Ancient seafloor signals in Pitcairn Island lavas and evidence for large amplitude, small length-scale mantle heterogeneities

Earth and Planetary Science Letters, 94 (1989) 257-213 Elsevier Science Publishers B.V., Amsterdam - Printed

257 in The Netherlands

Ancient seafloor signals in Pitcairn Island lavas and evidence for large amplitude, small length-scale mantle heterogeneities Jon D. Woodhead and Malcolm T. McCulloch Research School of Earth Sciences, Australian

Received

February

National

University, Canberra, A.C. T. 2601 (Australia)

13, 1989; revised version accepted

July 3, 1989

Isotopic data for Pitcaim island volcanic rocks reveal the presence of two markedly different sources which appear to be closely related both spatially and temporally. The isotopic variability (or amplitude ratio) of this suite relative to all other oceanic basalts (mid-ocean ridges and oceanic islands) is - 30% for Pb, - 40% for Sr and 50% for Nd. This confirms the findings of several recent studies indicating that, contrary to earlier conclusions, large amplitude variations can exist over limited ( - 10 km) length scales. The unusual, unradiogenic 2csPb/2w Pb and 143Nd/144 Nd and radiogenic 87Sr/s6Sr isotopic compositions, lack of strong covariation between isotopic and other geochemical parameters, and variable Nb/U ratios place strong constraints on any petrogenetic model. It is suggested that the two magma types may reflect the presence of variable quantities of ancient subducted oceanic crust and ancient subducted sedimentary material in the mantle source beneath Pitcairn. The critical role of subduction zone “processing” in modifying the chemical composition of such sources is stressed; thus, for example, the trace element composition of modem pelagic sediment cannot be used as a direct analogue of the subducted sediment residuum which may be important in the source of Pitcaim and some other ocean islands. The complementary trace element abundance patterns often noted between ocean island basalts (OIB) and island arc tholeiites (IAT) are seen as a direct result of this process.

1. Introduction

2. Geological background and previous work

It is now generally accepted that the mantle is both chemically and isotopically heterogeneous. A major problem, however, is constraining both the amplitude and length scales over which these heterogeneities exist. For example, a large-scale (> lo3 km) isotopic anomaly (Dupal) has been proposed by Hart [l] for the Southern Hemisphere mantle. With the notable exception of the Hawaiian Islands, evidence for this large-scale anomaly has been mainly based on analyses of isolated and chronologically poorly constrained samples from ocean islands. Here we present the results of a detailed isotopic study from a single ocean island, Pitcairn, in an attempt to constrain the amplitude and origin of small (- 10 km) length-scale heterogeneities in ocean islands. The geochemical and isotopic data suggest that a complementary relationship may exist between subduction-related and oceanic island volcanism.

Pitcaim lies in the remote southeast Pacific Ocean (20”04’S, 130”06’W), a small volcanic island, approximately 4 X 2 km in size, rising to an altitude of 347 m (Fig. 1). Its isolation is extreme-being located nearly 2100 km to the west of Easter Island, near the East Pacific Rise, and over 160 km from the nearest volcanic island, Mangareva, in the Gambier Group of French Polynesia. Pitcairn’s immediate neighbours, Oeno, Ducie and Henderson, are all coralline. Linear age progressions, recording plate motions, are well documented in the islands of this part of the Pacific [2]. The age progression of the PitcaimGambier-Duke of Gloucester lineament is less well-defined but still in accord with the “ hotspot” hypothesis and a plate migration rate of 11 cm/yr [3]. A recent tectonic investigation of this area, based upon SEASAT data [4], has revealed the presence of two fracture zones in the region, one

0012-821X/89/$03.50

0 1989 Elsevier Science Publishers

B.V.

258

25 Pitcairn Island lavas

135

130

120

125

115

'a

CHRISTIANS

9;;;i;:

CAVE

ADAMSTOWN

PULAWANA

TEDSIDE

FMN.

VOLCANIC9

VOLCANICS

VOLCANIC9

1.0 km

I

Fig. 1. Generalised geology of Pitcairn after Carter [lo]. Inset shows southeast Pacific tectonic Solid lines represent East Pacific Rise and two fracture zones (FZl and FZ2). Three “hotspot”

of which is speculated to control volcanism on the Oeno-Henderson-Ducie lineament (Fig. 1). However, Pitcairn lies well to the south of these features and they do not appear to have any influence on the island. The volcanic edifice of Pitcaim rises at least 3500 m from the surrounding sea floor [5], which has an age, poorly constrained by magnetic anomaly data, to around 30 Myr [6]. Very few isotopic data have been previously published for Pitcaim (e.g. [7]) and in this new study a

setting, after Okal and Cazenave tracks are marked by asterisks.

[4].

combination of isotopic and trace element geochemical data has been integrated with previously determined K-Ar ages to provide an insight into the nature and timing of oceanic island volcanism. Detailed isotopic data are, as yet, unavailable for the other islands in the chain, although work is currently in progress [8]. The lavas and pyroclastic rocks exposed on Pitcaim appear to be the remnants of a single shield volcano, with an annular caldera rim form-

259

ing a spine to the island. Pioneering chemical analyses by Lacroix [9] revealed a similarity between these lavas and those of the Marquesas to the northwest. Subsequently, a detailed survey by Carter [lo] established four geological formations: the Tedside volcanics, gently dipping lava flows exposed near the west coast; the Christians Cave Formation, an agglomeratic tuff unconformably overlying the Tedside volcanics; the Adamstown volcanics, horizontal caldera-filling flows; and the Pulawana volcanics, a sequence of flows occurring in the extreme west (Fig. 1). In addition, a number of dykes occur, mainly within the Tedside volcanics. Duncan et al.‘s [3] study provided K-Ar dates for these formations and thus established the following eruption chronology:

Christians Cave Formation Adamstown volcanics: intra-caldera fill Pulawana volcanics: extra-caldera caldera collapse with ca. 0.1 Myr hiatus in volcanic activity Tedside volcanics: shield-building phase

