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Role of the Kerguelen Plume in generating the eastern Indian Ocean seafloor. Weis, Dominique 1996

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. B6, PAGES 13,831-13,849, JUNE 10, 1996  Role of the Kerguelen Plume in generating the eastern  Indian  Ocean seafloor  DominiqueWeis D6partementdes Sciencesde la Terre et de l'Environnement,Universit6Libre de Bruxelles Brussels,Belgium  FrederickA. Frey Departmentof Earth, Atmosphericand PlanetarySciences Massachusetts Instituteof Technology,Cambridge  Abstract. Mid-oceanridgebasalts0VIORB)in the IndianOceanhave Sr-Nd-Pbisotopic characteristics thatdistinguishthemfrom seafloorbasaltsin the AtlanticandPacificOceans. Thesedifferenceshaveimportantimplicationsfor mantledynamics.We discussthe isotopic variationwith eruptionageof seafloorbasaltsrecoveredby deepseadrillingat 10 sitesin the easternIndian Oceanrangingin agefrom Eoceneto Late Jurassic.Exceptfor alkalicbasalts recovered from near Christmas Island in the northeastIndian Ocean, the basementlavas are  tholeiiticbasaltsthatare characterized by a wide rangein incompatibleelementabundance ratios, suchasLa/Yb andZr/Nb. Most of the tholeiiticbasaltsfrom sevensitesare geochemicallysimilar to recent Indian Ocean MORB, but the alkalic basaltsand tholeiitic lavas from two other siteshave  isotopicand incompatibleelementabundance ratiossimilarto lavasassociated with the Kerguelen Plume. Two of thesethreesites,however,are not closeto the trackof thisplume. The Dupal  isotopic signature (relatively high87Sr/86Sr andhigh208pb/204pb atagiven 206pb/204pb) is characteristic of lavasthathavebeenattributedto the KerguelenPlume,i.e., the Kerguelen Archipelago,NinetyeastRidge,andKerguelenPlateau.AmongeasternIndianOceanseafloor basalts,a Dupal componentis apparentin basementlavasfrom six of the sevendrill sitesin the easternIndian Oceanthatrangein inferredagefrom -57 to 125 Ma. The oldest(-155 Ma) seafloor lavas recovered from the Indian Ocean, derived , A• from , • a spreading center intheArgo  Abyssal Plain near northwest Australia, have highx,•Nd/ß'raNd andlow87Sr/86Sr similar tothe mostdepletedrecentIndianMORB. Becausethe oldestvolcanismon the KerguelenPlateau (-118 Ma) is the firstevidenceof theactivityof theKerguelenPlume,thisplumeis inferredto be the sourceof Dupal isotopiccharacteristics in IndianOceanMORBs. SomerecentIndianOcean  MORBarealsodistinctive because many have relatively low206pb/204pb (<17.4). Some ofthe oldest(110 to 155 Ma) seafloorlavasin theeasternIndianOceanalsohaverelativelylow  206pb/204pb ratios. Thislow206pb/204pb signature predates volcanism associated withthe KerguelenPlumeandmay reflecta significantrole for continentallithosphereasa long-term sourcecomponentfor IndianOceanMORB. Introduction  Basaltseruptedfrom active spreadingridge axesin the Indian Ocean define fields in Sr-Pb, Nd-Pb, and Pb-Pb isotopicspace that are distinct from the fields of mid-ocean ridge basalts (MORB) erupted in the Atlantic and Pacific Oceans [e.g., Subbaraoand Hedge, 1973;DuprdandAll•gre, 1983;Hamelinet a/., 1985/1986; Michard et al., 1986; Price et al., 1986; Ito et al.,  1987; Dosso et al., 1988; Mahoney et al., 1992]. These differencesrequire that the basalticIndian Oceancrustis derived from mantle  sources that are unlike  the sources of Atlantic  and  Pacific MORB. Hart [ 1988] proposedthat oceanicislandbasalts (OIB) in the Indian Ocean are an important part of a large distinctive mantle isotopic domain (Dupal anomaly) that is  centered at -30øSandis defined by 87Sr/86Sr >0.705and relatively high208pb/204pb at a given206pb/204pb. The distinctiveisotopiccharacteristics of Indian OceanMORB have been attributedto the influenceof Dupal componentsfrom the KerguelenPlume [Hatnelin et al., 1985/1986;Dossoet al., 1988; Storeyet al., 1989], perhapswith contributions from the Crozet and Marion plumes [Mahoney et al., 1992]. An alternative explanationfor the distinctiveisotopiccharacteristics of Indian OceanMORB, which is not mutuallyexclusive[Weis, 1992], is  thatancientGondwanaland continental lithosphere wasdispersed andincorporated into theIndianOceanMORB sourceduringthe breakupof Gondwanaland[Mahoneyet al., 1989, 1992]. The geochemicalcharacteristicsof Indian Ocean seaflooras a functionof eruptionage are importantin evaluatingalternative interpretations for the distinctivegeochemicalfeaturesof recent  IndianOceanMORB. The Deep SeaDrilling Project(DSDP) Copyright1996by the AmericanGeophysical Union.  andOceanDrilling Program(ODP) recoveredbasaltsof variable age, up to 155 Ma, from severalsiteswithin the easternIndian  Papernumber96JB00410.  Ocean.  0148-0227/96/96JB-00410509.00  roles of componentsderived from depleted mantle, mantle 13,831  We studied these basalts in order to assess the relative  13,832  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR  plumes, and continentallithospherein the sourcesof basalt at each DSDP site and to determinehow the proportionsof these components changedwith eruptionage. In thispaperwe focuson 10 sites that are not on the NinetyeastRidge; Prey and Weis [1995] focuson the NinetyeastRidge.  ResearchApproach Excludingthe NinetyeastRidge, the easternIndian Oceanwas sampled at nine DSDP sites and one ODP site (Figure 1). Petrographicand geochemicalcharacteristicsof basaltsfrom theseDSDP siteswere reportedin the initial reportsfor DSDP Legs 22, 26, and 27 [von der Borch et ed., 1974; Davies et ed., 1974; Veeverset ed.,1974], and a summarywasgivenby Prey et al. [1977]. However, these previous studiesdid not include isotopicdatafor St, Nd, andPb or preciseabundancedatafor the incompatibletraceelements,Rb, Ba, Nb, St, Zr, Hf, andY. This paper focuseson the isotopic ratios and abundanceratios of highly incompatibleelementsof easternIndian Ocean seafloor basaltsbecausetheseratiosare sensitivemeasuresof geochemical heterogeneityin the oceanicmantle;e.g., they distinguishMORB from OIB [e.g., Weaver, 1991; Hart et al., 1992]. Old ocean floor rocks have been affectedby postmagmaticalteration;thus abundancedata for Y, Zr, Nb, and rare earth elements(REE) are  important because theseelements arerelativelyimmobileduring postmagmatic alterationon the seafloor[Bienvenuet ed., 1990]. In our discussion, we use"depleted" to refer to basaltsthathave Rb/Sr,  Nb/Zr, La/Yb, Ce/Y and Nd/Sm ratios less than the  estimated bulkearthratios[e.g.,SunandMcDonough, 1989];that is, thesebasalts(andtheirmantlesources) arerelativelydepleted in the highly incompatibleelements,Rb, Nb, La, Ce, and Nd.  With time these depleted sources develo• 143Nd/144Nd greater thanthebulkearthestimate and87sr/g6srlessthanthebulk earth estimate. Conversely,relative to bulk earth, enriched basaltshave higher Rb/Sr, Nb/Zr, La/Yb, Ce/Y, Nd/Sm, and  87Sr/86Sr butlower143Nd/144Nd.  Analytical Techniques A subsetof previouslyanalyzed sampleswas selectedto encompass themajorelementcompositional rangeof theseocean floor lavas [Frey et ed., 1977]. Trace elementabundances were determinedby X ray fluorescenceand instrumentalneutron activation analysis (Table 1). Analytical procedures,and evaluation of dataaccuracy andprecisionaregivenby Preyet ed. [1991]. Trace element analyseswere done on unleached powders,but for isotopicanalysis,the sampleswere leachedin  acidto removesecondary alterationphases.We useda leaching  lOON  ß  217  218 758  SINGAPORE  717 0o  71•I 719  211 .  10 ø  h/HAlSTON  BAS/No 20"  255  752,  , 257  FREHANTLE  !  264  90 ø  100 ø  110ø  120 ø  Figure 1. locationmapfor theeasternIndianOceanshowingselected magneticanomalies, andmajorbathymetric featuressuchasthe NinetyeastandBrokenRidges. DSDP andODP drill sitesareindicatedas solidcircles.The 10 seafloorsitesdiscussed in thispaperareshownby largernumbers.  