HR: 0800h
AN: V41A-1427    [Abstracts]
TI: Subduction Input Flux of Nitrogen in Altered Oceanic Basalt
AU: * Li, L
EM: lol2@lehigh.edu
AF: Earth and Environmental Sciences, Lehigh University, 31 Williams Dr., Bethlehem, PA 18015 United States
AU: Bebout, G E
EM: geb0@lehigh.edu
AF: Earth and Environmental Sciences, Lehigh University, 31 Williams Dr., Bethlehem, PA 18015 United States
AU: Idleman, B D
EM: bdi2@lehigh.edu
AF: Earth and Environmental Sciences, Lehigh University, 31 Williams Dr., Bethlehem, PA 18015 United States
AB: We have employed sealed-tube, carrier-gas-based methods newly developed in our laboratory to examine the nitrogen (N) concentration and isotopic composition of altered oceanic crust (AOC). Knowledge of the subduction input flux of N in AOC is critical in any attempt to mass-balance N across arc-trench systems, either globally or on an individual-margin basis. Uncertainties (expressed as 1σ for >3 replicate analyses of both internal silicate standards and individual AOC samples) are less than 5% for N concentrations and on the order of 0.15‰ δ¹⁵N_air for samples with >5 ppm N. At current blank levels (overall system blank of 3.8±0.2 nmol N₂ with a δ¹⁵N value of -7.3±0.4‰), uncertainty in δ¹⁵N increases to about 0.6 ‰ for samples with 1-2 ppm N. Preliminary analyses of 33 AOC samples recovered on DSDP/ODP legs from the North and South Pacific, the North Atlantic, and the Philippine Sea (with a larger number of samples from Site 801 outboard of the Mariana trench) indicate N concentrations of 1.3 to 17.8 ppm and δ¹⁵N of -5.1 to +8.3‰. Combined, the data for all cores document a crude direct relationship between N concentration and isotopic composition, with the lowest-N samples characterized by mantle-like δ¹⁵N values of approx. -5 ‰, and more N-rich samples yielding progressively increasing δ¹⁵N. This relationship presumably reflects high- to low-temperature alteration on the seafloor. For the Izu-Bonin-Mariana trench, AOC samples from Leg 129 Site 801 with 1.3 to 17.8 ppm N have δ¹⁵N values ranging from -5 to +1.2‰ but most fall between -1.5 and +1.2‰. For Site 801 (thus far 14 samples representing core depths of 450-600 mbsf, within the uppermost part of the basement section), there is no obvious correlation between N and K₂O concentrations, indicating that the siting of N is more complicated than simple residence/sequestration in potassic alteration phases and likely reflects complex superposition of alteration effects produced over a large temperature range. Three samples from Site 417A/D contain 11 to 14 ppm N and show wide variation in δ¹⁵N (+1 to +8.3‰), whereas a small number of samples thus far analyzed for Site 1149 have extremely low N concentration (<3 ppm) and δ¹⁵N between -5 and 0‰. If altered basalt in the upper 1 km of the oceanic crust subducting along the Izu-Bonin-Mariana margin contains an average of 10 ppm N and the remaining 5 km of less-altered oceanic crust contains an average of 1 ppm N, subducting oceanic crust transfers 2.1×10⁶ g/km N annually into that trench. This N input flux in AOC is comparable to that in seafloor sediments subducting into the same margin (for the sediments, 2.5×10⁶ g/km N annually; see Sadofsky and Bebout, 2004, G-cubed), demonstrating that the input of N in AOC, and its isotopic consequences, must be considered in any assessment of convergent margin N flux. Our future work will include more complete coverage of the depth range sampled for Sites 801 and 1149 and will include analyses of various AOC composites (e.g., Site 801; see Plank et al., 2000, ODP Leg 185 Init. Rept.), in addition to work to further reduce the analytical blank.
DE: 0469 Nitrogen cycling
DE: 1021 Composition of the oceanic crust
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005