Transition Between Tholeiitic and Alkali Basalts: Petrographical and Geochemical Evidence from Fangataufa, Pacific Ocean, and Kerguelen, Indian Ocean

Home , Alkali basalt, Basanite, Macdonald seamount

Geochemical Journal, Vol. 28, pp. 489 to 515, 1994

Transition between tholeiitic and alkali basalts: Petrographical and geochemical evidence from Fangataufa, Pacific Ocean, and Kerguelen, Indian Ocean

JACQUES-MARIE BARDINTZEFF,1 HERVE LEYRIT,2 HERVE GUILLOU,3 GERARD GUILLE,4 BERNARD BONIN,1 ANDRE GIRET5 and ROBERT BROUSSE'

1Laboratoire de Petrographie-Volcanologie , CNRS-URA 1369, Universite de Paris-Sud, F-91405 Orsay, France 21 .G.A.L., 13 Boulevard de I'Hautil, F-95092 Cergy-Pontoise, France 3C .F.R., C.E.A.-C.N.R.S., F-91198 Gif-sur-Yvette, France 4C .E.A./L.D.G.-C.E.B. III, B.P.12, F-91680 Bruyeres le Chatel, France 5Laboratoire de Geologie-Petrologie , CNRS-URA 10, Universite Jean Monnet, 23 rue du Docteur Paul Michelon, F-42023 Saint-Etienne, France

(Received July 24, 1993; Accepted September 13 , 1994)

The transition from tholeiitic to alkali basalts, which is well established in the Hawaiian series, is exposed in numerous other oceanic islands. Two case studies are presented here, which illustrate the more general geochemical evolution with time from "subalkaline" to "alkaline" compositions: Fangataufa atoll (French Polynesia, Pacific Ocean) and Kerguelen archipelago (T.A.A.F., South Indian Ocean). At Fangataufa atoll, drillings by the French C.E.A. have reached the basaltic bedrock under coral cap rock formations. Two basalt types have been cored: tholeiites form the submarine volcanic sequence, while alkali basalts constitute the top of the submarine zone and the whole subaerial sequence. Tholeiites contain less than 0.9 wt% K20 with Na20/K20 ranging mostly between 4 and 8, while alkali basalts contain up to 1.8 wt% K20 with Na20/K2O ranging mostly between 2 and 4. Both suites underwent weak differentiation effects. In Kerguelen archipelago, two Miocene (26-6 Ma) magmatic episodes are recorded in the southeastern province (Ronarc'h and Jeanne d'Arc peninsulae). Lower Miocene mildly alkaline basalts (0.4-2.1 wt% K20 and Na20/K20 ranging between 2 and 4) are overlain by Upper Miocene highly alkaline basalts (1.4-3.7 wt% K20 and Na2O/K2O between 1 and 2). Lower Miocene magma type evolved towards trachyte compositions, whereas trachy-phonolite and phonolite compositions constitute the Upper Miocene residual melts. The thin transition zone with interstratified tholeiitic and alkali basalts, which is well exposed at Hawaii, was not observed in both Fangataufa and Kerguelen. These two case studies confirm that alkali contents increase with time within oceanic island basalts. Present data indicate that one single source is evidenced in the short-lived Fangataufa oceanic island, while more than one source was tapped in the long-lived Kerguelen magmatic system.

Project, 1981; hereafter referred to as BVSP). INTRODUCTION However, it obscures the complexity of mantle "Basalt" stands for "a volcanic rock consist sources and evolutionary processes involved in the ing essentially of calcic plagioclase and pyroxene. generation of basaltic melts. The status of basalt Olivine and minor foids or minor interstitial quartz as a primary melt, yet commonly assumed, has may also be present" (Le Maitre et al., 1989, even been questioned (for contrasting views, see p. 50). The simplicity of the definition accounts Stolper and Walker, 1980; McKenzie and Bickle, for the large distribution of basaltic volcanism on 1988). the terrestrial planets (Basaltic Volcanism Study The division of igneous rocks into subalkaline

489 490 J. M. Bardintzeff et al.

b m, t 4

FANGATAUFA

KERGUELEN

(a) Fig. 1. (a) Fangataufa atoll in Central Pacific Ocean and Kerguelen archipelago in South Indian Ocean. (b) Fangataufa atoll and location of drills mentioned in the present study. (c) Kerguelen archipelago. Areas of study are Ronarc'h and Jeanne d'Arc peninsulae.

and alkaline groups was first made by Iddings (Fig. 1(a)): Fangataufa atoll, French Polynesia, in (1892), followed by Harker (1896). The problem South-Central Pacific Ocean, and Kerguelen ar of different magma types is a long-lived matter of chipelago, T.A.A.F. (French Austral and Antarctic controversy (see Yoder and Tilley, 1962, p. 346 Territories), in South Indian Ocean. The two is 349). The concept of two or more separate primary lands originated through two contrasting processes: magmas was advocated by Kennedy and Anderson Fangataufa resulted from the activity about 13-10 (1938), followed by Kuno et al. (1957) in the Ma of a within-plate seamount, while Kerguelen specific case of Hawaiian lavas. was generated at the crest of the South-East Indian In oceanic islands, coexistence of tholeiite and Ridge 50 Ma and grew in within-plate settings alkali basalt was recognized and, since Powers through a long time-duration magmatic evolution. (1935), extensively studied in the Hawaiian ar chipelago. The term "tholeiite", though having THOLEIITE VS. ALKALI BASALT: caused considerable confusion (see Le Maitre et THE HAWAIIAN STATEMENT OF PROBLEM al., 1989, p. 121-122), is used hereafter with the sense of Kennedy (1933), formalizing the earlier The problem of the coexistence of two basaltic definition of Bailey et al. (1924). types (oceanic island tholeiite (OIT) and oceanic In this paper, we address the problem of alkali island alkali basalt (OIAB)) was studied in detail contents within basaltic rocks as a function of in Hawaiian islands. After the pioneering studies evolution with time. Two case studies illustrate of Powers (1935), Kuno et al. (1957) and Tilley the fundamental aspects of: (i) changing degrees and Scoon (1961) demonstrate that basaltic rocks of partial melting of one single mantle source, can be subdivided into two discrete groups. How (ii) tapping of discrete mantle sources. The two ever, through their extensive set of analytical data, islands studied are located in within-plate settings Macdonald and Katsura (1964) suggest that a

p Transition between tholeiitic and alkali basalts 491

138'47W 138°45W 138°43W

GEOEMPERUA

~ti! t KILO

22°13S r r

i r

mil \

1 \ l • MITRE 01

\ \ \ . GEOFO~C\• 1 1 1l

1 k / i ir v NATICE 01 / r r.~ / \ l 22°15 S / tk~~ 0 11 9r / (~/ r~ ~ llr \

<. l E 02 .3Gfi0 1

~ l\ r

TER ME S r 2 Km 22*17S N

(b)

Fig. 1. (continued).

complete chemical gradation from tholeiitic to al a highly alkaline "nephelinitic" suite is related kali basalts does exist and that both types are to the post-erosional stage. An additional feature interlayered in a thin transition zone. The two was provided by the determination of Loihi Sea magma suites evolved through fractional crystal mount as an early alkaline eruptive stage that pre lization. cedes the tholeiitic shield-building stage (Moore Clague et al. (1975) establish that: (i) the tho et al., 1982). This view was recently disputed by leiitic suite represents the main shield-building Chen et al. (1991) and Frey et al. (1991), who stage, (ii) the alkaline suite constitutes cap forma argue that the transition between tholeiitic and al tions attributed to the caldera-filling stage, and (iii) kali basalts can be related to the early stage of

