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The Earth's missing xenon: A combination of early degassing and of rare gas loss from the atmosphere
CHEMICAL GEOLOGY ,
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L";() T O P E (;I,:O.S'('IE.V( '1:
ELSEVIER
Chemical Geology 115 ( 1994 ) 1-6
Letter Section
The Earth's missing xenon: A combination of early degassing and of rare gas loss from the atmosphere 1 I.N. Tolstikhin a'b, R.K. O ' N i o n s a "Department of Earth Sciences, UniversiO,of Cambridge, Cambridge, CB2 3EQ, UK bGeological Institute, Kola Department, Russian Academy qf Sciences, ApatiO', Apatio' 184 200, Russia (Received March 17, 1994; revision accepted March 25, 1994)
Abstract The under-abundance of Xe in the Earth's atmosphere relative to solar system abundances has been long recog-
nised and is usually referred to as the "missing xenon problem". It is suggested here that this feature of the Earth's composition is the result of processes operative during its accretion and earliest degassing history,'. In essence, it arises from several competing processes. One of these is sorption of rare gases onto the accreting material, which favours an excess of Xe relative to lighter rare gases. Another is the subsequent fractional degassing of melts formed during or shortly after accretion, which favours preferential degassing of Xe over lighter rare gases from the Earth's interior. These two processes are combined with a major loss of rare gas from the Earth's early atmosphere, possibly accompanying hydrogen hydrodynamic escape.
1. Introduction
Considerations of rare gas abundances have been central to discussions of the accretion and degassing history of the terrestrial planets. A puzzling feature of both Earth and Mars has been the apparent under-abundance of primordial Xe relative to Kr and Ar in their atmospheres when compared with rare gases in meteorites (Krummenacher et al., 1962; Canalas et al., 1968; Fanale and Cannon, 1971 ; Pepin, 1991, 1992 ). Because proposed mechanisms of rare gas escape from planetary atmospheres should result in a relative over-abundance of the heaviest gas, Xe (Zahnle and Kasting, 1986; Hunten et al., 1987; Zahnle et al., 1990; Pepin, 1991, 1992), a sys[MB] IDepartrnenl of Earth Sciences Contribution No. 3772.
tematic search for an additional reservoir of Xe has been made (e.g., Podosek et al., 1981; Bernatowicz et al., 1984, 1985; Wacker and Anders, 1984; Matsuda and Matsubara, 1989; Matsuda et al., 1993 ). Such a reservoir has not been found. In this contribution we show that the underabundance of Xe may arise from the combination of processes that operated during planetary accretion. These are degassing of early formed melts favouring the loss of the heavier and less soluble gases and preferential escape of the lighter gases from an early atmosphere.
2. Terrestrial xenon
The ratios 13°Xe/36Arand t3°Xe/84K/"in the Earth's present-day atmosphere are a factor of 15 lower than those in carbonaceous chondrites
0009-2541/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
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I.N. Tolstikhin, R.K. O'Nions / Chemical Geology 115 (1994) 1-6
(Mazor et al., 1970; Wieler et al., 1991 ) and a factor of ~ 2 lower than the solar abundances (Anders and Grevesse, 1989). This deficit of primordial Xe (Fig. 1 ), although recognised 30 years ago by Krummenacher et al. (1962), remains an unresolved problem (Pepin, 1991, 1992; Matsuda et al., 1993 ). Because Xe relative to the lighter rare gases is more readily adsorbed onto various surfaces, missing Xe was searched for in sediments by Podosek et al. ( 1981 ) and Bernatowicz et al. (1984), in ice by Wacker and Anders (1984) and Bernatowicz et al. (1985), and in silica by Matsuda and Matsubara ( 1989 ), but in each case with negative results. Furthermore, the metal/silicate partition coefficient for Xe reported by Matsuda et al. (1993) suggests that the core itself cannot be the reservoir for missing Xe either. There is straightforward evidence which shows that the upper mantle is highly depleted in Xe and like the other rare gases its behaviour is similar to that of lithophile in-2 4C1 -3
Venus ~" -4 o t-E
Earth
-5
e3
-6 o t-
-7
t..Q
-8
O
rr
-9
O
.J -10 -11 3He
22Ne
36Ar
84Kr
130Xe
Fig. 1. Normalised abundances of primordial rare gases in various reservoirs of the solar system (compiled from Pepin, 1991 ).
