Mantleplumes.org - Evidence Against a Chondritic Earth CampbellChondriticEarth2012

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Evidence against a chondritic EarthIan H. Campbell

1

& Hugh St C. ONeill1

The 142Nd/144Nd ratio of the Earth is greater than the solar ratio as inferred from chondritic meteorites, which challengesa fundamental assumption of modern geochemistrythat the composition of the silicate Earth is chondritic, meaningthat it has refractory element ratios identical to those found in chondrites. The popular explanation for this and otherparadoxes of mantle geochemistry, a hidden layer deep in the mantle enriched in incompatible elements, is inconsistentwith the heat flux carried by mantle plumes. Either the matter from which the Earth formed was not chondritic, or theEarth has lost matter by collisional erosion in the later stages of planet formation.

T

he paradigm that underpins much of modern geochemistry isthat the integrated chemical composition of the whole Earth

should be that of the Sun, except for depletion in volatile ele-ments, according to their volatility under the conditions of the solarnebula. Similar solar-related compositions are found in chondriticmeteorites, which are fragments of small rocky bodies that escapedthe usual course of planetary differentiation into a metallic core, silicatemantleand crust.The composition of a chondritic meteorite is thereforepresumed to reflect its entire parent body. Although the solar composi-tion can be determined from spectroscopic measurements of the solarphotosphere, measurement is not possible or imprecise for many ele-ments, is model-dependent and does not give information on the iso-topic make-up of the elements1. Instead, a more complete picture of thesolar composition is inferred from chemical analyses of chondrites. Thecompositions of the chondrites vary, with at least 27 parent bodiessampled2, reflecting local differences in the solar-nebula-like density

or proportions of gas to solids, or different accretion processes. Thevarious chondrite compositions are distinguished by enrichment ordepletion of refractory elements, ratio of lithophile to siderophile ele-ments (for example, Mg/Fe), oxidation state, oxygen isotopic composi-tions, and their patterns of depletion of the volatile elements. Noexamples with volatile-element enrichment are known, except for aslight enrichment in a few of the least volatile of these elements in thehighly reduced enstatite chondrites. Yet all chondrites share one dis-tinctive compositional feature: their refractory lithophile elements(RLEs) are present in the same ratio relative to each other and to thesolar composition. The RLEs are defined by two properties: they arerefractory, because they condense from a gas of solar composition attemperatures higher than the main constituents of rocky planets, themagnesium silicates and iron metal; and they are lithophile, because

they do not enter metal or sulphide phases, either in chondrites or intothe metallic cores formed during planetary differentiation. There are 28RLEs that are stable or have long half-lives; they include Ca and Alamong the major elements, the entire suite of rare earth elements(REEs), and the radiogenic heat-producing elements U and Th.

The constant RLE ratio rule is ever challenged on several fronts: byexceptions due to terrestrial weathering or ad hoceffects on parentbodies such as impact brecciation, incipient melting or aqueous altera-tion; by the scale of heterogeneity in chondrites relative to the smallsample sizes available for analysis; by improvements in the precision ofchemical analysis; and by theincreasingnumbers of chondriticmeteoritesavailable for analysis. It is therefore difficult to quantify the precision towhich the rule holds, but variations fromthe solar ratios reflectingwhole-body chemistry that are larger than a few per cent are exceptional. The

REE pattern in the RLE-rich CV chondrite Allende is perhaps the largestwell-attesteddeviation3. New techniques of isotopic analysis are revealing

small anomalies in the isotopic make-up of heavy elements in bulksamples of chondrites, ascribed to less than perfect homogenization ofdifferent nucleosynthetic components in the solar nebula, such as Ti(ref. 4), Ni (ref. 5), Ba (refs 6 and 7) and Mo (ref. 8). This evidencechallenges the conceptual basis behind the constant RLE ratio rule, butas yet at no more than the few-per-cent level already accepted. Forexample, although the range in Lu/Hf and Sm/Nd in unequilibratedcarbonaceous, ordinary, and enstatite chondrites is as much as 7.9%and3.5% respectively, theaverage Lu/Hf andSm/Ndvaluesfor these threemain classes of chondrites agree within 1% and 0.3% respectively9.

