Isotopic evidence for the origin of the Manihiki and Ontong Java oceanic plateaus

Earth and Planetary Science Letters, 104 (1991) 196-210 Elsevier Science Publishers B.V., Amsterdam

196

[CL]

Isotopic evidence for the origin of the Manihiki and Ontong Java oceanic plateaus J.J. M a h o n e y a n d K.J. Spencer School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, H I 96822, USA Received July 12, 1990; revision accepted March 9, 1991

ABSTRACT Pb, Nd, and Sr isotopic results for lavas of the Cretaceous Ontong Java and Manihiki oceanic plateaus fall well within the modern-day oceanic island or hotspot field. The data provide no evidence of old continental basements but indicate a major involvement of "Kerguelen-type" or "EM-I"-like mantle in the sources of both plateaus, which appear to have probably been formed, at least in part, by hotspots. However, the presently active hotspots that Pacific plate reconstructions suggest might have been possible plateau sources lack Kerguelen-type isotopic compositions. Either these hotspots did not participate in the formation of the two plateaus, or if they did, Kerguelen-type material must have been volumetrically much more important early in their existence. Two hypotheses for the origins of these plateaus which involve hotspot sources are consistent with the sparse available geochemical, geochronological and geophysical data. The first holds that the plateaus formed cataclysmically in association with surfacing plume heads; the second posits a relatively steady but robust hotspot at or near a ridge crest and requires a much longer period of formation. A near-ridge origin appears to be indicated by evidence that most of the Pacific plateaus were built largely on relatively young ocean crust. However, we suggest that a near-ridge origin is also compatible with the plume head concept in that plume heads appear very likely to become associated with spreading axes through their influence on rift propagation, which should be substantially greater than for ordinary hotspots. In either case, the lack of hotspot tracks (seamount chains) attached to the two plateaus would be a consequence of ridge migration or rift propagation in a near-ridge setting.

1. Introduction

Oceanic plateaus have been estimated to cover 3% of the world's seafloor [1]. The western Pacific contains several (Fig. 1), including the Manihiki (ca. 550,000 km 2) and the gigantic Ontong Java (the world's largest, at ca. 1,500,000 km2). Standard oceanic plate-tectonic models do not explain the great individual extents of the Pacific plateaus, their 20-40 km-thick crusts [e.g., 2], weak or absent lineated magnetic anomalies [e.g., 3-5], or distinctive, elevated topography (typically 2-3 km above the adjacent ocean bottoms). Unfortunately, owing to a paucity of samples, knowledge of their basement compositions and ages is fragmentary at best; consequently, ideas about their origins tend to be quite speculative. Two general classes of hypotheses, both of which rely principally on geophysical data, have been advanced most frequently to explain the origin of these edifices. 0012-821X/91/$03.50

© 1991 - Elsevier Science Publishers B.V.

One argument holds that they were formed at Cretaceous ridge crests during a period of unusually intense volcanism a n d / o r ridge jumping in the vicinity of major transform offsets or slowly migrating triple junctions [e.g., 6-10,2]. This view predicts that the Pacific plateaus are composed mainly of thick accumulations of submarine flood basalts [e.g., 7], which should be broadly similar to mid-ocean ridge basalts (MORBs) in composition. Indeed, most of the few plateau basement rocks that have been recovered are tholeiitic basalts which tend to have roughly MORB-like incompatible element abundances. Also, whole-rock major elements and high Cr abundances in spinels of some Manihiki Plateau lavas indicate that they are the products of very high degrees of partial melting (actually higher than for most MORBs) [1114]. However, the analyzed Pacific plateau basalts for the most part do not possess MORB-like Nd or Sr isotopic ratios but, rather, values typical of

ORIGIN

OF THE MANIHIKI

140 °

AND ONTONG

160 °

JAVA OCEANIC

180 °

PLATEAUS

160 °

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>,'" Shatsky 40 °

465

° "*~lJ

20 ° -

-

Ontong dovo Plateou

o°i .

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Ls n

SOTW-I 1-78-

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Fig. 1. Map of the western Pacific showing locations of Cretaceous oceanic plateaus, N a u r u Basin, associated DSDP sites reaching basement, and the SOTW dredge haul on the Manihiki Plateau. Plateau outlines follow 5-km depth contours except for the Ontong Java Plateau, which is represented by its 4-km contour and its approximate boundary with the Solomon Islands in the southwest. Asterisk marks location of Malaita.

oceanic islands [15]; moreover, their ratios of highly to moderately incompatible elements tend to be elevated slightly (by a factor of 2 or less) over those in normal MORB [14-19]. These characteristics appear to require the presence of components in Pacific plateau mantle sources similar to those in oceanic island basalt (OIB) sources, and the available data are consistent with mixing between OIB-type and MORB-type mantle. Mahoney [15] proposed that a near-ridge hotspot environment, probably at slow relative spreading rates, would satisfy both the geochemical and geophysical evidence, providing a mix of MORB and OIB sources and the requisite voluminous melting. Comparatively small present-day analogs would be the Galapagos and Icelandic plateaus. Independently, Gordon and Henderson [20] also have suggested a near-ridge hotspot origin on the basis of the Icelandic Plateau analogy. More recently,

