q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.Geology; May 2003; v. 31; no. 5; p. 395–398; 3 figures; Data Repository item 2003055. 395

Giving birth to hotspot volcanoes: Distribution and composition ofyoung seamounts from the seafloor near Tahiti and Pitcairn islandsC.W. DeveyK.S. Lackschewitz

Fachbereich 5—Geowissenschaften, Universitat Bremen, D-28334 Bremen, Germany

D.F. Mertz Institut fur Geowissenschaften, Universitat Mainz, Becherweg 21, D-55099 Mainz, Germany, and Max PlanckInstitut fur Chemie, D-55128, Mainz, Germany

B. Bourdon Laboratoire de Geochemie, Institut de Physique du Globe Paris, place Jussieu, F-75252 Paris Cedex 05, FranceJ.-L. ChemineeJ. Dubois

Institut de Physique du Globe, place Jussieu, F-75252 Paris Cedex 05, France

C. Guivel Laboratoire Petrologie Structurale, Universite de Nantes, F-44322 Nantes, FranceR. Hekinian Institut Francais de Recherche pour l’Exploitation de la Mer, F-29280 Plouzane, FranceP. Stoffers Institut fur Geowissenschaften, Universitat Kiel, D-24118 Kiel, Germany

ABSTRACTApart from being popular holiday destinations, oceanic-island volcanoes such as Hawaii,

Tahiti, or the Canaries provide magmas that yield valuable information about the interiorof our planet. Until recently, studies have concentrated on the easily accessible, subaerialparts of the volcanoes, largely ignoring their earlier-formed, submarine parts. These sub-marine parts, however, provide critical information about how the mantle begins to meltand about the lowest-melting-point mantle components—information not available fromthe subaerial volcanoes but highly relevant for the chemical evolution of the whole mantle.We present here compositional information from small (,500 m) volcanoes on the seafloornear Tahiti and Pitcairn Islands and show that these small volcanoes erupt only highlydifferentiated magmas. These early melts are derived exclusively from the most traceelement–enriched, isotopically extreme mantle component, evidence that this componenthas the lowest melting temperature and is the first product of melting of a new batch ofmantle. The geochemical mantle components (enriched mantle EM-I, EM-II) proposed inthe 1980s to explain the compositional variations among oceanic volcanoes worldwideappear in reality to represent distinct rock masses in the mantle.

Keywords: Polynesia, geochemistry, plume, enriched mantle, EM-I, EM-II, melting, plum pud-ding, volcano evolution, hotspot.

INTRODUCTIONMany chains of intraplate oceanic-island

volcanoes are built as lithospheric plates moveover stationary melt sources (hotspots) in theasthenosphere. A hotspot is probably main-tained over long periods by an adiabaticallyupwelling mantle diapir or plume (Morgan,1971). The degree of partial melting of theplume mantle will vary both laterally (owingto radial gradients in plume temperature froma hot center to a cool rim; Loper and Stacey,1983) and vertically (owing to the effect ofpressure on the solidus; e.g., Farnetani andRichards, 1995). The volcanoes will thereforebe fed by melts formed at different tempera-tures and pressures during their growth. Par-ticularly interesting in this respect are the ini-tial phases of volcano growth, because theyshould be fed by melt formed either at the rimof the plume as it passes beneath previouslyunaffected lithosphere (Frey et al., 2000) ordeep in the plume as the upwelling mantle firstcrosses the solidus. Although deep drilling onoceanic islands (e.g., Stolper et al., 1996) canstart to examine the initial phases of volcanogrowth, it must be complemented by sampling

of present-day submarine volcanic activity.Although such sampling has been carried outon Loihi Seamount in the Hawaii chain, thisseamount is already relatively large and pres-ently erupting magmas similar to those foundon the islands (e.g., Garcia et al., 1995) andso is not ideal for studying the onset of vol-canism. We present here geochemical analysesfrom samples obtained from the submarine ed-ifices of the Pitcairn and Society hotspots(e.g., Stoffers et al., 1988; Stoffers et al.,1990a, 1990b; Binard et al., 1992a) and usethem to develop a petrogenetic model for theinitial stages of hotspot volcanism.

