Introduction to special section: Hawaii Scientific Drilling Project - Core

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. B5, PAGES 11,593-11,598, MAY 10, 1996

Introduction to special section: Hawaii Scientific Drilling Project EdwardM. Stolper Divisionof GeologicalandPlanetarySciences,CaliforniaInstituteof Technology,Pasadena

Donald

J. DePaolo

Centerfor IsotopeGeochemistry, Departmentof GeologyandGeophysics, Universityof California,Berkeley

Donald

M.

Thomas

Hawaii Instituteof Geophysics andPlanetology,Schoolof OceanandEarthScienceandTechnology University of Hawaii, Honolulu

Introduction

Intraplateor "hot spot" volcanic island chains, exemplified by Hawaii, play an importantrole in plate tectonic theory as referencepoints for absoluteplate motions,but the origin of these volcanoesis not explainedby the plate tectonic paradigm [Engebretson et al., 1985; Molnar and Stock, 1987; Morgan, 1971, 1981, 1983; Wilson, 1963]. The most widely held view is that these chains of volcanoes form from magma generatedby decompressionmelting of localized, buoyant upwellings in the mantle [Ribe and Christensen, 1994; Richards et al., 1988; Sleep, 1990; Watson and McKenzie, 1991]. These upwellings, or "plumes,"are believed to originate at boundary layers in the mantle (e.g., at the core-mantle boundaryor near the boundaryat-670 km between the upper and lower mantle), and the causeof the buoyancy may be both compositional and thermal [Campbell and Griffiths, 1990; Griffiths, 1986; Richards et al., 1988; Watson and McKenzie, 1991]. Mantle plumes are responsible for about 10% of the Earth's heat loss and constitute an important mechanism for cycling massfromthe deep mantle to the Earth's surface. Studies of the chemical and isotopic compositions of lavas fromintraplate volcanoes,especially oceanisland volcanoes, have contributed significantly to our knowledge of magma genesisin the mantle [Carmichael et al., 1974; Macdonald et al., 1983] and the compositional heterogeneity of the mantle [All•gre et al., 1983; Hart, 1988; Hart et al., 1986; Kurz et al., 1983]. Of particularimportanceis the identificationof distinct compositionalend membersin the mantle, the origin and distribution of which provide insight into the long-term differentiationof the mantle-crustsystem,the recyclingof oceanic crust and continentalsedimentinto the mantle, and the history of the lithosphere [All•gre et al., 1995; Farley et al., 1992; Hart, 1988; Hofmann and White, 1982; McKenzie and O'Nions, 1983; Weaver, 1991; Zindler and Hart, 1986]. A fundamentallimitation in the study of hot spot volcanoes is that the major volume of each volcano is inaccessibleto samplingand is consequentlyunknown. Erosion typically exposesonly hundredsof metersof an oceanicvolcano's inte-

Copyright1996 by the AmericanGeophysical Union. Paper number96JB00332. 0148-0227/96/96JB-00332509.00

rior, out of a total thicknessof 6 to 20 km, becauserapid subsidenceafter extinction preemptserosional downcutting. For example,the Hawaiian-Emperor chain has been active for at least 70 Myr, but all we can generally examinefor any individual Hawaiian volcano is that small fraction of its history (typically, the final 5-10%) that is exposedsubaerially. Thus, althougheach Hawaiian volcanoacts as a probe, sampling the plume outputas the Pacific plate carriesthe volcano over the hot spot and recordingthis output in stratigraphicsuccession in its lavas, the long-term evolution of any individual volcano during its -1 Myr passageacrossthe plume is almost entirely inaccessible. Continuouscore drilling through a sequenceof lavason the flank of an oceanicvolcano is probably the only way to obtain a stratigraphicsequencerepresentingthe complete traverseof the plume. If an extendedpart of such a successionof lava flows could be sampledby drilling and then analyzed, it would provide critical information on mantle plume structureand origin. In recognition of the essentially unique researchopportunities affordedby drilling throughthe flank of an oceanicvolcano, the Hawaii ScientificDrilling Project (HSDP) was conceivedin the mid-1980s to core drill continuously to a depth of several kilometers

in the flank of the Mauna

Kea volcano

[DePaolo et al., 1991]. Hawaii was a natural target since as the best studied volcanic construct on Earth, it is the arche-

type of oceanislandvolcanismand providesthe best possible scientificframeworkfor a major projectof this sort. This special section of the Journal of Geophysical Research reports the results of initial scientific characterization of the 1056-m-deepcorehole drilled in 1993 as the first phase of the HSDP. This goal of this introductionis to describethe project and to provide context for the diverse set of scientific studiesreported in the following papers.

