Earth and Planetary Science Letters 395 (2014) 159–167

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Earth and Planetary Science Letters

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An observed link between lithophile compositions and degassingof volatiles (He, Ar, CO2) in MORBs with implications for Re volatilityand the mantle C/Nb ratio

Pete Burnard a,∗, Laurie Reisberg a, Aurélia Colin b

a CRPG-CNRS UMR7358, Université de Lorraine, Vandoeuvre-lès-Nancy 54500, Franceb Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 May 2013Received in revised form 7 February 2014Accepted 18 March 2014Available online 12 April 2014Editor: T. Elliott

Keywords:noble gasesmagmatic degassingmantle carbon concentrationmantle heterogeneitySouth East Indian RidgeRe volatility

There are systematic variations between relative noble gas abundances and lithophile tracers such as87Sr/86Sr, εNd and La/Sm in a suite of basaltic glasses from the South East Indian Ridge (SEIR). 4He/40Ar∗(where 40Ar∗ is 40Ar corrected for atmospheric contamination) correlates positively with 87Sr/86Sr andLa/Sm but anticorrelates with εNd. The large range in 4He/40Ar∗ observed in the glasses is due tofractionation during magmatic degassing caused by the very different solubilities of He and Ar insilicate liquids, whereas 87Sr/86Sr, εNd, La/Sm, etc. are insensitive to magmatic processes but ratherreflect mantle heterogeneity. Thus, there is a curious situation in this suite of basalts where tracers ofmantle heterogeneity (87Sr/86Sr, εNd, La/Sm, etc.) correlate with a tracer of magmatic volatile processes(4He/40Ar∗).Here, we propose that “enriched” mantle (with high La/Sm and 87Sr/86Sr, low εNd) also has a higher Cconcentration than “depleted” mantle. Magmas derived from enriched mantle will therefore have higherinitial C concentrations, leading to a greater fraction of CO2 degassed and thus a higher 4He/40Ar∗ ratioon eruption. Simple solubility-determined fractional degassing models show that the range in 4He/40Ar∗observed in SEIR basaltic glasses can be generated if the mantle C concentration varies by a factor of2 over the length of the ridge, consistent with independent estimates of C concentration heterogeneityin the MORB mantle. The correlations between lithophile tracers and 4He/40Ar∗ can be reproduced bymixing between a depleted endmember with 87Sr/86Sr = 0.70275, εNd = 8.2 and [C] = 12 ppm and anenriched endmember with 87Sr/86Sr = 0.70360, εNd = 5 and [C] = 24 ppm, followed by degassing.The proposed degassing model allows us to estimate the initial C concentration (i.e. prior to degassing)of each SEIR basalt (for which Sr or Nd isotopes are available); using independent Nb concentration data(Mahoney et al., 2002), we show that C/Nb ratios prior to degassing along the SEIR are relatively constant,probably with a C/Nb ratio of 200 ± 100. However, although the constancy of C/Nb in these samples is arobust conclusion, the estimated C/Nb ratio itself is model dependent.We also use these data to evaluate volatility of Re during degassing of MORBs; Re is known to bemoderately volatile during subaerial and shallow marine volcanism, although it is not known if thiselement is also volatile at conditions appropriate to MORB emplacement. Although there is a (poor)correlation between Re/Yb (Yb being a non-volatile element of similar apparent bulk compatibility to Re)and 4He/40Ar∗ in these samples, it is more likely that this correlation results from Re/Yb variation in themantle source and is not due to loss of Re during magmatic degassing.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Studies of the geochemistry of volatile species in the mantle arecomplicated by magmatic processes which usually result in consid-erable loss of gases during transit between the mantle and the sur-

* Corresponding author.E-mail address: peteb@crpg.cnrs-nancy.fr (P. Burnard).

http://dx.doi.org/10.1016/j.epsl.2014.03.0450012-821X/© 2014 Elsevier B.V. All rights reserved.

face. It is therefore essential to constrain magmatic degassing in or-der to quantify the volatile content of the mantle. This can be doneby measuring the ratio 4He/40Ar∗ (where 40Ar∗ is the radiogenic40Ar present, corrected for atmospheric contributions assumingthat all 36Ar is atmospheric in origin and an atmospheric 40Ar/36Arratio of 298). In numerous studies of oceanic basalt glasses, thisratio has been shown to be strongly fractionated by magmaticdegassing due to the large differences between the solubilities

160 P. Burnard et al. / Earth and Planetary Science Letters 395 (2014) 159–167

Fig. 1. 4He/40Ar∗ vs. 40Ar∗ concentration in basaltic glass vesicles (determined by invacuo crushing) from the South East Indian Ridge (data from Burnard et al., 2004).Undegassed magma is assumed to have a 4He/40Ar∗ ratio similar to that of the man-tle (∼2) and high 40Ar∗ concentrations. Gas loss from the system reduces 40Ar∗ butalso produces an increase in 4He/40Ar∗ (because Ar is less soluble in basaltic liquidsthan He); the solid line indicates the vesicle compositions predicted for Rayleigh de-gassing (distillation) with SHe/SAr = 11 (Iacono-Marziano et al., 2010; Nuccio andPaonita, 2000), starting with a mantle 4He/40Ar∗ of 2; figures on the trajectory in-dicate [CO2]/[CO2]init values. It is thus clear that the primary control on 4He/40Ar∗in SEIR basaltic glasses is degassing during magmatic processes.

of He and Ar in silicate liquids (Burnard, 2001; Burnard et al.,2003; Colin et al., 2011; Gonnermann and Mukhopahdyay, 2007;Marty and Zimmermann, 1999; Moreira and Sarda, 2000). The4He/40Ar∗ ratio can thus be used as an index of the extent ofdegassing. Importantly, the 4He/40Ar ratio of the mantle sourceis well constrained at 2–3 from knowledge of the U + Th andK contents of the mantle, as these elements produce respec-tively 4He and 40Ar by radioactive decay (Burnard et al., 1998;Marty, 2011). Helium is approximately an order of magnitudemore soluble than Ar in basaltic liquids (Carroll and Stolper, 1993;Jambon et al., 1986; Nuccio and Paonita, 2000; Iacono-Marzianoet al., 2010) therefore an exsolved gas phase will contain more Arthan He, leaving a high 4He/40Ar∗ residual silicate liquid. Magmaticgases trapped in basaltic glasses commonly preserve 4He/40Ar∗ ra-tios that are consistently greater than that of the mantle source,and are also highly variable (Marty and Zimmermann, 1999). Thusthe noble gases trapped in basaltic glasses are the residue of vari-able extents of volatile loss during magma ascent.

Magmatic degassing along the South East Indian Ridge (SEIR)has been well documented using noble gas relative abundances(Burnard, 2004; Burnard et al., 2002). Good correlations between4He/40Ar∗ and noble gas abundances (40Ar∗,4He; Fig. 1) are con-sistent with solubility controlled distillation as the primary mech-anism of gas loss (Burnard, 2004).

