Unradiogenic lead in Earth’s upper mantle 12_Burton

LETTERSPUBLISHED ONLINE: 22 JULY 2012 | DOI: 10.1038/NGEO1531

Unradiogenic lead in Earth’s upper mantleKevin W. Burton*†, Bénédicte Cenki-Tok†, Fatima Mokadem†, Jason Harvey†,Abdelmouhcine Gannoun†, Olivier Alard† and Ian J. Parkinson*The mantle and continental crust—Earth’s main silicatereservoirs—have a lead isotope composition that is tooradiogenic to have evolved from primitive Solar Systemmaterial over 4.57 billion years1. To account for this imbalance,it has been suggested that unradiogenic lead may havepartitioned into the metallic core2–4 or lower continentalcrust5. Alternatively, radiogenic lead could have been addedto Earth later by meteorite impacts6. Unradiogenic lead wasdiscovered in fragments of mantle rocks exhumed in theHoroman massif, Japan, implying that the mantle itself mayprovide a complementary reservoir of unradiogenic lead7.However, it is unclear why this unradiogenic component isnot sampled by the melting that generates oceanic basalts8.Here we present double-spike lead isotope data for abyssalperidotite rocks, considered to represent suboceanic mantle,exposed on the Atlantic Ocean floor. We find that sulphidesdated at about 1.83 billion years old and trapped as inclusionsin silicate minerals preserve extremely unradiogenic leadisotope compositions. This unradiogenic lead could have beenprevented from adding significantly to oceanic basalts if eitherthe silicates shield the sulphide inclusions or if the sulphidesreside in refractory mantle rocks that are rarely sampled duringmelting. We conclude that the lead isotope composition of thesilicate Earth could be largely balanced by unradiogenic lead insulphide in the mantle.

Mid-ocean ridge basalts (MORBs) are the most abundantterrestrial magmas and are believed to form by partial meltingof ultramafic rocks in Earth’s upper mantle. A fundamentalassumption underlying the use of radiogenic isotopes in suchmantle-derived basalts is that they are in equilibrium withtheir mantle source. Consequently, the composition of long-livedisotopes of heavy elements inMORBs and the upper-mantle residueshould be the same. The decay of 187Re to 187Os provides anexceptional tracer of mantle melting, because Os behaves as acompatible element and is preferentially retained in the mantle,whereas Re is moderately incompatible and enters the melt (forexample, ref. 9). Therefore, mantle rocks usually develop low187Re/188Os (parent/daughter) ratios and evolve to unradiogenic Osisotope compositions over time9. Remarkably, over recent years,Os isotope data for abyssal peridotites has revealed the presenceof ancient segments of the mantle9–12, preserving evidence of meltdepletion at least 2 billion years (Gyr) ago, which are seemingly notsampled by MORBs (ref. 13).

The natural radioactive decay of 238U and 235U to the isotopes ofPb provides an isotope record of the fractionation of U/Pb throughgeological history and Earth’smain silicate reservoirs, themantle (assampled by oceanic basalts) and continental crust, seem to possess

Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK. †Present addresses: Department ofEarth Sciences, Durham University, Science Labs, Durham DH1 3LE, UK (K.W.B.); Géosciences Montpellier, UMR 5243-CC 60, Université Montpellier 2,Place E. Bataillon, 34095 Montpellier cedex 5, France (B.C-T., O.A.); Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX13AN, UK (F.M.); School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK (J.H.); Laboratoire Magmas et Volcans, Université Blaise Pascal,CNRS-IRD, BP 10448, 63000 Clermont Ferrand, France (A.G.). *e-mail: [email protected]; [email protected].

100101

Seawater enrichment

Os isotope data

Included sulphides

Interstital sulphidesWhole rock

1001010.1000.0100.001

1.5 Gyr

2.0

2.5 3.0 Gyr

187Re/188Os

187 O

s/18

8O

s

0.113

0.114

0.115

0.116

0.117

0.118

0.119

Figure 1 | 187Re–187Os isotope diagram for sulphide grains from amid-Atlantic abyssal peridotite. In sample 2R1-31-37; site 1274A,interstititial sulphides (yellow circles) yield 187Os/188Os isotopecompositions that are indistinguishable from the whole rock. The solidcurve illustrates the trajectory for mixing of sulphides with sea water. Alsoshown (dark grey curves) are Re–Os model ages indicating an age of about2 Gyr for the included sulphides (red circles). Regression of the includedsulphides11 (not shown) yields a best-fit line corresponding to an age of2.06±0.26 Gyr (mean square weighted deviation= 3.3; model 2; ref. 28)indistinguishable from the Re–Os model ages. Error bars are 2 s.e.

a Pb isotope composition that is far too radiogenic for evolutionfrom chondritic material over 4.57Gyr, the so called Pb paradox1.The simplest solution to this paradox is to invoke a complementaryreservoir with an unradiogenic Pb isotope composition. Lead ishighly siderophile (iron-loving) and the Earth’s metallic core couldconstitute such a reservoir, if Pbmigrated to the core (preferentiallyto U) ∼80–140million years after the start of the Solar System (forexample, refs 2–4). Lead is also chalcophile (sulphur-loving) andit has been suggested that segregation to the core may have beenfacilitated by a sulphide melt, either soon after oxidation of themantle2 or long term through subduction4. Recent experimentaldata were initially taken to indicate that the affinity of Pb formetal and sulphide is not strong enough for it to have beencarried to the core14; however, this study used carbon-saturatedalloys, which are inappropriate given the low carbon content ofthe core3. Nevertheless, because Pb is volatile, it may have beenlost during accretion and the radiogenic composition of the Earthhas been taken by some to mark the late addition of volatile-rich meteoritic material that provided much of the terrestrial Pb(refs 6,14). That all of Earth’s Pb was acquired in this fashionrequires an implausibly large amount of late-accreted material15,

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NATURE GEOSCIENCE DOI: 10.1038/NGEO1531 LETTERS

20191817161515.0

15.2

15.4

15.6

15.8

MORBs 10°¬17° N

Abyssal peridotitesite 1274¬¬15° 39’ Nsample 2R1-31-37

Included sulphidest = 1.83 ± 0.23 Gyr

Interstitial sulphides

Pb isotope data

CC

4.57

Gyr

207 P

b/20

4Pb

206Pb/204Pb

North Atlantic sea water

Figure 2 | 207Pb–206Pb isotope diagram for sulphide grains from amid-Atlantic abyssal peridotite. Interstitial sulphides (yellow circles)possess radiogenic Pb isotope compositions reflecting recent seawaterinteraction. In contrast, included sulphides (red circles) possessunradiogenic isotope compositions that yield a 207Pb–206Pb age of1.83±0.23 Gyr (mean square weighted deviation=0.18, model 2; ref. 27),consistent with the Os isotope ages in Fig. 1 (ref. 11). Also shown areMORBs close to the fifteen–twenty fracture zone23,24 (blue squares)average normal MORBs (ref. 29; light blue down-facing triangle)continental crust16 (CC; green triangle) and North Atlantic sea water(purple oval30; see text). Two s.e. bars are smaller than the symbol sizes inall cases.

or the accretion of material that is distinctly non-chondritic, bothat odds with the abundance of highly siderophile elements inthe mantle, but some degree of Pb volatile loss or late additioncannot be ruled out.

