strontium, neodymium and lead isotopic compositions of deep crustal xenoliths from the snake river...

354 Earth and Planetary Science Letters, 75 (1985) 354-368 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Strontium, neodymium and lead isotopic compositions of deep crustal xenoliths from the Snake River Plain: evidence for Archean basement

William P. Leeman 1, Mart in A. Menzies 2 David J. Mat ty i and Glen F. Embree 3

i Geology Department, Rice University, Houston, TX 77251 (U.S.A.) -" Earth Sciences Department, Open University, Milton Kevnes, MK7 6AA ( U.K.)

3 Geology Department, Ricks College, Rexburg, ID 83440 gU.S.A.)

Received October 10, 1984; revised version received June 25, 1985.

Xenoliths of intermediate to felsic granulites found in evolved lavas from the Snake River Plain have been analyzed for Nd, Pb, and Sr isotopic compositions and related trace element contents. Overall, they exhibit wide ranges in present-day values of S7Sr/86Sr (0.70235-0.83011), 143Nd/144Nd (0.51023-0.51148) and 2°6Pb/~°4pb (13,43-24.65). The Rb-Sr, U-Pb, Th-Pb, and Sm-Nd decay schemes have been variably affected by granulite facies metamorphism and possibly by transport in the host magmas. However, Pb-Pb and Sm-Nd isochron systematics seemingly are preserved in many of the xenoliths and indicate essentially concordant metamorphic ages of about 2.8 Ga. Nd model ages are significantly older (ca. 3,1-3.4 Ga) for many of the xenoliths, Precambrian metasediments exposed along the southern margin of the Snake River Plain have Sm-Nd systematics similar to those of the xenoliths. These results suggest that at least two significant thermal events occurred during Archean evolution of the crust in this region: (a) early (ca. 3.1-3.4 Ga) additions of mantle-derived magmas to the crust, and (b) regional metamorphism at 2.8 Ga accompanied at least locally by magmatism. Concordance between the Sm-Nd and Pb-Pb ages suggests that these isotopic systems are little affected by high-grade metamorphism in this case. The distribution of xenoliths in Snake River Plain lavas supports the presence of Archean crust beneath much of southern Idaho, although such rocks rarely are exposed at the surface. Thus, Nd and Pb isotopic studies of crustal xenoliths can provide a useful means of determining the extent of crustal age provinces where surface exposures are lacking.

1. Introduction

The petrologic and geochemical nature of deep continental crustal rocks can be inferred by anal- ogy with high-grade metamorphic terranes exposed at the Earth's surface, from various geo- physical constraints, and from studies of crustal- derived xenoliths in volcanic rocks [1]. Of these approaches, crustal xenoliths provide the most direct evidence of lithology and composition of the deep crust, although it may not be clear how these characteristics vary with depth. Such information is critical for evaluation of processes of crustal evolution and continental magmatism. In the context of a broader study of magmatism in the Snake River Plain (SRP)-Yellowstone Plateau (YP) province, we have undertaken petrologic and geochemical studies of crustal xenoliths which occur in several localities there. These xenoliths are dominantly granulite facies gneisses, and pre-

0012-821X/85/$03.30 ~ 1985 Elsevier Science Publishers B.V.

liminary Pb isotopic data on selected samples have indicated an Archean age [2,3]. Here we report St, Nd and Pb isotopic analyses for representative xenoliths and for metasediments from the Albion Range (southern Idaho) to evaluate the nature and age of the basement beneath the SRP province. These data confirm the antiquity of the xenolith population, document a wide compositional range for such material at depth, and provide constraints on early crustal evolution. Mineralogical details, geothermometry, geobarometry and related petrologic aspects will be presented elsewhere (Matty et al., in preparation).

2. Geologic setting

The SRP-YP province extends more than 500 km across southern Idaho to northwestern Wyo- ming (Fig. 1) and transects an area of ancient

355

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Fig. 1. Index map of part of the northwestern United States showing the geographic relationship of the Snake River Plain-Yellowstone National Park volcanic province (hachured pattern) to the Archean (black) and Proterozoic (stippled pattern) basement outcrops (adapted from [23]). The Yellowstone (YNP) and Island Park (IP) calderas are shown in the northeastern Snake River Plain. Crustal xenoliths studied herein were collected at the Spencer-Kilgore (SK) area, at Craters of the Moon National Monument (COM), and in the Square Mounta in-Magic Reservoir (SM) area. Western extent of continental basement indicated by 87Sr/S6Sr = 0.706 isopleths in (1) western Idaho [11] and (2) the northern Great Basin [11] shown for comparison. The western edge of the continental basement (3) as indicated by Nd-Sm isotopic studies [12] is also shown.

crust. Bimodal tholeiitic basalt-rhyolite volcanism characterizes the whole province and displays a time-dependent shift in locus of initial eruptive activity from mid-Miocene (15-16 m.y.) in the southwest to late Pliocene ( - 2 m.y.) at Yellow- stone [4]. Volumetrically minor evolved or hybrid lavas (ferrobasalt to ferrolatite) occur locally, par- ticularly near the northern margin of the SRP; these lavas have been variably contaminated by crustal material as evidenced by compositional data [5,6] and the presence of xenocrysts and xenoliths of crustal derivation. Isotopic studies of SRP-YP volcanic rocks reveal systematically high 87Sr/~6Sr (0.7055-0.7075) and low 143Nd/144Nd (0.5123-0.5125) in the tholeiitic basalts [7], and even more extreme values in the evolved lavas ([6,18]; Table 1), relative to estimated "bulk Earth" values [8] or basaltic rocks west of the Oregon- Idaho border [9,10]. Analogous geographic shifts in Sr [11,12] and Nd [12] isotopic compositions of pre-mid Tertiary granitic plutonic rocks in this region have been invoked as evidence for the pres-

ence of Precambrian crystalline basement beneath much of southern Idaho and northeastern Nevada. Fig. 1 shows isopleths of Sr and Nd isotopic compositions in Mesozoic or younger igneous rocks which have been suggested as marking approximate western limits of inferred Precambrian basement.

