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Origin of Ocean Island Basalts: a New Model Based on Lead and Helium Isotope Systematics

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Origin of Ocean Island Basalts: a New Model Based on Lead and Helium Isotope Systematics JOURNALOF GEOPHYSICALRESEARCH, VOL. 104,NO. B11,PAGES 25,479-25,491, NOVEMBER 10, 1999 Origin of ocean island basalts: A new model based on lead and helium isotope systematics Balz S. Kamber and Kenneth D. Collerson Departmentof Earth Sciences,University of Queensland,Brisbane, Australia Abstract. Currentmodels of oceanisland basalt (OIB) Pb isotopesystematics based on long- term isolationof recycledoceanic crust (with or withoutsediment) are not supportedby solutions to bothterrestrial Pb paradoxes.It followsthat the lineararrays of OIB datain Pb isotopediagrams are mixing lines andhave no age significance.A new modelis presentedthat takesinto account currentsolutions to bothterrestrial Pb paradoxesand that explains combined Pb andHe isotope evidencein termsof binarymixing. The key featureof thismodel is a two-stageevolution: first, long-termseparation of depletedmantle from undepletedlowermost lower mantle. Mixing between thesetwo reservoirs results in thewide spread in 2ø7pb/2ø4pbratios and generally high (but variable)3He/4He ratios that typify enriched mantle 1 (EM1)OIBs. The second stage involves metasomatismof depletedupper mantle by EM1 type,lowermost mantle-derived melts. Evolution in the metasomatizedenvironment is characterizedby variablebut generallyhigh (Th+U)/(Pb+He) ratiothat leads to a rapidincrease in 2ø8pb/2ø4pband2ø6pb/2ø4pb ratios and decrease in 3He/4He. Mixing betweendepleted mantle and melts from metasomatizedmantle portions reproduces the characteristicsof high g (HIMU) OIBs. The Sr versusNd isotopearray is compatiblewith binary mixingbetween depleted mantle and near-chondritic lowermost mantle because of the large variationin Sr/Nd ratiosobserved in EM1 andHIMU OIBs. OIBs contaminatedby subcontinental lithosphericmantle (EM2) exhibitmore complex isotope systematics that mask their primary geochemicalevolution. 1. Introduction important to note that SCLM can remain isolated from the convective astenospherefor billions of years and hence The vast majority of oceanisland basalts(OIB) are not only evolveto isotopicsignatures that deviatestrongly from those enrichedin incompatibletrace elementsbut also carry isotopic of thedepleted mantle. SCLM can extend past the present-day signatures interpreted to reflect evolution in enriched mantle shorelineof continents,and OIBs that erupt in theirproximity (variable but high (U+Th)/Pb, high Rb/Sr, low Sm/Nd, and low may be contaminated. Furthermore, subcontinental Lu/Hf). A plethora of models have been proposedto explain lithosphericmantle can be entrainedinto the convecting these features, but no consensushas been reached in spite of oceanic astenosphereupon continentalbreakup. The more than three decades of geochemical research into OIBs. geodynamicsignificance of a SCLMOIB sourcecomponent is Among the debated issues two questionsstand out: first, can thereforevery different from that of recycledcomponents. the isotopic signaturesof OIBs be interpreted to reflect the The claim for the existenceof (subducted)recycled existence of chemically distinct domains in the mantle that componentsthat have remainedconvectively isolated over escaped convective homogenization; and second, if such billionsof yearsis problematicif these domains are envisaged domains exist, where in the mantle are they located, what is to residein the depletedmantle. One very strongargument their origin, and how have they withstoodhomogenization? against the persistence of such domains is the second One popularview is basedon the work by Zindler and Hart terrestrialPb paradox.It is now firmly establishedthat the [1986], who proposed that chemically distinct domains do seculardecrease in Th/U ratioof the depletedmantle reflects exist in the mantle and that over time such domains would increasedrecycling of cootinent-derivedU (and Pb) backinto evolve characteristicisotopic signatures.Apart from the mid- the mantleafter establishmentof a pandemicoxidizing ocean ridge basalt (MORB)-source-depleted mantle, the hydrosphereand atmosphere at circa 2.2. Ga [e.g.Kramers and following mantle reservoirshave been suggestedto contribute Tolstikhin,1997; Collerson and Kamber,1999; Elliott et al., to OIB magmatism: (1) recycled oceanic lithosphere, both 1999].On the basisof the homogeneityof the Pb isotope- ancient(up to circa 2 Ga) and young; (2) recycled sediment;(3) inferredTh/U ratio and the observedNb/Th and Nb/U ratiosin subcontinental lithospheric mantle (SCLM); and (4) an MORB it hasto be concludedthat recycled components are enigmatic "plume component" probably derived from the well mixed into the MORB sourcemantle and that substantial lower mantle. chemicaland isotopic heterogeneity in the depleted mantle is There is now strongisotopic and trace element geochemical relativelyshort-lived [Collerson and Kamber,1999]. The evidence that SCLM is a component in some OIBs. It is rangein 2ø7pb/2øapbratios of OIBs(excluding those with a SCLM component)requires, however, that sourcesremained Copyright1999 by theAmerican Geophysical Union. isolatedfor billionsof years.The consensusthat has been reachedon the significance of the second terrestrial Pb paradox Papernumber 1999JB00258. thereforenecessitates a reevaluation of OIB Pb isotope 0148-0227/99/1999JB00258509.00 systematics. 