A critical evaluation of recent models for Lau–Tonga arc–backarc basin magmatic evolution

Chemical Geology 245 (2007) 9 – 44 www.elsevier.com/locate/chemgeo

A critical evaluation of recent models for Lau–Tonga arc–backarc basin magmatic evolution Janet M. Hergt ⁎, Jon D. Woodhead School of Earth Sciences, The University of Melbourne, Victoria, 3010, Australia Received 25 January 2007; received in revised form 11 July 2007; accepted 11 July 2007 Editor: R.L. Rudnick

Abstract New trace element, Sr-, Nd-, Pb- and Hf isotope data provide insights into the evolution of the Tonga–Lau Basin subduction system. The involvement of two separate mantle domains, namely Pacific MORB mantle in the pre-rift and early stages of back-arc basin formation, and Indian MORB mantle in the later stages, is confirmed by these results. Contrary to models proposed in recent studies on the basis of Pb isotope and other compositional data, this change in mantle wedge character best explains the shift in the isotopic composition, particularly 143Nd/144Nd ratios, of modern Tofua Arc magmas relative to all other arc products from this region. Nevertheless, significant changes in the slab-derived flux during the evolution of the arc system are also required to explain second order variations in magma chemistry. In this region, the slab-derived flux is dominated by fluid; however, these fluids carry Pb with sediment-influenced isotopic signatures, indicating that their source is not restricted to the subducting altered mafic oceanic crust. This has been the case from the earliest magmatic activity in the arc (Eocene) until the present time, with the exception of two periods of magmatic activity recorded in samples from the Lau Islands. Both the Lau Volcanic Group, and Korobasaga Volcanic Group lavas preserve trace element and isotope evidence for a contribution from subducted sediment that was not transported as a fluid, but possibly in the form of a melt. This component shares similarities with that influencing the chemistry of the northern Tofua Arc magmas, suggesting some caution may be required in the adoption of constraints for the latter dependent upon the involvement of sediments from the Louisville Ridge. A key outcome of this study is to demonstrate that the models proposed to explain subduction zone magmatism cannot afford to ignore the small but important contributions made by the mantle wedge to the incompatible trace element inventory of arc magmas. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. Keywords: Slab flux; Subduction magmatism; Lau Islands

1. Introduction Considerable effort has been expended, by numerous researchers, and over many decades, in the study of convergent margin magmatism. The Tonga–Kermadec ⁎ Corresponding author. E-mail address: [email protected] (J.M. Hergt).

island arc has provided a perfect natural laboratory for such studies as it preserves a record of arc magmatism and back-arc basin formation in a comparatively simple tectonic context (Karig, 1970). The consumption of the west-dipping Pacific plate beneath the Indo-Australian Plate in this region is only complicated by the progressive southward migration of the Louisville Ridge seamount chain during its subduction (Ruellan et al., 2003).

0009-2541/$ - see front matter. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2007.07.022

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J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

Fig. 1. Global-scale variations in Ba/La vs Th/Yb preserved in oceanic arc magmas. Variations are believed to reflect the influences exerted by different slab-derived components; that is, ‘fluids’ or ‘sediment’ (the latter possibly in the form of a partial melt). Modified from Woodhead et al. (2001).

This feature has also been used to some advantage, however, as Louisville Ridge crust and associated volcanic sediments are believed to provide a distinctive geochemical ‘marker’ that has been proposed to track the contributions of slab-derived material into the mantle wedge (e.g., Turner et al., 1997; Ewart et al., 1998). Moreover, the association of the active arc with continental crust in the south (Taupo Volcanic Zone; New Zealand) and ocean island magmatism in the north (Samoa) also provides a means of investigating how such materials may influence the magmatic products of arc volcanoes. It has been a widely held view that the composition of arc lavas records the influence of two chemically distinct components derived from the subducting slab (e.g., Hawkesworth et al., 1993). The first is a sedimentderived partial melt (e.g., Elliott et al., 1997; Turner et al., 1997; Johnson and Plank, 1999) which carries a range of incompatible elements into the mantle wedge and is responsible for observed enrichments in, for example, the light rare-earth elements. The second slab-derived flux is an aqueous fluid, which selectively transports only the highly fluid-soluble elements (e.g., Rb, Ba, Pb). The current paradigm suggests that, in cases of sediment involvement, the budget of almost all incompatible elements will effectively be dominated by this component and the fluid flux will have little impact. In contrast, for those arcs in which sediment subduction is minor or effectively absent, the fluid has the greatest influence in modifying the mantle wedge. The involvement of one or other of, or indeed a combination of, these components

in particular subduction zones is frequently illustrated in diagrams such as that shown in Fig. 1. More recently, some authors have further linked the compositions of the two components with physically separate sources such that the sediment contribution is delivered in the form of a melt, separate from any fluid flux, the latter being derived from an altered oceanic crust (mainly basalt) with the two contributions separated in both space and time (see Elliott, 2003 for an elegant review). In contrast, others have argued that slab-derived fluids will scavenge elements from all available components, including sediments and altered oceanic crust (e.g., Woodhead et al., 1998; Haase et al., 2002). Clearly, whatever the nature of the mass transfer process, both sediments and altered oceanic crust have the potential to profoundly modify the incompatible trace element and isotopic signatures of the mantle wedge and hence the lavas derived therefrom. In arc studies it is sometimes assumed that the slabderived flux exerts such an overwhelming influence on the mantle wedge, that, apart from concerns about prior melt depletion, the low concentrations of incompatible elements allow this reservoir to be largely ignored. In this contribution we focus our attention upon this question— namely what contribution, if any, does the depleted mantle wedge actually make to the incompatible trace element inventory of arc magmas? To address this question requires a critical re-examination of the way in which recent models have been applied to explain the geochemical variations in magmas from the Tofua Arc, particularly with respect to the implications of such models for the composition and evolution of the underlying asthenosphere in this region. 2. Background In order to examine the problem, our study draws together data from many different suites of rocks recording particular aspects of magmatic evolution within the Lau–Tonga region. It is therefore useful to place these in an appropriate geological context, and the following describes briefly the magmatic history of subduction in the area, as well as the tectonic development of the Lau back-arc basin. Fig. 2 illustrates the locations of key features and sampling sites referred to in the text. 2.1. Arc magmatism The earliest arc magmas exposed in the region are represented by Eocene basement preserved on the island of ʻEua (∼ 46–40 Ma; Duncan et al., 1985). ʻEua is located at the southern end of the largely submerged

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

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Fig. 2. Geography of the Lau Ridge, Lau Basin and Tonga Ridge, showing the location of sites drilled during ODP Leg 135 and other localities referred to in the text. Modified from Hergt and Hawkesworth (1994). LSC refers to the Louisville Ridge Seamount Chain (see inset).

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J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

Tonga Ridge, which extends northwards to the Vava'u Group of islands, and is a volcanically inactive feature that was formerly juxtaposed with the Lau Ridge (Hawkins and Falvey, 1985). Duncan et al. (1985) also document the occurrence of younger volcanism on ʻEua recorded in both flows (∼ 33–31 Ma) and dykes (∼19– 17 Ma). Evidence of Miocene–Pliocene magmatism along the Lau Ridge is preserved primarily as a series of magmatic groups exposed in the Lau Islands (Gill, 1976; Woodhall, 1985; Cole et al., 1990) while other materials have been recovered from sites drilled during Leg 135 of the Ocean Drilling Program (e.g., dykes at Site 840; Bloomer et al., 1994, and an ash record at Site 841; Clift and Vroon, 1996). At around 6 Ma the Lau Ridge began to split—the extension associated with the initial opening of the Lau Basin (Parson and Hawkins, 1994; Taylor et al., 1996; Zellmer and Taylor, 2001). Remnants of the early arc were abandoned to become the now-dormant Lau Islands, with other vestiges of the ridge (including ʻEua) carried eastward, eventually to be accompanied by migration of the locus of arc magmatism (the Tofua Arc; Karig, 1970; Gill, 1976). As a result, in the Lau Islands we are afforded a rare subaerial record of arc magmatism, including events predating back-arc basin formation. On the basis of work conducted by Gill (1976), later expanded upon by Woodhall (1985) and Whelan et al. (1985), magmas preserved in the Lau Islands represent volcanism (i) within the early arc (Lau Volcanic Group—LVG; 14–5.4 Ma), (ii) during the earliest stages of rifting and the onset of seafloor spreading (Korobasaga Volcanic Group—KVG; 4.4– 2.4 Ma), and (iii) continuing well after spreading in the Lau back-arc basin was established (Mago Volcanic Group—MVG; 2.0–0.3 Ma). As this study is mainly concerned with the causes of changes in arc magmatism, the MVG, which has an apparent ocean island basalt (OIB; e.g., Gill et al., 1989) affinities, will not be considered further here. The time at which magmatism was initiated in the Tofua Arc is not well-constrained (Gill, 1976). Evidence of Pliocene (∼ 3.5 Ma) eruptions, has been identified in the ash records of cores recovered from ODP Site 840 (Tappin et al., 1994) and volcanic rocks from the northern Tongan island of Niuatoputapu are also reported to have been dated at around 3 Ma (J. Gill, in Tappin, 1993). Magmatism along the Tofua Arc has however continued since this time, and eruptions less than 10,000 years old have been experienced on all islands, with the possible exceptions of Niuatoputapu (in the north) and the southernmost island of Ata (Turner et al., 1997).

2.2. Back-arc basin formation Parson et al. (1990) and Parson and Hawkins (1994) describe the regional tectonic history relating to the opening of the Lau Basin. In brief, the initial stages of back-arc basin formation involved extension of the arc crust of the Lau Ridge, with magmatism occurring within some graben structures (e.g., at ODP Site 834). True seafloor spreading was initiated at ∼3–5 Ma (e.g., Taylor et al., 1996; Zellmer and Taylor, 2001) and the tip of the propagating spreading ridge migrated southward from the Peggy Ridge along the western side of what was to become the Tonga Platform. Axial spreading resulted in a fan-shaped opening in the northeastern part of the basin, while horst-and-graben style extension continued in the southwest. The first spreading ridge to become established is now located closest to the Tofua Arc and is referred to as the Eastern Lau Spreading Centre (ELSC) except for the section in the vicinity of the propagating tip which is generally referred to as the Valu Fa Ridge. A second spreading centre, currently located more centrally in the Lau Basin is also believed to have initiated at the Peggy Ridge. This Central Lau Spreading Centre (CLSC) is propagating southwards at the expense of the ELSC, and a small ‘relay’ ridge (the intermediate spreading centre, or ILSC) is located between the two. A number of offaxis seamounts have also been documented close to these ridges (e.g., Pearce et al., 1995). The Lau Basin is therefore floored by extended crust related to the Lau Ridge in the west, and a triangular shaped region of new oceanic crust formed at a pair of southward propagating spreading centres in the east (Fig. 2). It has been argued recently that the development of the Lau Basin cannot be accounted for by extension and the propagation of spreading centres alone and that instead, the dynamics of basin opening are intimately linked to the subduction of the Louisville Ridge seamount chain (LSC; Ruellan et al., 2003). These authors argue that the subduction of the LSC effectively ‘locks up’ the system and prevents the transition from rifting to seafloor spreading. Thus, in their view, the southward migration of spreading centres in the Lau Basin follows the locus of oblique subduction of the LSC. The study of Ruellan et al. (2003) may explain the recent increase in spreading rates on both the ELSC and CLSC proposed by Taylor et al. (1996). 3. Samples and analytical techniques This investigation draws together data from the oldest arc rocks known in the region (ʻEua), younger arc

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

magmatism associated with the Lau Ridge (ʻEua, Lau Islands, ODP Sites 840 and 841), and volcanic rocks erupted during the opening of the Lau Basin including the extension phase (ODP Sites 834, 835 and 839) and seafloor spreading (CLSC, ELSC, ILSC, off-axis seamounts, ODP Sites 836 and 837). Descriptions of the petrographic and major element variations of most sample suites employed in this work are provided elsewhere and will not be re-iterated here. Instead, broad observations for the ʻEua and Lau Island magmatic groups, which have not been summarised before, are presented in Table 1 and the reader is referred to the wealth of more detailed information available in the contributions cited. Data for samples from the range of arc and back-arc localities listed above are compared with those from the Tofua Arc, particularly samples derived from volcanoes in the central part of the arc (i.e., excluding Ata in the south and Niuatoputapu and Tafahi in the north). The data upon which the following discussions are based are drawn from a range of sources including both published results (as indicated in figure and table captions) and data reported here for the first time. Major element data (Table 2) were published by Ewart and Bryan (1972; ʻEua) and Cole et al. (1985, 1990; Lau Islands) and are reported here for completeness. All of the trace element data are new and were acquired on sample solutions following digestion of powders in screw-top Savillex beakers. These solutions were analysed by ICPMS employing a Fisons PlasmaQuad 2STE at the

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Research School of Earth Sciences at the Australian National University following the protocols of Eggins et al. (1997) and readers are referred to this article for further sample preparation and instrumental details. Our data were acquired in the same laboratory at around the same time as this compilation and, as we reported in Eggins et al. (1997), “analytical precision is near or better than 1% RSD for most elements with mass greater than 80 amu and between 1 and 4% RSD for elements with mass less than 80 amu”. Based upon the analyses of the common USGS standards analytical accuracy is always within 5% of the preferred values included in Eggins et al. (1997), for elements of mass N77 amu and within 10% for elements of lower mass. The trace element analyses for 29-coded Lau Island samples, and the ʻEua samples were conducted on powders provided by previous authors, whereas all other samples were prepared in agate from rock chips. Although tungsten was not monitored during analysis, it is possible that the low Nb/Ta in both the 29coded samples and ʻEua reflects contamination from sample crushing which was likely conducted in tungsten carbide. Although much of the Pb, Sr and Nd data for some sample suites have been reported previously (e.g., in Hergt and Hawkesworth, 1994; Bloomer et al., 1994 and others as indicated in figure and table captions) additional analyses, including Hf isotope data, have been conducted on many of these same samples. Thus, for ease of reference, all isotope data have been compiled into Table 3. Lead, Sr and Nd isotope data were obtained

Table 1 Petrological summary of magma suites preserved on ʻEua and the Lau Islands Magmatic suite

Occurrence and distribution

Main eruption style

Rock-type (proportions present where available and phenocryst assemblage) Basalt/gabbro

ʻEua a

Eocene basement (∼40–46 Ma) ∼ 31–33 Ma ∼ 17–19 Ma

Conglomerates of gabbroic and volcanic rocks, tuffs mainly subaqueous Flows

Dykes; eroded boulders also occur Lau Volcanic 15 islands 80 km Lava flows and breccia mainly Group b wide 400 km along subaerial tuff and (LVG) N 6 Ma the Lau Ridge lapilli some dykes (NW–SE and NE–SW) Korobasaga 6 islands 35 km Pillow lavas, lapilli tuff and Volcanic wide 200 km along hyaloclastites mainly Group b (KVG) the Lau Ridge subaqueous followed by subaerial 4.5–2.5 Ma flows and dykes (N–S) a b

Mafic andesite

Felsic andesite

pl + cpx ± mt ± qtz (altered ol)

pl ± cpx (altered ol) 28% pl + ol ± cpx

74% pl + ol + cpx ± ct

Dacite

Rhyolite

pl + qtz + mt (Eocene?)

hbe + pl

pl + cpx + mt ± hbe pl + cpx + mt ± hbe 40% 10% pl + cpx + mt ± pl + mt + opx (no ol) cpx ± opx

20% pl + cpx + mt ± opx (altered ol) ∼ 25% rare pl + cpx + pl + hbe + mt opx + mt pl + opx + cpx + mt

Based on the work of Ewart and Bryan (1972), Hawkins and Falvey (1985), Duncan et al. (1985). Largely from Cole et al. (1985), Cole et al. (1990).

rare pl + hbe + mt

10% 2% pl + mt ± opx

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J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

Table 2 Major and trace element data for ʻEua and the Lau Islands (sources for the major element data are provided in the footnote) Sample code

Sample type

SiO2

TiO2

Al2O3

Fe2O3

FeO

MnO

MgO

CaO

Na2O

K2O

P2O5

Li

Be

Sc

ʻEua E11 E12 E15 E7 E8 E18G E18b E18c E20

Dyke rock Dyke rock Dyke rock Boulder Boulder Boulder Boulder Boulder (Miocene) Boulder

