10:45 AM - 11:00 AM
[SCG49-12] Structural analyses of ultramafic rocks used by X-ray CT core imaging (Oman Drilling Project Phase 2, D/V CHIKYU)
Keywords:Oman, XCT, Structural Analyses, Serpentinization
The X-ray CT scanner (Discovery CT 750HD, GE Medical Systems) scans and reconstructs an image of a 1.4 m section in 10 minutes and produces a series of scan images, each 0.625 mm thick. The X-ray source and detector are installed inside of the gantry opposite each other. The excitation voltage and current for X-ray tube are 140kV and 100 mA, respectively.
Core samples are scanned in the gantry with a scanning rate of 20 mm/sec. The distribution of attenuation values mapped to an individual slice comprises the raw data that are used for subsequent image processing. Successive two-dimensional (2D) slices of 512 x 512 pixels yield a representation of attenuation values in three-dimensional (3D) voxels of 512 x 512 by ~1600 in length. Data generated for each core consist of core-axis-normal planes (XY planes) of X-ray attenuation values with dimensions of 512 × 512 pixels in 9 cm × 9 cm cross-section, with a resolution of 0.176 mm/pixel.
X-ray intensity varies as a function of X-ray path length and the linear attenuation coefficient (LAC) of the target material, which is a function of the chemical composition and density of the target material. The basic measure of attenuation, radiodensity, is the CT number given in Hounsfield units (HU). CT numbers of air and water are -1000 and 0, respectively.
CT numbers of OmanDP cores reveal structural features such as the attitude of foliation in primary peridotites, defined by spinel and orthopyroxene crystal shape. Combined with microstructural analyses such as EBSD mapping, we can study petrophysical properties of the uppermost mantle in the mantle section. Moreover, comparing with density profiles measured onboard, we can also estimate the degree of serpentinization in core from Holes BA1B, BA3A and BA4A, ranging from 80 to almost 100%. Profiles of the degree of serpentinization with depth may provide insight into ongoing serpentinization processes.