The investigation of magnetic properties at Site 1117 included the measurement of point susceptibilities and remanent magnetization of archive sections from Core 180-1117A-1R, and the measurement of magnetic susceptibility and its anisotropy, acquisition of isothermal remanent magnetization (IRM), decay of anhysteretic remanent magnetization (ARM), and remanent magnetization of discrete samples.
Discrete samples measured were as follows: three samples from Core 180-1117A-1R (metamorphic-derived material interpreted as fault gouge; see "Hole 1117A"); one sample from Section 1117A-2R-CC (a clast from unconsolidated metamorphic material); one sample from interval 180-1117A-9R-1 (Piece 17) (epidote breccia); and one sample from interval 180-1117A-11R-1 (Piece 7) (gabbro; see "Igneous and Metamorphic Petrology").
Measurements of remanent magnetization were made on discrete samples taken from working half core sections (Fig. F17) and on archive sections from Core 180-1117A-1R (Fig. F18).
Initial natural remanent magnetization (NRM) intensities of discrete samples were on the order of 10-2 A·m-1 to 10-1 A·m-1. Demagnetization behavior of discrete samples from Cores 180-1117A-1R and 2R was erratic with no stable end-point reached (Fig. F17A, F17B). However, demagnetization behavior of samples from Cores 180-1117A-9R and 11R showed a linear decay toward the origin between ~20 and 50 mT on vector plots (Fig. F17C, F17D). Discrete sample data from Core 180-1117A-1R did not generally agree with the long core data (Fig. F18A, F18B).
Intensity of remanent magnetization of sections from Core 180-1117A-1R after AF demagnetization at 20 and 25 mT was on the order of 10-3A·m-1 (Fig. F18A, F18B); inclinations and declinations are shown but are considered unreliable because of the erratic demagnetization behavior of discrete samples. No attempt was made to interpret the data in terms of the magnetic polarity time scale.
Magnetic susceptibility measurements were made on archive sections of Core 180-1117A-1R as part of the archive multisensor track (AMST) analysis. The AMST susceptibilities (uncorrected for volume) were on the order of 10-3 SI (Fig. F19).
Mean susceptibility values and magnetic anisotropy parameters of discrete samples are listed in Table T6. A plot of the shape parameter (T) vs. the degree of anisotropy (Pj) is shown in Figure F20A. All samples showed T values greater than 0, which indicated an oblate magnetic fabric; samples from Core 180-1117A-1R, which recovered only fault gouge (see "Hole 1117A"), showed a higher degree of anisotropy than the other samples.
Experiments on the acquisition of IRM and the demagnetization of ARM were conducted on all six discrete samples. The ARMs were imparted to samples using a 0.05 mT DC field in an AC peak field of 100 mT.
All samples were saturated by 0.2 T during experiments of the acquisition of IRM (Fig. F20B); this suggested the presence of only magnetite in the measured samples. The sample from Section 180-1117A-11R-1 was most resistant to AF demagnetization of ARM, while the samples from Cores 180-1117A-2R and 9R were least resistant to AF demagnetization of ARM (Fig. F20C). Behavior during AF demagnetization of ARM suggested a finer grain size of magnetite in the sample from Core 180-1117A-11R, which was from a gabbro (see "Hole 1117A"), than in the fault gouge samples (Core 180-1117A-1R). The curves for the two samples of metamorphic clasts from Cores 180-1117A-2R and 9R reflected the largest magnetic grain size.