0.6 Myr 0.63-0.45

Myr

0.67-0.62 0.95-0.76

Myr Myr

Thus volcanism at Pitcairn island was confined to a period of around 0.5 Myr in the Pleistocene. The current location of the Pitcaim “ hotspot”, if still active, is estimated to be about 100 km to the southeast of the island assuming an average plate migration rate of 11 cm/yr. Petrographically, the lavas and pyroclastic rocks which make up the island range from alkali olivine basalts through hawaiites and mugearites to minor trachytes. Detailed petrographic analysis of these samples will be presented elsewhere. The samples used in this study were largely collected in 1986 specifically with isotopic work in mind and, with the addition of two samples from the Pulawana volcanics used in the Duncan et al. study, provide a complete coverage of the known volcanic formations of Pitcairn. Intrusive rocks are not considered further in this study, as their temporal relationships are not well established. 3. Analytical techniques Major and trace element concentrations were determined at the Department of Earth Sciences in Oxford by X-ray fluorescence (XRF) tech-

niques on fused glass beads and pressed powder pellets respectively, using a Philips PW1400 spectrometer with data processing by PDPll minicomputer. Calibration lines were constructed using up to 25 recommended USGS standards. Details of counting times, operating conditions and detection limits can be found in [ll]. For all major elements precision is substantially less than 1% and, for trace elements, generally better than 5%. Uranium concentrations were determined by isotope dilution (see below) and precision, based upon duplicate sample dissolutions, is estimated to be better than 1%. In addition, four representative samples were analysed by spark source mass spectrography (SSMS) at R.S.E.S., Canberra for rare earth elements (REE), Th, Hf and Cs. Analytical details with estimates of machine sensitivity, precision and accuracy can be found in [12]. Sr, Nd, and Pb were separated by conventional ion exchange techniques on 100 mg samples. Chemical and loading blanks for Sr, Nd and Pb are considered negligible (ratios of Pb in sample: Pb in blank averaged 1000 : 1). Uranium samples were separated on anion exchange columns in 7N HNO, and the U and Th eluted with H,O and 6N HCl. Sr, Nd, and Pb samples were run on single Ta, Re-Ta double, and Re single filaments respectively. Uraniums were run on single Ta filaments with H,PO, (as for Sr). All samples were run on a Finnigan MAT 261 mass spectrometer used in multi-collector mode. “Sr/*‘Sr and 143Nd/144Nd ratios are normalized to s6Sr/s8 Sr = 0.1194 and 144Nd/146Nd = 0.7219 respectively. Over the course of this study analysis of NBS 987 Sr standard and the La Jolla Nd standard provided mean values of 87Sr/86 Sr = 0.710208 f 4 and 143Nd/‘44Nd = 0.511869 + 2 respectively. 20 precision in Table 4 refers to within-run statistics. Pb isotope data were corrected for mass fractionation by using data for the NBS 981 standard (mass fractionation averaging -0.13% per a.m.u.); absolute values adopted for this standard were from Catanzaro et al. [13], but using the “preferred” value 208Pb/204Pb = 36.700 of Richards [14]. As a further check on the Pb data, absolute values for the isotope ratios were determined on three samples by the use of a 207Pb-204Pb double spike technique [15]; these showed good agreement with the ratios corrected by the usual method.

260

4. Results 4.1. Major and trace element data An extensive body of data is now available from many oceanic islands which shows a clear distinction between a shield-building phase, often of tholeiitic character, and a subsequent phase of alkaline volcanism, termed post-caldera or posterosional, since the two are frequently separated by a hiatus during which caldera collapse and erosion occur. Data from Loihi seamount in the Hawaiian islands [16], which is believed to be the current expression of the Hawaii hotspot, and Macdonald seamount in the Australs [17] also reveal an early alkaline phase, which is rarely sampled once shield-building volcanism has been initiated. However, the lavas exposed on Pitcaim do not conform to this general alkalinetholeiitic-alkaline trend. The eruption chronology recorded by K-Ar work reveals a hiatus in volcanism between the Tedside volcanics and overlying formations, but this is not accompanied by any distinctive changes in major element chemistry. All the Pitcaim lavas are alkaline in character, using the criteria of Macdonald and Katsura [18], for example. The Tedside volcanics appear to be generally less evolved than lavas from the overlying formations with Mg#‘s (molar ratio Mg/(Mg + Fe2’)) of 37-55 compared with 11-44 in the Adamstown volcanics and Christians Cave formation but, at comparable degrees of fractionation, the lavas are very similar in terms of absolute abundances of both major elements and many trace elements (Table 1). The only notable exceptions to this rule are the higher MgO, Ni and Cr, and lower Fe,O, in the Tedside volcanics, features which are entirely consistent with the petrographic observation that these lavas are olivine accumulative. There are, however, some marked variations in elemental ratios between the Tedside volcanics and the Adamstown volcanics and Christians Cave formations (henceforth referred to as the post-erosional volcanics). In particular the Th/U ratios in the Tedside samples (- 11) are a factor of two larger than those in the post-erosional volcanics (- 5), but also the Tedside lavas have significantly higher large ion lithophile/high field strength element (LIL/HFS) ratios e.g. La/Nb, Ba/Nb, Zr/Nb and are slightly more light rare earth en-

TABLE

1

Mean analyses for Tedside volcanics (n = 6) and post-erosional volcanics (n = 10) with silica wt.% between 47 and 49 and Mg# between 35 and 55 Element

T-V

P-e V

Element

T-V

P-e V

SiO,

48.37 49.36 3.85 15.31 12.12 0.17 5.45 8.56 3.62 1.82 0.68 37

47.77 39.03 3.81 15.76 14.40 0.21 4.16 7.53 3.98 1.63 0.73 39

Sr Y Zr Nb Pb Zn Cu Ni Co Cr v Ba

665 37 341 48 8 115 48 89 43 105 266 530

619 65 330 59 9 138 21 15 38 8 220 457

Mg# TiO, Al,03 Fez03 MnO MgO CaO Na,O KzO pzo5

Rb

riched with (La/Yb), of 15 compared with 12-13 in the post-erosional volcanics (Fig. 2 and Table 3). Hofmann et al. [19] have noted that Nb/U ratios are generally remarkably constant in OIB and Mid Ocean Ridge Basalts (MORB) at 47 + 10. This is certainly the case for the post-erosional volcanics on Pitcaim, with the exception of sample P14 which probably inherited excess U from exchange with seawater (see isotope data below).

