lu-hf geochronology applied to dating cenozoic events...

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ISOTOPE GEOSCIENCE ELSEVIER Chemical Geology 142 (1997) 63-78

Lu-Hf geochronology applied to dating Cenozoic events affecting lower crustal xenoliths from Kilbourne Hole, New

Mexico

Erik E. Scherer a3 * , Kenneth L. Cameron a, Clark M. Johnson b, Brian L. Beard b, Karin M. Barovich b3c, Kenneth D. Collerson d

a Earth Sciences Department, Unioersity qf California, Santa Cruz, CA 95064, USA b Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA

’ Department of Geology and Geophysics, University of Adelaide, Adelaide, S.A. 5005, Australia d Department of Earth Sciences, The University of Queensland, Brisbane, QLD. 4072, Australia

Received 4 October 1996; accepted 15 May 1997

Abstract

Melt-extraction or crystal accumulation events that affected garnet-bearing, granulite xenoliths from Kilboume Hole, New Mexico, have been dated using the Lu-Hf isotope system. Two garnet-bearing granulites from Kilboume Hole have extreme ‘76Lu/ ‘77Hf ratios of 0.95 and 1.3 (Lu/Hf = 28 and 39 X chondritic), but relatively ‘normal’ ~~~ values (+ 5, and + 12) necessitating either garnet accumulation or melt-extraction from a garnet-bearing protolith in the Cenozoic. Hf isotope evolution curves for these two samples intersect those of depleted mantle and Proterozoic crust at high angles and at similar times, demonstrating the potential of Hf isotope model ages to yield true age significance even if the initial Hf isotope composition is not well constrained. A third garnet-granulite xenolith (CKH63; ‘76Lu/ ‘77Hf = 0.025) contains zircon, which buffered this sample against changes in Lu/Hf during the Cenozoic differentiation event. The three garnet granulites lie closely about a 25 Ma Lu-Hf reference line, demonstrating the potential of the Lu-Hf system for detecting garnet-controlled differentiation events in the Cenozoic; given the range in Lu/Hf ratios measured, events as young as 5 Ma may be detected using the Lu/Hf system. Conventional U-PI, zircon data from sample CKH63 reveal both a ca. 1.4 Ga inherited component and a component of recent Pb-loss (or new zircon growth), supporting the Cenozoic event documented by Hf isotope model ages. 0 1997 Elsevier Science B.V.

1. Introduction crust (Unruh et al., 1983; Salters and Hart, 1989;

The Lu-Hf system is demonstrably sensitive to the mineralogy of magma sources in the mantle and

Beard and Johnson, 1993; Johnson and Beard, 1993; Johnson et al., 1996). As a result, the Lu-Hf system provides a view of crust-mantle evolution that is

* Corresponding author. Tel.: + 1 (408) 459-5228; fax: + 1 (408) 459-3074: e-mail: [email protected]

unique as compared to more commoniy used isotopic systems such as Rb-Sr, Sm-Nd, and U-Pb. How- ever, geochronological application of the Lu-Hf sys-

0009-2541/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOOOS-2541(97)00076-4

64 E.E. Scherer et al. / Chemical Geology 142 f1997J 63-78

tern has been restricted because of the generally limited range and low absolute values of 176Lu/ “’ Hf for ‘normal’ terrestrial rocks and mete-

orites (e.g., 0.001-0.12; Patchett and Tatsumoto, 1980a,b; Patchett et al., 19811, and numerous analytical challenges involved with sample dissolution,

Table 1 Kilboume Hole granulite xenolith data

Group-2 garnet granulites 2-px granulite

CM64 cKH39 CM63 CKH58

SiO, 42.5 45.1 45.3 53.9 TiO, 2.35 2.60 1.61 1.17

A1203 22.9 20.8 22.4 15.6 Fe0 17.3 16.8 15.8 9.42

MgG 6.97 5.36 6.20 8.45 CaO 5.68 5.75 6.26 7.73 Na,O 2.09 3.09 1.96 2.27 K20 0.28 0.44 0.44 0.93

p205 0.04 0.07 0.09 0.48

Mg# 44 39 44 64

Nb 21 17 9 3 Zr 50 52 412 134 Y 169 154 110 24.5 Sr 312 388 236 1044 Rb 3.1 2.0 5.9 5.5 Ba 283 308 371 1177 Cc 3.60 6.27 33.3 56.3 Nd 2.73 3.01 17.2 34.7 Sm 1.36 1.48 6.99 7.55 Eu 1.29 2.12 2.27 1.93 Gd 5.20 4.71 12.0 6.24

DY 16.0 14.9 16.4 4.85 Er 29.7 22.6 13.9 2.56 Yb 30.4 28.8 10.8 2.24

” 86 Sr/ Sr

“‘Pb/ ‘04Pb 207Pb/ 204Pb 208Pb/ 204Pb

0.705491 & 11 0.706866 + 23 0.716265 * 11 0.704094 * 10

18.1 17.2 17.0 16.2 15.6 15.4 15.4 15.3 37.9 37.4 37.1 35.9

‘43Nd/ ‘44Nd 0.512781 + 8 0.512511 f 9 0.512426 * 11 0.512155 k 10

14’Sm/ ‘44Nd 0.301 0.297 0.246 0.132

EN,’ (0) +2.8 -2.5 -4.1 - 9.4

Major element XRF analyses are recalculated to 100 wt% anhydrous. Trace element concentrations are in ppm. Mg# = lOO(Mg/(Mg + Fe2’)) with 0.9 Fe as Fe*+. REE concentrations were determined by isotope dilution. Measured Sr and Nd isotopic ratios were normalized to 86Sr/88Sr = 0.1194 and ‘46 Nd/ ‘44Nd = 0.7219, respectively. Mean values for NBS-987 and La Jolla Nd during the course of this study were “Sr/ 86Sr = 0.710230 f 17 (n = 251, and ‘43Nd/ ‘44Nd = 0.511835 f 8 (n = 271, respectively. Reported ‘43Nd/ ‘44Nd values are corrected by +37 ppm for instrumental bias (assuming ‘43Nd/ ‘44Nd = 0.511855 for the La Jolla standard). and values are therefore calculated with respect to a modem CHUR ‘43Nd/ ‘44Nd of 0.512638. No correction was applied to Sr ratios. Reported uncertainties for Sr and Nd isotopic ratios are based on 2 se. in-run statistics. Pb was run in static mode between 1250” and 1320°C. Pb isotopic ratios are corrected for Faraday collector bias and 0.1% per AMU fractionation based on runs of NBS-981 Pb standard. All analyses were. made at the University of California, Santa Cruz.

E.E. Scherer et al./ Chemical Geology 142 (1997) 63-78 65

spike equilibration, chemical separation, and mass analysis. Although initial ~~~ values can be well constrained in studies of old (> 1 Ga) rocks using zircons (e.g., Pettingill and Patchett, 1981; Stevenson and Patchett, 1990; Corfu and Noble, 1992; Corfu and Stott, 1993; Vervoort et al., 19961, relatively little work has been published using whole-rock samples, largely because of difficulties with spike- sample equilibration. Recent advances in chemical separation have been published by the University of Wisconsin-Madison group (Barovich et al., 1995), and we report here important improvements in spike-sample equilibration which are critical to the application of the Lu-Hf system to precise geochronological work on whole-rocks or minerals, as well as for making precise initial .ent determinations on ancient whole-rock samples.