Although most geochemists long ago abandoned the notion that theEarths composition mimics any particular type of chondrite10, the ideathat the Earth has solar ratios of RLEs has persisted, providing the fun-damental reference frame against which trace element and radiogenic iso-

topic ratios are compared. This reference frame has usually, if confusingly,been termed chondritic rather than solar in the literature, because of thehistory of fine-tuning the presumedsolar composition to the compositionsof chondrites.Emphasis hasbeen placed on theCI chondrites,which matchthe solarcomposition within uncertainty for manyelementsirrespective oftheir chemical properties, except for the most volatile elements; butCIs arerare, and useful analyses come from just three falls, Orgueil, Ivuna andAlais11, and mostly from Orgueil1. The implicitassumption is that thebulkchemical composition of rocky bodies is established at the earliest stages inthe planet-building process, of which the chondrite parent bodies arerelicts. In this view, the subsequent stages by which these small chondriticbodies collide and merge to form planetary embryos and ultimately Earth-sized planets, while resulting in extensive differentiation associated withmelting, does not affect the integrated whole-planet compositions: these, it

is assumed, remain chondritic.

Planetary accretion

The chondritic hypothesis for the Earths composition is a survivorfrom times when the understanding of how terrestrial planets formwas quite different. Current models can be traced back to Safronov,whose monograph on the subject was only translated into English in1974 (ref. 12). Our present understanding is that planetary accretionproceeds through several stages of ever increasing average size. Initially,runaway growth is from kilometre-sized bodies (approximately the sizeof parent bodies of chondrite meteorites) toform planetesimal oligarchsabout a thousand kilometres in diameter. This is the approximate size ofthe differentiated bodies thatare thought to be parental to the achondritemeteorites, like Vesta (diameter 500 km). The achondrite parent bodies

1Research School of Earth Science, Australian National University, Canberra, Australian Capital Territory 0200, Australia.

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all show non-chondritic patternsof volatileloss, for example,in their Mn/Na ratios13. This post-nebular volatile loss can be dated using the Rb/Srchronometer to several million years after the chondrite stage14. The laststages of planetary accretion see the assembly of these planetesimals intoMoon-to-Mars-sized planetary embryos, which then merge throughhighly energetic collisions to form a few terrestrial planets. The giantimpact that formed the Moon is the last of significance in the formationof the Earth. None of this was envisaged when the chondritic hypothesis

was first advanced. Maintaining the hypothesis in its current formrequires that the collisions between bodies during the several stages ofaccretion result in no net fractionation of RLEs, despite the bodies beingalreadydifferentiatedat theearliest stages,whichdoes not seem probable.

Challenge to the chondrite paradigm

Thechallenge to this paradigm started with twolandmarkpapers showingthat the 142Nd/144Nd ratio of chondritic meteorites is 2065 parts permillion lessthanrocks of terrestrial mantle origin15,16. 142Nd is thedaughterof146Sm, with a half-life generally assumed to be 103 million years (Myr)but which could be as short as 68Myr (ref. 17). Sm andNd are two typicalRLEs, and their isotopic relationships provide particularly powerfulconstraints on Earth differentiation models, because the short-lived146Sm/142Nd system is complemented by the long-lived 147Sm/143Nd

system (half-life 106 billion years, Gyr), which has long been usedto constrain the sizes of Earth reservoirs. Although there is somenucleosynthetic variability in the isotopic composition of Nd amongchondrites, this is unlikely to explain the difference in 142Nd/144Nd(ref. 18), which therefore requires the ratio of Sm to Nd to be about 6%above the average chondritic value15,16; because of its isotopic signifi-cance, this value is known to within 0.3% (refs 9 and 19).

