Download The argon constraints on mantle structure

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.)