SECULAR DECREASE IN ATMOSPHERE d15N: ATMOSPHERE ORIGIN AND CRUSTAL CYCLING

New N isotope data demonstrate a systematically ¹⁵N-enriched Archean crust. Kerogen separated from greenschist facies metasedimentary rocks, and carbonaceous schists, from 2.7 Ga greenstone belts both have d¹⁵N of 15 to 24 per mil. K micas in 2.7 Ga structurally hosted orogenic gold deposits, precipitated from hydrothermal fluids that sampled bulk continental crust, span 14 to 22 per mil. Proterozoic counterparts are characterized respectively by d¹⁵N=+9.5 ± 2.4 per mil, and 11.2 ± 2.7 per mil. Phanerozoic continental crust has d¹⁵N of 2 to 6 per mil, and K micas in orogenic gold deposits are also within this range, endorsing their use as proxies of bulk crust in the Archean. Nitrogen contents of metasedimentary rocks and hydrothermal K micas increase from a few 10's ppm in the Archean, through 103 ± 91 ppm for Paleoproterozoic examples, up to several 1,000 ppm for Phanerozoic sedimentary rocks and K micas.

Modern atmospheric N₂, defined to be 0 per mil, is sequestered by microorganisms to form organic N compounds having an average d¹⁵N of - 4 per mil. After maturation N is incorporated into sediments as kerogen with an average d¹⁵N of + 4 per mil. During diagenesis and metamorphism some N is transferred to the crustal silicate reservoir where N as NH₄ substitutes for K, also of about 4 per mil. Accordingly, the atmosphere crust fractionation is - 4 per mil. If similar processes were operating in the Archean, then the atmosphere at 2.7 Ga would have been 11 to 20 per mil. The secular trends of decreasing d¹⁵N but increasing N-content are interpreted as progressive transfer of N from the atmosphere to the crust. These results support the model for acquisition of the atmosphere from late accretion of a veneer of C1 carbonaceous chondrites and comets where (d¹⁵N=+30 to +42) (Javoy), ruling out the model of mantle degassing d¹⁵N=-5 and hydrodynamic fractionation (Marty). The mass of N in the initial atmosphere is estimated at 13 × 10¹⁸ kg and in the present atmosphere is 3.8 × 10¹⁸ kg. Given 2.1 × 10 ¹⁸ kg N in the continental crust, large quantities of continental crust must have been recycled into the mantle to account for isotopic and chemical mass balance.