MANTLE PLUMES and FLOOD BASALTS Scribedblob of Uniformtemperature, Rather Than Resultingfrom Startsat the Paranaflood Basalt Province,South America

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provided by Memorial University Research Repository JOURNALOF GEOPHYSICAL RESEARCH, VOL. 106, NO. B2, PAGES 2047-2059, FEBRUARY 10, 2001

Mantleplumes and flood basalts: Enhanced melting fromplume ascent and an eclogitecomponent A.M. Leitch•, andG. F.Davies ResearchSchool of EarthSciences, Australian National University, Canberra, ACT

Abstract.New numerical models of startingplumes reproduce the observed volumes and rates of floodbasalt eruptions, even for a plumeof moderatetemperature arriving under thick lithosphere. Thesemodels follow the growth of a newplume from a thermalboundary layer and its subsequent risethrough the mantle viscosity structure. They show that as a plumehead rises into the lower- viscosityupper mantle it narrows,and it isthus able to penetrate rapidly right to thebase of lithosphere,where it spreadsas a thinlayer. This behavior also brings the hottest plume matehal to the shallowestdepths. Both factors enhance melt production compared with previous plume models. Themodel plumes are also assumed to containeclogite bodies, inferred from geochemistry to be recycledoceanic crust. Previous numerical models have shown that the presence of nonreacting eclogitebodies may greatly enhance melt production. it hasbeen argued that the eclogite-derived meltwould react with surroundingperidotite and refreeze; however, recent experimental studies indicatethat eclogite-derived melts may have reached the Earth's surface with onlymoderate or evenminor modification. Combined with anassumed 15 vol % componentof eclogite,our models yielda sharppeak in meltingof about1-3 Myr durationand volumes of meltthat encompass those observedin floodbasalt provinces.

1. Introduction Traps)[Hooper, 1990]. The plume head model is quitecompati- ble with synrifteruptions, but initial estimatesof t_heamount of Flood basalts are vast, geologicallybrief outpouringsof preriftmagma that would be producedwere orders of magnitude basalticlava onto the surfaceof the Earth, and hot spottracks, less than observed flood basalt volumes. whichoften start at a flood basaltprovince, are lineartrails of Two factorsthat might overcome this discrepancy have been much less voluminousvolcanism. The "startingplume" or addressedin previousstudies. Farnetani and Richards[1994, "plumehead-plume tail" model has been proposedto explain 1995]proposed that plume heads are hotter than had previously floodbasalts and hot spottracks [Morgan, 1981;Richards et al., been inferred,350--400øC rather than ! 00-200øCabove ambient 1989;Campbell and Griffiths,1990]. Accordingto thismodel, a mantle.They found that they could obtain large volumes of melt, mantleplume begins as an instabilityin the thermalboundary butthe high plume temperature implies surface uplift of 2-4 km, layerat the core-mantleboundary (CMB) andrises through the whereasuplifts associated with flood basaltsare typicallyno mantleas a large,spherical head, with a thinnertail connectingit morethan 1 km [Campbell,1998]. Also suchhigh temperatures to the sourceregion. A floodbasalt eruption would then result wouldproduce highly magnesian picrites rather than basalts. Far- fromdecompression melting as a hotplume head approaches the netaniand Richards attribute the scarcityof picritesto fractional surface,while a hot spottrack would result as the lithosphere crystallization[Farnetani et al., 1996;Lassiter et al., 1995],but passesover the longer-livedtail. thisdoes not explain why pierites are confinedto theearly stages Thestarting plume model is basedon well-established physics of floodbasalts [Campbell, 1998]. Finally,their models included [Griffithsand Campbell,1990] and is consistentwith important a relativelysoft lower lithosphere which the plumes could dis- geophysicaland geochemical characteristics of flood basalts and placein orderto rise to shallowerdepths. However, there is little hotspot tracks [Campbell and Griffith&1990], but it hasbeen independentevidence that the lithosphere is so readily displaced criticizedbecause it hasseemed that little or no magmawould be [Davies,!994]. producedfrom a plumehead unless the lithosphererifted and On the otherhand, Cordcry et al. [1997], followingCampbell allowedthe plume material to riseto withinless than 100 km of et al. [1995],took note of well-knowngeochemical evidence that thesurface, where substantial pressure-release melting can occur plumecomposition is different from the sourceof mid-ocean [Whiteand McKenzie,1995]. On the otherhand, the rifting ridgebasalts (MORBs). Trace element and isotopic evidence is modelfor floodbasalts [White and McKenzie,1995] does not commonlyinterpreted as indicatingthat plumeshave a higher seemto be compatiblewith flood basalts that involve little or no proportionof recycledoceanic crust [Hofrnann and White, 1982; rifting(as in Siberiaand the ColumbiaRiver basalts) or wherea Hofmann,1997]. This old oceanic crust would exist as eclogite majorphase of the eruptionpreceeds firing (asin theDeccan in theupper mantle, and eclogite is expectedto meltat substan- tiallylower temperatures than the pyrolite composition of MORB 1Nowat Departmentof Earth Sciences, Memorial University of sourcemantle [Yasuda eta!., 1994]. Cordcryet al. [1997] Newfoundland,StJohn's, Canada. demonstratedthat a hotblob incorporating15% eclogite would producevolumes of eclogite-derivedmelt that approached the Copyright2001 by the American Geophysical Union. volumesof floodbasalts, even using conventional estimates of Papernumber 2000JB900307. plumetemperature and with a nonerodinglithosphere. However, 0148-0227/01/2000JB900307509.00 thosemodels were preliminary, in thatthe plume head was a pre-

