Contributions to ., Contrib Mineral Petrol (1986) 5)?:?60-268 Mineralogy and Petrology Springer-Verlag 1986 F . ,. Geochemistry and origin of basaltic lavas from Marquesas Archipelago, French Polynesfa J.M. Liotard', H.G. Barsczu~~.~,C. Dupuy3, and J. Dostal' Laboratoire de Pttrologic, U.S.T.L., Place E. Bataillon, 34060 Montpellier Cedex, France Ofice de la Récherche Scientifique et Technique Outre-Mcr Papeetc/Tahiti, French Polynesia Centre Geologiquc et Gkophysique, U.S.T.L., Montpellier, France Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada Abstract. The Marquesas Archipelago, a volcanic chain in and McDougall 1974) range rrom 3.85 my. nt Nuku Ifiw to 1.35 French Polynesia (south-central Pacific Ocean), is predomi- m.y. ;it Falu Hiva indicating :I decrease of age in ;I SE direction nantly composed of alkalic, transitional and tholeiitic ba- and a rate ol'migration of volcanic activity of9.9 cm,ycar. Further- salts. The variation trends in these intrnplate basaltic rocks more. in two islands (Nuku Hiva and Iliva Oa) for which both imply that the magmas were derived from different upper age determinations anti whole-rock analyses arc avnilable. the de- gree of undersaturation of the basalts increases with the decreasing mantle sources. Model calculations using the total inverse age of the volcanic rocks. method show that the peridotite source of most Marquesas The petrography and chemical mineralogy havc been described basalts was enriched in incompatible elements compared for several islands of the Archipelago (Bishop and Woolley 1973: to a primordial mantle and had higher than chondritic ra- Maury 1976: Brousse and Guille 1975. 1978: Brousse ct al. 1978). tios of several elements such as La/Yb, Ti/V and PlCe. The dominant rock type is olivine basalt with subordinate and A metasomatic enrichment event is suggested by the se- variable amounts of differentiated rocks including trachybasalt. quence of element enrichment in the source relative to the trachyte and phonolite (e.g. Ua Pou island). primordial mantle (Ba>Nb> La>Ce>Sr>Sm> Eu> In this study. the analyzed samples have basaltic composition Zr>Hf>Ti>Y>Yb). On the other hand, some lavas in- with either porphyritic or hyalophilic and subaphyric textures. The phenocryst phases are olivine (Fo 86-77) containing inclusions of cluding tholeiites of Ua Pou and alkalic basalts of Hiva Cr-spinel. and clinopyroxcnc with frequent oscillatory sector zon- Oa, were probably derived from relatively depleted upper ing. The compositon of clinopyroxene is highly variable (diopside, mantle. In some islands such as Hatutu. the different types augite and salite) but is compositionally distinct among the three of basalts were generated from sources with rather similar basaltic types (tholeiitic, transitional and alkalic) similar to clinopy- compositions. The residual phases of the Marquesas mag- roxenes of Hawaii (Fodor and Keil 1975). In addition to olivine mas included garnet. The sources of these magmas were and clinopyroxene. the groundmass contains mainly Fe-Ti oxides. similar in trace element chemistry to the oceanic mantle glass and occasionally plagioclase. In sonic samples. patches of below Hawaii. devitrified glass have been transformed to chlorite whilc in other rocks. alkali feldspar and:or phlogopite appear as interstitial phases. Introduction Analytic methods Mid-ocean ridge basalts (MORB) have been extensively Seventy-nine samplcs were analyzed for major elements and Li. studied during the last fifteen years but relatively little mod- Rb, Sr. Ra. V. Cr, Co, Ni, Cu and Zn by atomic absorption. _erntraceelem-ent_ta ue available on .oceanic island baL Twenty-six samples were then-selected from this-suite Tor thedeter--- -- salts (OIB). Since OIB are considered to be generated at mination of additional trace elements. Rare earth elmients (REE). greater depths, their study complements that of MORB. Sc, Hf and Th were dctcrmincd by instrumental neutron activation This paper presents some data on the trace element dis- and Y, Zr and Nb by X-ray fluorcsccnce. The prccision and accura- tribution in volcanic rocks of the main islands of the Mar- cy of the trace elemcnt data were given by Dostal et al. (1983). quesas Archipelago, a volcanic island chain in French Po- lynesia, and provides constraints for their mantle source. Geochemistry .. .. _- Majo,. c~l~Jl7l~~llIS Geological7 P.-.-,.- and petrographic. .*..