Atlantic Ridge - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B2, PAGES 2941-2962, FEBRUARY 10, 1999

Plume-ridge interactions of the Discovery and Shona mantle plumes with the southern Mid-Atlantic Ridge (40ø-55øS) Jill Douglassand Jean-GuySchilling GraduateSchoolof Oceanography,Universityof RhodeIsland, Narragansett

Denis Fontignie D6partementde Min6ralogie,Universit6de Gen•ve, Geneva,Switzerland

Abstract. We reporton 66 Pb, Sr, and Nd isotopeanalysesof basaltsdredgedalongthe MidAtlanticRidge (MAR) from 40ø to 55øS. The resultsstronglyindicateinteractionand mixing betweenthe off-ridgeDiscoveryandridge-centered Shonamantleplumesandthe ambient asthenosphere beneaththe MAR. In addition,the Bouvetmantleplume appearsto be feedingthe southernmost portionof the MAR as suggestedearlier by le Roexet al. [ 1987]. The Discovery

andShona plumes haveenriched mantle andhigh-•t(g=238U/2ø4pb) affinities, respectively. Their proximityto oneanothersuggests a geneticrelationship,probablyassociatedwith subducted alteredoceaniccrustrecycledthroughthe mantlewith somesediment(Discovery)or without sediment(Shona). The DiscoveryRidge AnomalyexhibitsPb, Sr, andNd isotopicdiscontinuities resultingfrom southwardpreferentialplumeflow acrossthe Agulhastransformbeginning-13 Ma.

Thepresence of a component withunusually low 2ø6pb/2ø4pb accompanied by high87Sr/86Sr and low 2øSpb/2ø4pb andt43Nd/t44Nd in theDiscoveryRidgeAnomalyandto a lesserextentin the ShonaRidge Anomalyindicatesthree-component mixing betweenthe ambientasthenosphere, the Discoveryand Shonaplumes,andthislow-•t (LOMU) componentwhichpossiblyrepresents subcontinental lithosphericmantlematerial. We alsonote that in Pb, Sr, and Nd isotopicspace, oceanislandbasaltsfrom the Tristan,Gough,andDiscoveryfamily of plumescouldbe interpreted asresultingfrom binarymixing betweena genericplumecomponentsimilarto Bouvetor the "C" component [Hananand Graham,1994]andtheLOMU component, whichprogressively increases southward.The LOMU component seemsto be a characteristic featureof the SouthAtlanticand IndianOceanmantlesandis thoughtto residepassivelyin the shallowmantlebecauseof delaminationof subcontinental lithosphericmantlefollowingthebreakupof Gondwana.

1. Introduction

Evidence of chemical heterogeneityin the mantle was first revealed from striking inter-island and intra-island basalt Sr and Pb isotopic differencesin the South Atlantic [Gast et al., 1964]. At least four principal end-member isotopic components are currently recognized in oceanic basalts: depleted mid-ocean ridge basalt (MORB) mantle, high U/Pb

mantlesource (highg, whereg=238U/2ø4pb) (HIMU)), andtwo distinctenrichedmantle sourcecomponents(EM-1 and EM-2) [Zindler and Hart, 1986; Hart, 1988]. Three of these endmember components are readily identified in the South Atlantic. Namely, the Mid-Atlantic Ridge (MAR) between 3ø and 7øSis a depletedMORB type [Schillinget al., 1994], and the oceanic

islands

of St. Helena

and Tristan

da Cunha

are

HIMU and EM1 type, respectively[Baker et al., 1964; O'Nions et al., 1977; Sun, 1980]. Pb, Sr, and Nd isotopic ratios of MORBs from the MAR near the Tristan and St. Helena plumes exhibit isotopic signaturessimilar to these plumes [Hanan et al., 1986]. The similarity between the off-ridge mantle plumes Copyright1999 by the AmericanGeophysicalUnion.

Papernumber98JB02642. 0148-0227/99/98JB-02642509.00

and the nearby MORBs provides evidence that upwelling material derived from these plumes is dynamically entrained and mixed with upper mantle material feeding the mid-ocean ridge accretionprocess[Schilling et al., 1985; Hanan et al., 1986; Fontignie and Schilling, 1996]. As a result, sampling along mid-oceanridges allows detectionof mantle sources superimposed on the normal mid-ocean ridge accretion processes.

