Petrogenesis of Boninites from the Betts Cove Ophiolite

JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 PAGES 1853–1889 1999

Petrogenesis of Boninites from the Betts CoveOphiolite, Newfoundland, Canada:Identification of Subducted SourceComponents

J. H. BEDARD∗GEOLOGICAL SURVEY OF CANADA, CENTRE GEOSCIENTIFIQUE DE QUEBEC, CP 7500, STE-FOY, QUE.,

CANADA, G1V 4C7

RECEIVED MARCH 9, 1998; REVISED TYPESCRIPT ACCEPTED MAY 27, 1999

The Betts Cove Ophiolite, Newfoundland, Canada, records the INTRODUCTIONinitiation of seafloor spreading in an Ordovician marginal basin. The origin of the distinctive geochemical signature thatEarly lavas and sheeted dykes are composed of Low-Ti (<0·3 wt characterizes arc magmas is a contentious issue. Geo-% TiO2) and Intermediate-Ti (0·3 to ~0·6 wt % TiO2) boninites.

chemical and isotopic data imply that the depleted peri-The boninites are overlain by arc tholeiites, and then by sequences

dotites of the supra-subduction mantle wedge areof calc-alkaline pyroclastics and tholeiitic lavas. Results of trace

refertilized by the influx of a slab-derived componentelement melting models suggest that the Betts Cove Low-Ti boninites

(SZ component), which is either an aqueous fluid or awere extracted from a mantle source residual after 20–22% meltinghydrous silicate melt enriched in incompatible traceof fertile mantle, subsequently refertilized with minor amountselements such as Rb, Ba, Cs, Th, U, Pb, Sr and the light(<0·25%) of incompatible-element enriched components. Inter-rare earth elements (LREE) (Hawkesworth et al., 1993;mediate-Ti boninites were derived from a less depleted sourcePearce & Parkinson, 1993; Arculus, 1994; Iwamori,(~12% previous melting), fluxed by similar fertile components.1998). However, considerable uncertainty remains con-The composition of the source mantle for different end-membercerning the extent of prior wedge depletion, and withboninite magmas is calculated, allowing the composition of theregard to the composition and source of this slab-derivedrefertilizing components to be derived. The compositions of theSZ component. Boninites are thought to originate asrefertilizing components are consistent with a mixture of fluid-mobilelow-pressure partial melts of extremely depleted mantleelements derived from dehydration of the subducting oceanic crust,wedges (Cameron et al., 1979; Coish et al., 1982; Hickeyby partial melting of that same crust, and by partial melting of& Frey, 1982; Kostopoulos & Murton, 1992; Sobolev &subducted sediments. The gradation from extremely incompatible-Danyushevsky, 1994; Taylor et al., 1994). As such, theelement depleted boninites to less depleted boninitic and tholeiitictrace element signatures of the SZ component shouldmagmas implies the progressive involvement of less depleted mantlestand out more clearly in boninites than in normal arcsources. This suggests a vertical compositional zonation of themagmas. This paper presents field, petrographic andmantle source, with less depleted mantle domains entering the wedge,

perhaps in response to slab rollback and extension of the overriding whole-rock geochemical data for a suite of boninites fromplate. the Ordovician Betts Cove ophiolite of Newfoundland,

Canada (Fig. 1). The composition of the mantle sourcesof these boninites is calculated, allowing the degree ofprior wedge depletion and the composition of the addedSZ component to be modelled, so addressing some ofthe controversies related to arc magma genesis.KEY WORDS: mantle; seafloor spreading; sediment; subduction

∗e-mail: [email protected] Oxford University Press 1999

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JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 DECEMBER 1999

Fig. 1. Simplified map of Betts Cove complex adapted from Bedard et al. (1999a). Inset shows location of map area in Newfoundland, Canada.The Mount Misery Formation is composed of arc tholeiites. The Snooks Arm Group upper tholeiites are evolved basaltic flows and pillows.The Bobby Cove and parts of the Balsam Bud Cove Formations are composed of basaltic, andesitic, dacitic and rhyolitic pyroclastic rocks ofcalc-alkaline affinity.

REGIONAL GEOLOGICAL FIELD RELATIONSA new geological map of the Betts Cove ophiolite complexFRAMEWORKis now available (Bedard et al. 1999a), and descriptionsThe Betts Cove ophiolite (Fig. 1) is inferred to haveof field relations and petrographic characteristics of theformed by seafloor spreading (Fig. 2) near the margin ofdifferent units have been given by Bedard et al. (2000).North America (Harris, 1992; Pinet & Tremblay, 1995;All sample locations, the complete structural and geo-Bedard et al., 1998) in the Ordovician (488·6 + 3·1/chemical database, and a digital version of the map have–1·8 Ma, Dunning & Krogh, 1985). The ophiolitic oceanbeen given by Bedard et al. (1999b).crust and the sub-conformably overlying lavas and sed-

The Betts Cove ophiolite complex (Fig. 1) is separatediments of the Snooks Arm Group were accreted on tofrom its country rocks by a marginal band of serpentinitethe continental margin of eastern North America duringand talc schists. Massive talc–magnesite–ankerite–the Taconian Orogeny (~470 Ma, Williams, 1979; Hib-magnetite schists may represent mantle rocks. The innerbard, 1983; Dallmeyer & Hibbard, 1984; Tremblay etpart of the ophiolite complex comprises a basal sequenceal., 1997). The ophiolitic and cover rocks at Betts Coveof layered cumulate peridotites, pyroxenites and gabbro–were then tilted and eroded before deposition of thenorites (Upadhyay, 1973; Bedard et al., 1999a, 2000).Cape St John Group in the Silurian (425 Ma, Coyle,The sequence of crystallization recorded in the cumulates1990). Coeval Silurian granitoids intrude the Betts Coveis chromite, olivine, orthopyroxene, clinopyroxene,ophiolite along its northwestern margin. The ophioliteplagioclase, Fe–Ti oxide, hornblende and quartz. Quartz-and its cover rocks were then folded into a large-scaleand Fe–Ti oxide-bearing gabbro–norites commonly in-syncline (Fig. 1), presumably during Acadian compression

in the Devonian (Tremblay et al., 1997). trude the cumulate–sheeted dyke interface (see Church

1854


BEDARD BONINITES FROM THE BETTS COVE OPHIOLITE

Fig. 2. Schematic representation of the tectonic setting inferred for the Betts Cove boninites. Low-Ti boninites constitute the new oceanic crustformed from depleted mantle during seafloor spreading. Progressive upwelling of undepleted mantle from depth generates the undepletedtholeiites of the Upper Snooks Arm Group. Melting of mixed sources generates the Intermediate-Ti boninites and arc tholeiites. The whitearrows are mantle streamlines. Black arrows are streamlines for fluids or melts. The field under the eruption axis is the zone of melting.Decompression melting of the shallow depleted wedge mantle is enhanced by flux-melting caused by addition of SZ components. SZ-Hydrousrepresents aqueous fluid derived by dehydration of subducting oceanic crust. SZ-ATT is a wet melt derived by melting of subducting oceaniccrust. SZ-Sediment is a wet melt of subducting sediments.

& Riccio, 1974), and envelope slivers of boninitic pillow Sheeted dykeslavas and sheeted dyke septa, yet are themselves cut by The sheeted dyke unit thins and eventually disappearsdykes of the sheeted dyke complex. The sheeted dyke completely north of Betts Cove (Fig. 1), possibly as thecomplex is composed almost entirely of rocks belonging result of excision by spreading-related normal faults andto the boninitic suite, and grades up into a sequence of decollements. The basal and upper contacts of the sheetedboninitic lavas locally >1 km in thickness, the Betts Head dyke unit are gradational, faulted, or have been intrudedFormation (Bedard et al., 1999a, 2000). This paper focuses by gabbro–norites. In many places the sheeted dykeson the boninitic sheeted dykes and lavas. grade up into a 50–200 m wide zone characterized by

Rocks of the Snooks Arm Group sub-conformably alternating dyke swarms and septa of pillow lavas. Moreoverlie the Betts Head boninites. The lowermost unit of commonly, however, the sheeted dykes are separatedthe Snooks Arm Group is the Mount Misery Formation, from the lavas by faults that are interpreted (Tremblayconsisting of ~1 km of submarine basalts with arc tholeiitic et al., 1997; Bedard et al., 1998, 1999a, 2000) to have

originally been steeply dipping normal faults. Fault-brec-composition. These are followed by (Fig. 1) interstratifiedcias may be impregnated with boninitic matrices, im-evolved tholeiitic basalts (Upper Snooks Arm Groupplying that extensional faulting was contemporaneoustholeiites; Bedard et al., 2000), calc-alkaline tuffs andwith seafloor spreading. Rare trondhjemitic dykes (~2 mlavas (Cousineau & Bedard, 2000), and volcanogenicwide) have shallow palaeo-dips.sedimentary rocks (Kessler & Bedard, 2000). The petro-

genesis and tectonic implications of Snooks Arm Groupmagmas will be considered in a subsequent paper.

Several deformation episodes overprint this area, butBetts Head Formation lavasmost of the strain was accommodated in the marginal

serpentinite and talc schists, and so the intrusive, extrusive Spherulitic, sparsely amygdaloidal, sparsely porphyriticand sedimentary rocks constituting the core of the ophi- (2–6%) boninitic pillow lavas and hyaloclastite brecciasolite rarely exhibit pervasive deformation fabrics. The constitute the Betts Head Formation in the Betts Coveophiolite stratigraphy is largely intact, although tilted to area (Coish & Church, 1979; Coish et al., 1982; Bedardthe vertical and folded, and most of the faults and shear et al., 1998, 1999a, 2000). The thickest accumulation (up

to 1·3 km) is in a ridge-related graben structure south ofzones are thought to be syn-oceanic structures.

1855



Betts Cove. Restricted exposures of boninitic volcanic interstitial feldspar+ quartz. Some boninitic dykes con-tain plagioclase and clinopyroxene phenocrysts (1 mm,rocks in the Long Pond and Tilt Cove areas (Fig. 1) are

pervasively brecciated. Intra-breccia dykes and im- <10%), and abundant groundmass plagioclase. Pink-weathering microgabbroic and ferro-gabbro–noriticpregnations are principally boninitic, with compositions

similar to those of Betts Head Formation lavas. A few dykes are marginal facies and apophyses of the gabbro–norite intrusions. Blue–green-weathering, sparsely phyricintra-breccia dykes have tholeiitic compositions, similar

to those of lavas from the overlying Mount Misery diabase dykes are dominated by clinopyroxene and feld-spar. Diabase dykes may contain clinopyroxene pheno-Formation (Snooks Arm Group). Locally heterolithic

breccias contain detrital chromite and clasts of basalt, crysts (~5%) and rare prismatic bastite pseudomorphsafter orthopyroxene(?). Trondhjemite dykes containperidotite and gabbro, and probably represent fault talus

or slump deposits developed during an amagmatic phase pseudomorphs after phenocrysts of feldspar, quartz, maficsilicates and Fe–Ti oxides.of extension associated with seafloor spreading. The

presence of plutonic clasts implies unroofing of oceanicbasement rocks during seafloor extensional faulting.

Betts Head Formation lavasThe Betts Head Formation lavas belong to two subtypes.The most common subtype contains olivine phenocrystPETROGRAPHY AND MINERALOGYpseudomorphs (<10%), sparse, euhedral, chromite micro-

Most of the rocks of the Betts Cove complex are affected phenocrysts and prismatic bastite pseudomorphs afterby greenschist facies hydrothermal metamorphism orthopyroxene phenocrysts. Rare clinopyroxene pheno-(Coish, 1977b), although textural pseudomorphism allows crysts are present locally (Coish, 1977b). The groundmassreliable identification of most original phases. Electron is similar to that of the boninite dykes.microprobe compositions of chromite and clinopyroxene Interbedded with the olivine + orthopyroxene +were obtained at McGill University with a JEOL super- chromite-phyric lavas are sequences (up to 200 m thick)probe. Methods and accuracy have been described by of spherulitic lavas that may also contain plagioclase andBedard & Hebert (1996). The full dataset has been given clinopyroxene microphenocrysts and that have feldsparby Bedard et al. (1999b, 2000). Additional data were microlites in their groundmasses.compiled from Coish (1977a).

Mineral chemistryMassive talc–magnesite–ankerite–magnetite Clinopyroxene phenocrysts in the dykes and lavas haveschists low Ti contents (Coish & Church, 1979; Bedard et al.,

1999b, 2000), which is typical of boninites (see CrawfordAlong the shores of Red Cliff Pond and Long Pond areet al., 1989). Chromite phenocrysts in dykes and lavasdomains of massive talc–magnesite–ankerite–magnetitehave high Cr/(Cr+ Al)= 0·70–0·85 (Coish & Church,schists. Within a felted talc–magnesite matrix one can1979; Bedard et al., 1999b, 2000), also typical of thoselocally recognize olivine and orthopyroxene pseudo-found in boninites (see Crawford et al., 1989), but with amorphs. Euhedral magnetite octahedra locally containwider than usual range of Fe2+/(Mg+ Fe 2+). Chromitesragged chromite relics. Holly-leaf chromites typical offrom the talc–magnesite–ankerite–magnetite schists aremantle harzburgites are recognizable in places.compositionally distinct from those of the layered cu-mulates (Bedard et al., 2000), with lower Cr/(Cr + Al).

Sheeted dykesSeveral types of dyke have been recognized at Betts Cove

WHOLE-ROCK GEOCHEMISTRY(see Upadhyay, 1973; Coish, 1977a, 1977b; Coish &Sample preparation, analytical methodsChurch, 1979). Boninite dykes contain euhedral micro-and data sourcesphenocrysts of chromite (0·1–1 mm, 1–2%), and pseudo-

morphs after olivine and orthopyroxene phenocrysts New analyses of boninites from Betts Cove include 22dykes or breccia matrices, 28 lavas and four clasts in(<5 mm, <10%). Clinopyroxene phenocrysts have also

been reported (Coish, 1977a). Porphyritic boninite dykes breccias (Table 1). Sample locations are given as Uni-versal Transverse Mercator coordinates in Table 1, and(described as perknites in the older literature) contain

larger (1–10 mm), more abundant (10–50%) phenocrysts have been presented graphically by Bedard et al. (1999b).Altered margins and prominent veins were removed fromof the same type. The matrix of the boninite dykes

is composed of prismatic pyroxene pseudomorphs and the samples with a saw. Sawmarks were removed with

1856



Table 1: Low-Ti boninite lavas and dykes, Betts Cove

North: 5519560 5519560 5519620 5519640 5519710 5519700 5519700 5519660 5519660 5519660

