Carbon and nitrogen isotopes in the mantle

Abstract Most of mantle carbon has δ 13 C between −5 and −8‰ in carbonatites, diamonds, MORB, deep crust and subduction zones. From this, it can acquire values from −2.5‰ in CO 2 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 2.7·10 13 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 δ 15 N [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-mantle's mass of C 1 or C 2 -type chondrites. This combined system explains the isotopic structure of the upper Earth (upper mantle, crust, sediments, hydrosphere and atmosphere). It implies that δ 15 N in the primary lower-mantle systems may be as low as −40‰.