noble gas constraints on degassing processes

Noble gas constraints on degassing processes

Manuel Moreira a;b;*, Philippe Sarda c

a Laboratoire de Geochimie et Cosmochimie, Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris Cedex 05, Franceb Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

c Groupe de Geochimie des Gaz Rares, Departement Sciences de la Terre et U.M.R., Orsayterre, Universite Paris 11 - Sud,91405 Orsay Cedex, France

Received 7 June 1999; received in revised form 6 January 2000; accepted 12 January 2000

Abstract

We compare the rare gas elemental ratios measured in glass samples of mid-oceanic ridges (MORs) and oceanicisland basalts to current estimates of the same ratios for upper and lower mantle reservoirs. We selected only sampleswith high 20Ne/22Ne (s 11) in order to minimize atmospheric contamination. The combined use of He, Ne and Arelemental systematics in these samples allows differentiation between the vesiculation process at mantle plumes, whichappears to be in open system, and at normal MORs, where vesiculation occurs in a closed system. Such an open systemvesiculation has important consequences for the interpretation of heavy rare gas isotopic ratios in primitive plume-derived materials. ß 2000 Elsevier Science B.V. All rights reserved.

Keywords: noble gases; degassing; mantle plumes; mid-ocean ridge basalts

1. Introduction

Rare gas systematics provide constraints on theformation of the Earth's atmosphere and onmantle structure [1^4]. The helium isotopic com-position of oceanic basalts shows a dichotomybetween mid-oceanic ridge basalts (MORB)and oceanic island basalts (OIB). The formerhave constant 4He/3He ratios around 90 000(R/Ra = 8, where Ra is the atmospheric ratio of1.384U1036) [5,6], much higher than most ofthe OIB which show 4He/3He ratios as low as20 000 (R/Ra = 32) at Loihi seamount [3,7]. Fig. 1

shows the 4He/3He (and R/Ra) ratio for MORBand Loihi glass samples against the measured 4Heconcentrations (data from the literature). One canobserve the very di¡erent distribution betweenMORB and Loihi populations for both the heli-um content and the helium isotopic ratio. Thisdi¡erence is often attributed to the existence oftwo reservoirs in the mantle. The source ofMORB, which is certainly the upper mantle, isdegassed and its helium isotopic signature resultsfrom the radiogenic production of 4He in a rela-tively high (U+Th)/3He source. The `primordial'helium isotopic signature of some OIB is inter-preted as due to the existence of a less degassedreservoir, probably located in the lower mantle[1,7,8].

The helium systematics are con¢rmed by thestudy of neon isotopes. Ridge samples have

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* Corresponding author. Tel. : +33-1-4427-4805;Fax: +33-1-4427-3752; E-mail: [email protected]

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21Ne/22Ne of up to V0.07 (corrected for atmos-pheric contamination) while in plume samples (i.e.with low 4He/3He ratio), this ratio is as low as0.04 [9^14]. Again, this is due to a di¡erent(U+Th)/22Ne ratio in the two reservoirs, wherethe degassed MORB source shows a higher ratiothan the less degassed plume source.

Therefore, the helium and neon systematics,and also the argon isotopes [15], show that aless degassed reservoir exists in the lower mantle.In this model, where the two reservoirs have notsu¡ered the same degree of degassing, the noblegas concentrations should be di¡erent (i.e. higherin the deep mantle).

Concentrations in mantle sources are very di¤-cult to determine due to degassing and contami-nation of magmas en route to the surface. A long-standing problem is that helium concentrationsmeasured in oceanic island submarine basaltglasses are generally lower than in MORB glassesdespite the lack of evidence for a further degass-ing (Fig. 1) [3,7,11,12]. This seems to contradictthe less degassed character of the OIB source andthis has been called the `helium paradox' [16]. Oneway to understand the problem of low heliumcontent is to study the noble gas elemental ratiosthat re£ect degassing processes.