East: 586060 586060 585950 585930 585920 585970 585970 586000 586000 586000

Diabase Dyke Dyke Dyke Dyke Core Margin Dyke Dyke Dyke

Dyke in gabb in gabb in gabb Dyke Dyke

Sample: BC-94 BC-94 BC-94 BC-94 BC-94 BC-94 BC-94 BC-94 BC-94 BC-94

360 362 701 706 753 756 757 759-A 759-B 760

SiO2 54·7 57·3 52·4 48·6 50·1 52·6 51·5 55·4 56·0 51·5

TiO2 0·17 0·14 0·10 0·11 0·09 0·10 0·12 0·15 0·15 0·10

Al2O3 15·7 13·6 12·8 12·8 8·30 12·7 13·8 15·4 15·6 13·2

FeO∗ 8·06 6·80 7·04 8·66 7·27 6·71 6·87 6·19 6·20 6·55

MnO 0·12 0·14 0·13 0·15 0·14 0·11 0·11 0·11 0·12 0·12

MgO 6·35 7·40 14·3 16·9 17·6 13·7 13·1 9·31 7·80 11·7

CaO 4·12 8·17 4·52 5·06 8·10 6·21 4·68 4·31 6·45 8·02

Na2O 4·84 4·61 3·79 2·72 1·51 3·37 3·07 5·74 4·48 3·54

K2O 0·08 0·20 0·10 0·13 0·11 0·07 0·08 0·25 0·56 0·07

P2O5 0·04 0·03 0·04 0·04 0·01 0·02 0·02 0·02 0·03 0·02

LOI 3·51 1·72 4·11 4·89 4·87 4·47 5·92 3·09 2·40 3·91

Total 97·7 100·1 99·4 100·1 98·0 100·0 99·3 100·0 99·8 98·7

FeO/MgO 1·27 0·92 0·49 0·51 0·41 0·49 0·53 0·67 0·80 0·56

Cs — — — — — — — — — —

Rb 3 5 3 4 5 6 3 3 7 —

Ba — — — — — — — — — —

Pb — — — — — — — — — —

Sr 116 81 132 89 36 72 79 120 169 60

Th — — — — — — — — — —

U — — — — — — — — — —

Nb — — — — — — — — — —

Ta — — — — — — — — — —

Zr 33 25 24 17 17 20 25 26 28 21

Hf — — — — — — — — — —

Y 8 7 5 5 6 6 8 7 4 5

La — — — — — — — — — —

Ce — — — — — — — — — —

Pr — — — — — — — — — —

Nd — — — — — — — — — —

Sm — — — — — — — — — —

Eu — — — — — — — — — —

Gd — — — — — — — — — —

Tb — — — — — — — — — —

Dy — — — — — — — — — —

Ho — — — — — — — — — —

Er — — — — — — — — — —

Tm — — — — — — — — — —

Yb — — — — — — — — — —

Lu — — — — — — — — — —

Cr 54 207 703 621 1038 615 662 326 64 512

Ni 52 74 239 210 367 219 260 167 74 209

Co 50 42 48 60 50 45 43 40 38 47

Cu 35 3 — — — — 4 — — 5

Zn 70 45 52 53 43 34 49 36 36 40

V 343 251 270 331 238 260 317 288 296 259

Sc — — — — — — — — — —

1857



Table 1: continued

North: 5519660 5519660 5519650 5519650 5519650 5519650 5527040 5518450 5524000 5522520

East: 586000 586000 586030 586030 586030 586030 593710 586750 589040 587790

Dyke Dyke Dyke Sheared Breccia Dyke Pillow Dyke Pillow Pillow

Dyke Dyke LoLaNd

Sample: BC-94 BC-94 BC-94 BC-94 BC-94 BC-94 BC-94 BC-95 BC-95 BC-95

761 762 766 767 771 773 962 522 741 767

SiO2 51·2 55·0 55·8 43·7 49·8 41·6 55·1 54·6 47·4 53·3

TiO2 0·10 0·12 0·13 0·08 0·09 0·09 0·29 0·11 0·24 0·24

Al2O3 13·1 11·9 15·1 9·30 10·4 8·55 14·5 14·1 12·1 13·6

FeO∗ 7·61 7·37 7·53 9·52 7·48 7·92 7·33 8·40 7·27 7·32

MnO 0·12 0·12 0·12 0·17 0·17 0·14 0·11 0·13 0·16 0·11

MgO 12·5 12·3 8·60 19·5 16·8 23·8 12·1 8·99 10·7 8·63

CaO 7·39 6·14 3·94 7·57 6·75 6·82 1·30 7·69 6·03 5·50

Na2O 2·25 2·43 4·93 0·09 1·41 0·09 2·28 3·44 3·36 4·98

K2O 0·08 0·15 0·05 0·02 0·05 0·01 0·02 0·07 0·03 0·09

P2O5 0·02 0·02 0·02 0·02 0·02 0·02 0·04 <d·l· 0·13 <d·l·

LOI 4·94 4·20 3·56 7·16 5·91 8·93 6·40 2·97 13·0 4·60

Total 99·3 99·8 99·8 97·1 98·8 98·0 99·4 100·5 100·3 98·4

FeO/MgO 0·61 0·60 0·88 0·49 0·45 0·33 0·61 0·93 0·68 0·85

Cs — — — — — — — 0·055 — 0·037

Rb 5 3 4 4 4 4 4 0·34 — 0·48

Ba — — — — — — 82 9·9 — 13·5

Pb — — — — — — — 0·279 — 0·830

Sr 81 62 77 7 33 49 26 185 — 77

Th — — — — — — — 0·151 — 0·157

U — — — — — — 0·665 0·086 — 0·089

Nb — — — — — — — 0·37 — 0·61

Ta — — — — — — — 0·029 — 0·051

Zr 22 21 23 15 20 22 19 7·5 — 10·5

Hf — — — — — — 0·32 0·207 — 0·363

Y 7 7 6 7 8 4 6 3·0 8 6·5

La — — — — — — — 0·660 — 0·736

Ce — — — — — — — 1·31 — 1·64

Pr — — — — — — — 0·157 — 0·211

Nd — — — — — — — 0·650 — 0·933

Sm — — — — — — 0·195 0·183 — 0·341

Eu — — — — — — 0·119 0·068 — 0·135

Gd — — — — — — — 0·197 — 0·590

Tb — — — — — — 0·178 0·048 — 0·122

Dy — — — — — — — 0·416 — 0·943

Ho — — — — — — — 0·113 — 0·229

Er — — — — — — — 0·450 — 0·712

Tm — — — — — — — 0·087 — 0·124

Yb — — — — — — 0·812 0·687 — 0·877

Lu — — — — — — 0·103 0·135 — 0·138

Cr 668 534 140 2657 177 1770 587 295 790 218

Co 51 50 49 88 56 67 51 45 — 26

Ni 233 168 93 1420 130 800 171 87 403 68

Cu 9 6 140 31 36 66 38 60 36 106

Zn 43 43 69 75 64 57 75 56 53 60

V 308 289 308 265 272 210 314 288 184 231

Sc — — — — — — 36 53 32 40

1858



North: 5522540 5527030 5526220 5526200 5522390 5522010 5522010 5522090 5522130 5522110East: 587780 599230 598840 598860 587830 587190 587190 587260 587480 587520

Pillow Lava Lava Pillow Breccia Dyke Dyke Pillow Pillow PillowHiLaNd LoLaNd HiNb HiTh Dyke HiNb HiNb Edge

HiNbSample: BC-95 BC-95 BC-96 BC-96 BC-96 BC-96 BC-96 BC-96 BC-96 BC-96

768 1012 172 173 213 214 215 216 220 221

SiO2 53·2 43·5 47·6 43·8 54·3 52·7 50·3 54·1 49·9 51·7TiO2 0·09 0·22 0·11 0·09 0·25 0·23 0·11 0·14 0·27 0·14Al2O3 11·1 13·1 11·9 9·70 14·1 13·2 11·9 10·3 12·6 14·3FeO∗ 7·83 6·63 8·69 8·04 7·25 7·95 8·20 8·80 8·32 7·42MnO 0·12 0·22 0·09 0·12 0·12 0·14 0·15 0·12 0·12 0·08MgO 9·43 11·7 12·7 12·6 10·2 10·2 12·9 11·9 11·5 9·30CaO 11·1 5·56 3·86 8·59 4·38 6·54 6·58 5·82 7·57 7·93Na2O 1·69 3·11 0·80 0·06 5·70 4·98 0·99 3·30 2·64 4·44K2O 0·26 0·06 0·05 0·04 0·13 0·07 1·67 0·07 0·78 0·04P2O5 <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l.LOI 2·67 13·2 10·9 13·7 3·95 3·31 4·58 4·14 3·94 3·16Total 97·4 97·2 96·6 96·6 100·3 99·4 97·4 98·7 97·7 98·5FeO/MgO 0·83 0·57 0·68 0·64 0·71 0·78 0·64 0·74 0·72 0·80Cs 0·050 0·054 0·059 0·049 0·078 0·035 0·344 0·066 0·149 0·017Rb 3·6 0·16 0·55 0·11 1·11 0·50 24 0·92 12·3 0·15Ba 24 4·6 7·5 14 17 20 178 16 24 11Pb 1·91 2·33 0·564 1·69 0·328 0·223 0·439 0·199 0·229 0·918Sr 349 254 74 181 78 131 48 68 58 59Th 0·144 0·001 0·159 0·270 0·071 0·063 0·166 0·245 0·138 0·171U 0·074 0·029 0·112 0·134 0·050 0·034 0·088 0·111 0·066 0·097Nb 0·36 0·18 1·27 0·41 0·79 0·81 0·96 0·57 0·53 0·54Ta 0·030 0·013 0·40 0·045 0·16 0·16 0·14 0·041 0·032 0·043Zr 8·9 3·9 13 11 9·0 14 9·8 11 7·6 8·0Hf 0·214 0·157 0·428 0·318 0·377 0·376 0·310 0·333 0·299 0·667Y 3·1 4·2 1·6 2·3 5·2 6·4 3·5 3·5 7·5 4·9La 0·616 0·246 0·426 0·338 0·670 0·794 0·973 0·960 0·840 0·954Ce 1·17 0·603 0·951 0·755 1·93 1·95 2·36 1·96 1·90 2·10Pr 0·147 0·100 0·111 0·110 0·272 0·277 0·298 0·251 0·270 0·322Nd 0·526 0·562 0·418 0·424 1·12 1·18 1·14 1·06 1·32 1·00Sm 0·152 0·275 0·123 0·127 0·407 0·446 0·288 0·270 0·470 0·278Eu 0·075 0·174 0·048 0·046 0·118 0·111 0·113 0·084 0·183 0·126Gd 0·242 0·424 0·157 0·190 0·510 0·585 0·375 0·331 0·757 0·387Tb 0·049 0·089 0·036 0·042 0·118 0·122 0·061 0·066 0·150 0·082Dy 0·393 0·662 0·263 0·352 0·864 1·03 0·552 0·570 1·053 0·748Ho 0·107 0·156 0·065 0·085 0·206 0·242 0·120 0·119 0·255 0·191Er 0·408 0·551 0·239 0·322 0·714 0·885 0·444 0·476 0·842 0·562Tm 0·070 0·081 0·050 0·059 0·124 0·148 0·084 0·074 0·138 0·091Yb 0·529 0·634 0·387 0·454 0·802 0·958 0·560 0·543 0·886 0·650Lu 0·097 0·110 0·067 0·090 0·115 0·150 0·090 0·097 0·148 0·093Cr 711 168 1380 1440 263 389 807 772 350 419Ni 218 77 455 433 81 165 239 202 109 130Co 33 24 52 — — — 39 — — —Cu 10 85 5 94 27 8 90 15 12 13Zn 48 — 82 69 64 44 62 78 41 41V 239 188 205 174 194 217 217 208 232 240Sc 43 38 37 — 38 38 40 44 40 37

1859



Table 1: continued

North: 5522070 5522010 5527510 5527710 5519360 5519400 5519430 5519490 5519510 5520230 5525340

East: 587560 587600 599450 599420 587000 587100 587160 587240 587310 586760 598710

Pillow Pillow Pillow Pillow Pillow Pillow Pillow Pillow Pillow Lava Mass

HiLaNd HiTh HiLaNd LoLaNd HiTh HiLaNd Breccia Flow

Sample: BC-96 BC-96 BC-96 BC-96 BC-96 BC-96 BC-96 BC-96 BC-96 BC-96 BC-96

222 223 243 244 250 251 252 253 255 298 426

SiO2 59·2 55·0 49·9 53·9 55·1 56·3 59·4 52·2 51·8 52·0 44·3

TiO2 0·08 0·16 0·09 0·21 0·09 0·13 0·12 0·13 0·29 0·27 0·23

Al2O3 10·1 13·6 9·20 12·5 10·4 13·9 13·8 12·6 14·4 14·5 8·02

FeO∗ 7·13 6·64 6·01 7·06 7·68 5·52 5·48 7·83 7·61 8·95 9·37

MnO 0·11 0·11 0·10 0·14 0·16 0·10 0·10 0·15 0·16 0·13 0·14

MgO 8·67 9·23 9·88 11·6 10·5 8·01 7·13 10·2 8·83 8·23 22·0

CaO 10·2 4·55 9·50 5·62 11·5 5·85 4·86 9·20 7·90 3·95 5·73

Na2O 1·85 4·67 2·67 4·37 0·54 5·80 5·08 1·32 2·59 5·16 0·30

K2O 0·02 0·20 0·08 0·07 0·27 0·07 0·53 0·29 0·16 0·10 0·03

P2O5 <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l.

LOI 3·08 4·57 9·86 5·66 3·31 5·12 4·02 4·45 4·77 4·04 6·79

Total 100·4 98·7 97·3 101·1 99·5 100·8 100·5 98·3 98·5 97·3 97·0

FeO/MgO 0·82 0·72 0·61 0·61 0·73 0·69 0·77 0·77 0·86 1·09 0·43

Cs 0·017 0·053 0·146 0·542 0·061 0·027 0·135 0·091 0·084 0·035 0·068

Rb 0·16 2·4 0·28 0·62 3·8 0·78 8·4 4·2 2·0 0·75 0·40

Ba 5·9 11 5·6 12 13 10 24 20 20 20 4·4

Pb 0·934 0·462 0·530 0·624 1·23 0·157 0·211 1·68 1·81 0·395 0·141

Sr 107 59 102 56 272 73 74 156 159 115 7·0

Th 0·091 0·196 0·214 0·053 0·182 0·132 0·146 0·128 0·140 0·179 0·117

U 0·090 0·095 0·156 0·068 0·058 0·056 0·114 0·052 0·053 0·100 0·054

Nb 0·36 0·51 0·47 0·19 0·29 0·41 0·39 0·34 0·39 0·50 0·36

Ta 0·021 0·046 0·051 0·019 0·018 0·037 0·034 0·031 0·032 0·036 0·029

Zr 6·8 6·0 7·7 6·6 7·0 7·8 8·6 16 14 11 11

Hf 0·15 0·23 0·25 0·24 0·19 0·26 0·26 0·41 0·45 0·38 0·34

Y 3·4 4·3 2·7 6·4 2·7 3·6 2·6 3·9 4·9 6·8 4·1

La 0·746 0·562 0·828 0·267 0·440 1·19 0·575 — 0·639 0·778 0·622

Ce 1·30 1·20 1·46 0·624 1·05 1·80 1·23 — 1·61 1·74 1·60

Pr 0·162 0·150 0·169 0·098 0·131 0·189 0·139 — 0·255 0·247 0·245

Nd 0·620 0·633 0·657 0·536 0·380 0·762 0·602 — 0·981 1·23 1·19

Sm 0·166 0·226 0·172 0·296 0·114 0·235 0·178 — 0·388 0·455 0·421

Eu 0·063 0·047 0·072 0·129 0·040 0·052 0·062 — 0·180 0·131 0·142

Gd 0·251 0·347 0·220 0·511 0·161 0·338 0·272 0·296 0·496 0·627 0·498

Tb 0·038 0·084 0·047 0·121 0·033 0·066 0·048 0·072 0·101 0·140 0·085

Dy 0·441 0·683 0·407 0·917 0·324 0·541 0·390 0·579 0·772 1·07 0·711

Ho 0·113 0·153 0·100 0·237 0·106 0·131 0·097 0·156 0·191 0·249 0·147

Er 0·442 0·605 0·387 0·816 0·424 0·460 0·310 0·480 0·607 0·849 0·505

Tm 0·076 0·101 0·064 0·135 0·070 0·080 0·053 0·083 0·097 0·144 0·077

Yb 0·530 0·679 0·510 0·901 0·587 0·526 0·450 0·576 0·709 1·034 0·562

Lu 0·098 0·109 0·094 0·154 0·111 0·089 0·063 0·095 0·118 0·158 0·086

Cr 625 551 1207 323 713 417 476 509 316 — 1520

Co — 25 35 — — — — — — — —

Ni 222 172 329 124 169 174 134 170 127 56 957

Cu 2 104 37 15 12 16 9 19 12 18 8

Zn 48 84 82 85 52 — 48 63 65 76 49

V 215 198 179 191 220 174 153 194 169 252 115

Sc 40 35 39 36 40 34 35 39 32 40 23

1860



North: 5526570 5526500 5526490 5526030 5526030 5527040 5525820

East: 598550 598510 598500 597300 597300 593710 596230

Massive Breccia Altered Lava Lava Pillow Pillow

Lava Matrix Lava Clast Clast

Sample: BC-94 BC-94 BC-94 BC-94 BC-94 BC-94 BC-95

28 29 37 95 97-F 960 1059

SiO2 49·9 45·8 45·8 37·8 38·4 48·5 51·8

TiO2 0·37 0·42 0·22 0·23 0·61 0·33 0·61

Al2O3 14·9 15·8 14·1 11·3 16·4 15·6 14·95

FeO∗ 7·71 9·53 7·61 6·46 12·1 9·72 7·19

MnO 0·25 0·31 0·10 0·15 0·14 0·11 0·13

MgO 10·0 10·6 12·3 16·1 17·6 13·2 8·53

CaO 5·49 4·07 4·95 9·47 1·36 0·07 4·09

Na2O 4·81 4·29 2·61 0·33 1·26 3·12 5·32

K2O 0·10 <d.l. <d.l. <d.l. <d.l. <d.l. 0·11

P2O5 0·03 0·02 0·02 0·02 0·04 0·03 <d.l.

