ORIGINAL PAPER

Thomas J. Lapen Æ L. Gordon Medaris Jr

Clark M. Johnson Æ Brian L. Beard

Archean to Middle Proterozoic evolution of Baltica subcontinentallithosphere: evidence from combined Sm–Nd and Lu–Hf isotopeanalyses of the Sandvik ultramafic body, Norway

Received: 10 January 2005 / Accepted: 5 July 2005 / Published online: 13 August 2005� Springer-Verlag 2005

Abstract Combined Sm–Nd and Lu–Hf age and isotopedata indicate that Mg- and Cr-rich ultramafic rocks atSandvik, Western Gneiss Region (WGR), Norway,originated from depleted Archean lithospheric mantlethat was chemically and physically modified in MiddleProterozoic time. The Sandvik outcrop consists of gar-net peridotite and garnet-olivine pyroxenite and thingarnet pyroxenite layers. These contain two principalmineral assemblages: an earlier porphyroclastic assem-blage of grt + opx + cpx ± ol (1,200–1,000�C,40–50 kbar) and a later kelyphitic assemblage of grt+ spl + am ± opx ± ol (700–750�C; 12–18 kbar). ACHUR Hf model age indicates a period of meltextraction at ca. 3.3 Ga for garnet peridotite, reflectingextremely high Lu/Hf ratios and very radiogenic pres-ent-day 176Hf/177Hf (eHf=+2,165). Lu–Hf garnet-cpx-whole rock ages of two olivine-bearing samples (garnetperidotite and garnet-olivine pyroxenite) from the out-crop are ca. 1,255 Ma, whereas two olivine-free garnetpyroxenites yield Lu–Hf ages of ca. 1,185 Ma. All Sm–Nd garnet-cpx-whole rock ages of these samples aresignificantly younger (ca. 1,150 Ma for garnet peridotiteand ca. 1,120 Ma for garnet pyroxenite). The isotopesystematics indicate that the Lu–Hf ages representcooling from an earlier period of formation/recrystalli-zation for garnet peridotite whereas they probably

reflect formation/recrystallization ages of the garnetpyroxenite. The Sm–Nd ages and isotope systematics ofthe garnet peridotite samples are consistent with anepisode of LREE metasomatism, perhaps facilitated bya fluid of carbonatitic composition that strongly de-coupled Sm–Nd and Lu–Hf. The Sm–Nd ages of thegarnet pyroxenite may represent either LREE metaso-matism or cooling, and, like the peridotites, Lu–Hf agesare older than Sm–Nd ages. The age data, as well as theinferred Nd isotope composition of the fluid that af-fected the olivine-bearing samples, suggest that theserocks were not in contact during the LREE metasomaticevent. Moreover, the pyroxenite layers cannot have beenemplaced as magmas into the host peridotite. Thepyroxenite layers are interpreted to be tectonically jux-taposed with the host olivine-bearing samples sometimeafter 1,150 Ma but before development of kelyphite.

Introduction

Orogenic garnet-bearing ultramafic rocks, which arecommonly exposed within the roots of continent–con-tinent collisional mountain belts, provide considerableinformation on the physical and chemical evolution ofthe subcontinental mantle beneath such regions (Med-aris 1999). The presence of garnet in these mantle-de-rived rocks is especially useful for thermobarometricanalysis (e.g. Carswell and Harley 1990), as well asgeochronological studies using Lu–Hf and Sm–Nd iso-tope systems. Garnet, therefore, provides a critical linkbetween pressure, temperature, and time (P–T–t) thatdoes not exist for geochronological approaches thatutilize, for example, zircon, whose presence is not typi-cally temperature- and pressure-dependent. The Lu–Hfand Sm–Nd isotope systems may also be used tounderstand the nature and timing of enrichment andmelting events that occur in the mantle, thus addingcomposition to the pressure–temperature–time path(P–T–t–x).

Editorial Responsibility: T. L. Grove

T. J. Lapen (&) Æ L. G. Medaris Jr Æ C. M. Johnson Æ B. L. BeardDepartment of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53704, USAE-mail: tjlapen@uh.eduTel.: +1-713-7433399Fax: +1-713-7437906

Present address: T. J. LapenGeosciences Department,University of Houston, Houston, TX 77204, USAE-mail: tjlapen@uh.eduTel.: +1-713-7433399Fax: +1-713-7437906

Contrib Mineral Petrol (2005) 150: 131–145DOI 10.1007/s00410-005-0021-z

The Western Gneiss Region (WGR) of Norway,which is composed mainly of amphibolite- to granulite-facies gneisses, contains pods and lenses of variably ret-rogressed Mg–Cr garnet peridotite, garnet pyroxenite,and eclogite (Krogh and Carswell 1995; Medaris 1984;Carswell 1981). The mode and timing of eclogite andperidotite emplacement in gneiss, relative to high pres-sure and ultrahigh pressure metamorphism associatedwith the Scandian phase of Caledonian orogeny, hasbeen the subject of considerable debate (e.g. Lappin andSmith 1978; Smith 1980; Griffin et al. 1985; Cuthbert andCarswell 1990; Wain 1997; Wain et al. 2000). Althoughrecent evidence favors in situ eclogite-facies metamor-phism of metabasic lenses within enclosing quartzo-

feldspathic gneisses (see Cuthbert and Carswell 1990;Krogh and Carswell 1995, for reviews), questions remainregarding the position of ultramafic rocks in the crust ormantle prior to the Caledonian orogeny.

A characteristic feature of the WGR is that measuredmineral ages of garnet peridotites (Proterozoic) aremuch older than those of eclogites (Silurian) within thesame tectonostratigraphic unit (see Brueckner andMedaris 2000, for a discussion). Preservation of Prote-rozoic mineral ages in Mg–Cr garnet peridotites offers arare opportunity to understand ancient mantle events.We present trace- and rare-earth element geochemicaldata and Lu–Hf and Sm–Nd geochronology of Mg–Crgarnet-bearing peridotite, pyroxenite and olivinepyroxenite exposed in a small outcrop at Sandvik, Islandof Gurskøy, WGR, Norway (Fig. 1), to understand thechemical and isotopic evolution of these rocks prior tothe Caledonian orogeny. Samarium-neodymium andLu–Hf geochronology of ultramafic lithologies is par-ticularly challenging due to the very low concentrationsof these elements. The ultramafic rocks at Sandvik,however, are important because they preserve several ofthe multistage mineral assemblages identified in perido-tites elsewhere in the WGR (Carswell et al. 1983;Carswell 1986; Medaris et al. 2003) and record a long

Fig. 1 Map of part of the WGR modified from Carswell andCuthbert (2003). Peridotite typically occurs as isolated bodieswithin schist and gneiss of the Western Gneiss Region (WGR). Fe–Ti type garnet peridotites contain considerable amounts of Fe andTi oxide phases and are considered to be derived from layeredintrusions of olivine norite-peridotite cumulates (Carswell et al.1983). The Mg–Cr type peridotites are interpreted to be tectonicallyintroduced into the host WGR, representing ‘alpine-type’ ultra-mafic bodies. See Carswell et al. (1983) for a detailed discussion ofthe differences between Fe–Ti and Mg–Cr type peridotites. TheSandvik ultramafic body is shown in large text

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history, �1.8 b.y., of melt extraction, metasomatism,deformation, and cooling from Archean to mid-Prote-rozoic time.