It has been shown for some ridge segments that the extent ofdegassing of MORBs (as traced by 4He/40Ar∗) correlates with in-dicators of crystal fractionation (e.g. MgO; Burnard et al., 2002;Colin et al., 2011; Marty and Zimmermann, 1999): in these cases,crystallization increases the concentration of CO2 in the liquid (asCO2 is incompatible) thus favoring degassing. However, there is noobvious relationship between MgO and 4He/40Ar∗ along this sec-tion of the SEIR (88–118°E) as noted in the original noble gasstudy of these samples (Burnard et al., 2004). Furthermore, thereis no relationship between 4He/40Ar∗ and depth of eruption (be-low sea level) in this suite of samples (Burnard et al., 2004). Thus,although volatile loss controls 4He/40Ar∗, unknown mechanisms

determine the fraction of volatile loss (degree of 4He/40Ar∗ frac-tionation) in the final erupted basalts along the SEIR between 88and 118°E.

Here, we show that 4He/40Ar∗ ratios from these samplesare correlated with Sr and Nd isotopic ratios and incompatiblelithophile element ratios; these ratios should be completely un-affected by magma degassing. We suggest that these correlationsreflect variable degrees of incorporation of a source componentthat is enriched in both incompatible lithophile elements and incarbon. The resulting primary liquids would have variable initialCO2 concentrations ([CO2]init), and would thus experience differentextents of volatile loss, with higher [CO2]init resulting in greater4He–40Ar∗ fractionation due to degassing.

2. Observations

The South East Indian Ridge is an intermediate spreading ridgelocated between the Rodrigues Triple Junction and the Australian-Antarctic Discordance. The study area (88 to 118°E) displays fairlyconstant full spreading rates of ∼7 mm/yr (Cochran and Sempere,1997), but the ridge depth increases gradually from west to east.This increasing ridge depth is associated with increasing Na8 anddecreasing Fe8 (fractionation corrected oxide contents; Klein andLangmuir, 1987) which have been interpreted to imply shallowerand lesser extents of melting related to decreasing mantle temper-atures towards the east (Graham et al., 2001). As Fe8, which is anindicator of depth of melting, is roughly correlated with He iso-topic composition along the ridge, Graham et al. (2001) suggestthat source material melting at greater depths has higher 3He/4Heratios. In addition, based on U-series disequilibria data, Russo etal. argue for higher proportions of pyroxenite veins in the easternpart of the ridge section (Russo et al., 2009). Sr, Nd and Pb iso-topic compositions throughout the studied section are typical ofIndian Ocean MORB (Mahoney et al., 2002), that is, compared toother MORB, they have generally high 87Sr/86Sr, low 143Nd/144Ndand elevated 208Pb/204Pb relative to their 206Pb/204Pb ratios. Thesecharacteristics have been attributed to incorporation of a higherproportion of subducted sediments and/or detached continentalcrust and mantle lithosphere in the mantle beneath the IndianOcean. No evidence has been found for an influence of the nearbyAmsterdam-St. Paul or the Heard-Kerguelen hot spots in the iso-topic compositions of these samples (Mahoney et al., 2002).

We combine the noble gas and CO2 data reported in Burnardet al. (2004) with major and trace element and isotopic data fromMahoney et al. (2002) in order to determine relationships betweenlithophile and volatile tracers along the SEIR. In addition, we havemeasured Re abundances on hand-picked glasses from a subset ofsamples (techniques described in Appendix B of the Supplemen-tary Materials) with the aim of determining whether Re contentsin MORB are affected by extent of degassing. A total of 22 samplesfrom between 88 and 118°E are available with all data (noble gas,Re abundance and lithophile tracers). All noble gas data were ob-tained by in vacuo crushing of basaltic glasses (see Burnard et al.,2013 for details of analytical techniques), and therefore representthe noble gases trapped in vesicles. Lithophile chemistry and iso-topes presented in Mahoney et al. (2002) were measured on freshbasaltic glasses from the same samples as the noble gases withthe exception of samples WW10-10-115 and WW10-126-007 thatwere measured on different samples from the same dredges; how-ever, given the overall within-dredge homogeneity of the samplesand the smooth evolution of trace elements and isotopic compo-sitions along this section of ridge (Mahoney et al., 2002), it isunlikely that this has a significant impact on the data interpre-tation.

These data define clear correlations between relative noble gasabundances (4He/40Ar∗) and lithophile tracers with (for example)

P. Burnard et al. / Earth and Planetary Science Letters 395 (2014) 159–167 161

Fig. 2. Correlations between log(4He/40Ar∗) and geochemical tracers of mantlesource heterogeneity: a) εNd, b) 87Sr/86Sr and c) La/Sm (similar correlations existfor example with K/Ti or Sm/Yb). εNd, 87Sr/86Sr and La/Sm data from Mahoney,2002. He and Ar data from Burnard et al. (2004); data plotted are the sum ofall crush extractions (there were between 2 and 5 extraction steps per sample).Uncertainties are smaller than symbol sizes except for the 4He/40Ar∗ ratio of sam-ple WW10-118-1 (error bar plotted). Open symbols: samples with one or moresteps with 40Ar∗ concentrations below detection limit. High 4He/40Ar∗ ratios, in-dicative of more extensive degassing, occur consistently in more enriched (highLa/Sm, 87Sr/86Sr, low εNd) basalts. Although there is no a priori reason that thesecorrelations should be linear, significant linear correlation coefficients (r2 values)are obtained (La/Sm: 0.75; εNd: 0.62; 87Sr/86Sr: 0.3).

positive correlations between La/Sm or 87Sr/86Sr vs. 4He/40Ar∗and a negative correlation between 143Nd/144Nd and 4He/40Ar∗(Fig. 2); in general, incompatible element ratios that trace mantleenrichment (Sm/Yb or K/Ti; not shown) correlate positively with4He/40Ar∗. There are also correlations between 40Ar∗ concentra-tion and CO2/40Ar∗ and these same ratios (not shown). Possibleorigins of these unexpected relationships between degassing trac-ers (4He/40Ar∗) and lithophile element ratios or isotope ratios arediscussed below.

3. Discussion

3.1. Possible causes for variations in 4He/40Ar∗ in basaltic glasses

The observed correlations are surprising: 4He/40Ar∗ is thoughtto trace magmatic degassing while Sr and Nd isotopes andlithophile trace element ratios should be completely insensitive tothis process. To resolve this enigma, we consider the possible pro-cesses that could produce 4He/40Ar∗ variations in basaltic glasses.

3.1.1. Fractional crystallization and partial meltingAs noted above, fractional crystallization can promote degassing

and thus fractionate 4He/40Ar∗ ratios. However, while fractionalcrystallization exerts a major control on incompatible lithophile el-ement concentrations, resulting in increasing incompatible elementcontent with decreasing Mg#, it has no effect on isotope or incom-patible element ratios. For example, as expected, there is no dis-cernable trend between Mg# and La/Sm, 87Sr/86Sr or 143Nd/144Nd(not shown). Partial melting of a homogeneous mantle source willalso have no effect on Sr or Nd isotope ratios, and very little effecton ratios of trace elements of similar incompatibility. There are nocorrelations between 4He/40Ar∗ and either Na8 or Fe8, thereforevariations in partial melting regime (depth or extent of melting)cannot account for the relations in Fig. 2. Instead, the observedvariations of 87Sr/86Sr, 143Nd/144Nd and incompatible element ra-tios must reflect the presence of source heterogeneities.

