Models of hydrothermal circulation within 106 Ma seafloor : constraints on the vigor of fluid circulation and crustal properties, below the Madeira Abyssal Plain

Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 6 (2005): Q11001, doi:10.1029/2005GC001013.Heat flow measurements colocated with seismic data across 106 Ma seafloor of the Madeira Abyssal Plain (MAP) reveal variations in seafloor heat flow of ±10–20% that are positively correlated with basement relief buried below thick sediments. Conductive finite element models of sediments and upper basement using reasonable thermal properties are capable of generating the observed positive correlation between basement relief and seafloor heat flow, but with variability of just ±4–8%. Conductive simulations using a high Nusselt number (Nu) proxy for vigorous local convection suggest that Nu = 2–10 within the upper 600–100 m of basement, respectively, is sufficient to achieve a reasonable match to observations. These Nu values are much lower than those inferred on younger ridge flanks where greater thermal homogeneity is achieved in upper basement. Fully coupled simulations suggest that permeability below the MAP is on the order of 10−12–10−10 m2 within the upper 300–600 m of basement. This permeability range is broadly consistent with values determined by single-hole experiments and from modeling studies at other (mostly younger) sites. We infer that the reduction in basement permeability with age that is thought to occur within younger seafloor may slow considerably within older seafloor, helping hydrothermal convection to continue as plates age.Funding in support of this work was provided by the U.S. National Science Foundation (OCE-0001892), the U.S. Science Support Program for IODP (T301A7), and the Institute for Geophysics and Planetary Physics/Los Alamos National Laboratory (1317)

Deep-sea corehead camera photography and piston coring

Cameras were mounted in a newly designed corehead of a piston corer and used to photograph coring operations during 36 stations on CHAIN cruise 75 and 28 stations on ATLANTIS II cruise 42. Through the analysis of these photographs, the deep-water operation of a piston corer during its descent, tripping, impact with the bottom, and ascent has been studied, providing information on the corer's stability, effectiveness in obtaining a bottom sample, and influence on the nearby sea-floor. Accurate determinations of the amount of penetration were possible, allowing comparisons to be made with the more indirect methods of determining penetration and with the length of core recovered. Sediment clouds produced by bottom currents were noticed in many of the bottom photographs. A number of suggestions are made for future piston coring operations. The corer descends with little rotation and swinging. Free-fall and penetration generally take place in less than 5 seconds, with a rotation of 20-60° and an increase of about 6° in vertical deviation. During penetration, the corer disturbs the surrounding sea floor, producing both mounds and depressions around the core barrels. While resting in the bottom, the corer is very stable although some wobbling does occur. Considerable rotation takes place during both pull-out and ascent; frequent sediment discharges from the piston corer occur. No consistent relationship was found between the amount of penetration and the length of core recovered, and thus with the degree of core shortening. Comparisons between piston and pilot cores indicate that the piston cores have been shortened and disturbed relative to the pilot cores, and that as much as a meter of the upper portion of the piston core has been lost. The position of the mud-mark appears to be a reliable indicator of the amount of penetration; estimates by extrapolation of the thermal gradient to the surface are less reliable. The vertical deviation of the corer in the bottom does not influence the amount of penetration. Stratigraphic dips in the recovered cores correspond poorly to this vertical deviation in the bottom.The National Science Foundation Grants GA-1077 and GA-1209 and submitted to the Office of Naval Research under Contract Nonr-4029(00); NR 260-101, and N00014-66-C0241; NR 083- 004

Ocean Drilling Program Scientific Results Volume 118

ABSTRACT The sedimentology of gravel intervals was studied in three cores recovered at Site 734 at a water depth of -3700 m on the eastern wall of the Atlantis II Fracture Zone. At two holes, coring recovered single beds <l m thick that grade upward from moderately well-sorted igneous gravel to foraminiferal ooze. The origin of the stratigraphy in the cores is problematic, but sedimentological arguments are used to infer that the recovered core is intact. The sediments are tentatively interpreted as turbidity current deposits that originated from a nearby cliff face. Lithology, particle size, and particle shape suggest that mass-wasting of the cliff face was influenced as much by hydrothermal alteration and brecciation at depth as jointing and fracturing of the cliff face caused by stress release

Deep crustal geothermal measurements, hole 504B, Costa Rica Rift

We report an extensive suite of geothermal measurements in the deepest borehole yet drilled into the oceanic crust, hole 504B of the Deep Sea Drilling Project. Located in 6.2‐m.y.‐old crust of the Costa Rica Rift, hole 504B was cored during legs 69 and 70 in late 1979 and leg 83 in late 1981, to a total depth of 1350 m beneath the seafloor, through 274.5 m of sediment and 1075.5 m of basalt. During the three drilling legs, downhole temperatures were logged 11 times, and the thermal conductivities of 239 sediment and basalt samples were measured. The results indicate a dominantly conductive mode of heat transfer through the complete section, at 190±10 mW/m2. This is consistent with the predicted plate heat transfer and the hypothesis that the thick sediment cover acts as a seal against hydrothermal circulation of seawater to basement. For over 2 years after this sediment seal was penetrated, borehole temperatures were nearly isothermal to about 350–370 m, indicating that ocean bottom water was flowing down the hole into the upper ∼100 m of basement. This downhole flow was driven by the underpressure of the basement pore fluids, which is of indefinite, but possibly hydrothermal, origin (Anderson and Zoback, 1982). The flow rate decreased from 6000–7000 1/h in late 1979 to about 1500 1/h 2 years later; altogether over 50×106 kg of seawater has been drawn into the basement. We estimate a permeability of ≳6×10−14 m2 for the reservoir in the upper ∼100 m of basement. This zone seems to correspond to a layer of high apparent porosity (Becker et al., 1982), which has been tentatively identified as a thin layer 2A (Anderson et al., 1982a)