This paper presents the results of the first Lu-Hf isotope study of lalwer crustal xenoliths, from the Kilboume Hole maar, New Mexico. Whereas the Lu-Hf system commonly follows the same sense of fractionation as Sm-Nd during differentiation processes (i.e., the daughter element is concentrated in the melt relative to the parent element), the degree of Lu/Hf fractionation can be greater than that of Sm/Nd when garnet is a residual phase. This property creates some advantages for Lu-Hf over Sm-Nd for dating melt-extraction or crystal-accumulation processes involving garnet. First, the range of

Table 2 Lu-Hf data, including replicate analyses

Sample ‘78Hf/ ‘77Hf

Majk gamer granulites 0x39 IC, ID .$.7950

IC, ID 4.9608 CKH63 IC, ID Y2.1739

ID only IC only 1.4672

CKH64 IC, ID 5.5908 IC, ID 5.7731 IC only L .4674

2-px granulite (control sample) CKH58 IC, ID I. ,956 1

IC, ID 1.9651 IC only 1.467 1

‘*‘Hf/ 177Hf ppm Hf ppm Lu ‘76Lu/ ‘77Hf ‘76Hf/ ‘77Hf EHf (0)

1.8866 1.12 7.67 0.971 0.282911 * 29 +4.4 1.8865 1.12 7.34 0.928 0.282940 f 56 +5.5 1.8869 11.35 2.02 0.0253 0.282511 * 14 -9.7

1.97 1.8869 0.282534 f 14 -8.9 1.8865 0.890 8.11 1.29 0.283157 f: 36 + 13.1 1.8864 0.887 8.09 1.30 0.283123 & 25 + 12.0 1.8868 0.283094 & 25 + 10.9

1.8869 3.83 0.341 0.0127 0.282316 f 29 - 16.6 1.8870 3.76 0.341 0.0129 0.282246 f 50 - 19.1 1.8866 0.2823 14 f 24 - 16.6

176Lu/ 17’Hf ratios can be extreme relative to 147Sm/ 144Nd in garnet-bearing whole-rock samples (Tables 1 and 2). We expect this for mineral sepa- rates as well, based on published trace element partitioning data (Fig. 1). Furthermore, although the errors in the measurement of 176Lu/ t7’Hf and 176Hf/ 177Hf are larger on a percentage basis than their Sm-Nd system counterparts, the greater range in 176Lu/ 177Hf, and thus ’ 6Hf/ 177Hf ratios, en- ables the Lu-Hf isotope system to resolve whole-rock isochron ages as young as N 5 m.y.

Another potential advantage of the Lu-Hf system over Sm-Nd lies in the use of Hf isotope model ages, which are analogous to the Nd model ages defined by DePaolo and Wasserburg (1976). That is, a Hf isotope model age represents the time before present at which the 176Hf/ 177Hf of a sample equalled that of its source. Amdt and Goldstein (1987) emphasized that the use of such model ages relies on the following assumptions. (1) The isotopic evolution of the source reservoir is well constrained. (2) The sample acquired its observed parent element/daughter element ratio soon after differentiation from its source reservoir. (3) The parent element/daughter element ratio has not changed since the separation of the sample from its source. The first assumption is often invalid because the source reservoir is generally assumed to be depleted mantle or bulk earth, whereas the actual source may contain

All Hf isotopic data are fractionation-corrected to 17’Hf/ ‘77Hf= 0.7325. ‘76Hf/‘77Hf and ‘*“Hf/‘77Hf are also corrected for spike contribution. Abbreviatiow IC, isotope composition; ID, isotope dilution concentration measurement. All reported errors are 2 s.e. about the mean.

66 E.E. Scherer et al. / Chemical Geology 142 (1997) 63-78

P IO

cl

-i

0” 1

0.1 1, ,,,,,,,, , ,,,,,, (I , ,,,,,,,, , ,,,,,,,, , ,,.-.J

0.01 0.1 1 IO 100 1000 DLU ’ Dlif

Fig. I. Variations in distribution coefficient ratios D,, /D,, and Ds, /D,, for selected mafic granulite mineral phases and estimated bulk D ratios for the Kilboume Hole xenoliths from this study. Shaded fields denote mineral-melt partitioning data from Irving and Frey (1976, 1978, 1984), Haskin and Korotev (19771, Dostal et al. (1983), Fujimaki et al. (1984a,b), Nakamura et al. (1986), Fujimaki (1986), Francalanci (1989), Hart and Dunn (1993), Kennedy et al. (19931, Hauri et al. (1994). Johnson (1994), and Skulski et al. (1994). A wide range in melt compositions is represented in the compiled data. Ilm = ilmenite, Z-c = zircon, Amp = amphibole, PI = plagioclase, Opx = orthopyroxene, Ap = apatite, Gt = garnet. Field shapes for mineral D ratios are defined only by data for which D,, /D,,- D,, /D,, pairs exist, or for which D,, /D,, data was available and D,, /D,, could be estimated, and thus they may not represent the complete ranges for natural samples. Solid black fields indicate ranges of D,, /D,, and D,, /D,, calculated for Kilboume Hole (CKHI xenoliths in equilibrium with hypothetical melt compositions ranging from basaltic to rhyolitic (Lu = 0.4 to 0.7 ppm, Hf = 2 to 8 ppm).

various components of crust or enriched mantle. However, even for cases where the isotopic evolution of the source is unknown, Hf isotope model ages may still bear real age significance: samples that have extremely high Lu/Hf will follow very steep E nf evolution curves relative to E Nd, resulting in Hf isotope model ages that are far less ambiguous than Nd model ages. The intersections of whole-rock Hf evolution curves for high-Lu/Hf samples with evolution curves for DM, CHUR, or a wide range of crustal protoliths all occur within a short time inter- val as compared to the Nd isotope system.

2. Geologic background and sample selection

Kilboume Hole is a Late Pleistocene maar located within the southern extension of the Rio Grande Rift

in south-central New Mexico. The locality is well known for its diverse population of relatively large crustal and upper mantle xenoliths (e.g., Padovani and Carter, 1977; Reid et al., 1989; Padovani and Reid, 1989; Cameron and McMillan, 1994). The garnet-bearing granulite xenoliths provide a direct, though probably incomplete, sampling of the lower- most crust to depths of N 28 km (Padovani and Carter, 1977; DeAngelo and Keller, 1988), and are valuable for evaluating models of the chemical and isotopic evolution of the lower crust within a rift setting (Baldridge et al., 1995). The basement rocks in this region underlie a relatively thin veneer of sedimentary rocks and comprise Early- to Middle- Proterozoic orthogneisses and paragneisses. The latter are believed to be derived by emplacement of pelitic material into the deep crust during northwest- dipping subduction in the Proterozoic (Reid et al., 1989). The mantle beneath the Rio Grande Rift varies significantly in Nd and Hf isotopic composition from very depleted MORB-like mantle to enriched compositions near that of bulk earth (Perry et al., 1988; Roden et al., 1988; Johnson and Beard, 1993; Beard and Johnson, 1993; Salters and Zindler, 1995). The compositions of some of the orthogneiss xenoliths from Kilboume Hole clearly represent mixtures between Cenozoic mantle-derived materials and various Proterozoic crustal components. However, the isotopic and trace element compositions of these components have been obscured by subsequent fractionation events. It is therefore important to establish the timing of these events in order to model the relative contributions of Cenozoic mantle-derived melts and Proterozoic crust to the lower crust beneath the Rio Grande Rift.

Three of the xenoliths chosen for study are ‘group-2 garnet granulites’ (nomenclature of Padovani and Carter, 1977), and they contain the assemblage gt + plag + opx + ilm + rt. None of these rocks contain graphite or Al,SiO, phases, and we interpret them to be orthogneisses. They are relatively large, ranging in size from 8 X 6 X 5 cm to 12 X 9 X 6 cm, and they are massive to weakly foliated. They have granoblastic microstructures and average grain sizes of 0.5 to 4 mm. The garnets are mantled by substantial zones of glass formed by decompression melting. These have locally recrystal- lized to symplectic intergrowths of opx + sp + glass.

E.E. Scherer et al./Chemical Geology 142 (1997) 63-78 67

180 r 160

140 1

E 100 B > 80

I I

-40 45 50 55 60 65

wt% SO2

Fig. 2. Yttiium concentration versus wt% SiO, for Kilboume Hole gram&e xenoliths. Symbols are as follows: dots, garnet granulites; circles, two-pyroxene granulites. The shaded area en- closes the high-Y group of garnet granulites inferred to be either cumulates or restites after partial melting.

Sample CKH63 (412 ppm Zr; Table 1) contains zircon, whereas the remaining group-2 granulites have lower Zr contents ( N 50 ppm) and contain little or no zircon. Sample CKH58, a garnet-free, two-pyroxene granulite, has an intermediate Zr content ( N 130 ppm) and trace zircon. The garnet granulites are generally distinguished from the two-pyroxene

II 1 I I I I I I I I

10 -I

Ce Nd 3nEuGd Dy Er Yb Lu

Fig. 3. Chondrite-normalized rare earth element patterns for Kil- bourne Hole mafic garnet granulite xenoliths.

granulites by their high Y contents and REE patterns that have positive slopes (Figs. 2 and 3). Based on the REE contents of these rocks, we interpret them to be either garnet-bearing cumulates or restites. In the latter case, the garnet may be either part of the original protolith mode, or produced by dehydration melting reactions that may occur in mafic protoliths of the middle or lower crust (Johnson et al., 1996, and references within).

