Nitrogen Isotopes in Olivine Separates from Volcanic Arcs, Hot Spots and Continental Mantle Xenoliths

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 ¹⁵N/¹⁴N ratios (expressed in the δ ¹⁵N notation where δ ¹⁵N sample = {[(¹⁵N/¹⁴N)sample/(¹⁵N/¹⁴N)Air]-1} X 1000) of olivine separates are distinctly different from air (0.0‰ ) and range from lower than mean MORB (- 5 ‰ ) to values characteristic of (subducted) oceanic sediments (+ 7 ‰ ). Positive δ ¹⁵N values are found in olivines from volcanic arcs: Cerro Negro 1992 ash (+ 6.2 ± 1.6‰ ), Izalco lava flow (+ 5.1 ± 0.7‰ ), Ichinomengata spinel lherzolite (+ 1.1 ± 0.5 ‰ ) with the exception of Turrialba lava (- 1.7 ± 2.5‰ ). Olivines from hot spots have both positive and negative δ ¹⁵N signatures: Iceland, Theistareykir - northern rift zone (- 8± 1.6 ‰ ), Hawaii, dunite from 1801 Kaupulehu flow of Hualuai volcano (+ 3.1 ± 0.3 ‰ ) and Reunion dunite (+ 0.2 ± 0.5‰ ). The San Carlos mantle xenolith has a value of - 1.5 ± 2.5‰ . ⁴⁰Ar/³⁶Ar 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 ⁴⁰Ar/³⁶Ar ratio of 306, close to that of air. The ³He/⁴He ratios of the samples are higher than the MORB value of 8.0 R_A (R_A is the ³He/⁴He of air), the exception being Cerro Negro (6.1 R_A). Hawaii, Reunion and Iceland have ³He/⁴He of 10.3, 12.9 and 12.3 R_A, respectively. δ ¹⁵N signatures of fumarole gas samples collected at Cerro Negro (+ 4.9 ±0.1 ‰ ), Turrialba (- 1.0 ±0.3 ‰ ) and localities close to Izalco (+ 4.0 ±0.4‰ ) 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.