JOURNALOF GEOPHYSICALRESEARCH,VOL. 95, NO. B8, PAGES12,697-12,711,
AUGUST 10, 1990
The Cobb-Eickelberg
SeamountChain:
Hotspot
Volcanism
withMid-Ocean
RidgeBasaltAffinity
DANA L. DESONtoAND ROBERTA. DUNCAN
Collegeof Oceanography,
OregonStateUniversity,
Corvallis
Cobbhotspot,
currently
located
beneath
Axialseamount
ontheJuandeFucaridge,hasthetemporal
butnottheisotopic
characteristics
usually
attributed
toa mantle
plume.Theearlier
volcanic
products
of
thehotspot,
fromeightvolcanoes
in theCobb-Eickelberg
seamount
(CES)chain,
showa westward
age
progression
awayfromthehotspot
anda westward
increase
in theagedifference
between
the seamounts
andthecrust
onwhich
theyformed.These
results
areconsistent
withmovement
ofthePacific
plateover
a fixedCobbhotspot
andeventual
encroachment
bythewestwardly
migrating
JuandeFucaridge.CES
lavasare slightlyenrichedin alkaliesandincompatible
elements
relativeto thoseof the Juande Fuca
ridgebuttheyhaveSt,Nd,andPbisotopic
compositions
virtually
identical
to those
foundalongthe
ridge.Therefore,
Cobbhotspot
is a stationary,
upper
mantle
melting
anomaly
whose
volcanic
products
showstrong
mid-ocean
ridgebasalt(MORB)affinity.Theseobservations
canbe explained
by low
degreesof partialmeltingof entrained
heterogeneous
uppermantleMORB sourcematerialwithina
thermally
drivenlowermantle
diapiror by anintrinsic
MORB-like
composition
of thedeeper
mantle
sourceregionfrom whichnortheast
Pacificplumesrise.
INTRODUCTION
the Endeavoursegmenthas been attributedto the contribution
to theoverriding
Juande Fuca
Sincehotspotswere first described
by Wilson [1963] and of theHecklemeltinganomaly
linked to convectivemantleplumesby Morgan [1972], both spreadingridge melts [Karsten, 1988].
Axial seamount, which straddles the JDFR axis at -46øN
temporaland chemicalcharacteristics
of thesefeatureshave
beenrecognized.The linear array of volcanoesand the age- (segment2) [Delaneyet al., 1981] is the youngestexpression
progressive
distribution
of hotspottracks[Morgan,1972]are of volcanismover an uppermantle melting anomalytermed
two of the more distinguishing
featuresof volcanismresulting the Cobbhotspot(Figure 1). Older volcanicproductsof the
from a fixed mantle plume. Some hotspots have been hotspot,the Cobb-Eickelbergseamount(CES) chain,includea
persistentover long periods,a featureclearly seenin the line of edifices on the Pacific plate, from Brown Bear
a group
volumesof lava producedby the Hawaiianhotspot,while other northwestthroughCorn, Cobb, and Pipe seamounts,
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,
usuallymore radiogenicSr, Nd, and Axial seamounton the Juande Fucaplate (ThompsonandSon
Pb and higher concentrationsof incompatible elements, of Brown Bear) are inferred to be related to this linear volcanic
relative to basaltseruptedat 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.Theseolder volcanoeshave beeninterpretedto be
basalts(OIB) as distinguished
from basaltsthat form at mid- earlierproductsof volcanicactivity at Cobb hotspot[Duncan
oceanridges(MORB). The Cobb-Eickelberg
seamount
chain, and Clague, 1985; Smoot, 1985], but this suggestion
has been
and other hotspot-associatedvolcanic lineaments in the questioned
by Dalrympleet al. [1987] becauseseamountages
northeastern Pacific, are unusual in that they exhibit the do not completelyagreewith rigid Pacificplatemotionover a
temporalcharacteristics
producedby othermantleplumes,but fixed hotspot.
not the OIB isotopicsignatures.
The Pacificmantlerotationsproposed
by DuncanandClague
Basedon differentmorphologicandtectoniccharacteristics,[1985] and Pollitz [1988] place Miller seamount(26 Ma
the Juande Fucaridge (]DFR) wasdividedinto 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-segment
variationsin basaltchemistrygive evidence relativelylow alkali contentsand flat rare-earthelement(REE)
of separate
magmaticsystems
beneaththeridge[Liias, 1986]. patterns[Dalrympleet 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 mostprimitive(highestMg-numbers)yet has 150-250km westof Cobbhotspot.The higheralkalicontents
the mostincompatible
elementenrichedlavasfoundalongthe and light-REE enriched nature of the Murray and Patton
ridge[Liias, 1986;Karsten,1988]. The unusualchemistry
of
Copyright 1990 by the American Geophysical Union.
Paper number 89JB03074.
0148-0227/90/89JB-03074505.()0
seamount basalts would be consistent with their formation as a
productof late-stagerejuvescentvolcanismdownstreamfrom
the Cobb hotspot, or with formation at another,now extinct,
hotspot. Whether or not the three older seamountsformed at
Cobb hotspot, there has been a lack of consistencyof
volcanismover the melting anomaly;it either becameactive
or it teacrivedafter a long periodof quiescence
more thannine
12,697
12,698
DESONIEAND DUNCAN:•
150 ø
145 ø
COBB-EICg•.T.nERO
SEAMOUN•CHAIN
140 ø
135 ø
125 ß
130 ø
NORTH
60 ß
AMERICAN
PLATE
Giacomini
.•mount
Pratt Guyot
Weaker Guyot
55 ß
Patton
DIckins
Seamount
8eamount
Denson
Guyot
4•
o o
Guyot
/Miller
Seamount
Davidson
Parker
Seamount
Gu
Bowie
Seamount
Tuzo
Wilson
Solmounts
ß
athflnder
PACIFIC
contour
•/'d•8..mount
PLATE
interval
T.Horton
Guyot••
50 ß
= 400
ß
Hock
8mt•.
