Carbon isotopes in submarine basalts

High-sensitivity stepped extraction reveals two isotopically distinct forms of carbon in submarine basalt glasses: an isotopically light carbon component released by combustion from 200 to 600°C and an isotopically heavy CO₂ liberated from vesicles (magmatic carbon) from 600 to 1200°C. The δ¹³C_PDB of the low release temperature carbon varies from ?24 to ?30‰ and is believed to be surficial organic contamination. A survey of various types of oceanic glasses demonstrates that the δ¹³C of magmatic CO₂ varies from ?4.2 to ?7.5‰ in mid-ocean ridge basalt (MORB), from ?2.8 to ?6.7‰ in glasses from Hawaii and Explorer Seamount and from ?7.7 to ?16.3‰ in glasses from the Scotia Sea and Mariana Trough. Magmatic CO₂ in back-arc basin basalts (BABB) is on average 5‰ lighter than equivalent CO₂ in MORB and can be explained by the mixing in the source regions for BABB magmas of juvenile (MORB-like) CO₂ with an organic carbon component from subducted pelagic sediments. It is inferred that significant amounts of pelagic carbonate carbon (δ¹³C 0‰) must be recycled into the mantle.