JOURNALOF GEOPHYSICALRESEARCH, VOL. 95, NO. B8, PAGES12,697-12,711, AUGUST 10, 1990

The Cobb-EickelbergSeamount Chain: HotspotVolcanism with Mid-Ocean Ridge Basalt Affinity

DANA L. DESONtoAND ROBERTA. DUNCAN

Collegeof Oceanography,Oregon State University, Corvallis

Cobbhotspot, currently located beneath Axial seamount onthe Juan de Fuca ridge, has the temporal butnot the isotopic characteristics usually attributed to a mantleplume. The earlier volcanic products of thehotspot, from eight volcanoes in the Cobb-Eickelberg seamount (CES) chain, show a westwardage progressionaway from the hotspot and a westwardincrease in theage difference between the seamounts andthe crust on which they formed. These results are consistent with movement of the Pacific plate over a fixedCobb hotspot and eventual encroachment bythe westwardly migrating Juan de Fuca ridge. CES lavasare slightlyenriched in alkaliesand incompatible elements relative to thoseof the Juande Fuca ridgebut they have St, Nd, andPb isotopic compositions virtually identical to thosefound along the ridge.Therefore, Cobb hotspot is a stationary,upper mantle melting anomaly whose volcanic products showstrong mid-ocean ridge basalt (MORB) affinity. These observations can be explainedby low degreesof partialmelting of entrainedheterogeneous upper mantle MORB sourcematerial within a thermallydriven lower mantle diapir or by anintrinsic MORB-like composition of the deeper mantle sourceregion from whichnortheast Pacific plumes rise.

INTRODUCTION the Endeavoursegment has been attributedto the contribution Sincehotspots were first describedby Wilson [1963] and of theHeckle melting anomaly to theoverriding Juan de Fuca linked to convectivemantle plumes by Morgan [1972], both spreadingridge melts [Karsten, 1988]. temporaland chemicalcharacteristics of thesefeatures have Axial seamount, which straddles the JDFR axis at -46øN beenrecognized. The linear array of volcanoesand the age- (segment2) [Delaneyet al., 1981] is the youngestexpression progressivedistribution of hotspottracks [Morgan, 1972] are of volcanismover an upper mantle melting anomalytermed two of the more distinguishingfeatures of volcanismresulting the Cobb hotspot(Figure 1). Older volcanicproducts of the from a fixed mantle plume. Some hotspots have been hotspot,the Cobb-Eickelbergseamount (CES) chain, includea persistentover long periods,a feature clearly seen in the line of edifices on the Pacific plate, from Brown Bear volumesof lava producedby the Hawaiianhotspot, while other northwestthrough Corn, Cobb, and Pipe seamounts,a group Pacific plate hotspots have produced shorter or more of small volcanoes known as the Seven Deadly Sins intermittent seamount or island chains. Thus far, volcanic seamounts,and finally the larger Warwick, Eickelberg,and rocks formed at hotspots have been characterized by Forster seamounts(Figure 1). Two seamountsto the east of distinctivecompositions, usually more radiogenicSr, Nd, and Axial seamounton the Juande Fucaplate (Thompsonand Son Pb and higher concentrationsof incompatible elements, of Brown Bear) are inferred to be related to this linear volcanic relative to basaltserupted at spreadingridges. The isotopic province. Beyond Forster seamount,the westernmostedifice compositionof lavas eruptingover a given hotspotmay not in the Eickelberggroup, is a gap of 800 km before a line of be constantwith time, nor are all hotspotsalike isotopically, seamounts which includes Miller, Murray, and Patton but as a group these rocks have been termed ocean island seamounts.These older volcanoeshave been interpretedto be basalts(OIB) as distinguishedfrom basaltsthat form at mid- earlier productsof volcanicactivity at Cobb hotspot[Duncan oceanridges (MORB). The Cobb-Eickelbergseamount chain, and Clague, 1985; Smoot, 1985], but this suggestionhas been and other hotspot-associatedvolcanic lineaments in the questionedby Dalrympleet al. [1987] becauseseamount ages northeastern Pacific, are unusual in that they exhibit the do not completelyagree with rigid Pacificplate motion over a temporalcharacteristics produced by othermantle plumes, but fixed hotspot. not the OIB isotopicsignatures. The Pacificmantle rotations proposed by Duncanand Clague Basedon differentmorphologic and tectoniccharacteristics, [1985] and Pollitz [1988] place Miller seamount(26 Ma the Juande Fuca ridge (]DFR) was dividedinto four spreading [Dalrymple et al., 1987]) directly over Cobb hotspotat the segmentsby Delaney et al. [1981]. Along-segmentand time of its formation. Basaltic rocks fxom this seamount have between-segmentvariations in basaltchemistry give evidence relativelylow alkali contentsand flat rare-earthelement (REE) of separatemagmatic systems beneath the ridge[Liias, 1986]. patterns[Dalrymple et al., 1987] similarto volcanicproducts The Endeavoursegment, northernmost of the four, whichlies of the CES lineament. Backtrackingof Murray and Patton north of the Cobb offset (Figure 1) [Delaneyet al., 1981] is seamounts,however, places their positions at origin some chemicallythe most primitive (highestMg-numbers) yet has 150-250km westof Cobbhotspot. The higheralkali contents the mostincompatible element enriched lavas found along the and light-REE enriched nature of the Murray and Patton ridge[Liias, 1986;Karsten, 1988]. The unusualchemistry of seamount basalts would be consistent with their formation as a product of late-stagerejuvescent volcanism downstream from the Cobb hotspot, or with formation at another,now extinct, Copyright 1990 by the American Geophysical Union. hotspot. Whether or not the three older seamountsformed at Cobb hotspot, there has been a lack of consistencyof Paper number 89JB03074. volcanismover the melting anomaly;it either becameactive 0148-0227/90/89JB-03074505.()0 or it teacrivedafter a long periodof quiescencemore thannine

12,697 12,698 DESONIEAND DUNCAN:• COBB-EICg•.T.nEROSEAMOUN• CHAIN

150 ø 145 ø 140 ø 135 ø 130 ø 125 ß 60 ß

NORTH AMERICAN

PLATE

Giacomini .•mount Pratt Guyot

Weaker Guyot

55 ß

Patton Seamount DIckins 8eamount o o DensonGuyot 4• Guyot /MillerSeamount Davidson Gu

Parker Seamount Bowie Seamount

Tuzo Wilson Solmounts ß

athflnder •/'d•8..mount PACIFIC PLATE

T.Horton Guyot•• 50 ß

contour interval = 400 ß Hock8mt•. '•

Unione• •G

DE FUCA 45 ø PLATE

Pros. Jackson Solmounts

40 ø

ntour interval

Fig. 1. Bathymetricmap showingseamounts, spreading ridges (double lines) andfracture zones (single lines) of the northeastPacific, after Chase et al. [1970]. Inset showsseamounts of the Cobb-Eickclberglineament and Explorerand Union volcanoes.