'•
Unione
•
•G
DE FUCA
PLATE
Pros.
45 ø
Jackson
Solmounts
40 ø
ntour
Fig. 1.
interval
Bathymetric
map showingseamounts,
spreading
ridges(doublelines)andfracturezones(singlelines)of the
northeastPacific, after Chaseet al. [1970]. Inset showsseamountsof the Cobb-Eickclberglineamentand Explorerand
Union
volcanoes.
million yearsago with the formationof Forsterand Eickelberg
seamounts. Since the hotspot began this latest pulse of
activity, volcanicoutputhas been continuousbut hasvaried up
to 20% [Karsten and Delaney, 1989] which is within the
known variability of other hotspots [Bargar and .lackson,
1974; Lonsdale, 1988].
Within
the northeast
Pacific
basin are several other linear
originatedby differentmechanisms.The longestand best
known of theseis the Pratt-Welkerseamountchain (Figure 1).
Dalrymple et al. [1987] suggesteda complex history
involving at least two hotspots;some of the seamounts
formed in a midplate setting while volcanismat or near a
spreading
ridgeformedothers[Turneret al., 1980]. ThePrattWelker 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
DESONIE
ANDDUNC•: Tm• COBB-EICKELBERG
SEAMOUNI'
CHAIN
12,699
1) are proposedto reflecthotspotactivity,althoughthe Tuzo lavas. We discussthesedata in termsof possibledynamic
Wilsonseamounts
and Dellwoodknollsmay haveoriginated models of mantle plume and asthenosphere
mixing, and
by partialmeltingin a pull-apartbasin[Allanet al., 1988]. A concludethat the deep mantle under the northeastPacific
moresoutherlyline of seamounts
in the northwestern
Gulf 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.
Severalshorterseamount
chainslie closeto andintersect
SAMPLE
DESCRIFrION$
the JDFRfromthe west. Theseappearto reflecta much Basaltsamples
from the Cobb-Eickelberg
andExplorershorter-lived
andshallower-origin
meltingphenomenon.
The Unionseamounts
were obtainedfrom J. Delaneyand P.
HeckandHeckleseamount
chains,
for example,
maybe the Johnson
of theUniversity
of Washington
andwereoriginally
result of passiveupwellingand partial melting of dredged
duringR/VThomasThompson
cmises
Tr063,Tr080,
heterogeneous
upper
mantle
in advance
of thenorthwestwardly
andTI'175. TheCobbseamount
sample
wasobtained
by a
migrating
JuandeFucaspreading
ridge[DavisandKarsten,diverfromthepinnacle
of thevolcano.Dredge
locations
are
1986]similarto thesetting
of theLamont
seamounts
nearthe givenin Table1.
EastPacificRise[Fornariet al., 1988a]. Otherseamounts
and
Samplesused in this study for age determinations
and
small seamountchains,includingExplorerand Union compositional
analyses
werethefreshest
available,
withlittle
seamounts
whichlie to thenorthof theCES(Figure1), areof to no interstitial
glassandonlyslightto moderate
alteration.
unknown
origin. All aforementioned
seamount
chainsare CES basaltsamplesare generallymicrocrystalline
and
foundon the Pacificplate;only two edifices,Thompson
and aphyric. In some samples,phenocrystsof plagioclase,
Son of Brown Bear, which are interpretedto be relatedto clinopyroxene,
andFe-Ti oxidesare foundin an intergranular
volcanismat Cobb hotspot[Desonie and Duncan, 1986; or intersertalmatrix. Plagioclasephenocrysts
are generally
KarstenandDelaney,1989]wereformedon theJuande Fuca freshbut may be slightlyresorbedor zoned. Both samples
plate.
from Eickelberg seamountused in this study contain
In this paper we report new nøAr-39Al'and K-Ar age plagioclase
glomerocrysts.
Severalbasaltsamplesarehighly
determinationsfrom eight volcanoesof the CES which vesicular,indicatingthat the magmasdegassed
as theycooled
documenta clear age progressionalong the lineament,in andwereprobablyeruptedat a relativelyshallowdepthor had
concertwith platemotionover a stationaryhotspot. Major high ascent rates [Dixon et al., 1988]. Secondary
andtraceelementcontentsare slightlyenrichedover thosefor mineralization is slight; the matrix of some samples is
heterogeneous
MORB eruptedalongthe Juande Fucaspreadingpartially 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. DredgeSampleLocationsandWholeRockDescriptions
for Seamounts
of theCobbEickelbergSeamount
ChainandExplorerandUnionSeamounts
Utilizedin This Study.
