TY - JOUR
T1 - Observations of natural-state fluid pressures and temperatures in young oceanic crust and inferences regarding hydrothermal circulation
AU - Davis, E. E.
AU - Becker, K.
N1 - Funding Information:
The CORK installations were carried out with the help of many on board the JOIDES Resolution under the expert guidance of Bob Caldow, Mike Storms, and Bill Rhinehart. Tom Pettigrew, Bob Macdonald, and Bob Meldrum provided engineering support at all stages of the work, including design, fabrication, and retrieval operations. Data recovery operations were done with the skilful help of Alvin pilots Pat Hickey, Phil Forte, ‘Blee’ Williams, Dudley Foster, Steve Faluotico, and Matt Heintz, and support of the captain and crew of RV Atlantis . Physical recovery and replacement of the CORK instruments were carried out with the help of Fred Spiess, Gary Austin, Richard Zimmerman, Dave Price, and Dave Jabson who capably piloted the Scripps Institution of Oceanography Wireline Control Vehicle. Electronics for the data loggers and acoustic data telemetry links were supplied by Richard Brancker Research, Ltd., and Datasonics, Inc. Pressure sensors were supplied by Paroscientific, Inc., and thermistor cables by Neptune Technologies, Inc. Jiangheng He completed new numerical model calculations for figure 7. Extensive discussions with Kelin Wang and Andy Fisher helped us to improve our description of formation pressures, we hope to the point that it is clear to the general reader. Helpful reviews of the manuscript were provided by Leslie Smith and Elizabeth Screaton. Funding for equipment and operations was provided through the Ocean Drilling Program, the National Science Foundation (OCE-9301995 and OCE-9530426), and the Geological Survey of Canada. [BOYLE]
PY - 2002/11/30
Y1 - 2002/11/30
N2 - Four boreholes, drilled a few tens of meters into igneous basement on the eastern flank of the Juan de Fuca Ridge during ODP Leg 168, were sealed and instrumented for long-term monitoring to observe the hydrologic state of young sediment-sealed oceanic crust. The thermal regime is dominated by the effects of rapid fluid circulation in uppermost igneous basement driven by very small non-hydrostatic pressure gradients. Upper basement temperatures are uniform laterally between pairs of holes over distances of hundreds of meters to kilometers. In the case of two holes drilled into a sediment-buried basement ridge and adjacent valley, basement temperatures differ by less than 2 K despite the 2.2 km lateral separation of the sites and the 2.5:1 contrast in sediment cover thickness. Under conductive conditions, upper basement temperatures would differ by roughly 50 K. By comparison with modeling results, the observed degree of isothermality suggests a fluid flux of at least 10-6 m s-1 (30 m yr-1), and an effective permeability in the range of 10-10-10-9 m2 in the uppermost igneous crust. The pressure difference available to drive this rapid flux between the ridge and valley, estimated by comparing the observed pressures via the isothermal upper basement hydrostat that is inferred to connect the two sites, is small (≈ 2 kPa) and also suggests high permeability. Relative to the hydrostats defined by the local conductive sediment geotherms, substantial super-hydrostatic pressure (+18 kPa) is present within the buried basement ridge, and sub-hydrostatic pressure is present in the adjacent valley (-26 kPa). Such pressure differentials are the direct consequence of the advection-dominated thermal regime and small pressure losses in high-permeability basement, and are available to drive fluid seepage through sediment sections vertically up above and horizontally away from buried ridges, and down above valleys. No constraints are provided by any of the observations on the depth in the crust to which thermally or chemically significant flow might extend, although just as in the overlying sediments, the pattern of deep flow may be affected by the near-isothermal and near-hydrostatic conditions present in the permeable uppermost crustal section.
AB - Four boreholes, drilled a few tens of meters into igneous basement on the eastern flank of the Juan de Fuca Ridge during ODP Leg 168, were sealed and instrumented for long-term monitoring to observe the hydrologic state of young sediment-sealed oceanic crust. The thermal regime is dominated by the effects of rapid fluid circulation in uppermost igneous basement driven by very small non-hydrostatic pressure gradients. Upper basement temperatures are uniform laterally between pairs of holes over distances of hundreds of meters to kilometers. In the case of two holes drilled into a sediment-buried basement ridge and adjacent valley, basement temperatures differ by less than 2 K despite the 2.2 km lateral separation of the sites and the 2.5:1 contrast in sediment cover thickness. Under conductive conditions, upper basement temperatures would differ by roughly 50 K. By comparison with modeling results, the observed degree of isothermality suggests a fluid flux of at least 10-6 m s-1 (30 m yr-1), and an effective permeability in the range of 10-10-10-9 m2 in the uppermost igneous crust. The pressure difference available to drive this rapid flux between the ridge and valley, estimated by comparing the observed pressures via the isothermal upper basement hydrostat that is inferred to connect the two sites, is small (≈ 2 kPa) and also suggests high permeability. Relative to the hydrostats defined by the local conductive sediment geotherms, substantial super-hydrostatic pressure (+18 kPa) is present within the buried basement ridge, and sub-hydrostatic pressure is present in the adjacent valley (-26 kPa). Such pressure differentials are the direct consequence of the advection-dominated thermal regime and small pressure losses in high-permeability basement, and are available to drive fluid seepage through sediment sections vertically up above and horizontally away from buried ridges, and down above valleys. No constraints are provided by any of the observations on the depth in the crust to which thermally or chemically significant flow might extend, although just as in the overlying sediments, the pattern of deep flow may be affected by the near-isothermal and near-hydrostatic conditions present in the permeable uppermost crustal section.
KW - Fluid flux
KW - Hydrothermal circulation
KW - Marine hydrogeology
KW - Oceanic crust
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U2 - 10.1016/S0012-821X(02)00982-2
DO - 10.1016/S0012-821X(02)00982-2
M3 - Article
AN - SCOPUS:0037202656
VL - 204
SP - 231
EP - 248
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
SN - 0012-821X
IS - 1-2
ER -