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Giving Birth to Hotspot Volcanoes: Distribution and Composition of Young Seamounts from the Seafloor Near Tahiti and Pitcairn Islands

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Giving Birth to Hotspot Volcanoes: Distribution and Composition of Young Seamounts from the Seafloor Near Tahiti and Pitcairn Islands Giving birth to hotspot volcanoes: Distribution and composition of young seamounts from the sea¯oor near Tahiti and Pitcairn islands C.W. Devey Fachbereich 5ÐGeowissenschaften, UniversitaÈt Bremen, D-28334 Bremen, Germany K.S. Lackschewitz D.F. Mertz Institut fuÈr Geowissenschaften, UniversitaÈt Mainz, Becherweg 21, D-55099 Mainz, Germany, and Max Planck Institut fuÈr Chemie, D-55128, Mainz, Germany B. Bourdon Laboratoire de GeÂochemie, Institut de Physique du Globe Paris, place Jussieu, F-75252 Paris Cedex 05, France J.-L. ChemineÂe Institut de Physique du Globe, place Jussieu, F-75252 Paris Cedex 05, France J. Dubois C. Guivel Laboratoire PeÂtrologie Structurale, Universite de Nantes, F-44322 Nantes, France R. HeÂkinian Institut Francais de Recherche pour l'Exploitation de la Mer, F-29280 PlouzaneÂ, France P. Stoffers Institut fuÈr Geowissenschaften, UniversitaÈt Kiel, D-24118 Kiel, Germany ABSTRACT samples produced by the hotspot. These youn- Apart from being popular holiday destinations, oceanic-island volcanoes such as Hawaii, ger volcanoes can be further subdivided into Tahiti, or the Canaries provide magmas that yield valuable information about the interior small isolated volcanoes (height above sea- of our planet. Until recently, studies have concentrated on the easily accessible, subaerial ¯oor, #500 m), large isolated volcanoes parts of the volcanoes, largely ignoring their earlier-formed, submarine parts. These sub- (height above sea¯oor, .1500 m) and small marine parts, however, provide critical information about how the mantle begins to melt parasite cones on the ¯anks of the large edi- and about the lowest-melting-point mantle componentsÐinformation not available from ®ces. In the Pitcairn area, eight recently the subaerial volcanoes but highly relevant for the chemical evolution of the whole mantle. formed small volcanoes and three large vol- We present here compositional information from small (,500 m) volcanoes on the sea¯oor canoes were mapped and sampled over an area near Tahiti and Pitcairn Islands and show that these small volcanoes erupt only highly of ;50 3 70 km. At the Society hotspot, ®ve differentiated magmas. These early melts are derived exclusively from the most trace small volcanoes and ®ve larger volcanoes element±enriched, isotopically extreme mantle component, evidence that this component have been mapped and sampled over an area has the lowest melting temperature and is the ®rst product of melting of a new batch of of 150 3 80 km. Two of these volcanoes, Ro- mantle. The geochemical mantle components (enriched mantle EM-I, EM-II) proposed in card and Turoi, rise some 1000 m above the the 1980s to explain the compositional variations among oceanic volcanoes worldwide sea¯oor, occupying a transitional place in our appear in reality to represent distinct rock masses in the mantle. size classi®cation, a situation also re¯ected by their lava compositions, as outlined Keywords: Polynesia, geochemistry, plume, enriched mantle, EM-I, EM-II, melting, plum pud- subsequently. ding, volcano evolution, hotspot. Major element analyses on fresh glass chips show a clear distinction in degree of magmatic INTRODUCTION of present-day submarine volcanic activity. differentiation between the large and small Many chains of intraplate oceanic-island Although such sampling has been carried out volcanoes (see Fig. 2A). This is particularly volcanoes are built as lithospheric plates move on Loihi Seamount in the Hawaii chain, this striking for the Society hotspot, where all over stationary melt sources (hotspots) in the seamount is already relatively large and pres- large-volcano magmas have MgO . 3%, asthenosphere. A hotspot is probably main- ently erupting magmas similar to those found whereas the smaller volcanoes all yield te- tained over long periods by an adiabatically on the islands (e.g., Garcia et al., 1995) and phritic phonolite or trachyte magmas with upwelling mantle diapir or plume (Morgan, so is not ideal for studying the onset of vol- MgO , 2% characterized by high volatile 1971). The degree of partial melting of the canism. We present here geochemical analyses contents. Rocard and Turoi volcanoes have plume mantle will vary both laterally (owing from samples obtained from the submarine ed- yielded many trachytes and a handful of ba- to radial gradients in plume temperature from i®ces of the Pitcairn and Society hotspots saltic samples. A similar situation, with more a hot center to a cool rim; Loper and Stacey, (e.g., Stoffers et al., 1988; Stoffers et al., basic magmas being found on the larger vol- 1983) and vertically (owing to the effect of 1990a, 1990b; Binard et al., 1992a) and use canoes, is seen at Pitcairn, although here some pressure on the solidus; e.g., Farnetani and them to develop a petrogenetic model for the evolved magmas are also found on the larger Richards, 1995). The volcanoes will therefore initial stages of hotspot volcanism. edi®ces, most notably as a trachytic dome cap- be fed by melts formed at different tempera- ping the apparently extinct Adams volcano tures and pressures during their growth. Par- RESULTS AND DISCUSSION (Stoffers et al., 1990a). Nevertheless, no sam- ticularly interesting in this respect are the ini- On bathymetric maps (Fig. 1) we distin- ple with MgO . 