Mantle dynamics and mantle melting beneath Niuafo’ou Island and the northern Lau back-arc basin

ORIGINAL PAPER

Mantle dynamics and mantle melting beneath Niuafo’ou Islandand the northern Lau back-arc basin

Marcel Regelous Æ Simon Turner Æ Trevor J. Falloon ÆPaul Taylor Æ John Gamble Æ Trevor Green

Received: 24 June 2007 / Accepted: 18 December 2007 / Published online: 8 January 2008

� Springer-Verlag 2008

Abstract A suite of young volcanic basaltic lavas erupted

on the intra-plate island of Niuafo’ou and at active rifts and

spreading centres (the King’s Triple Junction and the

Northeastern Lau Spreading Centre) in the northern Lau

Basin is used to examine the pattern of mantle flow and the

dynamics of melting beneath this complex back-arc sys-

tem. All lavas contain variable amounts of a subduction

related component inherited from the Tonga subduction

zone to the east. All lavas have higher 87Sr/86Sr, lower143Nd/144Nd and more radiogenic Pb isotope compositions

than basalts erupted at the Central Lau Spreading Centre in

the central Lau Basin, and are interpreted as variable

mixtures of subduction-modified, depleted upper mantle,

and mantle residues derived from melting beneath the

Samoan Islands which has leaked through a tear in the

subducting Pacific Plate beneath the Vitiaz Lineament at

the northern edge of the Lau Basin. Our data can be used to

map out the present-day distribution of Samoan mantle in

this region, and show that it influences the compositions of

lavas erupted as far as 400 km from the Samoan Islands.

The distribution of Samoan-influenced lavas implies south-

and southwest-wards mantle flow rates of [4 cm/year.

U-series disequilibria in historic Niuafo’ou lavas have

average (230Th/238U) = 1.13, (231Pa/235U) = 2.17,

(226Ra/230Th) = 2.11, and together with major and trace

element data require *5% partial melting of mantle at

between 2 and 3 GPa, with a residual porosity of 0.002 and

an upwelling rate of 1 cm year-1. We suggest that intra-

plate magmatism in the northern Lau Basin results from

decompression melting during southward flow of mantle

from beneath old (110–120 Ma), relatively thick Pacific

oceanic lithosphere to beneath young (\5 Ma), thinner

oceanic lithosphere beneath the northern Lau Basin.

Keywords Niuafo’ou � Samoa Islands � Lau Basin �Trace elements � Radiogenic isotopes �Back-arc basin basalts

Introduction

Actively spreading back-arc basins exist behind a number

of island arcs, including many of those surrounding the

southwest Pacific. The basalts erupted in these basins

(BABB) generally have very similar mineralogy and major

element characteristics to those erupted at mid-ocean rid-

ges (MORB). However, their trace element and isotopic

compositions often indicate involvement of fluid and/or

sediment components (see Pearce and Stern 2006, for a

review), which are inferred to have been left behind in the

Communicated by T.L. Grove.

M. Regelous

Department of Geology, Royal Holloway University of London,

Egham, Surrey, UK

S. Turner (&) � T. Green

GEMOC, Department of Earth and Planetary Sciences,

Macquarie University, Sydney, NSW 2109, Australia

e-mail: [email protected]

T. J. Falloon

School of Earth Sciences and Centre for Marine Sciences,

University of Tasmania, GPO Box 252-79, Hobart,

TAS 7001, Australia

P. Taylor

Australian Volcanological Investigations, PO Box 291, Pymble,

NSW 2073, Australia

J. Gamble

Department of Geology, National University of Ireland, Cork,

Ireland

123

Contrib Mineral Petrol (2008) 156:103–118

DOI 10.1007/s00410-007-0276-7

back-arc mantle wedge as the arc front migrated away.

Thus, the extent to which melting beneath back-arc basins

is the result of passive decompression, the presence of

volatiles, active upwelling due to induced convection in the

mantle wedge, or the presence of thermal anomalies, is not

well constrained (Taylor and Martinez 2003; Kelley et al.

2006; Langmuir et al. 2006; Wiens et al. 2006). In the Lau

back-arc basin, influx of relatively enriched, and possibly

hotter, mantle from beneath the Samoan Islands may

influence the degree and location of melting in the north-

ernmost part of the basin (Danyushevsky et al. 1995;

Wendt et al. 1997; Turner and Hawkesworth 1998).

Niuafo’ou volcano, located in the centre of the northern

Lau Basin (Fig. 1), represents one of the few subaerial

volcanoes from a back-arc setting and affords the oppor-

tunity to sample stratigraphically-controlled flows at a fine

spatial resolution. Here we use major and trace element

data together with radiogenic (Sr-Nd-Pb) and short-lived

(U-Th-Pa-Ra) isotope data to investigate the petrogenesis

of basalts erupted at Niuafo’ou. These data are combined

with new Sr, Nd and Pb isotope data for lavas from the

nearby King’s Triple Junction and the Northeastern Lau

Spreading Centre (Fig. 1), and with published data for

lavas from elsewhere in the Lau Basin, in order to examine

the nature of the melting process and the pattern of mantle

flow in this complex back-arc basin.

Geological setting and petrology of the samples

analysed

The Lau Basin has had a complex tectonic history begin-

ning *6 M years ago when what is the present day Tonga

(Tofua) Arc split and migrated eastward away from the

remnant Lau Ridge which now forms the western flank of

the Lau Basin (e.g. Hawkins 1995; Zellmer and Taylor

2001). The active Tonga Arc extends as far north as 15�S,

where the Pacific–Indo-Australian plate boundary becomes

the WNW-ESE trending Vitiaz Lineament (Fig. 1), a

transform boundary beneath which the Pacific Plate tears in

order to accommodate subduction (Millen and Hamburger

1998). The ‘‘slab window’’ thus formed may allow south-

ward flow of mantle from the nearby Samoan Islands

(Fig. 1) beneath the northern Lau Basin (Danyushevsky

et al. 1995; Smith et al. 2001; Turner et al. 1998; Wendt

et al. 1997).

The active volcanic island of Niuafo’ou lies in the centre

of the northern Lau Basin north of the Central Lau

Spreading Centre (CLSC), northeast of the Peggy Ridge

leaky transform and west of the western arm of the King’s

(Mangatolo) Triple Junction (Fig. 1). Unlike these broadly

linear, submarine spreading centres, Niuafo’ou is an 8 km

diameter subaerial volcano, which may be located on a

microplate (Chase 1971; Zellmer and Taylor 2001). The

island consists of a broad lava shield capped by the rem-

nants of a composite cone, which was destroyed during a

caldera-forming eruption (Fig. 2). Taylor (1991) identified

four main stratigraphic phases; following an initial sub-

marine phase, a pre-caldera phase involving mainly

massive lavas flows built a basal lava shield and cone. The

undated caldera-forming eruption involved summit col-

lapse and left behind a 14 km2, water-filled caldera. The

post-caldera phase has involved intra-caldera explosive and

extra-caldera effusive activity. At least 12 eruptions have

been recorded in the last 150 years, the most recent major

one occurring in 1946 when the island was evacuated

(McDonald 1948). Locations of the historic lavas are

shown in Fig. 2.

Niuafo’ou lavas range from porphyritic to glassy

vesicular (5–67%) tholeiites containing normally-zoned

plagioclase (An79-47) as the dominant phenocryst phase

(2–21%) accompanied by minor amounts of Fo83-66

olivine (\1%) and clinopyroxene (\2%). Titanomagnetite

occurs only as microphenocrysts along with plagioclase,

clinopyroxene and olivine in the groundmass and glom-

eroporphyritic aggregates comprise 1–4% of the rocks

179°W -175°W -171°W

-20°S

-15°S

-12°S

-22°SELSC

CLSC

KTJ

Niuafo'ou

NELSC

Tafahi

Fonualei

Savai'i

Upolu

SamoanIslands

Tong

a/To

fua

Arc

Tong

a Tr

ench

UoMamae

100 km

VL

Fig. 1 Bathymetric map of the northern Lau Basin and Tonga Arc

(adapted from Falloon et al. 2007) showing main tectonic features and

locations of samples analysed in this study. Abbreviations: CLSCCentral Lau Spreading Centre, ELSC Eastern Lau Spreading Centre;

KTJ King’s Triple Junction, NELSC Northeastern Lau Spreading

Centre, VL Vitiaz Lineament

104 Contrib Mineral Petrol (2008) 156:103–118

123

(Taylor 1991). Plagioclase rim-groundmass thermometry

suggests eruptive temperatures in the range 1,195–1,216�C

(Ewart 1976). Previous geochemical studies have noted the

combination of MORB-like major and trace element

characteristics with ocean island basalt (OIB) like isotope

ratios in Niuafo’ou lavas (Reay et al. 1974; Ewart 1976;

Ewart and Hawkesworth 1987; Ewart et al. 1994, 1998;

Turner et al. 1997). However, the cause of partial melting

beneath Niuafo’ou and how this may relate to magma

production at the major spreading centres in the Lau Basin

remains enigmatic.

In addition to the samples from Niuafo’ou, we have

carried out Sr, Nd and Pb isotope analyses of young vol-

canic glasses erupted at spreading centres elsewhere in the

northern Lau Basin. These include five samples from the

King’s Triple Junction (KTJ) situated to the east of Ni-

uafo’ou, and one sample from the Northeastern Lau

Spreading Centre (NELSC) northeast of the KTJ (Fig. 1).