LOI 5·28 7·20 9·98 16·2 9·47 6·58 4·62

Total 98·9 98·1 97·7 98·0 97·4 97·2 97·3

FeO/MgO 0·77 0·90 0·62 0·40 0·69 0·74 0·84

Cs 0·536 0·862 — 0·682 — — 0·087

Rb 7 8 — 7 4 3 1

Ba 152 167 — 64 98 — 30

Pb — — — — — — 0·504

Sr 240 108 229 78 18 24 120

Th — 0·275 — — — — 0·123

U — 0·872 — 0·549 — — 0·059

Nb — — — — — — 0·70

Ta — — — — — — 0·046

Zr 22 30 24 28 44 34 16

Hf 0·40 0·29 — 0·27 0·67 — 0·55

Y 17 21 10 10 20 11 11

La 0·886 1·05 — — 0·692 — 1·02

Ce — — — — 3·61 — 2·81

Pr — — — — — — 0·441

Nd — — — — 5·47 — 2·21

Sm 0·587 0·739 — 0·442 1·33 — 0·842

Eu 0·390 0·537 — 0·312 0·616 — 0·235

Gd — — — — — — 1·29

Tb 0·383 0·323 — 0·193 0·532 — 0·254

Dy — — — — — — 1·76

Ho — — — — — — 0·394

Er — — — — — — 1·26

Tm — — — — — — 0·185

Yb 1·51 1·96 — 0·944 1·43 — 1·20

Lu 0·247 0·240 — 0·121 0·184 — 0·163

Cr 298 287 398 482 394 53 189

Co 40 58 48 43 89 67 32

Ni 130 116 140 306 171 72 213

Cu 171 77 98 78 69 — 9

Zn 145 162 56 40 89 82 66

V 266 325 274 165 383 362 227

Sc 37 39 — 39 39 — 36

1861



Table 1: continued

North: 5522110 5522100 5522100 5527510 5520500 5523050East: 587340 587420 587420 599200 587030 593400

Pillow Pillow Pillow Pillow Sill? Clast inDebrite

Sample: BC-96 BC-96 BC-96 BC-96 BC-96 BC-97217 218 219 231 404 2

SiO2 50·1 52·0 54·3 50·9 50·6 44·3TiO2 0·48 0·30 0·39 0·46 0·39 0·40Al2O3 13·5 14·8 14·0 14·0 14·9 16·8FeO∗ 8·26 7·34 6·92 6·76 7·55 8·91MnO 0·13 0·12 0·06 0·30 0·15 0·30MgO 11·2 9·00 9·76 7·46 6·44 12·2CaO 6·84 5·31 6·18 6·54 8·39 3·89Na2O 4·56 5·85 4·30 4·76 5·14 3·69K2O 0·04 0·08 0·17 0·02 0·39 0·04P2O5 <d.l. <d.l. <d.l. <d.l. <d.l. <d.l.LOI 4·22 3·49 3·68 9·46 4·67 9·12Total 99·4 98·3 99·8 100·6 98·7 99·8FeO/MgO 0·74 0·82 0·71 0·91 1·17 0·73Cs 0·032 0·060 0·065 0·027 0·063 —Rb 0·18 0·44 1·79 0·09 2·25 —Ba 7·9 8·8 13·8 8·6 30 13Pb 0·367 0·253 0·578 2·49 0·916 —Sr 107 101 150 163 220 70Th 0·164 0·235 0·102 0·051 0·117 —U 0·083 0·130 0·094 0·159 0·057 —Nb 0·55 0·62 0·37 0·23 0·35 —Ta 0·055 0·090 0·048 0·017 0·025 —Zr 15·3 12·3 — 11·8 27·5 13Hf 0·63 0·60 — 0·43 0·82 —Y 9·63 7·59 9·79 4·94 9·86 —La 0·937 0·977 0·745 0·559 0·727 —Ce 2·45 2·26 2·22 1·78 1·81 —Pr 0·380 0·310 0·347 0·309 0·287 —Nd 2·11 1·48 1·71 1·565 1·505 —Sm 0·833 0·526 0·829 0·549 0·685 —Eu 0·254 0·178 0·259 0·263 0·365 —Gd 1·20 0·724 1·11 0·742 1·14 —Tb 0·239 0·157 0·220 0·115 0·225 —Dy 1·74 1·24 1·69 0·863 1·62 —Ho 0·363 0·271 0·404 0·488 0·374 —Er 1·17 0·922 1·195 1·41 1·17 —Tm 0·169 0·146 0·176 0·083 0·178 —Yb 1·08 0·911 1·155 1·03 1·23 —Lu 0·172 0·161 0·160 0·122 0·179 —Cr 42 398 73 195 203 149Co — — — — — 23Ni 224 142 72 102 92 93Cu 7 15 9 18 12 21Zn 59 61 46 76 78 121V 196 204 206 177 215 266Sc 37 46 36 27 38 41

Whole-rock major and trace element analyses of Betts Cove boninite lavas and dykes. Major elements and Rb, Sr, Zr, Y, Cr,Ni, Co, Cu, Zn and V were determined by XRF, with other trace elements by INAA for data with ‘94’ suffixes. Data with-95, -96 and -97 suffixes were obtained by ICP-ES and ICP-MS. Dykes labelled HiNb, HiTh, HiLaNd and LoLaNd are boninitesused to compute the High-Nb, High-Th/La, High-La/Nd and Low-La/Nd end members, respectively. Sample BC-97-2 is a clastin an epiclastic debris flow from the Balsam Bud Cove Formation. <d.l., below detection limits; —, not analysed; gabb,gabbro; FeO∗, all iron as FeO. North and East are Universal Transverse Mercator coordinates.

1862



sandpaper. Samples were then crushed in a steel jaw that are attributable to hydrothermal remobilization.Nevertheless, averaged values of these elements (ex-crusher, and subsequently reduced to powder in an agatecluding data from obvious metasomatic zones) do notshatterbox. Whole-rock powders were analysed for majorshow large anomalies in such diagrams (Fig. 3) in com-and trace elements at the Centre geoscientifique deparison with nearby, more immobile elements (Th, La),Quebec (CGQ) laboratories. Major elements, Cr, Ni,suggesting that Rb, Cs and K were redistributed on theCu, Zn, Sr, Rb and Ba were analysed by conventionalscale of map units, but were not systematically leachedX-ray fluorescence (XRF) methods for samples collectedor enriched from the ophiolitic complex. Comparison ofin 1994, and by inductively coupled plasma atomicU vs Th distributions in Betts Cove rocks (Fig. 4a) suggestsemission spectrometry (ICP-AES) for samples collectedthat only a few strongly perturbed rocks have beenin 1995, 1996 and 1997. Other trace elements wereenriched in U. In normalized trace element diagrams,studied by instrumental neutron activation analysisBetts Cove boninites always show positive anomalies for(INAA) for 1994 samples; and by ICP-MS on a VGBa, U, Sr and Pb, similar to those of recent fresh boninitesTurbo Plasma Quad 2+ instrument, using a method(Fig. 3a). One could speculate that these Betts Cove lavassimilar to that described by Varfalvy et al. (1997) fororiginally had immobile element abundances identical toother samples. Analytical precision for ICP-MS analysisthose of recent boninites from the Bonin Islands, butof tholeiitic lavas is better than 1% for La, Ce, Pr, Nd,different concentrations of Ba, U, Sr, Pb and SiO2; andEu, Tb, Lu and Ba; 2% for Sm, Gd, Dy, Ho, Er, Tm,that it is the action of pervasive hydrothermal meta-Yb, Th, Rb and Sr; 4% for Zr, Cs, Y and U; and ~10%somatism that has somehow replicated boninitic sig-for Nb and Pb. Precision is lower for the extremelynatures for these elements. However, considering all thedepleted boninites, in which elemental abundances ap-other evidence pointing towards a boninitic affinity forproach detection limits. Lafleche et al. (1998, appendixthe Betts Cove lavas, a hydrothermal origin for the1) have published analyses of international rock standardsresemblance to boninites in terms of Ba, U, Sr, Pb andobtained in the CGQ laboratories. The new data reportedSiO2 seems providential. Consequently, it is concludedhere are supplemented by previously published data fromthat the abundances of these elements are similar toUpadhyay (1973), DeGrace et al. (1976), Coish (1977a),those in the original magmas. This is not to say thatJenner (1977), Coish et al. (1982), Hurley (1982), Saundershydrothermal alteration has had no impact at all. For(1985), Al (1990) and Swinden et al. (1997), which wereexample, Ba (Fig. 4b), Sr and Pb (not shown) show rathercompiled to yield the ‘average’ analyses of Table 2. Mostpoor correlations when plotted against Th. Hydrothermalof the pre-1990 trace element data were not plotted, oralteration is almost certainly responsible for the largeincluded in the averages, however.degree of dispersion in these elements in geochemicalvariation diagrams, but the high abundances of Ba, U,Sr, Pb and SiO2 characteristic of most of the rocks are

Impact of hydrothermal metamorphism probably not in themselves of hydrothermal origin.Samples affected by obvious hydrothermal alterationrelated to ore deposition (i.e. marked S and Fe en-

Betts Cove lavas and dykesrichment), and extreme enrichment or depletion in Na2O,K2O or CaO, were excluded from the dataset used Betts Cove dyke complex rocks are compositionally in-for petrogenetic interpretation and from the computed distinguishable from Betts Head Formation lavas (Fig. 5),averages. Coish (1977b), Coish & Church (1979) and suggesting that the sheeted dykes and lavas belong to aCoish et al. (1982) concluded that Ti, P, Ni, Cr, Zr, Y, single comagmatic suite. The porphyritic dykes and lavasthe rare earth elements (REE), Al2O3 and FeO∗/MgO in (picrites and perknites) have very high Cr, Ni and MgOthe Betts Cove rocks largely reflect magmatic abundances. contents (Fig. 5a) indicative of phenocryst accumulationThe concentrations of elements such as CaO and Na2O (Coish & Church, 1979). The lavas of the Betts Headare commonly perturbed, however. Consequently, it is Formation have been subdivided by Coish & Churchnot possible to classify the Betts Cove boninites in a (1979) and Coish (1989) into ‘Low-Ti’ and ‘Intermediate-reliable way using the Crawford et al. (1989) scheme, Ti’ suites (Fig. 5c). This first-order subdivision is adoptedwhich requires accurate values for CaO, SiO2 and alkali here. Rocks of the Low-Ti suite correspond to the olivineelements. Instead, the Betts Cove lavas and dykes have + orthopyroxene + chromite ± clinopyroxene phyricbeen subdivided using relatively immobile trace elements, dykes and lavas, whereas rocks of the Intermediate-Tiupon which the petrogenetic interpretations outlined suite correspond to the clinopyroxene ± plagioclase ±below also principally depend. olivine phyric dykes and lavas. In practice, many rocks

The elements Rb, Cs and K display random positive or have been assigned to these suites on the basis of theirnegative anomalies on mid-ocean ridge basalt (MORB)- TiO2 vs FeO∗/MgO (Fig. 5c) and La/Nd vs La (Fig. 5d)

distributions.normalized trace element variation diagrams (Fig. 3)

1863



Table 2: Average end-member magmas

Low-Ti Betts Head boninites

Average n FeO/MgO High- n Low- n High- n High- n

>0·5 <0·9 Th/La La/Nd La/Nd Nb

SiO2 55·2 180 53·6 56·8 56·2 3 55·8 3 58·2 4 55·2 4

TiO2 0·14 173 0·12 0·17 0·12 3 0·26 3 0·10 4 0·19 4

Al2O3 12·7 180 10·9 15·4 12·3 3 14·7 3 11·8 4 13·7 4

FeO∗ 8·36 180 8·46 8·27 8·24 3 7·72 3 7·06 4 8·69 4

MnO 0·17 176 0·17 0·16 0·14 3 0·18 3 0·11 4 0·13 4

MgO 12·9 180 16·7 7·73 12·0 3 13·0 3 9·63 4 12·5 4

CaO 7·53 180 8·10 7·44 9·03 3 4·64 3 9·77 4 5·74 4

Na2O 2·83 179 1·69 4·00 1·86 3 3·58 3 3·20 4 3·27 4

K2O 0·26 174 0·27 0·27 0·18 3 0·06 3 0·11 4 0·52 4

P2O5 0·02 146 0·02 0·02 <d.l. 0·04 1 <d.l. <d.l.

