helium–neon systematics and the structure of the mantle

Ž .Chemical Geology 147 1998 53–59

Helium–neon systematics and the structure of the mantle

Manuel Moreira ), Claude Jean Allegre`Laboratoire de Geochimie et Cosmochimie, Institut de Physique du Globe de Paris et UniÕersite Paris 7, URA CNRS No. 1758, 4 Place´ ´

Jussieu, 75252 Paris cedex 05, France

Abstract

We present here a comparison of helium and neon isotopes in the mantle. These two elements provide importantconstraints on the structure of the mantle and the exchange between the upper and the lower mantle. Using two low4Her 3He on-ridge hotspots located in the south Atlantic we observe that radiogenic He correlates with nucleogenic Ne.

Ž .Using data from Loihi seamount and from the MORB glass 2 P D43 ‘popping rock’ , we estimate the ratio of thew3 22 x w3 22 xHer Ne in the upper mantle to the ratio Her Ne in the lower mantle to be close to 1. This result is a strongLM

Ž .argument it is not a proof in favour of the steady-state model for the upper mantle which suggests that all the primordialnoble gases observed at the ridges come from the lower mantle. q 1998 Published by Elsevier Science B.V. All rightsreserved.

Keywords: Helium; Neon; Steady-state model; Rare gas; Mantle; Geochemistry

1. Introduction

The 4 Her 3He ratios measured on Oceanic IslandŽ .Basalts OIB and Mid-Oceanic Ridge Basalts

Ž .MORB present quite different patterns. The his-tograms of Fig. 1 show that the helium isotopic ratiomeasured on MORB is very homogeneous with amean value of 88,000 and a standard deviation of

Ž . Ž9000 ns206 Staudacher et al., 1994; Allegre et`. 4 3al., 1995 . In contrast, the dispersion of the Her He

Ž . Žratio for the OIB is very large 45,000 the mean is. Ž .similar ;94,000 Allegre et al., 1995 . Values as`

Žlow as 25,000 are observed in Hawaii Craig andLupton, 1976; Kaneoka and Takaoka, 1978, 1980;

.Kurz et al., 1982a,b and some very radiogenic ratiosŽ .have been measured in Sao Miguel Azores , Tristan

) Corresponding author. E-mail: [email protected]

Ž4 3da Cunha, Gough or St. Helena islands Her He. Žup to 200,000 Kurz et al., 1982a,b; Vance et al.,

1989; Graham et al., 1993; Moreira and Allegre,`.1996 .

The high 4 Her 3He ratio observed in MORB isconsidered to be generated in a highly degassedreservoir where 4 He produced by U and Th decays isclearly apparent. This reservoir is the upper mantle.

The low 4 Her 3He observed in most oceanicŽ .islands Hawaii, Reunion, Samoa, Iceland, etc. . . . ,

compared to MORB, is considered to originate froma less degassed reservoir, rich in primordial 3He,

Ž . Žlocated deeper in the mantle lower mantle Craigand Lupton, 1976; Kaneoka and Takaoka, 1978,

.1980; Kurz et al., 1982a,b; Allegre et al., 1983 .`Ž .The existence of the high radiogenic ratios

Ž 4 3higher than the mean MORB ratio, i.e. Her He).90,000 in some OIB is the subject of many debates.

Ž .Kurz et al. 1982a,b proposed that the radiogenic

0009-2541r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 97 00171-X

( )M. Moreira, C.J. AllegrerChemical Geology 147 1998 53–59`54

Fig. 1. Histograms of 4 Her 3He ratios for MORB and OIB,showing the heterogeneity of the OIB compared to the MORB.The lowest 4 Her 3He for the OIB correspond to the Loihiseamount.

4 Her 3He reflects the source ratio of a plume, origi-nating at the 670 km seismic discontinuity, where a

Ž . 3high UqTh r He component was stored. As anŽ .alternative, Zindler and Hart 1986 , Hart and Zindler

Ž . Ž .1989 and Hilton et al. 1995 proposed that theradiogenic signature is due to shallow contaminationby lithosphere or production of 4 He in a degassedmagmatic chamber. We will suppose here that thiskind of hotspot is very peculiar and that most of thehotspots have low 4 Her 3He. We will consider inthis study that this kind of hotspot is uncommon andnot involved in the global geodynamic cycle.

In the present study, we will show how the he-lium isotope systematics can be likewise substanti-ated by neon isotope systematics.