Sample description

Most Mg–Cr peridotites in the WGR consist of chloriteperidotite, reflecting extensive recrystallization duringthe Caledonian orogeny, but pre-Caledonian garnetif-erous assemblages are preserved in about 15 differentbodies, including Sandvik (Fig. 1). Although comprisinga small percentage of total outcrop and often small insize, these relatively fresh peridotite bodies provide a keyview into mantle lithologies of the WGR. The garnet-bearing ultramafic rocks at Sandvik occur in a single,isolated outcrop (�5 m in diameter) in the vicinity ofseveral separate outcrops of chlorite peridotite. Theoutcrop consists predominantly of foliated garnet peri-dotite and subordinate amounts of garnet pyroxenite,which occur as centimeters-thick layers that are con-cordant with foliation, and contain a crude minerallamination that is suggestive of high-temperaturedeformation and recrystallization. The analyzed samples(Fig. 2) include garnet peridotite (sample 4A), garnetpyroxenite in contact with pyroxene-rich garnetperidotite (samples 4G-2 and -1, respectively), and gar-net pyroxenite adjacent to olivine-garnet pyroxenite(samples 4I-2 and -1, respectively). When not statedotherwise, we will use the terms olivine-bearing in ref-erence to samples 4A, 4G-1 and 4I-1, and olivine-free inreference to samples 4G-2 and 4I-2 throughout the text.

The Sandvik ultramafic rocks are characterized by apronounced porphyroclastic texture, in which porphyr-oclasts of garnet, orthopyroxene, and clinopyroxene(and olivine, in the case of peridotite and olivine pyrox-enite) occur in a very fine-grained recrystallizedgroundmass (Fig. 3). The greater degree of recrystalli-

zation in relatively incompetent peridotite compared tothat in more competent pyroxenite is illustrated in Fig. 3.Modal abundances vary significantly among the twoanalyzed pyroxenites (4G-2 and 4I-2) and their adjacentolivine-bearing lithologies (4G-1 and 4I-1; Table 1). Theabundance of porphyroclasts in sample 4A is very lowdue to extensive recrystallization to Stage 2 and 3assemblages; the relative abundances of Stage 1 por-phyroclasts listed in Table 1 are therefore imprecise dueto their low abundance. The mineral composition ofporphyroclasts among the peridotite and pyroxenite are,however, remarkably uniform (Table 2). The Sandvikpyroxenites (4G-2 and 4I-2) are unusual in that the for-sterite content in olivine (Fo) is higher than that of oli-vines within associated garnet peridotite (e.g., Fo 92.7 inpyroxenites vs. Fo 92.4 in peridotite; the pyrope contentin garnet is 74.2 in pyroxenites vs. 71.6 in peridotite).Elsewhere, such as the Almklovdalen peridotite in theWGR and numerous peridotite bodies in the BohemianMassif, pyroxenites are less magnesian than the associ-ated peridotites (Medaris 1980; Medaris et al. 1995).

Based on textural relations, the sequence of mineralassemblages in the Sandvik ultramafic rocks, from oldestto youngest, is:

– Stage 1: ol + grt + opx + cpx (porphyroclasts)– Stage 2: ol + grt + spl + opx + cpx ± amp

(kelyphite around garnet)– Stage 3: ol + spl + opx + pargasitic amp (fine-

grained granoblastic groundmass)– Stage 4: ol + chl + opx + tremolitic amp (chlorite

peridotite in nearby outcrops)

The Stage 1 porphyroclasts (as defined here), whichrepresent the earliest discernible mantle assemblage,equilibrated at 1,000–1,200�C and 50–65 kbar (Medariset al. 2003; Medaris et al. in prep.). The Stage 2 ke-lyphite equilibrated at lower temperatures and pres-sures, 700–750�C, 12–18 kbar (Medaris et al. 2003), but

Fig. 2 Composite photographof the samples from Sandvikanalyzed in this study. a Garnetperidotite (sample 4A);b sample 4G which consists ofan �8 cm thick garnetpyroxenite layer (sample 4G-2)within the host garnet peridotite(sample 4G-1); c sample 4I,which consists of an �5 cmthick garnet pyroxenite layer(sample 4I-2) within the hostgarnet-olivine pyroxenite(sample 4I-1). Scale bars are2 cm

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at pressures still high enough to stabilize a secondgeneration of garnet. It is unclear at present if the Stage2 kelyphite is a retrograde assemblage that is related tocooling and decompression from mantle conditions, orif it is a prograde product of Caledonian high-pressuremetamorphism. Stages 3 and 4 represent retrograde

mineral assemblages that were probably associated withregional Caledonian exhumation and cooling of theWGR.

Sample preparation

Approximately 500–1,000 g of sample were crushed, halfof which was split for whole-rock analysis, the other halfwas sieved and sorted by grain size for mineral picking;the mineral separates were picked directly from the53–63 lm size fraction. The purity of mineral separates,which consists of the porphyroclastic assemblage only,was near perfect with the exception that picking did notavoid microscopic inclusions of exsolved phases. Theoccurrence of exsolved phases in the analyzed samples ispreferred because, barring open-system behavior, theoriginal high P-T composition would be retained,whereas removal of inclusions (mechanically or chemi-cally) might fractionate the primary parent–daughterratios. Following hand picking, mineral separates werewashed twice in doubly distilled water. Whole rockmaterial was split into �50 g fractions, followed by

Fig. 3 Photomicrographs of Sandvik garnet peridotite (a), garnetpyroxenites (b, c), and olivine-garnet pyroxenites (d). Photographstaken with partly crossed polarizers. Scale bars are 5 mm. Mineralphases are identified using small letters: c clinopyroxene; g garnet; kkelyphite; o olivine; p orthopyroxene. Note that orthopyroxeneoccurs in garnet peridotite, 4A, but is absent from the field of view

Table 1 Modal proportions of whole rocks based on >1,000 pointscounted per thin section

Sample 4a 4g-1 4i-1 4g-2 4i-2

Stage 1 Gt 0.05 0.018 0.077 0.295 0.580CPX 0.01 0.081 0.341 0.166 0.085OPX 0.20 0.359 0.282 0.234 0.235Ol 0.40 0.409 0.046 0.000 0.000

Stage 2 Kelyphite 0.34 0.133 0.253 0.305 0.100Sum 1.00 1.00 1.00 1.00 1.00

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powdering in an alumina-lined rock mill. Mixed Sm–Ndand Lu–Hf isotope tracers were added to the whole-rockpowders and minerals prior to dissolution in PTFE-linedParr bombs, as outlined in Lapen et al. (2004). Rubid-ium-strontium and REE isotope tracers were added to adifferent whole-rock split than that used for Lu–Hf andSm–Nd. Typically, 100–200 mg of mineral and rockmaterial were used. Procedural blanks for the completechemical processing procedure were <55 pg and <45 pgfor Hf and Nd, respectively.

