CO2 FLUXES FROM MIDOCEAN RIDGES, ARCS AND PLUMES

Estimates of CO2 emissions at spreading centres, convergent margins, a nd plumes have been reviewed and upgraded using observed CO2/He-3 rati os in magmatic volatiles, He-3 content estimates in the magmatic sourc es, and magma emplacement rates in the different tectonic settings. Th e effect of volatile fractionation during magma degassing, investigate d using new rare gas and CO2 abundances determined simultaneously for a suite of Mid-Ocean Ridge (MOR) basalt glasses, is not the major fact or controlling the spread of data, which mainly result from volatile h eterogeneity in the mantle source. The computed C flux at ridges (2.2 +/- 0.9) x 10(12) mol/a, is essentially similar to previous estimates based on a mon restricted data base. Variation of the C flux in the pa st can be simply scaled to that of spreading rate since the computed C depends mainly on the volatile content of the mantle source, which ca n be considered constant during the last 10(8) a. The flux of CO2 from arcs may be approximated using the CO2/He-3 ratios of volcanic gases at arcs and the magma emplacement rate, assuming that the He-3 content of the mantle end-member is that of the MORE source. The resulting fl ux is similar to 2.5 x 10(12) mol/a, with approx. 80% of carbon being derived from the subducting plate. The flux of CO2 from plumes, based on time-averaged magma production rates and on estimated contributions of geochemical sources to plume magmatism, is less than or equal to 3 x 10(12) mol/a. Significant enhancements of the CO2 flux from plumes might have occurred in the past during giant magma emplacements, depen ding on the duration of these events, although the time-integrated eff ect does not appear important. The global magmatic flux of CO2 into th e atmosphere and the hydrosphere is found to be 6 x 10(12) mol/a, with a range of (4-10) x 10(12) mol/a. Improvement on the precision of thi s estimate is linked to a better understanding of the volatile invento ry at arcs on one hand, and on the dynamics of plumes and their mantle source contribution on the other hand. (C) 1998 Elsevier Science B.V. All rights reserved.