Thereare twopossible interpretations15.ThesimplestisthattheEarthisnot chondritic after all, and the measured 142Nd/144Nd ratio of terrestrialsamples is that of the bulk silicate Earth (BSE). Many geochemists haveopted for an alternative scenario, in which the Earths mantle underwentan early fractionation event intoan early-enriched reservoir withlow Sm/Nd and an early-depleted reservoir (EDR) with high Sm/Nd15,16,2022.Boyet andCarlson (refs 15 and16) suggest that thelow-Sm/Nd reservoir

was an incompatible-element-enriched basaltic crust that sank to thecoremantle boundary, becoming isolated in the seismically anomalousregionatthebaseofthemantlecalledD99, an irregular200250-km-thicklayer overlying the core. The seismic properties of D99 make it unlikely tobea simplethermal boundarylayer andit isinterpreted to bea stablelayerwith a density greater than that of the overlyingmantle23,24. Furthermore,because the half-life of 146Sm is no more than 103 Myr, the early frac-tionation event must have occurred well within the first 10 Myr of SolarSystem formation to prevent the average e143Nd value of the comple-mentary EDR rising above about 10, the observed average value of themid-ocean-ridge basalt (MORB)25,26.

The142Nd/144Nd ratios of lunarsamples,however, are indistinguishablefrom terrestrial values2729, so the Moon and the Earth developed their142Nd/144Nd enrichment before the Moons formation30. If the Moonformed by a giant impact, the collision would have melted andhomogenized the Earths mantle31, which would have destroyed anyhypothetical early-enriched reservoir30. Furthermore, the effect of extrac-tion of thecontinental crust from themantle on the SmNdmassbalancecan be modelled, assuming that the BSE has a composition similar to theEDR, without a hidden reservoir30.

The heat carried by mantle plumes provides an equally compellingargument against a hidden early-enriched reservoir32. Most hidden-reservoir advocates15,16,33,34 equate it with D99. I f D99 is the hiddenearly-enriched reservoir, then under the chondritic assumption, a massbalance requires it to contain over 40% of the Earths heat-producingelements (U, Th and K), which produce 9 terawatts (TW) of heat15.Regardless of whether D99is a stable layer or forms a double-diffusiveconvecting layer35, the heat it liberates can only be transmitted throughthe overlying mantle in plumes. The amount of heat transferred by

mantle plumes can be estimated from the dynamic topography they

generate on thesea floor. Early estimates36,37 placed the heat flow carriedby plumes at 3.5 TW, which has been revised38 to 7 TW or 15% of theEarths total heat flow39 of 476 2 TW, to take into account the mantlessubadiabatic thermal gradient. However, models of mantle heattransfer,which takes into account both the heat required to warm subductedlithosphere and the additional heat required to lift compositionallydense plumes, suggest a higher figure38,4042 of 714 TW. From this mustbe subtracted the heat transferred from the core to the mantle, which 41

must be at least 34 TW, the minimum amount of heat required tosustain the geodynamo, but is more likely to lie between 57 TW and1214 TW (ref. 38). As a consequence, if heat transfer from the core tothe mantle is greater than the low estimate of 5 TW or if the heat carriedby mantle plumes is less than the high estimate of 14 TW, 40% of theEarths heat-producing elements cannot be hidden in the D99layer.

A sudden drop in the maximum MgO content of plume-relatedkomatiites and picrites 2.5Gyr ago from 3035 wt% to 1823 wt%,which implies a temperature drop of 200 to 250 uC, has also been usedto argue that D99did not form until the end of the Archaean eon35. Thesimplest explanation for theobserveddrop in MgOis that D99 formed asa stable layer about 2.5 Gyr ago, which insulated the mantle from thecore. The predicted drop in plume temperatures, dependingon assump-tions such as whether D99is a stable layer or formed a double-diffusive

convecting layer, lies within the range 33% to 50%, which is consistentwith theobservedMgO drop35. I f D99 formedafter the first 10Myr of theEarths evolution it cannot be responsible for the Earths 142Nd/144Ndanomaly.