compatible elements (l-Iiyagon and Ozima, 1986; Azbel and Tolstikhin, 1990, 1993a; Valbracht et al., 1994). Such behaviour also means that the isolation of substantial amounts of Xe in the lower mantle is improbable and that the underabundance of Xe is a whole-planet feature. Also important is the observation that the isotope composition of atmospheric Xe differs from that of solar and planetary Xe in being relatively deficient in the lighter isotopes (Krummenacher et al., 1962; Pepin and Phinney, 1978; Pepin, 1991). Several authors (Zahnle and Kasting, 1986; Hunten et al., 1987; Zahnle et al., 1990; Pepin, 1991, 1992) have developed models for hydrodynamic escape of a hydrogen-rich primary atmosphere driven by an intense ultraviolet radiation from the young Sun. This radiation and related hydrodynamic escape are expected to operate during and after accretion of the terrestrial planets, and decay on the time scale of ~ 100 Ma. Rare gases are dissipated from the atmosphere by this flux with a preferential release of lighter species. These models are also able to reproduce the observed light-isotope-depleted character of terrestrial Xe. However, the observed relative under-abundance of Xe in the atmosphere is contrary to the predictions of these models. The observed isotopic fractionation of the atmospheric Xe, if the result of hydrodynamic escape, imposes some limits on the ratio of the Xe retained to that lost by the early atmosphere. This ratio has been estimated at < 0.02 by Hunten et al. (1987), and at <0.1 by Pepin (1991). This same ratio has also been estimated from a consideration of P u - U - I - X e isotopic systematics at <0.01 by Azbel and Tolstikhin (1993a, b). Taken together these estimates point to the former existence of a much more massive atmosphere than the present-day one. Any initial rare gas abundance in this early atmosphere, either planetary or solar, would have a 13°Xe/84K_r ratio greater than that in the present-day atmosphere. Hydrodynamic escape from this atmosphere would serve to increase this ratio and thereby further exacerbate the under-abundance of Xe.
I.N. Tolstikhin, R.K. O'Nions / (71emical Geology 115 (1994) 1-6
3. The role of melt degassing The situation changes radically if rare gases were released into the atmosphere from the Earth's mantle and/or from colliding planetesimals through partial degassing of molten silicates at or near the Earth surface. This is because rare gases have different solubilities in silicates melts with Xe being the least soluble as shown by Kirsten (1968), Hiyagon and Ozima (1986), Jambon et al. (1986), Lux (1987) and Broadhurst et al. (1992). Fractional degassing of silicate melts exploits these solubility differences to produce a preferential degassing of Xe into the atmosphere. Therefore, in contrast to hydrodynamic loss of the rare gases from the atmosphere which favours retention of Xe over lighter rare gases, fractional degassing of melts (Zhang and Zindler, 1989; Spasennykh and Tolstikhin, 1993 ) favours loss of Xe over that lighter rare gases from the silicate Earth to the atmosphere. The principal thesis of this contribution is that the "missing" primordial Xe has arisen through the combination of "competing" processes - degassing of melts and simultaneous escape of rare gases from an early atmosphere. This thesis is considered further assuming that: ( 1 ) the rare gas abundances in the solid Earth were established by the fractional degassing of melts; and (2) that a decreasing hydrogen hydrodynamic flux from the atmosphere progressively established the abundance of each rare gas in the atmosphere as the flux became too small to sustain its loss. According to these assumptions, the hydrogen hydrodynamic flux during accretion must have been sufficient to allow Kr to escape nearquantitatively whilst Xe attained the present-day abundances of the non-radiogenic isotopes. Subsequent to accretion, degassing of the mantle supplied Kr to the post-accretion atmosphere, but only a minor or negligible part of its Xe. This sequence of events is explored further in Fig. 2. Little attention has been paid so far to the problem of large-scale melt degassing both during and after Earth accretion. Therefore, at this stage only the consequences of a simple degassing process is considered. In the calculations embodied in Fig. 2 it is assumed that melts
3
undergo fractional degassing through separation and loss of gas bubbles triggered by the pressure release. The formalism used follows that published by Zhang and Zindler ( 1989 ) and Spasennykh and Tolstikhin (1993) for fractional degassing. Rare gas fractionation was calculated using:
log(z/Z,, ) = (S,,/Si- 1 )log(z,, )
(1)
where subscripts i and n denote different rare gas species; S is the solubility (Henry.) constant of rare gases in silicate melts, presented in the caption to Fig. 2; x=C/C(O)~< 1 is the ratio of the concentrations of species in the melt before, C(0), and after degassing, C; (L/X,,) is the fractionation factor for species i relative to n. The maximum fractionation of two species i and n is determined in Eq. 1 by the ratio of their solubility constants, S,,/S,. For example, values of SAt=5-10 -5 cm 3 STP g-i atm-~, SAJSxc=3.3 and Sne/Sx,.=33 (Lux, 1987: Zhang and Zintiler, 1989) give XXe/Z~,r=0.0027 and )¢xe/ Xn~---0.00028 for ZA~=0.08. Preferential depletion of Xe relative to all rare gases occurs in the melt through the process of fractional degassing. The calculation made so far (Fig. 2) is intended to be illustrative of the processes involved and to explore a fairly extreme case to make the point. The assumed initial planetary abundances of the rare gases in the accreting material (heavy dashed line) and the present-day atmospheric (heavy solid line) abundances are shown. Stage 1 of the degassing-dissipation process coincides with the end of accretion. The rare gas concentrations in the Earth's interior have been decreased by degassing to the point where the Kr abundance in the mantle is equal to the present atmosphere inventory and the K r / X e ratio in the mantle has increased substantially. It is assumed, following models of hydrodynamic escape proposed by Zahnle and Kasting (1986), Hunten et al. (1987), Zahnle et al. (1990) and Pepin ( 1991, 1992 ), that He, Ne, Ar and Kr are mostly lost from the atmosphere at this stage and that isotopic fractionation of the residual Xe has occurred. The Kr abundance in the atmosphere is small at this stage relative to that in the mantle. The abundance and isotope composition of Xe in the atmosphere are similar to those in the
4
I.N. Tolstikhin, R.K. O'Nions /Chemical Geology 115 (1994) 1-6
[Accreting material l
t..) ¢t~ "¢,.. 0 ,.,0 t~
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,
/
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~O
~ -
. -~ - . . . . . . . . . .
