Lithium Budget and Isotopic Characterization of Materials Entering the Izu-Mariana Subduction Zone

We present lithium-isotopic data for alteration products entering the Izu-Mariana subduction zone, as part of a larger study of the Li flux balance across the margin. Sediments and altered basaltic crust samples were collected during ODP Leg 185 at Site 801C, Pigafetta Basin outboard of the Mariana arc, and Site 1149, Nadezhda Basin outboard of the Izu arc. Dramatic variability between the two sites has been observed in sediment lithology and basement petrology, architecture and alteration styles. This study will address the extent to which these factors affect the along-strike heterogeneity of the Li-content and isotopic ratios of subducting material. A small, preliminary, data set shows that whole rock samples (altered alkali and tholeiitic basalt, altered basalt rich in smectite, or in early or late stage celadonites etc.) are characterized by Li concentrations ranging from 8 ppm to greater than 30 ppm and δ ⁶Li values of -3‰ to -14‰ . Many weathered samples appear to fall on a mixing line first observed by Chan et al. (EPSL, 1992, 108, 151-160) between fresh MORB and authengenic phyllosilicates in equilibrium with seawater. Deviations from this trend are observed for alkalic basalts and late stage celadonitic samples. The alkalic basalts may possess greater initial Li concentrations than tholeiitic basalts, but the intrusive nature of the alkali basalts into sediments must contribute to their enhanced Li content and elevated δ ⁶Li values. Preliminary results suggest that early celadonites are characterized by Li-systematics similar to smectite, while low Li-concentrations and extreme isotopic ratios characterize late-stage celadonites. δ ⁶Li depletions may be the result of higher temperature fluids, but considered in light of low Li concentrations we suggest that this mineral phase formed in equilibrium with previously reacted, relatively low-temperature seawater component.