3.1.2. 4He/40Ar∗ variations in the mantle sourceSince the lithophile element data require the existence of a

heterogeneous mantle source, the observed correlations could besimply explained if enriched source components have system-atically higher 4He/40Ar∗ than depleted components. However,the 4He/40Ar∗ ratio of the mantle is likely to be nearly con-stant (Burnard et al., 1998; Marty, 2011) because their parentelements (K and U + Th) do not significantly fractionate in themantle (Arevalo et al., 2009; Jochum et al., 1983). For exam-ple, if mantle heterogeneity were responsible for the range in4He/40Ar∗ observed, mantle K/U ratios lower than 500 are im-plied (in order to produce 4He/40Ar∗ of 160 at secular equilibrium,the maximum observed in this data set). This seems unlikely asmeasured mantle K/U values are > 10 000 (Arevalo et al., 2009;Jochum et al., 1983). Furthermore, there is no correlation between3He/4He and 4He/40Ar∗ (not shown) so minimal U/3He fraction-ation is implied for the enormous range in K/U that would berequired in the SEIR mantle source to explain the observed rangein 4He/40Ar∗. The lack of a consistent negative co-variation be-tween 4He/40Ar∗ and K content of the lavas (not shown) alsoargues against a source 4He/40Ar∗ variation. It therefore does notseem plausible that mantle source 4He/40Ar∗ heterogeneities canaccount for the observed noble gas relative abundances in thesesamples.

3.1.3. Magmatic fractionation of 4He/40Ar∗ resulting from melting ofenriched domains

While significant variations of the source 4He/40Ar∗ ratio canbe excluded, it could be suggested that the existence of man-tle heterogeneities could promote the magmatic fractionation ofthese noble gases. In other words, He and Ar could be fraction-ated during the melting process, in which case the primary liquid4He/40Ar∗ ratio would not be the same as that of the mantlesource; for example, if He were extracted more efficiently thanAr in enriched lithologies (compared to depleted mantle), thenthis could potentially result in correlations between 4He/40Ar∗and La/Sm, 87Sr/86Sr and 143Nd/144Nd. However, this seems un-likely as both He and Ar are highly incompatible elements (Heberet al., 2007; Marty and Lussiez, 1993) and, given the relatively

162 P. Burnard et al. / Earth and Planetary Science Letters 395 (2014) 159–167

large degrees of melting implied during MORB generation (>10%),both elements will be almost totally partitioned into the liquidphase. Thus liquid–crystal fractionation cannot sufficiently frac-tionate these elements. Although kinetic processes during man-tle melting have been invoked as a mechanism for fractionatingHe from Ar (Burnard, 2004; Yamamoto et al., 2009), the calcu-lations supporting this suggestion were performed using poorlyconstrained He and Ar diffusivities for the solid mantle: subse-quent experimental work has better defined these diffusivities andit seems more likely that any diffusive fractionation in the mantlewill be minor (Cassata et al., 2011). In summary, the 4He/40Ar ra-tio of a primary undegassed liquid will be very similar to that ofthe mantle itself.

3.1.4. Compositional control on extent of degassing and thus on4He/40Ar∗

It seems most probable that, as has been shown to be the casefor MORB glasses generally (Moreira and Kurz, 2013), 4He/40Ar∗ ofthese samples traces degassing of magmas. High 4He/40Ar∗ ratiosresult from large degrees of noble gas loss from the magma. Inorder to generate the correlations observed in Fig. 2, the chemistryof the mantle source itself must somehow control the degassing process.

One possibility is that the chemistry of the magmas varies, andthis affects parameters, in particular the noble gas solubilities inthe silicate liquid, that determine noble gas degassing. Althoughnoble gas solubilities do depend on the composition of the sili-cate liquid (Carroll and Stolper, 1993; Nuccio and Paonita, 2000),variations in the major element chemistry of primary MOR meltsdue to melting of a heterogeneous mantle will be small and can-not significantly affect noble gas solubilities. Therefore, variationsin these solubilities cannot account for the differences in 4He/40Ar∗observed.

Gonnermann and Mukhopahdyay (2007) propose a modelwhereby high initial CO2 contents ([CO2]init) provoke more ex-tensive degassing of the magma (compared to magmas with low[CO2]init). This arises because the final concentration of CO2 inthe liquids ([CO2]final) depends only on the pressure at the endof degassing, which does not vary much for MORBs. Thereforethe fraction CO2 lost (= [CO2]final/[CO2]init) – which determinesthe degree of 4He/40Ar∗ fractionation – is primarily controlledby [CO2]init. We show in the following discussion that if [CO2]initvariation were accompanied by changes in mantle chemical (e.g.La/Sm) and isotopic (e.g. 87Sr/86Sr and 143Nd/144Nd) compositions– which is intuitively plausible – then a correlation between extentof degassing (as traced by 4He/40Ar∗) and lithophile tracers will re-sult. This model satisfactorily explains the variations observed andin addition can be used to constrain the nature of the SEIR mantlesource.

The fraction of CO2 lost from a magma (= [CO2]final/[CO2]init)depends on the initial CO2 concentration of the magma (Gonner-mann and Mukhopahdyay, 2007); in turn, 4He/40Ar∗ measured ina basaltic glass depends on the fraction of gas lost (= [CO2]final/[CO2]init), therefore 4He/40Ar∗ ratios of the erupted lavas willbe sensitive to the CO2 concentration of the primary magma([CO2]init). We propose that primary liquids with high [CO2] willresult from melting of mantle enriched in recycled componentswith high carbon contents. A correlation between [CO2]init and (forexample) 87Sr/86Sr is therefore expected. Then, the high [CO2]initliquids undergo greater extents of degassing, thus producing lavasat the surface that have both high 4He/40Ar∗ and high 87Sr/86Sr,etc.

3.2. Degassing model

The degassing model developed below is modified from Colin etal. (2011) and is equivalent to the equilibrium version of the model

presented in Gonnermann and Mukhopahdyay (2007); magmawith a given CO2 content decompresses (during ascent), decreas-ing CO2 solubility until CO2 solubility is exceeded. At this point agas phase is nucleated, fractionating the gases between the volatilephase and the liquid depending on their solubilities. The gas phaseis lost from the system; this loss has to occur over several stagesas a single stage of bubble creation and loss will not produce thefractionation observed in these samples (Burnard et al., 2004).

Therefore we assume that a continuous process of Rayleighfractionation (distillation) occurs. The composition of the volatilesin the final, erupted magma is given by:

( 4He40Ar∗

)final

=( 4He

40Ar∗)

init∗

( [CO2]final

[CO2]init

)(SCO2 /SHe)−(SCO2 /SAr)

(1)

Bubbles in equilibrium with this liquid will have 4He/40Ar∗ ra-tios determined by their relative solubilities:( 4He

40Ar∗)

final vesicles=

( 4He40Ar∗

)final

∗(

SAr

SHe

)(2)

(Si = solubility of species i in the magmatic liquid, in mol g−1

bar−1). In common with previous work, the mantle 4He/40Ar∗ ratiois taken to be 2 (Burnard et al., 1998; Marty, 2011) and there is nofractionation of He from Ar during melting (i.e. the initial liquidalso has 4He/40Ar∗ of 2).