Both the Tristan and St. Helena plumes are associatedwith aseismicridges (e.g., (1) Walvis Ridge and Rio Grande Rise and (2) the St. Helena SeamountChain [O'Connor and Duncan, 1990], respectively). Aseismic ridges are one of the primary indicatorsof mantle plumesreflectingthe movementof crustal plates over hotspots[Wilson, 1965] and related deep mantle plumes [Morgan, 1971]. The seafloor in the southernmost part of the South Atlantic is dominatedby the presenceof the Discovery and Shona hotspot tracks (Figure 1), raising the question as to whether the plumes responsiblefor the tracks are interacting with the southernMAR. Sampling along the mid-ocean ridges is also useful for detecting heterogeneities associatedwith older events on a larger geographicscale. For example, Fontignie and Schilling [1996] and Hanan et al. [1986] have shown that long wavelength variations of isotopic ratios in MORBs unaffected by present-day plume

2941

2942

DOUGLASS ET AL.: DISCOVERY

AND SHONA PLUME-R•GE

"

INTERACTIONS

' •'•'. •':"---"-.-:'i::: :','

,•

,.

•

.

%:< .:..:...c

I::",-.:,... , "-......... v' ::i::.':-

'"

......................

I

I

':•L'.' ??'7 :'"/.:"'•:'. ':' • ...... •:..:.•::.:•:'; :•; ' ' • • '*•', • .-.•.,'.:-.- 9 ø can maintain preferentialplume flow toward the ridge to overcome plate drag. This implies that the dispersalof plume materi•il toward the ridge (southwest direction) should remain essentially radial (i.e., the stagnationfront) until it encounters

FZ

will

therefore

be reduced

or eliminated.

With

this

preferential plume flow model in mind the following chronologyfor the evolution of the Discovery plume/ridge system is proposed. 1. The Discoverymantle plume impactedon the baseof the lithosphere•- 35 Ma.

2. From 35 Ma to the presentthe Discoverymantleplume constructedthe main Discovery plume track north of the AgulhasFZ (Figure 1). 3. After the impact of the plume 35 Ma until •-13 Ma, dispersion of the plume material produced a series of seamountsalong the southernedge of the AgulhasFZ (Figure 1, Eastern Seamounts).

4. Around-13 Ma, the dispersalof the Discoveryplume material began directly feeding the MAR axis south of the

AgulhasFZ, creatinga hotspottrack of Morgan's[1978]

secondkind which is expressedon the seafloorby the "Little Ridge"chainof seamounts in Figure 1. slopesnear 9ø. Applied to our numericalmodel, this implies The dateof the Discoveryplumeheadimpactwasdetermined that the radial dispersal of the Discovery plume toward the using, at face value, the -25 Ma age of the basalt collected ridge should encounter the steep slopes associatedwith the from the DiscoveryTablemount[Baker et al., 1964; Kempe Agulhas FZ well before reaching the spreadingcenter. The and Schilling, 1974], the location of the easternmost steep slope acrossthe Agulhas FZ may preferentially induce seamountof the Discovery plume track, and the kinematic flow of the Discovery plume material south across of the plate reconstructionmodelsin the South Atlantic [Cande et al., AgulhasFZ. Plumeflow towardthe ridgenorthof the Agulhas 1988;Duncanand Richards,1991]. Symmetricspreading was

DOUGLASS

ET AL.: DISCOVERY

AND SHONA PLUME-RIDGE

assumed. Notice that the robust nature of the Discovery plume track seems to decline toward the western end of the track, near

the proposed present-day location of the Discovery plume (Figure 1). This change in the nature of the plume track is likely due to the diversion of the plume material across the AgulhasFZ toward the MAR axis. The origin of the EasternSeamounts(Figure 1) is somewhat complicated. Sleep [1996] and Morgan et al. [1995] indicated that the change in pressure associated with the cascade of buoyant plume material upslope across fracture zone discontinuities may induce some pressure release melting. Acrossthe Agulhas FZ the expectedexcessdegree of melting would be only -1%-5% assuming1.5%-2% melt fraction per kbar decompression [Cawthorn, 1975]. Morgan [1997] argued that the thinner depleted harzburgite layer near fracture zones results in enhancedupwelling and melting of off-axis plume material, particularly beneath the young side of the fracture zone.