Sample
Seamount
Name
LatitudeøN LongitudeøWNatureof sample
TI'080 DH 01-1
Eickelberg(E)
48.29
133.09
v, d
TI'080DH 02A
TI'080 DH 02-8
TI'080 DH 04-4
Eickelberg
(E)
Eickelberg
(E)
Warwick(W)
48.32
48.53
48.02
133.10
133.10
132.46
m, pp
v, pp
m, pm
TT080 DH 05-14
Warwick(W)
48.01
132.53
v, d
TI'080 DH 08-8
TI'080 DH 09-1
Waxwick(W)
Waxwick(W)
48.03
48.02
132.45
132.47
m, pp
pm
TI'080
TI'080
TI'175
TI'175
unnamed(X)
unnamed(X)
Sloth (S)
Lust(L)
48.28
48.28
47.62
47.50
133.22
133.22
131.70
131.51
fo
v, fo
v, mx
v, mx
TI'175 DH 74-17
Gluttony(G)
47.14
131.46
v, pp
CB-1
TT080 DH 14A
TI'080 DH 14-5
Cobb (C)
Thompson(T)
Thompson(T)
46.77
46.03
46.03
130.83
128.63
128.63
v, mx
v, mx
m, mx
TI'063 DH 34
TI'063 DH 35
TI'063 DH36A
Explorer(Ex)
Explorer(Ex)
Union(U)
48.96
48.98
49.55
131.03
130.98
132.73
op, pp
v, op, pp
v, pp
DH
DH
DH
DH
10A
10-7
70-2
71-4
A symbolfor eachseamount
is includedin parentheses
behindseamount
nameandwill be
used throughoutthis study.
Sampledescriptions:
v = vesicular,
m = massive,
mx=microcrystalline,
d = devitrified
glass,
pp = plagioclase
phenocrysts,
pm = plagioclase
microlites,op = olivinephenocrysts,
fo =
Fe-Ti
oxides.
12,700
DESONIE
ANDDUNCAN:Tim COBB-EICKELBERG
SEAMOUNT
CHAIN
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 elementanalysesfor 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 thoseelementsis availableby anotheranalytical heterogeneous source or dynamic partial melting of a
homogeneous
source[Langmuiret al., 1977].
technique,the isotopedilution valuesare preferred.
MgO
content
and Mg-number
Sr, Nd, and Pb isotopicratioswere measuredon a VG Sector The moderate
thermal ionization mass spectrometerat Cornell University, [=100Mg/(Mg+Fe2)] of many CES and Explorer and Union
following the procedureof Whiteet 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
However, low Ni contents and a constant
previously publishedstudiesof other northeastPacific basalts, seamount.
only samples that were similarly leached before isotopic CaO/A1203 with decreasingMg-number indicate significant
olivine removalfrom the mantle-derivedmelts (Figure 6). The
analysiswere considered.
absenceof a Eu anomalyin 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 althoughplagioclasewas on the liquidus,a large
seamounts)show a narrow range of variation with respectto 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 stabilityfield.
The basaltsfrom Thompsonseamount,consideredpart 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 chemicalparameters,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 seamountsconditionsacting on a heterogeneoussource. In their studies
(TI'063 DH 34 and 36A, respectively)have a far greater of the JDFR and near-ridgeseamounts(includingBrown 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 heterogeneous
on 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 Cobbstrongly enriched in alkali and incompatible elements and Eickelberg region. Overlapping REE patterns (Figure 5) in
ratiosrelativeto JDFR lavas[Cousenset 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 seamountsamplesstraddlethe 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,
andratiosas well (Table 2). In somechemicalparameters
(i.e., incompatible-element rich pods within a heterogeneous
Zr/Nb, Zr/Y, P20s, andK20) CES lavasresemblethe Endeavour mantle source.
segmentbasalts,which are enrichedrelativeto segments
1-3
Ratio-ratioplots involving four different elementsthat are
of theJDFR (Table3) [Liias, 1986;Karsten,1988]. However, incompatiblein fractionatingphases(i.e., Zr/Y versusTi/Nb)
ontheaverage
CESbasalts
areenriched
in St,Zr, andP205but indicate mixing of two sources or magmas if sample
aredepleted
in Nb andhavehigherZr/Nband(La/Sm)Nrelative compositionsare linked by a curve [Langmuiret al., 1978]. In
to Endcavour
segmentlavas(Table3). CES lavasderivefroma 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
resembles
but is distinguishable
from the Endcavour
segmentversus Ti/Nb), sample compositions will fall on a line if
source.
mixing has occurred. In both plots the samplecompositions
On a chondrite-normalized
REE plot (Figure5), the CES should lie in the same relative positionsunlessfractionation
COBB-EICKEI•ERG
SEAMOUNT COMPOSITIONS
DESONIE AND Dt•c•:
o
o
T•
COBB-EICKELBERGSEAMOUNTCHAIN
12,701
12,702
DESONIE AND DUNCAN: TI-m COBB-EICKELBERG SEAMOUNt CHAIN
o
1.4
0
O0
,t
basalts (Figures 8a and 8b). However, scatter within these
plots precludessimple mixing of two discreteend-members.
Rather, the patterns seen are more compatible with partial
melting of incompatible-element
enrichedsegregations
within
the heterogeneousnortheastern Pacific upper mantle and
variablemixing with a relatively depletedJDFR source. To a
first order,mixing betweenenrichedand depletedsourcescan
also be seen along strike of the CES lineament, with more
enriched values of incompatible elements (K20 + Na20) and
ratios(La/Sm)N at the olderendof the chainandmorevariable
(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 enrichedin incompatible
element concentrationsrelative to normal-MORB (n-MORB)
from the JDFR (Table 3), Sr, Nd, and Pb isotopicratiosfor the
CES and Explorer and Union seamountsplot well within the
rangeexpectedfor n-MORB. Samplesfrom the seamounts
fall
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 basaltsanalyzedand the Juande Fucaridge (Figures10a and
10b). All CES samplesfall in or near the JDFR field [Church
Distance from Cobb hotspot, km
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).Basaltsdredgedfrom 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
trianglesrepresentanalysesof Cobb, BrownBear, Axial, and Son of isotopic composition.
BrownBear seamounts
by Morgan [ 1985].