If the composition of the EDR is that of the BSE 20, or if it formedwithin 10 Myr of the Solar System25,26 and is unaffected by subsequentevents, then the e143Nd value for the EDR today30,43 is 7.062.0. Thesevalues are remarkably similar to the prevalent mantle (PREMA) value of76 1 in ocean island basalts (OIBs)44,45. A component with this Ndcharacteristic can be recognized in four flood basalts43 the BaffinIslandWest Greenland province, the Antarctic Karoo, the SiberianTraps and the Deccan Trapsand in two oceanic plateaustheKerguelen and Ontong Java plateaus35. The EDR component plotswithin 150 Myr of the 4.5-Gyr geochron on a 207Pb/204Pb versus206Pb/204Pb diagram, which suggests that it developed early in theEarths history43. It also has enriched 3He/4He, showing that the sourceregion to these plumes is less degassed than the MORB source30,43.

Torsvik et al. suggest that most plume-related basalts that eruptedover the last 320 Myr were above one of two large low-shear-wave-velocity provinces, which form part of D99and make up about 2% ofthe mantles mass46. However, this primitive component in plumes maybe much more extensive35,45. The isotopic trend in a number of oceanisland suites, produced by melting plume tails, converges towards acommon point called FOZO44. A remarkably similar component wasidentified in the two oceanic plateaus that are free from contaminationby continental crust, the Ontong Java and the Caribbean plateaus35,43.Basalts from these plateaus are characterized by flat REE patterns and inthis respect they are dissimilar to OIBs and MORBs but are isotopicallysimilar to the EDR recognized in flood basalts43. Archaean basalts asso-ciated with komatiites, whose Nb/U ratios indicate that they are free ofcrustal contamination4750, commonly have flat REE patterns4850, andbasalts from Kambalda in Western Australia have an e143Nd value of 3(ref. 47), which lies on the EDR growth curve for Nd at 2.7 Gyr ago.Jackson and Carlson43 interpret the EDR component to originate fromthe boundary layer source of plumes, whereas Campbell and Griffith35

suggest that it is lower mantle that was entrained into plumes duringtheir ascent. The first interpretation allows the early-enriched reservoirto be a minor componentin plumes, whereas the secondrequires it to bea dominant component in the lower mantle. Because this difference iscritical to the debate on whether the Earth has chondritic RLE ratios, wenow summarize the basis for the second interpretation35,51.

Plumes must originate from a thermal boundary layer; in the Earth,only the coremantle boundaryhas the properties requiredto sustain the

temperature dropimpliedby plume activityon geologicaltimescales52. A

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newplume hasa large head,which is followedby a smaller tail (Fig.1). Asthe head rises through the mantle it heats the adjacent mantle, loweringits density so that it is swept into the plume head by its recirculatingmotion. The plume head is therefore a mixture of material from the hotboundary-layer source of the plume (dark material in Fig. 1) and coolerentrained material (light material in Fig. 1). This entrained material,which makes up a large fraction of the head, comes from the lower halfofthelowermantle51,52. When a plumeheadreachesthetop of the mantle

it melts to produce a continental flood basalt or an oceanic plateau52,53

.Thefirst material to reach the top of the mantleand undergo decompressionalmelting is the hot mantle that originated from the boundary layer abovethe core (Fig. 1), which melts to produce picrites or komatiites. Later,when the head flattens against the overlying lithosphere, the coolerentrained material from the lower mantle, which is calculated to be threetimes as thick as the hot material from the boundary layer54, enters themelting zone. Production of high-temperature picrites or komatiitesshould therefore be followed by lower-temperature basalts formed bymelting a mixture of boundary layer and entrained lower-mantle withthe latter becoming increasingly important with time as the plume headcontinues to rise and flatten. It is this primitive entrained lower-mantlecomponent that has been identified in the Ontong Java and Caribbeanplateaus35,43,51 and in several flood basalt provinces43.