The Eastern Seamounts are located on the southern side

of the Agulhas FZ, the younger side. In addition, the lithosphere in contact with the plume is expected to be deflected upwards because of (1) thermal reheating and thinning of the lithosphere by hot plume material [Crough, 1978; Derrick and Crough, 1978], (2) thermal expansion of hot plume material that has ponded at the base of the lithosphere [Crough, 1983; Davies, 1988; Olson, 1990; Sleep, 1990], (3) compositionalbuoyancy resulting from the "underplating" of material which has experienced melt extraction [O'Hara, 1974; Boyd and McCallister, 1976; Oxburgh and Parmentier, 1977; Jordan, 1979], and (4) crustal thickening associated with plume volcanism [Burke and Whiteman, 1973]. These buoyant forces may uplift the lithosphere over the dispersed plume material enough to enhancedecompressionmelting resulting in the formation of the EasternSeamounts. The excessvolcanismrequiredto form the seamountswould have begun when the dispersal of the Discovery mantle plume first reachedthe Agulhas FZ. Such volcanism may be occurring today but in a more subdued fashionbecauseof the captureand deflection of plume material toward the MAR axis beginning -13 Ma. This deflection may account for the gap in constructive volcanism between the Eastern Seamountsand the Little Ridge (Figure 1). Testing this model would require sampling and analyzing these two seamount

chains.

INTERACTIONS

2953

plume flow toward the MAR, resultingin excessvolcanismat the ridge axis. This preferentialplume flow model adequatelyexplainswhy the basalts just south of the Agulhas FZ (i.e., the central

Discovery Ridge Anomaly basalts) show the greatestaffinity to the Discovery plume. However, basalts north of the Agulhas FZ exhibit Discovery plume influence as well. We interpret this to indicate that while most of the Discovery plume material is deflected south, some of the material is

presentlyreaching the MAR axis north of the Agulhas FZ. This northern segmentis further from the plumes than the centralsegment,resultingin a more dilutedplumesignature. 5.2.3. Heterogeneous sources. While the preferential mantle plume-MAR interaction model accountsfor the Discovery plume signatureof basaltsfrom the northernand

centralsegmentsof the DiscoveryRidge Anomaly,it doesnot

readilyexplaintheunusually low 2ø6pb/2ø4pb ratiosof basalts from the southernsegmentof the DiscoveryRidge Anomaly (48.5ø-49øS). These southernDiscovery Ridge Anomaly basaltsare systematicallyshifted toward low 2ø6Pb/:ø4pb relative to the other Discovery Ridge Anomaly basalts (Figures4a, 4b, 4c, 4d). Traditionally, this separatetrend would be interpreted as a binary mixing line between an ambient mantle source and another enriched source similar to

Discoverybut with lower2ø6pb/2ø4pb. However,undertwo particularmixing conditionsthe covariationof two isotope ratios in a three-componentmixture may also form a line, whichhasbeencalleda pseudobinary mixingline [e.g.,Hartan et al., 1986; Schilling et al., 1992]. The first condition requiresthat the mass fraction zi of one componentvaries linearly with the mass fraction of one of the other two

components. Second, in two-dimensional(2-D) isotope covariation diagrams (e.g.,

206

Pb/ 204 Pb versus87 Sr/ 86 Sr) the

normalized concentration of 86Srincomponent 1 relative to component 3, nl, and the normalizedconcentration of 2ø4pb in component1 relativeto component3, ml, mustbe the same (i.e. nl=ml). However,nl andml maydifferfromn2 andm2, respectively. Of course,in Pb-Pb isotope diagrams,this second condition is readily met. The orientation of this pseudobinarymixing line in such three-component mixing systemscan be in any direction, dependingon the relative mixing proportions considered for the first condition, and

