HR: 1340h
AN: T53B-1303    [Abstracts]
TI: Compressional-Wave Velocities of Discrete Samples at In-Situ Pressures From IODP Hole U1309D
AU: * Willson, S
AF: Texas A&M University, Department of Geology & Geophysics, College Station, TX 77843,
AU: Harris, A C
EM: aharris@gso.uri.edu
AF: University of Rhode Island, Graduate School of Oceanography, S Ferry Rd, Narragansett, RI 02882,
AU: Carlson, R L
AF: Texas A&M University, Department of Geology & Geophysics, College Station, TX 77843,
AU: Blackman, D K
AF: Scripps Institution of Oceanography, University of California, La Jolla, CA 92093,
AB: One of the exciting objectives of IODP Expeditions 304 and 305 was to recover a seismically fast section of rock within the Atlantis Massif on the Mid-Atlantic ridge 30°N. Initial refraction analyses suggest that velocities greater than 7.5 km/s, which would correspond to essentially fresh peridotite, occur within the upper km at this site. After drilling 1.4 km at Hole U1309D and measuring compressional-wave velocities of individual samples shipboard, no single sample measurement was found to be greater than 6.8 km/s. P-wave velocities in individual samples may not accurately represent regional velocity structure because of the effect of increasing pressure down-hole, which was not accounted for in shipboard measurements. Sonic logging data do measure velocities at in-situ pressure but hole conditions can affect these data and in Hole U1309D they were obtained only to 800 mbsf. To compensate for these deficiencies we measured discrete sample velocities at elevated pressures. Two intervals (300 - 400 mbsf and 700 - 900 mbsf) were selected, based on available logging data and petrologic analysis, as likely intervals for a rapid change in physical properties. Samples from these intervals (~ 30 from 300-400 mbsf and 70 from 700-900 mbsf) were pressurized and velocity measurements were recorded at regular pressure increments up to 200 MPa. Pressure down-hole is expected to increase at a rate between 20 and 30 MPa/km, depending on the pore pressure. Velocities of pressurized samples from Hole U1309D match closely with pressurized velocity measurements from ODP Hole 735B, which sampled similar lithology (gabbro). In-situ pressure velocities are higher than shipboard lab-pressure measurements by ~ 1 km/s (300-400 mbsf interval: min 5.8 km/s; max 7.5 km/s, ave 6.7 km/s. 700-900 mbsf interval: min 6.5 km/s, max 7.5 km/s, ave 7.0 km/s). Even including the effect of increasing pressure down-hole, we do not find evidence to support a high velocity (greater than 7.5 km/s) reflective layer on the scale of core samples in either interval. This suggests that regional reflectivity must come from another source, such as larger scale changes in porosity and/or alteration.
DE: 1859 Rocks: physical properties
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
SC: Tectonophysics [T]
MN: 2007 Fall Meeting