HR: 13:55h
AN: V53F-02    [Abstracts]
TI: Nitrogen Isotope Geochemistry of Volcanic Rocks from Central America: Implications for Subduction-zone N Cycling
AU: * Sadofsky, S J
EM: ssadofsky@ifm-geomar.de
AF: SFB 574, University of Kiel/IFM-GEOMAR, Wischhofstr. 1-3, Kiel, D-24148 Germany
AU: Hoernle, K
EM: khoernle@ifm-geomar.de
AF: SFB 574, University of Kiel/IFM-GEOMAR, Wischhofstr. 1-3, Kiel, D-24148 Germany
AU: Duggen, S
EM: sduggen@geol.ku.dk
AF: Geological Institute, University of Copenhagen, Oster Voldgade 10, Copenhagen, 1350 Denmark
AU: Bebout, G E
EM: geb0@lehigh.edu
AF: EES Department, Lehigh University, 31 Williams Drive, Bethlehem, PA 18015 United States
AB: Prior attempts to understand nitrogen geochemistry of volcanic arcs have generally been limited to study of active fumaroles. Here we analyze whole-rock N concentration and δ ¹⁵N for active (Central American) and older (Alboran) volcanic systems using sealed-tube combustion methods. Whole-rock N-isotope studies can sample N present in quenched glass, fluid inclusions, plagioclase, or potassic minerals from a range of volcanic systems. Six basaltic samples from the volcanic front of Nicaragua have δ ¹⁵N_air from -3.3 to +9.9‰, and 7 to 240 ppm N. Seven basalts from the Costa Rican volcanic front show a slightly smaller range of δ ¹⁵N_air (-2.0 to +6.0‰), with lower N concentrations (5 to 50 ppm). One backarc sample from Costa Rica shows considerably lower δ ¹⁵N_air. Five samples of Alboran Lamproites range from -3.9 to +11 per mil, with 5 to 250 ppm N. All samples likely experienced degassing at relatively high-T, which should minimize any associated N-isotope fractionation but suggests that N concentrations are not original magmatic values. Atmospheric contamination is unlikely, and would cause high N concentrations nearer 0 per mil. Basalts vary in δ ¹⁵N due to different proportions of the N budget derived from subducting sediment, subducting crust, and mantle. Similar to the results of fumarolic studies from Central America (summarized by Zimmer et al., 2004), Nicaraguan data are more variable than those from Costa Rica, including some values with higher δ ¹⁵N. However, the highest δ ¹⁵N in basalts from both Costa Rica and Nicaragua are several per mil higher than those previously measured in fumaroles. δ ¹⁵N of Alboran Lamproites correlates negatively with ⁸⁷Sr/⁸⁶Sr, and trace elements thought to derive from mantle that has been contaminated by slab fluids or melts (Duggen et al., 2005), suggesting a low δ ¹⁵N slab fluid or melt may be present at some stage of subduction. This may also explain the low δ ¹⁵N in the backarc of Costa Rica.
DE: 1030 Geochemical cycles (0330)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1040 Radiogenic isotope geochemistry
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005