3.5% has ever been recov- tial phases of volcano growth, because they guish two major types of volcanoes in these ered from the small Pitcairn volcanoes. Liquid should be fed by melt formed either at the rim hotspot areas. The ®rst type yields only Mn- line of descent modeling at both hotspots is of the plume as it passes beneath previously encrusted samples (e.g., Glasby et al., 1997; compatible with derivation of evolved mag- unaffected lithosphere (Frey et al., 2000) or Puteanus et al., 1989), with mid-oceanic-ridge mas by extensive crystal fractionation of basic deep in the plume as the upwelling mantle ®rst basalt (MORB) compositions (e.g., Devey et parent magmas similar to those found on the crosses the solidus. Although deep drilling on al., 1990). These volcanoes are old and not adjacent large volcanoes (Devey et al., 1990). oceanic islands (e.g., Stolper et al., 1996) can related to hotspot activity and will not be dis- The Pitcairn and Society hotspots are typi- start to examine the initial phases of volcano cussed further in this paper. The second type cal examples of EM-I and EM-II hotspots, re- growth, it must be complemented by sampling comprises volcanoes yielding young, fresh spectively (EM is enriched mantle; see Zindler q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; May 2003; v. 31; no. 5; p. 395±398; 3 ®gures; Data Repository item 2003055. 395 and Hart, 1986). Lava compositions from these volcanoes form linear arrays in a Sr ver- sus Nd isotope diagram (White and Hofmann, 1982; Woodhead and Devey, 1993; Woodhead and McCulloch, 1989). Isotopic data from the seamounts (Fig. 2B) show the evolved mag- mas from the small volcanoes and Rocard to lie at the extreme high 87Sr/86Sr and low 143Nd/144Nd end of the EM-I and EM-II arrays for the present-day Pitcairn and Society hot- spots, respectively. We note that older parts of the Society chain (e.g., Tahaa Island; White and Duncan, 1996) have yielded lavas with more extreme isotopic compositions than those presently erupting on the small Society volcanoes. We attribute this to long-term var- iations in the composition of the enriched So- ciety source component and as such not in con¯ict with our observations; we would ex- pect small volcano trachytes erupted at the time of formation of Tahaa to show 87Sr/86Sr ratios close to 0.707. MODEL FOR THE INITIATION OF HOTSPOT VOLCANISM Seismological observations from the Soci- ety area show that both large basaltic and small trachytic volcanoes are currently active (Talandier, 1989; Talandier and Kuster, 1976; Talandier and Okal, 1984). The systematic freshness of samples recovered at Pitcairn im- plies that the same is true there. The large number of small compared to large or subaer- ial volcanoes currently active at both hotspots suggests that many more volcanic systems are initiated than ever reach maturity. The mag- mas entering these initial systems from the mantle are most probably basic; the eruptive products appear, however, to be exclusively highly fractionated phonolites and trachytes. This ®nding points to extensive fractionation in the lithosphere, either because the magma conduit system is initially cold and saps the magmas 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 when fractionation has suf®ciently lowered their density or increased their volatile pressure. Whatever the reason for their extreme differ- Figure 1. Bathymetry of sea¯oor southeast of (A) Pitcairn Island (modi®ed after HeÂkinian entiation, the initial magmas seem to be de- et al., 2003) and (B) Tahiti Island. Volcano shading codes: Light grayÐold non-hotspot rived, without exception, from the mantle volcanoes. SpeckledÐlarge, recently active volcanoes associated with hotspot. Dark grayÐsmall, recently active volcanoes associated with hotspot. WhiteÐhotspot volca- source with the highest time-integrated incom- noes of intermediate height. A: Dredge sampling stations (PNDRÐfrom POLYNAUT patible trace element enrichment. If these ini- cruise, DSÐfrom cruise 65 of FS Sonne) are shown. Note that volcano at 258359S, tial magmas re¯ect the onset of melting in a 1298309W is large enough to be classi®ed as a large volcano (and hence falls in same particular mantle volume, then this observa- size classi®cation as Bounty and Adams); however, it has yielded only old samples. Volcano names and numbers after Binard et al. (1992b). Samples PNDR13 and 50DS come tion implies that the most trace element± from a parasitic cone on southern ¯ank of Bounty volcano.
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