59.98 55.21 51.40 49.19 65.62 48.85 52.13 58.64 49.18

0.75 1.17 0.96 0.42 0.46 0.73 0.88 0.51 0.63

17.02 13.97 16.76 20.61 15.83 18.93 18.76 17.20 19.75

3.29 6.25 5.75 2.93 3.44 3.30 4.90 2.76 3.54

3.38 6.61 4.90 5.63 1.08 6.57 5.10 3.91 6.10

0.10 0.22 0.34 0.18 0.09 0.19 0.21 0.16 0.17

2.87 4.28 5.46 5.93 2.24 6.35 4.46 3.24 4.30

5.93 7.13 9.32 11.78 3.22 11.32 9.77 6.04 6.48

4.00 3.23 2.61 1.21 5.36 1.42 2.68 3.56 4.96

0.31 0.13 0.21 0.24 0.30 0.13 0.19 0.45 0.55

0.14 0.11 0.11 0.04 0.06 0.05 0.10 0.08 0.07

5.13 1.42 3.87 5.99 7.10 2.47 1.81 6.90 20.34

0.56 0.34 0.37 0.15 0.46 0.14 0.28 0.32 0.12

20.9 41.8 36.7 35.4 21.0 36.0 36.4 31.0 44.3

Lau Islands 29 339 29 371 29 385 29 458 29 490 29 491 29 764 29 765 29 769 29 770 29 784 29 787 29 790 29 792 29 796 AK 108 AK 113 AK 16 AK 19 C 2220 C 3416B C2294 DW 145 29 348 29 350 29 448 29 487 29 488 AK 117 AK 118 AK 12 AK 24 AK 6 AK 7 AK 8 AK 9 C 2238 C 2283 DW 6

LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG KVG KVG KVG KVG KVG KVG KVG KVG KVG KVG KVG KVG KVG KVG KVG KVG

52.36 57.28 65.46 52.06 60.87 61.04 67.47 51.02 62.21 59.43 57.34 63.74 50.44 70.51 66.09 58.67 49.41 55.47 62.84 53.55 59.03 59.01 55.21 48.45 60.31 51.62 52.55 55.65 51.38 55.68 48.78 48.90 51.67 56.01 51.44 49.46 55.99 50.82 51.76

1.07 0.85 0.84 1.20 0.90 0.90 0.48 0.96 0.99 1.35 0.94 1.01 1.19 0.52 0.87 0.97 0.78 0.85 0.63 0.96 0.93 0.95 1.07 0.96 0.55 0.68 0.89 0.68 0.70 1.02 0.88 1.01 1.15 0.65 1.00 0.95 0.76 0.95 0.91

19.33 17.20 15.66 18.40 16.70 16.67 15.95 20.68 16.41 17.73 18.05 15.77 19.01 15.62 16.35 16.21 16.51 18.78 16.46 18.06 17.38 16.42 19.26 20.19 18.50 20.00 19.69 18.95 19.69 16.28 18.11 19.82 22.13 18.79 21.03 20.02 18.86 18.42 18.99

4.37 2.48 1.72 5.67 3.54 3.24 4.66 4.00 4.59 2.14 3.63 5.34 5.03 3.08 4.63 7.56 10.53 6.86 5.45 9.53 6.17 8.00 7.84 3.54 4.69 6.67 2.89 2.63 8.27 9.24 10.97 8.84 6.78 6.27 8.51 10.18 7.74 10.48 8.43

5.97 5.26 4.10 4.12 3.30 3.32 0.19 3.82 2.67 4.03 3.83 2.15 5.66 0.28 0.82

0.18 0.15 0.16 0.16 0.12 0.15 0.04 0.13 0.06 0.09 0.16 0.15 0.17 0.04 0.06 0.15 0.17 0.17 0.14 0.17 0.15 0.14 0.14 0.18 0.20 0.13 0.13 0.15 0.15 0.20 0.18 0.13 0.07 0.13 0.17 0.13 0.21 0.17 0.14

3.46 3.98 1.31 4.17 2.94 2.92 0.54 4.18 1.06 1.49 3.59 0.15 4.36 0.37 0.87 3.43 6.49 3.56 1.89 4.51 1.86 3.23 2.19 4.33 2.45 5.15 5.10 4.22 5.33 4.21 5.87 4.47 1.83 3.42 2.66 3.59 2.69 5.50 5.15

10.14 8.43 4.38 10.50 6.39 6.41 2.21 11.48 4.01 6.84 7.93 3.99 10.91 1.93 3.29 7.00 11.97 8.20 4.78 9.80 4.93 7.00 7.62 11.59 7.44 10.50 9.95 8.51 10.20 8.07 11.45 12.13 10.16 8.48 10.90 10.03 7.98 9.53 9.57

2.38 3.06 3.91 2.78 3.33 3.42 4.34 2.44 4.19 3.70 3.36 4.67 2.34 5.85 5.21 3.54 2.14 3.14 4.24 2.78 4.30 3.47 3.73 1.54 2.46 2.05 2.18 2.78 2.02 3.21 2.24 2.21 3.12 2.99 2.79 2.96 3.02 2.50 2.13

0.54 1.16 2.18 0.72 1.67 1.69 4.09 1.02 3.34 2.61 0.91 1.67 0.70 1.78 1.57 1.41 0.61 1.29 2.05 0.55 2.77 0.89 1.76 0.77 0.96 1.20 1.40 1.63 1.16 0.78 0.60 0.57 0.69 1.03 0.57 1.48 1.27 0.78 1.48

0.21 0.15 0.28 0.21 0.23 0.23 0.03 0.26 0.47 0.58 0.25 0.35 0.18 0.01 0.26 0.22 0.20 0.44 0.21 0.18 0.40 0.20 0.43 0.22 0.24 0.24 0.23 0.33 0.24 0.22 0.22 0.22 0.32 0.29 0.21 0.31 0.37 0.21 0.24

5.07 4.89 4.97 4.87 5.22 4.90 7.49 4.03 10.15 3.17 4.68 9.12 6.01 7.55 6.18

0.60 0.71 1.29 0.70 1.25 1.21 2.22 0.80 2.19 1.98 0.87 1.45 0.60 1.52 1.28

4.38 4.79 4.77 5.36 5.00

0.44 0.76 1.18 0.73 1.13

31.7 25.4 18.3 31.9 17.7 17.5 10.9 26.2 15.4 22.5 21.1 17.2 27.7 11.0 20.9 25.1 40.3 17.8 10.6 31.9 17.6 27.8 22.8 39.4 10.5 21.9 28.9 16.2 25.5 26.4 32.8 34.3 31.5 18.6 30.0 22.8 17.3 27.8 33.4

8.23 2.21 1.75 4.99 4.46

Note: Although the dyke rocks E11, E12 and E15 from ʻEua are broadly regarded to be Miocene (e.g., Ewart and Bryan, 1972; Duncan et al., 1985) the ages of the boulders are not well-constrained. Ewart and Bryan (1972) interpret E18c to be a boulder from one of these young dykes, similar to E11. Boulders of similar age have been reported by Duncan et al. (1985) but most boulders from Lakupo Beach (also called ‘Liku Beach’) are regarded to be Eocene (e.g., Cunningham and Anscombe, 1985; Duncan et al., 1985). Major element data are derived from: Ewart and Bryan (1972)—ʻEua. Cole et al. (1985, 1990)—Lau Islands. Note: spaces indicate that the element was not determined, a hyphen indicates the concentration is below detection limits.

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

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Table 2 (continued ) V

Cr

Co

130 421 357 261 87 287 332 135 405

5.8 12.5 76.8 80.4 21.3 46.2 45.8 19.4 9.7

15.1 34.3 22.9 29.4 9.2 32.8 30.7 17.1 30.9

309 228 30 301 162 161 10 281 50 172 166 27 325 8 25 241 352 231 150 311 111 244 234 382 101 227 305 160 267 255 380 347 358 234 365 335 157 309 342

13.9 35.4 – 88.6 135 15.1 0.1 93.3 – – 23.3 – 14.0 – – 34.5 128 19.3 9.4 70.9 7.5 20.3 9.8 2.8 0.8 24.4 28.4 32.2 32.9 54.0 41.1 53.1 22.6 28.9 22.5 21.6 6.3 31.2 35.0

35.5 37.9 21.8 35.8 28.3 28.2 23.5 44.7 25.8 47.1 39.3 15.3 36.4 16.1 10.5 23.0 38.0 17.0 8.0 30.0 12.0 18.0 18.0 39.9 19.2 40.3 41.1 47.0 25.0 32.0 35.0 27.0 27.0 17.0 32.0 32.0 11.0 32.0 25.0

Ni

Cu

Zn

Ga

Rb

Sr

Y

5.3 17.4 41.6 25.9 49.0 20.5 26.6 6.8 10.9

63.9 160 14.8 106 7.6 52.9 101 40.0 89.0

38.0 67.9 54.9 128 30.2 51.3 84.3 63.0 66.5

16.7 16.9 16.7 15.1 12.5 16.1 17.2 15.4 15.5

2.00 0.52 1.24 1.34 2.15 0.90 1.32 2.28 6.14

142 102 123 112 127 126 120 132 204

31.5 33.8 27.2 12.6 26.0 13.2 24.0 23.9 11.1

10.2 16.6 1.0 34.0 10.8 10.0 3.4 44.1 1.2 2.5 25.3 0.9 0.2 0.9 1.1 3.0 70.0 16.0 0.0 24.0 38.0 0.0 0.0 7.1 1.9 23.6 28.1 26.6 23.0 24.0 58.0 29.0 9.0 7.0 106 17.0 2.0 14.0 13.0

87.7 40.1 24.3 77.7 29.1 24.3 14.6 102 37.2 112 37.6 29.5 60.9 23.5 6.3 34.0 134 50.0 39.0 56.0 28.0 33.0 31.0 77.7 24.4 58.3 121 54.5 80.0 34.0 86.0 120 142 51.0 97.0 22.0 26.0 33.0 127

85.3 66.5 84.0 77.8 64.0 60.8 70.3 62.3 83.4 115 77.5 96.6 78.8 65.9 63.9 74.0 75.0 96.0 66.0 74.0 91.0 78.0 77.0 70.0 71.2 66.1 67.3 76.1 71.0 78.0 77.0 76.0 57.0 91.0 88.0 80.0 87.0 65.0 74.0

18.3 16.3 17.1 17.9 15.8 16.6 16.5 17.2 17.9 19.2 16.9 18.2 17.9 17.1 18.2 29.0 29.0 31.0 29.0 25.0 25.0 23.0 23.0 16.4 15.6 16.5 17.5 17.2 31.0 29.0 25.0 32.0 31.0 24.0 30.0 28.0 27.0 23.0 23.0

9.08 16.9 32.8 12.2 35.5 36.7 110 24.0 83.1 68.0 12.6 25.3 11.5 24.4 23.0 26.9 12.0 29.8 33.3 7.24 63.3 14.4 32.6 13.2 15.1 40.3 45.7 60.4 36.1 10.5 21.2 7.06 7.39 32.3 12.3 23.1 18.8 12.8 40.2

283 240 235 327 296 303 206 547 280 416 338 290 366 211 290 296 450 570 406 253 380 239 451 442 519 596 542 598 592 312 536 442 562 520 470 576 477 438 517

24.3 27.4 50.8 27.4 60.1 54.9 22.9 22.2 39.6 47.9 34.9 179 25.4 35.0 162 40.2 16.7 29.1 26.2 24.0 48.7 33.2 30.2 18.5 29.2 19.8 33.7 19.0 18.2 47.7 17.2 22.1 27.2 19.7 24.4 20.6 25.7 17.0 28.3

Zr

Nb

Cs

Ba

67.7 24.3 35.7 14.4 57.6 10.9 13.5 38.2 10.2

1.37 0.89 0.83 0.27 1.06 0.21 0.64 0.55 0.20

0.03 0.01 0.04 0.02 0.07 0.04 0.04 0.02 0.23

39.2 43.9 33.5 12.1 44.8 22.5 18.4 64.6 97.5

73.1 114 201 81.3 173 175 286 64.2 215 179 110 186 67.3 197 185 165 40.6 103 179 69.3 186 119 104 36.4 59.6 45.7 34.6 83.0 48.2 103 41.5 74.4 46.9 61.1 40.6 53.4 63.6 37.8 36.1

1.83 2.61 4.72 2.57 4.81 5.09 8.49 2.04 6.38 5.87 2.29 3.71 1.45 3.91 4.02 4.27 0.71 3.92 4.90 1.46 6.33 2.25 3.45 0.84 1.41 3.35 2.59 5.60 2.90 1.42 0.80 1.97 1.09 1.51 0.88 1.03 1.38 0.87 2.43

0.17 0.26 0.55 0.09 0.60 0.62 0.75 0.37 0.73 1.08 0.08 0.19 0.09 0.09 0.25 0.35 0.13 0.37 0.29 0.05 0.99 0.07 0.52 0.06 0.57 0.52 0.52 0.86 0.40 0.14 0.48 0.39 0.20 0.37 0.18 0.11 0.23 0.05 0.45

65.7 178 201 145 218 225 465 150 408 381 126 269 82.2 246 269 178 203 211 261 61.7 430 107 237 158 254 243 290 390 212 125 179 126 205 289 167 257 212 148 276

La

Ce

Pr

3.36 2.01 1.70 0.76 3.44 0.68 1.58 2.24 0.52

10.6 6.19 5.39 2.35 9.45 2.10 4.84 6.18 1.51

1.91 1.16 1.03 0.44 1.57 0.43 0.90 1.08 0.30

5.19 7.66 13.2 7.09 14.3 16.2 14.4 8.47 22.2 21.3 10.2 18.1 7.24 8.28 22.2 12.0 4.50 10.7 16.0 4.77 22.9 7.05 13.7 3.72 9.69 6.97 6.74 9.33 5.60 10.5 4.77 7.96 6.28 6.94 4.87 6.15 6.27 3.71 5.54

13.1 18.2 32.7 17.2 33.0 36.0 23.7 19.8 48.0 47.4 25.3 49.9 18.1 19.0 43.9 26.8 9.90 23.4 31.3 11.6 43.6 16.8 29.1 9.32 14.4 13.4 9.28 18.3 10.7 32.3 10.6 18.0 14.8 14.6 11.2 13.7 14.4 8.52 9.10

2.11 2.80 5.01 2.73 4.85 5.61 3.42 3.07 7.16 6.84 3.96 9.22 2.81 3.26 9.44 3.88 1.56 3.45 4.32 1.80 6.67 2.61 4.36 1.53 3.28 2.11 2.25 2.49 1.59 5.05 1.67 2.77 2.46 2.12 1.84 2.12 2.30 1.38 1.71

(continued on next page)

16

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

Table 2 (continued ) Nd ʻEua 10.1 6.78 6.01 2.51 7.79 2.67 5.16 5.80 1.85

Sm

Eu

Gd

Tb

Dy

Ho

Er

Yb

Lu

Hf

Ta

Pb

Th

U

3.41 2.73 2.31 1.01 2.54 1.10 1.97 2.02 0.78

1.11 0.93 0.91 0.41 0.73 0.51 0.78 0.76 0.38

4.41 4.04 3.33 1.48 3.24 1.67 2.92 2.84 1.22

0.80 0.76 0.62 0.29 0.60 0.31 0.54 0.52 0.24

5.10 5.06 4.18 1.95 3.92 2.06 3.63 3.43 1.65

1.06 1.14 0.92 0.43 0.85 0.46 0.81 0.76 0.38

3.05 3.41 2.77 1.32 2.57 1.36 2.48 2.25 1.14

2.78 3.22 2.66 1.25 2.59 1.27 2.34 2.12 1.12

0.388 0.487 0.397 0.190 0.349 0.191 0.350 0.319 0.168

1.94 1.01 1.15 0.52 1.60 0.40 0.57 1.32 0.37

0.216 0.161 0.244 0.201 0.188 0.308 0.211 0.221 0.180

0.65 1.34 1.61 8.11 0.85 0.78 0.62 1.06 0.78

0.109 0.185 0.146 0.056 0.419 0.023 0.116 0.108 0.037

0.115 0.097 0.083 0.032 0.234 0.016 0.057 0.092 0.174

Lau Islands 10.4 3.07 12.8 3.41 23.1 6.39 12.8 3.70 21.8 5.83 24.9 6.43 13.6 3.28 14.1 3.77 30.7 7.29 30.1 7.69 19.3 5.30 50.2 17.6 13.7 3.86 14.9 4.12 46.7 13.8 16.5 4.83 7.25 2.40 14.8 4.28 16.7 4.24 8.49 2.90 27.6 7.68 12.1 3.99 18.7 5.43 7.82 2.47 15.4 3.84 9.44 2.57 11.2 3.24 10.5 2.57 6.94 2.13 23.9 8.43 7.72 2.46 12.4 3.63 11.7 4.00 9.15 2.78 8.75 2.96 9.73 3.10 10.7 3.47 6.63 2.23 8.17 2.74

1.12 1.05 1.79 1.27 1.64 1.74 1.74 1.12 1.93 2.05 1.62 6.01 1.26 1.87 4.47 1.33 0.83 1.37 1.23 1.00 2.01 1.26 1.69 0.92 1.31 0.96 1.14 0.90 0.77 2.63 0.87 1.20 1.46 0.99 1.06 1.06 1.23 0.88 0.96

3.66 4.00 7.15 4.18 6.92 7.52 2.92 3.85 7.03 8.00 5.78 23.3 4.13 4.39 17.3 5.46 2.72 4.46 4.08 3.52 7.94 4.84 5.60 2.90 4.30 2.95 4.29 2.76 2.56 8.73 2.78 3.82 4.65 3.11 3.57 3.41 3.99 2.70 3.66