Alternatively, it has been suggested that the unradiogenic Pbmay be stored in some other part of the silicate Earth, such asthe lower continental crust5, although this reservoir is generallyconsidered to be too small to balance the rest of the silicateEarth16, or even the upper mantle itself7. Uranium is highlyincompatible and will be depleted in mantle rocks that haveexperienced melting, resulting in a low U/Pb ratio, which thenevolve to relatively unradiogenic Pb isotope compositions overtime, but the primary Pb isotope compositions of peridotites areoften masked by secondary processes. Nevertheless, recent datafor the Horoman peridotite in Japan, an orogenic massif thoughtto represent a fossil fragment of Earth’s mantle, suggest that the

Pb isotope composition of the mantle may be less radiogenicthan was previously thought7. This massif is thought to representresidual material after melting that producedMORBs (for example,ref. 17), but it may have been chemically affected during uplift andemplacement onto the continental crust. The Pb concentrationsof Horoman peridotites are significantly higher than mantle rocksproducing oceanic basalts7,8, and Os isotopes suggest infiltrationand alteration by externally derived melts17. Moreover, if the Pbis located in the silicate minerals in mantle rocks7 then it is notclear why this unradiogenic Pb isotope signature is not seen inpresent-day MORBs (ref. 8).

Osmium is chalcophile and its compatibility in the uppermantleis largely controlled by the mineral sulphide (with sulphide/meltpartition coefficients of ∼106), which typically contains >90% oftheOs, despite the fact that this phase constitutes only∼0.05–0.08%of most mantle rocks4. Single-grain and in situ Os isotope data forsulphides indicate that the age and isotope information in abyssalperidotites is dominantly preserved by this phase10,11. Lead is alsochalcophile and concentrated in sulphide, although not to the sameextent as Os (sulphide/melt partition coefficients remain poorlyconstrained, ranging from 10 to 2,000; see ref. 4 and SupplementaryInformation). Therefore, it can be anticipated thatmantle sulphidespossessing unradiogenic Os (ref. 11) will also possess unradiogenicPb compositions, and that where trapped as inclusions in silicates,this ancient Pb isotope signal will, like that of Os, be shielded fromequilibration or sampling by later melting.

Here we present double-spike Pb isotope data for single sulphidegrains (see Supplementary Information for a full descriptionof analytical techniques) separated from an abyssal peridotite(2R1-31-37) from the axial rift valley of the Mid-Atlantic Ridgeat 15◦ 39′N (Ocean Drilling Program leg 209, site 1274A). Theharzburgites, dunites and minor gabbros at this site possessprimary mineral compositions and modes consistent with extrememelt depletion18,19, although all rock types have been heavilyserpentinized and altered by sea water18,19. These rocks containsulphide, oxide and nativemetal assemblages that formed as a resultof melting and subsequent serpentinization, but primary Fe–Nisulphides are still present as inclusions trapped in silicates (seeSupplementary Information)11,20.

Previously published 187Re–187Os isotope data for these sam-ples indicate that most interstitial sulphides possess 187Os/188Oscompositions that are indistinguishable from that of the bulkrock11, but with variable 187Re/188Os ratios, considered to resultfrom recent recrystallization or diffusional modification owingto partial melting or seawater infiltration. In contrast, included

Table 1 | Pb isotope and elemental data for sulphides from the abyssal peridotite sample 2R1 31-37 (Ocean Drilling Program leg209, hole 1274A).

Weight (g) Concentration(ppm)

Percentage ofblankcorrection

206Pb/204Pb ±2 s.e. 207Pb/204Pb ±2 s.e. 208Pb/204Pb ±2 s.e.

Interstitial sulphidesSulphide 1 0.0000334 3.72 10.77 18.4659 0.0097 15.5212 0.0092 38.1684 0.0260Sulphide 2 0.0000166 4.54 16.61 18.4672 0.0129 15.5970 0.0123 38.3485 0.0348Sulphide 3 0.0000412 4.50 7.49 18.4521 0.0093 15.5457 0.0098 38.2473 0.0260Sulphide 10 0.0000758 4.09 4.61 18.4454 0.0117 15.5191 0.0107 38.2271 0.0332Sulphide 8 0.0000428 4.00 8.06 18.4063 0.0108 15.5175 0.0098 37.9850 0.0285Included sulphidesSulphide 5 0.0000528 7.24 4.25 16.4351 0.0178 15.2784 0.0162 36.2934 0.0384Sulphide 6 0.000076 7.72 4.00 16.2276 0.0165 15.2593 0.0159 35.9011 0.0375Sulphide 3 0.000062 2.15 10.12 17.7839 0.0187 15.4314 0.0182 37.3290 0.0536

All Pb isotope data have been corrected for the analytical uncertainty on the double-spike measurement for the sample and the uncertainty on the blank measurement (see Supplementary Informationfor full details) and are reported together with their two standard errors (s.e.)

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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1531

sulphides possess high Os concentrations, low 187Re/188Os ratiosand yield an age of 2.06± 0.26Gyr, with model ages that rangefrom 1.99 to 2.06Gyr (ref. 11; Fig. 1). For the new Pb isotope dataobtained here (see Supplementary Table S1) interstitial sulphidespossess relatively radiogenic Pb isotope compositions that, likeOs, are likely to reflect recent crystallization and/or interactionwith sea water. Included sulphides, however, possess unradiogenic207Pb/204Pb and 206Pb/204Pb isotope compositions that yield an ageof 1.83± 0.23Gyr (Fig. 2), consistent with the ancient Os isotopeages of sulphides found in the same sample.

These results indicate that some abyssal peridotites, consideredto represent asthenospheric mantle underlying present-day mid-ocean ridges, preserve both Pb and Os isotope evidence for ancientmelting events. Entrapment of sulphide as inclusions in silicatesprovides a means for the preservation of this isotope signature,where it is shielded from equilibration or subsequent remelting (seeSupplementary Information). It is also likely that these sulphides arepresent in refractory domains in the mantle that are little sampledby later melting events. Experimental data on water solubility inmantle minerals suggests that dehydration accompanying MORBgenesis will dramatically increase the viscosity of the mantleresidue21. In principle, such highly viscous residual mantle couldpersist relatively undeformed for long periods of time, even in amantle undergoing vigourous convection22.

