Nitrogen and Argon in Volcanicand Hydrothermal Fluids.
Implications for the Volatile Budget
in Subduction Zones
Yuri TaranInstitute of Geophysics, UNAM, 04510, Mexico

Nitrogen, argon and other aircomponents are inevitably recycled from thesubducted oceanic lithosphere to the surface
by magmatism and accompanying volcanicand hydrothermal activity. A part of N andAr are of the “old” atmosphere origin, and asignificant fraction of N can be derived fromthe subducted organic-rich sediments,
appearing as “excess nitrogen” in volcanicand geothermal gases. However, excessnitrogen can be released also from thecontinental sedimentary organics
ormetamorphic rocks, with the same isotopic
characteristics (δ15N ≥ +5‰) and the samehigh N2/Ar ratio in the gas discharge. On theother hand, gas with the N2/Ar ratio close tothe ASW (air-saturated water) values (4080), δ15N ~ 0 and 40Ar/36Ar ~ 296 can bereleased also from altered basalts of the
subducted oceanic plate, poor in pelagic
sediments. A complex interplay between
δ15N, 40Ar/36Ar, N2/Ar and absoluteconcentration of N2 in a surface thermal
manifestation has different issues for thevolcanic and hydrothermal vents withindifferent subduction zones. The excess
nitrogen in volcanic fumaroles may havemagmatic origin. In that case its amountshould correlate with fraction of magmaticwater. “Magmatic” in this context means“derived from the magma generation zone”.
The total nitrogen in a volcanic gas is a
mixture of air (analytical errors, badsampling or permeable fumarolic roots),
ASW,
crustalN2(metamorphic,sedimentary) and magmatic, which is in turnis a mixture between nitrogen from the “old”
recycled air (subducted air-saturated
seawater – ASSW), subducted oceanic
sediments, altered basalts, and the MORBlike nitrogen from the mantle wedge. Argon
with very high 40Ar/36Ar ratios can beproduced in the crust, as well as by the
mantle-wedge-MORB-like material. Argon

in volcanic gases with the air-like 40/36
ratio of 296 could be supplied by ASW,
ASSW and air contamination. The lower is areal “magmatic” N2 and Ar content in a
volcanic gas, the higher probability tocontaminate a sample by any sort of airderived gases.
There are no too many good sets ofvolcanic and hydrothermal gas data withN2/Ar. The first is by Magro&Pennisi(1991) for Vulcano, the second is byMizutani
forShowa-Shinzan(fromSymonds et al. 1996). Good, partiallypublished data of Chiodini and co-workers.
exist for Campi Flegrei, Vesuvius (Italy) andNisyros (Greece). The three last are typicalhydrothermal gases in spite of their location
inside volcano craters. The two first are arcmagmatic + hydrothermal. All five show
similar “weird” patterns on the N2&Ar vsN2/Ar plots: relatively constant N2
concentration, much higher than that for thecompletely
evaporatedASW,anddecreasing Ar from high concentrations at
close to the air N2/Ar to low Ar, close to its
ASW content at high N2/Ar ratios (seeFigure 1). Our data from El Chichon
volcano-hydrothermal system show sametrends.
The excess nitrogen in “meteorichydrothermal” systems as a rule has crustal
origin and correlates positively with themethane content.
Volatile outputs from volcanoes andgeothermal systems of a subduction zone arecomparable. (3.1 .1010 mol/a of CO2 byvolcanoes vs 2.3.10 10 mol/a by geothermal
systems in Kamchatka without diffusion soilflux). In order to estimate the gas output
from a hydrothermal system one needs tomeasure the total water discharge andgas/water ratio. However, direct estimationsof the gas discharge from a bubbling thermal
spring sometimes impossible. A simple
expression based on the phase equilibrium isderived for the calculation of the total gas
content using exclusively the gas analysis:
Xg = 78.1pr/[(%)N2β(r)] – 1/β(st),36

Ar/ Ar1000

40296100Ar ppm

101ASW

N2 ppm0.1
1 000

100ASW

10100

10 00

N2/Ar

Solfatara
2.0

Ar ppm1.5
1.00.5

ASW

0.0
1000

N2 ppmwhere Xg in mol/kg of water, (%)N2 is mol% of N2 in dry gas, p r is atmosphericpressure at the altitude of the recharge areaand β are solubility coefficients for N2 atrecharge and discharge temperatures,respectively. Nitrogen in this equation canbe replaced by Ar or Ne with theircorresponding solubility coefficients andconcentrations in air (See Taran, 2005 fordetails).
Different mechanisms of the ASW
degassing (open, closed, multi-step,continuous, partial, etc.) result in different
N2/Ar ratios, but all are in the range of 3684. The N2/Ar < 30 can be realized only byRayleigh-type degassing of a limited water
body, without any supply from an externalsource of ASW. Another, but little probablecause of low N2/Ar is a contribution ofradiogenic 40Ar from the rock matrix.
Suchapproachandmixing
relationships help to constrain ranges ofabsolute concentrations of N2 and Ar in afree gas phase of thermal ASW with an
external source of CO2 and to discriminate
the “net” excess nitrogen in thermalmanifestations. The method is applied to theestimation of the output of subducted
nitrogen and argon from the KamchatkaKurile volcanic arc.

100ASW
10100

N2/Ar

1000

Vesuvio
90References

CH4 ppm

80
70
60
50
40

Magro G, Pennisi M. (1991) JVGR,

30Taran Y.A. (2005) GRL,

1010.11000

N2 ppmSymonds R.B., et al (1996) JVGR,

Ar ppm100

100

1001000

N2/Ar
Figure 1. N2 and Ar in gases from Vesuvius andSolfatara (Chiodini, unpublished) and Vulcano(Magro&Pennisi, 1991). Concentrations in ppm inthe total discharge (water + gases).