2047 2048 LEITCH AND DAVIES: MANTLE PLUMES AND FLOOD BASALTS scribedblob of uniformtemperature, rather than resultingfrom startsat the Paranaflood basalt province,South America. Hot the ascentof a plumefrom the CMB. spot tracks are easiestto identify when they occur on oceanic In thispaper we demonstrate,using high-resolution numerical crust, where they manifest as a sequenceof volcanic ocean models,that the detailsof how a plumegrows from a thermal islands(e.g., Hawaii and the Emperor Seamount Chain), but they boundarylayer and rises throughthe mantlehave important havealso been traced on continents.Hot spotsare relatively sta- effectson the thermalstructure of the plumehead and on the tionarywith respectto one another,and this observationhas led closenessof the approachof the plume to the Earth's surface. to the argumentthat they must have their origin in thedeepest Theseeffects substantially enhance melt productionin a plume mantlewhere, because of highviscosity, lateral flow is veryslow. head. In combinationwith a 15 vol % eclogiticcomponent, the The strengthof hotspots, as indicatedby theirbuoyancy fluxes, resultingmodel plumes readily reproduceobserved volumes of variesa greatdeal [Davies, 1988; Sleep, 1990]. Forthe strongest floodbasalts even with moderateplume temperatures. intraplatehot spot,Hawaii, the volume of volcanicrock above the We continueour modellinglong past the arrivalof the plume oceanfloor gives a rate of mantle melt productionof 0.03 headto investigatemelting in plumetails. We find thatthe melt km3yr-• [C!agueand Dairytopic, 1989]. Subcrustalvolcanism ratesfor the Hawaiianhotspot are bestmatched with about3 vol probablyincreases this number by a factorof 2 or 3, butwe are % eclogitein the tail. This apparentdifference in the eclogite left with a melt productionrate for stronghotspots which is less fractionof headsand tails may be due to nonverticalascent and thanthat for floodbasalt provinces by 1 or 2 ordersof magnitude. entrainmentin real tails or to differentdegrees of eclogite-matrix Weakhot spots(e.g., Tasmanfid[Duncan and Richards,1991]) reaction. canbe anotherorder of magnitudeless productive. The estimatedvolumes of eclogite-derivedmelt providesan upperbound on the amountof magmalikely to reachthe Earth's 2.2. Eclogitein the Mantle surface,since some of the eclogitemelt might react with sur- roundingperidotitic mantle and temporarily refreeze [Yaxley and The geochemicalevidence for a largereclogite component in Green, 1998]. However the degree of interactionof eclogitic plumeheads (flood basalts) and tails (ocean island basalts) than in meltswith surroundingmantle is uncertainas it dependson the the MORB sourceis discussedby Cordcry et al. [1997]. This poorly understoodphysical processesof melt migration and evidenceconsists of trace elementand isotopicdata [Hofmann extraction. and White,1982], oxygenisotopic data and Re-Osmeasurements Using new experimentalevidence, Takahashi et al. [1998] of ocean island basalts,and the FeO content of flood basalts. proposethat the the main phase of eruptionof the Columbia Together,the evidencesuggests that some plumescontain a River Basaltsmay result from partial melting of an eclogitic significantexcess component of ancient(~2 Gyr) subduetedcrust sourcewith little or no reaction with peridotire. Other flood or sediment,although the proportionsof eclogiteand pyrolite in basaltsdo show evidenceof' reactionwith peridotite,but their the sourceregion and the degreesof partialmelting of eachcan- compositionscan still be accountedfor by assumingan eclogite- not be determineunambiguously [Cordcry et al., 1997]. derivedprimary melt. Thesefindings suggest that it is possible A currentunderstanding of mantle convection[e.g., Davies for muchof the eclogite-derivedmelt to reachthe Earth's surface. and Richards,1992] lends some support to the geochemicalevi- Sincethe main question addressed here is whetherthe models can dence.The presentsubduction rate of oceaniccrust into the mantle correspondsto 2% of the entiremantle volume every billion approachthe observedorder of magnitudeof floodbasalt volumes,we arguethat the eclogitemelting estimates are a sufficient years,so after 4.6 Gyr of Earthhistory the mantle may well contain 10%or moreof eclogite.Since the meltingtemperature of approximationand that our resultssupport the viabilityof the all mantleminerals increases strongly with pressure[Boehler, plumehead model. 1996]and ec!ogite is probablytoo cold to meltas it is subdueted, it will onlybe ableto meltand react with the surrounding mantle 2. Background materialswhen it is returnedto the nearsurface. Throughout its residencein the deepmantle, subdueted oceanic crust will be 2.1. Flood Basalts and Hot Spot Tracks deformedby mantlecirculation, but anychemical homogeniza- Floodbasalts occur roughly every 10 or 20 Myr, bothon conti- tion (by solid-statereaction) will be minimal. nents(e.g., SiberianTraps, Columbia River Basalts)and on the Thedistribution of eclogitewithin the mantle is a matterof oceanfloor (e.g., OntongJava Plateau, Kerguelen Plateau). Vol- debate.Seismic tomography [van der Hilst et al., 1997]indi- umesare in therange of 0.1- 10x 106km 3 forcontinental flood catesthat some subducted lithosphere descends very deep in the basaltsand 10- 60 x 106km 3 for oceanicflood basalts. Dating mantle.It hasbeen suggested that on the basis of differencesin measurementsindicate very rapid eraplacement times of 1-3 Myr, rheologyand density, the subdueted crust might separate from the givingeruption rates of 0.1-8km3yr -• oncontinents and 2-20 slabat the 660 km phase transition [Ringwood, 1982; van Keken km3yr-• onthe ocean floor [Coffin and Eldholm, 1994]. The pre- et al., 1996]or at thebase of themantle [Christensen and sentbasalt production rate for the entiremid-ocean ridge system mann,1994]. According to Irifune[1993] and Kesson et al. is 20 km3yr-•, andfor subduetionrelated volcanism it is about2 [1994,1998], the oceanic crust would be about 0.2 g/cm 3more km3yr-•. In a globalcontext, melt production from flood basalts densethan pyrolite in theupper mantle, 0.2 g/cm3 less dense is of secondaryimportance; however, flood basalts have excited between660 km andabout 1100 km, thenvarying from 0.04 much interestboth becauseof their dramaticimpact (e.g., they g/cm3more dense to possibly 0.03 g/cm 3less dense atthe bottom have been linked to mass extinctions[Richards et al., 1989; of themantle. The uncertainties increase with depth, and as yet, Campbellet al., 1992])and because they do not appearto be thereis no consensus.The relative rheologies of eclogite and generatedby platetectonic processes. pyroliteare even more uncertain. Hot spottracks are linear trails of volcanismwhich often start Mixingefficiency within the mantle is also a matterof debate. at a floodbasalt province: for example,the Reunion hot spot track Recentnumerical studies [e.g., van Keken and Zhong, 1999; van startsat the DeccanTraps, India, and the Tristanhot spottrack Kekenand Ballentine, 1999] suggest that the mantle should be LEITCHAND DAVIES:MANTLE PLUMES AND FLOOD BASALTS 2049 evenlymixed; however, this does not accordwith geochemical 400øC, they found about 20% partialmelting occurred in a flat- evidencefrom isotopes of bothrefractory elements and noble gas tenedoval with a radiusof about400 km at the top of theplume isotopes,which point to a moreheterogeneous, lessdepleted, and axis,between depths of about80 and 180 km. They foundthere lessdegassed reservoir in thelower mantle [Davies, 1990; Hof- was very little melting of entrainedmantle material: more than mann,1997; McDougall and Honda,1998]. A combinationof 90% of meltingmaterial originated from within 100 km of the factors,including lateral variations in rheology,higher viscosity CMB. andminor densitystratification, may accountfor lessefficient mixingand longer residence times in thelower mantle [Davies, 3. Numerical Models 1990;Christensen and Hofrnann,1994]. If thesefactors inhibit overturningnear the CMB, sothat there is onlyslow settling as materialis ablated off the bottom of the mantle and channeled 3.1. Equationsand BoundaryConditions intomantle plumes, settling rates are of theorder of 150km every We ran the simulationsusing the finite difference,multigrid billionyears [e.g., Leitch, 1995]. programCONMG, which has been successfullycompared with There is clear geochemicalevidence for differencesbetween publishedbenchmarks [Blankenbach et al., 1989] and unpub- theocean island basalt (OIB) source(s)and the MORB source.In lishedfinite elementsolutions [Davies, 1995]. CONMG solved ourmodels we look at the implicationsfor the meltingrate if the equationsfor infinite Prandtl number, incompressibleflow thesedifferences are due to a higherproportion of nonreacting with Newtonianrheology, and the heat equationincluding the eclogitein theOIB source.This may be dueto a differencein the effectsof latentheat. In axisymmetriccylindrical coordinates the bulkcomposition of the OIB sources,or it mayreflect differences momentumequations in dimensionlessunits are in the scaleof heterogeneityand mechanismsof melt extraction O.t (section6.5). We envisagethat eclogiteis presentin the mantle 0= •P' •a',• aa% a'•r- • (la) andin mantleplumes in discreteblobs and stringers which, in the at' +-•-r' + az' + r' lowermantle in particular,may havedimensions up to kilometer size. 0 = - ap'az---7+-•'7-z 3o"=' + 3er',z ar' +7cr"z - T' ' (lb) 23. Previous Work wherethe components o'0. of thedeviatoric stress tensor are Farnetaniand Richards[1994] modeleda plumehead as an isolatedisothermal sphere of pyrolite compositionreleased at 2000km depthand allowedto rise andflatten against the litho- O'trr= •, "at"'au;' O'"•= •, r--- 7, sphere.The lower mantle had a viscosityof about1022 Pa s, the uppermantle was ! or 2 ordersof magnitudeless viscous, and the rt'au; • au;. lithospherewas up to the sameviscosity as the lower mantle. For a'== 0-%,= r'(b7,+ 77/,). initial temperatureanomalies AT between300 and 400øC and Vertical distance z is measured downward. The dimension!ess lithospherethicknesses of 50 and 110 km, they obtainedmelt ratescomparable with floodbasalts. Melting occurred in a single heatequation can be written pulseof about10 Myr durationin a thin discalong the top of the flattenedplume head. The lower lithospherein mostof their (2) Dr' ' modelswas, however,very weak [of. Davies, 1994], and this z)t': 7ka')+aTJaSat' ) a'-r' 1 allowedthe plumeto rise to relativelyshallow depths. For a stifferlithosphere they found melt volumeswere reduced by a whereD/Dt' is the materialderivative, L' is the latentheat, and X factorof 2 to 3. is thedepletion fraction. Symbols are defined in theTable 1. Cordcryeta!. [ 1997] carriedout the first numericalsimula- The domainwas a cylinderwith radiusI500 km and depth tionsof themelting of eelogitein plumeheads, with models simi- 3000km, withthe plume rising up the axis. A uniform256x512 lar to thoseof Farnetaniand Richards[1994], except that the gridprovided a veryhigh resolution of 6 kin. Boundarycondi- mantlewas always topped by lithospherea thousand times more tionswere free-slipand reflecting on the sides,no-slip constant viscousthan the upper mantle. In generalagreement with Farne- temperature(0øC) at the top andfree-slip constant temperature taniand Richards[1994], they foundthat for pyroliteplumes (Tcm) on the bottom.The backgroundmantle had a constant withAT of 100-300øC,very little melt formed.A "squeeze"potential temperature Tpof 1300øC.The temperature anomaly at layerof normalmantle was trapped between the plume head and the core-mantleboundary ATcM• was set between 200 and520øC thebase of the lithosphere,limiting how high the plume head in differentsimulations in orderto yield reasonabletemperature couldrise, and the lithosphere was not thinned at all by reheating anomaliesATm for plumesin themelting zone of theupper man- or plume-relatedshear. For the hottestplume heads (AT = tle. 300øC)and the thinnest lithosphere (50 kin) of theirruns only Thetemperature in thetop boundary layer had an errorfunc- 0.15x 106km 3 of melt was produced. However, when the plume tionprofile headhad 15 wt % eclogitemingled into it themodels yielded up T(z) =Tm err(z/h), (3) to5 x 106km 3 of meltwithin 3 Myr. Themaximum degree of meltingexperienced by the eclogitewas between 10 and35%, whereh is the "lithospherethickness." For oceaniclithosphere andagain melting occurred in a thindisc at thetop of theflat- therelationship between thickness and age is tenedplume head. Farnetaniand Richards [1995] looked at meltingand entrain- h = 2-4•'•; (4) mentin pyrolitestarting plumes, again with a relativelyweak lithosphere.Fortemperature contrasts across the CMB ATcm of h wasreset to a desiredthickness (60 or 90 km, correspondingto 2050 LEITCH AND DAVIES: MANTLE PLUMES AND FLOOD BASALTS

Table 1. Notation and Values Used in Simulations glacial rebound[Forte and Mitrovica, 1996] and the relative motionof hot spots[Steinberger and O'Connell,1998], and a Parameter Definitions Values/Units factor of about 20 increase through the transition zone is indicatedby independentevidence from subductionzone geoids Dimensionless Variables [Hager, 1984]. The viscositywas cappedat 2000r/0. The effect of mineralphase transitions on density,latent heat, and thermal E* activationenergy E/[R(Tp+ 273)] lateatheat ASm(T + 273)/%Tp expansivitywas neglected. The Rayleighnumber Ra (Table1) p' dynamicpressure p d2/(Ra•o r) was5.5 x 108. Nu Nusseltnumber Q d/(k A aTo) Ra Rayleighnumber p g aATo d3hlo • 3.2. Melting of Pyroliteand EelogRe T' temperature T/Tp T* absolutetemperature ratio (T + 273)/(Tp+ 273) We assumeda plumecomposition of eitherpyrolite or 15vol t' time t Ra tc/d2 ! % eclogite.This eclogitefraction (15 vol %) was chosenas a ui velocitycomponent (i = r,z) ui d/(Rar) reasonableestimate of the quantityof eclogitewithin the mantle x depletionfraction radial distance r/d (section2.2) and also to facilitatecomparison with workby depth z/d Corderyet al. [ 1997],whose choice of 15 wt % (14.4vol %) was rt' dynamicviscosity •7/r/0 basedon how much excesseclogite component the plumecould o temperaturefraction ( T - Ts)/( TL - Ts) carry. Dimensional Variables The implementationof meltingin the code is explainedby A horizontalsurface area m2 Leitch et aL [!998]. Basically,melting began when material specificheat 1000J kg -! K-1 crossedthe solidus(Figure 1). Melting absorbedlatent heat and depthof the mantle 3 x 106m produceda volumeof partialmelt, both at ratesproportional to diffusioncreep activation energy 3 x 105J mol -• the melt rate, DX/Dt: g accelerationofgravity 9.8rn s -2 h lithospherethickness 60, 90 km DX dX DO thermalconductivity 4 W m-1 øC-• Dt '= •dO •. Dt (6) heat flux W R universalgas constant 8.317J K-• tool -• X is thedepletion fraction, that is, themelt fraction of themelting tl timefrom start of simulation s, Myr component(either pyrolite or eclogite),and 0 is thenormalized t2 time from startof melting Myr rp backgroundpotential temperature 1300oc temperaturewithin the liquidus-solidusinterval (Table 1). For zo referencedepth 3.3 x 10s m eclogite,in the absenceof firm data,we assumedfor simplicity entropyof melting 400J kg -I K-• that X variedlinearly from 0 to 1 betweensolidus and liquidus referencetemperature difference Tpor 2•Tc•m CMB temperaturejump 200 - 520øC thermalexpansivity 2 x 10-5 K-• r/o referenceviscosity 5 x 10•øPa s o thermaldiffusivity 10-6 m2 s -1 referencedensity 4000kgm -3 50 Subscripts o reference value CMB coremantle boundary lOO m melting H Hawaiianplume L liquidus 150 solidus 200 oceaniclithosphere ages of 28.5 and64 Myr) as theplume head 250 approached. Initially, the bottomboundary had an error functionprofile 300 wherethe layer thickness hc•m varied linearly, usually from 150 km at the axisto 1!3 km at the far edge. This variationensured 350 that flow alongthe bottomwas inwardstoward the axisand that off-axisring boundary instabilities were avoided. Viscositywas both temperature- and depth-dependent. It had 4OO the form 1000 1200 1400 1600 1800 2000 T (øC) r/(T,z) = r/0exp [E * (1/T * -1)] H(z, B) exp[(lnG)(z - z0)], (5) Figure1. Liquidusand solidus curves used in themelting simu- where% is the referenceviscosity. H(z,B) is a blurredstep lations.Curves for pyrolite from McKenzie and Bickle [1988] functionequal to 1 in theupper mantle and B in thelower mantle. (thicksolid lines), extrapolated assuming a constant liquidus- Thereforethere was a stepincrease by a factorof B at 660 km solidusinterval (thin solid lines). Curves for eclogitefrom depthsuperimposed ona continuousincrease with depth by a fac- Yasudaet al. [1994](dashed lines). Depth scale assumes mantle tor of G. We used B = 20 and G = I 0. An increasein viscosity istopped by 7 kmof oceanic crust. The influence ofthe adiabat with depthof about2 ordersof magnitudeis predictedby post- was removed in the simulations. LEITCHAND DAVIES:MANTLE PLUMES AND FLOOD BASALTS 2051