LOI 4·34 172 5·05 3·72 7·20 3 8·42 3 5·18 4 5·67 4

FeO/MgO 0·71 0·52 1·10 0·70 0·59 0·74 0·70

S 371 14 469 350 — — — —

Cs 0·093 18 0·25 0·045 0·055 3 0·30 2 0·060 4 0·129 4

Rb 3·2 34 3·3 2·3 2·1 3 1·6 3 1·2 4 7 4

Ba 41 129 45 40 13 3 33 3 12 4 56 4

Pb 0·91 18 1·16 0·34 1·13 3 1·48 2 0·88 4 0·39 4

Sr 89 155 70 108 171 3 112 3 158 4 83 4

Th 0·157 19 0·127 0·165 0·216 3 0·027 2 0·145 4 0·115 4

U 0·115 19 0·229 0·093 0·096 3 0·254 3 0·094 4 0·071 4

Nb 0·43 19 0·38 0·44 0·40 3 0·19 2 0·40 4 0·96 4

Ta 0·034 18 0·028 0·033 0·036 3 0·016 2 0·035 4 0·21 4

Zr 14 152 14 16 7·9 3 9·8 3 7·8 4 11·3 4

Y 5·1 36 5·6 4·9 3·1 3 5·5 3 3·2 4 4·2 4

Hf 0·31 20 0·24 0·29 0·25 3 0·24 3 0·22 4 0·37 4

La 0·82 28 0·92 0·90 0·45 3 0·257 2 0·84 4 0·72 4

Ce 1·81 29 2·03 2·01 1·00 3 0·614 2 1·43 4 1·80 4

Pr 0·191 18 0·174 0·202 0·130 3 0·099 2 0·167 4 0·239 4

Nd 0·93 29 0·92 1·19 0·48 3 0·55 2 0·64 4 0·96 4

Sm 0·274 30 0·268 0·373 0·156 3 0·255 3 0·181 4 0·316 4

Eu 0·108 30 0·128 0·136 0·044 3 0·141 3 0·066 4 0·098 4

Gd 0·384 21 0·354 0·412 0·233 3 0·468 2 0·263 4 0·407 4

Tb 0·091 30 0·103 0·122 0·053 3 0·129 3 0·050 4 0·084 4

Dy 0·63 20 0·63 0·74 0·453 3 0·79 2 0·445 4 0·68 4

Ho 0·157 20 0·160 0·181 0·115 3 0·196 2 0·113 4 0·158 4

Er 0·548 20 0·578 0·65 0·450 3 0·68 2 0·424 4 0·571 4

Tm 0·090 20 0·088 0·115 0·077 3 0·108 2 0·072 4 0·101 4

Yb 0·705 31 0·725 0·97 0·573 3 0·782 3 0·524 4 0·677 4

Lu 0·127 31 0·134 0·158 0·103 3 0·122 3 0·094 4 0·105 4

Cr 621 151 1012 123 902 3 359 3 740 4 710 4

Co 48 26 51 46 25 1 38 2 34 2 45 2

Ni 221 160 373 65 258 3 124 3 236 4 235 4

Cu 63 142 51 57 70 3 46 3 16 4 33 4

Zn 72 102 64 78 68 3 80 2 59 3 63 4

V 254 104 240 293 197 3 231 3 202 4 208

Sc 40 23 43 46 38 2 37 3 39 4 38

As 39 2 39 — — — — —

Ag — — — — — — —

Au 11·5 5 17·5 6·1 — — — —

1864



Intermediate-Ti boninitic Mt Misery tholeiites

Average n FeO/MgO Average n FeO/MgO

<0·74 >0·8 >1·14 <1·69

SiO2 53·9 46 52·8 55·1 52·8 53 52·2 53·6TiO2 0·45 46 0·44 0·46 1·07 53 0·76 1·45Al2O3 15·7 46 15·3 15·9 16·5 53 16·5 16·6FeO∗ 8·09 46 7·95 8·67 10·6 53 8·89 13·2MnO 0·18 46 0·14 0·23 0·23 53 0·25 0·26MgO 10·3 46 12·2 8·49 7·58 53 9·18 5·72CaO 7·05 46 7·21 6·61 6·43 53 7·39 4·23Na2O 4·19 46 3·57 4·73 3·82 49 3·27 4·22K2O 0·22 39 0·33 0·14 1·22 49 1·69 1·06P2O5 0·03 38 0·04 0·04 0·10 34 0·09 0·13LOI 4·25 41 3·96 5·02 5·74 48 6·18 4·96FeO/MgO 0·83 46 0·66 1·06 1·51 53 0·98 2·41S 950 5 — 937 900 2 900 —Cs 0·055 6 0·048 0·059 0·72 25 0·50 1·2Rb 1·6 8 2·2 0·95 21 36 22 33Ba 34 9 46 20 99 41 77 138Pb 0·85 6 0·47 1·04 0·98 19 0·76 1·4Sr 126 41 121 120 112 43 104 87Th 0·176 8 0·202 0·160 0·228 25 0·234 0·222U 0·097 6 0·088 0·101 0·286 20 0·092 0·568Nb 0·49 7 0·51 0·47 1·15 20 0·71 1·08Ta 0·047 6 0·052 0·044 0·12 19 0·13 0·12Zr 23 41 24 23 53 48 34 79Hf 0·59 10 0·52 0·66 1·46 26 0·93 2·2Y 13 41 12 12 21 46 17 25La 1·05 23 1·04 1·13 1·86 31 1·63 1·85Ce 3·23 21 3·06 3·33 6·18 31 4·86 7·23Pr 0·38 7 0·44 0·34 1·01 22 0·74 1·13Nd 2·74 20 3·05 2·56 5·74 28 3·99 7·18Sm 0·93 23 0·99 0·89 2·05 30 1·55 2·45Eu 0·36 23 0·38 0·37 0·80 30 0·63 0·95Gd 1·20 8 1·28 1·16 2·72 20 2·18 3·06Tb 0·275 19 0·296 0·248 0·53 30 0·41 0·65Dy 1·69 8 1·80 1·63 3·43 20 2·80 3·84Ho 0·421 8 0·409 0·428 0·703 24 0·595 0·760Er 1·27 8 1·20 1·31 2·12 20 1·86 2·32Tm 0·172 7 0·173 0·172 0·308 23 0·277 0·349Yb 1·25 20 1·18 1·28 2·16 30 1·73 2·61Lu 0·178 20 0·163 0·183 0·304 30 0·254 0·344Cr 342 41 380 282 216 43 361 61Co 73 11 70 77 40 21 39 36Ni 146 44 168 107 94 35 144 45Cu 76 41 69 85 40 31 55 19Zn 110 32 55 216 79 28 73 96V 247 29 241 255 248 29 234 279Sc 39 23 40 39 36 31 38 34As — — — 0·31 2 0·94 —Ag — — — 6·9 7 7·6 —Au 18·6 2 — 18·6 0·008 2 0·025 —Sb 0·23 1 — 0·23 0·65 2 0·65 —

Average end-member magma compositions. Acronyms as in Table 1. The average Low-Ti Betts Cove boninite includes bothlavas and dykes, but excludes the High-Nb end-member boninites. The numbers in the FeO/MgO columns refer to averagesof rocks having FeO/MgO ratios above or below the number given, and correspond to primitive and evolved compositions(respectively). End-member Low-Ti boninites described in text. n, number of analyses in the average; —, not analysed. Alltrace elements in ppm. Where data were below detection limits, a value of P2O5 ~0·01 was assumed.

1865



Fig. 3. (a, b) N-MORB-normalized trace element variation diagram Fig. 4. (a) U vs Th, and (b) Ba vs Th, in ppm. It should be noted thatfor Low-Ti and Intermediate-Ti boninites from Betts Cove. Logarithmic there is a reasonably good correlation of U vs Th for most Betts Covescale. The subdivision into Intermediate-Ti and Low-Ti boninites is boninites, aside from a few outliers affected by the intense hydrothermalexplained in the text. All N-MORB- and chondrite-normalized dia- alteration associated with the orebodies. The correlation for Ba vs Thgrams in this paper use the values from Sun & McDonough (1989). It is markedly poorer, but the high Ba contents of some of the tholeiitesshould be noted that individual profiles (identified by sample number) may be primary.show little variation for the relatively immobile elements (Th, Nb, La,Ce, Nd). In contrast, profiles are spiky for the most mobile elements(Cs, Ba, Rb, U, K, Pb, Sr), although the averaged profiles are relatively inites (Figs 5–7); and have similar mineralogical char-smooth even for ‘mobile’ elements. Averaged values exclude outliers

acteristics, including low-Ti clinopyroxene, high-Cr/(Craffected by unusual enrichment in Cs, Rb, Ba, K, U and Pb. AverageBonin Island boninite profile compiled from Hickey & Frey (1982) and + Al) chromites, and abundant orthopyroxene. TheTaylor et al. (1994). The average Bonin Island Nb content is probably mineralogical and geochemical data support the inferenceoverestimated.

that rocks of the Low-Ti suite are boninites, in accordwith Upadhyay (1980), Coish et al. (1982), Coish (1989),and Swinden et al. (1989). The strong resemblance toOverall, the Betts Cove Low-Ti lavas and dykes defineBonin Island lavas suggests that they should be classedtrends of decreasing Cr (Fig. 5a) and Ni (not shown) withas Type 3 Low-Ca boninites in the scheme of Crawfordincreasing FeO∗/MgO, and a steep, diffuse trend ofet al. (1989).SiO2 enrichment (Fig. 5b). Rocks of the Low-Ti suite

Intermediate-Ti suite lavas and dykes (clinopyroxenehave low contents of most incompatible elements (Figs± plagioclase ± olivine phyric) have flatter normalized6a and 7). Their normalized trace element profiles showtrace element profiles than do the Low-Ti boninites (Figsrelative enrichment in large ion lithophile elements (LILE)6b and 7b), with overall higher contents of moderatelyand LREE, in comparison with the middle REE (MREE),incompatible elements, but have similar LILE contents,which gives them ‘U’ shapes (Figs 6a and 7a). Enrichmentnegative Nb anomalies, and positive Pb and Sr anomalies.in LREE is variable, as reflected in the wide range ofWhether the Intermediate-Ti suite rocks are best classifiedLa/Nd ratios (Fig. 5d). Negative Nb anomalies, andas arc tholeiites or boninites is uncertain (see Coish,positive Pb, Sr and Zr anomalies are typical (Fig. 7a).1989). Swinden et al. (1997) referred to them as arcMost Low-Ti suite rocks are geochemically almost

indistinguishable from recent Bonin Island Low-Ca bon- tholeiites. The presence of feldspar phenocrysts in some

1866



Fig. 5. (a) Cr ppm vs FeO∗/MgO wt %, (b) SiO2 wt % vs FeO∗/MgO wt %, (c) TiO 2 wt % vs FeO∗/MgO wt %, (d) La/Nd vs La in ppm;for Betts Head lavas, Betts Cove sheeted dykes and Mount Misery Formation lavas. Sources of data are given in the text. Fields for BoninIslands and Troodos boninites are from Hickey & Frey (1982), Cameron (1985), Thy & Xenophontos (1991) and Taylor et al. (1994).

rocks does suggest an affinity to tholeiites, and they are chromite were developed to test this possibility (Ap-pendix 1). These models are inappropriate for frac-transitional towards Mount Misery tholeiites in many

respects (Figs 5–7). The Mount Misery tholeiites have tionation of plagioclase-bearing assemblages, so thethermodynamically based Weaver & Langmuir (1990)normalized incompatible-element profiles sub-parallel to

those of the Intermediate-Ti boninites (Figs 6b and crystal fractionation program was used to model differ-entiation of the locally plagioclase-phyric Intermediate-7b), but have consistently higher contents of moderately

incompatible elements in comparison with the boninitic Ti magmas. Parental magma compositions for the Low-Ti and Intermediate-Ti boninite series in these cal-rocks (Figs 5–7). Because the Intermediate-Ti suite rocks

are interbedded with Low-Ti boninites, and fall within culations are simply the averages of all analyses withFeO∗/MgO <0·61 and <0·74, respectively (Table 2).the compositional range of boninites in most variation

diagrams (e.g. Fig. 5), in this paper they will be referred The model results (Fig. 8) show that closed-system frac-tional crystallization cannot explain the observed rangesto as boninites.of SiO2, FeO∗, TiO2 or La/Nd ratios within the Low-Ti or Intermediate-Ti boninite suites. Nor can it generateresidual magmas similar to Intermediate-Ti lavas from

PETROGENESIS Low-Ti boninite parental melts, or Mount Misery thole-Fractional crystallization iites from Intermediate-Ti parents; in agreement with

the conclusions of Coish et al. (1982). FractionationCan fractional crystallization alone explain the com-coupled with assimilation of trace element depleted lower-positional spectrum observed within the Betts Cove bon-crustal or mantle lithologies does not yield significantlyinitic lavas and dykes? Fractionation models involving

incremental extraction of olivine, orthopyroxene and different results and so cannot change these conclusions.

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Fig. 7. N-MORB-normalized trace element variation diagrams, loga-Fig. 6. Chondrite-normalized REE plots, logarithmic scale. (a) Rep-rithmic scale. The profiles are for the same rocks as shown in Fig. 6,resentative Low-Ti, Betts Cove boninite lavas and dykes, comparedand use the same symbols. (a) Representative Low-Ti, Betts Covewith field of Bonin Islands boninites (sources of data as in captionboninites compared with Bonin Islands boninite field (sources of dataof Fig. 3). (b) Representative Intermediate-Ti, Betts Cove boninites,given in caption of Fig. 5). (b) Representative Intermediate-Ti, Bettscompared with field of Low-Ti boninites. The average Mount MiseryCove boninites, compared with field of Low-Ti boninites. The averagetholeiite is shown for comparison.Mount Misery tholeiite is also shown.

Betts Cove end-member magmasTherefore, the wide range in SiO2 and incompatibleThe first step in the modelling process is to define thetrace element ratios in each suite must reflect eitherdifferent end-member magmas. Stratigraphic relationsmelting processes or source heterogeneity.allow a first-order division into the Mount Misery tholei-ites and the Betts Head–sheeted dyke boninites. Theboninites are subdivided into Low-Ti and Intermediate-Ti suites, as proposed by Coish & Church (1979). TheLow-Ti boninites are further subdivided into end-mem-Partial melting models or sourcebers to allow quantitative modelling of source hetero-

heterogeneity? geneity. This is done by defining a number of end-Partial melting models are developed to determine member boninite ‘poles’ or subtypes that bracket thewhether or not mantle melting processes can explain range of observed compositions. These end-members docompositional variations among the Betts Cove lavas and not correspond to precise stratigraphic intervals, anddykes. Rather than use completely ad hoc compositions, should be viewed only as the most extreme poles of athe modelling uses mantle compositions calculated from continuous compositional spectrum.the Betts Cove data. The solutions are non-unique, but The La/Nd ratio should reflect the amount of thethey provide a realistic starting point for modelling intra- slab-derived SZ component added to the depleted mantle

wedge (Pearce & Parkinson, 1993), and this ratio is usedsuite geochemical variation.

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Table 3: Calculated model parental end-member magmas

Low- La/Nd Low-Ti Intermediate- High-Nb High-Th/La High-La/Nd

boninite boninite Ti boninite boninite boninite boninite

BF13% BF12% BF7% BF10% BF8% BF9%

SiO2 54·28 53·63 51·99 53·95 55·18 56·82

TiO2 0·20 0·11 0·38 0·15 0·11 0·09

Al2O3 13·20 11·48 14·32 12·60 11·09 10·90

FeO∗ 7·62 8·26 7·94 8·62 8·18 7·12

MnO 0·20 0·19 0·16 0·16 0·15 0·14

MgO 16·85 16·42 14·53 15·49 14·66 12·56

CaO 4·15 6·80 6·93 5·26 7·50 9·03

Na2O 3·20 2·55 3·27 3·00 2·69 2·96

K2O 0·05 0·24 0·33 0·47 0·10 0·11

P2O5 0·03 0·02 0·03 0·01 0·01 0·01

Cr2O3 0·20 0·31 0·11 0·29 0·33 0·27

Fo mol % 93·6 92·9 92·4 92·2 92·2 92·2

Ti 1225 645 2287 918 678 521

P 145 73·0 137 32·8 33·0 33·4

Cs 0·231 0·073 0·226 0·106 0·067 0·050

Rb 1·23 2·51 2·78 5·39 0·78 1·01

Ba 25·3 32·4 64·4 45·6 10·0 9·7

Pb 1·14 0·720 0·740 0·318 0·613 0·738

Sr 86·5 70·0 107 67·9 87·3 132

Th 0·021 0·120 0·116 0·094 0·197 0·121

U 0·196 0·091 0·210 0·058 0·106 0·078

Nb 0·143 0·325 0·400 0·784 0·418 0·335

Zr 7·58 10·8 27·6 9·28 7·49 6·51

Y 4·25 5·26 10·9 3·44 2·71 2·68

La 0·198 0·626 0·790 0·586 0·573 0·706

Ce 0·474 1·41 2·47 1·47 1·15 1·20

Pr 0·076 0·144 0·316 0·196 — 0·139

Nd 0·423 0·729 2·63 0·789 0·591 0·536

Sm 0·197 0·217 0·851 0·259 0·170 0·152

Eu 0·109 0·087 0·329 0·080 0·053 0·055

Gd 0·361 0·307 1·00 0·333 0·232 0·220

Tb 0·100 0·072 0·258 0·069 0·051 0·042

Dy 0·609 0·501 1·49 0·554 0·429 0·372

Ho 0·151 0·124 0·333 0·130 0·098 0·094

Er 0·528 0·431 1·03 0·468 0·381 0·354

Tm 0·083 0·072 0·150 0·083 0·064 0·061

Yb 0·604 0·550 1·02 0·555 0·466 0·438

Lu 0·095 0·099 0·141 0·087 0·083 0·079

Cr 1369 2134 777 1986 2261 1868

Co 66·2 81·0 95·0 70·1 42·6 50·4

Ni 1210 1813 589 1354 1154 1140

Cu 40·2 55·8 63·7 29·3 57·5 14·8

Zn 77·2 68·8 54·1 61·2 76·5 57·9

V 192 214 217 181 169 178

Sc 29·5 32·0 33·1 32·4 34·4 33·5

Calculated parental end-member magmas, normalized to 100%. BF, amount of back-fractionation of 99:1 olivine:spinel(Appendix 1); Fo, molar % forsterite of the olivine in equilibrium with these magmas, assuming 10% of the iron in the meltis in the Fe3+ form and that the olivine FeO/MgO exchange coefficient is 0·3 (Roeder & Emslie, 1970). All trace elements inppm.