Hawaii Scientific Drilling

Project

Core drilling of a "pilot hole" (namedKahi Puka 1, or KP1), funded by the National Science Foundation Continental DynamicsProgram, took place in Hilo, Hawaii, fromOctober to December1993 and was done by Tonto Drilling Services, Inc. (Salt Lake City, Utah). The following 18 monthswere devoted to petrological, geochemical,geomagnetic,and volcanological characterization of the recovered core, downhole logging, and the fluid samplingprogram. The primary scien-

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STOLPER ET AL.: INTRODUCTION

tific objectives of the pilot hole stageof the project were essentiallythoseof the larger project, to characterizethe petrology, geochemistry, and rock magnetismof a nearly continuous sequenceof Hawaiian lavas erupted over a long time period. Other objectivesof the pilot projectwere demonstration of the technicalfeasibility of the drilling programand of the ability of the diverse group of scientists involved in the project (covering many disciplines, institutions, and countries) to work togethereffectively. The HSDP pilot hole drill site was near Hilo Bay on the surfaceof the -1400-year-old Panaewa flow series from the Mauna Loa volcano (Figure 1). This particular site was chosen because(1) it is far frommagmaticriff zones and fromthe summitsof Mauna Loa and Mauna Kea, therebyminimizingthe chancesof encountering intrusives, alteration, and high temperaturefluids during drilling; (2) its proximity to the coastline maximizedthe probabilities of encounteringsubmarine flow unitsand relativelyold lavas in a shallow hole; and (3) drilling began in recent lavas erupted fromthe Mauna Loa volcanobut at depthpenetratedunderlyinglavaseruptedfrom Mauna Kea, which allowed us to test our ability to distinguish between lavas from different volcanoes. Other factors leading to this site were relatedto permitting,including appropriatezoningand land use designationas well as an ability to screenthe drilling activities from a nearby residential community. The drilling lasted46 days; the total depth achievedwas 1056m with an averagepenetrationrate>20 m/d. The average penetrationrate duringperiodsof drilling (excludingtime for logging,waiting on cement,rig repairs,etc.) was -30 m/d. By comparison,averagepenetrationratesfrom previouscore drilling on the Kilauea East Riff Zone rangedfrom lessthan 10 m/d to about22 m/d for coring time only (H. Olson, unpublished data, 1991). Corerecoveryfor the entire drill hole averaged about 90%, with the major loss zones being in unconsolidatedsediments,which were not effectivelycapturedby the corebarrel,andin zonesof rubble,whichjammedthe corebarrel.

ples usedin the geomagneticinvestigations and samplesother thanthe referencesuitethat were analyzedgeochemically.

Description of the Stratigraphic Section The core logging led to the designation of 227 units (numberedin stratigraphic succession, from the top to the bottom of the core), out of which 208 are lava flows;the remainder areashbeds,marineandbeachsediments, and soils. A generalized, compositelithologic sectionis shown in Plate 1. A more detailed

column

is

available

on

the

Internet

at

http://expet.gps. caltech.edu/Hawaii_project.html. Units 1-43 are interpretedto be part of the Mauna Loa volcano; units 44-227 have been assigned to the underlying Mauna Kea volcano.

The contact between these volcanoes at a

depthof 279.5 m is unambiguousand sharp. Abrupt changes are observedin trace element ratios [Hofmann and Jochum, this issue;Rhodes, this issue]and He, O, Sr, Pb, and Nd isotopic ratios [Eiter et at., this issue; Hauri et at., this issue; Kurz et at., this issue; Lassiter et at., this issue]. Other observationsstrongly support this identification of the boundary betweenlavas fromthe two volcanoes:(1) the projection of the exposedslope of Mauna Kea under the drilling site is preciselyat this depth;(2) lava flows from shallowerthan 280 m are interlayered with near-shoresedimentsand are systematicallythickerthanthosefrom deeperlevels (Plate 1) (G.P.L. Walker, written communication, 1995), consistent with expecteddifferencesbetweenMauna Loa lavaseruptedas gently sloping lava deltas extending into Hilo Bay and Mauna Kea lavaseruptedon steep slopes well above sea level [DePaoto and Stotper,this issue;Lipman and Moore, this issue;G.P.L. Walker, written communication,1995]; (3) a significant soil horizonis presentat the geochemicallydefinedboundary;(4) the intercalation

of alkalic

and tholeiitic

lavas in the 50 m be-

low the geochemically defined disconformity is consistent with the end of shield building of Mauna Kea [Rhodes,this issue]; and (5) the major elementcompositionsof the tholeiites aboveand below the chosencontact,although not defini-