GEOC/mONOLOGY
has affectedthe less incompatibleelements[Langmuiret al.,
1978]. On most ratio-ratio plots (i.e., Figures 8a and 8b),
CES lavas and near-ridgeCES [Morgan, 1985] plot closeto a
mixing curve or line. These data suggestmixing of a low
alkaliesand (La/Sm)N, high Zr/Nb end-memberexpressedmost
stronglynear the spreadingridge, with a higher alkaliesand
(La/Sm)N, lower Zr/Nb end-memberseenin the older CES
100
Eleven of the freshest basalt samples from the CobbEickelberg, Explorer, and Union seamountswere chosenfor
age determinations
by 4øAr/39Artotal fusion and conventional
K-Ar radiometric dating methods [McDougall and Harrison,
1988; Dalrymple and Lanphere, 1969]. Becauseof the young
ages and low potassium concentrationsin these lavas and
samplesize limitations for irradiation, not enoughradiogenic
ß
u
10
O--
DH01-1
0
DH02A
[-]--
DH02-8
l
m
DH04-4
]
DH05-14
X
DH08-8
A
DH10-7
ß
CB-1
[•--
DH14-5
•
DH34
[]]--
DH36A
6sl•bl•a
l•lbK I•a•e•r l•Idi• S'm•r•i•f •b
Fig. 3. Geochemical
patterns
normalized
to n-MORB[afterSunandNesbitt,1977];incompatibility
of an elementin nMORB decreases
to the right.
DESONIEAND DUNCAN:THE COBB-EICK•.RERG
SEAMOUNT
CHAIN
12,703
10
in Table 4. The relatively large age uncertaintiesresult from
measuringsmall amountsof radiogenic,,oArwithin a total ',OAr
signal of largely atmosphericorigin. A replicate 40Ar-39Ar
total fusion analysis was performed on one sample from
Eickelbergseamount(Table 4). Agescoincidewithin their two
sigma errors. Reactor irradiation is known to increase the
atmosphericargon component [McDougall and Harrison,
1988] so conventionalK-At ageswere determinedfor Explorer
seamountwhere sampleswere expected to be the youngest
analyzed(i.e., smallestradiogenic•øAr) and Explorer samples
were absolutelyfresh (no alteration). Crustal ageswere based
'
I
'
I
'
I
'
on magnetic anomaly interpretationsby Wilson et al. [1984]
30
40
50
60
70
and tectonicreconstructions
by Karsten and Delaney [ 1989].
Figure 11 shows the variation in age of eight seamounts
SiO
2wt.%
with distancealong the seamountchain from Cobb hotspot,
Fig. 4. Alkaliesvs. silicadiagramfor the CES, Explorer,andUnion
assumingits location is defined by the center of Axial volcano
seamounts. Northeast Pacific seamount field includes data from
seamounts
of the Pratt-Welker[Dairytopiceta!., 1987],Tuzo Wilson, (zero age). Distanceswere measuredalong the lineament,in
Bowie, and Dellwoodknoll seamountchains[Cousenset al., 1985; the directionof Pacific plate motion over the mantle [Duncan
Cousens,1988]. Near-ridgeCES field includesdatafrom 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 includesthe
Endcavour
segment.Alkali basalt-tholeiite
boundaryfromMacdonald plotted againstdistancefrom 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-Welkerseamounts[Turner et al., 1980]);
SOAr
waspresentfor analysis
by n0Ar_39Ar
incremental
heating a 2-m.y. spanwas recordedin the threesamplesanalyzedfrom
'•:•
'"'"'";",
"",,
-"'•:'
'
near-ridge
CES
/ ••"-Juan
de
Fuca
Ridge
experiments.
Basaltsamples
werecrushed
andsievedto obtain Eickelbergseamount.Therefore,we haveusedthe oldest
the0.5and1.0mmsizefraction,
thenwashed
ultrasonically
in sample
fromeachseamount
wheremorethanonesample
was
distilledwater. For '*0Ar-ZgArtotal fusion experiments,analyzedas an estimateof the time whenthe volcanowas
samples
were irradiatedfor 6-10 hoursin the coreof the locateddirectlyover the hotspot. Replicateanalyses
for
Oregon
StateUniversity
TRIGAreactor,
wheretheyreceived
a Eickelberg
sampleDH02Awereaveraged.Oldestseamount
neutronflux of 0.6-1.0x 1017nvt. Isotopes
of argonwere ageswereusedin a least-squares
linearregression
to determine
measured
on an AEI MS-10Smassspectrometer,
aftersample an averagerate of platemotionoverthe hotspotof 43 + 3
fusion via radio frequencyinductionheatingand gas km/Ma. A least-squares
regression
of all PacificplateCES
purification;
potassium
contents
weredetermined
by atomic includingall threeEickelberg
pointsgivesa rate of plate
absorptionspectrophotometry.Analytical data and age motionoverthehotspot
of 47 + 3 km/Ma. AgesfromUnion
calculations
fromthe'mAr-a9Ar
andK-Ar experiments
aregiven andExplorerseamounts
werenot includedin the regression,
TABLE 3. AverageCompositions
of SelectedChemicalElementsandRatiosfor Different
TectonicSettingsin the Juande FucaRegion
Chemical
JDFR
EndcavourNear-Ridge
Axial
Near-Ridge PacificPlate
ParameterSegments
1-3a Segment
b Seamounts
c Seamount
a CES
d
CES
e
K20 (wt.%)
P205(wt.%)
0.18
0.17
0.42
0.23
0.12
0.07
0.15
0.13
0.17
0.13
0.43
0.28
Sr ppm
Zr ppm
Nb ppm
113
104
5.3
211
134
13.9
158
54
2.0
136
92
4.3
159
93
4.2
271
145
10.4
Zr•
Zr/Y
22.1
3.10
9.7
4.58
28.1
2.17
23.3
3.26
28.1
3.66
14.0
3.84
(La/Sm)
N
0.86
a0.70
0.76
0.74
1.24
---
a FromLiias [1986].
