Carbon isotopes and concentrations in mid-oceanic ridge basalts

In order to estimate carbon fluxes at mid-ocean ridges and carbon isotopic compositions in the convective mantle, we have studied carbon concentrations and isotopic compositions in tholeiitic glasses from the FAMOUS zone (Mid-Atlantic Ridge at 360N) and East Pacific Rise from 210N (RITA zone) to 200S. These samples correspond essentially to the whole spectrum of spreading rates (2-16 cm/yr). The contain: -CO2 vesicles in various quantities (3-220 ppm C) with delta13C between -4 and -9per mille relative to PDB, in the range of carbonatites and diamonds. - Carbonate carbon (3-100 ppm C) with delta13C between -2.6 and -20.0per mille relative to PDB. - Dissolved carbon at a concentration of 170+-10 ppm under 250 bar pressure with delta13C from -9 to -21per mille relative to PDB. This dissolved carbon, not contained in large CO2 vesicles, corresponds to a variety of chemical forms among which part of the above carbonates, microscopic CO2 bubbles and graphite. The lightest portions of this dissolved carbon are extracted at low temperatures (400-6000C) whereas the CO2 from the vesicles is extracted near fusion temperature. These features can be explained by outgassing processes in two steps from the source region of the magma: (1) equilibrium outgassing before the second percolation threshold, where micron size bubbles are continuously reequilibrated with the magma; (2) distillation after the second percolation threshold when larger bubbles travel faster than magma concentrations to the surface. The second step may begin at different depths apparently related to the spreading rate, shallower for fast-spreading ridges than for slow-spreading ridges. (orig./WL)