The KTJ and NELSC glasses contain phenocrysts and

microphenocrysts of plagioclase + olivine ± clinopyro-

xene; major and trace element data for these samples are

published elsewhere (Sun et al. 2003; Falloon et al. 1992,

2007). All samples are less than about 1 Ma in age, and can

therefore be used to map out the present-day distribution of

mantle components in this region. For comparison, we

have also analysed one sample from Uo Mamae (Machias)

Seamount, which is located on the Pacific Plate south of the

Samoan Islands (Fig. 1) where it is being subducted into

the Tonga Trench. A phonolite dredged from the summit of

this seamount yielded a K-Ar age of 0.94 Ma (Hawkins

and Natland 1975), and major and trace element data for

the sample analysed in this study are given in Falloon et al.

(2007).

Analytical techniques

Major and trace element data for Niuafo’ou samples, some of

which have been partially published before, are listed in

Table 1. Major element data for samples 31461, 31462,

31463, 31477 and 31479 erupted between 1886 and 1946

were published by Turner et al. (1997) and are augmented

roads

trails

settlements

Big Lake(Vai Lahi)

22.86 m / 75 feetabove sea level

Motu Sil

Motu Lahl

Motu Molimoli(Vai

LittleLake

Motu

Angaha

Tataula

Taamot

Mataako

Vai

Niuafoou Island

0 1 km

0 1 mile

after T. A. Jaggar,

N

Betani

Cinder Cone1929

Cinder Cone1912

Mua

Tongamama

1946 lava1929 vents

1929 lava

1912 vents

1912 lava

1853 lava

old lava

escarpment

15°33' 42"S

175°37'46"W

1867, 1935-6, 1943 lavas

Fig. 2 Geological map of

Niuafo’ou island adapted from

Jaggar (1931) and Taylor

(1991), showing locations of

historic lava flows analysed in

this study

Contrib Mineral Petrol (2008) 156:103–118 105

123

Table 1 Major and trace element data for lavas from Niuafo’ou, Lau Basin

Sample 31461 31462 31463 31477 JA2 N107 N108 N110 N111 N131

SiO2 49.43 49.85 49.91 50.11 49.21 48.98 49.90 50.74 50.23

TiO2 1.33 1.35 1.42 1.53 1.78 1.53 1.67 1.37 1.41

Al2O3 16.57 16.31 15.59 14.69 15.70 15.50 13.38 14.50 15.00

Fe2O3 10.55 10.81 11.25 11.91 12.43 11.65 12.77 12.02 11.31

MnO 0.17 0.17 0.18 0.20 0.21 0.19 0.20 0.18 0.19

MgO 6.71 6.80 6.99 7.16 6.07 7.00 6.72 7.22 7.32

CaO 12.13 12.03 11.81 11.62 11.16 11.63 11.16 11.55 12.30

Na2O 2.78 2.86 2.85 2.89 3.48 3.07 3.19 3.23 3.00

K2O 0.16 0.16 0.12 0.20 0.24 0.17 0.19 0.17 0.17

P2O5 0.11 0.12 0.14 0.13 0.15 0.09 0.10 0.10 0.07

L.O.I. -0.41 -0.44 -0.33 -0.51 0.22 0.13 1.92 0.26 0.12

Total 99.53 100.02 99.96 99.93 100.65 99.94 101.20 101.34 101.12

Li 6.10 6.16 6.26 6.72 30.1 5.93 5.28

Be 0.525 0.532 0.558 0.571 2.50 0.668 0.461 0.505 0.486 0.495

Sc 43.2 44.5 45.4 47.6 18.7 46.0 45.9 46.7 45.6 48.2

V 254 261 267 284 117 300 265 280 288 301

Cr 282 286 285 250 401 207 303 179 319 345

Co 40.7 41.3 41.7 41.8 29.2 41.4 44.2 45.9 42.3 43.9

Ni 53.0 56.0 59.0 53.0 135 44.6 54.3 44.5 57.5 61.2

Cu 89.5 97.3 97.6 79.7 32.6 92.2 88.1

Zn 75.0 77.6 80.0 83.3 68.1 81.4 89.3

Ga 16.5 16.4 16.1 16.1 18.4 18.0 17.1 17.8 16.8 16.8

Rb 3.03 3.10 3.23 3.66 67.4 3.43 2.85 3.18 2.67 3.02

Sr 184 180 172 170 245 172 163 161 165 160

Y 29.6 30.1 31.3 32.8 16.7 41.1 28.9 31.4 32.9 33.5

Zr 92.6 94.8 99.0 106 110 128 92.7 102 95.8 96.9

Nb 3.25 3.35 3.49 3.98 9.54 4.68 3.46 3.96 3.53 3.75

Cs 0.020 0.022 0.023 0.030 4.95 0.037 0.037 0.041 0.035 0.038

Ba 31.8 32.8 33.2 38.2 313 45.6 35.0 38.4 32.8 35.3

La 4.03 4.10 4.23 4.53 14.5 5.56 4.46 4.96 4.21 4.27

Ce 11.7 11.94 12.4 13.1 31.4 15.9 12.9 14.2 12.1 12.2

Pr 1.93 1.98 2.04 2.15 3.60 2.58 2.15 2.35 1.98 2.01

Nd 9.85 9.96 10.3 10.9 13.5 13.2 10.9 11.8 10.2 10.1

Sm 3.21 3.23 3.53 3.65 2.87 4.31 3.49 3.79 3.41 3.43

Eu 1.25 1.27 1.29 1.35 0.874 1.50 1.36 1.44 1.23 1.24

Gd 4.51 4.50 4.74 4.98 2.91 5.40 4.63 5.04 4.32 4.31

Tb 0.823 0.805 0.874 0.896 0.475 0.981 0.806 0.882 0.785 0.795

Dy 5.23 5.34 5.543 5.81 2.74 6.38 5.45 5.93 5.11 5.13

Ho 1.15 1.19 1.23 1.28 0.565 1.39 1.18 1.28 1.12 1.13

Er 3.17 3.25 3.36 3.55 1.59 3.91 3.33 3.60 3.12 3.15

Tm 0.537 0.497 0.543 0.428 0.429

Yb 3.07 3.11 3.22 3.40 1.53 3.76 3.18 3.46 3.05 3.09

Lu 0.468 0.471 0.494 0.524 0.239 0.584 0.480 0.524 0.466 0.470

Hf 2.37 2.40 2.52 2.69 2.69 2.73 2.50 2.74 2.09 2.13

Ta 0.239 0.244 0.255 0.288 0.670 0.267 0.227 0.263 0.201 0.220

Pb 1.16 1.05 0.957 1.00 18.5 1.19 1.17 1.23 0.712 1.07

Th 0.256 0.260 0.305 0.309 4.68 0.444 0.311 0.363 0.306 0.338

U 0.080 0.085 0.093 0.095 2.13 0.106 0.375 0.127 0.098 0.090


123

Table 1 continued

Sample T078 T079 T080 T081 T082 T083 T084 BHVO-1 %RSD

SiO2 50.01 49.98 50.01 49.63 50.46 49.97 49.84

TiO2 1.50 1.03 1.52 1.46 1.51 1.34 1.52

Al2O3 14.57 18.60 14.60 14.69 14.35 16.18 14.45

Fe2O3 12.26 9.05 12.19 11.73 12.33 11.05 12.25

MnO 0.19 0.15 0.20 0.18 0.19 0.18 0.20

MgO 7.03 5.22 7.08 7.37 6.79 6.83 6.90

CaO 11.65 12.84 11.70 11.89 11.61 11.97 11.56

Na2O 3.16 2.43 3.07 2.77 2.76 2.88 3.14

K2O 0.22 0.15 0.21 0.18 0.18 0.16 0.20

P2O5 0.11 0.07 0.11 0.13 0.11 0.09 0.11

L.O.I. -0.54 0.04 -0.51 -0.27 -0.43 -0.52 -0.59

Total 100.16 99.56 100.18 99.76 99.86 100.13 99.58

Li 6.53 4.57 6.46 5.83 6.51 5.72 6.40 4.69 1.31

Be 0.476 0.357 0.460 0.426 0.476 0.406 0.462 0.837 2.25

Sc 46.5 35.0 46.9 47.3 47.4 42.2 47.4 29.9 1.68

V 270 197 272 264 273 241 274 286 1.39

Cr 269 275 257 318 287 292 254 295 2.37

Co 86.6 75.3 78.4 99.0 87.8 66.2 86.6 46.7 1.53

Ni 51.3 58.6 50.5 50.9 47.1 55.9 49.1 117 2.57

Cu 67.7 72.9 82.2 69.9 69.0 80.6 91.8 137 0.94

Zn 81.8 60.7 82.0 77.9 84.9 72.7 82.2 106 1.81

Ga 17.3 16.8 17.3 17.0 17.4 16.8 17.1 21.2 1.18

Rb 3.21 2.17 3.16 2.84 3.04 2.59 3.06 9.27 0.83

Sr 165 202 164 159 162 170 163 394 0.52

Y 28.9 20.8 29.5 27.6 29.4 26.1 29.9 22.9 0.76

Zr 94.4 67.8 95.0 87.5 96.0 83.2 96.1 165 0.94

Nb 3.86 2.64 3.86 3.38 3.70 3.10 3.66 18.5 0.86

Cs 0.041 0.027 0.040 0.036 0.037 0.032 0.040 0.097 1.98

Ba 37.1 30.1 37.4 33.2 35.8 30.9 35.7 132 0.75

La 4.65 3.50 4.77 4.12 4.55 4.20 4.70 15.1 0.74

Ce 13.2 9.85 13.5 12.0 13.1 11.7 13.5 37.7 0.65

Pr 2.20 1.62 2.23 1.99 2.18 1.93 2.24 5.46 0.55

Nd 11.0 8.18 11.3 10.2 11.1 9.89 11.3 24.1 0.52

Sm 3.55 2.62 3.61 3.36 3.63 3.16 3.66 5.95 0.79

Eu 1.38 1.08 1.39 1.31 1.40 1.23 1.41 2.04 0.98

Gd 4.74 3.40 4.76 4.44 4.71 4.15 4.77 5.97 0.68

Tb 0.821 0.595 0.831 0.786 0.837 0.732 0.849 0.886 0.97

Dy 5.50 3.95 5.57 5.26 5.61 4.91 5.62 5.12 0.85

Ho 1.18 0.852 1.20 1.13 1.20 1.05 1.21 0.954 0.84

Er 3.35 2.39 3.44 3.24 3.42 3.00 3.45 2.42 1.32

Tm 0.496 0.355 0.507 0.479 0.507 0.444 0.512 0.324 1.28

Yb 3.32 2.29 3.26 3.07 3.27 2.89 3.28 1.90 1.05

Lu 0.480 0.341 0.488 0.460 0.496 0.433 0.496 0.268 1.36

Hf 2.56 1.84 2.59 2.42 2.60 2.25 2.59 4.37 1.28

Ta 1.15 1.31

Pb 0.823 0.626 0.795 0.817 0.815 0.685 0.842 1.97 3.87


123

