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Degassing JAMES C. G. WALKER The University of Michigan ABSTRACT Measurements of the concentrations of rare gases and trace elements in oceanic basalts have provided new information concerning the structure of the Earth's mantle and its evolution. This review is based principally on papers by Allegre, Staudacher, Sarda, O'Nions, Oxburgh, and Jacobsen. Approximately 35% of the mantle lost more than 99% of its rare gas content in the first 100 million years of solar system history. A comparable volume of the mantle has also been depleted in radioactive and other large ion lithophile elements, the depleted elements being concentrated in continental crust. But depletion was a much slower process than degassing. The average age of continental crust is 1.8 billion years, but the average age of the rare gas atmosphere is 4.4 billion years. There has been very little mixing of material between the degassed and depleted portion (presumably the upper mantle) and the undegassed and relatively undepleted portion (presumably the lower mantle). Gas fluxes from the mantle indicate that degassing today is inefficient, affecting only the top few hundred meters of oceanic crust. It is not likely that sea floor spreading processes like those now operating could have degassed the entire upper mantle within a 100 million years, even given large initial heat fluxes. At the same time, it is not likely that sea floor spreading processes could have dissipated the initial heat of a nearly molten Earth. Lava flooding could have removed initial heat efficiently and at the same time degassed the upper mantle rapidly. Rare gases do not make an atmosphere, of course. There is new information concerning the release of carbon dioxide from the mantle. As 191
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192 PLANETARY SCIENCES pointed out most forcefully by Marty and Jambon, the exogenic system (atmosphere, ocean, and sedimentary rocks) is deficient in carbon by a factor of 100 relative to rare gases when present amounts are compared with present fluxes from the mantle. It appears that carbon dioxide did not participate in the initial rapid degassing that released rare gases from the upper mantle. Instead, carbon has been modestly concentrated into the continental crust like other incompatible, but not atmophile, elements. Less than 10% of upper mantle carbon has been transferred to the crust, and the total mantle amount may be 40 times the amount in the exogenic system. INTRODUCTION Important new information has become available in recent years con- cerning the release of gases from the interior of the Earth. The most fruitful source of information has been the measurement of rare gas concentrations in sea floor basalts. The results set important constraints that need to be incorporated into any comprehensive understanding of the early history of the planets. In my review here, I will describe some of the highlights of these results and give an indication of how they are derived. I cannot provide a complete description of all of the evidence that is used to reach the conclusions presented. RESERVOIRS Measurements on sea floor basalts have provided clear indications of two major reservoirs within the mantle. The larger reservoir, constituting about 65% of the mantle, is undegassed and relatively undepleted in incompatible elements. The remaining 35% of the mantle was degassed very early in Earth history (within 100 million years of the beginning), and more than 99% of the initial gas content of this reservoir was released. Throughout the whole of Earth history there has been very little mixing between these reservoirs (O'Nions 1987; Anderson 1989~. These conclusions are based on measurements of the concentrations in sea floor basalts of the radioactive parent elements shown in Figure 1, along with their radiogenic daughter isotopes and non-radiogenic cousin isotopes also shown in the figure (Allegre e' al. 1983~. The important feature of these isotope systems is that the ratio of daughter/cousin increases through time as a result of the radioactive decay of the parent, and that there are no other processes that will cause the ratio of daughter/cousin to change because they are chemically and physically almost identical. Figure 2 shows how the ratio of daughter/cousin, called ALPHA, increases at a rate that depends on the ratio of parent/cousin, called MU.
