Ocean Drilling Program (ODP) Leg 209 was devoted to drilling mantle peridotites and associated gabbroic rocks along the Mid-Atlantic Ridge (MAR) from 14° to 16°N. In this area, there are nearly continuous outcrops of residual mantle peridotite on both sides of the rift valley for at least 100 km along strike (Fig. F1; Table T1). We drilled 19 holes at 8 sites along the MAR from 14°43´ to 15°44´N (Fig. F2). At Sites 1269 and 1273, we penetrated 112 m of basaltic rubble; recovery was poor (a total of 3.7 m) and holes were unstable, so drilling was terminated. Lavas form nearly horizontal surfaces overlying cliffs exposing peridotite and gabbro. At six other sites, we drilled a mixture of residual peridotite and gabbroic rocks intrusive into peridotite. We penetrated 1075 m at these sites and recovered 354 m of core. Drilling at Sites 1268, 1270, 1271, and 1272 recovered 25% gabbroic rocks and 75% residual mantle peridotite. Core from Site 1274 is mainly residual peridotite, with a few meter-scale gabbroic intrusions. Core from Site 1275 is mainly gabbroic but contains 14% poikilitic dunite, troctolite, plagioclase harzburgite, and plagioclase lherzolite interpreted as residual peridotite "impregnated" by plagioclase and pyroxene crystallized from melt migrating along olivine grain boundaries. Impregnated peridotites are also common at Site 1271 and present at Sites 1268 and 1270.
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 ODP. In addition to identifying many suitable drill sites, these cruises completed shipboard bathymetric, gravity, and magnetics surveys.
In addition to the extensive outcrops of mantle peridotite, significant features of the area include:
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. F5). This type of seismic structure is typical for slow-spreading ridges near fracture zones (R.S. Detrick, pers. comm., 1998).
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 F1, and a summary of drill sites is shown in Figure F2. 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. F2).
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 F1 (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. F3). Escartin and Cannat (1999) report similar results, although with slightly different values for the magnitude of the anomalies. 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, relative to the anomaly along the fracture zone, 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 interpretation 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 purport to explain focused crustal accretion, or at least focused volcanic activity in regions with low gravity, along magmatic segments.
Extensive analytical work has been done on samples recovered by dredging in the 14°–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. F4). Perhaps related to this observation is the further 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. F4).