Lower-mantle material is also entrained along the side of rising plumetails by viscous drag (Fig. 1). As a consequence, when plume tails melt toproduce a chain of ocean islands, which progressivelyincreasein ageawayfrom the current position of the plume,the basalts that make up theislandsmay contain a component from the lower mantle. The existence of FOZOas a component in plume tail basalts is therefore consistent with theinterpretation45 that it was entrained from the lower mantleduring ascentof a plume tail (Fig. 1). Its presence in all or most plume tails providesfurther support that FOZO is a major component in the lower mantle.

FOZO45, EDR15, SCHEM30 (super-chondritic Earth model), andNCPM43 (non-chondritic primitive mantle) are therefore all the same:theprimordial componentof theBSE. Theevidencefromoceanic plateaus,flood basalts and OIBs is that this is a major component in the lower

mantle. The high 3He/4He ratios recognized in the EDR component areconsistent with this interpretation30. The displacement of most oceanicplateaus and flood basalts, with e143Nd between 5 and 9, slightly to theright of the geochron on a plot of207Pb/204Pb against 206Pb/204Pb, showsthat the lower mantle, as sampled by Phanerozoic plumes, includes somerecycled oceanic crust43. Basalts from Malaita Island, part of the OntongJava Plateau, have an average Nb/U of 42, which is between that of theaverage for both the OIB- and MORB-type mantles (47) and the BSE

value of 34, indicating that some continental crust has also been extractedfrom the lower mantle51.The picture of mantle convection that is emerging is one in which the

upper mantle differentiates into harzburgite and basalt at mid-oceanridges, which are subducted to the thermal boundary layer above thecore and returned to the upper mantle in plumes, mixed in varyingproportions, with perhaps a small sedimentary component. It appearsthat the bulk of the lower mantle is largely bypassed by this process, sothat it is less degassed, has had less subducted basalt mixed into it, andless continental crust extracted from it, compared to OIB- and MORB-source mantles.

Volatile elements

Although the comparison of Earths composition with chondrites has

focused on the RLEs, the differences in the abundances of the moderatelyvolatile elements between the Earth and chondrites provide another lineof evidence that chemical fractionations, which occurred in the latterstages of accretion, moved the final compositions of terrestrial planetsaway from the compositions encompassed by chondritic meteorites13,14.The moderately volatile elements are those calculated to condense fromthe solar nebula after magnesian silicates and Fe metal, but at tempera-tures abovethe ice-formingelements(H, C, N andthe noble gases).Manymoderately volatile elements arealso siderophileand were depleted in theBSE by partitioning into the core, making their bulk Earth abundancesinaccessible, but among the lithophile elements are the alkali metals, thehalogens and boron. For the other potentially siderophile elements likeZn and In, their BSE abundances provide useful constraintson minimumEarth abundances.The patternof depletion of themoderately siderophile

elements is unlike that found in any chondrite group (Fig. 2). In thechondrites, depletion correlates with calculated condensation tempera-tures,but in the Earth, elements that arevery highly incompatibleduringpartial melting, namely the heavy halogens and Cs, are much moredepleted than the nearly compatible Zn and In. This is highlighted byZn/Cl ratios, which are an order of magnitude greater in the BSE than inany chondrite group (Fig. 2). The dependence of volatility on incom-patibility suggests volatilization from early-formed crusts during thelatter stages of accretion13. In the case of the halogens, their volatilitiesare enhanced by relatively oxidizing conditions that prevailed after dis-persion of the H-rich solar nebula. Post-nebular oxidizing conditions arereflected in the ubiquitous fractionation of Na from Mn in small-differentiated planetary bodies13, although this particular fractionationis not seen in the nearly chondritic Na/Mn ratios of the BSE, perhapsbecause some Mn has also been partitioned into the core. Attempts todefine simple volatile-element depletion trends in the BSE have invari-ably omitted key elements from consideration, like the heavy halogens.