Chemical and isotopic analyses

Separation of Lu, Hf, Nd, and Sm from the samplematrix follows the chemical procedures outlined in La-pen et al. (2004) and Munker et al. (2001), and all iso-tope and concentration data for these elements are fromthe same dissolved sample solution; no aliquoting ofsample solutions prior to chemical separation of ele-ments was done. Isotopic analysis of Lu, Hf, and Ndwas performed on a GV Instruments IsoProbe at UW-Madison and data acquisition and reduction proceduresfollowed those of Lapen et al. (2004). Repeated analysisof our Ames Nd I and Ames Nd II in-house standardsby MC-ICP-MS during this study yielded 143Nd/144Nd=0.512146±25 (2-SD, n=17) and 143Nd/144Nd=0.511972±16 (2-SD, n=28), respectively. These valuescompare well with our long-term TIMS values for AmesNd I and Ames Nd II of 143Nd/144Nd=0.512149±22 (2-SD, n=24) and 143Nd/144Nd=0.511975±18 (2-SD,n=57), respectively. 143Nd/144Nd=0.512637±15 (2-SD,n=7) for the BCR-1 standard, also analyzed by TIMS,

which we take to be equal to the present-day CHURvalue (e.g. Wasserburg et al. 1981). Analysis of the Hfstandard JMC-475 by MC-ICP-MS yielded176Hf/177Hf=0.282163±14 (2-SD, n=14) over thecourse of this study. Samarium isotope analyses wereperformed on a VG Sector 54 TIMS at UW-Madison asSm+ on Re filaments that were loaded with silica gel andphosphoric acid. All Sm data were internally normalizedto 147Sm/152Sm ” 0.5608 (derived from Russ 1973) forisotope-dilution calculations. Based on replicate analy-ses of spiked standard solutions and rock standards, theprecisions of 176Lu/177Hf and 147Sm/144Nd isotope ratiomeasurements are ±0.2 and ±0.5% (2-SD) respectively.

Bulk separation of REEs from the sample matrixwas accomplished with cation exchange resin using2.5 M and 6 M HCl; a-hydroxyisobutyric acid usinganion exchange resin was used to separate the REEsfrom one another. The concentrations of REEs (Ce,Nd, Sm, Eu, Gd, Dy, Er, and Yb) and Rb were mea-sured by isotope dilution using single collector Dalyanalysis on the UW-Madison TIMS. No correction forinstrumental mass fractionation was applied to REEs(other than Nd, Sm, and Lu) or Rb analyses. Precisionof isotope-dilution measurements are estimated to beabout ±5%, based on replicate analyses of the samedissolved sample material.

Mineral compositions were determined by using aCameca SX-51 electron microprobe, an acceleratingvoltage of 15 kV, a beam current of 20 nA, and a suiteof analyzed natural minerals as standards; a /(qz) datareduction program was used to correct the data (Arm-strong 1988).

Table 2 Average mineral compositions from garnet pyroxenite and garnet peridotite/gt-ol pyroxenite

Garnet peridotite/gt-ol pyroxeinte Garnet pyroxenite

OPX CPX Ol Gt OPX CPX Gt

SiO2 57.88 54.26 41.53 42.33 57.79 54.90 42.51TiO2 0.04 0.15 – 0.13 0.07 0.28 0.04Al2O3 0.64 2.23 – 22.01 1.19 2.78 22.15Cr2O3 0.16 1.56 – 2.27 0.10 1.49 2.19FeO 4.64 1.21 7.13 6.88 4.16 1.11 6.61MnO 0.11 0.04 0.07 0.28 0.08 0.04 0.27MgO 37.10 16.87 50.79 21.38 37.24 16.48 21.87CaO 0.19 21.96 – 4.66 0.16 21.08 4.38NiO – – 0.36 – – – –Na2O 0.01 1.48 – – 0.01 1.99 –Sum 100.78 99.76 99.88 99.95 100.80 100.16 100.04

n=36 n=26 n=38 n=40 n=6 n=8 n=32Si 1.965±4 1.971±4 1.006±1 3.000±7 1.957±3 1.973±6 3.002±11Ti 0.001±1 0.006±1 – 0.007±3 0.002±1 0.007±1 0.002±8Al 0.025±2 0.109±4 – 1.838±9 0.047±5 0.117±4 1.844±13Cr 0.004±1 0.043±2 – 0.127±6 0.003±1 0.042±1 0.122±8Fe 0.132±3 0.035±2 0.144±14 0.426±2 0.118±2 0.033±2 0.391±9Mn 0.003±1 0.001±1 0.000 0.017±10 0.002±1 0.001±1 0.016±2Mg 1.878±8 0.894±15 1.834±17 2.259±2 1.880±6 0.883±10 2.303±20Ni – – 0.007±2 – – – –Ca 0.007±4 0.826±14 – 0.354±15 0.006±1 0.812±5 0.332±10Na 0.001±1 0.126±5 – – 0.000 0.138±4 –Sum 4.018 4.013 2.991 8.027 4.016 4.009 8.010Mg # 93.12 96.12 92.70 94.10 96.35

2r is derived from all (n) mineral analyses and refer to least significant digits

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Results

Trace element data

Trace-element abundances of whole-rock samples aregiven in Table 3 and are illustrated in Fig. 4. Chondrite-normalized trace-element patterns, plotted in order ofincreasing compatibility to the right, are highly variablebetween olivine-bearing samples (4G-1, 4I-1, and 4A),but are very similar for the olivine-free samples (4G-2and 4I-2; Fig. 4). The relative trace-element abundancesof samples 4G-1 and 4I-1 are quite similar, although theabsolute abundances of sample 4G-1 are significantlylower due to dilution by olivine. Samples 4G-2 and 4I-2,as well as 4I-1, have trace-element contents that aresimilar to those of primitive mantle (Sun and McDon-ough 1989), but samples 4G-2 and 4I-2 have lower Ce/Yb ratios than olivine-bearing samples, as well asprimitive mantle. The trace-element pattern of sample4A is saddle shaped and contains a strong negative Hfanomaly relative to Nd and Sm, very high Lu/Hf, andlow Sm/Nd. Samples 4G-1 and 4I-1 also have negativeHf anomalies, although they are smaller in magnitude ascompared to sample 4A. Samples 4G-2 and 4I-2 do nothave Hf anomalies (Fig. 4). All samples are stronglydepleted in Rb relative to C1 chondrite.

Sm–Nd and Lu–Hf ages

Lutetium-hafnium and Sm–Nd isotope analyses of Stage1 porphyroclastic garnet, diopside, and whole rock(which includes mineral Stages 1 and 2) yield Lu–Hfages of 1,190±6.2 and 1,175±6 Ma for garnet pyroxe-nite (samples 4I-2 and 4G-2, respectively), 1,266±6.3for garnet-olivine pyroxenite, and 1,253±6.7 Ma forgarnet peridotite (samples 4I-1 and 4G-1, respectively;Fig. 5; Table 4). Samarium-neodymium ages determined

on the same dissolved sample material that was used forLu–Hf analyses are 1,126±7.7 and 1,115±7.3 Ma forsamples 4I-2 and 4G-2, respectively, and 1,155±7.7 and1,149±7.6 Ma for samples 4I-1 and 4G-1, respectively.Age data for samples 4G and 4I consistently yield olderLu–Hf ages as compared to Sm–Nd ages (Fig. 5; Ta-ble 4). The difference in age between the Lu–Hf and Sm–Nd is about 100 Ma between samples 4G-1 and 4I-1(garnet peridotite and olivine-garnet pyroxenite) andabout 60 Ma between samples 4G-2 and 4I-2 (garnetpyroxenites). In addition to the age differences betweenSm–Nd and Lu–Hf for the same sample, the olivine-bearing samples (4G-1, 4I-1) are older than Lu–Hf andSm–Nd ages obtained on the pyroxenites (samples 4G-2and 4I-2). No mineral ages of sample 4A could be ob-tained in this study due to its very low proportions ofporphyroclastic clinopyroxene and garnet.