here by the addition of new ICPMS trace element data for

four of these samples (see Kelley et al. 2003 for analytical

details). Major and trace element data for samples with the

N- and T-prefix are from Ewart et al. (1998) and references

therein. Trace element concentrations for these samples were

determined by ICPMS; full analytical details are given in Niu

and Batiza (1997). Trace element data for international rocks

standards measured together with these samples are listed in

Table 1. In Fig. 3, these data are supplemented by XRF data

for older (caldera and post-caldera) samples from Taylor

(1991), six of which were also analysed for Sr, Nd and Pb

isotope composition (MU- prefix in Table 2).

48

50

52

54

56

58

60

62

Niuafo'ou (other)

Lau Basin spreading centresNiuafo'ou (historic: 1886-1946 AD)

0

1

2

3

12

14

16

18

20

4

6

8

10

12

14

0

100

200

300

108642010

100

1000

1086420

MgO MgO

Al 2

O3

TiO

2S

iO2

Zr

Sr

CaO

(b)

(c)

(d)

(e)

(f)

(a)Fig. 3 Major and trace element

compositions of Niuafo’ou

lavas. Large filled squaresrepresent lavas from known

historic eruptions (1886–1946

AD). Increasing Al2O3, CaO, Sr

and decreasing SiO2, TiO2, Zr

with decreasing MgO reflect

plagioclase accumulation. The

samples analysed are dominated

by historic and Holocene lavas,

and variable plagioclase

accumulation explains most of

the chemical variation in these

samples. At a given MgO, the

most primitive lavas from

Niuafo’ou have lower SiO2 and

higher TiO2 than basalts from

Lau Basin spreading centres

(data from Peate et al. 2001)

Table 1 continued

Sample T078 T079 T080 T081 T082 T083 T084 BHVO-1 %RSD

Th 0.340 0.244 0.342 0.302 0.329 0.282 0.328 1.19 1.09

U 0.101 0.074 0.103 0.093 0.100 0.086 0.099 0.433 1.09

Major element concentrations in weight %, trace element concentrations in ppm. Major element data are from Turner et al. (1997) and Ewart

et al. (1998) and references therein. The T0- samples were powdered in tungsten carbide; Ta concentrations for these samples are therefore not

reported. BHVO-1 values represent average of 82 measurements of this rock standard over a 2 year period during which sample analyses (N- and

T0-prefix) were carried out (University of Queensland). Data for JA-2 represent mean of two separate dissolutions of this standard measured

together with 314 samples at Boston University


123

Unless otherwise indicated in Table 2, all Sr, Nd and Pb

isotope measurements were carried out at Royal Holloway

University of London. Handpicked fresh glasses from

submarine samples were leached for 10 min at room tem-

perature in a 50:50 mix of H2O2 and 3 M HCl in an

ultrasonic bath before dissolution. Whole-rock powders for

Sr were leached in hot 6M HCl for 1 h before dissolution,

sample powders for Nd were unleached. As far as possible,

Pb analyses were carried out on rock chips; both chips and

powders were leached for 30 min in 6 M HCl before

dissolution.

Sr and Nd isotope measurements were carried out on a

VG354 multicollector mass spectrometer in dynamic

mode. Measured Sr and Nd isotope ratios were corrected

for instrumental mass fractionation using 86Sr/88Sr and142Nd/144Nd values of 0.1194 and 1.141870, respectively

Table 2 Sr, Nd and Pb isotope data for samples from the northern Lau Basin

Sample Location 87Sr/86Sr 143Nd/144Nd 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

31461 Niuafo’ou 0.704319 0.512827a 18.4616 ± 14 15.5977 ± 13 38.5514 ± 36

31462 Niuafo’ou 0.704321 0.512801 18.4638 ± 18 15.5986 ± 16 38.5548 ± 48

31463 Niuafo’ou 0.704323 0.512803 18.4635 ± 15 15.5995 ± 14 38.5580 ± 38

31477 Niuafo’ou 0.704355 0.512798 18.4622 ± 15 15.5983 ± 13 38.5496 ± 43

31479 Niuafo’ou 0.704288a 0.512803a

T078 Niuafo’ou 0.704391 0.512795 18.4564 ± 17 15.5996 ± 15 38.5505 ± 41

T079 Niuafo’ou 0.704338 0.512815 18.4096 ± 17 15.5948 ± 16 38.4789 ± 48

T080 Niuafo’ou 0.704370 0.512794 18.4599 ± 16 15.5983 ± 14 38.5508 ± 37

T081 Niuafo’ou 0.704390 0.512795 18.4748 ± 17 15.6005 ± 15 38.5705 ± 47

T082 Niuafo’ou 0.704358 18.4638 ± 11 15.5971 ± 10 38.5538 ± 27

T083 Niuafo’ou 0.704345 0.512804 18.4624 ± 20 15.5970 ± 18 38.5523 ± 47

T084 Niuafo’ou 0.704377 0.512808 18.4669 ± 19 15.5982 ± 17 38.5599 ± 49

N107 Niuafo’ou 0.512809

N108 Niuafo’ou 0.704358 0.512807 18.4662 ± 17 15.5988 ± 16 38.5520 ± 43

N110 Niuafo’ou 0.704366 0.512797

N111 Niuafo’ou 0.704342 0.512822

N131 Niuafo’ou 0.704365 0.512800 18.4621 ± 16 15.5991 ± 15 38.5525 ± 42

MU40422 Niuafo’ou 0.704329 0.512810 18.4393 ± 25 15.5981 ± 22 38.5274 ± 57

MU40454 Niuafo’ou 0.704281 0.512811 18.4480 ± 17 15.5959 ± 17 38.5290 ± 50

MU40457 Niuafo’ou 0.704358 0.512804 18.4115 ± 27 15.5937 ± 24 38.4767 ± 67

MU40458 Niuafo’ou 0.704264 0.512794 18.4305 ± 19 15.5954 ± 18 38.5050 ± 48

MU40461 Niuafo’ou 0.704272 0.512826

MU40462 Niuafo’ou 0.704302 0.512820 18.4527 ± 15 15.5910 ± 14 38.5353 ± 39

2212-2 (g) KTJ 0.703992 0.512838 18.6259 ± 19 15.5842 ± 17 38.6841 ± 51

2212-2b (g) KTJ 18.6266 ± 21 15.5848 ± 19 38.6856 ± 54

2218-7 (g) KTJ 0.703942 0.512861 18.5928 ± 11 15.5843 ± 11 38.6562 ± 28

2218-7b (g) KTJ 0.512857 18.5929 ± 14 15.5850 ± 13 38.6565 ± 34

2218-8 (g) KTJ 0.703865 0.512868 18.5719 ± 16 15.5781 ± 15 38.6286 ± 43

2218-8b (g) KTJ 18.5720 ± 18 15.5783 ± 16 38.6289 ± 46

2218-10 (g) KTJ 0.704083 0.512829 18.5559 ± 15 15.5895 ± 14 38.6398 ± 40

2218-12 (g) KTJ 0.703887 0.512861 18.5935 ± 19 15.5835 ± 17 38.6445 ± 46

D120-2-1 (g) NELSC 0.703930 0.512786 18.9877 ± 22 15.6169 ± 19 38.9232 ± 49

16-94/1 Uo Mamae 0.705215 0.512470 18.3317 ± 11 15.6372 ± 10 38.5210 ± 26

g Denotes fresh glass

Sr and Nd isotope measurements were carried out by TIMS at Royal Holloway University of London, except for those in italics (University of

Queensland), and those marked with superscript a (Open University). All Sr and Nd isotope data are normalised to values of 0.710245 and

0.511856 for the NBS987 and La Jolla standards, respectively. All Pb isotope measurements carried out at RHUL using double-spike MC-

ICPMS. See text for analytical detailsb Repeat measurements of same sample solution carried out on separate days with different instrument settings


123

(the 142Nd/144Nd ratio was used for normalisation in pref-

erence to 146Nd/144Nd, because the 142Nd peak is relatively

large, and masses 142 and 144 span the ratio of interest

(143Nd/144Nd)). During the period of analysis, the NBS987

Sr and Aldrich Nd standards yielded 87Sr/86Sr and143Nd/144Nd values of 0.710248 ± 15 and 0.511403 ± 8,

respectively. All Sr data in Table 2 are normalised to a87Sr/86Sr value of 0.710245 for the NBS987 standard; Nd

isotope data are reported relative to a 143Nd/144Nd value of

0.511856 for the La Jolla standard.