0.64 0.69 1.28 0.73 1.26 1.32 0.56 0.62 1.14 1.31 0.97 4.41 0.70 0.84 3.28

3.88 4.20 7.59 4.36 7.84 8.00 3.50 3.58 6.40 7.42 5.69 28.2 4.13 5.28 21.0 5.83 2.72 4.42 3.96 3.81 7.55 5.18 5.20 3.02 4.07 3.10 4.30 2.86 2.68 9.15 2.77 3.67 4.41 2.99 3.71 3.32 4.09 2.78 3.91

0.84 0.91 1.69 0.95 1.83 1.80 0.80 0.75 1.33 1.59 1.20 6.05 0.90 1.21 4.94

2.46 2.70 4.86 2.76 5.46 5.32 2.50 2.17 3.71 4.43 3.37 17.6 2.60 3.72 15.2 3.81 1.66 2.80 2.52 2.40 4.62 3.31 3.10 1.84 2.63 1.93 2.88 1.82 1.71 5.26 1.67 2.18 2.62 1.82 2.31 1.97 2.57 1.73 2.58

2.23 2.60 4.70 2.53 5.34 5.16 3.14 1.98 3.46 4.24 3.25 16.8 2.33 3.98 14.6 3.79 1.59 2.82 2.66 2.36 4.57 3.36 3.00 1.69 2.55 1.91 2.67 1.86 1.68 5.43 1.56 2.08 2.44 1.78 2.26 1.89 2.54 1.67 2.50

0.349 0.407 0.741 0.399 0.843 0.803 0.531 0.307 0.535 0.669 0.496 2.635 0.363 0.647 2.383 0.606 0.242 0.448 0.434 0.362 0.708 0.521 0.454 0.257 0.399 0.308 0.434 0.305 0.265 0.825 0.236 0.313 0.369 0.281 0.355 0.291 0.392 0.263 0.411

1.86 2.88 4.88 2.02 4.24 4.21 6.96 1.68 5.16 4.44 2.92 4.77 1.81 5.04 4.71 4.13 1.20 2.51 4.10 1.81 4.49 3.09 2.83 1.05 1.55 1.24 1.05 1.99 1.32 2.50 1.15 1.80 1.30 1.56 1.18 1.47 1.78 1.09 1.20

0.283 0.297 0.371 0.308 0.410 0.516 0.494 0.548 0.430 0.541 0.329 0.328 0.331 0.310 0.542 0.267 0.042 0.217 0.342 0.094 0.359 0.150 0.209 0.301 0.202 0.450 0.478 0.549 0.166 0.105 0.042 0.127 0.060 0.084 0.049 0.059 0.084 0.049 0.146

1.11 1.60 3.07 1.22 1.64 1.92 4.88 1.41 4.06 4.00 2.08 2.19 0.88 2.35 1.49 5.35 4.20 5.28 5.67 3.90 6.63 4.81 5.22 0.92 2.47 3.54 3.90 5.42 6.23 4.36 4.26 4.05 4.05 5.24 4.31 4.98 4.85 3.75 7.01

0.425 0.814 1.363 0.577 1.769 1.726 5.051 0.914 3.550 3.148 0.789 1.433 0.569 1.435 1.418 1.624 0.396 0.931 2.451 0.385 3.384 0.746 1.673 0.279 0.450 0.473 0.337 0.926 0.467 0.515 0.393 0.603 0.385 0.592 0.327 0.536 0.506 0.262 0.361

0.235 0.385 0.679 0.255 0.746 0.761 1.673 0.367 1.359 1.270 0.347 0.636 0.245 0.646 0.616 0.653 0.184 0.621 0.890 0.175 1.237 0.343 0.606 0.141 0.196 0.259 0.228 0.507 0.217 0.275 0.184 0.288 0.198 0.271 0.180 0.239 0.349 0.130 0.224

0.51 0.70 0.51 0.72 0.48

0.65 0.89 0.68 0.98 0.61

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

17

Table 3 Isotope data for this study Sample code

Island/unit

Alternative code

87Sr/ 86Sr

143Nd/ 144Nd

206Pb/ 207Pb/ 208Pb/ 176Hf/ 204Pb 204Pb 204Pb 177Hf

ʻEua E11 E12 E15 E7 E8 E18G E18b E18c E20

ʻEua ʻEua ʻEua ʻEua ʻEua ʻEua ʻEua ʻEua ʻEua

Dyke rock Dyke rock Dyke rock Boulder Boulder Boulder Boulder Boulder (Miocene) Boulder

0.703260 0.703424 0.703500 0.703449 0.704122 0.703246 0.703487 0.703523 0.704170

0.513118 0.513111 0.513107 0.513110 0.513108 0.513129 0.513119 0.513133 0.513111

18.820 18.837 18.804 18.790 18.877 18.762 18.815 18.787 18.758

15.573 15.566 15.556 15.561 15.566 15.555 15.554 15.551 15.557

38.455 38.482 38.427 38.419 38.484 38.387 38.426 38.387 38.339

18.789 18.746 18.754 18.785 18.791 18.619 18.732 18.739 18.772 18.730 18.739 18.742 18.630 18.732 18.887 18.740 18.728 18.763 18.741 18.807 18.875

15.551 15.550 15.544 15.551 15.564 15.524 15.557 15.548 15.560 15.552 15.543 15.548 15.518 15.542 15.551 15.540 15.536 15.551 15.483 15.550 15.551

38.358 38.325 38.308 38.364 38.493 38.124 38.347 38.340 38.393 38.332 38.322 38.254 38.106 38.309 38.361 38.304 38.291 38.352 38.132 38.387 38.349

18.902 18.900 18.883 18.869 18.894 18.826 18.862 18.939

15.560 15.558 15.564 15.556 15.562 15.558 15.565 15.558

18.851 18.840 18.821 18.824 18.821 18.742 18.809 18.866 18.733

15.581 15.569 15.567 15.573 15.570 15.541 15.554 15.566 15.541

Lau Volcanic Group DW 324 DW 166 DW 202 29 458 DW 70 AK 108 29 784 29 787 29 790 29 792 29 796 AK 114 AK 119 C 2342 C 2367B 29 339 29 371 29 385 C 2220 C 2255 AK 113 C2294 29 490 29 491 AK 16 AK 19 29 764 29 765 29 769 29 770 DW 145 C 3416B DW 174A

Cicia Cicia Cicia Mago Cikobia-I-Lau Komo Lakeba Lakeba Lakeba Lakeba Lakeba Nayau Oneata Tuvuca Tuvuca Vanua Balavu Vanua Balavu Vanua Balavu Vanua Balavu Vanua Balavu Yacata Yacata Yacata Naitauba Naitauba Ono-I-lau Ono-I-lau Ono-I-lau Ono-I-lau Ono-I-lau Ono-I-lau Ono-I-lau

Korobasaga Volcanic Group 29 448 Moce AK 118 Moce 29 487 Olorua 29 488 Olorua DW 6 AK 117 Kanacea AK 9 Kibobo AK 12 Kibobo AK 24 Mago

MG191

0.702999 0.513045

LK203 LK243 LK361 LK394 LK409B

0.702885 0.703069 0.703239 0.703159 0.702999 0.703109

0.513069 0.513057 0.513048 0.513043 0.513045

0.702935 0.513063

VB133 VB129 VB870A

YT33 YT43

OA64C OA73A OA149B OA160

MC19 OR6 OR11

0.703189 0.703219 0.703069 0.703100

0.513035 0.513042 0.513042 0.513074

0.702869 0.703190 0.702919 0.702759 0.703398 0.703487 0.703149 0.702919 0.702929 0.703219 0.702940 0.703012

0.513051 0.513054

0.513042 0.513034 0.513046 0.513043

0.703919 0.704010 0.704059 0.703679 0.703984 0.703315 0.703366 0.703419 0.703181

0.513020 0.513037 0.513010 0.513017 0.513034 0.513058 0.513044 0.513040 0.513071

0.513046 0.513068 0.513050

0.283140

eHf

0.283150 0.283199

9.32 9.19 9.11 9.17 9.13 9.54 9.34 9.62 9.19

0.283132

7.90 12.73

0.283153

8.37 13.47 8.13 7.96 7.86 12.63 7.90

0.283169

0.283129

13.01

14.04

13.37 15.10

0.283151

8.25 13.40

0.283131

0.283172

7.70 12.70 7.84 7.84 8.47 14.15

0.283151 0.283153

8.02 13.40 8.08 13.47

38.398 38.393 38.513 0.283159 38.526 0.283150 38.418 38.377 0.283155 38.414 38.432 0.283160 18.931 15.551 38.404 18.935 15.552 38.410

38.753 38.702 38.678 38.649 38.678 38.304 38.396 38.473 38.288

ɛNd

0.283141 0.283158 0.283142 0.283166 0.283192 0.283189 0.283171 0.283167

7.84 13.69 7.69 13.37 7.92 7.86 13.54 7.92 8.35 13.72 8.00

7.41 7.74 7.22 7.35 7.69 8.15 7.88 7.80 8.41

13.05 13.65 13.08 13.93 14.85 14.75 14.11 13.97

(continued on next page)

18

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

Table 3 (continued ) Sample code

Island/unit

Korobasaga Volcanic Group AK 6 Malima AK 7 Malima AK 8 Malima 29 348 Vanua Balavu 29 350 Vanua Balavu C 2238 Vanua Balavu C 2283 Vanua Balavu Modern Lau Basin 10.1.1 10.1.3 11.1.1 12.1.1 12.5.1 12.5.3 13.1 13.5 15.1.2 15.1.4 20.1.1 20.5.1 20.5.2 21.1.1 22.1.1 22.6.1 22.6.2 23.2.1 23.8.1 23.8.2 24.1.1 25.2.1 25.5.1 18.1.2 41.2.1 41.3.2 9.1.1 9.3.2 17.1.1 17.1.3

CLSC CLSC CLSC CLSC CLSC CLSC CLSC CLSC CLSC CLSC ELSC ELSC ELSC ELSC ELSC ELSC ELSC ELSC ELSC ELSC ELSC ELSC ELSC ILSC ILSC ILSC Seamount Seamount Seamount Seamount

Lau Basin ODP Leg 135 834B 8R-2 10–13 2 834B 11R-3 83–86 5 834B 13R-1 130–131 5 834B 14R-1 28–32 6 834B 15R-2 35–37 6 834B 15R-2 96–104 6 834B 18R-1 7–13 6 834B 26R-1 107–109 7 834B 31R-1 98–104 7 834B 31R-2 43–48 7 834B 33R-1 0–6 7 834B 33R-2 105–110 7 834B 34R-2 60–63 7 834B 35R-1 15–23 8 834B 35R-2 77–83 8 834B 36R-1 0–7 8

Alternative code

VB479 VB512

87Sr/ 86Sr

143Nd/ 144Nd

206Pb/ 207Pb/ 208Pb/ 176Hf/ 204Pb 204Pb 204Pb 177Hf

0.703255 0.703237 0.703323 0.703379 0.703419 0.703354 0.703227

0.513062 0.513055 0.513065 0.513053 0.513058 0.513056 0.513042

18.786 18.798 18.735 18.809 18.807 18.793 18.807

15.542 15.552 15.545 15.549 15.551 15.550 15.534

38.346 38.391 38.302 38.395 0.283162 38.409 0.283168 38.385 0.283200 38.361

18.112 18.088 18.119 18.091 18.160 18.156 18.149 18.171

15.447 15.462 15.472 15.545 15.484 15.474 15.475 15.499

37.864 37.859 37.911 37.999 37.977 37.948 37.951 38.037

18.161 18.147 18.269 18.223 18.169 18.205 18.216 18.228 18.208 18.131 18.295 18.080 18.285 18.399 18.142 18.263 18.341 18.097 18.391 18.167 18.139

15.495 15.488 15.510 15.511 15.504 15.525 15.494 15.473 15.467 15.498 15.472 15.525 15.533 15.555 15.450 15.463 15.511 15.483 15.542 15.476 15.487

37.973 37.981 38.116 38.092 37.980 38.088 38.054 37.944 37.796 37.990 37.869 37.983 38.049 38.132 37.940 38.015 38.170 37.636 38.101 37.766 37.758

0.513131 18.781 15.554 0.513141 18.738 15.529 18.730 15.528 0.513112 18.643 15.509 0.513142 18.617 15.480 0.513145 18.638 15.500 0.513143 18.584 15.505 0.513116 18.561 15.482 0.513129 18.572 15.476 0.513142 18.593 15.502 0.513133 18.572 15.467 18.605 15.494 0.513120 18.767 15.527 0.513104 18.769 15.534 0.513139 18.790 15.546 0.513111 18.776 15.534

38.428 38.324 38.333 38.177 38.093 38.147 38.095 38.027 38.026 38.035 38.028 38.081 38.384 38.398 38.425 38.395

0.703207 0.703272 0.703224 0.703292 0.703348 0.703329 0.703310 0.703179 0.703182 0.703216 0.703297 0.703212 0.703284 0.703315 0.703257 0.703218 0.703203

0.513157 0.513141 0.513144 0.513139 0.513130 0.513030 0.513141 0.513126 0.513127

0.513113 0.513104 0.513095 0.513086 0.513085 0.513087 0.513070 0.703235 0.513102 0.702918 0.513069 0.703191 0.513098 0.703297 0.703315 0.703186 0.513084 0.703413 0.513023 0.513064 0.702794 0.513053 0.513053 0.702857 0.513105 0.702916 0.513109

0.702871 0.702812 0.703102 0.702656 0.702704 0.702648 0.702667 0.702554 0.702572

0.702574 0.702804 0.702776 0.702806 0.702988

ɛNd

eHf

8.23 8.10 8.29 8.06 13.79 8.15 14.00 8.11 15.14 7.84

10.09 9.77 9.83 9.73 9.56 7.61 9.77 9.48 9.50 9.23 9.05 8.88 8.70 8.68 8.72 8.39 9.01 8.37 8.94

8.66 7.47 8.27 8.06 8.06 9.07 9.15

9.58 9.77 0.283166 0.283174 0.283171 0.283174

0.283160 0.283178 0.283168

9.21 9.79 9.85 9.81 9.29 9.54 9.79 9.62

13.93 14.22 14.11 14.22

13.72

9.37 14.36 9.05 9.73 14.00 9.19

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

19

Table 3 (continued ) Sample code

Island/unit

Lau Basin ODP Leg 135 834B 37R-2 42–44 10a 834B 39R-1 9–15 10a 834B 40R-1 45–47 10a 834B 46R-1 37–40 11 834B 47R-1 130–135 12 834B 49R-1 142–144 12 834B 53R-1 7–20 12 834B 56R-2 111–118 13 834B 59R-2 52–55 13 835B 3R-1 123–127 1 835B 3R-2 0–4 1 835B 4R-1 73–78 1 835B 5R-1 14–21 1 835B 7R-2 75–84 1 835B 7R-3 135–140 1 1 836A 3H-3 23–24 836A 3H-3 30–95 1 836A 3H-3 33–43 1 836A 3H-4 53–110 2 836A 3H-4 88–100 2 836A 3H-CC 0–7 3 836A 9X-1 54–58 4 836A 9X-2 16–24 4 836B 3R-1 47–52 4a 836B 5R-2 65–74 4b 836B 6R-2 20–25 4b 836B 7R-2 62–67 5 836B 9 M-1 97–102 837B 2R-1 59–63 1 837B 4R-1 101–109 1 837B 5R-1 86–92 1 838A 20H-1 50–110 1 839B 12R-2 84–87 1 839B 13R-3 100–104 1 839B 18R-1 0–6 2 839B 19R-1 0–5 2 839B 19R-1 15–19 3 839B 19R-1 38–41 3 839B 21R-1 53–56 3 839B 23R-1 32–35 3 839B 25R-1 48–53 4 839B 27R-1 7–11 5 839B 27R-1 29–33 6 839B 29R-1 39–43 9 839B 29R-1 121–126 9 839B 36R-1 32–37 9 839B 37R-1 1–5 9 839B 42R-1 0–6 9 841B 18R-1 137–140 1a 841B 25R-3 118–119 1d 841B 25R-4 21–22 1d 841B 50R-1 16–18 2b Sediments 595A 3-2, 94–100 595A 4-3, 90–96

Volcaniclastic sediments Volcaniclastic sediments

Alternative code

87Sr/ 86Sr

143Nd/ 144Nd

0.702810 0.513137 0.513141 0.702944 0.513131 0.702838 0.513129 0.702898 0.513180 0.702926 0.513133 0.702921 0.513130 0.702832 0.513136 0.702841 0.513117 0.702965 0.513099 0.702993 0.513106 0.702957 0.513086 0.703108 0.702970 0.703219 0.703267 0.703303 0.703280 0.703454 0.702997 0.703046 0.703139 0.703608 0.703587 0.703028 0.703266 0.703204 0.703224 0.703255 0.703255 0.703641