One of the key goals while the drilling was progressing tive, are consistentwith known differences between lavas from was to processeachsegmentof the corewithin 24-48 hoursaf- Mauna Loa and Mauna Kea [Rhodes,this issue]. ter recovery. A successfulprocedurewas developedthat built Weathered ash deposits are readily recognized as soils on experiencefromthe Cajon Pass and Creededrilling proj- throughout the core. Near-shore sedimentsare interspersed ects. The detailed corehandling and logging proceduresare with lava flows in the Mauna Loa part of the section,indicatpresentedalongwith the corelogsand photographs of the en- ing that the drill site was close to the shore during this time tire core in a separatevolume [Hawaii ScientificDritting Properiod and thereforethat aggradation due to lava flows and ject, 1994]; the logs and photographs are alsoavailableon the subsidence were roughlyin balance. A succession(-25 m)of Internet (at http://expet.gps.caltech.edu/Hawaii_project.html).carbonatesedimentsrich in coral fragmentswas encountered The corewas dividedinto a workingsplit (currentlystoredat immediatelybelow the Panaewalava flow at the surface.These the CaliforniaInstituteof Technology)from which all samples sedimentsare interpretedas Hilo Bay lagoonal depositsthat have so farbeentaken and an archival split (curatedby the recordthe rise of sealevel since 10 kyr ago. Other sediments include volcaniclastic sediments, beach and dune sands, U.S. Geological SurveyCore ResearchCenterin Denver). A setof referencesamplesfor geochemical analysiswas collected hyaloclastites, and a "bog" deposit rich in organic carbon. fromthe core(onlyfrominteriorsectionsof the core,avoiding Only one sand,interpretedas a wind-blown sediment,is presthe parts of the core that were in contact with the bit or the ent in the Mauna Kea part of the section(at a depthof 867 m). corebarrel)in conjunctionwith the logging program.These No intrusive units have been identified. Nearly all of the were prepared(crushedand cleaned)accordingto a common sampledlavas are subaerial(as opposedto submarine). That

procedureat the Massachusetts Institute of Technologyand

subaerial lavas are found more than a kilometer

the University of Massachusetts,Amherst, and then distrib-

sealevel is not surprising becauseHawaii is known to be subsiding at a rate of 2.0-2.5 mm/yr [Moore, 1987; Moore et at., thisissue]. The minimumage of the lavasat the baseof the core is thusestimatedto be •400 kyr, sincethis is how long it would take lava eruptedat sealevel to subsideto this depthat a rateof 2.5 mm/yr.

uted to the various investigatorsfor analysis. Thin sections were preparedfromsamplesadjacentto the parts of the core fromwhich the powdersfor geochemical analysiswere prepared. In addition to the reference suite,manyothersamples were later takenfrom the workingsplit, including all the sam-

below current

STOLPER ET AL.' INTRODUCTION

155005 '

155000 '

11,595

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19 ø

45'

.. iDrill Hole!i.".

::!!:::!i:::!!: Panaewa flow

ß

....

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40'

-"

NORTH

I

I 5km

Figure1. Location maps fortheHawaiiScientific Drilling Project. (top)TheHawaiian Islands (inset) andthe island of Hawaii, showingthe "Loa"and "Kea"trends,the volcanoeson the island of Hawaii (including Loihi), and the locationof the HSDP pilot hole at Hilo. (bottom)Detailedgeologicmapof the vicinity of Hilo andthe locationof the pilot hole. Volcanonamesindicatethe sourceof the surfaceflows. After Lipman and Moore [this issue].