197

Richards et al. [21] have proposed that large oceanic plateaus instead might be formed cataclysmically when the inflated heads of nascent mantle plumes reach the base of the l i t h o s p h e r e -whereas seamount chains or elongated aseismic ridges would reflect the much narrower but longlived plume tails that follow. This plume-initiation model does not specify a near-ridge setting; however, it also could account for mixing of MORB and OIB-type components because the rising plume head, presumably made up originally of OIB-type mantle, should entrain significant amounts of MORB-type mantle as it expands [22, 23]. An apparent problem with both of these hypotheses is that the Manihiki, Ontong Java and the other old Pacific plateaus are not connected physically today with obvious trailing or leading hotspot tracks in the form of major island or seamount chains or elongated aseismic ridges; nor are such associations evident in Pacific plate reconstructions. In contrast, the relationship of some plateaus in the Indian and Atlantic oceans with hotspot tracks is relatively well established (e.g., Icelandic Plateau with Greenland-Scotland Ridge, Kerguelen Plateau with the now distant Ninetyeast Ridge, etc.) Moreover, large continental flood basalt provinces, which may represent the on-land counterparts of oceanic plateaus, are in several cases clearly associated with trailing oceanic hotspot tracks (Deccan and Chagos-Laccadive Ridge, Etendeka-Parana and Walvis Ridge, North Atlantic Tertiary and Greenland-Scotland Ridge, etc.) [e.g., 24]. A s e c o n d - - a n d very different--class of model has been proposed in a series of papers by Nur and Ben-Avraham [e.g., 25] and by Carlson et al. [26], largely on the basis of crustal thickness measurements and intracrustal seismic P-wave data. These authors contend that the thick crust of many plateaus, including the Manihiki and Ontong Java, is principally continental. Here the plateaus are seen essentially as foundered continental blocks, intruded and at least partly capped by Cretaceous basaltic rocks. However, the same seismological data also can be interpreted to indicate thickened oceanic (basaltic) crust [2]. Thus far, the OIB-like isotopic results available and the low degrees of incompatible element enrichment in the plateau lavas give no indication of old continental crust [15,27], but isotopic determina-

198

tions have been made only for Sr and Nd. Pb isotopes, which are sensitive indicators of continental crustal contamination, have not been measured. The diversity of models that can be applied to the existing plateau data emphasizes that further study is needed. Here we report the first Pb isotopic measurements for the Pacific plateaus, specifically for the Manihiki and Ontong Java specimens analyzed for Sr and Nd isotopes by Mahoney [15]; in addition, we have determined Pb as well as Nd and Sr isotopes on several Manihiki samples not previously analyzed. 2. Samples and methods The Manihiki Plateau has been divided into three geomorphic subunits: the High, North and Western plateaus, separated from each other by deep troughs which may have a fault origin. This physiography, in part, led Winterer et al. [6] to suggest that the plateau was formed at a triple junction beetween the Pacific, Farallon and Antarctic plates. Nine of the Manihiki samples in this study are from the ten ( < 5 m thick) flows drilled at DSDP Site 317 on the High Plateau [28]; three are from a 4085 m-deep Scripps Institution of Oceanography dredge haul about 300 km to the northwest in the Danger Island Troughs (SOTW11-78) bordering the High Plateau in the north [11]. All of the Site 317 flows are highly vesicular and, although now 3500 m deep, appear to have been erupted at quite shallow levels ( < 400 m). They are slightly to moderately altered, characterized by flat chondrite-normalized rare earth element patterns, with low concentrations of incompatible elements in the range of normal MORB (e.g. K 2 0 avg. =0.09 wt.%) [28]. The SOTW dredge specimens contain no vesicles and are fresher than most of the Site 317 lavas. Two are high MgO (ca. 14 wt.%) basalts with high-Cr spinels and abundant olivine phenocrysts of Fo88; the third is a more ordinary basalt (MgO = 8.8 wt.%, olivine = Fo85). All three have very low TiO 2 and P205, and low K20 (0.47-0.72, 0.01-0.03 and 0.12-0.24 wt.%, respectively) [11]. The Ontong Java Plateau consists of a main, high plateau and an eastern lobe or "salient" [7]. Adjacent to the main plateau on the east is the Nauru Basin, which is filled with extensive Early

J.J. M A H O N E Y A N D K.J. S P E N C E R

to mid-Cretaceous basaltic flows and sills (total thickness > 600 m at DSDP Site 462), the origin of which is understood only poorly but may be linked to that of the plateau [e.g., 13]. In the southwest, the plateau is bounded by the so-called Pacific Province of the Solomon Islands [e.g., 29,30]. Field and geophysical evidence [e.g., 7,9,30-34] indicates that the Pacific Province (which includes the islands of Malaita, Small Malaita, Ulawa, and the northern half of Santa Isabel) represents the southwestern edge of the plateau, where late Tertiary and Quaternary uplift associated with the collision of the plateau against the old North Solomon Arc has exposed 10002000 m of igneous basement on Malaita, Small Malaita and Santa Isabel. Three of our samples, originally described by Hughes and Turner [30], are slightly to moderately altered tholeiites from the pillowed and massive (avg. > 10 m thick), low-K20 (ca. 0.10 wt.%) Lower Cretaceous flows on the island of Malaita; one is from the single, highly altered basement flow ( > 9 m thick) cored atop the central part of the high plateau at DSDP Site 289 [35]. These basalts are similar to the Manihiki Site 317 lavas in having chondritic or slightly lower S m / N d ratios, but they display somewhat greater (though still MORB-like) abundances of Nd, Sm and Sr [15]. Unlike the Manihiki Site 317 lavas, there is no evidence that any of these basalts were erupted near sea level [e.g.,

30,351. Pb, Sr, and Nd isotopic analyses (Tables 1 and 2) were carried out in the Isotope Laboratory of the University of Hawaii. To avoid drilling-related or other contamination, samples first were chipped to a size of 0.5-1.0 cm to obtain interior pieces; the freshest chips were cleaned thoroughly in three stages (in an ultrasonic bath) in ultra-pure H N O 3HF, HC1 and H20, and then chipped further to a size of ca. 0.3-0.5 cm; the freshest of these smaller chips were cleaned again in the same manner, and then powdered in a pre-cleaned boron carbide mortar. Subsequent dissolution, chemical separation and mass spectrometric procedures have been described elsewhere [36, this issue]. In Table 1, Pb isotopic results are listed as present-day ratios, whereas Nd and Sr isotope values are shown in Table 2 both for the present and 120 m.y. ago. Based on paleontological dates of basal sediments and 4°Ar/39Ar dating of lavas, the latter value is

199

ORIGIN OF THE MAN1HIKI AND ONTONG JAVA OCEANIC PLATEAUS TABLE 1 Pb isotopic and isotope dilution data for Ontong Java and Manihiki basalts Sample