Pb isotope data for all samples were determined by MC-

ICPMS (GV Isoprobe) in multidynamic mode using a204Pb-207Pb double spike, following methods described by

Thirlwall and Anczkiewitz (2004) and Thirlwall et al.

(2004). During the period of analysis, the NBS981 standard

yielded 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of

16.9430 ± 18, 15.5007 ± 21 and 36.7247 ± 53 (2sd,

n = 23).

The U-series analyses compiled in Table 3 comprise238U-230Th data from Turner et al. (1997) and Regelous

et al. (1997) supplemented by 226Ra data from Turner et al.

(2000). The 231Pa analyses were carried out at the Uni-

versity of Bristol using methods described in Regelous

et al. (2004).

Results

Most Niuafo’ou lavas are basalts with a restricted range in

major element composition (SiO2 = 48.98–50.74; MgO =

5.22–7.37) and, although there is some scatter, SiO2

appears to decrease with decreasing MgO, as exemplified

by the historic 1886–1946 AD samples (Fig. 3a). These and

the majority of the other samples show trends of decreasing

TiO2, Fe2O3, K2O and Na2O, and increasing CaO and

Al2O3 with decreasing MgO (Fig. 3b–d). There are poor

correlations between MgO and Cr and Ni in the majority of

samples. With the exception of Sr, which increases with

decreasing MgO (Fig. 3e), the concentrations of most

incompatible elements (e.g. Ba, Zr, Nb, etc) either decrease

or show no systematic variation with decreasing MgO

(Fig. 3f).

On primitive mantle-normalised multi-incompatible

element diagrams (Fig. 4), the Niuafo’ou basalts have

higher concentrations of the highly incompatible elements

(Rb, Ba, Th) compared to average N-MORB and basalts

from the Central Lau Spreading Centre (CLSC). The

Niuafo’ou lavas have negative Pb anomalies, which are

smaller than those of average N-MORB, but larger than

those of basalts from the KTJ. In common with KTJ and

CLSC lavas, Niuafo’ou basalts have primitive mantle-

normalised Ba/Th ratios [1 (Fig. 4), in contrast to

N-MORB which have values \1.

Sr and Nd isotope compositions of Niuafo’ou lavas

show little variation with most samples having 87Sr/86Sr

Table 3 U-series disequilibria data for historic Niuafo’ou basalts

Sample Year U (ppm) Th (ppm) 231Pa (fg/g) 226Ra (fg/g) (238U/232Th) (230Th/232Th) (230Th/238U) (231Pa/235U) (226Ra/230Th)

31461 1946 0.080 0.256 57.6 65.87 0.944 1.031 1.102 2.213 2.247

31462 1946 0.085 0.260 59.0 67.05 0.993 1.236 1.256 2.134 1.876

31463 1946 0.093 0.305 62.0 81.38 0.922 1.134 1.240 2.049 2.091

31477 1886 0.095 0.309 70.7 77.63 0.931 1.026 1.111 2.288 2.206

31479 1943 0.112 0.297 – – 1.148 1.074 0.944 – –

N107 – 0.110 0.410 – – 0.844 1.189 1.409 – –

N111 – 0.098 0.306 – – 1.006 1.140 1.133 – –

U-Th data from Regelous et al. (1997) and Turner et al. (1997), Ra data from Turner et al. (2000)

Niua fo'ouN-MORBCLSCKTJ

Prim

itive

man

tle n

orm

alis

ed

RbBa

ThU

NbLa

CePb

PrSr

NdSm

ZrHf

EuGd

TbDy

YHo

ErYb

Lu

10

1

20

0.7

Fig. 4 Incompatible trace element concentrations of representative

Niuafo’ou lavas, normalised to the primitive mantle composition of

Sun and McDonough (1989). Data for representative mafic lavas from

the KTJ and CLSC are from Sun et al. (2003) and Peate et al. (2001),

respectively. Average N-MORB composition from Hofmann (1988)


123

and 143Nd/144Nd ratios in the range 0.70426 to 0.70439,

and 0.51279 to 0.51283, respectively (Fig. 5d). Prehistoric

shield lavas (MU40454-63) tend to have less radiogenic

and more variable Sr (and Pb) isotope compositions, but

there are no systematic isotopic or chemical variations with

time within the historic samples analysed. The Niuafo’ou

basalts have higher 87Sr/86Sr and lower 143Nd/144Nd than

basalts from the CLSC (Fig. 5d). The new double spike Pb

isotope data for Niuafo’ou lavas show much less variation

than existing conventional TIMS data. Pb isotope compo-

sitions of Niuafo’ou lavas determined in this study are

similar to those reported by Ewart et al. (1998), but sig-

nificantly different from the values given by Turner et al.

(1997) and Turner and Hawkesworth (1998) for the same

samples. The Pb isotope compositions of Niuafo’ou lavas

are more radiogenic than those of CLSC lavas, and extend

towards the field for Samoan post-erosional lavas in

Fig. 5b, c. Lavas from the KTJ and NELSC have lower87Sr/86Sr and higher 143Nd/144Nd than Niuafo’ou lavas, but

more radiogenic Pb isotope compositions (Fig. 5a–d).

U-series data for historic Niuafo’ou lavas are given in

Table 3. (230Th/232Th) ranges from 1.03–1.24 and all but

one sample have 230Th-excess with (230Th/238U) = 0.94–

1.41. Thus, they overlap with published data for lavas

from spreading centres in the Lau Basin (Peate et al. 2001).

All samples have 231Pa-excesses (2.05–2.29) and

226Ra-excesss (1.88–2.25), which indicate that the U-Th

and U-Pa data have not been modified by decay. The Lau

Basin samples analysed by Peate et al. (2001) do not have

age constraints and thus their (226Ra/230Th) ratios represent

minima. Nevertheless, the higher (226Ra/230Th) ratios

reported by Peate et al. (2001) are very comparable to those

observed here from Niuafo’ou. We note that there is sig-

nificant variation in the measured disequilibria within the

historic Niuafo’ou lavas, including different samples from

the 1946 eruption series. It is possible that some of the

U-series data may be affected by alteration and plagioclase

19.519.018.518.0

206Pb/204Pb

207 P

b/20

4 Pb

208 P

b/20

4 Pb

Samoa (Tau, shield)

Samoa (Upolu, shield)

87Sr/86Sr

143 N

d/14

4 Nd

206 P

b/20

4 Pb

0.7080.7070.7060.7050.7040.7030.5124

0.5126

0.5128

0.5130

0.5132

T

U

18.0

18.5

19.0

19.5

Samoa (Malumalu,shield)

Niuafo'ouCLSCKTJTonga-KermadecNELSCUo Mamae (Machias) SeamountSavaii (post-erosional)

37.5

38.0

38.5

39.0

39.5

40.0

M

15.45

15.50

15.55

15.60

15.65

(a)

(d)

(b)

(c)

LSC

M

M

U

U

T

T

LSC

LSC

LSC

X

YZ

X

Y

Z

X

Y

Z

X

Z

Fig. 5 Sr, Nd and Pb isotope

compositions of Lau Basin

lavas. a Sr-Pb, b, c Pb-Pb, d Sr-

Nd. Data from Samoa

(Workman et al. 2004), CLSC

(Peate et al. 2001), Louisville

Seamount Chain (Cheng et al.

1987) and Tonga Arc (Ewart

et al. 1998) shown for

comparison. Additional data for

lavas from the KTJ, NELSC and

Uo Mamae (Machias) Seamount

from Loock et al. (1990), Volpe

et al. (1988), Falloon et al.

(2007) and Pearce et al. (2007).

Also shown are representative

mixing curves between

subduction modified Lau mantle

(X) and ‘‘high 206Pb/204Pb

Samoan post-erosional’’ (Y) and

‘‘Uo Mamae-type Samoan post-

erosional’’ (Z) endmembers.

Tick marks at 20% intervals;

mixing endmember

compositions are listed in

Table 4

Table 4 Endmember compositions used in mixing calculations

X

(Subduction

modified Lau)

Z (Uo

Mamae

post-erosional)

Y

(High 206Pb/204Pb

post-erosional)

Pb (ppm) 1.60 4.95 1.50

Sr (ppm) 350 820 450

Nd (ppm) 20.7 57.8 35.087Sr/86Sr 0.7032 0.7054 0.7052143Nd/144Nd 0.5130 0.51257 0.5128206Pb/204Pb 18.340 18.500 18.830207Pb/204Pb 15.532 15.630 15.625208Pb/204Pb 38.300 38.700 39.000


123

accumulation (see below), and so we consider the inter-

pretation of these data as preliminary.

Discussion

In the following sections we examine the roles of crystal

fractionation and accumulation on the major and trace

element compositions of the lavas, and use major and trace

element and U-series data to constrain the dynamics of

mantle melting beneath Niuafo’ou (i.e. depth, upwelling

rate and residual porosity). We compare trace element and

radiogenic isotope data for the lavas from Niuafo’ou, the

KTJ and the NELSC, with published data for lavas from

the Tonga Arc and from spreading centres elsewhere in the

Lau Basin in order to examine the pattern and rate of

mantle flow beneath the northern Lau Basin, and develop a

tectonic model that may explain the composition and

location of intraplate magmatism in this area.

Fractional crystallisation and crystal accumulation

Basalts from the Lau Basin exhibit a broad compositional

range which, within individual segments, such as the eastern

and central Lau spreading centres (ELSC and CLSC), can be

largely ascribed to fractional crystallisation (Pearce et al.