Sm–Nd isotope data

Samarium-neodymium isotope data for whole rock,garnet, and diopside are listed in Table 4 and ages andinitial 143Nd/144Nd ratios are illustrated in Fig. 6.Samarium-neodymium ages form two populations, agroup yielding 1,118 Ma (garnet pyroxenites) and an-other yielding 1,149 Ma (garnet peridotites and garnet-olivine pyroxenite; Fig. 6). Both groups are older than a2-point garnet-cpx Sm–Nd age of 1,011±8 Ma from agarnet peridotite also from Sandvik (Brueckner et al.1996). The inset of Fig. 6 shows the initial eNd values ofthe whole rocks at their corresponding isochron ages aswell as the initial eNd values that are defined by themineral isochrons. The inset shows that all olivine-bearing whole-rock samples (including sample 4A), aswell as their constituent minerals, have the same eNd of+3.3 at their common mineral-whole rock isochron ageof 1,149±11 Ma. Moreover, the garnet pyroxenites

Table 3 Whole rock trace and rare earth element contents (ppm)determined by isotope dilution

Sample 4a 4g-1 4i-1 4g-2 4i-2Gt perid Gt perid Gt-ol pxite Gt pxite Gt pxite

Rb 0.0419 0.0548 0.1763 0.0924 0.0790Ce 0.2694 0.3097 3.092 2.123 1.409Sr 3.337 4.331 41.19 25.95 17.40Nd 0.0940 0.2841 2.673 1.565 1.295Hf 0.0032 0.0564 0.4508 0.4579 0.3549Sm 0.0095 0.0953 0.8579 0.7165 0.6011Eu 0.0012 0.0369 0.3165 0.2829 0.3138Gd 0.0088 0.1369 1.017 1.134 1.135Dy 0.0396 0.1608 1.376 1.551 1.727Er 0.0699 0.1059 0.9714 1.203 1.350Yb 0.1083 – 1.010 1.290 1.410Lu 0.0207 0.0179 0.1672 0.21* 0.2306

*Imprecise due to anomolously low element yield fromion-exchange chemistry

Fig. 4 Chondrite-normalized rare-earth and trace element compat-ibility diagram of whole rock samples analyzed at Sandvik. Solidcircles olivine-bearing samples; open squares garnet pyroxenite.Note the highly unusual element abundance pattern of sample 4A

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have a distinct initial eNd value of +6.4 at their commonmineral isochron age of 1,118±11 Ma. The difference ininitial eNd values between the olivine-bearing samplesand the olivine-free samples is a factor of six greaterthan the analytical uncertainty of ±0.5 eNd units.Therefore, the Sm–Nd isotope data indicate that theolivine-bearing samples had identical 143Nd/144Nd ratiosat their measured Sm–Nd ages but that the olivine-free

samples (4G-2 and 4I-2) were derived from sources thathad different 143Nd/144Nd ratios.

Whole rock 147Sm/144Nd ratios as well as f(Sm/Nd),which are equal to [(147Sm/144Nd(sample)/

147Sm/144Nd(CHUR))-1], are listed in Table 4. The f(Sm/Nd) valuesindicate that the olivine-free samples have a higher par-ent–daughter ratio than CHUR, which is consistent withtheir high Mg content, but the olivine-bearing samples

Fig. 5 Isochron diagrams foranalyzed samples fromSandvik. Regressions werecalculated using ISOPLOT V.2.49 (Ludwig 2001). Thedifferences in ages betweenLu–Hf and Sm–Nd for eachsample are: 111 Ma, 4I-1;104 Ma, 4G-1; 65 Ma, 4I-2;and 60 Ma, 4G-2. Decayconstants and analyticalprecisions used for agecalculations are listed inTable 4. W whole rock; Ggarnet; Px clinopyroxene. Errorbars are smaller than the size ofthe symbols

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have parent–daughter ratios less than or close to CHUR(Table 4; Fig. 6) despite their high Mg contents.

Lu–Hf isotope data

Lutetium-hafnium data of whole rock, garnet, anddiopside are listed in Table 4 and whole-rock Hf isotopedata are illustrated on a Hf evolution diagram relative toCHUR in Fig. 7. The present-day eHf values for all whole-rock samples are very high and quite variable, especiallycompared to their present-day eNd (see sample 4A, Ta-ble 4). Comparison of measured and initial eHf and eNd

values indicates that Hf isotope compositions are signif-icantly more radiogenic than expected for their Nd iso-tope compositions (Table 4), lying far off theHf-Nd trenddefined by oceanic basalts (Patchett 1983; Salters andHart 1991; Johnson and Beard 1993). This indicates that,even in the Proterozoic, Hf and Nd element and isotopesystematics were strongly decoupled at Sandvik.

The large range in initial Hf isotope compositions ofthe olivine-bearing samples (samples 4A, 4G-1, and 4I-1;Table 4, Fig. 7), indicates a diverse pre-Caledonianhistory. Sample 4A, which has the highest present-day176Hf/177Hf and 176Lu/177Hf ratios but the lowest143Nd/144Nd and 147Sm/144Nd ratios (Table 4), has a

single-stage Hf model age of �3.3 Ga, assuming deri-vation from a CHUR reservoir (Fig. 7). This age issimilar to Re-Os and Sm–Nd model ages and whole-rock isochron ages obtained from peridotite bodieselsewhere in the WGR (Jamtveit et al. 1991; Brueckneret al. 2002; Beyer et al. 2004). Samples 4G-1 and 4I-1,however, have relatively radiogenic present-day176Hf/177Hf ratios, but their 176Lu/177Hf ratios are toolow for these rocks to have been directly derived fromCHUR (Fig. 7).

Samples 4G-2 and 4I-2 (olivine-free samples) havehigh measured 176Hf/177Hf and 176Lu/177Hf ratios (f(Lu/Hf)=1.4 – 1.8) relative to CHUR. Their high Lu/Hfratios are consistent with an overall depletion inincompatible elements (Fig. 4) and could be related tomelt extraction. In contrast to the olivine-bearing sam-ples, the garnet pyroxenites have identical mineral iso-chron initial 176Hf/177Hf ratios.

Origin of Lu–Hf and Sm–Nd age differences

The difference between Lu–Hf and Sm–Nd ages that isseen in all samples cannot be related to heterogeneitieswithin each sample because the Sm–Nd and Lu–Hf datawere acquired from the same dissolved sample material.