1 492 J. M. Bardintzeff et al.

m a O a W p

s LORANCHET _n~5

PLATEAU CENTRAL

. 49° 30'S RONARC H

20 Km I JEANNE D ARC

(c)

Fig. 1. (continued).

the caldera formation. lapping of the two fields (Chen et al., 1991; Frey Both tholeiitic and alkali representative fields et al., 1991). CIPW-normative hypersthene can can be distinguished in the total alkalies versus not be completely discriminative, as some alkali silica (TAS) diagram with a diagonal boundary basalts may be hypersthene-bearing, and a con line expressed by: Si02 = 39 + 2.7 (Na20 + K20), tinuum exists between tholeiitic and alkaline for a range of Si02 contents of 42 to 55 wt% compositions. Nearly identical Tb/Ta, Hf/Ta, (Macdonald and Katsura, 1964; hereafter referred Nb/La, La/Ce, La/Sr, Hf/Sm, P/Nd, Ti/Eu, Zr/Hf to as M & K). Chemical contrasts between tho ratios in tholeiites and alkali basalts at Mauna leiites and alkali basalts induce, respectively, Loa (Frey et al., 1991) and Haleakala (Chen et al., presence and absence of CIPW-normative hyper 1991) have been argued as evidence of one single sthene. When plotted onto the Yoder and Tilley mantle source. (1962) nepheline-olivine-diopside-hypersthene quartz tetrahedron, tholeiites are located in the FANGATAUFA ATOLL: A CASE FOR ONE CIPW-normative hypersthene ± quartz field, while SINGLE SOURCE AND VARYING DEGREES OF alkali basalts spread in the CIPW-normative oli PARTIAL MELTING vine ± nepheline field (BVSP, 1981; see Fig. 7(a)). Since the M & K reference work, additional In French Polynesia, four NW-SE-trending sets of data on Hawaii archipelago resulted in the linear arrays of archipelagoes: Marquesas, enlargement and the more or less complete over Tuamotu (including Fangataufa)-Gambier, Society Transition between tholeiitic and alkali basalts 493

(including Tahiti), and Australes, result from the Pitcairn. The origin of the twin atoll of Mururoa, activity of four hot spots located at their south where only the alkaline suite is known, is a mat eastern edges. The younger islands in the south ter of debate (Bardintzeff et al., 1986; Guillou et eastern part are emerged volcanic islands, some al., 1990). of which could undergo renewed activity in the future (i.e., Mehetia, Society archipelago) . The Coexistence of alkaline and tholeiitic basalts older islands in the northern part are atolls covered Samples have been selected from eight drill by a calcareous coral cap. cores (Fig. 1(b)), situated either on the coral ring From the petrological point of view, French (Terme Sud, Echo 1, Echo 2, Geokilo, Geofox, Polynesia constitutes a rather complex area, where Geoempereur) or in the lagoon (Natice 01, Mitre various and contrasting magma types can coexist 01). Wholerock analyses have been performed on (Brousse, 1985; Brousse et al., 1990; Bardintzeff, 134 samples (Bardintzeff, 1981; Guillou, 1990; 1991). Some islands are purely alkaline (i.e., unpublished data, 1992), which yield losses on Mururoa atoll, situated at only 45 km in the NNW ignition (L.O.I.) lower than 3.5 wt%, with one of Fangataufa: Bardintzeff, 1981; Bardintzeff et exception at 4 wt%. Volcanic rocks have silica al., 1986; Maury et al., 1992), while others are contents of 45.2 to 50.2 wt% and Thornton and only tholeiitic (i.e., Eiao in Marquesas archipelago: Tuttle (1960) differentiation indices (D.I.) ranging Liotard et al., 1986; Gambier archipelago: Caroff between 18.1 and 45.3. They have been classified et al., 1993). The transition between tholeiitic and as basalts (D.I. < 35) and hawaiites (D.I. > 35). alkaline lavas has been observed in Marquesas and According to the chemical classification scheme Australes archipelagoes. of Le Bas et al. (1986) and percentage frequency distribution studies (Le Bas et al., 1992), all Geological setting samples are basalts, except for two trachybasalts Fangataufa is a rather small atoll, about 10 km which contain total alkali abundances higher than in size (Fig. 1(b)). Its polygonal shape could be 5 wt%. Only a weak differentiation can be dis related to the coexistence of several volcanic cerned. Cumulative oceanites, containing up to 16 centres active during the build up of the seamount vol.% olivine phenocrysts and 13.2 to 17.5 wt% (Bardintzeff, 1981; Guillou, 1990; Guille et al., MgO, have been found in drill holes at Terme 1993). The volcano rises above the 4000 m-deep Sud, Natice 01 and Mitre 01. They will not be Pacific ocean floor. considered in the present study. Numerous drillings performed by the French Tholeiitic and alkaline basalt-trachybasalt Atomic Energy Commission (C.E.A.) have reached suites were defined on the basis of: (i) petro the basaltic bedrock under the biocalcareous for graphical and mineralogical features of basic rocks, mations composed of 170 in-thick limestones (ii) major element chemistry (especially alkali and overlying 200 in-thick dolomites. The top of the TiO2 contents), (iii) their stratigraphic position basaltic bedrock is composed of a subaerial 60 within the volcanic pile, and (iv) the TAS fields m-thick volcanic sequence. Below, typically sub defined by Macdonald and Katsura (1964) at Ha marine volcanic rocks were cored: pillow lavas, waii. In the followings, we shall use preferentially autoclastic breccias and hyaloclastites (Varet and the criteria of petrographical and mineralogical Demange, 1980). features to distinguish tholeiitic, transitional and The aerial episodes yield eruptive ages of 11.5 alkali basalts. to 10.1 Ma, the submarine episodes 13.4 to 11.5 Both suites contain olivine, clinopyroxene and Ma and the total activity lasted 3.3 Myr (Guillou plagioclase phenocrysts and in the groundmass, et al., 1993a, b). The genesis of the atoll of olivine, clinopyroxene, plagioclase, spinel, mag Fangataufa is directly connected to the activity of netite and ilmenite (Bardintzeff, 1981; Guillou, a hot spot, presently located 100 km southwest of 1990). In alkaline rocks, olivine phenocrysts are 494 J. M. Bardintzeff et al.

Table 1. Selected chemical analyses of basalts from Fangataufa and CIPW normative compositions calculated with Fe203/FeO = 0.20, according to Middlemost (1989)

Sample 1 2 3 4 5 TS 391.8 TS 420.5 TS 608.0 TS 730.5 TS 808.0

SiO2 47.90 45.50 47.30 48.00 49.20 TiO2 3.15 3.31 3.00 2.90 2.54 A1203 14.50 15.83 14.40 14.78 15.91 Fe203t 11.80 12.29 12.07 12.15 10.95 MnO 0.18 0.16 0.15 0.16 0.14 MgO 7.13 4.30 6.13 6.15 6.01 CaO 8.93 9.50 11.08 11.95 11.02 Na2O 3.14 3.37 2.67 2.50 2.59 K20 1.22 1.51 0.73 0.42 0.49 P205 0.50 0.60 0.45 0.35 0.18 L.O.I. 0.98 2.24 1.42 1.48 0.62

Total 99.43 98.41 99.40 100.84 99.65

Qz 0.63 Or 7.21 8.92 4.31 2.48 2.89 Ab 26.56 24.10 22.59 21.15 21.91 An 21.88 23.62 25.16 27.88 30.35 Ne 2.39 Di 15.60 16.22 22.01 23.87 18.93 Hy 6.14 8.87 12.19 15.73 01 10.35 9.73 4.64 1.78 Mt 2.62 2.72 2.67 2.69 2.43 Il 6.00 6.30 5.71 5.52 4.83 Ap 1.10 1.32 0.99 0.77 0.40