2°6pb/2°4 Pb

2°7pb/2°4pb

2°8pb/2°4 Pb

18.355 (3) 18.398 (5) 18.270 (3) 18.285 (3) 18.302 (19) 18.223 (5) 18.232 (7) 18.131 (4) 18.160 (12) 18.095 (4) 18.136 (4) 19.113 (6) 19.112 (3) 19.172 (9) 19.174 (4) 19.345 (4)

15.495 (3) 15.492 (4) 15.495 (3) 15.482 (3) 15.496 (18) 15.495 (5) 15.499 (6) 15.470 (3) 15.489 (11) 15.480 (3) 15.483 (4) 15.588 (5) 15.574 (3) 15.576 (8) 15.570 (3) 15.587 (4)

38.316 (10) 38.322 (12) 38.354 (7) 38.299 (7) 38.407 (50) 38.362 (12) 38.357 (18) 38.361 (8) 38.405 (30) 38.316 (10) 38.411 (10) 38.976 (14) 38.937 (7) 39.034 (21) 39.041 (8) 39.138 (10)

18.709 (4) 18.708 (12) 18.245 (8) 18.521 (6) 18.377 (4)

15.538 (4) 15.541 (10) 15.525 (7) 15.502 (6) 15.514 (5)

38.559 (11) 38.579 (30) 38.247 (18) 38.436 (15) 38.486 (14)

Pb (ppm)

Manihiki Plateau

317A-31-1 (133-135)

U L

317A-31-2 (52-56)

U

317A-31-3 (40-42)

U

317A-31-3 (108-110)

U

317A-32-1 (27-29)

U

317A-33-3 (124-126)

U

317A-34-1 (141-143)

U

317A-34-2 (115-117)

U L

317A-34-4 (72-74)

U

SOTW-11, 78D-14

U L

SOTW-11, 78D-15

U L

SOTW-11, 78D-16

L

0.28 0.24

0.30

0.44 O.23

0.21 0.21 0.16 0.34 0.31

Ontong Java Plateau

289-132-3 (52-55)

U L

P-43

U

P-384

U

8393

U

0.31 0.19

Pb isotopic ratios are present-day values. Within-run error on individual measurements is given in parentheses beneath the respective ratio, and corresponds to the last digit(s) of number above. Pb isotopic ratios are corrected for fractionation using the NBS 981 standard values of Todt et al. [68]; the total ranges measured are _+0.008 for 2°6pb/Z°4Pb, +_0.008 for 2°7pb/2°4pb, and _+0.030 for 2°8pb/2°4pb. Total procedural blanks are negligible at 10-40 picograms. Estimated uncertainty in Pb abundances is < 1%.

r o u g h l y t h e a g e o f t h e t o p o f b a s e m e n t a t Sites 317 a n d 289 [e.g., 28,35; R . D u n c a n , p e r s . c o m m u n . , 1989], a n d i n t h e r a n g e o f b a s e m e n t a g e e s t i m a t e s

f o r M a l a i t a (ca. 9 0 - 1 2 0 M y r [e.g., 2,30; R. D u n c a n , p e r s . c o m m . , 1989]). To test for possible effects of seawater alter-

200

J.J. M A H O N E Y A N D K.J. S P E N C E R

TABLE 2 Nd and Sr isotopic and isotope dilution data for selected Manihiki and Ontong Java basalts Sample

CNd (0)

(87Sr/86Sr)0

Nd (ppm)

Sm

U L

+ 4.2

0.70445 0.70403

0.8920

0.4302

L

+3.6

0.70413

3.573

1.220

L U L U L

+ 3.5

1.345

0.5766

+ 6.6 + 6.4 +6.7

0.70427 0.70365 0.70345 0.70377 0.70359

3.042 2.593 3.012

L

+6.9

0.70350

5.136

U b L U h L

+3.9

0.70423 0.70405 0.70456 0.70435

Sr

Rb

eNa (T) a

(87Sr/86Sr)T a

0.39 0.28

+ 2.8

0.70440 0.70399

105.3

0.81

+3.4

0.70409

1.00 2.15 1.89 2.20 1.71

+ 2.6

1.299 1.014 1.185

107.3 39.38 45.84 57.37 42.49

+ 5.6 + 5.8 +6.0

0.70422 0.70338 0.70325 0.70358 0.70339

1.726

63.13

3.45

+6.8

0.70323

1.63 0.14 6.00 3.20

+4.1

0.70417 0.70404 0.70435 0.70423

Manihiki 317A-31-1 (133-135) 317A-31-3 (108-110) 317A-34-2 (115-117) SOTW-1178D-14 SOTW-1178D-15 SOTW-1178D-16

40.93 38.83

Ontong Java P-43 8393

+ 3.7 + 4.0

10.63 8.448 4.571

3.291 2.581 1.500

137.5 112.2 138.5 133.0

+ 3.9 + 4.0

~ Nd and Sr isotope ratios are age-adjusted to T = 1 2 0 m.y. b Unleached data for P-43 and 8393 are from [15]. Isotopic fractionation corrections are 148NdO/144NdO = 0.242436 (148Nd/144Nd = 0.241572) and 86Sr/S~Sr = 0.1194. D a t a are reported relative to University of Hawaii standard values: for La Jolla Nd, 143Nd/la4Nd = 0.511855; for BCR-1 1 4 3 N d / l ' ~ N d = 0.512630; for NBS 987Sr, 87Sr/86Sr = 0.71025; for E&A Sr, SVSr/86Sr = 0.70803. The total range measured for La Jolla Nd is +0.000012 (0.2e units); for NBS 987 it is _+0.000022. Within-run errors on the isotopic data above are less than or equal to the external uncertainties on these standards. Total blanks are negligible: < 20 picograms for N d and < 200 picograms for Sr. Uncertainties in Nd and Sm abundances are estimated at < 0.2%; in Sr and Rb, < 0.5% and ca. 2% respectively. eNd(T) = 0 today corresponds to 143Nd/144Nd = 0.512640; CNd(T ) = 0 at 120 m.y. corresponds to (143Nd/144Nd)v = 0.512486 for ~4VSm/144Nd = 0.1967.

