Chemical Geology, 77 (1989) 1-18 1 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands Subducted and recycled lithosphere as the mantle - source of ocean island basalts from southern Polynesia, I central Pacific C. DUPUYl, H.G. BARSCZUS”’, J. DOSTAL3, P. VIDAL4 and J.-M. LIOTARD’ ‘Centre Géologique et Géophysique, C.N.R.S. et Université des Sciences et Techniques du Languedoc, F-34060 Montpellier Cédex (France) ‘Centre ORSTOM de Tahiti, Papeete (French Polynesia) 3Department of Geology, Saint Mary’s University, Halifax, N.S. B3H 3C3 (Canada) 4UA 10 C.N.R.S. et Université, F-63018 Clermont-Ferrand Cédex (France) (Received October 13,1988; revised and accepted April 4,1989) Abstract Dupuy, C., Barsczus, H.G., Dostal, J., Vidal, P. and Liotard, J.-M., 1989. Subducted and recycled lithosphere as the mantle source of ocean island basalts from southern Polynesia, central Pacific. Chem. Geol., 77: 1-18. The Marquesas, Society and Austral-Cook Islands, three volcanic chains in the central Pacific Ocean (French Polynesia), are composed mainly of alkali basalts, basanites and tholeiites, which have geochemical characteristics typical of ocean island basalts. The lavas from the Marquesas and Society Islands display generally chondritic ratios of highly incompatible trace elements and have higher s7Sr/s6Srthan the basalts from the Austral-Cook Islands which have many trace-element ratios similar to those of mid-ocean ridge basalts. This grouping probably reflects differences in the composition of an ancient subducted and recycled lithosphere incorporated into the mantle source of the Po- lynesian basalts. Compared to Marquesas and Society Islands basalts, the mantle source of the Austral-Cook Islands basalts contains refractory oceanic lithosphere from which a larger amount of basaltic melt was extracted during subduction. 1. Introduction the generation of OIB. Ringwood (1982,1986) has argued that the subducted oceanic litho- Ocean island basalts (OIB) are usually con- sphere made out of oceanic crust and refractory sidered to be generated by melting of an upper- harzburgite buckled and thickened at the base mantle source enriched in incompatible ele- of the upper mantle and subsequently formed ments by CO,-rich fluids or undersaturated large megaliths. Such peridotite diapirs which melts derived from the low-velocity zone (LVZ) are enriched in incompatible trace elements (e.g., Green, 1971; McCulloch et al., 1983; Hart, (ITE) by melts derived from the subducted 1988; Nelson et al., 1988). Alternatively, it has oceanic crust may become the source of OIB. been suggested (e.g., Hofmann and White, 1980, Although some recent findings suggest a more‘ 1982) that subduction of the oceanic litho- complex scenario (Kato et al., 1988), the latter sphere can produce compositionally distinct re- model has been invoked for basalts of the Aus- gions within the convecting mantle involved in tral Islands (French Polynesia); their mantle 0009-2541/89/$03.50 O 1989 Elsevier Science Publishers B.V. OR3TOM Fonds Documentaire 2 C. DUPUY ET AL. source was probably formed by mixing of the lands: at Rurutu both old (12 Ma) and young depleted upper mantle with subducted oceanic (1Ma) basalts have been reported (Dalrymple crust from which melts with island-arc basalt et al., 1975; Duncan and McDougall, 1976) as (IAB ) composition were previously extracted well as at Aitutaki ( N 1 and -8 Ma; Turner (Dupuy et al., 1988). and Jarrard, 1982). In this paper we present data which indicate The age of sea floor inferred from magnetic I I- that basalts from other Polynesian island chains lineations 6-34 present in this region might also be generated from an ancient sub- (CPCEMR, 1981) ranges from 20 Ma (east of ducted and recycled lithosphere. The new anal- Pitcairn) to more than 80 Ma (west of the So- yses confirm the existence of geochemical het- ciety Islands). Various geophysical data sug- erogeneities of the mantle source of basaltic gest that the mantle which underlies southern í rocks from the different Polynesian island Polynesia has anomalously high temperatures chains, which can be explained by the mixing (Nishimura and Forsyth, 1985; Calmant and of the oceanic crust and residual peridotite. Cazenave, 1986, 1987; Haxby and Weissel, 1986) and that the source fÔr the excess heat is 2. Geological notes located in the asthenosphere (McNutt and Fischer, 1987). Southern Polynesia in the central Pacific 3. Samples and analytical methods Ocean is composed of the Marquesas, Tua- motu, Pitcairn-Gambier, Society and Austral- From the set of MA and AC samples which Cook archipelagos (Fig. 1).These roughly NW- were previously analyzed for major and some SE-trending island chains are cross-cut by two trace elements (Liotard et al., 1986; Dupuy et major fossil fracture zones: the Marquesas and al., 1988), 32 representative basalts were se- Austral fracture zones. The islands are pre- lected for the determination of U and Ta. In dominantly made up of basalt (except the Tua- addition, 114 other basaltic samples