3. Analytical methods

It is widely acknowledged by all groups involved in Lu-Hf isotope work that complete spike-sample equilibration is difficult to achieve, yet it is essential for determining meaningful initial 17’I-If/ 177Hf ratios for old ( > 1 Ga) rocks. Moreover, precise Lu-Hf geochronologic work on rocks of any age also re- quires complete spike-sample equilibration. How- ever, achieving equilibration presents a serious analytical challenge due to the large amounts of sample required to yield enough Hf for the TIMS analysis, and the markedly different chemical behaviors of Lu and Hf during sample processing. Even for samples that have relatively high Hf contents, several grams may be required to average the effects of the hetero- geneous distribution of Lu- and I-If-rich trace phases in rock powders (Pettingill and Patchett, 1981). To optimize spike-sample equilibration in large samples, the entire sample is spiked before a two-stage dissolution in high-pressure bombs. Measurement of isotope compositions from samples that have been ‘totally spiked’ in this manner has long been routine for the Rb-Sr and Sm-Nd isotope systems, but has been avoided in Lu-Hf work because the 176Hf/ 177Hf ratios of existing Hf spikes are extremely anomalous (e.g., Oak Ridge National Labo- ratory i7*Hf spikes: .eur = - 4870, this study; U.S.G.S. lsOHf spike: .anf = -2232, Patchett and Tatsumoto, 198Oa), and large corrections must be made to remove the effects of the spike from the measured 176Hf/ 177Hf ratio. We avoid this problem by using a 178Hf s ‘ke that has been corrected to a near-chondritic “&f, 177Hf ratio (enr = + 14) which permits robust spike subtraction of the totally spiked samples.


3.1. Preparation of spike solutions

Concentrated spike solutions (“‘Hf, 176Hf, and ‘76Lu) were prepared by dissolving enriched-isotope oxides (HfO, or Lu,O,) in warm, concentrated HF for the Hf spikes, and HBr-H,SO, for the Lu spike. Both spikes were diluted with 2.5 M HCl. All handling of the ‘76Hf spike was done in a separate clean lab facility to avoid potential high-s,, contamination of the Univ. of Wise. Madison radiogenic isotope lab.

The isotopic compositions of the 176Hf and ‘78Hf spikes were estimated by several static measurements on a multi-collector thermal ionization mass spectrometer (TIMS). Isotope ratios were corrected for Faraday cup collector bias and mass fractionation according to trends observed for standards of known isotopic compositions. Small amounts of ‘76Hf were added to the “* Hf spike, adjusting the latter to a ‘76Hf/ “‘Hf ratio equivalent to en’ = + 14 (shifted from the original cuf = -4870), but maintaining a favorably high 17*Hf/ “‘Hf ratio (61.18). The isotopic composition of the mixed spike was measured statically as before, then again using a dynamic routine that corrected for mass fractionation using the mean ‘79Hf,/ “‘Hf obtained from the static runs. In this way, the dynamic routine corrects for variations in efficiency among Faraday cups and applies an approximate fractionation correction that is inter- nally consistent. To fine tune this measurement, we analyzed various spiked normal solutions (“*Hf/ “‘Hf = 2 to 4.5), then varied the 179Hf/ “‘Hf ratio used for the mass fractionation correction, such that all of the mixtures simultane- ously gave the correct normal Hf isotopic composition. Fig. 4 shows the highly robust nature of spike subtraction that is achieved by using the total spike method and a “‘Hf spike that has a near-chondritic ‘76Hf/ “‘Hf ratio. Pairs of spiked and non-spiked samples agree to within error over a wide range of spike : sample ratios, as well as a wide range in ‘76Hf/ 17’Hf that nearly spans that observed in the Earth.

Measurement of the isotopic composition of the ‘76Lu spike was relatively straightforward. Several static TIMS measurements of normal Lu were made under consistent run conditions to establish corrections for Faraday cup bias and mass fractionation,

-50 -40 -30 -20 -10 0 +10 +20

non-spiked &Hf

H +1

g O 6 -1

Lb!= I w -2

-3

2 3 4 5 6

“sHf I 17’Hf of spiked sample

Fig. 4. (a) Comparison of ‘spike-subtracted’ ~nr. and em of non-spiked replicates. Error boxes represent 20 about the mean (in-run statistics). CKH = Kilboume Hole crustal xenoliths (this study); unlabeled boxes are crustal xenoliths from La Olivina (K.L. Cameron and E.E. Scherer, unpubl. data), and Bearpaw Mountains, Montana (E.E. Scherer unpubl. data). AMES = Ames normal Hf. spiked to a ‘78Hf/ “‘Hf ratio indicated in the brack- ets. (These points are the means of 5 separate spiked normals each; relatively large 213 error boxes represent the external reproducibility of the 5 runs.) The isotopic composition of 180A (Portman Lake Granite; Barovich, 1991) determined at the Uni- versity of Arizona is compared to the spike-corrected analysis made at U.W. Madison. (b) Deviation of spike-corrected cur from non-spiked snf versus ‘78Hf/ “‘Hf of the spiked sample. Sam- ples are those shown in (a). Error bars represent the 2a uncertainty for spike-corrected cm measurements. The horizontal dashed lines enclose the area within typical external reproducibility of non-spiked sample runs (i.e., + 1 epsilon unit). The relatively large error bars on spiked Ames normal Hf points represent the 2a ex?emal reproducibility of the 5 runs.

which were applied to static measurements of the 176Lu spike. The mixed ‘76L~-‘78Hf spike was made by combining our stock ‘76Lu and ‘78Hf spikes using

E.E. Scherer et al./Chemical Geology 142 (1997) 63-78 69

standard gravimetric techniques and diluting with 2.5 M HCl. The final mixed spike solution had an I-IF molarity of 0.1, which was sufficient to keep Hf in solution without causing the precipitation of Lu fluorides.

The spike concentrations were calibrated using normal solutions made from 1 g Ames ultrapure metal ingots, which were weighed, dissolved in 2.5 M HCl (Lu metal:1 or HF-HNO, (Hf metal), and diluted to 1 liter with additional HCl. Mixed Lu-Hf normal solutions were prepared from these stock Lu and Hf normals, three with a chondritic Lu/Hf ratio (0.2251, and one with a zircon Lu/Hf ratio (0.00209). The acid matrix of these solutions is: 2.1 M HCl-0.25 M HF-0.11 M HNO,. Although a small amount of hydrofluoric acid is required to keep the Hf in solution once it has been dissolved, it is very important that a sufficiently high HCl : HF ratio be main- tained to prevent precipitation of Lu fluorides.

3.2. Sample digestion and spike-sample equilibration

The following m(ethod was designed to digest up to 5 g of sample powder, and to achieve complete spike-sample equilibration without the use of per- chloric acid. Samples are initially ‘pre-reacted’ in open vessels using concentrated HF-HNO,, fol- lowed by HF-HNG, dissolution in high-pressure Teflon bombs at 180-210°C for 4 days. Fluorides are then attacked with repeated 6 M HCI bomb digestions, heating to 180°C for 24 h each step and decanting the dissolved, centrifuged sample into a 60 ml PFA Teflon vial. This process is repeated until no precipitates remain. A few ml of 4 M HF are added to the empty bomb and heated (capped) to leach any Hf that may have precipitated during the 6 M HCl application. This HF is added to the 60 ml vial containing the sample. At this point, the sample should be in complere solution and equilibrated with the spike. The samples are evaporated to dryness, then converted to fluorides by two evaporations with concentrated HF. Further preparation of the sample for ion exchange chromatography follows the Hf- leaching procedure of Barovich et al. (1995). Our leaching process typically recovers > 98% of the Hf for 2 g granite samplIes. However, 100% recovery is

not required because we use the total spike method and equilibrate the samples before the leaching step.

Lutetium is insoluble in concentrated HF and is precipitated during the Hf-leaching step. A few ml of 6 M HCl are added to the precipitates and the mixture is evaporated to dryness. Lu is then leached from the precipitates by adding additional HCl and heating at 100°C overnight. Again, because we use the total spike method, complete extraction of Lu from the rock is not necessary, although we find that nearly 100% of the Lu is recovered using this procedure.

3.3. Ion exchange chemistry

Hafnium is extracted using the three-stage separation technique of Barovich et al. (1995), with modifi- cations made by Scherer et al. (1995) to improve the separation of Ti from Hf on the second-stage H,SO, columns. These columns have been lengthened to 25 cm and the molarity of the eluting acid decreased to 0.2 M. The overall recovery of Hf using this modi- fied three-stage technique is typically 90%. We have subsequently enlarged the diameter of the columns to 8 mm, minimizing flow difficulties associated with 0, bubble generation that occurs when processing high-Ti samples.