11,596


7OO

8OO

un

v

auna

g00

Ke

mmm

mm

mm

mm

emro

.emro

1000

1060

5•

Plate1. Simplified lithologic column of theHSDPpilotcore.BlueandgreenindicateMaunaLoa andMaunaKeabasalticlava flows,respectively. Notethegreater thickness of flowsin MaunaLoapartof thesectionrelativeto theMaunaKeapart. The intensityof the shadingis proportional to the phenocryst content.Dashedlines are internalflow units. Orangeindicates sedimentary units. All butoneof thesedimentary unitsin theMaunaKeapartof thesection arethinsoilsor ashbeds.Sediments in the MaunaLoa partof the sectionalso includesoils and ashbedsbut areprimarilyclasticcarbonates, beachsands,or hyaloclastites. Depthsat whichradiometric agesareavailableare indicatedby a solidcircleadjacentto the section[Beesonet al., thisissue;Mooreet al., thisissue;Sharpet al., thisissue].Depthsof excursions in the geomagnetic field [Holt et al., this issue]are indicatedby a circledcrossadjacentto the section.


11,597

In choosingthe Hilo area for the site, one factor was the expectation that there would be minimal interaction of the section with hydrothermal solutions, based on distance fromrift

tionalvariationsof mantleplumes;(3) the growth and subsidenceratesof volcanoesduringshieldbuildingandtheir relations to magmacompositionandisotopicratios,to the total

zonesandpreviousstudiesof well waters. The overallimpression of the recoveredsamplesis of remarkablyfreshlava, althoughthere is somealterationassociated with weatheringor with eruptive conditions (e.g., thin iddingsite rims on oliv-

duration of shield building in Hawaiian volcanoes,and to the expectedratesfor even earlier stagesof shield building of Hawaiian volcanoes; (4)the ages and nature of short-duration geomagneticpolarity events in the first half of the Brunhes Normal Chron and long-term patterns in paleointensity of the local magneticfield; and (5) downhole geophysicalobservations (e.g., temperature,resistivity, fracture orientation) and the relations between groundwater hydrology (e.g., the age, composition,source,and flow of water) and volcanostructure. In our view, the papers presented here demonstrate that critical issues in mantle geochemistryand geodynamics,volcanology, and paleomagnetismcan be addressedin a unique and powerful way by drilling in Hawaii and that these issues cannotbe adequatelyaddressedin the absenceof drilling. On

ines, oxidized groundmass, low K/P ratiosin somelavas). The key point is that geochemicaland petrological studies have not been compromisedby alterationand metasomatism.In the deepestpart of the core, minor zeolite precipitation is observed.

Samplesin the following articles are identified either by their unit numbers,the "corerun number"(i.e., each cored interval was given a number, R1 to R467, where lower numbers referto sectionsof corerecoveredearlier in the drilling), the positionof the top of the sample(in feet)relative to the top of an individual core run (e.g., R303-1.85A refersto a sample fromcorerun 303, wherethetopof the samplewas 1.85 feet(1 foot = 0.3048 m) from the top of the corerun; letterssuchas the "A" are used when severalsampleswere taken from the same depth),or the depthin metersrelativeto the rotarytableon the drill rig (unlessotherwiseindicate). Details of thesevarious designationsare explainedin the core log volume [Hawaii ScientificDrilling Project, 1994]. The rotary table on the drill rig was4.22 m abovesealevel,so if depthsrelativeto sea

this basiswe suggestthat even deeperdrilling to samplecontinuously the long-term history and deep structureof Hawaiian volcanoes can present extraordinaryopportunities for increasedunderstandingof hot spot volcanism and magmageneration. Moreover, the availability of core fromsuch drilling would provide sample suitesof lasting value to future generations of scientists.

levelarerequired,4.22 m mustbe subtractedfromthe reported References

depth.

Al16gre,C.J., T. Staudacher,P. Sarda,and M. Kurz, Constraintson evolution of the Earth from rare gas systematics,Nature, 303, 762-766, 1983.

Value

of the HSDP

Al16gre, C.J., P. Schiano,and E. Lewin, Differences between oceanic basaltsby multitraceelementratio topology,Earth Planet. $ci. Lett.,

core

The articles included in this special section provide a wide-ranging view of the evolution of Hawaiian volcanoes and demonstrate

how much can be learned

about

mantle and

higherlevel magmaticprocesses and volcano evolution froma long, continuoussequenceof lavas when it is systematically examinedwith the full rangeof modemanalytical techniques. Moreover,when properly logged and curated,such a suite of rocks can serveas a valuable resourcefor future generations. The particular value of the HSDP core relates to several factors: (1) The essentiallycontinuous nature of the core, which was also typically very fresh,yields informationunavailable from reconstructions basedon surfaceexposures.(2) For both the Mauna Loa and Mauna Kea sectionsof the core, the recovered samplesfill in previously unsampledparts of the volcano'shistory; i.e., for eachvolcano,the sampledlavas span

the gap betweenthe oldestknown subaeriallyexposedlavas (exceptinga few very old subaerial lavas fromMauna Loa ex-

humedalong fault scarps)and the limited samplingof older submarinelavas dredged from the volcano's submarinerifts.

