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1 km
                                           Tracer injection                      GeoB00-483
                             N20°E        Hole                                                               3.4
                                    Hole
              380                   U1362A  U1362B
                                           Hole
                                Hole U1301A  1026B
              400  Pillow basalt  Hole U1301B         Hole                   U1362B U1362A             Seafloor
                                                      1027B  Seafloor =   U1301B    1026B           1027C   3.6
                                                              2,600 m
              420
                   Massive basalt
              440                                             Depth (mbsl)  Sediment                       3.8
             Depth (mbsf)  460  Basalt top                               3.5                              4.0 Two-way travel time (s)
              480
              500                        Tracer                          3.7
                      Dominant fluid flow direction transport                                             4.2
              520                                                  Basaltic ocean crust
                                        Tracer                           3.9 Two-way travel time (s)
              540                     detection                          4.1                             4.4
              560                                                                            Basalt top
                   Basalt flow                         0      1 km       4.3                            4.6

          Figure 3. Schematic (not to scale) of borehole locations and experimental design for a tracer injection experiment conducted during and after Expedi-
          tion 327 to investigate transport rates in the upper ocean crust. Boreholes observatories were placed in ~3.5–3.6-m.y.-old crust on the eastern flank of
          the Juan de Fuca Ridge. Thicker sections of the drill pipe indicate open intervals where tracer was injected or sampled. The stratigraphy of the upper
          volcanic crust at Hole U1301B is shown on the left. Main diagram modified from Fisher et al. (2011), and stratigraphic column is from Becker et al. (2013).
          Experimental results indicate very rapid tracer transport (meters/day), with most of the flow occurring in a small fraction of the rock (Neira et al., 2016).
          See Figure S7 for additional information (see text footnote 1).


          152, 163) with segments along the East   submersible observations, and ophiolites   spreading crust. These faults are also con-
          Greenland–Norwegian rifted margins    was perceived as fairly simple: basalts on   duits for hydrothermal fluids that lead to
          (Fig. S6 [see footnote 1]), an area that con-  the seafloor were fed by sheeted dike com-  the formation of massive sulfide deposits
          stitutes part of the North Atlantic Igneous   plexes that emanated from a magma cham-  as observed in the Trans-Atlantic Geo-
          Province. An early surprise in drilling   ber where gabbro crystalized and accumu-  traverse hydrothermal field (Leg 158).
          these margins was that some of the well-  lated over underlying mantle. There was,   A critical component of modeling crustal
          defined, seaward-dipping reflectors west   as there is today, discussion about the dif-  accretion (and subduction) is understanding
          of Norway consisted of subaerial extrusive   ferences between fast and slow spreading   the role of hydrothermal circulation. To
          basalts. Later,  Ar- Ar dating of volcanic   and the distinguishing features of backarc   learn more about this plumbing system,
                     40
                        39
          rocks in southeast Greenland showed that   basins. Now parts of the ocean crust have   many holes have been instrumented with
          the breakup evolution spanned ~12 m.y.   been sampled, boreholes instrumented,   circulation obviation retrofit kits (CORKs).
          (Tegner and Duncan, 1999).         and geophysical and submersible observa-  CORKs seal off the borehole from the sea
            Magma-poor margin drilling has taken   tions collected. The ocean floor is much   floor, allowing in situ measurements of
          place on the once contiguous Newfound-   more varied and complex than the early   pressure and temperature, and show the
          land-Iberian margins (Legs 47, 149, and   models suggested.           extent of fluid circulation in young ocean
          173, and Expedition 210), and in the South   Multiple expeditions at Hole 504B (Alt et   crust, which may be one of the most hydro-
          China Sea (Expeditions 349, 367, and 368).   al., 1996) and Hole 1256D (Wilson et al.,   logically active formations on Earth
          These expeditions, with sites carefully   2006) in the eastern Pacific partially sup-  (Becker and Fisher, 2000). Advanced
          located along seismic lines, have recovered   port the model of layered ocean crust, with   CORKs allow for additional measurements
          sediments, continental crust, a wide vari-  basalts overlying sheeted dikes, which in   (e.g., seismometers and fluid sampling),
          ety of igneous rocks, and even serpen-  their deeper parts contain lenses of gabbro.   ocean observatories, and experiments
          tinized mantle. Most recently, MSP   The structure and composition of the    (Fisher et al., 2011; Figs. 3 and S7
          Expedition 381 cored the Corinth Rift in   deeper crustal components was addressed   [see footnote 1]).
          the Mediterranean. There, a syn-rift   at the Hess Deep Rift on the East Pacific
          sequence is accessible, allowing the fault   Rise (Leg 147 and Expedition 345). There,   Convergent Boundaries
          and rift evolutionary history, including the   the variety of textures and composition of   Convergent boundaries (subduction
          deformation rates, to be determined.  the plutonic rocks was larger than expected.  zones) include island arcs, forearcs, backarc
                                               In the Atlantic, where spreading is   basins, and accretionary prisms. Processes
          Ocean Crust and Lithosphere        slower, investigators were initially sur-  at these plate boundaries are more likely to
            Understanding the composition and   prised by the presence of serpentine mixed   impact humans than any other component
          structure of ocean crust, how it varies, and   with basalts (Site 395, Leg 45). Many sub-  of plate tectonics, generating large earth-
          how it is accreted is fundamental to under-  sequent expeditions showed that mantle   quakes, explosive volcanoes, and tsunamis.
          standing Earth’s basic evolution. In the late   and lower crustal rocks, displaced upward   The Chikyu has focused on the deeper por-
          1970s, the structure of ocean crust gleaned   along major normal faults, are integral   tions of this plate boundary, especially in
          from geophysical surveys, dredge hauls,   components of slow and ultra-slow    the Nankai Trough and Japan Trench.

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