Separation of U, Pb, Lu, and Hf from single zircons for combined U–Pb dating and Hf isotope measurements by TIMS and MC-ICPMS

Chemical Geology 220 (2005) 105 – 120 www.elsevier.com/locate/chemgeo

Separation of U, Pb, Lu, and Hf from single zircons for combined U–Pb dating and Hf isotope measurements by TIMS and MC-ICPMS Yona Nebel-Jacobsena,*, Erik E. Scherera, Carsten Mu¨nkera,b, Klaus Mezgera a

Zentrallabor fu¨r Geochronologie, Institut fu¨r Mineralogie, Universita¨t Mu¨nster, Corrensstrasse 24, 48149 Mu¨nster, Germany b Mineralogisch-Petrologisches Institut, Universita¨t Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany Received 9 November 2004; received in revised form 3 March 2005; accepted 14 March 2005

Abstract The U–Pb system has long been used to precisely date zircons because the high U-to-common-Pb ratio in zircon produces very radiogenic Pb isotope compositions over time. In contrast to U/Pb, zircon has very low Lu/Hf and therefore unradiogenic Hf, making this mineral ideally suited for determining the initial Hf composition of its original host rock. A new chemical separation technique presented here enables the determination of both U–Pb age and initial Hf isotope composition of individual zircon grains. The acquisition of such complementary information for single detrital zircons is especially useful for provenance analyses and crustal growth studies. Zircons are spiked with mixed 176Lu–180Hf and 233U–205Pb tracers and then digested in HF–HNO3. Lead, Lu, U, and Hf are sequentially separated from the zircon matrix on a single ion exchange column filled with EichromR Ln Spec resin. Using only ~100 Al of resin for the separation keeps Pb blanks low (~5 pg) while achieving better than 90% yields for each of the four elements. Hafnium isotope compositions and Lu concentrations are measured with multiple collectorinductively coupled plasma-mass spectrometry (MC-ICPMS), whereas U and Pb are analyzed by thermal ionization mass spectrometry (TIMS). The minimum grain size that can be processed is dictated by the amounts of Pb and Hf needed for an analysis. The smallest grains we currently analyze, as small as 50 Am (~12pg of Pb and ~3ng of Hf), can be analyzed with an external 176Hf/177Hf precision of ~100 ppm (2 s.d.). The utility of this method is demonstrated with a population of detrital zircons from a Cambrian sediment of the Takaka Terrane, New Zealand. In addition, the technique has been used for 14 analyses of the standard zircon 91500, which yield a mean present-day 176Hf/177Hf of 0.282305 F 12 (2 s.d., i.e., an external reproducibility of 43 ppm). The Hf isotopic compositions and U–Pb ages presented here are in good agreement with those of previous studies (e.g., Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., von Quadt, A., Roddick, J.C., Spiegel, W.,

* Corresponding author. Tel.: +49 0251 83 33463. E-mail address: [email protected] (Y. Nebel-Jacobsen). 0009-2541/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2005.03.009

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1995. Three natural zircon standards for U-Th-Pb, Lu-Hf, Trace element and REE analyses. Geostandards Newsletter 19, 1-23.), but our data suggest that the 91500 zircon is heterogeneous with respect to Lu and Hf contents and Lu/Hf. D 2005 Elsevier B.V. All rights reserved. Keywords: Zircon; Lu–Hf; U–Pb; MC-ICPMS; TIMS; 91500

1. Introduction Zircon is one of the most durable phases in the geologic environment, often withstanding weathering, transport, diagenesis, and metamorphism without completely losing its primary age information (e.g., Gulson and Krogh, 1973; Pankhurst and Pidgeon, 1976; Ho¨lzl et al., 1994). Thus, zircon is ideally suited for studying the origins of rocks that have complex geologic backgrounds, such as sandstones and siltstones (e.g., Ireland, 1992; Wysoczanski et al., 1997; Berry et al., 2001; Wysoczanski and Allibone, 2004). The U–Pb system has long been used to obtain highprecision ages for the growth of zircon because this mineral incorporates significant amounts of U, which substitutes for Zr and later decays to radiogenic Pb. Zircon’s robustness has made it the most commonly used mineral for obtaining precise U–Pb ages of hightemperature rocks or rocks that have undergone multiple geologic events (e.g., Kro¨ner et al., 2001). Due to the generally high U-to-common-Pb ratio of zircon, the U–Pb system provides a precise age for mineral growth or recrystallization (e.g., Mezger and Krogstad, 1997). In addition, zircon generally contains ~1 wt.% Hf, an element that is chemically almost identical to Zr and, once incorporated into the zircon lattice, is extremely resistant to any kind of disturbance. Although the rare earth element (REE) patterns of zircons often show enrichments in the heavy REE (e.g., Lu), the Lu/Hf of zircon is so low that corrections of Hf isotopic compositions for age are minor. Thus, this mineral is commonly used to provide robust estimates of the Hf isotope composition (IC) of the host rock at the time indicated by the zircon’s U– Pb age. In addition, two-stage Hf model ages, which provide information about the crustal history of a zircon’s host rock, can be calculated from the initial Hf isotopic composition of a zircon and typical crustal Lu/Hf. Such model ages indicate the time at which the host rock’s precursor was extracted from its mantle source and became part of the crust. The ability to