In this paper, we use the neon isotopes to con-strain the proportion of atmospheric contamina-tion in basalts [3,9,13,14,17] and to study vesicu-lation processes using two rare gas elementalratios: He/Ar and He/Ne. Using only sampleswith high 20Ne/22Ne ratios (i.e. those with littleatmospheric contamination), we show that theprocess of degassing is not the same for normalmid-oceanic ridge (MOR) as for oceanic island.This has important consequences for the concen-trations of all the noble gases in plume-derivedbasalts and gives constraints on the `atmospheric'xenon isotopic composition in lower mantle-derived samples.

2. The noble gas elemental ratios in the mantlesamples

As many mantle samples have su¡ered atmos-pheric contamination during eruption, the mea-

sured elemental ratios He/Ne or He/Ar do notnecessarily represent the mantle ratio. This poten-tial problem is usually addressed using the4He/40Ar* [18,19], where the 40Ar* is the meas-ured 40Ar `corrected' for atmospheric contamina-tion (40Ar* = (40Ar/36Ar340Ar/36Aratm) 36Ar). Asthere is very little helium in the atmosphere, the4He/40Ar* ratio is therefore virtually free of at-mospheric contamination. Even in this case, themeasured ratios are often very high in MORB(15^150), which is not compatible with the K/Uratio of the mantle (the 4He/40Ar* ratio should bebetween 1.5 and 4) [2]. This has led to the sugges-tion of an elemental fractionation in magma byvesiculation after melting in the mantle [19,20].Therefore, we need to know what the `true' ele-mental ratios in the mantle are.

2.1. The `true' mantle elemental ratios

Moreira et al. [21] have used the popping rock`22D43' to determine the rare gas isotopic andelemental ratios in the upper mantle. This sampleis very volatile rich [18,19,22,23]. Its high vesicu-larity (V17%) suggests that minimal vesicle losshas occurred; gases in the vesicles represent mag-ma gases before vesiculation [20], and thus prob-ably the mantle, since melting appears to fraction-ate the rare gases weakly [2,18,19].

We will use in this study 4He/40Ar* = 1.5 for theupper mantle value, as measured in this sample22D43 [21]. This is also consistent with the closedsystem production ratio [2,18,20,24]. The correla-tion found between the 20Ne/22Ne and 3He/22Neratios in this sample (by step crushing) was usedto determine a 3He/22Ne value of 7.3 for the uppermantle [21], assuming a solar-like 20Ne/22Ne ratioof 13.8 [25]. This ratio is in agreement with theestimate of Honda and Mc Dougall [26], using theHe^Ne isotopic systematics. A lower than solar20Ne/22Ne ratio leads to lower 3He/22Ne, but, inthe following, we will use the solar 20Ne/22Ne ra-tio of 13.8 [25] for consistency.

No similar inferences can be made for the lowermantle because unfractionated samples have notbeen found. However, there is no obvious reasonto think that the stable 3He/22Ne ratio in the low-er mantle is very di¡erent from that of the upper

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mantle. We can estimate this ratio using the equa-tion:

4He=3HeLoihi34He=3Hesolar � �4He=21Ne��

U �21Ne=22NeLoihi321Ne=22Nesolar� 22Ne=3He

Taking (4He/21Ne)* = 2.2U107 [26], one can get3He/22NeW7.5, which is close to the MORB val-ue. The 4He/40Ar* ratio in the lower mantle canonly be modeled to be 1.8, assuming a closedsystem evolution [2].

2.2. MORB samples

Measured 3He/22Ne ratios in MORB glasses arehighly variable (from 0 up to 50), mainly due toatmospheric contamination in magma chambersor during eruption. One way to estimate thepre-contamination ratios is to use a mixing dia-gram 20Ne/22Ne^3He/22Ne, where mixing is repre-sented by a straight line. For each data point,the measured 3He/22Ne can be extrapolated tothat value, (3He/22Ne)corr, which corresponds to

Fig. 1. 4He/3He and R/Ra (R = 3He/4He and Ra is the atmos-pheric ratio) vs. the 4He concentrations measured in MORBand Loihi glasses. The two populations are very distinctboth in concentrations and isotopic ratios. The measuredconcentrations of Loihi samples are much lower than ex-pected if the Hawaiian plume comes from an undegassedlower mantle. Data are from the literature.