1995, 2001). Differences in the trace element and isotopic

compositions between ELSC and CLSC lavas are attributed

to variations in the contribution from remnant subduction

components (Pearce et al. 1995, 2001). The major element

variation in Lau Basin spreading centre basalts (Fig. 3) can

be accounted for by fractional crystallisation of a low-

pressure crystal assemblage composed of olivine ± clino-

pyroxene ± plagioclase (Pearce et al. 1995). In contrast,

most of the Niuafo’ou lavas have a very restricted range in

major element composition. The unusual trends of

increasing CaO, Al2O3 and decreasing Na2O and FeO with

decreasing MgO observed in historic Niuafo’ou lavas

(Fig. 3) can be explained by variable plagioclase accumu-

lation. Accumulation of approximately 15% plagioclase can

account for the variation in CaO and Al2O3 with MgO in

Fig. 3, and explains qualitatively the increase in Sr, and the

decrease in the concentration of Zr and other incompatible

elements with decreasing MgO observed in the historic

Niuafo’ou lavas. Thus, although the relatively low Ni con-

centrations and Mg values of these basalts indicates that all

samples have undergone crystal fractionation of the

observed phenocryst phases (olivine, plagioclase, clino-

pyroxene, magnetite), much of the chemical variation

within our sample suite, which is dominated by historic and

Holocene lavas (and are therefore of similar age), results

from variable plagioclase accumulation.

Partial melting processes beneath the northern Lau

Basin

Although all of the Niuafo’ou lavas have undergone low-

pressure crystal fractionation and plagioclase accumulation

and do not represent primary mantle melts, some constraints

can be placed on the degree and depth of melting. The least

evolved Niuafo’ou lavas, which have undergone minimal

plagioclase accumulation, tend to have lower SiO2, and

higher TiO2 (Fig. 3) and Na2O at a given MgO compared to

basalts from the Central and Eastern Lau spreading centres,

suggesting that the former represent smaller degree mantle

melts from beneath thicker lithosphere. Lavas from

Niuafo’ou have slightly lower heavy rare-earth element

concentrations, and higher Tb/Yb ratios than the least

evolved CLSC lavas and average MORB, indicating that

their parental magmas were generated mainly within the

stability field of spinel, but probably at slightly greater

average depth than the primary melts of the CLSC basalts.

U-series isotopes are sensitive to the physical processes

of partial melting such as the upwelling rate and porosity in

the melting zone (see Bourdon et al. 2003 for a recent

review) as well as the relevant partition coefficients which

vary with pressure, especially in the case of clinopyroxene

(e.g. Blundy and Wood 2003). On Fig. 6 we compare the

Niuafo’ou and Lau U-series data with the results of simple

dynamic melting models that are intended to be illustrative

only. The results show that partial melting between about 2

and 3 GPa can broadly reproduce the 238U-230Th-226Ra and231Pa-235U disequilibria in the Niuafo’ou basalts (Fig. 6).

There is a hint of a negative slope in the data in Fig. 6b, c

and, if this were confirmed with further data, then our

preliminary modelling suggests that these might reflect

extraction of melts from different depths within the melting

column (see Fig. 6). Assuming that decompression is the

dominant cause of melting, Fig. 6b, c also broadly con-

strain the upwelling velocity beneath Niuafo’ou to about

1 cm/year and the porosity of the melting region to

*0.002. As noted earlier, a number of the Lau basalts have

similar (226Ra/230Th) ratios to those of the historic Ni-

uafo’ou basalts suggesting that these may be relatively

primary melt signatures; those samples with lower

(226Ra/230Th) ratios probably reflect an unknown amount

of 226Ra decay since eruption (Fig. 6c).

At pressures of *1 GPa or less, Th is more compatible

than U in clinopyroxene and so melting at these depths

produces 238U excesses (Fig. 6a) and a number of the Lau

Basin basalts and one sample from Niuafo’ou have 238U

excesses. However, varying the pressure of melting and/or

the upwelling rate produces vertical displacements on the

equiline diagram (Fig. 6a) whereas varying the porosity has

little effect on 238U-230Th disequilibria. Thus, these samples

must have formed from sources with higher U/Th ratios


123

probably reflecting the effects of fluid addition of U from

the retreating Tonga subduction system to the east. Peate

et al. (2001) observed that there is a systematic spatial

variation with U/Th ratios and H2O contents increasing in

samples nearer to the arc and the closest samples from the

Valu Fa ridge have significant 238U excesses. Many of the

Lau basalts lie around a horizontal trend, which extends

from the 2–3 GPa model melts and Niuafo’ou basalts

towards the Valu Fa basalts. Accordingly, we suggest that

most of the Lau basalts also formed by melting between 2

and 3 GPa of sources variably enriched in U by prior fluid

addition (Fig. 6a). This evidence for a quite deep onset of

melting is consistent with recent suggestions that the

potential temperature of the mantle beneath the Lau Basin

may be as high as 1,449�C (Wiens et al. 2006).

Influence of the Tonga subduction zone

In order to be able to use the trace element and Pb and Sr

isotope compositions of Lau Basin lavas to examine mantle

dynamics and mixing processes in the upper mantle in the

region, the influence of subduction on the compositions of

these lavas must first be assessed. Ce/Pb (11.0–17.3) and

Nb/U (31.3–41.7) ratios of Niuafo’ou lavas extend to lower

values than those of MORB and OIB (most of which have

Ce/Pb and Nb/U ratios of 25 ± 5 and 45 ± 10, respec-

tively), indicating that fluid-soluble elements such as Pb and

U in the mantle beneath Niuafo’ou are inherited partly from

fluids derived from the subducting Pacific Plate. Glasses

from the CLSC, which is far removed from the active arc,

have a very similar range in Ce/Pb and Nb/U (11–17 and

39–44, respectively) to the Niuafo’ou lavas (Fig. 7), sug-

gesting that a subduction component has been widely

dispersed throughout the Lau Basin as a result of back-arc

rifting. Glasses from the KTJ and NELSC, which are situ-

ated closer to the Tonga Arc, have lower Ce/Pb and Nb/U

ratios (10.2–12.1 and 8.4–29.5, respectively, Fig. 7), than

CLSC lavas (Sun et al. 2003; Falloon et al. 2007), which

may reflect more recent contamination by Pb- and U-bear-

ing subduction-derived fluids from the active Tonga arc.

2 GPa

0.9

1.0

1.1

1.2

1.3

1.4

0.8 0.9 1.0 1.1 1.2 1.3 1.4

1.0

1.5

2.0

2.5

3.0

3.5

4.0

(238U/232Th)

(230

Th/

232T

h)

(230Th/238U)

(231

Pa/

235U

)

0.8 0.9 1.0 1.1 1.2 1.3 1.4

(230Th/238U)

1.8

2.0

2.2

2.4

2.6

2.8

0.8 0.9 1.0 1.1 1.2 1.3 1.4

(226

Ra/

230T

h)

equil

ine

V alu Fa

1 GPa

1.5 GPa

3 GPa

W=0.5

W=2

2 GPa

1 GPa

1.5 GPa

3 GPa

W=0.5

W=2

φ =0.003

φ =0.004

fluid addition

2 GPa

1 GPa

1.5 GPa

3 GPa

W=0.5

W=2

φ =0.003

φ =0.004

226

Ra-

deca

y

(a)

(b)

(c)

Fig. 6 a U-Th equiline diagram, b (231Pa/235U) versus (230Th/238U)

and c (226Ra/230Th) versus (230Th/238U) showing Niuafo’ou data from

Table 3 and Lau Basin and Valu Fa data from Peate et al. (2001).

Also shown are the results of dynamic melting models simulating

melting across a range of pressures. We assumed that melting did not

extend far beyond the spinel-garnet transition at 2.8 GPa, and that the

lower limit of melting was GPa based on the likely thickness

(*30 km) of the lithosphere beneath Niuafo’ou (Wiens et al. 2006;

Wiens pers. comm. 2006). The models assumed 5% total melting over

a 20 km long melt column (at variable absolute depths) and were

based on the equations of Williams and Gill (1989) using the

following input parameters (unless otherwise indicated): qs = 3,300

kg/m3, qf = 2,800 kg/m3, W = 1 cm/year, / = 0.002. The source

was assumed to be in secular equilibrium with a (238U/232Th) ratio of

0.95 and the modes and partition coefficients used are given in

Table 5 (the 2 and 3 GPa models assumed 4 and 8% residual garnet,

respectively). Dashed lines illustrate the effect of varying the

upwelling velocity at 2 GPa and the porosity at 1.5 GPa (see text

for discussion)

b


123

Assuming an initial mantle composition with Ce/Pb =

25, the proportion of slab-derived Pb in the sources of these

lavas can be estimated at between approximately 31 and

56% (CLSC, Niuafo’ou) and 48 to 62% (KTJ, NELSC, see

Fig. 7b). This ‘‘excess’’ Pb may be derived from either

subducted sediment or subducted Pacific oceanic crust.

However, the Pb isotope heterogeneity within northern Lau

Basin lavas is unlikely to be the sole result of subduction

contamination, because lavas from the CLSC and

Niuafo’ou (and also from the KTJ and NELSC) have similar

Ce/Pb ratios yet very different Pb isotope compositions,

which are unlike that of material subducted at the Tonga

Trench. Instead, we argue below that the higher 207Pb/204Pb

and 208Pb/204Pb ratios of northern Lau Basin lavas result

from the presence of Samoan mantle beneath the region.

The Ce/Pb ratios of Uo Mamae (Machias) Seamount

lavas overlap with those of lavas from the main Samoan

Islands (Fig. 7). The former therefore do not appear to

contain a subduction contribution, as would be expected if

mantle from beneath the northern edge of the Indian-

Australian Plate was able to flow northwards across the

Vitiaz Lineament through the torn Pacific Plate at depth.

