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Zn isotopic composition of fumarolic gases from
Merapi volcano (Indonesia), preliminary results.
I) General Framework
Because of selective distillation, volcanoes
release to the atmosphere and the hydrosphere
trace gases and metallic trace elements (MTE)
potentially involved in global geochemical
cycles. En route to the surface, steep physical
and chemical gradients (TC, fO2 ) affect the
composition of volcanic fluids. The main
objective of this study is to better understand
volatilisation, deposition processes and origins
of MTE in magmatic fluids by identifying new
isotopic tracers. Isotopic variations in the
representative components of volcanoes
fumaroles, solid sublimates and igneous rocks
should also increase the knowledge of
mass-dependent isotopic fractionation of heavy
stable isotopes in volcanic areas.
II) Why Zinc ?
III) Study area an sampling technique
GAS SAMPLING Condensates
Fumarolic gas is carried through a silica tube to
a condensor constituted by a reception flask
coupled with an acetone cooler. Zn concentrations
obtained during 200X-200X campain vary from
XXng/g to XXng/g.
IV) Chemistry / Mass Spectrometry
CHEMICAL SEPARATION Elution of 500ng sample Zn
was performed on AGMP-1 anionic resin. Yields of
chemical separation were 100 10. Total
chemistry blank contribution was lt 2. MASS
SPECTROMETRY Instrumentation Zn isotopic
compositions were determined by MC-ICP-MS. All
samples were measured on a Plasma 54 at the
E.N.S. Lyon. Three of them were replicated on a
Thermo-Finnigan Neptune at the L.M.T.G.,
Toulouse. Internal precision range from XX to XX
ppm at the XX confidence level. The overall
reproductibility (Xs) was usually better than
0,1 at the XX confidence level. Corrections
for instrumental mass fractionation -Sample-Stand
ard Bracketing (SSB) - slow mass bias drift
over the measurement session -External
Normalisation - mass discrimination
relationship between Cu and Zn
V) Results/Validation
VI) First observations
The overall variation in Zn isotopic
composition of gas condensates is found to be on
the order of 1. This range is similar to
published data on the isotopic variability of Zn
in terrestrial samples. A correlation is observed
between d66Zn and the sampling temperature of the
fumarolic vent suggesting an impoverishment of
the heavy Zn isotopes in volcanic gas upon
cooling.
VII) Conclusion
The first Zn isotopic measurements performed in
volcanic gas displays an overall variation in the
magnitude of 1 (conventional d66Zn notation).
Moreover, Zn isotopic composition is correlated
with gas temperature, suggesting that heavy Zn
isotopes are preferentially partioned in solid
phases upon gas cooling. With the hypothesis of a
negligible isotopic fractionation of Zn isotopes
at the magma chamber level (high TC), these data
also suggest a high Zn isotopic composition of
the deep source of volcanic fluids. These early
interpretations have to be confirmed by Zn
isotopic caraterisation of solid sublimates and
igneous rocks from Merapi volcano.