Fig. 2. A: Neon isotopic diagram showing our MORB data base, restricted to total 20Ne/22Ne ratios higher than 11. The dataplot on the MORB trend ¢rst observed by Sarda et al. [9]. Solar corresponds to the neon isotopic ratios measured in solar wind[25]. Loihi line is from [10] and [12]. B: 20Ne/22Ne versus 3He/22Ne ratios for the same samples. Data from the popping rock22D43 (step crushing) are also reported (black squares) and represent unfractionated samples [21]. White dots represent the3He/22Ne corrected for air contamination, assuming that each sample is a mixing between two components (MORB and air) witha MORB 20Ne/22Ne ratio of 13.8 (see Appendix).

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a solar 20Ne/22Ne ratio (13.8 [25]). Because thereis no 3He in the atmosphere compared to 22Ne,the equation used for the correction is (3He/22Ne)corr = (13.839.8)/(20Ne/22Nesample39.8) (3He/22Ne)sample (see Appendix). This approach is sim-ilar to the one of Honda et al. [3], who assume amixing between three components (air, solar andnucleogenic). The only di¡erence between the twomethods is that we neglect the 22Ne coming fromnucleogenic production, as illustrated in theAppendix. Note that the neon correction implies,in theory, that the atmospheric contaminationoccurs after the elemental fractionation (due todegassing). This is certainly the case since theatmospheric component is generally degassed atlow temperature during step-heating. This con-tamination probably occurs during the eruption,but after the vesiculation (produced at relativelyhigher depth) or in the degassed magma cham-ber.

Fig. 2B shows the total 20Ne/22Ne ratios versusthe total 3He/22Ne ratios measured on normalMORB samples (4He/3Hes 80 000; R/Ra6 9)[13,27]. We have only plotted samples with total20Ne/22Ne ratios higher than 11, which indicateslittle atmospheric contamination (Fig. 2A). In thiscase, the extrapolation to solar 20Ne/22Ne is nottoo far from the sample data, implying relativelysmall uncertainties. The obtained corrected3He/22Ne values are between 25 and 70, very frac-tionated from the upper mantle 3He/22Ne ratio of7.3 [21]. The 4He/40Ar* ratios for the same sam-ples are also very fractionated, as they vary be-tween 15 and more than 50 compared to the man-tle value of 1.5. We have plotted the (3He/22Ne)corr ratios versus the 4He/40Ar* ratios onFig. 3 (black dots).

2.3. Plume samples

It is also useful to consider the 3He/22Ne and4He/40Ar* elemental ratios of MORB samplesdredged on the South Atlantic Ridge where4He/3He ratios are lower than 60 000 (R/Ras 12) (Fig. 4). The so-called Shona and Dis-covery Ridge anomalies are characterized bytopographic highs, gravity anomalies [28,29] and`high 3He anomalies', with 4He/3He ratios as low

as 48 000 (R/Ra = 15) (Fig. 4) [13,14,27]. Theseanomalies probably represent, at least for raregases, a plume coming from the lower mantle,mixed with upper mantle material. Fig. 3 displaysthose plume-in£uenced MORB samples (R/Ras 12 only) (Fig. 4), again selecting sampleswith total 20Ne/22Ne ratios higher than 11 (whitedots). In this case, the samples de¢ne a lineartrend which is clearly di¡erent from the `normal'MORB array. They show little 3He/22Ne varia-tion, while their He/Ar ratios may be highly frac-tionated, with values of up to V60. Fig. 3 also