The geochemistry of the abyssal peridotites at site 1274 suggeststhat they have experienced recent remelting and melt infiltration19.The sulphides measured here seem to possess an indistinguishableage and initial isotope composition to the MORBs in this area23,24(Fig. 2). This might be taken to suggest that the abyssal peridotitesare the actual residues of the melting that produced the MORBsand that the Pb isotope compositions of the mantle sulphidesand MORBs reflect the preservation of material with differentU/Pb ratios in the mantle (that is, that this age reflects anancient differentiation event). Under normal circumstances isotopeequilibrium is expected between the different minerals in mantlerocks, however, if much of the Pb is located in sulphide andisolated from equilibration with silicates that contain most of theU (and Th), then this potentially provides a means of decouplingU from Pb in the mantle (see Supplementary Fig. S1 and TableS5). Nevertheless, the data here also lie close to the slope definedby all Northern Hemisphere MORBs and the global MORB array25.These arrays are taken to reflect mixing between distinct chemicaldomains in the mantle, where the slope in 207Pb–206Pb correspondsto the time for these reservoirs to attain secular equilibrium25, ratherthan a differentiation age. In either case, the data here indicate thaton the mineral scale, at least, some parts of the mantle preserveextremely unradiogenic Pb isotope compositions that relate toancient mantle melting events.

Quantifying the contribution of mantle sulphide to the compo-sition of the bulk silicate Earth (BSE) depends on the concentra-tion, composition and abundance of sulphide (see SupplementaryInformation). Taking the average Pb concentration of 20.6 ppm forincluded sulphides obtained here (n= 10; see Table 1 and Supple-mentary Table S3) and the least radiogenic Pb isotope composition(Table 1), this phase could account for 50% of the Pb in the mantleand result in a shift in the 206Pb/204Pb of the BSE from 18.29 to 18.05(corresponding to a difference in the age of the BSE of∼50millionyears; Supplementary Fig. S2). If the concentration is ∼47 ppm,within the range observed for sulphides at site 1274A (Supplemen-tary Table S3), then this could entirely balance the compositionof the BSE (see Supplementary Information), thereby resolvingthe Pb paradox. However, mantle sulphides are likely to possessesvariable Pb isotope compositions reflecting melt extraction overmuch of the age of the Earth. Indeed, osmium isotope data forplatinum-group alloy grains from abyssal and cratonic peridotiteshave been taken to indicate thatmantle depletion ages coincide with

peaks in generation of continental crust26, providing evidence forcoupled globalmantle–crust differentiation. In this case it is difficultto quantify the exact composition or concentration of sulphideneeded to balance the Pb in the silicate Earth.

Although Pb may well have been affected by core formation(for example, refs 2–4) and late meteorite addition6, our datashow that a significant proportion of Earth’s missing Pb mustbe trapped in refractory domains in the mantle. The growingevidence for the existence of extremely refractory domains in themantle7,9–12,27 has important consequences for the interpretationof the chemistry of mantle-derived melts and models of crust–mantle differentiation. If MORBs do not sample these refractorydomains then neither do they constrain the overall chemistry ofthe asthenosphere9 and the Earth’s upper mantle is likely to beconsiderably more depleted and heterogeneous than indicated byMORB chemistry alone.

Received 4 May 2012; accepted 26 June 2012; published online22 July 2012

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10. Alard, O. et al. In situ Os isotopes in abyssal peridotites bridge theisotopic gap between MORBs and their source mantle. Nature 436,1005–1008 (2005).

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12. Liu, C-Z. et al. Ancient, highly heterogenous mantle beneath Gakkel ridge,Arctic Ocean. Nature 452, 311–316 (2008).

13. Gannoun, A. et al. The scale and origin of the osmium isotope variations inmid-ocean ridge basalts. Earth Planet. Sci. Lett. 259, 541–556 (2007).

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15. Wood, B. J., Halliday, A. N. & Rehkämper, M. Volatile accretion history of theEarth. Nature 467, E6–E7 (2010).

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17. Saal, A. E. et al. Re–Os isotopes in the Horoman peridotite: Evidence forrefertilization? J. Petrol. 42, 25–37 (2001).

18. Bach, W. et al. Seawater-peridotite interactions: First insights from ODP Leg209, MAR 15◦N. Geochem. Geophys. Geosyst. 5, Q09F26 (2004).

19. Godard, M., Lagabrielle, Y., Alard, O. & Harvey, J. Geochemistry of the highlydepleted peridotites drilled at ODP Sites 1272 and 1274 (Fifteen-TwentyFracture Zone, Mid-Atlantic Ridge): Implications for mantle dynamics beneatha slow spreading ridge. Earth Planet. Sci. Lett. 267, 410–425 (2008).

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NATURE GEOSCIENCE DOI: 10.1038/NGEO1531 LETTERS24. Agranier, A. et al. The spectra of isotopic heterogeneities along the mid-Atlantic

ridge. Earth Planet. Sci. Lett. 238, 96–109 (2005).25. Albarède, F. Radiogenic ingrowth in systems with multiple reservoirs:

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29. Workman, R. K. & Hart, S. R. Major and trace element composition of thedepleted MORB mantle (DMM). Earth Planet. Sci. Lett. 231, 53–72 (2005).

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AcknowledgementsThis research used samples provided by the Ocean Drilling Program, sponsored by theUS National Science Foundation and participating countries under the management of

the Joint Oceanographic Institutions. Financial support for this research was providedby Ocean Drilling Program UK, NERC (grant ref. NER/A/S/2001/00538) and the ECMarie Curie Research and Training Network EUROMELT (HPRN-CT-2002-00211).A.G. acknowledges financial support from the Laboratory of Excellence ClerVolc. Wewould like to thank B. Wood, A. Halliday and J. Wade for discussions and W. M. Whitefor a constructive and thoughtful review.

Author contributionsK.W.B., A.G. and J.H. prepared the samples for analysis. I.J.P., B.C-T. and F.M. designedthe analytical procedure. B.C-T., F.M. and O.A. carried out the chemistry andmeasurements. K.W.B. and I.J.P. wrote the main paper and the SupplementaryInformation. All authors discussed the results and commented on the manuscript atall stages.

Additional informationSupplementary information is available in the online version of the paper. Reprints andpermissions information is available online at www.nature.com/reprints.Correspondence and requests for materials should be addressed to K.W.B. or I.J.P.

Competing financial interestsThe authors declare no competing financial interests.