Influence of Samoa on compositions of lavas

from the Lau Basin

Sr, Nd and Pb isotope constraints

Several previous geochemical studies of basalts from the

northern Lau Basin and Tonga Arc have found evidence

that Samoan mantle has leaked southward beneath this

region through the tear in the subducting Pacific Plate

(Danyushevsky et al. 1995; Wendt et al. 1997; Turner and

Hawkesworth 1998; Ewart et al. 1998; Falloon et al. 2007;

Pearce et al. 2007). Wendt et al. (1997) suggested that

lavas from both the northern Lau Basin and Tonga Arc are

derived from a mantle source containing a contribution

from Samoan mantle. Turner and Hawkesworth (1998)

suggested that the Samoan mantle has infiltrated the

northwestern Lau Basin but did not play a role further east

(including Niuafo’ou) or in the northern Tonga arc, partly

Table 5 Melting parameters

Partition coefficients based on

Blundy and Wood (2003),

Landwehr et al. (2001),

McDade et al. (2003)

Mode DU DTh DRa DPa

Olivine 60 6.00E-05 9.52E-06 5.75E-08 6.00E-08

Orthopyroxene 20 7.70E-04 1.70E-03 6.00E-07 7.70E-07

Clinopyroxene (1 Ga) 12–20 2.25E-02 2.30E-02 4.13E-06 2.25E-09

Clinopyroxene (1.5 Ga) 2.16E-02 2.00E-02

Clinopyroxene (2 Ga) 1.74E-02 1.50E-02

Clinopyroxene (3 Ga) 1.22E-02 9.00E-03

Garnet 0–8 1.65E-02 3.30E-03 7.00E-09 5.80E-04

Niua fo'ouCLSCSamoa PETonga-Kermadec ArcKTJNELSCUo Mamae

208Pb/204Pb

143 N

d/14

4 Nd

Ce/

Pb

50%

75%

90%

0.5124

0.5126

0.5128

0.5130

0.5132

Tau

Malumalu

Upolu

40.039.539.038.538.037.50

10

20

30

40

50

TauUpolu

(a)

(b)

0%

Y

Z

Fig. 7 Variation of 208Pb/204Pb with a 143Nd/144Nd and b Ce/Pb for

Niuafo’ou and other Lau Basin lavas. All samples have lower Ce/Pb

than average MORB, reflecting presence of a widespread subduction

component. Ce/Pb ratios generally decrease towards the active Tonga

Arc. Approximate % of slab-derived Pb calculated assuming a Ce/Pb

ratio of 25 for uncontaminated mantle. Niuafo’ou lavas have Nd-Pb

isotope compositions which lie close to mixing lines between basalts

from the CLSC and from Samoa (compare with Fig. 5 of Turner and

Hawkesworth, 1998). Data are from Ewart et al. (1998), Peate et al.

(2001), Sun et al. (2003), Workman et al. (2004), and this study.

Mixing endmember compositions used in calculating mixing lines in

(a) listed in Table 4


123

on the basis of Pb-Nd isotope systematics. However, the

new double-spike Pb isotope data show that the Niuafo’ou

samples do lie on potential mixing lines between the Lau

Basin and Samoan lavas in the Pb-Nd isotope space

(Fig. 7a). Such mixing lines are expected to be approxi-

mately linear if both the endmembers have similar Pb/Ce

and Pb/Nd ratios (in contrast to Turner and Hawkesworth’s

Fig. 2). Since the northern Lau Basin lavas contain up to

75% subduction-derived Pb (Fig. 7b), the Pb isotope

composition of the Lau Basin mantle ‘‘endmember’’ is

likely to represent a mixture of Indian MORB mantle with

subducted, altered Pacific MORB crust and sedimentary Pb

and to have lower Ce/Pb ratios, in which case mixing lines

will be curved in the opposite sense.

In detail, lavas from the Samoan islands display a large

range in Sr, Nd and Pb isotope compositions (Workman

et al. 2004; Jackson et al. (2007), see Fig. 5a–d). Falloon

et al. (2007) argued that Niuafo’ou lavas reflect mixing of

Lau Basin mantle with a component similar in composition

to that of Uo Mamae (Machias) Seamount lavas, and this is

supported by our new data. Lavas from Uo Mamae Se-

amount have low 143Nd/144Nd for a given 87Sr/86Sr, and

high 207Pb/204Pb for a given 207Pb/204Pb, compared to lavas

from the main Samoan Islands (Fig. 5a–d). The Uo Mamae

‘‘endmember’’ also apparently contributes to post-erosional

magmatism on Savaii and Upolu Islands; this is most

clearly shown by the unusual positive correlation between87Sr/86Sr and 143Nd/144Nd for a subset (mostly from Upolu)

of the post-erosional lavas (Fig. 5d). It is unclear whether

the Uo Mamae component is present beneath much of the

Samoan island chain, but is only sampled during post-

erosional magmatism and at Uo Mamae, or whether Uo

Mamae-type mantle has recently flowed northwards

beneath Savaii/Upolu. However, the ultimate origin of the

Uo Mamae component is unimportant for purposes of this

study. The presence of this component in the Niuafo’ou

lavas indicates southward flow of the mantle beneath the

Vitiaz Lineament through a tear in the subducting Pacific

Plate. The age of Uo Mamae Seamount is not well con-

strained. Although a phonolite dredged from the summit of

this volcano yielded an age of 0.94 Ma (Hawkins and

Natland 1975), it is possible this young magmatism results

from reactivation as the volcano is torn apart during sub-

duction, and the age of the Uo Mamae sample analysed in

this study is unknown.

The Sr, Nd and Pb isotope compositions of Niuafo’ou

lavas can be explained by mixing of the Lau mantle (con-

taining a contribution from subducted materials, as required

by the low Ce/Pb of Niuafo’ou lavas), with the mantle

similar to that feeding post-erosional magmatism on Savaii

(situated on the Pacific Plate approximately 390 km to the

northeast) and having a contribution from the Uo Mamae

‘‘component’’ (Fig. 5). The Pb isotope compositions of

Niuafo’ou lavas are difficult to explain by subduction of

oceanic crust or sediments, as none of these have suffi-

ciently low 206Pb/204Pb at high 207Pb/204Pb.

The Uo Mamae component is apparently not present in

lavas from the nearby NELSC and KTJ. As noted by Fal-

loon et al. (2007), lavas from the King’s Triple Junction

have less radiogenic Sr, but significantly more radiogenic

Pb isotope compositions than lavas from Niuafo’ou

(Fig. 5b, c). This observation could be explained by mixing

of subduction-modified Lau mantle with a Samoan com-

ponent with more radiogenic Pb. Calculated mixing lines

between subduction-modified Lau mantle and high206Pb/204Pb post-erosional lavas from Upolu and Savaii

provide a close fit to the KTJ data in Fig. 5. The NELSC

lavas have more radiogenic Pb compositions, but relatively

low 87Sr/86Sr values which might be explained by mixing

with a high 206Pb/204Pb, low 87Sr/86Sr Samoan mantle such

as underlies Tau (Fig. 5). Falloon et al. (2007) suggested

that the radiogenic Pb compositions of NELSC lavas and

some Tongan boninites might be explained by mixing with

mantle derived from the Cook-Austral Seamount Chain.

Another possible explanation for the high 206Pb/204Pb

ratios of the NELSC lavas is that they contain material

from the Louisville Seamount Chain (LSC). The subducted

portion of this seamount chain contributes Pb (via a fluid

phase) to the source of lavas from the nearby Tongan

islands of Tafahi and Niuatoputapu (Wendt et al. 1997;

Turner and Hawkesworth 1998). The NELSC lavas have

isotope compositions, which lie close to the field for LSC

lavas in Fig. 5. If a Louisville component does contribute

to NELSC lavas, it is most likely in the form of ‘‘remnant’’

LSC mantle, which has flowed southwards from beneath

the Pacific Plate (rather than from fluids derived from the

subducted part of the LSC). This is because NELSC lavas

have 143Nd/144Nd values, which are lower than those of

northern Tonga lavas, and overlap with those of LSC lavas,

and Nd is much less soluble than Pb in aqueous fluids.

In summary, Sr, Nd and Pb isotope data indicate that

Samoan mantle of several different ‘‘flavours’’, and possi-

bly also Louisville-type mantle from beneath the Pacific

Plate, appear to contribute to magmatism in the northern

Lau Basin. This is further supported by the trace element

systematics of these samples, as discussed below.

High-field strength element constraints

Since the Pb and Sr budgets of most northern Lau Basin

lavas contain a contribution from subducted materials (see

above), it is not straightforward to compare the Pb and Sr

isotope compositions of lavas with different Pb/Ce ratios to

identify the presence of Samoan-type mantle. The high

field strength elements provide another way to do this,


123

because these elements are insoluble in fluids and are

therefore insensitive to subduction fluid input. For instance,

both Zr and Nb are insoluble in fluids, so that assuming

broadly similar degrees of melting; Nb/Zr ratios of arc and

back-arc lavas can be compared directly. Shield and post-

erosional lavas from Samoa have similar Nb/Zr ratios

(Fig. 8), suggesting that the effect of variations in the

degree of melting on the Nb/Zr of melts is minor.

Nb/Zr ratios of lavas from Niuafo’ou, the KTJ, NELSC

and from volcanoes of the Tonga Arc, increase systemati-

cally with decreasing distance from the Samoan Islands and

Uo Mamae (Fig. 8), suggesting that more enriched, high

Nb/Zr mantle underlies the northern Lau Basin and Tonga

Arc. The Nb/Zr variation is consistent with the presence of

Samoan mantle beneath the northern Lau Basin and Tonga

Arc, extending 400–500 km southwards and southwest-

wards from the Samoan Islands. The systematic geographic

variation in Nb/Zr supports the interpretation of Wendt et al.