million yearsago with the formationof Forsterand Eickelberg originatedby different mechanisms.The longestand best seamounts. Since the hotspot began this latest pulse of known of theseis the Pratt-Welkerseamount chain (Figure 1). activity, volcanicoutput has been continuousbut has varied up Dalrymple et al. [1987] suggesteda complex history to 20% [Karsten and Delaney, 1989] which is within the involving at least two hotspots;some of the seamounts known variability of other hotspots [Bargar and .lackson, formed in a midplate setting while volcanismat or near a 1974; Lonsdale, 1988]. spreadingridge formed others [Turner et al., 1980]. The Pratt- Within the northeast Pacific basin are several other linear Welker hotspot(s)may be located near the Tuzo Wilson volcanic chains, all of which are generally subparallelwith seamounts [Chase, 1977; Cousens et al., 1985], Bowie more prominent Pacific chains to the southwest. These seamount[Turner et al., 1980], or the Dellwoodknolls [Silver lineaments differ in length and volume and may have et al., 1974]. All three of theseyoung volcanicsites (Figure DESONIEAND DUNC•: Tm• COBB-EICKELBERGSEAMOUNI' CHAIN 12,699

1) are proposedto reflecthotspot activity, although the Tuzo lavas. We discussthese data in termsof possibledynamic Wilson seamountsand Dellwoodknolls may haveoriginated models of mantle plume and asthenospheremixing, and by partialmelting in a pull-apartbasin [Allan et al., 1988]. A concludethat the deep mantle under the northeastPacific more southerlyline of seamountsin the northwesternGulf of region is intrinsicallymore MORB-like than the centraland Alaska, the Horton-Pathfinder-Parker group, cannot be southPacific [Hart, 1984, 1988]. attributedto a currently active hotspot. Severalshorter seamount chains lie closeto andintersect SAMPLEDESCRIFrION$ the JDFRfrom the west. Theseappear to reflecta much Basaltsamples from the Cobb-Eickelbergand Explorer- shorter-livedand shallower-origin melting phenomenon. The Unionseamounts were obtainedfrom J. Delaneyand P. Heckand Heckle seamount chains, for example,may be the Johnsonof theUniversity of Washingtonand were originally result of passiveupwelling and partial melting of dredgedduringR/VThomasThompson cmises Tr063, Tr080, heterogeneousupper mantle in advanceof thenorthwestwardly and TI'175. TheCobb seamount sample was obtained by a migratingJuan de Fucaspreading ridge [Davis and Karsten, diver from the pinnacle of thevolcano. Dredge locations are 1986]similar to thesetting of theLamont seamounts near the givenin Table1. EastPacific Rise [Fornari et al., 1988a]. Otherseamounts and Samplesused in this study for age determinationsand small seamountchains, including Explorer and Union compositionalanalyses were the freshest available, with little seamountswhich lie to thenorth of theCES (Figure 1), areof to no interstitialglass and only slight to moderatealteration. unknownorigin. All aforementionedseamount chains are CES basalt samplesare generallymicrocrystalline and foundon the Pacificplate; only two edifices,Thompson and aphyric. In some samples,phenocrysts of plagioclase, Son of Brown Bear, which are interpretedto be relatedto clinopyroxene,and Fe-Ti oxidesare foundin an intergranular volcanismat Cobb hotspot[Desonie and Duncan, 1986; or intersertalmatrix. Plagioclasephenocrysts are generally Karstenand Delaney,1989] were formed on theJuan de Fuca freshbut may be slightlyresorbed or zoned. Both samples plate. from Eickelberg seamount used in this study contain In this paper we report new nøAr-39Al'and K-Ar age plagioclaseglomerocrysts. Several basalt samples are highly determinationsfrom eight volcanoesof the CES which vesicular,indicating that the magmasdegassed as they cooled documenta clear age progressionalong the lineament,in andwere probably erupted at a relativelyshallow depth or had concertwith plate motionover a stationaryhotspot. Major high ascent rates [Dixon et al., 1988]. Secondary and traceelement contents are slightlyenriched over thosefor mineralization is slight; the matrix of some samples is heterogeneousMORB eruptedalong the Juande Fucaspreading partially altered to reddish-brown clays. One Warwick ridge. In St, Nd, and Pb isotopiccompositions, the CES seamountsample (TI'080 DH04-4) has zeoliteslining some basalts are indistinguishable from nearby spreading ridge vesicles.

TABLE 1. DredgeSample Locations and Whole Rock Descriptions for Seamountsof theCobb- EickelbergSeamount Chain and Explorer and Union Seamounts Utilized in This Study.

Sample SeamountName LatitudeøN LongitudeøWNature of sample

TI'080 DH 01-1 Eickelberg(E) 48.29 133.09 v, d TI'080 DH 02A Eickelberg(E) 48.32 133.10 m, pp TI'080 DH 02-8 Eickelberg(E) 48.5 3 133.10 v, pp TI'080 DH 04-4 Warwick(W) 48.02 132.46 m, pm TT080 DH 05-14 Warwick(W) 48.01 132.53 v, d TI'080 DH 08-8 Waxwick(W) 48.03 132.45 m, pp TI'080 DH 09-1 Waxwick(W) 48.02 132.47 pm TI'080 DH 10A unnamed(X) 48.28 133.22 fo TI'080 DH 10-7 unnamed(X) 48.28 133.22 v, fo TI'175 DH 70-2 Sloth (S) 47.62 131.70 v, mx TI'175 DH 71-4 Lust(L) 47.50 131.51 v, mx TI'175 DH 74-17 Gluttony(G) 47.14 131.46 v, pp CB-1 Cobb (C) 46.77 130.83 v, mx TT080 DH 14A Thompson(T) 46.03 128.63 v, mx TI'080 DH 14-5 Thompson(T) 46.03 128.63 m, mx TI'063 DH 34 Explorer(Ex) 48.96 131.03 op, pp TI'063 DH 35 Explorer(Ex) 48.98 130.98 v, op, pp TI'063 DH36A Union (U) 49.55 132.73 v, pp

A symbolfor eachseamount is includedin parenthesesbehind seamount name and will be used throughoutthis study. Sampledescriptions: v = vesicular,m = massive,mx=microcrystalline, d = devitrifiedglass, pp = plagioclasephenocrysts, pm = plagioclasemicrolites, op = olivinephenocrysts, fo = Fe-Ti oxides. 12,700 DESONIEAND DUNCAN: Tim COBB-EICKELBERGSEAMOUNT CHAIN