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AMERICAN AND SOVIET RESEARCH P are nt D laughter ~ outran K40 Ar40 Ar36 U,Th He4 He3 Il29 Xel29 Xel30 193 · ALPHA (=DAUGHTER/COUSIN) increased by decay · MU (=PARENT/COUSIN) determines rate of 1ncre ase FIGURE 1 Isotope taxonomy. The solid line in the top panel of the figure shows the evolution of the amount of radiogenic 40Ar resulting from the decay of radioactive 40K The bottom panel shows the evolution of the ratio of 40Ar/36Ar, ALPHA The effect of a degassing episode fairly early in Earth history is indicated by the left hand arrow labeled Degas 50%. The degassing episode reduces the concentration of 40Ar by a factor of two, as shown in the top panel. Because 36Ar concentration is also reduced by a factor of two there is initially no change in ALPHA The rate of increase of ALPHA with time is larger after the degassing episode, however, because there is less 36Ar in the denominator of the ALPHA ratio. This evolution is shown by the dashed line in the figure. The effect of a second degassing event at -1 billion years is also shown in the figure. The impact of the second degassing event on the evolution of ALPHA is smaller because, later in Earth history, there is less radioactive 40K left to decay. Thus, early degassing leads to large increases in ALPHA; late degassing has a smaller effect. The event in the middle of Figure 2 shows the effect of a depletion by 50% in the concentration of radioactive 40K Depletion reduces the rate of increase of ALPHA in the manner indicated by the dashed line. In this way it is possible to deduce the history of MU from measurements of ALPHA The basic data concerning mantle degassing appear in Figure 3. They are ALPHA values measured for He, Ar, and Xe in mid-ocean ridge basalts and in ocean island basalts. The mid-ocean ridge basalts appear to sample the upper mantle, whereas the ocean island basalts are assumed
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194 PLANETARY SCIENCES _ 1.5 ·_. A: 0 a.) P C: 0 5 i_ 5- o 400 ID ~ 200 lo Degas /~,.~- /6' 1 Deplete 50% j Degas 50% , -, , 1 - . ~~ - '' - -73 -1 BILLION YEARS FIGURE 2 Measure ALPHA to deduce history of MU. 1 to sample plumes of material rising from the lower mantle. There is a range of compositions of ocean island basalts representing various degrees of mixing between lower mantle material and upper mantle material. As representative of the least contaminated material I show results for Loihi sea mount in Hawaii. The point is that ALPHA is larger in MORB than in Loihi material, which indicates that MORB material is more degassed. The enhancement in ALPHA has been large for He and Ar. From data such as these it is now possible to derive important results concerning mantle reservoirs. First, the bulk Earth concentration of K gives the 40AT concentration in undegassed mantle material. The ALPHA value observed in Loihi basalts gives the 36Ar concentration in undegassed mantle material. The mass of 36Ar in the atmosphere then gives the mass of the mantle that has been degassed. From a comprehensive study of rare gas isotope systematics Allegre et al. (1987) deduce that 46% of the mantle has been degassed. To increase the ALPHA value of Ar from the Loihi value to the MORB value it is necessary that no more than 390/25000 = 1.6% of the initial 36Ar complement be retained in degassed mantle material. This value would apply in the case of early degassing from undepleted material. Delayed degassing or prior depletion of 40K would reduce the permitted degree of
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AMERICAN AND SOVIET RESEARCH He ALP HA Ar , Xe MORB 86~000 Loihi 25,000 195 25~000 6.95 390 6.48 MORB samples degassed reservoir (upper mantle ~ Loihi samples undegassed flower mantle ~ FIGURE 3 Data that constrain degassing (AIlegre et al. 197. retention. The conclusion is that degassing has been very thorough indeed. At the same time, because the difference between ALPHA values in Loihi and MORB is so great, it is possible to conclude that just 2% contamination of MORB material by Loihi material would reduce the ALPHA value of the degassed mantle by a factor of two. There is therefore evidence for strong isolation of the mantle reservoirs from one another. The increase in the ALPHA value for Xe between Loihi and MORB, although modest, demonstrates that degassing took place very early in Earth history. For ALPHA to have changed, degassing must have occurred before all of the parent i29I had decayed away. But the half life of t29I is only 17 million years. Therefore, the division of the mantle into two reservoirs, the very thorough degassing of one of these reservoirs, and the nearly total isolation of the two reservoirs all took place very early in Earth history. At this time it is not clear to me how to reconcile these surprising conclusions with our current understanding of the growth of the Earth by planetesimal impact, in which planetesimals were vaporized and degassed, at least during the later stages of accretion. Neither is it clear how to reconcile with these data the current thinking concerning the formation of the Moon by a giant impact event occurring near the end of Earth
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196 PLANETARY SCIENCES accretion. It appears likely that such an impact would have completely remixed and homogenized the mantle. On the other hand, it is not clear that such an impact event would have led to complete degassing of the mantle or to complete removal from the Earth of any atmosphere released during the course of previous accretion. Also unclear is what physical process causes the separation of the mantle into two distinct reservoirs. In my further analysis I shall assume that degassing of the upper mantle was a consequence of mantle convection, possibly driven by accretional energy, but that most of the impacts, and in particular the giant Moon-forming impact had already occurred before the processes that brought about the presently observable state had begun. In this interpretation then, degassing should be related to continental growth and the depletion of the upper mantle in incompatible elements. Studies