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 the agedifference 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