COBB-EICKEI•ERG SEAMOUNT COMPOSITIONS exhibit greater variability than the JDFR basalts (stippled field), althoughthe older Pacific plate CES have ratheruniform AnalyticalProcedures REE concentrations. Cobb through Eickelberg seamount Major and some trace elementanalyses for dredgedbasalts basalts show slight enrichment in light-REEs relative to the were determined from pressed powder pellets by X-ray JDFR field (including the Endcavour segment which lies fluoresencespectroscopy [Hooper, 1981]. Rare-earthelement virtually within the field for segments1-3) and a steeperlight- (REE) and Sc concentrations were determined from to heavy-REE pattern (and thus greater (La/Sm)N) than the instrumentalneutron activation analysis(INAA) [Laul, 1979]. spreadingridge samples. CrossingREE patternswithin the Isotopedilution techniqueswere employedto measureSt, Rb, Pacific plate CES (Figure 5, inset) may indicatethat the basalts Cs, K, Sm, and Nd concentrations[White et al., in press]. originated from variable partial melting of a mildly Where any of thoseelements is availableby anotheranalytical heterogeneous source or dynamic partial melting of a technique,the isotopedilution values are preferred. homogeneoussource [Langmuir et al., 1977]. Sr, Nd, and Pb isotopicratios were measuredon a VG Sector The moderate MgO content and Mg-number thermal ionization mass spectrometerat Cornell University, [=100Mg/(Mg+Fe2)] of many CES and Explorer and Union following the procedureof White et al. [in press]. All samples samplesindicate that the basaltshave undergonelow pressure were leached in 6 N HC1 to eliminate effects of seawater fractionationaway from primary melt compositions. Olivine alteration. In order to comparethe CES resultswith thosefrom phenocrystsare present only in the sample from Explorer previously publishedstudies of other northeastPacific basalts, seamount. However, low Ni contents and a constant only samples that were similarly leached before isotopic CaO/A1203 with decreasingMg-number indicate significant analysiswere considered. olivine removal from the mantle-derivedmelts (Figure 6). The absenceof a Eu anomaly in the REE patternsof CES samples Major and Trace Elements (Figure 5) and roughly constantSr/Zr ratios (Table 2) provide The older Pacific plate CES lavas (Cobb throughEickelberg evidencethat althoughplagioclase was on the liquidus,a large seamounts)show a narrow range of variation with respect to amount of it was not removed from the melt during many chemical parameters, often smaller than previously crystallization. From the lack of correlation between Sc and analyzed samples from the JDFR (Table 2) [Liias, 1986; CaO/A1203 with Mg-number (Table 2 and Figure 6) it appears Karsten, 1988]. However, a larger variation in chemical that clinopyroxenewas not an important fractionatingphase. parametersexists in seamountsof the CES chain that lie close REE patterns (Figure 5) and heavy-REE ratios show no to the spreadingridge (Figure 2) [Morgan, 1985]. Variations evidence for garnet in the CES source. Probably, as with in Brown Bear seamountare reportedby Rhodeset al. [this basalts of the JDFR [Liias, 1986], the depth of melt issue]. segregationfor CES lavas was within the spinel stability field. The basaltsfrom Thompsonseamount, considered part of Determining the petrogenesis of this basaltic suite is the CES chain [Delaney et al., 1981] although moving limited by the small numberof analysesfrom severalvolcanic eastwardwith the Juande Fuca plate, are more primitive and edifices erupted in a changing tectonic setting over 9 m.y. depleted in incompatible elements than other CES lavas However, the compositional similarity of the older Pacific (Figure 3) and, in some chemical parameters,JDFR lavas. plate CES allow a commonhistory for theseseamounts, which Chemical analyses of the Seven Deadly Sins cluster are could range from multiple stages of melting of a single reportedby Rhodeset al. [this issue]. For almostall chemical homogeneoussource (dynamic partial melting) to similar melt criteria, the two samplesfrom Explorer and Union seamounts conditionsacting on a heterogeneoussource. In their studies (TI'063 DH 34 and 36A, respectively)have a far greater of the JDFR and near-ridge seamounts(including Brown Bear compositionalrange than do the Pacific plate CES; e.g., the and Son of Brown Bear), Morgan [1985], Liias [1986], and most extreme Mg-numbers and (La/Sm)N analyzedare from Karsten [1988] concluded that the mantle beneath the Juan de thosetwo volcanoes(Table 2), and they encompassa greater Fuca region must be heterogeneouson a small scale (hundreds range than the older CES on a plot of incompatibleelements of metersto a few kilometers). The variability of lavaswithin (Figure 3). Brown Bear seamount[Morgan, 1985] is one line of evidence Although many northeast Pacific seamount basalts are for heterogeneity of the upper mantle beneath the Cobb- strongly enriched in alkali and incompatible elements and Eickelberg region. Overlapping REE patterns (Figure 5) in ratiosrelative to JDFR lavas[Cousens et al., 1985;Dalrymple CES basaltswith similar chemical characteristicsand a range et al., 1987; Cousens, 1988], the Cobb-Eickelberg,Explorer, in TiO 2 and other chemical parametersat constantM gO for and Union seamountsamples straddle the line dividingalkalic basalts of the Juan de Fuca region (Figure 7) allow us to from tholeiitic basalts on a plot of alkalies versus silica conclude that the entire CES lineament was producedby (Figure 4) and are transitionalin their incompatibleelements preferential melting of compositionally similar, andratios as well (Table 2). In somechemical parameters (i.e., incompatible-element rich pods within a heterogeneous Zr/Nb, Zr/Y, P20s, and K20 ) CES lavasresemble the Endeavour mantle source. segmentbasalts, which are enrichedrelative to segments1-3 Ratio-ratio plots involving four different elementsthat are of the JDFR (Table3) [Liias, 1986;Karsten, 1988]. However, incompatiblein fractionatingphases (i.e., Zr/Y versusTi/Nb) onthe average CES basalts are enriched in St, Zr, andP205 but indicate mixing of two sources or magmas if sample aredepleted in Nb andhave higher Zr/Nb and (La/Sm)N relative compositionsare linked by a curve [Langmuiret al., 1978]. In to Endcavoursegment lavas (Table 3). CES lavasderive from a a companion plot, one in which the ratios shown have the source (or sources) enriched over the JDFR source that same incompatible element in the denominator (i.e., Zr/Nb resemblesbut is distinguishablefrom the Endcavoursegment versus Ti/Nb), sample compositions will fall on a line if source. mixing has occurred. In both plots the samplecompositions On a chondrite-normalizedREE plot (Figure5), the CES should lie in the same relative positionsunless fractionation DESONIE AND Dt•c•: T• COBB-EICKELBERGSEAMOUNT CHAIN 12,701