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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NGEO1531

NATURE GEOSCIENCE | www.nature.com/naturegeoscience 1

Unradiogenic lead in Earth’s upper mantle

Supplementary Information

Kevin W. Burton1,2, Bénédicte Cenki-Tok1,3, Fatima Mokadem1,4, Jason Harvey1,5, Abdelmouhcine Gannoun1,6, Olivier Alard1,3, Ian J. Parkinson1

1 Department of Earth and Environmental Sciences, The Open University, Walton Hall,

Milton Keynes, MK7 6AA, UK. 2 Present address: Department of Earth Sciences, Durham University, Science Labs,

Durham DH1 3LE, United Kingdom 3 Present address: Géosciences Montpellier, UMR 5243 - CC 60, Université Montpellier 2,

Place E. Bataillon, 34095 Montpellier cedex 5, France. 4 Present address: Department of Earth Sciences, University of Oxford, South Parks Road,

Oxford OX1 3AN, UK. 5 Present address: School of Earth & Environment, University of Leeds, Leeds LS2 9JT,

UK. 6 Present address: Laboratoire Magmas et Volcans, Université Blaise Pascal, CNRS-IRD,

BP 10448, 63000 Clermont Ferrand, France.



2

1. Sulphide mineralogy and petrography in abyssal peridotites from ODP Site

1274A

The distribution of sulphides in abyssal peridotites at Site 1274A is highly variable,

ranging from no visible grains to up to ≈ 20 or more grains (>50 µm) in a single thin

section. The typical size of base metal sulphides in 1274A abyssal peridotites is

between 70 and 150 µm, however, large sulphides up to ≈500 µm occasionally occur.

The textural relationship of sulphide with other minerals in the rock, and

compositional variations, allow 3-types of sulphide to be distinguished.

Type 1, sulphides are included either in relict primary olivine or clinopyroxene, or

form isolated grains of sulphide within the serpentine matrix. Their mineralogical and

microstructural features are characteristic of sulphide that is residual after melting

(Alard et al., 2005; Luguet et al., 2001), and they comprise uniquely of pentlandite

(Pn-1; Table S1).

Type 2A, sulphides occur at sites of orthopyroxene reaction, embayed in the margin

of orthopyroxene porphyroclasts. They comprise 90-95% pentlandite (Pn-2; Table S1)

and 5 to 10% chalcopyrite (Cp; Table S1). Such Cu-rich compositions are more akin

to the crystallisation product of a sulphide partial melt (Ballhaus et al., 2001), and

Seyler et al. (2006) note that 40% of the sulphides in these abyssal peridotites share a

grain boundary with orthopyroxene, although orthopyroxene itself only accounts for

ca. 20% of the silicate phases. Such unusual sulphides, embedded in orthopyroxene

porphyroclasts, are commonly observed in ophiolitic harzburgites and have been

interpreted to represent sulphide only partially extracted through melting (e.g. Lorand,

1988).

Type 2B, sulphides occur mainly as large (100-500 µm) convoluted patches

intimately associated with clinopyroxene and spinel. They show low dihedral angles

with surrounding silicates, and often grade into vein-like extensions. They can also be

found as sprays of droplets (1-10 µm), within the clinopyroxene cleavage planes.

They consist of pentlandite (Pn-3, Table 1) and primary bornite (±native copper) (Pn-

3 and Cu-n, Table S1). While the proportion of the Cu-rich phase is variable, it can



3

reach values as high as 35%. These microstructural and compositional features

suggest that the Sulphide-2B–clinopyroxene (±spinel) assemblage represents the

crystallization product of a Cu-Ni-rich sulphide-bearing melt, trapped under

lithospheric conditions and post-dating the crystallization of Type-2A sulphides.

Finally, due to serpentinisation and steatization (seawater weathering) secondary

phases such as awaruite, hezelewoodite or native Cu and Fe-oxide/hydroxide are also

found, and are thought to have been formed at the expense of the magmatic

assemblages described above (see also Klein and Bach, 2009). The extent of this

alteration is highly variable (0 to 72%), however, there is no relationship between the

occurrence and abundance of these secondary phases and the abundances of platinum

group elements and Pb. The same observation is made for the osmium isotopic

composition of sulphide grains (Alard et al., 2005).

Table S1. Major element compositions of typical sulphide phases.

Primary Sulphide Phases

Secondary-Alteration

Phases Wt.% Pn-1 Pn-2 Pn-3 Cp Bn Aw Hz Cu-n

S 33.65 33.33 33.62 34.77 24.70 0.39 28.08 0.56

Fe 30.89 29.78 30.90 30.39 8.50 24.91 2.16 2.56

Co 0.64 2.74 0.97 0.03 0.30 0.42 0.09 0.06

Ni 33.54 33.46 33.10 0.01 1.07 72.57 69.08 6.35

Cu 0.04 0.23 0.75 34.09 64.78 0.64 0.23 90.14

Zn 0.03 0.04 0.014 0.18 0.00 0.01 0.02 0.00

O 0.57 0.65 0.33 0.35 0.51 0.71 0.50 0.24

Si 0.08 0.11 0.02 0.03 0.10 0.23 0.01 0.10

Tot 99.44 100.34 99.71 99.85 99.96 99.88 100.15 100.00 Pn: Pentlandite; Cp, chalcopyrite; Bn, Bornite; Aw, awaruite; Hz, Heazelwoodite; Cu-‐n, native copper. Data obatined by electron probe (Cameca SX100, 15kV, 10nA) at the Université de Montpellier II.

2. Methods

2.1 Pb isotope and elemental measurements

Individual sulphides were hand picked from crushed fragments of the abyssal

peridotite sample 2R1-37, cleaned in ultrapure water and weighed using a micro-

balance. The sulphides were dissolved in new PFA Savillex® beakers using a HF-

HNO3 mixture and by heating the sample beakers overnight on a hotplate. A 5%



4

aliquot of the dissolved sample was spiked with a tracer enriched in 206Pb for isotope

dilution concentration measurements. This aliquot and the remaining solution were

then dried down and converted to Br- form using HBr. Separation of the Pb was

performed using a column separation procedure similar to the technique described in

Lugmair and Galer (1992), which is based on the methods developed by Strelow and

co-workers (Strelow, 1978; Strelow and Van Der Walt, 1981; Van Der Walt et al.,

1982). The columns were fabricated from shrink-fit Teflon and fitted with a

polypropylene frit. Each column was loaded with 50 µl of AG-1 X8 anion resin and

the Pb separation was effected using dilute HBr-HN03 mixtures that produce a clean

Pb fraction (Lugmair and Galer, 1992). All the reagents used in the chemistry were

Ro-Mil ultra-pure reagents. Total procedural blanks were measured throughout this

study, and yield an average Pb blank of 15 pg.