Actual outcrops of Precambrian basement are considerably more restricted and occur primarily to the north and east of the SRP-YP province (Fig. 1). To the north, basement consists mainly of Proterozoic to Palaeozoic sediments and metasediments with limited exposures of Proterozoic ortho- gneiss [13]. Archean high-grade metamorphic rocks, which are part of the widespread Wyoming Province [22], occur near YP and in south-central Idaho, northwest Utah, and northeastern Nevada. In the vicinity of Yellowstone at least two main stages of metamorphism have been documented mainly by Rb-Sr dating; these are an early phase of amphibolite to granulite facies metamorphism around 3.4 Ga [15,43] and a later amphibolite facies event around 2.8 Ga [16,17], both of which are associated with significant magmatism. Amphibolite facies metasediments occur in the Albion range, south-central Idaho [14]. The cur- rent study provides direct evidence for the presence of such Archean basement beneath at least the eastern two-thirds of the SRP-YP province.

3. Sampling and petrography

Crustal-derived xenoliths have been noted in numerous hybrid lavas (or their vents) across the SRP (Leeman, unpublished data) and are notably abundant at three localities in the eastern and central part of the province (Fig. 1). Near the hamlets of Spencer and Kilgore, Idaho (SK locality), we have examined over 200 xenoliths from Swan Butte and Crystal Butte vents. Approxi- mately 30 xenoliths were collected at Craters of the Moon lava field (COM locality), principally from the Devils Orchard flow. Finally, another 30 or so xenoliths were obtained from three localities in the Square Mountain lavas (SM locality) near Magic Reservoir. In each case the host lavas (or their vents) are located along the northern margin of the SRP, and all are evolved and contaminated relative to typical SRP olivine tholeiites [19]. Rep- resentative isotopic analyses of these lavas are

356

T A B L E 1

N d a n d Sr i so topic d a t a for S R P xenoliths and Albion Range metasediments

Nd (~)r TLNHdll R Ref. Sample Sm N d 1478m 143Nd R b Sr ~7Rb SVSr %

144Nd 144Nd S~Sr S6Sr

S K h)cality 1 BI 3.83 ~ 16.40

2 B2 1.45 6.76

3 B3 1.11 6.05

4 BI0 5.91 32.16

5 BI1 1.29 7.69

6 R4 1.64 7.34

7 73-68X 4.87 22.28

C O M locality 8 CKI-1 1.94 13.51

9 70-40 1.15 9.75

10 SI-I 13.3 85.0

11 C O M - 1 1.14 9.23

S M Iocali O' 12 S M - 2 A 0.873 4.88

13 S M - 2 F 2.40 13.0

14 S M - 2 G 1.18 7.14

15 D M - 1 0 3 4.40 25.7

AIbion Range b

16 Y A G - 7 9 9 4.30 19.0

17 Y A G - 8 0 0 9.85 59.0

18 Y A G - 8 2 2 13.1 94.0

Host lavas

73-106G (SK) 18.8

V-31 ( C O M ) 32 SM-1B (SM) 11.5 62

0.1413 0 .511467 ± 12 1.9 220 0.025 0 .70579 +_ 5 - 22.9 15.5 3.20

0 .1300 0 . 5 1 0 9 1 8 ± 22 0.3 634 0 .0014 0 . 7 0 2 3 5 + 3 - 33.6 - 33.3 3.90

0 .1108 0 .510883 +_ 26 0.3 466 0 .0019 0 .70382 + 3 - 34.3 - 12.5 3.10

0.1111 0 .510809 ± 14 1.8 616 0.0085 0 .70246 +_ 2 - 35.7 - 31.8 3.24

0.1017 0.51t449+_ 20 22 338 0.188 0 .71362+_4 23.2 126.6 1.91

0.1351 0 .511154 +_ 16 0.2 321 0.0018 0 .70274 ± 2 - 29.0 - 27.8 3.64

0.1321 0.510895 ± 20 14 175 0.232 0 .72550 ± 3 - 34.0 295.2 4.08

0.0868 0 .510499+_30 29.3 210.9 0.403

0.0710 0.510454+_ 22 22.5 324.4 0.201

0.095 0.510590+_ 8 6.34 69.8 0.263

0 .0749 0 .510228 +_ 12 66 97 1.99

0.1082 0 .511484+_14 99 480 0 .598

0.112 0.510874+_ 36 142 205 2.03

0.0997 0 .509973 +_ 14 45 559 0.233

0.104 0 . 5 1 0 8 5 3 ± 1 8 61 310 0.571

0.137 0 .511278+_24 114 115 2.90

0.101 0 .510675+_16 305 181 4.96

0 .084 0 .510392+_24 188 147 3.75

0.112

0.512179+- 10 52 277 0.542

0 .512266+_10 97 175 1.60

0 .511914+_34 60 287 0 .604

0 .73359 ± 3 - 41.8 409.9 2.95

0 .71525+-3 42.6 149.7 2.64

0 .71795+-3 40.0 188.0 3.05

0.81728_4.3 - 4 7 . 1 1597.6 3.00

0.73138+_5 22.6 378.6 1.98

0.83011 +_ 3 - 34.4 1779.6 3.16

0.72092+_4 52.0 230.2 4.15

0 . 7 3 8 7 8 ! 2 - 34.9 483.6 2.92

0.8060 26.6 1437.5 3.45

0 .89260 4 4 - 38.3 2666.4 3.11

0 .83452+_4 - 4 3 . 9 1842.2 3.02

0 .70890 +- 3 9.0 59.6

0.71112+_3 7.3 91.1 0 .71387+_2 - 1 4 . 1 130.1

Isotope dilution concentration measurements are underlined. b Rb and Sr con ten t s for all samples and SVSr/S6Sr in Y A G - 7 9 9 are f rom

given in Table 1; further details have been pub- lished for some of them [5,6,18].

The xenolith populations are dominated by a variety of granulite facies gneisses, but in addition include fragments of basalt, rhyolite pumice and welded tuff, unmetamorphosed sediment, and cognate cumulate clots. The gneissic xenoliths invari- ably contain intergranular glass (in situ partial melts as determined by microprobe analysis), and some have been oxidized and display textural evidence for mineralogical re-equilibration during ascent. The least modified xenoliths appear to have equilibrated under granulite facies conditions (ca. 700-800°C, 5 kbar) based on application of various geothermometers and geobarometers [20,44]. Thus, these rocks are inferred to represent portions of the deep crust. The majority of sam-

[14]. Qz Qz

m o •

AIk Plag Plag

Fig. 2. I.U.G.S. mineralogical classification of metamorphic xenoliths from the SRP. Fields are as follows: C = charnockite (hypcrsthene granite), O = opdalite (hypersthene granodiorite), E = enderbitc (hypersthene tonalite), N = gabbronori te (norite, hypersthene diroJte, anorthosite). Left: C O M locality; right: SM and SK localities. Note that quartz, alkali feldspar, and plagioclase modal proport ions arc normalized to total 100%; a major i ty of SK xenoliths plots at the plagioclase apex.

pies studied appear to be metaplutonic rocks, but we use here the I.U.G.S. alternative metamorphic classification [21] to subdivide xenolith populations mainly by relative proportions of quartz, plagioclase, and alkali feldspar as shown in Fig. 2. As the lithologic constitution of xenolith populations differs from one suite to another, it is con- venient to give general descriptions by locality in the following sections.