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR  procedurecomparable to Mahoney's [1987], i.e., "cold" acid leaching(HC1 6 N), with eliminationof the finesby removingthe acid immediately after 30 min in an ultrasonicbath (the time period of 30 rain is critical as it allows for a slight increasein  13,833  change in87Sr/86Sr, 143Nd/144Nd ' 206pb/204 Pb,207pb/204pb,  and208pb/204pb. Forexample, in120Myra238U/204pb = 20 (a relatively high value for unalteredoceanicbasalts[White,  1993])creates a change in 206pb/204pb of only0.38. In  addition,tholeiitic basaltsare commonlyinterpretedto result Assuming This leachingprocedurewas repeateduntil a colorlesssolution from relatively high extents of melting. was obtained[Weis and Frey, 1991], up to 10 stepsfor the most parent/daughter abundance ratiosarenot stronglyaffectedby the alteredsamples.After acid leaching,the remainingpowderwas partial melting process,the isotopicratios in the sourcesand rinsed with quartz-distilled water at least three times. The tholeiitic lavasevolvesimilarlywithtime. Therefore Mahoney powder was dried on a hot plate until a constantweight was andSpencer[1991]concluded thatover100Myr, agecorrections achieved. The difference between this final weight and the are relatively small, especially when compared to isotopic startingweightis theweightpercentlosscausedby acidleaching; differencesamongOIB, MORB, andoceanicplateaubasalts. typically, this was between50 and 60%. The large weight loss However, postmagmaticalteration processesincrease the temperature, whichis necessary to strengthen theleachingeffect).  reflects dissolutionof mineralsformed during low-temperature complexity of inferring magmatic isotoperatios of old altered alteration. seafloor lavas, becausepostmagmaticalteration may affect The sampleswere processedfollowing standardchemical isotopicratiosandparent/daughter abundance ratios. Typically, separationprocedures(i.e., HF-HC104 dissolutionand anion the isotopic ratios of Nd and Pb in oceanic basalts are not exchangecolumn separationof the different isotopesfollowing significantly changed during postmagmatic alteration, but the methoddescribedby Weiset al. [1987]). The blanksfor the formation ofsecondary phases withrelatively high87Sr/86Sr is columnswere below 3 ng Sr, and the total blanksfor the whole common[e.g., Mahoney, 1987; Weis and Frey, 1991; Mahoney procedurewere below 6 ng for Sr andbelow2 ng for Nd. These and Spencer,1991; Staudigelet al., 1995]. As discussed earlier, blanksare negligiblerelativeto the concentrations in the samples. inaneffort toremove such secondary phases, thesampl,e.,powders For Pb isotopes,the sampleswere processedin a clean, over were repeatedly acid-leached before determination of8'Sr/86Sr. pressurized(>3 mm Hg) laboratory,usingreagentspurified in a Although mostofthesamples define aninverse 143Nd/144Ndoceanicbasalts subboilingstill. Pb was separatedon artionexchangecolumnsin 87Sr/86Srtrendsimilarto that of unaltered a ItBr-HC1 medium,following a methodderivedfrom Manhdset (Figure2), we cannotbe certainthattheseprocedures completely a/. [1978]. Pb and U concentrationswere measuredon the same removeall effectsof postmagmatic alteration. In contrast to unaltered tholeiitic basalts, corrections for samplesolution (aliquots were split before loading on columns andsp'iked with a 235U- 206Pb mixed spike). U was separatedin radiogenicgrowthaftereruptionmay be relativelylargein highly a HNO3 medium. Total blank values for Pb for the whole alteredlavas.This is especiallytrue for Sr andPb isotopicratios, chemicalprocedurewere typicallybelow 1 ng. becauseRb/Sr and U/Pb ratios may be significantlychanged Sr isotopiccompositions were measuredon singleTa filaments during alteration [e.g., Staudigel et al., 1995]. If alteration in the dynamicmodeon a VG Sector54 massspectrometer.The occurredsoonafter eruption,thenover 100 Myr, themeasuredSr internal precision formeasured 87Sr/86Sr isbetter than1x 10'5. and Pb isotopicratiosmay differ considerablyfrom thoseof the Themeasured values arenormalized to 86Sr/88Sr = 0.1194.For unaltered lavas at the time of eruption. In contrast,neither eachbarrelof 20 filaments,four analysesof bIBS 987 Sr standard Sm/Nd nor143Nd/144Nd in ocean floorbasalts areusually were made. The averageof 87Sr/86Sr over the time period the significantlychangedby postmagmaticalteration. In addition, DSDP analyseswere made is 0.710232q-8 (2c•m for 18). An thelong half-life of 147Sm andlowSm/Nd leadtoarelatively evaluationof between-runprecisionis alsogivenby the replicate small agecorrection. analysesreportedin Table 2. Nd isotopiccompositionswere For each DSDP site studied,Figure 2a showsfields for the measuredon triple Ta-Re filaments with the VG Sector 54 present-day 87Sr/86Sr and143Nd/144Nd values measured onthe multicollector mass spectrometer(analysesof the Merck Nd acid-leachedresidues.It also showsthe age-correctedvalues  standard 143Nd/144Nd= 0.51173 + calculated yielded _ 1and145Nd/144Nd  = 0.348417q-5 (2c•m for 12)). Nd wasrun as a metal, andfor eachrun the 146, 145, 144, and 143 isotopeswere measuredwith  all values normalized to 146Nd/144Nd = 0.7219. Pb isotopiccompositionsand Pb and U concentrationsby the isotopedilution (ID) techniquewere measuredon single Re filamentswith a FinniganMAT 260 massspectrometer, usingthe H3PO4-silicagel technique[e.g.,Cameroneta/., 1969]. All the resultswerecorrectedfor massfractionation(0.13% q-0.00% per ainu) on the basisof 72 analysesof the BIBS 981 Pb standard [Catanzaroet al., 1968] for a temperaturerange of 1090ø to 1200øC. Between-run precisionsare better than --0.1% for  with the unleached whole rock Rb/Sr and Sm/Nd and  ages inferred from magnetic anomalies or the age of the sediments overlying basaltic basement (Table 2). These correctionsare likely to be too large for the Sr isotopicratios becausethe Rb/Sr of the wholerock is probablylargerthanthat of the acid-leachedresidueand the alterationprocesses did not occur instantaneouslyupon eruption. Moreover, there is an inherentuncertaintyin calculatinginitial isotopicratiosbecause the age of the basaltsis not preciselyknown. However, it is  si,•nificant thatexcept for onesample fromSite212(high 8•Sr/86Sr),age-corrected datapoints for 87Sr/86Sr and 143Nd/144Nd inacid-leached residues define a general inverse  206pb/204pband 207pb/204pband betterthan•0.15% for trend that largely overlapsthe trend for recentMORB and OIB 208pb/204pb. ThePbandU concentrations havebetterthan2% from theIndian Ocean(Figure2b).  precision. Figure 3ashows theeffects ofagecorrection on206pb/200pb The evolutionof isotopicratioswith time must be considered and207pb/204pb based ontheinferred minimum ageof the measured ontheacid-leached residues whencomparingpresent-dayradiogenicisotopicratiosin lavasof basaltandthe238U/204pb 232Thabundance dataarenotavailable different ages. Mahoney and Spencer [1991] discussedthis (Table2). Accurate problemin regard to lavas from the OntongJavaplateau. They (below INAA detectionlimit in MORB); thusno correctionwas noted that the Rb/Sr, Sm/Nd, U/Pb, and Th/Pb in tholeiitic basalts  madeto 208pb/200pb ratios.The238U/200Pbratiosof Indian  are relativelylow so that over 100 Myr thereis relativelylittle  OceanMORB are typically5 to 10 [White,1993], butin the acid-  13,834  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 0.5135  143Nd/144Nd 261  0.5133  257  0.5131  212 0.5129  259  '  215  0.5127  260  211  87Sr/86Sr  0.5125  0.702  0.704  0.706  0.708  Figure 2a. Theratio of87Sr/86Sr versus 143Nd/144Nd plotforbasalts fromtheeastem Indian Ocean seafloor. Foreach site, thetwofields indicate theeffects ofagecorrections on87Sr/86Sr and143Nd/144Nd using measured Rb/SrandSm/Ndonunleached samples. Fieldstotherightareforacid-leached residues andfieldstotheleftshow age-corrected values.Ages(Table2) wereinferred fromtheageof thesediments overlying basaltic basement or nearbymagneticanomalies. 0.5134  !  1431• d/144Nd  ß  i  -  i  ß  i  ß  i  ß  N-MORB 0.5132  Ridges .)61  St. Paul  0.5130  NinetyeastRidge 213  0.5128  Q•'212  257•  216  259  0.5126  211  EMII  Kerguelen Plume  215  (•  Christmas Isl. Kerguelen Archipelago  0.5124  ß  Sediments //'•  O•EMI ,  0.5122  0.702  0.703  0.704  0.705  0.706  (•1 .  87 Sr/•lP6 ts•  ,87Sr/8.6Sr •= 0.707  0.715  own  = 0.5120  Figure 2b.Theratio of87Sr/86Sr versus 143Nd/144Nd plot comparing age-corrected fields foroldeastern Indian Oceancrust(thispaper)to lavasfromNinetyeast RidgedrillsiteswithdataforSite216shownasa distinct field  [Mahoney etal., 1983;Hart,1988;Saunders eta/., 1991;WeisandFrey,1991;FreyandWeis,1995],fieldsfor recent Indian Ocean MORB(seereferences inworks byWeisetal.