Reviews of deep drilling into oceanic crust

Author(s): Umino, Susumu
Author Affiliation(s): Primary:
Shizuoka University, Department of Biology and Geosciences, Shizuoka, Japan
Volume Title: Recent development of oceanic petrology
Source: Recent development of oceanic petrology. Chigaku Zasshi = Journal of Geography, 112(5), p.650-667. Publisher: Tokyo Geographical Society, Tokyo, Japan. ISSN: 0022-135X CODEN: CGZAAL
Note: In Japanese with English summary. 19 refs.; illus., incl. 1 table
Summary: Deep drill holes bored into in situ oceanic crust were reviewed. Since the beginning of the Deep Sea Drilling Project, 45 drill holes have penetrated over ca. 50 m into normal oceanic crust, however, most are concentrated in the north Atlantic and east Pacific Oceans. Basement ages of most holes are younger than 20 Ma, and are rather biased toward younger crust compared to the average age of the oceanic crust (61 Ma). Only five deep drill holes have penetrated over 500 m into the basement, three of which drilled into slow spread crust formed at <4 cm/yr, with only one hole is fast spread crust formed at >8 cm/yr and one in intermediately spread crust. Three deep holes (332B, 395A, 418A) drilled into slow spread crust formed at Mid-Atlantic Ridge gave the first evidence of magnetic reversals through the vertical oceanic crust, and showed that the slow spread upper oceanic crust away from hot spots is dominantly composed of pillow lavas with a normal MORB-like affinity. Younger 332B and 395A holes (3.5 and 7.3 Ma) gave poor core recovery (18-21%), while the oldest Hole 418A (110 Ma) yielded a fairly high recovery of 72%. Hole 504B is the only hole to penetrate the extrusive rocks and most of the way through the sheeted dike complex (1836.5 m sub-basement). Average core recovery dropped from 29.8-25.3% in the lava and transition zone down to 14.3% in the sheeted dike complex. Unfortunately, the 504B lava is the depleted extremity of MORBs from intermediate-fast spread ridges. One of the most important findings of Hole 504B is a discrepancy between the seismic velocity structure and the downhole lithology in that the Layer 2/3 boundary resides in the middle of the sheeted dikes, as interpreted by the difference in porosity and bulk density. Hole 1256D is dedicated to coring typical oceanic crust and ultimately penetrates the entire crust into the upper mantle. The site is located on the 15-Ma Cocos plate generated at a superfast rate (22 cm/yr). 502-m-long cores of basement (48% recovery) are lavas showing moderately evolved MORB-like compositions similar to those from the present fast spread ridges. The hole has been cleaned and left ready for future drilling, possibly into Layer 3. The above examples of deep drill holes show that the major obstacles to ultradeep drilling are hole collapse and poor core recovery. Riser drilling is expected to overcome these obstacles for "the 21-century Mohole".
Year of Publication: 2003
Research Program: DSDP Deep Sea Drilling Project
IODP Integrated Ocean Drilling Program
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 05 Petrology, Igneous and Metamorphic; 18 Geophysics, Solid-Earth; Atlantic Ocean; Basalts; Crust; DSDP Site 332; DSDP Site 395; DSDP Site 418; DSDP Site 504; Deep Sea Drilling Project; Deep drilling; Drilling; IPOD; Igneous rocks; Integrated Ocean Drilling Program; Leg 37; Mid-Atlantic Ridge; Mid-ocean ridge basalts; Mid-ocean ridges; ODP Site 1256; Ocean Drilling Program; Ocean floors; Oceanic crust; Review; Volcanic rocks
Coordinates: N224521 N224521 W0460454 W0460454
N250205 N250207 W0680326 W0680327
N011335 N011338 W0834348 W0834357
N064400 N064400 W0915600 W0915600
Record ID: 2007127298
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