Thus thesesamplesrepresentthe longest nearly continuous, detailedrecordof the historyof Hawaiianvolcanoes. (3) Perhapsmostsignificantly,the integrated,multidisciplinaryapproachtakenhereyieldsa moredetailedview than had previously been achievable. Among the topics coveredin this special section are the

following: (1) the temporalevolution of the petrology and geochemistryof Mauna Kea and Mauna Loa lavas and sources

129, 1-12, 1995.

Beeson,M.H., D.A. Clague,and J.P. Lockwood,Origin and depositional environmentof clasticdepositsin the Hilo drill hole, Hawaii, J. Geophys.Res.,this issue. Campbell,I.H., and R.W. Griffiths, hnplicationsof mantle plume structure for the evolution of flood basalts,Earth Planet. Sci. Lett., 99, 7993, 1990.

Carmichael,I.S.E., F.J. Turner, and F. Verhoogen,Igneous Petrology, 739 pp., McGraw-Hill, New York, 1974. DePaolo, D.J., and E.M. Stolper,Models of Hawaiian volcano growth andplume structure:Implicationsof resultsfrom the Hawaii Scientific Drilling Project,J. Geophys.Res., this issue. DePaolo,D.J., E.M. Stolper,and D.M. Thomas,Physicsand chemistryof mantleplumes,Eos Trans.AGU, 72, 236-237, 1991. Eiler, J.M., J.W. Valley, andE.M. Stolper,Oxygen isotoperatios in olivine from the Hawaii ScientificDrilling Project,J. Geophys.Res., this issue.

Engebretson,D.C., A. Cox, and R.G. Gordon, Relative motionsbetween oceanicand continentalplatesin the Pacific Basin,Spec. Pap. Geol. $oc. Am., 206, 1-59, 1985. Farley, K.A., J.H. Natland, and H. Craig, Binary mixing of enriched and undegassed (primitive?)mantle components(He, Sr, Nd, Pb) in Samoan lavas, Earth Planet. Sci. Lett., 111, 183-199, 1992. Griffiths, R.W., The differing effects of compositionaland thermal buoyancieson the evolutionof mantle diapirs, Phys. Earth Planet. Inter., 43, 261-273, 1986. Hart, S.R., Heterogeneous mantledomains:signatures,genesisand mixing chronologies,Earth Planet. Sci. Lett., 90, 273-296, 1988. Hart, S.R., D.C. Gerlach, and W.M. White, A possiblenew Sr-Nd-Pb mantle array and consequencesfor mantle mixing, Geochim. Cosmochim.Acta, 50, 1551-1559, 1986. Hauri, E.H., J.C. Lassiter,andD.J. DePaolo,Osmiumisotopesystematics of drilled lavasfrom Mauna Loa, Hawaii, J. Geophys.Res., this issue.

and their correlationswith magmaflux, sourcedepth, and the Hawaii ScientificDrilling Project,Core-Logs,editedby E.M. Stolperand stageof shieldbuilding;(2) the long-termvariabilityof stable M.B. Baker, 471 pp., Calif. Inst. of Technol.,Pasadena,1994. and radiogenic isotope ratios of Mauna Kea and Mauna Loa Hofmann,A.W., and K.P. Jochum,Sourcecharacteristicsderived from volcanoes,and their relation to the structureand composivery incompatibletrace elements in Mauna Loa and Mauna Kea

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Rhodes,J.M., Geochemicalstratigraphyof lava flows sampledby the Hawaii ScientificDrilling Project,J. Geophys. Res.,thisissue. Three-dimensional modelingof plumeHolmann,A.W., andW.M. White, Mantle plumesfrom ancientoceanic Ribe, N., and U. Christensen, lithosphere interaction, J. Geophys. Res.,99, 669-682,1994. crust,Earth Planet. Sci. Lett., 57, 421-436, 1982. Holt, J.W., J.L. Kirschvink,and F. Garnier, Geomagneticfield inclina- Richards,M.A., B.H. Hager, and N.H. Sleep,Dynamicallysupported geoidhighsoverhotspots: Observation andtheory,J. Geophys. Res., tionsfor thepast400 kyr fromthe 1-kmcoreof theHawaiiScientific 93, 7690-7708, 1988. Drilling Project,J. Geophys. Res.,thisissue. Kurz, M.D., W.J. Jenkins,S.R. Hart, and D. Clague, Helium isotopic Sharp,W.D., B.D. Turrin, P.R. Renne, and M.A. Lanphere,The basalts,Hawaii Scientific Drilling Project,J. Geophys.Res., this issue.