[1992]andLeRoexetal. [1983], Hamelin etal. [1985/86], Michard eta/.[1986], Priceeta/.[1986], Dosso eta/.[1988], andMahoney eta/.[1992]), themantle components normal MORB(NMORB),enriched mantle1 (EM1),andEM2[ZindlerandHart, 1986]andfieldsfor  theKerguelen Archipelago [Gautier eta/., 1990;Weiseta/., 1993a, b],Christmas [Hart,1988;Falloon etal., 1989],andSt.PaulIslands [Dosso etal.,1988].TheKerguelen Plume composition isasdefined byWeis etal. [1993a]. TheBanda Seasediment fieldisfromVroon eta/.[1993].Compared tothepresent-day fields forIndian  Ridges andKer•u•elen Archipelago, thefields ofmost DSDP sites (e.g., Site259)areoffset tolower 143Nd/144Nd  atagiven 87Sr/ø6Sr. This offset reflects aging ofthemantle source; e.g., thearrow labeled "200 Matrend" shows  aging ofaMORB-like source for200Ma(parent/daughter ratios forthissource areaverages fordepleted MORB, Rb/Sr = 0.011andSm/Nd = 0.336[Hofmann, 1988]; because ofchanges caused bymelting these aremaximum and minimumratios,respectively.  to  0.708 143Nd/144Nd  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 15.7  207pb/204pb  259 21  l•)•212  260 15.6  15.5  257  15.4  15.3  ,  ,  ß• 7.0  i  ,  I  ,  ,  17.5  •  ß ,  .........  18.0  18.5  19.o  Figure 3a. Theratio207pb/204pb versus 206pb/204pb plotfor old eastern Indian Ocean basalts showing the effects of age corrections for in situU decay(measuredfieldson right andagecorrectedfields on left; U/Pb measuredon acid-leachedresidues).  The age-corrections are especiallylargefor Site 257 lavas,and  13,835  Results: Specific Site 215 (8ø7.30'S, 84ø47.50'E)  This site was drilled 240 km west of the NinetyeastRidge and is off the ridge at >5000m water depth (Figure 1). Approximately25 m of tholeiiticbasalt,composedof at least 14 pillowed flows, were penetratedbeneath-59-60 Ma sediments [Hekinian, 1974]. Unaltered glassis abundantin this core, and the high K20 (-1%) and P20 5 (-0.25%) contentsof these glassesindicate that thesebasaltsare enrichedin incompatible elementsrelativeto depletedMORB [Melsonet al., 1975;Frey et a/., 1977]. Theseglasses(15 samples)aresimilarin composition, and our studyof four additionalwhole rocksfrom differentcore sectionsconfirmsthe geochemicalhomogeneityof lavas at this site (Table 1). Site 215 basaltsare more enrichedin incompatibleelements (i.e., higherCe/Y and La/Yb and lower Zr/Nb) thanbasalticlavas recoveredfrom the NinetyeastRidge (Figure 5), a linear volcanic ridge (Figure 1) thatis interpretedto be the traceof the Kerguelen Plumeon theIndian Plate [e.g., Weiset al., 1992]. Relativeto the transitionalbasaltsof the KerguelenArchipelago[Gautier et al., 1990], the Site 215 basalts have similar Ce/Y but tend to lower  La/Yb and Zr/Nb (Figure 5). In addition,Site 215 basaltshave thespread in206pb/204pb isdiminished bytheagecorrection.  Agesusedfor eachsiteare givenin Table2. The truemagmatic ratios are intermediatebetweenthe measuredand age-corrected ratios (seediscussionin text).  87Sr/86Sr,143Nd/144Nd, 207pb/204pb and 208pb/204pb  similarto thesetransitionalKerguelenbasalts(Figures2b, 3c and 3d). Although thecalculatedinitial 206Pb/204Pb (17.94-17.99) are lower than in lavas from the Kerguelen Archipelago, the  relatively lowmeasured 206pb/204pb (18.10-18.15)overlap with leached residues of these DSDPsamples, 238U/204pb rangesthose of the upper Miocene alkaline lavas of the southeast from4.8to 163andonly9 of 18samples have238U/204pbKerguelenArchipelago,which havebeeninterpretedby Weiset below30. Consequently, in Figure 3a the206pb/204pb age a/. [1993a] to be representativeof the KerguelenPlume(Figures  3c and 3d). Therefore basaltsfrom Site 215 have the high A7/4 corrected 87Sr/86Sr,we inferthatthe truemagmatic(8-9) and A8/4 (81-85) that define the Dupal isotopic anomaly 206pb/204pb ratiosat the timeof formationare intermediate andcharacterizethe KerguelenPlume[e.g., Gautier et al., 1990;  correctionsfor some samplesare significant. As with the age-  between the measuredand age-correctedratios. An important  Weis etal.,1989a, b],where A7/4 = [(207pb/204pb)sampl e-  O113from the Indian Ocean (Figures3c and3d). Becauseof the  [(208pb/204pb)sample(208pb/204pb)NHRL] x 100, with the  x 100,withtheNHRLequation being resultis thattheage-corrected 206pb/204pb ratiosfor these (207pb/204pb)NHRL] 207pb/204pb = 0.1084 (206pb/204pb) + 13.491) and A8/4 = DSDP basaltsare within the rangedefinedby youngMORB and lowabundance of235Utheagecorrections for207pb/204pb are lesssignificant(Figure 3a). Results:  208pb/204pb  General  211256  Although these basaltshave been affected by postmagmatic alteration,major elementanalysesof the least alteredlavas,and in somecasesfresh glasses,showthat the basaltsare tholeiitic at eightof the nine DSDP sitesstudied[Frey et al., 1977]. Alkalic basaltscontainingtitanaugite,amphibole,and small amountsof biotiteand high abundances of relativelyimmobileincompatible elements(Figure4) wererecoveredonly at Site 211 [Frey et al., 1977]. Abundancesof Zr and Ce are positivelycorrelatedwith Nb abundanceand reflect magmatic characteristics(Figure 4). Abundances  of Rb and Ba are not as well  correlated  with Nb  213  t•.• 212 215  37.5  257  abundancewhich probably reflects the effects of postmagmatic alteration;the scattershowsthe difficulty in making a reliable age correction on the basis of measured Rb/Sr. The Sr, Nd, and Pb  259  206pb/204pb 37.0  isotopic ratios in these DSDP basaltsrange widely, but they generallyoverlapwith therangedefinedby IndianOceanMORB, lavasfrom the NinetyeastRidge, and lavasfrom the Kerguelen Archipelago(Figures2b, 3c and3d). In the followingsection,we discusseach site proceedingfrom west to eastacrossthe eastern  3acaption.). Nocorrections areindicated for208pb/204pb asno  Indian Ocean.  accurate 232Thabundance dataareavailable.  ß• 7.0  17.5  18.0  18.5  19.0  Figure3b. Theratio208pb/204pb versus 206pb/204pb plot showing effects ofagecorrections on206pb/204pb. (SeeFigure  13,836  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR  15.70 ß  !  ß  !  ß  !  ß  I  -  !  ß  ß  207 pb/204pb Kerguelen Plume  15.65  i  2'12  ß  ß  !  ß  E•  3  259  260  1-2-3  256  15.60  216  •  Track sedime(• Paul Christmas Isl.  15.55  NinetyeastRidge 15.50  211  I  N-MORB  15.45  Kerguelen Archipelago 15.40  IndianRidges  15.35  206pb/204pb 15.3o  17.2  .  ,  ß  ,  17.4  ß  ,  17.6  .  i  17.8  18.0  ,  i  ,  18.2  i  ,  18.4  i  ,  i  18.6  ,  18.8  i  ,  19.0  19.2  Figure 3c.Theratio 207pb/204pb versus 206pb/204pb plot comparing age-corrected fields foroldIndian Ocean crustbasalts (Figure 3a)tofieldsfortheNinetyeast Ridge(dataforSite216areshown asa separate field),recent IndianOceanMORB,mantlecomponents, andtheKerguelen, Christmas andSt.PaulIslands.Datasources areas  given forFigure 2b.Onesample ofSite260(20-1, 16-18 cm)has a206pb/204pb > 20andhas notbeen plotted. Track1-2-3sediment fieldsaredataforsediments collected fromthreetransects in theBanda Sea[Vroon etal., 1993].  40.0  208pb/204pb  Kerguelen  39.5  256  Archipelago  Kerguelen Plume  EM II  SeTrack diments 1-2-3  •  39.0  216 &  Christmas Isl.  254 215  EMI  261  211  259  Ninetyeast Ridge  N-MORB  257 213 37.5  IndianRidges  206pb/204pb  37.o  17.2  17.4  17.6  17.8  18.o  18.2  18.4  18.6  18.8  19.o  19.2  Figure 3d.The ratio 208pb/204pb versus 206pb/204pb plot (only 206pb/204pb data have been age corrected, see Figure3b). Fieldsareasin Figure3c,except thatdataforthetwoNinetyeast Ridgesites(216and254)with relatively low206pb/204pb areindicated.  WEIS AND FREY: EASTERNINDIAN OCEAN SEAFLOOR 100  400  ß  I  '  I  13,837 I  '  '  I  '  I  '  I  '  ß  30O  211  256  200  4o  215  100  ß  211  []  257  $  212  ß  259  213  a  260  "  215  o  261  x  256  +  20 0  '  I  '  .  I '  ,  I I  , '  I  ß  I I  , '  I  .  I I  , '  I  .  