D.I. 33.77 35.41 26.90 23.63 25.43

more abundant and additional alkali feldspar oc poor (always less than 4 wt%). The actual bound curs in the groundmass. Chemical compositions ary line is expressed as: Si02 = 40.5 + 1.95 (Na2O of mineral phases are differing: higher Ca, Ti, Na + K20), which is slightly different from the M & levels in clinopyroxene of alkali than in tholeiitic K line (see discussion below). basalts; less Cr contents in spinel (Cr203 ranging Tholeiitic and alkaline suites display noticeable from 16 to 26 wt% in alkali basalts, 42 to 44 differences. Alkaline rocks contain up to 4.1 wt% wt% in tholeiitic basalts, to be compared with the Na2O (Fig. 3(a)) and up to 1.8 wt% K20 (Fig. 36-48 wt% range in Hawaiian tholeiites); plagio 4(a)), which results in Na2O/K20 ranging mostly clase in alkali basalts slightly K-enriched com between 2 and 4 (Fig. 5(a)). Tholeiitic rocks con pared to tholeiites. tain less than 3.2 wt% Na2O, less than 0.9 wt% When classified according to mineralogical K20, and Na20/K20 ranging between 4 and 8. criteria, 87 samples yield alkaline affinities and Na20 and K20 contents in both suites are similar 37 tholeiitic ones, while 10 bearing intermediate to the values described at Hawaii for alkali basalt characters have been called "transitional". In the and tholeiite suites (BVSP, 1981). TAS diagram (Fig. 2(a)), alkaline rocks can be As far as Ti contents are concerned (Fig. 6(a)), rather silica-poor (down to 45 wt%) and alkali most alkali basalts contain more than 3 wt% Ti02, rich (up to 5.9 wt%); tholeiitic rocks are alkali while most tholeiitic basalts contain less. "Tran Transition between tholeiitic and alkali basalts 495

Table 1. (continued)

Sample 1 2 3 4 5 TS 391.8 TS 420.5 TS 608.0 TS 730.5 TS 808.0

Li 6 10 6 5 Rb 22 51 12 10 8 Sr 580 590 495 467 414 Ba 260 301 170 142 111 V 226 237 285 280 240 Cr 208 54 144 153 235 CO 43 42 43 42 35 Ni 142 77 76 90 95 Cu 51 63 85 79 Zn 106 112 125 95 Cs 0.16 1.91 0.08 0.19 0.07 Sc 24.20 26.70 28.80 32.00 30.20 Sb 0.06 0.06 0.05 0.05 0.04 Y 25.80 32.04 26.92 25.00 21.78 Zr 275.00 319.00 185.00 165.00 151.00 Hf 5.85 6.54 4.57 4.16 3.70 Nb 26.00 Ta 3.54 3.74 2.11 2.04 1.79 Th 3.47 3.69 1.98 1.77 1.49 U 0.98 0.94 0.63 0.44 0.31

La 33.20 34.05 20.43 20.00 17.09 Ce 67.30 81.17 49.10 46.00 42.51 Nd 34.48 40.08 29.32 28.00 21.71 Sm 7.42 8.19 6.08 5.50 5.24 Eu 2.65 3.00 2.37 2.00 1.97 Gd 6.74 7.95 6.45 5.16 Th 0.86 0.96 0.80 0.76 0.67 Dy 4.70 5.79 4.94 5.10 4.02 Er 2.02 2.48 2.14 2.50 1.77 Yb 1.71 1.88 1.64 1.75 1.31 Lu 0.29 0.34 0.30 0.25 All samples come from the Terme Sud drill. Depth is indicated by: TS-391.8 = 391.8 m. 1 and 2: alkali, 3: transitional, 4 and 5: tholeiitic basalts. 1, 2, 3 and 5 are taken from Guillou (1990), 4 is an unpublished result, major and minor elements by J. Cotten analyst, U.B.O. Brest, trace elements by J.L. Joron analyst, C.E.N. Saclay, C.R.P. G. Nancy, and J. Cotten analyst , U.B.O. Brest.

sitional basalts" are characterized by (Na20 + value of the ratio is modified. A ratio of 0.20 has K20) between 3 and 3.7 wt%, slightly more than been used for all rocks, except for the two 3 wt% Ti02, and only 0.5-0.7 wt% K20. Ti con trachybasalts of the alkaline suite for which a value tents are close to alkaline types, while low K of 0.30 has been used. contents are close to tholeiitic types. All 87 samples of the alkaline suite contain CIPW-normative compositions have been cal normative olivine (0.6-19.1 wt%), among which culated with a fixed Fe203/FeO ratio, as sug 30 contain normative nepheline (up to 4.4 wt%) gested by Middlemost (1989). The critical value while the 57 others contain normative hyperthene of the ratio should be carefully chosen because (up to 14.3 wt%). Among the 37 samples of the basaltic rocks may be CIPW-normative olivine ± tholeiitic suite, 34 contain normative hypersthene nepheline or hypersthene ± quartz-bearing, if the (up to 19 wt%), 26 normative olivine (up to 12.2 496 J. M. Bardintzeff et al.

8 Na2O + K20

6 •

• 5 •• %•• •

4 • a •y 0 oa • 0 3 0 0 • • CT 0 D 2 MD-K Si02

1 41 42 43 44 45 46 47 48 49 50 51 (a)

Na20 +K20 8

• • 7 • • 0 • 0 0 6 • 0

5 0 0 • 0 0 0 0 DO 0 0 0 D O 4 0 0 A D i

3 0

2 MD-K Si02

1 41 42 43 44 45 46 47 48 49 50 51 (b)

Fig. 2. Total alkali vs. silica wt% diagram for volcanic rocks. MD-K is the boundary line defined for Hawaii by Macdonald and Katsura (1964). For mineralogical criteria of classification, see text. (a) Fangataufa. Symbols: open square = tholeiitic, star = transitional, full square = alkali. For comparison, full circle = tholeiite from Kilauea, Hawaii, active lava front near Kamoamoa, collected on March 24, 1993, sample J.M.B., analyse U.B.O., Brest. (b) Kerguelen. Symbols: triangle = transitional from Loranchet peninsula and Plateau Central (Gautier et al., 1990), open square = mildly alkaline from Ronarc'h and Jeanne d'Arc peninsulae, full square = highly alkaline from Ronarc'h and Jeanne d'Arc peninsulae. Transition between tholeiitic and alkali basalts 497

5 Na2O

• 4 • • ~• •

3 • •• .• ^~ • °D• r)Kr ' •• N • • o• • • • C] OED ¶ !IJ • • 00 0A 0 0 2 0

Mg0 1

3 4 5 6 7 8 9 10 11 12 (a)

5 Na2O

•

0 0 0 0 4 0 • 0 O S 0 MIN • 0 0 0 00 ~ • 3 • A El • 0 A 0 0 0

Mg0

1 3 4 5 6 7 8 9 10 11 12 (b) Fig. 3. Na20 vs. MgO wt% diagram for volcanic rocks from Fangataufa (a) and Kerguelen (b). Same symbols as in Fig. 2.

wt%), 11 normative quartz (up to 4.6 wt%), and most tholeiites contain more than 10 wt% norma 3 normative nepheline (less than 0.2 wt%). All tive hypersthene. The 10 wt% normative hyper the 10 transitional rock samples are normative sthene limit corresponds fairly to that described hypersthene (6.4-12.7 wt%) and olivine (0.4-8.2 by Bass (1972) in abyssal basalts. wt%)-bearing. As in the case of Hawaii, alkali When plotted onto the Yoder and Tilley (1962) basalts may contain normative hypersthene but, tetrahedron, the two fields overlap each other. The for most of them, less than 10 wt%. Reversely, field of "transitional" lavas is precisely located at 498 J. M. Bardintzeff et al.