For elements that are both volatile and siderophile, the complexitiesof the volatile-element depletion in the Earth, as indicated by the fullpattern (Fig. 2), prevents us from assigning how much of an elementsloss is dueto volatility andhow much is dueto partitioninginto the core.This is particularly vexing for Pb. Was the extent of Pb depletion byvolatility in the material that formed the Earth like that of Zn and In, orwas it like Cl and the other heavy halogens, or was it somewhere inbetween? On the 207Pb/204Pb versus 206Pb/204Pb diagram, it is wellestablished that both the mantle and crust of the Earth plot well to theright of the 4.57-Gyr geochron, which, neglecting the hidden reservoirexplanations, is usually taken to imply loss of Pb relative to parental Usometime later, at about 4.45 Gyr ago55. Because feasible hypotheses can

be constructed to argue for Pb loss by both mechanisms at several stages

Hot mantle from

boundary-layer source

(OIB-type mantle)

Entrained

lower mantle

Hot mantle from

boundary-layer source

(OIB-type mantle)

Figure 1| Laboratory model of a mantle plume. The dark-coloured fluid isfrom the hot boundary-layer source of the plume, whereas the light material iscooleroverlyingfluidthat wasentrained into therising plume.In thecase of themantle the dark-coloured material is from the thermal boundary layer abovethecore,whereas thelightmaterial is entrainedlowermantle (afterref.54). Theentrained material makes up a large fraction of the plume head54 and comesmainlyfrom near thebottom of thelowermantle butabovethe boundary-layersource of the plume. The critical parameter in the present context is thethickness of the upper entrained layer (light material near the top of the plumehead), which is about three times the thickness of layer at the top of the head

(dark material) that originates from the boundary-layer source of the plume54.

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of the planet-building process, the significance of this age informationremains ambiguous.

Likewise, theinterpretation of other early-Earth geochronometers is alsoaffected if theEarth has a non-chondriticcomposition. For example,short-lived 182Hf (half-life 9 Myr) decays to 182W, providing a chronometer with

which to constrain the duration of core formation, because W is amoderately siderophile element that partitioned incompletely into thecore. Evaluating the time significance of this chronometer for the Earthdepends on knowing the W/Hf ratio in the BSE56. The W content of theBSE has been estimated by noting that W/Th (or W/Ba) ratios remainconstant in igneous processes, hence W/Hf5 (W/Th)3 (Th/Hf).Because Hf and Th (or Ba) are both RLEs, it is then assumed that theirratio is the same as in chondrites. But the non-chondritic Earth model ofref. 13 predicts that Th/Hf would be only around 70% of the chondriticratio; for a simple two-stage model of coreformation, thiswould increasethe calculated time from about 30 to about 35 Myr.

Alternative hypotheses

There are two classes of explanation for the Earth not being chondritic.

Most simply, the compositions of chondrites may not reflect that of the

Solar System preciselyenough to deduce detailed element ratios for RLE.It needs to be remembered,however,that theaverage Sm/Nd ratio of thethree main classes of chondrites agrees to within 0.3%.

Alternatively, the Earth could have been assembled from initiallychondritic material that was then modified during the subsequent stagesof the planet-building process by collisional erosion13,57,58. Current esti-mates of the Earths Fe/Mg ratio are consistent with about 10% of itssilicate part having been lost by this mechanism relative to its Fe-rich

metallic core. The meteorite record attests to the differentiation of smallrocky bodies into metal and silicate being inevitably associated withpartial melting and hence also the formation of an incompatible-element-enriched crust. If material from these crusts were preferentiallylost during the collisions, it would deplete the Earth systematically inincompatible elements according to their incompatibility (Fig. 3). Oneweakness of the hypothesis is that it implies the loss of incompatible-element-enriched material to space that no class of meteorite hassampled. However, no meteorites sample the moderately volatile ele-ments missing from all chondrites (apart from the CIs) and from theachondrites andterrestrial planets. Didthe gravitation field of theSun orJupiter capture this missing material?