doesnot necessarily"point"to the compositionof any end-

Our reconstruction model for the position of the MAR member(J. Douglassand J.-G. Schilling,Systematicsof threerelative to the presumed location of the Discovery plume component,pseudobinarymixing lines in 2-D isotope ratio (assuming symmetric spreading and using Duncan and spacerepresentations and implicationsfor mantle plume ridge Richards' [1991] poles of rotation for the South American and interaction,submittedto Chemical Geology, 1998, hereinafter African plates) indicates that it would have been moving referredto as Douglassand Schilling,submittedmanuscript, easterly towards the Discovery plume for at least the past 36 1998). Myr (Figure 1 inset). The plume dischargeto the MAR south Four end-member caseshave been identified, one of which is of the AgulhasFZ first startedbetween 12 and 14 Ma, judging directly applicable to the South Atlantic (Douglass and from Cande et al.'s [1988] magnetic lineation map and the Schilling, submitted manuscript, 1998). For this case the easternterminusof the Little Ridge (Figure 1). The changein pseudobinarymixing line extendsfrom a single end-member behavior from intraplate volcanism to volcanism of Morgan's (end-member 1) toward a composition intermediate between [1978] "secondkind" (i.e., dischargeof plume material at the the other two end-members(end-members2 and 3) (Figure 4b ridge axis) was precipitatedby the MAR's approachtoward the inset). This geometryrequiresthat the ratio between the mass Discovery plume. As the MAR came closer to the Discovery fraction of end-members 2 and 3 remain constant while the mantle plume, the plume material crossingthe AgulhasFZ was absolutevalues vary. For the origin of the southernDiscovery encountering steeper and steeper sublithospheric slopes Ridge Anomaly basaltsthe three mixing end-membersare the orthogonalto the ridge axis. Slopes orthogonal to the ridge ambient depleted mantle source,the Discovery plume source, south of the Agulhas FZ are steeper than those north of the anda low 2ø6pb/2ø4pb mantle heterogeneitypassively Agulhas FZ. Most likely, around 13 Ma these orthogonal embeddedin the asthenosphere,similar to that sampled by slopes south of the Agulhas FZ were steep enough to induce basalts from the 39ø-41øEsegmentof the SouthwestIndian

2954

DOUGLASS ET AL.: DISCOVERY

AND SHONA PLUME-RIDGE

Ridge,which we refer to as the LOMU component[Douglasset al.,

1996].

INTERACTIONS

For the first model the predicted isotopic variation associatedwith the Shonamantle plume matcheswell with the actual Pb, Sr, and Nd (not shown) isotopicdata for the most part (Figure 6). However, the model doesnot producethe low

The ratio of the mass fraction of the LOMU

componentto the Discovery plume componentwould remain constantbecauseof the dependenceof the LOMU material on the hot plume material to lower its viscosity and induce 87Sr/86Sr andhigh143Nd/144Nd ratiosof thebasalts in the melting (Douglass and Schilling, submitted manuscript, northernmostpart of the Shona Ridge Anomaly. The second 1988). The nature of this LOMU componentwill be discussed and more significant problem with the radial model is that it further in section 5.3. requires the ratio k of the normalized concentrations (i.e., component relative to component 2, for example, 5.3. Shona Ridge Anomaly kpb/sr=(Pbl/Pb2)/(Srl/Sr2))to be much differentthan 1. Figure 8a showsmodel mixing curvesfor constantk correspondingto 5.3.1. Plume-ridge interaction radial mixing model. The composition of the Shona plume is difficult to mixing end-membersfor radial model 1. For many of the constrainbecauseof the poor correlationof the Pb with the St, basalts,the mixing curvature-controllingratio for Pb and Sr Nd, and He isotopes. Consequently, two end-member cases system,kpb/S r wouldhaveto be > 10 or < 0.5. were considered. The first model uses an ambient mantle and The secondmixing model uses end-membercompositions Shona end-member compositionswhich are compositionally which are isotopicallymore extremethan any of the northern as close to basaltsfrom the Shona Ridge Anomaly as possible Shona Ridge Anomaly basalts(Table 2). As a result, the k to still bracket the data (Table 2). The second model uses an neededto spanthe data are closerto unity (0.1 - 10) (Figure averageof depletedbasaltsfrom the 3ø-7øSsegmentof MAR, 8b). However, the along-ridge profiles yield higher Pb which are the most depleted basaltsfound along the MAR, for isotoperatios than are shownby the basalts. Reconcilingthe the ambient mantle end-member. The model 2 Shona endPb isotope data with the along-ridge profile would require member is based on a best fit line through the northern Shona moving the plume sourcefurther from the ridge (off-ridge) and Ridge Anomaly basalts forced through the 3ø-7øSdepleted consequentlylowering the along-ridge Pb isotope profile. mantleend-member. The Shonamantleplume is assumedto be However,both the along-ridgegravity and the 3He/4He centeredon dredgestation20D, on the basisof the peak of the variation suggest that the Shona plume is ridge-centered along-ridge3He/4He[Moreiraet al., 1995]andthe centerof (Figure 6b). A more obvious problem is the very low the residual mantle bouguer anomaly [Small et al., 1995]. The 2ø8pb/2ø4pb ratioswhichwouldbe produced by usingthe3øradial extent of the Shona plume material is difficult to 7øSMAR segmentfor the ambientmantleend-member.Again, determine becausethe Shona Ridge Anomaly is bound to the as for the Discovery region, this clearly indicates that the mantle beneath the southernmost MAR is north by the Discovery Ridge Anomaly and to the southby the ambient central segment of the Shona Ridge Anomaly. As a result, a characterized by relativelyradiogenic 2øspb/2ø4pb perhaps causedby some early, regional contaminationevent. radiusof 300 km was chosenas a best compromise.

