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Drilling the Crust at Mid-Ocean Ridges

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Drilling the Crust at Mid-Ocean Ridges or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of Th e Oceanography Society. Send all correspondence to: [email protected] or Th eOceanography PO Box 1931, Rockville, USA.Society, MD 20849-1931, or e to: [email protected] Oceanography correspondence all Society. Send of Th approval portionthe ofwith any articlepermitted only photocopy by is of machine, reposting, this means or collective or other redistirbution articleis has been published in Th SPECIAL ISSUE FEATURE Oceanography Drilling the Crust Threproduction, Republication, systemmatic research. for this and teaching article copy to use in reserved. e is rights granted All OceanographyPermission Society. by 2007 eCopyright Oceanography Society. journal of Th 20, Number 1, a quarterly , Volume at Mid-Ocean Ridges An “in Depth” perspective BY BENOÎT ILDEFONSE, PETER A. RONA, AND DONNA BLACKMAN In April 1961, 13.5 m of basalts were drilled off Guadalupe In the early 1970s, almost 15 years after the fi rst Mohole Island about 240 km west of Mexico’s Baja California, to- attempt, attendees of a Penrose fi eld conference (Confer- gether with a few hundred meters of Miocene sediments, in ence Participants, 1972) formulated the concept of a layered about 3500 m of water. This fi rst-time exploit, reported by oceanic crust composed of lavas, underlain by sheeted dikes, John Steinbeck for Life magazine, aimed to be the test phase then gabbros (corresponding to the seismic layers 2A, 2B, for the considerably more ambitious Mohole project, whose and 3, respectively), which themselves overlay mantle perido- objective was to drill through the oceanic crust down to tites. Deep drilling into the oceanic crust would provide the Earth’s mantle (Lill and Bascom, 1959; Bascom, 1961). Born ground truth for the Penrose model; the fl ame of the Mohole in the late 1950s, the Mohole project unfortunately ended project still burned. This article draws from results of more in muddy waters and was terminated by the United States than 30 years of ocean drilling at mid-ocean ridges or in Congress in 1965 (Shor, 1985; Greenberg, 1971). Undeterred, older igneous oceanic crust and briefl y reviews some im- the scientifi c community rallied again to launch the Deep portant milestones that improved understanding of crustal- Sea Drilling Project (DSDP) in 1968, followed by the Ocean accretion processes at mid-ocean ridges. Figure 1 shows the Drilling Program (ODP) in 1985, and the Integrated Ocean locations of the numerous DSDP, ODP, and IODP expedi- Drilling Program (IODP) in 2003. These programs have pro- tions to which we refer; Table 1 lists site locations. vided solutions to some of the most pressing and interest- ing problems in ocean and earth science (see, for example, Oceanography 19-4, December 2006). 66 Oceanography Vol. 20, No. 1 Mohole Project DSDP ODP 60 IODP 139/169 301 169 82 37 Mohole 51/53 158 304/305 30 test site 45 46/109/153 206/309/312 209 147 193 69-148 0 34 -30 118/176/179 -60 90 120 150 180 210 240 270 300 330 0 30 60 90 Figure 1. Map of principal drilling sites at mid-ocean ridges and in igneous oceanic crust, with related DSDP/ODP leg and IODP expedition numbers (see also Table 1). ODP leg reports are available online at www-odp.tamu.edu/publications/pubs.htm. Old volumes, prior to Web- based publication, are currently being scanned and will progressively be available on the same site. DSDP leg reports are only available on paper. IODP expedition preliminary reports and proceedings are available online at iodp.tamu.edu/publications/. Oceanography March 2007 67 DSDP: THE EARLY YEARS drilling of the oceanic basement was then cluded by poor drilling conditions in The fi rst confi rmation that oceanic attempted at several sites in the North basalts at depth. The most spectacular layer 2A was made of basaltic fl ows and Atlantic Ocean (e.g., DSDP Legs 45, 46, failure was when the derrick of the drill- pillow lavas came in 1973 from drilling 51, and 52). Core recovery, although ing vessel Glomar Challenger bent during on the Nazca Plate (DSDP Leg 34) in the good enough to allow high-quality sci- DSDP Leg 46 as the hole reached 255 m eastern Pacifi c Ocean, and on the western entifi c investigation of the samples, was in basaltic crust at Site 396. fl ank of the Mid-Atlantic Ridge, south of limited, typically less than 40 percent. DSDP boreholes yielded the fi rst in- the Azores Plateau (DSDP Leg 36). Deep Deep penetration of the crust was pre- dications that the Penrose model cannot Table 1. Site Numbers, Locations, and Main Characteristics Leg/Exp. * Year Site # Location Comment 34 1973/74 319–321 Eastern Pacifi c, Nazca plate Early drilling in basalt 13.02°S, 101.52°W 9.01°S, 83.53°W 