ation on Pb isotopes, five sample powders were split prior to dissolution; one fraction was subjected to the harsh, multi-step acid leaching procedure [15,37] used previously for removing lowtemperature alteration phases and recovering near-magmatic Sr isotopic ratios. This procedure, which produces a residue of mainly well-crystallized plagioclase and clinopyroxene in moderately altered tholeiitic specimens, was also applied for Sr isotopes to several samples, including two Malaitan lavas which Mahoney [15] did not leach. Comparisons of leached and unleached ( L and U, respectively) sample pairs are shown in Tables 1 and 2. In the earlier study, leaching produced no measurable changes in calculated initial CNd between leached and unleached splits of Manihiki and Ontong Java powders, but reduced (sometimes dramatically) the calculated (SVSr/86Sr)l. Where it has been possible (not with any of the present samples) to compare results on fresh glass

or plagioclase separates with those on leached powders of crystalline, moderately altered tholeiitic basalt from the same rock chip, the calculated initial isotopic ratios have generally been identical, within errors, for both Sr and Nd [15; Mahoney and Spencer, unpublished data]. Note, however, that because leaching tends to modify bulk composition, elemental abundances and interelement ratios in leached samples should not be taken as representative of magmatic values [15]. 3. Results

As with Nd, Sr, Sm, etc., Pb concentrations in the unleached samples (0.16-0.44 ppm) are within the range of values for normal MORB. Acid leaching did not cause significant or consistent changes in Pb isotopic values (although Pb abundances plummeted by a factor of about two in the leached residues of two of the more altered specimens).

ORIGIN

OF THE MANIHIKI

AND ONTONG

JAVA OCEANIC

201

PLATEAUS

40.0

JFSF

~_

Samoan / ~

a_ 59,0

Kerguelen 38,0

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ONTONG JAVA •

t~-~'/

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MANIHIKI A

r5.8- B

Louisville Nauru a ~ . j . m ~ / _ ~ . . .

o 04

o 04

pc" , ~ /e/~.~/ ~ . . . . ~ - - ~ - ~ - ~/'~/f),~A~ ~

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PAC-N.ATL MORB I

18

L

19

I

I

20 206pb / 204 Pb

]

I

21

I

I

22

Fig. 2. (A) Present-day 2°8pb/2°4pb vs. 2°6pb/2°4pb values for samples from the Ontong Java (circles and dots) and Manihiki (triangles) plateaus. References for other data fields are: MORB [58 and references therein], Cook-Austral Islands [39,40], Louisville Ridge [37], Koolau, Hawaii [41,42], Pitcairn (PC) [38], Rarotonga (RA) [39,40], Samoan shields [39,69], Easter and Sala y Gomez (Easter) [54], Kerguelen Islands [55], Nauru Basin [43], Juan Fernardez and San Felix ( J F - S F ) [53]. Five leached-unleached sample pairs are shown; each pair is connected by a bracket, with the leached data represented by white symbols. (B) Present-day 2°7pb/2°4 Pb vs. 2°6pb/Z°4pb values for the same samples; fields and data symbols as in (A). Error bars are for data in this paper.

This was true even in quite altered samples where the Sr isotope ratio dropped substantially, such as the Site 289 basalt [(87Sr/86Sr)T = 0.70426 before, vs. 0.70342 after leaching [15]]. This result suggests that, like Nd isotopes, Pb isotopes have been affected little by seawater in these rocks. The present-day 2°6pb/Z°4pb ratios of the Manihiki samples range from 18.095 to 18.398 for the Site 317 lavas; their 2°sPb/z°apb varies only slightly, between 38.299 and 38.411 (Fig. 2). The SOTW samples have substantially higher 2°6pb/Z°4pb (19.112-19.345), however, and concomitantly greater 2°spb/Z°apb (38.937-39.138). Values for

the four Ontong Java basalts are similar to those of the Site 317 lavas but extend to somewhat greater 2°6pb/Z°4pb (18.245-18.708) and to slightly higher 2°spb/2°4pb (38.247-to 38.579). Most of the data for both plateaus are distinctly OIB-like, plotting well above the field for Pacific MORB in Fig. 2A. With the exception of the SOTW samples, their 2°6pb/Z°4pb ratios are significantly lower for a given 2°spb/2°4pb than found in the majority of South Pacific oceanic islands. Broadly similar compositions occur on Pitcairn Island [38] and Rarotonga [39,40], however, and in the North Pacific among the Koolau,

JJ. MAHONEYAND KJ. SPENCER

202

12 EPR

ONTONG-JAVA MANIHIKI

I0 8 OE~Nauru 6

B

Louisville ~

~°x~,lau~

~4 2 0

J-

-2

~

~.~erguelen

A

-4 L

I

0.705

0.704

I

0.705 (87Sr/86Sr) T

i

0.706

0.707

0.706 Somoan Shields

+ Kerguelen

0.705

)

FO3 ~0 O3 p_CO0.704 o3

Koolau

B

0.703 I/

12

f

t

8

ej~y ~

~4 kuz pc~

L)Kerguelen

Cook-Austral

~SF

C

-4

17

, i8

! 19

I 20

] 21

22

2o6pb/2O4pb Fig. 3. (A) Age-adjusted (to 120 m.y.) Nd and Sr isotopes for Ontong Java and Manihiki Plateau samples from this study and [15]. Sr isotopic data are for leached samples. Other data fields are as in Fig. 2 except the East Pacific Rise (EPR) field, which is from [70]. (B) 2°6pb/2°apb (present-day ratios) vs. age-corrected Sr isotopic ratios. (C) 2°6pb/a°4pb vs. age-adjusted Nd isotope ratios. Symbols and fields are as in Fig. 2. Error bars are for data in this paper.

