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Secular variations of N-isotopes in terrestrial reservoirs and ore deposits

R. Kerrich, Y. Jia, C. Manikyamba and S. M. Naqvi
Secular variations of N-isotopes in terrestrial reservoirs and ore deposits (in Evolution of early Earth's atmosphere, hydrosphere, and biosphere; constraints from ore deposits, Stephen E. Kesler (editor) and Hiroshi Ohmoto (editor))
Memoir - Geological Society of America (2006) 198: 81-104


New delta (super 15) N analyses combined with a literature compilation reveal that shale kerogen, VMS-micas, and late-metamorphic vein micas show a secular trend from enriched values in the Archean, through intermediate values in Proterozoic terranes, to the Phanerozoic mode of 3 per mil-4 per mil. Kerogen in metashales from the 2.7 Ga Sandur Greenstone Belt, eastern Dharwar Craton, India, is characterized by delta (super 15) N 13.1 per mil+ or -1.3 per mil, and C/N 303+ or -93. A second population has delta (super 15) N 3.5 per mil+ or -0.9 per mil, and C/N 8+ or -0.4, close to the Redfield ratio of modern microorganisms, and is interpreted as precipitates of Proterozoic or Phanerozoic oilfield brines that penetrated the Archean basement. Kerogen from 1.7 Ga carbonaceous shales of the Cuddapah Basin average 5.0 per mil+ or -1.2 per mil, close to the mode at 3 per mil-4 per mil for kerogen and bulk rock of Phanerozoic sediments. Biotites from late-metamorphic quartz-vein systems of the 2.6 Ga Kolar gold province, E. Dharwar Craton, that proxy for average crust, are also enriched at 14 per mil-21 per mil for three samples, confirming that the N-budget of the hydrothermal fluids is dominated by sedimentary rocks. Muscovites from altered volcanic rocks in 2.7 Ga Abitibi belt VMS deposits have delta (super 15) N 12 per mil-20 per mil, in keeping with published data for shale kerogen from the same terrane, whereas equivalents in the 1.8 Ga Jerome VMS span 11.7 per mil-14.1 per mil. (super 15) N-enriched values in Precambrian rocks cannot be caused by N-isotopic shifts due to metamorphism or Rayleigh fractionation because (1) pre-, and post-metamorphic samples from the same terrane are both enriched in (super 15) N; (2) there is no covariation of delta (super 15) N with N, C/N ratios, or metamorphic grade; and (3) the magnitude of fractionations of 1 per mil (greenschist) to 3 per mil (amphibolite facies) during progressive metamorphism of sedimentary rocks, as constrained from empirical observations and experimental studies, is very small. Nor can (super 15) N-enriched values stem from long-term preferential diffusional loss of (super 14) N, as samples were selected from terranes where (super 40) Ar/ (super 39) Ar ages are within a few million years of concordant U-Pb ages; nitrogen is structurally bound in micas, whereas Ar is not. It is possible that the (super 15) N-enriched values stem from a different N-cycle in the Archean, with large biologically mediated fractionations, yet the magnitude of the fractionations between atmospheric N (sub 2) and organic nitrogen observed exceeds any presently known, and chemoautotrophic communities tend to depleted values. Earlier results on Archean cherts show a range of delta (super 15) N from -6 per mil to 30 per mil. Given the temporal association of chert-banded iron formation (BIF) with mantle plumes, the range is consistent with mixing between mantle N (sub 2) of -5 per mil and the (super 15) N-enriched marine reservoir identified in this study. The (super 15) N-enriched Archean atmosphere-hydrosphere reservoir does not robustly constrain Archean redox-state. We attribute the (super 15) N-enriched reservoir to a secondary atmosphere derived from CI-chondrite-like material and comets with delta (super 15) N of +30 per mil to +42 per mil. Shifts of delta (super 15) N to its present atmospheric value of 0 per mil can be accounted for by a combination of early growth of the continents with sequestration of atmospheric N (sub 2) into crustal rocks, and degassing of mantle N approximately -5 per mil. If Earth's surface environment became oxygenated ca. 2 Ga, then there were no associated large N-isotope excursions.

ISSN: 0072-1069
Serial Title: Memoir - Geological Society of America
Serial Volume: 198
Title: Secular variations of N-isotopes in terrestrial reservoirs and ore deposits
Title: Evolution of early Earth's atmosphere, hydrosphere, and biosphere; constraints from ore deposits
Author(s): Kerrich, R.Jia, Y.Manikyamba, C.Naqvi, S. M.
Author(s): Kesler, Stephen E.editor
Author(s): Ohmoto, Hiroshieditor
Affiliation: University of Saskatchewan, Department of Geological Sciences, Saskatoon, SK, Canada
Affiliation: University of Michigan, Department of Geological Sciences, Ann Arbor, MI, United States
Pages: 81-104
Published: 2006
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
ISBN: 0-8137-1198-3
References: 149
Accession Number: 2007-006042
Categories: StratigraphyIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables, sketch map
Secondary Affiliation: Pennsylvania State University, USA, United StatesMonash University, AUS, AustraliaNational Geophysical Research Institute, IND, India
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200703
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