Mass spectrometry involves making three measurements. One measurement is

required to determine the Pb concentration using the aliquot spiked with 206Pb and this

in turn allowed us to produce an optimal spike-sample mixture for the double spike

(DS) measurement. Lead isotopes were analysed using the DS technique and thus

requires two measurements; one spiked and one unspiked. In all cases a H3PO4-

colloidal silica emitter is used to enhance thermal ionisation of the Pb. Our H3PO4-

colloidal silica emitter was produced following the recipe of (Gerstenberger and

Haase, 1997) using Merck Suprapure H3PO4 and the same batch of Merck colloidal

silica used by Gerstenberger and Haase, (1997), which was provided by Professor

Matthew Thirlwall. The aliquot for isotopic analyses was dried down after the column

separation and taken up in ~1 µl of emitter solution, to minimize any differential

blank effects from the loading solution. Two-thirds of the solution was loaded directly

onto a single Re filament and dried at 0.5 A, before ‘flashing’ the filament at 1.8 A

for a second so that any H3PO4 was evaporated from the filament. The remaining one-

third of the solution was taken up in a pipette tip and mixed with an appropriate

amount of a 207Pb-204Pb double-spike before loading, thereby avoiding contamination

of the beakers with 204Pb.

All mass spectrometric measurements were made on ThermoScientific Triton thermal

ionization mass spectrometer (TIMS) in the Department of Earth and Environmental



5

Sciences at the Open University. Sample or standard loads with greater than 1 ng of

Pb were analysed statically using the Faraday cups, whereas those with less than 1 ng

were analysed statically using the secondary electron multiplier (SEM). For the

Faraday runs, a gain calibration was completed at the start of each day. Each analysis

consisted of 18 blocks of 10 ratios, with each ratio a 4s integration, with a 60s

baseline measurement made before each block. For the unspiked runs a 204Pb beam of

>100 mV was aimed for, as 204Pb counting statistics are major source of uncertainty

in DS Pb isotopic measurements (Powell et al., 1998), enabling 2 s.e. uncertainties of

~40 ppm for the Pb isotope ratios. Isotope dilution measurements where all made on

the Faraday cups. Initially the low level standards were analysed using both the

Faraday cups and the SEM. This required performing a gain calibration at the start of

each day and then a Faraday/SEM yield calibration prior to each measurement,

derived by switching a 204Pb beam of ~ 7x105 counts per second between the SEM

and Faraday. While this method achieved good internal uncertainties, the external

uncertainty was poor. Thereafter the 0.05ng NBS 981 standards and the sulphide

samples were analysed on the SEM in dynamic mode, which yielded 2 s.e.

uncertainties of ~400-900 ppm for the 206Pb/204Pb and 207Pb-204Pb ratios and 500-1100

ppm for the 208Pb/204Pb ratio. Richter et al., (2001) have demonstrated that some

SEMs have issues with non-linearity. In this study, the unspiked 0.05ng NBS 981

standards were measured with 208Pb beam intensities that range from 2x105 to

6.5x105, and we were careful to analyse the sulphide samples at similar beam

intensities. We note that there was no variation in the internally normalised value for

NBS 981 as a function of intensity. This suggests that there is no issue of non-

linearity of the SEM used in this study, which is not surprising as Richter et al. (2001)

report significant variations only when the beam intensity varied by two orders of

magnitude.

Double-spike Pb isotope TIMS measurements have significant advantages over

traditional TIMS Pb isotope measurements because the instrumental mass

fractionation factor for each specific sample is determined and does not rely on using

mass fractionation data determined on standards (see Galer, 1999). Therefore the

technique has the ability to produce both accurate and precise data for both standards

and samples (Galer, 1999; Thirlwall, 2000). The 207Pb-204Pb double spike used in this



6

study is the same as that used by Thirlwall (2000), but was recalibrated on our Triton

against NBS 982 assuming a 208Pb/206Pb ratio of 1.00016 (e.g., Thirlwall, 2000). An

assessment of the accuracy and precision of our measurements was undertaken by

determining the composition of the NBS 981 standard using 50, 20, 10 and 0.05 ng

sample sizes of Pb. There is little difference in averages for the sets of 50, 20 and 10

ng loads, because they were run on the Faraday cups and in each case we were able to

obtain >100 mV of 204Pb. Combining all the NBS 981 data from the Faraday cup runs

yields Pb isotopic ratios that overlap those of other DS Pb isotope studies and with

comparable precision (see Table S2). Data collected using the SEM method are

obviously less precise, with external and internal precisions that are an order of

magnitude greater than the Faraday cup analyses, due to counting statistics. However,

even considering these differences in sample size our data are of comparable quality

to many other studies, using both TIMS and MC-ICP-MS, due to the use of a double

spike. Firstly, because many traditional TIMS studies that did not use a double spike,

produce 2 s.e. uncertainties of <100 ppm, but have 2 s.d. external precisions of 800-

1000 ppm, comparable to our external precision, because the standards do not recover

realistic instrumental mass fractionation values for samples (see Galer, 1999;

Thirlwall, 2000). Secondly, our sample data is likely to be accurate; our low

concentration NBS 981 data are within error of the accepted value for NBS 981

derived during this study and other DS Pb isotope studies (see Table S2) and it has

been demonstrated that although non-DS techniques can produce precise data there is

still considerably debate about the accuracy of many studies (see Thirlwall, 2002;

Baker et al., 2004, 2005). Thirdly, the rock standard JP-1 (Horoman peridotite) run at

similar concentrations to the sulphides (~0.1 ng) gives the following isotope

composition; 206Pb/204Pb = 18.347 ± 0.0275, 207Pb/204Pb = 15.562 ± 0.0340, 208Pb/204Pb = 38.337 ± 0.136, indistinguishable to that obtained elsewhere using

double spike techniques on larger sample sizes (Kuritani et al., 2006). Critically,

while we are confident that our data is accurate, in this study we report variations in

the Pb isotopes of the sulphide minerals of up to ~3% for the 206Pb-204Pb ratio, which

is large compared to both the precision and accuracy of our standard data.



7

Table S2. Lead isotope data for NBS 981 from this and previous studies

Sample weights range from 0.0166-0.256 mg of sulfide, which equates to sample

loads on the filament of between 0.06 and 0.22 ng for the unspiked runs. Both the

concentration and the isotope data have been blank corrected using an average Pb

blank of 15 pg which has the following isotope composition; 206Pb/204Pb = 17.7444 ±

0.02485, 207Pb/204Pb = 15.6223 ± 0.02389, 208Pb/204Pb = 37.7438 ± 0.06371. The

blank correction produces a difference of 4-16% in the Pb concentration, although the

correction is around ~10% for most samples. In the case of the isotope values, the

blank correction produces shifts of 0.02-0.88% for the 206Pb/204Pb ratios, 0.03-0.25%

for the 207Pb/204Pb ratios and 0.014-0.44% for the 206Pb/204Pb ratio, depending on the

isotopic composition of the sample. However, it is important to note, that the

uncertainty on the isotope measurements cited in Table S2 incorporates both the

analytical uncertainty on the double-spike measurement for the sample and the

uncertainty on the blank measurement, which have been propagated through the blank

correction calculation using a Monte Carlo method.