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to define one set of end-members. Thus, ‘High-La/Nd’ Recalculation to bring end-member magma compositions intoand ‘Low-La/Nd’ end-members are defined as the Low- equilibrium with the mantleTi boninites with the highest and lowest La/Nd ratios, All of these end-member magma compositions haverespectively (Table 2). During the course of the modelling FeO∗/MgO ratios that are too high to coexist withit became apparent that some Low-Ti boninites had normal or depleted peridotite mantle, and so a parentalhigher Th abundances than most of the others, whrereas magma composition was calculated for each end-memberothers were richer in Nb. This suggests that additional magma (Table 3) assuming fractional crystallization (Ap-components or processes might be recorded by these Th- pendix 1) of a mineral assemblage comprising 99% olivineor Nb-rich boninites, and, for this reason, a pair of ‘High- and 1% chromite. The Intermediate-Ti boninite parentTh/La’ and ‘High-Nb’ end-members were also defined. and most Low-Ti boninite parents were brought into

equilibrium with Fo~92 olivine, which would be ap-propriate for a refractory wedge peridotite. The Low-La/Nd end-member is extremely depleted in in-compatible trace elements, which might reflect a greaterextent of prior mantle depletion. For this reason, theLow-La/Nd end-member was recalculated to bring itinto equilibrium with a slightly more refractory olivineof Fo93·6 composition. In all cases, the degree of calculatedback-fractionation is small (8–13%, Table 3), and theassumptions made about the extent of fractionation andthe phases involved (olivine or orthopyroxene) have onlya small effect on the absolute abundances of incompatibletrace elements in the calculated parental magmas (e.g.see Fig. 8b). Furthermore, fractionation of small amountsof olivine, spinel or orthopyroxene does not significantlychange the characteristic shape of incompatible traceelement profiles, and so all calculated parental magmaprofiles (Fig. 9) retain the small positive Zr anomalies,large positive Pb+ Sr anomalies, negative Nb anomalies,and the enrichment in Th + LILE of the uncorrecteddata.

Fig. 8. (a) SiO2 vs FeO∗/MgO, (b) TiO 2 vs FeO∗/MgO, all in wt%. Fractionation vectors compared with fields of data from Fig. 5(b,c). The point labelled ‘B’ is the average Low-Ti Betts Cove boninitewith FeO∗/MgO < 0·61. The point labelled ‘I’ is the average Inter-mediate-Ti Betts Head boninite with FeO∗/MgO < 0·74 (Table 2).Curves labelled ‘opx25’, ‘ol25’ and ‘ol20’ refer to fractionation of25% orthopyroxene, 25% olivine and 20% olivine, respectively (seeAppendix 1 for method). Curve labelled ‘wl40’ is for 40% fractionalcrystallization of the Intermediate-Ti boninite parent ‘I’ calculated withthe program of Weaver & Langmuir (1990), run at 2 kbar, with theCaO and Na2O contents of the parent magma adjusted to 10 and 2wt %, respectively, to correct for the effects of hydrothermal alteration,which has perturbed these elements in the Betts Cove lavas. (c) La/Nd vs La, showing fractionation and partial melting vectors comparedwith field of data of Betts Cove boninites from Fig. 5(d). Numbers nextto the tick marks represent percent melting or fractionation. The curvelabelled ‘C’ tracks 80% fractional crystallization of olivine + spinel(99/1) from the Low-La/Nd boninite model parent, labelled ‘L’. Curve‘FMM PCM’ tracks the evolution of pooled critical melts (one-thirdmelt retention assumed) from the fertile MORB mantle (FMM) source.Curves labelled ‘BM’, ‘PFM’ and ‘PCM’ track the evolution of batch(equilibrium), pooled fractional and pooled critical melts, respectively,of the High-La/Nd source mantle ‘H’. Curves ‘IFM’ and ‘ICM’ are,respectively, instantaneous fractional and critical melts of the High-La/Nd source. (See Appendix 3 and text for methods. Note that pooledfractional and batch melts are similar, and that neither shows asignificant range in La/Nd.)

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melting suggests that the form of the moderately in-compatible trace element profile in the residues is notstrongly dependent on the melting process. The abund-ances of the very highly incompatible elements (Cs, Th,U, Nb, La, Ce), in contrast, are strongly dependent onthe nature of the melting process. However, they areeven more sensitive to source composition (Pearce &Parkinson, 1993), and so the source mantle profilescalculated from different end-member magmas (assumingsimilar residual modes and degrees of fusion) shouldprovide a useful baseline for comparison.

The trace element composition of the mantle residuein equilibrium with a given magma can be calculated bysummation:

CRM = RuDCL. (1)

The modal proportion of the different residual mineralsis u. The concentration of a trace element in the meltis CL, and CRM is the composition of a trace element inthe residual mantle. The mineral–liquid trace elementpartition coefficient for a given element is D (see Ap-pendix 2, Table A1).

The composition of the pre-melting source mantle(CSM) can then be reconstructed by adding appropriateproportions of the melt (Table 3) to its assumed residualmantle assemblage (Table 4):

CSM = (RuDCL) + (FCL). (2)

The composition of the melt (CL) dominates the traceFig. 9. N-MORB-normalized trace element variation diagrams forcalculated parental magma compositions for different types of Betts element budget of the model sources [equation (2)] atCove parental magmas (see Table 3). Logarithmic scale. All model degrees of melting >1%. Varying the assumed value ofmagmas were corrected for low-pressure fractionation of olivine and

F (degree of melting) from 5 to 10% has a negligiblechromite, as described in Appendix 1. Extents of back-fractionationeffect on the shape of the model source’s trace element(BF) are also given in Table 3. (a) The ‘High-La/Nd’ and ‘Low-La/

Nd’ end-members are the averages of Low-Ti Betts Cove boninites profile (Fig. 10), although it does have a small effect onwith the highest and lowest La/Nd ratios, respectively. (b) ‘High-Th/ the absolute abundances of trace elements. AlthoughLa’ and ‘High-Nb’ are the averages of Low-Ti Betts Cove boninites

small errors in the assumed value of F cannot changewith the highest Th/La ratios and Nb contents, respectively.the conclusions obtained, the models will be more realisticif accurate estimates of F are used. Van der Laan etal. (1989) compared boninite compositional data withexperimentally determined liquidus phase boundaries

Source composition and inversion models and proposed that Low-Ca boninites formed through7–13% mantle melting. Consequently, most Low-Ti BettsTo reconstruct the composition(s) of the mantle source

from a lava, it is necessary to make assumptions about Cove parental boninite end-members are assumed torepresent 10% melting. A lower degree of melting isthe nature of the melting process (equilibrium, fractional,

or critical melting), to constrain the modal composition assumed for the depleted Low-La/Nd parental magma(6%), so as to constrain its model source mantle to haveof the residue in equilibrium with the parental magmas,

and to estimate the degree of fusion involved. a moderately incompatible trace element content lessthan that of the model High-La/Nd source mantle, andThe inversion calculations presented here assume equi-

librium, or batch melting, as there are no unique inversion thus permit mass balance calculation of SZ componentcompositions (see below). The Intermediate-Ti boninitesolutions for fractional or critical melting. This necessary

simplification is probably not a fatal flaw, as equilibrium parental magma, which is less depleted in incompatibleelements, is assumed to have formed through a slightlyand pooled fractional melts have similar compositions

for moderately incompatible elements (Fig. 8c, and see greater degree of fusion (12%).The modal composition of the residue (Ru) must alsobelow, Fig. 13). Furthermore, comparison of profiles

representing the residues of fractional, critical and batch be assumed in equations (1) and (2). Although small

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Table 4: Model mantle source compositions

Low- High- High- High- Low- Low- Intermediate- Intermediate-Ti La/Nd Th/La Nb La/Nd La/Nd(I) Ti Ti(I)BF10% BF9% BF9% BF10% BF13% BF13% BF9% BF9%

Ti 71 55 66 97 85 45 297 297P 7·6 3·3 3·7 3·3 8·7 1·83 16·0 15·9Cs 0·008 0·005 0·005 0·011 0·014 0·000036 0·026 0·0002Rb 0·26 0·10 0·18 0·54 0·074 0·0011 0·32 0·0065Ba 3·4 0·97 1·1 4·6 1·5 0·031 7·4 0·183Pb 0·075 0·074 0·094 0·032 0·069 0·0009 0·086 0·007Sr 7·3 13·2 14·3 6·8 5·2 0·335 12·5 2·81Th 0·012 0·012 0·018 0·009 0·001 0·00043 0·013 0·003U 0·009 0·008 0·008 0·006 0·012 0·00017 0·024 0·001Nb 0·034 0·034 0·034 0·079 0·009 0·007 0·047 0·048Ta 0·003 0·003 0·003 0·018 0·001 0·0005 0·005 0·003Zr 1·14 0·66 0·67 0·94 0·47 0·30 3·29 2·37Hf 0·026 0·019 0·021 0·031 0·012 0·010 0·141 0·071Y 0·46 0·27 0·26 0·35 0·26 0·26 1·31 1·31La 0·065 0·071 0·037 0·059 0·012 0·008 0·092 0·062Ce 0·147 0·120 0·084 0·148 0·029 0·023 0·287 0·177Pr 0·015 0·014 0·011 0·020 0·005 0·0043 0·037 0·033Nd 0·076 0·054 0·040 0·079 0·026 0·026 0·307 0·221Sm 0·023 0·015 0·013 0·026 0·012 0·010 0·100 0·081Eu 0·009 0·005 0·004 0·008 0·007 0·007 0·039 0·039Gd 0·032 0·022 0·019 0·033 0·022 0·022 0·118 0·118Tb 0·008 0·004 0·004 0·007 0·006 0·006 0·031 0·031Dy 0·052 0·037 0·038 0·056 0·037 0·037 0·177 0·177Ho 0·013 0·010 0·010 0·013 0·009 0·009 0·040 0·040Er 0·046 0·036 0·038 0·048 0·033 0·033 0·124 0·124Tm 0·008 0·006 0·007 0·009 0·005 0·005 0·018 0·018Yb 0·060 0·046 0·050 0·058 0·040 0·040 0·127 0·127Lu 0·011 0·008 0·009 0·009 0·007 0·007 0·018 0·018Cr 32200 32900 16000 36700 24200 24200 15200 15200Co 223 151 167 211 201 201 291 291Ni 11600 10400 5460 12300 11100 11100 6900 6900Cu 31 8 23 16 20 20 33 33Zn 66 55 74 59 73 73 51 51V 134 105 120 110 108 108 122 122Sc 8 8 8 8 7 7 9 9Residual modesCPX 0 0 0 0 0 0 0·0056 0·0056OPX 0 0 0 0 0·0417 0·0417 0·0954 0·0954OL 0·8654 0·8671 0·8671 0·8654 0·8653 0·8653 0·7496 0·7496SP 0·0346 0·0329 0·0329 0·0346 0·0330 0·0330 0·0294 0·0294TMF 0·1 0·1 0·1 0·1 0·06 0·06 0·12 0·12

EM10Bon EM5Bon DM10Bon

CPX 0·032 0·034 0OPX 0·125 0·132 0·087OL 0·714 0·754 0·782SP 0·029 0·03 0·030TMF 0·1 0·05 0·1

Calculated compositions and model mineralogies of model source mantles. Method is explained in text. Notes as in Table2. ‘Low-La/Nd(I)’ and ‘Intermediate-Ti(I)’ are the concentrations of the smoothed (interpolated) profiles of the Low-La/Ndand Intermediate-Ti mantles (Fig. 12) calculated as in Pearce (1983). The first row of residual modes represents the solidassemblage in equilibrium with the trapped melt fraction (TMF) for each end-member mantle. The profiles of Fig. 10 werecalculated with the second row of residual modes, all of which were assumed to be in equilibrium with the average Low-Ti boninite, where EM10Bon and EM5Bon are the residual modes for 10 and 5% melting of an enriched source, respectively,and DM10Bon is the residual mode for 10% melting of a depleted source. All values in ppm.

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Fig. 11. FMM-normalized trace element variation diagrams comparingFig. 10. Fertile MORB mantle (FMM)-normalized trace element model source mantles calculated from Betts Cove parental magmasvariation diagrams illustrating how the composition of the model mantle (values in Table 4), with residues calculated from the fusion of FMMinversion calculation depends on variations in the assumed source mantle (dotted lines). Logarithmic scale. Only the more compatiblemode and degree of melting. Logarithmic scale, normalized to FMM part of the profile is shown. Dotted curves labelled with a percentageof Table A2. Model source mantles assume equilibrium with the average are residues of batch (equilibrium) melting of FMM. The numberBetts Cove Low-Ti boninite model parent (Table 3). If the model adjacent is the percent melting involved.source mode remains constant, the assumption of larger extents ofmelting (e.g. 10%) produces model sources (EM-10%) with higherincompatible element contents than if 5% melting is assumed (EM- mantle, Table A2, see Appendix 3). The modal as-5%), although the profiles have similar shapes. If the extent of melting semblage of the most closely matching theoretical FMMis kept constant, then an ‘enriched’ model mantle (EM-10%) with

residue is dunitic for most Low-Ti boninite source mantles~3·3% residual clinopyroxene has only slightly higher incompatibleelement contents than a clinopyroxene-free ‘depleted’ model mantle and is lherzolitic for the Intermediate-Ti boninite source(DM-10%). mantle (Table 4). These residual modes were adopted

for a second (and final) inversion calculation using equa-tions (1) and (2). The first- and second-pass results of

variations in the assumed residual mode (e.g. clino- the calculations are essentially identical, and no furtherpyroxene 0–3·5%, orthopyroxene 8·7–13%) have only a iterations are necessary.small effect on the calculated source profiles (Fig. 10), Comparison of the model source mantles calculatedthe results will be more accurate if a realistic residual from the Betts Cove model parental magmas with theassemblage is used. The residual mode used in the models theoretical residues of FMM allows the degree of prioris determined as follows. First, an initial modal estimate melt extraction from their source mantles to be de-is obtained for each end-member magma type on the termined (Fig. 11; the full profiles are shown in Fig. 12,basis of phase equilibria studies, which imply that boninite and the values are given in Table 4). The comparisonmagmas separate from extremely depleted, clino- implies that the Betts Cove Low-Ti boninite source hadpyroxene-free dunitic to harzburgitic residues, in contrast already lost 19–22% equilibrium melt; whereas the sourceto most tholeiites and arc basalts, which separate from of the Intermediate-Ti boninites was less depleted, havingless depleted harzburgite to lherzolite residues (Howard lost only ~12% equilibrium melt. These estimates of the& Stolper, 1981; Dick et al., 1984; Crawford et al., extent of prior source depletion are similar to published1989; Falloon et al., 1989; Parkinson & Pearce, 1998). estimates for wedge mantle depletion (Ewart & Hawkes-Consequently, a residual harzburgite assemblage (DM- worth, 1987; McCulloch & Gamble, 1991; Pearce &10Bon residual mode in Table 4) is assumed for an Parkinson, 1993; Woodhead et al., 1993, 1998; Pearce etinitial calculation of all Low-Ti boninite residual mantle al., 1995; Ewart et al., 1998; Parkinson & Pearce, 1998).subtypes using equation (1), and pre-melting model source This depletion may be related to seafloor spreading atmantles using equation (2). Because the Intermediate-Ti an oceanic ridge, to back-arc spreading, or to earlierboninites are less depleted in incompatible trace elements, arc magmatism. The lack of relative HREE/MREEa more fertile clinopyroxene harzburgite residue (clino- fractionation among the different model sources (Figs 11pyroxene:orthopyroxene:olivine:spinel = CPX:OPX: and 12) suggests that this prior depletion took place inOL:SP = 1:10:74:3) is assumed for them. the spinel lherzolite field.

