Contributions to Contrib Mineral Petrol (1988) 98: 293-302. Mineralogy and Petrology O Springer-Verlag 1988 Trace element evidence for the origin of ocean island basalts: an example from the Austral Islands (French Polynesia) 1 C. Dupuy', H.G. Barsczus2*I, J.M. Liotard3, and J. Dosta14 ' Centre Géologique et Géophysique, CNRS et Université des Sciences et Techniques du Languedoc, Eugène Bataillon, F-34060 Montpellier Cedex, France Centre ORSTOM de Tahiti, BP 529, Papeete, French Polynesia Laboratoire de Petrologie, Université des Sciences et Techniques du Languedoc, place Eugène Bataillon, F-34060 Montpellier Cedex, France ' Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3. Canada Abstract. The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are com- posed mainly of alkali basalts and basanites with subordi- nate amounts of olivine tholeiites and strongly undersatur- ated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace ele- ments indicate that the lavas were derived from a heteroge- neous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted. Introduction According to Hofmann and White (1980, 1982), Chase (1981) and Ringwood (1982, 1986), some within-plate ba- salts (WPB) including ocean island basalts (OIB) may have been generated by the melting of a large megalith formed by the accumulation of subducted oceanic lithosphere (ba- saltic crust and harzburgite) in the mantle for 0.5-2.0 b.y. Such a model postulates a connection between the petrogen- esis of WPB and island arc basalts (IAB). The worldwide isotopic similarity between IAB and OIB supports such a AUSTRAL ISLANDS genetic relationship (Morris and Hart 1983; White and Pat- chett 1984). The distribution of trace elements in OIB from the Austral Islands, French Polynesia, presented in this Fig. i A, B. Maps of the Austral Islands (after Duncan and study, provides further evidence for the close relationship McDougall 1976) showing their location with respect to other ar- between the two types of basalts. chipelagos in the South Central Pacific Ocean (A) and distribution of the islands within the chain (B). Dashed lines represent the frac- ture zones. The contour lines are 4000 m isobaths on Map A and Geological setting isobaths of 4000,2000 and 1000 m on Map B The Austral Islands are located in the South Central Pacific Ocean (Fig. 1) where they form the south-eastern part of Austral-Cook Islands chain. The Austral Islands stretch over more than 1500 km in a ESE-WNW direction, from about 29" S and 140" W to about a sea-floor of Paleocene age (Pitman et al. 1974) at a depth of 22" S and 155" W, subparallel to the Society and Tuamotu Islands more than 4000 m. A fracture zone running from ENE to WSW chains. They are composed of seven main islands and several sea- crosses the chain between the islands of Raivavae and Tubuai mounts, guyots and shoals. With the exception of the currently (Mammerickx et al. 1975). The K/Ar dating of the Austral Islands active submarine volcano, MacDonald, discovered in 1967 (Norris (Krummenacher and Noetzlin 1966; Dalrymple et al. 1975; Dun- and Johnson 1969), all the islands are extinct volcanoes rising from can and McDougall 1976; Mottay 1976; Barsczus 1980; Bellon et al. 1980; Matsuda et al. 1984; Turner and Jarrard 1982) show an overall age progression (Duncan and Clague 1985) from the Offprint requests to: C. Dupuy MacDonald seamount located at the southeastern end of the chain ORSTOM Fonds Documentaire 294 Table 1. Representative major and trace element compositions of basalts from Austral Islands MacDonald Marotiri MCD-202 MCD-064 MCD-201 MCD-079 MCD-130 MRT-004 MRT-203 MRT-001 6984 6970 6983 6974 6980 6991 6996 6988 TH TH AB BSN BSN AB BSN BSN Si02 47.12 46.81 45.86 42.60 42.77 42.60 41.45 43.75 A1203 33.53 15.16 10.95 10.54 13.38 10.37 10.10 