variations in volcanic rocks from Loihi Seamount and the island of

4øAr/39Ar andK/Ar datingof lavasfromtheHilo 1-kmcorehole,

Hawaii, Earth Planet. Sci. Lett., 66, 388-406, 1983. Ku.rz, M.D., T.C. Kenna, J.C. Lassiter,and D.J. DePaolo,Helium iso-

Hawaii ScientificDrilling Project,J. Geophys. Res.,thisissue. Sleep,N.H., Hotspotsand mantleplumes:Some phenomenology, J.

topicevolutionof MaunaKea volcano: Firstresultsfrom the 1-km drill core,J. Geophys.Res.,thisissue.

Geophys.Res.,95, 6715-6736, 1990. Watson, S., and D. McKenzie, Melt generationby plumes:A studyof

Lassiter,J.C., D.J. DePaolo, and M. Tatsumoto,Isotopicevolutionof Mauna Kea volcano: Resultsfrom the initial phaseof the Hawaii ScientificDrilling Project,J. Geophys. Res.,thisissue. Lipman,P.W., and J.G. Moore, Mauna Loa lava accumulation ratesat the Hilo drill site: Formationof lava deltasduringa period of decliningoverallvolcanicgrowth,J. Geophys. Res.,thisissue. Macdonald,G.A., A.T. Abbott, and F.L. Peterson,Volcanoesin the Sea, 517 pp., Univ. of Hawaii Press,Honolulu,1983. McKenzie, D., and R.K. O'Nions,Mantle reservoirsand ocean island basalts,Nature, 301,229-231, 1983. Molnar, P., andJ. Stock,Relativemotionsof hotspotsin the Pacific, Atlantic, and Indian Oceans since late Cretaceoustime, Nature, 327, 587-591, 1987.

Moore, J.G., Subsidenceof the Hawaiian Ridge, U.S. Geol. Surv. Prof Pap., 1350, 85-100, 1987. Moore, J.G., B.L. Ingram, K.R. Ludwig, and D.A. Clague,Coral ages and islandsubsidence, Hilo drill hole, J. Geophys.Res.,this issue. Morgan,W.J., Convectionplumesin the lower mantle,Nature, 230, 4243, 1971.

Hawaiian volcanism,J. Petrol., 32, 501-537, 1991.

Weaver,B.L., The origin of ocean islandbasaltend-membercompositions:Traceelementandisotopicconstraints, Earth Planet.Sci. Lett., 104, 381-397, 1991.

Wilson,J.T., A possibleoriginof the Hawaiian Islands,Can.J. Phys., 41, 863-870, 1963.

Zindler, A., and S.R. Hart, Chemicalgeodynamics,Annu. Rev. Earth. Planet. Sci., 14, 493-571, 1986.

D.J. DePaolo,Centerfor IsotopeGeochemistry, Departmentof Geologyand Geophysics, Universityof California,Berkeley,CA 947204767 (e-mail: depaolo•garnet.berkeley.edu) E.M. Stolper,Divisionof Geologicaland PlanetarySciences,California Institute of Technology, Pasadena, CA 91125 (e-mail: ems•expet.gps.caltech.edu) D.M. Thomas, Hawaii Institute of Geophysicsand Planetology, Schoolof Oceanand Earth Scienceand Technology,Univresityof Hawaii, Honolulu,HI 96822;(e-mail:dthomas•soest.hawaii.edu)

Morgan,W.J., Hotspottracksandtheopeningof theAtlanticand Indian oceans,in The Sea, vol. 7, The Oceanic Lithosphere,edited by C.

Emiliani,pp. 443-475,Wiley-Interscience, New York, 1981. Morgan,W.J., Hotspottracksand the early rifting of the Atlantic, Tectonophysics, 94, !23-139, 1983.

(Received January 19, 1996;revised January 26, 1996; accepted January 26, 1996.)