The trace element profiles of these initial models are Most model source mantle profiles calculated usingthen compared with theoretical residual mantle profiles equation (2) for the different Low-Ti boninite end-mem-

bers are very similar. Like the melt compositions fromformed by equilibrium melting of FMM (fertile MORB

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Fig. 12. (a–c) FMM-normalized trace element variation diagrams showing calculated source mantles (values in Table 4). Logarithmic scale.The different end-member model mantles were calculated from their respective end-member boninite parental magma compositions (Table 3;see Appendix 3 and text for method). All profiles assume 10% melting, except those for the Low-La/Nd end-member (6%) and the Intermediate-Ti boninite (12%). The average Low-Ti boninite profile does not include the Low-La/Nd or High-Nb boninites. The ‘Low-La/Nd(I)’ and‘Intermediate-Ti Interpolated, or (I)’ profiles are smoothed, pre-fertilization values inferred for the mantle wedge (method of Pearce, 1983). (d)Comparison of the average Low-Ti boninite model mantle and Low-La/Nd model mantle with the average talc-magnesite schist analysed byAl (1990). Data for Zr, Pb and P from Al (1990) are not shown, because of inferred analytical problems. Elements such as U (not shown), Baand Eu have been affected by hydrothermal remobilization, and show considerable scatter. The more immobile elements (Th, REE) as well asSr resemble the Low-Ti boninite model mantle, except for Th in the Al (1990) data, which is always 0·1 ppm and may be overestimated.

which they were computed, the model source mantle talc–magnesite schists has remobilized some elements,leading to considerable scatter in Ba, U, Sr and Eu.profiles (Fig. 12) typically show enrichment in Ba, Th,

U, LREE, Sr and Pb (also Rb and Cs, not shown), When the less mobile elements are considered, however,variable Zr enrichment, and Nb depletion. The Low- the profile of the average talc–magnesite schists is ratherLa/Nd source mantle profile differs from the others in similar to the model mantle compositions computed herehaving a smaller positive Zr peak and in lacking Th and (Fig. 12d). The overall similarity of composition betweenLREE enrichment. Like the Low-Ti boninite model the analyses of Al (1990) and the models presented heremantles, the Intermediate-Ti model mantle is enriched suggests that the model results shown in Fig. 12 are atin LILE, Pb and Sr, and depleted in Nb, but it is notably least approximately correct. The largest discrepancy ismore enriched in MREE to HREE. for Th, which is systematically high in the rocks analysed

The massive talc-magnesite schists along the shore by Al (1990). This probably reflects the relative im-of Red Cliff and Long Ponds are mineralogically and precision of the Th data (all are 0·1 ppm) from Al (1990).compositionally distinct from the layered cumulates andmay represent slivers of mantle rock. Model mantles

Closed-system melting modelscalculated from the lavas are compared with analysesof these talc–magnesite schists in Fig. 12d. Pervasive Melting models were developed (Appendix 3, Figs 8c

and 13) to test whether variable extents of melting of ahydrothermal alteration and addition of carbonate to the

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Multiple SZ components added to thedepleted wedgeHow many subduction zone components?

To create the characteristic trace element and isotopicpatterns of arc magmas, one or more SZ componentsderived from the subducting slab need to be added tothe depleted mantle wedge (Rogers et al., 1985; Ellam &Hawkesworth, 1988; Elliott et al., 1997; Kepezhinskas etal., 1997; Turner et al., 1997; Ewart et al., 1998; Gribbleet al., 1998). There are considerable data implying thatthe elements B, Pb, Ba, Rb, U and Sr are largely carriedby a hydrous fluid (SZ-Hydrous) produced when thesubducting, hydrothermally altered oceanic crust isheated and devolatilizes (Hickey & Frey, 1982; White &Dupre, 1986; Davidson, 1987; Tatsumi, 1989; Lin et al.,

Fig. 13. N-MORB-normalized trace element variation diagrams com- 1990; McDermott et al., 1993; Miller et al., 1994; Brenanparing the calculated parental Low-La/Nd boninite end-member et al., 1995; Elliott et al., 1997; Turner et al., 1997;magma (Table 3) with partial melts produced from the High-La/Nd

Iwamori, 1998; Schmidt & Poli, 1998; Stalder et al., 1998;model source mantle. Logarithmic scale. Equilibrium, batch and pooledWoodhead et al., 1998).fractional melts of the High-La/Nd source are nearly indistinguishable,

and none resemble the Low-La/Nd magma, implying that Low-La/ Experimental data and numerical modelling (PeacockNd magmas cannot originate from a High-La/Nd source.

et al., 1994; Iwamori, 1998; Schmidt & Poli, 1998) implythat when young, hot, oceanic crust is subducted, thehydrated basaltic–gabbroic part of the subducting slab

single source mantle could have produced the range of may begin to melt at shallow depths, yielding wet meltscompositions (Figs 5d, 6 and 7) observed in the boninites of adakitic, trondhjemitic or tonalitic composition (e.g.at Betts Cove. Given that the model sources defined Cameron, 1985; Defant & Drummond, 1990; Schianoabove approximate the composition of the actual sources, et al., 1995). These melts will be referred to as SZ-ATTthen the results of the melting models imply that frac- henceforth. Pearce et al. (1992) and Taylor et al. (1994)tional, equilibrium or critical melting of a single source have suggested that SZ-ATT was involved in boninitemantle cannot generate the range of trace element ratios genesis. The isotopic signatures of SZ-Hydrous and SZ-seen in the Betts Cove boninites. ATT are probably indistinguishable, as they have the

same source, but trace element chemistry should allowdiscrimination, because SZ-ATT is a silicate melt and

Metasomatic models so should contain significant concentrations of Zr, ThIt has been proposed that percolation of partial melts and LREE (Fig. 14a). In contrast, because of the relativethrough their own mantle residues generates arc-like insolubility of these elements in water (Brenan et al.,geochemical signatures (depletion in Nb and Ti, and 1995; Keppler, 1996; Ayers et al., 1997; Stalder et al.,relative LREE/MREE enrichment) in the magmas (Kele- 1998), SZ-Hydrous should contain little or no Zr, Th ormen et al., 1990, 1993). A powerful argument against an LREE.autometasomatic model is the heterogeneity of isotopic Analysis of in situ partial melts of oceanic crust fromsignatures from arc-related volcanic suites, including bon- ophiolites (plagiogranites or trondhjemites) and ofinites, which implies the involvement of an external SZ modern adakites can also help to constrain the com-component (Brown et al., 1982; White & Dupre, 1986; position of the SZ-ATT end-member (Figs 14a and 15).Woodhead, 1989; Olive et al., 1997). Limited isotopic There are basically two types of ophiolitic trondhjemitesdata from Betts Cove (Coish et al., 1982; Swinden et al., (e.g. Pederson & Malpas, 1984; Elthon, 1991; Flagler &1997) also imply an external SZ component. There may Spray, 1991; Jenner et al., 1991): those that formed bybe complex metasomatic reactions associated with the anatexis of amphibolitized gabbros, and those derivedinflux of such a fluid-rich SZ component into the depleted through fractionation of basalts. Typical anatectic ophi-mantle wedge (e.g. Bodinier et al., 1990; Van der Wal & olitic trondhjemites have rather flat REE profilesBodinier, 1996), but these processes cannot easily be (Fig. 14a), with low La and La/Nd (Fig. 15a), and highreconstructed from the lava chemistry. The simplifying Zr and Zr/Sm (Fig. 15b). The negative Nb and positive

U anomalies of ophiolitic trondhjemites probably reflectassumption that will be made here is that the Betts Covelava sources can be modelled by simple addition of an the arc affinity of their protoliths (e.g. Elthon, 1991).

Adakites differ from trondhjemites in showing LILE–SZ component to a depleted mantle wedge.

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Pb–Sr enrichment (Fig. 14a). Although the overall sim- et al., 1994; Brenan et al., 1995; Pearce et al., 1995; Elliottilarity to trondhjemites (Fig. 14a) supports the concept et al., 1997; Turner et al., 1997; Plank & Langmuir, 1998).that adakites are indeed melts of the oceanic crust, many I will refer to this component as SZ-Sediment henceforth.of their trace element characteristics (Fig. 15) more closely Oceanic sediments typically show prominent positiveresemble those of sediments, which implies that a sed- LILE and Pb anomalies, and negative Ti and Nb anom-imentary component must also be involved in their alies (Fig. 14a). They rarely show Sr or Zr enrichmentgenesis (e.g. Defant & Drummond, 1990; Maury et al., relative to the REE. SZ-Sediment has been identified as1996; Stern & Kilian, 1996). the principal source of enrichment in 10Be, LREE and

In many arcs, distinctive trace element and isotopic Th in arc magmas, and may also contribute to the Zr,data appear to require the involvement of a component U and Pb budget.derived from wet melting of subducted sediments (White Many of the trace element signatures interpreted by& Dupre, 1986; Davidson, 1987; Ellam & Hawkesworth, some as SZ-Sediment have been attributed to in-1988; Woodhead, 1989; McDermott et al., 1993; Cousens volvement of an SZ component similar to ocean island

basalts (SZ-OIB) by others (Fig. 14a). It was originallysuggested that SZ-OIB resides in the supra-subductionzone wedge as dispersed melt or veins (e.g. Morris &Hart, 1983; Stern & Ito, 1983; Falloon & Crawford,1991; Stern et al., 1991; Kostopoulos & Murton, 1992;Lin, 1992). Stern et al. (1991) proposed that SZ-Hydrousderived from the slab leaches the dispersed SZ-OIBcomponent from the mantle wedge and carries it up tothe melting zone. More recently, others have proposedthat SZ-OIB enters the arc source through subductionof hotspot-related seamounts (Turner et al., 1997; Ewartet al., 1998), or by movement of plume-related materialaround the edge of a subducting slab (Wendt et al., 1997;Ewart et al., 1998), or by refusion of OIB-related residues(Danyushevsky et al., 1995). Ascent of undepleted mantleduring back-arc extension is another possible mechanism

Fig. 14. N-MORB-normalized trace element variation diagrams, loga-rithmic scale. (a) Typical profiles of potential SZ components. Theaverage ‘adakite’ is from Drummond et al. (1996). The OIB (oceanisland basalt) is from Sun & McDonough (1989). The ‘av. PL Sediment’is the average of all the integrated sediment columns of Plank &Langmuir (1998), ‘Bulk Mariana Sediment’ is from Elliot et al. (1997).Average trondhjemites marked ‘BOI’ from the Bay of Islands [Bedard,unpublished data, 1998; also Elthon et al. (1991) and Jenner et al.(1991)], and ‘TM’ from Thetford Mines (Olive et al., 1997). (b) Variationof elemental abundances of the model SZ-Total [SZ-Total = SZ-Hydrous + SZ-(Sediment/ATT/OIB)] component calculated usingequation (4) with the assumed mixing proportion z. The percentagesrepresent the amount (z) of SZ-Total added to the Low-La/Nd(I)mantle to create the High-La/Nd mantle. It should be noted how theshape of the model SZ component profiles does not change as z varies,although the absolute trace element abundances increase as the assumedvalue of z decreases. Thus, trace element ratios are not sensitive to theassumed value of z. It should be noted also that the profiles shownegative Nb and Ti troughs similar to those of sediments, but dissimilarto typical OIB. ‘Bulk Sumatra Sediment’ is from Plank & Langmuir(1998). (c) The different SZ-Total components calculated in this paper(values in Table 5). SZ-Total components were calculated with equations(4) and (5) from the Betts Cove boninite end-member mantles assumingz = 0·25%. The overall similarity of all the profiles should be noted,and how they provide reasonable matches to a typical sediment (BMS,Bulk Mariana Sediment) for most elements more incompatible thanNd. Conversely, there is a better match to Thetford Mines trondhjemite(TM) for the P–Nd–Sm–Zr profile segment.

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so that it would be reassuring if an independent argumentcould be constructed using the trace element data alone.In the following sections I use the model mantle traceelement compositions calculated previously to determinethe compositions of the SZ component involved in thepetrogenesis of the Betts Cove boninites, in an attemptto discriminate between competing hypotheses. The firststep is to calculate the composition of the depletedwedge component from the least enriched model mantlecomposition (Low-La/Nd) defined above. Then, mixingcalculations between the depleted end-member and themost enriched end-members (High-La/Nd, High-Th/La, High-Nb) allow the composition of the SZ com-ponent(s) to be calculated. The procedure should be validfor any suite of primitive magmas where there is a rangeof trace element profile shape.

The mantle wedge: the depleted component

The Low-La/Nd model mantle has the most depletedtrace element profile of all the Betts Cove model mantlescalculated above (Fig. 12a), and so is assumed to bestrepresent the pre-fertilization depleted mantle wedge(Pearce, 1983; Pearce & Parkinson, 1993). However, eventhis depleted Low-La/Nd mantle has prominent positiveBa + U + Sr + Pb anomalies (Fig. 12a). As these areprecisely the elements thought to be associated with SZ-Hydrous, it seems reasonable to attribute this enrichmentpattern to addition of SZ-Hydrous to the depleted wedge.

Fig. 15. (a) La/Nd vs La ppm, and (b) Zr/Sm vs Zr ppm. Trace The Low-La/Nd mantle lacks enrichment in Th +element ratio plots comparing the values of SZ-Total in Table 5 with LREE + Zr, however, and so it is logical to infer thatpotential SZ-components. The curves are solutions to equations (4)

other SZ components did not affect the Low-La/Ndand (5) for different values of assumed z, shown as percentages. Itboninite mantle source. Conversely, the ubiquitous pres-should be noted that the LREE signatures of SZ-Total from this paper

more closely resemble values of sediments, whereas the moderately ence of prominent positive Ba+U+ Sr+ Pb anomaliesincompatible MREE and Zr signatures more closely resemble values in all Betts Cove boninites and Mount Misery tholeiitesof oceanic trondhjemites. Adakite data from Defant & Drummond

(Fig. 7) suggests that SZ-Hydrous may have more widely(1990), Defant et al. (1991), Drummond et al. (1996), Maury et al. (1996)and Prouteau et al. (1996); Cook Island data from Stern & Kilian dispersed in the mantle under Betts Cove.(1996). Residual trondhjemites are those interpreted to be residues from The composition of the pre-metasomatic wedge mantlefractional crystallization of basalts, whereas anatectic trondhjemites are

can be approximated in a manner analogous to thatthose interpreted to be results of partial melting of crustal amphibolites.proposed by Pearce (1983) and Pearce & ParkinsonTrondhjemite data are from Elthon (1991), Flagler & Spray (1991) and

Jenner et al. (1991), as well as unpublished data (Bedard, 1998) from (1993), by removing the prominent spikes (SZ-Hydrousthe Bay of Islands; and the average plagiogranite is from Drummond component) from the model Low-La/Nd mantle by in-et al. (1996). Sediment data from Woodhead (1989), McCulloch &

terpolation and extrapolation (Fig. 12a). This pre-SZGamble (1991), McDermott et al. (1993), Cousens et al. (1994), Elliot etal. (1997) and Plank & Langmuir (1998). Low-Ti boninite wedge source is referred to as the Low-

La/Nd(I) mantle (I for interpolated), henceforth, and itscomposition is given in Table 4. A flat profile that extends

to explain the presence of OIB-like signatures in arc from Nd and passes just below Pr and Nb is inferred formagmas (Lin et al., 1990; Gribble et al., 1998). Low-La/Nd(I), with a shallow, positively sloped profile

Determining which of these numerous potential SZ linking Nd to Eu (Fig. 12a). Use of a differently slopedcomponents is responsible for the distinctive geochemical profile for the Nd–Cs segment of Low-La/Nd(I) wouldand isotopic signatures of a given arc suite is a difficult change the absolute abundances and slopes of the modelproblem. Typically, the identity and proportion of these SZ-component profiles calculated below, but would notcomponents are identified with isotopic data. However, affect their shapes (peaks and troughs), and would onlythere is always the suspicion that some isotopic systematics slightly change their ratios. An Intermediate-Ti(I) profile

can be calculated in a similar fashion, by passing amay be decoupled from the trace element concentrations,

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straight line from Ti through Nb (Fig. 12c, Table 4), Table 5: Model SZ componentsthus providing an estimate of the pre-SZ Intermediate-Ti boninite wedge source. Trace element abundances SZ-Total SZfor Low-La/Nd(I) and Intermediate-Ti(I) source mantlesfall on the depleted mantle curve of Pearce et al. (1995,

Intermediate- High- High- High- Hydroussee Fig. 16c), suggesting that the estimates are good.

