Properties of Smectite in the Kumano Basin, Japan: New Results from NanTroSEIZE Expedition 319

Smectite clays are common in the sediments of the Kumano basin, offshore Japan, but their properties and the role they may play in influencing the mechanical and hydrological behavior of sediments and the splay-fault system is not well understood. In this study, samples from NanTroSEIZE Expedition 319 and 316 were studied for their mineralogical characteristics, hydration behavior and textural properties. We focused on the fabric development and swelling behavior of smectite at ~1500 mbsf, in order to evaluate the occurrence and intake of water in smectite, and its relation to digenesis and fault weakening processes. Rock chips were measured by X-ray texture goniometry (XTG), revealing an overall weak fabric that changes to a slightly stronger fabric with depth and a weak fabric at the splay-fault. The clay-size fraction (< 2 micron) of the sediments, as measured by X-ray diffraction (XRD), shows smectite, illite-smectite and illite together with chlorite, quartz, and calcite. The air-dried sample shows a smectitic phase with a characteristic interlayer distance (d-value) of 1.3 nm that increases to 1.8 nm after classic ethylene glycolization, suggesting the presence of 1-3 water layers in the interlayer. In addition, novel temperature/humidity measurements were conducted under controlled conditions, with temperatures at 25°C, 50°C, 75°C, and 95°C, and humidity ranging from 10% RH (relative humidity) to 98% RH. The x-ray patterns show no breakdown of smectite due to hydration and/or heating. With increasing humidity under isothermal conditions, the hydration of interlayer cations and the particle orientation increases, whereas under stable humidity the intensities decrease with increasing temperature. At 25°C and with increasing humidity, illite-smectite changes from 1.1 to 1.3 nm, whereas smectite changes from 1.25 to 1.55nm (1-2 water layers). The increase is less pronounced at 50°C and 75°C, with changes from 1.25 to 1.45nm (1-2 water layers). At all temperatures, the patterns show a maximum of 2 water layers, in contrast to the ethylene glycol treated sample with up to 3 water layers, indicating additional change in the interlayer structure during chemical treatments. All hydration measurements are fully reversible. Preliminary high-resolution TEM analysis of impregnated rock chips from the same depth confirms the presence of two water layers at all depths. Based on our experiments we conclude that i) the fabric of smectite increases slightly with depth, reflecting compaction, ii) swelling is not significantly affected by increasing temperature, iii) preferred orientation increases with increasing humidity, suggesting higher mobility of the fundamental smectite particles cause by hydration of interlayer cations. This characterization of the hydration state and associated fabric development of smectite in natural conditions is critical for our understanding of clay-fluid interaction and mechanical properties during fault displacements and seismogenesis.