ELEMENTAL AND ISOTOPIC FRACTIONATION OF NOBLE GASES IN PLASMAS. IMPLICATIONS
FOR CHONDRITIC PRIMORDIAL NOBLE GASES. Maia Kuga1, Bernard Marty2, Guy Cernogora3, Yves
Marrocchi2 and Laurent Tissandier2, 1ETH Zürich, Institute of Geochemistry and Petrology, Clausiusstrasse 25,
8092 Zürich (Switzerland), [email protected], 2CRPG-CNRS, 15 rue Notre-Dame des Pauvres, 54500 Vandoeuvre-les-Nancy (France), 3LATMOS-UVSQ-CNRS, 11 Bd d'Alembert, 78280 Guyancourt (France).
Introduction: Most heavy noble gases (Ar, Kr and
Xe) trapped in chondrites are hosted in the so-called
“Phase Q”, which is thought to be part of the Insoluble
Organic Matter (IOM) of meteorites [1]. Q-noble gases
are significantly fractionated relative to solar
composition : heavy noble gases are enriched by orders
of magnitude relative to light ones, and their isotopic
compositions show mass-dependent enrichments in
heavy isotopes [2]. Several studies have reproduced
some of the Q-Xe characteristics by trapping Xe ions
in various solids [3-6]. From these experiments, ionization of noble gases comes out to be the necessary condition to account for isotopic fractionation of experimentally trapped noble gases. However, the physical
process behind remains unclear.
Experimental methods: We modified the Nebulotron plasma experiment used in [6]. Our Nebulotron
version consists of a glass line in which a mixture of
gases is flowing continuously at ~ 1 mbar during the
whole duration of the experiment [8]. We used mixtures of gases composed of CO (+H2(O) contamination) with the addition of a mixture of noble gases
(NGM in the following, composed of He, Ne, Ar, Kr,
Xe = 75:15:5:3:2) and/or N2. A microwave plasma
discharge is generated in the quartz reactor (2.45 GHz)
where the molecular and atomic gas species are processed by ionization and dissociation, eventually ending in the deposition of organic dust on the reactor’s
surfaces. This dust was gently recovered and characterized ex-situ and analysed for their noble gas content
[7]. The noble gases trapped in the synthesized organics were extracted at 1400 °C in a double-walled induction quartz furnace at CRPG. Extracted noble gases
were purified and separated according to standards
noble gas methods. Analysis was performed with sector-type mass spectrometers. Only Kr and Xe content
and isotopes could be measured, while Ar, Ne and He
displayed signals below the blank contribution. Several
experiments were driven, either varying the proportions of CO and NGM (11-50% of NGM in CO), or the
duration of the experiment (2-7 h), or the power of the
electric discharge (30-65 W).
Results: Over the whole range of experiments, the
Xe elemental concentration is large, but highly variable. This sheds light on the complexity of these experiments and this plasma setup, which inherently produces heterogeneous deposits. Consequently, this high
variability in the trapped Xe concentration prevents
any conclusion on the dependence on experimental
conditions, such as the Xe partial pressure in the setup,
the duration of the experiment, and the power input to
the discharge. The trapping of Xe is also accompanied
with the trapping of Kr, and probably of Ar, but in a
much less efficient way [7]. All Nebulotron samples
present large isotopic fractionation factors for Xe and
Kr isotopes, of +1.0±0.4%/amu and +1.3±0.7%/amu,
in average, respectively [7]. These isotopic fractionation factors are mass-dependent according to m1, and
are clearly enhanced when increasing the power input
to the discharge.
Discussion: The particular elemental depletion pattern of noble gases in plasma experiments is best explained by the Saha ionization balance in plasma. This
ionization equilibrium predicts the ratio of two ionized
species, for instance 84Kr+/132Xe+, to be a function of
(i) the initial neutral ratio (84Kr/132Xe), (ii) the ionization potentials of these two species, and (iii) the average energy of electrons in the plasma. Because the
different noble gases have contrasted ionization potentials (24.58 eV for He and 12.13 eV for Xe), this results in a strong depletion of ionized light noble gases
in a poorly ionized gas such as laboratory plasmas. The
resulting Saha depletion pattern for noble gases is consistent with the elemental fractionation of noble gases
trapped in plasma-synthesized organics.
This fractionation due to ionization in the gas does
not work for isotopes because the difference of ionization potential between two isotopes of a same noble
gas is too small. We thus suggest that the isotopic fractionation for noble gases in plasmas is due to ambipolar diffusion, which results from the diffusion of
charged species with opposite electric charges caused
by their interaction via an electric field. Electrons diffuse faster than ions which causes locally a disequilibrium of charges, balanced by the apparition of an electric field. The resulting electric field will tend to slow
down the electrons and to accelerate the ions. We calculated a diffusion model for our Nebulotron plasma,
mainly composed of CO. We derived and estimated the
velocity of ions in the plasma, which turns out to be of
the same order of magnitude whatever the mass of the
considered ion. Consequently, in this case, the kinetic
energy of ions is propotional to their mass, i.e.,
Ea/Eb=ma/mb, consistent with the m1 fractionation law
observed for noble gases trapped in plasma synthesized
solids, including ours.
Conclusions: In the light of our elemental and isotopic fractionation models for noble gases in plasmas,
we suggest that the process governing their trapping
and fractionation is low-energy ion implantation. This
process is likely responsible for the formation of the
chondritic Q-component from the ionized gas in the
solar nebula [7].
References: [1] Lewis et al. (1979) Science 190,
1251-1262. [2] Busemann et al. (2000) Met. Planet.
Sci. 35, 949-973. [3] Frick et al. (1979) LPSC 10th,
1961-1972. [4] Dziczkaniec et al. (1981) LPSC 12th,
246-248. [5] Hohenberg et al. (2002) Met. Planet. Sci.
37, 257-267. [6] Marrocchi et al. (2011) GCA 75,
6255-6266. [7] Kuga et al. PNAS, 112, 23.