Geology

doi: 10.1130/0091-7613(1997)025<0611:EFACFT>2.3.CO;2 1997;25;611-614Geology

 J. I. Wendt, M. Regelous, K. D. Collerson and A. Ewart northern TongaEvidence for a contribution from two mantle plumes to island-arc lavas from  

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ABSTRACTLavas from the islands of Tafahi and Niuatoputapu, at the north-

ern end of the active Tonga-Kermadec arc in the southwest Pacific,were erupted at a convergent plate margin, yet they can be shown tocontain a contribution from two different mantle plumes. High con-centrations of Nb relative to other high field strength elements in theselavas, compared to other Tonga lavas, reflect an ocean island basaltcomponent in the mantle wedge derived from the nearby Samoa man-tle plume. Pb isotope compositions indicate that most of the Pb in theselavas is derived from the oceanic crust of the plume-generatedLouisville Seamount Chain, which is being subducted beneath theTonga arc. These two plume components were thus introduced into thearc lavas in very different ways and provide insight into upper-mantledynamics and magma-generation processes occurring in an activearc–back-arc system.

INTRODUCTIONThe Tonga-Kermadec arc is an ~3000-km-long chain of active volca-

noes in the southwest Pacific (Fig. 1). Volcanism is related to the subductionof the Pacific plate westward beneath the Indo-Australian plate, at a rate of5 to 20 cm/yr. The northern (Tonga) and southern (Kermadec) segments ofthe arc are separated by a nonvolcanic zone corresponding to the intersec-tion of the trench with an aseismic ridge (the Louisville Seamount Chain)that is being subducted beneath the Tonga arc. Behind the arc, active spread-ing is taking place in the Lau Basin–Havre Trough back-arc system. Theplume-generated intraplate volcanic islands of Samoa are situated on the Pa-cific plate at the northern end of the Tonga arc.

The geochemistry of Pleistocene-Holocene lavas from the Tonga-Ker-madec arc was discussed in detail by Ewart and Hawkesworth (1987).Tonga lavas include low-K andesites and basaltic andesites, and lesseramounts of low-K rhyolite and dacite. All lavas have unusually low concen-trations of incompatible elements, and this characteristic has been attributedto previous melt-extraction events occurring in the back arc and resulting indepletion of the mantle wedge beneath the arc (Ewart and Hawkesworth,1987). The degree of depletion increases northward along the arc, presum-ably reflecting the increasing degree of extension in the northern part of theLau Basin. Compared to mid-ocean ridge basalt (MORB), all Tonga andKermadec lavas display the enrichment in large-ion lithophile elements(LILEs) and light rare earth elements relative to high field strength elements(HFSEs), which is characteristic of convergent plate margin lavas.

This paper focuses on the geochemistry and petrogenesis of lavas fromthe two islands at the northernmost end of the Tonga segment of the arc,Tafahi and Niuatoputapu. The lavas from these islands are highly unusual inthat they preserve the geochemical fingerprints of two different mantleplumes. New trace element and Sr and Pb isotope data for Tafahi and Niu-atoputapu lavas are presented in Table 1.

INFLUENCE OF THE LOUISVILLE PLUMESubduction-related magmatism is thought to occur in response to the

addition of fluids derived from dehydrating sediments and/or altered oce-anic crust in the subducting slab to the subarc mantle wedge. Water-solubletrace elements, in particular the LILEs, are transported from the slab into themantle wedge by fluids, and melting occurs when these fluids reach mantlethat is hot enough to melt (e.g., Gill, 1981). This model can account for the

characteristic LILE enrichment and the presence of 10Be derived from sub-ducted sediment in many arc lavas.

Pb is highly soluble in aqueous fluids (e.g., Keppler, 1996). During thesubduction process, Pb is added to the mantle wedge by the fluids derivedfrom the subducting plate, resulting in the characteristic high Pb/Ce andhigh 207Pb/204Pb ratios of most arc lavas compared to oceanic basalts. Thehigh Pb/Ce ratios of Tafahi and Niuatoputapu lavas compared to those ofMORB (Fig. 2) imply that ~90% of the total Pb in these lavas is derivedfrom the slab, assuming that all of the Ce in the lavas is derived from themantle wedge. Tonga lavas containing the most slab-derived Pb have theleast radiogenic Pb isotope compositions, in particular low 207Pb/204Pb ra-tios (Regelous et al., 1996). In contrast, sediments cored from the Pacificplate close to the Tonga trench have high 207Pb/204Pb (Fig. 3) and high Pbconcentrations, indicating that subducted sediment does not contribute sig-nificantly to the Pb budget of Tonga arc lavas. This finding may reflect therelatively thin layer of sediment (~200 m) that is being subducted along withthe Pacific plate at the Tonga Trench. The slab-derived Pb component in

