HR: 0800h
AN: T41D-1338    [Abstracts]
TI: Physical Properties of Samples Cored From Atlantis Oceanic Core Complex, Mid-Atlantic Ridge 30 N
AU: * Searle, R
EM: r.c.searle@durham.ac.uk
AF: Earth Sciences, Durham University, Durham, DH1 3LE United Kingdom
AU: Blackman, D
EM: dblackman@ucsd.edu
AF: IGPP, Scripps Institution of Oceanography, La Jolla, CA 92093-0225 United States
AU: Karner, G
EM: garry@ldeo.columbia.edu
AF: MGG, Lamont-Dohert Earth Observatory, Palisades, NY 10964-8000 United States
AU: Harris, A
EM: aharris@gso.uri.edu
AF: GSO, U. Rhode Island, Narragansett, RI 02882 United States
AU: Frost, R
EM: rfrost@uwyo.edu
AF: Geology/Geophysics, U. Wyoming, Laramie, WY 82071 United States
AB: IODP expedition 304/305 penetrated 1415 m into Atlantis Oceanic Core Complex, in 1.5 - 2.0 My crust, 12 km W of the Mid-Atlantic Ridge axis. Bulk magnetic susceptibility (MS), non-contact resistivity (NCR), P-wave velocity (Vp), bulk density, porosity and thermal properties were measured on recovered samples of peridotite, olivine-rich troctolite, olivine gabbro, gabbro, oxide gabbro, diabase and basalt. The most variable properties were MS and NCR, which were highly correlated, implying that the same minerals carry each signal, most likely Fe-Ti oxides such as magnetite and ilmenite and possibly minor sulfides. MS generally increased with iron content and decreased in intervals where magnetite had altered to ilmenite in diabase. High MS tends to concentrate in narrow bands and correlates with oxide- and sulfide-bearing gabbros and serpentinized zones (reflecting magnetite production during alteration). It exceeds 0.00001 SI in some oxide gabbros, equivalent to 8% magnetite by volume. MS is thus a valuable aid for mapping zones of oxide injection and serpentinization or other alteration and for stratigraphic correlation between holes. Core sample Vp is generally constant at about 5.5 km/s to 350 mbsf, increases to around 6.0 km/s at 450 mbsf and maintains this value to 750 mbsf, below which there is a steady decrease to about 5.8 km/s at 1200 mbsf, then a sharper decrease to 5.5 km/s at the bottom of the hole. The initial increase may be caused by closing cracks. The 10% decrease from 750 mbsf is a surprise: it may be real or perhaps due to overburden stress release. However, there is no corresponding reduction in bulk density with depth to indicate microscopic cracking during recovery. The final sharp decrease may reflect progressively increasing alteration. The largest local Vp variations are associated with massive olivine gabbros and troctolitic gabbros. A minimum between 300 and 350 mbsf reflects a zone of serpentinization. There is no significant seismic anisotropy. Bulk sample density is 2.93 +/- 0.1 g/cc with a general increase down hole except below 1230 mbsf where it is constant or slightly decreases downwards. High bulk densities correlate with oxide gabbros and some olivine-rich troctolites. Scatter tends to decrease with depth except for local zones of alternating lower density gabbros and higher density olivine-rich rocks. Porosity is generally low, though relatively higher, with low bulk density, in cataclastic deformation and fault zones. Velocities and densities are consistent with ODP Leg 209 results. Thermal conductivity is highest in olivine-rich troctolites and low in basalts and diabases. Combined with a measured bottom-hole temperature of ~120°C, these measurements help define a temperature structure consistent with a simple conductive cooling model for 2 Ma oceanic crust, apparently contrary to evidence of significant fluid flow in the upper part of hole 1309D.
DE: 3035 Midocean ridge processes
DE: 3036 Ocean drilling
DE: 5102 Acoustic properties
DE: 5109 Magnetic and electrical properties (0925)
DE: 5199 General or miscellaneous
SC: Tectonophysics [T]
MN: Fall Meeting 2005