ORIGIN

OF THE MANIHIKI

AND ONTONG

JAVA OCEANIC

PLATEAUS

West Maui, Kahoolawe, and Lanai tholeiites of the Hawaiian chain [41,42]. It is important to note that the relative positions between the plateau and various island fields in Fig, 2 continue to hold even when changes in Pb isotope ratios over the last 120 m.y. (caused by decay of Th and U) in both the plateau lavas and the mantle sources of the modern oceanic islands are taken into account. Considering 2°6Pb/2°4Pb, for example, the 23~U/ 2°4Pb of the plateau samples is probably similar to or somewhat more than a typical MORB-like value of ca. 10 (and could be larger due to U uptake from seawater during alteration soon after eruption), as is the case for the chemically rather similar Nauru Basin lavas [43]. For typical oceanic island s o u r c e s , 238u/Z°4pb should resemble the values measured in fresh island basalts (e.g., ca. 10-12 for most Pitcairn samples [38]; a more typical oceanic island tholeiite value is ca. 15-20 [e.g., 44]). Values of 10 and 20, respectively, correspond to changes in 2°6pb/2°4pb of 0.19 and 0.38 in 120 m.y. Sr and Nd isotopic ratios of the new Manihiki Site 317 samples analyzed here are in the range previously measured, but the SOTW lavas exhibit significantly greater (though still OIB-like) c ya ( T ) and lesser (SVSr/S6Sr)l ( + 6.8 to + 5.6 and 0.70323 to 0.70339, respectively; Fig. 3A). Unlike most of the Site 317 or 289 basalts, leaching did not cause a marked reduction in Nd, Sin, Rb ([15]; see also the leached Site 317 data in Table 2) or Pb (Table 1) concentrations in the SOTW samples, probably because they are comparatively fresh, with glassy groundmasses and with alteration confined principally to their olivine phenocrysts [11]. The reduction with leaching in 87Sr/86Sr for SOTW-1178-14 and -15 was comparatively modest as well. Sr isotopic values for the newly leached lavas from Malaita are lower than for their unleached counterparts (Table 1), but the difference is much less than for the highly altered basalt from Site 289 (where a change of -0.00084 occurred [15]). The Malaitan samples are fresher than the Site 289 basalt; however, it also is possible that prolonged natural leaching by abundant, fresh groundwater in the tropical environment of Malaita has removed some seawater Sr from the rocks. On a Pb-Sr isotope diagram (Fig. 3B), most of the data again lie well above the Pacific (and

203 Atlantic) MORB field, and in the Pacific only Pitcairn, Rarotonga and the aforementioned Hawaiian volcanoes have comparable isotopic compositions. On a Nd-Pb isotope plot (Fig. 3C), the plateau data fields fall well away from fields for both MORB and the majority of Pacific Ocean islands (again, with the exception of Rarotonga, several Hawaiian shields, and Pitcairn). Compared to the Ontong Java samples, lavas from the extensive flow and sill complex filling the Nauru Basin to the east of the Ontong Java Plateau possess somewhat higher 2°6pb/2°4pb, greater eNd(T) and lower (S7Sr/86Sr), [43,15] than all but the Site 289 basalts. Also, unlike either the Ontong Java or Manihiki plateau rocks, the Nauru Basin lavas show very little variation in Nd and Sr isotopes. Note that in Fig. 3 age-corrected Nd and Sr isotopic data for the plateaus are plotted together with fields for recent oceanic ridges and islands (other than the Cretaceous Nauru Basin and 0-70 m.y. Louisville Ridge fields); such a comparison is meaningful despite the difference in ages. Even with the undoubted elevation of Rb by seawater interaction, ~7Rb/86Sr is quite low in the fresher unleached plateau samples [15]; and 87Rb/S6Sr in their sources should be comparable or lower. Among the fresher samples, the highest values by far are in the SOTW lavas (0.1109-0.1580) and correspond to changes in ~TSr/~6Sr of only 0.00019-0.00027 in 120 m.y. The ~47Sm/~44Nd ratio in the unleached samples is significantly different from the chondritic value (0.1967) only for the unleached SOTW basalt (0.2378). Because the sample reflects a very large degree of shallow melting [11,12] (and because 14VSm/14'~Nd is not affected appreciably by moderate levels of seawater alteration), its 1478m/ln4Nd should mirror that in its source fairly closely; yet this value corresponds to a change in end of only 0.6 in 120 m.y. Thus, if for some reason the plateau sources had not melted until today, the available evidence suggests that their resulting end and S7Sr/S6Sr would not have been greatly different than at 120 m.y. Similarly, the isotopic compositions of seamounts formed in the South Pacific between 120 and 80 m.y. ago closely resemble those of present-day South Pacific hotspot volcanoes, indicating that isotopic evolution of the hotspot sources since the Cretaceous has been limited [45].

204 4. Discussion

Because the plateau Pb isotopic data, as well as the new and earlier Nd and Sr isotopic results, are well within the range encompassed by oceanic, mantle-derived rocks, they offer no obvious support to the case for old continental basements under either plateau. On the continents, flood basalt lavas possessing broadly similar isotopic characteristics are common, but so are those with values completely outside the oceanic mantle field, reflecting the influence of ancient continental crust or lithosperic mantle; indeed, in several major provinces the vast majority of flows exhibit such compositions [e.g., 46]. Interestingly, independent evidence for a lack of continental material beneath Malaita comes from two well-studied 34 m.y.-old alnoite intrusions on the island which postdate the eruption of the basement lavas by 60-80 m.y. Crustal igneous xenoliths in these intrusions are, exclusively, altered and metamorphosed basalt [e.g., 47], whereas mantle xenocrysts and the alnoites themselves have initial Nd and Sr isotopic values [48] quite similar to those of the upper-level basaltic basement flows reported here. Although consistent with an oceanic origin, the isotopic results now available require sources for both plateaus with characteristics distinct from those of modern Pacific MORB and most South Pacific Oceanic islands. Most present-day oceanic islands in the South Pacific, as well as many Cretaceous seamounts now north of the Equator, were formed between 10°-29°S, 170°-110°W in a long-lived region of hotspot volcanism that has been termed the " S O P I T A " (South Pacific Isotope and Thermal Anomaly) [45]. To a large extent, the mantle sources of volcanoes formed in this region can be considered as variable mixtures of two hypothetical end-member components: primarily a very high 2°6pb/2°4pb ( > 21.5), low 87Sr/86Sr (<0.7030), relatively high eya (ca. +5) "St. Helena" [49] or " H I M U " [e.g., 50] type, and a very high 8VSr/86Sr (>0.707), low ~Na ( < 0 ) "Samoan" or " E M I I " type with 2°6pb/2°4pb around 19.0. A third "Kerguelen-type" [49] or " E M I-type" [e.g., 50] end-member, characterized by lower 2°6pb/Z°4pb and 87Sr/S6Sr than the Samoan-type component, is typically associated with the Indian Ocean and appears relatively unimportant in most recent and all measured Creta-