1

# La

I Ce

R

N3

Sm Eu Gd l-n

Dy

Ho

Er

Yb

Fig. 2: Chondrite-normalized REE plot for lavas of the Tedside volcanics and post-erosional volcanics. Normalization factors from Taylor and McClennan [58].

261 TABLE 2 Major (wt.%) and trace element (ppm) compositions of volcanic rocks from Pitcaim island. All data by XRF except U (determined by isotope dilution) Pl SiO TiO: AW, Fez03 MnO MgO CaO Na,O KzO P,O, Rb Sr Y Zr Nb Pb Zn CU Ni Co Cr V Ba U

TiO: SiO Al@, Fe@, MnO MgO CaO Na,O K,O P,O, Rb Sr Y Zr Nb Pb Zn cu Ni co Cr V Ba U

P3

P4

P5

P7

P8

P9

PlO

Pll

P12

51.53 0.98 17.32 8.97 0.22 1.18 3.43 6.34 3.67 0.37

48.58 3.62 15.33 11.94 0.16 5.66 8.73 3.71 1.65 0.59

48.89 4.28 15.26 12.42 0.18 3.97 8.13 3.65 2.29 0.89

48.76 4.27 15.36 12.50 0.18 3.86 8.29 3.62 2.26 0.88

47.88 3.15 14.93 12.21 0.17 6.54 8.44 3.68 1.64 0.57

48.74 3.59 15.93 10.98 0.15 5.55 8.97 3.81 1.65 0.61

47.37 3.57 15.06 12.68 0.17 7.09 8.82 3.24 1.43 0.53

49.92 3.95 16.17 10.80 0.15 3.82 8.54 3.99 1.93 0.71

47.51 3.86 15.69 14.54 0.20 3.96 7.85 4.05 1.63 0.72

47.08 3.85 15.42 14.46 0.18 5.15 7.62 3.88 1.64 0.69

19 454 60 610 108 10 146 7 2 9 2 26 1012 2.52

38 602 36 318 43 I 116 43 97 42 143 262 416 1.20

46 673 45 412 60 10 121 63 28 33 6 281 653 0.85

44 675 45 425 61 9 126 68 28 43 6 280 629 0.89

34 591 35 308 43 8 107 39 127 41 139 263 484 0.88

35 736 32 301 43 1 99 32 89 45 119 239 530 0.69

28 591 30 282 39 7 120 42 166 50 214 269 410 1.03

40 647 40 366 50 6 116 27 42 33 59 282 523 1.12

38 636 36 318 52 8 132 24 17 39 9 229 438 1.15

38 578 36 304 49 7 121 22 18 40 9 215 412 1.15

P14

P18

P20

P22

P26

P28

P30

P31

P34

642

641

62 503 60 592 86 34 174 12 10 21 6 109 340 1.51

50 498 138 509 74 7 231 14 10 28 4 170 422 2.01

62.64 0.60 16.36 6.47 0.19 0.42 1.93 5.62 5.66 0.11

47.45 4.28 17.54 16.12 0.23 2.04 5.56 4.21 1.81 0.78

84 59 64 703 93 12 143 4 2 3 3 14 1845 3.00

38 651 52 359 58 8 155 26 16 39 8 248 476 1.26

45.61 4.77 15.58 15.21 0.19 4.51 8.84 3.71 1.10 0.47

49.55 3.37 16.40 13.49 0.18 3.16 6.89 4.24 1.91 0.81

27 654 33 254 41 4 124 29 35 45 14 291 291 0.85

43 595 253 376 61 9 154 13 8 32 5 183 447 1.40

41.36 3.92 15.69 14.69 0.21 4.20 1.61 3.98 1.58 0.72

48.65 3.67 16.07 14.05 0.30 3.44 7.02 4.27 1.77 0.78

31 654 40 315 49 6 127 24 15 38 10 227 412 1.14

40 638 171 358 58 33 167 20 12 43 6 200 551 1.30

49.52 3.26 15.69 13.00 0.20 3.67 1.49 4.55 1.84 0.77

47.52 3.85 15.66 14.45 0.20 4.53 8.10 3.55 1.41 0.72

49.68 2.86 15.30 13.59 0.22 3.42 7.16 4.54 1.98 1.24

44 615 43 361 58 5 137 7 4 30 1 204 433 1.47

34 628 37 332 53 8 145 20 15 46 6 235 316 1.06

42 611 51 396 63 8 154 13 2 22 2 142 506 1.55

262 TABLE 3 Additional trace element data by spark source mass spectrography (all values in ppm) Pl

P4

PlO

P31

DY Ho Er Yb

87.83 205.12 21.28 79.83 15.88 4.45 11.13 2.02 11.05 2.29 6.13 5.06

63.46 135.10 15.75 64.37 12.95 3.69 9.87 1.49 8.52 1.54 3.73 2.74

57.71 124.86 12.90 55.64 10.93 3.00 8.54 1.38 7.60 1.46 3.45 2.46

43.12 95.02 12.03 51.73 10.09 3.07 8.09 1.18 6.63 1.21 2.84 2.19

Th Hf CS

15.0 19.7 0.3

10.9 10.3 0.2

11.7 10.5 0.05

7.2 8.5 0.25

La Ce Pr Nd Sm EU Gd Tb

However, the Tedside volcanics show rather more variation, both within the specified range, and notably to higher Nb/U ratios (maximum 72). No systematic correlation of Nb/U with fractionation indices (e.g. Mg#, SiO,) is observed and hence this feature is assumed to be source related.