o o 12,702 DESONIE AND DUNCAN: TI-m COBB-EICKELBERG SEAMOUNt CHAIN

basalts (Figures 8a and 8b). However, scatter within these

1.4 o plots precludessimple mixing of two discreteend-members. Rather, the patterns seen are more compatible with partial melting of incompatible-elementenriched segregations within 0 the heterogeneousnortheastern Pacific upper mantle and O0 variable mixing with a relatively depletedJDFR source. To a first order, mixing between enrichedand depletedsources can also be seen along strike of the CES lineament, with more ,t enriched values of incompatible elements (K20 + Na20) and ratios(La/Sm)N at the olderend of the chainand more variable (Brown Bear seamount)or less enrichedvalues (Cobb or Axial seamounts)at the youngerend (Figures2a and 2b). 0.2 Sr, Nd, and Pb Isotopes Although CES lavas are slightly enriched in incompatible element concentrationsrelative to normal-MORB (n-MORB) from the JDFR (Table 3), Sr, Nd, and Pb isotopicratios for the CES and Explorer and Union seamountsplot well within the rangeexpected for n-MORB. Samplesfrom the seamountsfall virtually within the JDFR field in their Sr and Nd isotopic ratios (Figure 9a) [Eaby et al., 1984; Morgan, 1985; Liias, ß ß 8 1986], and there is little isotopic variation along the seamountlineament (Figure 9b). Only in Pb isotopicratios is there a difference in composition between the seamount 400 3•0 2•0 1•0 6 160 200 basaltsanalyzed and the Juande Fuca ridge (Figures10a and Distance from Cobb hotspot, km 10b). All CES samplesfall in or near the JDFR field [Church and Tatsumoto,1975; Hegner and Tatsumoto,1987] but range Fig. 2. Variation in alkali and light-REE enrichmentwith distance to somewhat elevated 206Pb/204pbbut well within the larger from Cobbhotspot (Axial seamount).Basalts dredged from seamounts farthest west of Cobb hotspot show greater enrichment and less field for northeastPacific seamounts. The single samplefrom variability than seamountbasalts dredged near the ridge. Solid Union seamountis clearly distinctfrom the CES basaltsin Pb trianglesrepresent analyses of Cobb, Brown Bear, Axial, and Son of isotopic composition. Brown Bear seamountsby Morgan [ 1985].

GEOC/mONOLOGY has affected the less incompatibleelements [Langmuir et al., Eleven of the freshest basalt samples from the Cobb- 1978]. On most ratio-ratio plots (i.e., Figures 8a and 8b), Eickelberg, Explorer, and Union seamountswere chosenfor CES lavas and near-ridgeCES [Morgan, 1985] plot closeto a age determinationsby 4øAr/39Artotal fusion and conventional mixing curve or line. These data suggestmixing of a low K-Ar radiometric dating methods [McDougall and Harrison, alkalies and (La/Sm)N, high Zr/Nb end-memberexpressed most 1988; Dalrymple and Lanphere, 1969]. Becauseof the young stronglynear the spreadingridge, with a higher alkaliesand ages and low potassium concentrationsin these lavas and (La/Sm)N, lower Zr/Nb end-memberseen in the older CES sample size limitations for irradiation, not enoughradiogenic

100 ß

O-- DH01-1

0 DH02A

u [-]-- DH02-8 10 l m DH04-4

] DH05-14

X DH08-8

A DH10-7

ß CB-1

[•-- DH14-5

• DH34

[]]-- DH36A

6sl•bl•a l•lb K I•a•e•r l•Idi• S'm•r•i•f •b Fig. 3. Geochemicalpatterns normalized to n-MORB [afterSun and Nesbitt, 1977]; incompatibility of an elementin n- MORB decreasesto the right. DESONIEAND DUNCAN:THE COBB-EICK•.RERGSEAMOUNT CHAIN 12,703

10 in Table 4. The relatively large age uncertaintiesresult from measuringsmall amountsof radiogenic,,oAr within a total ',OAr signal of largely atmosphericorigin. A replicate 40Ar-39Ar total fusion analysis was performed on one sample from Eickelbergseamount (Table 4). Ages coincidewithin their two sigma errors. Reactor irradiation is known to increase the atmosphericargon component [McDougall and Harrison, 1988] so conventionalK-At ageswere determinedfor Explorer '•:•'"'"'";","",,-"'•:'' near-ridge CES seamountwhere samples were expected to be the youngest analyzed(i.e., smallestradiogenic •øAr) and Explorer samples / ••"-JuandeFuca Ridge were absolutelyfresh (no alteration). Crustal ages were based ' I ' I ' I ' on magnetic anomaly interpretationsby Wilson et al. [1984] 30 40 50 60 70 and tectonicreconstructions by Karsten and Delaney [ 1989]. SiO2wt. % Figure 11 shows the variation in age of eight seamounts with distancealong the seamountchain from Cobb hotspot, Fig. 4. Alkaliesvs. silicadiagram for the CES, Explorer,and Union seamounts. Northeast Pacific seamount field includes data from assumingits location is defined by the center of Axial volcano seamountsof the Pratt-Welker[Dairytopic eta!., 1987], Tuzo Wilson, (zero age). Distanceswere measuredalong the lineament,in Bowie, and Dellwoodknoll seamountchains [Cousens et al., 1985; the direction of Pacific plate motion over the mantle [Duncan Cousens,1988]. Near-ridgeCES field includesdata from BrownBear, and Clague, 1985; Pollitz, 1988]. Also shownon this figure Sonof BrownBear, and Axial seamounts[Morgan, 1985]. TheJDFR are the ages determinedfor Union and Explorer seamounts field is from Liias [1986] and Karsten [1988] and includes the Endcavoursegment. Alkali basalt-tholeiiteboundary from Macdonald plotted againstdistance from Endcavourand Middle Valley and Katsura [ 1964]. spreading segments, respectively (Figure 1). Volcanic activity may continue for as long as 3 m.y. at a given seamount(e.g., Pratt-Welker seamounts[Turner et al., 1980]); SOArwas present for analysisby n0Ar_39Arincremental heating a 2-m.y. spanwas recordedin the threesamples analyzed from experiments.Basalt samples were crushed and sieved to obtain Eickelbergseamount. Therefore, we haveused the oldest the0.5 and1.0 mm size fraction, then washed ultrasonically in samplefrom each seamount where more than one sample was distilledwater. For '*0Ar-ZgArtotal fusion experiments,analyzed as an estimateof the time whenthe volcanowas sampleswere irradiatedfor 6-10 hoursin the coreof the locateddirectly over the hotspot. Replicateanalyses for OregonState University TRIGA reactor, where they received a Eickelbergsample DH02A were averaged. Oldest seamount neutronflux of 0.6-1.0x 1017nvt. Isotopesof argonwere ageswere used in a least-squareslinear regression to determine measuredon an AEI MS-10Smass spectrometer, after sample an averagerate of platemotion over the hotspotof 43 + 3 fusion via radio frequencyinduction heating and gas km/Ma. A least-squaresregression of all Pacificplate CES purification;potassium contents were determined by atomic includingall threeEickelberg points gives a rate of plate absorptionspectrophotometry. Analytical data and age motionover the hotspot of 47 + 3 km/Ma. Agesfrom Union calculationsfrom the 'mAr-a9Ar and K-Ar experimentsare given andExplorer seamounts were not includedin the regression,