Ti La/Nd Th/La Nb

Calculation of SZ component compositions Ti — — — 4215 20300

The composition of the different SZ components at Betts P 247 540 662 513 40

Cove can be determined by mass balance using the Cs — — — — 5·87model mantle compositions calculated above. As argued Rb 57 33 63 208 7·17above, the Low-La/Nd model mantle is identified as the Ba 1200 228 262 1670 147depleted wedge + SZ-Hydrous. Therefore, the com- Pb 4·0 0·43 8·59 — 28·8position of SZ-Hydrous can be calculated from the Low- Sr 1130 2215 2650 — 2940La/Nd and Low-La/Nd(I) model mantles if the pro- Th 4·13 4·34 6·71 3·24 0·35portions of the components in the mixture are fixed. If

U 2·72 2·19 2·25 1·38 0·89z is the fraction of SZ-Hydrous in the mixture, and Ci

Nb — 9·8 9·9 28 0·9is the concentration of a trace element in a given reservoir,

Ta 0·78 0·86 0·91 6·8 0·13then

Zr 420 158 162 270 —

La 11·9 23·2 9·89 18·4 1·73CiLow-La/Nd(I) + zCiSD-Hydrous = CiLow-La/Nd. (3)Ce 46·1 35·8 21·5 46·9 2·76

This calculation probably yields results that are onlyPr 1·81 3·63 2·42 5·91 0·24

approximately correct for the Betts Cove suite for twoNd 38·7 10·6 5·19 20·8 0·63

reasons. First, the Low-La/Nd(I) profile is inferred, andSm 7·24 0·43 — 4·73 1·92

small shifts in the real position of this profile will haveEu — — — 0·20 0·81large effects on the computed value of SZ-Hydrous.

Second, the two rocks constituting the Low-La/Nd end-Model SZ components calculated from the different Bettsmember are rather altered, and most of the elements Cove end-member boninites using equations (4) and (5) (SZ-

that define the prominent peaks (Sr, Pb, Ba, U) have Total) and (3) (SZ-Hydrous). The amount of SZ componentin mixture is fixed at z= 0·25% for all calculations. All valuesprobably been perturbed somewhat. The calculation ofin ppm.SZ-Hydrous would be more applicable to fresher, modern

rocks.By comparing the model Low-La/Nd(I) mantle with the model SZ-Total component associated with the High-

the different Low-Ti boninite end-member model La/Nd end-member change with the mixing proportionmantles, it is possible to calculate the composition of the (z). Changing z does not affect the characteristic profiletotal SZ contribution to the depleted wedge for each shapes or incompatible element ratios, although assumingend-member. This calculation is more robust than the a smaller value of z does cause the absolute abundancecalculation of SZ-Hydrous, as the end-member profiles of the calculated SZ-Total component to increase. Theare computed from larger numbers of relatively fresh SZ-Total components calculated from the different bon-rocks. The value of SZ-Total is the sum of all the SZ inite end-members (Intermediate-Ti, High-La/Nd, High-components involved (Hydrous, Sediment, ATT or OIB). Th/La, and High-Nb) are all rather similar (Fig. 14bTaking the High-La/Nd boninite model mantle as an and Table 5), and for values of z = 0·25% provideexample, approximate matches for immobile trace element abund-

ances with putative SZ components (OIB, Sediment,CiLow-La/Nd(I) + zCiSZ-Total High-La/Nd = CiHigh-La/Nd. (4)ATT, Fig. 14b). Such small proportions of the SZ com-

The compositions of SZ-Total components involved ponent are consistent with the proportions inferred fromin the genesis of the High-Nb and High-Th/La end- isotope data for Tertiary boninites and other arc suitesmembers are calculated in the same way. The com- in modern environments, and for boninites in otherposition of SZ-Total involved in generation of Inter- Appalachian ophiolites (McDermott et al., 1993; Pearcemediate-Ti boninites can be calculated in a similar & Parkinson, 1993; Olive et al. 1997; Turner et al., 1997).fashion: The model SZ component profiles calculated with

equations (4) and (5) (except for the SZ componentCiIntermediate-Ti(I) + zCiSZ-Total Intermediate-Ti = CiIntermediate-Ti. (5)associated with the High-Nb end-member) have char-acteristic troughs at Nb and Ti (Fig. 14b); this feature isFigure 14b shows how the abundances and profiles of

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probably inconsistent with the involvement of a dispersed (Figs 14a and 15). The model SZ-Total componentsSZ-OIB component at Betts Cove. Comparison of the calculated from the Betts Cove Low-Ti boninites closelyModel SZ-Total incompatible trace element profiles with resemble typical sediments in terms of the LILE, LREE,published data on putative SZ components allows the Nd and Pb (Figs 14b, c and 15a); but more closelyidentity of the SZ component at Betts Cove to be further resemble trondhjemites in terms of the MREE and Zrrefined (Figs 14c and 15). Typical trondhjemites have (Fig. 15b). This strongly suggests that both SZ-Sedimentflatter REE profiles than typical sediments (Fig. 14a), and SZ-ATT were involved in refertilizing the depletedwith lower La and La/Nd, and higher Zr and Zr/Sm mantle wedge source of the Low-Ti boninites. The SZ-

Total calculated from Intermediate-Ti boninites, in con-trast to the Low-Ti end-members, shows a much greaterresemblance to the trondhjemites for most incompatibleelements (Figs 14 and 15), and one could posit that SZ-ATT is the dominant influence in their genesis.

The pronounced enrichment in Pb and LILE shownby the model SZ-Total components also suggests theinvolvement of SZ-Sediment, but as the Betts Cove lavasare altered, less confidence can be placed on variationsof these elements. Nevertheless, it is interesting thatthe so-called mobile elements suggest exactly the sameconclusions as do the immobile ones. Elements such asBa, U, Pb and Sr are very enriched in most SZ-Totalmodels, with abundances greater than typical terrigenousSZ-Sediment components (Fig. 14b). This is interpretedto be due in part to addition of a ubiquitous SZ-Hydrouscomponent. Alternatively, some of these enrichmentsmight reflect the presence of carbonate (Sr) in the sub-ducted sediment, or simply the natural heterogeneity to

Fig. 16. Trace element ratio plots constructed by adding the BettsCove model SZ-Total components calculated (assuming z = 0·25%),to the depleted mantle component ‘L’ (L is Low-La/Nd model mantle).The percentages near the tick marks refer to the percentage of SZ-Total added to L. Same legend as for Fig. 4. Dykes that are geo-chemically similar to typical Low-Ti or Intermediate-Ti boninite lavashave been given the same symbols as the corresponding lavas. Fieldsfor Tertiary and ophiolitic boninites delimited from the sources givenin Fig. 5, with additional data from Jenner (1981), Crawford et al.(1989), Falloon et al. (1989), Falloon & Crawford (1991) and Sobolev& Danyushevsky (1994). (a) Ba/La vs La/Sm (normalized to N-MORB);(b) Ba/La vs Th/La. The Ba/La ratio is thought to be a good reflectionof the SZ-Hydrous component, whereas La/Sm and Th/La betterreflect addition of either SZ-ATT or SZ-Sediment. It should be notedthat both SZ-Hydrous and either SZ-ATT or SZ-Sediment are neededto reproduce the trends of Betts Cove and other boninites, and thatthe Betts Head Intermediate-Ti boninites seem to contain a higherproportion of SZ-Hydrous. (c) Th/Yb vs Nb/Yb diagram (logarithmicscales) adapted from Pearce et al. (1995). The ‘mantle’ line is the rangeof possible mantle compositions, where MORB is the FMM mantle,and Int-Ti and L are the Betts Cove Intermediate-Ti(I) and Low-La/Nd(I) boninite model mantles, respectively. The two Low-La/Ndboninites from Betts Cove are labelled, and a field encloses the High-Nb Betts Cove boninites. The percentages near the tick marks representaddition of SZ-Total calculated from the High-La/Nd end-member,to the Low-La/Nd(I) mantle, L. A Yb content of 0·0001 ppm wasassumed for SZ-Total for this calculation. The dotted curve ‘90%sz’represents 90% added subduction zone component to the mantleaccording to Pearce et al. (1995). It should be noted that addition ofonly 0·2–0·5% of the SZ-Total fertile component can account for 90%of the Th budget in the boninites.

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be expected in sediments. Support for the interpreted & Tremblay, 1995). This would account for the commonoccurrence of ophiolitic boninites along the margin ofpresence of small amounts of SZ-Sediment also comesNorth America (Church, 1977; Bedard & Hebert, 1996).from published Nd-isotopic data for these rocks (Coish

A detailed discussion of the interrelation of Taconicet al., 1982; Swinden et al., 1997).deformation and the accretion of ophiolites to NorthThe computed values for the SZ components calculatedAmerica is beyond the scope of this paper. However, thefrom the Betts Cove data are used as end-members fordata from Betts Cove provide constraints on the sequencemixing calculations shown in Fig. 16. Values for Ba inof events attendant on formation of one of these accretedSZ-Total were calculated with equation (4). Ba is shown,Ordovician peri-continental marginal basins. The narrowas it may be less mobile than the other LILE. High-active volcanic zone of arcs is commonly interpreted toBa/La in arc lavas is typically interpreted to signifyreflect focused transfer of an LILE- and volatile-rich SZinvolvement of SZ-Hydrous, whereas high values ofcomponent from the subduction zone into the mantleLa/Sm (or La/Nd) signify higher proportions of SZ-wedge above (Fig. 2) (Gill, 1981; Tatsumi, 1989; Arculus,Sediment. It should be noted that involvement of both1994; Keppler, 1996; Iwamori, 1998). Any change inSZ-Hydrous and SZ-Sediment are needed to explain thethe geometry of subduction (e.g. in response to trenchcompositions of Betts Cove and other boninitic suitesrollback) should disrupt the focused volatile-rich efflux(Fig. 15a). In addition, the presence of High-Th/Laderived from the slab and distribute it over a largerboninites (Fig. 15b) suggests that some of these lavasvolume of the depleted mantle wedge (Bedard et al.,involve a High-Th sediment component as well.1998). Considering the small proportion of volatile-richSZ component (<0·5%) calculated for Betts Cove bon-Intermediate-Ti boninitesinites, pure flux melting alone is probably not capable

Intermediate-Ti boninites at Betts Cove cannot form of accounting for the huge volumes of boninitic meltthrough partial melting of the same sources as the Low- required, and so decompression melting (Fig. 2) mustTi boninites (Figs 8c and 12b), because their La/Nd also be involved (see Pearce & Peate, 1995). This impliesratios are too low, and their HREE contents are too high that extension of the overriding plate must have ac-for this to be plausible. Model calculations (Figs 11 and companied subduction. If slab rollback and extension of14c) imply that the Intermediate-Ti boninites formed in the overriding plate are synchronous, then the com-much the same way as the Low-Ti boninites, i.e. through bination of a dispersed volatile component, de-melting of a depleted source coupled with an influx of a compression-melting of the mantle at fairly low pressures,trace element enriched SZ component. However, the and a depleted wedge source, would all favour productionIntermediate-Ti source mantle was less depleted initially of boninitic magmas, in addition to providing the op-than was the source of the Low-Ti boninites (Fig. 11). portunity for magma-dominated extension (seafloorModel results imply that the genesis of both Low-Ti and spreading).Intermediate-Ti boninites involved similar SZ com- At Betts Cove, there is a systematic progression fromponents (Figs 14c and 15), but that the Intermediate-Ti extremely depleted boninites (Low-Ti Betts Head For-boninites were refertilized by a greater relative proportion mation), to less depleted boninites (Intermediate-Ti), toof SZ-Hydrous (higher Ba/La) and SZ-ATT (Figs 15 still less depleted arc tholeiites (Mount Misery Formation),and 16). This inference is consistent with the isotopic and eventually to the fairly enriched Upper Snooks Armdata of Swinden et al. (1997), which indicate that less Group tholeiites. These fertile tholeiites are interstratifiedSZ-Sediment was involved in genesis of the Intermediate- with calc-alkaline lavas and pyroclastic rocks, and theTi boninites (their Island Arc Tholeiites). sedimentary and volcanic facies (Bedard et al., 2000;

Cousineau & Bedard, 2000; Kessler & Bedard, 2000)indicate that both tholeiites and calc-alkaline volcanicsare extremely proximal to their respective vents. ThisSEAFLOOR SPREADING IN Asystematic stratigraphic relationship is inconsistent with

MARGINAL BASIN a model where the chemical signatures of the differentAt Betts Cove, the sheeted dykes, the cumulates and lava types reflect the heterogeneous distribution of a fossilthe Betts Head lavas constitute a comagmatic suite of SZ-OIB component in the wedge, as the most fertileboninitic affinity (Bedard et al., 1998, 2000). As Tertiary mantle domains should melt first, not the most refractoryboninites appear to be restricted to forearcs (Cameron et ones. One must therefore posit a spatial compositionalal., 1979; Hawkins et al., 1984; Murton, 1989; Johnson zonation in the mantle beneath the Betts Cove ophiolite& Fryer, 1990; Stern & Bloomer, 1992), this suggests in the Ordovician. As the magmas derived from the morethat seafloor spreading at Betts Cove may have been fertile mantle domains post-date the onset of boniniticinitiated in a forearc environment (Bedard et al., 1998), seafloor spreading, it is reasonable to infer that the more

fertile mantle domains were located at greater depth,probably in a peri-continental setting (Harris, 1992; Pinet

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experimentally produced amphiboles and silicate melts with variableand were entrained into the extending supra-subductionF content. Chemical Geology 109, 29–49.zone wedge to fill the space created by slab rollback and

Al, T. A. (1990). The character and setting of gold mineralizationseafloor spreading (Fig. 2).associated with the Betts Cove complex. M.Sc. Thesis, MemorialUniversity of Newfoundland, St John’s, 163 pp.

Arculus, R. J. (1994). Aspects of magma genesis in arcs. Lithos 33,189–208.CONCLUSIONS

Ariskin, A. A. & Nikolaev, G. S. (1996). An empirical model for theThe Betts Cove Ophiolite records the initiation of seafloor calculation of spinel–melt equilibria in mafic igneous systems atspreading in a marginal basin characterized by magmas atmospheric pressure: 1. Chromian spinels. Contributions to Mineralogy

and Petrology 123, 282–292.of boninitic affinity. The source mantle of the Low-TiAyers, J. C., Dittmer, S. K. & Layne, G. D. (1997). Partitioningboninites was a refractory harzburgite or dunite residual

of elements between peridotite and H2O at 2·0–3·0 GPa andafter 19–22% melting of a fertile MORB mantle (FMM).900–1100°C, and application to models of subduction zone processes.Decompression melting was assisted by the addition ofEarth and Planetary Science Letters 150, 381–398.

small proportions (<0·25%) of a fluxing SZ component, Barnes, S. J. (1986a). The distribution of chromium among ortho-identified as a mixture of volatiles derived from the pyroxene, spinel and silicate liquid at atmospheric pressure. Geochimicasubducting oceanic crust (SZ-Hydrous), siliceous melts et Cosmochimica Acta 50, 1889–1909.derived from fusion of subducted sediments (SZ-Sed- Barnes, S. J. (1986b). The effect of trapped liquid crystallization on

cumulus mineral compositions in layered intrusions. Contributions toiment), and partial melts of the subducting oceanic crustMineralogy and Petrology 93, 524–531.(SZ-ATT). There is no evidence for the involvement of

Beattie, P. (1993). The generation of uranium series disequilibria byOIB-like components at Betts Cove. Intermediate-Tipartial melting of spinel peridotite: constraints from partitioningboninites were derived from less depleted mantle sourcesstudies. Earth and Planetary Science Letters 117, 379–391.(~12% prior melting of FMM), fluxed with a greater

Bedard, J. H. (1989). Disequilibrium mantle melting. Earth and Planetaryproportion of SZ-Hydrous and SZ-ATT, and less SZ- Science Letters 91, 359–366.Sediment (in comparison with the Low-Ti boninites). Bedard, J. H. & Hebert, R. (1996). The lower crust of the Bay ofThe gradation from extremely depleted boninites to Islands ophiolite, Canada: petrology, mineralogy, and the importance

of syntexis in magmatic differentiation in ophiolites and at oceanless depleted boninitic and tholeiitic magmas with timeridges. Journal of Geophysical Research 101, 25105–25124.implies a change in source composition, with less depleted

Bedard, J. H. & Hebert, R. (1998). Formation of chromitites bysources being entrained into the zone of melting, perhapsassimilation of crustal pyroxenites and gabbros into peridotitic in-in response to slab rollback and extension of the over-trusions: North Arm Mountain Massif, Bay of Islands ophiolite,riding plate.Newfoundland, Canada. Journal of Geophysical Research 103, 5165–5184.