•j~--...........~._.,,
~
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-6
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. . . . . .
~..::::
lID
8I
n"
-9
0 _J
-10
-o("v
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,..
-7
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.'"
..
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' ~ ~stages of degassingl ~ " .'. ./ //
"'J~.. . '" ~ . .
.......
[Earth's atmosphere~ I 3He
~
.
I 22Ne
".,
:3) I 36Ar
I • MKr
.
I 13°Xe
Fig. 2. Rare gas fractionation during degassing of the Earth's interior and rare-gas escape from an early atmosphere. Rare gas abundances are expressed relative to solar abundances ( Pepin, 1991 ). The accreting material is assumed to contain rare gases in chondritic abundances. A typical chondritic abundance pattern for rare gases in the accreting material is used here as an example. Faint dotted lines designate rare gas abundances in the Earth's interior at successive stages of degassing ( 1 to 3). The abundances in the mantle-atmosphere system decrease with time by degassing of silicate melts and simultaneous escape of rare gases from an early atmosphere driven by a decreasing hydrodynamic hydrogen flux. Rare gas solubilities are compiled from Lux (1987); the values are the same as those used by Zhang and Zindler (1989): SAt= 5" 10-5 cm 3 STP g - i atm-~, SHe/SAr 10, SNe/SAf = 4, SKr/SAr 0.5 5 and Sxe/S~r = 0.3. Ideal gas behaviour, equilibrium partitioning between melt and gas phases, and infinitesimal small gas/melt ratio are envisaged in this simple model. Stage l of the degassing-dissipation process extends from the start of accretion until the calculated 8"Kr abundance in the mantle equals that in the present-day atmosphere. The calculated abundances of all gases as the end of Stage 1 (end of accretion) are shown. At that time the abundance of atmospheric Xe is set. During Stage 2 Kr is accumulated in the atmosphere, whilst Ar and the lighter rare gases continue escape. At the end of Stage 2 Kr is also set in the atmosphere. During Stage 3 Ar accumulates and is set at the end of this stage. Similar considerations hold for Ne. Escape of He from the atmosphere is a permanent feature of the Earth's history. =
present-day atmosphere, while the mantle abundance of Xe is already negligibly small. Following the terminology of Pepin (1991 ), the atmospheric abundance of Xe has now been set, as the hydrodynamic flux becomes too small to cause further loss. At Stage 2 (Fig. 2) further mantle degassing occurs but the hydrodynamic flux from the atmosphere is now lower such that Kr accumulates in the atmosphere. At the end of this stage the abundance of atmospheric Kr approaches that in the present-day atmosphere, and its mantle abundance becomes relatively small. The lighter rare gases, Ar, Ne, He, are set in a similar way, depending upon the detailed relationship of the degassing and dissipative fluxes (Stage 3).
=
4. Concluding remarks A net consequence of this process is that Xe is degassed from the mantle much more efficiently than He (Fig. 2 ). It is emphasised, that so far we have only attempted to examine qualitatively those physical processes that are likely to exert the dominant control on the final rare gas inventory of the Earth. The calculations carded out are only illustrative of these processes and are a preliminary to the development of a fuller model. A feature of this process is that the Earth's interior should remain relatively enriched in He over Xe. The present-day abundance of 3He in the upper mantle can be maintained by a small (O'Nions and Tolstikhin, 1994) or negligibly small (Azbel
I.N. Tolstikhin, R.K. O'Nions / Chemical Geology 115 (1994) I-6
and Tolstikhin, 1990) flux from the lower mantle. A prediction of the simple model explored above is that the small amount of residual Xe in the upper mantle should have non-radiogenic isotope abundances similar to those in the initial accreting material. Analysis of Xe from ocean ridge basalts (Staudacher and All~gre, 1982; Marty, 1989; Hiyagon et al., 1992) and continental CO2 gases (Phinney et al., 1978; Caffee et al., 1994) show that mantle Xe is in fact much closer to atmospheric Xe in its isotope composition. The extent to which recycling of atmospheric Xe into the upper mantle (which need only be very small, e.g., Azbel and Tolstikhin, 1993a, b) on the one hand, or an initial Xe composition in the accreting material different to that in C1 chondrites (Ozima and Nakazawa, 1980; Sugiura, 1993) on the other, is responsible for this observation is an open question. However, in either case the central thesis expressed here that fractional degassing of melt is a key part of the missing Xe problem is unaffected. The above discussion enables us to conclude that the rare gas abundances in the Earth's atmosphere are to result of: (1) sorption and/or implantation to provide initial concentrations in accreting materials; (2) fractional degassing of early formed melts with a consequent fractionation of rare gas abundances in the Earth's interior; and ( 3 ) hydrodynamic escape of rare gases accompanying a decreasing hydrogen flux. It would be interesting to assess weather these processes have also been important in the establishment of the rare gas abundances in the Martian and Venusian atmospheres.