500 8O ß  400 ß  6O  ß  ß  300  ß  4O  200  q-  o  20 0  0  ß I  10  I , 20  I , I ß I 30  40  50  ß I , 60  ß  I , 70  80  100  0 - ,Oi , • , 0  l0  20  I  30  ,  I  40  •  50  60  70  8O  Nb  Nb  Figure4. Abundance ofvarious incompatible elements (inppm)versus Nbcontent (inppm)inbasalts fromthe. eastern Indian Ocean seafloor. Thehighest abundances areinthealkalic basalts fromSite211.Except forRband Ba,whichhavebeenaffected bypostmagmatic alteration, theabundances ofincompatible elements arepositively correlated.  NHRL equation being 208pb/204pb = 1.209 (206pb/204pb) + IndianOceanMORB field (Table2 andFigure2b). Comparedto 15.627 [Han, 1984].  Indian Ocean MORB, this samplehas anomalouslyhigh Pb  Site 213 (10ø12.71'S,93ø53.77'E)  thanthoseof lavasfromtheKerguelen Archipelago (Table2 and  This site is in the westemWharton Basin 500 km eastof the  isotopic ratios, e.g.,207pb/204pb = 15.65, which isalso higher Figures3c and3d).  Ninetyeast Ridgeandseparated fromtheNinetyeast Ridgebythe Site 212 (19ø1134'S, 99ø17.84'E) Ninetyeast Fracture Zone.It isneartheeast-west trending Chron 25;if thesebasalts formedata spreading ridgeaxis,theyerupted This sitewasdrilledin thedeepestpartof theWhartonBasin endof a longlineartopographic high, at theeast-west strikingspreading centerthatbecame extinctin (6233m) at the southern the Investigator Ridge, and near the east-west trending Chron 34 themiddleEocene (Figure1). Eighteen metersof pillowbasalt overlying thebasaltlacksfossils,and were recoveredbeneath56-58 Ma sediments.In contrastto Site (Figure1). The sediment is poorlyconstrained to be ~100Ma 215,glasses fromthissitehavelow K20 (0.06%)andP205 the ageof the basement (0.09%)contents [Melson et al., 1975]. Freyet al. [1977] [Sclater et al., 1974]; Powell et al. [1988] used magnetic concluded thatSite213basalts aredepleted MORB. Ourstudyof  anomaliesto infer a basementage of 90 Ma. Five metersof  pillowbasaltwerepenetrated. Thesebasalts areveryaltered, typically5-10% weight loss on ignition [Hekinian,1974]. Analysis of a singleglasschipandwholerocksshows thatthese basalts haveverylow TiO2 andZr abundances [Melsonet al., 1975;Freyeta/., 1977]. Ourdatafor sixbasalts showthatthey (~800 ppm,Table 1) whichis REE(Table 1and Figure 4)and has higher 87Sr/86Sr and lower havevery highCr abundances with the highMgO (9.0%) andCaO (13.5%)of the 143Nd/144Nd thanthesample withlowerincompatible element consistent of abundances. However,its Sr andNd isotopicratiosarewithinthe glass. The whole rocks also have low abundances  five samples confirms thatmostof thebasalts in thiscoreare depleted MORBin composition (Figures 4 and5) andin Srand Nd isotopic ratios(Figure2b). However, thelowermost basalt studied fromthiscoreis geochemically distinct.It is relatively enriched in theincompatible elements Ba, Nb, St, Zr, andlight  13,838  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR Table 1. Abundances of Trace Elements in DSDP Basalts From the Eastern Indian Ocean Site 215  18-2  18-2  18-3  19-2  47-53  106-110  110-112  145-150  Rb Sr Ba  15.5 284 285  14.5 284 269  10.6 282 262  294 274  V  213  187  174  192  Cr Ni Zn Ga  258 105 72 17.2  237 101 64 18.4  299 100 58 17.6  215 92 64 17.7  Y Zr Nb Hf  27.4 134 22.5  26.6 127 21.6 2.82  27.0 118 20.2 3.1  27.3 122 20.6 -  14.5 32.6 16.8 4.17 1.41 0.75 2.56 0.37  16.2 36 20.1 4.5 1.5 0.8 2.5 0.51  La Ce Nd Sm Eu Tb Yb Lu  47  12.7  43  Site 213  Rb Sr Ba  V  Cr Ni Zn Ga Y Zr Nb Hf Ca Ce  17-2 108-110  17-3 90-99  18-2 115-117  19-2 54-56  19-2 127-130  12.3 127 7  10.5 136 15  6.5 121  13.3 122 14  13.3 260 176  230 327 90 106 15.8  256 319 104 84 17.0  236 330 82 151 16.2  248 354 101 132 16.5  219  22.5 61 1.6  26.7 62 2.0  23.6 64 1.1  26.7 68 2.1  3  19  2.2 7.6 6.2  6.3  Nd Sm Eu Tb Yb Lu  2.18 0.85 0.64 2.55 0.41  225 71 146 18.6 29.0 135 22 3.2 18.8 30.8 16.4 4.23 1.65 0.74 2.75 0.39  Pb isotopicratios,this sampleis similar to the lowermostbasalt studiedat Site 213; that is, it has Pb isotopicratios higher than  incompatibleelementssuchas Y, Zr, Nb, and REE; the erratic and high contentsof Rb and Ba reflect the high extent of alteration (Table 1 and Figure 4). However, these low abundances are not accompanied by MORB-like Zr/Nb, Ce/Y,  Indian Ocean MORBandits207pb/204pb (15.66) ishigher than  and La/Yb, which are more similar to the ratios in lavas from the NinetyeastRidge (Figure5).  A7/4of14.4,although its208pb/204pb fallswithin theKerguelen  One samplefrom Site 212 was analyzedfor its isotopic  lavas from the KerguelenArchipelago,comparableto thoseof Indian Oceansediments[Vroonet al., 1993]. Henceit hasa high field (Figures 3c and3d).  compositions. It hasa 143Nd/144Nd typical ofdepleted MORB, buteven theacid-leached residue hasstillaveryhigh87Sr/86SrSite 211 (9ø46.53'S, 102ø41.95'E) (0.70755) that mustreflect the extremelyalterednatureof these This sitewasdrilled in deepwater(5525 m) westof Christmas basalts[Hekinian,1974]. Apparently,someof thepostmagmatic phaseswere not removedby the acid leaching. With respectto Island. ChristmasIslandis composedof alkalinebasaltsranging  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR  13,839  Table1. (continued) Site 212  39-1 134-136  39-1 146-149  39-2 0-14  39-2 39-42  39-2 60-64  39-3 145-147  39.0 75  368 112 153  24.9 72 11  18.7 67  20.2 68 9  31.1 93 116  V Cr Ni Zn Ga  224 820 144 86 -  189 782 138 78 11.9  240 881 148 93 15.0  206 764 144 71 13.6  221 786 141 84 13.9  206 872 160 76 12.6  Y Zr Nb Hf  16 45 3  13.8 31 2.1  11.9 30 1.2  15.2 24 1.3  16.9 26 1.3  15.7 32 2.1 0.64  La Ce  1.9 5  7.6  4.2  9.6  7.3  Nd  3.4  3.5  Sm  Eu  1.08 0.47  1.29 0.49  Tb  0.32  0.30  Yb  1.17 0.18  1.72 0.28  Rb Sr  Ba  Lu ,  2.39 6.2  ,  site 211  Diabase Sill  Basement  12-1 23-25  12-1 143-145  12-2 100-102  14-2 55-61  15-2 14-16  15-2 95-97  15-3 40-46  15-3 40-45  15-3 67-70  42.0 447 469  41.0 540 534  45.1 514 488  42.0 447 489  18.9 265 151  70.4 535 501  61.7 595 526  59.3 482 432  91.4 530 508  411  v  154  Cr Ni  299 116  Zn  94  Ga  19.9  147 274 136 82 18.0  165 222 101 90 18.2  215 92 107 113 19.9  163 94 103 111 20.1  119 53 68 87 20.0  118 57 63 80 20.5  102 58 79 107 20.0  146 72 118 101 21.2  137 85 111 85 19.0  183 52  201 57  342 70  368 76  323 67  304 63  338 70  333 68  300  75  65  84  86  84  75  81  74  Rb Sr  Ba  15-4 70-73  52.4 426  Hf  4.12  Zr Nb  193 54  Hf  4.12  La  35.3  Ce  70.7  Nd Sm  29.3 6.01  Eu  1.93  2.29  Tb Yb  0.69 1.75  0.78 2.28  Lu  0.26  0.33  5.82 61 5.82 37.3 77.3 32.4  7.04  in age from Eoceneto Miocene [Smith and Mountain, 1925; Falloon eta/., 1989]. ThesebasaltshaverelativelyradiogenicPb  mildly alkaline suite of the Kerguelen Archipelago (Figure 5). Theseare the only alkalic lavasrecoveredfrom the easternIndian  isotopic compositions with206pb/204pb > 18.8[Hart, 1988].At Site211a 10-m-thick diabase sill(40Ar/39Ar ageof 71Ma  Ocean sea floor.  [McDougall, 1974]) occurs 18 m above an amphibole-bearing basalticbasementthat is inferredto be >76 Ma. Althoughthe sill has significantly lower abundancesof Nb and Zr than the basementlavas (Table 1 and Figure 4), all of the lavas from this site are alkalic basaltsthat are very enriched in incompatible elementsrelative to the lavas from the other sites(Figure 4). In terms of La/Yb, Ce/Y and Zr/Nb, Site 211 lavas are similar to the  The Sr andNd isotopicratiosof a basementlava from Site 211 are close to those of Christmas Island lavas (Figure 2b). Comparedto lavasfrom ChristmasIsland,this Site 211 basalthas  lower 206pb/204pb and207pb/204pb (Figures 3cand3d),butits 206pb/204pb ratioisnevertheless thehighest value measured on samplesof the northeastern Indian Oceanseafloor(exceptfor an anomaloussample from Site 260, Table 2). The Pb isotopic ratiosof the sill sampleoverlapwith the upperMiocene alkalic  13,840  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR  Table1. {continued• Site 256  9-3 15-17  9-3 52-54  9-3 138-140  10-1 141-143  10-4 62-64  10-4 114-116  11-1 27-30  11-3 148-150  2.8 168  Ba  5.1 191 60  5.1 176 59  6.6 172 64  1.8 186 56  2.6 172 28  4.4 166 46  3.6 170 51  V Cr Ni Zn Ga  545 116 73 134 21.3  424 110 78 136 20.5  426 100 73 127 21.2  486 120 76 139 21.9  416 130 85 134 20.0  410 155 90 141 19.0  426 102 77 127 19.9  Y Zr Nb  33.7 164 17.0  35.2 140 13.6  34.8 146 14.8  33.8 154 15.3  34.7 137 13.3  33.7 133 12.5  35.0 143 14.4  10.4 30 17 4.73 1.59 