3.1. Spencer-Kilgore (SK) locality

Examination in hand sample of more than 200 lithic inclusions (nearly all from Swan Butte) reveals a variety of rock types. General categories and relative proportions are as follows: rhyolite pumice and welded tuff (9%), fresh and partly fused felsic gneisses (67%), mafic gneisses and metaplutonic rocks (11%) and cumulates (13%). Modal analyses of thin sections and stained slabs of representative samples indicate that most are free of alkali feldspar and are either enderbites (hypersthene tonalites) or gabbronorites (norites, hypersthene diorites, anorthosites) (Fig. 2). Opda- lite (hypersthene granodiorite) occurs as veins in a few composite xenotiths and may represent relatively late influx or segregation of silicic magma. The gabbronorites contain little or no quartz and are essentially plag + opx + cpx bearing rocks with minor Fe-Ti oxides and rare biotite (partly or completely reacted to opx and oxides) in one sample. These rocks typically contain about sub- equal amounts of plagioclase and ferromagnesian minerals. A few anorthositic gabbro xenoliths contain mainly plagioclase with small amounts (20%) of opx + oxides + quartz. All of the gabbronorites contain intergranular glass in amounts ranging up to 10-15%. The enderbites are foliated rocks which typically contain about 60% plagioclase, 25% quartz, and the remainder comprises opx + oxides + cpx + glass. Some highly fused enderbites contain up to 90% vesicular glass. With increasing degree of fusion it appears that the constituent minerals are progressively consumed in the relative sequence: pyroxenes, oxides, plagioclase, quartz; i.e., quartz is more refractory than plagioclase, for example, as indicated by increasing quar tz / plagioclase ratios with increasing degrees of fusion. Although not analyzed here, the cumulate xenoliths comprise mainly plagioclase, ortho- and

357

clinopyroxene, Fe-Ti oxides and, in some, minor olivine. These rocks yield two-pyroxene tempera- tures near l l00°C, appear to have crystallized from magmas similar to the host basalts, and provide evidence for crustal-level magma cham- bers as immediate reservoirs for these magmas (Matty and Leeman, in preparation).

3.2. Craters of the Moon (COM) locality

Some 30 metamorphic xenoliths were studied in detail, comprising mainly felsic variants (charnockite, opdalite, and enderbite) with fewer norites and a single mylonitized metasediment (biotite- garnet gneiss). The xenolith population also includes rare volcanic and sedimentary clasts; a single cumulate xenolith was found. Mineralogi- cally, these rocks are broadly similar to their cohn- terparts described above (Fig. 2). The norites are free of alkali feldspar and consist dominantly of ptagioclase and pyroxenes with interstitial glass. The felsic gneisses consist of quartz, plagioclase and alkali feldspar with varied amounts of pyroxenes, oxides, rare biotite, and interstitial glass; hypersthene is present in nearly all and clinopyroxene occurs in many. Major differences from the SK suite are the apparent lack of anorthositic lithologies and dominance of alkali feldspar- bearing lithologies. Brown intergranular glass occurs as an injected phase along foliation planes, but a clear glass phase is more common and results from in situ partial fusion.

3.3. Square Mountain (SM) locality

Xenoliths from this locality are locally abundant ( - 3 0 % of outcrop) and remarkable in size (many > 1 m across). Nearly all are felsic gneisses which have either been fused extensively during decompression (intergranular glass typically amounts to 25%) a n d / o r injected along foliation planes by the host magma (brown quenched glass with abundant microlites). Ferromagnesian minerals originally present in these xenoliths have been strongly oxidized and in many cases either fused or reacted. Consequently, modal proportions of the ferromagnesian minerals may be underesti- mated and most of the xenoliths now contain dominantly quartz, alkali feldspar and plagioclase, with no more than a few percent oxides and

358

hypersthene and traces of zircon in a network of interstitial glass. Modal analyses of representative examples are portrayed in Fig. 2. One cognate mesocumulate norite (plagioclase + opx + cpx + oxides) was found; other lithologies present include granitic xenoliths from the underlying Idaho batholith and clasts of Tertiary rhyolite. The SM xenolith suite differs from the others notably in its preponderance of felsic gneisses and their relatively higher degree of fusion.

Representative lithologies from each of these localities were selected for geochemical study with an attempt to use the freshest possible samples. Detailed descriptions and mineralogical analyses, geothermometry, etc., are available [20,44] for the samples used in this study.

3.4. Geochemical characteristics

Major and trace element analyses of representative xenoliths and Albion Range metasediments display a wide range, relative to estimates of aver-

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Fig. 3. SiO 2 variation diagram for SRP xenoliths and estimates of average lower (LC) and middle (MC) crust and a Canadian Shield (upper crust; CS) composite [31]. Key to symbols: O = C O M , ~ = S M , v = S K .

age crustal composition (cf. [31 and references therein]), from intermediate to felsic compositions (cf. Figs. 3 and 4). Mafic rocks of basaltic composition have not been found, and only at the SK locality have we observed xenoliths with less than 65% SiO 2. Most of the analyzed xenoliths are characterized by relatively low contents of Rb and U and high K / R b and T h / U , in which respects they are typical of granulite facies rocks worldwide [32-35]. In general, the SK suite shows the great- est depletion of Rb, but nearly all of the analyzed xenoliths appear to have been strongly depleted in U. As will be shown by our Pb isotopic results, these depletions must have occurred long ago and cannot have accompanied transport in the host magmas or other relatively young events. Repre- sentative REE profiles for each xenolith suite and for Albion Range metapelites are shown in Fig. 4. All the analyzed xenoliths are LREE-enriched, but the felsic rocks are generally more so than the norites and commonly have very low HREE contents and positive Eu anomalies. S m / N d ratios in the xenoliths are very similar to those in most other granulites as well as most crustal granitoid rocks (cf. [36]). The low HREE, Rb, and U contents preclude significant contamination of the xenoliths by their host lavas, which are considerably more enriched in all of these elements. REE profiles for the felsic xenoliths (charnockites, opdalites, enderbites) are very similar to those calculated for partial melts of quartz dioritic source rocks [37]. Thus, it is possible that these xenoliths could represent metaigneous rocks produced by anatexis of pre-existing mafic-intermediate composition crustal rocks. These and other details of the petrochemistry of SRP xenoliths will be developed elsewhere (Leeman and Matty, in preparation).