2. Terrestrial neology

Neon has three stable isotopes: 20 Ne, 21 Ne and22 Ne. In the atmosphere, the abundance is 90.5% for20 21 22 ŽNe, 0.268% for Ne and 9.23% for Ne Nier,

.1950 .In terrestrial rock samples, neon isotopic anoma-

lies are very hard to detect with a mass spectrometerbecause of different problems:

Ž . Ž1 experimental blanks up to 60–70% for some.MORB glasses or OIB xenoliths phenocrysts ;

Ž .2 atmospheric contamination of the samples dur-ing the eruption, masking the primordial signature;

Ž . Ž3 mass interferences during analysis with dou-40 20 22 .bly charged Ar on Ne and CO on Ne .2

This is the reason, until the work of Sarda et al.Ž . 20 221988 , why the value of the Ner Ne ratio in themantle has been the subject of many debates. Themajority of researchers considered the ratio to besimilar to the AIR ratio of 9.8. Craig and LuptonŽ . 201976 first demonstrated an excess of Ne and21 Ne by comparison to AIR in basalts and in gasesfrom Kilauea, but the anomalies were very small andfurther studies by other laboratories did not confirm

Ž .clearly their initial proposition Fig. 2 or proposedmass fractionation for the 20 Ner 22 Ne isotopic ratioŽKyser and Rison, 1982; Ozima and Zashu, 1983;

.Poreda and Radicati di Brozolo, 1984 . Sarda et al.Ž .1988 have shown that MORB samples are alignedin a 20 Ner 22 Ne– 21 Ner 22 Ne neon isotopic diagramŽ . 20 22Figs. 2 and 3 with some Ner Ne up to 13.2 and21 Ner 22 Ne up to 0.07. The interpretation of thisalignment is that it results from mixing between the

Ž 20 22MORB source with Ner Ne ) 13 and21 22 . ŽNer Ne)0.07 and an AIR component 9.8 and

. 210.029 . The origin of Ne excess is attributed to theŽ24 Ž . 18 Ž .nucleogenic reaction Mg n, a and O a , n ;

. Ž .Wetherill, 1954 . Marty 1989 and Hiyagon et al.Ž .1992 have confirmed these anomalies in MORB,but with very large uncertainties.

Ž .Staudacher et al. 1990 measured neon anomaliesŽ20 22in xenoliths from Reunion Island Ner Ne up to

21 22 .12 and Ner Ne up to 0.04 and interpreted theseanomalies as due to mass fractionation. Honda et al.Ž .1991 measured some glasses from Loihi and ob-

20 21 Ž20 22served some Ne and Ne excess Ner Ne). 20 2211 . They have proposed that the Ner Ne of the

lower mantle has a solar value but that the21 Ner 22 Ne ratio is less nucleogenic. Since the work

Ž . 20 22of Honda et al. 1991 , high values of Ner Ne4 3 Žhave been observed in low Her He OIB Poreda

and Farley, 1992; Hiyagon et al., 1992; Valbracht et. 4 3al., 1996 . Some low Her He on-ridge hotspots

show neon isotopic ratios confirming that the lowerŽmantle has solar-like neon isotopic ratios Moreira et

.al., 1995, 1996; Sarda et al., 1996 .From these observations, one can conclude that

the MORB source reservoir and the OIB reservoirhave 20 Ner 22 Ne ratios between the maximum value

( )M. Moreira, C.J. AllegrerChemical Geology 147 1998 53–59` 55

Ž . Ž .Fig. 2. Evolution with time of neon anomalies in basalts. Data from Craig and Lupton 1976 , Kyser and Rison 1982 , Ozima and ZashuŽ . Ž . Ž . Ž . Ž . Ž .1983 , Poreda and Radicati di Brozolo 1984 , Sarda et al. 1988 , Staudacher et al. 1990 , Honda et al. 1991 , Poreda and Farley 1992 ,

Ž . Ž . Ž .Hiyagon et al. 1992 , Moreira et al. 1995 , Valbracht et al. 1996, 1997 .


Fig. 3. Three-isotope Ne–Ne diagram. The Loihi line is fromŽ . Ž .Honda et al. 1991 and Valbracht et al. 1996 . The MORB line

Ž .is from Sarda et al. 1988 . mflsmass fractionation line. Thisfigure shows that the reservoir source of MORB is more nucle-ogenic than the reservoir source of OIB, whereas they may havethe same 20 Ner 22 Ne close to solar value. The schematic pictureshows the structure of the mantle from both the He and Neisotopes systematics.

Žmeasured for each 13.2 for MORB and 13.1 forŽ .Loihi Sarda et al., 1988; Valbracht et al., 1997 and

Ž .the solar value 13.8, Benkert et al., 1993 . Thedifference between MORB and OIB is the21 Ner 22 Ne ratio which has more nucleogenic 21 Ne

Ž .in the MORB mantle upper mantle than in the OIBsource mantle, in the same way as the helium iso-topic ratio in MORB source is more ‘radiogenic’than in the OIB source mantle. This supports themodel of a two-layer mantle with an upper degassed

Žmantle and a lower, less degassed mantle Allegre et`.al., 1983, 1986r87 .

