Chemistry of waters sampled from oceanic basement boreholes, 1979-1988

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doi: 10.1029/JB095iB06p09327
Author(s): Mottl, Michael J.; Gieskes, Joris M.
Author Affiliation(s): Primary:
Univ. Hawaii, Dep. Oceanogr., Honolulu, HI, United States
Other:
Univ. Calif. at San Diego, United States
Volume Title: Special section on Logging and downhole measurements in Deep Sea Drilling Project/Ocean Drilling Program deep crustal holes
Source: Special section on Logging and downhole measurements in Deep Sea Drilling Project/Ocean Drilling Program deep crustal holes. Journal of Geophysical Research, 95(B6), p.9327-9342. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. Hawaii Inst. Geophys., Contrib. No. 2211. 55 refs.; illus. incl. 3 tables, sketch map
Summary: Waters have been sampled from the basement section of four Deep Sea Drilling Project holes (395A, 396B, 418A, and 504B) since 1979, in an attempt to determine the composition of oceanic basement formation waters. Samples have been obtained by two methods: active, in which water is drawn under negative pressure from the interstices of the wall rocks, and passive, in which it is collected from the open borehole. The most successful sampling has been done on re-entering the holes months to years after they were last disturbed by drilling and/or pumping of surface seawater downhole. Most of the samples show large changes in composition from seawater that are consistent with alteration of basalts at the in situ temperatures of 10° to 146°C. The samples are typically depleted in Mg2+, K+, Na+, sulfate, and 87Sr, and enriched in Ca2+ and Si. However, all of the samples are contaminated to some extent, mainly by seawater which is either introduced during drilling or sampling, flows downhole due to underpressure in the upper basement, or mixes downhole due to convective overturn in the hole itself. The latter process is apparently common and dilutes any formation water which may enter the hole via diffusion and horizontal convection. In hole 504B, the tritium content of the waters indicates that 24 to 35% of the surface seawater which originally filled the hole is still present after periods of 470 to 1233 days. The more general problem of what has displaced the surface seawater and produced the large chemical changes observed in the borehole waters, whether exchange with formation water or simply reaction of seawater with the wall rocks of the hole, remains unsolved. As a result, no component of true basement formation water has been identified unambiguously in any of the holes. This problem may be solved by (1) compartmentalizing a hole with a series of plugs to limit downhole mixing or (2) using a straddle packer to draw a series of samples from the interstices of the wall rocks of a hole, either shortly after drilling or later following re-entry. In either case, an array of chemical tracers will have to be measured, either in situ or in the laboratory, to infer the nature of the waters obtained. Copyright 1990 by the American Geophysical Union.
Year of Publication: 1990
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; Basaltic layer; Basalts; Boreholes; Connate waters; Crust; DSDP Site 395; DSDP Site 396; DSDP Site 418; DSDP Site 504; Deep Sea Drilling Project; Geochemistry; IPOD; Igneous rocks; Leg 102; Leg 109; Leg 111; Leg 137; Leg 140; Leg 148; Leg 45; Leg 46; Leg 52; Leg 53; Leg 69; Leg 70; Leg 78B; Leg 83; Leg 92; Ocean Drilling Program; Oceanic crust; Sampling; Sea water; Volcanic rocks
Record ID: 1990053613
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute.

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