from the motu Islands which are coral atolls) with sub- AC, MA and SO archipelagos, particularly from ordinate amounts of differentiated products the islands for which only very few data are such as phonolites and trachytes. The ages of available, were analyzed for major and 19 trace these volcanics are variable according to the is- elements. The samples were selected according land chain. The oldest recorded ages are 19.5 to their degree of freshness after inspection of Ma in the Austral-Cook (AC) Islands (Turner thin sections. The powders to be analyzed were and Jarrard, 1982), 4.5 Ma in the Society (SO) prepared by extracting centimeter-size frag- Islands (Duncan and McDougall, 1976), 6 Ma ments from coarsely crushed material. The in the Pitcairn-Gambier Islands (Bellon, 1974) fragments were washed in cold distilled water and -6 Ma in the Marquesas (MA) Islands in an ultrasonic bath. Up to 60 g of such rock (J.H. Cantagrel, pers. commun., 1988). In all fragments were ground to powder in an agate the island chains the ages decrease from NW to mill. SE and two of them have still active submarine Major elements and Li were determined by volcanism: Mehetia-Teahitia seamount region atomic absorption spectrometry; Rb, Sr, Ba, Zr, in the SO Islands (e.g., Talandier and Okal, Y and Nb by X-ray fluorescence and rare-earth 1984) and Macdonald Seamount in the AC Is- elements (REE), Hf, Th, Ta and U by instru- lands (Norris and Johnson, 1969). In most mental neutron activation. The precision and cases a typical correlation between island age accuracy of the trace-element analyses have 3 and distance to hot spot is apparent (Mc- been discussed elsewhere (Dostal et al., 1986). Dougall and Duncan, 1980),but two noticeable For most elements the precision is better than b exceptions have been observed in the AC Is- 4 5%.New analyses are reported in Table I. MANTLE SOURCE OF OCEAN ISLAND BASALTS 3 5' $ 10 20 3c I I ! 30' 160' 150' 140' 130'W Fig. 1. Location of the various Polynesian archipelagos in the south central Pacific Ocean. 4. Geochemistry olivine tholeiites, alkali basalts and basanites. However, the proportion of the basaltic types The geochemical characteristics of the ba- are variable: in the SO and MA Islands the salts from the Marquesas, Austral-Cook and amounts of olivine tholeiites and alkali basalts Society volcanic island chains have been de- appear to be equal, whereas alkali basalts and scribed by Dostal et al. (1982), Liotard et al. basanites are more abundant in the AC. All ba- (1986), Palacz and Saunders (1986), Dupuy et salts have trace-element features characteristic al. (1987,1988), and Vidal et al. (1987) and are of OIB: high contents of incompatible trace ele- briefly reviewed here. On the other hand, as only ments (ITE) and REE patterns marked by an very few data for the Cook Islands have been enrichment in light REE (LREE) and frac- previously reported (Palacz and Saunders, tionation of heavy REE (HREE). The ITE 1986),a more detailed description for the Cook concentrations vary within a large range and Islands basalts is given. The three island groups exhibit an increase both with differentiation will be referred to as AC (Austral-Cook), MA and degree of undersaturation (e.g., Liotard et (Marquesas) and SO (Society), respectively. al., 1986). The chondrite-normalized ITE pat-, In each island chain, the normative compo- terns and the corresponding element ratios are sition of the basalts usually indicates the pres- variable. For example, the Ba/Nb ratio varies ence of four magmatic types: quartz tholeiites, between 3 and 13 and distinguishes MA basalts 4 C. DUPUY ET AL. MANTLE SOURCE OF OCEAN ISLAND BASALTS 5 ow R w Y 6 C. DUPUY ET AL. a 8 C. DUPUY ET AL. Q5m m $23 Y i Co 2 m3 $23 2 N 1 1 s 1 MANTLE SOURCE OF OCEAN ISLAND BASALTS 9 TABLE I (continued) 7995 7996 7997 7998 7999 8000 8001 8002 8003 8004 8005 8006 8007 ATTl3l ATT132 ATT136 ATT138 ATT139 ATT144 ATT145 ATT146 ATT148 ATT149 ATT149X ATT152 ATT154 NE NE BSN NE BSN BSN BSN BSN BSN BSN NE NE NE SiOB 41.03 41.52 41.33 40.64 45.62 45.13 44.94 45.32 44.93 44.93 39.5 41.22 39.76 AIA 11.41 11.38 11.17 11.33 11.27 11.5 11.35 11.46 11.71 11.57 11.07 11.45 11.12 13.3 13.07 12.75 12.8 11.83 11.95 12.04 12.02 12 11.91 14.06 13 14.05 MnO 0.19 0.19 0.19 0.19 0.16 0.16 0.16 0.16 0.16 0.16 0.21 0.2 0.2 MgO 11.12 11.45 11.66 12.18 12.86 12.26 11.95 11.55 11.58 11.93 10.9 11.31 10.58 Ca0 11.56 11.8 11.83 12.1 9.85 10.05 10.2 9.94 10 10 12.56 11.75 11.66 NaaO 4.2 3.92 3.34 3.37 2.16 3.21 4 3.87 3.13 3.9 4.45 3.75 4.67 KZO 1.52 1.34 1.29 1.33 1.06 0.7 1.11 0.94 0.6 0.87 1.45 1.5 1.2 Tio, 2.3 2.23 2.2 2.21 2 2.15 2.13 2.13 2.14 2.06 2.35 2.19 2.58 Pz05 1.05 1.01 1.05 1.07 0.64 0.68 0.65 0.67 0.64 0.65 1.03 1.08 1.17 LOI 1.52 1.23 2.32 --1.95 2.19 1.8 0.7 -1.41 -2.43 1.18 1.5 1.65 2.13 Total 99.2 99.14 99.13 99.17 99.64 99.59 99.23 99.47 99.32 99.16 99.08 99.1 99.12 [Mgl 0.65 0.66 0.67 0.68 0.71 0.69 0.69 0.68 0.68 0.69 0.63 0.66 0.62 Li (ppm) 12 9 15 11 9 10 10 10 10 9 17 14 15 Rb 50 48 48 54 16 31 60 70 28 25 .