Fig. 3. 4He/40Ar* versus the 3He/22Ne corrected for AIR (seetext). Dot labeled 25D is sample EW9309-25D [27] which,despite of its 4He/3He of 84 156 þ 799, belongs to the Dis-covery Ridge anomaly based on neon isotopes, Sr, Nd, Pbisotopes, REE composition and topography [27,30] (see Fig.4). Sample 2D was analyzed by melting in [27]. We re-analyzed this sample by crushing and get the followingvalues: 4He = 5.8U1037, 20Ne = 5.0U10311, 36Ar = 6.0U10311

ccSTP/g, 4He/3He = 48 000 þ 700, 20Ne/22Ne = 11.16 þ 0.18,21Ne/22Ne = 0.0342 þ 0.0017, 40Ar/36Ar = 476 þ 6. The 3He/22Necorr = 7.9 and 4He/40Ar* = 54. Upper mantle composition(3He/22Ne = 7.3 and 4He/40Ar* = 1.5) is from [21]. Lowermantle is represented here by 3He/22Ne = 5 and 4He/40Ar*= 1.8. We report two lines corresponding to Rayleigh distilla-tion (magma) calculated with measured solubilities [20] andwith our estimate of the solubilities (see text). `Normal'MORB are samples with a 4He/3He ratio higher than 80 000.E-MORB samples are samples with 4He/3He ratios lowerthan 60 000. With the exception of sample 25D, `normal'MORB fall on the MORB line in a three isotope neon dia-gram (Fig. 2A).

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shows samples from Loihi seamount, with total20Ne/22Ne ratios higher than 11 [11,12]. One cansee that they plot on the same trend as samplesfrom Shona and Discovery anomalies.

Note that sample 25D, which has a MORB-likehelium signature (4He/3He = 84 156 þ 800, R/Ra =8.6), falls on the plume line in Fig. 3. This samplebelongs to the Discovery Ridge anomaly based onSr^Nd^Pb systematics and on topography (Fig.4). In the three isotope neon diagram, it falls be-tween the MORB line and the Loihi line, re£ect-ing a plume component in its neon composition(Fig. 2A). The helium isotopic ratio re£ects hyper-bolic mixing in a helium^neon isotopic diagram(see [14] or Fig. 6). The neon and Sr^Nd^Pb sys-tematics, and the geographical location of thissample identify it as an E-MORB rather than ann-MORB [30].

Hence, rare gas elemental fractionation appearsvery di¡erent in plume-related areas compared tonon-plume, ridge basalts. This observation sug-

gests that di¡erent degassing mechanisms occurin the di¡erent geodynamic settings.

3. Discussion

3.1. Origin of the elemental fractionations

The origin of the rare gas elemental fractiona-tion might be related to di¡erent degrees of in-compatibility during melting for He, Ne and Ar,which could possibly fractionate the elemental ra-tios. We reject this hypothesis because the pop-ping rock 22D43, which also results from melt-ing, but shows no elemental fractionationcompared to predicted ratios. Our view is thatthe model of vesiculation and vesicle loss, thathas already been proposed by Sarda and Grahamfor normal MORB [19], is the simplest way toexplain rare gas elemental fractionations inMORB. Vesiculation and vesicle loss are also gen-

Fig. 4. Depth and R/Ra vs. latitude for South Atlantic MORBs (from [27]). D is the Discovery anomaly and S is the Shonaanomaly. Sample 25D is also indicated (see legend of Fig. 3 and text). It is located to the south of the Agulhas fracture zone(FZ) in the Discovery Ridge anomaly. The two high 3He hotspots located near or on the Mid Atlantic Ridge. For E-MORB ofFig. 3, we have chosen the samples with R/Ra higher than 12.

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erally advanced to explain variations of the C andN isotopic ratios [22,23,31,32]. Such a process canfractionate the rare gases because of di¡erent sol-ubilities in magma: the lighter the gas, the moresoluble in magma [20]. However, the e¡ect on therare gas elemental ratios depends on the mecha-nism of vesicle creation and loss.

One degassing scheme is closed system vesicu-larity where the vesicles are created in equilibriumwith the magma, then lost during, for example,residence in a magma chamber. The maximumHe/Ne ratio permitted in the residual magma aftera vesiculation in closed system is (He/Ne) =(He/Ne)0 (SHe/SNe), whereas the ratio in thevesicles is close to the initial ratio before vesicu-lation (SHe and SNe are the solubilities of He andNe in the magma).

