0016-7630
                1349-9963
                2017
                2017
                : 123
                : 4
                : 185-205
                Article
                10.5575/geosoc.2016.0063
                6a4d013f-b745-4b3e-9cf9-8c2162095f72
                © 2017
                History

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