determine such crustal residence ages, as well as the U–Pb age and the Hf isotope compositions of single zircons, benefits both crustal evolution studies and provenance analysis. Despite the utility of zircon as a combined chronometer and geochemical indicator, several complications can arise and must be avoided. Zircons often preserve very complex growth histories and it is essential to consider their internal structures when selecting grains for analysis. Common structural features, such as inclusions, inherited cores, and metamorphic overgrowths can be identified by cathodoluminescence (CL) and backscattered electron (BSE) imaging of individual grains. Grains that contain obvious inclusions or those having multistage growth histories should be avoided for U–Pb dating and determination of Hf isotope composition because their isotope compositions are mixtures of several components (e.g., Patchett, 1983; Amelin et al., 2000). Metamictisation, which often results in Pb loss, is primarily a problem for U–Pb dating and cannot be identified by CL or BSE imaging alone. To avoid the complexities of interpreting the ages and Hf compositions of discordant zircons, Patchett (1983) recommended using the most concordant zircons possible for determining initial eHf values. Due to the low total amounts of U, Pb, Lu, and Hf present in single zircon grains, determining their Lu– Hf and U–Pb systematics pushes the limits of currently established analytical techniques. For example, a 100-Am diameter spherical zircon may contain about 25 ng of Hf, 50 pg of Lu, 250 pg of U, and 100 pg of Pb. Whereas larger amounts of Pb (a few tens of nanograms) can be routinely analyzed by TIMS in static mode, analyses of sub-nanogram Pb loads generally require peak-hopping on an ion counter. Furthermore, the high first ionization potential of Hf makes its analysis by TIMS difficult because at least 1–2 Ag of Hf are required to obtain useful isotope data. The advent of multiple collector-inductively coupled plasma source mass spectrometry (MC-

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ICPMS) has facilitated the measurement of Hf isotope ratios (e.g., Blichert-Toft et al., 1997; Blichert-Toft, 2001) and now an external precision of approximately 50 ppm can be achieved for as little as 5 ng of Hf with the Micromass IsoProbe MC-ICPMS at Mu¨nster (Mu¨nker et al., 2001). The improved precision of modern Hf isotopic analyses has allowed Lu–Hf measurements to be made on the same tiny amounts of material that have long been possible to analyze for U–Pb. Here we present a new separation technique that enables the determination of a U–Pb age and corresponding initial Hf isotopic composition of the same single zircon grain. This method facilitates the use of single zircons from sediments and magmatic rocks for determining the provenance of sediments and studying crustal evolution (e.g., Bodet and Scha¨rer, 2000; Samson et al., 2003, 2004).

2. Previously used separation techniques and measurement protocols Combining elements of earlier separation techniques (e.g., Faris, 1960; Benedict et al., 1954), Patchett and Tatsumoto (1980) established the first protocol for separating Lu and Hf from geological materials using three different cation and anion resin columns. Subsequently, additional methods were developed (e.g., Salters and Hart, 1991; Barovich et al., 1995; Scherer et al., 1995; Blichert-Toft et al., 1997; Kleinhanns et al., 2002) that employed cation and anion resins or TEVA resin (Ulfbeck et al., 2003) in two to four column steps. Performing combined U–Pb and Lu– Hf analyses with any of the preceding procedures requires two or more ion-exchange steps and, in some cases, splitting the digested sample into aliquots. Nevertheless, a combined analysis of U–Pb and Lu– Hf of single grains has been accomplished by combining various chemistries for U–Pb and Lu–Hf (e.g., Corfu and Noble, 1992; Amelin et al., 1999). The new technique presented here is based on a previously published single-column separation scheme for Lu and Hf (Mu¨nker et al., 2001), which has been developed further to meet the specific requirements of zircon analyses, namely the efficient separation of U, Pb, Hf, and Lu from the zircon matrix while maintaining low procedural blanks. Effective separation of these elements is necessary to maximize