2.2 In-situ Pb concentration measurements

In-situ sulphide Pb-data (Table S3) were obtained using a GEOLAS excimer UV

laser operating at 193 nm, coupled with a Thermo-Finnigan XR-Element 2

inductively coupled plasma mass spectrometer (ICP-MS) at Geosciences Montpellier.

Ablation was carried out using a helium carrier gas (≈0.6 l/min) and ablated products

206Pb/204Pb 2 s.d. 206Pb/204Pb 2 s.d. 206Pb/204Pb 2 s.d.

NBS 981 - this study

Faraday cups (50 ng, n = 19) 16.9418 0.0017 15.4995 0.0016 36.7263 0.0049

Faraday cups (20 ng, n = 8) 16.9412 0.0009 15.4991 0.0009 36.7247 0.0025

Faraday cups (10 ng, n = 5) 16.9401 0.0015 15.4975 0.0015 36.7219 0.0023

Faraday cups (all data, n = 32) 16.9415 0.0021 15.4992 0.0021 36.7251 0.0050

SEM (0.05 ng, n = 4) 16.9399 0.0232 15.4928 0.0390 36.7455 0.0993

NBS 981 - other studies

Galer and Abouchami, 1998 16.9406 0.0022 15.4963 0.0016 36.7219 0.0044

Thirlwall, 2000 16.9409 0.0022 15.4956 0.0026 36.7228 0.0080

Baker et al., 2004 16.9416 0.0013 15.5000 0.0013 36.7262 0.0031



8

were transferred with Ar (≈0.8 l/min) to the plasma.

Analytical conditions included a ≈50 µm beam diameter, 5 Hz laser frequency and

a beam energy ca. 15 mJ/pulse. These instrumental conditions were found to produce

a near steady signal with minimum inter-element fractionation. Data reduction was

carried out using the GLITTER software (Griffin et al., 2008). The analyses were first

normalised using S as internal standard (determined by electron microprobe) and then

to Pb-doped NiS beads (see Table S4) following methods used previously for

Platinum Group Elements (Alard et al., 2000). This double standardization allows

correction for variations in ablation yield and instrumental drift (e.g. Longerich et al.,

1996), and the accuracy and precision of the method is reported in Tables S4and S5

below.

Table S4. Cross calibration of in-house standards - Pb doped-NiS beads

Table S3. In-situ Pb and PGE data for sulphides from abyssal peridotites from Hole 1274A (ODP Site 209)

OA-ID ODP-name Sulf. Type Sulf N#

1274-3 1274A-1R1(Pc10B)-27-31 T1 #21274-3 1274A-1R1(Pc10B)-27-31 T1 #111274-3 1274A-1R1(Pc10B)-27-31 T1? #X1274-5 1274A-4-R2(Pc)-1-3 T1 #51274-5 1274A-4-R2(Pc)-1-3 T2A #X1274-6 1274A-5-R2(Pc2)-130-135 T1 #11274-6 1274A-5-R2(Pc2)-130-135 ? #X1274-6 1274A-5-R2(Pc2)-130-135 ? #Y1274-7 1274A-7R1(pc3)-18-20 T1 #41274-7 1274A-7R1(pc3)-18-20 T2A #2

See text for anlytical details. Sulphide types: T1 = included in primary silicates; T2A = interstitial sulphide

Sample Sulphide

Table S3. In-situ Pb and PGE data for sulphides from abyssal peridotites from Hole 1274A (ODP Site 209)

Os Ir Pd Re Pb (Pd/Ir)N

ppm

12.34 12.29 0.76 0.161 52.12 0.0515.52 5.64 2.02 0.276 22.31 0.2964.16 4.09 1.82 1.137 18.12 0.368

12.34 12.29 0.76 0.112 45.63 0.0510.16 0.11 0.93 0.258 1.53 7.31

10.32 9.96 0.86 0.291 17.22 0.0710.98 0.93 0.92 0.25 7.43 0.8181.10 1.14 1.45 0.14 6.23 1.0493.97 3.69 1.88 0.219 13.33 0.4210.55 0.67 1.16 0.1125 5.19 1.432

See text for anlytical details. Sulphide types: T1 = included in primary silicates; T2A = interstitial sulphide

LA-ICP-MS

NiS-OA#0 NiS-OA#4 N Avg ±1SD N Avg ±1SD

Sol. ICP-MS 2 29.21 0.71 3 1338 61

PIXIE 4 34.2 8.5 2 1478 187

LA-‐ICP-‐MS 12 28.9 3.9 6 1341 102



9

The PIXIE Pb analyses were obtained using the CSIRO-GEMOC PIXE (Sydney,

Australia) by C.G.Ryan using techniques described in Ryan (2001). The solution ICP-

MS, were obtained using standard techniques. In this several aliquots of the two Pb-

doped NiS-Buttons were initially dissolved in mixture of HCl:HNO3 (1:2) this was

evaporated and brought back into solution with concentrated HNO3. The solution

obtained was then diluted 2000 fold (2 hours before analysis, and made up to 2%

HNO3) and calibrated against diluted pure elemental Pb (with dilutions of 0.113, 12.6

and 130.1 ppm). The possibility of a matrix effect and/or polybaric interferences was

checked using a Ni-S doped synthetic solutions (ca. 400 and 200 ppm respectively).

Analyses were performed using the XR-elemental2 ICP-MS at Géosciences

Montpellier. 206Pb, 207Pb and 208Pb were measured in low resolution and high

resolution mode. Bismuth was used as an internal standard to monitor and correct for

any ICP-MS drift.

Table S5. Lead concentration data for Certified Reference Material JK37 - AB Sandvik Steel

3. The distribution and behavior of Th, U and Pb in mantle minerals.

A number of studies have shown that the Pb content of mantle rocks cannot be

accounted for by the primary silicate phases alone (e.g. Meijer et al., 1990; Tatsumoto

et al., 1992; Carignan et al., 1996; Ionov et al., 2006) and this missing Pb is

commonly thought to be present in sulphide. Similarly our own unpublished data

suggest that primary silicate phases can only account for between 5 to 10% of the Pb

in spinel peridotites from Kilbourne Hole, USA (Jason Harvey, unpublished data).

