Drilling deep into young oceanic crust, Hole 504B, Costa Rica Rift

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doi: 10.1029/RG027i001p00079
Author(s): Becker, Keir; Sakai, Hitoshi; Adamson, Andrew C.; Alexandrovich, Joanne; Alt, Jeffrey C.; Anderson, Roger N.; Bideau, Daniel; Gable, Robert; Herzig, Peter M.; Houghton, Simon D.; Ishizuka, Hideo; Kawahata, Hodaka; Kinoshita, Hajimu; Langseth, M. G.; Lovell, Michael A.; Malpas, John; Masuda, Harue; Merrill, R. B.; Morin, Roger H.; Mottl, Michael J.; Pariso, Janet E.; Pezard, Philippe A.; Phillips, Joseph D.; Sparks, Joel W.; Uhlig, Stefan
Author Affiliation(s): Primary:
Rosenstiel Sch. Mar. and Atmos. Sci., Div. Mar. Geol. and Geophys., Miami, FL, United States
Other:
Ocean Res. Inst. Tokyo, Japan
Tex. A&M Univ., United States
Lamont-Doherty Geol. Obs., United States
Wash. Univ., United States
Inst. Fr. Rech. Exploit. Mer, France
BRGM, France
Aachen Univ. Technol., Federal Republic of Germany
Open Univ., United Kingdom
Kochi Univ., Japan
Univ. Toronto, Canada
Chiba Univ., Japan
Univ. Niottingham, United Kingdom
Mem. Univ. Newfoundland, Canada
U. S. Geol. Surv., United States
Hawaii Inst. Geophys., United States
Univ. Wash., United States
Univ. Austin, United States
Univ. Mass., United States
Univ. Giessen, Federal Republic of Germany
Volume Title: Reviews of Geophysics
Source: Reviews of Geophysics, 27(1), p.79-102. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 8755-1209 CODEN: RGPSBL
Note: In English. 130 refs.; illus. incl. 3 tables, sketch maps
Summary: Hole 504B is by far the deepest hole yet drilled into the oceanic crust in situ, and it therefore provides the most complete "ground truth" now available to test our models of the structure and evolution of the upper oceanic crust. Cored in the eastern equatorial Pacific Ocean in 5.9-m.y.-old crust that formed at the Costa Rica Rift, hole 504B now extends to a total depth of 1562.3 m below seafloor, penetrating 274.5 m of sediments and 1287.8 m of basalts. The site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust. As a result, the conductive heat flow approaches the value of about 200 mW/m2 predicted by plate tectonic theory, and the in situ temperature at the total depth of the hole is about 165°C. The igneous section was continuously cored, but recovery was poor, averaging about 20%. The recovered core indicates that this section includes about 575 m of extrusive lavas, underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes; the latter have been sampled in situ only in hole 504B. The igneous section is composed predominantly of magnesium-rich olivine tholeiites with marked depletions in incompatible trace elements. Nearly all of the basalts have been altered to some degree, but the geochemistry of the freshest basalts is remarkably uniform throughout the hole. Successive stages of on-axis and off-axis alteration have produced three depth zones characterized by different assemblages of secondary minerals: (1) the upper 310 m of extrusives, characterized by oxidative "seafloor weathering"; (2) the lower extrusive section, characterized by smectite and pyrite; and (3) the combined transition zone and sheeted dikes, characterized by greenschist-facies minerals. A comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology, alteration, and porosity into three zones that are analogous to layers 2A, 2B, and 2C described by marine seismologists on the basis of characteristic seismic velocities. Many of the logs and experiments suggest the presence of a 100- to 200-m-thick layer 2A comprising the uppermost, rubbly pillow lavas, which is the only significantly permeable interval in the entire cored section. Layer 2B apparently corresponds to the lower section of extrusive lavas, in which original porosity is partially sealed as a result of alteration. Nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900-1000 m below seafloor, within the transition between extrusives and sheeted dikes, indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2B and 2C and confirming that layer 2C consists of intrusive sheeted dikes. A vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends--that between the sheeted dikes of seismic layer 2C and the gabbros of seismic layer 3, which has never been sampled in situ--may be within reach of the next drilling expedition to hole 504B. Therefore despite recent drilling problems deep in the hole, current plans now include revisiting hole 504B for further drilling and experiments when the Ocean Drilling Program returns to the eastern Pacific in 1991. Copyright 1989 by the American Geophysical Union.
Year of Publication: 1989
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; Cores; Costa Rica Rift; Crust; DSDP Site 504; Deep Sea Drilling Project; Deep drilling; Drilling; East Pacific; Equatorial Pacific; Halmyrolysis; Heat flow; IPOD; Igneous rocks; Interpretation; Lava; Leg 111; Leg 137; Leg 140; Leg 148; Leg 69; Leg 70; Leg 83; Leg 92; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Tectonophysics; Velocity structure; Well-logging
Coordinates: N011338 N011338 W0834349 W0834349
Record ID: 1989052373
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