The pressure at which degassing starts and the final extent ofdegassing (e.g. [CO2]final/[CO2]init) depend on the amount of initialCO2 in the magma:

[CO2]final

[CO2]init= SCO2 ∗ Pmin

[CO2]init(3)

where Pmin is the pressure at the end of degassing; althoughthis might be taken to be the hydrostatic seawater pressure atthe eruption depth (i.e. ∼300 bars) we show below that Pminmust be higher than 300 bars. SCO2 used in the model is fromJendrzejewski et al. (1997) (= 1.14 × 10−8 mol g−1 bar−1).

An alternative model for degassing involves an initial stage ofclosed system (or “batch”) degassing resulting from residence in amagma chamber, followed by Rayleigh degassing during the erup-tion phase (Cartigny et al., 2001; Gerlach, 1989; Pineau and Javoy,1994). A stage of batch degassing would imply considerably higher[CO2]init values for the same range in 4He/40Ar∗; a combined batch– Rayleigh degassing model is described in Appendix A (Supple-mentary Materials). However, the noble gas data presented hereare readily modeled by distillation (Burnard et al., 2004) and inde-pendent arguments suggest that a batch stage (or at least, a sig-nificant amount of degassing via a batch stage) is unlikely (seeAppendix A), therefore our preferred model is pure Rayleigh de-gassing as described by Eqs. (1)–(3) above.

In our model, the concentration of C in the initial magma isvaried between 10 and 20 μmol g−1. These are reasonable con-centrations for MORB primary liquids, for example, these wouldcorrespond to C concentrations in the mantle source of between12 and 24 ppm (for 10% melting), entirely consistent with Marty’s(2011) estimate of 20 ± 8 ppm in the mantle.

3.2.1. Equilibrium vs. disequilibrium degassingOur model considers that the bubbles and liquid are at equi-

librium during degassing. Although previous workers have de-veloped non-equilibrium degassing models (Aubaud et al., 2004;Cartigny et al., 2008; Gonnermann and Mukhopahdyay, 2007),there is strong evidence that noble gas fractionation occurs at equi-librium in mid-ocean ridge magma chambers. Burnard (1999a) andColin et al. (2011) measured noble gases in individual vesiclestrapped in MORB basaltic glasses and found that the variations

P. Burnard et al. / Earth and Planetary Science Letters 395 (2014) 159–167 163

in He–Ar–CO2 compositions of individual vesicles cannot be theresult of kinetic fractionation during magmatic degassing but in-stead are entirely consistent with solubility determined degassing.Numerical simulations (Aubry et al., 2013) also show that effectsof disequilibrium on He–Ar fractionation during MORB degassingare negligible. Additionally, the degassing trend in Fig. 1 fits thatpredicted for solubility controlled degassing extremely well: anycontributions from non-equilibrium processes are apparently mi-nor compared to the fractionation due to distillation at equilib-rium. One of the principal arguments for disequilibrium degassingcomes from studies that demonstrate that CO2 concentrations inbasaltic glasses are often higher than predicted for CO2 solubil-ity at seafloor pressure (Aubaud et al., 2004; Chavrit et al., 2013;Dixon et al., 1988; Jendrzejewski et al., 1997; Stolper and Hol-loway, 1988). However, this assumes that lava emplacement on theseafloor occurs at hydrostatic pressure (equivalent to the columnof seawater above the eruption site) which is probably incorrect:the liquid magma inside the lava flow needs to be overpressured(relative to the overlying water column) during extrusion of pil-low lavas in order to overcome the yield strength of the crustof solidified lava. Colin et al. (2013) showed that this is indeedthe case. Because of the low viscosity of basaltic liquids, the pres-sures inside vesicles trapped in MORB glasses (the vesicle internalpressure or VIP) will faithfully record the magma pressure dur-ing the quenching of the glass (Burnard, 1999b; Colin et al., 2013;Sparks, 1978). Colin et al. (2013) show that VIPs of the five MORBglasses analyzed were all 20–40% higher than the seawater pres-sure at eruption, demonstrating that some degree of magmaticoverpressure existed at the time of eruption. As a result, CO2 su-persaturation in MORB glasses has been overestimated to somedegree and disequilibrium degassing may be less important thanpresumed.

3.2.2. Source composition modelVariations in mantle C content will presumably be accompa-

nied by variations in trace element and isotope ratios, with high[C] associated with more enriched (high 87Sr/86Sr, etc.) signatures.Therefore, lavas sourced from enriched mantle will degas more ex-tensively (greater [CO2]final/[CO2]init) as these have higher [CO2]initand yet [CO2]final is more-or-less constant, determined only bythe CO2 solubility at Pmin. The relationship between CO2 con-centration and trace element/isotopic enrichment in the primaryliquids results in the observed correlations between tracers of de-gassing (4He/40Ar∗) and lithophile element and isotopic indicatorsof source composition (Fig. 2).

Using Nd isotopes as an example (but the reasoning appliesequally to Sr isotopes or trace element ratios such as La/Sm)we consider simple two component mixing between two pri-mary liquids derived from N-MORB (εNd = 8.2 and [CO2]init= 10 μmol g−1) and a more enriched mantle (εNd = 5 and[CO2]init = 20 μmol g−1) respectively (Fig. 3A). The curvature ofthe mixing trend depends on the ratio (Nd/C)N-MORB/(Nd/C)E-MORB.Both primary liquids have the same 4He/40Ar∗ ratio of 2. Aftermixing, decompression during ascent of the magmas results in de-gassing which fractionates 4He/40Ar∗ according to Eqs. (1) and (2);the CO2 concentration in the final liquid is calculated followingEq. (3).

The 4He/40Ar∗ ratio of the erupted material is calculated fol-lowing Eqs. (1)–(3) for variable [CO2]init; as can be seen in Fig. 3B,the final 4He/40Ar∗ ratio is highly dependent on the CO2 content ofthe primary magma, with a factor 2 increase in [CO2]init producing∼100 increase in 4He/40Ar∗ in the final liquid.