Experimental data are sparse and not relevant to mantle peridotite melting,

nevertheless partition coefficients for Pb between sulphide and basaltic melts are

consistent with compatible behavior for Pb in sulphide, ranging from 1.6 to 40

(Oversby and Ringwood, 1971; Shimizaki and MacLean, 1976). Gaetani and Grove

(1997, 1999) demonstrated that Co, Cu and Ni are all highly compatible in sulphide

Pb in ppm ±1SD N-analyses Ref

Recommended value 1.29 0.18

Solution ICP-TOF-MS 1.32 0.02 n=9 Granfors & Gustavsson, 2001

LA-ICP-MS 1.30 0.07 n=5 This study



10

melts, relative to olivine, and estimated a sulphide/olivine partition coefficient for Pb

of 2,000.

Although Pb is strongly partitioned into sulphide, Th and U concentrations in this

phase are extremely low (e.g. Hegner & Tatsumoto, 1987) and these elements are

hosted in coexisting silicates (e.g. Blundy & Wood, 2003). Our own unpublished data

for Th, U and Pb and Pb isotopes for both silicates and sulphide from a garnet-

peridotite from Lashaine, Tanzania, indicates that sulphide contains much higher Pb

concentrations than any of the silicate phases. Sulphide contains 5.75 ppm Pb,

compared to 1.17 ppm for clinopyroxene, 515 ppb for phlogopite, 49.8 ppb for garnet,

22.4 ppb for orthopyroxene and 2.99 ppb for olivine (Table S6). If partitioning

between sulphide and silicate is taken as K = CSulphide/Csilicate, then this yields partition

coefficients that range from 5 (DPb sulphide-clinopyroxene) to ~1,900 (DPb sulphide-olivine), the

latter being similar to the estimate of Gaetani and Grove (1999). Given the modal

abundance of clinopyroxene (2%, Table S5) in this rock, this phase accounts for 68%

of the Pb in the primary mantle minerals. These data illustrate that the Pb budget of

mantle rocks will be largely controlled by clinopyroxene and sulphide, and it can be

anticipated that in highly depleted rocks where clinopyroxene has been removed by

melting, sulphide will dominate the Pb content of the rock (cf. Meijer et al., 1990). In

contrast, most of the U and Th is hosted in the coexisting silicate phases (Table S6).

If much of the Pb is hosted in sulphide, and isolated from the U and Th hosted in

silicates then this provides a means of decoupling Th and U from Pb in the mantle.

The Pb isotope data for the silicates and sulphide from the garnet-peridotite BD738

from Lashaine, Tanzania, clearly indicates that the sulphide inclusions are not in

equilibrium with their silicate hosts. The silicates yield a best-fit line corresponding to

a 207Pb-206Pb age of ~500 Myr, whereas the sulphides yield a much older age of

3.06±0.39 billion years (Fig. S1). This old Pb age for the sulphides is

indistinguishable from Os model ages for sulphides from the same sample (Burton et

al., 2000). This indicates that both Pb and Os in sulphides from this rock preserve

ages greater then 3.0 billion years, being shielded from equilibration with their host

silicates. The principal difference in the behaviour of Os and Pb is that 90% of the Os

is hosted in sulphide and this dominates the signature of the “whole-rock” (Burton et



11

al., 2000). Whereas, only 10% of the Pb is in the sulphide, and the “whole-rock” has a

composition that is only slightly displaced from that of the silicates (Fig. S1).

For the sample studied here Pb concentrations in sulphide range from 2.2 to 7.7 ppm,

where the ancient included sulphides possesses higher concentrations than the

interstitial sulphides. The Pb concentrations for sulphides, determined in-situ by LA-

ICP-MS, from a number of other samples at the same site (1274A) yield a range from

1.5 to 52 ppm (Table S2). Again it is evident that the included, primary, sulphides

(type T1 in Table S2) generally have higher Pb concentrations than the interstitial

sulphide (type T2A). There is also a clear positive covariation between the Pb and Os

content in these sulphides, with the high Pb grains possessing low Re/Os ratios,

consistent with those observed in primary included sulphides from sub-continental

xenoliths (cf. Burton et al., 1999, 2000). The average Pb content of all of the

sulphides is ∼12.6 ppm, but taking only those sulphides that are considered to be

primary and trapped as inclusions, then the average is ∼20.6 ppm. By comparison,

sulphides from peridotite assemblages found as inclusions in diamonds possess Pb

Table S6. Pb isotope data for silicates and sulphides from garnet peridotite (BD738) from Lashaine, Tanzania

Proportion of rock, % 206Pb/204Pb ± 2 s.e. 207Pb/204Pb ± 2 s.e. 208Pb/204Pb 2 s.e. err U Th Pb

garnet 7 16.161 0.012 15.424 0.016 36.444 0.036 8.32 31.65 49.82clinopyroxene 2 15.43 0.012 15.404 0.016 36.003 0.036 11.96 70.79 1117orthopyroxene 17.9 17.563 0.02 15.519 0.018 37.472 0.044 1.13 3.32 22.38phlogopite 3 17.382 0.012 15.513 0.016 37.523 0.036 128.7 956.4 515olivine 65.2 0.54 1.43 2.99

whole-rock 15.536 0.012 15.284 0.016 36.241 0.036 14.9 59.3 2113

sulphide 1 0.017 14.297 0.036 14.73 0.036 36.241 0.094 11.01 42.11 5747sulphide 2 0.017 13.664 0.012 14.584 0.016 35.03 0.036 7.15 35.78 --

Concentrations in ppb by weight. U, Th and Pb concentrations determined by isotope dilution.

Note that these data were measured without the use of a double spike.

All Pb isotope ratios relative to the NBS981 standard: 16.932 0.012 15.486 0.016 36.691 0.036(n = 35)

The internal errors of the Pb isotope ratios for the minerals separates were usually better than the external reproducibility of the NBS981 standard (run at similar beam intensities).Hence the given uncertainties are those determined for NBS981 except where they are larger.

18171615141314.4

14.6

14.8

15.0

15.2

15.4

15.6

Pb/

P

b20

720

4

Pb/ Pb206 204

t = 3.06±0.39 Gyr

whole-rock

cpxgarnet

opx phl

sulphide

t = 500 Myr

Garnet-peridotiteLashaine, Tanzania

Fig. S1



12

concentrations that vary from 35 to 147 ppm (Eldridge et al. 1991) (excluding two

sulsphides with a concentrations >1400 ppm), which overlaps the range seen here in

abyssal peridotites.

4. Diffusional isolation of sulphide inclusions

The available data, presented both here (section 3 above) and elsewhere, suggest that

Pb, like Os, will be highly concentrated in sulphide, relative to most primary silicates

in mantle rocks. The Pb partition coefficient between sulphide and silicates ranges

from around 5 (DPb sulphide/clinopyroxene) to 2000 (DPb sulphide/olivine) (cf. Gaetani and Grove,

1999), and thus in a highly depleted mantle rock (with little or no clinopyroxne) most

of the Pb will be contained in the sulphide. Even in the presence of clinopyroxene the

limited solubility of Pb in olivine and orthopyroxene will significantly impair

diffusional equilibration between Pb located in sulphide grains in those phases and

interstitial sulphide or melt.