b From
Karsten
[1988].
c Includes
Heck,HeckleandSpringfield
seamount
chains,
fromKarsten[1988].
d Includes
Cobb,
Brown
Bear,
andSon
ofBrown
Bear
seamounts,
data
from
Morgan
[1985],
averagedby Liias [1986].
e Includes
Eickelberg,
unnamed
(except
forP205wt.%),Warwick,
andCobb
seamounts,
from this study.
12,704
DESONIEAND DUNCAN:THE COBB-EICKF•BERG
SEAMOUNT
CHAIN
100
O--
1o
i C½
i
i
La
I
I
i
La Ce
Nd
I
Nd
I
Sm Eu
Smi Eui
t
DH01-1
•
DH02A
{"]•
DH02-8
m_
DH04-4
i
DH05-14
X
DH08-8
A
DH10-7
ß
CB 1
[]
DH14-5
•
v
DH34
[]
DH36A
t Lui
Tb
Yb
I
I
Tb
I
Yb Lu
Fig. 5. Chondrite-normalized
[afterNakamura,1974]REEpatterns
for seamounts
of theCES,Explorer,
andUnion
seamounts.
JDFR(includingEndeavour
segment)
field shownin stipplepauem[Wakeham,1978;Liias, 1986]. Analyses
fromsamples
dredged
neartheBlanco
fracture
zone(dredges
6 and7) fromWakeham
[1978]werenotincluded.
Insetshows
REE patterns
for threebasalts
fromWarwick seamount.
beingfromseparate
provinces,
but thesedataaregenerallyThis changein the crustalage at the time of seamount
consistent
with thePacificplatemotioninferredfromtheCES volcanismis due to the westwardmigrationof the JDFR,
age-distance
relationship.
toward
Cobbhotspot,
sinceatleast10Ma [Riddihough,
1984;
Two agepatterns
canbe seenin Figure11:
KarstenandDelaney,1989].
1. Agesincrease
tothenorthwest
alongtheCESchainfrom The line of seamounts
from Axial throughEickelberg,
Axialseamount
at zeroageonthespreading
ridge,to 3.3Ma at althoughshort,is subparallelwith other more prominent
Cobbseamount,
some105 km from the ridge,andon to 9.0 Pacificchainsthat are associated
with stationarylong-lived
Ma at Eickelberg
seamount,
about340 km fromthe ridge. mantlehotspots.
Thevolcanic
migration
rateof 43+ 3 km/Ma
Union seamountis one memberof a shortPacific plate is consistentwith a plate-mantlevelocity of 44 km/Ma
volcanicchainfartherto thenorth. The zeroagepositionof predictedfrom the latestPacificplaterotationpole at 62øN,
theassociated
meltinganomaly
is unknown;
we haveassumed94øWandrotationrateof 0.95ø/Ma[Pollitz, 1988], which
it tobetheEndearour
ridgesegment
(Table4).
applies
fortheperiod0-9Ma. NewagedatafromtheCES,as
2. Thereis a northwesterly
increase
in thedifference
in the wellasthePratt-Welker
seamounts
[Dalrymple
et al., 1987],
ageof theseamounts
andthecruston whichtheyformed.Samoa
[McDougall,
1985],andtheLouisville
seamount
chain
Axialseamount
is currently
forming
ontheridgeonzero-age[Wattset al., 1988]support
the ideaof rigidPacificplate
crustbutEickelberg
seamount
formed
oncrustthatwas2-3Ma. motionovera fixedconstellation
of hotspots
(Figure12).
1.0
3.0
•ø!•
cpxPlag
o
0.9
o o
2.0-
0.8
o
o
o
o
fl
o
o
ß
ß
[]0 *•1
oO
0.7
o
0.6
40
1.0-
o
'
5'0
'
Mg-number
6'0
'
70
MgO wt. %
MgOvariation
diagram
forbasalts
fromtheJDFR
Fig. 6. Fractionation
trendsfor olivine(ol), clinopyroxene
(cpx), Fig.7. TiO2 versus
andplagioclase
(plag)andvariation
of of CaO/A1203
withMg-number (solid squares)[Liias, 1986], Axial and the near-ridgeCES (open
squares)
[Morgan,1985],andtheolderCES(circles,thisstudy).
for Cobb-Eickelberg,
Explorer,andUnionseamount
basalts.
DESONIE AND DUNCAN: THE COBB-EICKELBEROSEAMOUNTCHA•
7
(a)
6o
5-
4-
3-
12,705
usually distinguishhotspot from spreadingridge lavas, the
CES basaltscannotbe separatedfrom MORB compositions.
The lack of distinguishing
St, Nd, Pb, andHe isotopicratiosis
a feature of other northeast Pacific seamountand spreading
ridgelavas[Churchand Tatsumoto,1975; Lupton, 1982;Eaby
et al., 1984; Morgan, 1985; Liias, 1986]. No significant
isotopicvariationhas yet been found in basaltsfrom the PrattWelker [Churchand Tatsumoto,1975;Hegner and Tatsumoto,
1989], Heckle [Hegner and Tatsumoto, 1989], Tuzo Wilson
[Cousenset al., 1985], Bowie [Cousenset al., 1985; Cousens,
1988], Dellwood knolls [Cousenset al., 1984], or President
Jackson (W. M. White, personal communication, 1988)
2
seamount groups.
