Carbon and oxygen isotopic equilibrium in fumarolic CO: Applications forremote temperature measurements
Urumu Tsunogai, Daisuke D. Komatsu, Akira Ijiri, and Fumiko Nakagawa
Earth and Planetary Sciences, Graduate School of Science, Hokkaido University
N10 W8, Kita-ku, Sapporo, 060-0810, Japan. (E-mail: urumu@ep.sci.hokudai.ac.jp)

The stable carbon and oxygen isotopic compositions (δ13C and δ18O) of CO were determinedfor high temperature fumarolic gases on Satsuma-Iwo-Jima volcano, Japan, as well as δ13Cand/or δ18O of co-existing CO2 and H2O. It turns out that both δ13C and δ18O of CO reached tothe isotope exchange equilibrium with CO2 and H2O, respectively, under the temperature of morethan 800 °C (Fig. 1). We conclude that following chemical reactions is so rapid in both directionsin the fumarolic gases at the temperature.
CO + H2O = CO2 + H2Besides, the results also suggest that the reactions fall into sufficiently slow at the sampling
temperature(surrounding

airtemperature),

because the isotopic compositions were quenchedat the temperature just before the sampling. As aresult, we anticipate similar quenched state forthe components in volcanic plume, highly diluted
fumarolic gas floating around volcano.
In principle, the chemical and/or isotope
equilibrium in volcanic gases enables us tomonitor

remotetemperatureofvolcanic

fumaroles at a distant site, if we could deduce thechemical/isotopic compositions of every related
components in fumaroles from a distant siteusing the chemical/isotopic compositions involcanic plume. In practical, however, it isdifficult

todeducechemical/isotopic

compositions in fumaroles from those in volcanic

Fig.1 The δ13C difference between CO2and CO (Δ13C(CO2–CO)) in fumarolicgases plotted against the fumarolictemperature,together
withtheΔ13C(CO2–CO) under isotopic equilibrium
condition (Richet et al., 1977).plume, because most components in fumaroles are condensable (H2O, SO2, and H2S, etc.), and/ormajor in atmosphere (H2O and CO2, etc.).
In case of the stable isotopic compositions of CO, however, it would be highly possible todeduce those in fumaroles using those in volcanic plume, because it is less condensable andminor in atmosphere (less than 0.3 ppmv). Besides, because both H2O and CO2 are major in
volcanic fumaroles, their stable isotopic compositions can be regarded as constant irrespective tothe temperature change. That is to say, the variation range in the stable isotopic compositions infumarolic CO can be attributed to the total variation in the isotopic equilibrium state in fumaroles,
which mostly corresponds to change in the fumarolic temperature.
To check this hypothesis, addition to fumarolic gases, we also determined δ13C and δ18O ofCO in volcanic plume emitted from Satsuma-Iwo-Jima volcano, as well as mixing ratios. By
using the isotopic compositions of CO in volcanic plume sampled at a distant site from thefumaroles (several to several hundred meters), we verified the possibility that we could deducethe isotopic compositions in high temperature fumaroles or not. Recent advances in stable isotope
measurements using CF-IRMS enable us such highly sensitive measurements of CO even inatmospheric level of several 100 ppbv (Tsunogai et al., 2002).
We found that the reciprocal of CO mixing ratio (1/[CO]) in the volcanic plume exhibitedlinear correlation with δ13C of CO (Fig.2). The
linear correlation suggests that both mixing
ratio and δ13C of CO in the plume were
determined through simple mixing between
two endmembers: CO in ambient atmosphere(high1/[CO]endmember)

and

that

infumaroles (low 1/[CO] endmember). By
extrapolating the linear correlation to 1/[CO] =0, we estimated the δ13C of CO in hightemperature fumaroles as -10.8±1.1, whichcoincide well with that actually emitted fromthe high temperature fumaroles. The presentFig.2 δ13CCO vs 1/(mixing ratio) plot for
CO in volcanic plume (‫)۔‬, together withthose in fumarolic gases (‫)ې‬. The dottedline corresponds to the linear regression tothe volcanic plume.

result suggest that determination of bothmixing ratio and δ13C of CO in volcanic plume,
we can deduce the temperature of fumaroles, if
the temperature exceed 800 °C.