__-."I notes-"-I. .e- --I - ... -- "__ -- -- The Marquesas Archipelago, located in the south-central Pacific According to the normative composition (assuming Fe3+i Ocean consists of 12 islands and several sea-mounts which are Fe2+ --0.15), the rocks (Table 1) range from basunites the tops of Inrge subterranean volcanoes rising more than 4,000 m (Ne>S%) to olivine tholeiite, and in Ua Pou island to from the Paleocene oceanic crust (Fig. 1). Their arrangement is quartz normative basalts (Q - 1 'YO).Although some samples approximately parallel to other volcanic archipelagoes of the east- have high content of H,O, most rocks are without any ern Pacific and is perpendiculnr to the East Pacific rise. mineralogical features of alteriition. The positive correla- With the exception of Eiao island, which has been dated at 6.3 to 5.3 m.y. (Brousse and Bellon 1974), the K/Ar ages (Duncan tion between mobile and immobile incompntible elcments suggests that the rocks retained their primary inaginatic distribution of elements. ORSTOM Fonds Documentaire NOE cc34344- ex4 Cote t u 261 k J > 1 142"W 140" 1 I' 136"V I I te u.. 1 Fig. 1. Map of the Marquesas Archipelago. Contour lines drawn at 1,000 m intervals show simplified bathymetry (after CEBCO Map 297, Centre National Exploitation des Oceans, Paris, 1973). Square on insert map shows location of Marquesas islands in the Pacific Ocean - "_ --Using the-classification of Feigenson et a1,(1383), the tholeiites can be subdivided on the (Na20+K20) vs. SO2 diagram into two groups: a) tholeiitic basalts which lie in Ni and Co decrease with the decreasing [Mg] ratio. but the tholeiitic field and b) transitional basalts which plot for a given value of this parameter, the contents of these into the alkalic field. Tholeiites are mainly found in the elements are not distinct for individual islands and mag- northern islands (Eiao, Hatutu and Ua Pou) while transi- matic types. Most samples have a 10" x Ni/Mg ratio be- tional basalts predominate in the southern ones (Hiva Oa tween 50 and 65 and may represent a near-primary magma and Fatu Huku). The [Mg] ratio (Mg/Mg+FeZ+with Fe3* derived from a mantle source with olivine Fo 87-90. For /Fe2*=0.15) of the basalts ranges between 0.78 and 0.39 rocks with [Mg] 50.55, the Cr/Ni ratio varies between 1.2 - indicating that they have undergone variable dëgrees-of and 2.0 -iñ-tiäni;ifibna'l"âÏGd~lFb?ZZltsd' i: 'higxe? u low-pressure fractionation. A decrease of [Mg] up to 0.55 in tholeiites (Cr/Ni= 1.4 to 3.0). In rocks with [Mg]<0.55. is accompanied by an increase of AI, Ca, Ti and P while the Cr/Ni ratio decreases with differentiation in all three Fe remains constant suggesting that the differentiation is magmatic series indicating that Cr becomes more compati- dominated by olivine. The influence of clinopyroxene is ble than Ni and thus implying the influence of clinopyrox- indicated by the steeper increase of AI compared to that ene and/or Cr-spinel. of Ca. In the latter stages of differentiation in the transition- 'J increases with differentiation in tholeiitic and transi- al and alkalic basalts, clinopyroxene fractionation predomi- tional basalts from 240 to 380 ppm. It also increases in nates as implied by the steady decrease of Ca while AI alkalic basalts from 260 to 400 ppm up to [Mg]=0.55 then continues to increase with differentiation. decreases, indicating some Fe-Ti oxide fractionation. The , 262 I I 11 Table 1. Representative major and trace element compositions or basalts from Marquesas Archipelago *Y EIAO HATUTU HIVAOA Th Th Alk Tr Alk REF: 6567 6559 6558 6465 6458 6466 6456 6457 6575 6577 6589 6570 6574 sio* 44.75 47.90 47.90 45.88 48.47 46.68 45.08 46.48 45.79 46.18 43.33 43.45 45.6 I AlzOj 10.70 12.40 12.80 9.05 12.58 13.00 10.45 12.00 9.71 11.44 10.57 10.73 14.25 Fe203 13.54 12.73 13.02 11.97 12.50 12.1 1 13.33 12.28 14.15 13.33 14.33 13.9 1 12.92 M nO 0.1 7 0.17 0.17 0.18 0.18 0.17 0.19 0.17 0.18 0.17 0.18 0.18 o. 16 MgO 14.63 8.50 8.00 16.15 7.18 7.30 15.20 11.31 15.60 10.80 13.49 12.10 6.56 Ca0 7.85 10.87 10.95 9.80 1 1.70 11.73 8.51 10.82 8.68 9.47 9.43 10.97 8.Y8 Na20 1.88 2.29 2.45 1.79 2.50 2.57 2.04 2.47 2.02 2.37 2.59 1.81 2.71 K2O 0.73 0.67 0.76 0.85 0.76 0.71 1.10 1 .O4 1 .o0 1.23 0.76 1.31 1.92 Tio2 2.65 2.98 3.00 2.40 3.09 3.25 2.78 3.03 2.70 3.28 3.59 3.35 3.68 P2Os 0.43 0.44 0.44 0.34 0.40 0.50 0.42 0.42 0.35 0.45 0.50 0.51 0.64 H20C 1 .so 0.02 0.23 0.92 0.33 0.67 0.07 0.02 0.02 0.60 0.21 0.83 1.22 H2O- 0.50 0.49 0.35 0.38 0.80 0.46 0.56 0.21 0.21 0.36 O.