Bedard, J. H., Lauziere, K., Tremblay, A., Sangster, A. & Tellier, M.(1998). Evidence from Betts Cove ophiolite boninites for forearc

ACKNOWLEDGEMENTS seafloor-spreading. Tectonophysics 284, 233–245.A. Tremblay and K. Lauziere contributed to all phases of Bedard, J. H., Lauziere, K., Boisvert, E., Sangster, A., Tellier, M.,

Tremblay, A. & Dec, T. (1999a). Geological map of the Betts Covethis study and their assistance is gratefully acknowledged.Ophiolitic Massif and its cover rocks. Geological Survey of Canada, A-Charles Langmuir supplied modelling software. LouiseSeries Map 1969A, 1:20 000 scale.Corriveau, Marc Lafleche, Ray Coish, Julian Pearce, Ian

Bedard, J. H., Lauziere, K., Boisvert, E., Deblonde, C., Sangster, A.,Parkinson, Robert J. Stern, Rosemary Hickey-VargasTremblay, A. & Dec, T. (1999b). Betts Cove geological dataset forand Marjorie Wilson commented on earlier versions ofGIS applications. Geological Survey of Canada Open File D3623 (in press).

the manuscript. Their contributions were invaluable. Bedard, J. H., Lauziere, K., Tremblay, A., Sangster, A., Douma, S. L.K. Lauziere prepared Fig. 1. The people of Tilt Cove, & Dec, T. (2000). The Betts Cove ophiolite and its cover rocks.LaScie and Snooks Arm made the fieldwork un- Geological Survey of Canada Bulletin 550 (in press).

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evidence. Geology 7, 499–503.Varfalvy, V., Hebert, R., Bedard, J. H. & Lafleche, M. R. (1997). Woodhead, J. D. (1989). Geochemistry of the Mariana arc (western

Occurrence of primitive boninitic intrusive magmas in upper mantle Pacific): source composition and processes. Chemical Geology 76, 1–24.rocks of the North Arm Mountain massif, Bay of Islands, New- Woodhead, J., Eggins, S. & Gamble, J. (1993). High field strength andfoundland, Canada: evidence for an arc environment. Canadian transition element systematics in island arc and back-arc basinMineralogist, 35, 543–570. basalts: evidence for multi-phase melt extraction and a depleted

Watson, E. B., Ben Othman, D., Luck, J. M. & Hofmann, A. W. mantle wedge. Earth and Planetary Science Letters 114, 491–504.(1987). Partitioning of U, Pb, Cs, Yb, Hf, Re and Os between Woodhead, J. D., Eggins, S. M. & Johnson, R. W. (1998). Magmachromian diopsidic pyroxene and haplobasaltic liquid. Chemical Geo- genesis in the New Britain island arc: further insights into meltinglogy 62, 191–208. and mass transfer processes. Journal of Petrology 39, 1641–1668.

Weaver, J. S. & Langmuir, C. (1990). Calculation of phase equilibrium Zindler, A. & Jagoutz, E. (1988). Mantle cryptology. Geochimica et

Cosmochimica Acta 52, 319–333.in mineral–melt systems. Computers and Geoscience 16, 1–19.

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was calculated from the liquid Al2O3 content using anAPPENDIX 1: FRACTIONATION equation derived by fitting a line through the ex-

perimental data of Barnes (1986a):MODELLINGFractional crystallization was modelled through in-

Alopx =cremental subtraction of equilibrium olivine, ortho-[(Al2O3(liquid)× 0·0148) – 0·14178] – 0·00167.pyroxene and spinel in steps of 0·5%. Comparisons

(A1)with results calculated with the Rayleigh fractionationequation showed negligible deviations after 90% frac- Values of Al calculated to be <0·01 were set at 0·01.tionation. Melt Fe3+/Fe2+ ratios were calculated from Orthopyroxene Cr2O3 was calculated using equation (4)liquid compositions and temperatures using the SPIN- of Barnes (1986b):MELT program of Ariskin & Nikolaev (1996) with theprocedure of Sack et al. (1980), assuming that oxygen Cr2O3orthopyroxene =fugacity was buffered at quartz–fayalite–magnetite Cr2O3(liquid)× [(20400/T ) – 11·67)]. (A2)(QFM) conditions. An initial liquidus temperature of

Temperature T is in degrees Kelvin. Orthopyroxene1300°C was assumed, with the temperature–FeO/MgO was calculated using an exchange coefficientcrystallization curve as described by Bedard & Hebertof 0·27 (Barnes, 1986a). Ferric iron was ignored. As-(1998) for the Tongan boninite parent. The averagesuming stoichiometry, (Fe + Mg)orthopyroxene = 1 – (Al –phenocryst spinel composition from Betts Cove lavas and0·01) – Ca – Mn – Cr – Ti. Knowing both (Fe/dykes was used in all calculations. Olivine FeO/MgOMg)orthopyroxene and (Fe + Mg)orthopyroxene, Fe and Mg con-compositions were calculated with an exchange co-tents of orthopyroxene can be calculated.efficient of 0·3 (Roeder & Emslie, 1970); Ni in olivine

To avoid the problems of non-linearity associated withwith ol/liqDNi = 10 (Kinzler et al., 1990), and Cr2O3 inthe equations of DePaolo (1981), coupled assimilationolivine with ol/liqDCr = 1. Olivine MnO was fixed at 0·3and fractionation was also modelled in a stepwise fashion.wt %. Orthopyroxene Si was fixed at 1·99 f.u. (formulaModel contaminants were added to the melt, the com-units), Mn at 0·005, Ca at 0·04, and Ti at 0·001.positions were normalized to 100%, and then a smallOrthopyroxene NiO was set equal to 0·2 olivine NiOfractionation step (0·5%) was calculated, and so on.[equation (2) of Barnes (1986b)]. Al in orthopyroxene

Fig. A1. Partition coefficients used in this paper, corresponding to values given in Table A1. Logarithmic scale.

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Table A1: Mineral–liquid partition coefficients

CPX OPX OL SP

Cs 0·0003 s 0·00015 s 0·00015 s 0·0001 s

Rb 0·0004 19 0·0002 17 0·0002 20 0·0002 20

Ba 0·00068 1 0·0005 s 0·0003 s 0·0005 3

Th 0·0021 10 0·001 s 0·0003 s 0·001 3

U 0·0028 i 0·002 s 0·0003 s 0·001 3

Nb 0·01 21 0·003 15 0·001 s 0·01 is

Ta 0·013 s 0·0036 s 0·001 s 0·01 is

La 0·0536 1 0·0031 18 0·0003 s 0·0006 6

Ce 0·0858 1 0·0021 18 0·0003 5 0·0006 6

Pr 0·1 i 0·0026 18 0·0003 10 0·0006 i

Pb 0·0075 10 0·0014 14 0·0003 16 0·0006 i

Sr 0·1283 1 0·002 is 0·00036 7 0·0006 i

P 0·13 i 0·002 is 0·0002 i 0·0006 i

Nd 0·1873 1 0·0023 18 0·0002 5 0·0006 6

Sm 0·291 1 0·0037 18 0·00018 5 0·0006 6

Zr 0·26 11 0·012 18 0·001 18 0·015 21

Hf 0·33 13 0·019 18 0·0029 18 0·015 22

Ti 0·34 23 0·086 18 0·002 7 0·125 21

Eu 0·3288 1 0·009 18 0·0002 5 0·0006 6

Gd 0·367 i 0·0065 18 0·00025 5 0·0006 6

Tb 0·404 i 0·008 i 0·000475 i 0·00105 i

Dy 0·38 23 0·011 18 0·0007 5 0·0015 6

Y 0·412 23 0·015 18 0·001 is 0·002 i

Ho 0·4145 i 0·016 18 0·00122 i 0·0023 i

Er 0·387 1 0·021 18 0·00174 5 0·003 6

Tm 0·4085 i 0·029 i 0·00348 i 0·00375 i

Yb 0·43 1 0·038 19 0·00522 5 0·0045 6

Lu 0·433 1 0·046 i 0·00852 5 0·0045 i

Ga 0·35 21 0·3 21 0·05 21 4 21

Cr 3·8 1 1·9 6 1·25 21 500 21

Co 1·2 1 1·7 21 3·2 21 4 21

Ni 2·0 21 3·5 6 10 9 10 6

Cu 0·36 1 0·2 A 0·47 2 1 A

Zn 0·79 4 0·79 8 0·83 8 4 12

V 0·5 1 0·2 21 0·189 7 10 12

Sc 0·85 1 0·5 21 0·16 21 0·1 21

Mineral–liquid partition coefficients (D) used in calculations, given in same order as the normalized trace element plots. Theadjoining columns indicate the data sources. A, approximate value assumed; i, interpolated from well-constrained valuesof adjacent trace elements; s, smoothed extrapolations of moderately well-constrained data. 1, Hart & Dunn (1993); 2, Irving(1978); 3, Hawkesworth et al. (1993); 4, Liotard et al. (1988); 5, Prinzhofer & Allegre (1985); 6, Kelemen et al. (1990); 7, Skulskiet al. (1994); 8, Sweeney et al. (1995); 9, Kinzler et al. (1990); 10, Beattie (1993); 11, Kuehner et al. (1989) and Adam et al.(1993); 12, Horn et al. (1994); 13, Watson et al. (1987) and Kuehner et al. (1989); 14, calculated from data of Lee et al. (1996);15, Kelemen et al. (1993); 16, Dunn & Sen (1994); 17, Zindler & Jagoutz (1988); 18, Kennedy et al. (1993); 19, Halliday et al.(1995) interpolated value; 20, set equal to D of Opx; 21, synthesis of Pearce & Parkinson (1993), average of 1200–1300°Cvalues; 22, set equal to D of Zr; 23, Johnson (1998).

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Table A2: Mantle reservoirs Table A3: Melting modes

FMM PUM 1 2 3

Cs 0·0014 1 0·032 1 CPX 0·60 0 0

OPX 0·714 2·010 0Rb 0·044 6 0·635 1

Ba 1·2 3 6·989 2 OL −0·3125 −1·005 0·98

SP −0·0015 −0·005 0·02Th 0·017 1 0·085 2

U 0·0066 1 0·021 2

Nb 0·28 A 0·713 2 Melting modes used in melting calculations. Mode 1 is fromPearce & Parkinson (1993). Mode 2 assumes that one olivineTa 0·018 A 0·041 2forms for each two orthopyroxenes (2MgSiO3 = Mg2SiO4 +

La 0·334 1 0·687 2 SiO2 liq) that react out during melting, and that 0·0025 chromiteforms for each orthopyroxene, which is roughly equivalentCe 0·93 1 1·775 2to the amount of Cr in typical mantle orthopyroxenes. ModePr 0·17 A 0·276 23 assumes a cotectic ratio of 98:2 olivine:chromite.

Pb 0·035 1 0·071 2

Sr 13·2 3 21·1 2

P 73·3 3 95 2 probably resides in a combination of kinetic factors, andNd 0·992 1 1·354 2 in the dependence of D values on melt composition andSm 0·349 7 0·4404 1 temperature [references given by Maaløe (1995)]. KineticZr 9·8 7 11·2 2 effects are important because rapid rates of melting orHf 0·2835 7 0·309 2 crystallization prevent full equilibration, causing D toTi 1177 3 1300 2 tend towards a value of unity [references given by BedardEu 0·156 1 0·168 2 (1989)]. Published olivine–liquid D values are poorly

constrained because trace elements in olivine are hardGd 0·55 1 0·596 2

to analyse, and being so small, the D values are sensitiveTb 0·1 A 0·108 2

to disequilibrium effects. Despite these complexities, thereDy 0·7 1 0·737 2is broad agreement on certain trends. The ‘conventional’Y 4·36 1 4·52 1order of incompatibility of trace elements (e.g. Pearce,Ho 0·158 A 0·164 21983; Pearce & Parkinson, 1993) reflects a systematicEr 0·465 A 0·48 2trend in D values, allowing interpolation of some DTm 0·071 A 0·074 2values. Care must be taken, however, because, for crystalYb 0·475 1 0·493 2chemical reasons, olivine–liquid and orthopyroxene–Lu 0·072 1 0·074 2liquid D values for high field strength cations such as Zr,

Ga 4 4 4·5 4Ti and Nb are probably about an order of magnitude

Cr 2628 5 2500 5higher than those of immediately adjacent REE (Kelemen

Co 106 4 102 4et al., 1990). The partition coefficients (Table A1) used

Ni 2020 4 1990 5 here were compiled from published experimental data,Cu 27 5 35 5 with poorly constrained values adjusted to yield smoothZn 50 5 59 5 profiles (Fig. A1).V 78 4 85 5

Sc 15·5 4 17·1 4

APPENDIX 3: MANTLE MELTING,Values of FMM (fertile MORB mantle) and PUM (primitiveupper mantle) reservoirs used in this paper. The adjacent EQUATIONS AND METHODOLOGYcolumns record the data source: A, interpolated or assumed;

The trace element compositions of equilibrium melts1, McCulloch & Bennett (1994); 2, Sun & McDonough (1989);3, Wood (1979); 4, Pearce & Parkinson (1993); 5, Hartman & were calculated using the batch melting equation ofWedepohl (1993); 6, Jochum et al. (1983); 7, average of values Hanson (1980):of Wood (1979) and McCulloch & Bennett (1979).

CL/C0 = 1/[RuD (1 – F ) + F ] (A3)

where CL is the concentration of the element in the melt,APPENDIX 2: PARTITIONC0 is the initial concentration in the system, F is the

COEFFICIENTS fraction of melt, D is mineral–melt partition coefficient,RuD is bulk partition coefficient, and u is normalizedDifferent experimental studies rarely yield the same values

for mineral–melt partition coefficients D. The problem weight fraction of the residual phases at a given F. The

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composition of the solid residue is CS, and is equal to primitive mantle mode of 15% Cpx, 25·5% Opx, 57%Ol, and 2·5% Sp was assumed to correspond to PUMCLRuD.

Fractional melts and residues were calculated by ex- (Williamson et al., 1995). Values of the melting mode(Table A3) were adjusted to force disappearance of clino-tracting small increments (0·1% steps) of equilibrium

melt from the source mantle, recalculating C0, and then pyroxene after 25% melting (Pearce & Parkinson, 1993).Using these values, and the data of Table A1, the residualrepeating the process. Critical melts were modelled as

per the fractional melts, except that one-third of the mantle data trends shown as fig. 5 of Pearce & Parkinson(1993) were bracketed by fractional and critical meltingmelt was retained in the source at each melt extraction

step. (assuming one-third melt retention) simulations. The traceelement profile of FMM closely resembles the residue ofPreferred compositions of the primitive upper mantle

(PUM) and fertile MORB mantle (FMM) compiled from 0·2% melting of primitive mantle (PUM), and so thestarting FMM mode was set as the residue of 0·2%the literature are given in Table A2. FMM is normal N-

MORB-source mantle (Pearce & Parkinson, 1993). A melting of the PUM mode.

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Petrogenesis of Boninites from the Betts Cove Ophiolite

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