Carbon and water have been recycled over geologic time through subduction of oceanic crust plus sediments, possibly in significant quantities to the deep mantle. In contrast, helium isotope systematics of oceanic rocks support the hypothesis of mantle plumes rising from regions deep in the Earth which are relatively undegassed of their primordial volatile inventory. The extent to which major volatiles released during volcanism, such as carbon dioxide and water, are juvenile rather than recycled by plate tectonics is therefore a fundamental question in geochemistry. The geodynamic cycle of helium is closely linked to that of carbon. There are systematic geographic variations in 3He/4He ratio along the mid-ocean ridge system, but their exact cause is not always well understood. In some cases they may be tied to localized upwelling of high 3He/4He material from the deep mantle. In other cases they may be due to differences in the addition of 4He from the radioactive decay of U and Th. In principle, an accurate knowledge of variations in the carbon dioxide/helium ration cna be used to distinguish between these possibilities, but the full development of such an approach depends on a better definition of fine-scale variations in the mantle CO2/He ratio. The purpose of this study is to accurately determine the 3He/4He and CO2/He rations for upper mantle regions beneath the Atlantic, Pacific and Indian Ocean from measurements on mid-ocean ridge basalts.