In addition to [CO2]init, the extent of degassing ([CO2]final/[CO2]init) also depends on the pressure at the end of degassing,Pmin (Eq. (3)). Fig. 3B shows the effect of Pmin on (4He/40Ar∗)final:low Pmin corresponds to lower [CO2]final, more extensive degassing

Fig. 3. Model input parameters: (A) Nd isotope–CO2 concentration relationshipin primary liquids (i.e. prior to degassing). Variations in Nd isotope composi-tion are accompanied by changes in C concentration such that high C concen-trations occur in enriched (low εNd) mantle. The range in εNd matches thatfound along the SEIR while the variations in primary liquid CO2 concentrationspan the range of estimates for undegassed MORB liquids (Cartigny et al., 2008;Marty, 2011; Saal et al., 2002); D = depleted endmember, E = enriched endmem-ber. The three curves represent variable (Nd/C)N-MORB/(Nd/C)E-MORB ratios: solid line(Nd/C)N-MORB/(Nd/C)E-MORB = 1; dashed line (red) (Nd/C)N-MORB/(Nd/C)E-MORB = 0.2;dotted line (blue) (Nd/C)N-MORB/(Nd/C)E-MORB = 5. (B) 4He/40Ar∗ vs. [CO2]init. Eachinitial CO2 concentration results in a different degree of degassing and thereforea different 4He/40Ar∗ in the erupted lavas, therefore there is a relation between[CO2]init and the (4He/40Ar∗)final as defined by the individual curves in the figure.The different curves correspond to different final pressures (Pmin) because a lowerfinal pressure will produce more degassing and therefore higher (4He/40Ar∗)final.4He/40Ar∗ ratios are calculated according to Eqs. (1) and (2) with [CO2]final/[CO2]initdefined by Eq. (3) for Pmin between 675 and 300 bars; He, Ar and CO2 solubili-ties from (Jendrzejewski et al., 1997; Nuccio and Paonita, 2000). (For interpretationof the references to color in this figure legend, the reader is referred to the webversion of this article.)

and higher (4He/40Ar∗)final for a given [CO2]init. If we fix Pminat 300 bars (the average hydrostatic pressure at eruption), then[CO2]init in the range 0.5–1 μmol is required in order to reproducethe observed SEIR 4He/40Ar∗; these concentrations (equivalent to0.6–1.2 ppm C in the mantle) appear unrealistically low for the

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Fig. 4. εNd vs. 4He/40Ar∗ with modeled curves; these curves simply combineFigs. 3A and 3B (for Pmin = 550 bars). Variations in initial CO2 concentration canresult in co-variations between lithophile isotope tracers (εNd shown here, but sim-ilar models can be constructed for Sr isotopes or for trace element ratios such asLa/Sm) and extent of degassing in the erupted basalts as traced by 4He/40Ar∗ . Theprimary magmas are assumed to have 4He/40Ar∗ = 2 and CO2 contents that varybetween 10 and 20 μmol g−1 and Pmin = 550 bars. εNd varies with [CO2]init asshown in Fig. 3A. The extent of degassing of the final liquids depends on the ini-tial CO2 concentration (see text) such that liquids with high [CO2]init will degas agreater proportion of their CO2, resulting in high 4He/40Ar∗ in the erupted lavas asshown in Fig. 3B. This then creates the correlation between 4He/40Ar∗ and lithophiletracers such as 143Nd/144Nd and 87Sr/86Sr. The different curves represent different(Nd/C)N-MORB/(Nd/C)E-MORB ratios in the primary liquids, as defined in Fig. 3A.

MORB mantle. Consequently Pmin must be greater than 300 bars.As both Pmin and [CO2]init are poorly constrained, here we havechosen a range in [CO2]init that corresponds to independent esti-mates for C in the mantle (see above) and fixed Pmin such thatthe range in 4He/40Ar∗ output by the model corresponds to thatobserved in SEIR lavas. This corresponds to Pmin = 550 bar, butwe note that there is a series of Pmin – [CO2]init solutions thatare possible. Fig. 4 shows that the co-variations between 4He/40Ar∗and εNd observed in this suite of samples are readily reproducedby the models above, with the ratio (Nd/C)N-MORB/(Nd/C)E-MORB be-tween 0.2 and 5.

3.3. CO2/Nb ratio

Saal et al. (2002) suggested that CO2/Nb of the mantle is rela-tively constant (based on analyses of undersaturated MORB glasses,and consistent with the similar incompatibilities of C and Nbduring mantle processes), resulting in a global MORB CO2/Nb ra-tio of 239 ± 46 prior to degassing; this was re-examined for awider dataset by Cartigny et al. (2008), this time using CO2 –saturated samples but correcting for CO2 loss during degassingusing δ13C fractionation. Despite uncertainties resulting from thedegassing correction, the variability in the CO2/Nb ratio was con-siderably less than that of either Nb or CO2 concentrations, sug-gesting that the C/Nb ratio of the mantle is buffered to someextent. However, the MORB CO2/Nb ratio obtained by Cartignyet al. of ∼450 was considerably higher than that of Saal et al.,bringing into question the global homogeneity of the mantle C/Nb.Furthermore, Helo et al. (2011) observe a large range in CO2/Nb

Fig. 5. Histogram of CO2/Nb ratios for SEIR basalts; [CO2]init is calculated using therelation in Fig. 4 (εNd vs. 4He/40Ar∗) for the filled bars, whereas the dashed linehistogram was obtained using the correlation with Sr rather than Nd isotopes. Nbconcentrations from (Mahoney et al., 2002). In general, a relatively constant CO2/Nbratio is observed with a mode of ∼200. The value of the mode is model depen-dent with at least factor 2 permissible. However, the dispersion in CO2/Nb is modelindependent and suggests that this ratio is relatively constant in the SEIR sourceregion.

ratios in melt inclusions trapped in plagioclase phenocrysts fromthe Juan de Fuca ridge typically extending to higher CO2/Nb ra-tios, suggesting that the CO2/Nb of the MORB source region isof the order ∼4000. The lower values measured by Cartigny andSaal would therefore represent post-degassing modification to theCO2/Nb ratio.

We address this issue using the data from the SEIR samples. Itis possible to calculate an initial CO2 concentration for each sam-ple using the model outlined above (initial CO2 concentration isassumed to be a linear function of 87Sr/86Sr or 143Nd/144Nd). Fromthe Nb data reported for these samples by Mahoney et al. (2002),we calculate pre-degassing CO2/Nb ratios for each sample: thesedata are presented in Fig. 5. As noted previously, there is a suite ofdifferent Pmin – ([CO2]init range) solutions that would fit the SEIRdata and our chosen parameters have been ‘tuned’ using previousestimates of mantle C concentrations, therefore there is a certaincircularity in this method for estimating CO2/Nb. Nevertheless, the[CO2]init and consequently CO2/Nb estimates are constrained inthe model to within ±50%; high [CO2]init (e.g. >30 μmol g−1 inthe enriched endmember) will result in degassing at unrealisticallyhigh pressure (for example, Pmin > 1000 bars is implied in orderto satisfy the highest observed 4He/40Ar∗) whereas low [CO2]init(<9 μmol g−1 in the enriched endmember) would not produce asufficient extent of degassing (even with Pmin = 300 bars) and cannot reproduce the highest 4He/40Ar∗ ratios measured in the SEIRbasaltic glasses.

Aside from these uncertainties, the average CO2/Nb for the SEIRfor the model outlined above is 170 ± 100 (1σ , based only onthe scatter in the data and ignoring 1 outlier); given that dif-ferent model parameters (different combinations of [CO2]init andPmin, for example) will produce different CO2/Nb estimates (witha factor ∼2), these estimates are also within error of the CO2/Nbestimated by Cartigny et al. (2008). However, the dispersion in theratio (170 ± 100 or factor ∼0.6) is considerably smaller than thatfor Nb concentrations (factor ∼20), suggesting that the C/Nb ratioof the mantle is indeed constant (relative to the mantle C or Nbconcentrations).