TABLE 3. AverageCompositions of SelectedChemical Elements and Ratios for Different TectonicSettings in the Juande FucaRegion

Chemical JDFR EndcavourNear-Ridge Axial Near-Ridge PacificPlate ParameterSegments 1-3a Segmentb Seamounts c Seamount a CES d CESe

K20 (wt.%) 0.18 0.42 0.12 0.15 0.17 0.43 P205(wt.%) 0.17 0.23 0.07 0.13 0.13 0.28 Sr ppm 113 211 158 136 159 271 Zr ppm 104 134 54 92 93 145 Nb ppm 5.3 13.9 2.0 4.3 4.2 10.4 Zr• 22.1 9.7 28.1 23.3 28.1 14.0 Zr/Y 3.10 4.58 2.17 3.26 3.66 3.84 (La/Sm)N 0.86 a0.70 --- 0.76 0.74 1.24

a FromLiias [1986]. b FromKarsten [1988]. c IncludesHeck, Heckle and Springfield seamount chains, from Karsten [1988]. d IncludesCobb, Brown Bear, and Son of Brown Bear seamounts, datafrom Morgan [1985], averagedby Liias [1986]. e IncludesEickelberg, unnamed (except for P205 wt. %), Warwick, and Cobb seamounts, from this study. 12,704 DESONIEAND DUNCAN:THE COBB-EICKF•BERGSEAMOUNT CHAIN

100

O-- DH01-1

• DH02A

{"]• DH02-8

m_ DH04-4

i DH05-14

X DH08-8

A DH10-7

ß CB 1

[] DH14-5

v• DH34

1o [] DH36A Lai C½i Ndi Smi Eui Tbt Ybt Lui I I i I I I I I La Ce Nd Sm Eu Tb Yb Lu

Fig. 5. Chondrite-normalized[after Nakamura, 1974] REE patternsfor seamountsof theCES, Explorer, and Union seamounts.JDFR (includingEndeavour segment) field shownin stipplepauem [Wakeham, 1978; Liias, 1986]. Analyses fromsamples dredged near the Blanco fracture zone (dredges 6 and 7) fromWakeham [1978] were not included. Inset shows REE patternsfor threebasalts from Warwick seamount. beingfrom separateprovinces, but thesedata are generallyThis changein the crustalage at the time of seamount consistentwith the Pacificplate motion inferred from the CES volcanismis due to the westwardmigration of the JDFR, age-distancerelationship. towardCobb hotspot, since at least10 Ma [Riddihough,1984; Two agepatterns can be seenin Figure11: Karstenand Delaney, 1989]. 1. Agesincrease to thenorthwest along the CES chain from The line of seamountsfrom Axial throughEickelberg, Axialseamount at zeroage on the spreading ridge, to 3.3 Ma at althoughshort, is subparallelwith other more prominent Cobbseamount, some 105 km from the ridge, andon to 9.0 Pacific chainsthat are associatedwith stationarylong-lived Ma at Eickelbergseamount, about 340 km from the ridge. mantlehotspots. The volcanic migration rate of 43+ 3 km/Ma Union seamountis one memberof a short Pacific plate is consistentwith a plate-mantlevelocity of 44 km/Ma volcanicchain farther to thenorth. The zeroage position of predictedfrom the latestPacific plate rotation pole at 62øN, theassociated melting anomaly is unknown;we haveassumed 94øW and rotation rate of 0.95ø/Ma[Pollitz, 1988], which it tobe the Endearour ridge segment (Table 4). appliesfor the period 0-9 Ma. Newage data from the CES, as 2. Thereis a northwesterlyincrease in thedifference in the well asthe Pratt-Welker seamounts [Dalrymple et al., 1987], ageof theseamounts and the crust on whichthey formed. Samoa [McDougall, 1985], and the Louisville seamount chain Axialseamount is currently forming on theridge on zero-age[Watts et al., 1988]support the ideaof rigid Pacificplate crustbut Eickelberg seamount formed on crust that was 2-3 Ma. motionover a fixedconstellation of hotspots (Figure 12).

1.0 3.0

0.9 o ß •ø!•cpxPlag o o ß 2.0- 0.8 o o oO[]0 *•1 o o fl o o

0.7 o o 1.0-

0.6 40 ' 5'0 ' 6'0 ' 70 MgO wt. % Mg-number Fig. 6. Fractionationtrends for olivine(ol), clinopyroxene(cpx), Fig.7. TiO2 versusMgO variation diagram for basaltsfrom the JDFR andplagioclase (plag) and variation of of CaO/A1203with Mg-number (solid squares)[Liias, 1986], Axial and the near-ridgeCES (open for Cobb-Eickelberg,Explorer, and Union seamount basalts. squares)[Morgan, 1985], and the older CES (circles, this study). DESONIE AND DUNCAN: THE COBB-EICKELBEROSEAMOUNT CHA• 12,705

7 usually distinguishhotspot from spreadingridge lavas, the CES basaltscannot be separatedfrom MORB compositions. (a) 6- The lack of distinguishingSt, Nd, Pb, and He isotopicratios is o a feature of other northeast Pacific seamountand spreading

5- ridge lavas [Churchand Tatsumoto,1975; Lupton, 1982;Eaby et al., 1984; Morgan, 1985; Liias, 1986]. No significant isotopicvariation has yet been found in basaltsfrom the Pratt- 4- Welker [Churchand Tatsumoto,1975; Hegner and Tatsumoto, 1989], Heckle [Hegner and Tatsumoto, 1989], Tuzo Wilson 3- [Cousenset al., 1985], Bowie [Cousenset al., 1985; Cousens, 1988], Dellwood knolls [Cousenset al., 1984], or President 2 Jackson (W. M. White, personal communication, 1988)

60- seamount groups. Models 50- Several models may explain the age distribution and 40- chemical compositionsof the CES basalts. 30- Passivelyupwelling heterogeneous mantle and a migrating spreading ridge. The CES may be the result of the same 20- processesthat form near-ridge seamounts along the East 10- (b) Pacific Rise [Batiza and Vanko, 1983, 1984; Zindler et al., 1984; Fornail et al., 1988a, b] and Juan de Fuca and Gorda 0 ridges[Davis and Karsten,1986; Karstenand Delaney, 1989]. 0 1000 2000 3000 4000 5000 6000 Most of thesesmall near-ridgeseamount clusters are thought Ti/Nb to be the result of volcanism above a short-lived melting anomalyembedded in the uppermanfie lateral flow, rather than Fig. 8. Incompatibleelement ratio-ratio mixing plots [after Langmuir a deep mantle plume. Following the Batiza and Vanko [1983, et al., 1978]. Axial seamountsamples are shownin solid squares [Morgan, 1985], near-ridgeCES (includingBrown Bear, Cobb, Axial, and Son of Brown Bear seamounts)are shown in open squares [Morgan, 1985], and older CES analysesare shownin opencircles 0.5133 . (this study). JDFR and Endcavoursegment fields are labelled[Liias, 1986; Karsten, 1988]. NE Pacific Se