4.2. Isotopic data Isotopic data for the Pitcairn lavas (Table 4, Figs. 3 and 4) show extreme variations in character between the Tedside volcanics and overlying formations. In the familiar Sr-Nd diagram (Fig. 3) the “shield” lavas of the Tedside volcanics lie parallel to the so-called “LoNd” mantle array [20], close to the proposed “EM I” mantle component [21]. Such extremely low Nd isotopic compositions have no direct correlatives in the Pacific with the possible exception of San Felix [22], but are similar to lavas from Kerguelen, Gough and Tristan da Cunha islands [23] in the Indian and Atlantic Oceans and to basalts recovered by Deep Sea Drilling Project drilling on the Walvis Ridge [24]. In contrast, the post-erosional volcanics form a very homogeneous grouping near to the “HIMU” end-member of the LoNd array [21]. In this respect they are almost identical to lavas from the Juan Fernandez archipelago to the east [25]. One sample, P14, an obsidian of unknown affinity (obsidians are found only as large boulders on the shore at Down Rope) has a 143Nd/‘44Nd ratio equivalent to the post-erosional volcanics but higher 87Sr/s6Sr (Fig. 3). However, following leaching in hot 4N HCl for several hours, its Sr

TABLE 4 Sr, Nd and Pb isotopic compositions for volcanic rocks from Pitcaim island

Pl P3 P4 P5 P7 P8 P9 PlO Pll P12 P14 P18 P20 P22 P26 P28 P30 P31 P34 642 647

Formation

“Sr/“Sr

143Nd/‘44Nd

*06Pb/204 Pb

*” Pb/*04 Pb

208Pb/204 Pb


Chr. Cave Tedside Tedside Tedside Tedside Tedside Tedside Tedside Adamstown Adamstown Chr. Cave? Adamstown Adamstown Adamstown Adamstown Adamstown Adamstown Adamstown Chr. Cave Pulawana Pulawana

0.703784 f 11 0.704744 f 10 0.704475 + 13 0.704481+ 10 0.704734* 15 0.704853 f 13 0.704682 f 12 0.705120 f 11 0.703507 f 14 0.703513 It 13 0.704202 f 25 0.703522 f 13 0.703625 f 11 0.703519 f 14 0.703536 f 13 0.703526 * 19 0.703505 + 12 0.703483 f 12 0.703497 f 13 0.703690 f 11 0.703681 f 12

0.512813 + 0.512502 + 0.512610* 0X2620+ 0X2524* 0.512469 + 0.512545 f 0.512439 f 0.512855 + 0.512866 * 0.512840+ 0.512866 + 0.512835 + 0.512853 * 0.512867 f 0.512861* 0.512858 + 0.512887 f 0.512847 f 0.512655 f 0.512732 5

18.411 17.782 17.827 17.832 17.761 17.635 17.795 17.640 18.449 18.427 18.393 18.515 18.424 18.475 18.489 18.484 18.406 18.459 18.419 17.832 17.849

15.516 15.477 15.486 15.477 15.464 15.460 15.475 15.459 15.492 15.501 15.502 15.523 15.494 15.517 15.498 15.502 15.499 15.494 15.499 15.508 15.477

39.074 38.872 38.898 38.908 38.823 38.725 38.844 38.913 38.896 38.876 38.982 39.047 38.912 39.035 38.958 39.009 38.900 38.920 38.967 38.503 38.698

+ 3.2 - 2.9 -0.8 -0.6 - 2.5 - 3.5 -2.1 - 4.1 + 4.0 +4.2 + 3.7 +4.2 +3.6 + 4.0 + 4.2 +4.1 +4.1 + 4.6 + 3.8 +0.1 +1.6

5 9 9 7 8 8 9 7 6 8 2 12 10 8 8 7 8 9 8 8 8

Pitcairn Island Lavas .

Adamstown

0

Pulawana

x

Tedside

VolcanicslChristians

Cave

Fmn

Volcanics Volcanics

0.45-0.64

Myr

0.62-0.67

Myr

0.76-0.95

Myr

‘EM II’ -

‘EM I’

W.Pacific

p&a@ sediment

0.5122

t

0.7020

0.7030

0.7040

0.7050

0.7060

0.7070

0.7060

0.7090

*7Sr/*6Sr Fig. 3. Sr and Nd isotopic compositions of Pitcaim lavas in relation to MORB [62] and selected Pacific Ocean islands [22,25,32,63,64,65]. West Pacific sediment compositions from Woodhead [55]. HIMU, EM “I” and EM “II” refer to the mantle components designated by Zindler and Hart [21]. Age ranges, from [3], and symbols used for the different formations on Pitcaim are shown in the figure.

isotopic composition was reduced markedly and thus it is likely that this sample formed part of the post-erosional sequence (Christians Cave Formation?), and had suffered a degree of seawater alteration (see discussion). The two samples from the Pulawana volcanics which, considering their K-Ar ages [3], were probably among the first post-erosional lavas erupted, fall on a gentle curve linking these two sequences, which may be interpreted as a mixing line and, if so, would suggest a close spatial relationship between the two sources. Pb isotopic compositions are also unusual (Fig. 4); the post-erosional lavas have compositions within the MORB array with 207Pb/204Pb and

206Pb/204Pb ratios similar to samples from Tristan da Cunha [26,27] and Kerguelen [l], whereas the Tedside volcanics have remarkably unradiogenic 207Pb/204Pb and 206Pb/204Pb ratios, lying close to the 4.55 Ga Geochron, and comparable to the least radiogenic Walvis Ridge [24] and Hawaiian [28] samples. Once again the two Pulawana samples could be interpreted as a mixture of these two sources. Interestingly, however, the 208Pb/204Pb ratios are approximately equivalent in all formations and relatively high compared to the MORB field, suggesting evolution with a considerably elevated Th/Pb ratio relative to MORB.