4 K20

2 • • • • ••U • •. ' • ' 1 •rr T • •o• • p Vd~J • n O 0 0 0 r~ ED 00 pe 0 MgO

3 4 5 6 7 8 9 10 11 12 (a)

4 K20 • • • •

• • 2 OW El

0 0 0 0C:D0 00 DO 0 • 0 0 0 1 0 s 0 0

LA 0 Mg0

0 3 4 5 6 7 8 9 1 0 11 1 2 (b)

Fig. 4. K20 vs. MgO wt% diagram for volcanic rocks from Fangataufa (a) and Kerguelen (b). Same symbols as in Fig. 2.

the intersection of the alkaline and tholeiitic fields with space and time can be defined. In Terme Sud, (Fig. 7(a)). tholeiitic basalts have been reached at the depth of 610.7 m and transitional basalts at 608 m. In Chemical evolution with space and time Echo 1, tholeiitic basalts are exposed at 647.5 m From detailed studies of the four drill cores and transitional basalts at 577 m. In Natice O1, (Terme Sud, Echo 1, Natice 01, Mitre 01) where tholeiitic basalts were cored at 551 m, and only tholeiites have been found, the chemical variation one single sample of tholeiitic basalt was found

0 Transition between tholeiitic and alkali basalts 499

10 Na20/K20 0

9 0 8 8

• 0 0 O 0 7 • 0 00 0 00 0 6 00 Nc 0 5 0 0 0

• x x Xu 0 0 D 4 • • • 3 1P ••• SkIr 46 2

1 Si02 0

41 42 43 44 45 46 47 48 49 50 51 (a)

10 Na20/K20

6 0 5

4 0 0 00 3 A 0 0 OEPEI • Oo 0 2 0 n0 • 1 •• • • Si02

0 41 42 43 44 45 46 47 48 49 50 51

(b) Fig. 5. Na20/K20 vs. Si02 wt% diagram for volcanic rocks from Fangataufa (a) and Kerguelen (b). Same sym bols as in Fig. 2.

at 820.2 m at Mitre 01. These depths cannot be been obtained, with 34 alkali basalts, 20 tholeiitic used as precise values of the tholeiite-alkali ba basalts and 2 intermediate basalts. Four levels of salt transition, because numerous younger alka aerial/subaerial volcanic episodes have been cored line dykes and sills intrude older tholeiitic lava between 372.5 m and 432.5 m depths (Schiano, flows. 1988; Guillou et al., 1990). Below, submarine The Terme Sud drill (926.8 m) is probably the volcanic levels were drilled down to 957.6 m. most promising: a rather complete sampling has Approximate proportions of basalt types are esti 500 J. M. Bardintzeff et al.

6 Ti02

4 IC

• • 3 • I" 0 Cis a`~ • D 0

• D0 2 O 0

Si02 1

41 42 43 44 45 46 47 48 49 50 51 (a)

6 Ti02 0

•• • 4 0 0 0 0 El 0 • 00 D 0 3 • 0D 0 0 0 Q] 00 0 DA • 0 0 • 0 2 D Si02

1 41 43 45 47 49 51 (b) Fig. 6. TiO2 vs. SiO2 wt% diagram for volcanic rocks from Fangataufa (a) and Kerguelen (b). Same symbols as in Fig. 2.

mated at more than 95% tholeiites, less than 0.1% terns. The enrichment is more pronounced for Ba transitional basalts and only few % alkali basalts. (from 100 to 300 ppm; Fig. 9) and LREE than for As far as the tholeiite-alkali basalt connection Ti and HREE. Rb, K, Ba and Sr contents in both is concerned, further constraints are provided by tholeiitic and alkaline lavas are relatively low chondrite-normalized spidergrams (Fig. 8(a)). compared to alkaline rocks of Society and From tholeiites to alkali basalts, absolute element Marquesas archipelagoes (Liotard et al., 1986), abundances increase and follow subparallel pat which could suggest phlogopite as a refractory Transition between tholeiitic and alkali basalts 501

Ne Di Qz FANGATAUFA

alkaline tholeiitic

Hawaii alkali basalt suite Hawaii tholeiite suite

transitional

01 Hy

(a)

Ne Di Qz KERGUELEN

highly alkaline transitional

mildly alkaline

01 Hy

(b) Fig. 7. Basalt normative compositions projected onto the basalt tetrahedron of Yoder and Tilley (1962). (a) Fangataufa compared with Hawaii (dashed fields) and (b) Kerguelen. Contoured fields enclosed most data, except 3 nepheline-bearing tholeiites of Fangataufa and 1 quartz-bearing mildly alkaline basalt of Kerguelen, which are omitted.

and/or fractionating phase in the mantle source The identical linear correlations in both tholeiitic region. Th (1.37-1.96 to 1.90-3.69 ppm), Ta and alkaline suites suggest the same source rock (1.65-2.24 to 2.20-3.74 ppm) and La (12.64-25.93 which underwent decreasing degrees of partial to 20.50-35.50 ppm) contents increase from tho melting with time. Co vs. Rb and La/Sm vs. La leiites to alkali basalts. plots evidence again one single source and em Element/element ratios, such as La/Th (re phasize the roles of: (i) decreasing partial melt spectively 9.45-11.48 and 9.23-11.02), Ta/Th ing, (ii) subsequent fractional crystallization (1.05-1.22 and 1.00-1.21) and La/Ta (8.92-10.04 (Guille et al., 1993). The homogeneous range of and 8.95-9.99; Fig. 10), are nearly identical. The 87Sr/86Sr, 143Nd/144Nd,206Pb/204Pb, 207Pb/204Pb, respective ranges of values of (Ce/Yb)„ (4.8-9.6 and 208Pb/204Pbratios (Fig. 11, Table 3) affords and 7.9-10.3) are strongly overlapping each over. an additional argument. 502 J. M. Bardintzeff et al.

Thompson plot (rock/chondrites) 1000 0 TS-391.8 TS-420.5 0 TS-608.0

• TS-730.5 100 TS-808.0

1 Ba Rb Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm Yb

(a)

Thompson plot (rock/chondrites) 1000

0 85-12 ARC 215

0 ARC 573

• ARC156 100

1 Ba Rb Th K Nb Ta La Ce Sr Nd PSmZrHf 11 Tb Y TmYb (b)

Fig. 8. Chondrite-normalized spidergrams using Thompson (1982) values, except for Rb, K and P, normalized to Sun (1980) estimates of undepleted mantle, and drawn with the program of Wheatley and Rock (1988). (a) Fangataufa (Table 1) and (b) Kerguelen (Table 2).

For the whole French Polynesia, Vidal et al. pic compositions (Chauvel et al., 1992). As far as (1984) argue cogently that rocks come from a Fangataufa is concerned, chondrite-normalized heterogeneous source which can be characterized trace element patterns (Fig. 8(a)) can be interpreted isotopically by a combination of a MORB-like by a mixture of MORB and HIMU components, depleted component and enriched components. as defined by Weaver (1991a, b). Isotopic data HIMU end-member was defined in Tubuai and (Table 3; Fig. 11) are in agreement with this con other islands display either EM I, or EM II isoto clusion. Transition between tholeiitic and alkali basalts 503

Ba content (ppm)

0 100 300 400 500

300

f.TI: N CO

CO r.Is: 1* Q

800

700

• H a 800 a) O L !I • H

r.Is:

1000

Depth (m)

Fig. 9. Evolution with depth of Ba (ppm) abundances in the Terme Sud drill core, Fangataufa, emphasizing the transition from tholeiitic to basaltic lavas.