a

Shona1

B

.001 0 .1

.7035-

• .7030

•

.5

Model

1

.7025 ambient asthenosphere 1

1000

I

b

Shona 2 B .7O35-

o3ß7030-

Model 2

r,D

-

2& .7025-

1 ooo

ambient asthenosphere2

.7020-•

I

18.0

'

I

18.5

'

I

19.0

19.5

2o6pb/2O4pb Figure 8. The 2ø6pb/2ø4pb ratio versus87Sr/86Sr showingmodelmixing curvesfor constantk

((Pb/Sr)plume/(Pb/Sr)asthenosphere) usingthemodel1 andmodel 2 end-members (Table2). Alsoplotted are the ShonaRidge Anomaly basaltcompositions.Numbersnext to the mixing curvesindicatethe value of k. (a) model 1 requiresa largerangeof k to spanthe data. (b) model2 requiresa smallerrangeof k but requiresmore extreme end-membercompositions.

DOUGLASS ET AL.: DISCOVERY

AND SHONA PLUME-RIDGE

Both models clearly indicate that a k > 1 is required to adequately encompass the northern Shona Ridge Anomaly basalts. The fact that the Sr, Nd, and He isotopesare strongly coupled, whereasthe Pb isotopesare only correlatedwith each other suggestssome variation in the Pb concentrationsin one or both of the mixing end-members. The larger range in k values required to span the data might be producedby mixing of variably evolved melts resulting from variable melting and crystallization conditions [Verma et al., 1983]. However, trace

element

ratios

in these basalts

indicate

curvature

ratios

restrictedbetween 0.3 and 3.5 (J. Douglass, unpublisheddata, 1998). Both the along-ridge profiles and the mixing curves suggest that most of the data can be explained by such curvature. If the end-membercompositionsshow some degree of isotopic heterogeneity,we can reduce the need for extreme k values. However, some of the basaltsmay still require extreme k values (0.1 respectively).

to 100 or 0.1 to 10, models

1 and 2,

INTERACTIONS

2955

However, increasing the radius would only inflate the northern Shona Ridge Anomaly profiles such that they would show extremely poor matcheswith the data. In Sr versusNd isotopic space(Figure 5) these southernShonaRidge Anomaly basalts are offset from basaltsfrom the northernsegmentof the Shona Ridge Anomaly and lie in the field of basalts associatedwith the Bouvet plume. Because of the isotopic and geographic closenessof the southern Shona Ridge Anomaly basalts and the Bouvet plume basalts, we assume that the Bouvet mantle plume is feeding the accretion process at the southernmost portion of the MAR as suggestedby le Roex et al. [1987].