4. Analytical details

Pb isotopic compositions and U, Th and Pb contents were determined by W.P.L. at the U.S. Geological Survey (Denver) using procedures described by Doe et al. [24]. Analytical uncertainties are about 0.1% for the isotopic ratios and about 2% for concentrations of U, Th and Pb. Sr and Nd isotopic compositions were determined by M.A.M. and W.P.L. at the Open University using the procedures of Hawkesworth et al. [25]. After correc-

359

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I I I I I I La Co Nd Sm Eu Tb

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Y - 7 7 9

I I I I I I I I O' I I I I I I I I La Ce Nd Sm Eu Tb Yb Lu La Ce Nd Sm Eu Tb Yb Lu

Fig. 4. Representative REE profiles for (a-c) SRP xenoliths and their host lavas, and (d) Albion Range metasediments.

360

tions for mass fractionation were applied (normal- ization to ~68r/87Sr = 0.1194 and 146Nd/laaNd = 0.7219), Sr isotopic ratios were adjusted to 8VSr/S6Sr=0.71028 for NBS-987 standard. Nd isotopic ratios have not been adjusted; 143Nd/ 144Nd was measured as 0.51262 + 2 for BCR-I (n = 18) and 0.51182_+ 2 for Johnson and Mat- they NdO (n = 8).

Sm and Nd concentrations were determined by INAA for all samples and redetermined in most of the xenoliths by isotope dilution with errors less than 0.5% for each element. Errors in the INAA analyses are about 3% for Sm and 5% for Nd. Rb and Sr contents were determined by XRF spec- trometry and checked by isotope dilution for three samples. Analytical results are reported in Tables 1, 3, and 4.

Sr and Nd isotopic compositions also are presented using the c-notation (cf. [12]) and T~.,u R ages [26] calculated using "bulk earth" parameters as follows: STSr/S6Sr=0.7045, 143Nd/ln4Nd = 0.512638, SVRb/86Sr = 0.0827, 14VSm/la4Nd = 0.1967 [8]. Decay constants used are as follows: XRb=0.0142 AE -1, XSm=0.00654 AE-1; X23 s = 0.155125 AE l; X235=0.98485 AE 1, X232 = 0.049475 AE -1. All isochron calculations follow the method of York [38].

5. Isotopic results

Results of our isotopic analyses have direct bearing on possible magma-crust interactions and crustal anatexis associated with SRP-YP magmatism. However, these aspects of our study will be presented elsewhere in context with our studies of the volcanic rocks (Leeman and Menzies, in preparation). Here we focus on aspects of crustal composition, age and evolution.

5.1. Srn-Nd svstematics

Nd isotopic compositions in the analyzed samples vary widely and are unradiogenic (c y d = - 2 2 . 9 to -52 .0) compared to modern "bulk Earth" or " C H U R " [12] values. 147Sm/ln4Nd ratios range between 0.075 and 0.141, averaging about 0.111, and are similar to values reported for many crustal rocks [36]. TcNdu R ages [26] calculated for these rocks range between 1.9 and 4.2 Ga (Table 1); however, the majority of the xenoliths

yield similar model ages (3.08 +_ 0.24 Ga, 2o; n = 8). The three Albion range metasediments yield similar model ages (3.02-3.45 Ga). Among those xenoliths having notably distinct model ages, one (B-11) is a felsic vein in a norite xenolith; its model age of 1.9 Ga may reflect a younger igneous event. Two of the SM xenoliths (SM-2A and SM- 2G) yield extreme model ages (1.98 and 4.15 Ga), but considering their high degree of fusion ( - 35%), it is possible that the Sm-Nd systematics in these samples have been modified. The same is potentially true for most of the analyzed samples, as all contain intergranular glass. Thus, it is per- haps surprising that so many of the xenoliths yield consistent model ages.

Considering those samples with the most extreme Nd model ages, we have calculated 1478m/144Nd ratios required to produce model ages (ca. 3.0 Ga) close to median values for the other xenoliths; fSm/Nd ( = observed/calculated S m / N d ) ranges from about 0.75 for samples B-11 and SM-2A to 1.53 for sample SM-2G. Thus, if all of these samples are related and have true crustal residence ages near 3.0 Ga, 147Sm/144Nd ratios must have decreased by about 25% in B-11 and SM-2A and increased by about 50% in SM-2G. Decreases in S m / N d correspond to relative enrichment of LREE, and vice versa. Theoretically, LREE enrichment could result from introduction of a LREE-enriched melt phase, such as a partial melt from the xenolith protolith. Contamination by host lavas is not a likely explanation because these liquids have relatively high contents of HREE and lower N d / S m ratios (Fig. 4). Likewise, LREE depletion required for SM-2G, could plausibly result from extraction from it of a LREE enriched partial melt. Examination of individual chondrite- normalized REE profiles shows very similar slopes for LREE in all of the SM xenoliths, and there is little indication for relative LREE enrichment in SM-2A or depletion in SM-2G. Sample B-11 is more LREE enriched than other SK xenoliths, but its bulk composition is more felsic, so again it is not clear that this enrichment necessarily is related to a recent disturbance. Thus, qualitative examination of the REE profiles does not provide a ready explanation for the extreme Nd model ages in these samples. We note that similar variations in Nd model ages are observed in other granulite facies terranes (e.g. [45]) and could be inherent in

0.5115 5 e [312 1 •

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18

11

Loo 0.5100 ~ '

0 ~14

I I I 0 0.05 0.10 0.15

1 4 7 S m / 1 4 4 N d

Fig. 5. Samarium-neodymium isochron diagram for SRP xenoliths and Albion Range metasediments. Data points are keyed to reference numbers in Table I. Best fit isochron is shown for all samples with consistent model ages (case 5, Table 2).

the xenolith source. Causes for such variations remain obscure. We emphasize that Sm and Nd contents were determined by isotope dilution for most xenoliths, including those discussed above, so extreme Nd model ages cannot be attributed to analytical uncertainties.