The pattern of depletion caused by preferential collisional erosion isgeochemical rather than cosmochemical, and its effect on the composi-

tion of the Earths mantle is essentially the same as that which subse-quently occurred throughout the Earths history by crust formation.Detecting the effects of collisional erosion therefore depends on obser-vations that can sum all the reservoirs in the BSE to see whether they addup to chondritic ratios of RLEs. The Sm/Nd ratio provides the mostcompelling evidence, but once the idea of a non-chondritic Earth isallowed, the resolution of other so-called geochemical paradoxesbecomes achievable.

Geochemists are fascinated by the many paradoxes of the Earths man-tle,which aresummarized in Table 1. Allof these paradoxesare predicatedon geochemistrys most fundamental paradigm; that the Earth was pro-duced by the accretion of meteorites with the same ratios of RLEs as inchondrites. Most of these paradoxes disappear if this assumption isrelaxed, but one existing paradox becomes worse: the low value of the

Earths Urey ratio. The Urey ratio is the Earths radiogenic heat produc-tion dividedby its surface heat flux, whichunderthe assumption of BSEU, Th and K concentrations given by the chondritic hypothesisis about0.5. The difference must be accounted for by secular cooling. Collisionalerosion lowers the heat-producing element content of the BSE by up to

Mantle/crust partition coefcient

U

La

Sm

HfNd

Th

Lu

Ca Al

104 1103 102 1010.4

0.5

0.6

0.7

0.8

0.9

1.0

Ba

Norm

alizeddepletion

Figure 3| Depletion of some RLEs in the BSE by preferential collisionalerosion of early-formedbasaltic crust during accretion of planetesimals andplanetary embryos.The figure is based on themodelof ref. 13,assumingthreeconstraints: (1) loss of silicate (crusts plus mantles) relative to metallic cores is10%; (2) the most incompatible RLEs (here represented by Ba) are depleted to50% of their chondritic abundance; and (3) Sm/Nd is 6% above the chondriticratio. The partition coefficients during crust formation are from ref. 67 for the

production of oceanic crust.

Pb

0.001

0.01

0.1

1

400 600 800 1,000 1,200

CV

EH

50% condensation temperature (K)

10

100

1,000

0.5 1 1.5 2 2.5 3

Zn/Mg

Cd

Cl

In

RbB

Li

MnNa K

I

Zn

Cs

Br

F

?

Chondrites

BSE

Zn/Br

BSE/Mg

Zn/Mg

Figure 2| The pattern of volatileelement depletionin the BSE for lithophileelements compared to CV carbonaceous chondrites and EH enstatitechondrites. CV carbonaceous chondrites are the most volatile-depleted of thechondrites and EH enstatite chondrites are a class of chondrites sometimesconsidered to have affinities to the Earth because of their stable isotope ratios.Elements are normalized to CI abundances and Mg. The calculated 50%condensation temperatures of elements for the solar nebula are from ref. 10.Someelementsin theBSE mayhave been additionallydepleted by coreformation(forexample, Zn,In andPb), in which casetheir depletionsdue to volatilityalonewill be overestimated. The chondrites form smooth depletion trends withcalculated condensation temperatures, but the BSE is not only more depleted inmoderately volatile elements than any known chondrite, but the pattern ofdepletion is qualitatively different, probably owing to post-nebula volatile lossundermore oxidizing conditions13. This isshown, forexample,by theBSEZn/Br

ratios (inset), which are about an order of magnitude greater than that found inany class of unmetamorphosed chondrites (black circles). The lack of any clearvolatility trendin the BSE means that it is not at present possible to constrain howmuch Pb, if any,was partitionedinto the Earths core, making the interpretationof Pb isotopic systematics in terms of Earth accretion and core formationuncertain. Meteorite abundances are fromref. 66, and BSE abundances fromref.13, which was derivedunder the chondritic assumption. A non-chondritic BSEwould havelowerabundances ofB, K,Rb,Cs, Cl, BrandI but not Mn, Na, F,Znor In (ref. 12), increasing the discrepancy with the chondritic trends.