La /Ta

16 KERGUELEN

a 14 0 ao * o x IC o 12

10 FANGATAUFA o . •. • • .!

8 Ta 0 1 2 3 4 5 6

Fig. 10. La/Ta vs. Ta (in ppm) contents of volcanic rocks from Fangataufa and Kerguelen. Same symbols as in Fig. 2. Sample No. 81-19 (transitional basalt of Kerguelen), with La/Ta = 24.1 and Ta = 0.87, is omitted. 504 J. M. Bardintzeff et al.

143Nd/144 Nd

05130 FANGATAUFA

0.5128 KERGUELEN

x 8 0.5126 O

•

0.5124 87 Sr/ 86 Sr 0702 0.703 0.704 0.705 0.706 Fig. 11. 143Nd/144Ndvs. 87Sr/86Srfor volcanic rocks from Fangataufa and Kerguelen (data from Table 3). Same symbols as in Fig. 2. Sample No. ARC 156 (highly alkaline basalt of Kerguelen), with 143Nd/144Nd= 0.51251 and 87Sr/86Sr = 0. 72385, is omitted.

Ma-old alkali basalts can be observed on the KERGUELEN: A LONG-STANDING MAGMATIC heights. The lava flows related to the first stage REGIME AND TAPPING OF DIFFERENT and to the beginning of the second stage have been SOURCES emplaced as huge trapps which are exposed within On the South Indian ocean floor, the subma up to 700 m-high cliffs and suffered from heavy rine Kerguelen-Heard plateau was generated 110 zeolitization of hydrothermal origin (Verdier et al., 120 Ma and grew rapidly at 50-60 Ma, when the 1988). The climax of zeolitization processes oc two Indian ridges were connected together and a curred at 13-15 Ma (Giret et al., 1992). .The more triple junction was created (Royer and Coffin, recent lavas have been emitted from distinct vol 1992). The northern part of the plateau comprises canic centres and are devoid of alteration products. the emerged Heard island, with its active volcano Complexes of associated volcanic and plutonic Big Ben, and the large Kerguelen archipelago (Fig. rocks were emplaced since 14 Ma ago to present, 1(c)). The archipelago covers an area of 6500 km2. such as Mont Ross, the highest peak (1850 m) of Born near or at the crest of the embryonic South the archipelago. In the southeastern province, the East Indian Ridge (Fig. 1) about 50 Ma, it has summits are made up of phonolitic or trachytic drifted southwestwards to its present location. necks or intrusions, related to the volcanic-plutonic Huge production of magma represented the result complexes. The whole archipelago have been of hotspot prolonged activity. The last eruptive slightly tilted southwestwards. Migration of the activity at Mont Ross is dated at about 0.2-0.4 magmatic activity from south to north was re Ma (Giret et al., 1988) and presently active fu corded during Upper Miocene (10-6 Ma) in vol marolic and hot spring areas are known in differ canic centres and plutons of the Rallier-du-Baty ent places. Peninsula (Lameyre et al., 1976). This paper deals with alkali basalts of the Geological setting southeastern province, which comprises the Several magmatic provinces were defined Ronarc'h and Jeanne d'Arc peninsulae (550 km2). (Nougier, 1970; Giret, 1983; Gautier, 1987; This area was chosen because it displays clearly Gautier et al., 1990; Leyrit et al., 1990) with two the transition of mildly to highly alkaline basalts, major stages in the build up of the archipelago: while, in other parts (northern parts of the archi (i) more than 26 Ma-old transitional basalts are pelago), transitional and alkali basalts are exposed at the Loranchet peninsula and throughout interstratified. Two magmatic periods were rec the Plateau Central (Fig. 1(c)), (ii) less than 26 ognized: Lower Miocene (26-20 Ma) alkali basalts Transition between tholeiitic and alkali basalts 505

and hawaiites are overlain by Upper Miocene (10 from 29.8 to 45.7, 22.8 to 46.7 and 25.1 to 30.0, 6 Ma) alkali basalts, basanites and tephrites (Leyrit respectively. According to Le Bas et al. (1986), 7 et al., 1990; Leyrit, 1992). The mildly alkaline samples among the 28 mildly alkaline, 5 among suite evolved towards trachyte end-members, while the 7 highly alkaline, and none among the 7 tran the highly alkaline and locally K-rich suite evolved sitional samples are trachy-basalts, the other rocks towards trachy-phonolite and phonolite end are basalts and, more rarely, basanites and members. During this 20 Myr-long period, a 10 tephrites. Myr-long interval of no magmatic activity was Na2O contents in both suites are poorly dis recorded by deposition of marine fossil-bearing criminative (Fig. 3(b)) and the range of values is sandstones in the eastern part of the Ronarc'h rather similar to that found at Fangataufa, while peninsula (Carriol et al., 1992; Giret et al., 1994). K2O contents (Fig. 4(b)) vary considerably. In The 10 Myr-long period of no activity is charac highly alkaline basalts, K20 contents, generally teristic the southeastern part of Kerguelen archi higher than 2 wt%, can reach up to 3.7 wt%, which pelago and remains unknown elsewhere so far. corresponds to twice the values obtained at Fangataufa. In mildly alkaline basalts, K2O con From mildly to highly alkaline basalts tents range between 0.4 and 2.1 wt% and only In the Ronarc'h and Jeanne d'Arc peninsulae, between 0.5 and 1.0 K20 wt% in transitional all alkali basalts contain olivine and clinopyroxene basalts. Na2O/K2Oratios (Fig. 5(b)) are lower than as phenocrysts and groundmass components. those obtained at Fangataufa, with a range of about Oceanites (up to 20.4 wt% MgO) are crowded with 1-2 for the highly alkaline suite and 2-4 for the olivine phenocrysts and ankaramites include mildly alkaline one. Ti02 contents appear to be not abundant clinopyroxene + olivine phenocrysts: discriminative (Fig. 6(b)). CIPW-normative com these cumulative rock types will not be considered positions have been computed with a fixed Fe203/ further. FeO ratio of 0.20 for basalts, basanites and Mildly alkaline basalts display Fe-Ti oxides, tephrites and 0.30 for trachybasalts (Middlemost, scarce biotite and apatite in their groundmass. 1989). All highly alkaline rocks are normative Plagioclase, always present in the groundmass, can nepheline-bearing (6.1 to 14.9 wt%). The mildly occur as phenocrysts in the more evolved basalts alkaline trend (28 samples) straddles the silica and in hawaiites, where olivine phenocrysts de saturation limit with 11 samples yielding a little crease in volume. amount of normative nepheline (less than 5.3 wt%) Highly alkaline basanites carry Fe-Ti oxides, and 1 sample, where plagioclase is a cumulus scarce apatite and analcime in their groundmass. phase, containing 0.3 wt% of normative quartz. In contrast, tephrites contain phenocrysts of Normative olivine (2.2 to 21.2 wt%) and/or hy kaersutite + clinopyroxene + magnetite + ilmenite persthene (0.5 to 8.9 wt%) are always present. The ± (mica + plagioclase). 7 "transitional" samples (Gautier et al., 1990) are In the area, 28 Lower Miocene mildly alkaline all normative hypersthene-bearing (5.8 to 18 wt%): rocks and 7 Upper Miocene highly alkaline rocks 4 samples contain normative quartz (up to 2.4 have been sampled for chemical analyses (Leyrit, wt%) and 3 normative olivine (7.5 to 8.1 wt%). 1992). Additional 7 "transitional basalts" analyses The occurrence of modal pigeonite and normative from the Loranchet peninsula and the Plateau hypersthene within the "transitional basalts" im Central (Gautier et al., 1990) will be used for plies that they are more akin to tholeiites. comparison. In the TAS diagram (Fig. 2(b)), highly Highly alkaline rocks plot in a well defined alkaline rocks contain 42.0-45.7 wt% Si02 and field in the Ne-01-Di triangle within the Yoder 4.6-7.3 wt% (Na20 + K20), mildly alkaline rocks and Tilley tetrahedron (Fig. 7(b)). The mildly al 45.1-49.7 and 2.8-6.4, and transitional basalts kaline field straddles the boundary between Ne 44.9-49.5 and 3.1-3.6, respectively. D.I. range Ol-Di and 01-Di-Hy triangles and lies in the same 506 J. M. Bardintzeff et al.

