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Tahiti: Geochemical Evolution of a French Polynesian Volcano

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Tahiti: Geochemical Evolution of a French Polynesian Volcano JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B12, PAGES 24,341-24,357, DECEMBER 10, 1994 Tahiti: Geochemical evolution of a French Polynesian volcano Robert A. Duncan and Martin R. Fisk Collegeof Oceanicand AtmosphericSciences, Oregon State University, Corvallis William M. White Departmentof GeologicalSciences, Cornell University, Ithaca, New York Roger L. Nielsen Collegeof Oceanicand Atmospheric Sciences, Oregon State University, Corvallis Abstract. The island of Tahiti, the largestin French Polynesia,comprises two major volcanoes alignedNW-SE, parallelwith the generaltrend of the SocietyIslands hotspot track. Rocksfrom this volcanicsystem are basaltstransitional to tholeiites,alkali basalts,basanites, picrites, and evolvedlavas. Through K-Ar radiometricdating we haveestablished the ageof volcanicactivity. The oldestlavas (--1.7 Ma) crop out in deeplyeroded valleys in the centerof the NW volcano (Tahiti Nui), while the main exposedshield phase erupted between 1.3 and 0.6 Ma, and a late- stage,valley-filling phase occurred between 0.7 and0.3 Ma. The SW volcano(Tahiti Iti) was active between0.9 and 0.3 Ma. There is a clear changein the compositionof lavas throughtime. The earliestlavas are moderatelyhigh SiO2,evolved basalts (Mg number(Mg# = Mg/Mg+Fe2+) 42-49), probablyderived from parentalliquids of compositiontransitional between those of tholeiitesand alkali basalts.The main shieldlavas are predominantlymore primitive olivine and clinopyroxene-phyricalkali basalts(Mg# 60-64), while the latervalley-filling lavas are basanitic (Mg# 64-68) andcommonly contain peridotitic xenoliths (olivine+orthopyroxene+ clinopyroxene+spinel). Isotopic compositions also change systematically with time to more depletedsignatures. Rare earthelement patterns and incompatible element ratios, however, show no systematicvariation with time.We focusedon a particularlywell exposedsequence of shield- buildinglavas in the PunaruuValley, on the westernside of Tahiti Nui. CombinedK-Ar agesand magnetostratigraphicboundaries allow high-resolutionage assignments to this -0.7-km-thick flow section.We identifiedan early periodof intensevolcanic activity, from 1.3 to 0.9 Ma, followedby a periodof moreintermittent activity, from 0.9 to 0.6 Ma. Flow accumulationrates droppedby a factorof 4 at about0.9 Ma. This changein rateof magmasupply corresponds to a shiftin activityto Tahiti Iti. We calculatedthe compositionof the parentmagma for the shield- buildingstage of volcanism,assuming that it was in equilibriumwith Fo89olivine and that the mostprimitive aphyric lavas were derived from thisparent by the crystallizationof olivine alone. The majorityof the shieldlavas represent 25 to 50% crystallizationof thisparent magma, but the mostevolved lavas represent about 70% crystallization.From over 50 analyzedflow unitswe recognizea quasi-periodicevolution of lava compositionswithin the early,robust period of volcanicactivity, which we interpretas regular recharge of themagma chamber (approximately every25 + 10 kyr). Volcanicevolution on Tahiti is similarto the classicHawaiian pattern. As the shield-buildingstage waned, the lavasbecame more silica undersaturated and isotopic ratios of the lavasbecame more MORB-like. We proposethat the Societyplume is radially zoneddue to entrainmentof a sheathof viscouslycoupled, depleted mantle surrounding a centralcore of deeper mantlematerial. All partsof the risingplume melt, but the thermaland compositionalradial gradientensures that greaterproportions of meltingoccur over the plume center than its margins. The changingcomposition of Tahitianmagmas results from lithosphericmotion over this zoned plume.Magmas erupted during the main shield-buildingstage are derived mainly from the hot, incompatibleelement-enriched central zone of theplume; late-stage magmas are derived from the cooler,incompatible element-depleted, viscously coupled sheath. A correlationbetween Pb/Ce and isotoperatios suggests that the Societyplume contains deeply recycled continental material. Introduction southcentral region known as French Polynesia.Here, several separate, subparallel island chains strike west-northwest, Linear chains of volcanic islands and seamounts are a aligned in the direction of Pacific plate motion. Young to conspicuousfeature of the Pacific Oceanfloor, especiallyits active volcanism occurs at the southeast end of each of these groups. It is now generally accepted that such volcanic Copyright 1994 by the AmericanGeophysical Union. lineaments are the consequenceof plate motion over long- Paper number 94JB00991. lived hotspots, which are proposed to be sublithospheric 0148-0227/94/94JB-00991 $05.00 thermal anomaliesmaintained by rising convectiveplumes of deeper mantle material [Morgan, 1971, 1972]. 24,341 24,342 DUNCAN ET AL.: GEOCHEMICAL EVOLUTION OF TAHITI Hotspot-related oceanic islands are also significant as radiometric dating has shown that island ages become geochemical records of mantle-lithosphereinteraction, mantle progressively older to the northwest, at a rate of 10 to 11 melting and magma transportprocesses, and the temporal and cm/yr [Duncan and McDougall, 1976]. The orientationof the spatial scales of mantle heterogeneity.In contrastto the first- chain and the age distribution of the volcanism are compatible order uniformity of compositionsof mid-ocean ridge basalts with Pacific plate motion over a stationary hotspot [Duncan (MORBs), ocean island basalts (OIBs) show tremendous and Clague, 1985]. variability. The compositionalrange of volcanic productsin Tahiti, the largest of these islands, lies near the southeast French Polynesia reflects melt contributions from several end of the chain. This island comprisestwo major coalesced mantle reservoirs that have remained isolated from one another volcanoes,aligned NW-SE, parallel with the general trend of for periods appropriate to mixing times for whole mantle the Society hotspot track. Volcanic activity here ceased at convection [Gurnis and Davies, 1986]. The origin of mantle about0.2 Ma [Dymond, 1975; Duncan and McDougall, 1976; heterogeneities may lie in melting and fractionation Becker et al., 1974; Gillot et al., 1993]. The present-day processes, recycling of lithosphere, metasomatism, and position of the hotspot southeastof Tahiti is indicated by the mantle-lithosphere assimilation. However, understandingthe young island of Mehetia (0.0 to 0.3 Ma [Binard et al., 1993; nature and significanceof distinct sourcesfor melt generation R. Duncan, unpublisheddata, 1993]), intense seismicactivity requires resolution of the volcanic histories of individual [Talandier and Okal, 1984], and active submarine volcanoes oceanic islands. [Chemineeet al., 1989; Devey et al., 1990]. This paper describes the volcanic history of Tahiti, the Tahiti is roughly 30 km wide and 55 km along the axis of largest island in the Society Islands chain. We begin with the two volcanoes and has a total volume of about 6 x 104 km3, radiometric dating to establish the time frame of volcanic of which about 2% lies above sea level (Figure 1). The activity. Next, we present whole rock and mineral major and volcanoesare composedpredominantly of basaltic lava flows, trace element abundancesfrom which we infer parent magma but pyroclastic rocks related to localized phreatic eruptions compositions.Isotopic data are used to evaluate mantle source occur in coastal outcrops,and subordinatedifferentiated rocks variability throughout the constructionof Tahiti. Finally, we (hawaiite-mugearite-trachyte-phonoliteseries) are common as assess fractionation, recharge, and assimilation effects in a plugs, sills, and irregular flows [DeNeufbourg, 1965]. 700-m-thick section in the Punaruu Valley. We combine Remnants of the original shield ("planeze") are separatedby compositional variations with age determinations to estimate deeply incised, radial valleys that provide spectacular recharge and fractionation rates. topographic relief (-2 km) and offer extraordinary accessto the volcanic stratigraphy. Gillot et al. [1993] reported east- west trending rift zones that cut the shield and evidence of Geological Setting and Previous Work caldera collapse at the main eruptive centers. Posterosional eruptions from these centers have partially filled old river The Society Islands chain is a -700-km lineament valleys ("remplissage des vallees"). Tahiti is remarkable in composedof islands, atolls, and seamounts,all of which are displaying the most extensive suite of plutonic rocks among predominantly volcanic. Individual volcanic centers rise Pacific islands. These are found in the eroded centers of both separatelyfrom abyssaldepths of 3.5 to 4 km below sea level volcanoes, having crystallized some 2 to 3 km below the and were erupted onto -60 Ma oceanic lithosphere.Previous original summitsof the shields[Williams, 1933]. Tahiti Nui i Tahiti Iti ß .. ß . ß ß ß ß 10 km "'. ." ß , Figure 1. Map of Tahiti showingmajor featuresmentioned in the text. Heavy shadingindicates areas of plutonic rocks. Light shadingis the area of oldest central lavas. Dotted lines are flinging reefs. Sample locations(latitude, longitude,and elevation)are listed in Table A1. Crossmarks a xenolith locationdiscussed in the text. DUNCAN ET AL.: GEOCHEMICAL EVOLUTION OF TAHITI 24,343 Early geologic and petrologic studies [Lacroix, 1910; (sample numbers other than T73-, T74-, and T85-) are those Marshall, 1915; Williams, 1933; DeNeufbourg, 1965; describedby Chauvin et al. [1990]. McBirney and Aoki, 1968] established that most Tahitian The thick sequenceof seawarddipping
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