DISCUSSION 0.5132

Summaryof Temporaland ChemicalCharacteristics of the CES The CES chain has the temporal and morphologic 0.5131 characteristicsof volcanismgenerated over a hotspotfed by a mantle plume. The seamountchain is age progressiveand the (a) linear geometry and age distribution of volcanoes are 0.5130 compatible with the motion of the Pacific plate over other 0.7023 0.7024i ß0.7 b25 ' 0.7026I ß0.7027 I ß0.7028 well-establishedhotspots. Age relationshipsalong the CES 40 39 chain, determined by Ar- Ar and K-Ar analyses, in 87Sr/86Sr 0.7028 conjunctionwith the tectonic reconstructionof the Juan de Fuca region of Karsten and Delaney [1989] require a fixed (b) source for the CES lineament and a westwardly migrating 0.7027 spreading ridge. Our data demonstrate that the melting anomaly currently beneath Axial seamount is a stationary, 0.7026 persistentfocus of unusuallyhigh magma supply,even though coø ß o volcanic activity along the lineament may have been 0.7025 o o intermittent, with the Eickelberg to Axial seamountsection being the latest "pulse"of plume flow. 0.7024 In most respects,CES lavas are transitionalbetween basalts characteristicof hotspotsand those of segments1-3 of the 0.7023 .... JDFR; they are similar to those of the Endeavour segment. 400 30'0 2to 0'0 6 ' ' 200 Compositional variability among CES basalts probably Distance from Cobb hotspot,km reflectsa range of partial melting of themantle supplied tothe Fig.9. (a)143Nd/•44Nd versusS7Sr/S6Sr forCES and Explorer and melting anomaly,which is heterogeneouswith respectto Unionsamples with JDFR [Hegner and Tatsumoto, 1987] and other NE incompatibleelements [Morgan, 1985; Liias, 1986; Karsten, Pacificseamount [Hegner and Tatsumoto,1985; Cousens et al., 1985; 1988]. The eastwardtrend toward depleted compositions Cousens, 1988) fields superimposed. Range in isotopicratios shown indicatesthat progressively greater proportions of the source is(b)equal 87Sr/a6Srton-MORB versus field.distance from Cobb hotspot for samplesfrom the for JDFRbasalts participated in melting for CESbasalts. In CESlineament. Solid triangles represent analyses ofBrown Bear and terms of the St, Nd, and Pb isotopic compositions,which Axialseamounts by Eabyet al., [1984]. 12,706 DESONIEnN• DUN•: T•m COBB-EIC•:•ERO SEAMOUNTCHAIN

15.60 1984] and Zindler et al. [1984] model for small seamountsnear the East Pacific Rise, a somewhat smaller degree of partial melting of the same heterogeneousupper mantle sourcethat is 15.55 producing the JDFR lavas would result in melts somewhat enriched in incompatible elements and alkalies but with essentially MORB-like isotopic composition. This 15.50' mechanism could easily explain the modest enrichment in alkalies of the CES lavas and might. with an even smaller

15.45 degree of partial melting, explain the alkali basalts and hawaiites of other northeastern Pacific seamounts. The scale of volcanic activity at such near-ridge seamounts 15.40 is,however, much smaller than in the CES province. A •q•ical near-ridgeseamount •s 100 km, comparedwith 1000 km for a Cobb-Eickelberg cone. The duration of volcanism for an 38.50 entire near-ridge seamountchain can be brief: the Heckle chain near the Juande Fuca ridge beganactivity at ~2 Ma and ceasedat 0.5 Ma [Karsten and Delaney, 1989]. Volcanism •.•38.10 along the CES chainhas persistedfor at least9 m.y. Although it is possible that a large passive upper mantle "-':• •'...... 'NEPacific melting anomalycould accountfor the increasedvolume and 37.70 '"• Seamountsduration of volcanism at Cobb hotspot, we believe that a melting anomaly that has been fixed with respect to the hotspotreference frame for at least 9 m.y. must be rooted 37.30' , , , , - , ' 18.4 18•618.8 19.019.2 19.4 19.6 below the asthenosphere. A melting anomaly, of the type responsible for volcanism at small near-ridge seamount 206pb•04pb chains,would be swept along with uppermantle flow over the time scalerequired. Therefore, we believe that Cobb hotspot ' 10.(a) •7pb/2ø4Pbversus •6pb/•ø•Pb and (b) •Pb/•ø•Pbversus has been maintained by a mantle plume during the CES Pbfor CES,Explorer, and Union samples compared with Juan formation. Also, near-ridge seamountbasalts in the Juan de de Fuca ridge [Churchand Tatsumoto,1975; Hegner and Tatsumoto, Fuca region, i.e. Heck, Heckle and Springfield [Karsten, 1987] and NE Pacific seamounts[Church and Tatsumoto,1975; Hegner and Tatsumoto,1989; Cousens,1988] fields. The samplefrom Union 1988], are more depletedin alkali and incompatibleelements seamount(DH36A) is distinct. Range in isotopic ratios shown is at a given MgO content than the spreadingridge basalts,in slightlylarger than values seenin n-MORB basalts. contrast with the CES lavas.

TABLE4. 40Ar-39AxandK-At Total Fusion Ages and Analytical Data for Whole Rock Basalt Samples From the Cobb- Eickelbergand Explorerand Union Seamounts

Sample J-Factor 40Ar/39Ar 36Ar/39Ar37ArC/39Ar %K *40Armol/g %40At Age(:L 1 sigma)Ma (xlO-9)

•r080 DHOl-1 0.001957 21.269 0.06435 22.052 11.91 9.03 (0.41) •r080 DH02A 0.002446 15.865 0.05083 15.574 12.69 8.98 (0.75) replicate 0.002446 40.990 0.13615 12.812 4.16 7.52 (0.46) •r080 DH02-8 0.002446 21.768 0.07212 14.542 7.35 7.05 (0.15) •r080 DH08-8 0.001957 28.394 0.09047 16.109 6.85 6.91 (0.30) •r080 DH 10-7 0.001292 23.862 0.77980 22.765 12.61 7.73 (0.33) •r175 DH 70-2 0.003050 35.811 0.12171 14.064 2.64 5.20 (0.32) •r175 DH 71-4 0.003050 29.516 0.10218 18.908 2.72 4.40 (1.07) •r175 DH 74-17 0.003050 55.674 0.19423 25.713 0.51 1.55 (1.40) CB-1 0.003050 11.161 0.04158 22.013 3.27 (0.30) •r080 DH 14-5 0.002446 163.580 0.56704 60.010 0.44 3.19 (1.30) •r063 DH 34 0.003050 30.274 0.11003 29.057 0.07 0.12 (0.57) 0.666 4.375 0.17 (0.03) •r063 DH 35 0.003050 35.303 0.13252 50.211 0.16 0.31 (0.37) 0.489 6.510 0.35 (0.04) Tr063 DH 36A 0.003050 8.875 0.29922 8.696 1.14 5.78 (0.65)