Another model is vesiculation in an open sys-tem, where vesicles leave the magma shortly aftertheir generation. In this case, a distillation e¡ectarises: an elemental ratio in the magma can bemodeled by a Rayleigh distillation law. For theHe^Ne couple, we can write : (He/Ne)magma = (He/Ne)0 g�13K�, where K= SHe/SNe, (He/Ne)0 is thestarting elemental ratio and g = He/He0 is the he-lium fraction remaining in the magma. A similarequation can be written for He/Ar, replacing Neby Ar. In the logarithmic diagram of Fig. 3, sucha distillation is represented by a straight line,which is indeed what is observed for plume sam-ples.

The closed system vesiculation, followed by de-gassing, has been applied with success to MORBs(e.g. [19,20]). Fig. 5 shows the expected negativecorrelation between 40Ar* and 4He/40Ar* in thecase of solubility-controlled degassing (closed sys-tem vesiculation) where argon is less soluble inmagma than helium. For the normal MORB sam-ples, we will assume that the rare gas elementalratios of the magma after vesiculation are givenby the end of the data array in Fig. 3: this is 3He/22NeV70 and 4He/40Ar*V60. The data disper-sion between original and fractionated magma isthen attributed to variable vesicle loss. In otherwords, what is seen is mixing between vesiclesfrom the residual magma, very depleted in raregases and created in a late vesiculation near thesurface, and a variable number of vesicles, which

record the rare gas elemental ratios in the unve-siculated magma because they contain most of therare gas inventory.

Using upper mantle ratios of 7.3 and 1.5 [21],we can determine solubility ratios SHe/SArV40and SHe/SNeV10. These two values are 4^5 timeshigher than solubility ratios measured in the lab-oratory, for example by Jambon et al. [20] (SHe/SArV10 and SHe/SNe = 2) or Lux [33] (SHe/SAr = 7and SHe/SNe = 2). Using the measured value, onecannot explain the MORB data even with twostages of vesiculation since the He/Ar ratio willbe too high compared to the He/Ne ratio. Becauseboth He/Ar and He/Ne solubility ratios are di¡er-ent by a factor 4^5, this indicates that only thehelium solubility is higher by this factor. The rea-son that the observed helium solubility is di¡erentfrom the laboratory measurements is unclear. Thechemistry of the basalts cannot explain the di¡er-ence between MORB and E-MORB+plume ele-mental ratios since they are all tholeites. Fast he-lium di¡usion might explain this apparent highersolubility.

Fig. 3 shows the corresponding Rayleigh distil-lation curve (residue of vesicle extraction), usingthe above determined SHe/SAr and SHe/SNe valuesand initial values of 3He/22Ne = 5 and 4He/40Ar* = 1.8. The 3He/22Ne of the lower mantle istaken here to be 5 to ¢t the data best. A distilla-tion curve starting with a value of 7.3 as for theupper mantle would not ¢t the data correctly. Thedi¡erence between the 3He/22Ne of the upper andlower mantles could be ascribed to a di¡erence inthe 20Ne/22Ne ratio since some authors proposethis ratio is di¡erent in the two reservoirs (13.5for lower mantle and 12.5 for upper mantle)[21,34]. Taking 12.5 for the 20Ne/22Ne ratio ofthe upper mantle (the highest ratio measured inMORB), Moreira et al. [21] got a 3He/22Ne ratioof 4.9, very similar to 5 used here for the lowermantle. The 4He/40Ar* is taken here to be 1.8,assuming the lower mantle had a closed systemevolution with K/U = 12 700 [35]. With these pa-rameters, one can observe that the line in Fig. 3perfectly ¢ts the data with low 4He/3He ratios(triangles and white dots). The same calculationcould be done with the measured solubility ratios[20] and gives a similar distillation curve (Fig. 3).

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This is because the distillation curve is mainlycontrolled by the ratio SNe/SAr which is similarto the measured one (we obtain 4 and the mea-sured values are between 5 and 3.5).