In a Sm-Nd isochron diagram (Fig. 5; see Table 2), there is remarkably little scatter, considering the diverse origins of the xenoliths, when the three samples (B-11, SM-2G, and SM-2A) noted above are excluded. In fact, those xenoliths having similar model ages (case 5, Table 2) produce an accept- able isochron (MSWD = 2.5) which indicates an age of 2.76 ___ 0.22 Ga, (R 0 = 0.50886 + 0.00016;

361

C N d = - 3 . 9 _+ 2.2). The three Albion Range samples yield a slightly lower age (2.55 + 0.79 Ga, case 4) with fairly large uncertainty owing to the small range in composition. Combinat ion of the metasediments and the eight xenoliths with consistent model ages produces results similar to that for the xenoliths alone (case 3).

We conclude from these observations that xenoliths from all three localities and the Albion Range samples could have been affected by a common metamorphic event about 2.8 Ga ago; this age is virtually identical to the Pb-Pb age determined for the xenoliths (see next section) and to Rb-Sr ages for lower-grade basement rocks to the east. The "discordant" Sm-Nd ages for certain xenoliths could (a) reflect distinctly different metamorphic events, (b) represent a range in initial Nd composition, or (c) result from in situ melting or contamination from host magmas during transport to the surface. Of these choices, (b) seems most plausible.

The isochron age is distinctly lower than the model ages, and suggests a complicated history for the basement sampled. The initial 143Nd/144Nd ratio (0.50886) determined for the xenoliths is statistically indistinguishable from that for the metasediments alone. So, although distinction is possible between respective source materials, we treat them together for simplicity. Fig. 6 shows that this isotopic composition is less radiogenic than C H U R sources at 2.8 Ga (~NO = _ 3.9 + 2.2). Thus, if the average T(F~u R age (3.1 Ga) is taken as the approximate time that igneous material (with average 1475m/144Nd = 0.111) was extracted from a C H U R source in the mantle and emplaced in the crust, then subsequent isotopic evolution would produce the required initial ratios for the xenoliths at 2.8 Ga. Regional metamorphism could have

TABLE 2

Sm-Nd ages for SRP crustal xenoliths and Albion Range metasediments

Case Data included N MSWD Age_+ 2 o (Ga) R o -+ 2 o

1 all data 18 150 3.88 -+ 1.52 0.5081 -+ 0.0011 2 as above, excl. SM-2A, SM-2G, Bll 15 22 2.25 -+0.42 0.5092 -+0.0003 3 as above, excl. B2, 73-68X, R4, 70-40

(includes samples with similar T(-N~tj a ) 11 1.7 Albion only 3 - 0.0 xenoliths only as in case 3 8 2.5

2.75+0.18 4 2.55_+0.79 5 2.76 4- 0.22

0.50886 + 0.00012 0.50898 + 0.00054 0.50886 + 0.00016

362

+10

0

- 1 0

~Nd

- 2 0

-:30

- 4 0

- 5 0

~" ....................... DEPLETING ~ . - ~ ...M_.AN ZLE..._.7...~.7.._.7.,,..~ ~,~: ~

CHUB EVOLUTION ~ =o / n ~'/~"'/'//

Albion ,,'" / ~ COM

/" / . El KS

/ / [] SM

I I I 1 3 4

T(Ga)

Fig. 6. Neodymium isotopic evolution diagram showing present-day ~yd and changes in c TM with time for selected representative samples. C H U R and Depleting Mantle evolution curves (cf. [39]) are shown for reference, lnitial isotopic composition and inferred metamorphic age for samples with consistent model ages is shown (O) with 20 error bars. Note that modal ages relative to the Depleting Mantle curve will only be about 0.1 Ga older than TCHUR ages.

reset Sm-Nd ages at the latter time. The two-stage history suggested would be little modified by assumptions that the early igneous protolith was derived from a "depleting mantle" rather than a " C H U R " reservoir (Fig. 6; cf. [39]).

5.2. Pb isotopes

Pb isotopic analyses have proved successful in dating ancient metamorphosed crustal rocks because of the unique coupling between the 235U- 20v Pb and 238 U- 2o6 Pb decay schemes [27]. Whereas the parent-daughter elements may be "fraction- ated" (i.e., their intrinsic ratios modified) in the course of high-grade metamorphism (e.g., U depletion or extraction from granulite grade rocks), it is far less likely that the daughter isotopes will be modified. Thus, 2°vpb/2°6pb (or Pb-Pb) ages can be determined solely from Pb isotopic analyses of rock suites initially characterized by uniform (or nearly so) isotopic compositions. The validity of such Pb-Pb ages has been demonstrated in many cases by comparisons with other dating methods (e.g. [28,29]). On the other hand, rehomogenization of Pb isotopic compositions is not always effected by high-grade metamorphism (e.g. [45,46]), in which cases Pb-Pb ages may exceed those by other dating methods.

Preliminary Pb isotopic analyses of three COM xenoliths [2] suggested a Pb-Pb age of about 2.9 Ga. One sampe (CKI-1) with extremely low U / P b contained unradiogenic Pb similar to 2.8 Ga common lead relative to the Stacey-Kramers [30] evolution curve. Because the range in Pb isotopic composition in these samples was small, additional samples were analyzed to improve the age estimate. Furthermore, because the xenoliths were subject to potential contamination from their host lavas, leaching studies were conducted to evaluate the proportion and composition of labile Pb contained therein. Analyses of whole-rock xenoliths and host lavas are given in Table 3 and results of the leaching study are given in Table 4.

The analyzed samples vary widely in 2°6pb/ 2°4pb (13.43-24.65), 2°7pb/2°4pb (14.68-16.87), and 2°SPb/2°4pb (33.73-59.07) isotopic ratios and in U (0.01-2.8), Th (0.02-33.2), and Pb (3.6-60.4) contents (in ppm). U / P b and Th /Pb ratios correlate only crudely with the respective daughter isotopic ratios, which precludes direct dating, but 2oTpb/204pb and 206pb/204pb ratios are nearly collinear and suggestive of a Pb-Pb isochron (Fig. 7). The SK and COM suites contain the least radiogenic Pb and lowest U contents, which suggests that these samples suffered a U / P b reduc- tion early in their history--most likely at the time of granulite metamorphism. Contamination by the relatively radiogenic Pb in the host lavas could

42

L59.07 o SK ~ WR Am ~ M j XENOLITHS • LEACH RESIDUE + LEACHEATE

J 40 /

/ Host Lavas / . r]

/ ~ > " ~ / / "

3 6 ~ ....