60-
Models
50-
Several models may explain the age distribution and
chemicalcompositionsof the CES basalts.
40-
Passivelyupwellingheterogeneous
mantleand a migrating
spreading ridge. The CES may be the result of the same
processesthat form near-ridge seamountsalong the East
3020-
(b)
100
0
1000
2000
3000
4000
5000
6000
Ti/Nb
Pacific Rise [Batiza and Vanko, 1983, 1984; Zindler et al.,
1984; Fornail et al., 1988a, b] and Juan de Fuca and Gorda
ridges[Davis and Karsten,1986; Karstenand Delaney, 1989].
Most of thesesmall near-ridgeseamountclustersare thought
to be the result of volcanism above a short-lived melting
anomalyembeddedin the uppermanfielateral flow, ratherthan
Fig. 8. Incompatible
elementratio-ratiomixingplots[afterLangmuir
a deep mantle plume. Following the Batiza and Vanko [1983,
et al., 1978]. Axial seamountsamplesare shownin solid squares
[Morgan,1985],near-ridgeCES (includingBrownBear,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 Endcavoursegmentfields are labelled[Liias,
1986; Karsten, 1988].
NE Pacific
Se
DISCUSSION
Summaryof Temporaland ChemicalCharacteristics
of the CES
The CES chain has the temporal and morphologic
characteristicsof volcanismgeneratedover a hotspotfed by a
mantleplume. The seamountchain is age progressiveand the
linear geometry and age distribution of volcanoes are
compatiblewith the motion of the Pacific plate over other
well-establishedhotspots.
Age
relationshipsalong the CES
40
39
chain, determined by
Ar- Ar and K-Ar analyses, in
conjunctionwith the tectonic reconstructionof the Juan de
Fuca region of Karsten and Delaney [1989] require a fixed
source for the CES lineament and a westwardly migrating
spreading ridge. Our data demonstrate that the melting
anomaly currently beneath Axial seamountis a stationary,
persistentfocusof unusuallyhigh magmasupply,even though
volcanic activity along the lineament may have been
intermittent, with the Eickelberg to Axial seamountsection
being the latest "pulse"of plume flow.
In most respects,CES lavas are transitionalbetweenbasalts
characteristicof hotspotsand those of segments1-3 of the
JDFR; they are similar to those of the Endeavoursegment.
Compositional variability among CES basalts probably
0.5132
0.5131
(a)
0.5130
i
0.7024
0.7023
ß0.7 b25 ' 0.7026
I ß0.7027
I ß0.7028
87Sr/86Sr
0.7028
(b)
0.7027
0.7026
0.7025
coø
ß
o
o
o
0.7024
0.7023
....
400 30'0 2to
0'0
6 '
' 200
Distance from Cobb hotspot,km
reflects
arange
ofpartial
melting
ofthemantle
supplied
tothe Fig.9. (a)143Nd/•44Nd
versus
S7Sr/S6Sr
forCES
and
Explorer
and
meltinganomaly,which is heterogeneous
with respectto Union
samples
withJDFR[Hegner
andTatsumoto,
1987]andother
NE
incompatibleelements[Morgan, 1985;Liias, 1986; Karsten, Pacificseamount
[HegnerandTatsumoto,
1985;Cousens
et al., 1985;
1988]. The eastward
trendtowarddepletedcompositions
Cousens,
1988)fieldssuperimposed.
Range
inisotopic
ratios
shown
indicates
thatprogressively
greater
proportions
ofthesource
is
ton-MORB
field.
(b)equal
87Sr/a6Sr
versus
distance
fromCobbhotspotfor samples
fromthe
for JDFRbasalts
participated
in meltingfor 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 somewhatsmaller degree of partial
melting of the sameheterogeneous
upper mantle sourcethat is
producing the JDFR lavas would result in melts somewhat
enriched in incompatible elements and alkalies but with
essentially MORB-like
isotopic composition.
This
mechanismcould easily explain the modest enrichment in
alkalies of the CES lavas and might. with an even smaller
degree of partial melting, explain the alkali basalts and
15.55
15.50'
15.45
hawaiites
of other northeastern
Pacific
seamounts.
The scale of volcanic activity at such near-ridge seamounts
is,however,
much
smaller
than
intheCESprovince.
A •q•ical
15.40
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
chainnear the Juande Fuca ridge beganactivity at ~2 Ma and
ceasedat 0.5 Ma [Karsten and Delaney, 1989]. Volcanism
alongthe CES chainhaspersistedfor at least9 m.y.
Although it is possible that a large passiveupper mantle
melting anomalycould accountfor the increasedvolume and
duration 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•618.819.019.2 19.4 19.6 below the asthenosphere.A melting anomaly, of the type
responsible for volcanism at small near-ridge seamount
chains,wouldbe sweptalong with uppermantleflow over the
time scalerequired. Therefore,we believe that Cobb hotspot
' 10.(a) •7pb/2ø4Pb
versus
•6pb/•ø•Pb
and(b) •Pb/•ø•Pbversus has been maintained by a mantle plume during the CES
PbforCES,Explorer,
andUnionsamples
compared
withJuan formation. Also, near-ridge seamountbasaltsin the Juan de
de Fucaridge [Churchand Tatsumoto,1975; Hegner and Tatsumoto, Fuca region, i.e. Heck, Heckle and Springfield [Karsten,
1987] andNE Pacificseamounts
[Churchand Tatsumoto,1975;Hegner
1988], are more depletedin alkali and incompatibleelements
and Tatsumoto,1989; Cousens,1988] fields. The samplefrom Union
seamount(DH36A) is distinct. Range in isotopicratios shown is at a given MgO content than the spreadingridge basalts,in
•.•38.10
"-':•
37.70
•'
....................