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element yields for analyses (important for smaller zircons) and to minimize 1) isobaric interferences during the MC-ICPMS measurements and 2) ionization-inhibiting species such as Zr during TIMS measurements. For TIMS Pb analyses removing Zr first can result in a two- to five-fold increase in Pb signal intensity, which allows single-grain analyses to be performed in static mode (with only 204Pb in an ioncounter), rather than by peak-hopping on all Pb peaks into an ion-counter. The single-column chemistry can be completed in a day and avoids the additional sample handling required by multi-column methods.

3. Sample selection and preparation To evaluate the precision and accuracy of the procedure described here, we made several combined U–Pb–Lu–Hf analyses of the standard zircon 91500. Although it has been shown to be heterogeneous with respect to its REE composition, zircon 91500 was considered to be relatively homogeneous in its U– Th–Pb and Lu–Hf systematics (e.g., Wiedenbeck et al., 1995), is one of the most widely used standards for U–Pb-chronology and Hf isotope measurements (e.g., Wiedenbeck et al., 1995; Chen et al., 2003), and has been especially useful as a reference for laser ablation analyses of these elements (Griffin et al., 2000; Andersen and Griffin, 2004). Four fragments of the 91500 zircon weighing 1–18 mg each were provided to ZLG Mu¨nster by M. Wiedenbeck. Two of these fragments (3 and 4) were divided further into fourteen 1–2 mg pieces for multiple replicate analyses. Because the technique was ultimately being developed for the analysis of single detrital zircons, we also processed a set of typical detrital zircons from Cambrian sandstone of the Takaka Terrane, New Zealand. Metamorphic and sedimentary rocks commonly contain multiple zircon populations that differ in age and origin. Previous studies that used multi-grain analyses attempted to isolate these populations by grouping zircons according to their physical characteristics such as size, shape, color, and magnetic properties (e.g., Lopez et al., 2001; Chen et al., 2003). Nevertheless, there is always the risk that more than one distinctive population is included in a multigrain analysis. Though this problem can be avoided by

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performing single-grain analyses as described here, single grains can have internal complexities that make the interpretation of the U–Pb or Lu–Hf systematics of a zircon difficult. Hence, after the zircons are extracted from their host rock with standard methods (crushing, sieving and separation with heavy liquids) they are inspected optically under a binocular microscope. Compound grains and those having conspicuous inclusions are rejected. The remaining grains are prepared for CL and BSE imaging by embedding them in an epoxy resin plug which, after hardening, is ground and polished to expose the interiors of the grains. This process results in a loss of approximately 1 / 3 of the zircon mass. To prevent contamination, Pbfree tools and materials have to be used throughout. Cathodoluminescence and BSE images are used as guides to select only those grains having the simplest growth histories and no visible inclusions for isotopic analysis. These zircons are plucked from the grain mount with a needle and then washed in acetone to remove epoxy residue. Detrital zircon grains are generally not weighed before digestion to minimize sample handling. Instead, grain weights are estimated from their dimensions in CL and BSE images, such that the absolute concentrations of U, Pb, Lu, and Hf are not well constrained, even though U/Pb and Lu/Hf are measured precisely. In contrast to this typical procedure, the 14 relatively large pieces of the 91500 standard zircon were weighed on a microbalance before digestion, allowing the concentrations of these elements to be determined precisely.

4. Separation technique 4.1. Reagents and procedural blanks Due to the extremely low amounts of Pb present in individual zircon grains, it is particularly important to reduce Pb-blanks to a minimum. To help minimize blanks, all acids used in the procedure were distilled once from p.a. grade in either a two-bottle TeflonR still (HF) or in a quartz still (HNO3, HBr, and HCl). All SavillexR vials used in the chemistry were cleaned in HCl and HNO3, bombed with HF– HNO3, and then stored in weak HBr until ready for use. The chemical separation described below is based on the different partition coefficients of Pb,