P. Burnard et al. / Earth and Planetary Science Letters 395 (2014) 159–167 165

Fig. 6. Comparison of Re/Yb variations with depth for Mauna Kea lavas (open circles;data from Lassiter, 2003) and the SEIR (filled triangles; Table A1 for Re concentra-tions and Mahoney et al., 2002 for Yb). There is no discernable decrease in Re/Ybin the shallowest erupted SEIR lavas, in contrast with Mauna Loa where eruptiondepth plays an important role in determining Re/Yb.

3.4. Re volatility in submarine eruptions

An additional incentive for trying to improve our understandingof MORB degassing processes is that they could potentially affectthe concentrations of a large number of moderately volatile ele-ments. A particularly important example is Re, the parent isotopein the Re–Os isotopic system. Re usually behaves in a moderatelyincompatible manner during mantle melting, unlike Os, which ishighly compatible. Because of this contrast in behavior, mafic rockstypically have very high Re/Os, and develop with time very high187Os/188Os ratios, which can then be used to trace recycling ofocean crust in the mantle. However, the complexity of Re geo-chemistry complicates the interpretation of Os isotopic systematicsin basalts. Experimental studies have shown that Re abundances inmagmas not only reflect oxygen and sulfur fugacity during mantlemelting (Brenan, 2008; Mallmann and O’Neill, 2007) but also maybe modified by magma degassing (MacKenzie and Canil, 2006).This element is known to behave in a volatile manner in subaerialvolcanism (e.g. Taran and Hedenquist, 1997), probably explainingthe low concentrations of Re in ocean island and island arc basaltsrelative to those in MORB (e.g. Sun et al., 2003). This effect hasbeen well studied in Hawaiian lavas (Bennett et al., 2000); Lassiter(2003) demonstrated that Re concentrations and Re/Yb ratios inthe HSDP-2 Mauna Kea drillcore increase with depth of eruption(depth below sea level). Re concentrations are normalized to thoseof Yb because the two elements, although controlled by differentphases, are believed to display similar degrees of incompatibilityduring mantle melting (Hauri and Hart, 1997), but unlike Re, Ybis not volatile. Based on the results of these earlier studies, it isnow widely acknowledged that volatile Re loss in arcs and oceanislands occurs, but the possibility that similar processes might takeplace to a lesser extent in MORB has not been explored. Such pro-cesses could have an effect not only on the Re content of oceaniccrust but also on the Re concentration of seawater.

Fig. 7. Re/Yb (in ppb/ppm) variations with 4He/40Ar∗ in SEIR basaltic glasses.A poor negative correlation exists between Re/Yb and 4He/40Ar∗ (e.g. r2 = 0.3 forlog(Re/Yb) ∝ log(4He/40Ar∗)). The curves show different trajectories for Rayleighdegassing assuming that Re/Yb in the mantle is constant (0.41 ppb/ppm used here,but this is not a unique solution) and that Yb is perfectly non-volatile. DifferentSCO2 /SRe have been modeled: solid line = 0.5; dashed line = 1; dotted line = 1.5(solubilities expressed as mol CO2 g−1 bar−1/mol ReO2 g−1 bar−1). While the poorcorrelation can be modeled as volatile Re loss, given that the relative ReO2 and CO2

solubilities required to reproduce the correlation seem extremely unlikely and thatRe/Yb also correlates with εNd, it seems more likely that the variability in Re/Yb inthese samples reflects mantle heterogeneity. Any Re volatile loss in these samplesis likely to have been very minor.

Given the large range in extent of degassing implied for theSEIR (as indicated by their 4He/40Ar∗ ratios), we can test forvolatile loss of Re during MORB magmatic processes in this suiteof lavas. Re contents in the SEIR samples range from 0.363 to0.984 ppb (Appendix B, supplementary online material) and areanticorrelated with Mg# and positively correlated with Yb con-tents (Figs. B1 and B2 in Appendix B; Yb data from Mahoneyet al., 2002), as expected for a moderately incompatible element.Unlike in the Hawaiian samples, in the SEIR data there is no sys-tematic variation of Re/Yb with eruption depth (Fig. 6). However,Re/Yb does vary (albeit weakly) with 4He/40Ar∗ (Fig. 7) such thathigh Re/Yb ratios are associated with relatively undegassed (low4He/40Ar∗) samples. This could be consistent with Re loss duringdegassing. A simple Rayleigh degassing model is presented in Fig. 7where the source Re/Yb ratio is assumed to be constant and therelation between 4He/40Ar∗ and Re/Yb results only from degassing(using Eqs. (1) and (2), and assuming that Yb has zero volatility).It is possible to fit the data using SCO2/SRe between 0.5 and 1.5(molar ratios, assuming that Re in the volatile phase is presentas ReO2), implying that Re and CO2 have similar volatile behav-ior. This does not seem plausible given the extremely differentgeochemistry of ReO2 and CO2. Instead, the Re/Yb vs. 4He/40Ar∗ re-lation could equally result from minor mantle heterogeneity suchthat low Re/Yb ratios are associated with enriched mantle compo-sitions: indeed, Re/Yb also correlates weakly with εNd (Fig. B3 inAppendix B). Therefore it seems more plausible that these varia-tions in Re/Yb are not the result of Re degassing but rather theresult of heterogeneities in the mantle itself. It is worth notingthat even if Re is lost during degassing of MORBs, this effect is mi-nor relative to that observed during shallow eruptions at MaunaKea (Lassiter, 2003): Re/Yb varies little along the SEIR compared towhat was observed in the Lassiter study (see Fig. 6). This probablymostly reflects the fact that because of the difference in eruptiondepths, the extent of gas lost in the HSDP-2 glasses is consider-ably greater than that along the SEIR: very low CO2 concentrationsin the HSDP glasses (<20–100 ppm) suggest that 75 to >95% of

166 P. Burnard et al. / Earth and Planetary Science Letters 395 (2014) 159–167

the CO2 has been degassed (Seaman et al., 2004) (assuming that[CO2]init

MaunaLoa was equal to or less than the lowest CO2 contentinferred for any liquid from the SEIR, i.e. 10 μmol g−1 ≡ 440 ppmCO2). In addition, the lower oxygen fugacity of the MORB sourcerelative to those of island arc or ocean island basalts will affect theoxidation state of the Re dissolved in the magma (Mallmann andO’Neill, 2007) which in turn might affect its volatility. Thus botheruption depth and redox conditions might explain the apparentabsence of substantial Re loss during MORB degassing.

4. Conclusions

The ratio 4He/40Ar∗ of gases trapped in vesicles in basalticglasses from the SEIR correlates with lithophile tracers such as87Sr/86Sr, εNd and La/Sm (amongst others). This is surprising as4He/40Ar∗ is thought to be near-constant in the mantle (due tolittle K/U fractionation during mantle processes) yet is highly sen-sitive to magmatic degassing processes. By contrast, the lithophiletracers reflect mantle heterogeneities and are deliberately chosento be little affected by magmatic processes.

