HR: 1330h
AN: V42D-0379    [PDF]
TI: $^{15}$N Enriched Archean Crust, or $^{15}$N Depleted Crust Recycled into -6 \permil Upper Mantle?
AU: * Kerrich, R
EM: robert.kerrich@usask.ca
AF: University of Saskatchewan, Department of Geological Sciences, 114 Science Place,, Saskatoon, SK S7N 5E2 Canada
AU: Jia, Y
EM: yiefei.jia@anu.edu.au
AF: Australian National University, Research School of Earth Sciences, Canberra, ACT 0200 Australia
AB: Nitrogen concentrations and isotopic compositions have been measured on 2.7 to 1.0 Ga Precambrian metasedimentary rocks from Botswana, Canada and Ghana; pre-metamorphic 2.7 Ga VMS deposits in Canada; and on post-metamorphic hydrothermal micas from 2.7 to 1.8 Ga quartz vein systems in Zimbabwe, Canada, and Ghana, to investigate the origin and evolution of nitrogen in the atmosphere, crust, and mantle systems during Earth's early history. Archean sedimentary kerogens, in K-silicates in VMS deposits, and K-micas in hydrothermal systems that sample average crust, yield $\delta^{15}$N values of 15-24$\permil$, compared to 2 to 6$\permil$ in Phanerozoic counterparts. Paleoproterozoic equivalents have intermediate $\delta^{15}$N of 7 to 12$\permil$, implying a secular decrease in crustal $\delta^{15}$N. In parallel, N contents increase from tens to hundreds ppm in the Archean to hundreds up to thousands ppm since the Paleoproterozoic. The $^{15}$N-enriched nitrogen in both Archean sedimentary rocks and hydrothermal vein systems cannot be caused either by long-term diffusional loss of $^{14}$N, or by N-isotopic shifts due to metamorphism; such fractionations are $<$2$\permil$ as established from empirical studies of Phanerozoic terranes with progressive metamorphism, and experiments. Moreover, pre- and post-metamorphic Archean samples have $^{15}$N enriched values in common. Retention of near-primary $^{15}$N-enriched values is endorsed by the lack of covariation of C/N with $\delta^{13}$C or $\delta^{15}$N, or with metamorphic grade. It is possible that the $^{15}$N-enriched values stem from a different N-cycle in the Archean, with large biologically mediated fractionations, yet the magnitude of the fractionations observed exceeds any presently known. The $^{15}$N-enriched nitrogen does not robustly constrain Archean redox-state. We attribute the $^{15}$N-enrichment to a secondary atmosphere derived from CI-chondrite-like material and comets with $\delta^{15}$N of +30 to +42$\permil$. Shifts of atmospheric $\delta^{15}$N to its present values of 0$\permil$ can be accounted for by a combination of early growth of the continents, and sequestering of atmospheric N$_{2}$ into crustal rocks; recycling into the mantle; and mantle degassing. Consequently, these shifts are tracked by the secular change of $\delta^{15}$N in continental crust. If Earth's surface environment became oxygenated at ~ 2 Ga, then there were no associated large N-isotope excursions. Based on a few $^{15}$N depleted Archean cherts Marty and Dauphas (2003) proposed that recycling of $^{15}$N depleted Archean crust could shift the upper mantle from a primordial value of +6 to +8 $\permil$ to the observed value of -5$\permil$G85. The new data rule out this model.
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting