JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. BI0, PAGES 17,649-17,665, OCTOBER 10, 1993 Temporaland GeochemicalVariability of VolcanicProducts of the MarquesasHotspot D. L. DESONIE1 AND R. A. DUNCAN Collegeof Oceanicand AtmosphericSciences, Oregon State University, Corvallis J. H. NATLAND 2 ScrippsInstitution of Oceanography,University of California,La Jolla The Marquesasarchipelago is a short,NW-SE trendingcluster of islandsand seamounts that formedas a result of volcanicactivity over a weak hotspot. This volcanicchain lies at the northernmargin of a broad regionof warm and compositionallydiverse mantle that meltsto build severalother subparallel volcanic lineaments.Basalts dredged from submergedportions of volcanoesalong the Marquesaslineament decrease in age from northwestto southeast.The new sampleage distributionyields a volcanicmigration rate significantlyslower than that expectedfor Pacificplate motion over a stationaryMarquesas hotspot. This and the aberrantorientation of the chain indicate deflectionof the plume by westwardupper mantle flow. The interactionof this weak plumewith uppermantle flow accountsfor the temporaland spatialpatterns in Marquesanvolcanism. The compositionsof subaerialand submarine basalts reflect the mixingof at least two mantle sources,distinguished by Sr, Nd, and Pb isotopeand trace elementcompositions. There is a consistentevolutionary pattern at each volcano,from early tholeiitic to later alkalic basalt eruptions. Tholeiitic and transitionallava compositionscan be derivedby variabledegrees of partial melting of a source composedof depletedmid-ocean ridge basalt mantle (DMM) and mantle characterizedby radiogenicPb (HIMU). Alkaliclava compositions appear to be dominantlythe resultof smallerdegrees of meltingof a moreradiogenic mantle source (EMII). Large-scalemelting of the lowerlithosphere or upper mantle (DMM+HIMU) entrainedwithin a sheared,thermally buoyant plume (EMII) could producethe tholeiitic and transitional basalts found in the main shields of the volcanoes, while alkalic basalts could result from meltingof mantleof EMII compositionat theedges of thehotspot. THE MARQUESASVOLCANIC LINEAMENT In this paperwe reportnew 4øAr-39Arand K-Ar age The Marquesas lineament, the northernmostof the many determinationsfrom eight dredgehauls and the island of Fatu hotspot lineaments of French Polynesia, includes 12 islands Hiva in the Marquesas lineament, which revise the age and at least eight seamountsthat extend for nearly 500 km progressionpreviously determined from studies of island north of the Marquesas Fracture Zone (Figure 1). These samplesonly. Major and trace elementcontents are consistent youthful volcaniccones rise up to 5000 m from 50- to 65-m.y.- with fractional crystallizationfrom a range of primary basalt old oceanic lithosphere [Kruse, 1988] that formed at the melt compositions. Isotopic ratios identify the parental ancestral East Pacific Rise (EPR). The Marquesas, and the mantle compositionsas having depletedmid-ocean ridge basalt other islandsof FrenchPolynesia, are on the westernside of an mantle plus mantle characterized by radiogenic Pb unusually shallow region of thinner than normal lithosphere, (DMM+HIMU) and more radiogenic mantle source (EMII) termed the South Pacific isotopic and thermal anomaly signatures,in Zindler and Hart's [1986] terminology. We (SOPITA), thought to be a zone of broad-scale mantle discussthese data in terms of possible dynamic models of upwelling [McNutt and Fischer, 1987; Smith et al., 1989; mantle plume and lithosphere/asthenospheremelting and McNutt and Judge, 1990; Staudigelet al., 1991]. Extreme mixing. compositionalvariations are found for basaltson a variety of scales: between and among island groups, and within Age Relationshipsof Marquesas Volcanoes individual volcanoes[Bishop and Woolley, 1973; Liotard et al., 1986; Duncan et al., 1986; Vidal et al., 1987]. Volcanism Previousresults of K-Ar age determinationsfrom the islands above the Marquesas hotspot has been intermittent, is of the Marquesas[Duncan and McDougall,1974; Duncan et al., somewhat oblique to volcanism at other south Pacific 1986; Brousse et al., 1990] have shown a southeastward lineaments[Crough and Jarrard, 1981], and is the only French migrationof volcanismalong the Marquesaschain from 6 Ma Polynesianhotspot track for which there is no known site of at Eiao [Brousseet al., 1990; Caroff et al., 1990] through 1.3 active volcanism. Ma at Fatu Hiva [Duncanand McDougall, 1974]. The resultant volcanic migration rate of 104 + 18 km/m.y. closely matches 1Nowat Lamont-Doherty EarthObservatory, Palisades, New York. those calculated for other French Polynesian volcanic chains 2Nowat Rosenstiel School ofMarine and Atmospheric Sciences, [McDougalland Duncan, 1980]. Universityof Miami, Miami, Florida. It has been suggestedthat volcanism over the Marquesas hotspotdid not initiate at Eiao (Figure 1). Volcanismat ridges Copyright1993 by the AmericanGeophysical Union. to the west of the MarquesasIslands along the Galapagos Papernumber 93JB01562. FractureZone and the Nova-Cantontrough, with agesof 13 and 0148-0227/93/93JB-01562505.00 30 Ma, respectively, has been proposedto have originated 17,649 17,650 DESONIEET AL.: VOLCANICPRODUCTS OF THE MARQUESASHOTS POT . ark Iti Huka Pou Huku Nuku Hiv; O8 ane Nao /FatuHiva 12 12 I I 141' 140' 139' 138' 137' ,6' Fig. 1. Bathymetricmap of the Marquesaslineament. Arrows indicatelocations of dredgehauls; after GEBCO Map 297, Centre National pour rExploitation des Oceans,Paris, 1973. Contour interval = 500 m, dotted !in, contour= 200 m. Insert indicatesthe location of the Marquesasbathymetric map and showsmajor tectonic featuresof the Pacific Ocean basin: GalapagosFracture Zone (GFZ), MarquesasFracture Zone (MFZ), and the EastPacific Rise. over the Marquesas hotspot [McNutt et al., 1989]. A long, margin of the MarquesasFracture Zone (MFZ). Furtherstudy of broad topographicand geoid swell extendswestward from the this ridge has shown that it is an old feature unrelated to Marquesas lineament through the center of the north-south hotspot activity (M. K. McNutt, personal communication, trending Line Islands volcanic lineament. This feature, termed 1991). In September1991, however,fresh glassybasalts were the Line-Crosstrend, has allowed speculationthat at least some dredgedfrom a seamountat the southeastend of the Marquesas of the centralLine Islandsformed above the Marquesashotspot lineament, 20 km north of the MFZ, now thoughtto be the [Croughand Jarrard, 1981]. Three of the volcanoes,with ages present location of the hotspot (M. K. McNutt, personal between 38 and 60 Ma, show an age progressionwhich could communication,1991); radiometricages for these samplesare be related to volcanism along the Marquesas-Line trend in progress(R. A. Duncan, unpublishedresults, 1993). [Schlanger et al., 1984]. If so, the Marquesashotspot has The duration of volcanism at a single edifice within this producedvolcanic activity only intermittently. lineament is longer than that measured at other hotspot In contrast to the three other volcanic chains of the French locations. Duncan et al. [1986] discoveredthat the 3.8-m.y. Polynesian region (Society, Austral-Cook, Pitcairn-Gambier), history of volcanism at the island of Ua Pou followed a no active volcanism has been found at the southeastern end of petrogeneticsequence similar to Hawaiian volcanoes. Shield the Marquesasline [McNutt et al., 1989]. McNutt et al. [1989] lavas exposedin stream valleys, road cuts and wave-cut cliffs proposed that the most recent expressionof volcanism above around the perimeter of Ua Pou are part of a tholeiitic basalt the Marquesas hotspot may be found at the northern ridge phase of volcanism which occurred from 5.6 to 4.5 Ma. DESONIEET AL.: VOLCANIC PRODUCTSOF THE MARQUESASHOTSPOT 17,651 Volcanism began again after a 1.6 m.y. hiatus with eruption of alkali basalts from 2.9 to 2.7 Ma, and finally more evolved Samoa lavas from 2.5 to 1.8 Ma [Duncan et al., 1986]. In contrast 0.707 with Hawaiian volcanism, the magmatic production rate is independentof composition and does not diminish with time, based on the age relationshipsof the compositional phaseson 0.706 each of the Marquesanvolcanoes [Brousse et al., 1990]. 0.705 GeochemicalVariability of SOPITA Volcanic Chains The south-central Pacific basin presents several remarkable 0.704 geochemical and geophysical characteristics. This region (essentially French Polynesia) has thinner lithosphere Pitcairn Is. 0.703 [Nishimura and Forsyth, 1985] and shallowerdepths [McNutt PREMA and Fischer, 1987] than comparablyold lithosphereelsewhere. ter Is. Slow lower mantle seismic velocities [Dziewonski and 0.702 Woodhouse, 1987] directly beneaththe region correlate with 16.0 17.0 18.0 19.0 20.0 21.0 22.0 an unusually high density of hotspot tracks. These volcanoes exhibit some of the most extreme isotopic compositions 206pb/204pb among oceanic basalts [Duncan and Compston,1976], which Fig.2. 87Sr/86Srversus 206pb/204pb for southPacific spreading ridge Hart [1984] and Castillo [1988] have related to large-scale basaltsand hotspotvolcanic lineaments with mantle componentsafter convectiveupwelling from the deep mantle. Staudigel et al. Zindler and Hart [1986] shownin hatchuredboxes. Althoughdata from [1991] have shown that the compositional identity of the most island chains are grouped,data from each volcano in the Cook- Austral lineamentis shownas a separatefield but with the samedotted volcanism has persistedat least since mid-Cretaceoustime and patternand print.