Another constraint to the problem can be ob-tained from the isotopic data. A 4He/3He^21Ne/22NeC diagram where 21Ne/22NeC is the 21Ne/22Necorrected for atmospheric contamination (using asolar 20Ne/22Ne) can yield information on the3He/22Ne ratios of the two sources. This is shownon Fig. 6, for MORBs from South Atlantic andSouth Paci¢c. In this diagram, mixing is hyper-bolic and the `curvature' of the hyperbola is givenby the ratio r = (3He/22Ne)MORB/(3He/22Ne)PLUME,if we assume a binary mixing between the normaldepleted mantle and a plume source (data arefrom [13,27] and Moreira and Kurz, unpublisheddata). The best ¢t is given in both cases by a valueof r = 10 which means that the 3He/22Ne ratio ishigher by a factor 10 in the MORB magma thanin the plume magma, whereas the measured 4He/40Ar* are roughly the same [13,27,34]. That con-¢rms the conclusion obtained from the He/Ar^He/Ne diagram of Fig. 3.

To summarize, we have shown evidence thatnormal and plume-in£uenced MORB have suf-fered di¡erent degassing processes. NormalMORB glasses have probably undergone closedsystem vesiculation, then lost vesicles in a magmachamber located near the surface (V5 km). Thismodel was previously advanced based on carbonisotope systematics [22,23,32], and proposed to ¢trare gases as well [19]. Conversely, for samplesin£uenced by primitive plumes, there is good evi-dence for open system vesiculation, where vesicleswere lost immediately and distillation e¡ects oc-cur. This observation is highly dependent on thechoice of the sample set. We selected only sampleswith high 20Ne/22Ne (s 11) to avoid contamina-tion problems.

3.2. Consequence for noble gas contents

Upper mantle rare gas concentrations have al-ready been discussed by Sarda and Graham [19],and the model of magma chamber degassing fol-lowing a vesiculation in closed system could ac-count for noble gas concentrations measured in

normal MORB (4He = 1037^1034 ccSTP/g) [19].However, plume or E-MORB glasses have Heconcentrations between V1037 and 2.5U1035

ccSTP/g, i.e. similar or lower than normalMORB samples, which seem to contradict the un-degassed lower mantle model [7,12,13,27,36].

We have calculated the He/He0 ratio, i.e. thefraction of helium remaining in the magma, forthe most fractionated plume-in£uenced sampleson the Rayleigh distillation line in Fig. 3: theHe/He0 ratio is not lower than 91%. These valuesindicate that distillation does not e¤ciently ex-tract helium from the magma because of its highsolubility. However, the same estimate for argonshows that the argon remaining is very low, downto 2%. The case of xenon is even more drasticsince the distillation leaves 2U1036 of initial xe-non in the magma for He/He0 = 0.91. In contrast,the Ne/Ne0 ratio is 40% when He/He0 = 0.91,which indicates that neon is not extensively de-gassed.

An important consequence of the distillationprocess in plume-like magma is the considerableloss of argon and xenon. We have shown thatwhen only 5% of the helium was lost, 87% ofthe Ar and more than 99.9% of the Xe was lost,but only 40% of the neon. Such an important Ar^Xe loss, if followed by only small atmosphericcontamination, could explain the large dispersionof 40Ar/36Ar ratios and the atmospheric values ofXe isotopic ratios measured in primitive plume-derived samples, even when 20Ne/22Ne ratios arehigh [3,10^13,37]. This makes the search for Xeisotopic anomalies in lower mantle-derived mate-rial very di¤cult, which has important consequen-ces for the mantle structure and the evolution ofthe atmosphere [2,4,38,39].

