Degassing of carbon dioxide from basaltic magma at spreading centers: II. mid-oceanic ridge basalts

Abstract

This study examines the hypothesis that a significant fraction of the CO2 in basalt supplied to mid-oceanic spreading centers escapes by degassing from magma chambers of the oceanic crust. The approach employs mass balance calculations, stepped-heating data for carbon in sea-floor mid-oceanic ridge basalts (MORBs), and the CO2 content determined in part I for transitional basalt supplied to the Erta'Ale spreading center in the Afar depression. It is also shown as part of the analysis that carbon data acquired for sea-floor MORBs by single-heating techniques give systematically high values that are difficult to reconcile with solubility data for CO2 in MORB liquid and the depths of subridge magma chambers. The results confirm the hypothesis. A conservative estimate of the average loss of CO2 during degassing from subridge magma chambers is between 30 and 65% of the CO2 initially present in the magma. The conservative estimate for CO2 degassing at depth is between two and seven times greater than the maximum amount of volcanic CO2 degassing from MORB. CO2 degassing from subridge magma chambers is sufficient to supply much (perhaps most) of the mantle carbon discharged from the global mid-oceanic spreading system to the oceans. CO2 degassing from subridge magma chambers has several important implications. It may be more important than hydrothermal stripping in supplying mantle carbon to mid-oceanic hydrothermal vents. It is an effective mechanism for removing most of the heavier rare gases (Ar, Kr, Xe, and Rn) and much of the He and Ne in magma supplied to mid-oceanic spreading centers and for transporting them and possibly other volatiles from subridge magma chambers to the ocean-atmosphere system. It should also be an effective mechanism for fractionating carbon isotopes in MORB.