RESULTS AND DISCUSSIONOn bathymetric maps (Fig. 1) we distin-

guish two major types of volcanoes in thesehotspot areas. The first type yields only Mn-encrusted samples (e.g., Glasby et al., 1997;Puteanus et al., 1989), with mid-oceanic-ridgebasalt (MORB) compositions (e.g., Devey etal., 1990). These volcanoes are old and notrelated to hotspot activity and will not be dis-cussed further in this paper. The second typecomprises volcanoes yielding young, fresh

samples produced by the hotspot. These youn-ger volcanoes can be further subdivided intosmall isolated volcanoes (height above sea-floor, #500 m), large isolated volcanoes(height above seafloor, .1500 m) and smallparasite cones on the flanks of the large edi-fices. In the Pitcairn area, eight recentlyformed small volcanoes and three large vol-canoes were mapped and sampled over an areaof ;50 3 70 km. At the Society hotspot, fivesmall volcanoes and five larger volcanoeshave been mapped and sampled over an areaof 150 3 80 km. Two of these volcanoes, Ro-card and Turoi, rise some 1000 m above theseafloor, occupying a transitional place in oursize classification, a situation also reflected bytheir lava compositions, as outlinedsubsequently.

Major element analyses on fresh glass chipsshow a clear distinction in degree of magmaticdifferentiation between the large and smallvolcanoes (see Fig. 2A). This is particularlystriking for the Society hotspot, where alllarge-volcano magmas have MgO . 3%,whereas the smaller volcanoes all yield te-phritic phonolite or trachyte magmas withMgO , 2% characterized by high volatilecontents. Rocard and Turoi volcanoes haveyielded many trachytes and a handful of ba-saltic samples. A similar situation, with morebasic magmas being found on the larger vol-canoes, is seen at Pitcairn, although here someevolved magmas are also found on the largeredifices, most notably as a trachytic dome cap-ping the apparently extinct Adams volcano(Stoffers et al., 1990a). Nevertheless, no sam-ple with MgO . 3.5% has ever been recov-ered from the small Pitcairn volcanoes. Liquidline of descent modeling at both hotspots iscompatible with derivation of evolved mag-mas by extensive crystal fractionation of basicparent magmas similar to those found on theadjacent large volcanoes (Devey et al., 1990).

The Pitcairn and Society hotspots are typi-cal examples of EM-I and EM-II hotspots, re-spectively (EM is enriched mantle; see Zindler

396 GEOLOGY, May 2003

Figure 1. Bathymetry of seafloor southeast of (A) Pitcairn Island (modified after Hekinianet al., 2003) and (B) Tahiti Island. Volcano shading codes: Light gray—old non-hotspotvolcanoes. Speckled—large, recently active volcanoes associated with hotspot. Darkgray—small, recently active volcanoes associated with hotspot. White—hotspot volca-noes of intermediate height. A: Dredge sampling stations (PNDR—from POLYNAUTcruise, DS—from cruise 65 of FS Sonne) are shown. Note that volcano at 258359S,1298309W is large enough to be classified as a large volcano (and hence falls in samesize classification as Bounty and Adams); however, it has yielded only old samples.Volcano names and numbers after Binard et al. (1992b). Samples PNDR13 and 50DS comefrom a parasitic cone on southern flank of Bounty volcano. Inset: Location of Polynesianhotspots in South Pacific. East Pacific Rise (EPR) and Easter microplate are visible nearright-hand margin. B: Dredge sampling stations (DTH—from 1989 CYAPOL submersiblecruise, DR—from 1985 cruise of NO Charcot, 47 and 65 from cruises 47 and 65 of FSSonne, respectively) and dive tracks (TH) are shown. Volcano names and numbers afterBinard et al. (1992a). Asterisk on the summit of Teahitia is the location of samples 65-119GTV, 65-120GTV, 47-5DS, 47-20DS, 47-22GTV.

