HR: 0830h
AN: V11E-0545    [PDF]
TI: Physical Properties of Gabbros and Serpentinized Peridotites at the MAR, $15\deg$N. Shipboard Results from ODP Leg 209
AU: * Ildefonse, B
EM: benoit@dstu.univ-montp2.fr
AF: CNRS - Lab. de Tectonophysique, ISTEEM, Universit\'{e} Montpellier 2, Montpellier cedex 5, 34095 France
AU: Carlson, R L
EM:
AF: Dpt of Geology & Geophysics, Texas A\&M University, College Station, TX 77843-3115 United States
AU: Garc\'{e}s, M
EM:
AF: Group of Geodynamics and Basin Analysis, Universitat de Barcelona, Facultat de Geologia, Campus de Pedralbes, Barcelona, 08028 Spain
AU: Gee, J S
EM:
AF: Scripps Institution of Oceanography, UCSD, Geosciences Research Division - 0220, La Jolla, CA 92093-0220 United States
AU: Iturrino, G J
EM:
AF: Lamont-Doherty Earth Observatory, Columbia University, Borehole Research Group, Route 9W, Palisades, NY 10964 United States
AB: In May-July 2003, ODP Leg 209 recovered gabbroic rocks and serpentinized peridotites at several sites along the Mid-Atlantic Ridge, close to the $15\deg$20' fracture zone. Gabbroic rocks are variously altered, generally evolved and oxide-rich. Peridotites and troctolites are strongly altered, and reveal various types and intensities of alteration. A series of physical properties were measured on board at ambient pressure and temperature, including bulk magnetic susceptibility, thermal conductivity, bulk and grain densities, porosity and compressional velocity. 235 values of thermal conductivity were measured on 92 samples, using the needle probe technique in the half-space mode. They are similar to values measured in gabbros and peridotites during previous ODP Legs. Gabbroic rocks and diabases range from 1.71 to 3.83 W/mK, with a mean of 2.16 W/mK. Serpentinized peridotites and troctolites range from 1.98 to 3.51 W/mK, with a mean of 2.64 W/mK, close to the serpentine and talc values. We measured an apparent anisotropy of the thermal conductivity by using different probe orientations on the split face of the half core. Estimated apparent anisotropies range from 0.06 to 12.60 %, with a mean of 3.92 %. In most samples, the thermal conductivity seems to be higher in the foliation plane. Bulk densities of gabbroic rocks and diabases range from 2.44 to 3.28 g/cm$^{3}$, with a mean of 3.00 g/cm$^{3}$. Such high densities reflect the abundance of oxides in many samples. In contrast, the densities of serpentinized peridotites and troctolites are very low, due to their strong alteration. They range from 2.26 to 2.90 g/cm$^{3}$, with a mean of 2.55 g/cm$^{3}$. Porosities are poorly constrained because of experimental limitations; they are high on average, in the order of 2 % for the gabbros, and 5 % for the peridotites. Except for one site, in which the late alteration is marked by abundant talc, the density of serpentinite samples is inversely correlated to their bulk magnetic susceptibility. The slope of the correlation is different at each site, and may be related to the type of hydrothermal alteration reactions. As expected in a highly altered environment, the P-wave velocities in serpentinized peridotites and troctolites are very low (2.82 to 5.14 km/s, with a mean of 3.88 km/s), and lower than in the gabbros and diabases (3.41 to 5.86 km/s, with a mean of 5.13 km/s). Most gabbros have slightly lower velocities than those sampled earlier at MARK (ODP Leg 153). Peridotites and troctolites are similar to those sampled earlier, or have lower densities and velocities. Our data are compared to seismic profiles (J.A. Collins \& R.S. Detrick, pers. com.) in the same region, and to velocity-depth profiles calculated for gabbros and variously serpentinized peridotites. The in-situ velocities are lower than the velocities in completely serpentinized rocks, suggesting that the uppermost crust contains large-scale fractures. The seismic velocities near the seafloor are consistent with a composite crust, composed of serpentinized peridotites and altered gabbros.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
DE: 5102 Acoustic properties
DE: 5109 Magnetic and electrical properties
DE: 5134 Thermal properties
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting