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Sleep, N.H., Hotspots and Mantle Plumes: Some Phenomenology, J

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Sleep, N.H., Hotspots and Mantle Plumes: Some Phenomenology, J JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. B5, PAGES 6715-6736, MAY 10, 1990 Hotspotsand Mantle Plumes'Some Phenomenology NORMAN H. SLEEP Departmentsof Geologyand Geophysics,Stanford University, Stanford, California The availabledata, mainly topography, geoid, and heat flow, describinghotspots worldwide are examined to constrainthe mechanismsfor swelluplift and to obtainfluxes and excess temperatures of mantleplumes. Swelluplift is causedmainly by excesstemperatures that move with the lithosphereplate and to a lesser extenthot asthenospherenear the hotspot.The volume,heat, and buoyancy fluxes of hotspotsare computed fromthe cross-sectionalareas of swells,the shapes of nosesof swells,and, for on ridgehotspots, the amount of ascendingmaterial needed to supplythe lengthof ridgeaxis whichhas abnormallyhigh elevationand thickcrust. The buoyancy fluxes range over a factorof 20 withHawaii, 8.7 Mg s -1, thelargest. The buoy- ancyflux for Iceland is 1.4Mg s -1 whichis similarto theflux of CapeVerde. The excess temperature of both on-ridgeand off-ridgehotspots is aroundthe 200øCvalue inferred from petrologybut is not tightly constrainedby geophysicalconsiderations. This observation,the similarityof the fluxesof on-ridgeand off- ridgeplumes, and the tendency for hotspotsto crossthe ridge indicate that similar plumes are likely to cause both typesof hotspots.The buoyancyfluxes of 37 hotspotsare estimated;the globalbuoyancy flux is 50 Mgs -1, whichis equivalentto a globallyaveraged surface heat flow of 4 mWm-2 fromcore sources and wouldcool the core at a rateof 50ø C b.y. -1. Basedon a thermalmodel and the assumption that the likeli- hoodof subductionis independentof age,most of the heatfrom hotspotsis implacedin the lower litho- sphereand later subducted. I.NTRODUCWION ridge plumesusing Iceland as an example. The geometryof flow implied by the assumed existence of a low viscosity Linearseamount chains, such as the Hawaiian Islands, are asthenosphericchannelis illustrated bythis exercise. Then the frequentlyattributed tomantle plumes which ascend from deep methods forobtaining theflux of plumes ona rapidlymoving inthe Earth, perhaps thecore-mantle boundary. Theexcessive plate are discussed withHawaii as an example. These methods volcanismof on-ridge hotspots, such as Iceland, is also often involve determining theflux from the plume from the cross- attributedto plumes. If on-ridgeand midplate hotspots are sectionalareaof the swell and taking advantage ofthe kinemat- reallymanifestations of the same phenomenon, onewould ics of theinteraction of asthenospheric flowaway from the expectthat the temperature andflux of the upwelling material plume and asthenospheric flowinduced by thedrag of the wouldbe similar under both features. In particular, thecore- lithospheric plate. The methods forextending thisapproach to mantleboundary isexpected tobe nearly isothermal sothat the hotspotsonslowly moving plates are then discussed which Cape temperatureofplumes ascending fromthe basal boundary layer Verde as an example. Anestimate ofthe global mass and heat shouldbethe same globally provided that cooling byentrain- transfer byplumes isthen obtained byapplying themethods to mentof nearbymaterial and thermal conduction areminor. 34 additional hotspots. The magnitude ofthis total estimated Finally,theglobal heat loss from plumes should imply areason- flux is compatible withthe heat flux expected fromcooling the ablecooling rate of the core [Davies, 1988a]. corein agreement withDavies [1988a]. Finally, the results are Themost important point of this paper isthat the properties used to obtain general properties ofplumes and inferences on ofmantle plumes areconstrained byobservations, mainlytopog- the underlying mantle dynamics. "• raphy, geoid, and heat flow. I follow Davies [1988a], Richards eta!. [1988],and Sleep [1987a, b] in assumingthat the ICELANDICPLUM•FLUX geometryof plumesconsists of narrowvertical pipes which sup- plieshot mantle to a thinlow-viscosity asthenosphere beneath TheIcelandic hotspot is directlyon theMid-Atlantic Ridge. the movingplate (Figure 1). Suchplumes provide a muchMethods developed for off-ridge hotspots are thus inappropriate. betterexplanation for geoidand topographic anomalies than Theexcess volcanism of theridge near the hotspot is probably doessecondary convection from the cooling of theoceanic plate the best way to constrainthe excess temperature andvolume of Davies[1988a, b]. Thisgeometry allows simplification of the materialsupplied by theplume. discussionand the physicsbecause the model plume is much narrowerthan the hotspot swell and the rapidly flowing layer of ExcessCrustal Thickness andTemperature asthenosphereis thin compared to the thickness of the mantle of Icelandic Plume and the width of the swell. The expenseis somemodel depen- dencyof the resultsprimarily because the existence of a thin Highertemperatures beneath hotspot ridges lead to morepar- low-viscositychannel has not been established. tial meltingand explain the 20-km-thickcrust beneath Iceland I beginby developingmethods for estimating the flux of on- [McKenzie,1984]. An excesstemperature of the upwelling materialbetween 200 ø C and250 ø C is neededto generatethe.... excess15 km of crustin the computationsof McKenzie[1984]. This averageexcess temperature is also compatiblewith the Copyfight1990 by the American Geophysical Union. depthand volume of extensivemelting beneath Hawaii [McKen- Papernumber 89IB03587. zie,1984] and with the maximum excess temperature of 300øC 0148-0227/90/89JB-03587505.00 of Wyllie [1988]. 6715 6716 SL•,: Ho?s•?s • M•rrL• PLU•_• Ridge axis Lithosphere t \ 4 ! \ Lithosphere Asthenosphere Asthenosphere Plume material Fig. 2. The geometryof platemotion away from the Icelandhotspot is shownschematically. At large distancesaway from the hotspot,the flux of the plumeis balancedby the flow in the lithosphereat the platevelo- city Vt. and the flow in the entrainedasthenosphere. For convenience, both the lithosphereand the asthenospherethickness are assumedto be 100 km in this paper. velocity at the top to zero at the bottom. The volume flux is then Qp = Vs(L + A/2) Y (1) where V$ is the full spreadingrate, L and A are the thicknessesof the lithosphereand the asthenosphereaway from the ridge, here takento be 100 km, and Y is the along-strike Fig. 1. Mantleplumes are envisionedto tap a basalchannel near the distancesupplied by the plume. core-mantleboundary and to dischargeinto the low-viscosityastheno- Axial topographyappears to correlateon a globalbasis with sphere. The lithospheremoves over the plumecreating a hotspottrack. The radialflow in the asthenosphereaway from the plumecarries heat to crustal thicknessand magma chemistryvariations attributed to the flanks of the swell. higher temperaturesin the ascendingsource material [Klein and Langmuir, 1987; 1989; cf. Brodholtand Batiza, 1989; Viereck et al., 1989]. The distancesupplied by the plumeY is therefore obtainedfrom topography.An along-axisdistance of 500 km of Furtherquantification of theseestimates would give more Icelandis suppliedmainly by theplume, as it is eithersubaerial informationon the temperatureof the upwellingmaterial. In or shallowshelf. Severalhundred additional kilometers where addition,the geochemical anomaly provides information on the elevationand crustal thickness decrease away from Iceland are motionof theplume material [Schilling, 1986]. Unfommately,parfly suppliedby the plume. The geochemicalanomaly the methodof generationof mid-oceanicridge basalts is not extendsover about 700 km [Schilling,1986]. I obtaina fluxof agreedupon. For example, Stolper [1980] and Elthon et al. 63 m3 s -1 byassuming analong-strike length of 800 km and a [1982]state that mid-ocean ridge basalt (MORB) forms from a full spreadingof 16.5 mm yr -• [Gordonand Jurdy, 1986]. picriticmelt generated around 70 km depth.In contrast,The volume flux of magma Qv is 8 m3s-1. Presnalland Hoover [1984] state that extensive partial melting It is usefulto definea buoyancyflux for latercomparison beginsat 30 km depth. The formerhypothesis implies that withoff-ridge hotspots miningextensiveexcess melttemperatureremains inthefrom manfie crustal andthickness. thus precludesForms ofdeter- the B = p,,,otAT Q/, (2) latter hypothesisare thus used by McKenzie [1984]. For- whereAT is the averageexcess temperature, Pm is the density tunarely,the various melting relationships used by McKenzieof themanfie, about 3300 kg m -3, and 0t is the thermal expan- ß 5o 1 [1984] and a simplifiedrelationship by Sleepand Windley s•oncoefficient, about 3 x 10- C-. Thebuoyancy flux of the [1982]give similar excess temperatures. I thus make no Icelandplume is thusabout 1.4 Mg s -1. Asshown below this attemptto improveestimates of meltingrelationships. An esti- flux is similarto the flux of CapeVerde and much less than the mate of 225øC for the averageexcess temperature of the Ice- flux of Hawaii. landplume is usedbelow. The Jan Mayen hotspotis supposedto exist alongthe ridge north of Iceland [e.g., Vink, 1984]. However, it has litfie topo- Flux of IcelandicPlume graphicexpression along the ridge axis. It is possiblyrelated to southwardpropagation of the Moins ridge axis [Saemundsson, The volume flux of the Icelandic plume may be computed 1986]. The flux of this plume if it exists is includedhere in from the kinematicsof spreading. The plume needs to supply that of the Icelandhotspot. the oceaniclithosphere at least down to the depth of extensive melting, about 80 km, and probably the underlying astheno- HAWAUAN I•UM•
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