Table 2. Selected chemical analyses of basalts (1, 2), trachybasalt (3) and basanite (4) from Kerguelen and CIPW normative compositions calculated with Fe203/FeO = 0.20, and 0.30 for trachybasalt (3), according to Middlemost (1989)

Sample 1 2 3 4 85-12 ARC 215 ARC 573 ARC 156

Si02 49.50 47.86 47.50 42.89 TiO2 2.45 2.71 2.99 3.21 A1203 16.47 15.13 16.38 11.33 Fe203t 11.40 12.10 13.33 13.12 MnO 0.16 0.16 0.16 0.19 MgO 4.36 5.72 4.37 10.58 CaO 10.44 9.41 6.63 10.30 Na20 2.86 3.24 4.30 3.58 K20 0.72 1.35 2.12 2.33 P205 0.37 0.41 0.80 1.04 L.O.I. 1.43 1.20 0.84 1.44

Total 100.16 99.29 99.43 100.01

Qz 1.56 Or 4.25 7.97 12.53 13.76 Ab 24.20 27.40 31.43 2.73 An 29.99 22.76 19.15 7.98 Ne 2.69 14.92 Di 16.06 17.46 7.05 29.38 Hy 13.69 1.17 01 11.55 13.11 17.37 Mt 2.53 2.68 4.12 2.91 Il 4.66 5.16 5.70 6.11 Ap 0.81 0.90 1.76 2.29

D.I. 30.01 35.38 46.65 31.42

area as the alkaline field of Fangataufa. The 7.16-9.25 to 6.57-9.88), Ta/Th (from 0.50-0.64 "transitional" basalts (Gautier , 1987) contain large to 0.43-0.56, respectively), while others differ amounts of normative hypersthene and sometimes completely, e.g., La/Ta (from 12.1-14.3 to 15.4 normative quartz. They lie in the field of "tholei 17.5, Fig. 10), Zr/Nb (from 6-7 to 4-5), and itic" basalts defined at Hawaii and at Fangataufa (Ce/Yb)n (from 6.8-8.3 to 21.1-23.3, respectively) and, in the TAS diagram, they plot near the (Leyrit, 1992). boundary of tholeiitic and alkaline fields. It is difficult to relate by simple models of partial melting of a single source and/or differen Mantle sources tiation from the same parent melt the transitional As far as trace element contents are concerned, basalts (Th = 1.82-2.60; Ta = 0.94-1.87; La = noticeable variations can be evidenced. From the 15.6-24.4; La/Th = 8.17-9.80; Ta/Th = 0.49-0.75; mildly alkaline to the highly alkaline basalts, Th La/Ta = 12.1-16.6) and the mildly and highly al increases from 2.42-6.4 to 9.9-12.3, Ta from kaline basalts, so that a heterogeneous mantle 1.54-2.20 to 5.25-5.59, and La from 21.2-39.2 to source should be advocated. At the scale of the 80.8-97.8, respectively. Element/element ratios whole Kerguelen archipelago, Th, Ta and La con display contrasting features: some of them are tents systematically increase from the transitional nearly identical in both suites, e.g., La/Th (from through the mildly alkaline to the highly alkaline Transition between tholeiitic and alkali basalts 507

Table 2. (continued)

Sample 1 2 3 4 85-12 ARC 215 ARC 573 ARC 156

Rb 12 30.4 42 42.9 Sr 380 467 877 1116 Ba 174 348 588 1064 V 215 Cr 124 335 CO 33 57.2 Ni 44 58 6 214 Cu 45 23.4 77.2 Zn 79 86 91 Pb 9 9.1 10.9 Ga 21 19.3 19 Y 28 27 33.5 30.3 Zr 165 176 265 329 Hf 5 6.7 5.8 Nb 23 27 41.2 73.3 Ta 1.44 5.25 Th 2.01 4 6.4 12.3

La 17.4 37 39.2 80.8 Ce 39 72 103.2 167 Nd 23.49 68.3 Sm 5.47 11.5 Eu 1.9 3.53 Tb 1.22 Yb 1.29 1.99 Lu 0.3

1: transitional basalt, Plateau Central (Gautier et al., 1990); 2 and 3: Lower Miocene mildly alkaline lavas, Jeanne d'Arc peninsula; 4: Upper Miocene highly alkaline basanite, Ronarc'h peninsula. Analyses 2-4 were performed at Universite Jean Monnet, Saint-Etienne (Leyrit, 1992), except for 4: V,, Cr, Co, Ta and REE (Weis et al ., 1993).

Table 3. Isotopic data on volcanic rocks from Fangataufa (C. Chauvel analyst, Rennes) and the southeastern province of Kerguelen (Weis et al., 1993)

Sample *87Sr/86Sr 143Nd/144Nd 2o6Pb/ 2o4Pb 207Pb/ 2o4Pb 208Pb/ 2o4Pb

Fangataufa TS 611.5 tholeiitic 0.70263 0.512971 19.320 15.539 38.976 TS 730.5 tholeiitic 0.70299 0.512945 19.492 15.597 39.231 TS 395.5 alkali 0.70294 0.512948 19.252 15.561 38.962 GK 435.95 alkali 0.70289 0.512956 19.389 15.567 39.054 TS 483.4 alkali 0.70341 0.512901 19.501 15.616 39.257 TS 672.85 alkali (dyke) 0.70313 0.512937 19.414 15.569 39.088

