24. Pb ISOTOPE RATIOS OF NORTH PACIFIC SEDIMENTS, SITES 881,883, AND 884: IMPLICATIONS FOR SEDIMENT RECYCLING IN THE KAMCHATKAN ARC

The Pb isotope ratios of 13 sediment samples, collected during Ocean Drilling Program Leg 145 at Sites 881, 883, and 884 in the North Pacific east of the Kamchatkan trench, were determined. The sediments were selected from continuously recovered cores and are representative of the range of sediment compositions recorded. The sediments analyzed are predominantly pelagic oozes, representing approximately 50 m.y. of depositional history, and have an age span from Pliocene to Eocene. Pb isotope covariations of all the samples are more radiogenic than Pacific mid-ocean ridge basalt. These sediments provide the best analog for previously subducted sediments beneath the Kamchatkan arc. Previously analyzed basalts from Klyuchevskoy volcano, Kamchatka, have Pb isotope ratios that fall within the MORB field. The sediments analyzed in this study are significantly elevated in their Pb isotope composition to preclude their addition to the source of the Klyuchevskoy volcano. Sediments or sediment-derived fluids are not involved in the generation of Klyuchevskoy magmas, and thus are not required for arc magmagenesis.