Table 4 Measured Lu–Hf and Sm–Nd isotope data and summary of ages and initial isotope compositions

Sample # Lithology Material Luppm

Hfppm

176Lu/177Hf 176Hf/177Hf Smppm

Ndppm

147Sm/144Nd 143Nd/144Nd

4A Gt-peridotite WR 0.0206 0.0032 0.9946* 0.34401 ±167* 0.0175 0.0940 0.1126 0.512186 ±134G-1 Gt-peridotite WR 0.0179 0.0564 0.0452 0.283878 ±9 0.0998 0.2841 0.2129 0.512919 ±10

CPX 0.0077 0.4710 0.0023 0.282871 ±8 1.884 7.547 0.1513 0.512434 ±9GT 0.1436 0.1232 0.1657 0.286734 ±8 0.4873 0.3486 0.8480 0.517690 ±13

4I-1 Gt-ol-pyroxenite WR 0.1672 0.4508 0.0527 0.284188 ±7 0.8535 2.673 0.1935 0.512791 ±12CPX 0.0241 0.5365 0.0064 0.283077 ±9 3.030 12.18 0.1508 0.512463 ±11GT 0.3994 0.3104 0.1830 0.287298 ±6 0.5630 0.4129 0.8272 0.517584 ±12

4G-2 Gt-pyroxenite WR – 0.4579 – 0.284954 ±8 0.5984 1.565 0.2317 0.513225 ±11CPX 0.0511 1.283 0.0057 0.283282 ±9 2.180 8.330 0.1586 0.512672 ±13GT 0.3772 0.2771 0.1936 0.287444 ±10 0.5329 0.3639 0.8887 0.518014 ±12

4I-2 Gt-pyroxenite WR 0.2305 0.3549 0.0923 0.285219 ±6 0.5993 1.295 0.2804 0.513580 +11CPX 0.0323 0.7722 0.0060 0.283289 ±8 2.196 8.402 0.1584 0.512688 +10GT 0.3862 0.2971 0.1849 0.287306 ±7 0.5328 0.3716 0.8699 0.517934 ±13

Summary of ages and initial isotope compositions

Sample Lithology Lu–Hf eHf(CHUR, t) Sm–Nd ENd(CHUR, t) f(Lu/Hf) f(Sm/Nd)

4G-1 Gt-peridotite 1,253±7 +28.3 1,149±7 +3.1 0.35 0.084I-1 Gt-ol-pyroxenite 1,266±6 +32.3 1,155±8 +3.5 0.57 -0.024G-2 Gt-pyroxenite 1,175±6 +40.4 1,115±7 +6.4 ~1.4*** 0.184I-2 Gt-pyroxenite 1,190±6 +40.3 1,126±8 +6.4 1.76 0.434A Gt-peridotite – +2,165** – �8.8** 28.77 -0.43

Decay constants: 176Lu: 1.865·10�11 (Scherer et al. 2001); 147Sm:

6.54·10�12176. Hf/177Hf and 143Nd/144Nd are corrected to179Hf/177Hf=0.7325 and 146Nd/144Nd=0.7219, respectively. Allerrors in isotope ratios shown here are 2 SE from in-run statistics.Errors used in age calculations are based on external reproduc-ibility of spiked standards and whole rock samples as stated in

Lapen et al. (2004): 176Hf/177Hf=0.005%, 176Lu/177Hf=0.2%.

Precisions of 143Nd/144Nd and 147Sm/144Nd are based on externalreproducibility of standard solutions analyzed throughout an

analytical session: 143Nd/144Nd=0.005%, 147Sm/144Nd=0.5%.WR whole rock powder; GT garnet; CPX clinopyroxene. Initialisotope compositions are calculated using the CHUR parameters:176Hf/ 177Hf=0.282772; 176Lu/177Hf=0.0334 (Blichert-Toft and

Albarede 1997), and 143Nd/144Nd=0.512636; 147Sm/144Nd=

0.1967. *Corrected for blank assuming a 176Hf/177Hf=0.2822;**Present-day values; ***f(Lu/Hf) is imprecise due to anomolouslylow Lu yield from ion-exchange chemistry

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The differences in ages are not related to inaccuratecharacterization of the isotope tracers because a sys-tematic error in the Lu–Hf and Sm–Nd spikes usedwould produce a systematic offset, which was not ob-served. Moreover, the spikes were recently calibratedagainst standard solutions of well-known concentration.In particular, the Lu–Hf and Sm–Nd spikes producedcorrect ages for a Proterozoic sample (garnet amphibo-lite from Gore Mountain, New York, USA; Lapen et al.2004; Lapen, unpublished data) that has been measuredin other labs by Lu–Hf and Sm–Nd (Mezger et al. 1992;Scherer et al. 2000). After considering the issues of ele-mental zoning and diffusion, as well as isotopic equili-bration during slow cooling, we conclude that the mostlikely explanation for the age differences lie in the rela-tive element mobility of Lu, Hf, Sm, and Nd during ametasomatic event.

Decoupling of Sm–Nd and Lu–Hf in garnet-bearingsamples may occur if prograde REE zoning is preservedin garnet and the garnet grew over a protracted timeinterval. In such samples, garnet cores may have veryhigh Lu/Hf ratios (>20), whereas the rim may have Lu/Hf ratios that are close to that of the whole rock; this is incontrast to Sm/Nd ratios, which can be nearly constantfrom core to rim (e.g., Lapen et al. 2003). The high Lu/Hfratio in the earlier-grown garnet core, as compared to thegarnet rim, will produce Lu–Hf ages that are older thanSm–Nd ages because Sm–Nd ages reflect the average ageof the bulk garnet but Lu–Hf ages are weighted towardthe cores (Lapen et al. 2003). Brueckner et al. (1996)documented variations of Sm/Nd ratios up to a factor oftwo across a garnet within peridotite from Sandvik, butno variation in HREE (e.g. Yb) that would be indicativeof prograde element zoning. Furthermore, there is noevidence for prograde zoning of major elements in garnetfrom the samples studied here (Medaris et al. in prep.),reflecting the fact that these mantle-derived rocks cooledfrom mantle temperatures. Because the zoning in Sm/Ndratios documented by Brueckner et al. (1996) are stronglyasymmetric across the diameter of the garnet, in additionto the lack of evidence for prograde zoning in HREEs,we conclude that garnet growth processes cannot explainthe observed differences between the Lu–Hf and Sm–Ndisochron ages.

Given the fact that the peridotites and pyroxenitescooled through REE and Hf blocking temperaturesfrom mantle temperatures, an additional explanation forthe age discrepancy between Sm–Nd and Lu–Hf iso-chron ages could be different blocking temperatures forHf4+ and Nd3+. The Stage 1 mineralogy of the samplesindicate that isotopic equilibration of Hf and Nd wouldhave occurred primarily between garnet and diopside;exchange between garnet and low-Nd and -Hf mineralssuch as orthopyroxene and olivine would be very minor,especially considering their modest modal abundances.The diffusion rate of Hf4+ of garnet and diopside ispresently unknown, although Van Orman et al. (2001,2002) note that diffusion rates are, in general, inverselyproportional to cation charge in garnet, but to a much

Fig. 6 Sm–Nd isochron diagram and detail of the initial eNd valuesof each age regression (inset). The olivine-bearing samples (filledcircles) define an array on the isochron diagram that is distinct fromthe array defined by the garnet pyroxenites (open squares). Thedifferences in initial 143Nd/144Nd between garnet pyroxenite andgarnet peridotite/garnet-olivine pyroxenite whole rocks are high-lighted in the inset for each group of samples. It is important to notethat the spread in initial eNd values is 6 times greater than theanalytical uncertainties. The inset also shows the 147Sm/144Nd of thewhole rock samples relative to the 147Sm/144Nd of CHUR (verticalline). Symbols on the y-axis of the inset are initial eNd valuescalculated from the age regressions. eNd values are based on CHURparameters: 143Nd/144Nd(0)=0.512636 and 147Sm/144Nd = 0.1967