Agescalculated usingthe following decay and abundance constants: •œ=0.581x10 -10yr'l; )•[1=4.963x10-10 yr-1; 40K/K=l.167x10-4mol/mol. Neutron flux monitored with hornblende standard MMhb-1. Correctedfor decaysince irradiation. DESON[E AND DU•C•: TI• COBB-EICKELBERG S.EAMOUNT CHAIN 12,707

10.0

Eickelberg ß

unnamed 8.0 x\\ Warwick \xxx"x

6.0 U t xx 4.0

2.0

GluttonyJ • Exp•'or,er

Kilometers from ridge

Fig.11. Seamount age versus distance from spreading ridge for CES and Explorer and Union seamounts. K-Ar age of Cobb seamountby Dymondet al. [1968]shown as triangle. Measured distance for CESis fromCobb hotspot, defined as the centerof Axial volcano(46øN, 130øW), along the direction of Pacificplate motion [Pollitz, 1988]. Crustalages [after Wilsonet al., 1984]are shown by thedashed line; a breakindicates location of a pseudofault.Average rate of motionof thePacific plate over the hotspot was calculated to be 43 +_.3 km/Ma. Replicateanalyses of sampleDH02A from Eickelbergseamount were averaged.

Compositionally buoyant mantle plume and a migrating spreading ridge. Compositionalbuoyancy occurs when plume flow is driven by intrinsic density differencesbetween the diapir and its surroundings [Griffiths, 1986a]. Chemical diffusion proceeds extremely slowly so that compositional -16 diapirs maintain their chemical identity relative to the upper mantle material throughwhich they rise [Griffiths, 1986a].

-14 A compositionalplume that has moderatelyenriched trace element contentsbut a nonradiogenicisotopic charactercould

-12 originate in several ways. For example, the MORB-like isotopic composition of all seamounts of the northeastern Pacific may reflect a broad regional difference in mantle compositionfrom the equatorial oceanicregions where Dupal hotspotsare found [Hart, 1984]. Hart [1988], who noticed a poleward trend toward less radiogenic isotopic compositions in hotspot-associatedbasalts, termed the regional isotopic homogeneity of the northeastern Pacific the anti-Dupal anomaly. He proposed that the lower mantle which feeds plumes may be compositionally variable on a very long lengthscale(10 '• kin) and someregions may be isotopically MORB-like if large amounts of melts have been previously extracted from them.

i i i i i i i i A metasomaticevent that enriched the mantle in tb.eregion 20 40 60 80 100 120 140 160 in Rb, Sm, and U, along with other incompatible elements AngularDistance from RotationPole (62o N, 94ø W) [Menzies and Murthy, 1980], but so recently that their Fig. 12. Volcanicmigration rate as a functionof rotationpole located elevated concentrationshave not yet significantly shifted the at 62ø N, 94ø W [Pollitz, 1988]. The dottedcurve is a least-squares Sr and Nd isotoperatios from the MORB field was suggestedby bestfit withan angularrotation rate of 0.95+ 0.02ø/Ma. Cousenset al. [1985] to explain the alkalic but isotopically 12,708 DESONIE AND DUNCAN: T}ta COBB-EICK•RERG SEAMOUNTCHAIN