Acknowledgements
We are grateful to R.O. Pepin and Y. Zhang for their comments. This research has been supported by the Royal Society. Igor Tolstikhin acknowledges support from the Newton Trust and Clare Hall, Cambridge.
5
References Anders, E. and Grevesse, N., 1989. Abundances of the elements: Meteoritic and solar. Geochim. Cosmochim. Acta, 53: 197-214. Azbel, l.Ya. and Tolstikhin, 1.N., 1990. Geodynamics, magmatism, and degassing of t;hc Earth. Geochim. Cosmochim. Acta, 54: 139-154. Azbel, l.Ya. and Tolstikhin, I.N., 1993a. Accretion and early degassing of the Earth: constraints from P u - U - I - X e isotopic systematics. Meteoritics, 28:609-621. Azbel, l.Ya. and Tolstikhin, I.N., 1993b. Accretion and early degassing of the earth: constraints from modelling of PuU - I - X e isotopic systematics. Preprint, Department of Earth Sciences, University of Cambridge, Cambridge, 100 PP. Bernatowicz, T.J., Podosek, F.A., Honda, M. and Kramer, F.E., 1984. The atmospheric inventory of xenon and noble gases in shales: the plastic bag experiment. J. Geophys. Res., 89B6:4597-4611. Bernatowicz, T.J., Kennedy, B.M. and Podosek, F.A., 1985. Xe in glacial ice and the atmospheric inventory of noble gases. Geochim. Cosmochim. Acta, 49:2561-2564. Broadhurst, C.L., Drake, M.J., Hagee, B.E. and Bernatowicz, T.J., 1992. Solubility and partitioning ofNe, Ar, Kr, and Xe in minerals and synthetic basaltic melts. Geochim. Cosmochim. Acta, 56: 709-'723. Caffee, M.W., Hudson, G.B., Velsko, C., Alexander, Jr., E.C., Huss, G.R. and Chivas, A.R., 1994. Xe and Ne in CO2 well gases: A meteoritic component in the mantle. Nature (London) (submitted). Canalas, R.A., Alexander, Jr., E.C. and Manuel, O.K., 1968. Terrestrial abundance of noble gases. J. Geophys. Res., 73: 3332-3334. Fanale, F.P. and Cannon, W.A., 1971. Adsorption on the Martian regolilh. Nature (London), 230: 502-504. Hiyagon, H. and Ozima, M., 1986. Noble gas distribution between basalt and crystal. Earth Planet. Sci. Lelt., 58: 255264. Hiyagon, H., Ozima, M., Marty, B., Zashu, S. and Sakai, H., 1992. Noble gases in submarine glasses from mid-oceanic ridges and Loihi seamount: Constraints on the early history of the Earth. Geochim. Cosmochim. Acta, 56:130113016. Hunten, D.M., Pepin, R.O. and Walker, J.C.G., 1987. Mass fractionation in hydrodynamic escape. Icarus, 69: 532549. Jambon, A., Weber, H. and Braun, O., 1986. Solubility of He, Ne, At, Kr and Xe in basalt melt in the range 12501600°C. Geochemical implications. Geochim. Cosmochim. Acta, 50: 401-408. Kirsten, T., 1968. Incorporation of rare gases in solidifying enstatite melts. J. Geophys. Res., 73: 2807-2810. Krummenacher, D., Merrihue, C.M., Pepin, R.O. and Reynolds, J.H., 1962. Meteoritic krypton and barium versus the general isotopic anomaliies in xenon. Geochim. Cosmochim. Acta, 26:231-249.