Fig. 7 Hafnium isotope evolution diagram. Filled circles representthe suite of olivine-bearing samples and open symbols representthe garnet pyroxenites. Sample 4A has a very high 176Lu/177Hf(Table 2), resulting in a steep intersection with CHUR at�3.3 Ga. Samples 4G-1 and 4I-1 do not intersect CHUR nordepleted mantle in the span of Earth’s history and are thus likelyto be refertilized remnants of previously depleted mantle material.It is possible that the refertilization event occurred at �2,150 Ma,the time at which the two samples share a common 176Hf/177Hf.Samples 4G-2 and 4I-2 have a very radiogenic present-day176Hf/177Hf and high 176Lu/177Hf relative to CHUR. Bothsamples share a common 176Hf/177Hf at their measured ages,suggesting that they have either recrystallized formed, at thattime. CHUR parameters are: 176Hf/177Hf=0.282772 and176Lu/177Hf=0.0332, which corresponds to 176Hf/177Hf of JMC-475=0.282163 (Blichert-Toft and Albarede 1997)

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lesser extent in diopside. This suggests that the blockingtemperature of Hf should be higher than that of Nd, atleast for pyrope-rich garnet. Rare-earth element diffu-sion in diopside is much slower than in garnet at 1,000–1,200�C, 5 GPa (Van Orman et al. 2001, 2002), indi-cating that Nd3+ diffusion would be limited by clino-pyroxene. It is unclear if Hf4+ diffusion will be limitedby garnet or clinopyroxene, although it is expected thatin garnet, Hf4+ diffusion rates will be less than that ofREEs. We conclude that although, in principle, expecteddifferences in diffusion of Nd and Hf might offer anexplanation for the consistently older Lu–Hf ages ascompared to Sm–Nd, consideration of mineral andwhole-rock isochrons and calculated initial isotope ra-tios point to another explanation, which is exploredbelow.

Isotopic re-equilibration may be tested throughcomparison of ages and initial ratios determined forwhole-rock samples versus minerals. Because isotopic re-equilibration on a mineral scale produces clockwiserotation of mineral isochrons, resulting in mineral agesthat are younger than whole-rock ages (e.g. Faure 1986),calculated initial isotope ratios will be more radiogenicusing mineral data as compared to whole-rock data.Whole rocks and minerals lie along similar isochrons(Fig. 6), where, for example, whole-rock samples 4A,4G-1, and 4I-1 define an age of 1,121±51 Ma (2r,MSWD=0.64) that lies within error of the garnet-clin-opyroxene-whole rock age of 1,149±11 (garnet-olivinepyroxenite and garnet peridotite; Fig. 6). It is importantto note that there is no correlation between 143Nd/144Ndand 1/Nd (not shown), suggesting that the apparent is-ochrons are not mixing lines (e.g. Faure 1986) but reflecttrue ages. We therefore conclude that diffusional equil-ibration during slow cooling cannot explain the Sm–Ndisotopic systematics, as might be envisioned, for exam-ple, if the Sandvik units were collectively exhumed fromthe mantle at the same time.

In principle, the maximum time these samples couldhave resided in the mantle and preserved their mineral-whole rock associations may be calculated based ontheir blocking temperatures. Unfortunately, estimatesof Nd3+ diffusion into and out of diopside at uppermantle conditions are highly variable. Using publisheddiffusion coefficients and activation energies (correctedfor pressure using an activation volume of �9 cc/mol;Van Orman et al. 2001), coupled with uncertainties incooling and exhumation rate from �1,200 Ma toCaledonian orogeny at �400 Ma, produces ‘closuretemperatures’ for diopside that vary from �1,300 to750�C, using either the formulation of Dodson (1973)or Ganguly and Tirone (1999). Therefore, at this timewe are unable to assess whether these rocks were em-placed into the crust prior to Caledonian orogenybased simply on the relations shown in (Fig. 6). Futurework, however, will be aimed at dating the Stage 2(kelyphite) assemblage, which records pressure andtemperature conditions consistent with lower-crustalenvirons.

Chemical and lithologic evolution of the Sandvikultramafic body

We interpret the relatively young Sm–Nd ages to reflecta REE metasomatic event, given the difficulties inexplaining the results through elemental zoning or iso-topic re-equilibration between garnet and clinopyroxene.The older Lu–Hf ages, as well as wide range in initial176Hf/177Hf ratios, suggest that the metasomatic fluidwas impoverished in Hf so that the Lu–Hf systematicsremained largely unaffected.

Melting and metasomatism of olivine-bearing ultramaficrocks at Sandvik

The unusual trace-element pattern of sample 4A sug-gests depletion of Hf, MREE, and Rb, and relativeenrichment in LREE (Fig. 4), consistent with the de-coupling of Sm–Nd and Lu–Hf isotope systematics. InFig. 8, the relative proportions of REEs in a metaso-matizing agent are explored via non-modal incrementalbatch melting (e.g., Ottonello et al. 1984; Shaw 2000) ofa garnet or spinel lherzolite (Fig. 8a) that containedprimitive mantle REE contents (Sun and McDonough1989). High degrees of melt depletion are indicated by anaverage Mg#=92.3 for Stage 1 olivine porphyroclasts(Table 2) as well as a high 176Lu/177Hf ratio (Table 4).

Figure 8b shows the REE pattern of a mixture (fluid/rock(wt) of mixture=0.0002) between restite (triangles)and carbonatite liquid (diamonds; Hornig-Kjarsgaard1998). A similar result (fluid/rock(wt) of mixture=0.004)can be achieved using a silicate liquid that would be inequilibrium with samples 4G-1 and 4I-1 (unlabeled linein Fig. 8b). It is important to note that the fluid/rockratio of the mixture is related to the absolute abundanceof REEs in the fluid and that only fluids that containvery high Ce/Yb ratios (C1-chondrite normalized: 30–60) will produce REE abundances in the ‘fluid-restitemixture’ (squares, Fig. 8b) that will match the REEpattern of sample 4A. The difference in Eu contentbetween the mixture and sample 4A (Fig. 8b) is inverselyproportional to the Ce/Yb ratio in the fluid atCe(mix) = Ce(sample 4A), suggesting that the Ce/Yb ratioused in the calculations is likely to be a minimum esti-mate of the natural fluid composition.

It is unlikely that the metasomatizing agent was ahydrous fluid derived from surrounding crustal rocksbecause the clinopyroxenes from samples 4G-1,2 and 4I-1,2 have very low 87Sr/86Sr ratios of 0.7013–0.7014(T. Lapen, unpublished data) which are inconsistentwith crustal derived fluids. It is also unlikely that themetasomatizing agent was a silicate liquid. Sample 4Ahas an Sm/Hf ratio of 2.9, which is much higher than theSm/Hf ratio of most silicate melts. If the metasomatizingagent was a silicate melt, the Sm/Hf ratio of sample 4Ashould be more similar to the fluid, even at a low fluid/rock(wt) ratio of 0.004 (above). The negative Hf

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concentration anomaly relative to Nd and Sm in sample4A, coupled with its extremely radiogenic 176Hf/177Hfratios relative to 143Nd/144Nd (Table 4; Fig. 4), suggeststhat the fluid had a low concentration of Hf relative toNd and Sm. We therefore conclude that the metaso-matizing agent was not a silicate melt but may have beensimilar to carbonatite magmas, which are generally de-pleted in Hf (e.g., Nelson et al. 1988; Veksler et al. 1998).

