Françoise Pineau

Carbon and nitrogen isotopes in the mantle

Most of mantle carbon has δ13C between −5 and −8‰ in carbonatites, diamonds, MORB, deep crust and subduction zones. From this, it can acquire values from −2.5‰ in CO2 outgassed from subduction zones down to −32.5‰ in placer diamonds, either by contamination from sedimentary carbonate and organic matter or by outgassing effects. We presently think that the second mechanism plays a very important role. Mantle carbon fluxes are greater than 2.7·1013 g a−1, implying recycling of sedimentary carbon of the same mean isotopic composition. This recycling can be the cause of interferences in the two above-mentioned mechanisms. It insures a more or less constant carbon isotopic composition in the upper mantle. Results on nitrogen mantle are more scarce. Very negative δ15N [down to −12.5‰ (ATM)] are seen in diamonds, and also distinctly negative values [down to −6.4‰ (ATM)] in volcanic gases. Residual nitrogen in the depleted mantle is eventually enriched up to +20‰ by outgassing although this upper limit has to be further investigated. These results strongly favor a model of the Earth based on enstatite chondrite-type material, topped by a veneer of less than 2% of the upper-mantles mass of C1 or C2-type chondrites. This combined system explains the isotopic structure of the upper Earth (upper mantle, crust, sediments, hydrosphere and atmosphere). It implies that δ15N in the primary lower-mantle systems may be as low as −40‰.

Volatiles and Stable Isotopes in Recycling

The use of stable isotopes is pertinent to the study of recycling in several ways, among which the fact that oxygen is the main constituent of terrestrial materials, and the fact also that volatiles, which can be both outgassed from the mantle and recycled, are traced by three more stable isotope ratios, namely those of carbon, nitrogen and hydrogen. In addition to stable isotope ratios, some chemical ratios in the volatiles can be used to link their fluxes if they are not too much affected by the processes of dissolution and outgassing. We shall review in the next pages, first some of the isotope aspects, then some chemical characteristics of the fluids involved in outgassing of the mantle, and of those possibly recycled.