0.513099 0.513093

0.513077 0.513045 0.513062 0.513072

0.513085 0.513081 0.513069 0.513078 0.513034 0.513136

0.703231 0.512933 0.513082 0.703111 0.513079 0.513094 0.703070 0.513094 0.513109 0.513104 0.703123 0.513112 0.703205 0.513102 0.703073 0.513099 0.703203 0.513104 0.703189 0.513076 0.703437 0.513114 0.703146 0.513116 0.513106 0.703410 0.513116 0.513105 0.703496 0.513105 0.513136

206Pb/ 207Pb/ 208Pb/ 176Hf/ 204Pb 204Pb 204Pb 177Hf 18.677 18.722 18.732 18.680 18.731 18.712 18.738 18.701 18.700 18.704 18.688 18.705 18.686 18.655 18.680 18.485 18.458 18.502 18.485 18.511 18.357 18.330 18.346 18.401 18.356 18.335 18.279 18.530 18.519 18.543 18.547 18.910 18.684 18.710 18.674 18.689 18.659 18.654 18.659 18.676 18.664 18.712 18.667 18.712 18.692 18.663 18.659 18.669 18.707 18.716 18.709 18.750

15.534 15.523 15.533 15.514 15.516 15.495 15.525 15.518 15.512 15.547 15.540 15.560 15.546 15.503 15.530 15.548 15.523 15.542 15.523 15.536 15.523 15.497 15.513 15.528 15.510 15.505 15.536 15.533 15.516 15.538 15.535 15.559 15.539 15.548 15.519 15.536 15.518 15.514 15.519 15.535 15.536 15.545 15.523 15.540 15.522 15.537 15.530 15.515 15.536 15.537 15.547 15.539

38.395 38.297 38.328 38.245 38.293 38.243 38.320 38.275 38.270 38.340 38.302 38.351 38.321 38.194 38.282 38.154 38.075 38.139 38.086 38.140 38.041 37.974 38.000 38.089 38.004 38.001 37.958 38.170 38.142 38.202 38.189 38.548 38.261 38.293 38.192 38.247 38.182 38.168 38.170 38.222 38.198 38.280 38.193 38.264 38.209 38.219 38.193 38.176 38.296 38.324 38.310 38.341

ɛNd

eHf

9.70 9.77 9.58 9.54 10.53 9.62 9.56 9.68 9.31 0.283160 8.95 13.72 0.283161 9.09 13.76 8.70

0.283213

8.95 8.84 15.60

0.283206

8.52 15.35

0.283165 0.283163

7.90 13.90 8.23 13.83 8.43

0.283197 0.283196 0.283188 0.283185

0.283157

0.283180 0.283162

8.68 8.60 8.37 8.54 7.69 9.68

15.03 14.99 14.71 14.61

5.72 8.62 8.56 8.86 8.86 9.15 9.05 9.21 9.01 13.62 8.95 9.05 8.51 9.25 14.43 9.29 9.09 13.79 9.29 9.07 9.07 9.68

0.512343 18.813 15.644 38.744 0.282988 −5.79 0.512405 18.773 15.658 38.762 0.282996 −4.58

7.64 7.92

(continued on next page)

20

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

Table 3 (continued ) Alternative code

87Sr/ 86Sr

143Nd/ 144Nd

Island/unit

Sediments 595A 8-1, 110–115 204-4-3, 75–76 204-4-4, 140–141 204-5-1, 60–61D 204-5-4, 60–61 204-6-2, 60–61 204-6-3, 48–50 204-6-4, 99–100 204-7-1, 92–93 204-8-1, 127–128 204-8-3, 86–87 204-9-1, 108–110 204-9-3, 24–27 204-9-4, 111–112

Volcaniclastic sediments Pelagic clays Pelagic clays Pelagic clays Pelagic clays Tuffaceous sandstone Tuffaceous sandstone Tuffaceous sandstone Tuffaceous sandstone Tuffaceous sandstone Tuffaceous sandstone Vitric tuff Vitric tuff Vitric tuff

0.708581 0.512426 0.512491 18.782 15.628 0.706535 0.512909 19.176 15.626 18.775 15.701 0.512372 18.768 15.652 0.512700 0.512916 19.088 15.586 0.707015 0.512928 19.150 15.558 19.218 15.569 0.707321 0.512899 19.066 15.538 18.989 15.536 0.512287

0.283009 − 4.17 38.703 0.282957 − 2.91 38.942 5.25 38.966 38.794 0.282979 − 5.23 0.283030 1.17 0.283022 5.38 38.836 5.62 38.697 38.858 38.695 0.283023 5.05 38.677 0.283016 − 6.89

Tofua Arc HH basal HH upper F30-69 F39-69 Metis Late 13 Late 7-69 Late 21-69 T102 T103C 64T6 T113 NT64-T2 NT64-T8

Hunga Haʻapai Hunga Haʻapai Fonualei Fonualei Metis shoal Late Late Late Kao Kao Kao Tafahi Niuatoputapu Niuatoputapu

0.703688 0.703822 0.703745 0.703812 0.703561 0.703646 0.703627 0.703663 0.703312 0.703339 0.703290 0.703926 0.703993 0.704020

38.287 38.232 38.233 38.235 38.255 38.210 38.212 38.208 38.216 38.219 38.201 38.949 38.718 38.711

0.513028 0.512949 0.512943 0.512949 0.512992 0.512967 0.512966 0.512975 0.513038 0.513041 0.513040 0.512917 0.512891 0.512885

206Pb/ 207Pb/ 208Pb/ 176Hf/ 204Pb 204Pb 204Pb 177Hf

ɛNd

Sample code

18.660 18.592 18.593 18.593 18.607 18.577 18.580 18.577 18.616 18.617 18.607 19.332 18.987 18.983

15.566 15.554 15.554 15.554 15.563 15.558 15.557 15.556 15.557 15.557 15.553 15.616 15.592 15.590

0.283197 0.283176 0.283174 0.283180 0.283180 0.283181 0.283191 0.283192 0.283176 0.283184 0.283172 0.283227 0.283219 0.283223

7.57 6.03 5.91 6.03 6.87 6.38 6.36 6.53 7.76 7.82 7.80 5.40 4.90 4.78

eHf

8.38 6.53

7.33 9.12 8.84

8.88 8.63

15.03 14.29 14.22 14.43 14.43 14.46 14.82 14.85 14.29 14.57 14.15 16.09 15.81 15.95

Note the Pb, Sr and Nd isotope data for Leg 135 samples and the sediments have been previously published (Hergt and Hawkesworth, 1994; Bloomer et al., 1994; Turner et al., 1997), but new Hf data are included for some samples so the data have been kept together here. Data published previously are indicated in italics. All Pb, Sr and Nd data for Leg 135 samples, other Lau Basin samples and the sediments were conducted at the Open University using TIMS. All other data (i.e., Pb, Sr, Nd and Hf for ʻEua, the Tofua Arc and the Lau Island samples) were conducted at the ANU and University of Melbourne using TIMS and MC-ICPMS. In the case of the Pb isotope data, the ANU TIMS results employed the double-spike technique of Woodhead et al. (1995) whereas the data acquired using MC-ICPMS utilised Tl correction as described in Woodhead (2002). See text for further details.

employing a combination of facilities at the Open University, UK and the ANU, Australia (both using MAT Finnigan 261 TIMS instruments) and the University of Melbourne, Australia (Nu Instruments MC-ICPMS). In order to facilitate comparisons between datasets, including those retrieved from the literature (Appendix A), all Sr and Nd isotope data (i.e., new results and those from the literature) have been re-normalised to an NBS 987 87Sr/86Sr value of 0.71025 and La Jolla 143Nd/144Nd of 0.511860 respectively. Similar corrections have not been applied to the Pb isotope data in Appendix A because 1) the standard values employed for mass fractionation correction are often not reported (e.g., Ewart et al., 1998), 2) the differences between NBS 981 ratios employed (∼ 0.005) are within analytical uncertainties (e.g., Turner et al., 1997) and 3) an assessment of

an existing Pb isotope data has encouraged us to restrict our use of results to broad fields, or a small number of datasets (see later discussion). Of the new Pb isotope data reported here, the Open University data are corrected for mass fractionation using deviations in NBS 981 from 206 Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of 16.937, 15.491 and 36.70 respectively (ODP Site 841, CLSC, ELSC, ILSC and associated seamounts) and the other data were acquired either using a 207Pb–204Pb double-spike following the method of Woodhead et al. (1995; Lau Islands, ʻEua), or are Tl corrected as described in Woodhead (2002; Tofua Arc), both referenced to the same SRM 981 values noted above. Both double-spike and Tl corrected Pb isotope data are accurate to b ±0.02% (with respect to the SRM981 Pb standard), with an external (2sd) precision of ≈±0.02%. The Hf isotope data

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

were all acquired employing the Nu Instruments facility at Melbourne according to the techniques described in Woodhead et al. (2001) and are reported relative to a JMC 475 176Hf/177Hf value of 0.282160. In terms of precision, typical in-run uncertainties (2se) are ±0.000010 (Nd and Hf) and ±0.000015 (Sr). External (2sd) precision, or reproducibility, is ±0.000015 (Nd and Hf) and ±0.000030 (Sr), based on the results for secondary standards. 4. Results 4.1. Trace elements New trace element data for ʻEua, the LVG and KVG magmas are presented in Table 2 and illustrated in Fig. 3. Data from ʻEua are readily distinguished from the

21

other two groups by the LREE depletion shown by all samples (across the age range recovered). Based on the description of sampling sites (Ewart and Bryan, 1972), subsequent geochronology of similar samples (Duncan et al., 1985), and trace element patterns, we have allocated the ʻEua samples to Eocene and Miocene groups. The assignment is tentative however, and a more detailed geochronological analysis of ʻEua materials would be welcomed. Overall, the mantle normalised incompatible trace element patterns for all samples, including those from ʻEua, LVG and KVG, are typical of arc magmas inasmuch as they display strong depletions in Nb and enrichments in Pb relative to adjacent elements. In detail, the distinction between the LVG and KVG is supported by mantle (N-MORB) normalised trace element patterns. For example, and as noted by Gill (1976) the abundances of HFSE and REE are

Fig. 3. N-MORB normalised incompatible trace element diagrams for representative samples from (a) LVG: black filled squares, KVG: open squares and (b) ʻEua dyke rocks: black filled circles, ʻEua boulders: open circles, and Site 841: stars. Note the more depleted compositions of the KVG rocks compared with the LVG and LREE enrichment in both the LVG and KVG samples compared with the flat or LREE depleted patterns from ʻEua and Site 841. The examples illustrated from Site 841 appear to more closely resemble the dyke rocks from ʻEua. Normalising values are from Sun and McDonough (1989).

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J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

generally lower in the KVG and Cole et al. (1990) suggested that the mantle wedge supplying the KVG magmas was depleted compared with the source of the LVG. Indeed in absolute terms, all but Sr and Pb are generally lower in the KVG samples compared with the LVG (Fig. 3). Gill (1976) and Cole et al. (1985) noted that the fluid-mobile elements are relatively more enriched in the KVG samples compared with the LVG, and this is further confirmed by our new results (e.g., note the difference in Rb/Th between the two groups in Fig. 3). As noted in Table 1 the KVG are, in general, more basaltic in composition than the LVG; however, the lower incompatible trace element concentrations are unlikely to reflect less fractional crystallisation of these magmas, or higher degrees of partial melting. This is because the ratios between elements of similar incompatibility (e.g., Th/U, Nb/La) also tend towards lower values in the KVG, although both groups preserve significant variation. According to the reported age information, the younger dyke rocks on ʻEua, members of the LVG and mafic igneous rocks preserved at ODP Site 841 all predate rifting. Fig. 3(b) suggests that the ʻEua dyke rocks and Site 841 samples are similar to each other, and lack the LREE enrichment of LVG magmas. Volcanic sediments at ODP Site 840 were deposited during both LVG and KVG magmatism. Comparisons between these three groups are restricted by both the number of samples (particularly for Site 840), and breadth of trace element (and isotope) data available for these. Nevertheless, the trace element variations of these groups

share some similarities, such as the LREE enrichment, absent in most other rocks of this investigation. Exceptions to this include some of the volcanic debris retrieved from the ash deposits at ODP Site 840 that contain low abundances of LREE, otherwise only observed in the Eocene samples from ‘Eua and the northern Tofua Arc. 4.2. Isotopes All new isotope data acquired in this study are listed in Table 3 and illustrated in Figs. 4, 5, 6 and 7. Existing data derived from the literature have been re-normalised (as described in the analytical techniques section) and are included in Appendix A to assist the reader in ascertaining precisely what values have been used. As some of the Nd and all Hf data for ODP samples and DSDP sediments are new, the entire dataset is reproduced in Table 3 to provide the results in a single location rather than splitting the data between Table 3 and Appendix A. One observation essential to the discussions that follow is that substantial differences in isotope ratios have been reported for rocks of the Tofua Arc by previous studies, often for the same samples and particularly noticeable in the Pb isotope data. In the worst cases published isotope ratios for the same samples can differ by up to 1%; variations which far exceed modern analytical errors, with significant ramifications for the interpretation of the subtle variations seen between the different magmatic products of the region. In order to examine this issue more closely, 14 previously studied

Fig. 4. New Pb isotope data acquired on splits of 14 samples common to previous studies. New data (Table 3) are shown in filled black circles. Data for the same samples from Ewart et al. (1998) and a subset of 7 samples in common reported by Turner et al. (1997) are illustrated using open circles and stars respectively. Additional samples analysed by Ewart et al (1998) and Regelous et al (1997) are shown as crosses. Note the 7 samples reported by Turner et al. (1997) plot with significantly different 206Pb/204Pb values in this study and that of Ewart et al. (1998).

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

23

Fig. 5. Sr–Nd isotope diagram illustrating the distribution of new data for the LVG, KVG, ʻEua and Lau Basin relative to other rocks of the region. Site 834 samples represent the lowest Sr isotope compositions so far reported for the Lau Basin and overlap with Pacific N-MORB. ʻEua samples plot at a similar Nd isotope composition to the Site 834 group, but are displaced to higher Sr values. These closely match the compositions of igneous rocks recovered from Site 841. LVG and KVG samples are displaced towards lower Nd isotope compositions and higher Sr isotope values than Site 834 lavas. Other samples illustrated in this diagram include those from: the modern Lau Basin spreading centres, Site 835, Sites 836 and 837, the central Tofua Arc, the northern Tofua Arc islands of Tafahi and Niuatoputapu and volcanic fragments recovered from Site 840. Data are from Table 3 and Appendix A and symbols are as illustrated in the legend. MORB fields are based on data from White and Hofmann (1982), Hamelin et al. (1986), Michard et al. (1986), Price et al. (1986), Ito et al. (1987), White et al. (1987), Dosso et al. (1988) and Mahoney et al. (1989).

samples for which uncrushed rock is still available were sampled for re-analysis during the course of this study. Chips were hand-picked for new Pb, Sr, Nd and Hf isotope determinations with the data reported in Table 3. Fig. 4 illustrates an example of the comparison between the new dataset and those summarised in two previous publications (Turner et al., 1997; Ewart et al., 1998). Note that the values reported in these papers include those derived from pre-existing literature and no judgements are made here regarding the quality of data generated in either of these laboratories at the present time. Rather, the purpose of the comparison is to highlight the difficulties in employing previously published work, despite efforts to re-normalise the results to the same values for international standards. The subset of 7 samples from central Tofua common to all three studies range from 206 Pb/204 Pb values of ∼ 18.55 to 18.65 in our new dataset and that of Ewart et al. (1998), substantially higher than the values of ∼ 18.45–18.53 reported in Turner et al. (1997). It is unlikely that these differences are related to the sample heterogeneity since most of these materials are fresh volcanic products and thus we conclude that they are most likely analytical artefacts. In fact, upon further inspection, this is perhaps not surprising since some of these data are now over 3 decades old and, although they were considered pioneering for the time (e.g. Oversby and Ewart, 1972), there have been dramatic analytical advances over this period. This is particularly the case

for Pb isotope determinations where chemical separation methods now produce blanks many orders of magnitude lower and improved control over instrumental mass fractionation is routinely available (note that although many of these early Pb isotope determinations were the products of ‘double-spike’ methods there were a number of problems with the initial implementation of this method e.g., Oversby, 1973; Woodhead et al 1995). The potential difficulties inherent in high-precision Pb isotope analysis are well-documented and will not be reiterated here but readers may wish to refer to McDonough and Chauvel (1991) and Woodhead (2002) for cautionary tales demonstrating how such analytical difficulties can significantly influence analytical interpretations. That such discrepancies have emerged at all is of course testament to the importance of the Tonga–Kermadec ‘subduction factory’ and hence the large number of studies that it has generated. Nevertheless, after a detailed consideration, we would suggest that many of the older data gathered for samples collected from this region appear to be inappropriate to address current issues in arc geochemistry and should be superseded by the data obtained with modern methods and instrumentation. The new dataset for the full suite of 14 Tofua Arc samples shows almost complete overlap in the 206 Pb/204Pb values with those of Ewart et al. (1998); however, the offset to higher 207Pb/204Pb values in the latter is striking. Nevertheless, when all other data

24 J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44 Fig. 6. Variation diagrams illustrating the new Pb isotope data relative to other rocks from the area. On the basis of the discussions surrounding Fig. 4, the Tofua Arc Pb isotope data from Ewart et al. (1998) have been included to increase the number of islands represented. a) and c) provide regional overviews illustrating the locations of samples described in this study relative to MORB mantle fields, modern Lau Basin compositions, the arrays defined by samples from Site 834 and Sites 836 and 837, fields for central and northern Tofua Arc magmas, and Pacific pelagic and volcanic sediments. b) and d) provide a more detailed view of the areas in which LVG, KVG and other data are clustered as illustrated by the boxes in a) and c). In both panels (b and d) the new data for ʻEua, LVG and KVG lie long and extend the ‘Site 834 array’ and plot towards the field for volcanic sediments (and northern Tofua Arc magmas). In contrast, a small number of KVG lavas plot at very high 208 Pb/204Pb values, within the field for pelagic sediments. Other data illustrated include those from: Site 835, Site 841, and volcanic fragments recovered from Site 840. Data for Pacific and more locally-derived sediments (Sites 204 and 595A) are from Ben Othman et al. (1989), McDermott and Hawkesworth (1991), Turner et al. (1997), Ewart et al. (1998). Symbols are as illustrated in the accompanying legend. MORB fields are based on data from the same sources listed in the caption for Fig. 5.

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

25

Fig. 7. An illustration of ɛNd vs ɛHf for some of the samples examined in this study. ʻEua, Site 834, Site 835 and Site 839 samples fall in the field for Pacific MORB mantle (PMM) whereas samples from Sites 836 and 837 lie in the Indian MORB mantle field (IMM). All LVG and KVG lie in the field for IMM based on the boundary of Kempton et al. (2002; dashed black line); whereas half of the LVG and one of the KVG samples plot within the PMM based on the more recent discrimination line of Pearce et al. (2007; solid black line). The boundary lines dividing the two mantle domains are based on the data from the Australian–Antarctic Discordance (Kempton et al., 2002; ɛHf = (ɛNd × 3.03) − 12.03) and studies of SW Pacific magmatism (Pearce et al., 2007; ɛHf = (ɛNd × 1.522) + 1.26). The field for the modern Lau Basin (ML) spreading ridges is based on Kempton et al. (2000). Three mixing lines between PMM (2) and IMM (1) and volcanic sedimentary compositions are illustrated in blue, purple and orange respectively. These seek to reconcile the higher sediment input in the LVG and KVG lavas (based on trace element variations) compared with the northern Tofua Arc samples that display lower Nd isotope compositions. A mixing curve (red) between mantle with characteristics of Site 836 mantle and pelagic sediments better explains the field for central Tofua Arc magmas. See text for details.

reported by Ewart et al. (1998) are included (vertical crosses in Fig. 4) the overlap with our new dataset is substantial. For the purposes of the discussions that follow, and in future research, we suggest that our most recent data be used in place of existing results for the same samples as these new results display significantly less scatter and are validated by a large body of standard data obtained on both NBS 981 and matrix matched reference materials (e.g., Woodhead et al., 1995; Woodhead, 2002). Where a larger database is required, we suggest the use of the results reported in Ewart et al. (1998) owing to the closer coherence between their data and our own. Similar observations can be made between our new Sr and Nd isotope data for the Tofua Arc and

the data reported in Ewart et al. (1998) and Turner et al. (1997) and, while the reasons for these discrepancies are beyond the scope of this study, our recommendations on the use of existing results are the same. Fig. 5 shows the new Sr- and Nd isotope ratios reported in this study together with similar results for other samples from the region. Both the LVG and KVG have lower 143Nd/144Nd and similar or higher 87Sr/86Sr compared with the most MORB-like compositions preserved within the Lau Basin (e.g., modern Lau Basin samples, and some samples from Sites 834 and 836; see later discussion). The KVG have slightly higher 87 Sr/86Sr compared with the LVG (noted previously by Gill, 1976, 1987; Cole et al., 1990) and most closely

26

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

resemble samples from Ata Island (Jenner et al., 1987; Turner et al., 1997). Fig. 6 illustrates the Pb isotope compositions of the samples from each site and a number of other fields have been included for comparison. Lead isotope compositions of basalts from back-arc sites drilled during ODP Leg 135 (Hergt and Hawkesworth, 1994) define two linear trends which converge at high values of 206 Pb/204Pb and extend, at lower values, into either the field for Pacific MORB (Site 834) or Indian MORB (Site 836). Data for new samples shown here are more variable, but appear to indicate that the magmas from ʻEua, LVG and KVG lie on the trend with Pacific MORB affinities. Samples from ODP Site 841 also plot broadly within this group. In contrast, the compositions for most samples from ODP Site 840 project into the Pacific MORB Mantle (PMM) field, but define a steeper slope on 206Pb/204Pb vs 207Pb/204Pb and 206Pb/204Pb vs 208 Pb/204Pb diagrams, similar to the data from Site 839. Finally, as noted by Clift and Vroon (1996) five samples from Site 840 plot close to the field defined by the northern Tofua Arc magmas (Tafahi and Niuatoputapu). It is worth noting the similarity between this small group of Site 840 samples and rocks from the LVG and KVG displaying high 206Pb/204 Pb ratios. New Hf isotope data for the Lau Basin, Tofua Arc, ʻEua and Lau Ridge samples are illustrated in Fig. 7. The hafnium compositions appear to be relatively uniform across the different samples (∼+12.5 to +15.5), but the back-arc magmas erupted onto the old extended Lau Ridge crust (Sites 834 and 835) display higher Nd isotope ratios compared with those erupted at the spreading centres (Sites 836 and 837). The ancient arc rocks of ʻEua and strongly arc-like magmas preserved at ODP Site 839 also plot towards higher Nd isotope compositions compared with the LVG and KVG lavas. New data for both the central and northern Tofua Arc magmas plot with elevated Hf, but lower 143Nd/144Nd compared with all other arc magmas. This observation is consistent with the displacement of the Tofua Arc magmas towards unusually low 143Nd/144Nd on Sr–Nd isotope diagrams (e.g., Fig. 5), a feature recognised in previous studies. 5. Discussion A number of excellent reviews have aimed at contributing to, and synthesising, the considerable body of data now available for the Tonga–Kermadec arc and back-arc systems (e.g., Turner et al., 1997; Ewart et al., 1998). Turner et al. (1997) propose the involvement of four components to explain the petrogenesis of the Tofua Arc magmas; 1. the mantle wedge, 2. a partial melt of

pelagic sediments, 3. a partial melt of volcanic sediments derived from the subducting Louisville Ridge, and 4. a fluid derived from the subducting altered oceanic crust. Employing a similar dataset, Ewart et al. (1998) arrive at similar conclusions but with some important differences. These authors agree that the majority of features in the Tofua Arc lavas can be explained by the influx of fluids derived from the downgoing Pacific altered oceanic crust. They also agree that the Pb isotope signatures in the northern Tongan islands (Tafahi and Niuatoputapu) reflect a contribution from the subducting Louisville Ridge Seamount Chain (LSC). In contrast with Turner et al. (1997), however, Ewart et al. (1998) propose that all slab contributions (from both sediments and altered oceanic crust) are delivered to the mantle wedge in the form of a fluid and employ the models of Stolper and Newman (1994) to provide constraints on the compositions of such fluids. They also suggest that a small contribution is required from the influx of Samoan plume material into the mantle wedge beneath the northern Tofua Arc islands in order to explain the concentration of certain elements such as Nb in these otherwise highly depleted lavas. Finally, Ewart et al. (1998) highlight the significant variations in sediment thickness being subducted beneath the Tonga and Kermadec arc segments, and level of mantle wedge depletion along the length of the arc system. Thus, in their view, both the amount of sediment (contributing to the fluid flux) and the degree of mantle wedge depletion are thought to exert a strong control on the resultant magmas. 5.1. Proposed constraints on the slab-derived flux composition for the modern Tofua Arc It is not the intention of this study to reproduce or reinvent the comprehensive syntheses provided by previous authors. Instead, here we examine the consequences of some of these hypotheses, particularly with respect to any implications they may have for the mantle wedge composition. Turner et al. (1997) employed a range of techniques to characterise the trace element and isotopic composition of the fluid, concluding that it had been “… derived by dehydration of the subducting altered oceanic crust rather than the subducted sediments”. Among the diagrams employed in such reasoning are plots such as that illustrated in Fig. 8 which show the ratio of a fluid-mobile element (e.g., Ba) relative to an element believed to be conservative (e.g., Th) against a parameter thought to provide an independent assessment of the slab-contribution to the mantle wedge (e.g., an isotope ratio).

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

27

Fig. 8. Illustration after that employed by Turner et al. (1997) to determine the Pb isotope composition of the slab-derived fluid flux. These authors propose that the progressive increase in Ba/Th, without an accompanying change in Pb isotope composition effectively constrains the flux composition to a value of ∼ 18.5. Although this may be a reasonable conclusion, the scatter in the data could permit 206Pb/204Pb values anywhere between ∼ 18.4 and ∼ 18.7. The new data from this study (filled black circles) would support a flux composition closer to ∼ 18.6.

Employing this approach, Turner et al. (1997) concluded that the fluid component had an estimated isotopic composition of ∼0.7035 and ∼18.5 for 87Sr/86Sr and 206Pb/204Pb respectively. Similar diagrams (not shown) can be constructed for the other Pb isotope ratios, yielding 207Pb/204Pb ratio of ∼15.56 and 208Pb/204Pb ratio of ∼38.1, a composition that lies close to the central Tofua Arc magmas themselves. In detail however, the flux composition calculated in this way actually has lower 206 Pb/204Pb and higher 207Pb/204Pb and 208Pb/204Pb than we now consider appropriate for the Tofua Arc lavas (see discussion of Fig. 4 above) and plots within the field defined by the Indian MORB rather than the Pacific MORB. Clearly, the subducting oceanic crust is Pacific rather than Indian in character requiring either that alteration of the subducting oceanic crust resulted in the significant shift in the Pb isotope composition, or that the fluid itself is a mixture between that derived from the altered oceanic crust (with a Pacific MORB Pb isotope composition) and some other component with an unusually low 206Pb/204Pb composition (i.e., lower than might be expected for the PMM at the required 207 Pb/204Pb value). One means of reconciling the Pb isotope composition of the fluid flux with the subducting Pacific plate is provided by Regelous et al. (1997). These authors conclude that the fluid responsible for most of the Tofua Arc lavas, while dominated by a component derived from altered Pacific MORB crust, also has a small

contribution from pelagic sediments and therefore plots at more radiogenic Pb isotope values than most or all of the Tofua Arc magmas. Regelous et al. (1997) suggested that the small but significant variations in, for example, Pb isotope compositions of the magmas reflected differences between the relative contributions of fluids from the pelagic sediment and altered oceanic crust at various locations along the arc. Elliott (2003) also noted that the Tofua Arc magmas with the least radiogenic Pb isotope ratios still preserve high Pb contents (i.e. Pb/Ce high compared with MORB) while the more radiogenic Pb compositions project towards sediments. Instead of a mixed fluid of variable composition, Elliott (2003) suggested that the two components were added separately, the pelagic component as a sediment melt and a more dominant fluid derived from the altered mafic oceanic crust in the subducted Pacific plate. Despite some differences in the proposed origin or physical nature of the slab flux, the general consensus appears to be that its isotopic composition lies within, or close to, the Pb (and Sr) isotope values of the central Tongan Arc lavas and involves a small contribution from the subducted sediments. The revision of Pb isotope values for many of the Tofua Arc samples proposed above results in a far smaller range than previously assumed, with samples plotting entirely within, or close to, the Pacific MORB. As a consequence, the requirement for a sedimentary component based on Pb isotope data alone is removed (but not precluded); however, the elevated Sr

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and lower Nd isotope compositions in the central Tongan Arc lavas are not explained by a component solely derived from the altered oceanic crust. We concur with previous authors that it is more consistent with the addition of a sedimentary component in some form. 5.2. Proposed constraints on the mantle wedge composition In all of these models, the Pb isotope composition of the mantle wedge is assumed to have been completely overprinted by the slab flux (melt and/or fluid). Indeed a number of authors have suggested that the Pacific MORB component, proposed by Hergt and Hawkesworth (1994) to underlie the region prior to seafloor spreading in the Lau back-arc basin, can be explained more simply by the prolonged addition of fluid from the subducting Pacific oceanic crust; the implication being, that a composition more typical of Indian MORB mantle reflects the true nature of the unmodified mantle wedge (e.g., Bach et al., 1998; Ewart et al., 1998). In order to clarify the confusion that appears to have arisen in this regard, the evidence for a true Pacific MORB mantle (PMM) reservoir beneath at least some parts of this region at some time in the past is re-iterated below. Magmas at ODP Site 834 were erupted onto rifted Lau Ridge crust at around 5 Ma, in close proximity to the (then still active) arc. Hergt and Nilsson-Farley (1994) describe the down-hole chemical variations in magmas at this site; notably, the variation in both incompatible trace element and Sr-, Nd- and Pb isotope compositions from arc-like magmas through to the most MORB-like compositions so far recovered in the Lau Basin. Fig. 9(a) illustrates examples of the incompatible trace element compositions of the most MORB-like materials (Unit 7). Importantly, the comparatively smooth ‘MORB-like’ patterns of these key samples are accompanied by isotopic features more typical of Pacific upper mantle (Fig. 6; Hergt and Hawkesworth, 1994) than the Indian MORB mantle (IMM) known to occur at the spreading centres (e.g., Loock et al., 1990; Boespflug et al., 1990; Table 3 and Appendix A). At least in this one instance then, mantle-derived magmas from the Lau Basin, lacking the pronounced enrichment in Pb (e.g., relative to Ce) so typical of all arc lavas, have isotopic signatures that clearly identify the mantle wedge as having PMM affinities. In other words, this PMM signature cannot have been added from the slab in any form and must therefore be a feature inherent to the mantle source. The PMM character of Site 834 magmas is further confirmed by both the low 87Sr/86Sr values and the Hf–Nd isotope relationships as illustrated in Fig. 7.

A similar inspection of other MORB-like samples available from the Lau Basin reveals that these are more typical of the IMM. At ODP Site 836 for example there are essentially two main groups of magmas in terms of trace element abundances (Fig. 9(b)). Units 1, 2, 5 and 6 have higher overall levels of incompatible trace elements compared with Units 3 and 4. In both cases the MORB-normalised trace element patterns are approximately horizontal between La and Lu, complicated by striking Nb depletions and enrichments in K and Sr. In contrast, unlike Units 1, 2, 5 and 6, samples from Units 3 and 4 exhibit little Pb enrichment (although Nb and Sr anomalies persist). These samples also help define the least radiogenic end of the Pb–Pb isotope arrays and lie within fields for the IMM (Fig. 6). Perhaps the samples best placed to constrain the IMM composition are those erupted at the CLSC as this spreading ridge has propagated into new oceanic crust (i.e., not extended arc crust) and lies at the greatest distance from the active arc (Fig. 2). Unfortunately, of the rocks dredged from the CLSC, the most primitive samples with lowest relative enrichment in Pb contents (Pearce et al., 1995) lack Pb isotope data. Of the CLSC samples for which both extensive incompatible element contents and Pb isotope data do exist (Table 3), those with the smallest Pb enrichments are 10-1-1 and 10-1-3. Although these magmas are more evolved, and sufficiently enriched in Fe and Ti to be referred to as ferrobasalts (Pearce et al., 1995), they lie on a projection of the Pb isotope array defined by ODP Site 836 rocks at less radiogenic, IMM compositions (i.e., the small enrichment in Pb content is derived from within the mantle wedge and not from the subducted slab). Indeed the Nd/Pb values of 18 and 15 respectively for these two samples are similar to the values for the IMM proposed by Rehkämper and Hofmann (1997; 18–22). These are rather lower than the value of ∼ 25 for PMM and OIB and reflect the elevated Pb believed to characterise the IMM source. Finally, the Hf vs Nd isotope ratios of both the Site 836 samples, and data from the CLSC, plot within the IMM field (Fig. 7). 5.3. The role of back-arc magmas in constraining the nature of the slab flux Having established that both PMM- and IMM-like magmas, largely unaffected by a slab-derived flux, have erupted at various stages of back-arc basin formation, we can now employ these results to our advantage. Back-arc basin magmas arguably provide the least ambiguous means of constraining the composition of the slab-derived flux, particularly in cases such as the

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Fig. 9. Mantle normalised incompatible trace element patterns for (a) basalts from Site 834 with the most MORB-like features (note the lack of Pb enrichment relative to Ce and Nd) and (b) samples from Site 836. The Site 836 magmas fall into two main groups, one with no Pb enrichment and lower abundances of most elements (units 3 and 4; open circles), and another with higher incompatible element contents and a pronounced enrichment in Pb (units 1, 2 and 5; black filled circles). Data are from Hergt and Nilsson-Farley (1994) and Hergt and Hawkesworth (1994). Normalising values are from Sun and McDonough (1989).