of continental growth and depletion are based on precisely the same kind of isotopic arguments as the studies of degassing already described. The only difference in depletion is that the daughter and cousin isotopes are concentrated in the continents instead of in the atmosphere. Analyses of Sm-Nd, Lu-Hf, and Rb-Sr isotopes in sea floor basalts, summarized in Figure 4, indicate that 30% of the mantle has been depleted to form the continents (Jacobsen 1988~. The average age of the continents is 1.8 Ga. Allegre e! al. (1983, 1988), in a similar analysis, conclude that 35% of the mantle has been depleted while 47% of it has been degassed (Sarda et al. 1985~. The average age of the rare gas atmosphere deduced in their analysis is 4.4 Ga. My tentative conclusion is that the degassed and depleted reservoirs are probably the same, but that degassing occurred much earlier than depletion. FLUXES Fluxes of gases from the mantle to the atmosphere can be deduced from the measured flux of 3He and the concentration ratios in sea floor basalts. These fluxes lead to the very interesting conclusion that heat is released much more readily from the mantle than are the rare gasses (O'Nions and Oxburgh 1983; Oxburgh and O'Nions 1987~. Further it can be argued that degassing today is inefficient. Processes now operating could not have degassed the upper mantle rapidly and thoroughly. A comparison of the fluxes of heat, helium, and argon is presented in Figure 5. The sources are mainly concentrated in the lower mantle because the upper mantle is depleted in radioactive incompatible elements. The heat flux through the surface of the Earth exceeds the sum of upper and lower mantle sources because the interior of the Earth is cooling down. This fact is reflected in the Urey ratio of source/flux. For heat this ratio has a value of about 0.6 (Pollack 19&0~. For 4He the Urey ratio is 6.8, indicating that
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AMERICAN AND SOVIET RESEARCH He3 Ar36 Xel30 He4 Ar40 Xel29 U238 Th232 K40 HEAT Q DE GASSED DEPLETED DEPLETED Ndl44 Hfl77 Sr86 /////////////////////~ A//////////// a///////// IW/////~ ///////////////////1~\\\~ ,///////////\ ///////////AK//////~ //////////////~V//////////~ //////'E~///////////~ //////////////~IY/////////// _ ///////////////~//////////// ////////// //////////////~\\\\\\\\\\\\~ ,///////////////~\\\\\\\\\~ ///////////////~\\\\\\~///// , . . . . . . . . . . , . . , . . ~ . . .. /////~//////////1~\~\\\\~/~ 0 0.5 t.O 197 `~79 LOWER `~ MANTLE MANTLE CONTINENTS ATMOSPHERE Allegre et al. "Jacobsen FIGURE 4 Comparison of deductions concerning degassing and depletion (Allegre et al. 1983, 1987; Jacobsen 19883. Ibe team indicate what fraction of the terrestrial complement of each isotope is in the indicated reservoir. HEAT Q refers to heat source. most radiogenic helium is retained within the Earth and that the flux from mantle to atmosphere is much less than the production within the Earth. However, the flux does exceed upper mantle production. Helium must be flowing from the lower mantle to the upper mantle at a significant rate. For 40Ar, on the other hand, the flux is less than the upper mantle source. There is no evidence of flow from lower mantle to upper mantle; the Urey ratio is 23, and 40Ar is accumulating even in the depleted upper mantle. These observations provide strong support for the notion of a two-layer convective structure in the mantle. It is entirely reasonable to suppose that heat is more mobile than helium which is in turn more mobile than argon. The argon flux from the mantle is 6.2 x 106 molely. The 40Ar concentration in the upper mantle is 3 x 10-~° mole/g. Therefore, the rate at which upper mantle material is degassed, calculated from the ratio of these two numbers, is 2 x 10~6 g/y. Since the mass of the upper mantle (35% of the total mantle) is 1.4 X 1027 g, it would take 70 Ga to degas the upper mantle at this rate. But the xenon isotope data indicate that the upper mantle was degassed in less than .1 Ga. Therefore, the present rate of degassing is too slow to explain the observations by a factor of 1000. Furthermore, degassing today is inefficient, in the sense illustrated in Figure 6. Ocean crust is formed by the partial melting of upper-mantle
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198 PLANETARY SCIENCES UREY RATIO 0.6 SOURCE/FLUX FLUX: 3.6E15} Heat He4 6.8 9.6E7 ~ Ar40 ~ .62E7 ~ Upper mantle SOURCE: .09E 1 5 4E7 watt mole/y ~ 5.~7 ~ 1E7 tower mantle SOURCE: 1.4E15 63E7 13E7 FIGURE 5 Fluxes of heat and gases and ratios of sources to fluxes. material. The degree of partial melting can be deduced from the con- centrations of the completely incompatible element potassium. Potassium concentration is enhanced in ocean crust by a factor of 10, more or less, so we have approximately a 10% partial melt of 60 kilometers of upper mantle material to produce six kilometers of ocean crust. About the same increase by a factor of 10 can be expected in the concentration of 40Al, also presumably a completely incompatible element. New ocean crust is generated at the rate of 3 km2 per year. 1b produce the 40Ar degassing flux of 6.2 x 106 mole per year it would be necessary to extract 40Ar from just the top 250 meters of ocean crust. This extraction presumably occurs by in- teraction between sea water and the ocean crust. The 40Ar does not diffuse directly out of the crust or bubble out of the magma. It must be extracted by leaching at relatively shallow depths in the crust. During the lifetime of the sea floor before subduction, heat will be extracted from a lithospheric layer approximately 60 kilometers thick, but Ar will be extracted only from 250 meters of ocean crust. This thickness of crust is equivalent, before partial melting, to Z5 kilometers of upper mantle, so the release of Ar is about 25 times less efficient than the release of heat. The flux data indicate that radiogenic rare gases are accumulating in the mantle. The degassing process now operating is inefficient and slow. It seems that the process that originally degassed the upper mantle completely and rapidly must have been markedly different from the process now operating.
