Download ORIGIN AND EVOLUTION OF ATMOSPHERIC XENON

ORIGIN AND EVOLUTION OF ATMOSPHERIC XENON: ATMOSPHERIC AND GEODYNAMICAL
IMPLICATIONS G. Avice1, B. Marty1, R. Burgess2, 1CRPG-CNRS, Université de Lorraine, Vandoeuvre-lèsNancy, France ([email protected], [email protected]). 2School of Earth, Atmospheric and Environmental Sciences, University of Manchester ([email protected]).
Introduction: The origin of volatile elements on
Earth remains poorly understood. Among other noble
gases, atmospheric xenon presents striking features.
Firstly, it is elementaly depleted relative to lighter noble gases (Ne, Ar, Kr) which follow a similar depletion
pattern than chondrites relative to the solar composition. Secondly, atmospheric Xe is strongly isotopically
fractionated (30-40 ‰.u-1) relative to cosmochemical
components (SW-Xe, U-Xe, Q-Xe). These two features form the xenon paradox [1]. It has also been recognized for decades that neither Solar (SW-Xe) nor
Chondritic (Q-Xe) cosmochemical components can be
the ancestors of atmospheric xenon due to remaining
134
Xe & 136Xe excesses after correction for the massdependent fractionation described above. U-Xe, a theoretical component has thus been defined as the starting
isotopic composition of the Earth's atmosphere [2,3].
Recent studies demonstrated that Archean atmospheric Xe had an isotopic composition intermediate
between potential primordial components and the
modern atmosphere [4,5]. These results suggest a longterm escape and isotopic fractionation of atmospheric
Xe [6]. However, some key points remain to be clarified: i) What is the starting isotopic composition of the
Earth's atmosphere? ii) What is exactly the isotopic
evolution of atmospheric with time? iii) When did atmospheric xenon reach its modern isotopic fractionation?
Samples & Methods: Samples from the Barberton
greenstone belt (South Africa) are quartz crystals from
hydrothermal veins intruding surrounding mafic lavas
and cherts. A total of 27 analyses on 7 different samples by using a recent Helix MC Plus (Thermofisher)
mass spectrometer permitted to determine the isotopic
composition of xenon in these samples at the permil
level. The isotopic compositions of other noble gases
(Ne, Ar, Kr) and nitrogen were also determined in these samples. Samples duplicates from Barberton were
neutron-irradiated and dated following the Ar-Ar
method. We also analyzed quartz samples of various
ages (3.2 Ga to 500 Ma) in order to follow the evolution of the isotopic composition of atmospheric Xe
with time.
Results:
The 3.2 Ga-old atmosphere. Ar-Ar results demonstrate a time of fluid entrapment of 3.2 Ga for Barberton samples together with an atmospheric 40Ar/36Ar
ratio of 210 ± 29 (1σ) at this time. N2-Ar correlations
on the same samples indicate a partial pressure of atmopheric nitrogen in the Archean atmosphere lower or
similar to the modern one. Neon isotopic ratios are
atmospheric-like. Importantly, krypton is isotopically
normal and show no deviation relative to the isotopic
composition of the modern atmosphere. These two
results confirm the absence of any mantle-derived
component in these samples that thus recorded the isotopic composition of ancient atmospheres.
Isotopic fractionation of Xe at the permil level.
Barberton Xe presents an isotopic fractionation of 13.3
± 1.8 ‰.u-1 (2σ) relative to the isotopic composition of
the modern atmosphere (Fig. 1). This confirms that
Archean Xe had an isotopic composition intermediate
between cosmochemical ancestors and the modern
atmosphere [4,5]. Furthermore, this result was obtained
on samples originating from a different geological area
compared to previous studies [4,5] and thus demonstrates that the isotopic fractionation of Archean Xe
reflects a global atmospheric signal and not a local
effect. Interestingly, after correction for this massdependent fractionation, the 129Xe* excess, due to the
radioactive decay of extinct 129I (t1/2=15.7 Ma), is lower than in the modern atmosphere and allows us to
compute a degassing rate of 9 ± 5 mol.a-1 for 129Xe*
degassed from the silicate Earth during the last 3.2 Ga.
This degassing rate is at least one order of magnitude
higher than the modern one (≈ 0.45 mol.a-1 [7,8]). Such
a discrepancy might be due to a higher convection regime in the past sustained by a higher heat flux in the
early Earth's interior [9].
Fig. 1: Isotopic composition of Xe in Barberton quartz samples
(3.2 Ga) in delta notation normalized to
composition of the modern atmosphere.
130
Xe and to the isotopic
U-Xe as the starting isotopic composition. The
precise determination of the isotopic ratios of Xe in
Barberton samples allows us to compute what was the
starting isotopic composition of atmospheric Xe (Fig.
2). This starting isotopic composition was similar to
the theoretical U-Xe [2,3]. This observation calls for a
contribution to the Earth's atmosphere by some extraterrestrial material different from chondrites. Comets
are promising candidates for carrying this unusual
starting isotopic composition to the Earth since these
objects present high noble gases to H2O ratios [10].
from the atmosphere to the outer space [12]. In these
conditions, atmospheric xenon was probably also easily ionized. K. Zahnle proposed a model in which Xe
ions escape ar lifted up by H ions and escape along
open lines of the terrestrial magnetic field [13]. If this
model is correct, the progressive isotopic fractionation
of atmospheric xenon might reflect the escape of hydrogen from the early atmosphere that finally led to a
global oxidation of the Earth's atmosphere [14].
Fig. 3: Evolution of the isotopic fractionation (in delta notation
relative to the isotopic composition of the modern atmosphere) of
atmospheric Xe with time. The starting isotopic composition is U-Xe
(red star). The question mark represents the lack of knowledge on
the mode of transition from U-Xe to the 3.5 Ga-old atmosphere.
Fig. 2: Three-isotope plot of Xe demonstrating that only the massdependent isotopic fractionation of an isotopic composition similar
to U-Xe (red range) (1), followed by the addition of products of the
spontaneous fission of
238
U (2) are able to reproduce the isotopic
composition of Xe measured in Barberton samples (black dot).
Evolution of the isotopic composition of atmospheric Xe. Results obtained on quartz with ages bracketed between 2.7 Ga and 400 Ma allow us to build the
curve of the evolution of the isotopic composition of
atmospheric Xe with a much higher resolution than in
previous studies (Fig. 3). Our results confirm that the
isotopic fractionation of atmospheric xenon was established through long-term geological processes acting
over more than 2.5 Ga. Interestingly, the isotopic fractionation marked a pause of 500 Ma, between 3.2 Ga
and 2.7 Ga. The modern isotopic composition of atmospheric Xe was established around 2 Ga. Previous
models calling for early episodes of hydrodynamic
escape, preferential Xe retention in the mantle and/or
cometary contribution must be revisited.
How to selectively escape Xe during several Ga?
The EUV flux from the Sun was several orders of
magnitude higher in the Archean [11]. Such a high flux
promoted the ionization and escape of hydrogen ions
Conclusions: This study demonstrates the need for
U-Xe as the starting isotopic composition of atmospheric xenon. This component may have been brought
to the Earth by volatile-rich (cometary?) bodies. Furthermore, the evolution of the isotopic composition of
atmospheric xenon with time is now documented with
a good temporal and isotopic resolution. Even if the
mechanism responsible for the long-term escape of Xe
remains unknown, this evolution might be linked to
hydrogen escape episodes and thus to major changes in
the composition of the Earth's atmosphere.
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