Oregon State University, United States

Deep subseafloor sediment in the South Pacific Gyre was recovered for the first time by Integrated Ocean Drilling Program (IODP) Expedition 329 (2010.10.10-2010.12.13). In the present study, the variations of clay minerals with increasing burial depth down to the contact with mid-ocean ridge basalt (MORB) in sites U1365 and U1369 were investigated with respect to diagenetic clay mineral process and chemical mineral reactions utilizing a combination of microscopy and spectroscopy. Site U1369 is on relatively young basalt (13.5 Ma) close to the oceanic ridge and Site U1365 is on much older basalt (84-120 Ma) distant from the ridge center. Major clay mineral components by X-ray diffractometer (XRD) were smectite, mica, and chlorite at U1369, whereas smectite is the dominant phase at U1365; this difference between these assemblages indicates that the predominant diagenetic processes depend on the location. The pore water chemistry as well as petrophysical properties measured on board were interpreted in terms of clay mineral formation processes. The formation of smectite may be associated with alteration of MORB. A smectite-like mineral with low crystallinity was also observed with metalliferous sediment in XRD pattern, suggesting that amorphous Fe/Mn-(oxyhydr)oxides may be the origin of the smectite formation. The illite peak mixed with smectite in U1369 may indicate diagenetic formation by hydrothermal fluid.