The lutetium-bearing HCI is first processed through cation exchange columns that are typically used to separate Sr, Rb, and the REE from the bulk rock matrix (e.g., Shirey et al., 1987). Lutetium is separated from the rest of the REE using a 0.08 M rw-HIBA (2-methyllactic acid) cation exchange column, following the general procedure for ‘column 2’ of Gruau et al. (1988).

Total procedural blanks measured during the pe- riod of this study were < 200 pg for Lu and 0.6-1.2 ng for Hf. Although this Hf blank is near the high end of published values, its effect on me measured Hf ratios is negligible for samples containing > 0.5 l,r.g Hf. For this study, the amount of Hf processed per sample ranged from N 2 to 20 pg.

3.4. TIMS analysis of Lu and Hf

All TIMS analyses of Lu and Hf were made at the University of Wisconsin, Madison Radiogenic Iso- tope Laboratory on a VG Sector 54 mass spectrome-

70 E.E. Scherer et al. /Chemical Geology 142 (1997) 63-78

ter. Single Re filaments of N 15 mm free length were used for both Lu and Hf loads; zone-refined Re was used for Hf. Hafnium filament loads were made using a few beads of cation exchange resin in H,O, boric acid (equivalent to 6 pg of boron), 1 p,l 2 A4 HF plus sample, and l-2 p,l 1 M H3P0,. The Hf ionization efficiency achieved using this technique is approximately 0.003-0.004%. Data were collected using a three-jump dynamic multicollector routine that normalized isotopic ratios to 179Hf/ 17’Hf = 0.7325 using a power-law fractionation correction (e.g., Wasserburg et al., 1981). Data blocks for this study consisted of 15 ratios with baseline measurements taken at the beginning of each block. The 176Hf/ 17’Hf of the JMC-475 Hf standard averaged 0.282125 f 16 (2 s.e., n = 25) over the duration of this study, corresponding to a 176Hf/ 17’Hf ratio of 0.282785 for CHUR, which was used for calculating epsilon parameters. Better precision has been subsequently achieved by measuring baselines before each ratio measurement cycle. Our JMC-475 and thus CHUR ‘76Hf/ 177Hf values are 0.000075 lower than those reported by Patchett (1983; JMC-475 = 0.28220, CHUR = 0.28286). The means of the ‘78Hf/ 177Hf, “‘Hf/ 17’Hf (raw), and 18’Hf/ “‘Hf for the JMC-475 runs were 1.46721 + 4, 2 s.e., 0.7356 f 7, and 1.88684 5 14, respectively.

Lutetium loads for isotope dilution measurements consist of resin, 1 ~1 1 A4 H,PO,, and l-2 ~1 1 M HCl plus sample. The 176Lu/ 175Lu ratio was mea-

Table 3 Nd and Hf model ages

sured statically, while monitoring 174Yb to correct for an occasional isobaric interference of 176Yb on 176Lu. If Yb was present during a run, it was burned off until the correction to 176Lu became insignificant (‘74Yb/ ‘75Lu < 0.002) prior to data collection.

4. Results for lower crustal xenoliths

The Lu-Hf data for the four Kilboume Hole xenoliths analyzed in this study are presented in Table 2. The most significant feature of the garnet- bearing granulites is that they have an extremely large range in Lu/Hf ratios. It is striking that 176Lu/ 177Hf differs by a factor of 100 between CKH63 and CKH64, whereas 14’Sm/ 144Nd varies by only a factor of 2.3. The ‘76Lu/ 17’Hf ratios for CKH39 (N 0.95) and CKH64 (N 1.3) are the highest whole-rock values reported to date using high-precision isotope dilution, and are far greater than the range of 0.001 to 0.01 that is observed in erogenic crustal rocks (e.g., Johnson et al., 1996; Vervoort and Patchett, 1996). These extraordinary Lu/Hf ratios (up to 39 X chondritic; Lu > 200 X chondritic) are accompanied by relatively normal ~“r values that lie between present-day depleted mantle compositions and those expected for Proterozoic crust, essentially requiring a Cenozoic age for the event that elevated the Lu/Hf in these rocks. CKH63, a zircon-bearing garnet granulite, has moderately high

Sample &Nd co) TNd, CHUR TNd, DM

Low-Zr garner gram&es cKH39 -2.5 (- 194) (- 1051) CKH64 f2.8 209 (-514)

Zircon-bearing garnet granulite CKH63 -4.1 (-659) ( - 3297)

Two-pyroxene gram&e (garner-free, contains trace zircon) CKH58 - 9.4 1137 1649

-?Hf (0) THf, CHUR T Hf. DM A”Hf (0)

+ 4.4 7 (- 18) + 6.2 + 12.0 14 (-5) -k&5

-9.7 1706 2478 -5.7

- 16.6 1152 1719 -5.4

Model ages in Ma calculated using Sm-Nd data from Table 1 and Lu-Hf data from Table 2. Future (i.e., negative) model ages are enclosed in parentheses. Model ages were calculated assuming a linear isotopic evolution for Nd and Hf using the following parameters: k14’Sm = 6.54 x lo-” yr- ‘, A’76Lu = 1.94 X lo-” yr-‘; ‘47Sm/ ‘44Nd,HoR = 0.1967; ‘76Lu/ “‘Hfcm,a = 0.0334; DM eNd(0) = + 8.34, 14’Sm/ ‘44Nd oM = 0.216, from a linear approximation of the DM curve of DePaolo (1981) between 0 and 2 Ga; DM EHf(0) = + 16, ‘76Lu/ “‘Hf,, = 0.04, from average of values given in Corfu and Noble (1992) and Corfu and Stott (1993). Deviation of eHf from the OIB reference line is given by AEON = ~~~ - [1.36eN, + 1.631 (Johnson and Beard, 1993).

E.E. Scherer et al/Chemical Geology 142 (1997) 63-78 11

concentrations of both Lu and Hf and a Lu/Hf ratio less than that of CHUR, but a Sm/Nd ratio greater than that of CHUR. In contrast to the garnet granulites, two-pyroxene granulite CKH58 does not ex- hibit significant dec,oupling of the Nd and Hf isotopic systems, as evidenced by its present-day rela- tionship of E nf = 2 E Nd, which is common for juve- nile crustal rocks (P.atchett et al., 1981; Vervoort et al., 19961, and very similar Nd and Hf isotope model ages which are in general agreement with the inferred basement age in the area (Table 3).

Uranium-lead ages were obtained on zircons from xenolith CKH63; zircons in this sample were rounded and elongate grains averaging N 200 pm in length (before abrasion). U-Pb data from three multigrain

fractions and five single grain fractions are presented in Table 4 and Fig. 5. Seven fractions are highly discordant and yield Middle Proterozoic ‘07Pb/ 206Pb ages. An eighth fraction, ‘h,’ is dominated by common Pb, and the large corrections for blank and initial common Pb preclude meaningful 207Pb/ 235 U and 207Pb/ ‘06 Pb age determinations. This fraction was excluded from the calculation of discordia intercepts. However, the 206Pb/ 238U age of this sample is relatively insensitive to the common Pb corrections, and constrains the lower intercept to 2-13 Ma. We interpret these fractions to represent either Mid- dle Proterozoic zircon that has recently lost Pb, or, more likely, a mixture of Middle Proterozoic and late Cenozoic zircon components; distinction between

Table 4 CKH63 zircon data

Fraction a Concentrations b Atomic ratios ’ Ages, Ma

Weight d U Pb corn. Pb e *06Pb/ *07Pb/ ‘08Pb/ *“Pb/ *07Pb/ “‘Pb/

(IL?) (ppm) @pm) (pg) *04 Pb *06 Pb *“Pb 238~ 235~ “‘Pb Round, clear 63a m + 5 abr (20) 139 139.5 10.5 12 4396 0.08749 0.1881 426 611 1371 f 2 63d dm - 2 abr (1) 29 187.7 10.8 17 1053 0.08928 0.2075 317 490 1410 f 4 63edm-2abr(l) 15 238.3 4.5 11 347 0.08170 0.3320 98 161 1238 + 35 63j dm - 2 (15) 12 268.8 21.0 25 556 0.08890 0.1505 430 622 1402 + 7

Round, turbid 63g dm - 2 (1) 19 152.8 4.1 18 239 0.09150 0.2258 136 242 1457 + 14