Kerguelen LVLK 70 mildly alkaline 0.70514 0.51266 18.526 15.591 39.165 LVLK 88 mildly alkaline 0.70522 0.51272 18.539 15.591 39.193 LVLK 87 mildly alkaline 0.70513 0.51265 18.440 15.553 39.012 LVLK 108 mildly alkaline 0.70511 0.51270 18.543 15.566 39.116 LVLK 130 mildly alkaline 0.70493 0.51275 18.607 15.579 39.212 LVLK 123 mildly alkaline 0.70521 0.51261 18.379 15.559 39.030 ARC 156 highly alkaline 0.72385 0.51251 18.050 15.556 38.630 ARC 195 highly alkaline 0.70560 0.51254 18.263 15.569 38.923 ARC 321 highly alkaline 0.70573 0.51249 18.071 15.560 38.879 508 J. M. Bardintzeff et al. basalts, and the highest values have been deter displayed in numerous oceanic islands. Elsewhere, mined in basanites of the southeastern region. only the restricted OITB -* OIAB transition was MORB-normalized Th levels, ranging from 9 observed. up to 62, determine a large positive anomaly, which was observed also at La Reunion, and Geochemical evolution in oceanic islands therefore appears to represent a typical feature of In the Hawaiian archipelago, the coexistence Indian OIBs, compared to Atlantic and Pacific of OIT and OIAB is well established at Mauna OIBs (Gautier et al., 1990). Though Kerguelen Kea (Frey et al., 1991), Kohala (Feigenson et al., lavas have been classified as belonging to the EM 1983), Haleakala (Chen et al., 1991), East Molokai I OIB type (Weaver, 1991a, b), chondrite-nor (Clague and Beeson, 1980), and Hualalai (Moore malized trace element patterns are consistent with et al., 1987). Only OIT lavas are known at Kilauea a combination of EM I and EM II sources. The and Mauna Loa (Clague, 1987; Wilkinson, 1991). large range of 87Sr/86Sr,143Nd/144Nd, 206Pb/204Pb, In French Polynesia, several examples of the 207Pb/204Pb, and 208Pb/204Pbratios (Fig. 11; Table same evolution with time are provided. In the 3) further confirms that the mantle source was Marquesas archipelago, coexistence of OIT and highly heterogeneous with time and again that EM OIAB is evidenced in Hatutu and Ua Pou, whereas I and EM II constituted the major components OITB coexist with OIAB at Nuku-Hiva (Liotard (Weis et al., 1989a, b, 1993; Gautier et al., 1990). et al., 1986). In the Macdonald seamount In this scheme, basalts were generated from mixed (Australes archipelago), chemical studies reveal source regions comprised of a depleted MORB coexisting olivine tholeiites, alkali basalts, and component and the Kerguelen plume component, basanites (Barsczus and Liotard, 1985). At Tahiti characterized by a very enriched composition in (Society archipelago), a volcanic transitional suite termediate between EM I and EM II. Lower Mio characterized the pre-caldera stage while the vol cene mildly alkaline lavas contain a higher pro canic and plutonic silica-undersaturated suites were portion of the depleted component than Upper emplaced during both syn-caldera and post-caldera Miocene highly alkaline lavas which sampled the stages (Gelugne, 1988; Bardintzeff et al., 1988; pure Kerguelen plume (Weis et al., 1993). Bonin and Bardintzeff, 1989). Cheng et al. (1993) argue for a progressive decrease with time in the degree of partial melting from "transitional" basalts DISCUSSION (series A, >1.3 Ma) to more and more alkaline Both Fangataufa and Kerguelen case studies ones (series B, 0.6-1.3 Ma and series C, <0.6 Ma). confirm that the volcanic suites in oceanic islands In the Indian Ocean, at La Reunion, the poly evolve through alkali enrichment: from "tholeiitic" genetic Piton des Neiges strato-volcano has been to "alkali" basalts at Fangataufa, from "mildly built in two stages (Upton and Wadsworth, 1972; alkaline" to "highly alkaline" in the southeastern Nativel, 1978): (i) a transitional basaltic suite be province of Kerguelen. However, the ca. 200 in tween 2.1 and 0.43 Ma and (ii) a differentiated thick transition zone made up of interlayered tho alkaline suite between 0.35 and less than 0.030 leiitic and alkali basalts, which was described in Ma (McDougall, 1971; Gillot and Nativel, 1982). detail in the Hawaiian archipelago at Mauna Kea (Frey et al., 1991), Haleakala (Chen et al., 1991) Modal vs. normative mineralogy and East Molokai (Clague and Beeson, 1980), but In order to provide a general model which can not at Kohala, was not observed at Fangataufa and be applied to different volcanic oceanic islands, Kerguelen, as well (Bardintzeff et al., 1992). mineralogical and chemical criteria should be The complete Oceanic Island Tholeiite (OIT) carefully considered. Modal mineralogy was the -4 Oceanic Island Transitional Basalt (OITB) -* first criterion used for the distinction of basaltic Oceanic Island Alkali Basalt (OIAB) transition is types (Kennedy, 1933; Kennedy and Anderson, Transition between tholefitici s andan alkalia a basalt asa is 509