(i.e.,dX/dO was constant). For pyrolite we used the empirical 3500 kg/m 3 andP0 = 3330kg/m 3, JT is 85øC.If thereis no relationship[McKenzie and Bickle, 1988] excesseclogite, only a differencein the scaleof heterogeneity, dX thenthere is no compositionalbuoyancy. Sinking rates of kilometer-scaleeclogite blobs within the plume are negligibly slow. dO = 2.068 - 8.1130 + 8.964 02 . (7) In eclogite-bearingplumes melting of pyrolitewas not consid- Weassumed that there was no reaction between eclogite melt and ered:melt ratesof pyrolitewere always orders of magnitudesless thesurrounding mantle matrix. The issueof melt-matrixreaction thanthose for eclogite.Time wasmeasured from the beginning is discussedin section 6.1. Becausekilometer-scale ec!ogite of thesimulations (q) andfrom the startof melting(t2). blobswould rapidly thermally equilibrate with their surroundings [Cordcryet el., 1997],the mantle material was treated as a con- 4. Plume Heads and Flood Basalts tinuumwhere the latent heat was that of eclogitemultiplied by thevolume fraction of eclogite.The totalintegrated melt was A summaryof resultsis givenin Table2 anddiscussed in sec- presumedto appear instantly on the surface. We neglect advec- tion4.2. We firstdescribe the rise and melting of theplume head tionof heat due to percolationof the melt and variationsin melt over a time period of tens of millions of years(section 4.1). In volumedue to reactionor freezing. In termsof melt volumes sections4.3 and4.4 we focuson the meltingin the first few mil- produced,these terms are of secondaryimportance, and more- lion years after the start of melting for plume compositionsof over,they depend on the method of melt extraction (e.g., through pyroliteand 15vol % eclogite. porousflow or a networkof fissures), and this has not been firmly established.Therefore we restrict ourselves to a simplerepresen- 4.1. Rising Plume tationof meltingand do notclaim an accuracyon absolutemelt Figure 2 is a time sequenceshowing a plume head rising volume of better than about 50%. throughthe mantle and impingingon the bottom of the litho- Theupward motion of theplume heads was driven solely by sphere. Figure 2f shows the backgroundviscosity structure thebuoyancy forces resulting from the temperatureanomaly of definedin (5). A plumehead forms from an instabilityin the bot- theplume. For simplicity,although these are not negligible, tom boundarylayer and risesup the cylinder axis (Figure 2a). buoyancyforces due to thepresence of eclogite,depleted residue, The plume headis elongatedin the vertiea!(Figure 2b) because or the presenceof melt were ignored.In the uppermantle at the backgroundviscosity decreases with height. At the viscosity least,eclogite is denserthan pyrolite. A crudeestimate of the step the plume head necksdown (Figure 2c), and the centre!, temperatureexcess 8T in the plumerelative to the surrounding hottestpart of theplume rises quickly through the lower viscosity mantlenecessary to counteractthis negative buoyancy is uppermantle and impinges on the bottomof the lithosphere(Fig- ure 2d), trappingvery little normalmantle above it. The outer = aœ.... ß (8) part of the plumehead follows through more slowly (Figure 2e) tx p0 andspreads out in a thin, flat layer. If weassume the surrounding upper mantle contains 10% eclog- The risetimethrough the uppermantle is very quick, and the itc,then for an excess eclogite component Afe of 0.05,given Pe = plumehead spreads out overa timescaleof about5 Myr. In Fig-

Table 2. Summaryof Resultsa

Run Conditions Results for Heads Results for Tails

Run ATcMB h tim t2raax ATto Zm Zave tarmax V2 ATto zm Zave mr/mrH

Pyrolite BBb 520 60 34.15 0.12 450 570-390 470 1.0 0.65 370 190-80 140 2.7 BEb 90 34.16 0.12 450 570-390 470 1.0 0.30 190-105 150 1.3 GG 400 60 57.5 1.2 335 160-1 I0 135 0.06 0.085 305 140- 80 120 1.0 GF 90 58.0 0.8 345 165-145 155 0.0008 0.0007 140-105 125 0.24 IB 300 60 96.1 2.7 250 100-125 110 0.006 0.008 235 115-80 100 0.!3

15%Eclogite BH 520 60 34.15 0.35 445 400- 80 260 18 20 370 365- 60 240 8 BG 90 34.16 0.4 445 510-170 300 11.5 13 365- 80 250 6 GH 400 60 57.25 0.45 345 340- 80 225 5.9 7.6 305 295-115 205 5.5 GI 90 57.26 0.5 345 340-170 245 2.7 4.0 295-130 215 4 IA 300 60 95.0 0.5 260 270- 90 190 2.3 2.9 235 260-105 185 <4.6 I 90 95.12 1-2 255 260-150 200 0.6 1.0 245-115 185 2.3 J 200 60 179.0 0.8 175 210- 90 165 0.83 0.8 115 200-105 160 1.0 JA 90 179.15 I 175 210-150 180 0.17 0.28

ah is lithospherethickness (kin); t •mis timeafter beginning of mn at whichmelting starts; t2max is timeof maximummelt rate, measuredfrom tim for 15%eclogite and h of 60 kin;Zm is melting depth range at truax(kin); ATto is temperatureanomaly at which meltingstarts; Zave is average melting depth attruax (kin); tarmax ismaximum melt rate (krn3yr -1); V2 is melt volume infirst 2 Myr (106kin3); mr/mr}• isratio of tail melt rate to that of Hawaiian plume (about 0.1 km3yr-1). bMeltingrates and depths forplume heads are not quantitatively reliable (see text, section 4.2). 2052 LEITCH AND DAVIES' MANTLE PLUMES AND FLOOD BASALTS

0 i 2 3

! log•o(Wi/qO) 1ooo 13•o 1700 T (øC)