'NEPacific
'"•
Seamounts
206pb•04pb
contrast with the CES lavas.
slightlylarger thanvaluesseenin n-MORB basalts.
TABLE
4. 40Ar-39Ax
andK-AtTotal
Fusion
Ages
andAnalytical
DataforWhole
Rock
Basalt
Samples
From
theCobbEickelbergand ExplorerandUnion Seamounts
Sample
J-Factor 40Ar/39Ar 36Ar/39Ar37ArC/39Ar
%K
•r080 DHOl-1
•r080 DH02A
replicate
•r080 DH02-8
•r080 DH08-8
•r080 DH 10-7
•r175 DH 70-2
•r175 DH 71-4
•r175 DH 74-17
CB-1
•r080 DH 14-5
•r063 DH 34
0.001957
0.002446
0.002446
0.002446
0.001957
0.001292
0.003050
0.003050
0.003050
0.003050
0.002446
0.003050
21.269
15.865
40.990
21.768
28.394
23.862
35.811
29.516
55.674
11.161
163.580
30.274
0.06435
0.05083
0.13615
0.07212
0.09047
0.77980
0.12171
0.10218
0.19423
0.04158
0.56704
0.11003
22.052
15.574
12.812
14.542
16.109
22.765
14.064
18.908
25.713
22.013
60.010
29.057
•r063 DH 35
0.003050
35.303
0.13252
50.211
Tr063 DH 36A
0.003050
8.875
0.29922
*40Armol/g%40At Age(:L1sigma)Ma
(xlO-9)
11.91
12.69
4.16
7.35
6.85
12.61
2.64
2.72
0.51
0.44
0.07
0.666
4.375
0.489
6.510
0.16
8.696
1.14
9.03
8.98
7.52
7.05
6.91
7.73
5.20
4.40
1.55
3.27
3.19
0.12
0.17
0.31
0.35
5.78
(0.41)
(0.75)
(0.46)
(0.15)
(0.30)
(0.33)
(0.32)
(1.07)
(1.40)
(0.30)
(1.30)
(0.57)
(0.03)
(0.37)
(0.04)
(0.65)
Ages
calculated
using
thefollowing
decay
and
abundance
constants:
•œ=0.581x10
-10yr'l;)•[1=4.963x10-10
yr-1;
40K/K=l.167x10-4
mol/mol.Neutronfluxmonitored
withhornblende
standard
MMhb-1.
Correctedfor decaysinceirradiation.
DESON[E AND DU•C•:
TI•
COBB-EICKELBERG S.EAMOUNTCHAIN
12,707
10.0
Eickelberg
ß
unnamed
8.0
x\\
Warwick \xxx"x
6.0
U
t
xx
4.0
2.0
Gluttony
J •
Kilometers
Exp•'or,er
from ridge
Fig.11.Seamount
ageversus
distance
fromspreading
ridgeforCESandExplorer
andUnionseamounts.
K-ArageofCobb
seamount
by Dymond
et al. [1968]shown
astriangle.Measured
distance
forCESisfromCobbhotspot,
defined
asthe
centerof Axial volcano(46øN,130øW),alongthedirection
of Pacificplatemotion[Pollitz,1988]. Crustalages[after
Wilsonet al., 1984]areshown
by thedashed
line;a breakindicates
location
of a pseudofault.
Average
rateof motionof
thePacificplateoverthehotspot
wascalculated
to be 43 +_.3 km/Ma. Replicate
analyses
of sampleDH02Afrom
Eickelbergseamountwere averaged.
Compositionally buoyant mantle plume and a migrating
spreading ridge. Compositionalbuoyancyoccurswhen plume
flow is driven by intrinsic density differencesbetween the
diapir and its surroundings[Griffiths, 1986a]. Chemical
diffusion proceeds extremely slowly so that compositional
diapirs maintain their chemical identity relative to the upper
mantle material throughwhich they rise [Griffiths, 1986a].
A compositionalplume that has moderatelyenrichedtrace
element contentsbut a nonradiogenicisotopiccharactercould
originate in several ways. For example, the MORB-like
isotopic composition of all seamountsof the northeastern
Pacific may reflect a broad regional difference in mantle
compositionfrom the equatorialoceanicregions where Dupal
hotspotsare found [Hart, 1984]. Hart [1988], who noticed a
poleward trend toward less radiogenicisotopic 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
-16
-14
-12
lengthscale(10'• kin) and someregionsmay be isotopically
MORB-like if large amountsof melts have been previously
extracted
from them.
A metasomaticevent that enrichedthe 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. Volcanicmigrationrate as a functionof rotationpole located elevated concentrationshave not yet significantly shifted the
by
at 62ø N, 94ø W [Pollitz,1988]. Thedottedcurveis a least-squares Sr andNd isotoperatiosfrom the MORB field wassuggested
bestfit withan angular
rotationrateof 0.95+ 0.02ø/Ma.
Cousenset al. [1985] to explain the alkalic but isotopically
i
i
i
i
i
i
i
i
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 transitionallavas of archipelago,in which Sr isotopic ratios decreasetoward the
the CES chain.
centerof the hotspot,may result from the thermalentraimment
The proximity of Cobb hotspotto the Juan de Fuca ridge processproposedby Griffiths [ 1986a, b].