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AMERICAN AND SOVIET RESEARCH UPPER MANTLE K=42 ppm Ar40-3E- 10 mole/g 60 km 199 3 km^2/y OCEAN (RIOT ~ 6km / K-500 ppm / Ar40=3E-9 /t 096 p artial melt Ar40 degassing flux = 6.2E6 mole/y Leaching 250 m DE GASSED C RUST FIGURE 6 Degassing is inefficient compared with the extraction of heat. CARBON DIOXIDE ~ what extent can the rare gas results be applied to more important constituents of the atmosphere? It turns out that there is significant infor- mation concerning carbon dioxide (Des Marais 1985; Marty and Jambon 1987~. The data and results are summarized in Figure 7. The flux of carbon dioxide from mantle to atmosphere today is 2 x 10~2 mole per year. The flux of 36Ar is 250 mole per year, so the ratio of the fluxes is 8 x 109. On the other hand, the ratio of the amounts in atmosphere, ocean, and continental crust is 1.8 x 106. The ratio of fluxes is very much larger than the ratio of amounts now present in the surface layers of the Earth. Carbon is missing from the surface layers relative to argon. This conclusion can be seen also in the accumulation times calculated by dividing the flux into the amount. Carbon would accumulate at present rates in 5 x 109 years, but it would take 2.2 x 10~3 years for the 36AT now in the atmosphere to accumulate at the present flux The conclusion is that while 36Ar was massively degassed earlier in Earth history, carbon did not participate in this early degassing. If carbon was rapidly released from the mantle early in Earth history it was just as rapidly returned to the mantle. Carrying this analysis further it can be concluded that carbon is a lithophile and not an atmophile element. From the flux ratio of carbon to
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200 Accumulation times: 5E9 AMOUNT/FLUX PLANETARY SCIENCES 2.2E13 y Amounts: lE22 5.6ElC mole Ratio C/Ar = t.BE6 CRUST AND ~ ~ ATMOSPHERE C ~ T Arab MANTLE Fluxes: HE ~ 2 250 mole/y Flux ratio C/Ar = BE9 FIGURE 7 Compared to argon, carbon is deficient in the crust and atmosphere. 36Ar and the concentration of 36Ar in the upper mantle we can calculate the concentration of carbon in the upper mantle. The value is 1 x 10-4 mole/g. This calculation assumes that carbon is not more mobile than Ar, surely a reasonable assumption. If carbon is less incompatible than Ar the required upper mantle concentration of carbon would be larger. From the concentration and the mass of the upper mantle I calculate that there are 1.4 x 1023 moles of carbon in the upper mantle. The amount in the crust and atmosphere and ocean is only 1 x 1022 mole (Wilkinson and Walker 1989~. Therefore less than 10% of upper-mantle carbon has been degassed. By way of contrast, more than 99% of upper mantle 36Ar has been degassed. Continuing the analysis and assuming that the lower- mantle concentration is given by the upper-mantle concentration augmented by crustal carbon mixed back in, the total amount of carbon in the mantle is 4.2 x 1023 mole, which is 42 times the amount in crust, ocean, and atmosphere. It seems that the fate of most carbon dioxide released from the mantle is to be incorporated into oceanic crust in weathering reactions and to be carried back into the mantle on subduction. Only a small fraction of the carbon is captured in the exogenic system as cratonic carbonate rocks. The average carbon concentration in continental crust is 5 x 104 mole/g. The concentration in the upper mantle is 1 x 104 mole/g. Therefore, the crust is only moderately enriched in carbon dioxide relative to the upper mantle and, unlike the rare gases, carbon is a modestly incompatible element.