A carbonate metasomatizing agent that was enriched inLREE but depleted in Hf would preserve the highlyradiogenic 176Hf/177Hf ratios in whole rocks andstrongly alter their 143Nd/144Nd ratios.

In contrast, samples 4G-1 and 4I-1 have Sm/Hf ratiosof 1.7–1.9, much closer to that of primitive mantle (Sm/Nd=1.4) than the Sm/Hf ratio of sample 4A. Metaso-matism by a silicate melt, which would have substantialquantities of Hf, seems the most likely explanation forthe less radiogenic initial 176Hf/177Hf ratios of thesesamples relative to the highly depleted sample 4A(Fig. 7). However, because all of the olivine-bearingsamples equilibrated to the same 143Nd/144Nd at�1,150 Ma, it is extremely unlikely that one area of theoutcrop underwent carbonatite melt metasomatism andanother area underwent silicate melt metasomatism atthe same time. Because we would expect the initial176Hf/177Hf ratios to reflect silicate melt metasomatism,it is more likely that samples 4G-1 and 4I-1 underwentthis type of metasomatism at or prior to their measuredLu–Hf isochron ages, most likely at 2.15 Ga, the time atwhich both samples share a common 176Hf/177Hf ratio.Therefore, if 2.15 Ga is the age of silicate melt metaso-matism that affected samples 4G-1 and 4I-1, their Lu–Hfisochron ages represent a time between 2.15 Ga and thetime at which the rocks cooled below the blockingtemperature for Hf4+ exchange between garnet and cpx.The Sm–Nd isochron ages for samples 4G-1 and 4I-1represent a time of homogenization of 143Nd/144Ndacross the scale of the outcrop (samples 4A, 4G-1, and4I-1), probably as a result infiltration of very smallamounts of carbonatitic melt (fluid/rock(wt) ra-tio=0.0002).

Emplacement of garnet pyroxenite layers into the hostolivine-bearing lithologies at Sandvik

It is unlikely that the garnet pyroxenites (olivine-freesamples) are related to the metasomatic event that isinferred to have affected the olivine-bearing samplesstudied here because of their younger ages and signifi-cantly different initial 143Nd/144Nd ratios (Fig. 6). Manygarnet pyroxenite layers that are hosted in peridotite areinterpreted to be emplaced as crystallized high-pressuremelts or reflect high-pressure crystal accumulations(± trapped melt) from transient magmas which passedthrough the host peridotite (e.g. Medaris et al. 1995;Becker 1996; Bodinier et al. 2004). If the pyroxenites hadoriginated as a high-pressure crystal residue or melt thatwas intruded into the exposed peridotite, we would ex-pect the wall rocks to be altered by this intrusion (e.g.,Bodinier et al. 2004), but there are no mixing relationsbetween the peridotites and pyroxenites for either Hf orNd. Crude layering defined by alternating garnet- andpyroxene-rich domains within the garnet pyroxenitesand host olivine-bearing rocks, which is coplanar to thecontact between lithologies, is suggestive of high-tem-perature deformation. Furthermore, the strong compe-

Fig. 8 Possible explanation of REE compositions of sample 4Athrough non-modal, incremental batch melting calculations (a)followed by incompatible element enrichment (b). a Melting ofeither a garnet or spinel-lherzolite that had primitive-mantleelement contents (Sun and Mcdonough 1989) can reproduce theHREE pattern of sample 4A. Percent melt shown for severalillustrative curves. Mineral–liquid distribution coefficients used are:Green et al. (2000), Irving and Frey (1984), Kennedy et al. (1993),and Kelemen et al. (1992). Melting coefficients from Walter (1998).Melting equations follow Ottonello et al. (1984) and Shaw (2000). bMixing of previously depleted garnet peridotite (triangles) andcarbonatitic fluid (diamonds) or a fluid in equilibrium with samples4G-1 and 4I-1 (unlabeled line). The resulting mixture, obtainedfrom a carbonatitic fluid or fluid in equilibrium with samples 4G-1and 4I-1, fits the present-day composition of sample 4A andsuggests that the metasomatizing agent was probably LREE richand Hf poor (see Fig. 4)

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tency contrast between the olivine-bearing samples andthe olivine-free garnet-pyroxenites at mantle conditionswould tend to localize strain along the lithologicboundaries between them (Fig. 2). Recrystallization andphase transitions associated with high strain duringdevelopment of the kelyphite has, unfortunately, over-printed much of the evidence for the high-temperaturedeformation (>1,000�C). Given the chemical evidenceagainst emplacement as a melt product into the hostperidotite, however, as well as the mineral and lithologictextures, we propose that the garnet pyroxenites weretectonically juxtaposed with the host olivine-bearinglithologies under mantle conditions at Sandvik.

Archean to Proterozoic evolution of the Sandvik peridotitebody

The Sm–Nd and Lu–Hf ages, as well as the range ininitial 143Nd/144Nd and 176Hf/177Hf ratios, point to aprotracted history for the Sandvik body, possiblyspanning more than 2 b.y. The highly depleted perido-tite (sample 4A) requires significant melt extraction,perhaps up to 24% at �3.3 Ga. This interpretation isconsistent with a developing Re-Os and Sm–Nd datasetof Archean mantle extraction ages determined elsewhereon peridotites of the WGR (Jamtveit et al. 1991; Brue-ckner et al. 2002; Beyer et al. 2004; Fig. 9). Pressure andtemperature conditions of this depletion event are un-known. Preservation of ancient depleted mantle materialis possible because of its refractory nature and buoyancy

(Oxburgh and Parmentier 1978; Jordan 1978; PoudjomDjomani et al. 2001), and such material is unlikely to bereincorporated into the asthenosphere (O’Reilly et al.2001).

Following the Archean depletion event, the evolutionof the Sandvik rocks between 3 and about 1.25 Ga isuncertain. It is likely that samples 4G-1 and 4I-1 wereenriched in LREEs and Hf, sometime during thisinterval, perhaps by a silicate melt. If this enrichmentevent was sufficiently intense to homogenize Hf isotopecompositions on the outcrop scale, a possible age for thisenrichment is 2.15 Ga, the point at which samples 4G-1and 4I-1 share a common initial 176Hf/177Hf ratio(Fig. 7); this, however does not explain the distinct Hfisotope composition of the peridotite sample 4A, so thisinterpretation must be considered preliminary. Althoughthe timing of this enrichment is unclear, the Hf evolutiontrajectory (Fig. 7) indicates that samples 4G-1 and 4I-1originated from a precursor that had higher Lu/Hfratios than are observed today.