We demonstrate here that 4He/40Ar∗ of the erupted liquids de-pends strongly on the CO2 content of the primary liquid (prior toany degassing). The CO2 concentration of the erupted magmaticliquids is relatively constant (as it depends only on the pressureof the magma at eruption), therefore the amount of degassing(≡ [CO2]final/[CO2]initial), and thus the 4He/40Ar∗ of erupted prod-ucts, depend primarily on the CO2 concentrations of the primaryliquids. We hypothesize here that magmas sourced from “enriched”MORB mantle with high 87Sr/86Sr, La/Sm and low εNd will havehigh initial CO2 concentrations compared to “depleted” MORBs.These enriched magmas undergo greater degassing, resulting inhigher 4He/40Ar∗ in the erupted products. We present a quantita-tive model describing this process and showing how the observedcorrelations between 4He/40Ar∗ and lithophile tracers could de-velop.

Loss of Re by magmatic degassing at shallow marine depths hasbeen proposed elsewhere (Lassiter, 2003). Re/Yb ratios along theSEIR show a weak anti-correlation with 4He/40Ar∗; however, fittingthe data to a Rayleigh degassing model can only work if CO2 andRe have similar volatile behavior, which does not seem plausible.Instead, mantle heterogeneity may be a more plausible explanationfor this relationship. Magmas derived from more enriched sourcecompositions would have lower Re/Yb ratios but higher [CO2]init,and would thus tend to lose greater amounts of gas during ascent,resulting in coincidental co-variations with indicators of magmaticdegassing (4He/40Ar∗) and lithophile tracers.

We can use our model in order to estimate the CO2/Nb ratio ofthe SEIR primary liquids (prior to degassing). Consistent with pre-vious work on CO2/Nb in MORBs, we find that this ratio is alsorelatively constant along the SEIR. The model constrains the vari-ability in C/Nb ratio while the absolute C/Nb ratios of the primaryliquids are model dependent. However, the model parameters areconsistent with previous estimates of mantle CO2/Nb of 239 ± 46(Saal et al., 2002) or ∼450 (Cartigny et al., 2008).

Acknowledgements

We thank Catherine Zimmermann for the dissolution and Reanalyses of the SEIR glasses. The comments of two anonymous re-viewers and the editor considerably improved the final manuscript.This work was funded by the Centre National de Recherche Sci-entifique “SEDIT” programme. The samples for Re analysis wereprovided by the Oregon State University rock curation facility. Thisis CRPG contribution number 2293.

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2014.03.045.

References

Arevalo Jr., R., McDonough, W.F., Luong, M., 2009. The K/U ratio of the silicate Earth:insights into mantle composition, structure and thermal evolution. Earth Planet.Sci. Lett. 278, 361–369.

Aubaud, C., Pineau, F., Jambon, A., Javoy, M., 2004. Kinetic disequilibrium of C, He, Arand carbon isotopes during degassing of mid-ocean ridge basalts. Earth Planet.Sci. Lett. 222, 391–406.

Aubry, G.J., Sator, N., Guillot, B., 2013. Vesicularity, bubble formation and noble gasfractionation during MORB degassing. Chem. Geol. 343, 85–98.

Bennett, V.C., Norman, M.D., Garcia, M.O., 2000. Rhenium and platinum group el-ement abundances correlated with mantle source components in Hawaiian pi-crites: sulphides in the plume. Earth Planet. Sci. Lett. 183, 513–526.

Brenan, J.M., 2008. Re–Os fractionation by sulfide melt-silicate melt partitioning:a new spin. Chem. Geol. 248, 140–165.

Burnard, P., 1999a. The bubble-by-bubble volatile evolution of two mid-ocean ridgebasalts. Earth Planet. Sci. Lett. 174, 199–211.

Burnard, P.G., 1999b. Eruption dynamics of “popping rock” from vesicle morpholo-gies. J. Volcanol. Geotherm. Res. 92, 247–258.

Burnard, P., 2001. Correction for volatile fractionation in ascending magmas; no-ble gas abundances in primary mantle melts. Geochim. Cosmochim. Acta 65,2605–2614.

Burnard, P., 2004. Diffusive fractionation of noble gases and helium isotopes duringmantle melting. Earth Planet. Sci. Lett. 220, 287–295.

Burnard, P.G., Farley, K.A., Turner, G., 1998. Multiple fluid pulses in a Samoanharzburgite. Chem. Geol. 148, 99–114.

Burnard, P.G., Graham, D.W., Farley, K.A., 2002. Mechanisms of magmatic gas lossalong the Southeast Indian Ridge and the Amsterdam-St. Paul Plateau. EarthPlanet. Sci. Lett. 203, 131–148.

Burnard, P.G., Harrison, D.W., Turner, G., Nesbitt, R., 2003. Degassing and contamina-tion of noble gases in Mid-Atlantic Ridge basalts. Geochem. Geophys. Geosyst. 4.http://dx.doi.org/10.1029/2002GC000326.

Burnard, P., Graham, D.W., Farley, K.A., 2004. Fractionation of noble gases (He, Ar)during MORB mantle melting: a case study on the Southeast Indian Ridge. EarthPlanet. Sci. Lett. 227, 457–472.

Burnard, P., Zimmermann, L., Sano, Y., 2013. The noble gases as geochemical tracers:history and background. In: Burnard, P. (Ed.), The Noble Gases as GeochemicalTracers. Springer, Berlin, Heidelberg, pp. 1–15.

Carroll, M.R., Stolper, E.M., 1993. Noble gas solubilities in silicate melts and glasses:new experimental results for argon and the relationship between solubility andionic porosity. Geochim. Cosmochim. Acta 57, 5039–5052.

Cartigny, P., Jendrzejewski, N., Pineau, F., Petit, E., Javoy, M., 2001. Volatile (C, N, Ar)variability in MORB and the respective roles of mantle source heterogeneity anddegassing; the case of the Southwest Indian Ridge. Earth Planet. Sci. Lett. 194,241–257.

Cartigny, P., Pineau, F., Aubaud, C., Javoy, M., 2008. Towards a consistent mantle car-bon flux estimate: insights from volatile systematics (H2O/Ce, δD, CO2/Nb) inthe North Atlantic mantle (14°N and 34°N). Earth Planet. Sci. Lett. 265, 672–685.

Cassata, W.S., Renne, P.R., Shuster, D.L., 2011. Argon diffusion in pyroxenes: implica-tions for thermochronometry and mantle degassing. Earth Planet. Sci. Lett. 304,407–416.

Chavrit, D., Humler, E., Morizet, Y., Laporte, D., 2013. Influence of magma ascentrate on carbon dioxide degassing at oceanic ridges: message in a bubble. EarthPlanet. Sci. Lett. 357, 376–385.

Cochran, J.R., Sempere, J.C., 1997. The southeast Indian ridge between 88°E and118°E: gravity anomalies and crustal accretion at intermediate spreading rates.J. Geophys. Res., Solid Earth 102, 15463–15487.

Colin, A., Burnard, P., Graham, D.W., Marrocchi, Y., 2011. Plume–ridge interac-tion along the Galapagos Spreading Center: discerning between gas loss andsource effects using neon isotopic compositions and 4He–40Ar–CO2 relativeabundances. Geochim. Cosmochim. Acta, 1145–1160. http://dx.doi.org/10.1016/j.gca.2010.11.018.

Colin, A., Burnard, P., Marty, B., 2013. Mechanisms of magma degassing at mid-oceanic ridges and the local volatile composition (4He–40Ar∗–CO2) of the man-tle by laser ablation analysis of individual MORB vesicles. Earth Planet. Sci.Lett. 361, 183–194.