and Hart, 1986). Lava compositions fromthese volcanoes form linear arrays in a Sr ver-sus Nd isotope diagram (White and Hofmann,1982; Woodhead and Devey, 1993; Woodheadand McCulloch, 1989). Isotopic data from theseamounts (Fig. 2B) show the evolved mag-mas from the small volcanoes and Rocard tolie at the extreme high 87Sr/86Sr and low143Nd/144Nd end of the EM-I and EM-II arraysfor the present-day Pitcairn and Society hot-spots, respectively. We note that older parts ofthe Society chain (e.g., Tahaa Island; Whiteand Duncan, 1996) have yielded lavas withmore extreme isotopic compositions thanthose presently erupting on the small Societyvolcanoes. We attribute this to long-term var-iations in the composition of the enriched So-ciety source component and as such not inconflict with our observations; we would ex-pect small volcano trachytes erupted at thetime of formation of Tahaa to show 87Sr/86Srratios close to 0.707.

MODEL FOR THE INITIATION OFHOTSPOT VOLCANISM

Seismological observations from the Soci-ety area show that both large basaltic andsmall trachytic volcanoes are currently active(Talandier, 1989; Talandier and Kuster, 1976;Talandier and Okal, 1984). The systematicfreshness of samples recovered at Pitcairn im-plies that the same is true there. The largenumber of small compared to large or subaer-ial volcanoes currently active at both hotspotssuggests that many more volcanic systems areinitiated than ever reach maturity. The mag-mas entering these initial systems from themantle are most probably basic; the eruptiveproducts appear, however, to be exclusivelyhighly fractionated phonolites and trachytes.This finding points to extensive fractionationin the lithosphere, either because the magmaconduit system is initially cold and saps themagmas of heat before they can reach the sur-face, or because, in the absence of a continu-ous magma supply from the mantle, the mag-mas can reach the surface only whenfractionation has sufficiently lowered theirdensity or increased their volatile pressure.Whatever the reason for their extreme differ-entiation, the initial magmas seem to be de-rived, without exception, from the mantlesource with the highest time-integrated incom-patible trace element enrichment. If these ini-tial magmas reflect the onset of melting in aparticular mantle volume, then this observa-tion implies that the most trace element–enriched mantle component has the lowestmelting point. This inference lends some di-rect observational support to the suggestion,based either on theoretical or trace-elementmodeling considerations (e.g., Batiza, 1984;Zindler et al., 1984; Graham et al., 1988;

GEOLOGY, May 2003 397

Figure 2. Composition of volcanic samples from submarine Pitcairn and Society hotspots.A: SiO2 vs. MgO plot using previously published data (Binard et al., 1992a, 1992b; Devey etal., 1990; Hemond et al., 1994; Woodhead and Devey, 1993) and our own analyses (TableDR11). Different volcano types are described in text and are visible in Figure 1. Fields showrange of compositions of large volcanoes from Pitcairn and Society hotspots. B: 87Sr/86Srvs. 143Nd/144Nd isotope diagram. As previously shown (White and Hofmann, 1982; Woodheadand McCulloch, 1989), Pitcairn and Society hotspots lie on EM-I and EM-II isotopic trends(Zindler and Hart, 1986) as marked by trends of samples from neighboring islands (Duncanet al., 1994; White, 1985; White and Duncan, 1996), respectively. Data from Devey et al.(1990), Hemond et al. (1994), Woodhead and Devey (1993), and present work (Table DR2;see footnote one).

Figure 3. Model for magma-plumbing system beneath Society and Pitcairn hotspots. See text for discussion.

