The major volatile elements of the Earth: Their origin, behavior, and fate

Abstract

Stable isotope constraints impose a heterogeneous accretion of the earth's volatiles: the primary, reduced material, 99.8% of the planet, brought 6% of the hydrogen, 48% of the carbon and 70% of the nitrogen of the Upper Earth (upper mantle, crust, ocean‐atmosphere). A late veneer (1.4 per mil of the bulk Earth's mass), of CI composition, supplied 94% of the hydrogen, 52% of the carbon and 30% of the nitrogen.The present distribution of the major volatiles in the Upper Earth corresponds to a negligible supply from the lower mantle. Measured in units of the superficial reservoirs the upper mantle contains: 0.21 ocean masses of water, ∼10 atmosphere masses of nitrogen, 5.4 crustal carbon masses of carbon.This corresponds to a very significant trapping of carbon and nitrogen by the mantle during the Earth's history, whereas water repartition stayed about unchanged and strongly favoured the superficial reservoir. There is, however, a very significant difference between carbon and nitrogen, the present outgassing and subducting fluxes of carbon being equilibrated both in masses and isotopically, whereas there is still a gross isotopic disequilibrium for nitrogen, the large, isotopically heavy, subducted flux, still outweighing the small, isotopically light, outgassed flux.This is due to the fact that, while carbon and water are essentially incompatible components, nitrogen is a compatible element because of the presence of very stable mantle nitrides (probably osbornite).