14 11 I f " t I I ~ I I 16 118 /0 22 214

2O6 Pb/2o4 Pb

Fig. 7. Lead isotopic compositions of SRP xenoliths and host lavas. Stacey-Kramers growth curve (S-K) shown for reference with ticks showing time in Ga before present. Tie lines connect leacheate and whole-rock xenoliths from leaching study. Best fit isochron corresponds to case 7, Table 5.

TABLE 3

Pb, U, and Th contents and Pb isotopic composition of xenoliths and host lavas

363

Sample description Pb (ppm) U (ppm) Th (ppm) 206 Pb 207 Pb 208 Pb

T h / U 2o4 Pb 2o4 Pb 2o4 Pb

Island Park Rift Zone 73-106G Swan Butte basalt 17.7 2.44 8.91 B1 norite-enderbite 5.76 0.094 0.217 B3 enderbite-norite 3.63 0.012 0.021 B 10 nori te 7.88 0.120 0.45 B11 charnockite band in

layered gneiss 17.2 0.114 0.693 73-68X norite 9.04 - -

Craters of the Moon laoa field 69-20 Devils Orchard flow 33.7 4.6 13.0 V-31 Devils Orchard flow 31.1 3.25 11.8 SI-I norite 9.02 0.36 33.2 70-40 opdalite 22.4 < 0.2 6.09 CKI rim opdalite 16.3 - - CKI core opdalite 15.8 0.061 0.104

Square Mountain SM-1B host lava 18.2 1.24 6.78 SM-2A charnockite-opdalite 46.3 0.31 1.09 SM-2F opdalite-enderbite 60.4 2.80 15.9 SM-2G charnockite-opdalite 43.1 0.39 1.36

3.65 17.75 15.55 38.24 2.3 16.73 15.45 37.25 1.8 14.18 14.98 34.71 3.8 14.80 14.81 33.73

6.1 16.23 15.32 37.54 - 14.95 15.18 43.18

2.8 17.81 15.60 38.43 3.6 17.81 15.60 38.52

92.2 16.09 15.64 59.07 - 14.11 14.82 35.73

- 13.43 14.68 34.70 1.6 13.56 14.72 35.01

5.5 19.02 15.75 39.06 3.5 16.36 15.50 35.22 5.7 24.65 16.87 39.29 3.5 18.76 15.88 36.90

o n l y s h i f t t h e x e n o l i t h P b c o m p o s i t i o n s to m o r e

r a d i o g e n i c v a l u e s w h i c h t h e m s e l v e s l ie v i r t u a l l y o n

t h e x e n o l i t h a r r a y . H e n c e , s u c h c o n t a m i n a t i o n

w o u l d h a v e l i t t l e e f f e c t o n c a l c u l a t e d a g e s .

T h e l e a c h i n g s t u d y i n v o l v e d r e a c t i o n o f t h r e e

x e n o l i t h p o w d e r s w i t h c o l d 6 N HC 1 . A n a l y s i s o f

t h e l e a c h s o l u t i o n a n d r e s i d u e i n d i c a t e s t h a t o n l y a

f e w p e r c e n t o f t h e P b i n e a c h x e n o l i t h is l a b i l e .

A l t h o u g h t h e 2 ° 6 p b / 2 ° 4 p b c o m p o s i t i o n s o f t h i s

l a b i l e c o m p o n e n t a r e i n t e r m e d i a t e b e t w e e n t h e

w h o l e - r o c k a n d h o s t l a v a i n t w o c a s e s (B3 a n d

7 0 - 4 0 ) , i n t h e t h i r d c a s e ( S M - 2 G ) i t is m o r e r a d i o -

TABLE 4

Pb isotopic analyses of leach and residue fractions of selected xenoliths

206 Pb 207 Pb 208 Pb Sample Material " Pb (ppm)

204 Pb 204 Pb 204 Pb

B3 whole-rock 3.63 14.18 14.98 34.71 leach - 15.90 15.25 35.80 residue (99.2%) b 3.45 14.13 14.89 34.37

70-40 whole-rock 22.4 14.11 14.82 35.73 leach - 15.28 15.18 41.36 residue (99.0%) 17.4 14.10 14.81 35.85

S M-2G whole-rock 43.1 18.76 15.88 36.90 leach - 20.05 16.50 39.22 residue (99.4%) 18.72 15.83 36.73

a Fine powder of each sample was leached for 4 hours in cold 6N HCI. The supernate and washings (with pure H20) were decanted and, after evaporation, processed for analysis taking splits for isotopic composition and Pb content. The residue was dried and weighed, then processed for analysis as above. Whole-rock analyses are from Table 3.

b Weight percent remaining of original sample subjected to leaching.

364

genic than the host lava. Furthermore, the 20~ Pb/204 Pb- 206 Pb/204 Pb leach compositions do not lie on the tie lines between the bulk xenolith and host lava (Fig. 7). Thus the labile components do not appear to contain a high proportion of Pb from the host lavas, nor are they a significant fraction of the total xenolith Pb budget. For these reasons we infer that the xenolith isotopic compositions are indeed representative of their respective crustal source regions.

Pb-Pb isochron ages have been calculated for various subsets of the available data, including the three leach residues. Resulting ages are shown in Table 5 for the COM, SK, and SM suites individu- ally, and for all collectively. Two samples (B10 and SI-1) contribute most heavily to the overall scatter and calculated ages are presented with and without these samples. Uncertainties in calculated ages are relatively large for the low population subsets, even though MSWD values are low (cases 2, 4 and 5), because of the restricted range in isotopic compositions. The pooled data indicate a best estimate of age (2.81 __% 0.04 Ga, MSWD = 14). This age is interpreted as the time of granulite metamorphism. The scatter about the best-fit isochron exceeds analytical uncertainty, but is surprisingly small considering the random nature of xenolith sampling and the distance between sample locations. Given these factors, it is prob- able that the assumptions involved in the Pb-Pb isochron approach may not be satisfied strictly (e.g. slight isotopic heterogeneity probably pre-

vailed at 2.8 Ga in the crustal reservoirs sampled). However, the age obtained is similar to Rb-Sr ages for basement exposed further east and is geologically reasonable.