The distillation process suggested here cannotexplain the low He concentrations of primitiveplume material, unless further degassing occursat the surface when vesiculation occurs at rela-tively low pressure. We have no evidence forsuch a shallow degassing for the samples analyzedsince the 4He/40Ar* is not very di¡erent in plumebasalts than in MORBs. Another possibility forthe low He concentrations in plume-like materialwas recently proposed by Chamorro-Perez et al.[40], who showed that Ar becomes insoluble in

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magma at high pressure. This could explain whyrare gases are less abundant in plume-like materi-al as melting begins at higher pressure than forMORB. However, in this context, the melting rateis probably very low, and melts move very weakly

because they are not inter-connected, while thesolid matter continues to rise to the surface whereAr and other gases become soluble in magma, andcan be extracted.

3.3. Possible mechanisms for the distillation

The mechanism of such a di¡erence betweenthe degassing process occurring in normal MORand plume contexts is the key issue. The degassingat MORs is now relatively well understood anddegassing of the magma chamber after vesicula-tion in a closed system is a relatively simple pro-cess [19,22,23,41]. However, distillation (vesicula-tion in open system) that seems to occur in aplume context, whether the plume is near a ridgeor not, is more di¤cult to explain.

We have done the same calculations for 3He/22Ne and 4He/40Ar* ratios for lithospheric xeno-liths from Samoa and Kerguelen archipelago[42,43]. Kerguelen and Samoa archipelago bothhave a high 3He/4He ratio [42,43]. The resultsare shown in Fig. 7. As above, we have consid-ered only samples with 20Ne/22Ne ratios higherthan 11. In the case of Kerguelen, the samplesare subaerial and there is a cosmogenic 3He con-tribution to the measured helium since the sam-ples were melted. We used the helium obtained by

Fig. 6. 21Ne/22Ne (corrected of atmospheric contamination, see [13] or [14]) against the 4He/3He ratios for MORB from SouthAtlantic [13,27] and South Paci¢c (Moreira and Kurz, unpublished data). The hyperbolic mixing can be interpreted as a mixingbetween a MORB magma (UM) and a plume magma (LM) with a ratio (3He/22Ne)MORB/(3He/22Ne)PLUME = 10. This con¢rmsthe di¡erence between MORB and plumes in the 3He/22Ne ratios observed on Fig. 3.

Fig. 5. 4He/40Ar* vs. 40Ar* for MORB with high 20Ne/22Neratios (s 11). The negative correlation between 4He/40Ar*and 40Ar* can be interpreted as solubility-controlled degass-ing [19,20].

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crushing (no cosmogenic) to get the `true' 3Hecontent. Fig. 7 shows that the olivines from xen-oliths fall near a line that corresponds to thevesicles extracted from the magma in an opensystem vesiculation. This line was calculated as-suming low vesicularity (V* = 1034) at each stepof vesicle extraction. It is the complement of thedistillation line for magma shown in Fig. 3.

The fact that such samples have a low heliumcontent compared to other noble gases has al-ready been noted and generally attributed to apreferential loss of helium [42,43]. We suggestthat the elemental rare gas ratios correspond tovesicles formed at low vesicularity.

Thus, Fig. 7 shows that the vesicles extractedby plume-like magma (in open system) may havebeen trapped in the residual lithospheric mantle(these xenoliths are extracted from the lithospher-ic mantle). One possibility for the distillation (anddi¡erence with MORB degassing) could be melttransport through the lithosphere in the case ofplumes, if the vesiculation starts at the base of thelithospheric mantle or deeper. Such percolation ina solid material could store the CO2 rich gas inmineral interfaces or grain boundaries by surfacetension.

However, the samples from the South AtlanticRidge (Shona and Discovery anomalies), where

Fig. 7. Same diagram as Fig. 3, but olivines from lithospheric xenoliths from Samoa and Kerguelen have been added. Data from[42,43] and Moreira (unpublished data). Only the samples with total 20Ne/22Ne higher than 11 have been chosen. Note that thecrushed olivines fall near the Rayleigh distillation curve (extracted vesicles), indicating that the gases trapped in these xenolithsmay come from vesicles extracted in open system from the magma.

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two hotspots are located on or near the ridge,show the same trend as for Loihi where the litho-sphere is relatively thick. If the proposed process(percolation) is correct, we must also assume thatthe process occurs when the hotspot is close to aridge.