1938; Tilley, 1950; Coombs, 1963; Kuno, 1960). Bardintzeff et al., 1986; Guillou, 1990; Guillou et Bass (1972) defined transitional basalts as bearing al., 1990) and Kerguelen (Gautier, 1987): olivine and low-Ca pyroxene in the groundmass clinopyroxene within alkali basalts is mostly Ca and points out that transitional basalts can be re Ti-Al-Na-rich diopside, while it is Si-rich augite lated either to alkali basalts or to tholeiites. within tholeiites. Plagioclase yields fairly identical In the specific case of Kohala where all basal compositions in basic rocks, as pointed out by Keil tic types are displayed, Feigenson et al. (1983) et al. (1972); however, we observed K-rich pla observed the following preferential phenocryst gioclase in alkali basalts. Apatite is a minor com assemblages: (i) only olivine within tholeiites, ponent in alkali basalts. The mineralogical char (ii) small but increasing amounts of plagioclase acteristics outlined above result clearly from along with major olivine within tholeiites, whole-rock chemistry, i.e., higher Ti02, Na20, (iii) plagioclase dominant over olivine in transi K2O, and P2O5 contents in alkali basalts. tional basalts, and (iv) little plagioclase, no olivine, and even no phenocryst at all in alkali basalts. Nomenclature of basalts: The "transitional" According to data collected from the literature debate and from our own studies, we determined the Nomenclature of igneous rocks was a matter distinctive mineralogical features: (i) the occur of lively debates and especially for the term rence of Ti-augite as phenocrysts as well as Ti "transitional" . Two chemical parameters are clas augite, olivine, alkali feldspar and apatite in the sically considered: silica saturation and alkali groundmass is typical of alkali suites, (ii) hyper contents. In the classification scheme of Yoder and sthene or pigeonite occur in the groundmass and Tilley (1962), alkali basalts are basically silica olivine is absent in silica-oversaturated tholeiitic undersaturated and tholeiites silica-(over)saturated, basalts, and (iii) both olivine and hypersthene co which was later confirmed for the Hawaiian ar exist generally in the groundmass of transitional chipelago by M & K (1964). Since then, this gen basalts. In that way, some pigeonite-bearing eral simple scheme was obscured by the increas "transitional" basalts in Kerguelen should be ingly large amounts of chemical data. For example, considered as tholeiites, while the olivine-bearing at Mauna Kea (Frey et al., 1991), the chemical "tholeiites" of Fangataufa and the "tholeiites" criteria used for distinguish the various types are lacking modal hypersthene or pigeonite in Kohala, those defined by M & K. But, in East Molokai Hawaii, appear to be not so typical. (Clague and Beeson, 1980), transitional basalts Our general conclusion that olivine is basically contain the olivine + clinopyroxene paragenesis absent from the phenocryst assemblage of tholei typical of alkali basalts but resemble chemically ites agree with Yoder and Tilley (1962) and tholeiites. At Kohala (Feigenson et al., 1983), Miyashiro (1978) definitions, but is at variance transitional lavas plot in the M & K alkaline field with Feigenson et al. (1983) observations on and have the typical paragenesis of alkali basalts. Kohala. This type of discrepancy is frequently At Haleakala (Chen and Frey, 1985), olivine and encountered in the literature and the result was pigeonite occur in the groundmass of transitional that nomenclature may change noticeably accord lavas. ing to local considerations. As a result, boundary lines in the TAS diagram As far as mineral chemistry is concerned, vary notably according to the different authors spinel yields Cr/(Cr + Al + Fe3+) ratios of about (Macdonald and Katsura, 1964; Kuno, 1966; Irvine 0.6 in tholeiites (Hawaii and Fangataufa) and only and Baragar, 1971; Miyashiro, 1978; Guillou, 0.4 in alkali basalts. That clinopyroxene and melt 1990; Hoernle and Schmincke, 1993). Uncertain chemistries are closely related (Leterrier et al., ties due to analytical errors cannot alone explain 1982) was evidenced at Hawaii (Fodor et al., 1975; the discrepancies and it became clear that the TAS BVSP, 1981), Fangataufa (Bardintzeff, 1981; boundary lines differ according to the different 510 J. M. Bardintzeff et al. geological settings. In his synthesis of the bound transition is ascribed to decreasing degrees of ary lines available in the literature, Rickwood partial melting and, therefore, to decline of the (1989) concludes that, as chemical variations heat source. For example, an average value of within single or multiple magmatic centres are 6.6% of partial melting of a zone which has a continuous, any sharp boundary line is inevitably vertical extent of ca. 55 km and a radial extent of artificial and that, rather than a line, a narrow band ca. 130 km to produce 0.16 km3-y 1 of tholeiitic should be preferred. melt is assumed beneath Hawaii by Watson and That a great care is needed when comparing McKenzie (1991). different volcanic islands is illustrated by the fact (ii) Mantle heterogeneity models are based on that the same normative hypersthene-bearing basalt the notion that melt compositions are derived di can be named "tholeiitic" (Liotard et al., 1986; rectly from the mineralogical and chemical com Caroff et al., 1993; Hoemle and Schmincke, 1993), positions of source rocks. The transition implies "transitional" and even "alkali" (Gautier et al., that melting process occurred in progressively less 1990; Guillou et al., 1990; Chen et al., 1991). The depleted and/or more enriched sources, which "transitional" stage defined at Tahiti (Gelugne, means incorporation of either oceanic lithosphere 1988) resembles the mildly alkaline stage of previously metasomatized, or a deep mantle plume. Kerguelen. Likewise, other terms have been ap Any general model can be subjected to large plied to transitional basalts: "high-alumina tholei local variations, according to mantle compositions, ite", "hypersthene-normative alkali basalt"..., and geodynamic settings, and subsequent magma appear unnecessary and misleading (see discussion evolution through mineral fractionation. For the in Bass, 1972, p. 62). only example of Hawaiian archipelago, which The vexed question of nomenclature is also could appear at a first glance as a relatively simple addressed by Middlemost (1991), who proposes a case, more and more complexity has been devel new and expanded classification into three asso oped to explain the OIT -4 OIAB transition. At ciations defining discrete fields in the TAS dia Kauai, Feigenson (1984) invokes mixing of a small gram. The suites are named as follows: subalkali fraction of MORB source material with a large (tholeiite and calk-alkaline basalts), transalkali fraction of an "enriched" mantle component. Stille (hypersthene-bearing basalts) and alkali (highly et al. (1986) suggest that the mantle source is a alkaline basalts). Following this scheme, the ex combination of oceanic lithosphere, depleted as ample of Fangataufa concerns the subalkali --* thenosphere, and a deep mantle plume. Whether transalkali transition, while the southeastern region phlogopite and/or pargasite are present in impor of Kerguelen emphasizes the transalkali -* alkali tant amounts (up to about 20%), or not in the transition. source influence strongly melt compositions, es pecially Na2O/K20 ratios and Ti02 contents, as Partial melting vs. mantle heterogeneity suggested by Wilkinson (1991). The variations The magmatic OIT -* OIAB sequence, with or observed among the lavas can be explained equally without OITB, is a common feature of oceanic by subsequent high to low-pressure mineral islands. It constitutes therefore a major constraint fractionation of different batches of magma within on the build up of large shield volcanoes. Many deep to shallow reservoirs (Nicholls and Russell, petrological models have attempted to explain this 1991). evolution with time. They can be grouped into Fangataufa and Kerguelen are both located in two contrasting approaches: similar oceanic within-plate contexts and their (i) Partial melting models assume that melt mantle sources are now famous for their large compositions are controlled by the thermodynamic isotopic heterogeneity. A basic boundary exists parameters which govern the degree of melting of between the Indian and Pacific domains. one single mantle source. The OIT -* OIAB Beneath French Polynesia, the main geo Transition between tholeiitic and alkali basalts 511

chemical mantle components that give rise to OIBs are represented (Vidal et al., 1984). Fangataufa can CONCLUDING REMARKS be explained by a mixture of MORB and HIMU 1. Different criteria should be used jointly to components (Table 3; Fig. 11). The pure HIMU typify alkaline or tholeiitic affinities of basalts: signature was found in Tubuai, Mangaia and critical modal mineralogy: alkali basalts Rurutu, EM I is present in the source of Rarotonga contain Ti-augite as phenocrysts as well as in the and Pitcairn, and EM II dominates the composition groundmass, olivine, alkali feldspar and apatite in of most Society Islands (Chauvel et al., 1992). The the groundmass; tholeiitic basalts display hyper compositions of the end-members have been as sthene or pigeonite in the groundmass, and olivine cribed to various amounts of: (i) incorporation into is absent or scarce. the MORB source of an ancient subducted and • chemical compositions of mineral recycled lithosphere (Dupuy et al., 1989), and/or phases: higher Ca, Ti, Na levels in clinopyroxene (ii) metasomatism of oceanic lithosphere by prior of alkali than in tholeiitic basalts; variations of passage over the Easter hot spot (Dupuy et al., Cr, Al, Fe, Mg contents in spinel; plagioclase in 1993). alkali basalts K-enriched compared to plagioclase In the model developed recently by Chauvel in tholeiites. et al. (1992), HIMU represents oceanic crust al • CIPW-normative mineralogy (quartz , tered prior to and during subduction, while EM I hypersthene, olivine, nepheline), with reference to and EM II have acquired their characteristics the Yoder and Tilley tetrahedron. through entrainment of sediments, respectively • alkali contents and ratios (K20, Na20/ pelagic and terrigenous, that were subducted to K20): in the TAS diagram, individual boundary gether with the oceanic crust. An additional and lines between the alkaline and tholeiitic fields may important point is that the French Polynesian change, but, in any case, "transitional" basalts plot plumes have tapped at different times source re close to the boundary lines. gions containing both HIMU and EMs, suggest • other element contents can be used for ing that the components are not large, isolated, discrimination, e.g., Ti at Fangataufa. At independently generated reservoirs, but instead are Kerguelen, however, Ti levels are nearly identical physically related in the mantle. and Fe is a more useful tool. By contrast, HIMU seems to be completely 2. Prior to any comparison between different missing beneath the Indian Ocean. Kerguelen and suites, definitions of the terms used afterwards Heard islands are the latest products of a wide should be labelled as clearly and precisely as and long-lived plume system, which has contami possible. The problem is particularly acute for the "transitional basalts" nated large volumes of the Indian asthenosphere. , because they have characters Its composition is largely dominated by EM I and in common with both tholeiites (modal EM II (Storey et al., 1988, 1989; Weis et al., orthopyroxene or pigeonite) and alkali basalts 1993). The Kerguelen Upper Miocene highly al (high K20 and/or Na20 contents), as observed at kaline basalts are K-enriched compared to Kerguelen. Fangataufa (Figs. 4(a) and (b)) and represent the 3. Any general model of magma evolution signature of the pure Kerguelen plume (Weis et al., with time should take into account a combination 1993). of the following factors: At a more global scale, the whole southern • tectonic settings: within-plate oceanic hemisphere is K-enriched compared to the north island-oceanic island emerged from an aseismic ern one, which could explain the differences ob plateau-oceanic island nearby a mid-oceanic served between Fangataufa and Hawaii, and the ridge. various boundary lines proposed between the al • mantle heterogeneity , with at least four kaline and tholeiitic fields. isotopic reservoirs recognized so far: MORB, 512 J. M. Bardintzeff et al.

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