Pb Systematics of Klyuchevskoy Volcano, KamChatka and North Pacific Sediments: Implications for Magma Genesis and Crustal Recycling in the Kamchatkan Arc

Abstract

The continuous interaction between a subducting slab, overlying mantle wedge and crustal lithosphere profoundly affects the chemical evolution of the Earth. In terms of chemical mass balance, the interactions between the subducting components (oceanic crust, sediment and fluid) and the overlying mantle wedge are far from understood and their precise role in the genesis of arc magmas much debated. Controversy surrounds: 1) whether or not oceanic sediments are subducted, and if so, whether they are melted and recycled into the arc crust via magmatism, or continue into the deep mantle, 2) whether the oceanic slab is partially melted and incorporated into the arc crust, and 3) the role and composition of metasomatic fluids resulting from both the dehydration of sediments and altered oceanic slab. Determination of the chemical fluxes in island arc systems is central to debates concerning global geochemical recycling, modeling island arc magma genesis, and ultimately understanding the differentiation of the mantle and generation of the crust. We have completed a Pb radiogenic isotope study on the volcanic rocks from Klyuchevskoy, Kamchatka as well as oceanic sediments collected near the Kamchatkan trench during Ocean Drilling Program, Leg 145. Klyuchevskoy volcano was chosen as it is well characterized and has erupted some of the most primitive basaltic magmas found in island arc settings thereby minimizing the effects of lithospheric involvement in magma generation. Continuously cored sediments collected parallel to the Kamchatkan trench provide the best analog for sediments previously subducted beneath the Kamchatkan arc. Klyuchevskoy is the world's most active volcano, producing primarily basaltic material from high-MgO (~ 12 wt%) to high-Al203 (~ 18 wt%) basalts (Kersting and Arculus, 1994). The samples chosen for this study span the entire chemical range of erupted lavas. The overwhelming majority of sediments recovered from Leg 145 were diatomaceous oozes. The majority of sediments analyzed in this study were oozes, although two claystone and two nannofossil chalk samples were also analyzed. The North Pacific sediments analyzed represent approximately 50 million years of sedimentary deposition from the Eocene through the Pliocene. The Pb isotope ratios of the Klyuchevskoy