MORB-like compositions of the Tuzo Wilson seamounts. argued that the unusual isotopic structure of the Galapagos This explanation could also apply to the transitional lavas of archipelago,in which Sr isotopic ratios decreasetoward the the CES chain. centerof the hotspot,may result from the thermal entraimment The proximity of Cobb hotspot to the Juan de Fuca ridge processproposed by Griffiths [ 1986a, b]. may provide anotherexplanation for the unusualchemistry of Thermal entrainment can also occur in a continuousplume the CES basalts. A weak supply of relatively enrichedplume which is deflected by mantle shear flow beneatha spreading material to a hotspot in a spreading ridge setting may be ridge [Richards, 1988]. In this case, as well as for discrete diluted by mixing with the MORB partial melt zone. That is, diapirs,the most MORB-like magmasare to be expectedfrom the zone of partial melting below the ridge may extend at least the centerof the plume. Therefore, the isotopiccomposition 130 km away from the spreadingaxis. This model of hotspot of erupted magmasmay be an important distinctionbetween and spreadingridge interactionis easily testable. Older Gulf of chemical and thermal plumes, regardless of possible small- Alaska seamounts(e.g., Miller, Patton, and Murray) proposed scale heterogeneity and variable source. A large volume of to have formed at hotspotsfar removedfrom a spreadingridge entrained material implies a sharply reduced temperature at the time of seamount formation should exhibit a more contrastbetween plume and normal manfie. typical OIB signatureif they formed from a plume with OIB In reality plumes may occur from a combinationof thermal composition. If basaltsfrom such seamountsare found to have and compositionalinstabilities, but one effect may dominate. MORB compositions they would almost certainly have The hotspotthat producedthe Louisville seamountchain (LSC) resulted from a plume with MORB chemical characteristics may have been supplied by a largely compositionalplume. since melts from those hotspotscould not easily have mixed Like many hotspot tracks, the LSC has an associated with spreadingridge melts. The alkalic lavas from Murray and topographic swell, in this case several hundreds of meters Patton seamounts,which erupted through 11-12 Ma oceanic high, and a volcanic eruption rate of 3-4000 km3/Ma from 70 crust [Dalrymple et al., 1987], would have been 550 km west Ma until 20 Ma, when it waned sharply[Lonsdale, 1988]. The of the spreading ridge system and isotopic data could be LSC shows a very narrow range of St, Nd, and Pb isotopic expected to detect an OIB isotopic plume signature, if one values over the 65-m.y. range of available samples. These existed. basaltsshow radiogenicSr, Nd, and Pb isotopiccompositions, Interaction of a plume of enriched mantle compositionwith typical of OIB lavas [Cheng et al., 1987]. Based on the oceanic lithosphere cannot explain CES chemistry. For the consistently enriched compositions of Louisville samples, Hawaiian Islands, a model of mixing between the enriched Cheng et al. [1987] suggestedthat the LSC was formed by a mantle (EM) plume and a small degreeof melting of oceanic compositional plume that originated from a long-lived, lithospherethrough which the plume rises, can explain the stationaryand homogeneousmantle source. This plume was distinct incompatible element and isotopic compositionsof not noticeably contaminated by overlying asthenosphereor tholeiitic and alkalic lavas [Chen and Frey, 1983]. A similar lithosphere as were plumes that form other hotspot chains mixing model can also be appliedto the CES lavas and other [Cheng et al., 1987, and referencestherein]. seamountlavas of the northeast Pacific [Cousenset al., 1985]. In contrast,the more intermittentand consistentlyMORB- A plot of La/Ce vs 87Sr/86Sr[Chen and Frey, 1983, Figure 3] like melts from Cobb hotspot perhaps manifest a thermal shows that CES lavas could result from 0.25% to more than plume. Such a plume may rise throughand entrain the MORB- 3.0% partial melt of lithospheric(MORB) material mixed with like upper mantle, supplyingthe melting anomalyat the base EM, in proportionsof about 1:4, a larger MORB component of the lithosphere. To produce the observed isotopic than is seen in Hawaii. Samplessuch as the basalt from Cobb compositions,we would expect the proportion of MORB- seamountcould be generatedonly if the La/Ce of the upper componentto be large. If a large amount of the final plume mantle beneath the northeast Pacific is lower than the one volume is entrained material, the plume might be broad but postulatedin the Chen and Frey [1983] model for Hawaii. would be thermally dilute (cooled); thus, resulting volcanism However, the required variation in degree of melting of the might be volumetrically small compared with that of other lithosphere, followed by mixing of nearly constant hotspots. Furthermore, thermal entrainment increases proportions of MORB and EM componentswould result in strongly with decreasing thermal plume Rayleigh number 87Sr/86Srenrichment with increasingdegree of melting of the [Griffiths, 1986b]. Plumes that start off weaker (smaller lithosphere, a trend not seen in the CES lineament. The diameter and lower temperature contrast) suffer stronger observedcompositions seem to point strongly to the absence entrainment. This is consistentwith a relatively "weak" of a long-termenriched mantle componentin lavas of the CES (thermal)plume sourcefor the CES. and other northeast Pacific lineaments. Thermally buoyantplume and a migratingspreading ridge. SummaryModel Formation of a plume that has MORB isotopiccharacteristics The major and trace element chemical characteristicsof the might result from thermal entrainmentof MORB upper mantle CES are best explainedby mixing of enrichedand depleted by a plume of lower mantle composition. Thermally buoyant lavasresulting from partial meltingof a heterogeneousupper plumesare driven entirely by viscosityand densitydifferences mantle in both a hotspotand spreadingridge meltingregime. createdby a temperaturegradient acrossa thermal boundary Like JDFR melts,CES liquidsprobably segregate in the spinel layer [Griffiths, 1986a]. Diffusion of heat from suchthermal lherzolite stability field. CES lavas are also similar to all diapirs warms the surroundingmantle and lowers its density northeasternPacific seamountand spreadingridge basalts sufficiently for the plume to entrain its initially cooler analyzed thus far in that they show no enrichment in surroundingsbut conserve its total heat content, buoyancy, radiogenic isotopesover normal-MORB. Thus, there is no and driving force. The thermalplume will be a mixture of the evidenceto suggestthat Cobb hotspotis a compositionally diapiric material and surroundingmaterial entrainedtoward its buoyantmantle plume. Although,the chemicalcharacteristics center [Griffiths, 1986a]. Recently, Geist et al. [1988] have of the lavascan be explainedby a largepassive upper mantle DESONIEAND DUNCAN:• COBB-EICKELBERGSEAMOUNT CHAIN 12,709

melting anomaly, we believe that, to have remained 1984]. At 8 Ma the JDFR lay about 130 km east of Cobb stationary,the sourceof the CES lineamentmust be rooted hotspot(Figure 13). Eickelbergseamount was then forming beneath upper mantle lateral flow. The most likely abovethe hotspot on seaflooraged 2-3 m.y. Proximityof the explanationfor the unusualgeochemical characteristics of Cobb hotspotto the JDFR may have resultedin dilution of Cobb hotspotis a deeplyrooted, thermally buoyant diapir plumematerial by mixingwith the MORB partialmelt. The whichhas entrained nonradiogenic upper mantle MORB source next seamount to theeast (unnamed) had begun forming while material. The large, thermallydilute plumesupplies the Eickelbergseamount was still active,-7.8 Ma. By 6 Ma, hotspotwith volumesof warm mantlein excessof that Eickelbergseamount had moved 88 km westof the hotspot, upwellingalong the spreading ridge, resulting in thebuildup theJuan de Fucaridge had moved 35 km closerto thehotspot of thevolcanic cones of theCES chain. Progressively greater andWarwick seamount was forming on-2 Ma crust.At -4 Ma, mixingof plumeand spreadingridge mantle sources has Slothseamount had formed on 1.6-m.y.crust and had moved occurredas the IDFR approachedthe hotspot. morethan 50 km to the west;Lust seamounthad formedon Age relationshipsalong the CESlineament since 9 Ma 1.8-m.y.crust while the ridgewas only 65 km fromthe permitgreater resolution of theKarsten and Delaney [1989] hotspot.Although a large uncertainty is associated with the modelfor hotspot-spreading ridgeinteraction. In thismodel a ageof Thompsonseamount it is consistentwith an originat westwardmigration of theIDFR toward a fixedCobb hotspot the ridge at ~3.2 Ma, followed by transport east at a rateof -34 wasdetermined from analysis of thepropogating rift and km/Mawith the Juan de Fuca plate [Desonie and Duncan, 1986; spreadinghistory of theridge [Wilson, 1984; Riddihough, Karsten and Delaney,1989]. Cobbseamount formed at approximatelythe same time as Thompsonseamount on crust of age 0.4 Ma. Interaction of spreading ridge and hotspot E partial melt zonesresulted in less enriched(Cobb seamount)or more variable (Brown Bear seamount)chemical characteristics in the seamountsnear the Juan de Fuca spreadingridge. Axial seamountis currentlyforming over the ridge-centeredhotspot. • 8MaI • 400 300 200 100 0 100 200 Acknowledgments. We are grateful to M. Perfit of the University of Florida (OCE-8716827) and P. Hooper of EXW WashingtonState University for providingXRF data. W.M. White allowedus the use of his massspectrometry facilities at Cornell University. Isotopic analyseswere performedunder ?-•,','.....'•?•'4'•:,:•:•'?•'•'..... the guidance of M. Cheatham. We thank R. Walker of the 6 Ma Radiation Center at OSU for prompt INAA results. Age I I I-'•:' I ".....:'•-.."!•t::•'""'"'1•'"'' ' I I determinations were obtained with the assistance of L. G. 400 300 200 100 0 100 200 Hogan, also of OSU. Application of the thermal entrainment model developed from discussionswith M. Richards of the EXW S L University of California, Berkeley. Reviews by A. Grunder, M. Richards, and W. Bryan were extremely valuable. An especially thoroughreview by J. Karsten led to substantial improvementof the manuscript.

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