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I.N. Tolstikhin, R.IC O'Nions / Chemical Geology 115 (1994) I-6
Lux, G., 1987. The behaviour of noble gases in silicate solution, diffusion, bubbles and surface effects, with implications to natural samples. Geochim. Cosmochim. Acta, 51 : 1549-1560. Marry, B., 1989. Neon and xenon isotopes in MORB: Implications for the earth-atmosphere evolution. Earth Planet. Sci. Lett., 94: 45-56. Matsuda, J. and Matsubara, K., 1989. Noble gases in silica and their implication for the terrestrial " missing" Xe. Geophys. Res. Lett., 16: 81-84. Matsuda, J., Sudo, M., Ozima, M., Ito, K., Ohtaka, O. and Ito, E., 1993. Noble gas partitioning between metal and silicate under high pressures. Science, 259: 788-790. Mazor, E., Heymann, D. and Anders, E., 1970. Noble gases in carbonaceous chondrites. Geochim. Cosmochim. Acta, 34: 781-824. O'Nions, R.K. and Tolstikhin, I.N., 1994. Behaviour and residence times of lithophile and rare gas tracers in the upper mantle. Earth Planet. Sci. Lett., 124 (in press). Ozima, M. and Nakazawa, K., 1980. Origin of rare gases on the Earth. Nature (London), 284:313-316. Pepin, R.O., 1991. On the origin and early evolution of terrestrial planet atmospheres and meteoritic volatiles. Icarus, 92: 1-79. Pepin, R.O., 1992. Origin of noble gases in the terrestrial planets. Annu. Rev. Earth Planet. Sci., 20: 389-430. Pepin, R.O. and Phinney, D., 1978. Components of xenon in the Solar system. Dep. Phys., Univ. of Minnesota, Minneapolis, Minn. Unpublished preprint Pbinney, D., Tennyson, J. and Frick, U., 1978. Xenon in CO2 well gas revisited. J. Geophys. Res., 83:2313-2319. Podosek, F.A., Bernatowicz, T.J. and Kramer, F.E,, 1981.
Adsorption and excess fission xenon. Proc. Lunar Planet. Sci., 12B: 891-901. Spasennykh, M.Yu. and Tolstikhin, I.N., 1993. Noble gas fractionation during degassing of melts. Geochem. J., 27: 213-217. Staudacher, Th. and Allhgre, C.J., 1982. Terrestrial xeno[ogy. Earth Planet. Sci. Lett., 60: 389-406. Sugiura, N., 1993. Formation of atmospheres of terrestrial planets from volatiles in solid (meteorite-like) material. In: H. Oya (Editor), Primitive Solar nebula and origin of planets. Terra Sci. Pub. Co., Tokyo, pp. 527-543. Valbracht, P.J., Honda, M., Staudigel, H., McDougall, [. and Trost, A.P., 1994. Noble gas partitioning in natural samples: results from coexisting glass and olivine phenocrysts in four Hawaiian basalts. Abstr., Int. Workshop on Noble Gas Geochemistry and Cosmochemistry, Kyoto, pp. 1920. Wacker, J.F. and Anders, E., 1984. Trapping of xenon in ice: implications for the origin of the Earth's noble gases. Geochim. Cosmochim. Acta, 48: 2373-2380. Wieler, R., Anders, E., Baur, H., Lewis, R.S. and Signer, P., 1991. Noble gases in "phase Q": Closed-system etching of an Allende residue. Geochim. Cosmochim. Acta, 55: 1709-1722. Zahnle, K. and Kasting, J.F., 1986. Mass fractionation during transonic escape and implications for loss of water from Mars and Venus. Icarus, 68:62-97 Zahnle, K., Kasting, J.F. and Pollack, J.B., 1990. Mass fractionation of noble gases in diffusion-limited hydrodynamic hydrogen escape. Icarus, 69: 532-549. Zhang, Y. and Zindler, A., 1989. Noble gas constraints on the evolution of the Earth's atmosphere. J. Geophys. Res., 94B10: 13719-13737.
,,
,v
,,
L";() T O P E (;I,:O.S'('IE.V( '1:
ELSEVIER
Chemical Geology 115 ( 1994 ) 1-6
Letter Section
The Earth's missing xenon: A combination of early degassing and of rare gas loss from the atmosphere 1 I.N. Tolstikhin a'b, R.K. O ' N i o n s a "Department of Earth Sciences, UniversiO,of Cambridge, Cambridge, CB2 3EQ, UK bGeological Institute, Kola Department, Russian Academy qf Sciences, ApatiO', Apatio' 184 200, Russia (Received March 17, 1994; revision accepted March 25, 1994)
Abstract The under-abundance of Xe in the Earth's atmosphere relative to solar system abundances has been long recog-
nised and is usually referred to as the "missing xenon problem". It is suggested here that this feature of the Earth's composition is the result of processes operative during its accretion and earliest degassing history,'. In essence, it arises from several competing processes. One of these is sorption of rare gases onto the accreting material, which favours an excess of Xe relative to lighter rare gases. Another is the subsequent fractional degassing of melts formed during or shortly after accretion, which favours preferential degassing of Xe over lighter rare gases from the Earth's interior. These two processes are combined with a major loss of rare gas from the Earth's early atmosphere, possibly accompanying hydrogen hydrodynamic escape.