Phipps Morgan and Morgan, 1999) that melt-ing in the mantle initially fuses a low-melting-point, trace element–enriched component pre-sent as ‘‘plums’’ in the mantle. Furthermore,the correlation of initial melting with extremeisotopic compositions implies that the mantleend members EM-I and EM-II originally pro-posed to explain hotspot isotope geochemistry(Zindler and Hart, 1986) do actually corre-spond to distinctive mantle rock types withrelatively low solidus temperatures.

Figure 3 shows our model for the crust andmantle around the Polynesian hotspots. At theonset of melting deep in the plume, traceelement–enriched melts from the enrichedmantle plums are formed. These melts either(1) rise into the lithosphere where they stag-nate at a density boundary (Moho?) or (2) arechanneled into preexisting magma-pathwaysrelated to the larger volcanoes in the area. Incase 1, the magmas will fractionate, and theresidual magma will eventually erupt as tra-chytes or phonolites on the small volcanoes aspreviously outlined. In case 2, the enrichedmagmas become mixed with melts generatedshallower in the plume at larger degrees ofmelting. They provide the enriched mantleend member for the range of isotopic valuesseen in the larger volcanoes.

The facts that the larger volcanoes are (1)fewer in number than the small volcanoes and(2) predominantly basaltic or basanitic incomposition suggest that at some point, oneor a few of the initial magma pathways willbecome more established. Magma will then bechanneled into them regularly, and the thermalconditions for extensive fractionation will nolonger (or in the case of Pitcairn less often)be met. With time, one of these volcanoes may

1GSA Data Repository item 2003055, micro-probe analyses of volcanic glasses from the Pitcairnand Society hotspots, is available on request fromDocuments Secretary, GSA, P.O. Box 9140, Boul-der, CO 80301-9140, USA, editing@geosociety.org,or at www.geosociety.org/pubs/ft2003.htm.

398 GEOLOGY, May 2003

become the focus of almost all eruptions—inthis case an island may be formed. The oc-casional, more evolved magmas occurringeven on the larger Pitcairn edifices may reflecta generally lower rate of magma supply at Pit-cairn (as indicated by the intermittent natureof the hotspot trace since 15 Ma [Duncan etal., 1974] when compared to the Societies)that allows extensive cooling and fractionationof the magma to occur even in a large vol-cano. It is interesting to note, however, thatthe trachyandesites and trachytes on theselarge Pitcairn volcanoes do not show the ex-treme isotopic values characteristic of evolvedmagmas from the smaller edifices; they areisotopically indistinguishable from the basaltserupted on the same volcano (Woodhead andDevey, 1993). Our model for the evolution ofPolynesian hotspot volcanoes therefore in-volves a two-stage process—the initiation ofmany small-volume volcanic systems derivedfrom melting of the enriched mantle plums,followed by focusing of magmatic activity inonly a few of these systems. Whether thesmall volcanoes that we currently observe atboth hotspots are coeval with the initiation ofthe larger volcanoes or whether they representa more recent renewed phase of magmatic sys-tem initiation is at present not known. The So-ciety volcanoes Rocard and Turoi, from whichsome basalts and numerous trachytes havebeen recovered, may currently be in the pro-cess of focusing magmas and will perhaps bethe next large underwater Society volcanoes.

ACKNOWLEDGMENTSThe research cruises that yielded the samples and

maps forming the basis for this project were fundedby the German Bundesministerium fur Bildung undForschung and Deutsche Forschungsgemeinschaftand the French Centre National de la RechercheScientifique. We thank the captains and crews of theresearch vessels Sonne and Atalante and the Nautilesubmersible team for their unflagging assistance. R.Duncan and P. Castillo provided thoughtful, con-structive reviews that helped improve the manu-script markedly. We also thank A.W. Hofmann ofthe Max-Planck-Institut fur Chemie, Mainz, for ac-cess to isotope facilities.

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Manuscript received 2 October 2002Revised manuscript received 20 January 2003Manuscript accepted 28 January 2003

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