Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v11d0614k&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #V11D-0614
Other
1610 Atmosphere (0315, 0325), 8125 Evolution Of The Earth (0325)
Scientific paper
Nitrogen isotope fractionation between atmosphere and Phanerozoic average kerogen and shale is +3‰. Fractionation between kerogen and diagenetic to metamorphic K-silicates is small up to lower amphibolit facies, as is the fractionation between average Phanerozoic shale and K-silicates precipitated from metamorphic fluids that therefore proxy for average crustal siliciclastic rocks. Accordingly, kerogens, and K- silicates from metamorphic fluids of Archean terranes may in turn proxy for Archean atmospheric nitrogen. Data are reported for these two types of samples from 3.0 to 2.7 Ga supracrustal terranes of the Superior Province. Terranes were selected to have been at sub-, to mid-greenschist facies, and to have upper plateau 40Ar/39Ar ages of amphiboles within ~20 Ma of concordant zircon ages in late-metamorphic granitoids; consequently those terranes, unlike Isua, cannot have experienced multiple metamorphic events. An absence of significant post-formation N-isotopic disturbance is endorsed by pre-metamorphic kerogens and late-metamorphic hydrothermal K-silicates having similar secular trends, and there is no covariation between δ15N, N content, and C/N ratios of kerogen, or metamorphic grade. Shales were selected to be distal from chert-banded iron formations, that have a strong secular association with mantle plumes that erupted intraoceanic plateau komaiite-basalt sequences, and which likely have a pronounced mantle signature. New, and compilations of published, data show maximum and median values for shales as follows: Archean ( 30‰; 16‰), Proterozoic (18‰; 12‰ and 4‰), Paleozoic (11‰; 4‰o), Mesozoic-Cenozoic (6‰; 3‰). Corresponding values for hydrothermal K-silicates are: Archean (24‰; 17‰), Proterozoic (17 ‰; 13‰), Paleozoic (6‰; 4‰), Mesozoic-Cenozoic (7‰; 3‰). Earth's volatile inter-element ratios are near carbonaceous chondrite, allowing for hydrodynamic fractionation of H, Ne excepted (Kramers, 2003, Precambrian Research), whereas the Si/Mg ratio, and isotopic composition of the mantle is unlike CI, E, or ordinary chondrites. CI chondrites are characterized by δ15N 30‰ to 42‰, and Earth's mantle ~ - 6‰ to -3‰. Accordingly, the common secular trends in δ15N of kerogens and hydrothermal K-silicates is consistent with the the late vaneer hypothesis for Earth's volatile and mantle siderophile element budgets. The trend involves some combination of processes: drawdown of relatively enriched N by microorganisms transferring atmospheric nitrogen into the crust, and outgassing of depleted mantle N. The former process is analogous to drawdown of atmospheric CO2 by microorganisms, storing it as carbonate or reduced C in the crust.
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