Tohoku University, Japan

We report on the Sr isotopic composition of pore fluids recovered during Integrated Ocean Drilling Program Expeditions 322 and 333 at Sites C0011 and C0012, drilled on the Philippine Sea plate before subduction along the Nankai Trough. Acidified pore fluids were directly loaded into EICHROM Sr-specific columns, and isotopic analysis was performed using a Nu multicollector inductively coupled mass spectrometer. At both Sites C0011 and C0012, (super 87) Sr/ (super 86) Sr decreases with depth from the seawater value to approximately 0.7069 at a depth that corresponds to the bottom of lithologic Unit I, which consists of ash-bearing hemipelagic sediments. This decrease is consistent with the reported observation of increasing ash alteration with depth within Subunit IB. Pore fluids recovered from lithologic Units II-IV show a relatively constant (super 87) Sr/ (super 86) Sr ratio, which ranges between 0.7069 and 0.7065 at both sites. The volcaniclastic turbidites of Unit V and the red clay (Unit IV) directly overlying the basement were only recovered at Site C0012. The (super 87) Sr/ (super 86) Sr ratio below Unit IV shows a marked decrease, reaching the lowest value of 0.704367 in the deepest pore fluids recovered (529 m below seafloor), suggesting a diffusional communication with an (super 87) Sr-depleted fluid in the igneous upper crust.