18.17 Fe,03 11.67 12.08 11.70 13.20 13.70 12.71 12.63 11.43 MnO 0.16 0.1 6 0.15 0.17 0.17 0.16 0.18 0.16 Mg0 8.00 4.75 13.18 13.74 9.83 16.16 12.92 3.50 Ca0 12.65 10.53 11.90 11.23 10.43 10.55 12.40 9.91 Na,O 2.31 3.07 2.16 3.60 3.54 3.82 2.26 4.88 K2O 0.47 0.43 0.47 0.90 1.36 0.81 0.75 1.98 Tio2 2.33 2.52 2.59 3.69 3.84 2.56 3.06 2.97 P2OS 0.35 0.26 0.31 0.50 0.69 0.55 1.25 0.90 L.O.I. 0.56 3.82 0.64 0.04 0.04 1.98 2.31 1.66 Total 99.15 99.59 99.91 100.21 99.75 100.27 99.30 99.31 IMgl 0.61 0.46 0.72 0.70 0.62 0.74 0.69 0.41 Nenor - - 1.9 15.5 12.6 4.9 7.8 17.1 HYnor 2.8 6.7 - - - - - - Li (PPm) 9 4 8 6 7 6 9 8 Rb 10 9 6 16 30 20 5 39 Sr 41 1 341 368 575 757 600 545 1215 Ba 155 114 151 284 381 492 242 642 sc 34 24 35 28 22 23 30 5 v 271 238 237 297 289 240 251 96 Cr 268 160 770 765 350 650 825 9 eo 123 45 63 70 56 89 99 60 Ni 73 102 304 362 173 497 356 32 CU 82 69 3 o9 70 37 65 76 108 Zn 110 118 110 128 139 1O0 117 315 Y 27 26 21 23 27 21 39 31 Zr 178 136 3 50 228 299 152 191 239 Nb 31 21 27 44 63 43 42 76 La 23.0 14.0 18.9 31.2 45.9 25.4 27.2 49.4 Ce 47.9 31.4 41.3 68.3 97.2 55.0 63.3 lQ5.7 Nd 27.1 18.8 22.4 35.7 47.8 27.0 30.5 51.8 Sm 6.51 5.03 5.51 7.67 9.65 5.99 6.22 9.72 Eu 2.10 1.72 1.78 2.36 2.90 1.98 1.97 3.25 Tb 0.98 0.93 0.82 0.98 I .31 0.91 0.83 I .40 Yb 1.72 1 .83 1.39 1.29 1.62 1.36 1.28 2.18 Lu 0.27 0.29 0.23 0.20 0.25 0.20 0.19 0.29 Hf 4.5 3.6 4.0 5.5 6.9 3.4 4.4 4.3 Th 2.5 3.4 3.9 3.3 5.2 3.2 3.2 5.8 TH= tholeiites: AB= alkali basalts: BSN= basanites; PT=phonolitic tephrites. PF=phonolitic foidites. [Mg]= (Mg/Mg+Fe2+) with Fe3*/Fe2+ assumed to 0.15. Ne,,,=nonnative nepheline. Hj.,,,=normative hypersthene. W-1 : Lo= 10.7, Ce=22.8, Ard=34.8, S?n=3.29. Eu=1.06, Tb=O.60, Yb=.2.05, Lu=0.34 to Rimatara located to the northwest. The latter island has a mini- spot theory may be observed in the same island or among different mum age in excess of 21 m.y. (Turner and Jarrard 1982). On the islands. basis of these ages, a hotspot origin for these islands was proposed (Dalrympk et al. 1975; Duncan and McDougall 1976). However, Petrography and mineralogy this age progression is far from regular especially at Rurutu where everal groups of ages (1.1-1.9, 8.4 and 31.7-12.2 m.y.) have been Descriptions of the petrography and mineralogy for most of the btained (Duncan and McDougall 1976) and at Tubuai where vol- islands have been prevously given by Lacroix (1927, 1928), Smith anic activity appears to span several millions of years (Bellon and Chubb (1927), Jeremine (1959), Aubert de la Rue (1959), Mot- 1et al. 1980). At least two hotspots. aligned with the direction of tay (3976), Maury et al. (1978), Brousse and Maury (1980) and the Pacific plate movement, would be required in order to explain Berger (3985). In general, the rocks closely resemble volcanic suites the observed jumps in ages in these islands. The age pattern can from Tahiti and Hawaii. The dominant rock type is olivine basalts also be explained by the activities of randomly distributed imma- associated with subordinate and variable amounts of difkrentiated ture mantle plumes in that region (Barsczus 1980; Keating et al. rocks such as phonolites and trachytes particularly in Tubuai, Rai- 1982). If different plumes tap different mantle reservoirs, rocks vavae and Rapa. Coarse-grained rocks have also been described with varying isotopic and geochemical features which are not in from the MacDonald seamount (Brousse and Richer des Forges accordance with the general age progression predicted by the hot- 1980) and Rapa (Chubb, 1927). c _”- ‘E‘ 295 Olivine basalts have either subaphyric, hyalophilic or porphyr- itic textures with phenocrysts of olivine (Fo 75-88) and clinopyrox- Rapa ene which has frequent oscillatory or sector zoning and highly variable core compositions (diopside - salite - augite).