1.1 3.4 0.54  9.8 30 18 4.85 1.57 1.1 3.3 0.53  Rb  Sr  La  Ce Nd  28  Sm Eu Tb Yb Lu  20.5 34.3  9.0 33 17 4.31 1.37 1.1 3.0 0.47  29  10.1 29 18 4.86 1.55 0.83 3.4 0.65  28  30  Site 257  11-1 122-124  11-3 25-27  14-5 25-27  16-1 65-67  16-3 31-33  17-1 70-72  17-1 97-99  17-5 67-69  Ba  16.1 204 229  9.5 76 114  13.0 76 11  6.3 93 41  0.5 80 12  31 80 11  50 87 33  0.5 83 34  V Cr Ni Zn Ga  217 425 83 75 14.0  285 492 117 92 13.3  325 183 87 93 17.0  288 365 116 81 16.0  256 435 100 70 15.7  243 179 77 79 14.9  274 232 121 82 15.0  264 226 88 83 15.7  Y Zr Nb  18.5 82 7.6  15.9 39 1.6  24.3 52 2.1  17.6 51 2.0  21.8 43 1.3  17.8 43 2.5  22.0 48 2.9  21.4 47 2.6  Hf  1.89  1.09  1.78  La Ce Nd Sm Eu Tb Yb Lu  6.11 16.1  1.60 5.4 4.5 1.64 0.64 0.37 1.95 0.30  1.78 6.6 5.6 2.06 0.81 0.49 2.84 0.43  Rb Sr  10.5 2.75 1.03 0.54 1.93 0.29  2.7  1.41 4.9 4.9 1.60 0.71 0.36 2.5 0.42  4.9  3.1  lavasof the KerguelenArchipelago.Thereforeamongall lavas Bataviaknoll, andZeewkknoll (Figure1) (seeFigure5 of Powell recoveredby drilling from the easternIndian Ocean seafloor, et al. [1988] for details). Based on fossils, the minimum theseSite 211 alkalic basaltsare geochemicallythe most similar basementage is 102 Ma; usingmagneticanomalies,Powell et al. to recentlavaseruptedin the KerguelenArchipelago[Weiset al., [ 1988]inferreda basement ageof 125Ma. Basement penetration 1993a, b]. However, Site 211 is not close to the track of the of 19 m recoveredFe-Ti rich tholeiiticbasalts.Both the major of Site256 basalts KerguelenPlumeor anyotherrecognized plume. It is likely that elementandincompatibleelementabundances lavas from this site are related to the volcanism  that created the  aresimilar tobasalts recovered fromtheNinetyeast Ridge qFi•ure  87Sr/86Sr, 143Nd/t4'*Nd, northeast trendingbathymetrichighsthatformthe Cocos-Keeling 5 andFreyet al. [1977].Moreover, 206pb/204 a 207Pb/204Pb in Site 256 1avas overlap with Plateau-Christmas Islandcomplex(Figure 1). Pb,nd therangeof NinetyeastRidgebasalts(Figures2b, 3c and3d). Site 256 (23ø27.35'S, 100ø46.46'E)  This site is in the southernWhartonBasin. Althoughdrilled in deepwater (5361 m), the siteis neara trendof bathymetrichighs extendingnortheastfrom BrokenRidge,i.e., GoldenDraakknoll,  Site 257 (39ø59'S, 108ø21'E) DSDP Sites 257, 259, and 260 are near the western coast of  Australia (Figure 1). This basaltic seaflooris inferred to have  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR  13,841  Table1. (continued) Site 259  Site 260  Site 261  38-1 65-67  41-1 101-104  18-2 140-142  20-1 16-18  33-1 101-105  34-1 75-77  35-2 120-123  36-1 60-63  39-1 11-13  Rb Sr Ba  7.2 91 7  12.1 99  3.6 129 64  3.1 112 35  20.3 81  10.9 87  14.2 98 20  24.4 91 12  6.1 85  V Cr Ni Zn Ga  270 148 59 112 16.8  260 185 45 132 17.0  479 125 210 178 21.9  391 106 51 151 19.8  333 172 85 104 17.1  323 181 76 92 17.5  431 38 55 213 21.1  539 22 53 166 23.2  321 165 78 94 18.0  Y Zr Nb Hf  29.9 72 2.1 1.92  33.1 74 1.3 2.04  31.8 138 5.3 3.14  37.1 119 5.5  30.6 72 0.9 1.83  33.1 77 1.1  61.8 205 7.5 5.75  47.3 176 7.1  32.4 78 2.0  La Ce Nd Sm Eu Tb Yb Lu  5.07 10.6 8.9 3.09  4.39 12.8 8.9 3.20  7.60 14.9 10.1 3.95  6.0 17 9.7 3.75  2.34 11 7.1 3.11  1.03 0.63 3.04 0.43  1.17 0.83 3.27 0.50  1.46 0.79 3.31 0.47  1.21 4.1 0.65  1.99 7.7 7.1 2.76 1.04 0.70 3.45 0.53  8.35 29.6 23.5 7.84 2.62 1.63 5.96 0.85  1.03 0.73 3.4 0.62  17.9  2.45 9 7.8 3.16 1.17 0.79 3.90 0.60  In partspermillion. Sampledesignation indicatescoreandsectionnumberfollowedby an intervalin centimeters.Datafor REE (Lu throughLu) andHf are by instrumental neutronactivationat MIT; datafor otherelementsare determinedby X ray fluorescence at Universityof Massachusetts, Amherst.WhenCe abundances arenot accompanied by otherREE data,Ce wasdetermined by XRF. For discussion of precision andaccuracy, seeFreyeta/. [ 1991].  formed at the northeast-southwest orientedspreadingridge that  separatedGreaterIndia from Australia[e.g., Markl, 1974; Veeverset al., 1974; Rundleet al., 1974; Fullerton et al., 1989]. At Site 257, Middle Albian -106-110 Ma sedimentsoccur 13 m  abovethebasalticbasement.Basement penetration was64.5m. Althoughbasaltsfrom core1! have-100 Ma K/Ar agesroughly consistentwith the age of the overlyingsediments,mucholder K/Ar ages(157 to 196 Ma) wereobtainedfrom basaltslowerin the core [Rundle et al., 1974]. Based on extrapolation of magneticanomalies, inferredbasement agesrangefrom 110 to 130 Ma (Powellet al. [1988] andLuyendykand Davies[1974], respectively). Most of the basalts from this core have incompatibleelement abundanceratios intermediatebetween SEIR MORB and lavas from the NinetyeastRidge (Figure 5). Their isotopicratiosoverlapwith the Indian OceanMORB field (Figures2b, 3c and3d). In contrast,the uppermost basalts(core 11, section1, Table 1) have La/Yb, Ce/Y and Zr/Nb similar to basaltsfrom Site 256 and lavas from the NinetyeastRidge (see Fleet et al. [1976] and Figure 5). The uppermostlava is also similar to NinetyeastRidge lavas in Sr and Nd isotopicratios, althoughits combinationof Sr-Nd is not withinNinetyeastRidge field (Table 2 andFigure2b). It alsohashigherPb isotopicratios than the depletedlavas from Site 257 (Table 2). Although the accuracyof the K-Ar agesis unknown,it is intriguingthatthe KAr age(-100 Ma [Rundleet al., 1974]) of the uppermostbasalts is also similar to the minimum age inferred for the enriched basalts at Site 256.  However, like other Site 257 lavas, the  enriched basalt hasrelatively low206pb/204pb (17.57).Thisis  eruptedon the NinetyeastRidge. However,all basaltsat Site 257 plot within the Indian Ridgesfield in Pb-Pbdiagrams(Figures3c  and3d)andhavethelow206pb/204pb thatischaracteristic of many Indian Ocean MORB and which has not been found in lavasrelatedto the KerguelenPlume. Site 259 (29ø37'S, 112ø42'E)  Based on the age of overlying earliest Aptian sediments, basementat this site is older than ~112 Ma; Powell et al. [1988]  usedmagneticanomaliesto infer a basementageof 125 Ma. As at Site 257, the oceaniccrust at Site 259 is presumedto have formed at a northeast-southwest orientedridge. The Site 259 lavas have La/Yb, Ce/Y, and Zr/Nb typical of depleted SEIR MORB (Figure5). The two analyzedsamplesaregeochemically similar, exceptfor a differencein Zr/Nb which probablyreflects analyticalerror at theselow Nb contents(--2 + 0.6 ppm, Table 1  [Rhodes etal.,1990]).The87Sr/86Sr and143Nd/144Nd overlap withthehigh87Sr/86Sr endoftherecent Indian Ocean MORB field; the offset of the age-corrected values to lower  143Nd/144Nd and87Sr/86Sr (Figure 2b)isconsistent withaging of a depletedmantle(MORB) source.In Pb isotopes,bothlavas have206Pb/ 204pb at the high end of the Indian Ocean MORB field. However,like the depletedbasaltsfrom Site 212 andthe enriched basalt from Site 213, these Site 259 lavas have  207pb/204pb greater thananylavafromthe Kerguelen Archipelago(Figures3c and3d), with A7/4 of 14.0-14.3 andA8/4 of 32.7-60.7.  much lower than that found in lavas from the NinetyeastRidge Site 260 (16ø9'S, 110ø18'E) Like the basement at Sites 257 and 259, oceanic crust at Site andIndian0]]3. Thusthe youngestIndianoceancrustsampledat Site 257 is geochemicallyenriched,similarto lavassubsequently 260 in the northeastIndian Ocean (Figure 1) is inferred to have  13,842  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR  50  • I' I' I' I' I' I'I' I Kerguelen Archipelago  40  i © 212I I + 213I  30  •  I I'  20  e57I 2591  I ø 26al I ß 765I SEIR  10  ,n•a• :•-,, .. o, Do . , ..  [ I • I ß , :--, :  KerguelenArchipelago  NER  SEIR  q-  o  0  !0  20  30  40  50  60  70  80  90  Zr/Nb Figure 5. Ce/Y andLa/Yb versusZr/Nb in basaltsfromtheeasternIndianOceanseafloor(datafromTable 1 and averagefor Site 765 lavasfrom Ishiwatari[1992] comparedto fieldsdefinedby lavasfrom the Kerguelen Archipelago [Storeyet al., 1988;Gautieret al., 1990;Weiset al., 1993a],Ninetyeast Ridge[Freyet al., 1991],and Southeast IndianRidge(SEIRMOP,B) [Priceet al., 1986];therangefor Kerguelen Archipelago lavasfromlow to highLa/Yb andCePs r reflectstheevolutionfromolder,--25 Ma, transitional basaltsto younger,<10 Ma, highly alkalinelavas. We usetheseelementsbecausethey are relativelyunaffectedby postmagmatic processes.The selectedratiosinvolveelementsof differentincompatibility andtheyillustratethe diversityof theseDSDP lavas. Theseratiosclearlydistinguish depletedMORB fromO11tandenrichedMOP,B; La/Yb indicatesthe slopeof a chondrite-normalized REE plot(La/Ybis =1.48in chondrites); Ce/Y (=0.39in chondrites) is alsoplottedbecause there are more data for theseelements(Table 1). Theseincompatibleelementratios show that mostof the basementsitesin the easternIndianOceanhaverecoveredbasaltswhichareintermediatebetweendepletedMORB andNinetyeastRidgelavas. Only Site211 andSite215 basaltsarewithinthefield of lavasfrom the Kerguelen Archipelago.  separatedGreater India from Australia. At Site 260, the  0.5 m of core was recovered. Similar to basalts from Site 257, Ce/Y, Zr/Nb and La/Yb in the Site 260 lava are intermediate  recoveredbasalt is interpretedto be a sill that is overlain by 105 Ma sediments.Although9 m of basaltwaspenetrated, only  between SEIRMORBandlavasfromtheNinetyeast Ridge (Figure 5). Consistent withthisresult, measured 8'Sr/86Sr and  formed from the northeast-southwestspreading center that  WEIS AND FREY: EASTERN INDIAN  OCEAN SEAFLOOR  13,843  Table 2. St, Nd, andPb IsotopicData andPb andU Concentrations by IsotopeDilutionin DSDP BasaltsFromthe EasternIndianOcean Leg  Site  22  213  Sample  Age, Ma  end 87Rb/86Sr87Sr/86sra 2C•m 87Sff86Sr 143Nd/144Nda 2Om end 143Nd/144Nd  Initialb 18-2 115-117  0.155  57  (0.702696) 0.702809  213  19-2 127-130  0.148  57  22  215  18-2 106-110  0.148  60  215  18-3 110-112  0.109  60  211  12-1 23-25  76  0.272  211  15-4 70-73  0.369  76  (0.512977)  (14)  6.6  0.51292  6.9  0.70433  (0.512738)  (9)  2.0  0.51268  2.3  (6) 0.70437  (0.512723)  (8)  1.7  0.51267  2.1  (0.512656)  (24) 0.4  0.51259  1.1  0.70370  (0.512824)  (18)  3.6  0.51276  4.3  0.70562 0.70359  (0.513055) (0.512910)  (34) (10)  8.1 5.3  0.51292 0.51276  4.8  0.70369  (0.512872)  (17)  4.6  0.51272  4.1  0.703109  7  0.703173  6  (0.704451)  (6)  (0.704462)  (6) 0.70390 22  (0.704096)  (7)  0.704118  212  39-1 134-136  90  27  260  20-1 16-18  105  1.51 0.080  (0.707548) (0.703702)  27  260  18-2 140-142  105  0.081  (0.703805)  0.70299  8  (0.704194)  22  0.70296  7  0.704167 22  7.8  8  0.704420  22  0.51296  (7)  0.704461  22  7.9  14  (0.703082) 0.703172  0.148  Initial c  (13)  33  0.702611  22  Initialb (0.513043)  (10) 0.70257  ,  21  (42) (8)  0.703768  7  (6)  0.512853 26  257  16-3 31-33  110  0.0181  (0.703232)  (7)  0.703194  257 11-3 25-27  110  0.362  257 11-1 122-124 256 10-1 141-143  110 125  0.228 0.111  O)  256  10-4 114-116  125  0.0437  (0.704114) (0.704188)  (5) (8)  0.704151  9  259  38-1 65-67  125  0.229  (0.704205)  (8)  259  41-1 101-104  125  0.354  (30)  9.6  0.51299  9.6  0.70320  0.513104d  168d  9.1  0.51294  8.7  0.70376 0.70399  (0.512835) (0.512903)  (19) (16)  3.8 5.2  0.51272 0.51277  4.4  0.70412  (0.512855)  (7)  4.2  0.51272  4.7  25  4.3  0.51271  4.6  0.51276  5.5  0.512860  9  (11)  0.704198  27  (0.513128)  0.70380  (0.512885)  0.70317  (0.512938)  261 33-1101-105  152  0.73  0.704257  11  0.704240  24  (0.703802)  (0.703982)  (5)  261 35-2120-123  152  0.419  (0.703340)  4.8  0.512886  35  (31) 49  4.8  5.9 6.4  0.70242  (0.513208)  (22)  11.1  0.51297  10.4  (10) 0.70243  (0.513380)  (50) 14.5  0.51318  14.4  (12)  0.703979  27  (33)  11  0.512965  27  5.6  9  (0.704203) 0.704259  27  0.70320  8  0.704227 26  4.2  8  (0.703761) 0.703721  26 26  21  10  0.703233  26  7.8  18  0.703341  20  0.703328  25  a Thedifferent numbers correspond toduplicate analysis ontheVG54 mass spectrometer and show thebetween-run reproducibility. Thenumber in parentheses istherunwiththebetter precision and stability and istheone used inthis 1•or. •' "Initial" corrected values, i.e.,measured ratios forinsitu decay of87Rb,147Sin,238U,and 235U, respectively, fortheage given See analytical section in the text for discussion.  c eNdcalculated forthe"initial" values and relative tobulk earth values attheage given foreach sample (BE(O): 143Nd/144Nd =0.512638 and 147Sm/144Nd = 0.1967). a Verylowintensity analysis, poor precision. This143 Nd/144Nd value isused intheplots because itisnotsignificantly different from thevalue from anothersampleat thesamesite(Leg26, Site257, 16-331-33).  143Nd/144Nd overlap withtheenriched endoftheMORB field  o  t  Site 261 (12ø57'S, 117 54 E)  i•ure 2b) and like the Site 259 lavas,the offsetof agecorrected Nd/  144Nd  and  87Sr/ 86Sr  from the fileld for recen  t MOR B  This sitein the northernArgo AbyssalPlainpenetrated47 m of basaltbelow sedimentsof 152 Ma (Figure 1). Thus thesebasalts are the oldest recoveredby DSDP in the easternIndian Ocean, and they are similar in age to the 155 Ma basaltsrecoveredat 206pb/204pb (-17.5)anda high207pb/204pb (15.5),whereasODP Site 765 in the southernArgo AbyssalPlain [Luddenand  (Figure 2b) is consistentwith agingof a depletedmantlesource. The Pb isotopiccharacteristics of thesetwo Site 260 samplesare anusua! (Figure 3), one sample has relatively low initial  theotherhasunusually highinitial206pb/204pb (20.3)and 207pb/204 Pb(15.8)(notplotted onFigure 3).  Dionne, 1992]. The basaltic core can be divided into three units  [Robinsonand Whi•ord, 1974]. The uppermostunit A is a 10-m  13,844  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR Table 2. (continued) Ce/Pb PbppmlJppm2381j/204pb 206pb/204pb 207pb/204pb 208pb/204pb 206pb/204pb 207pb/204pb  Initialb  Initialb  0.07  0.01  9.0  18.17  15.491  38.01  18.09  15.49  108.6  0.29  0.08  17.7  18.66  15.66  38.84  18.50  15.65  21.7  0.37  0.1  0.76  0.21  17.5  18.15  15.548  38.42  17.99  15.54  42.9  1.02  0.28  17.3  18.10  15.533  38.32  17.94  15.53  35.3  0.94  0.29  19.7  18.48  15.582  38.77  18.25  15.57  75.2  1.2  0.09  18.65  15.557  38.90  18.59  15.55  64.4  0.17  0.06  0.18  0.44  4.80 22.6 163  18.80  15.673  38.77  18.48  15.66  29.4  22.85  15.911  38.07  20.29  15.79  94.4  53.2  0.28  0.15  33.6  18.05  15.579  37.96  17.52  15.55  0.3  0.15  31.3  18.03  15.544  37.92  17.54  15.52  0.06  0.01  14.3  17.72  15.386  37.35  17.47  15.37  81.7  0.12  0.07  36.3  18.06  15.439  37.47  17.44  15.41  45.0  0.76  0.95  80  19.19  15.627  38.01  17.82  15.56  21.2  0.13  0.12  61  19.17  15.621  39.33  17.98  15.56  223.1  0.21  0.11  33.9  19.05  15.58  39.2  18.39  15.55  142.9  0.41  0.05  7.7  18.26  15.61  38.31  18.11  15.60  25.8  38.8  0.33  0.03  4.82  18.58  15.648  38.42  18.49  15.64  0.33  0.03  4.82  18.58  15.635  38.44  18.50  15.63  0.12  0.06  30.9  18.46  15.537  37.62  17.72  15.50  64.2  0.7  0.38  34.6  18.72  15.634  38.39  17.89  15.59  42.3  Discussion coarse-grained sillwitha highl•depleted MORBcomposition  (Figure5) whose87Sr/86Sr and143Nd/144Nd areat thedepleted  end of the Indian OceanMORB field (Figure2b). Althoughthe older,underlyingunitsB andC are lessdepletedin incompatible  Occurrence  of Enriched  MORB  At three of the studied DSDP sites, the recovered basalts are  highly enrichedin incompatibleelementsrelative to MORB. At equally low87Sr/86Sr andeven higher 14ONd/144Nd (Table 2 Sites 215 and 256, the tholeiitic lavas have isotopic and andFigure2b). TheseSite 261 lavashaveSr andNd isotopic incompatibleelementratios similar to lavas associatedwith the signaturessimilar to thoseof Site 765 basalts[Luddenand Kerguelen Plume (Figures 2b, 3c, 3d and 5). Because the Dionne,1992],andtheyhavelower87Sr/86Sr andhigher Ninetyeast Ridge, which is interpreted to be a hotspottrack  element abundances thanunitA, thes,a•le fromunitB has  143Nd/144Nd thanbasalts fromtheothereastern IndianOcean relatedto the KerguelenPlume[e.g., Weiseta/., 1992], is only  sitesstudiedin this paper(Figure2b). Site 261 lavashavethe  low206pb/204pb ratios typical of some Indian Ocean MORB,  240 km eastof Site 215, it is conceivablethat the >60 Ma basaltic  basementat Site 215 is relatedto the NinetyeastRidge. Site 256 buttheyhaverelatively high207pb/204pb (15.50to 15.59), is locatedon a northeasttrendingseriesof bathymetrichighs althoughnot as high as in depletedMORB samplesfrom Sites emanatingfrom Broken Ridge (Figure 1), which formedas the northern portion of the Kerguelen Plateau. This very large 212 and259 (Figure3c).  