Geology; July 1997; v. 25; no. 7; p. 611–614; 4 figures; 1 table. 611

Evidence for a contribution from two mantle plumes to island-arclavas from northern TongaJ. I. WendtM. RegelousK. D. CollersonmmDepartment of Earth Sciences, University of Queensland, St. Lucia, Queensland 4072, Australia

A. Ewart

Figure 1.Simplified tectonic map of Tonga region,showing locations men-tioned in text.1—Tafahi,2—Niuatoputapu,3—Fonualei,4—Niua fo‘ou,5—Peggy Ridge, 6—Horne Island, 7—Rochambeau Bank, 8—Tutuila.

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Tonga lavas has a Pacific Ocean–type MORB Pb isotope signature(Regelous et al., 1996), whereas geochemical studies of lavas from the LauBasin behind the Tonga arc have shown that the uppermost mantle beneaththe Lau Basin consists of Indian Ocean–type MORB mantle (Hergt andHawkesworth, 1994). The dominant source of the slab-derived Pb in Tongalavas is thus the altered basaltic crust of the subducting Pacific plate.

Compared to other Tonga lavas, lavas from Tafahi and Niuatoputapuhave higher 206Pb/204Pb ratios (Fig. 3), unlike Pacific sediments or sub-ducting Pacific ocean crust, but similar to basalts dredged from theLouisville Seamount Chain (Cheng et al., 1987), the northern end of whichhas been subducted beneath the Tonga segment of the arc. The Pb budget ofTafahi and Niuatoputapu lavas is therefore dominated by the contributionfrom the altered oceanic crust of the plume-generated Louisville SeamountChain.

Volcanic activity associated with the Louisville plume has waned overthe past 10 m.y., and as a result, the present-day location of the Louisvilleplume is unclear. However, plate reconstructions place the plume at ~lat52°S, long 140°W, close to the Heezen fracture zone (Watts et al., 1988).The age variation of the seamounts along the Louisville Seamount Chain(Watts et al., 1988) implies that the seamounts currently beneath the north-ern end of the Tonga arc formed at 80–90 Ma. The Pb in Tafahi and Niu-atoputapu lavas is thus derived indirectly from the Louisville plume, via

612 GEOLOGY, July 1997

Figure 2. Zr/Nb and Pb/Ce ratios of Tonga lavas. Lavas from islandssouth of Tafahi and Niuatoputapu have Zr/Nb ratios similar to those oflavas from Lau Basin, but higher Pb/Ce ratios because of subductioninput of Pb.Vertical lines indicate percentage of slab-derived Pb. Lavasfrom back-arc volcano of Niua fo‘ou, and from Tafahi and Niuatoputapuat northern end of Tonga arc have lower Zr/Nb ratios, reflecting ocean-island basalt component in mantle wedge beneath northern end ofTonga-Lau arc–back-arc system. Data for Lau Basin spreading centersare from Pearce et al. (1995); Samoa data are from Newsom et al. (1986)and Palacz and Saunders (1986).

Figure 3. Pb isotope data for Tafahi and Niuatoputapu lavas and Pacificsediments. Data for lavas from Samoa (Wright and White, 1987) andLouisville Seamount Chain (Cheng et al., 1987) are shown for compari-son. Lavas from Tafahi and Niuatoputapu have Pb isotope compositionssimilar to those of basalts from Louisville Seamount Chain. Lavas fromother Tonga islands have Pb isotope compositions that reflect mixingbetween subducting Pacific oceanic crust and Pacific sediment.MORB—mid-ocean ridge basalt.

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plume-generated seamount lavas that were erupted onto the Pacific plate atabout 85 Ma and subducted at the Tonga trench at about 7–9 Ma.

INFLUENCE OF THE SAMOA MANTLE PLUMEIn contrast to the LILEs, the HFSEs such as Zr and Nb are relatively

insoluble in aqueous fluids (e.g., Keppler, 1996). In arc lavas, therefore,these elements are derived predominantly from the mantle wedge, and theirrelative concentrations reflect the composition of the mantle wedge beneaththe arc. Zr/Nb ratios for Tonga lavas from all islands other than those ofTafahi and Niuatoputapu range from 40 to 120, similar to the range for lavasfrom Lau Basin spreading centers (Fig. 2), and similar to MORB world-wide. Lavas from Tafahi and Niuatoputapu at the northern end of the archave significantly lower Zr/Nb ratios (16–21). This Zr-Nb fractionation can-not be attributed to the earlier melt-extraction events that resulted in the in-creasingly depleted mantle wedge beneath the northern end of the Tonga arc(Ewart and Hawkesworth, 1987) because Nb is more incompatible than Zrin mantle minerals and thus residues of partial melting will have higherZr/Nb ratios than fertile mantle. That no correlation of Zr/Nb with Pb/Ce isseen in Tonga lavas (Fig. 2) indicates that the low Zr/Nb ratios of Tafahi andNiuatoputapu lavas were not created during the subduction process. In ad-dition, lavas from the back-arc volcano of Niua fo‘ou in the northern LauBasin also have lower Zr/Nb ratios than lavas from the Lau Basin spreadingcenters to the south (Fig. 2). These back-arc lavas have low Pb/Ce, consis-tent with much less trace element imput from the slab (Ewart andHawkesworth, 1987), and therefore the low Zr/Nb ratios of Niua fo‘ou lavasmust have been inherited from the mantle. The low Zr/Nb ratios of Tafahi,Niuatoputapu, and Niua fo‘ou lavas therefore reflect low Zr/Nb in the up-permost mantle beneath the northern end of the arc–back-arc system.

The low Zr/Nb ratios of northern Tonga arc–back-arc lavas are inter-preted to reflect the influence of the mantle plume responsible for magma-tism on the Samoa Islands, which lie on the Pacific plate at the northern endof the Tonga arc. Ocean island basalts (OIBs) typically have high concen-trations of Nb relative to other HFSEs compared to MORB and island-arclavas. Shield and posterosional lavas from the Samoa islands have Zr/Nb ra-tios of 3–7 (Fig. 2). Northern Tonga arc lavas have intermediate ratios, dueto mixing of Samoa OIB mantle with normal MORB mantle beneath thenorthern end of the arc.

The age progression of shield volcanism along the length of the Samoaisland chain (Duncan, 1985; Natland and Turner, 1985) suggests that theSamoa plume is located ~150 km east of the island of Tutuila (Fig. 1). Thepresence of a Samoa plume component in the highly depleted northernTonga arc lavas suggests that depletion of the mantle wedge occurred afterthe OIB mantle component was mixed with the normal-MORB upper man-tle. As a result, the geochemical signature of the Samoa plume can be de-tected only by those elements such as the HFSEs that are derived from themantle wedge, not by the Pb or Sr isotope systematics that are dominated bythe contribution from the subducting plate. Thus the different input from theSamoa and Louisville plumes to these arc lavas can be resolved because ofthe decoupled behavior of water-soluble and water-insoluble elements insubduction zones.

REGIONAL DISTRIBUTION OF SAMOA OIB MANTLEThe OIB signal observed in arc lavas from the northern end of the

Tonga arc has also been detected in lavas from the back-arc region. 3He/4He(R) ratios of up to 22 times the atmospheric value (i.e., R/RA ≤ 22) have beenreported for lavas from Rochambeau Bank in the northern Lau Basin(Poreda, 1985), similar to the high values reported for Samoan lavas (R/RA= 10 to 24, Farley et al., 1992), and distinct from values for lavas from else-where in the basin (R/RA < 10, Honda et al., 1993, and references therein).Volpe et al. (1988) showed that young lavas from the northeastern part of theLau Basin (east of Peggy Ridge) have more radiogenic Sr and Nd isotopecompositions than lavas from elsewhere in the basin, and suggested that thisdifference reflected the influence of the Samoa plume. In addition to theirlow Zr/Nb ratios, lavas from the back-arc island of Niua fo‘ou have rela-

tively radiogenic Sr and high 208Pb/204Pb ratios (Fig. 3), which has been ar-gued to reflect a contribution from the Samoa mantle plume (Ewart andHawkesworth, 1987). An OIB signature in post–3 Ma lavas from the islandof Fiji has also been attributed to the influence of the Samoa plume (Gill andWhelan, 1989), and Price et al. (1990) and Bach et al. (1996) argued for anOIB component in lavas from as far west as the North Fiji Basin. Cole et al.(1990) showed that the youngest lavas erupted at the northern end of the LauRidge (the Mago Volcanic Group; 0.3–2.0 Ma) have OIB characteristics.New data presented in this paper show that the Samoa plume componentcan also be detected in northern Tonga arc lavas and extends southward tobetween the islands of Niuatoputapu and Fonualei.