Lau Basin, where PMM and IMM have both been available. In the Lau Basin magmas, arrays between the MORB-like mantle endmembers, and ‘subduction’ components, will necessarily reflect wedge-flux mixtures. This contrasts with the more subtle variations preserved between Tofua Arc magmas that might reflect differences in the contributions of distinct slab-derived components which dominate any mantle signal (e.g., Regelous et al., 1997; Elliott, 2003) notwithstanding the fact that some variations in current Tofua Arc datasets appear to be analytical in origin (Fig. 4). The Pb isotope arrays defined by Sites 836 and 834 (Fig. 6) clearly indicate that, although the IMM trend passes into the Tofua Arc field as would be expected (based on the composition of the flux proposed by Turner et al., 1997; Regelous et al., 1997), the PMM array does not. Two explanations could account for this observation.

The first possibility is that the slab-derived flux has changed in composition, from one lying at higher 206 Pb/204Pb (circa 18.7) during early back-arc basin formation, to that constrained by the Tofua Arc magmas at the present day. An alternative hypothesis is that, as this shift coincides with a change in the underlying mantle wedge from PMM to IMM, it reflects the varying composition of this source (Hergt and Hawkesworth, 1994). The implication of the second alternative is profound as it would require that, in contrast with recent studies of the Tofua Arc and constraints proposed for the slab-derived flux (e.g., Turner et al., 1997; Elliott, 2003), the mantle source itself exerts a small but important level of control on the Pb isotopic composition of arc magmas. The extent to which the mantle and flux influence the changes in Pb isotope compositions in the evolving arc is now explored in more detail.

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As noted in Section 4.2, the Pb isotope arrays defined by the samples from ODP Site 836 represent mixtures between an endmember similar to the source of IMM and a slab-derived flux. Furthermore, the most arc-like magmas at this site are similar in their Pb isotope compositions to the samples from the Tofua Arc. It is therefore possible to use the constraints available from the mantle source and arc rocks to assess the range of compositions possible for the slab flux. First, using the 206 Pb/204Pb vs 208Pb/204Pb array for Site 836 samples (as it shows less scatter compared with the 206Pb/204Pb vs 207 Pb/204Pb plot), a best-fit line has been determined as illustrated in Fig. 10a (i.e., 208 Pb/ 204 Pb = (0.821 × 206 Pb/204 Pb) + 22.946). Second, initial estimates of

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Pb/204Pb for the IMM source (18.15) and slab flux (18.65) have been made, with the 208Pb/204Pb calculated employing the relationship established above (that is, the IMM source and slab flux are constrained to lie along the same array as that defined by Site 836 lavas). Although the value of 18.15 is lower than the lowest value at Site 836, it was selected for the IMM source as it is more typical of CLSC and ELSC magmas. The value of 18.65 was selected for the slab-derived flux as it is very similar to the more radiogenic of the central Tofua Arc lavas. An important constraint employed in selecting these initial parameters is that the mixing trend should pass through the main cluster of Tofua Arc magmas based on the new data acquired in this study, and the broader array

Fig. 10. The results of mixing calculations conducted using the well-defined trend in Site 836 data, assumed mantle and flux compositions and the requirement that the mixing trend should pass through the main cluster of the Tofua Arc data (see text for a full description of the approach). Panel a) illustrates the locations of components employed and fit of the proposed mixing line relative to Site 836 lavas (asterisks) and central Tofua Arc magmas (solid circles are data from this study, open circles are the data from additional samples from Ewart et al., 1998). b) Mixing trends calculated assuming a slab-derived flux carries Pb, but little Nd into the mantle wedge. The Nd/Pb of the IMM wedge is assumed to be between 18 and ∼ 22 (shaded region) after Rehkämper and Hofmann (1997) whereas PMM is given a value of 25 for the purposes of illustration. Site 836 data are from Hergt and Hawkesworth (1994), and trace element data for the two CLSC samples are from Pearce et al. (1995).

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defined by the larger dataset of Ewart et al. (1998; Fig. 10a). In addition to the Pb isotope ratios, deviation of the Site 836 magma compositions away from N-MORB and towards magmas from the Tofua Arc can be assessed using variations in certain trace element ratios. It is wellknown that partial melts of the mantle yield remarkably uniform values for Ce/Pb and Nb/U (e.g., Newsom et al., 1986) and ratios employing Pb in the denominator will be used here as these generate linear mixing relationships with Pb isotope data. More recently, Rehkämper and Hofmann (1997) have proposed the use of Nd in place of Ce and argued that the ratio for Nd/Pb is around 24 ± 5 for most MORB and OIB magmas; although, as noted earlier, values of 18–22 were proposed for the IMM. A plot of Nd/Pb vs 206Pb/204Pb is therefore used here as a measure of the change in isotopic composition, with the progressive increase in magnitude of the positive Pb anomaly observed in the mantle normalised trace element patterns, over and above that which might be attributed to the mantle source (e.g., IMM). Using the Pb isotope data from Fig. 10a, and estimating the MORB source to have Nd/Pb values of 18 or 20 (for IMM) and 25 (for PMM-like mantle), mixing lines have been calculated (Fig. 10b). Note that in this case, we have allowed for a small addition of Nd to the wedge by the slab-derived flux (i.e., Nd/Pb of the flux is 0.1). This value is likely a maximum in so much as most researchers consider the REE far less mobile than Pb (e.g., by more than an order of magnitude, Keppler, 1996). Even so, the change in Nd/Pb effectively reflects the progressive addition of Pb only (indeed using a flux with Nd/Pb of 0 does not change the results). In short, because the flux has such low Nd/Pb, if a sample has twice the Pb it should (i.e., the Nd/Pb is half that of the mantle source value) this means the flux, whatever its Pb concentration, will have contributed half of the Pb in the sample. For the Site 836 rocks there is a broadly negative correlation preserved between most samples, although the scatter in the array is considerable (Fig. 10b). Some samples plot away from the ‘main’ trend; however, the extent to which those above the mixing line may simply indicate variations in the 206Pb/204Pb and/or Nd/Pb of the IMM source remains unclear. Such source heterogeneity would not be surprising as, for example, isotopic variations clearly exist both between and within the MORB-like samples from Site 836 and the modern spreading centres (Fig. 6). Other samples in Fig. 10b plot below the mixing lines and this might indicate 1) the IMM in this region has even lower Nd/Pb than proposed by Rehkämper and Hofmann (1997), 2) that

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there has been a sequestering of Pb from the IMM source by the slab flux, or 3) that partial re-equilibration of the flux with Pb from the mantle wedge has taken place. That the mantle wedge here is capable of generating magmas with unusually low Nd/Pb at low 206 Pb/204Pb is also confirmed by samples from the CLSC (Pearce et al., 1995). Two samples with the highest Nd/Pb are used to illustrate this in Fig. 10b. The purpose of the calculations described here is to assess the extent to which Pb isotope and trace element data constrain the proportion of Pb in the Tofua Arc lavas that must be derived from the mantle wedge. The first observation to make is that the wedge composition can be varied between Nd/Pb values of ∼18 and ∼ 25 and a 206 Pb/ 204 Pb range of ∼ 18.1–18.3, without changing substantially the results depicted by the mixing trends. The calculations illustrated in Fig. 10b employing IMM compositions (i.e., low Nd/Pb and 206 Pb/204Pb) encompass most of the data from Site 836 and pass through the field for the Tofua Arc lavas, providing a better fit than a wedge with PMM composition. Mixing calculations indicate that, in this case, between 10–15% of the Pb in the arc magmas must be derived from the wedge component. In contrast, the composition of the slab-derived flux appears to be more tightly constrained. If the 206Pb/204Pb of this component were much higher than 18.65 (e.g., 18.75) most of the Tofua Arc data would lie below the mixing lines that would be generated (unless the Nd/Pb of the IMM component were also reduced to a value of ∼10, at which point the Pb derived from the wedge would increase to ∼20–30%). Values of 206Pb/204Pb lower than ∼18.65 could be accommodated (e.g., 18.6) but because many of the Tofua Arc lavas display more radiogenic compositions than this, it would be necessary to appeal to a series of fluxes with differing isotopic compositions to explain the arc data. This is not impossible, and, if it occurred, would reduce the proportion of Pb derived from the mantle wedge to 5–10%. To summarise, using this approach a number of broad constraints are revealed. First, the mixing lines that best describe the variations in Site 836 lavas and Tofua Arc lavas require a slab flux with 206Pb/204Pb value close to 18.65 (consistent with the vertical array defined by the new Pb data in Fig. 8, and corresponding 207 Pb/204Pb and 208Pb/204Pb of 15.57 and 38.25 respectively based on regressions through data arrays), slightly different from those proposed by other researchers. Second, the best match to the Tofua Arc data is obtained by using a pre-flux wedge with Nd/Pb around 18–20, similar to that proposed for IMM. Third, although the proportion of Pb in the arc magmas that can be attributed to the mantle

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wedge is poorly constrained, estimates of ∼10–15% appear reasonable, and could result in a shift of ∼ 0.05 in 206 Pb/204Pb with the move in the mantle source from PMM to IMM. Thus, although the influence of the Pb isotope composition of the flux predominates, the contribution from the mantle wedge cannot be ignored as this alone can explain at least half of the observed difference in 206Pb/204 Pb values between the central Tofua Arc magmas and some of the other arc groups illustrated in Fig. 6. Finally, an important observation then is that, as originally proposed by Regelous et al. (1997), the flux responsible for the composition of the central Tofua Arc magmas is consistent with one derived from a mixture of fluids from the altered oceanic crust (with PMM affinities) and sediments. 5.4. The extent of variation in the slab-derived flux through time Hergt and Hawkesworth (1994) argued that if the slab flux had remained approximately constant through time, and the variations in the back-arc basin magmas resulted from a change in the mantle wedge from PMM to IMM, the Pb isotope composition of this flux would be constrained to lie somewhere close to a point of intersection between the IMM-Site 836 and PMM-Site 834 Pb isotope arrays. It is clear that this can no longer be considered the case as significant changes in the slopes of mixing arrays projecting towards fields for volcanic and pelagic sediments are apparent (Fig. 6) and it has just been demonstrated above that the central Tofua Arc magmas reflect a change in both their mantle wedge and flux compositions. Furthermore, it has been proposed that subduction of the LSC is responsible for the unusual compositions of the northern Tofua Arc magmas (i.e., Tafahi and Niuatoputapu; Turner et al., 1997; Ewart et al., 1998). Consequently, the constraints on the slab-derived flux used by Hergt and Hawkesworth (1994) employing the compositions from these islands, are no longer valid. The oldest magmas in the region (e.g., ʻEua, LVG) preserve evidence for the involvement of volcanic sediments in their petrogenesis (e.g., Fig. 6). In contrast, pre-rift and syn-rift volcanic debris deposited at ODP Site 840 and a small number of samples from the KVG have trajectories clearly displaced towards the field for pelagic sediments. Such a shift is not ubiquitous as most KVG lavas, and magmas erupted during the earliest stages of back-arc opening at ODP Site 834, continued to preserve arrays between PMM and volcanic sediments. Five samples in the uppermost levels of ODP Site 840 (≤∼3 Ma) are displaced to far higher 206Pb/204Pb values

than those deeper in the section, and lie close to the field defined by Tafahi and Niuatoputapu. The KVG rocks share similar Pb isotope compositions with these islands, and therefore it is feasible that the material in the uppermost sections of the Site 840 core represents products of volcanism derived from the KVG or the northernmost Tofua Arc. The Sr–Nd isotope compositions of Site 840 (Fig. 5) would appear to be more consistent with an origin within the KVG. Although the distance between these islands and Site 840 would have been considerable, explosively erupted materials, or pumices swept away by surface currents, are known to travel such distances without difficulty (e.g., Carter et al., 2003). It would appear that pre- to syn-rift magmatism in this region is consistent with a slab flux changing in response to variations in the nature of the subducting sediment (e.g., pelagic vs volcanic, Fig. 6). Importantly, although the slab flux varied significantly with time, it appears to have ‘cycled’ as the composition postulated to explain the central Tofua Arc magmas is similar to that required in the earlier stages of arc magmatism recorded at some sites (e.g., the calculated flux composition is close to that apparently required to explain the older Site 840 ash samples, or lavas from Site 835). Irrespective of any contribution from the subducting altered Pacific oceanic crust during the history of this arc–backarc system, this flux composition appears to have always required at least some contribution from pelagic or volcanic sediments. 5.5. The nature of the sediment contribution in the slabderived flux If we accept that the broad composition of the slab flux can be identified from the arguments above, and that there is clearly a requirement for some sediment contribution in all of the arc magmas, it remains for us to assess the mass transfer process by which this signature was imparted (i.e., melt or fluid). Plank (2005) has argued convincingly that the subducted sediment is delivered to the mantle wedge in the form of a melt, and that plots of Th/La vs Sm/La form linear arrays that constrain both the composition of the sediment melt (Th/La) and the degree of depletion in the mantle wedge (Sm/La). In Fig. 5c of her article, Plank (2005) illustrates an array for Tonga that employs a sediment melt Th/La of around 0.12 ± 0.2 and a highly depleted wedge (Sm/La ∼ 3 ± 1). Note that the Th/La of the sediment melt is enhanced by partial melting and therefore reflects subducted sediment with a somewhat lower Th/La (see Plank (2005) for further discussion).

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Fig. 11. a) Plot of Sm/La vs Th/La for the ʻEua (open stars), LVG (filled squares) and KVG (open squares) magmas with SiO2 b 58 wt.% shown relative to the regression through the Tonga Arc data (depicted by the elliptical field) reported by Plank (2005). Sediment data from Site 204 (Turner et al., 1997; Ewart et al., 1998) are also illustrated and display low Sm/La but Th/La ranging from ∼ 0.8 to ∼ 0.4. b) Plot of ΔNd vs ΔɛNdP/I for the arc data from this study. The SW Pacific IMM–PMM discrimination line of Pearce et al. (2007) plots as a horizontal line at zero, with positive values of ΔɛNdP/I placing samples in the IMM region, and negative values lying in the PMM domain. The fields surrounding each magmatic suite have been drawn using the data from this study, and results for the same groups reported in Pearce et al. (2007). The dashed lines represent regressions with a slope of ∼ 3.5 proposed by Pearce et al. (2007) to correct for Nd contributions from the slab flux in each case. The lowermost regression reported by these authors included the data for samples from ʻEua, the Lau Islands (LVG and KVG suites) as well as the Kermadec Arc and Valu Fa Ridge. Note that although our new data for the Nth Tofua arc magmas are consistent with the proposed regression, the results from this study suggest that horizontal arrays are more appropriate for the other suites.

