Carbon and its isotopes in mid-oceanic basaltic glasses

Abstract

Three carbon components are evident in eleven analyzed mid-oceanic basalts: carbon on sample surfaces (resembling adsorbed gases, organic matter, or other non-magmatic carbon species acquired by the glasses subsequent to their eruption), mantle carbon dioxide in vesicles, and mantle carbon dissolved in the glasses. The combustion technique employed recovered only reduced sulfur, all of which appears to be indigenous to the glasses. The dissolved carbon concentration (measured in vesicle-free glass) increases with the eruption depth of the spreading ridge, and is consistent with earlier data which show that magma carbon solubility increases with pressure. The total glass carbon content (dissolved plus vesicular carbon) may be controlled by the depth of the shallowest ridge magma chamber. Carbon isotopic fractionation accompanies magma degassing; vesicle CO 2 is about 3.8‰ enriched in 13 C, relative to dissolved carbon. Despite this fractionation, δ 13 C PDB values for all spreading ridge glasses lie within the range −5.6 and −7.5, and the δ 13 C PDB of mantle carbon likely lies between −5 and −7. The carbon abundances and δ 13 C PDB values of Kilauea East Rift glasses apparently are influenced by the differentiation and movement of magma within that Hawaiian volcano. Using 3 He and carbon data for submarine hydrothermal fluids, the present-day mid-oceanic ridge mantle carbon flux is estimated very roughly to be about 1.0 × 10 13 g C/yr. Such a flux requires 8 Gyr to accumulate the earth's present crustal carbon inventory.