Blocky, clear 63h dm - 2 (1) 61 56.1 0.15 11 41 - 1.66 7 - -

Elongate, clear 63b m + 5 abr (14) 55 229.5 12.3 4 565 1 0.08708 0.1328 319 483 1362rt2 63cdm-2abr(l) 6 172.3 14.6 13 392 0.09017 0.3098 425 624 1429 f 9

a Fraction designations: magnetic (ml, diamagnetic (dm), degrees of tilt on Frantz LB- 1 magnetic separator (+ 5 or - 21, air abraded (abr), numbers in parentheses indicate the number of grains analyzed. b Concentrations do not include blank. ’ The ‘06Pb/ *04Pb ratios are corrected for mass fractionation and spike (for “‘Pb-spiked samples). *“Pb/ *06Pb and *‘*Pb/ *06Pb are corrected for mass fractionation, spike, blank, and initial Pb. Initial Pb compositions are from Stacey and Kramers (1975) two-stage Pb evolution model at 1.4 Ga. Air abrasion technique follows that of Krogh, 1982. Dissolution and chemical procedures are based on Krogh (1973), using Parrish (198’7) style microcapsules. Single grain fractions were spiked with 205Pb-235U before dissolution. All U-PI, isotope measurements were made -with a VG 54-30 Sector TIMS at the University of California, Santa Cruz. Isotope ratios were measured in static mode using a pulse-counting Daly detector for the ‘04Pb peak, or by peak hopping on the Daly detector. Mass fractionation for U and Pb was 0.1% f 0.03% per AMU. Procedural blanks were 7 pg f 50% for Pb, and I 1 pg for U. Decay constants used in age calculations: A*‘*U= 1.55125 X lo-” yr-‘, Az3’U = 9.8485 X lo-” yr-‘. Natural 23sU/ 235U = 137.88. Data regression and estimation of 7/6-age uncertainties (20) were done using method of Ludwig (1988). Typical 20 uncertainties on the *06Pb/ z38U and ‘07Pb/ 235U ratios are estimated to be +OS% based on replicate analyses of a single zircon fraction. d Weights of single grain fractions 63c, 63d, and 63e were calculated using dimensions measured from photographs, with an estimated uncertainty of +20%. All other fractions were weighed to within +2 kg on a microbalance. e Includes initial Pb and Pb blank.


CKH63 zircon

1373 f 56 and 2 f 31 Ma M.S.W.D. = 210

I I I I I

0 0.2 0.4 0.6 0.6

a’Pb I 235U

Fig. 5. Conventional U-Pb concordia plot of CKH63 zircon data from Table 4. Error ellipses are 2~. The intercepts and errors shown are for a chord that includes all fractions except ‘h’. Zircon data were regressed using software of Ludwig (1988, 1991).

these two models is not possible with conventional U-Pb zircon analysis. In either case, these U-Pb systematics strongly suggest that xenolitb CKH63, like CKH39 and CKH64 was affected by a mid- to late Cenozoic granulite-facies metamorphic event or partial melt extraction.

5. Discussion

5.1. Roles of garnet and zircon during intracrustal differentiation processes

The importance of garnet and zircon in controlling Lu/Hf fractionation has been previously recog- nized (e.g., Patchett, 1983; Barovich and Patchett, 1992; Vervoort and Patchett, 1996). Fig. 1 compares the ratios of published partition coefficients,

D,,/Dn, and DSm/DNd, for some minerals that are commonly found in mafic granulites. The two axes were drawn at the same scale to emphasize the large ranges in D,,/D,, relative to D,,/D,,, which result in elongated fields for several of the minerals in Fig. 1. For some minerals, such as garnet, this property may be due to variations in individual partition coefficients as a function of the bulk compositions of the mineral and melt. For example,

HREE partition coefficients can increase ten-fold between pyrope-basic magma and almandine-felsic magma pairs, whereas D,, and presumably D,,, do not follow this trend (Irving and Frey, 1978; Sisson and Bacon, 1992). Also shown in this Fig. are the bulk D ratios for the xenoliths in this study, estimated using whole-rock trace element contents and assuming equilibrium with melt compositions that range from basaltic to rhyolitic. The positions of the xenolith fields relative to those for the minerals, together with approximate modal abundances and typical Lu and Hf contents of individual phases, indicate that garnet and zircon are the most significant phases in terms of controlling Lu/Hf fractionation during differentiation of these rocks.

Three styles of differentiation could produce rocks such as CM39 and CKH64, which have extremely high Lu/Hf ratios, but only slightly elevated Sm/Nd ratios: (1) high-pressure fractional crystallization involving garnet as a cumulate phase, (2) partial melt extraction from a garnet-bearing assemblage, and (3) subsolidus open-system growth of metamorphic garnet. The first two mechanisms involve the partitioning of Hf into a melt phase, which is subsequently separated from Lu-rich solids. Zircon has a high D,, and a D,,/DHf < 1, and will act in opposition to garnet during differentiation by adding Hf to cumulates or strongly retaining Hf during partial melt extraction. Among the garnet-bearing samples in this study, Lu/Hf and 176Hf/ ‘77 Hf decrease with increasing Zr concentration, perhaps reflecting the in- fluence of different amounts of modal zircon.

The third mechanism might operate by the migra- tion of Lu and Hf into garnet-rich and garnet-poor layers, respectively, with the high Lu/Hf xenoliths representing the garnet-rich layers. Given the size of the xenoliths in this study, such layering would have to be on a scale greater than 5 cm. Subsolidus segregation of garnet-rich layers would indeed frac- tionate trace elements, but the newly formed layers would be isotopically homogeneous. Since the Lu-Hf systematics require that the formation metamorphic layering occurred less than 40 m.y. ago, both garnet-rich and garnet-poor layers should have present-day Sr, Nd, and Pb isotopic compositions that are similar to those of CM39 and CKH64. Further- more, given the extreme Lu concentrations in CKH39 and CKH64 (i.e., > 10 X normal crustal rocks), mass

E.E. Scherer et al. / Chemical Geology 142 (1997) 63-78 73

balance considerations dictate that the volume of the complimentary garnet-poor layers would be large relative to that of the garnet-rich layers. Importantly, no garnet-poor xenoliths or layers with relatively low 87Sr/ 86Sr ratios, high eNd, and high *08Pb/ *04Pb like CKH39 and CKH64 have been identified by any study to date. Therefore, there is no evidence that the Lu-Hf fractionation observed in the group-2 garnet granulites is due to local metamorphic layering.

5.2. Hf and Nd isotopic evolution and model ages

The Nd isotope model ages presented in Table 3 are generally negative for the garnet granulites. A future age, or one that is older than the actual differentiation event, represents a rotation of a sample’s apparent Nd isotope evolution curve, caused by increasing the Sm/Nd ratio at some time in the past. This may occur in cumulates that separate from a melt, or it may occur during subsequent melt extraction or metamorphism (e.g., Nelson and DePaolo, 1985; DePaolo et al., 1991). The difference between the model age and the true age of differentiation is minimized if the rock had an .sNd which was close to that of the source reservoir at the time of rotation. That is, changes in tbe Sm/Nd ratio do not significantly affect the model age if the change occurs just after separation from the source. In contrast, a rock that separated from the mantle in the Proterozoic that has its Sm/Nd ratio increased by, for example, Cenozoic melt extraction would yield Nd model ages that exceed the age of the differentiation event. Further increase in Sm/Nd ratios to values that are greater than those of the source reservoir would produce future model ages in the Sm-Nd system. In many cases, the behavior of natural systems is more complex, owing to polymetamorphism. Furthermore, the Nd isotopic characteristics and Sm/Nd of the source are usually assumed, resulting in ambiguous interpretations of Nd model ages. A depleted mantle (DM) source is often assumed, but the actual source may have included components of enriched mantle or crust, or may have been entirely crustal. In these cases, the significance of the model ‘age’ is difficult to interpret without independent constraints on cvs- tallization age such as U-Pb zircon data (Arndt and Goldstein, 1987).

These concepts also apply to hafnium isotope model ages. Specifically, melt extraction or crystal accumulation can elevate Lu/Hf in the residual solids if, for example, garnet is residual, causing model ages that are older than the differentiation event, or

&Nd

%f

%f

0 10 20 30 40 50

kx W)

Fig. 6. (a, b, c) Nd and Hf model evolution curves for xenoliths. Depleted mantle (DM) curve for Hf is that of Corfu and Noble (1992) and Corfu and Stott (1993). The DM curve for Nd is a linear fit to the O-2 Ga portion of the DM curve of DePaolo (1981). ‘Representative Pelitic Paragneiss,’ (RPP) Nd trajectory from Reid et al. (1989). The Hf RPP curve was approximated by calculating the Nd DM model age, and using the isotopic composition of the Hf DM curve at that time for the initial ‘76Hf/ “‘Hf ratio. This was traced to present-day using RPP Lu/Hf of Reid et al. (1989). (c) Cenozoic detail from (b).