(1997) that southward flow of Samoan-type mantle is

responsible for the relatively high Nb/Zr ratios of lavas

from the northernmost Tongan islands of Tafahi and Niu-

atoputapu. Lavas from these islands have among the lowest

concentrations of highly incompatible elements (e.g. Th) of

all Tonga-Kermadec lavas, indicating derivation from a

depleted source, which would therefore be expected to have

low Nb/Zr ratios. Another possible source of high Nb/Zr in

northern Tonga lavas is from the subducted Louisville

Seamount Chain (Turner and Hawkesworth 1998). How-

ever, elevated Nb/Zr in Fonualei lavas (Fig. 8) is not

accompanied by Louisville Seamount Chain Pb (Ewart

et al. 1998), suggesting that a Samoan origin of the high Nb/

Zr ratios of northern Tonga arc lavas is more likely.

Several recent studies have reported Hf isotope data for

Tonga-Lau lavas including two of the Niuafo’ou lavas from

this study (Hergt and Woodhead 2007; Pearce et al. 2007).

Since Hf is also insoluble in aqueous fluids, Hf isotopes

should provide a useful tracer of Samoan mantle beneath the

northern Tonga Arc and Lau Basin, which is less sensitive to

subduction contamination. Niuafo’ou, KTJ and NELSC

lavas have relatively unradiogenic Nd and Hf isotope

compositions, consistent with a contribution from Samoan

mantle (Pearce et al. 2007). However, lavas from the North

Tongan islands of Tafahi and Niutoputapu have more

radiogenic Hf than any lavas from the Lau Basin or from

central and southern Tonga, which is not consistent with the

mantle beneath the northern Tonga islands containing a

Samoan contribution. This might be a consequence of a

greater proportion of DUPAL-like mantle beneath northern

Tonga (Pearce et al. 2007). Furthermore, lavas from Tafahi

have low 3He/4He ratios of R/Ra *3.7 (Turner and van

Soest, unpublished data) that do not demand a Samoan

plume input, unlike the high values in the northwestern Lau

Basin (Turner and Hawkesworth, 1998). On the other hand,

the He budget of arc lavas is likely to be dominated by a low3He/4He component from the subducted slab.

The southward extent of Samoan plume mantle inferred

from Fig. 8 can also be used to constrain mantle-flow

velocities, if the time of removal of the barrier to mantle

flow between the Pacific and Indian Plates at the Vitiaz

Lineament can be estimated. Figure 8 shows that Samoan

type mantle extends 350–400 km southwards of the Paci-

fic-Indian plate boundary. A maximum age of Pacific Plate

tearing is given by the onset of active spreading in the Lau

Basin at 4 Ma (Taylor et al. 1996). However, at 4 Ma the

northern termination of the Tonga Trench was situated

*1,200 km west of active Samoan magmatism (Hart et al.

2004), and so flow of Samoan-type mantle beneath the

northern Lau Basin at that time is unlikely. A more likely

maximum age for penetration of Pacific mantle beneath the

northern Lau Basin is 1 Ma, which implies a southward

mantle flow rate of[4 cm/year consistent with the estimate

of Turner and Hawkesworth (1998), though those authors

did not have the extent of Samoan influence extending as

far east as has been recognised in this study. If, as proposed

by Pearce et al. (2007), the southward gradient in Samoan

mantle influence is a result of continuous melt extraction

during mantle flow, this estimate of flow rate represents a

lower limit. Condor and Wiens (2007) inferred along-strike

upper mantle flow velocities of up to 50 cm/year beneath

the northern Tonga Arc from measurements of seismic

0.3

0.01

0.1

0 400 800 1200 1600 2000 2400

Nb/

Zr

Distance from Savaii (km)

1

2 3

4

5

6

7

810

9

11

12

13

CLSCand ELSC

Fig. 8 Variation of Nb/Zr with approximate distance from the

Samoan Islands for young lavas from the Lau Basin and Tonga

Arc. Numbers refer to sample locations: 1 Samoa (post-erosional), 2Samoa (shield), 3 Uo Mamae Seamount, 4 NELSC, 5 Tafahi, 6 KTJ, 7Niuafo’ou, 8 Fonualei, 9 Late, 10 Hunga Ha’apai, 11 Raoul, 12Macauley, 13 L’Esperance. Data are from Pearce et al. (1995, 2007),

Ewart et al. (1998), Peate et al. (2001), Workman et al. (2004),

Falloon et al. (2007), and this study. Error bars represent 1 s.d. of the

mean


123

anisotropy, which would imply a minimum age of 80 ka

for the time at which Samoan-type mantle was first able to

flow southwards beneath the Vitiaz Lineament.

A tectonic model for intraplate magmatism

in the northern Lau Basin

Decompression melting of subduction-modified mantle at

active spreading centres in the Lau Basin (e.g. KTJ, CLSC,

ELSC and Valu Fa Ridge) can explain the major and trace

element and isotope compositions of these lavas (Peate et al.

2001; Pearce 1995). However, this process does not account

for the presence of magmatism remote from areas of active

rifting. Intraplate magmatism occurs not only at Niuafo’ou,

but also at other locations in the northern Lau Basin, notably

at 15�20’S, 174�W, where young silicic lavas have been

recovered from a large submarine caldera (Falloon et al.

2007). Nevertheless, as discussed above, the Niuafo’ou

U-series isotope data are consistent with melting, between 2

and 3 GPa, of mantle that is upwelling at *1 cm/year.

Thus, decompression appears to be the principle cause of

melting rather than the presence of mantle of unusual (e.g.

volatile-rich) composition, particularly because the

Niuafo’ou lavas have similar Ce/Pb to the KTJ lavas. The

reasons for melting beneath the Niuafo’ou microplate, away

from any clear zone of rifting, may in part reflect the ele-

vated mantle potential temperature inferred for the Lau

Basin (Wiens et al. 2006). Boninites from the northern Lau

Basin and Tonga Arc also indicate high temperatures of

melting, possibly related to inflow of Samoan mantle

(Sobolev and Danyushevsky 1994).

Both geochemical (Wendt et al. 1997; Turner and Haw-

kesworth 1998, and this study) and seismological (Smith

et al. 2001) data have been used to infer southward flow of

mantle beneath the northern Lau Basin and Tonga Arc. In

the northern Lau Basin, relatively young oceanic crust is

juxtaposed against much older Pacific oceanic crust north of

the Vitiaz Lineament. Tearing of the Pacific slab allows

mantle to flow southwards from beneath old (110–120 Ma),

relatively thick Pacific lithosphere, to beneath much

younger (\5 Ma), thinner lithosphere in the northern Lau

Basin, as shown by the widespread occurrence of Samoan

mantle in this area. We suggest that recent magmatism on

Niuafo’ou may at least partly result from decompression

melting as a result of southward flow of mantle from beneath

thicker to thinner oceanic lithosphere (e.g. Demidjuk et al.

2007). We predict that diffuse intraplate magmatism may

also occur in other back-arc basins, for example the South

Scotia Sea back-arc behind the South Sandwich island

arc (Livermore 2003; Leat et al. 2004) where mantle is able

to flow beneath a plate boundary that separates oceanic

lithosphere of very different age and thickness.

Acknowledgments We thank Tony Reay and Tony Ewart for pro-

viding some of the samples on which this research was based, and

Immo Wendt, Yaoling Niu and Alan Greig for their help with the

trace element analyses and isotope measurements (University of

Queensland). Matthew Thirlwall and Christina Manning are thanked

for help with the isotope measurements at Royal Holloway. Terry

Plank kindly performed the trace element analyses of the 314-series

samples. We thank the three reviewers for their helpful comments.

This study used instrumentation funded by ARC LIEF and DEST

Systemic Infrastructure Grants, Macquarie University and Industry.

Simon Turner is funded by an ARC Federation Fellowship and this is

GEMOC publication #503.

References

Blundy J, Wood B (2003) Mineral-melt partitioning of uranium,

thorium and their daughters. Rev Mineral Geochem 52:59–123

Bourdon B, Henderson G, Lundstron CC, Turner SP (2003) Uranium-

series geochemistry. Rev Mineral Geochem 52:1–656

Chase C (1971) Tectonic history of the Fiji Plateau. Geol Soc Am

Bull 82:3087–3110

Cheng Q, Park K-H, Macdougall JD, Zindler A, Lugmair GW,

Staudigel H, Hawkins J, Lonsdale P (1987) Isotopic evidence for

a hotspot origin of the Louisville Seamount Chain. Am Geophy

Union Geophys Monogr 43:283–296

Conder JA, Wiens DA (2007) Rapid mantle flow beneath the Tonga

volcanic arc. Earth Planet Sci Lett 264:299–307

Danyushevsky LV, Sobolev AV, Falloon TJ (1995) North Tongan

high-Ca boninite petrogenesis: the role of Samoan plume

and subduction zone––transform fault transition. J Geodyn

20:219–241

Demidjuk Z, Turner S, Sandiford M, George R, Foden J, Etheridge M

(2007) U-series isotope and geodynamic constraints on mantle

melting processes beneath the Newer Volcanic Province in South

Australia. Earth Planet Sci Lett 261:517–533

Ewart A (1976) A petrological study of the younger Tongan andesites

and dacites, and the olivine tholeiites of Niuafo’ou island, S.W.