J.J. M A H O N E Y

A N D K.J. S P E N C E R

ceous SOPITA volcanoes (as old as 120 m.y.) studied to date [45]. Previously, the Nd and Sr isotope systematics of the Pacific plateau lavas were interpreted in terms of mixing between MORB-like and OIB-like mantle [15]. Our Pb isotope data are consistent with mixing but indicate that mixtures of MORB and Samoan-type mantle, for example, cannot explain the Manihiki or Ontong Java data (see Fig. 2A and 3C). Instead, the relatively low 2°6pb/ZO4pb, high 2°sPb/ 2°4Pb and moderate 87Sr/86Sr values of most of the samples point to the presence of a Kerguelentype component in the source of each plateau. Moreover, any MORB-like end-member involved must have been near the high 2°6pb/2°4pb end of the Pacific MORB spectrum; alternatively, a second, higher 2°6pb/2°4Pb OIB-like component was also involved. What might be the specific origin of the rather similar, OIB-like isotope signatures in the two plateaus? In particular, can they be linked to any long-lived, still-active hotspots? There are very large uncertainties inherent in estimates of pre-80 m.y. Pacific plate positions, hotspot fixity, and the amount of Cretaceous true polar wander. However, the present-day Pacific oceanic island sources with low 2°6pb/2°4Pb characteristics generally resembling those of the plateau lavas (Hawaii, and in the SOPITA, Pitcairn and Rarotonga) are all situated far away from the postulated original (ca. 120 m.y.) locations of either the Manihiki or Ontong Java plateaus. Even without taking the possibly large degree of true polar wander into account, the paleolatitude of ca. 33°S measured for the basement basalt and lowermost sediments at Site 289 [51] would seem to put the Ontong Java Plateau's origin outside o f - - o r at least a t - the southern limit of the SOPITA. Some paleogeographic reconstructions of the Pacific place the plateau as far south as the present locus of the Louisville hotspot (50°S, 139°W) at about 120 m.y. (Fig. 4); and Gordon and Henderson [20], Mahoney [15], and Richards et al. [21] have previously considered the possibility that the Louisville hotspot could have been involved in the formation of the plateau. If correct, then isotopic similarities between Ontong Java basalts and more recent products of the Louisville hotspot might be expected, especially in view of the fact that the Louisville hotspot has displayed a remarkably uni-

ORIGIN OF THE MANIHIKI AND ONTONG JAVA OCEANIC PLATEAUS

205

180

5 I "

'U

~

~ "':~,

',."5c~,. -~',. ',

\

.

--

\

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~

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/

...... '

>,~

~

2.'5o~,

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Fig. 4. Plate reconstruction at 120 m.y. from Firstbrook et al. [71] showing approximate locations of Manihiki and Ontong Java Plateau, and present-day hotspot locations. Note that the paleopositions of the plateaus (and other Pacific seafloor features) become increasingly uncertain for ages greater than about 80 m.y.

form isotopic signature for at least the last 70 m.y. [37]. However, the 0 - 7 0 m.y. Louisville fields in Fig. 2 and 3 are markedly different from those defined by the Ontong Java samples, even after allowance for differences in age; in particular, the Louisville Ridge is characterized by much higher 2°6pb/2°4pb (19.1-19.5). Nor do the Ontong Java data lie along possible mixing trends between Louisville and modern Pacific M O R B compositions. For the Manihiki Plateau the situation is even more problematic, as paleogeographic reconstructions do not place it unambiguously near any presently active hotspots. Paleomagnetic data (unadjusted for true polar wander) for the basalts and lower sediments from Site 317 allow a wide possible range of paleolatitudes, from 19°S to 48°S [52]. The closest hotspots (Fig. 4), which are all probably within the large range of uncertainty in the paleoposition of the plateau, are Juan Fernandez, San Felix (both well outside the

SOPITA), and Easter or Sala y G o m e z (at the southeastern edge of the SOPITA) [20; B. Keating, pers. commun., 1989]. Earlier volcanic products of these hotspots have not yet been studied, but all recent examples have rather high 2°6pb/ 2°4pb (18.9-19.3 for San Felix and Juan Fernandez [53], 19.3-19.9 for Easter and Sala y Gomez [54]) and low 87Sr/86Sr. Interestingly, Pb, Nd and Sr isotopic results for the SOTW rocks plot reasonably close to available values for Easter, and are not too distant from the Juan Fernandez field; however, the Site 317 samples do not appear to have any isotopic counterparts in these hotspots. Although data for San Felix can be interpreted as reflecting a mixed source containing Kerguelentype and high 2°6pb/Z°4pb Saint Helena-type components [53], the Kerguelen-like component in the San Felix hotspot today must be much less prevalent than in the source of the Site 317 rocks. The simplest explanation, of course, is that the two plateaus are completely unrelated to the above