IMPLICATIONS FOR MANTLE DYNAMICSSeveral lines of evidence suggest that the influx of Samoa OIB mantle

beneath the northern end of the Lau Basin and Tonga arc may have occurredrelatively recently. Gill and Whelan (1989) showed that the geochemistry ofmagmatism on Fiji changed from arc-like to OIB-like at about 3 Ma, and ar-gued that this change was due to influx of Samoa OIB mantle at that time.In contrast, an older lava dated as 4.9 Ma, dredged from close to Horne Is-land to the northwest of Fiji (and thus closer to the Samoa plume), has aZr/Nb ratio of ~40, similar to MORB (Duncan, 1985; Sinton et al., 1985).

The presence of small islands and seamounts on the Pacific plate to thewest of the Samoa islands indicates that the Samoa plume was active at leastat 15 Ma (Duncan, 1985). Why then is the Samoa plume signature only seenin lavas younger than 3–4 Ma in the Lau-Tonga region? Prior to collision ofthe Ontong-Java Plateau with the Vitiaz Trench in the late Miocene, the Pa-cific plate subducting at the Vitiaz Trench would have prevented the flow ofSamoa plume mantle southward and westward beneath the Tonga arc–LauBasin (Fig. 4). Lavas containing a Samoa plume component are restricted tothe area immediately south of the former Vitiaz Trench (Gill and Whelan,1989; Bach et al., 1996). Bach et al. (1996) proposed that the Pacific and In-dian plates, which are being subducted at the Tonga and New Hebrides

GEOLOGY, July 1997 613

Figure 4. Schematic tec-tonic evolution of Tongaregion, from 20 to 1 Ma(modified from Gill andWhelan, 1989). Prior to20 Ma, single westward-dipping subduction zoneexisted in area, prevent-ing east-west movementof upper mantle material.After subduction ceasedat Vitiaz Trench, mantlecontaining Samoa plumecomponent was able tomigrate westward be-neath northern part ofTonga-Lau arc–back-arcsystem (arrows).

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trenches, respectively, actively control the flow of material in the uppermantle. However, the subducting slabs will limit the flow of upper-mantlematerial in the direction normal to the trench axis (Gill and Whelan, 1989),and this restriction explains why Samoa plume mantle is seen only in lavasyounger than about 4 Ma. At that time, subduction of the Pacific plate at theVitiaz Trench segment immediately to the north of the Tonga Trench hadceased, and Pacific upper-mantle material was able to cross the Pacific-In-dian plate margin at the transform boundary to the northwest of the north-ern end of the Tonga Trench. The data presented in this paper place a south-ern limit on the penetration of the Samoa OIB mantle beneath theIndo-Australian plate (between the islands of Fonualei and Niuatoputapu).Assuming that the Samoa OIB mantle has been introduced since 4 Ma, thisimplies a mantle flow rate of 5–7 cm/yr, which is similar to the rate of5 cm/yr for north-south mantle shear flow beneath the Lau Basin that wasestimated from seismic data by Giardini and Woodhouse (1986).

CONCLUSIONSLavas from the northern Tonga islands of Tafahi and Niuatoputapu are

highly unusual in that they contain geochemical evidence for a contributionfrom two different mantle plumes and yet were erupted at a convergent platemargin. These components are derived from the Samoa and Louisvilleplumes and were introduced into the arc lavas in very different ways. Thegeochemical signature of the two plume components can be resolved be-cause of the decoupled behavior of water-soluble and water-insoluble ele-ments in subduction zones. The mantle wedge beneath the northern end ofthe Tonga arc contains a component of OIB mantle from the Samoa mantleplume, and this is reflected in high concentrations of Nb relative to otherHFSEs. This Samoa-type mantle was introduced into the upper mantle be-neath the Lau-Tonga region since 3–4 Ma, after subduction at the VitiazTrench ceased. The second plume component was introduced into the arclavas by fluids derived from the 80–90 Ma plume-generated crust of theLouisville Seamount Chain, which was subducted beneath the northern endof the Tonga-Kermadec arc at about 8 Ma.

ACKNOWLEDGMENTSFunded by grants from the Australian Research Council and University of

Queensland to Collerson and Ewart. We thank Y. Niu, J. X. Zhao, R. Frankland, andM. Bruce for help with the trace element analyses, and J. W. Hawkins and K. P.Jochum for reviews.

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Manuscript received November 8, 1996Revised manuscript received March 10, 1997Manuscript accepted March 26, 1997

614 Printed in U.S.A. GEOLOGY, July 1997

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