Thermal entrainmentcan also occur in a continuousplume
may provide anotherexplanationfor the unusualchemistryof
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 extendat least the centerof the plume. Therefore,the isotopiccomposition
130 km away from the spreadingaxis. This model of hotspot of eruptedmagmasmay be an important distinctionbetween
and spreadingridge interactionis easilytestable. Older Gulf of chemical and thermal plumes, regardlessof possible smallAlaska 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
contrastbetweenplume and normal manfie.
In reality plumesmay occur from a combinationof thermal
typical OIB signatureif they formed from a plume with OIB
composition. If basaltsfrom suchseamountsare 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 lavasfrom Murray and topographic swell, in this case several hundredsof 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 enrichedmantle 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,
mantle source. This plume was
lithospherethrough which the plume rises, can explain the stationaryand homogeneous
distinct incompatibleelement and isotopic compositionsof not noticeably contaminated by overlying asthenosphereor
tholeiitic and alkalic lavas [Chen and Frey, 1983]. A similar lithosphereas were plumes that form other hotspot chains
mixing model can also be appliedto the CES lavas and other [Cheng et al., 1987, and referencestherein].
In contrast,the more intermittentand consistentlyMORBseamountlavas of the northeastPacific [Cousenset al., 1985].
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 entrainthe MORB3.0% 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 MORBseamountcould 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/86Srenrichmentwith increasingdegreeof 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
observedcompositionsseem to point strongly to the absence entrainment. This is consistentwith a relatively "weak"
of a long-termenrichedmantle componentin lavasof the CES (thermal)plume sourcefor the CES.
and other northeast
Pacific
lineaments.
Thermally buoyantplume and a migratingspreadingridge. SummaryModel
of the
Formation of a plume that has MORB isotopiccharacteristics The major and trace element chemicalcharacteristics
might result from thermal entrainmentof MORB uppermantle CES are best explainedby mixing of enrichedand depleted
by a plume of lower mantle composition. Thermally buoyant lavasresultingfrom partial meltingof a heterogeneous
upper
plumesare driven entirelyby viscosityand densitydifferences mantle in both a hotspotand spreadingridge meltingregime.
createdby a temperaturegradient acrossa thermal boundary Like JDFRmelts,CES liquidsprobablysegregate
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 buoyantmantleplume. Although,the chemicalcharacteristics
center [Griffiths, 1986a]. Recently, Geist et al. [1988] have of the lavascan be explainedby a largepassiveuppermantle
DESONIEAND DUNCAN:•
COBB-EICKELBERG
SEAMOUNT
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 lineamentmustbe rooted hotspot(Figure 13). Eickelbergseamount
was thenforming
beneath upper mantle lateral flow. The most likely abovethehotspoton seaflooraged2-3 m.y. Proximityof the
explanationfor the unusualgeochemicalcharacteristics
of Cobb hotspotto the JDFR may have resultedin dilutionof
Cobb hotspotis a deeplyrooted,thermallybuoyantdiapir plumematerialby mixingwith the MORB partialmelt. The
whichhasentrained
nonradiogenic
uppermantleMORBsourcenextseamount
to theeast(unnamed)
hadbegunformingwhile
material. The large,thermallydiluteplumesuppliesthe Eickelberg
seamount
was still active,-7.8 Ma. By 6 Ma,
hotspotwith volumesof warm mantlein excessof that Eickelberg
seamount
hadmoved88 km westof thehotspot,
upwelling
alongthespreading
ridge,resulting
in thebuildup theJuande Fucaridgehadmoved35 km closerto thehotspot
of thevolcanic
cones
of theCESchain.Progressively
greaterandWarwickseamount
wasforming
on-2 Ma crust.At -4 Ma,
mixingof plumeand spreading
ridgemantlesources
has Slothseamount
hadformedon 1.6-m.y.crustandhadmoved
occurred
astheIDFR approached
thehotspot.
morethan50 km to the west;Lustseamount
had formedon
Agerelationships
alongtheCESlineament
since9 Ma 1.8-m.y.crustwhiletheridgewasonly65 km fromthe
permitgreater
resolution
of theKarsten
andDelaney
[1989] hotspot.Although
a largeuncertainty
is associated
withthe
modelforhotspot-spreading
ridgeinteraction.
In thismodela ageof Thompson
seamount
it is consistent
withanoriginat
westward
migration
of theIDFRtoward
a fixedCobbhotspottheridgeat~3.2Ma,followed
bytransport
eastatarateof -34
wasdetermined
fromanalysis
of thepropogating
rift and km/MawiththeJuan
deFuca
plate[Desonie
andDuncan,
1986;
spreading
history
of theridge[Wilson,1984;Riddihough,
Karstenand Delaney,1989]. Cobbseamount
formedat
approximatelythe same time as Thompsonseamounton crust
of age 0.4 Ma. Interaction of spreading ridge and hotspot
partial melt zonesresultedin lessenriched(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.
E
• 8MaI
400
•
300
200
100
0
100
200
EXW
?-•,','.....'•?•'4'•:,:•:•'?•'•'
.....
I
400
I
300
I-'•:'
200
I
100
EXW
300
200
EXW
....
the guidance of M. Cheatham. We thank R. Walker of the
Radiation Center at OSU for prompt INAA results. Age
6 Ma
400
".....:'•-.."!•t
::•'""
'"'1•'"'
' '
I
0
100
S
100
S L
I
200
0
C
100
were
obtained
with
the assistance
of L. G.
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,,,•,•,,,,,,,,,,,,,,,...,,•,,,,
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(ReceivedApril 10, 1989;
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acceptedSeptember26, 1989.)