A mathematical analysis of a similar problem, that of the reequilbration of a melt

inclusion in a host crystal, has been presented previously (Qin et al., 1992). The key

to understanding the timescales for Pb loss or gain to the sulphide grain is to evaluate

the flux across the surface between the host crystal (in this case olivine) and the

sulphide,

€

dC a, t( )dt

= βDol

a∂Col

∂r r= a (1)

where a is the radius of the sulphide inclusion, Dol is the diffusion rate in the host

crystal, Col is the concentration in the host crystal, r is a radial geometry coordinate

and t is time and

€

β = 3Kρolρ sulph

(2)

where ρol and ρsulph are the densities of olivine and sulphide respectively and K is the

partition cefficient between the two phases where

€

K =Col

Csulph

(3)

Qin et al. (1992) demonstrate that the equilibration time is inversely proportional to

the partition coefficient, K, such that elements with K values <<1 (e.g. Pb in olivine



13

with respect to sulphide) take longer to equilibrate. Although the analyses presented

in Qin et al. (1992) gives an indication of the equilibration time for a sulphide in

detail it cannot be directly applied to the problem because it requires the inclusion and

the grain boundary both being the same phase (i.e. a melt phase) with the same K

value, which is not appropriate from sulphides in the mantle. Numerical modeling

(which will be presented elsewhere) indicates that critical to the diffusive isolation of

the sulphide inclusion is that host olivine will have very low Pb concentrations,

because Pb is preferentially partitioned into clinopyroxene, interstitial sulphides and

any melt phase present on the grain boundaries. This strongly restricts the ability of

Pb to be diffusively fluxed into the included sulphide from the surrounding olivine.

Therefore a combination of DPb sulphide/olivine ~ 103 (Gaetani and Grove, 1999; this

study) and the incompatibility of Pb into olivine (see Table S6 above), drastically

reduces the intercommunication between sulphides enclosed in olivine grains and

grain boundary phases. For Os partitioning into sulphide and Os-Ir alloys, it is likely

that DOs sulphide/olivine >>106, and Os is known to be highly incompatible in mantle

olivine (Burton et al., 2002) such that diffusional equilibration will be even more

impaired. Consequently, if sulphide inclusions are preserved in mantle rocks, and

these record ancient melting events, then extreme Pb and Os isotopic heterogeneity on

a grain scale can be preserved. In contrast, diffusional equilibration between

interstitial sulphides and melts on grain boundaries, and coexisting silicates, is likely

to be more rapid be more rapid (cf. Burton et al., 1999, 2000).

5. Contribution of unradiogenic mantle sulphide to the Bulk Silicate Earth

Estimating the contribution of unradiogenic mantle sulphide to the overall Pb isotope

composition of the Bulk Silicate Earth (BSE) is challenging, not least, because

estimates of the BSE vary considerably (for example, see Table 1 in Halliday (2004))

yielding model ages that range from 40 to 180 Myr after the start of the solar system.

However, the Earth and the Moon have identical tungsten isotope compositions,

which constrains their age to >50 Myr after the start of the solar system (Touboul et

al. 2007). In this case the Pb isotope composition of the BSE must be more radiogenic

than might be expected from the Geochron at 4.57 Gyr (Figure S2). It is possible to



14

calculate the composition of the silicate Earth using estimates of the Pb isotope

composition for the lower and upper continental crust (Kramers and Tolstikin, 1997)

and assuming that the upper mantle has a composition close to depleted mantle

estimates (Workmann & Hart, 2005). Taking the proportion of sulphide in the mantle

to be 0.08% (Hart & Gaetani, 2006), with a Pb isotope composition similar to that

obtained here (Table 1), it is then possible to calculate the contribution of mantle

sulphide to the overall composition of the silicate Earth (Figure S2). If the

concentration of Pb in mantle sulphide is 7.5 ppm (based on the data from sample

2R1-31-37 studied here) then sulphide contains 25% of the Pb in the mantle and 5%

in the BSE. If sulphide contains 20.6 ppm Pb (based on the average of all the included

sulphides analysed in this study from samples at Site 1274A, given in Table 1 and

Table S3) then mantle sulphide contains 50% of the Pb in the mantle and contributes

12% to the BSE. If the sulphide contains 47 ppm Pb (within the range of Pb

concentrations measured in this study, Table 1 and Table S3) then it accounts for 70%

of the Pb in the upper mantle and could entirely balance the composition of the

silicate Earth. It is important to note that the effect of mantle sulphide remains broadly

the same irrespective of the value taken for the Pb isotope composition of the BSE.

However, mantle sulphides are likely to possess variable Pb isotope compositions

reflecting melt extraction over much of the age of the Earth, rather than the singular

value used in this model (see main text for discussion). In this case it is difficult to

quantify the exact concentration or proportion of sulphide needed to balance the Pb in

the silicate Earth.

Pb

/

Pb

20

72

04

18.6518.2517.8517.45

15.3

15.4

15.5

15.6

15.7

Pb/ Pb206 204

1020

3040

5060

ppm Pb in

mantle sulphide

Geochro

n

50 M

yr (H

f-W

age)

100 M

yr

150 M

yr

Silicate

Earth

Fig. S2



15

Table S7. Calculation of contribution of mantle sulphide required to balance the Bulk Silicate Earth Reservoir 206Pb/204Pb 207Pb/204Pb Pb, ppm Massa (1024 g)

Upper crustb 19.33 15.73 7.01 6.9 Lower crustb 17.62 15.35 4.64 15.7 Upper mantlec 18.28 15.49 0.018 1170

Silicate Earthd

18.29 15.50 0.12

Mantle sulphidee

16.23 15.26 7.5

Calculated BSE for a given concentration of Pb in mantle sulphidef

Pb in mantle Sulphide, ppm

Contribution to BSE, %

18.20 15.49 7.5 4.7 18.05 15.47 20.65 11.9 17.81 15.44 47.00 23.6 a – Masses of reservoirs from Asmerom and Jacobsen (1993) b – Average composition given by Kramers and Tolstikin (1997) c – Assumes that the upper mantle has a similar composition to the depleted mantle composition of Workman & Hart (2005) d – Composition of the Silicate Earth, based on a, b & c above e – Mantle sulphide measured in this study (Table 1). f – Contribution of mantle sulphide to the BSE for a given concentration in mantle sulphide assuming 0.08% sulphide in the mantle that is hidden from mantle melting by entrapment in silicates



16

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Unradiogenic lead in Earth’s upper mantle 12_Burton