CARBON-SOURCES IN ARC VOLCANISM, WITH IMPLICATIONS FOR THE CARBON-CYCLE

New CO2/He-3 data from the East Sunda Arc (Indonesia) confirm earlier observations that arc volcanic gases have higher CO2/He-3 ratios than MOR environments. On average, > 80% of arc volcanic CO2 is recycled, exogene carbon. Addition of a few percent of carbonate-bearing sediments to the mantle wedge explains much of the carbon abundance and carbon isotopic data of arc gases, but can not explain the He isotope observations. The CO2/He-3 in arc volcanoes is not strongly dependent on the composition of modern trough sediments (e.g. deep sea clays vs carbonate-rich sequences), and calcite veins in the hydrothermally altered subducted slab may provide a contribution to the recycled carbon flux of arcs. The sum of globally deep-subducted sediment and slab carbon exceeds the estimated arc CO2 flux, and approximately 3.5 teramole of carbon may return annually to the mantle in convergent zones. The modern combined processes of MOR volcanism, slab alteration, and subduction volcanism do not produce a substantial carbon flux into the exosphere, and rate-changes in ocean floor spreading are unlikely to cause major changes in atmospheric CO2 as a result of changes in the volcanic CO2 fluxes. Intense pulses of flood basalt volcanism, however, may alter the CO2 contents of the atmosphere over the course of a millenium or so, and influence global climate.