Paper No. 226-1
Presentation Time: 1:30 PM-1:50 PM
SECULAR DECREASE IN ATMOSPHERE d15N: ATMOSPHERE ORIGIN AND CRUSTAL CYCLING
JIA, Yiefei, Research School of Earth Sciences, Australian Univ of Canberra, Canberra, ACT 0200, Australia, yiefei.jia@anu.edu.au and KERRICH, Robert, University of Saskatchewan, 114 Science Pl, Saskatoon, SK S7N 5E2, Canada, robert.kerrich@usask.ca

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.

2002 Denver Annual Meeting (October 27-30, 2002)
Session No. 226
Evolution of the Early Atmosphere, Hydrosphere, and Biosphere II: Constraints from Ore Deposits
Colorado Convention Center: Ballroom 4
1:30 PM-5:30 PM, Wednesday, October 30, 2002
 

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