264 I

I

I

Mangaia 0

Ua POU (alkaline)

Ua Pou (tholeilte)

15.8

15.8

Ua POU (tholeiite)

17

18

20

19

21

22

20sPb/204Pb Fig. 4. Pb isotopic compositions of Pitcairn lavas. Data sources and symbols for Pulawana volcanics as for Fig. 3. T-V= volcanics, P-V = post-erosional volcanics, NHRL = Northern Hemisphere Reference line of Hart [l].

5. Discussion DuprC and Allegre [29] and Hart [l] have suggested that widespread areas of isotopically anomalous mantle (termed the “Dupal” anomaly) may underlie parts of the ocean basins. Clearly, the presence of two radically different sources beneath Pitcairn is not easy to reconcile with such a general proposal, a conclusion also reached in studies of other Pacific islands, e.g. Marquesas [30]. Thus the extension of the proposed Dupal anomaly into the Pacific region appears to require re-evaluation in the light of both regional and local heterogeneity in the area. In addition, the conclusion of Zindler and Hart [21], that large amplitude ratios occur only at large scale lengths,

Tedside

also now seems to require modification, considering the Pitcairn data and results obtained from Ua Pou in the Marquesas [30] (Fig. 5). Considerable isotopic variability within individual volcanic centres is becoming apparent in many of the most recent studies of ocean islands (e.g. [30-321). Although in the past it has been usual to draw fields around the data for individual islands or island groups, it is now clear that we should now start considering the isotopic variability inherent within such centres, especially in view of the increased precision now available with the advent of multicollector mass spectrometers. Further, the data do not conform to the recently proposed model of Park and Zindler [33], which suggests that Pacific OIB erupted on young oceanic lithosphere show

80 20. SMNT#6-#8

1. SMNT#6

60

‘;;

a

‘;;

21. WALVIS

3. CYAMEX

22. KANE

4. REYKYANES

23. GALAPAGOS

5. LOlHl

24. SOCIETIES

6. SWIR

25. MAR

7. OAHU

50

A Nd

9. HALEAKALA 40

IO. KERGUELEN

G c

FZ

l Sr

8. St HELENA

‘= z!!

a 5

2. FAMOUS

n

F/C

Pb

11. HAWAII 12. AZORES

30

Oceanic

13. MARQUESAS 14. CANARIES 20

volcanism

15. ROSS 16,. FIJI 17. SAMOA IS. ICELAND 19. KERGUELEN

Scale

length

Is.

(km)

Fig. 5. Plot of amplitude ratio vs. scale length. Diagram from [21], with the addition Curve shows possible upper boundary to the data, also from [21].

limited within-suite heterogeneity, i.e. that heterogeneity is proportional to the age of the oceanic lithosphere at the time of eruption. The total range in isotopic variation observed between the Pitcaim shield and post-erosional volcanics is - 30%, - 40% and 50% of the total range observed in oceanic lavas (both MORB and OIB) for Pb, Sr and Nd respectively, a figure referred to as the “amplitude ratio”, after Zindler and Hart [21]. These extreme isotopic variations (which, in the case of Nd, span almost the entire range of compositions observed in OIB) within the source of a single volcano over a time period of around 100,000 years are quite exceptional and, in view of the lack of correlated major element data noted above, place strong constraints on any petrogenetic model. The data from Pitcaim are unique in that large shifts in source composition, as revealed by the isotopic data, do not appear to be mirrored by changes in the major (and most trace) element compositional data. An important aspect of current research concerns the alkaline-tholeiitic-alkaline transition in oceanic islands and its rela-

of Pitcairn

(this study)

and Ua Pou [30] data.

tionship, if any, with the nature of the source. The Pitcaim data demonstrate that the large scale isotopic variations within the source need not be associated with shifts in the major and trace element chemistry of the eruptive products. It is possible, therefore, that the occurrence of tholeiitic vs. alkaline compositions in OIB may be controlled purely by melting phenomena (e.g. [34]) upon which isotopic variation may be imposed by tapping of sources with distinctive histories. Such a suggestion finds further support in the observation, from the Hawaiian islands, that tholeiitic-alkaline transitions occur both with [35,36] and without [34,37] concomitant isotopic variation. The similarity in the major element chemistry of the shield building and post-erosional phases suggests that the isotopically distinctive components at Pitcairn cannot be simply related by variable degrees of partial melting and that two or more sources are involved. The data do not provide any distinction between different models for mantle heterogeneities (e.g. “marble-cake” or “plum-pudding” vs. layered mantle). However, the temporal relationship be-

266

tween the two sources may provide some constraint. If we assume a simple “end-member” situation in which the plume components are separated vertically, it is possible to make some rough estimates of the scale of these heterogeneities. For example, if the plume has an ascent velocity similar to large-scale plate motions (i.e. - 11 cm/yr in this region), then the - 100,000 yr hiatus between the shield-building and post-erosional phases would correspond to a physical separation of the plume segof - 10 km. The dimension ments from which each of the components were derived is not well constrained, but using a similar reasoning the - 0.2 Myr period of volcanism for both the shield-building and post-erosional phases would imply a melt diapir of diameter - 20 km. For derivation from a source by < 10% partial melting, this would imply sources of diameter > 20 km. Thus, if the temporal sequence of volcanism is translated into vertical dimensions, a - 50 km in size with the shield-building plume phase in the upper 20 km and the younger posterosional phase in the lower 20 km ascending at a rate of - 11 cm/yr could account for the volcanic history of Pitcairn Island. This is clearly a gross oversimplification but allows us to place the volcanic activity at Pitcairn into some kind of physical perspective. Thus, for example, if the plume originated from the upper-lower mantle transition zone (i.e. a depth of - 650 km) this would imply that heterogeneities on the scale of - lo-20 km are preserved during ascents of 650 km. The ultimate origin of OIB sources still remains a matter of considerable conjecture. A current consensus suggests that all known compositions can be explained in terms of multi-component mixing. Presently, up to five “end-member” components have been tentatively assigned [21,23], although their exact nature remains controversial. At Pitcairn the post-erosional magma source has a “typical” OIB composition lying close to the main mantle array in Sr-Nd space, whereas the Tedside volcanics have an isotopically anomalous composition which is far less frequently observed. The Pitcairn data do form remarkably linear arrays in both Sr-Nd and Pb-Pb plots, a feature suggestive of mixing processes. However, it seems unlikely that mixing of one enriched plume component (e.g. the Tedside source) with surrounding MORB

