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GEOPHYSICALRESEARCHLETTERS,VOL. 23, NO. 24, PAGES3555-3557,DECEMBER 1, 1996 The Argon constraints on mantle structure. Claude J.All•gre, • Albrecht Hofmann,: KeithO'Nions • Mid Ocean Ridge Basalts (upper mantle sourse) Ocean Island Basalts (lower mantle source) Abstract. The4øArbudgetof the Earthoffersa powerfuland Hofmann et al. (1986) showed that the trace element ratios Nb/U andCe/Pb are indistinguishable between the MORB and structure to the mantle. OIB sourceregions and distinct to the respectivite primitive WhereasK-At-isotope systematicshave been includedas mantle values. This observation is important because it demonstratesthat these lithophile elements in OIB are not part of various discussionsof mantle structurepreviously a simple and straightforwardargumentis presentedhere. Because derivedfrom a primitive reservoir. A mass fraction for the 4øAris produced by 4øKtheamount of K in the Earthmaybe "depleted"reservoirincludingMORB andOIB sourceswithin constrained from4øArbudgets. The atmospheric 4øAtbudget the range 30 to 75 % of the mantle, was estimatedfrom the impliesthatapproximately halfof all 4øArproduced withinthe budgetsof theseelements[Hofmannet al., op.cit.] Earth since its formation is retained within the solid Earth. It The presentcontributionhas its origins in the early work straightforwardargumentin favour of the existenceof a layered of Turekian(1959) on the 4øArbudgetof the Earth,wich was lower is argued thatthisadditional 4øAtis locatedprincipallyin the mantle. K - Potassium extendedby All•gre et al. (1983) and Tumer (1989). The Introduction in abstract form [All•gre et al., 1994] reaffirms the geochemical arguments in favour of two-layer mantle present analysis of the40K-4øArbudget,previously presented convection. The abundancesof radiogenicisotopes and trace elements have been used to constrain the structure and evolution of the Earth'smantlefor a long time. Early effortsusedthe budgetsof SrandNdandS7Sr/S6Sr and•43NdflnnNd isotope ratios[Allbgre et al., 1979; Jacobsen et al., 1979; O'Nions et al., 1979; De Paolo, 1980] to argue for a layered mantle with separate convection in the upper and lower parts as suggestedon the basis of geophysical arguments by Richter and McKenzie (1978). Subsequently,other radiogenicisotopes of lithophile The K-Ar systemof the Earth 4øArin theEarthis almostentirelyproduced by thedecayof 4øK.Its atomicweightis toohighto allowits escapefromthe Earth's atmosphere,and under normal conditions it will be retained aftertheEarthaccreted. Because 4øKhasdecayed over 4.5 109yr of Earthhistory, theamount of 4øAtproduced during this time can be calculated if the K content of the Earth is elements including 2ø8'2ø7'2ø6pb, •76Hf,187Os andtraceelement known.Theamountof 4øAtproduced in 4.5 109yr is equalto abundances were also usedto addressthis question,and inverse 1.164øKpresent in thebulkEarthtoday.TheK contentof the techniqueswere employed to obtain refined estimates for the amount of depleted mantle [All•gre et al., 1983]. Finally, isotopic composition of rare gases were also found to consistent with a layered mantle structure(e.g. O'Nions and Oxburgh, 1983; All•gre et al., 1983; Allbgre et al., 1986). All of the above budget calculations were made with the assumptionthat the silicate Earth consistsof three reservoirs: the continentalcrustof known massand chemicaland isotopic composition, a depletedmantle reservoir of known isotopic (though not necessarily chemical) composition and an undepletedor "primitive" reservoir of known chemical and isotopic composition. It should be noted that most of the estimatesfor the mass fraction of depletedmantle exceed the massof the upper mantle, suggestingthat convection in the upper mantle down to 670 km was probably not completely isolatedfrom exchangewith the lower mantle. In this casethe lower mantle cannot have a primitive composition. •Laboratoire deG6ochimie et Cosmochimie, InstitutdePhysique du Globe de Paris, Paris, France. 