1. Introduction
Considerations of rare gas abundances have been central to discussions of the accretion and degassing history of the terrestrial planets. A puzzling feature of both Earth and Mars has been the apparent under-abundance of primordial Xe relative to Kr and Ar in their atmospheres when compared with rare gases in meteorites (Krummenacher et al., 1962; Canalas et al., 1968; Fanale and Cannon, 1971 ; Pepin, 1991, 1992 ). Because proposed mechanisms of rare gas escape from planetary atmospheres should result in a relative over-abundance of the heaviest gas, Xe (Zahnle and Kasting, 1986; Hunten et al., 1987; Zahnle et al., 1990; Pepin, 1991, 1992), a sys[MB] IDepartrnenl of Earth Sciences Contribution No. 3772.
tematic search for an additional reservoir of Xe has been made (e.g., Podosek et al., 1981; Bernatowicz et al., 1984, 1985; Wacker and Anders, 1984; Matsuda and Matsubara, 1989; Matsuda et al., 1993 ). Such a reservoir has not been found. In this contribution we show that the underabundance of Xe may arise from the combination of processes that operated during planetary accretion. These are degassing of early formed melts favouring the loss of the heavier and less soluble gases and preferential escape of the lighter gases from an early atmosphere.
2. Terrestrial xenon
The ratios 13°Xe/36Arand t3°Xe/84K/"in the Earth's present-day atmosphere are a factor of 15 lower than those in carbonaceous chondrites
0009-2541/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
SSDI0009-2541 (94)00028-7
2
I.N. Tolstikhin, R.K. O'Nions / Chemical Geology 115 (1994) 1-6
(Mazor et al., 1970; Wieler et al., 1991 ) and a factor of ~ 2 lower than the solar abundances (Anders and Grevesse, 1989). This deficit of primordial Xe (Fig. 1 ), although recognised 30 years ago by Krummenacher et al. (1962), remains an unresolved problem (Pepin, 1991, 1992; Matsuda et al., 1993 ). Because Xe relative to the lighter rare gases is more readily adsorbed onto various surfaces, missing Xe was searched for in sediments by Podosek et al. ( 1981 ) and Bernatowicz et al. (1984), in ice by Wacker and Anders (1984) and Bernatowicz et al. (1985), and in silica by Matsuda and Matsubara ( 1989 ), but in each case with negative results. Furthermore, the metal/silicate partition coefficient for Xe reported by Matsuda et al. (1993) suggests that the core itself cannot be the reservoir for missing Xe either. There is straightforward evidence which shows that the upper mantle is highly depleted in Xe and like the other rare gases its behaviour is similar to that of lithophile in-2 4C1 -3
Venus ~" -4 o t-E
Earth
-5
e3
-6 o t-
-7
t..Q
-8
O
rr
-9
O
.J -10 -11 3He
22Ne
36Ar
84Kr
130Xe
Fig. 1. Normalised abundances of primordial rare gases in various reservoirs of the solar system (compiled from Pepin, 1991 ).
compatible elements (l-Iiyagon and Ozima, 1986; Azbel and Tolstikhin, 1990, 1993a; Valbracht et al., 1994). Such behaviour also means that the isolation of substantial amounts of Xe in the lower mantle is improbable and that the underabundance of Xe is a whole-planet feature. Also important is the observation that the isotope composition of atmospheric Xe differs from that of solar and planetary Xe in being relatively deficient in the lighter isotopes (Krummenacher et al., 1962; Pepin and Phinney, 1978; Pepin, 1991). Several authors (Zahnle and Kasting, 1986; Hunten et al., 1987; Zahnle et al., 1990; Pepin, 1991, 1992) have developed models for hydrodynamic escape of a hydrogen-rich primary atmosphere driven by an intense ultraviolet radiation from the young Sun. This radiation and related hydrodynamic escape are expected to operate during and after accretion of the terrestrial planets, and decay on the time scale of ~ 100 Ma. Rare gases are dissipated from the atmosphere by this flux with a preferential release of lighter species. These models are also able to reproduce the observed light-isotope-depleted character of terrestrial Xe. However, the observed relative under-abundance of Xe in the atmosphere is contrary to the predictions of these models. The observed isotopic fractionation of the atmospheric Xe, if the result of hydrodynamic escape, imposes some limits on the ratio of the Xe retained to that lost by the early atmosphere. This ratio has been estimated at < 0.02 by Hunten et al. (1987), and at <0.1 by Pepin (1991). This same ratio has also been estimated from a consideration of P u - U - I - X e isotopic systematics at <0.01 by Azbel and Tolstikhin (1993a, b). Taken together these estimates point to the former existence of a much more massive atmosphere than the present-day one. Any initial rare gas abundance in this early atmosphere, either planetary or solar, would have a 13°Xe/84K_r ratio greater than that in the present-day atmosphere. Hydrodynamic escape from this atmosphere would serve to increase this ratio and thereby further exacerbate the under-abundance of Xe.
