CHe in volatile fluxes from the solid Earth: implications for carbon geodynamics

In order to picture C geodynamics past and present, theC³He ratios of the relevant reservoirs are considered. Evaluation of publishedC³He ratio in conjunction with new results for MORB glasses worldwide, suggests that this ratio is unfractionated during magma outgassing, a best estimate being 2 × 10⁹.C³He ratios from other volcanic emissions (hot spots and arcs) do not appear significantly different when the subducted component is omitted.This result permits scaling of the CO₂ degassing flux to that of³He and yields a value of 2 × 10¹² mol/yr which corresponds to a model degassing duration of 3.9 Gyr when recycling to the mantle is disregarded.A bulk Earth chondritic ratio of about 2 × 10⁹ is calculated, very close to the MORB value. On the other hand the reconstructed exospheric (“Rubey inventory”) value of4 ± 1 × 10⁷ is very different from both basaltic and chondritic values.Among the possible interpretations of these results the following two are retained: (1) CO₂ was not released in the early age of the Earth because of the reducing conditions prevailing at that time in the mantle. Formation of the core changed this picture and permitted subsequent degassing of CO₂. (2) Carbonates need a continental crust of significant size to become stabilized in the exosphere. Therefore accumulation in the exosphere was delayed until crustal formation.Alternatively, a similar degassing behaviour for both He and CO₂ requires a massive recycling of carbonates throughout time. This possibility is in contradiction with the present-day maximum recycling rate and the severe imbalance with the observed outgassing flux on one hand and with the small fraction of carbon now present in the exosphere on the other.We conclude that carbon has never been severely degassed. The mantle acts as a buffer for C and most carbon is still retained there, possibly as graphite (or diamond?) or dissolved in minerals.