14 E.E. Schereretal./ChemicalGeology 142 (1997163-78

future ages if the Lu/Hf ratio becomes slightly greater than that of its source. However, changes in the Lu/Hf ratio of the residual solids are generally more dependent on mineralogy as compared to changes in the Sm/Nd ratio. Fig. 6 shows the xenolith Nd and Hf isotope evolution paths in relation to various possible parental reservoirs, including CHUR, DM and Proterozoic crust (‘Representative Pelitic Paragneiss’ or ‘RPP’ of Reid et al., 1989). An important property of the low-Zr (and possibly zircon-free) garnet granulites (samples CKH39 and CKH64) is that their Hf isotope evolution paths intersect all of these likely parent curves at high angles, producing a very restricted range in Hf isotope model ages relative to the Nd system (compare Fig. 6a with Fig. 6b). This is attributed to the large, garnet-dominated fractionation of Lu/Hf relative to Sm/Nd. The exact chemical and isotopic characteristics of the parent reservoir need not be known in order to generally constrain the age of the fractionation event.

Although Hf isotope model ages are not as precise as U-Pb ages determined for old, concordant zircons, they may be as precise or better than ages determined for Cenozoic zircons, especially if the zircons have a significant inherited component (e.g., CKH63 zircons). Furthermore, hafnium isotope model ages are likely to be far more precise and accurate than Nd isotope model ages for rocks of any age which have high Lu/Hf ratios like those of samples CKH39 and CKH64. For example, the age of the Lu/Hf fractionation event that affected these samples has been constrained to O-40 Ma, based solely on Hf isotope model ages, in agreement with the lower intercept U-Pb zircon age for CKH63. By contrast, the Sm-Nd system yields future model ages for these two samples. Meaningful Nd isotope model ages for such samples could only be determined if the chemical and isotopic characteristics of the protolith were well-constrained, which would rarely be the case.

5.3. Lu-Hf isochron ages

176Hf/ 17’Hf_1’6Lu/ “‘Hf variations (Fig. 7) indicate that the three group-2 garnet granulites lie closely about a 25 Ma reference line. Although the three xenoliths represent a petrographically distinct

0.2832

0.2831

0.2830

z 0.2829 I= : 0.2828 5 ? 0.2027

0.2826

0.2825

0.2824 --I2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

‘76~~ / ‘77Hf

Fig. 7. Lu-Hf isochron diagram for Kilboume Hole ma& garnet granulites. The shaded region on the left edge of the plot shows the range in ‘76Lu/ ‘77Hf observed in ‘normal’ terrestrial rocks and meteorites. Vertical error bars represent 2a TIMS mn statistics, and may underestimate true external reproducibility.

suite, it is uncertain whether they are cogenetic and the correlation may not represent an isochron. Never- theless, these three samples demonstrate that Lu-Hf whole-rock isochrons may be used to date events as young as mid- to late Cenozoic, a time scale in which the Sm-Nd whole-rock and U-Pb garnet methods may yield very imprecise ages.

Our preliminary data indicate that for rocks which have ranges in Sm/Nd and Lu/Hf ratios that are similar to those of the garnet granulites in this study, Lu-Hf isochrons yield better age resolution than Sm-Nd whole-rock isochrons, even though analytical errors on ‘76L~/177Hf and ‘76Hf/‘77Hf measurements are currently greater than for their Sm-Nd counterparts. Fig. 8 demonstrates the difference in resolution between Lu-Hf and Sm-Nd isochrons for hypothetical cogenetic garnet granulites shown at 25 Ma. For simplicity, we use two-point isochrons in our example, and assume that all scatter is analytical rather than geological (i.e., no open-system behavior). Furthermore, we use the long-term external reproducibility of standards, rather than 20 TLMS in-run statistics, when calculating age uncertainties. After 25 m.y. of evolution, the age uncertainty for the Lu-Hf system is about f 2 Ma (&- 8%), whereas the uncertainty of the Sm-Nd system is k 100 Ma (+400%) and cannot be distinguished from zero age. This simplistic scenario demonstrates that Lu-Hf

E.E. Scherer et al. /Chemical Geology 142 (1997) 63-78 15

0.2838

E 0.2834

k

$ 0.2832

i?

0.2830

0.2828 0 0.4 0.8 1.2 1.6

‘78~~ / ‘77Hf b

.51280~ , I I I

31276

$ .51272

; 51288

f

.51264

.51260 0 0.1 0.2 0.3 0.4

14’Srn I ‘&Nd

Fig. 8. Comparison of age resolution between Lu-Hf and Sm-Nd isochrons for garnet-bearing gram&es. Whole-rock isochrons (solid lines) are shown al 25 Ma for two hypothetical cogenetic samples X and Y with ‘76Lu/ “‘Hf and 14’Sm/ ‘44Nd values that bracket those of the xenoliths from this study, and initial isotopic ratios arbitrarily set to present-day CHUR. Error boxes represent the following typical analytical uncertainties: ‘43Nd/ ‘44Nd, +0.005% (kO.5 .zNd); ‘47Sm/‘44Nd, +O.l%; ‘76Hf/177Hf, +O.Ol% f&l eHf); ‘76Lu/‘77Hf, fl%. Age errors are graphically illustrated with 2a error envelopes.

can be used to precisely date Cenozoic differentiation events which produce wide ranges in ‘76Lu/ “‘Hf. Finally, we note that the uncertainty in the ‘76Lu decay constant produces a systematic uncertainty of &-4% in all calculated Lu-Hf ages (Patchett et al., 1981), which is minor compared to the analytical uncertainty for Cenozoic rocks.

5.4. Petrogenetic implications for the lower crust beneath Kilboume Hole

The anomalously high present-day 176Hf/ 17’Hf ratios for samples CKH39 and CKH64, as expressed

by positive As, values (Table 3), are likely to be the isotopic fingerprints of continental crust that has experienced open-system garnet growth, such as that which may be produced by removal of a partial melt in the presence of residual garnet, or produced by garnet accumulation. As recently reviewed by John- son et al. (19961, dehydration melting of amphibolite or tonalite in the middle or lower crust will produce significant quantities of garnet. Over time, the high Lu/Hf ratios of such crust will generate very high

Aam values, which are unusual as compared to the strongly negative present-day A.snf values that are typical for crustal erogenic terranes (Johnson et al., 1996). That the Kilboume Hole garnet granulites produced markedly high A&,, values in less than 25 m.y. indicates that lower crustal or middle crustal reservoirs that have gained garnet during ancient dehydration melting may have extraordinarily high present-day Aenf values, consistent with the calculations presented by Johnson et al. (1996). Subsequent melting of such crust will produce silicic magmas that inherit these high Aaur values, and Johnson et al. (1996) have suggested that the Hf isotope compositions of N 1.4 Ga ‘anorogenic’ granites from the southwestern U.S.A. may reflect derivation from such distinctive crust. However, Vervoort and Patchett (1996) found little or no evidence for elevated initial cuf values in a series of granitoids derived by melting ancient crust. Thus the extreme Lu/Hf fractionation observed in the lower crust at Kilboume Hole may be a local effect.

Variations in and and *06Pb/ *04Pb among the garnet granulites of this study, a pelitic paragneiss (eNd = -9.3 + 1, ‘06Pb/ *04Pb = 16.83 f 0.23; Reid et al., 1989) and local Cenozoic basalts (aNd = +4.7 to +7.1, 206Pb/207Pb = 18.46 to 19.01; Ro- den et al., 1988; Kempton et al., 1991; N.J. McMil- lan, unpubl. data) indicate that the Nd and Pb isotopic compositions of the group-2 garnet granulites have been strongly influenced by a mid- to late Cenozoic basaltic component, although two of the three have lower ~~~ and *06Pb/ *04Pb values than the basal&, suggesting the presence of an older crustal component as well. As noted earlier, the *“Pb/ *06Pb ages for zircons in sample CKH63 provide direct evidence for the existence of such a component. Possible candidates for the Proterozoic protolith include pelitic paragneiss (‘group- 1’ garnet

76 E.E. Scherer et al./ Chemical Geology 142 (1997) 63-78

granulites of Padovani and Carter, 1977; ‘RPP’ of Reid et al., 1989) and mafic to silicic orthogneisses, both of which have been inferred to lie at lower- crustal depths between 22 and 28 km (Padovani and Carter, 1977). The mineralogy of the group-2 garnet granulites is comparable to that of assemblages formed during the experimental reaction of a high- alumina basalt with a metapelite at 0.9 GPa and 1000°C (e.g., Patico Deuce, 1995). We interpret group-2 garnet granulites to represent cumulates which formed at the base of the crust by assimila- tion-fractional crystallization (AFC) processes that involved the incorporation of one or both of the Proterozoic components by Cenozoic basalts.