Dixon, J.E., Stolper, E., Delaney, J.R., 1988. Infrared spectroscopic measurements ofCO2 and H2O in Juan de Fuca Ridge basaltic glasses. Earth Planet. Sci. Lett. 90,87–104.

Gerlach, T.M., 1989. Degassing of carbon dioxide from basaltic magma at spreadingcenters: II. Mid-oceanic ridge basalts. J. Volcanol. Geotherm. Res. 39.

Gonnermann, H.M., Mukhopahdyay, S., 2007. Non-equilibrium degassing and a pri-mordial source for helium in ocean-island volcanism. Nature 449, 1037–1040.

P. Burnard et al. / Earth and Planetary Science Letters 395 (2014) 159–167 167

Graham, D.W., Lupton, J.E., Spera, F.J., Christie, D.M., 2001. Upper-mantle dynamicsrevealed by helium isotope variations along the southeast Indian ridge. Na-ture 409, 701–703.

Hauri, E.H., Hart, S.R., 1997. Rhenium abundances and systematics in oceanic basalts.Chem. Geol. 139, 185–205.

Heber, V., Brooker, R.A., Kelley, S.P., Wood, B.J., 2007. Crystal–melt partitioning ofnoble gases (helium, neon, argon, krypton, and xenon) for olivine and clinopy-roxene. Geochim. Cosmochim. Acta 71, 1041–1061.

Helo, C., Longpre, M.A., Shimizu, N., Clague, D.A., Stix, J., 2011. Explosive eruptionsat mid-ocean ridges driven by CO2-rich magmas. Nat. Geosci. 4, 260–263.

Iacono-Marziano, G., Paonita, A., Rizzo, A., Scaillet, B., Gaillard, F., 2010. Noble gassolubilities in silicate melts: new experimental results and a comprehensivemodel of the effects of liquid composition, temperature and pressure. Chem.Geol. 279, 145–157.

Jambon, A., Weber, H., Braun, O., 1986. Solubility of He, Ne, Ar, Kr and Xe in abasalt melt in the range 1250–1600 °C. Geochemical implications. Geochim. Cos-mochim. Acta 50, 401–408.

Jendrzejewski, N., Trull, T.W., Pineau, F., Javoy, M., 1997. Carbon solubility in mid-ocean ridge basaltic melt at low pressures (250–1950 bar). Chem. Geol. 138,81–92.

Jochum, K.P., Hofmann, A.W., Ito, E., Seufert, H.M., White, W.M., 1983. K, U, andTh in mid-ocean ridge basalt glasses and heat production, K/U and K/Rb in themantle. Nature 306, 431–436.

Klein, E.M., Langmuir, C.H., 1987. Global correlations of ocean ridge basalt chemistrywith axial depth and crustal thickness. J. Geophys. Res. 92, 8089–8115.

Lassiter, J.C., 2003. Rhenium volatility in subaerial lavas: constraints from subaerialand submarine portions of the HSDP-2 Mauna Kea drillcore. Earth Planet. Sci.Lett. 214, 311–325.

MacKenzie, J.M., Canil, D., 2006. Experimental constraints on the mobility of rhe-nium in silicate liquids. Geochim. Cosmochim. Acta 70, 5236–5245.

Mahoney, J.J., Graham, D.W., Christie, D.M., Johnson, K.T.M., Hall, L.S., Vonderhaar,D.L., 2002. Between a hotspot and a cold spot; isotopic variation in the South-east Indian Ridge asthenosphere, 86°E–118°E. J. Petrol. 43, 1155–1176.

Mallmann, G., O’Neill, H.S.C., 2007. The effect of oxygen fugacity on the partition-ing of Re between crystals and silicate melt during mantle melting. Geochim.Cosmochim. Acta 71, 2837–2857.

Marty, B., 2011. The origins and concentrations of water, carbon, nitrogen and noblegases on Earth. Earth Planet. Sci. Lett. 313, 56–66.

Marty, B., Lussiez, P., 1993. Constraints on rare gas partition coefficients fromanalysis of olivine-glass from a picritic mid-ocean ridge basalt. Chem. Geol. 106,1–7.

Marty, B., Zimmermann, L., 1999. Volatiles (He, C, N, Ar) in mid-ocean ridge basalts:assesment of shallow level fractionation and characterization of source compo-sition. Geochim. Cosmochim. Acta 63, 3619–3633.

Moreira, M., Sarda, P., 2000. Noble gas constraints on degassing processes. EarthPlanet. Sci. Lett. 176, 375–386.

Moreira, M.A., Kurz, M.D., 2013. Noble gases as tracers of mantle processes. In:Burnard, P. (Ed.), The Noble Gases as Geochemical Tracers. Springer, Heidelberg,pp. 371–391.

Nuccio, P.M., Paonita, A., 2000. Investigation of the noble gas solubility in H2O–CO2

bearing silicate liquids at moderate pressure; II, The extended ionic porosity(EIP) model. Earth Planet. Sci. Lett. 183, 499–512.

Pineau, F., Javoy, M., 1994. Strong degassing at ridge crests: the behaviour of dis-solved carbon and water in basalt glasses at 14°N, Mid-Atlantic Ridge. EarthPlanet. Sci. Lett. 123, 179–198.

Russo, C.J., Rubin, K.H., Graham, D.W., 2009. Mantle melting and magma supply tothe Southeast Indian Ridge: the roles of lithology and melting conditions fromU-series disequilibria. Earth Planet. Sci. Lett. 278, 55–66.

Saal, A.E., Hauri, E.H., Langmuir, C.H., Perfit, M.R., 2002. Vapour undersaturation inprimitive mid-ocean-ridge basalt and the volatile content of Earth’s upper man-tle. Nature 419, 451–455.

Seaman, C., Sherman, S.B., Garcia, M.O., Baker, M.B., Balta, B., Stolper, E., 2004.Volatiles in glasses from the HSDP2 drill core. Geochem. Geophys. Geosyst. 5.

Sparks, R.S.J., 1978. The dynamics of bubble formation and growth in magmas: a re-view and analysis. J. Volcanol. Geotherm. Res. 3, 1–37.

Stolper, E.M., Holloway, J.R., 1988. Experimental determination of the solubility ofcarbon dioxide in molten basalt at low pressure. Earth Planet. Sci. Lett. 87,397–408.

Sun, W.D., Bennett, V.C., Eggins, S.M., Kamenetsky, V.S., Arculus, R.J., 2003. Enhancedmantle-to-crust rhenium transfer in undegassed arc magmas. Nature 422,294–297.

Taran, Y.A., Hedenquist, J.W., 1997. Geochemistry of magmatic gases from Kudryavyvolcano, Iturup, Kuril Islands – Reply. Geochim. Cosmochim. Acta 61, 3267.

Yamamoto, J., Nishimura, K., Sugimoto, T., Takemura, K., Takahata, N., Sano, Y., 2009.Diffusive fractionation of noble gases in mantle with magma channels: origin oflow He/Ar in mantle-derived rocks. Earth Planet. Sci. Lett. 280, 167–174.