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AMERICAN AND SOVIET RESEARCH 21)1 CONCLUSION The rare gas data indicate that there was early, thorough degassing of the upper mantle, but that there remain large amounts of primordial rare gases in the undegassed, lower mantle reservoir. The time scales and rates of degassing and depletion are very different. Depletion and continental growth occurred much later in Earth history than degassing. Degassing today, by weathering of the sea floor, is a slow and inefficient process and could hardly have provided the rapid and total early degassing that apparently occurred. Carbon dioxide did not degas like the rare gasses and is only modestly incompatible in the upper mantle. With the example of carbon dioxide in mind, we must be cautious about deducing degassing histories of other important atmospheric gases like nitrogen or water from the rare gas data. In the absence of relevant observations it is not immediately clear whether other atmospheric gases have behaved more like argon or more like carbon dioxide. By analogy with the Earth, it does seem likely that large amounts of both rare gases and carbon dioxide may be retained within the interiors of Mars and Venus. This possibility must be kept in mind in the study of the origin of planetary atmospheres. I do not feel that we yet have a satisfactory description even in qualitative terms of the origin of the Earth and the atmosphere. The challenge is to reconcile the ideas of planetary growth by accretion, impact degassing during the course of accretion, the origin of the Moon by a giant impact, and the data described in this paper concerning the degassing history of the mantle. ACKNOWLEDGEMENTS This research was supported in part by the National Aeronautics and Space Administration under Grant NAGW-176. I am grateful to Alex Halliday and Richard Arculus for guidance and advice. During the course of the conference my ideas were significantly influenced by discussions with D. Weidenschilling, L. Mukhin, Jim Kasting, Dave Stevenson, and V.N. Zharkov. I am grateful to all of them. REFERENCES Allegre, CJ., S.R. Hart, and J.-F. Minster. 1983. Chemical structure and evolution of the mantle and continents determined by inversion of Nd and Sr isotopic data, II. Numerical experiments and discussion. EZarth Planetary Sci. Letters 66:191-213. Allegre, CJ., T. Staudacher, P. Sarda, and M. Kurd. 1983. Constraints on evolution of Eanh's mantle from rare gas systematic Nature 303:762-766. Allegre, CJ., 1: Staudacher, and P. Sarda. 1987. Rare gas systematics. formation of the atmosphere, evolution and structure of the Earth's mantle. Earth Planetary Sci. Letters 81:127-150. Anderson, D.L. 1989. Composition of the Earth. Science 243: 367-370.
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202 PLANETARY SCIENCES Des Marais, DJ. 1985. Carbon exchange between the mantle and the crust and its effect upon the atmosphere: today compared to Archean time. Pages 602-611. In: Sundquist, E.T., and W.S. Broecker (eds.~. Natural Variations in Carbon Dioxide and the Carbon Cycle. American Geophysical Union, Washington, D. C. Jacobsen, S.B. 1988. Isotopic and chemical constraints on mantle-crust evolution. Geochim. Cosmochim. Acta 52 1341-1350. Marty, B., and A. Jambon. 1987. C/3 He in volatile fluxes from the solid Earth: implications for carbon geodynamics. Earth Planetary Sci. Letters 83:16-26. Oxburgh, E.R., and R.K. O'Nion~ 1987. Helium loss, tectonics, and the terrestrial helium budget. Science 237:1583-1588. O'Nions, R.K, and E.R. Oxburgh. 1983. Relationships between chemical and convective layering in the Earth. J. Geological Soc. London 144:259-274. O'Nions, R.K, and E.R. Oxburgh. 1983. Heat and helium in the Earth. Nature 306:429431. Pollack, H.N. 1980. The heat flow from the Earth: a review. Pages 183-192. In: Davies, P.A., and S.K Runcorn (eds.~. Mechanisms of Continental Drift and Plate Tectonics. Academic Press, New YorL Sarda, P., T. Staudacher, and CJ. Allegre. 1985. 40Ari36Ar in MORB glasses: constraints on atmosphere and mantle evolution. Earth Planeta~y Sci. Letters 72:357-375. W~lkinson, B.H., and J.CG. Walker. 1989. Phanerozoic cycling of sedimenta~y carbonate. American J. Sci. 289:525-5413.
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