206 present-day hotspots. Alternatively, the compositions of the possible source hotspots could have changed substantially over time; i.e., a previously important Kerguelen-type influence could have dwindled to insignificance since the Cretaceous. Clearly, such a component was not distributed uniformly even in the sources of the plateaus themselves, as attested by the comparatively high ~Nd(T) and high 2°6pb/Z°4pb SOTW and Site 289 data (e.g., Fig. 3C). Campbell and Griffiths [23] have postulated that mantle plumes may be chemically zoned, and that major changes in the OIB components of a hotspot could occur commonly, particularly between the initial plume-head phase and the ensuing plume-tail stage. Indeed, the isotopic differences between the Ontong Java and Louisville Ridge samples indicate that if the Louisville hotspot was involved in plateau genesis, loss of a Kerguelen-type component would have had to take place fairly soon after the formation of the uppermost lavas at Site 289 and on Malaita (i.e., between about 120-90 m.y.) and 70 m.y. ago (the age of the oldest Louisville seamount not subducted along the T o n g a - K e r m a d e c trench [e.g., 37]). Although major temporal changes in the isotopic composition of hotspots are certainly possible (see [42] for example), they appear not to be universal; the Kerguelen hotspot itself, for instance, has retained a distinctive (although somewhat variable) signature since its apparent birth more than 115 m.y. ago [e.g., 55-57]. Some workers have suggested that the distinctive mantle endmember at Kerguelen may have been introduced (entrained) into the underlying mantle plume at shallow levels [55,58]; in contrast, others prefer a deep origin within the plume itself [e.g., 57]. Whether such material was already abundant at shallow depths beneath the old Pacific [cf. 43] (as it appears to be today beneath much of the Indian Ocean) or was brought up from deeper levels around the time of plateau formation is an important question; unfortunately, its resolution must await the sampling and isotopic characterization of sufficient amounts of early (older than the plateaus) south Pacific abyssal seafloor not associated with plateaus. We note here, however, that the Nauru Basin complex, adjacent to and apparently roughly contemporaneous with the Ontong Java Plateau, and west of the Manihiki,

J.J. MAHONEY AND K.J. SPENCER

possesses a much weaker Kerguelen-type signal [43] than the Ontong Java or Manihiki lavas, possibly favoring the latter alternative. In any event, the available isotopic results do not provide a simple test of the connection between specific, presently active hotspots and the origin of the Manihiki or Ontong Java plateaus. They do indicate that isotopically OIB-type mantle fed at least the upper lavas on both plateaus, which were in widely separated locations 120 m.y. ago. In conjunction with the immense crustal volumes of the plateaus and the very large degrees of partial melting recorded by the SOTW basalts [11,12], in particular, this evidence strongly indicates the involvement of hotspots or mantle plumes.

4.1. Plume head and near-ridge hotspot hypotheses for plateau origin With regard to the plume-initiation model, we note that the head volume suggested by Richards et al. [21] (ca. 11 x ] 0 6 km~), though sufficient to explain the volumes of most continental flood basalts, is far too small to account for the much vaster crustal volumes of either the Manihiki or Ontong Java plateaus. The Ontong Java Plateau, for example, has a volume greater than 50 x 106 km s [15,59]; assuming that its crust is composed of basalts which reflect an average degree of source melting of 20%, a source of at least 250 x 106 km 3 is necessary. This requirement can be satisfied if the volume of a typical plume head is significantly larger than the value used by Richards et al. [21], a possibility these authors admit. Indeed, Campbell and Griffiths [23] contend that a volume of ca. 500 X 106 km s is more reasonable. A testable corollary of the plume-initiation model is that the great bulk of plateau-building volcanism should take place in only a few million years [21,23]. On the other hand, the simplest incarnation of the near-ridge hotspot hypothesis, wherein the hotspot is robust but more or less steady-state, predicts that substantial basement age progressions should exist across large plateaus. For example, a range of as much as 60 m.y. could be represented on the Ontong Java Plateau if it formed at a 20 m m / y r spreading rate, as argued by Hussong et al. [2]. As a comparison, the present Icelandic Plateau, which has an area roughly six times less than that of the Ontong Java, spans

ORIGIN OF T H E MANIHIKI A N D O N T O N G JAVA OCEANIC PLATEAUS

ca. 25 m.y. in age [e.g., 60]. Unfortunately, the necessary samples and geochronological data to make this test are not yet available for any of the old Pacific plateaus. Interestingly, however, seafloor magnetic anomalies adjacent to the ca. 600,000 km z Shatsky Rise indicate that it was probably constructed over a period of at least 24 m.y. at a triple junction between the Pacific, Farallon and Izanagi plates [8,61]. As noted earlier, non-geochemical evidence also suggests that the other Pacific plateaus were formed during times of relatively slow spreading on comparatively young, lithosphere, thus also favoring a near-ridge origin [e.g., 6,7,2,10,20]. Moreover, there is evidence that once a ridge axis becomes established near a robust hotspot, the ridge can be anchored or trapped in the vicinity for many millions of years, as required in the near-ridge hotspot hypothesis; this tendency is well-illustrated today by the Galapagos [62] and Icelandic [24,63] hotspots. The lack of obvious seamount chains attached to the Ontong Java and Manihiki plateaus is also explained most easily in a near-ridge context, as it could simply be a consequence of a ridge jump (new propagation episode) or ridge migration over the hotspot, shifting the center of volcanism to the adjacent plate and severing the plateau from the subsequent trace of the hotspot. Examples are provided by the Galapagos again [e.g., 62], and by Kerguelen [e.g., 24]. We emphasize, however, that the near-ridge hotspot and plume-initiation hypotheses are not at all mutually exclusive, inasmuch as the former requires a bigger melting anomaly the larger the plateau, whereas the latter does not exclude plume heads from rising under or n e a r - - o r becoming associated with--spreading centers. With regard to the first point, the Ontong Java Plateau, for example, would reflect a plume some 3-4 times larger than that beneath the Icelandic Plateau, assuming formation times of 60 and 25 m.y. respectively, and an average Icelandic Plateau thickness of 20-25 km. As for the second p o i n t - - a n d perhaps more importantly--it seems highly likely that a plume head encountering the base of the lithosphere at a location even a fairly large distance away from a ridge crest would nonetheless become associated with a spreading center quite quickly, because it would induce rifts to propagate

