CO2 FLUXES FROM MID-OCEAN RIDGES, ARCS AND PLUMES

Abstract

Estimates of CO2 emissions at spreading centres, convergent margins and plumes have been reviewed and upgraded using observed CO2/3He ratios in magmatic volatiles, 3He content estimates in the magmatic sources, and magma emplacement rates in the different tectonic settings. The effect of volatile fractionation during magma degassing, investigated using new rare gas and CO2 abundances determined simultaneously for a suite of Mid-Ocean Rdige (MOR) basalt glasses, is not the major factor controlling the spread of data, which mainly result from volatile heterogeneity in the mantle source. The computed C flux at ridges (2.2 ± 0.9) x 1012 mol/a, is essentially similar to previous estimates based on a more restricted database. Variation of the C flux in the past can be simply scaled to that of spreading rate since the computed C depends mainly on the volatile content of the mantle source, which can be considered constant during the last 108 a. The flux of CO2 from arcs may be approximated using the CO2/3He ratios of volcanic gases at arcs and the magma emplacement rate, assuming that the 3He content of the mantle end-member is that of the MORB source. The resulting flux is ~ 2.5 x 1012 mol/a. Significant enhancements of the CO2 flux from plumes might have occurred in the past during giant magma emplacements, depending on the duration of these events, although the time-integrated effect does not appear important. The global magmatic flux of CO2 into the atmosphere and the hydrosphere is found to be 6 x 1012 mol/a, with a range of (4-10) x 1012 mol/a. Improvement on the precision of this estimate is linked to a better understanding of the volatile inventory at arcs on one hand, and on the dynamics of plumes and their mantle source contribution on the other hand.