could produce the very homogeneous post-erosional volcanics (and indeed these would seem to have 208Pb/204Pb too high to account for such a process). Thus it is likely that at least two different plume-related sources are involved. At this point the two Pulawana samples become critical to the discussion. It has been shown from K-Ar dating [3] that these samples were among the first of the post-erosional volcanics to be erupted, i.e. they were erupted between the Tedside volcanics and the bulk of the post-erosional volcanics (Adamstown volcanics and Christians Cave Formation). Isotopically, and in terms of some trace element ratios (e.g. Zr/Nb), they also appear to be intermediate in chemical properties between the two sources, suggesting that a mixing relationship exists between the Tedside and post-erosional sources, with the Pulawana volcanics representing an intermediate product. It is clear that this is not a perfect relationship in terms of the isotopes (the samples do not fall exactly along a proposed mixing line) and in a future expedition to Pitcairn one of the aims will be to sample this formation more extensively to test this hypothesis. However, at this stage, the weight of evidence from the Pulawana samples suggests that these two sources may be related temporally and spatially by a simple mixing process, indicating that the Pitcairn lavas could be modelled in terms of the addition of an “enriched” component to an ambient plume composition (post-erosional lavas) to produce the Tedside source. Clearly, such an enriched component must possess high LIL/HFS and La/Yb, ratios, high 87Sr/86 Sr and low 143Nd/itiNd (Fig. 3) and, most importantly, must have evolved with a history of high Th/U ratio. In previous studies, various candidates have been proposed for the “EM I” component including subducted crust or sediments, delaminated sub-continental lithosphere and the effects of mantle metasomatic processes [28,38-411, with very little evidence to distinguish between these possibilities. We believe that considerable evidence is now mounting to favour the first of these alternatives (e.g. 42-44) and so, at this time, prefer a model involving subducted oceanic crust and minor sediment but, as discussed in the following section, only after substantial modification of these sources by passage through the subduction zone.

261 ,o

19

I8 208Pb\204Pb 17

Fig. 6. Pb evolution model based upon [45], but extended to consider Thorogenic Pb. Clay and ooze represent two pelagic sediments from the West Pacific [55], and illustrate the difficulty in mixing modem sediment compositions into the mantle to produce the Pitcaim lavas. Pelagic sediment compositions at 2.0, 1.6, 1.2, and 0.8 Ga (large circles) are derived from “orogene” in the plumbotectonics version IV model [46]. Upon lowering of the U/Pb ratio to 2 by passage through a palaeosubduction zone, these sediments (now residues) evolve to isotopic compositions at the present day represented by small circles. It is thus possible to mix a small proportion of this ancient sediment residuum (generally ca. 2 Ga old) into the post-erosional (ambient plume) source to generate the distinctive Tedside volcanics source. The model can also be applied to 208Pb/204Pb variations, but in this case the Th/Pb ratio does not appear to be affected by passage through the subduction zone (see text), resulting in a relatively high 208Pb/204Pb ratio with concomitant reduction in the 207Pb/204Pb and 206Pb/ 2w Pb ratios.

Pb isotopic compositions of modern pelagic sediment have inappropriate 206Pb/204Pb and 207Pb/204Pb ratios to be considered as a possible source for the “enriched” characteristics of the Tedside source (Fig. 6). However, in a study of South Atlantic ocean islands, Weaver et al. [45] noted that ancient sedimentary material (of the order of l-2 Ga old) could form a suitable candidate for “EM I” [l]. The basis for their model is illustrated in Fig. 6. Hypothetical pelagic sediment compositions at 0.8-2.0 Ga age are derived from “orogene” in version IV of the

plumbotectonics model [46]. Consider the plot of 207Pb/204Pb vs. “‘Pb/‘“Pb. If at some time the U/Pb ratio of these sediments was to be reduced to around 2, subsequent isotopic evolution would be retarded sufficiently to provide a component which, if mixed into the mantle, would form a very reasonable approximation to EM I at the present day. Weaver et al. were concerned primarily with trace element variations in OIB and did not suggest how such a reduction in U/Pb might be effected, although they noted that modem pelagic sediments may have low U/Pb ratios, hence implying that this lowering may be an inherent feature of the sedimentation process. However, the data presently available reveal that this is by no means always the case; the U/Pb ratios of pelagic sediments are very variable and can be quite high (e.g. [26,47]) and so it is not clear at present whether lowering of the U/Pb ratio could be brought about by a sedimentation process alone. In addition, if we consider variations in Thorogenie Pb (Fig. 6) again using the orogene compositions from plumbotectonics IV, it is apparent that, although a reduction in U/Pb ratio is necessary to account for the *O’Pb/204 Pb_206Pb/204Pb composition of the Tedside source, the high 2o8Pb/ 204Pb indicates that the U/Pb fractionation is not accompanied by a reduction in Th/Pb ratio (a high Th/Pb ratio must be maintained to account for the high 208Pb/204Pb in these samples). Thus, in the source region of the Tedside volcanics, we are seeing a fundamental fractionation of U from Th with an increase in the Th/U ratio. Marine sediments do show variable Th/U ratios and pelagic sediments generally have high Th/U ratios (due to the short residence time of Th and long residence time of U in seawater [48,49]). Low Th/U ratios are restricted to organic-rich sediments occurring on continental shelves and hence generally immune to subduction. Thus, subducted sediment could be expected to have a high Th/U ratio (W.M. White, personal communication). However, we suggest that a fundamental fractionation of Th/U ratio (and the lowering of the U/Pb ratio) is brought about by “processing” of the slab through a palaeo-subduction zone. The relative mobility of U and Pb in slab-derived fluids or melts is still a matter of some debate (e.g. [50]) although most authors would suggest that U is more mobile under such circumstances. In ad-