Figure 2. Time sequenceshowing the temperaturefield of a plumehead rising through the mantle,assuming the depth-and temperature-dependentrheology given in equation(5). CaseGG. Times t• after startof simulationare (a) 45.8 Myr; (b) 56.8 Myr; (c) 57.4 Myr; (d) 57.7 Myr; (e) 62.8 Myr. Melting startsat t• = 57.5 Myr. Contour intervalis 70øC. Only partof the domainclosest to theaxis is shown.Alternate flames show tracer particles, origi- nally in horizontallines at I0, 100, 200, and 500 km from the CMB and 660 km from the surfhce. (f) Background viscositystructure and the original positionsof the lines of tracerparticles. ure 2e, the outer part of the plume head is still feedingthrough 4.2. Comparison of Models into the upper mantle but only very slowly: at this point the The resultsfor plumehead melting are given in theleft side of dynamicsis similar to that of the quasi-steadyplume tail. The Table 2. For both pyroliteand eclogitecompositions, the peak actualti•nes tbr the rise and spreadingof the plume headdepend meltrate rnrmax, and the total melt volume V 2 produced•n the first on the buoyancyof the head and the viscosityof the mantle,but 2 Myr after the startof meltingdecrease sharply as the plume the samequalitative behavior was observedin all models. temperatureATcM e decreasesor the lithospherethickness h Figure3 showsthe melting behaviorfor a plume similarto that increases.Because the soliduscurve for pyroliteis steeperthan in Figure 2 but with a lower initial buoyancy(see Figure 3 cap- tion) overa periodof 65 Myr, for plume compositionsof pyrolite (dashed line) and 15% eclogite, starting from when eclogite beginsto melt. The curvesboth rise from zero to an initial peak within about half a million years. This peak, which produceshigh melt rates for about3 Myr, coincideswith the arrival of the centralpart of the plumehead, which hasquickly risenthrough the uppermantle (Figure 2d). A second,broader peak, which in the case of the pyroliteplume is actuallyhigher than the first, is associatedwith the slow draining, over about 15 Myr, of the outer part of the 0'lr/ / -"! plume head into the upper mantle (Figure 2e). At still longer time, melting is due to quasi-steadystate upflow of the plumetail. 0.01 [• [ 15%eclogite • =.7 Plume heads arise from a boundary layer instability at the CMB, and their initial buoyancymay be quite variable because [ IIf pyrolite...... the boundarylayer may vary in thicknessand compositiondue to o.ool unsteadyflow in the deepmantle, which is affectedby the irregu- 0.1 I 10 lar subductionof oceanic plates. A plume tail arises from the drainingof the boundarylayer into an existingconduit: its buoy- t2 (Myr) ancyflux is determinedby its "catchmentarea" and the coreheat Figure3. Meltrate versus time. Plume compositions ofpyrolite flux, which may vary from place to place. Thusthe buoyancyof (dashedline) and 15 vol % eclogite.Time t 2 countedfrom the a plumehead is not necessarilyrelated to thebuoyancy flux in its startof meltingof theeclogite-bearing plume. Cases similar to tail, and so when lookingat the melt rate producedspecifically by GG andGH (ATcM•= 400øC,h = 60 kin), exceptwith thinner plumeheads, we restrictourselves to the meltingpeak observed initialbottom boundary layer thickness (80 km on axis to 60 kin in the first 4.5 Myr. onfar edge ) andlower resolution of 128x256cells (12 krn). LEITCHAND DAVIES:MANTLE PLUMES AND FLOODBASALTS 2053

• ' I ' [ ' I ' I" beforethe plume encountersthe lithosphere(see z•, in Table 2) ATcMB= 520øC ..... _] becausethe solidushas a shallowslope at depthand plumehead (x",. =400 øC ..... ,.---•] is very hot when it first rises into the upper mantle and cools quickly with time. Eclogitemay becomeexhausted along the 0.1 ,,,:--_...._= 300 øc ...... ,.;.,', plume axis. The shallowmelting for eclogitewith h = 60 km occursin the outerpart of the plume stem. 0.01 Melting ratesand volumesfor plumesof pyrolitecomposition only approachthose of flood basaltprovinces for caseBB with . AT,,, = 450øC and h = 60 km. Hot eclogite-bearingplumes, par- 0.001 ticularlyunder thin lithosphere(BH, BG, GH) generatemelt rates appropriateto oceanicplateaus, and only the coolestplume and the thickestlithosphere (JA) producelow melt rates. O.0001 . 0 I 2 3 4 4.3. Pyrolite Plumes t2 (Myr) Figure 4 showsthe total melt rate versustime for plumesof pyrolite composition. Except for the hottest case where the Figure4. Meltrate versus time for plumes with pyrolite compo- plume head intersectsthe solidusat depth (see above), the melt sition. For eachvalue of ATcMB,time countedfrom the startof rate is relativelyflat, and it is very sensitiveto both plume tem- meltingof eclogite-bearingplumes with h = 60 km. Thin lines indicateh = 60 km, and thick lines indicate h -- 90 kin. Circles peratureand lithosphericthickness, decreasing by roughly an indicatetimes of snapshotsin Figure5. order of magnitudefor each 100øC decreasein plume temperature, or as the lithospherethickness increases from 60 to 90 km. A clue to the low magnitudeand relatively uniform melt rate thatfor eclogite (Figure 1), melt rates change more rapidly with h with time can be found by examiningthe melt rate field in the forpyrolite plumes. plume head. Figure 5 showstemperature and melt rate fields for Asthe head rises and melts, the depth range of meltingZm and plume head GG at the two times indicatedby the circleson Fig- theaxial temperatureanomaly when the risingmaterial starts to ure 4. The region in which melting occursis very small, along meltATm vary greatlywith time: the valuesin Table2 applyat the axis at the top of the plume,and there is little differencein the thetime of peakmelt rate trax. Eclogite-bearingplumes usually positionor magnitudeof melting in the 3 Myr interval. Only the startto meltdeeper than pyrolite plumes. For the hottest plumes, hottestpart of the plume actually melts, and material in this pyrolitemelts deeper because the liquidus-soliduscurves cross at regionsoriginates from very close to the CMB [Farnetani and depth(Figure 1): However, these curves were determinedfor Richards, 1995]. The tracer particles that outline the melting depthsless than 250 km [McKenzieand Bickle,1988] and so the regionin Figure5 originatefrom 10 km abovethe CMB. Melting resultsare not quantitativelyreliable. For pyrolite,tmax occurs in these plume heads is virtually indistinguishablefrom the

200 100 0 0 300 600 km 0 100 ...... '

[ . ";.•.--...... ,,q,, , 200 ß .,?",5<."/: 300 ß [ ,::, (km)

300 ß i , ß ' 600 '0

ll 0 0.5 1.0 01000 1300 1600 dX/dt(Ma -•) T (øC) Figure5. Temperatureand melt rate fields for plume heads with pyrolite composition. Case GG, ATcMB = 400øC, h = 60 kin. (fight)Temperature in 600 km x 600km region at topof axis.(left) Closeups of meltrate and temperaturein a regionof depthand radius 285 km, indicated by the white squares on the fight. Scaleson frames show hor- izontaland vertical distances in km fromthe topof theaxis of thedomain. Tracer particles as in Figure2. (a) tl = 57.7 Myr, t2 = 0.5 Myr; (b) t• = 59.5, t2 = 2.3. 2054 LEITCH AND DAVIES: MANTLE PLUMES AND FLOOD BASALTS

encesbetween the melt fields in Figures7 and 5. Eclogitemelts over a much deeperand wider region. The melt rates dX/dt in Figure7 arean orderof magnitudehigher than those in Figure5, 10• •/c-,'"•;'---.. =300 øC ..... :: but dX/dt in Figure 7 is for the eclogitecomponent only (15% of themantle). The melt rates of thetotal mantle material (dX•,/dt and f•dXe/dt) are comparable.The mostsignificant differences are in the spatial extent of melting. Eclogite melts at much greaterdepths than pyrolite (about 130 km deeper,see Table 2), andalong the plumeaxis it finishesmelting, because the eclogite componenthas been exhausted,at aboutthe sametime as the pyrolitestarts to melt. This canbe understoodfrom examining Figure 1: the liquidusfor the eclogitelies belowthe solidusfor 0.1 the pyrolite.(Note thatthe latentheat of meltingof theeclogite reducesthe temperaturein the plume.) While a pyroliteplume 0 I 2 3 4 only meltsat the top of the plumeaxis, the melting region for a t2 (Myr) plumewith an eclogitecomponent is muchwider, although the highestmelt rates are still restricted to theaxis of theplumeß Figure 6. Melt rate versustime for plumeswith 15 vol % eclog- Figure7a alsoshows that there is a minordegree of meltingin itc component.For eachATCMB, time t2 countedfrom the startof themantle just abovethe plume head as the headheats the mantle melting of plume with h = 60 kin. Thin linesindicate h = 60 km, just aboveit and erodesthe bottomof the lithospherea littleso and thick lines indicate h = 90 kin. Circles indicate times of thatthe eclogitecomponent in the mantleat its "normal"potential snapshotsin Figure 7. temperature(1300øC) just startsto meltdue to decompression. Figure6 showsthat as ATcMt3 (and ATto) increases, the melting peakis morepronounced because at higherplume temperatures longer term "plume tail" melting, exceptthat as tail flow becomes moreof the plumehead outside the plumeaxis is hot enoughto establishedthere is a fasterflux up the axis and hencethe melt melt at sublithospheredepths. rate actually increaseswith time.

4.4. Eclogite-Bearing Plumes 5. Plume Tail Melting and Hawaiian Hot Spot Figure 6 showscurves of melt rate as a functionof time for Hot spotsare thoughtto arisefrom a long-term,quasi-steady plumescontaining 15 vol % cclogite. As for a pyroliteplume, the drainingof' the boundary layer at theCM B. To lookat plumetail melt rate is highly dependenton both h •tndATeMrs. Most impor- melting,we continuedour simulationstUr longertirne at lower tant, melt productionrates comparablewith flood basalts(0.5-18 resolution(256x128 elements, t•)r a cell size of about12 kin). km3yr"•) areachieved over reasonable timescales (1-3 Myr). Resultsare summarizedand compared with resultsfor Hawaiiin Figure7 showsa closeup of the plume headas it approaches Tables2 and 3. For Hawaii, we assurnethat the total heatflux the lithosphereßThe tYalneSare at similar time intervalst2 after Qt• = 2x I0TM W, themelt rate mr• = 0.1 km3yr-• [Davies, the startof melting •tsthose in Figure 5. There are strikingdiffer- 1988],and the lithosphere thickness h• = 105kin [Clagueand

200 1O0 0 0 300 600 km i ! ! 0

.... ß" 200

,..

,oo ...... 600 ,, ":- i i *'i'i 3000 100 :'.'::...... ß o . . .,.;::,::,.., .... 200 ß":"'; •"' ". '""•;" "•:::'::"' ' :'::•'"" ":" ':'::...... •'" ;'*':"' ":'';'..... 300

'% .:. •,•..• 3OO •'" •. ' ß - ' 4 • ,'•,,•:,.. • ...... '•'•" '•*'• 600

0 8 16 1000 1300 1600 dX/dt (Ma-•) T (øC) Figure7. Greyscaleplots of temperatureand melt rate in plumeheads with 15 vol % eclogitecomponent. Case GH,ATo4n = 400øC,h = 60km. Seecaption for Figure 5. (a)t• = 57.7Myr, t 2 = 0.5Myr; (b) t• = 59.6,12 = 2.3. LEITCHAND DAVIES: MANTLE PLUMES AND FLOOD BASALTS 2O55

Table3. Conditionsfor Tail Melting Runs a

ATCM• tm t2 QcM• /Qa QuM/Q.

520 34 20 1.1 1.3 5OO 400 57 26 0.7 1.0 300 95 35 0.5 0.7 200 179 52 0.3 0.4

1000 atm is time after start when plume head starts to melt (Myr); t2 istime a!tertm when tail melting is measured(Myr); Qc•m/Q}• is ratio of total basalfluxto 2 x 10TM W(flux for Hawaiian plume); QuM isflux into upper mantleat time tm+ t2. 1500

Dalrymple,1989]. The asymptoticsteady state in oursimulations is notparticularly realistic (it involvesring subducfionof the lithosphere)sowe measuredthe melt rate at timet2 whenthe 2000 plumehead had spread out to the edgesof thecomputational domain.Since the plume head spreads at a rateroughly propor- tionalto its buoyancy, t2 isinversely proportional toATcm •. In a steadystate the flux of heatQua{ up theplume tail is the sameas the flux of heatQcm intothe source boundary layer 250O fromthe core. At highRayleigh numbers the steadystate heat fluxfrom a horizontalboundary is, from dimensional arguments,

Nu = cRa ]/3 , (9) 3000 250 300 350 400 wherethe Nusselt number Nu is thenormalized heat flux (Table 1),and the constant c is about0.1 [Turner,1979]. Wecalculated A T (øC) Qcmby integratingthe conductiveflux (k dT/dz)over the bottom boundary and by integrating the advected heat Figure 8. Centerlinetemperature profile for plumetail, caseGK. (pcpv(T-Tp)) overthe plume tail as it passesinto the upper mantle.In oursimulations, Qcm approachesa steady value soon showstemperature and melt rate fields for plume tail melting. afterthe plume head reaches the bottom of thelithosphere. Using Eclogitemelts much deeperthan pyrolite, and finishesmelting appropriatevalues AT0 = 2ATc•mand r/0 = 102]Pa s in thedefini- (becausethe eclogiteis exhausted)before the pyrolitestarts to tionsof Nu andRa (seeTable 1), we obtainc = 0.079_+0.002.melt. In both cases,melting is concentratedin the centerof the Qcmvaries from 0.3 to 1.1times the flux of the Hawaiian plume plume tail, thoughit doesextend radially at the top. Melting andso within reasonable values for the Earth. There is a timelag occursat the top of the plumewhere the flow is divergingand betweenthe flux across the CMB and the flux in theupper part of upwardflow is slowing.The averagevertical flow rate is about themantle, so the tail heat flux QuM at thetime we measured the 850km Myr -• (85cm yr -•) inthe eclogite melt range and 120 km meltrate is somewhathigher than the base flux Qcm (Table3). Myr-] (12cm yr -•) in thepyrolite melt range. Thetemperatures ATto are comparable with estimates of hot spot The"catchment area" of ourmodel plumes is 7 x 106km 2, temperatures.These temperatures are significantly lower than the whichis about5% of the surfacearea of the CMB. Sleep[1990] CMBtemperatures and also lower than the plume head tempera- estimatesthat the Hawaiianhot spotcontributes about a tenthof tures(Table 2). Figure8 showsa plotof thecenterline tempera- the total flux from 40 hot spots. Assumingheat flux is propor- tureof a plumetail, whichdecreases steadily, due to horizontal tional to catchmentarea, the catchmentarea in the models is rea- diffusion,asit risesthrough the lower mantle, then drops more sonable. abruptlyasthe tail passes into the upper mantle and thins. In summary,we were able to producea reasonablematch to Table2 liststhe melt rates and the depth range of meltingfor many of the featuresof the Hawaiian hot spot, includingthe pyroliteand 15% eclogite plume compositions. Melt ratevaries buoyancyflux, temperature,lithosphere thickness, and CMB aboutan order of magnitudeon either side of theHawaiian melt catchmentarea. To match the melt rate as well requiresa few rate,with eclogite-bearing plumes having consistently higher val- percentof eclogitein theplume. ues.The closest match to theHawaiian plume for heat flux and lithospherethickness is withcases GF andGI. GF hasa meltrate only20% of the Hawaiian melt rate, whereas GI hasa meltrate a Table 4. ExtraTail MeltingRuns a factorof 4 greater. Seekinga closer match, we ran two further simulations with a Run Composition ATcM• h Zm Zave mr/mr}• lithospherethickness of 105km to match that underneath Hawaii. Weused plume compositions ofpyrolite (GK) and 3 vol% ec!og- GK pyrolite 400 105 140-106 130 0.12 itc(GN) (Table 4). Forthe pyrolite composition, themelt rate GN 3% eclogite 400 105 305-150 225 1.0 waslower than Hawaiianmelt rateseven if we assumeno sub- crustalmelting. For 3% eclogiteit wascomparable. Figure 9 aATCMB,h, 2m, .gave, mr/mr}•' see Table 2. 21•56 LEITCH AND DAVIES: MANTLE PLUMES AND FLOOD BASALTS

0 1000 2000 km i i i i

500 km , a,;""''"."'?•';;"';":; .>,....,-' I ./

1000 ' •' :....,::55._,_':3.'::=...... '"":' '

1000 13 O0 1600 w (oc)

0 100 200 km 0 100 200 km -•______! 1oo c 1oo

200 200 km km

300 300

0.0 dX/dt(Ma -•) 0.5 0 dX/dt(Ma -I) 10 Figure 9. Plume tail melting. (a) Temperaturefield (greyscale)and streamlinesin the uppermantle •br caseGK (ATc• = 400øC,h = 105kin). Contourinterval for streamlines 4 x 10-8 (dimensionlessunits). White square indi- cates the area shown in Figures 9b and 9c. (b) Melt rate (greyscale),contours of temperature(fine lines) and streamlines(bold dashed lines) for case GK (pyrolite).Streamline contour interval 2 x 10-8, halfthat in Figure9a. (c) As for Figure9b for caseGN (3 vol % eclogite). Distancesmarked on framesare measured from top of domain axis.

6. Discussion of themain eruptivephase of the ColumbiaRiver Basalts (CRB). Thusthe CRBs can be explainedby partialmelting of theeclogite 6.1. Reaction of Eclogite with Surrounding Peridotite componentof a plumesource at low temperaturesand pressures, There is no questionthat primary ½clogite-dcrivcdmelt would with minimal reaction with peridotite. Other flood basaltsare be om of equilibrium with peridotire. However,the degreeof moretypically less siliceous (olivine tholeiites), and Takahashi e! reaction and the ultimate amount and compositionof the melt aL [1998]argue that these flood basalts can be accountedfor by dependson how much of the original melt comes into contact eclogitemelting at highertemperatures and pressures, with some with peridotiteand on the conditionsunder which meltingoccurs. reactionwith peridotite.Kogiso et aL [ 1998]show that melting a Yaxleyand Green[1998] foundthat at 3.5 GPa (110 km depth) homogeneousmixture of basaltand peridotite yields low-silica eclogite-derivedmelts are very siliceous (dacitic) at low melt alkalibasalts that are typical of manyOIBs. Thusit is argued fractions,and these react with pyrolite to form a more "fertile" thatthe full rangeof OIBsand flood basalts can be explained by peridotire. At temperaturesat or below the pyrolite solidus,this meltingof heterogeneousplume heads and tails containing fertile peridotiteproduces a volume of low-silica(nepheline-nor- eclogiticbodies in a peridotitematrix. mative) picritic melt comparablewith the original volume of •e explanationof the full rangeof floodbasalt compositions eclogite. Picritic mantle melts may pond and fractionatenear the is likely to involvesome additional processes. Others have base of the crust [Lassiter et al., 1995; Farnetani et al., 1996], arguedthat ponding and fractionation (mentioned above) and but nephelinenormafive picrites would fractionateto produce crustalcontmnination, which has been inferred on thebasis of low-silica basalts. Yaxleyand Green [1998] argued that flood orthopyroxeneabundance and other chemical signatures [Irvine, basalts are more siliceous than this. 1970;Campbell, 1985], would account for thehigher silica con- On the other hand, Takahashiet aL [ 1998] found experimen- tent of many continentalflood basalts. tally thatmelting a basalticcomposition at 2 GPa (70 km depth) Thesepetrological questions remain to be fullyresolved, but by 30-50% (at a temperatureof 1300-1350øC)yields a basaltic theresults of Takahashiet al. [1998]indicate that it isplausible andesitethat matchesthe main major and trace elementfeatures thateclogite-derived melts are only moderately modified by reac- LEITCHAND DAVIES: MANTLE PLUMES AND FLOOD BASALTS 2057 tionwith peridotitc during their ascent to thesurface. We con- sic.) The diameterof thecylindrical features (about 200 karocor- eludethat our use of eclogite-derivedmeltvolumes, though responds well with both the centralregion of the plume in our approximate,isadequate for deciding whether aplume model can models(where most intensemelting takes place (see Figure7)) reproducetheobserved order of magnitude offlood basalts. and recentseismic observations of the plumetail underIceland [Allen et al., 1999]. 6.2. Melt Sourcesand Entrainment for PlumeHeads This concentrationof melting near the centralpart of the Fora startingplume, in contrastto a uniformtemperature plume is alsoin agreementwith observationsfrom giant radiating blob,the highest temperatures and therefore the most favourable dike swarms,which are associatedwith flood basalts[Ernst and positionfor melting occur along the plume axis. The models of Buchan,1997]. Flow patternsin the dikesshow magma upfiow Farnetaniand Richards [1995] showed that for a pyrolite starting through the crustin a centralarea with a radiusof a few hundred plume,all melting material originates from a sourceregion within kilometersand then lateraltransport of magmafor distancesof 100km of the CMB. Our high-resolutionsimulations show that morethan a thousandkilometers [Ernst andBaragar, 1992]. thesource region may be evenmore restricted. The tracer parti- clessurrounding the pyrolite melting region in Figure5 origi- 6.4. Melt Volumes From Plume Heads and Tails natedfrom within 10 km of theCMB. Forec!ogite melting (Fig- In this studywe lookedat melting of plumeheads and tails ure7) mostmaterial originated from within 10 km of theCMB underthe lithosphere.We found that for a pyrolitecomposition andalmost all from within 100 kin. melting rateswere very low unlessthe plume temperaturewas Uponpassing through a significantstep in viscositybetween very high and the lithospherethin (Table 2). Moreover,for a thelower and upper mantle, the outerpart of theplume head is pyrolitecomposition, there was very little differencebetween the folded,as canbe seenfrom the foldingof the linesof tracersin melt rate associatedwith the arrival of the plume headand that Figures2e, 5, and7, sothat material from different depths in the associatedwith the longer-termplume tail melting. This was mantleis mingledon a mediumscale. If mostmelting occurs as becauseonly the hottestplume material, situated along the axisof theplume head first approaches the lithosphere,then these mod- the plume,was able to melt. The large, warm plumehead spread elsindicate that flood basaltsarise from the centralpart of the withoutmelting. plume,which has its sourcevery closeto the CMB as described For a plume containing15 vol % eclogite,in contrast,there above.However, if meltingof the plumehead occurs some time was a large peak in the melt rate as the plume head arrivedat afteremplacement as a resultof rifting[e.g., Leitch et al., 1998], shallowdepth because the outerpart of the plumehead was warm thenthe meltingmaterial may haveits soumefrom a rangeof enoughfor the ec!ogitecomponent to melt (Figure7). The melt depthsin themantle. Thesemodels have not yet spannedthe full ratesof a few million cubickilometers per million years,and the rangeof plausibleplume temperaturesand headsizes, so more peakwidth of a few millionyears (Table 2), matchobservations extensivemelting of plumeheads remains a possibility. of floodbasalt provinces. The melt rates of plumemils containing15 vol % eclogite 6.3. Melting Column werea few timesless than the peak melt ratesbut still higherthan Forpyrolite compositions, melting occured only at thetop of observedin hotspot tracks. The observedmelt rate of theHawai- theplume axis (Figure 5), whereasan eclogitecomponent may ian plumewas matched by a plumetail containing3 vol % eclog- startto meltat nearly300 km depthand have completely melted itc. It is conceivablebut unlikelythat plumetails actually contain somedistance below the lithosphere(Figure 7). Sincepyrolite a lowerproportion of eclogitethan plume heads, since they arise startsto meltroughly when eclogite has melted completely (Fig- from the sameregion of the mantle. On the otherhand, the cir- ure1), the melting column may be 200 km in extent. cumstancesin which magmaascends and eruptsare substantially Thelong, narrow melting column of aneclogite-bearing plume differentfor headsthan for mils. The degreeof reactionwith sur- wouldhave an importantimplication if the melt becamesuffi- roundingmantle might well be systematicallydifferent, as we cientlyinterconnected and concentratedthat viscousresistance discussin section6.5. In addition,our axisymmetricplume tails fromadjacent solid matrix is small.Such an interconnected melt- are vertical,but real plumetails will be tilted fromthe verticalby ingcolumn of height H exertsan overpressure AP= ApgHat the theplate-scale flow in themantle, and this will causesome ther- topof thecolumn. For Ap = 300 kg/m3 thisgives AP/H = mal entrainmentinto the tail [Richardsand Griffiths,1988, 1989; 3x 103PaYm or 30 bar/km. Kelemen et al. [1997]suggest that Griffithsand Richards, 1989]. The tail temperaturewould thus be dikegeneration beneath mid-ocean ridges might occur for over- coolerand the melting substantially reduced by thiseffect. pressuresof about500 bars. Once this pressureis exceeded, 6.5. Differences Between Heads, Tails and MORBs dikeswould form and quickly drain the melt. Aninterconnected melting column of 150km would generate When consideringeclogite-periodite reaction there are two anoverpressure of 0.45 GPa(4.5 kbar). Althoughthere is consid- "end-member"extremes. In one extreme,fine-scale stringers of erableuncertainty about the strength of thelithosphere, it cer- eclogitemight partially melt and the meltspropogate slowly by tainlycould not sustain such an enormous overpressure, andwe porousflow, maximizing contact and reaction with the matrix. If wouldexpect the lithosphere at the top of sucha meltingcolumn the neighboringmatrix was residualharzburgite from the slab, torupture as the plume head first approached. This process may reactionmay evengenerate a pyrolite composition.In another accountfor the narrow,cylindrical low seismicvelocity features extreme,large bodies of eclogiticmelt might propagaterapidly seenin thelithosphere under the Deccan [Kennett and Widiyan- upwardthrough existing dike pathways [Maaloe, 1998] with little toro,1999] and the Parana [VanDecar et al., 1995]flood basalt alteration. provinces.These features have been interpreted to be warm Whileboth extremes are unlikely, we mightexpect the MORB and/orcompositionally distinct remnants of the plume responsi- source to be closestto the first scenario,plume heads closest to blefor the flood basalts. (In the case of theParana, the thermal the second,and plume tails to be in between.This mighthelp signaturewould have dissipated since plume arrival inthe Juras- explain the differentquantities of nonreactingeclogite that our 2058 LEITCH AND DAVIES: MANTLE PLUMES AND FLOOD BASALTS numericalmodels require to matchobserved melt ratesin MORB Campbell,I.H., M.J.Cordcry, and G.F. Davies, The relationship between C0%), plumetails (~3%), and plume heads(~15%) without the mantleplumes and continental flood basalts, in Proceedingsof the InternationalField Conferenceand Symposium on Petrology and Met- needfor the total eclogitefraction to be different.We arguedin allogenyof Volcanicand Intrusive Rocks of the Midcontinent RiftSys- section6.4 that real plumetails may be coolerthan our axisym- tem,pp. 23-24, Contin.Educ. and Ext., Univ. of Minn., Duluth,1995. metric models. Lower overall melting ratesmay also lead to a Christensen,U.R., andA.W. Hofmann,Segregation of subductedoceanic crustin theconvecting mantle, J. Geophys.Res., 99, 19,867-19,884, significantdifference in theupward propogration of themelt. 1994. Providedthat eclogitein plumesis not mixed on a fine scale, Clague,D.A., and G.B. Dalrymple, Tectonics, geochronology andorigin the highermelting rates (of eclogite)in plumeheads might lead of the Hawaiian-Emperorvolcanic chain, in The Geologyof North to more rapid propagationof the melt, to greaterpropagation America,vol. N, TheEastern Pacific Ocean and Hawaii, edited by throughdikes, and thusto lessreaction with peridotitc,particu- E.L. Winterer,D.M. Hussong,and R.W. Decker,pp. 188-217,Geol. Soc. of Am., Boulder,Colo., 1989. larly if the eclogitestringers are alignedwith the plumeaxis by Coffin,M.F., and O. Eldholm,Large igneous provinces: crustal structure, flow and particularlyif the melt can punchthrough the litho- dimensions,and externalconsequences, Rev. Geophys.,32, 1-36, spherein onelocation (section 6.3). 1994. There are three factors which would tend to mute the effect of Cordcry,M.C., G.F. Davies,and I.H. Campbell,Genesis of floodbasalts eclogitein the sourceof MORBsrelative to plumes[Hofmann, from eclogite-bearingmantle plumes, J. GeophysRes, 102, 20,179-20,198, 1997. 1997]. First,the MORB source,the upper part of theupper man- Davies,G.F., Oceanbathymetry and mantleconvection, 1, Large-scale tle, is arguablymore intimatelymixed thanthe CMB, where flow andhotspots, J. Geophys.Res., 93, 10,467-10,480,1988. plumesoriginate [Davies, 1990] (although the scale of mixingin Davies,G.F., Mantleplumes, mantle stirring and hot-spotchemistry, plumesnear the surface has yet to be determined).Second, the Earth Planet. Sci. Lett., 99, 96-109, 1990. Davies,G.F., Thermomechanica!erosion of the lithosphereby mantle resultsof Yaxleyand Green[!998] suggestthat eclogitewould plumes,J. 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