2Laboratoirede G6ochimieet Cosmochimieand Max Plank far Chemie, Mainz, Germany. Earth may be estimated assuming that K/U = 12,700, by weight [Jochumet al., 1983], which is the MORB ratio, taken with the uranium content of primitive mantle. For a U abundancein the range of 20 and 22.5 ppb (e.g. Ganapathyand Anders, 1974), then the correspondingK-content is between 250 and 285 ppm. These valuescorrespondrespectivelyto the production of 140 10•8g and156 10•8g of 4øArin the Earth since4.5 109yr. Theamount of 4øAr residing in theatmosphere is 66 10•8g [Turekian,1959]. The 4øArcontentof the continentalcrustis unknown from observation but if, for example, it has been a closed system witha meanageof 2 109yr thentheamount of 4øAr produced in thecontinental crustis around 9 10•8g fora Kcontent of 1.7 %. An upper bound would be a mean age of 2.7 109yranda K-content of 2 %, leading to 4øAr= 12 10•8g. [All•gre et al., 1986]. A lower bound should allow for degassing, which may be as much as 50 % and lead to proportionately lower estimates.In effect therefore, much of the 4øArproduced in the continentalcrustis now part of the atmosphere inventory. Therefore,some63 10•8 to 80 10•8 g of 4øArresides 3Department of EarthSciences, University of Oxford,Oxford,England. elsewherein the interior of the Earth and this representsabout 50 % of the total 4øArproduced geologicaltime. Recent Copyright1996bytheAmericanGeophysical Union. experiments [Matsuda et al., 1993] have shown that the partitioning of rare gasesbetweenmetal and silicate even at high pressuresstrongly favours the silicate phase. Indeed if Papernumber96GL03373. 0094-8534/96/96GL-03373 $05.00 3555 3 556 ALLI•GRE ET AL.: THE ARGON CONSTRAINTS ON MANTLE STRUCTURE ATMOSPHERE M 4øAr = 66x10 F4OAr-5xl 07-0.6xl 07mol/y gr F4OAr=4x109mol/y Continental crust M4øAr- 4-6x1018g K=1.7% M4OAr =2.8+3.5x •.018 g K••4•0..m Upper 670 km mantle ; .... ½..__..._.•..._.•._.•.•,••••••• Figure1. Thisfiguregivesa representation of a coherent modelfor distributing 4øAtand4øKin theEarth.F andM represent the fluxes in moles/yearand massin grams,respectively. the "missing"4øAris in the corethenit shouldhave a K- 81 10•Sg of 4øArassumed to be presentin the mantlefromthe contentof 500 ppm - well outsideany estimatesmadeso far. above Therefore the mantle should contain about 50 % of the considerations. Alternativelyif the mantle is layered at the 670 km seismic thentheamount of 10•Sg of 4øArproduced in the terrestrial4øArbudget.The distributionof 4øAtis now discontinuity, upperlayeris only7.3 to 9.3 10•8g. In thiscasesome54-74 t0•Sg of missing4øAtwill be in the lower mantle-this consideredin the framework of two layer and single layer mantleconvectionusingtwo independantestimatesfor mantle 4øAr,onedevidedfromthe uppermantleK content,the other correspondsto about 230 ppm of K in the lower mantle. fromestimates of the4øAtflux through theoceanridges. 4øAr flux Potassium content Anestimate of the4øArflux fromtheworld'sridgesystem The K contentof MORB is typically 500 ppm [Jochumet at., 1983]. For a degreeof partial melting between8 to 10 %, as estimated from petrotogy or geochemistry [Klein et at., 1987; Hofmann, 1988], we obtain a present day mantle source K concentrationof 40 to 50 ppm. If this value is usedfor the can also be used to constrain the 4øAtabundancein the MORB source. Thetotal3Heflux is 1.1 103motes/yr[Craiget at., t975],andthecorresponding 4Hefluxis 9.46 107moles/yrfor a 4He/3He = 8.6 104at theridges[Craiget at.,1976;Kurzet at., 1982]. The 4He/4øAr ratio in MORB variesbetween2 and 15 wholemantle,it leadsto some22 to 28 1028 g of 4øArproduced[Sardaet at., 1985; Altb•greand Lewin, 1989; Staudacheret at., to an4øAtflux of 5 107and0.63 107 in themantleover4.5 109yr, considerably lessthanthe63 to 1989],whichcorresponds ALLI•GRE ET AL.: THE ARGONCONSTRAINTSON MANTLE STRUCTURE 3557 moles/yr, respectively. This Ar flux corresponds to the outgassingof the oceanic lithosphere by melting at the ridge C.J. All•gre, Th. Staudacher,Ph. Sarda, Rare gas systematics: formationof the atmosphere,evolutionand structureof the Earth's crests.Oceaniccrust is produced at 3 km2/yr and for a mantle, Earth Planet. Sci. Lett., 81,127-150, 1986. lithosphere thickness of 60 km with a mean density of 3.2 C.J. All•gre, R.K O'Nions, A.W. Hofmann,Two layer mantlewith intermittentconvectionsupported by geochemistry, EOS 75, 61, 1994. H. Craig, J. Lupton,Primoridalneon, helium, and hydrogenin g/cm3.Thusthe massof oceaniclithosphere passingthrough theridgesystemis 5.76 1027g/yr.If thisvolumeof materialis oceanicbasalts,Earth Planet. Sci. Lett., 31,369-385, 1976. completelyoutgassed at the ridgecrest,then the 4øArflux D.J. De Paolo,Crustalgrowth and mantleevolution:inferencesfrom estimate maybe usedto compute theconcentration of 4øArin modelsof element transport and Nd and Sr isotopes,Geochim. mantlematerial.Givena mantlemassof 4 1027g the 4øAr Cosmochim.Acta, 44, 1185-1196, 1980. contentof themantleis 1.4 10•8gand1.8 10TM g respectively, R. Ganapapthyand E. Anders. Bulk composition of the Moon and for 4He/4øAr ratiosof 2 and15 respectively.Both valuesare Earth estimatedfrom meteorites.Proc. Fifth. Lunar. Sci. Conf., 1181muchlowerthan the 63 to 81 10TM g required from above. 