268

dition, we know from the composition of arc basalts that extraction of components from the subducted slab (especially from the sedimentary component) is relatively efficient. Thus it is possible to visualise a form of “zone refining” process occurring within the subducting slab as melts are generated transporting U into the mantle wedge more readily than Pb and resulting in a sedimentary “residue” being subducted into the deep mantle with lowered U/Pb ratio. Further, it has been inferred from U-Th disequilibrium studies of arc terrains [51] that U is transported from the slab more efficiently than Th, thus providing the necessary enrichment in Th/U observed for the Tedside source, i.e. the Th/Pb ratio in the sediment residuum returned to the mantle is approximately constant (or increased) whilst the U/Pb ratio is reduced. It is believed that such a model fulfills the requirements of the rather unusual Pb isotopic ratios found in the Tedside volcanics source, which would otherwise be quite difficult to account for. The model described by Fig. 6 requires mixing with a component (the subducted sediment residuum) having a high Th/U ratio of around 17.5. This might seem unrealistically high but appears to be confirmed by the high Th/U ratios observed in the Tedside volcanics ( - ll), reflecting a mixing process with a small proportion of such a component. Ancient sedimentary material with low Nd and high Sr isotopic ratios [39], and high Ba/Nb and Zr/Nb [52] ratios would further satisfy the geochemical requirements of this source and, as sediments generally contain much higher concentrations of Sr, Nd, Pb and Ba than typical mantle sources (even after passage through the subduction zone), only a very small proportion of sediment would be required to produce the observed isotopic and trace element ratio shifts (ca. a few percent at most). This would not be expected to seriously affect major element abundances, which would be dominated by the non-sedimentary component, and any shifts in absolute trace element abundances (which might amount to a few tens of percent) could easily be masked by subsequent magmatic processes e.g. fractional crystallization. It is suggested therefore that the source represented by the Tedside volcanics on Pitcaim island may represent an admixture of an ambient plume component (similar to the source of the Adams-

town and Christians Cave Formations) with a small proportion of ancient sedimentary material modified by its passage through the subduction zone, a component we refer to as the subducted sediment residuum. If this is the case, then considering the close spatial and temporal relationship inferred between the two sources, it would be logical to suggest that the ambient plume source may derive from ancient subducted oceanic crust itself which is likely to have an initially high U/Pb ratio [53] and hence higher 206Pb/204Pb and 207Pb/204Pb ratios and, as noted by Hofmann and White [38], lower 143Nd/‘44Nd and higher *7Sr/86 Sr than present-day MORB sources. Given the thin nature of the oceanic crustal profile (- 5 km crust and < 1 km sediment), it is highly likely that subduction of such material could result in the small-scale, large-amplitude heterogeneity noted in the Pitcairn lavas. It is not immediately obvious that OIB could be a product of partial melting of a source which is essentially basaltic i.e. subducted oceanic crust, as basalts are generally considered to be the products of partial fusion of peridotite. However, as Hofmann and White [38] have pointed out, this is not necessarily an unreasonable proposition since, in reality, it is likely that the source eclogite will be “contaminated” to some extent by local peridotite and in this case it is probable that phase relationships during melting will be indistinguishable from those of an ordinary “fertile” peridotite. For further discussion of this point readers are referred to [38]. A number of geochemical arguments have recently been levied against the large-scale subduction of sedimentary material into the sub-oceanic mantle, based upon Nd/Hf ratios in pelagic sediments [54] and the apparent uniformity of Pb/Ce and Nb/U ratios in MORB and OIB [19] (Fig. 7). These models depend to a large degree on the assumption that sedimentary material subducted into the deep mantle retains elemental ratios identical to those observed in pelagic sediments at the Earth’s surface today. Although it is considered that these elemental pairs must remain unfractionated by simple partial melting or fractional crystallization processes, studies of subduction related volcanics [55] and experimental studies of slab dehydration processes [56] suggest that this is very unlikely to be the case in the subduc-

269

0.5124

0.5125

0.5126

0.5127

0.5126

0.5159

Fig. 7. Nb/U ratio vs. Nd isotopic composition for the Pitcaim lavas (symbols as in previous figures). See text for discussion.

tion zone environment. If the agent of mass transfer between the slab and mantle wedge is a hydrous fluid or silicate melt containing a significant hydrous component it is extremely unlikely that ratios such as Pb/Ce, Nb/U, and almost certainly Nd/Hf, will retain their integrity. To illustrate this problem we have chosen the ratio Nb/U since the effects of transit through a hypothetical subduction zone can be most readily predicted for this elemental pair. U, as evidenced by U-Th disequilibrium studies [51] is readily transported from the slab into the mantle wedge. In contrast, Nb concentrations in IAT are similar to MORB or lower, arguing against any significant transport of Nb from the slab, a conclusion also supported by experimental work [56]. As a result, slab derived fluids are likely to have very low Nb/U (even lower than that of pelagic sediments with Nb/U - 10) and it is expected that this signature would be subsequently inherited by arc lavas. As indicated in Fig. 7, this is indeed the case; most oceanic arc lavas have Nb/U lower than 5 and many lower than 2 (Nb/U in Mariana basalts, for example, averages 4-5: Woodhead, unpublished data). A somewhat analogous, although admittedly much lower temperature process, is seen in Pitcaim sample P14 which has seemingly interacted with a brine (based on Sr isotope data) of very low Nb/U ratio. However, in the case of OIB sources, we must consider the fate of the slab residuum, after “leaching” by

