Abstract
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 CO 2 liberated from vesicles (magmatic carbon) from 600 to 1200°C. The δ 13C 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 δ 13C of magmatic CO 2 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 CO 2 in back-arc basin basalts (BABB) is on average 5‰ lighter than equivalent CO 2 in MORB and can be explained by the mixing in the source regions for BABB magmas of juvenile (MORB-like) CO 2 with an organic carbon component from subducted pelagic sediments. It is inferred that significant amounts of pelagic carbonate carbon (δ 13C ⋍ 0‰) must be recycled into the mantle.
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