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR  plateauis alsointerpretedto be a manifestation of the Kerguelen Plume[e.g.,Davieset al., 1989;Weiset al., 1989a;Salterset al., 1991;Storeyet al., 1992; Miiller et al., 1993]. Agesfor lavas fromtheKerguelenPlateaurangefrom85 to 118Ma [Leclaireet a/., 1987; Whitechurch et al., 1992;Pringleet al., 1994],whereas lavas from Broken Ridge have agesrangingfrom 63 to 89 Ma [Duncan, 1991]. Becauseof their geochemicalsimilaritieswith lavas relatedto the KerguelenPlume and their minimum -102 Ma and maximum 125 Ma age, Site 256 lavas may represent earlyvolcanismrelatedto theKerguelenPlume. In contrast, the enriched alkalic lavas at Site 211 in the northern Wharton Basin cannot be directly related to the KerguelenPlume. Site 211 is locatedon a seriesof northeast trendingbathymetrichighswhoseorigin is unknown,and their strike is not consistentwith the trace of a known hotspot. The alkalic basalts at this site, however, have compositional similaritiesto the much youngerbasaltsformingthe Kerguelen Archipelago(Figure5). Althoughlavasfrom nearbyChristmas  Island have higher 206pb/204pb thanKerguelen lavas, thePb isotopic fields defined by lavas from Site 211 overlap the Kerguelenfield (Figures3b and 3c). In addition,a basement samplefrom this site has Sr and Nd isotopicratios close to the  low87Sr/86Sr-high 143Nd/144Nd endof therange defined by  13,845  range (Figure 6a). These lavas contrastwith somerecentIndian OceanMORB, somelavasrecoveredfrom the KerguelenPlateau, and some of the lavas from the oldest eastern Indian Ocean drill  sites, Sites 257and765,which range tomuch lower 206pb/204pb (to 17.30,Figure 6a). Thusunusually low206pb/204pb is characteristic of bothrecentIndianOceanMORB andrelatively old seafloor in the eastern Indian Ocean, but it is not  characteristicof <82 Ma lavas associatedwith the Kerguelen Plume (Figure 6a). Similarly, relatively high A8/4 valuesare associated with lavasfrom the KerguelenPlumeandlavasat the sevenDSDP sitesrangingin agefrom -56 to 125 Ma (Sites213, 215, 211, 212, 260, 256, and 259). In contrast,lavasfrom DSDP Sites 257 and 261 have much lower A8/4 (Figure 6b). Lavas from DSDP  Site 261 and ODP Site 765 are the oldest Indian  Ocean seafloor studied, and they also have higher  (143Nd/144Nd), >0.51285, thanlavasassociated withthe KerguelenPlume;thusthey are similar to recentIndian MORB (Figure 6c).  Source ofAnomalously High207pb/204pb At three of the DSDP sites studied, some of the basalts have  anomalously high207pb/204pb (Sites 212,213,and259),i.e.,  higher than Indian OceanMORB and lavas from the Kerguelen lavas from the Ninetyeast Ridge and Kerguelen Archipelago Archipelago (Figure 3c).Conversely, inthe208pb/204pb versus (Figure 2b). Thereforelavas with geochemicalcharacteristics 206pb/204pb diagram (Figure 3d),most of these samples plot similar to lavas associatedwith the Kerguelen Plume have within the Kerguelenand NinetyeastRidgefields. In an oceanic erupted in locations where the volcanismcannot be directly environment, onlysediments have such high207pb/204pb. The relatedto the KerguelenPlume. Site212basalt hasalso ahighage-corrected 87Sr/86Sr (0.70741) Lavas at two sites (257 and 213) range widely in ratios of indicating that some of the postmagmaticphaseswere not incompatible elementsbuthaveSr andNd isotopicratioscloseto removedby acidleaching. or within the Indian MORB field. At Site 257, close to the In Pb-Pb diagrams(Figure 3c), the sampleswith anomalous southwestcoastof Australia (Figure 1), the uppermostlavas are 207pb/204pb plotontrends between theIndian MORBfieldand compositionallyvery similar to the enrichedlavas at site 256. the field for Indian Ocean sediments[Ben Othman et al., 1989; An importantdifferenceis that all of the relativelyold Site 257 Vroon et al., 1993]. In general,abundanceratiosof Ce/Pb are  basalts havelowinitial206pb/204pb (<17.8).Thisfeature isa  distinctive characteristic of some recent Indian Ocean MORB,  unusuallyhigh in these easternIndian Ocean basalts(12 of 18 sampleshave Ce/Pb>40, Table 2, comparedto a typicalMORB ratio of -25 [Hofmannet al., 1986]). Sampleswith anomalously  which has not been observed in lavas from the Kerguelen Archipelago(Figures3c and3d). Site 213 is locatedin a region high207pb/204pb, however, havelowerCe/Pb, <40(Table 2). where the east-westmagnetic lineations are remarkably clear Mixing calculations by BenOthmaneta/. [ 1989] in theirstudyof (Figure 1), andthe basementis inferredto havebeenformedfrom sedimentrecycling into the mantle indicate that the additionof an east-westspreadingridge axis well north of the Kerguelen only 1% sediment to the mantle leads to low Ce/Pb and Plume [e.g., Royer et al., 1991]. Consistent with this anomalously high207pb/204pb. Weconclude thatthese samples interpretation,most of the Site 213 core is depleted MORB. may containsmallamountsof sedimentthat were not removedby However, the lowermost basalt in this core is enriched in  incompatibleelements(Table I and Figures 4 and 5) and has anomalousPb isotoperatios that are much higher than Indian  acidleaching. Therefore wedonotusethese high207pb/204pb valuesin our discussion of sourcecomponents.  Ocean MORB.In fact,207pb/204pb evenexceeds thatmeasuredOrigin of the Dupal Anomaly in lavasfrom the KerguelenArchipelago(Figure3c andTable 2).  Followingnumerous previousstudiesstartingwith Subbarao Occurrenceof Depleted MORB Most of the basaltsfrom six DSDP sites(Sites 212, 213, 257,  259, 260, and261) andODP Site 765, rangingin inferredages from -56 to 155 Ma, are geochemicallysimilarto Indian Ocean  andHedge[1973],DuprgandAlldgre[1983]showed thatmany oceanicislandbasaltsin theIndianOceanhavedistinctiveSt, Nd,  and Pb isotopicratios,whichHart [1984] termedthe Dupal anomaly. This large distinctiveisotopicdomainis centeredat  -30øSandisdefined by87Sr/86Sr >0.705 andA8/4> +60[Hart, MORB.They have relatively lowLa/YbandCe/Y,(87Sr/86Sr)i1988]. Hart [1988]notedthatonlythreelocalitiesin thenorthern hemisphere havelavaswith a Dupalsignature.The distinctive isotopiccharacteristics of IndianOceanMORB in comparison to Thesebasalts,however,have diversePb isotopiccharacteristics AtlanticandPacificMORBareinterpreted toresultfroma Dupal (Figure 3). Lavas associatedwith the KerguelenPlume, i.e., component incorporated into the Indian Oceanasthenosphere. hasalsobeeninferred those formin• theKerguelen Archipelago andNinetyeast Ridge, Thisisotopicallydistinctiveasthenosphere have initial 206pb/204pb of17.67 to18.71 (Figure 6a).Enrichedto be the sourcefor lavaseruptedin themarginalbasinsof the <0.7038  (the Site 212 sample is an exception) and  (143Nd/144Nd)i > 0.5127 and(eNd)i> 4.1(Figures 2 and5).  anddepletedlavasfrom DSDP siteswith agesof <125 Ma (Sites  westernPacific. Hickey-Vargaseta/. [ 1995]concludethatIndian  213,215,211,212,256,and259)have206pb/204pb within this Oceanasthenosphere, perhapsflowing in alongthe northern  19.25  (206pb!204pb)i St. Paul  Ninetyeast  Kerguelen Archipelago  18.75  Ridge 211 212  259  18.25  765  Kerguelen Plateau drilled -Plume 17.75  261  dredged-  Kerguelen  256  215  ßSWIR  !,[=8 ITJ  !,[=20 257  17.25  20  0  40  60  80  1O0  120  160  140  Age Ma 20O  !  Delta 8/4  Kerguelen Plume  150  Kerguelen Archipelago  Kerguelen Plateau  Ninetyeast Ridge  drilled  dredged 100  259  260  St. Paul  211 ITJ  256  [] 212 D  765  215  213  257  261  0  0  20  40  60  80  100  120  16o  140  Age Ma 0.5134  ß  *  !  C,  143Nd/144Nd  261 0.5132  ITJ  213  o.513oSEIRKerguelen  Ridge  Archi  257  Ninetyeast  212 D  SWlR  256  o.5128  259 215  0.5128  Rajmahal  211 0.5124  Kerguelen Plume  o.5122 0  ,  I 20  ,  I 40  ,  I 60  •  I 80  a  I 100  •  I 120  •  I 140  • 160  Age Ma  Figure 6. (opposite) Isotopic parameters (206pb/204pb)i, A8/4and (143Nd/144Nd)i versus age foreastern Indian ected. The upwardarrowfor Site260 indicates Oceanseafloor.