Gas hydrates have been reported to exist in marine sediments from various parts of the world ocean. The hydrates start decomposing soon after recovery of the sediments through coring operations due to changes in ambient pressure and temperature. This decomposition leads to changes in sedimentary structures, and thus complicates physical property related measurements of the sediments by conventional methods. In this study, we used a medical X-ray CT scanner to quickly scan the recovered cores, and then used raw data from the CT, and thus avoided image processing steps, to estimate porosity and density of the sediments. The raw data were in terms of CT numbers, which were obtained by drawing a circular region of interest (ROI) to cover most of the sediments visible in a cross section XCT image of the sediments. The data were weighted for relative contribution of liquid and solid in sediments before estimating porosity. On the other hand, density was estimated by using an average CT number that was automatically calculated by the Osirix software used for drawing the ROI on an XCT image, and by using a calibration equation based on a set of standards. Although some uncertainty in estimation of relative volumes of solid, liquid and gas could not be avoided, the results obtained by this new procedure were in good agreement with those obtained by conventional methods. Since porosity and density estimates by the new procedure can be made in a matter of minutes after core recovery, it can guide progress of coring operation and further processing of hydrate-bearing sediments.

We evaluated the mineral fractions of listvenite (completely carbonated peridotite) in Hole BT1B drilled by the ICDP Oman Drilling Project from 3D X-ray Computed Tomography (XCT) images. Total >250,000 XCT images from continuous ~200 m listvenite core samples were analyzed. Histograms of the intensity of X-ray attenuation (XCT number) of each XCT core-slice image were fitted assuming that the CT histogram is composed of magnesite, quartz, and dolomite peaks. These mineral peaks were confirmed by comparison of XCT numbers with chemical mapping data obtained using an XRF core scanner. In most core sections, XCT data indicate that listvenite matrix is composed of magnesite and quartz, consistent with discrete XRD and XRF data. Veins are composed mostly of dolomite. The mean abundance of dolomite in listvenite from BT1B is 11 vol.%, whereas that in core sections within 15 m of the basal thrust is >50 vol.%, suggesting that the basal thrust acted as a pathway for Ca- and CO fraction than that of Oman peridotite (39:60:1), indicating enrichment of Si during carbonation. P- and S-wave velocities and density of listvenite is close to that of peridotite while higher than that of serpentinites. These results suggest that limited material transfer during carbonation and hydration of the Oman Ophiolite, except for Si, Ca, CO2 and H2O, but potential as an overlooked carrier of CO2 into the deep of Earth’s interior.