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half13, which halves the already low Urey ratio, implying unlikely coolingrates extrapolated over geological time. Perhaps the Earth is currently in aphase of abnormally fast ocean crust formation and subduction59.

It is apparent that the only reliable way of determining the composi-tion of the Earth is by sampling the Earth itself. As argued in this study,the heads of mantle plumes entrain primitive lower mantle. By studyingbasalts produced by melting this material, especially Archaean basaltsassociated with komatiites, provided they are not affected by crustalcontamination, we are sampling basalts derived from the Earths earliestand most primitive mantle. It may also be possible to obtain the inte-grated BSE composition of two of the RLEs most susceptible to col-lisional erosionTh and Ufrom the geoneutrino flux60. It wouldthen be possible to see whether the ratios of these elements with otherRLEs, such as Ca and Al, were indeed within the range found in chon-dritic meteorites or that predicted by collisional erosion.

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Table 1| The geochemical paradoxes of the mantle

Paradox Chondritic solution Non-chondritic solution

The 142Nd/144Nd ratio of chondritic meteorites is2065 parts per millionlessthan that of rocks ofterrestrial mantle origin.

A low-Sm/Nd hidden reservoir became isolated from theconvectingmantle within 10Myr of the Earthsformation15,61.

TheSm/Ndratioof theprimitive Earth wasabout6% above the chondritic value13.

Earths oldest rocks show evidence of being derivedfrom a mantle with positive eNdand eHf before theformation of the first preserved continental crust.

Extensive continental crust formed before the first preservedcontinental crust and was recycled through the mantle62 orthere is a hidden basaltic low-Sm/Nd reservoir15.

TheSm/Ndratioof theprimitive Earth wasabout6% above the chondritic value13.

The Ar concentration in the mantle is abouthalfthe value predicted from the chondritic model63.

Only half of the mantle is degassed63. The collisional erosion hypothesis13 predicts a Kcontent of the mantle appreciably below thatexpected from the chondritic model.Alternatively, the Earth is not chondritic16.

Nb/Ta and Nb/La values of both continentalcrustand depleted mantle lie below (Nb/Ta) and above(Nb/La) the primitive mantle values of 17.5 forNb/Taand 0.9 for Nb/La.

Hidden reservoir enriched in Nb, Ta and Nb with super-chondritic Nb/Ta and sub-chondritic Nb/La64.

The Nb/Ta and La/Nb values of the primitivemantle lie between those of the depleted mantle(15.5 and 1.2) and the continental crust (12.5and 2.2).

4He production in oceans is less than thatpredicted from observed heat flow and abouthalf that predicted from chondritic Earth model.

4He stored in lower mantle that is separated by a boundarylayer that transmits heat but not 4He (ref. 65).

Collisional erosion model predicts the ThUcontent of theBSE to be about half thechondriticvalue13.

PERSPECTIVE RESEARCH

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Acknowledgements We thankY. Amelinand R. Taylor fortheir comments on an earlydraftof the paper, C. Allenfor preparing Fig. 1 andR. Carlson andB. Bourdon fortheirdetailed and constructive reviews.

Author ContributionsBoth authors contributed equally to the manuscript.

Author InformationReprints and permissions information is available atwww.nature.com/reprints. The authors declare no competing financial interests.Readers are welcome to comment on the online version of this article atwww.nature.com/nature. Correspondence and requests for materials should beaddressed to I.H.C. ([email protected]).


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