5.4. Probable Nature

of the LOMU

Component

The LOMU componentis genericallydefined [Douglasset al., 1996] as a mantle component characterized by low

2ø6pb/2ø4pb, 2øSpb/2ø4pb, and143Nd/144Nd, high87Sr/86Sr, and variable2ø7pb/2ø4pb (Table2) thoughtto haveoriginated in the subcontinentallithospheric mantle. The LOMU component

As an alternative to the binary mixing model, the distinctly nonlinear field of the northern and southern Shona Ridge

sharesthe low 2ø6pb/2ø4pb ratiosof the EM-1 component but alsohasthe veryhigh87Sr/86Sr andverylow 143Nd/•44Nd ratios

Anomaly basalts in 2ø6pb/2ø4pbversus87Sr/86Sr and 143Nd/144Nd isotopic space may be explained by

more typical of the EM-2 component. The LOMU component is defined both in terms of its isotopic composition, as described above, and its geological associationwith mantle plumes in the South Atlantic and Indian Oceans, as described

contaminationof the ShonaRidge Anomaly basaltsby a third component similar to the LOMU component evident in the southern Discovery Ridge Anomaly basalts. This additional mixing end-member would cause increased dispersionof the Shona depleted mantle binary mixing array towards high

below.

material

significantlylower 2ø6pb/2ø4pb.At this time no suchend-

5.4.1. LOMU

setting. The following observations

on the occurrenceof the LOMU componentmust be taken into 87Sr/86Sr andlow143Nd/144Nd asseenin Figure4. consideration when modeling its possible origin and 5.3.2. Heterogeneous sources. Neither of the two relationship with the South Atlantic mantle, the Indian Ocean binary mixing modelsconsideredadequatelypredictsthe high mantle, and the DUPAL anomaly. 87Sr/86Sr of thebasalts fromthecentralsegment of theShona 5.4.1.1. MORBs with a LOMU affinity: Basalts Ridge Anomaly (Figure 6). These basaltsare offset from the with a LOMU affinity are found along the Walvis Ridge mainShonaRidgeAnomalyfield towardlower2ø6pb/2ø4pb[Richardson et al., 1982; Milner and le Roex, 1996], the 39øand lie very near or within the field for the Discovery Ridge 41øE segment of the SWIR [Hamelin and All•gre, 1985; Anomaly basalts (Figure 4). If these basaltsare related to the Mahoney et al., 1992], the 85øE Ridge at the Afanasy-Nikitin Discovery plume, this would imply that the Discovery Ridge seamounts[Mahoney et al., 1995, 1996], and basaltic dikes Anomaly and Shona Ridge Anomaly overlap and that the from southwesternMadagascar[Mahoneyet al., 1991]. All of Shona Ridge Anomaly basaltsclosestto the Discovery Ridge thesebasalts arecharacterized by unusually low 2ø6pb/2ø4pb, Anomaly (i.e., northern segment of the Shona Ridge 2ø8pb/2ø4pb, and143Nd/144Nd andhigh87Sr/86Sr relativeto Anomaly) contain significant fractions of Discovery plume normal depleted MORBs or geographicallyassociatedbasalts material. However, basalts from the northernmostportion of (Figure9). The 2ø7pb/2ø4pb ratio of theseLOMU basalts the Shona Ridge Anomaly show no affinity for the Discovery appearsto be variable from one locality to another. plume(Figure4), indicating thatthelow 2ø6pb/2ø4pb of the Previously, the linear isotopic trend of the southern central Shona Ridge Anomaly basalts are not related to the Discovery Ridge Anomaly basalts was interpreted as a Discovery plume. However, this does not preclude the central pseudobinary mixing line produced by ternary mixing Discovery Ridge Anomaly basalts'having been affected by the involving depleted mantle, Discovery plume material, and a presenceof a detached "blob" of Discovery plume material, LOMU component. Invoking a true binary model requires assuming that a discontinuous train of Discovery plume another end-member similar to Discovery but with reaches the MAR.