1206, 1974. Indeed this is even smaller than the estimate based on potassiumcontent. For a two layer model,the amountof 4øArstoredin the uppermantle (abovethe 670 km seismicdiscontinuity)may be A. W. Hofmann, K.P. Jochum,M. Seifert and W.N. White, Nb and Pb in oceanicbasalts:new constraintson mantle evolution, Earth Planet. Sci. Lett., 79, 33-45, 1986. A.W. Hofmann, Chemical differentiation of the Earth: the relationshipbetweenmantle, continentalcrust and oceanic crust,Earth Planet. Sci. Lett., 90, 297-314, 1988. estimated in thesameway,to givea rangeof 0.6 to 4.6 10TM g. Thecorresponding amount of 4øArin the lowermantleis then J.B. Jacobsenand G.J. Wasserburg,The mean age of mantle and at least59 10•8g4øArwhichcorresponds to a K concentration crustalreservoir,J. Geophys.Res., 84, 7411-7429, 1979. of about230 ppm as obtainedpreviously. K.D. Jochum,A.W. Hofmann,E. Ito, H.M. Seifert and W.M. White, K, U andTh in mid-oceanridgebasaltglassesandheat production,K/U and K/Rb in the mantle, Nature, 306, 431-436, 1983. Conclusions and discussion In summary,two different argumentsbasedon K and Ar in M.D. Kurz, W.J. Jenkins,S.R. Hart, Helium isotopicsystematics of oceanicislandsand mantleheterogeneity, Nature, 297, 43-47, 1982. J. Matsuda,M. Sudo,M. Ozima,K. Ito, O. Ohtaka,E. Ito, Noble gas partitioning betweenmetal and silicateunder high pressure,Science, MOR basalts andconsideration of 4øArflux at ridgessuggest a 259, 788-790, 1993. low 4øArmantlewhileconsiderations of the total4øArbudget R. K. O'Nions, N.M. Evensen, P.J. Hamilton, Geochemical necessitate a relativelylargeconcentration of 4øArinsidethe modelingof mantledifferentiation andcrustalgrowth,J. Geophys.Res., Earth.Because it is unlikelythat the corehas a high 4øAr 6091-6101, 1979. content, this mismatch is most readily reconciled by a two layermantlewithanuppermantleoutgassed in 4øAranda lower mantlerelativelyundegassed in 4øAr. R. K. O'Nions and E.R. Oxburgh,Heat and helium in the Earth, Nature, 306, 429-431, 1983. F. Richter, D. P. McKenzie, Simple plate models of mantle convection,Geophys.Res., 44, 441-471, 1978. This is IPGP contribution number 1446 Ph.Sarda,Th. Staudacher, C.J.All•gre,4øAr/3aAr in MORBglasses: constraintson atmosphereand mantleevolution, Earth Planet. Sci. Lett., 72, 357-375, 1985. References. Th. Staudacher, Ph. Sarda,S.H. Richardson, C.J. All•gre, I. Sagma and L.V. Dmitriev, Noble gasesin basaltglassesfrom a Mid-Atlantic Ridge topographichigh at 14øN: geodynamicconsequences, Earth C.J. All•gre, D. Ben Othman, M. Polv6, P. Richard, The Nd-Sr Planet. Sci. Lett., 96, 119-133, 1989. isotopiccorrelationin mantlematerialsand geodynamicconsequences, K.K. Turckian,The terrestrialeconomyof heliumand argon, Phys.Earth Planet. lnt., 19, 293-306, 1979. Geochim. Cosmochim.Acta, 17, 37, 1959. C.J. All•gre, S. R. Hart, J.F. Minster, Chemical structureof the G. Turner, The outgassing historyof the Earth's atmosphere, J. mantle and continentsdeterminedby inversionof Nd and Sr isotopic Geol. London, 146, 147-154, 1989. data: Theoretical methods, Part I Earth Planet. Sci. Lett., 66, 177-190, 1983. C.J. All•gre, S.R. Hart, J.F. Minster, Chemical structure of the mantle and continentsdeterminedby inversionof Nd and Sr isotopic data: Numerical experimentsand discussion, Part II Earth Planet. Sci. ClaudeJ. All•gre, Laboratoirede G6ochimieet Cosmochimie,Institutde Physiquedu Globe de Paris, 4 Place Jussieu75252 Paris Cedex 05, France. Lett., 66, 191-213, 1983. Albrecht Hofmann, Laboratoire de G6ochimie et Cosmochimieand Max C.J. All•gre, E. Lewin, Chemicalstructureand historyof the Earth: evidencefrom globalnon-linearinversionof isotopicdatain a three box Plankfiir Chemic,Mainz - Germany. Keith O'Nions,Departmentof Earth Sciences,Universityof Oxford ParksRoad,Oxford,OX 1 3PR, England. model, Earth Planet. Sci. Lett., 96, 61-88, 1989. C.J. All•gre, Th. Staudacher,Ph. Sarda, M. Kunz, Constrainson evolutionof Earth's mantlefrom rare gassystematics, Nature, 303, 762766, 1983. (ReceivedJanuary17, 1996;revisedJuly2, 1996; acceptedAugust20, 1996.)