HR: 0800h
AN: V51C-0594    [Abstracts]
TI: Nitrogen Isotopes in Olivine Separates from Volcanic Arcs, Hot Spots and Continental Mantle Xenoliths
AU: * Fischer, T P
EM: fischer@unm.edu
AF: Dept. of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, NM 87131-1116 United States
AU: Takahata, N
EM: ntaka@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, The University of Tokyo, Tokyo, 164-8639 Japan
AU: Sano, Y
EM: ysano@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, The University of Tokyo, Tokyo, 164-8639 Japan
AU: Hilton, D R
EM: drhilton@ucsd.edu
AF: Fluids and Volatiles Laboratory, Scripps Institution of Oceanography, La Jolla, CA 92093-0244 United States
AB: We report the first nitrogen isotopic data of olivine separates from volcanic arcs (Cerro Negro, Nicaragua; Izalco, El Salvador; Turrialba, Costa Rica; Ichinomegata, Japan). In addition, we report nitrogen isotopic data of olivine separates from ocean islands (Hawaii, Reunion, Iceland) and continental mantle xenoliths (San Carlos, Arizona). Samples were processed by crushing and analyzed using a modified noble gas mass spectrometer (VG3400). N concentrations range from 0.6 to 22 micro ccSTP/g olivine. The $^{15}$N/$^{14}$N ratios (expressed in the $\delta$$^{15}$N notation where $\delta$$^{15}$N sample = {[($^{15}$N/$^{14}$N)sample/($^{15}$N/$^{14}$N)Air]-1} X 1000) of olivine separates are distinctly different from air (0.0$\permil$) and range from lower than mean MORB (- 5 $\permil$) to values characteristic of (subducted) oceanic sediments (+ 7 $\permil$). Positive $\delta$$^{15}$N values are found in olivines from volcanic arcs: Cerro Negro 1992 ash (+ 6.2 $\pm$ 1.6$\permil$), Izalco lava flow (+ 5.1 $\pm$ 0.7$\permil$), Ichinomengata spinel lherzolite (+ 1.1 $\pm$ 0.5 $\permil$) with the exception of Turrialba lava (- 1.7 $\pm$ 2.5$\permil$). Olivines from hot spots have both positive and negative $\delta$$^{15}$N signatures: Iceland, Theistareykir - northern rift zone (- 8$\pm$ 1.6 $\permil$), Hawaii, dunite from 1801 Kaupulehu flow of Hualuai volcano (+ 3.1 $\pm$ 0.3 $\permil$) and Reunion dunite (+ 0.2 $\pm$ 0.5$\permil$). The San Carlos mantle xenolith has a value of - 1.5 $\pm$ 2.5$\permil$. $^{40}$Ar/$^{36}$Ar ratios of the samples as determined in this study or reported in the literature are significantly higher than air (295.5) in olivines from Ichinomegata, San Carlos, Iceland, Reunion and Hawaii. The olivines from Cerro Negro have a $^{40}$Ar/$^{36}$Ar ratio of 306, close to that of air. The $^{3}$He/$^{4}$He ratios of the samples are higher than the MORB value of 8.0 R$_{A}$ (R$_{A}$ is the $^{3}$He/$^{4}$He of air), the exception being Cerro Negro (6.1 R$_{A}$). Hawaii, Reunion and Iceland have $^{3}$He/$^{4}$He of 10.3, 12.9 and 12.3 R$_{A}$, respectively. $\delta$$^{15}$N signatures of fumarole gas samples collected at Cerro Negro (+ 4.9 $\pm$0.1 $\permil$), Turrialba (- 1.0 $\pm$0.3 $\permil$) and localities close to Izalco (+ 4.0 $\pm$0.4$\permil$) are identical (within error) to those of the olivine separates. The results obtained in this study are significant and show that 1) N isotopes in olivine separates and volcanic/hydrothermal gas emissions both sample volatiles that are primarily derived from the magma source. 2) subduction of oceanic sediments and transfer of N through the mantle wedge controls the N isotopic composition in volcanic arcs as sampled by olivine separates and gas discharges. 3) the N isotopic signature of hot spot related volatiles is variable but the samples from Hawaii, and to a lesser extent from Reunion, support the idea that surficial N (from subducted sediments or oceanic basement) is recycled into the lower mantle. 4) N as sampled by the San Carlos mantle xenolith may record a greater extent of air contamination than the other samples. Alternatively, N data of San Carlos and the Iceland northern rift zone suggests that the N isotopic signature of the upper mantle may be more variable than previously inferred from MORB samples.
DE: 3670 Minor and trace element composition
DE: 3699 General or miscellaneous
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting