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

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MANTLE PLUMES and FLOOD BASALTS Scribedblob of Uniformtemperature, Rather Than Resultingfrom Startsat the Paranaflood Basalt Province,South America View metadata, citation and similar papers at core.ac.uk brought to you by CORE 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 litho- sphere,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 plumescould 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 the deepest 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
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