An important remaining problem is: how can the20 Ner 22 Ne ratio of the atmosphere be 9.8 if it wasdegassed from the mantle with a 20 Ner 22 Ne ratio

close to 13.8? After the exploration of several possi-bilities, one possible answer is provided by the model

Žof hydrodynamic escape Hunten et al., 1987; Pepin,.1991 . This model proposes that the young atmo-

Ž .sphere composed mainly of H escaped from the2

Earth during the T-Tauri phase of the Sun, entrainingŽ .the other gases e.g. Ne . This dynamic escape has

the capability to fractionate the isotopes sufficientlyto cause some important change in isotopic ratios oflight elements, in particular the 20 Ner 22 Ne ratiowhich may have decreased from solar values to 9.8

Žin the residual atmosphere Hunten et al., 1987;.Ozima and Zahnle, 1993 .

To compare more quantitatively the He–Ne iso-topes we will examine the so-called ‘steady-statemodel’, initially proposed for helium by Allegre et`

Ž . Ž .al. 1983 and Kellog and Wasserburg 1990 , andextended to other noble gases by O’Nions and Tol-

Ž .stikhin 1994 and Porcelli and WasserburgŽ .1995a,b .

3. The steady-state model: the HerrrrrNe test

The mantle is supposed to be divided in twolayers: an upper and a lower mantle. We can assumethat there is a flux from the lower mantle to uppermantle and from the upper mantle to atmosphere.

The steady-state model proposes that the fluxes ofŽ3 22 .primordial noble gases He, Ne are equal and

that the measured values of these gases in MORBcome directly from the lower mantle and stay in theupper mantle with a residence time of ;1 Ga beforethey escape to the atmosphere at mid-oceanic ridgesŽO’Nions and Tolstikhin, 1994; Porcelli and Wasser-

. Ž4 21 .burg, 1995a,b . The radiogenic isotopes He, Nehave two origins. The first is from the lower mantle.The second is the radioactive decay of U and Th forhelium and nucleogenic neon in the upper mantle.

Ž .An important constraint not proof in this model isthe 3Her 22 Ne ratio, that must be similar in the tworeservoirs as proposed by O’Nions and TolstikhinŽ . 3 221994 . They proposed a Her Ne in the mantle of;3. In the following, we will estimate here the3Her 22 Ne ratio using two different methods.

3.1. The South Atlantic hotspots

Ž . Ž .Moreira et al. 1995 and Sarda et al. 1996 havestudied two low 4 Her 3He on-ridge hotspots located


in the South Atlantic, the so called Shona and Dis-covery ‘anomalies’. This region provides a goodillustration of mixing between material from theupper mantle and a plume coming from the lowermantle in the environment of the ridge crest. Theminimum measured 4 Her 3He ratios for these two

Žhotspots are similar 58,000 for Shona and 48,000. 4 3for Discovery . Fig. 4 shows the Her He varia-

tions along the Mid-Atlantic ridge. The two peaksare clearly defined at 51.18S and 488S. Next toShona and Discovery, the 4 Her 3He in N-MORB is90,000–100,000, which is a typical range for South

Ž .Atlantic MORB Graham et al., 1992 .In a Ne–Ne isotopic diagram, the samples are

spread between the MORB line and the Loihi lineŽ .Moreira et al., 1995; Sarda et al., 1996 . Thus,

Fig. 4. Variations of 4 Her 3He ratios and 21 Ner 22 Ne alongextrap

the Mid-Atlantic Ridge between 42 and 548S. Two low 4 Her 3Hehotspots are injected on the ridge or are very close to the ridge.These anomalies are called Shona and Discovery anomalies. Thisfigure shows that Ne has the same isotopic systematics as He.

Ž .From Sarda et al. 1996 .

Fig. 5. 4 Her 3He vs. 21 Ner 22 Ne for the same samples as inextrap

Fig. 4. This figure shows the very good correlation between thesetwo ratios, interpreted as a mixing between lower mantle andMORB. The ‘curvature’ of the mixing hyperbole is given by

w3 22 x w3 22 xr s Her Ne r Her Ne . Here r is close to 10 if weUM LM

suppose that the lower mantle is represented by Loihi.

mixing between a plume component from the lowermantle, with solar-like Ne isotopic and low 4 Her 3Heratios and the upper mantle can explain the resultsfor helium and neon. The 21 Ner 22 Ne corrected foratmospheric contamination using a solar 20 Ner 22 Ne