Closer examination of the Pb isotopic data suggests the possibility that the xenoliths may sample multiple reservoirs with varied 2 3 8 U / 2 ° 4 p b (/~)

values, which may account for observed deviations from a single ideal Pb-Pb isochron. For example, if it is valid to pool subgroups of xenoliths from different localities, certain such groups (e.g., cases 8 and 9, Table 5) define well-fitted subparallel isochrons that correspond to significantly different

values; these two isochrons are steeper than that shown in Fig. 7 and yield older ages (ca. 3.1 Ga) similar to the median Nd model age. Possible effects of pooling samples (as in cases 1-7, Table 5) that may lie on families of parallel isochrons with different /~ values include a worsening of the isochron fit and possible rotation of the apparent best fit isochron to greater or lower ages, depend- ing upon the distribution of data points on the respective isochrons. The latter effect, if real, would be a rotation to a lower apparent age for the SRP xenoliths because some of the most radiogenic Pb analyses are on or below the isochron (case 9) with lowest /~. According to this view, samples B-10 and SI-1 could lie on "isochrons" (for which no other samples have been analyzed) corresponding to lower and higher /~, respectively; another explanation for their compositions involves U-Pb fractionation long ago (e.g., U ga in /Pb loss and U

TABLE 5

Calculated Pb-Pb ages for SRP crustal xenoliths

Case Data included N MSWD Age-+ 2o (Ga) a t~l h

1 COM only 5 8.5 3.78 _+ 0.09 10.58 2 COM, excl. SI-1 4 0.15 2.76+_0.52 8.04 3 SK only 6 34 2.90 + 0.14 8.32 4 SK, excl. BI0 5 6.7 2.76_+0.16 8.33 5 SM only 5 2.0 2.53 + 0.08 8.46 6 All data 15 36 2.84 + 0.04 8.31 7 All data, excl. SI-I and B10 13 14 2.81 _+0.04 8.26 8 SM-2A, 73-68X, B3 3 0.97 3.10+_0.12 8.81 9 BI, Bll , 70-40, SM-2G, CK1

core, CKI rim 6 1.77 3.05 -+ 0.04 8.38

Isochrons were fit using the method of York [38]. ~' pq is 238U/2°4pb ratio for time-integrated first-stage Pb isotopic evolution prior to the

of the Earth is taken as 4.57 Ga [27] for these calculations. "event" dated by the Pb-Pb isochrons. Age

loss /Pb gain, respectively) if they were formerly related to either of the isochrons in cases 8 and 9. Unfortunately, there are no independent means to determine which xenoliths should be pooled for an isochron fit because geological relationships between them cannot be determined. The interpre- tations noted in this paragraph are noteworthy because they suggest possible Pb isotopic corrobo- ration of the Nd model ages.

A brief comment is in order concerning the generally systematic Pb-Pb relations in xenoliths that yield atypical Nd model ages (e.g., B-11, SM-2A, SM-2G). The simplest explanation for the discrepancies noted is that recent chemical disturbances, possibly related to decompressional melting during ascent in magmas, has variably modified S m / N d ratios in some of the xenoliths. Such disturbances could significantly modify the Nd model ages, as discussed in the previous section. Similar disturbances, because they are so recent and involve elemental rather than isotopic fractionations, would have relatively little effect on Pb-Pb systematics.

5.3. Rb-Sr systematics

Present-day 87Sr/86Sr ratios in the xenoliths range widely from 0.70235 to greater than 0.83 and correlate only roughly with observed R b / S r ratios (Fig. 8). A linear regression through all the data yields a geologically improbable age of 4.4 + 0.3 Ga, and scatter in the data (MSWD = 210)

o.892e J"

• KS 0 . 8 5 ~. COM

. ; , ; ,oo o , 3

o,o, . ,

• o . , oo l , - ' , , , , ~ 0 " 0 0 . 0 1 0 0 . 0 2 0

I I I 3 I 4 0 . 7 0 1 2 87F ib /86Sr

Fig. 8. Rubidium-strontium isochron diagram for SRP xenoliths and Albion Range metasediments with 4.46 Ga and 2.8 Ga reference isochrons. Individual data points are keyed to reference numbers in Table 1.

365

precludes any confidence in this "age". Rather the scatter likely reflects (a) sampling of the xenolith population from diverse crustal reservoirs with varied initial 87Sr/86Sr and possibly distinct ages, and (b) effects of differential Rb loss during granulite facies metamorphism.

The above observations apply even when xenolith suites from individual localities are considered separately. Samples from the SM locality have generally higher R b / S r and 87Sr/~6Sr (which accord with their more pelitic compositions), whereas many of the SK (meta-igneous) xenoliths have exceptionally low values for these ratios. Low e sr values (ca. - 3 0 ) for several of the SK xenoliths are comparable to those in present-day mid- ocean ridge basalts and indicate both (a) their derivation from a relatively primitive source, and (b) early Rb depletion to preserve such low values. Given the complex effects of metamorphism on R b / S r ratios [35] and the possibility of varied initial 87Sr/86Sr ratios , no quantitative age information can be derived for the xenoliths. TS~ model ages [26] scatter widely and are not reported here. However, the Rb-Sr data are consistent with original derivation of some of the xenoliths from the mantle whereas some undoubtedly were derived from existing crustal sources prior to granulite facies metamorphism.

As a measure of the extent of Rb depletion that accompanied granulite facies metamorphism, we calculated time-integrated 87Rb/86Sr ratios required to generate the present-day 87Sr/86Sr of the xenoliths from c sr = 0 at 3.0 Ga. Values of fRb/Sr ( = observed/calculated Rb /S r ) for individual samples range from 0.03 to 0.85. Assuming that differences between observed and calculated 87Rb/86Sr ratios are due solely to Rb depletion, these results imply Rb losses between 97 and 15%, respectively. On average, the SK xenoliths appear to be more depleted (77 + 23%) than those at the other localities (34 + 11% Rb loss). These estimates of Rb loss are likely to represent upper extremes for xenoliths which (a) were derived initially as magmas from mantle reservoirs, but had a significant residence in the crust prior to granulite metamorphism, or (b) that originally were derived from crustal protoliths.