Samarium-neodymium mineral ages of most Mg–Crperidotites throughout the WGR scatter between -1,700to 1,000 Ma (Fig. 9). Despite uncertainties on the originof these ages (e.g. cooling, LREE metasomatism, and/orblocking temperature variations), the ages roughly cor-respond to tectonic-magmatic events that are recordedin the surrounding gneiss (Fig. 9). Rhenium-osmiummodel ages also correspond to ages of known tectonic-magmatic events in the WGR (Beyer et al. 2004), lendingfurther evidence that the mantle and crust were coupledduring much of the Middle Proterozoic (e.g. Medaris

Fig. 9 Summary of ages andtectonomagmatic eventsrecorded in the WGR. Thelower section of the figuresummarizes published agesfrom WGR peridotites: (1)Re-Os model ages (Beyer et al.2004), (2) Re-Os isochron age(Brueckner et al. 2002), (3)Sm–Nd model age (Jamtveit etal. 1991), (4) Lu–Hf model age(this study), and (5) Sm–Ndmineral-whole rock ages(Jacobsen and Wasserburg1980; Mearns and Lappin 1982;Mearns 1986; Rubenstone et al.1986; Jamtveit et al. 1991;Brueckner et al. 1996; Medarisand Brueckner 2003; thisstudy). The upper part of thediagram summarizes ourinterpreted Proterozoic geologichistory of the Sandvikultramafic body duringSveconorwegian time

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and Brueckner 2003). Lutetium-hafnium mineral-wholerock isochron ages for samples 4G-1 and 4I-1 likely re-flect the effects of partial retention of Hf4+ over the timeinterval between formation/recrystallization/Hf homog-enization and the time at which diffusion and exchangeof Hf4+ between garnet and cpx essentially ceased. TheLu–Hf mineral-whole rock isochron ages for samples4G-2 and 4I-2 likely represent either formation/recrys-tallization or the age of homogenization of Hf isotopeswithin the garnet-free samples during Sveconorwegiantime (Fig. 9). The Sm–Nd ages of 1,120 Ma for theolivine-free garnet pyroxenites may be cooling ages aswell, but cooling fails to explain the strong decouplingbetween Sm–Nd and Lu–Hf systematics in these samplesas well as the co-linearity between whole rock andmineral data on an isochron diagram (Fig. 6). Althoughthe Sm–Nd ages are slightly younger than Lu–Hf agesfor garnet pyroxenite, perhaps because exchange ofNd3+ is inferred to be more rapid between garnet andclinopyroxene than Hf4+, an LREE-enrichment event islikely to have affected these rocks at some point in theirearly history, as discussed above. Light rare-earth ele-ment enrichment of the olivine-bearing samples, whilethey were still at mantle conditions, resulted in the1,150 Ma Sm–Nd ages for these samples, as evidencedby their 147Sm-144Nd and 143Nd/144Nd relations depictedin Fig. 6. The differences in ages and initial 143Nd/144Ndratios between the olivine-bearing samples (samples 4A,4G-1, and 4I-1) and the adjacent olivine-free samples(4G-2 and 4I-2; Fig. 6) indicate that these two groups ofsamples were not in contact during LREE metasoma-tism at 1,150 Ma, nor do the olivine-free pyroxenitelayers represent trapped melt that was injected into thehost olivine-bearing unit. Tectonic juxtaposition of thegarnet pyroxenite (olivine-free samples) with the olivine-bearing units likely occurred after about 1,150 Ma, butbefore lower-temperature deformation which was asso-ciated with formation of Stage 2 kelyphite at 700–750�C,12–18 kbar (Medaris et al. 2003).

Concluding remarks

Subcontinental lithospheric mantle, especially low-den-sity depleted mantle, may commonly survive tectonicevents and remain isolated from the convecting mantle(e.g., O’Reilly et al. 2001), thus providing an excellentrecord of mantle, and perhaps crust/mantle chemicaland tectonic processes. The very long history recorded atSandvik is an excellent example of how fragments ofsubcontinental lithosphere can preserve evidence ofevents that have been either obscured or erased in thecrustal rocks in which they now reside. At Sandvik,garnet peridotite (sample 4A) experienced ancient(�3.3 Ga) melt depletion resulting in extremely high Lu/Hf and present-day 176Hf/177Hf ratios relative to Sm/Ndand 143Nd/144Nd ratios. Subsequent Hf enrichment ofthe olivine-bearing samples 4G-1 and 4I-1, which

lowered the Lu/Hf and 176Hf/177Hf ratios relative tosample 4A, likely occurred at 2.15 Ga or sometime be-tween the �3.3 Ga depletion event and their measuredLu–Hf ages (-1,255 Ma). The olivine-bearing rocks weremetasomatized by a fluid with a high Ce/Yb ratio andlow Hf content at about 1,150 Ma resulting in low Sm/Nd and 143Nd/144Nd ratios relative to Lu/Hf and176Hf/177Hf ratios. Based on the Lu–Hf age data, thegarnet pyroxenites probably formed or recrystallized atabout 1,190 Ma. It is unclear at present if the Sm–Ndages of the olivine-free garnet pyroxenites representcooling or LREE metasomatism at 1,120 Ma, but ineither case, Nd and Hf isotope compositions are de-coupled in these samples.

A tectonic mode of emplacement for the garnetpyroxenite layers into the host peridotite is proposedbased on the initial 143Nd/144Nd and 176Hf/177Hf ratiosof the garnet pyroxenites (olivine-free samples) relativeto that of the host peridotite (olivine-bearing), the lackof any mixing relations between the pyroxenites andhost rock for Nd or Hf, the different ages recorded inthese lithologies, and the mineral textures within thesamples. A tectonic origin for the juxtaposition of py-roxenites and peridotite layers is different from tradi-tional interpretations of such relations, which haveoften been interpreted to reflect trapped melts orcrystal residues from passing magmas. In the case atSandvik, a magmatic origin for the pyroxenite layers isuntenable.

The Sandvik ultramafic body offers a rare glimpseinto the long history of Baltic sub-continental litho-spheric mantle afforded by combined Lu–Hf and Sm–Nd isotope analyses of whole rock and constituentminerals, as well as trace-element and REE abundancesof whole-rock samples. The very strong decoupling be-tween the Lu–Hf and Sm–Nd isotope systems in theserocks offers an opportunity to understand the chemicalaspects and timing of mantle metasomatism and meltextraction events. The Lu–Hf system is especially usefulin understanding the nature and timing of ancientmantle melting events because the Lu–Hf system is lesssensitive to ‘cryptic’ (non-silicate) LREE metasomatic orenrichment processes that are seemingly commonplacein the mantle. The Sm–Nd system is well suited to dating‘cryptic’ metasomatic events, as long as the whole rockdata define an isochron which is similar to that definedby their constituent minerals. Combined Lu–Hf andSm–Nd analyses of the same samples can thereforeprovide a more detailed history of these rocks than ei-ther system alone.

Acknowledgements We thank Rene Wiesli for maintaining the MC-ICP-MS at UW-Madison. We also thank Dr. John Fournelle whoassisted us with electron microprobe analyses at UW-Madison.This research was supported by National Science Foundation grantEAR-0309853 (CMJ) and the UW-Madison Morgridge GraduateFellowship (TJL). Reviews and criticisms by Hannes Bruecknerand Aaron Cavosie and journal reviewers P. Jonathan Patchett andPeter Kelemen resulted in significant improvements to this manu-script.

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