25,479 25,480 KAMBER AND COLLERSON:ORIGIN OF OCEAN ISLAND BASALTS An independenttest of OIB Pb isotopemodels is possible ratio.It is concludedthat OIBs are a mixtureof onlytwo long- by combinedconsideration of Pb and He isotopesystematics lived components,one beingthe undegasseddeepest portion since these are coupled through their parent isotopes.At of the lower mantleand the otherbeing the depletedMORB present, the He isotope characteristicsof those OIBs with source mantle. We discussimplications of this model for radiogenicPb can only be explainedby opensystem behavior chemicalevolution of the mantleand convection dynamics. [Hanyu and Kaneoka,1998]. In thispaper we presenta reviewof OIB Pb andHe isotope 2. Selection of OIB Database systematicsand show that they requirea two-stageevolution. First, long-termseparation of primitive undegassedlowermost Like continentalcrust, its underlyingSCLM root remains mantlefrom depletedmantle leads to a significantdifference in isolatedfrom the astenospherefor substantialperiods of time the 2ø7pb/2ø4pbratio. Second,short-term modification of Pb and plays a key role in stabilizingand preservingcontinental and He isotope systematics in localized, metasomatized crust. Even though SCLM may be entrained into the portions of the upper mantle, characterizedby variable but astenosphereby delamination,this processhas no bearingon high (Th+U)/(Pb+He) ratio that leads to an increase in the Pb isotopecomposition of the oceanicmantle [Kramers 2ø8pb/2ø4pband 2ø6pb/2ø4pbratios and a decreasein 3He/4He and Tolstikhin,1997]. The effectof SCLM on OIB isotopeand trace elementgeochemistry is contaminationduring magma ascent.SCLM is thus not a sourcecomponent in the stricter sense,and we have excludedfrom the global OIB databaseall SCLM-contaminatedocean islands. Using the interpretationof isotopic and trace element data by original authorswe have omitted data from the following ocean islands: Canaries [Elliott, 1991; Hoernle and Schmincke, 1993]; Comores [Spath et al., 1996]; Heard [Barling et al., 1994]; and Inaccessible [Cliff et al., 1991]. The distinctionbetween subducted sediment component and SCLM is difficult. Figure la shows that the omitted OIBs define a field at high Th/Yb ratios for a given Nb/Yb ratio which extends over almost the entire range of enrichment indicated by the Nb/Yb ratio. This could either reflect i , i i I i ] , , i i , i , i , , , , i , , , i I i i i i contaminationwith sedimentor SCLM. Accordingto Hilton et .... i ' ' ' ' i ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' al. [1995] the HeardIsland OIBs werecontaminated with 4He at shallowlevels due to interactionwith buoyantSCLM. They arguedthat otherocean islands, whose sources were previously consideredto carry the signatureof subductedsediment, may only havebeen contaminated at very shallowlevels by SCLM. Theseconclusions have recently been supported by Os isotope :3GLOSS data from Kerguelen [Hassler and Shimizu, 1998] and the Canaries [Widom et al., 1999]. In fact, many oceanislands (Comores,Cape Verde, and CanaryIslands) that occurclose to the African continenthave geochemicalcharacteristics which can be interpretedto reflect involvementof SCLM [Sp•ithet al., 1996].As neitherAfrica nor Antarcticaare surroundedby 0 10 20 30 40 50 60 subductionzones which could inject sedimentinto the mantle, Nb/Yb the SCLM signatureis likely to be due to contaminationby remnantsof sub-Gondwanalithosphere. For these reasonswe Figure 1. Th/Yb versus Nb/Yb plot for global ocean island excludethe abovementioned data from our investigation. basalt (OIB) dataset. All samples have MgO > 6 wt % to minimize effects of fractional crystallization. (a) OIBs with subcontinental lithospheric mantle (SCLM) contamination 3. Pb Isotope Characteristicsof OIBs (solid diamonds)are Canaries[Elliott, 1991], Comores[Spath et al., 1996], Heard [Barling et al., 1994], and Inaccessible Pb isotope systematicsof OIBs show two prominent [Cliff et al., 1991]. On average, these clearly plot at high featuresin commonPb space(Figure 2a). First, all OIBs plot Th/Yb values for a given Nb/Yb ratio, irrespective of the to the right of the meteoritic isochron. Second,OIBs define a degree of melting. (b) Data sourcesfor OIBs without SCLM linear array that is emphasized by data from individual
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