We cannot exclude that magma chamber mech-anisms are di¡erent for MORs and plumes evenwhen they are located on a ridge. In this case, wehave to invoke an open magma chamber duringvesiculation for plumes and a closed one forMORs. This could be due to di¡erent residencetimes in the magma chamber (short for hotspots)or the depth of this magma chamber. However, inthis case, gas trapped in £uid inclusions in litho-spheric xenoliths is still derived at low vesicular-ity.

4. Conclusions

We have shown that the process of degassing isnot the same when it occurs in a MOR contextthan when a mantle plume is at the surface.MORB noble gas data can be explained by de-gassing of a magma chamber after a vesiculationin a closed system, whereas ocean island/plumenoble gases suggest an open system (the vesiclesare lost as soon as they are created). The noblegas trapped in lithospheric xenoliths seems to con-tain the gases extracted from open system vesicu-lation. It may indicate that the process of distil-lation occurs in the lithospheric mantle, perhapsby percolation in the lherzolite. The CO2 vesiclesmay stay trapped, whereas the liquid continues torise to the surface. This distillation can also occurin an open magma chamber at plumes, whereasthe magma chambers are closed on MORs. Thisdistillation process is important to understandingthe heavy noble gas content and isotopic compo-sition of plumes, because it would imply that noprimordial xenon stays in the magma. This wouldmake xenon anomalies very di¤cult to ¢nd insamples from the lower mantle. The low heliumcontent problem in plumes is not resolved sincesuch a distillation is not a process that can re-move the helium from magma, due to its rela-tively high solubility.

Acknowledgements

We thank J. Kunz, C. Alle©gre, M. Kurz and P.Cartigny for discussions. Two anonymous re-viewers and M. Honda helped to improve the¢nal version of the manuscript.[AH]

Appendix. Atmospheric contamination correction

Honda et al. [3] propose that each sample is amixing between three components (air, solar andnucleogenic) :

R20 � KRs20 � L Rn

20 � Q Rat20

R21 � KRs21 � L Rn

21 � Q Rat21

where K= 22Nes/(22Nesample) and K+L+Q= 1Sum of solar (s), atmospheric (at) and nucleo-

genic (n).R20 is the 20Ne/22Ne ratio and R21 is the 21Ne/

22Ne ratio.Knowing all the ratios, we can obtain 22Nes :

R20 � KRs20 � L Rn

20 � �13K3L �Rat20

R203Rat20 � K �Rs

203Rat20� � L �Rn

203Rat20�

R213Rat21 � K �Rs

213Rat21� � L �Rn

213Rat21�

R2039:8 � 4K35:1L

R2130:029 � 0:0038K � 47:971L

R2039:8 � 4K30:106�R2130:02930:0038K �

DK � 0:25U�R2039:8� 0:106�R2130:029��A�

After to get the 3He/22Nes, they do:

3He=22Nes � 3He=K �22Nesample� �

4�3He=22Ne�sample=�R2039:8� 0:106�R2130:029��B�

EPSL 5370 29-2-00


Our method

We propose in the present study that the 3He/22Ne and the 20Ne/22Ne ratios re£ect a mixingbetween two components (air and solar) :

R20 � aRs20 � bRat

20 � a�Rs203Rat

20� � Rat20

�3He=22Ne�sample � a�3He=22Ne�s

� b�3He=22Ne�atWa�3He=22Ne�sbecause (3He/22Ne)atW0 (no 3He in the atmos-phere), thus we get:

�3He=22Ne�s � 4�3He=22Ne�sample=�R2039:8� �C�

Comparison between the two methods

We are now comparing eqs. B and C.In all mantle rocks, R21 6 0.06.D0.106(R2130.029) is negligible compared to

(R2039.8).Deq. B can thus be written:

3He=22NesW4�3He=22Ne�sample=�R2039:8�

This result is exactly the same as obtained in eq.C. This indicates that the nucleogenic 22Ne is neg-ligible in mantle.

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noble gas constraints on degassing processes

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