CO 2 fluxes from mid-ocean ridges, arcs and plumes

Abstract

Estimates of CO 2 emissions at spreading centres, convergent margins, and plumes have been reviewed and upgraded using observed CO 2/ 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 CO 2 abundances determined simultaneously for a suite of Mid-Ocean Ridge (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)×10 12 mol/a, is essentially similar to previous estimates based on a more restricted data base. 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 10 8 a. The flux of CO 2 from arcs may be approximated using the CO 2/ 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 ∼2.5×10 12 mol/a, with approx. 80% of carbon being derived from the subducting plate. The flux of CO 2 from plumes, based on time-averaged magma production rates and on estimated contributions of geochemical sources to plume magmatism, is ≤3×10 12 mol/a. Significant enhancements of the CO 2 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 CO 2 into the atmosphere and the hydrosphere is found to be 6×10 12 mol/a, with a range of (4–10)×10 12 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.