An age of 2.46 + 0.3 Ga (R 0 = 0.703) has been reported for Albion Range metasedirrients by

366

Armstrong and Hills [14]. Using the same samples (and our new data for two of them) we arrive at an age of 2.93 + 0.43 Ga and a substantially lower initial 87Sr/86Sr value of 0.678 _+ 0.027 (MSWD = 4.9). Including their sample (143) with highest R b / S r reduces the calculated age to 2.63 + 0.47 Ga (R 0 = 0.696 _+ 0.030, MSWD = 13). From these results it is clear that the Rb-Sr age is only approximate and that these rocks have either experi- enced complex open system behavior with respect to Rb-Sr systematics and /o r the rocks may have been characterized by varied initial isotopic compositions. However, TS~ model ages [26] for the three samples we studied range only slightly (2.45-2.66 Ga) and are similar to the errorchron "ages" noted above. These metasediments have considerably higher 87Sr/86Sr than all of the analyzed xenoliths except SM-2F, and notably distinct trace element characteristics (cf. Fig. 4d). If such metasedimentary rocks extend in the sub- surface below :he SRP, they have not to our knowledge been sampled by ascending magmas which carry the granulites.

6. Discuss ion anc:~ conclusions

Although there are limitations with respect to sampling (i.e., ent~'ainment of random xenoliths in erupted magmas) and in our sample selection (de- signed to characterize potential crustal contami- nants), our study has revealed the following significant observations. The crustal xenoliths are dominantly granulite facies gneisses which seemingly equilibrated at mid-crustal levels (say 15 km or so). Their occurrence with plagioclase-phyric cognate xenoliths indicates that the host magmas were derived from mid to lower crustal reservoirs. Despite possible disturbances of the isotopic systems in the xenoliths during transport to the surface, and possible scatter resulting from the wide geographic representation, we find that both Pb-Pb and Sm-Nd dating methods suggest essentially identical age estimates (excluding aberrant samples as discussed earlier). We interpret the apparent isochron ages obtained (ca. 2.8 Ga) as the approximate time of granulite metamorphism. This concept is supported rather convincingly by the presence in some xenoliths of unradiogenic Pb, very similar to 2.8 Ga common Pb, coupled with the U-depleted character of most xenoliths which

is commonly attributed to high-grade metamorphic conditions [35]. Although Rb-Sr systematics have been disturbed and provide no reliable age information, the preservation of very unradiogenic Sr in some xenoliths can be due to early (i.e., 2.8 Ga) Rb depletion accompanying granutite metamorphism. Evidence for earlier crustal evolution comes primarily from Nd model ages which are older than 2.8 Ga for most samples and support an age of about 3.1 Ga for the granulite protolith. This result is supported by Pb-Pb ages on two subsets of xenoliths, provided that such groupings are valid. Although not developed here, the REE patterns for the felsic granulite xenoliths are consistent with these rocks having developed as magmas produced by anatexis of pre-existing mafic to intermediate crustal rocks (cf. [37,40]).

Thus, we infer that Archean crustal evolution proceeded in at least two main stages, namely an early igneous period at ca. 3.1 Ga and a later metamorphic episode at ca. 2.8 Ga. This inference is supported by geological and geochronological studies in dominantly amphibolite grade terranes exposed to the east [15-17]. Even the Albion Range metasediments seem to reflect such a history as they have model Sr ages (about 2.4-2.6 Ga) and a Sm-Nd isochron age (2.6 Ga) consistent with most of the granulite xenoliths, but their Nd model ages (3.0-3.5 Ga) are older and may represent an average age for the sediment provenance terrane. De- tails of timing of major igneous and metamorphic events in the Archean crust of this region may be defined more precisely by studies of exposed basement rocks, but the crustal xenoliths provide tangible evidence that such old rocks indeed extend beneath much of the SRP.

The occurrence of crustal xenoliths in petrologi- cally and geochemically evolved lavas from the SRP attests to the likelihood that magma-crust interactions play an important role in these and possibly other magmas in the region. Although we will present quantitative models of such interactions elsewhere (Leeman and Menzies, in preparation), some comment is warranted concerning the geochemical nature of the xenoliths studied. Most notable from an isotopic standpoint is their U-depleted and LREE-enriched nature, which are common features world wide in granulites of intermediate to silicic composition (e.g. [35,36,40]). Given that these characteristics have persisted in

m u c h of the crus t s ince A r c h e a n t ime, it fo l lows

tha t m u c h of the d e e p crus t m u s t be cha r ac t e r i z ed

by u n r a d i o g e n i c Pb and Nd . A l s o n o t a b l e is the

fac t tha t low R b / S r g ranu l i t es also m a y c o n t a i n

ve ry u n r a d i o g e n i c Sr (0.703); this f ea tu re has been

o b s e r v e d in ce r ta in maf i c g ranu l i t e s (cf. [41,42]), bu t m a y be c o m m o n even in felsic g ranu l i t es that

h a v e expe r i enced severe R b - d e p l e t i o n . Clear ly , the

a f o r e m e n t i o n e d charac te r i s t i c s m a y he lp e luc ida t e

the role o f g ranu l i t e - f ac i e s crus ta l c o n t a m i n a t i o n

o f c o n t i n e n t a l m a g m a s , bu t on ly wi th due rega rd

to the range o f i so top ic c o m p o s i t i o n s that resul t

f r o m po t en t i a l l y large va r i a t i ons in p a r e n t /

d a u g h t e r e l e m e n t ra t ios a n d decay t ime.

Acknowledgements

This research was m a d e poss ib le by a N a t i o n a l

R e s e a r c h C o u n c i l f e l lowship and by N a t i o n a l Sci-

ence F o u n d a t i o n g ran t s ( E A R 80-18580, E A R 82-

14876 and E A R 83-20358) a w a r d e d to W.P .L , W e

thank p e r s o n n e l at the U.S. G e o l o g i c a l Survey,

D e n v e r , and the O p e n U n i v e r s i t y for access to

faci l i t ies and for s u p p o r t in m a n y w a y s - - s p e c i f i -

cal ly, B.R. Doe , C.J. H a w k e s w o r t h , P.V. Ca ls te ren ,

a n d A. Gledhi l l . T h e m a n u s c r i p t was grea t ly im-

p r o v e d t h rough the carefu l rev iew of P.N. Tay lo r .

A l b i o n R a n g e m e t a s e d i m e n t s were p r o v i d e d by

R.L . A r m s t r o n g . W e thank A. Wa i t e r s for

m a n u s c r i p t p r e p a r a t i o n and J. T a y l o r for the

graphics .

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strontium, neodymium and lead isotopic compositions of deep crustal xenoliths from the snake river...

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