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Site Survey Data and Other Geological Background

The Mid-Atlantic Ridge near the 15°20' Fracture Zone has been the focus of a long-term cooperative French-American and allied Russian research program. During the summer of 1998 the area was visited by a Japanese/American team, funded in part as a site survey for the Ocean Drilling Program (ODP). In addition to identifying many suitable drill sites, these cruises identified many suitable drill sites and completed shipboard bathymetric, gravity, and magnetics surveys (Fig. F1).

In addition to nearly continuous outcrops of mantle peridotite on both walls of the rift valley for at least 100 km from 14°40' to 15°40'N (Fig. F2), significant features of the area include:

  1. Large "gravity bulls-eyes," concentric negative residual Bouger and mantle Bouger gravity anomalies, centered at ~14° and 16°N (Fig. F1);
  2. A regional chemical anomaly with "hotspot" characteristics centered at ~14°N (Fig. F3);
  3. "Megamullion" structures, interpreted to be long-lived low-angle faults exposed on the seafloor over regions of ~100 km2 (e.g., 46°54'W, 15°44'N) (Fig. F2); and
  4. At least three areas with high methane signatures in the water column, including one active hydrothermal field within mantle peridotites.

Seismic Studies

In June 1997, a seismic refraction experiment was carried out north of the 15°20' Fracture Zone from the Ewing, led by John Collins of Woods Hole Oceanographic Institute (WHOI). Using NOBEL (Near Ocean Bottom Explosives Launcher), refraction profiles were shot over areas previously mapped using the submersible Nautile. Source and receiver were on the seafloor for determination of seismic velocity structure at length scales of 10 to 100 m instead of 100 m to 1 km with conventional surveys. NOBEL profiles were taken at 15°37'N on (1) an ultramafic outcrop, (2) a gabbro/wehrlite outcrop, and (3) basalt, to determine whether seismic velocities can be used to map the extent of gabbro and peridotite emplaced at or near the seafloor. In addition, a 100-km-long conventional refraction profile was shot along the median valley of the Mid-Atlantic Ridge north of the 15°20' Fracture Zone. Results show anomalous seismic structure in the crust with pronounced gradients in velocity rather than the layered structure typical for fast-spreading ridges (Fig. F4). This type of seismic structure is typical for slow-spreading ridges near fracture zones (R. Detrick, pers. comm., 1998).

Submersible Studies

Many possible drill sites were identified during the Faranaut cruise with the French Nautile submersible in 1992 (e.g., Cannat et al., 1995, 1997b). In 1998, the joint Japan Marine Science and Technology Center (JAMSTEC)/WHOI MODE 98, Leg 1 cruise with the Japanese Shinkai 6500 submersible completed the survey for possible drill sites. A summary of lithologic observations from dredging and diving is shown in Figure F2, and a summary of drill sites is shown in Figure F5. In addition, it is worthy of note that extensive exposures of moderate- to low-angle fault surfaces underlain by peridotite were observed on the seafloor, particularly at Sites 1275 and 1270 (Fig. F5).

Shipboard Geophysics

Although the 1992 Faranaut cruise included shipboard bathymetric, gravity, and magnetics surveys, the quality of the gravity and magnetics data was less than optimal. The 1998 MODE 98, Leg 1 cruise conducted additional surveys. The combined Faranaut and MODE 98 survey coverage is illustrated in Figure F2 (Cannat et al., 1995, 1997b; Casey et al., 1998; Kelemen et al., 1998b; Matsumoto et al., 1998; Escartin and Cannat, 1999; Fujiwara et al., 2003). For the purposes of this report, the most important result is the identification of large "gravity bulls-eyes," concentric negative residual Bouger and mantle Bouger gravity anomalies, centered at ~14° and 16°N (Fig. F1). These gravity lows correspond to areas with well-organized seafloor magnetic anomalies and ridge-parallel abyssal hill topography, whereas the relative gravity highs correspond to known areas with outcrops of serpentinized peridotites along the ridge axis and to areas with poorly organized seafloor magnetic anomalies and chaotic topography. Also note that the negative gravity anomaly at 14°N is about twice as large as that at 16°N, in keeping with geochemical indications that the 14°N area resembles a "hotspot."

The gravity lows have been interpreted as centers of magmatic segments where there is accretion of thick igneous crust, whereas the gravity highs on the periphery of these magmatic segments were thought to be magma starved. This idea was central to our drilling plan because it provides a potential explanation for the extensive outcrops of peridotite along the Mid-Atlantic Ridge between 14°40' and 15°40'N, but it has never been tested. This region was ideal for testing hypotheses that explain focused crustal accretion, or at least focused volcanic activity in regions with low gravity, along magmatic segments.

Geochemical Background

Extensive analytical work has been done on samples recovered by dredging in the 14° to 16°N region along the Mid-Atlantic Ridge (Bonatti et al., 1992; Bougault et al., 1988, 1990; Casey, 1997; Casey et al., 1992, 1994, 1995; Dick and Kelemen, 1992; Dosso et al., 1991; Peyve et al., 1988; Silantyev et al., 1996; Sobolev et al., 1992a, 1992b, 1992c; Staudacher et al., 1989; Xia et al., 1991, 1992; C. Xia et al., unpubl. data). This work reveals that the mantle source of basalts south of the 15°20' Fracture Zone is geochemically "enriched," similar to the source of hotspot-related mid-ocean-ridge basalts (MORB) elsewhere along the Mid-Atlantic Ridge (Fig. F3). Perhaps related to this is the observation that mantle peridotites seem to have undergone unusually high degrees of melting (mantle olivines have molar Mg/(Mg + Fe) up to 0.92, and spinels have molar Cr/(Cr + Al) up to 0.7, forming the depleted end-members for peridotites recovered from mid-ocean ridges (Fig. F6). North of the fracture zone, however, basalts and peridotites sampled by dredging and submersible have compositions typical for the Mid-Atlantic Ridge away from hotspots (Fig. F3).

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