6. Summary and conclusions

Determining the ages of protolith crystallization, metamorphic episodes, and melt extraction events is crucial to the study of lower crustal reservoirs. It is commonly difficult to apply geochronologic techniques to xenoliths because the samples are generally small and may have experienced various degrees of contamination and isotopic re-equilibration during transport to the surface. Our data demonstrate the value of the Lu-Hf system for geochronological studies of the lower crust, particularly for samples that contain little or no zircon.

There are two significant advantages of the Lu-Hf system over Sm-Nd for dating relatively young differentiation processes involving garnet as a residual phase. (1) The greater range in fractionation between Lu and Hf provides both rapidly evolving systems (in the case of high-Lu/Hf rocks) and retarded isotopic systems (low-Lu/Hf rocks or zircon). This range in Lu/Hf yields good ‘leverage’ on isochrons, even though analytical errors for 176Hf/ ‘77Hf and 176Lu/ 177Hf are currently greater than their Sm-Nd analogs. (2) Samples that have extremely high

176Lu/ 177 Hf ratios will have very steep Hf isotope evolution curves that intersect CHUR, depleted mantle (DM), and most crustal protolith evolution curves at high angles and within a short time-span relative to the Sm-Nd system. Hafnium isotope model ages for high Lu/Hf rocks are thus more likely to have geochronologic significance as compared to Nd model ages because they are relatively insensitive to

uncertainties in the isotopic compositions of crustal or mantle sources, and to minor isotopic contamination. Depending on the range of Lu/Hf ratios available in the rocks, Lu-Hf geochronology applied to garnet-bearing whole-rock samples has the potential to provide better precision than the Sm-Nd system over a greater age range. In addition, the Lu-Hf isotope system may produce more precise ages for garnet-bearing rocks than U/P\, dating of garnet, depending upon uncertainties in common Pb components.

The inherited (high *07Pb/ ‘06Pb) component of the CKH63 zircons, as well as the coherent Nd and Hf model ages of two-pyroxene granulite CKH58, confirm that the lower crust that was sampled by the Kilboume Hole xenoliths contained at least one component of Proterozoic crust. However, the Nd and Pb isotope compositions of the group-2 garnet granulites indicate the presence of a significant Cenozoic basaltic component as well. The Hf isotope model ages of samples CKH64 and CKH39, together with the U-Pb zircon data for CKH63 support a mid- to late-Cenozoic age for the differentiation event (or last equilibration event) which formed these rocks. This age corresponds to rift-related magmatism and heating of the lower crust. Our data are consistent with an AFC cumulate origin for the group-2 garnet granulites in which the parent melt was a Cenozoic basalt that assimilated varying amounts of Protero- zoic crust.

Acknowledgements

We acknowledge support from NSF grants EAR 9318687 to Cameron, EAR 9304455, EAR 9316277, and EAR 9406684 to Johnson, and EAR 9018561 to Collerson. We are indebted to Robert Lopez for his outstanding work in setting up the zircon geochronology lab at UCSC and assisting with the analyses of CKH63 zircons. We also thank Peter Holden, who gave constructive comments on an early draft of this paper, and P. Jonathan Patchett, whose helpful review improved the manuscript.

References

Amdt, N.T., Goldstein, S.L., 1987. Use and abuse of crust-formation ages. Geology (Boulder) 15, 893-895.

E.E. Scherer et al./ Chemical Geology 142 (1997) 63-78 77

Baldridge, W.S., Keller, G.W., Haak, V., Wendlandt, E., Jiracek, G.R., Olsen, K.H., 1995. The Rio Grande Rift. In: Olsen, K.H. (Ed.), Continental Rifts: Evolution, Structure, Tectonics. Else- vier, Amsterdam, pp. 233-275.

Barovich, K.M., 1991. 13ehavior of Lu-Hf, Sm-Nd and Rb-Sr Isotopic Systems During Processes Affecting Continental Crust. Doctoral thesis, Univ. Arizona, Tucson, AZ.

Barovich, K.M., Patchett, P.J., 1992. The role of garnet and other phases in generating unusual Hf isotopic variations during crustal melting and differentiation. Eos, Trans. Am. Geophys. Union 73, 370.

Barovich, K.M., Beard, B.L., Cappel, J.B., Johnson, CM., Kyser, T.K., Morgan, B.E., 1995. A chemical method for hafnium separation from high-Ti whole-rock and zircon samples. Chem. Geol. 121, 303-308.

Beard, B.L., Johnson, C.M., 1993. Hf isotope composition of late Cenozoic basaltic rocks from northwestern Colorado, USA; new constraints on mantle enrichment processes. Earth Planet Sci. L&t. 119, 495-5~09.

Cameron, K.L., McMillzn, N.J., 1994. Mafic Proterozoic basement beneath the Kilboume Hole xenolith locality, southwest- em New Mexico. Geol. Sot. Am. Abstr. Progr. 26, 7.

Corfu, F., Noble, S.R., 1992. Genesis of the southern Abitibi greenstone belt, Superior Province, Canada; evidence from zircon Hf isotope analyses using a single filament technique. Geochim. Cosmochim. Acta 56, 208 l-2097.

Corfu, F., Stott, G.M., 1993. Age and petrogenesis of two late Archean magmatic suites, northwestern Superior Province, Canada; zircon U-Pb and Lu-Hf isotopic relations. J. Petrol. 34, 817-838.

DeAngelo, M.V., Keller, G.R., 1988. Geophysical anomalies in southwestern New Mexico. Cretaceous and Laramide tectonic evolution of southwestern New Mexico. Guidebook, New Mexico Geological Society 39, pp. 71-75.

DePaolo, D.J., 1981. Neodymium isotopes in the Colorado Front Range and crust-mantle evolution in the Proterozoic. Nature (London) 291, 193-196.

DePaolo, D.J., Wasserburg, G.J., 1976. Inferences about magma sources and mantle structure from variations of Nd-143/Nd- 144. Geophys. Res. Lett. 3, 743-746.

DePaolo, D.J., Linn, A.M., Schubert, G., 1991. The continental crustal age distribution: methods of determining mantle separation ages from Sm-Nd isotopic data and application to the Southwestern United States. J. Geophys. Res. 968,2071-2088.

Dostal, J., Dupuy, C., Carron, J.P., Le Guen de Kerneizon, M., Maury, R.C., 1983. Partition coefficients of trace elements; application to volcanic rocks of St. Vincent, West Indies. Geochim. Cosmochim. Acta 47.525-533.

Francalanci, L., 1989. Trace element partition coefficients for minerals in shoshonitic and talc-alkaline rocks from Stromboli Island (Aeolian Arc). Neues Jahrh. Mineral. Abh. 160, 229- 247.

Fujimaki, H., 1986. Partition coefficients of Hf, Zr and REE between zircon, apatite and liquid. Contrib. Mineral. Petrol. 94, 42-45.

Fujimaki, H., Tatsumoto, M., Aoki, K., 1984a. Partition coeffl- cients of Hf. Zr and REE between phenocrysts and ground-

masses. Proc. 14th Lunar and Planetary Science Conf., Part 2. J. Geophys. Res. 89 (Suppl.), B662-B672.

Fujimaki, H., Tatsumoto, M., McKay, G., Wagstaff, J., 1984b. Partition coefficients of Hf. Zr, and REE between ilmenite and liquid. Abstr. Pap. Lunar Planet. Sci. Conf. 15, 282-283.

Gruau, G., Comichet, J., Le Coz-Bouhnik, M., 1988. Improved determination of Lu/Hf ratio by chemical separation of Lu from Yb. Chem. Geol. 72, 353-356.

Hart, S.R., Dunn, T., 1993. Experimental cpx/melt partitioning of 24 trace elements. Contrib. Mineral. Petrol. 113, l-8.

Haskin, L.A., Korotev, R.L., 1977. Test of a model for trace element partition during closed-system solidification of a sili- cate liquid. Geochim. Cosmochim. Acta 41, 921-939.