I.N. Tolstikhin, R.K. O'Nions / (71emical Geology 115 (1994) 1-6
3. The role of melt degassing The situation changes radically if rare gases were released into the atmosphere from the Earth's mantle and/or from colliding planetesimals through partial degassing of molten silicates at or near the Earth surface. This is because rare gases have different solubilities in silicates melts with Xe being the least soluble as shown by Kirsten (1968), Hiyagon and Ozima (1986), Jambon et al. (1986), Lux (1987) and Broadhurst et al. (1992). Fractional degassing of silicate melts exploits these solubility differences to produce a preferential degassing of Xe into the atmosphere. Therefore, in contrast to hydrodynamic loss of the rare gases from the atmosphere which favours retention of Xe over lighter rare gases, fractional degassing of melts (Zhang and Zindler, 1989; Spasennykh and Tolstikhin, 1993 ) favours loss of Xe over that lighter rare gases from the silicate Earth to the atmosphere. The principal thesis of this contribution is that the "missing" primordial Xe has arisen through the combination of "competing" processes - degassing of melts and simultaneous escape of rare gases from an early atmosphere. This thesis is considered further assuming that: ( 1 ) the rare gas abundances in the solid Earth were established by the fractional degassing of melts; and (2) that a decreasing hydrogen hydrodynamic flux from the atmosphere progressively established the abundance of each rare gas in the atmosphere as the flux became too small to sustain its loss. According to these assumptions, the hydrogen hydrodynamic flux during accretion must have been sufficient to allow Kr to escape nearquantitatively whilst Xe attained the present-day abundances of the non-radiogenic isotopes. Subsequent to accretion, degassing of the mantle supplied Kr to the post-accretion atmosphere, but only a minor or negligible part of its Xe. This sequence of events is explored further in Fig. 2. Little attention has been paid so far to the problem of large-scale melt degassing both during and after Earth accretion. Therefore, at this stage only the consequences of a simple degassing process is considered. In the calculations embodied in Fig. 2 it is assumed that melts
3
undergo fractional degassing through separation and loss of gas bubbles triggered by the pressure release. The formalism used follows that published by Zhang and Zindler ( 1989 ) and Spasennykh and Tolstikhin (1993) for fractional degassing. Rare gas fractionation was calculated using:
log(z/Z,, ) = (S,,/Si- 1 )log(z,, )
(1)
where subscripts i and n denote different rare gas species; S is the solubility (Henry.) constant of rare gases in silicate melts, presented in the caption to Fig. 2; x=C/C(O)~< 1 is the ratio of the concentrations of species in the melt before, C(0), and after degassing, C; (L/X,,) is the fractionation factor for species i relative to n. The maximum fractionation of two species i and n is determined in Eq. 1 by the ratio of their solubility constants, S,,/S,. For example, values of SAt=5-10 -5 cm 3 STP g-i atm-~, SAJSxc=3.3 and Sne/Sx,.=33 (Lux, 1987: Zhang and Zintiler, 1989) give XXe/Z~,r=0.0027 and )¢xe/ Xn~---0.00028 for ZA~=0.08. Preferential depletion of Xe relative to all rare gases occurs in the melt through the process of fractional degassing. The calculation made so far (Fig. 2) is intended to be illustrative of the processes involved and to explore a fairly extreme case to make the point. The assumed initial planetary abundances of the rare gases in the accreting material (heavy dashed line) and the present-day atmospheric (heavy solid line) abundances are shown. Stage 1 of the degassing-dissipation process coincides with the end of accretion. The rare gas concentrations in the Earth's interior have been decreased by degassing to the point where the Kr abundance in the mantle is equal to the present atmosphere inventory and the K r / X e ratio in the mantle has increased substantially. It is assumed, following models of hydrodynamic escape proposed by Zahnle and Kasting (1986), Hunten et al. (1987), Zahnle et al. (1990) and Pepin ( 1991, 1992 ), that He, Ne, Ar and Kr are mostly lost from the atmosphere at this stage and that isotopic fractionation of the residual Xe has occurred. The Kr abundance in the atmosphere is small at this stage relative to that in the mantle. The abundance and isotope composition of Xe in the atmosphere are similar to those in the
4
I.N. Tolstikhin, R.K. O'Nions /Chemical Geology 115 (1994) 1-6
[Accreting material l
t..) ¢t~ "¢,.. 0 ,.,0 t~
/
"4
,
/
I 0 GO ¢3 t"13 t.£1 t~
~O
~ -
. -~ - . . . . . . . . . .
•j~--...........~._.,,
~
I I /
-6
L'"
. . . . . .
~..::::
lID
8I
n"
-9
0 _J
-10
-o("v
"'''-. /
,..
-7
0
.'"
..
J
...