Alternatively, the low 2ø6pb/2ø4pb and moderately high member has been found in the South Atlantic. A ternary 87Sr/86Sr andlow 143Nd/144Nd of the centralShonaRidge mixing model involving a LOMU componentis now justified Anomaly basaltsare consistentwith the presenceof a LOMU component. The similar morphotectonic setting (short segmentconstrainedbetween fracture zones with a well-defined axial valley (Figure 1)) and isotopic displacement of the southern segment of the Discovery Ridge Anomaly and the central segment of the Shona Ridge Anomaly indicate that they may be samplinga LOMU componentof similar origin. The nature and distribution of this LOMU componentwill be

by the multiple occurrences of the LOMU component in MORBs and basalts from the Indian Ocean. Similarly, the LOMU component is invoked for the central Shona Ridge Anomaly basaltsbecauseof the lack of any other appropriate end-member and the increasingly frequent occurrenceof the LOMU componentin the SouthernOcean. Consequently,the basalts from the Discovery Ridge Anomaly and the Shona Ridge Anomaly are added to the list of basalts associatedwith further discussed in section 5.4. the LOMU component. The shortradial extent of the Shonaplume materialprevents This collection of LOMU-type basalts all exhibit common the radial model from predictingthe high Pb and Sr isotope characteristics. Each of these LOMU-type basalts are ratios of the southern Shona Ridge Anomaly basalts. associatedwith mantle plumes. The 39ø-41øEsegmentof the

2956

DOUGLASS ET AL.: DISCOVERY AND SHONA PLUME-RIDGE INTERACTIONS

Madagascar, WalvisRidge,and SWIR and southwestern Madagascar basalts are spatially (Figure10). The southwestern related to the Marion and/or Crozet plume activity [Mahoney Afanasy-Nikitin basalts are appear to be geographically et al., 1991]. The Afanasy-Nikitin Seamountis associated limited as well, thoughadditionalsamplingwould be required with the Crozetplume[Curray and Munasinghe,1991;Muller et al., 1993; Mahoney et al., 1996]. The Walvis Ridge is the mantle plume trace of the Tristan plume [Wilson, 1965; Morgan, 1971, 1981, 1983; O'Connorand Duncan, 1990]. Finally, basaltsfrom the southernsegmentof the Discovery Ridge Anomaly are relatedto the off-ridgeDiscoverymantle plume,and the central-Shona Anomalybasaltsare relatedto the ridge-centered Shonamantleplume. All of thesebasalts were producedwhile their associated plumewas either ridge centered,near ridge-centered, or activelyfeedingthe accretion processat the ridge spreadingcenter (same referencesas

for confirmation.

5.4.1.2. Off-ridge South Atlantic plumes with a LOMU affinity: The Tristan, Gough, and Discovery family of off-ridge, EM-type hotspotsindicate the presenceof a LOMU mixing component. The observationthat the Tristan

plume may have a LOMU-type componentwas initially

observed by Milner and le Roex [1996]. The isotopic compositionof the Discovery plume is thoughtto be more extreme than any MORB recovered in the Discovery Ridge Anomaly. Our estimate of the Discovery plume composition (Figure 9, "D", and Table 2) lies on the end of the Discovery above). These observations indicate that the LOMU Ridge Anomaly mixing array, which includes the Discovery componentis intimately related to mantle plume activities, Tablemount. Using this compositionfor the Discovery plume specifically,while interactingwith spreadingridges. and averagesof basalts from the Tristan and Gough islands, The "outcrops"of these LOMU type basaltsare typically these three off-ridge hotspotsform a tight linear mixing trend small and discrete. In the Shonaand Discovery Anomalies and suggestingthat the three plumes may be the result of binary the 39ø-41øESWIR locationsthe LOMU signatureis confined mixing between an intermediate2ø6pb/2ø4pb plume to a single, small segmentof the mid-ocean ridge system component similar to Bouvet or the "C" component [Hanan

andGraham,1994]anda low 2ø6pb/2ø4pb component (LOMU a

component) (Figure 9 inset). The trend of the Tristan, Gough, and Discovery family of plumes and the fields for the Tristan

41

Group I

plumeandWalvisRidgebasalts indicates thatthe2ø7pb/2ø4pb

."

