GOLDSCHMIDT CONFERENCE TOULOUSE 1998 Rhenium-osmium isotopic investigation in subduction zone lavas

The Re-Os isotopic system provides a unique perspective on the origin of arc lavas. Due to the behaviour of these elements during melting, MORB (i.e. oceanic crust) have significantly higher Re/Os ratios than their source, and will rapidly develop highly radiogenic Os isotopic ratios. Recycling such radiogenic material will thus increase the Os isotopic ratios of island arc mantle sources. We measured Os isotopic ratios and Re and Os concentrations on 13 volcanic rocks from West Java island (Sunda island arc). The samples, from 2 to 9.5 Ma old (Marcoux and Mil~si, 1994), are calc-alkaline rhyolitic to andesitic rocks and dacitic ignimbrites. The Os concentrations of the samples are extremely variable, ranging from 0.10 to 17.18 ppt; Re concentrations range from 26 to 1617 ppt. Several analyses of some samples revealed that the Os content can also vary significantly within a single rock. An extremely large variation in the osmium isotopic composition is observed for the whole sample set and also within some single rocks, with 1870S/1880S ratio ranging from 0.241 to 3.704. The ratios are still very radiogenic when corrected for sample ages (up to 3.146) and significantly higher than in both MORB and OIB (e.g. Schiano, et al