Pacific. Contrib Mineral Petrol 58:1–21

Ewart A, Hawkesworth CJ (1987) The pleistocene-recent Tonga-

Kermadec arc lavas: interpretation of new isotopic and rare earth

data in terms of a depleted mantle source model. J Petrol

28:495–530

Ewart A, Bryan WB, Chappell BW, Rudnick RL (1994) Regional

geochemistry of the Lau-Tonga arc and back-arc systems. Proc

ODP Sci Res 135:385–425

Ewart A, Collerson KD, Regelous M, Wendt JI, Niu Y (1998)

Geochemical evolution within the Tonga-Kermadec-Lau arc-

back-arc systems: the role of varying mantle wedge composition

in space and time. J Petrol 39:331–368

Falloon TJ, Malahoff A, Zorenshain LP, Bogdanov Y (1992)

Petrology and geochemistry of back-arc basin basalt from Lau

Basin spreading ridges at 15�, 18� and 19�S. Mineral Petrol

47:1–35

Falloon TJ, Danyushevsky LV, Crawford TJ, Maas R, Woodhead JD,

Eggins SM, Bloomer SH, Wright DJ, Zlobin SK, Stacey AR

(2007) Multiple mantle plume components involved in the

petrogenesis of subduction-related lavas from the northern

termination of the Tonga Arc and Lau Basin: evidence from

the geochemistry of arc and back-arc submarine volcanics.

Geophys Geochem Geosys 8:Q09003

Hart SR, Coetzee M, Workman RK, Blusztajn J, Johnson KTM,

Sinton JM, Steinberger B, Hawkins JW (2004) Genesis of the

Western Samoa seamount province: age, geochemical fingerprint

and tectonics. Earth Planet Sci Lett 227:37–56


123

Hawkins JW (1995) The geology of the Lau Basin. In: Taylor B (ed)

Backarc basins: tectonics and magmatism. Plenium, New York,

pp 63–138

Hawkins JW, Natland JH (1975) Nephelinites and basanites of the

Samoan volcanic chain––their possible tectonic significance.

Earth Planet Sci Lett 24:427–439

Hergt JM, Woodhead JD (2007) A critical evaluation of recent

models for Lau-Tonga arc-backarc basin magmatic evolution.

Chem Geol 245:9–44

Hofmann AW (1988) Chemical differentiation of the Earth: the

relationship between mantle, crust, and oceanic crust. Earth

Planet Sci Lett 90:297–314

Jackson MG, Hart SR, Koppers AAP, Staudigel H, Konter J,

Blusztajn J, Kurz M, Russell JA (2007) The return of subducted

continental crust in Samoan lavas. Nature 448:684–687

Jaggar TA (1931) Geology and geography of Niuafo’ou volcano.

Volcano Lett 318:1–3

Kelley KA, Plank T, Ludden J, Staudigel H (2003) Composition of

altered oceanic crust at ODP sites 801 and 1149. Geochem

Geophys Geosys 4. doi:10.1029/2002GC000435

Kelley KA, Plank T, Grove TL, Stolper EM, Newman S, Hauri E

(2006) Mantle melting as a function of water content beneath

back-arc basins. J Geophys Res 111:B09208

Landwehr D, Blundy J, Chamorro-Perez EM, Hill E., Wood BJ

(2001) U-series disequilibria generated by partial melting of

spinel lherzolite. Earth Planet Sci Lett 188:329–348

Langmuir CH, Bezos A, Escrig S, Parman SW (2006) Chemical

systematics and hydrous melting of the mantle in back-arc

basins. Am Geophys Monogr 166:87–146

Leat PT, Pearce JA, Barker PF, Millar IL, Barry TL, Larter RD (2004)

Magma genesis and mantle flow at a subducting slab edge: the

South Sandwich arc-basin system. Earth Planet Sci Lett

227:17–35

Livermore R (2003) Back-arc spreading and mantle flow in the east

Scotia Sea. Geol Soc Lond Spec Publ 219:315–331

Loock G, McDonough WF, Goldstein SL, Hofmann AW (1990)

Isotopic compositions of volcanic glasses from the Lau Basin.

Mar Min 9:235–245

McDade P, Blundy JD, Wood BJ (2003) Trace element partitioning

on the Tinaquillo lherzolite solidus at 1.5 GPa. Phys Earth Planet

Int 139:129–147

McDonald GA (1948) Notes on Niuafo’ou. Am J Sci 246:65–77

Millen DW, Hamburger MW (1998) Seismological evidence for

tearing of the Pacific plate at the northern termination of the

Tonga subduction zone. Geology 26:659–662

Niu Y, Batiza R (1997) Trace element evidence from seamounts for

recycled oceanic crust in the eastern Pacific mantle. Earth Planet

Sci Lett 148:471–483

Pearce JA, Stern RJ (2006) Origin of back-arc basin magmas: trace

element and isotope perspectives. Am Geophys Monogr

166:63–86

Pearce JA, Ernewein M, Bloomer SH, Parson LM, Murton BJ,

Johnson LE (1995) Geochemistry of Lau Basin volcanic rocks:

influence of ridge segmentation and arc proximity. Geol Soc

Lond Spec Publ 81:53–76

Pearce JA, Kempton PD, Gill JB (2007) Hf-Nd evidence for the origin

and distribution of mantle domains in the SW Pacific. Earth


Peate DW, Kokfelt TF, Hawkesworth CJ, van Calsteren PW, Hergt

JM, Pearce JA (2001) U-series isotope data on Lau Basin

glasses: the role of subduction related fluids during melt

generation in back-arc basins. J Petrol 42:1449–1470

Reay A, Rooke JM, Wallace RC, Whelan P (1974) Lavas from

Niuafo’ou Island, Tonga, resemble ocean-floor basalts. Geology

2:605–606

Regelous M, Collerson KD, Ewart A, Wendt JI (1997) Trace element

transport rates in subduction zones: evidence from Th, Sr and Pb

isotope data for Tonga-Kermadec arc lavas. Earth Planet Sci Lett

150:291–302

Regelous M, Turner S, Elliott TR, Rostami K, Hawkesworth CJ

(2004) Measurement of femtogram quantities of protactinium in

silicate rock samples by multicollector inductively coupled

plasma mass spectrometry. Anal Chem 76:3584–3589

Smith GP, Weins DA, Fischer KM, Dorman LM, Webb SC,

Hildebrand JA (2001) A complex pattern of mantle flow in the

Lau backarc. Science 292:713–716

Sobolev AV, Danyushevsky LV (1994) Petrology and geochemistry

of boninites from the northern termination of the Tonga Trench:

constraints on the general conditions of primary high-Ca

boninite magmas. J Petrol 35:1183–1211

Sun S-s, McDonough WF (1989) Chemical and isotopic systematics

of oceanic basalts: implications for mantle composition and

processes. Geol Soc Spec Publ 42:313–345

Sun W, Bennett VC, Eggins SM, Arculus RJ, Perfit MR (2003)

Rhenium systematics in submarine MORB and back-arc basin

glasses: laser ablation ICPMS results. Chem Geol 196:259–281

Taylor PW (1991) The geology and petrology of Niuafo’ou island,

Tonga: subaerial volcanism in an active back-arc basin. PhD

Thesis, Macquarie University, pp. 150

Taylor B, Zellmer K, Martinez F, Goodliffe A (1996) Sea-floor

spreading in the Lau back-arc basin. Earth Planet Sci Lett

144:35–40

Taylor B, Martinez F (2003) Back-arc basin basalt systematics. Earth


Thirlwall MF, Anczkiewitz R (2004) Multidynamic isotope ratio

analysis using MC-ICP-MS and the causes of secular drift in Hf,

Nd and Pb isotope ratios. Int J Mass Spectrom 235:59–81

Thirlwall MF, Gee MAM, Taylor RN, Murton BJ (2004) Mantle

components in Iceland and adjacent ridges investigated using

double-spike Pb isotope ratios. Geochim Cosmochim Acta

68:361–386

Turner S, Hawkesworth CJ (1998) Using geochemistry to map mantle

flow beneath the Lau Basin. Geology 26:1019–1022

Turner S, Hawkesworth CJ, Rogers N, Bartlett J, Worthington T,

Hergt J, Pearce JA, Smith I (1997) 238U-230Th disequilibria,

magma petrogenesis and flux rates beneath the depleted Tonga-

Kermadec island arc. Geochim Cosmochim Acta 61:4855–4884

Turner S, Bourdon B, Hawkesworth CJ, Evans P (2000) 226Ra-230Th

evidence for multiple dehydration events, rapid melt ascent and

the time scales of differentiation beneath the Tonga-Kermadec

island arc. Earth Planet Sci Lett 179:581–593

Volpe AM, Macdougall JD, Hawkins J (1988) Lau Basin basalts (LBB):

trace element and Sr-Nd isotopic evidence for heterogeneity in

back-arc basin mantle. Earth Planet Sci Lett 90:174–186

Wendt JI, Regelous M, Collerson KD, Ewart A (1997) Evidence for a

contribution from two mantle plumes to island-arc lavas from

northern Tonga. Geology 25:611–614

Wiens DA, Kelley KA, Plank T (2006) Mantle temperature variations

beneath back-arc spreading centres inferred from seismology,

petrology and bathymetry. Earth Planet Sci Lett 248:30–42

Williams RW, Gill JB (1989) Effects of partial melting on the

uranium decay series. Geochim Cosmochim Acta 53:1607–1619

Workman RK, Hart SR, Jackson M, Regelous M, Farley KA,

Bluszatajn J, Kurz M, Staudigel H (2004) Recycled metasoma-

tised lithosphere as the origin of the enriched mantle II (EM2)

end-member: evidence from the Samoan Volcanic Chain.

Geochem Geophys Geosys 5:Q04008

Zellmer KE, Taylor B (2001) A three-plate kinematic model for Lau

Basin opening. Geochem Geophys Geosys 2. doi:2000GC000106


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