Kimberlites, ' ß

St.H

ß

4O

composition of the LOMU end-members becomes higher toward the south, perhaps reflecting progressively older subcontinental lithospheric mantle southward. If such a mixing scenariois valid, the fraction of the LOMU component appears to increase southward, with the Tristan plume having the smaller fraction and the Discovery plume having the greater fraction of the LOMU component.

,

ß

,

,

,

Trista Crozet" . '.•• 39-

Kimberlites

. ust '• ,• ½•••;;

38

.....

Lamproites,' • •;•icaceous•T

oo o c•

e Figure 9. The2ø6pb/2ø4pb ratioversus(a) 2øSpb/2ø4pb, (b)

37

Smoky Butte

36

878r/86Sr,(c) 2ø7pb/2ø4pb, and (d) 143Nd/144Nd, and (e) the 87Sr/86Srratio versus 143Nd/144Ndfor LOMU-type basalts, kimberlites,and lamproites. D is Discovery(Table 2). S, B, and St. H as definedin Figure 4. The thick dashedfield is for

16 17 18 19

i

i

i

W. Australia,

'

Lamproites'

'

.710-

,'

'

Micaceous ,'

Indian MORB (referenceslisted in Figure 4). The gray field is

•LOMU

for North Atlantic and Pacific MORB

Kimberlites ' ' ', ' , '

.706 ,

.710- '

kimberlites [Fraser et al., 1985], and Western Australia

.704

Indian

' p

lamproitesand kimberlites [McCulloch et al., 1983; Fraser et al., 1985]). Thickly outlined fields are basaltswith a LOMU affinity (Marion [Hart, 1988], Crozet [Mahoney et al., 1996], Walvis Ridge [Richardsonet al., 1982], Madagascar[Mahoney et al., 1991]; A. N. is Afanasy-Nikitin [Mahoneyet al., 1996]; Tristan [le Roex et al., 1990], Gough [Sun, 1980], and NHRL from [Hart, 1984]). LOMU-type basalts all show general trends toward the fields of the Smoky Butte lamproites, Western Australian lamproites, and Group 2 kimberlites consistent with a subcontinental lithospheric mantle origin

,

Group 2

,

,' DM tA.A-• .702' 17 18

Kimberlites,,' ,, ,

,

•

..___,0A'N-

2'O

,

D

,'

Gough

•'n•l•; Butte

\ Tristan ,'.......

Madagascar Walvis\

.7O5

19

,

ß

I

(references listed in

Figure 4). The thin dashed fields are kimberlites and lamproites associated with the subcontinentallithospheric mantle (Smoky Butte lamproites[Fraser et al., 1985], Group 1 kimberlites and Group 2 kimberlites [Smith, 1983], micaceous

.708-

,' Group 1

for the LOMU basalts. The insets show schematic mixing models for producing the intraplate Tristan, Gough, and

39_41OE ** •,•,:i•berlites

SWIR • '

Discoveryfamily of plumesand the DUPAL-type MORBs. The thick

Crozet

/.,.. ........ OMario nB

-

16

!

17

18

1'9

2o6pb/2O4pb

lines

are

the

North

Atlantic

and

Pacific

MORB

or

Northern Hemisphere Reference Line (NHRL) [Hart, 1984]. The

St.H

2'0

21

thick

dashed

line is the Indian

MORB.

The

thin dashed

lines are binary mixing lines for the Tristan, Gough, and Discovery family of plumes. T, G, Discovery as defined in Figure 4. P is the composition of the generic plume component.

DOUGLASS

ET AL.: DISCOVERY

AND SHONA PLUME-RIDGE

INTERACTIONS

St. H

c

W. AustraliaKimberlites

15.8-

_

Group 1 Kimberlites _

.

"- -'Group 2

Crozet ,'

Micaceous Kimbe. rlites Gough

Kimberlites-l

o %

15.6-

O•

[ ,_

•,

' .....

•.:.::?,•.,:;:.:...::::,:,,•r-..:::,:,:•:,.•::•::.::ii.'.-,::::::• ::.

39-41øE • •','%,•;,.•,-' •....T ':• ...; , SWIR••{•'2 -,.• .....'•.. -'...... '

r-.

, ', , ,

-

'

• •

.

•

•

•2••?', /•••-.,.

........... •