Fig. 11a provides a comparison between the regression of Plank (2005) for Tonga, and the data from this study for ʻEua, the LVG and KVG. Although Plank (2005) restricted the Tonga dataset to those samples with SiO2 b 55 wt.%, we have found no correlation between

Th/La and SiO2 at higher silica values and have included samples with SiO2 b 58 wt.% in the data illustrated here. Despite the scatter, initial impressions are that this diagram illustrates well the observations made earlier in the discussion. For example, the regressions through the

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data would suggest a higher Sm/La of the KVG mantle endmember compared with that of the LVG (i.e., circa 0.9 and 0.7 respectively) consistent with greater depletion in the mantle endmember in the KVG and an even more depleted mantle source for many of the ʻEua magmas (see also Fig. 3 and Section 4.1). Also, although variations are evident, volcaniclastic sediment endmembers with Th/La around 0.1–0.15 are most consistent with the LVG and KVG data arrays. Finally, at least some of the LVG and KVG data plot close to the low Sm/La high Th/La endmembers consistent with a strong sediment influence on some magmas in these groups. Notwithstanding these results, an examination of the mantle normalised incompatible trace element diagrams indicates that all patterns with an arc-like affinity, even some from the modern spreading ridges (Pearce et al., 1995), show an increase in Th relative to elements such as La. It has also been noted that at least the modern Lau Basin magmas provide clear evidence for a fluid flux with high Cl contents (Kent et al., 2002) and it is known that under such conditions, Th can be mobilised, perhaps to a similar extent as Sr (Brenan et al., 1995; Keppler, 1996; Stalder et al., 1998). Thus, although the involvement of a sediment component is clear, there are some concerns in employing trace element ratios incorporating Th as a measure of sediment melt contributions since the exact conditions of Th mobility in fluids remain unclear. Pearce et al. (1999) present a model for quantifying the enrichment in Nd abundance over Hf in arc magmas (ΔNd) and removing this ‘slab component’ from the Nd isotope composition of the magmas. In this way they are able to examine the source of the slab-derived Nd (e.g., pelagic or volcanic sediments, altered oceanic crust) and constrain the isotopic affinity of the mantle wedge (IMM vs PMM) from which the magmas were derived. This model relies on Hf being immobile, with the implication that Nd derived from subducted sediments is lost to the mantle wedge via dehydration processes rather than partial melting. Consequently, employing their model may help unravel the roles of fluids vs melts in the arc magmas of this study. More recently, Pearce et al. (2007) used this approach to propose that the mantle wedge beneath ʻEua and the Lau Islands had PMM affinities with a move to IMM beneath the modern Tofua Arc and Lau Basin during basin opening. In plots of ΔNd vs ΔɛNdP/I (i.e., the contribution from Nd from the slab vs the isotopic composition of the magmas relative to the IMM–PMM discrimination line; their Fig. 6) these authors project positive trends through their datasets to zero ΔNd (i.e., no anomaly) to constrain the mantle wedge compositions. The slope of these trends

is approximately 3.5, believed to reflect the progressive incorporation (up to around 50%) of sediment-derived Nd with increasing ΔNd. Fig. 11b reproduces Fig. 6a from Pearce et al. (2007) with the addition of the new data from our study. With the possible exception of the array from the northern Tofua Arc, our data do not support the positive arrays proposed using the smaller dataset of Pearce et al (2007); rather, the arrays for ʻEua, the LVG, KVG and central Tofua Arc would appear to be scattered but horizontal. ʻEua remains at slightly negative values and hence in the PMM field whereas LVG straddles the IMM–PMM discrimination line, and most KVG data lie close to but slightly above this boundary. The Tofua Arc data are clearly within the IMM field. Given the evidence from the Pb isotope data and other parameters for sediment involvement in all of the arc magmas from this study, this result is surprising. If increasing ΔNd represents the progressive addition of a slab-derived flux to the mantle wedge, it would be expected that this would also result in variations in the Nd isotope composition at a fixed Hf isotope composition (and hence generate a line with some slope on Fig. 11b). The only way to avoid this would be for the slab component to have the same Nd isotope composition of the wedge, which would be remarkable in this instance given the changing composition of the wedge relative to the IMM–PMM boundary for at least 3 of these datasets (i.e., around 0 for ʻEua, LVG and KVG compared with ∼2 for the central Tofua Arc and ∼ 4 for the northern Tofua Arc magmas). The relationship between ΔNd and ΔɛNdP/I is worthy of further investigation beyond the scope of this contribution, but we make the following observations. First, although there is no correlation between ΔNd and SiO2 for the ʻEua, LVG and KVG datasets illustrated in Fig. 11b (particularly when restricted to SiO2 less than 58 wt.%), magma suites from individual volcanoes in other locations often display strong negative covariations even when restricted to the more mafic endmembers (e.g., Sulu Ranges, New Britain; Pagan Volcano, Marianas; not shown). The implication is that the ΔNd parameter may be influenced by magma chamber processes. Second, much of the variation in Nd–Hf isotope compositions for LVG and KVG rocks trend parallel to the IMM–PMM boundary. Thus, despite variations in both Nd and Hf isotope compositions, the deviation from the discrimination line (i.e., ΔɛNdP/I) is constant which goes some way to explaining the horizontal arrays in Fig. 11b. Nonetheless, we do not observe correlations between ΔNd and either Nd or Hf isotope composition. Finally, and perhaps even more curiously, we observe a strong negative

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correlation between ΔNd (thought to reflect fluid addition of Nd) and Th/La (thought to reflect the addition of a sediment melt) for the KVG samples (not shown) but not with either the ʻEua or LVG data. The negative covariation in the KVG data is also the reverse of any trend that might be expected as the most ‘mantle-like’ rocks (lowest Th/La values) display the greatest ΔNd (i.e., largest enrichments in Nd relative to Hf), whereas the rocks with the highest Th/La (greatest addition of sediment melt?) preserve the least ‘subduction influence’ based on ΔNd.

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Given the complexities (and apparent contradictions?) in assessments of Th/La vs Sm/La (Plank, 2005) and ΔNd vs ΔɛNdP/I (Pearce et al., 2007) we have resorted to a more conventional approach to assess the relative roles of sediment-derived fluids and melts. Fig. 12a illustrates how 206Pb/204Pb varies with Ba/La for magmas of this study and reveals a broad positive correlation in the data from Sites 836 and 834. This indicates that an indisputable measure of fluid addition (Ba/La) correlates with a gauge of ‘sediment’ signature (206Pb/204Pb) suggesting that the

Fig. 12. Trace element ratios vs 206Pb/204Pb. a) Ba/La is well-correlated with 206Pb/204Pb for samples from Sites 834, 836 and 837 indicating that the fluid carries Pb into the mantle sources of these samples, and that this fluid has a composition influenced by sediments (i.e., comparing the variations in 11a with those in Fig. 6). Mixing produces curved trends on this diagram, and a series of arrays could be generated from mantle source regions with a low Ba/La ratio and a range of Pb isotope compositions. Samples from ʻEua fall at the end of the Site 834 data array and also at higher Ba/La. LVG and KVG samples display a similar behaviour, and extend to even greater Ba/La values. Other data illustrated include those from Site 835, Site 839, Site 841, and volcanic fragments recovered from Site 840. Central Tofua Arc lavas plot at lower 206Pb/204Pb and higher Ba/La consistent with their derivation from an IMM source prior to the influx of the slab-derived fluid. b) La/Sm should not vary significantly with the addition of a fluid flux, but is likely to be sensitive to the introduction of a sediment melt. Most samples, including most of the ʻEua rocks, retain near-MORB values (i.e., close to 1) irrespective of their Pb isotope compositions. Clear exceptions to this are the LVG and KVG samples. Both groups appear to require an additional sediment component to explain the high La/Sm (and overall LREE enrichment, as noted in Fig. 3). Note that this input is more substantial compared with the central Tofua Arc magmas, and most of the northern Tofua Arc compositions. Additional data employed are from Hergt and Hawkesworth (1994), and Ewart et al. (1998).

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fluid does in fact transport Pb from the sedimentary component in this system. Although significantly scattered, samples from the older arc magmas appear to fall towards the ends of these arrays, particularly those defined by the data from Site 834. Scatter in this diagram might be attributed to a number of processes. First, as indicated from the Pb isotope diagrams (Fig. 6), the trajectories towards the fields for sediment vary between magmatic suites. Thus, although an increase in Ba relative to La is a common feature of the arc magmas relative to the back-arc basin basalts, the composition of the sedimentary component in the flux is not (as noted above, this varies between volcanic and pelagic sediment). It should also be noted that mixing trends are not straight lines on Fig. 11 so that a series of hyperbolic mixing curves are to be expected. Finally, as Ba is notoriously prone to variations during alteration, the Ba/La in many of the older rocks employed in this study (e.g., ʻEua in which metamorphism is apparent) may have been disturbed. The important observation to note is that suites clearly influenced by the addition of fluids show correlations between Ba/La and their Pb isotope compositions. Such trends extend from their respective mantle wedge source (IMM or PMM) towards sediments of different compositions, indicating that at least in some cases, sedimentderived fluid (with or without an input from altered mafic oceanic crust) controls the trace element and isotope features of the more arc-like magmas. Unlike Ba/La, a ratio that is expected to show dramatic changes with the addition of a fluid, La/Sm can be employed as a measure of sediment (melt?) addition. Fig. 12b illustrates the same dataset as shown in Fig. 12a, and indicates that most suites have a La/Sm ratio indistinguishable from N-MORB (i.e., close to 1) over a wide range in Pb isotope compositions. This provides further, convincing evidence that the variations in Pb isotope composition correlate with a fluid flux and not sediment melt. The fact that this fluid lies at higher 207Pb/204Pb and 208Pb/204Pb values than might be expected from the mafic altered PMM oceanic crust requires that sediments contribute Pb to this fluid (e.g., Regelous et al., 1997). Two notable exceptions in Fig. 12b are the LVG and KVG, where both groups have samples clearly displaced towards higher La/Sm. Thus, while most suites appear to show an influence from fluid only, the LVG and KVG appear to require an additional sediment component, possibly in the form of a melt. Cole et al. (1990) reported that both the LVG and KVG could be subdivided, with each displaying a high-K group, although the distinction is less evident in magmas with low silica contents. From Fig. 12b it is clear that, although most LVG and KVG lavas have La/Sm values

greater than 1 (∼ 1.5–2.5), a small number approach values of 4.5–5. It appears that the samples with highest La/Sm are also those belonging to the sub-groups with high-K affinities; furthermore, these are also the samples with the greatest displacement towards fields for sediments in Pb isotope plots (Fig. 6). As noted earlier, an interesting observation is that the sediment compositions appear to be different within the KVG. In Fig. 6, the LVG and most of the KVG samples are displaced towards volcanic sediments (i.e., in the direction of northern Tofua Arc rocks) whereas a subset of the KVG samples follow a trajectory towards pelagic sediments and are displaced to high 208Pb/204Pb. This latter group is represented by the islands of Olorua and Moce (that lie between Lakeba and Kabara at a latitude of ∼18.6– 18.7°S; Fig. 2). No other data for the KVG are available from this region and all other results are from samples from the northern Lau Island group. Olorua and Moce also display the lowest Hf and Nd isotope compositions and highest 87Sr/86Sr. A more comprehensive study of the KVG at different latitudes is underway to help determine if this apparent geographic variation persists with a larger dataset. Other samples showing some scatter towards higher La/ Sm include volcanic sediments from Site 840 and some of the Tofua Arc lavas. In both cases the concentrations of these elements should be sufficiently high for reliable analysis, so the cause of this variation requires explanation. In the case of the ash record this is straightforward as it is to be expected that these deposits have preserved the products of the LVG and KVG magmatism occurring at the time of their deposition (∼3–7 Ma). In contrast, the explanation for elevated La/Sm in the central Tofua Arc data is unclear. Closer inspection of the data in Fig. 12b reveals that values of La/Sm N ∼1.5 belong almost exclusively to samples from Fonualei. This island is the northernmost of the central Tofua Arc group and it is tempting to suggest that the mantle source region is either experiencing some involvement from the Samoan plume (as suggested by Ewart et al., 1998 for the more northern islands) or perhaps that an increase in the influence of a sedimentary component (LSC?) is starting to appear. On the other hand, the Fonualei rocks tend to be high in SiO2 (60 + wt. %) so an explanation may be found in some differentiation process. Whatever the cause, it would be interesting to assess whether there exists any progressive change in this, and other parameters, with time or silica content. 5.6. Nd–Hf constraints on the mantle wedge? Based largely upon Pb isotope compositions, a strong case has been made above for the presence of a MORB-

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

like mantle wedge with either Pacific or Indian affinities, according to the tectonic setting at the time of magmatism, as originally proposed by Hergt and Hawkesworth (1994). The work of Pearce et al. (1999, 2007) referred to previously explores the possibility that Hf isotope compositions of magmas, when combined with Nd, may also distinguish the Indian vs Pacific character of the mantle source, and that, as Hf is less prone to alteration than Pb, its use may be preferable in some circumstances. Indeed these authors noted that Pb can be an unreliable discriminant of the wedge composition in subduction zone environments where considerable ‘mantle’ Pb may be derived from the downgoing slab. The broad subdivision of IMM and PMM domains is illustrated in Fig. 7, which also displays the Nd–Hf isotope compositions derived in this study for a range of rock suites from this region. In contrast with the observation made by Pearce et al. (2007), we found that the choice of discrimination boundary in this plot is central to allocating some samples to a specific mantle affinity. As noted previously and, as predicted, the backarc basin magmas from Sites 834, 835 and 839 clearly plot in the PMM field, whereas those from Sites and 836 and 837 lie well inside the IMM domain. This remains true regardless of whether the discrimination boundary based on the Australian–Antarctic Discordance is used (Kempton et al., 2002) or that of Pearce et al. (2007) based on the data from the SW Pacific. As expected from the previous discussion, samples of various ages from ʻEua also plot within the PMM field but, depending upon the discrimination line employed, between zero and half of the LVG and zero to one of the KVG rocks also plot within the PMM with the remaining samples falling within the IMM field (Figs. 7 and 11b). Salters and Hart (1991) published the Nd–Hf isotope data for samples from Fiji spanning early arc basalts and andesites, to mature arc tholeiites and rift phase magmas. These were obtained using TIMS analysis but have been excluded from the current comparison owing to highly variable Hf isotope values (ɛHf ∼ 10–17) and the fact that far more precise Hf isotope datasets can now be acquired by MC-ICPMS analysis. Notwithstanding such scatter, the samples from their study are also distributed between the PMM and IMM fields rather than solely within the PMM as might be anticipated. A point worth noting is that all arc data from this study, other than those from the Tofua Arc, lie within ∼1 epsilon unit of the IMM–PMM boundary. Nevertheless, at least some of the LVG and KVG samples lie within the IMM domain and appear to require an IMM mantle provenance. One difficulty in applying the approach of Pearce et al. (1999) is that Hf is presumed to be completely immobile in the slab-derived fluid flux. Woodhead et al (2001)

37

suggested that need not be the case, and hence such calculations may provide misleading results. Furthermore, the addition of any sediment melt would certainly carry Hf into the wedge. A notable feature of Fig. 7 is that some suites of samples appear to generate arrays that broadly parallel the IMM–PMM boundary, indeed samples from Site 834 generate two such arrays. Thus, in addition to significant variations in the Hf isotope composition within each suite, variations in the Nd isotope ratios between suites are also apparent, presumably reflecting mantle heterogeneity beneath the region. The marked shifts in Nd isotope composition are also clear from Fig. 5 in which sub-horizontal arrays are observed, particularly from Site 834, to Site 836 and LVG, with the KVG preserving a weakly correlated trend towards lower 143Nd/144Nd with higher 87Sr/86Sr values rather than the horizontal arrays observed in the other suites. It is our contention that the first order variations illustrated in Fig. 7 can be ascribed to a shift in the mantle wedge composition from PMM to IMM character. Although variations within each field can partially be attributed to local heterogeneities in the mantle wedge, this cannot explain why some of the LVG and KVG samples plot within the IMM field if the mantle beneath the arc at this time had PMM affinities. As indicated in Fig. 12b, it is apparent that the LVG and KVG suites differ from others in the region by showing evidence for the more substantial involvement of subducted sediments, probably a sediment melt, and this may well contribute to their apparent IMM character. In order to test this hypothesis, Hf isotope compositions were determined on sediments from DSDP sites 595A and 204 (Table 3). The mixing calculations employed here are used as a means of illustration only. No attempt has been made to vary the Nd/Hf ratios of pelagic and volcanic sedimentary components to account for fractionation during partial melting which would be particularly important if residual accessory phases such as zircon were stable (e.g., Tollstrup and Gill, 2005). Instead, representative isotopic compositions of Nd and Hf, and concentrations of these elements have been selected for mantle and sediment endmembers on the basis of bulk-rock data acquired in this and previous studies (i.e., Nd/Hf less than 10). Some attempt has been made to allow for the greater fluid mobility of Nd compared with Hf in the case of the mixing curves relating to the Tofua Arc (i.e., by using Nd/Hf of 20–50). The tick marks are provided at 5% intervals and, although we do not suggest that these reflect true measures, they may provide some guide as to the relative proportions required for different sample suites. On this basis, for example, the LVG and KVG require between twice to three times the sediment influence of the Tofua Arc magmas.