12.02°S, 81.90°W 37 1974 332–335 MAR Early drilling in basalt at the Mid-Atlantic Ridge (MAR); gabbro and 36.88°N, 33.64°W serpentinized peridotite in Hole 334 36.84°N, 33.68°W 37.03°N, 34.41°W, 37.29°N, 35.20°W 45 1975/76 395 MAR Basalts, a few gabbro and serpentinized peridotite cobbles in 576.5-m- 22.76°N, 46.08°W deep Hole 395A 46 1976 396 MAR Drilled 255 m in basalt 22.99°N 43.51°W 51, 52, 53 1976/77 417, 418 Northern Atlantic 108 million year old basaltic upper crust, better recovery (~ 70 percent); 25.11°N, 68.04°W massive fl ows and pillow lava 25.03°N, 68.06°W 82 1981 556, 558, MAR A few tens of meters of metamorphosed gabbro and serpentinized 560 38.94°N, 34.68°W peridotite 37.77°N, 37.34°W 34.72°N, 38.84°W 109 1986 670 MAR First intentionally drilled peridotite; 92.5-m-deep hole, 7 percent recovery 23.17°N, 45.03°W 139 1991 856 Northern Juan de Fuca Ridge 8 holes to a maximum of 122 meters below seafl oor through inactive 48.44°N, 128.7°W massive sulfi de body and sediment into a basalt sill (Bent hill) 147 1992/93 894–895 Eastern Pacifi c, Hess Deep Upper mantle peridotites and upper crust gabbros in fast-spreading EPR 2.30°N, 101.5°W crust 2.28°N, 101.4°W 69, 70, 83, 1979–93 504 Eastern Pacifi c, 504B is the deepest scientifi c oceanic borehole (drilled to 2111 meters 111, 137, Guatemala Basin below seafl oor); upper rocks are basaltic; 5.9-million-year-old 140, 148 1.23°N, 83.73°W intermediate-spreading crust 153 1993/94 920–924 MAR Kane Fracture Zone area 200-m-deep Hole 920D in peridotites (57 percent recovery); ten short 23.34°N, 45.02°W holes (a few tens of meters) drilled in gabbros in the inside corner high 23.54°N, 45.03°W south of the Kane Fracture Zone 23.52°N, 45.03°W 23.54°N, 45.01°W 68 Oceanography Vol. 20, No. 1 be applied along the entire mid-ocean style crust appeared to be missing. Dur- Kane Fracture Zone, a few gabbro cob- ridge system and that the oceanic crust ing DSDP Leg 45, on the western fl ank bles were recovered from the bottom is much more heterogeneous, at least of the Mid-Atlantic Ridge south of the of a 588-m-deep hole in sediments and locally. At Site 334 (DSDP Leg 37), gab- bros and serpentinized (water-altered) BENOÎT ILDEFONSE ([email protected]) is CNRS Researcher, Géosciences peridotites were recovered at shallow Montpellier, Université Montpellier 2, Montpellier, France. PETER A. RONA is Professor, Insti- depth, directly underlying 50 meters of tute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ, USA. DONNA basalts; several kilometers of Penrose- BLACKMAN is Research Geophysicist, Scripps Institution of Oceanography, La Jolla, CA, USA. Leg/Exp. * Year Site # Location Comment 158 1994 957 MAR TAG hydrothermal fi eld, active high-temperature sulfi de mound 26.14°N, 44.83°W 169 1996 1035–1038 Northern Juan de Fuca Ridge Sediment hosted, inactive massive sulfi de deposit (Bent Hill) and ac- 48.43°N, 128.68°W tive hydrothermal fi eld (Dead Dog); 9 holes to a maximum of 404 m Escanaba Trough, terminating in basalt, indicating that massive sulfi de forms only a thin southern Gorda Ridge veneer (5–15 m) over the sediment sequence along the faulted margin 41.00°N 127.49°W of Central Hill 118, 176 1987, 735 SWIR 1508-m-deep Hole 735B in the Atlantis Bank, Southwest Indian 1997 32.7°S, 57.27°E Ridge (SWIR); this fi rst deep hole in slow-spreading crust recovered ~ 86 percent gabbroic rocks 179 1998 1105 SWIR 158-m-deep Hole 1105A, ~ 1.3 km east of Hole 735B (Atlantis Bank) 32.7°S, 57.28°E 193 2000/01 1188–1191 Manus Basin 13 holes to a maximum of 387 meters below seafl oor in PACManus 03.72°S, 151.67°E active high-temperature hydrothermal system, with minor sulfi des hosted in altered dacitic to rhyodacitic rocks 209 2003 1268, MAR 13 holes in mantle peridotites and gabbroic rocks; 209-m-deep 1270–1272, 14.85°N, 45.08°W Hole 1275D in 14.75°N oceanic core complex, recovered gabbros 1274, 14.72°N, 44.89°W and troctolites 1275 15.04°N, 44.95°W 15.09°N, 44.97°W 15.65°N, 46.68°W 15.74°N, 46.90°W 301 2004 1026, 1301 Juan de Fuca Ridge Installation of seafl oor observatory network on the eastern ridge 47.76°N, 127.76°W fl ank in preparation for long-tem monitoring and active hydrological 47.75°N, 127.77°W experiment 304, 305 2004/05 1309 MAR Hole U1309D is the second deepest (1415-m) gabbroic section recovered 30.17°N, 42.12°W in an oceanic core complex at a slow-spreading ridge 206, 309, 2002/03, 1256 Eastern Pacifi c, 1255-m-deep Hole 1256D (1005 m in basement) reached the sheeted 312 2005 Cocos Plate dike/gabbro transition zone in 15 million-year-old, superfast-spreading 6.74°N, 91.93°W East Pacifi c Rise crust * Details of these legs and expeditions can be found in the Initial Reports of the Deep Sea Drilling Project, the Proceedings of the Ocean Drilling Program, and the Proceedings of the Integrated Ocean Drilling Program.
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