slab-derived fluids. Thus, from the model, we predict that any sedimentary material with Nb/U of ca. 10 would have its Nb/U ratio substantially modified (increased) by passage through a hypothetical subduction zone. Nb/U data for the Pitcairn lavas are shown in Fig. 7. Although the post-erosional volcanics retain Nb/U ratios within the range specified by Hofmann et al. [19] as normal for OIB and MORB (and as would be expected from subducted oceanic crust), it is clear that the Tedside volcanics do not. Nb/U is variable in these lavas and notably extends to higher Nb/U (no component with high Nb/U ratio is identified in [19]). This is thought to be a clear indication of the involvement of a hydrous component at some stage in the history of the Tedside source and, in view of the unusual isotopic data noted above, we consider that this involvement most likely took place in a subduction zone. Thus we suggest that the Tedside source contains a proportion of a component derived from subducted sediment, but modified by transit through the subduction zone (subducted sediment residuum). In the light of recent data which suggests that the ratios Pb/Ce and Nb/U do vary considerably for some ocean islands, e.g. Societies [44], we concur with Loubet et al. [57] that there is now a need for a detailed review of these ratios in OIB in order to assess these effects more thoroughly. A very similar model to this has been proposed by Dupuy et al. [42], who note that the chondrite-normalized patterns of incompatible elements in basaltic rocks from the Austral islands are complementary to those of island arc tholeiites, suggesting that these lavas represented subducted material residual after extraction of components into the arc source. In an attempt to extend this model, Fig. 8 shows the variation in a number of elemental ratios between a typical oceanic arc basalt from the Marianas ([55], and Woodhead, unpublished data), the Tedside and post-erosional volcanics on Pitcaim and an estimated “bulk slab” composition. Calculation of the latter assumes a mixture of 19 parts oceanic crust to 1 part upper continental crust (used as an approximation to pelagic sediment plus altered oceanic crust, assuming net input to the oceans equals net output). All concentration data were obtained from Taylor and McClennan [58]. It is clear that in most cases considered the Pitcaim

270

I

20

40

60

2

6

Zr/Nb

La/4Nb

I

*

0.15

0.10

BalNb

El A”\

U/Pb

‘\L 1

2

ThlPb

0.04

0.05

I

1 .\-. 10

5

Th/U

0.06

RblSr

Sm/Nd

5

LulHf

*

10

15

Ce/Pb

Fig. 8. Key trace element ratios for Pitcaim lavas (symbols as in previous figures), a typical island arc basalt from the Marianas (triangle) and an estimated “bulk slab” composition (square). The figure illustrates a preferred model in which the slab is fractionated during passage through the palaeo-subduction zone into two components. One, a slab-derived fluid, enters the arc source and the other, a slab residuum, is subducted to greater depths, stored in the mantle, and subsequently involved in the genesis of some ocean island basalts. Marianas analysis from [55] and Woodhead, unpublished data.

271

lavas are complementary to arc basalts in the sense that a bulk slab composition can be “fractionated” to produce components for the arc source and the OIB source. It is admitted that the model is very dependent upon the assumed slab composition for Rb/Sr and Sm/Nd although this may be due to some extent to analytical uncertainties in the measurement of such small ratios (these were not analysed by isotope dilution). However, for all other ratios considered, this is not a problem and the data clearly suggest a complementary relationship between the arc source and the source of the Pitcaim lavas. These data, therefore, tend to reinforce the suggestion of Ringwood [59], based on experimental work, that the subducted slab, residual after the extraction of components in the subduction zone, is incorporated into the mantle to form a “megalith”. Some doubt remains, especially among geophysicists (e.g. [60,61]) about the feasibility of creating a megalith at the 670 km discontinuity. We do not intend to contribute to this debate but note that the trace element and isotopic data do seem to be consistent with the involvement of ancient oceanic crust and sediment residues in the genesis of some OIB’s. 6. Conclusions Isotopic data for Pitcairn island lavas indicate the presence of two radically different mantle sources and provide a number of new constraints on ocean island genesis and evolution. Firstly, it is now apparent that extreme isotopic variations can occur over relatively small scale lengths (- 10 km). These observations at Pitcaim and other South Pacific islands suggest a need for a reevaluation of the nature of the proposed Dupal anomaly in this region. Isotopic variations at Pitcairn do not appear to be coupled to any significant major element parameters in eruptive products. This suggests that the alkaline-tholeiitic-alkaline transitions observed in some oceanic islands (and possibly inherent, but un-sampled in many others) may be largely independent of source geochemistry and a product of variable partial melting histories. The observed temporal and geochemical relations in the Pitcaim island lavas place severe constraints on any petrogenetic model. We suggest

that the following scenario most readily accounts for these data. Initial melts of 1-2 Ga old subducted oceanic crust, with a small proportion of entrained, subducted sediment residuum, produced the geochemically very distinctive Tedside volcanics. After a period of about 200,000 years the small sedimentary component probably became exhausted, its waning phase recorded in the Pulawana volcanics. The ambient plume composition, corresponding to a partial melt of altered oceanic crust alone then became dominant for the remaining life of the volcano, producing the Adamstown volcanics and Christians Cave Formations. We emphasise the critical role in our model of subduction zone processes, which substantially modify the composition of slab components; the complementary nature of trace element abundance patterns in OIB and IAT are presumed to be a direct consequence of this process. We believe that quantifying these phenomena must remain a major goal for future research. Acknowledgements Collection of samples from Pitcaim would not have been possible without the assistance of “Operation Raleigh” and the crew of the “Sir Walter Raleigh’. Ian McDougall is thanked for the two Pulawana samples. All XRF major and trace element analyses were performed at the Department of Earth Sciences, University of Oxford; Keith Parrish is thanked for technical assistance at Oxford. Ross Taylor kindly provided access to the Spark Source laboratory at R.S.E.S. and Marc Norman an introduction to the machine. Mike Shelley and Pat Oswald-Sealy provided valuable technical assistance. The senior author gratefully acknowledges the Rothmans University Endowment scheme for funding, and Roland Maas and two anonymous reviewers for commenting on the manuscript. Finally, many thanks to the small community of Adamstown on Pitcairn who remains cheerful in the face of increasing uncertainty as to their future. References 1 S.R. Hart, Large scale isotopic anomaly in the southern hemisphere mantle, Nature 309, 753-757, 1984.

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