Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front

Abstract

We report new chemical and isotopic data from 26 volcanic and geothermal gases, vapor condensates, and thermal water samples, collected along the Nicaraguan volcanic front. The samples were analyzed for chemical abundances and stable isotope compositions, with a focus on nitrogen abundances and isotope ratios. These data are used to evaluate samples for volatile contributions from magma, air, air-saturated water, and the crust. Samples devoid of crustal contamination (based upon He isotope composition) but slightly contaminated by air or air-saturated water are corrected using N 2 /Ar ratios in order to obtain primary magmatic values, composed of contributions from upper mantle and subducted hemipelagic sediment on the down-going plate. Using a mantle endmember with d 15 N = À5& and N 2 /He = 100 and a subducted sediment component with d 15 N = +7& and N 2 /He = 10,500, the average sediment contribution to Nicaraguan volcanic and geothermal gases was determined to be 71%. Most of the gases were dominated by sediment-derived nitrogen, but gas from Volcán Mombacho, the southernmost sampling location, had a mantle signature (46% from subducted sediment, or 54% from the mantle) and an affinity with mantle-dominated gases discharging from Costa Rica localities to the south. High CO 2 /N 2 exc. ratios (N 2 exc. is the N 2 abundance corrected for contributions from air) in the south are similar to those in Costa Rica, and reflect the predominant mantle wedge input, whereas low ratios in the north indicate contribution by altered oceanic crust and/or preferential release of nitrogen over carbon from the subducting slab. Sediment-derived nitrogen fluxes at the Nicaraguan volcanic front, estimated by three methods, are 7.8 · 10 8 mol N/a from 3 He flux, 6.9 · 10 8 mol/a from SO 2 flux, and 2.1 · 10 8 and 1.3 · 10 9 mol/a from CO 2 fluxes calculated from 3 He and SO 2 , respectively. These flux results are higher than previous estimates for Central America, reflecting the high sediment-derived volatile contribution and the high nitrogen content of geothermal and volcanic gases in Nicaragua. The fluxes are also similar to but higher than estimated hemipelagic nitrogen inputs at the trench, suggesting addition of N from altered oceanic basement is needed to satisfy these flux estimates. The similarity of the calculated input of N via the trench to our calculated outputs suggests that little or none of the subducted nitrogen is being recycled into the deeper mantle, and that it is, instead, returned to the surface via arc volcanism.

Igneous Rock Associations 13. Focusing on the Central American Subduction Zone

“…An enhanced flux from subducted (hemipelagic) sediments can also explain the along-arc peaks in B/La, U/Th, Ba/Th, and ( 230 Th/ 232 Th) seen in Nicaragua (Leeman et al 1994;Patino et al 2000;Carr et al 2003;Walker et al 2007;Sadofsky et al 2008;Jicha et al 2010). The He -CO 2 relationships and the δ 15 N ratios of volcanic and geothermal fluids in Nicaragua also suggest elevated contributions from subducted sediments (Shaw et al 2003;Elkins et al 2006), although δ 15 N ratios in Nicaragua are similar to those in Guatemala (Fischer et al 2002;Elkins et al 2006). Jicha et al (2010) point out that 238 U excesses do not peak in Nicaragua, as might be expected if fluid input is maximized in this segment of the CASZ.…”

Origins of N2 in the Dongtai Depression of the Subei Basin

“…7a). It is unexpected that the data do not fit the two component model with the MORB mantle (MM) endmember of δ 15 N=-5‰ (Marty and Humbert 1997;Cartigny et al, 1997;Sano et al, 2001;Elkins et al, 2006;Palot et al, 2009). The MM endmember composition could be improper (Clor et al, 2005).…”

“…The MM endmember composition could be improper (Clor et al, 2005). Elkins et al (2006) have investigated this possibility by selecting -15‰ for the δ 15 N value of the degassed mantle (DM). Figure 7b shows the results from using the DM endmember value (Mohapatra and Murty, 2004;Elkins et al, 2006).…”

“…Therefore, the discovery of atmospheric N 2 with high contents in gas reservoirs provides a new clue to improve the understanding of natural gas and to study meteoric water and its influence on hydrocarbons in basins. Sano et al (2001), Bräuer et al (2004) and Elkins et al (2006). (a) Mixing line calculated using a mantle δ 15 N=-5‰; (b) using a mantle δ 15 N=-15‰.…”

“…N 2 from immature and mature organic matter presents δ 15 N values of -19‰ to -10 ‰ and -10 to -2 ‰, respectively (Zhu et al, 2000). Atmospheric N 2 shows δ 15 N ratios around 0 ‰ (Zhu et al, 2000;Elkins et al, 2006). Therefore, nitrogen cannot be distinguished between mantle-derived and organic origins and between their mixtures with atmospheric origin, on the basis of δ 15 N values alone.…”