21 22 Ž .is called Ner Ne Moreira et al., 1995 . Fig.extrap

4 shows the 21 Ner 22 Ne variations along theextrap

Mid-Atlantic ridge. In comparison with the4 Her 3He, we find the lowest 21 Ner 22 Ne forextrap

the lowest 4 Her 3He. This shows that the plumeshave both low 4 Her 3He and 21 Ner 22 Ne. Fig. 5shows the correlation between 21 Ner 22 Ne andextrap4 Her 3He. In such a diagram, the mixing is repre-sented by hyperbole and the ‘curvature’ of this hy-perbole is given by the r parameter: r sw3 22 x w3 22 x ŽHer Ne r Her Ne LM is for lowerUM LM

.mantlesLoihi . Note that there is no atmosphericcontamination to consider in this diagram. In thecase of a Loihi-like lower mantle, rf10.

3.2. The 20Ner 22Ne– 3Her 22Ne diagram

We have plotted the 20 Ner 22 Ne vs. the3 22 Ž .Her Ne for Loihi glasses Valbracht et al., 1997and for a stepwise crushing of the so-called ‘popping


Fig. 6. 20 Ner 22 Ne vs. 3Her 22 Ne for Loihi seamount samplesŽ . Žsquares and a stepwise crushed MORB sample ‘popping rock’

. Ž . Ž .2 P D43 dots . Data are from Valbracht et al. 1997 for LoihiŽ .and from Moreira unpubl. data for MORB. This figure shows

Ž3 22 . Ž3 22 .that the ratio Her Ne r Her Ne is close to 1.UM LM

Ž . Ž .rock’ 2 P D43 Moreira, unpubl. data Fig. 6 that isconsidered to be one of the best representative sam-ples of the depleted upper mantle. Extrapolations to asolar 20 Ner 22 Ne ratio give the same 3Her 22 Neratio obtained for Loihi and MORB, thus ratiow 3 22 x w 3 22 xHer Ne r Her Ne f 1 with aU M L M3Her 22 Nef7.

3.3. Discussion

The two methods do not give the same result.While the comparison of the two best representatives

Ž . Žof upper mantle 2 P D43 and lower mantle Loihi. 3 22 Ž .samples yields similar Her Ne ;7 which

agrees with the constraint of the steady-state model.The results for the South Atlantic on-ridge hotspotsindicate quite different 3Her 22 Ne for the upper andlower mantle if the mixing in the Shona–Discoveryarea is between a Loihi-like lower mantle materialand MORB material.

To explain this difference, we have two possibili-ties:

Ž .1 The upper mantle has not exactly a solar20Ner 22Ne as used here. This ratio is probably closeto 13 or lower rather than being close to 13.8because of a possible re-injection of atmosphericneon in the upper mantle. Thus the correction forAIR is overestimated for the normal MORB samples.An extrapolation to lower 20 Ner 22 Ne would givelower 21 Ner 22 Ne for the samples with highextrap4 H er 3 H e ratios w ithout changing the

21 Ner 22 Ne of the samples with low 4 Her 3He.extrap

Thus, in Fig. 4, the curvature would become lessŽ .pronounced with rf4–5 with mixing with Loihi

but will not become linear. The same calculation canbe done for the second method where an extrapola-tion to 13.8 for the Loihi samples gives a 3Her 22 Neclose to 7 for the lower mantle and 6 for the upper

Ž .mantle Fig. 5 with an extrapolation to 13. Thus ther value will be 6r7f0.9, close to 1, thus does notchange a lot.

Ž .2 Possible fractionation during melting. Themixing is not between solid materials but betweenmagma. The melt coming from a normal MORBregion is enriched in He, relative to Ne, by compari-son to a melt coming from an on-ridge plume regionbecause of fractionation between He and Ne duringmelting or vesiculation. Thus the hyperbolic mixing

Ž .does not reflect the ratio of the two solid sources.

4. Conclusion

He and Ne isotopes systematics provide strongarguments for a two-layer mantle. One layer is thesource of MORB, degassed, with radiogenic He andnucleogenic Ne signatures. The second is the lower

Ž3 22 .mantle, the source of primordial gases He, Nebecause it is less degassed. Using data from Loihiand from a popping rock, we estimate thew3 22 x w3 22 xHer Ne r Her Ne ratio to be close toUM LMŽ 3 22 .1 with Her Nef7 and it is a strong argument

in favour of the steady-state model for the uppermantle, which suggests that all primordial gasesmeasured in MORB come from the lower mantle.

Acknowledgements

The authors would like to thank J. Kunz fordiscussions, K. Burton for improving the English, K.O’Nions and an anonymous reviewer for reviews.This is IPG contribution No. 1471.

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helium–neon systematics and the structure of the mantle

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