207

toward or away from it. Okal and Cazenave [64] have suggested that an ordinary hotspot may be able to influence rift propagation as far away as 500 km; for a large plume head, the radius of influence should be correspondingly much greater. Another model, proposed recently by White and McKenzie [65] to account for continental flood basalt and related continental margin volcanism, depends on rifting of continental lithosphere above the hot, spread-out top of a steadystate, ordinary mantle plume, followed by rapid decompressional melting of the plume top (although the authors also mention a possible role for a small ca. 5 × 1 0 6 km 3 initial plume head). Like the near-ridge hotspot hypothesis for oceanic plateaus, this scheme features a rift near a hotspot, but is more like the plume-initiation model in that it predicts the bulk of edifice-building volcanism to occur cataclysmically in only a few million years. Taken literally, this model is not obviously relevant to the Ontong Java or Manihiki (or other old Pacific) oceanic plateaus because no evidence exists for the presence of sizeable continental lithospheric lids a b o v e - - o r anywhere n e a r - - t h e original locations of either plateau. There is no a priori reason, however, why mature oceanic lithosphere could not substitute for continental lithosphere [cf. 65]. The utility of this model is difficult to assess for the plateaus, but the chief requirement appears to be a long period of time prior to rifting during which there is very little plate motion above the plume, so that an extensive, flattened plume top may be created. This feature suggests that the volume of a plateau should reflect the size of the plume top and therefore the period of time over which the thermal anomaly was emplaced. To form a > 2.5 × 108 km 3 plume top of minimum volume suitable for the source of the Ontong Java Plateau appears to require at least 40 m.y., assuming a 200 m3/s plume flux comparable to that estimated for large present-day plumes [23]. Moreover, the plume top presumably would have to be emplaced beneath already fairly old lithosphere, i.e., far away from any spreading center which would otherwise tap rising plume material and inhibit growth of the top. These considerations suggest that the Ontong Java Plateau should be bounded by and partly overlie much older crust, probably substantially more than 40 m.y. older in places. Also, because volcanism

208

commences with a new spreading center cutting through older lithosphere above the plume top, a discontinuity would be expected in seafloor ages adjacent to the plateau. The oldest magnetic anomalies in the general region (M25-28) lie northeast of the plateau's northern margin [e.g., 66], and are ca. 40 m.y. older than the top of basement at Site 289. Other lineations close to the plateau in the east [e.g., 66] and west [2] are younger, however, and as far south as they have been well-documented (to roughly the latitude of Site 289), show a continuous southward decrease in crustal age to about 130 m.y., rather than an age jump. As for the Manihiki Plateau, the oldest magnetic anomalies bordering it ( M 0 - M 3 ) [67] are virtually the same age as the basement at Site 317. With the presently available information, it seems difficult to avoid the need for either (1) a large plume head (particularly for an immense plateau like the Ontong Java) or (2) protracted near-ridge volcanism at slow relative (to the plume) spreading rates above a robust but ordinary plume (tail), or a small plume head and plume tail combination. Further understanding requires sampling, high-precision 4°Ar/3°Ar dating, and geochemical study of basement rocks from various locations across each plateau. 5. Conclusions Pb isotopic values for basalts from the Manihiki and Ontong Java plateaus are similar and, like the corresponding Nd and Sr isotopic ratios, well within the range of oceanic island or hotspot lavas. The data thus provide no evidence of continental affinities, but strongly support the hypothesis of major hotspot involvement in the origin of these plateaus, either as plume heads [21] (implying plateau formation times of only a few million years) or as robust, near-ridge hotspots [15] (requiring much longer periods). However, the plateau samples are isotopically unlike most modern Pacific Ocean islands (and all Pacific MORB) in that most have relatively low 2°6pb/2°4pb, high 2°spb/2°4pb, and moderate (SVSr/86Sr)~ indicative of mantle sources with Kerguelen-like (or EM-I) isotopic attributes. Such characteristics appear not to be present today, to any extent, at the currently active hotspots that plate reconstructions suggest

J.J. M A H O N E Y

A N D K.J. S P E N C E R

could have been possible plateau sources; nor have they been for at least the last 70 m.y. in the case of the Louisville hotspot, a putative Ontong Java source. Therefore, these particular hotspots may not have been involved in the formation of the plateaus. Alternatively, assuming that the presumed association between these hotspots and plateaus is correct, the hotspots must have changed compositionally over time, possibly during transitions from plume-head dominated melting to melting within plume tails soon after plateau formation. In view of the wide range of Pacific plateau sizes, the near-ridge hotspot and plume-initiation concepts both may be valid. In particular, we suggest that plume heads are very likely to become associated with spreading axes via their influence on rift propagation, which should be considerably greater than that of ordinary plumes (tails), thus reconciling the plume-initiation model with evidence indicating the plateaus originated near ridges.

Acknowledgements Constructive reviews by Jim Natland, Andy Saunders and Hubert Staudigel are much appreciated. We thank Dave Clague and the Deep Sea Drilling Project for providing samples, Nancy Hulbirt for doing the illustrations, and Angie Lau and Carol Koyanagi for typing several versions of text and tables. Part of this research was supported by N S F grant EAR88-16192. This is SOEST contribution 2407.

References 1 D.T. Sandwell and K.R. MacKenzie, Geoid height versus topography for oceanic plateaus and swells, J, Geophys. Res. 94, 7403 7418, 1989. 2 D.M. Hussong, L.K. Wipperman and L.W. Kroenke, The crustal structure of the Ontong Java and Manihiki plateaus, J. Geophys. Res. 84, 6003 6010, 1979. 3 S. Uyeda and V. Vacquier, Geothermal and geomagnetic data in and around the island of Japan, in: The Crust and Upper Mantle of the Pacific Area, L. Knopoff, G.L. Drake and P.J. Hart, eds., pp. 349-366, Am. Geophys. Union, Washington, D.C., 1968. 4 R.L. Larson, S.M. Smith and C.G. Chase, Magnetic lineations of early Cretaceous age in the Western Equatorial Pacific Ocean, Earth Planet. Sci. Lett. 15, 315-319, 1972.

ORIGIN

OF THE MANIHIKI

AND ONTONG

JAVA OCEANIC

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