38

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

Clearly the results illustrated are non-unique as significant variations in the isotopic compositions of both mantle and sediment endmembers are available. Nevertheless, such curves serve to demonstrate that the addition of sedimentary material to PMM composition mantle can succeed in passing through the LVG and KVG data lying in the IMM field. The example curves shown employ volcanogenic sedimentary compositions because the trajectories observed in Fig. 6 are most consistent with this endmember. At least some of the KVG samples are displaced towards pelagic sediments in Fig. 6 and, as noted previously, it is possible that the composition of the sediment component was different at different latitudes in the arc during KVG magmatism. Variations in the PMM source are also required given the range in Hf isotope compositions, but this is consistent with the variations observed in the data that plot in the PMM field. The close proximity of the LVG and KVG data and distribution parallel to the discriminant boundary indicates that, either the sedimentary component involved in the generation of the Lau Ridge magmas had a comparatively high ɛNd, so that the mixing trajectories were steep, and/or the sediment was added to heterogeneous PMM source rocks with a wide range in ɛHf (e.g., as exemplified by the Sites 824, 835 and 839 results, and the data for ʻEua and the LVG samples that plot naturally in the PMM domain). An intriguing observation is that, despite trace element evidence for a greater sediment influence in the LVG and KVG magmas, the data for samples from the central Tofua Arc plot at significantly lower Nd isotope compositions (e.g., Figs. 5 and 7). This is consistent with the proposal by Hergt and Hawkesworth (1994) that the central volcanoes of the Tofua Arc are now underlain by a wedge of IMM composition and the mixing curves illustrated in Fig. 7 are consistent with such a hypothesis. Indeed without a change in the mantle wedge composition from PMM-like to IMMlike during the evolution of this arc–backarc system it would be difficult to reconcile the larger sediment input required by the trace element compositions of LVG and KVG rocks with the lower Nd isotope composition of both the northern and central Tofua Arc rocks. 6. Conclusions From a crustal evolution perspective, the most important lavas in the subduction zone environment are arguably those occurring at the arc front. These are the rocks most likely to survive collisions and become incorporated into continental landmasses, and are also the sites at which the greatest rates of elemental transfer and recycling are observed. A potential difficulty in studying

arc magmas in isolation however, is that detailed information on the volumetrically largest component, the mantle wedge, can be completely obscured, or at least rendered ambiguous. As a consequence, models may easily be developed to explain magmatic processes occurring in subduction zones that overlook important constraints provided by the underlying mantle wedge. Close examination of the Lau Basin back-arc magmas has made it possible to determine the trajectories defined by wedge-flux mixing on Pb isotope and trace element ratio diagrams. Employing this approach, and combining our observations with new data, including Nd and Hf isotope results for both the Lau Basin, Tofua Arc, Lau Ridge magmas and sediments, we formulate the following conclusions: 1. There are likely significant analytical errors associated with some pre-existing data published for the Tonga–Kermadec system. Interpretations based upon such data can be misleading. 2. There is conclusive evidence for the occurrence of MORB-like magmas with PMM affinities in this region. The PMM Pb isotope values in at least some Lau Basin magmas cannot be attributed to Pb derived from the downgoing Pacific oceanic crust and must be an inherent feature of the mantle wedge. Further support is provided by the Nd–Hf isotope signatures of the same samples that plot within the PMM field. 3. Arc magmas from ʻEua that precede the rift phase of back-arc basin formation have compositions that show a strong influence from a PMM mantle source. At least some samples reported for the same period through to the rift phase from Fiji (Salters and Hart, 1991) are also consistent with a PMM mantle source as are many of the LVG and perhaps one of the KVG samples from this study. The evidence that the mantle wedge beneath the Lau Islands had a PMM character prior to the opening of the Lau Basin is convincing. 4. At some stage arc magmatism was re-established and the modern Tofua Arc became active; however, compositional features of the arc magmas changed, particularly evident in their 206Pb/204Pb, Nd and Hf isotope signatures. That this change occurred around the same time as IMM back-arc magmatism was taking place is unlikely to be coincidental. It is argued here that, even though the arc magmas derive a relatively small proportion of their Pb from the mantle source (∼ 10–15%), much of the compositional shift in Pb, Nd and Hf isotope signatures in the Tofua Arc magmas is best explained by a change from PMM to IMM source in the mantle wedge. 5. Suites of samples employed in this study preserve trajectories in Pb isotope diagrams that extend from their mantle source regions, towards a range of possible

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

sediments (e.g., from volcanic to pelagic). While it is highly likely that the altered oceanic crust provides a significant contribution to the slab flux (e.g., central Tofua Arc), the radiogenic compositions require a sedimentary component in most, if not all, of these magmatic groups. 6. The sediment composition contributing to the KVG magmas appears to have been dominated by volcaniclastic material in the northern Lau Islands, and a more pelagic composition further south in the vicinity of Olorua and Moce islands. 7. The positive correlation between trace element ratios that track the addition of fluids, and the Pb isotope composition that provides a measure of the sedimentary influence, indicates that much of the sedimentary signal observed in arc magmas has been delivered via a fluid phase and that a sediment melt is not necessarily a requirement. 8. The preservation of mantle-like values of ratios such as La/Sm that are little affected by the addition of fluids but sensitive to the incorporation of sediment melts confirms the important role of fluids in these magmas. Thus, for most of the arc and back-arc magmas in this system, there is a strong evidence for a fluid control (in which the fluid carries a tangible sediment signature) and a little evidence for the involvement of sediment melts. 9. Although some magmatism associated with the original Lau Ridge is consistent with a slab flux dominated by fluids (e.g., ʻEua, Site 841) samples from the LVG and KVG show clear evidence for the greater involvement of subducted sediment, possibly in the form of a melt. This is reflected in ratios such as La/Sm, as well as the Nd–Hf isotope systematics for these magmas. 10. Although Hf data are unavailable for samples from the Site 840 ash record, the La/Sm, 206Pb/204Pb and 143 Nd/144Nd ratios these rocks preserve are consistent with derivation of volcanic fragments from eruptions of LVG Appendix A Appendix

39

and KVG magmas known to have been occurring during the time these sediments were deposited (b3.5–7 Ma). 11. Both LVG and KVG magmas display a significant influence from volcanic sedimentary compositions similar to those proposed for Tafahi and Niuatoputapu lavas. This suggests that sediments similar to those from the LSC were available well before ∼ 4 Ma (as implied by plate tectonic models involving the LSC), possibly as long ago as 14 Ma, indicating that additional sources of similar composition were locally available. This raises questions concerning the unique impact of the LSC on the northern Tofua Arc magmas and therefore the timing relationships derived for arc processes assuming LSC involvement. Acknowledgements The authors thank Jim Cole for allowing us to analyse splits of some of his Lau Island samples and Peter Rodda for his assistance in sourcing Lau Island samples from collections held in Suva. Tony Ewart kindly provided samples from the Tofua Arc for reanalysis. Data from the modern Lau Basin were obtained by JMH on samples generously provided by Julian Pearce and carried out many years ago at the Open University with the support of Chris Hawkesworth. The authors are most grateful for the detailed and helpful reviews from Jim Gill and John Gamble. The manuscript has been substantially improved on the basis of their suggestions and curly questions! Frantic emails to Julian Pearce were responded to calmly and thoroughly and we are most grateful for his generosity in helping us negotiate our way around ΔNd vs ΔɛNdP/I space during the final stages of revision. Outstanding editorial support was provided by Roberta Rudnick and the team at Chemical Geology. This work was partly funded by the Australian Research Council.

A

Supplementary data Isotope data compiled from the literature and employed in some form in this study (e.g., as points of discussion in the text, as data on diagrams or to help define fields). All Sr and Nd data have been re-normalised to an NBS987 87Sr/86Sr value of 0.71025 and La Jolla 143 Nd/144Nd of 0.511860 respectively. No re-normalisation of Pb isotope ratios has been performed. Data employed from the literature Sample code

Group/island

Source

87Sr/86Sr

143Nd/144Nd

Modern Lau Basin STO 64 PPTU 28-2 PPTU 30-1 Modern Lau Basin

CLSC CLSC CLSC

Volpe et al. (1988) Volpe et al. (1988) Volpe et al. (1988)

0.703202 0.703654 0.703233

0.513097 0.512971 0.513102

206Pb/204Pb

207Pb/204Pb

208Pb/204Pb

(continued on next page)

40

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

Appendix A (continued ) Data employed from the literature Sample code

Group/island

Source

87Sr/86Sr

143Nd/144Nd

PPTU 31-1 PPTU 32-1 PPTU 33-1 PPTU 36-1 PPTU 36-2 2KD2 8KD1 9KD1 9KD2 10KD2 13KD1 SO48/7 GC SO48/18 GA-2 SO48/42 GC DR1-1A D1-5 D1-7 D2-2 D2-4 D2-5 Lau 1-10 Lau 2-3 67KD3 73KD1 83KD1 94KD 121KD3 127KD 128KD2 DR2-1C DR3-1C DR4-1C SO48/54 GC SO48/127 KD SO48/114 KD/1 SO48/79 KD/1

CLSC CLSC CLSC CLSC CLSC CLSC CLSC CLSC CLSC CLSC CLSC CLSC CLSC CLSC ELSC Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa Valu Fa

Volpe et al. (1988) Volpe et al. (1988) Volpe et al. (1988) Volpe et al. (1988) Volpe et al. (1988) Boespflug et al. (1990) Boespflug et al. (1990) Boespflug et al. (1990) Boespflug et al. (1990) Boespflug et al. (1990) Boespflug et al. (1990) Loock et al. (1990) Loock et al. (1990) Loock et al. (1990) Boespflug et al. (1990) Jenner et al. (1987) Jenner et al. (1987) Jenner et al. (1987) Jenner et al. (1987) Jenner et al. (1987) Volpe et al. (1988) Volpe et al. (1988) Boespflug et al. (1990) Boespflug et al. (1990) Boespflug et al. (1990) Boespflug et al. (1990) Boespflug et al. (1990) Boespflug et al. (1990) Boespflug et al. (1990) Boespflug et al. (1990) Boespflug et al. (1990) Boespflug et al. (1990) Loock et al. (1990) Loock et al. (1990) Loock et al. (1990) Loock et al. (1990)

0.703179 0.703174 0.703030 0.703425 0.703441 0.703450 0.703243 0.703208 0.703225 0.703354 0.703341 0.703146 0.703114 0.703196 0.703482 0.70330 0.70334 0.70331 0.70331 0.70334 0.703313 0.703369 0.703348 0.703366 0.703351 0.703342 0.703574 0.703332 0.703415 0.703224 0.703388 0.703297 0.703243 0.703217 0.703409 0.703306

0.513082 0.513109 0.513124 0.513063 0.513041 0.513075 0.513085 0.513115 0.513155 0.513089 0.513036 0.513086 0.513102 0.513090 0.513016 0.512994 0.513023 0.513013 0.513010 0.513014 0.513047 0.513048 0.513066 0.513065 0.513073 0.513073 0.513077 0.513086 0.513033 0.513042 0.513075 0.513040 0.513033 0.513054 0.513035 0.513055

Tofua Arc Ata 8-9 482-8-11 482-8-12 482-8-1 482-8-8 482-8-3 482-8-4 Fon20 Fon30 Fon31 Fon31 Fon39 Fon41 Fon8 HHBF HHMF HHUF 38983 Tofua Arc

Ata Ata Ata Ata Ata Ata Ata Fonualei Fonualei Fonualei Fonualei Fonualei Fonualei Fonualei Hunga Haʻapai Hunga Haʻapai Hunga Haʻapai Hunga Haʻapai

Jenner et al. (1987) Turner et al. (1997) Turner et al. (1997) Turner et al. (1997) Turner et al. (1997) Turner et al. (1997) Turner et al. (1997) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Regelous et al. (1997) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998)

0.70334 0.703400 0.703470 0.703450 0.703390 0.703400 0.703370 0.703731 0.703520 0.703929

0.513024 0.513064 0.513107

0.703784 0.703829 0.703843 0.703774 0.703704 0.703537 0.703657

0.513103 0.513098 0.513081 0.512969 0.512969

0.512969 0.512956 0.512961 0.513054

206Pb/204Pb

207Pb/204Pb

208Pb/204Pb

18.173 18.099 18.151

15.504 15.476 15.475

38.045 37.928 37.966

18.640 18.647 18.665 18.660 18.669

15.551 15.556 15.557 15.555 15.560

38.327 38.352 38.364 38.357 38.375

18.634 18.768 18.681 18.659

15.561 15.553 15.543 15.558

38.330 38.363 38.313 38.363

18.714 18.701 18.686 18.692 18.764 18.726 18.714 18.597 18.589 18.589 18.542 18.609 18.573 18.580 18.651 18.648 18.649

15.558 15.529 15.532 15.534 15.520 15.537 15.514 15.554 15.578 15.578 15.535 15.573 15.548 15.549 15.574 15.565 15.584

38.346 38.244 38.240 38.243 38.286 38.290 38.232 38.229 38.271 38.271 38.131 38.285 38.200 38.189 38.293 38.268 38.308

J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

41

Appendix A (continued ) Data employed from the literature Sample code

Group/island

Source

87Sr/86Sr

38984 64T4C 64T6 104c T101p T102 T103c Late13 Late20 Late21 Late3 Late7 11108 Tof32 NT051 NT052a NT053 NT054 NT64-T2 NT64-T8 NTT25/4 NTT29/3 T113 T114 T116 T068 T069 T072 T073 TAF43/6 TAF18/10 TAF45/7

Hunga Haʻapai Kao Kao Kao Kao Kao Kao Late Late Late Late Late Metis Tofua Niuatoputapu Niuatoputapu Niuatoputapu Niuatoputapu Niuatoputapu Niuatoputapu Niuatoputapu Niuatoputapu Tafahi Tafahi Tafahi Tafahi Tafahi Tafahi Tafahi Tafahi Tafahi Tafahi

Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Turner et al. (1997) Turner et al. (1997) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Ewart et al. (1998) Turner et al. (1997) Turner et al. (1997) Turner et al. (1997)

0.703750 0.703273 0.703281 0.703206 0.703354 0.703299 0.703347 0.703654 0.703557 0.703832 0.703676 0.703622 0.703653 0.703513 0.703988 0.704050 0.704016 0.704045 0.704104 0.704083 0.704000 0.704000 0.704310 0.704002 0.704002 0.703908 0.703934 0.703908 0.703940 0.703910 0.703900 0.703920

0.512950 0.512970 0.512980

Lau Islands 29 339 29 371 29 385 29 790 29 784 29 787 29 792 29 458 29 765 29 770 29 764 29 487 29 488 29 348 29 350

LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG LVG KVG KVG KVG KVG

Cole et al. Cole et al. Cole et al. Cole et al. Cole et al. Cole et al. Cole et al. Cole et al. Cole et al. Cole et al. Cole et al. Cole et al. Cole et al. Cole et al. Cole et al.

0.70319 0.70322 0.70307 0.70316 0.70307 0.70324 0.70300 0.70300 0.70292 0.70322 0.70315 0.70406 0.70368 0.70338 0.70342

0.513035 0.513042 0.513042 0.513043 0.513057 0.513048 0.513045 0.513045 0.513043 0.513046 0.513046 0.513010 0.513017 0.513053 0.513058

Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics

Clift and Vroon (1996) Clift and Vroon (1996) Clift and Vroon (1996) Clift and Vroon (1996)

0.703335 0.703766 0.703763

0.513039 0.513115 0.513116

Site 840 Ash record 840A-1H-1, 102 cm 840A-1H-3, 45 cm 840C-1H-2, 114 cm 840C-2H-1, 6 cm Site 840 Ash record

(1990) (1990) (1990) (1990) (1990) (1990) (1990) (1990) (1990) (1990) (1990) (1990) (1990) (1990) (1990)

143Nd/144Nd

0.513039

0.513038

0.512971 0.512984 0.513035 0.513025

0.512894 0.512910 0.512899 0.512959 0.512943

206Pb/204Pb

207Pb/204Pb

208Pb/204Pb

18.619 18.622 18.616 18.602 18.602 18.518 18.651 18.579 18.603 18.571 18.590 18.570 18.562 18.640 19.250 19.209 19.003 19.258 19.005 18.925 19.020 18.980 19.306 19.103 18.970 19.311 19.041 19.258 18.926 18.978 18.991 18.938

15.573 15.570 15.571 15.572 15.560 15.571 15.574 15.573 15.583 15.559 15.548 15.566 15.567 15.582 15.623 15.621 15.602 15.621 15.618 15.605 15.580 15.580 15.619 15.612 15.600 15.629 15.592 15.618 15.593 15.559 15.564 15.583

38.263 38.236 38.229 38.224 38.196 38.164 38.293 38.225 38.279 38.199 38.129 38.194 38.214 38.291 38.956 38.934 38.742 38.965 38.780 38.672 38.720 38.670 38.934 38.782 38.632 38.964 38.682 38.900 38.608 38.560 38.587 38.580

18.733 18.883 18.899 18.893

15.549 15.578 15.583 15.577

38.450 38.559 38.591 38.561

(continued on next page)

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J.M. Hergt, J.D. Woodhead / Chemical Geology 245 (2007) 9–44

Appendix A (continued ) Data employed from the literature Sample code

Group/island

Source

87Sr/86Sr

143Nd/144Nd

206Pb/204Pb

207Pb/204Pb

208Pb/204Pb

840C-4H-6, 34 cm 840C-4H-6, 34 cm 840B-10X-CC, 4 cm 840B-10X-CC, 4 cm rpt 840B-11X-1, 122 cm 840C-5H-2, 106 cm 840C-6H-4, 123 cm 840C-7H-4, 128 cm 840C-8H-1, 92 cm 840C-8H-1, 97 cm 840C-8H-1, 97 cm rpt 840C-8H-1, 97 cm rpt 2 840C-9H-1, 92 cm 840C-10H-1, 3 cm 840B-47X-CC, 19 cm 840B-57X-3, 81 cm

Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics Volcaniclastics

Clift Clift Clift Clift Clift Clift Clift Clift Clift Clift Clift Clift Clift Clift Clift Clift

0.703702 0.703714 0.703423 0.703466

0.513091 0.513092 0.513096 0.513102

0.703291 0.703331 0.703320 0.703316 0.703379 0.703352 0.703380 0.703296

0.513097 0.513082 0.513067 0.513069 0.513077 0.513086 0.513053 0.513090

0.703438

0.513079

18.909 18.872 18.724 18.746 18.663 18.703 18.699 18.707 18.684 18.665 18.691 18.680 18.685 18.738 18.738 18.690

15.577 15.586 15.597 15.579 15.553 15.556 15.569 15.569 15.551 15.534 15.564 15.548 15.544 15.594 15.568 15.554

38.593 38.591 38.451 38.460 38.258 38.274 38.299 38.270 38.238 38.191 38.296 38.241 38.225 38.403 38.383 38.318

and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996) and Vroon (1996)

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