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

Abstract

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/m 2 predicted by plate tectonic theory, and the in situ temperature at the total depth of the hole is about 165øC. 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 extrasives andsheeted 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...

Three‐dimensional upper crustal heterogeneity and anisotropy around Hole 504B from seismic tomography

“…In the underlying sheeted dike section, P wave velocities increase more gradually, reaching-6.5 km s-' between 0.9 and 1.2 km into the crust. This increase in P wave velocity with depth closely correlates with a large decrease in apparent bulk porosity with depth in Hole 504B [Becker et al, 1989[Becker et al, , 1992]. An increase in fracture density in the lowermost part of Hole 504B and a decrease in sonic velocities at -1600 m depth have been interpreted as evidence that a subhorizontal fault zone was penetrated near the base of the hole [Altet al, 1993] A comparison of the in situ physical properties measured in Hole 504B with available seismic data from in and around the drill site indicates that at this location the seismic layer 2/3 boundary is not associated with a lithostratigraphic transition from sheeted dikes to gabbro as is commonly assumed for oceanic crust [Detrick et al, 1994;Salisbury et al, 1996;Swift et al, 1998a].…”

CORK: A Hydrologic Seal and Downhole Observatory for Deep-Ocean Boreholes

“…In many cases where permeable parts of young oceanic crust have been penetrated, the formations have been found to be underpressured (Anderson and Zoback, 1982), at least with respect to the hydrostatic conditions that are produced within the holes that are cooled by circulation during drilling. Downhole fluid flow is stimulated, and in those cases where holes have been later reentered, this stimulated flow has been found to be relatively stable and long lived (e.g., Hyndman et al, 1976;Becker et al, 1983Becker et al, , 1984Becker et al, , 1989Gable et al, in press;Morin et al, in press). This causes serious formation contamination problems and severely limits the quality of many downhole measurements and samples, both over short and long periods of time.…”

Stable and Strontium Isotopic Profiles through Hydrothermally Altered Upper Oceanic Crust, Hole 504B

“…The Fe 3+ /Fe Tot ratio is fairly constant with depth in the lower dikes ( 0.25), but is slightly elevated above the initial igneous value (0.12, vertical line), which indicates a moderate pervasive oxidation. The average secondary mineral proportions were calculated per 25 m depth from shipboard and published point counting and petrographic estimates (Becker et al, 1989;Dick, Erzinger, Stokking, et al, 1992;Alt, Kinoshita, Stokking, et al, 1993;Alt et al, 1995;Zuleger et al, 1995 . Strontium, oxygen, and sulfur isotopic compositions for whole-rock samples and selected secondary minerals with depth for Hole 504B.…”

“…It is the only hole to penetrate through the volcanic section, altered at low temperatures, into the underlying hydrothermally altered sheeted dike complex. The site has become a reference section for the petrology, geochemistry, hydrothermal alteration, and magnetic and physical properties of the upper oceanic crust (Anderson et al, 1982;Becker et al, 1989;Dick, Erzinger, Stokking, et al, 1991;Alt, Kinoshita, Stokking et al, 1993).…”