' ~ ~stages of degassingl ~ " .'. ./ //
"'J~.. . '" ~ . .
.......
[Earth's atmosphere~ I 3He
~
.
I 22Ne
".,
:3) I 36Ar
I • MKr
.
I 13°Xe
Fig. 2. Rare gas fractionation during degassing of the Earth's interior and rare-gas escape from an early atmosphere. Rare gas abundances are expressed relative to solar abundances ( Pepin, 1991 ). The accreting material is assumed to contain rare gases in chondritic abundances. A typical chondritic abundance pattern for rare gases in the accreting material is used here as an example. Faint dotted lines designate rare gas abundances in the Earth's interior at successive stages of degassing ( 1 to 3). The abundances in the mantle-atmosphere system decrease with time by degassing of silicate melts and simultaneous escape of rare gases from an early atmosphere driven by a decreasing hydrodynamic hydrogen flux. Rare gas solubilities are compiled from Lux (1987); the values are the same as those used by Zhang and Zindler (1989): SAt= 5" 10-5 cm 3 STP g - i atm-~, SHe/SAr 10, SNe/SAf = 4, SKr/SAr 0.5 5 and Sxe/S~r = 0.3. Ideal gas behaviour, equilibrium partitioning between melt and gas phases, and infinitesimal small gas/melt ratio are envisaged in this simple model. Stage l of the degassing-dissipation process extends from the start of accretion until the calculated 8"Kr abundance in the mantle equals that in the present-day atmosphere. The calculated abundances of all gases as the end of Stage 1 (end of accretion) are shown. At that time the abundance of atmospheric Xe is set. During Stage 2 Kr is accumulated in the atmosphere, whilst Ar and the lighter rare gases continue escape. At the end of Stage 2 Kr is also set in the atmosphere. During Stage 3 Ar accumulates and is set at the end of this stage. Similar considerations hold for Ne. Escape of He from the atmosphere is a permanent feature of the Earth's history. =
present-day atmosphere, while the mantle abundance of Xe is already negligibly small. Following the terminology of Pepin (1991 ), the atmospheric abundance of Xe has now been set, as the hydrodynamic flux becomes too small to cause further loss. At Stage 2 (Fig. 2) further mantle degassing occurs but the hydrodynamic flux from the atmosphere is now lower such that Kr accumulates in the atmosphere. At the end of this stage the abundance of atmospheric Kr approaches that in the present-day atmosphere, and its mantle abundance becomes relatively small. The lighter rare gases, Ar, Ne, He, are set in a similar way, depending upon the detailed relationship of the degassing and dissipative fluxes (Stage 3).
=
4. Concluding remarks A net consequence of this process is that Xe is degassed from the mantle much more efficiently than He (Fig. 2 ). It is emphasised, that so far we have only attempted to examine qualitatively those physical processes that are likely to exert the dominant control on the final rare gas inventory of the Earth. The calculations carded out are only illustrative of these processes and are a preliminary to the development of a fuller model. A feature of this process is that the Earth's interior should remain relatively enriched in He over Xe. The present-day abundance of 3He in the upper mantle can be maintained by a small (O'Nions and Tolstikhin, 1994) or negligibly small (Azbel
I.N. Tolstikhin, R.K. O'Nions / Chemical Geology 115 (1994) I-6
and Tolstikhin, 1990) flux from the lower mantle. A prediction of the simple model explored above is that the small amount of residual Xe in the upper mantle should have non-radiogenic isotope abundances similar to those in the initial accreting material. Analysis of Xe from ocean ridge basalts (Staudacher and All~gre, 1982; Marty, 1989; Hiyagon et al., 1992) and continental CO2 gases (Phinney et al., 1978; Caffee et al., 1994) show that mantle Xe is in fact much closer to atmospheric Xe in its isotope composition. The extent to which recycling of atmospheric Xe into the upper mantle (which need only be very small, e.g., Azbel and Tolstikhin, 1993a, b) on the one hand, or an initial Xe composition in the accreting material different to that in C1 chondrites (Ozima and Nakazawa, 1980; Sugiura, 1993) on the other, is responsible for this observation is an open question. However, in either case the central thesis expressed here that fractional degassing of melt is a key part of the missing Xe problem is unaffected. The above discussion enables us to conclude that the rare gas abundances in the Earth's atmosphere are to result of: (1) sorption and/or implantation to provide initial concentrations in accreting materials; (2) fractional degassing of early formed melts with a consequent fractionation of rare gas abundances in the Earth's interior; and ( 3 ) hydrodynamic escape of rare gases accompanying a decreasing hydrogen flux. It would be interesting to assess weather these processes have also been important in the establishment of the rare gas abundances in the Martian and Venusian atmospheres.
Acknowledgements
We are grateful to R.O. Pepin and Y. Zhang for their comments. This research has been supported by the Royal Society. Igor Tolstikhin acknowledges support from the Newton Trust and Clare Hall, Cambridge.
5
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