Hauri, E.H., Wagner, T.P., Grove, T.L., 1994. Experimental and natural partitioning of Th, U, Pb and other trace elements between garnet, clinopyroxene and basaltic melts. Chem. Geol. 117, 149-166.

Irving, A.J., Frey, F.A., 1976. Effect of composition on the partitioning of rare earth elements, Hf, SC and Co between garnet and liquid; experimental and natural evidence. Eos, Trans. Am. Geophys. Union 57, 339.

Irving, A.J., Frey, F.A., 1978. Distribution of trace elements between garnet megacrysts and host volcanic liquids of kim- berlitic to rhyolitic composition. Geochim. Cosmochim. Acta 42, 771-787.

Irving, A.J., Frey, F.A., 1984. Trace element abundances in megacrysts and their host basalts; constraints on partition coefficients and megacryst genesis. Geochim. Cosmochim. Acta 48, 1201-1221.

Johnson, K.T.M., 1994. Experimental cpx/ and garnet/melt partitioning of REE and other trace elements at high pressures; petrogenetic implications. Mineral. Mag. 58A, 454-455.

Johnson, C.M., Beard, B.L., 1993. Evidence from hafnium isotopes for ancient sub-oceanic mantle beneath the Rio Grande Rift. Nature 362, 441-444.

Johnson, C.M., Shirey, S.B., Barovich, K.M., 1996. New ap- proaches to crustal evolution studies and the origin of granitic rocks; what can the Lu-Hf and Re-0s isotope systems tell us?. Spec. Pap. Geol. Sot. Am. 315, 339-352.

Kempton, P.D., Fitton, J.G., Hawkesworth, C.J., Ormerod, D.S., 1991. Isotopic and trace element constraints on the composition and evolution of the lithosphere beneath the southwestern United States. J. Geophys. Res. 96, 13713-13735.

Kennedy, A.K., Lofgren, G.E., Wasserburg, G.J., 1993. An experimental study of trace element partitioning between olivine, orthopyroxene and melt in chondrules; equilibrium values and kinetic effects. Earth Planet Sci. Lett. 115, 177-195.

Krogh, T.E., 1973. A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations. Geochem. Cosmochem. Acta 37, 485-494.

Krogh, T.E., 1982. Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique. Geochim. Cosmochim. Acta 46, 637-649.

Ludwig, K.R., 1988. PBDAT for MS-DOS; a computer program for IBM-PC compatibles for processing raw Pb-U-Th isotope data, version l.OOa. Open-File Rep. U.S. Geol. Surv. 88-0542.

78 E.E. Scherer et al. / Chemical Geology 142 (1997163-78

Ludwig, K.R., 1991. ISOPLOT; a plotting and regression program for radiogenic-isotope data; version 2.53. Open-File Rep. U.S. Geol. Surv. 91-0445.

Nakamura, Y., Fujimaki, H., Nakamura, N., Tatsumoto, M., McKay, G.A., Wagstaff, J., 1986. Hf. Zr, and REE partition coefficients between ilmenite and liquid; implications for lunar petrogenesis. Proc. 16th Lunar and Planetary Science Conf., Part 2. J. Geophys. Res. 91B, D239-D250.

Nelson, B.K., DePaolo, D.J., 1985. Rapid production of continental crust 1.7 to 1.9 b.y. ago: Nd isotopic evidence from the basement of the North American mid-continent. Geol. Sot. Am. Bull. 96, 746-754.

Padovani, E.R., Carter, J.L., 1977. Aspects of the deep crustal evolution beneath south central New Mexico. In: Heacock, J.G. (Ed.), The Earth’s Crust. Am. Geophys. Union Monogr. 20, 19-55.

Padovani, E.R., Reid, M.R., 1989. Field guide to Kilboume Hole maar, Dona Ana County, New Mexico. In: Region, Chapin C.E., Zidek, J. (Eds.), Field Excursions to Volcanic Terranes in the Western United States, Vol. I. Southern Rocky Moun- tain. N.M. Bureau Mines Miner. Resour. Mem. 46, 174-179.

Parrish, R.R., 1987. An improved micro-capsule for zircon dissolution in U-Pb geochronology. Chem. Geol. (Isot. Geosci. Sect.) 66, 99-102.

Patchett, P.J., 1983. Importance of the Lu-Hf isotopic system in studies of planetary chronology and chemical evolution. Geochim. Cosmochim. Acta 47, 81-91.

Patchett, P.J., Tatsumoto, M., 1980a. A routine high-precision method for Lu-Hf isotope geochemistry and chronology. Con- trib. Mineral. Petrol. 75, 263-267.

Patchett, P.J., Tatsumoto, M., 1980b. Lu-Hf total-rock isochron for the eucrite meteorites. Nature 288, 571-574.

Patchett, P.J., Kouvo, O., Hedge, C.E., Tatsumoto, M., 1981. Evolution of continental crust and mantle heterogeneity: evidence from Hf isotopes. Co&b. Mineral. Petrol. 78,279-297.

Patiiio Deuce, A.E., 1995. Experimental generation of hybrid silicic melts by reaction of high-Al basalt with metamorphic rocks. J. Geophys. Res. BlOO, 15623-15639.

Perry, F.V., Baldridge, W.S., DePaolo, D.J., 1988. Chemical and isotopic evidence for lithospheric thinning beneath the Rio Grande Rift. Nature (London) 332, 432-434.

Pettingill, H.S., Patchett, P.J., 1981. Lu-Hf total-rock age for the Amitsoq gneisses, west Greenland. Earth Planet. Sci. Len. 55, 150-156.

Reid, M.R., Hart, S.R., Padovani, E.R., Wandless, G.A., 1989. Contribution of metapelitic sediments to the composition, heat

production, and seismic velocity of the lower crust of southern New Mexico, USA. Earth Planet. Sci. Lett. 95, 367-381.

Roden, M.F., Irving, A.J., Murthy, V.R., 1988. Isotopic and trace element composition of the upper mantle beneath a young continental rift, results from Kilboume Hole, New Mexico. Geochim. Cosmochim. Acta 52, 461-473.

Salters, V.J.M., Hart, S.R., 1989. The hafnium paradox and the role of garnet in the source of mid-ocean-ridge basalts. Nature 342, 420-422.

Salters, V.J.M., Zindler, A., 1995. Extreme ‘76Hf,/‘77Hf in the sub-oceanic mantle. Earth Planet Sci. L&t. 129, 13-30.

Scherer, E.E., Beard, B.L., Barovich, K.M., Johnson, C.M., Tay- lor, L.A., 1995. An improved method for determining the Hf isotopic composition of lunar basalts. Abstr. Pap. Lunar Planet. Sci. Conf. 26, 1235-1236.

Shirey, S.B., Banner, J.L., Hanson, G.N., 1987. Cation-exchange column calibration for Sr and the REE by EDTA titration. Chem. Geol. 65, 183-187.

Sisson, T.W., Bacon, C.R., 1992. Garnet/high-silica rhyolite trace element partition coefficients measured by ion micro- probe. Geochim. Cosmochim. Acta 56, 2133-2136.

Skulski, T., Minarik, W., Watson, E.B., 1994. High-pressure experimental trace-element partitioning between clinopyroxene and basaltic melts. Chem. Geol. 117, 127-147.

Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Len. 26, 207-221.

Stevenson, R.K., Patchett, P.J., 1990. Implications for the evolution of continental crust from Hf isotope systematics of Archean detrital zircons. Geochim. Cosmochim. Acta 54, 1683-1697.

Umuh, D.M., Stille, P., Patchett, P.J., Tatsumoto, M., 1983. Lu-Hf and Sm-Nd evolution in lunar mare basalts. Proc. 14th Lunar and Planetary Science Conf., Part 2. J. Geophys. Res. B89 (suppl.), 459-477.

Vervoort, J.D., Patchett, P.J., 1996. Behavior of hafnium and neodymium isotopes in the crust: Constraints from Precam- brian crustally derived granites. Geochim. Cosmochim. Acta 60, 3717-3733.

Vervoort, J.D., Patchett, P.J., Gehrels, G.E., Nutman, A.P., 1996. Constraints on early Earth differentiation from hafnium and neodymium isotopes. Nature (London) 379, 624-627.

Wasserburg, G.J., Jacobsen, S.B., DePaolo, D.J., McCulloch, M.T., Wen, T., 1981. Precise determination of Sm/Nd ratios, Sm and Nd isotopic abundances in standard solutions. Geochim. Cosmochim. Acta 45, 231 l-2323.

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