(a) All 206Pb/204Pb ratios areage-corr thatthe ratio for a sill in this core is off-scale at 20.3. Because U and Pb data are not available for all lavas from Site 765,  corrected ratios areshown fortwoassumed 238U/204pb ratios (u- 8 and20). (b)TheAvalues fortheolder  basalts aremaximum values because 206pb/204pb ratios have been age corrected, butthe208pb/204pb ratios have notbeen corrected because precise Thabundance data arenotavailable. (c)All143Nd/144Nd ratios areagecorrected.ITJ, SWIR, andSEIR indicateIndianOceantriplejunction,Southwest IndianRidge,andSoutheast IndianRidge,respectively.Dashedlinesin Ninetyeast Ridgefield(Figures6a and6b) aredefinedby datafrom DSDPSite216 [FreyandWeis,1995]. FieldlabeledKerguelen Plumeis fromWeiset al. [1993a]. Datafor Site 765 arerevisedfrom LuddenandDionne[ 1992;J.N. Ludden,personalcommunication, 1995];datafrom Heard IslandarefromBarlinget al. [1994];all otherdatasources areasindicated in captionfor Figure2b.  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR  13,847  etal.,1996]haveunusually low206pb/204pb; (2)some of the  boundaryof Australia prior to -50 Ma, was an importantsource componentfor thesewesternPacific lavas. Of the 10 drill sites  oldestbasalts(110 to 155 Ma) in the easternIndian Ocean have  that recovered basalts from the eastern Indian Ocean, lavas at  lower206pb/204pb thanlavasassociated withIndianOcean  seven sites (<125 Ma) have A8/4 >33 (range 33 to 85). In contrast,the presenceof a Dupal componentis not obviousin the oldestlavasfrom the easternIndian Oceanseafloor;for example, lavasfrom Sites261 and 765 (-152 to 155 Ma) have very high  plumes(e.g.,Figure6a andMahoneyet al. [1995]) and(c) the  low206pb/204pb component inIndian Ocean MORBhasbeen presentsincethe initial formationof theocean,and,although minor in volume,it is widely distributedon eachof the Indian  143Nd/144Nd (Figure 6c),andlow87Sr/86Sr (Figure 2band Oceanridgesystems. Luddenand Dionne [1992]) and lavasfrom Site 261 have very low A8/4 (Figure 6b). Therefore, in the easternIndian Ocean Summary seafloor, there is evidence for a Dupal componentthat has persistedsince--125 Ma. We emphasizethat this 125 Ma age is EasternIndian Oceanseafloorbasaltsrangingin age from not rigorouslyconstrainedbecausethe agesof basementlavasat Eoceneto late Jurassic aretholeiiticbasalts,exceptat onesite theseDSDP siteshavenot beenreliablydetermined.The earliest near Christmas Island where alkalic basalts were recovered. Both manifestationof Dupal characteristicsin dated lavas is in the enriched anddepleted MORB havebeenrecovered.Theisotopic oldestlavas associatedwith the KerguelenPlume; i.e., the 110- characteristics of basaltsyoungerthan 125 Ma indicate the 118 Ma lavasforming the KerguelenPlateau. Because=118 Ma presenceof a Dupal component(lavas from Site 257 are an corresponds to the first unambiguousevidenceof activity of the exception) thatis absentin theoldest(155 Ma) seafloorsamples. KerguelenPlume, we infer that the Dupal anomalyis carriedby The first evidenceof activityof the KerguelenPlumeis at 118 the KerguelenPlume and that the sourceof this anomalyis deep Ma with volcanismon the KerguelenPlateau. This leadsus to withinthemantle[e.g., Castillo,1988;Weiset al., 1989a]. concludethat the KerguelenPlumeis the carrierof the Dupal The origin of the geochemicalcharacteristicsof the Dupal anomalyin theIndian Ocean. In addition,we proposethat the anomaly [Duprd and Alldgre, 1983; Hart, 1984], with its concentrationof this anomalyin the southernhemisphereis subequatorialconcentration,is an unresolvedproblem [Hart, relatedto thenearlyfixed locationof theAfricancontinentabove 1988]. Most geochemicalmodelsrequirea longisolationtime (1 the mantle. This situationfavored recyclingof continental to 2 Gyr) to develop the isotopic characteristicsof Oltl. A lithosphere into the mantle via delamination. Delamination commonlyproposedoriginfor theDupalanomalyis recyclingof resulted eitherfromthermalblanketing or underplating. subcontinentallithosphereinto the mantle by a delamination Someof the oldestseafloorlavaswhichpredatevolcanism process[Hart et al., 1986; Hawkesworthet al., 1986;Har• 1988; associated withtheKerguelen Plume havethelow206pb/204pb SunandMcDonough,1989]. We proposethattheconcentration values that are characteristic of some recent Indian Ocean of the Dupal anomalyto the subequatorial southernhemisphere MORB.Relatively low206pb/204pb istypical ofcontinental may be directly connectedto the nearly fixed location of the basaltsin Madagascarand westernAustralia;thereforewe infer African continent (and by extension the Gondwana thatwidely di?ersed continental lithosphere isthesource ofthe in IndianOceanMORB. supercontinent). This wouldalloweitherfor a thermalblanketing low206pb/20'*pb [Anderson, 1982] or an underplating(thickening) of the Relativelyold, >45 Ma, easternIndianOceanseafloorresulted lithosphere,which would favor delamination. An important fromtheactivityof threedifferentspreading systems whichhave aspectof the Indian Ocean and the easternAtlantic Ocean is the beenactiveat differenttimeperiods; in orderof decreasing age occurrenceof major flood basaltprovinceson the surrounding theseare a nearlyeast-weststrikingridgein the Argo Abyssal continents[White and McKenzie, 1989]. Because continental plainbordering Northwest Australia,a northeast-southwest ridge flood basalt provincesreflect a large magmaticoutput in a bordering southwest Australia,andaneast-west spreading system relativelyshortintervalof time, theycouldgeneratea thickened in the Wharton Basin (Figure 1). Basalts from the oldest lithosphericmantle. This thickenedlithosphericmantle could spreadingcenter,sampledat Sites 261 and 765 in the Argo subsequently delaminatecoldlithosphere into the asthenospheric AbyssalPlain,aredepletedMORB thatareverymuchlike lavas mantlethrougha processof thermalerosionor delamination.The eruptedalongthe active Southeast Indian Ridge. In contrast oldestflood basaltsin thisregionare the KarooandFerrar,which basaltsfrom the WhartonBasin,sampledat Sites213, 212, and eruptedin the middle to early Jurassic.Delaminationresulting 256, haveisotopicratiosindicatingthe presence of a Dupal from thesevolcanic eventsprecededthe first manifestationsof componentwhichwasderivedfrom the KerguelenPlume. the KerguelenPlumeby <100 Myr. Acknowledgments.We thank J. M. Rhodesfor use of the XRF facility at the Universityof Massachusetts, Amherst;P. Ila for assistance  Source ofAnomalously Low206pb/204pb A distinctive feature of some recent Indian Ocean MORB  is  in dataacquisition; J.-P.Mennessier forhelpwiththeisotope chemistry;  relatively low206pb/204pb, e.g.,lavas fromtheTriple Junction J.Scoates foreditorial assistance; and H.-J. Yang forgraphics. Wealso andportions oftheSWIR(Figure 6a).Lavas fromSites 257and thank J.Ryan, theJGRAssociate Editor andananonymous reviewer for  765also range tolow206pb/204pb (Figure 6a).Theorigin ofthe constructive reviews, and wehave benefited from discussions with M.  mantle component with low206pb/20d:pb isuncertain. Mahoney Coffin and J.Veevers. This research was supported byU.S. NSF grants  eta/.[1992] discussed two alternative possibilities: 1)Gondwana OCE-8823028 and EAR-9303535 and Belgian FRFC grant 2.9002.90. Precambrian lithosphere or(2)mantle plumes. The206pb/204pb valuesof IndianOceanbasalts asa function of eruption ageare References  important inevaluating these alternatives. Evidence against a Anderson, D.L., Isotopic evolution ofthe mantle: Amodel, Earth Planet.  plume origin isthatrecent lavas related toIndian Ocean plumes $ci. Lett., 57,13-24, 1982.  donot have similarly low 206pb/204pb [Mahoney et' al.,1992]. Barling, J.,S.L. Goldstein, and I.A. Nicholls, Geochemistry ofHeard  We favora continental lithosphere originfor the low  Island (southern Indian Ocean): Characterization ofanenriched mantle  Madagascar [Mahoney et al., 1992]andwestern Australia •Frey  manfie, J.Petrol., 35,1017-1053, 1994.  206pb/204pb because (1) some continental basalts from component and implications forenrichment ofthe sub-Indian Ocean  13,848  WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR  Hawkesworth,C.J., M.S.M. Mantovani, P.N. Taylor, and Z. Palacz, Ben Othman, D., W.M. White, and J. 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