Lu, U, and Hf on EichromR Ln Spec resin in HCl– HF mixtures (Mu¨nker et al., 2001). This resin consists of an HDEHP (di[2-ethylhexyl]phosphoric acid) coating on an inert polymer carrier (Amberchrom CG71, 100–150 Am). Unlike resins used in previous methods (e.g., Patchett and Tatsumoto, 1980; Blichert-Toft, 2001), Ln Spec is relatively insensitive to different sample matrices and the amounts of sample loaded (Mu¨nker et al., 2001). In addition to the known properties of Lu and Hf, the partitioning behavior of U and Pb on Ln Spec resin was evaluated during the course of this study. Lead was found to have a low partition coefficient in 1–3 M HCl, whereas the U partition coefficient is similarly low in N 3 M HCl (see also Braun and Ghershini, 1975). Prior to the first use, the EichromR Ln Spec resin was cleaned in a TeflonR jar with 1 M HCl followed by a water rinse and a 1 M HNO3 step. Subsequently, 100–120 Al of the resin were loaded into columns made of heat-shrink TeflonR. To keep Pb blanks sufficiently low and eliminate the chance of radiogenic Pb cross-contamination among samples, the resin bed should be used only once and then discarded. The average procedural Pb blank was 4.7 pg with a 3–8 pg range, whereas the Lu and Hf blanks averaged 5 and 10 pg, respectively. The U blank was less than 1 pg. The sample/blank for Hf is usually above 1000, so blank corrections are insignificant (i.e., b 0.1 eunit on 176Hf/177Hf for the extreme case of a 100 eunit difference between sample and blank). Even a sample/blank of ~300 for the smallest zircons we can analyze would only result in a maximum blank correction of 0.35 e-units. Due to the low concentration of Lu in zircon, the sample/blank for Lu is typically ~10 in analyses of single detrital grains but may approach ~1 for the smallest grains. In addition to sample/blank, the effect of the Lu blank correction on initial 176Hf/177Hf also depends on the age of the grain and its 176Lu/177Hf ratio. For a 2 Ga zircon having a 176Lu/177Hf of 0.0003 and a sample/blank ratio of 10 for Lu, correcting for Lu blank would change the initial 176Hf/177Hf ratio by only 0.04 eunits. Thus, the contributions of the Lu and Hf blank corrections to the uncertainty of calculated initial 176 Hf/177Hf values are generally expected to be minor relative to the uncertainty of the 176Hf/177Hf measurement itself, which can be estimated from the

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external reproducibility vs. signal intensity relationship of Hf standard solutions. 4.2. Spiking and sample digestion Single zircon grains were placed in either 3-ml SavillexR vials or Parrish (1987)-style TeflonR microcapsules, then washed in several steps, first with warm HCl, then warm HNO3, pipetting the acid away between steps. Next, mixed 205Pb–233U and 180Hf–176Lu tracer solutions were pipetted into the vials for measuring the concentrations of U, Pb, Lu, and Hf by isotope dilution. (These tracers were calibrated multiple times against mixed U–Pb and Lu– Hf normal solutions prepared from 99.9% pure AMES Lu and Hf metals, NBS 982 Pb metal, and an IRMM056 U standard solution.) After adding ~5 : 1 HF– HNO3 (both concentrated) to the samples, the vials were closed, placed inside ParrR bombs, and heated at 180 8C for at least 5 days. (Complete digestion of the relatively large, low-U fragments of the 91500 zircon took up to 8 days.) After digestion, the samples were dried down in a filtered laminar flow box and then refluxed overnight with 6 M HCl in sealed capsules on an 80 8C hotplate. Early in the development of our method, this step was found to be critical for achieving consistent spike-sample equilibration (see also Amelin et al., 1999). After this step, the sample was dried down again. 4.3. Single column Pb–Lu–U–Hf separation The sample residue is dissolved in ~500 Al of 1 M HCl and then loaded onto the column. Lead elution starts immediately during the loading step. After the sample has passed through the column, Pb elution is completed by adding another ml of 1 M HCl, with a nearly quantitative yield of N 95%. At such high yields, the magnitude of Pb isotope fractionation produced on the column itself (e.g., Blichert-Toft et al., 2003; Baker et al., 2004) is expected to be ~10–40 times smaller than the uncertainty of mass fractionation during TIMS Pb measurements (e.g., at a yield of N 95% the maximum effect on 206Pb/204Pb, 207Pb/204Pb, and 208 Pb/204Pb is less than 0.1x on these ratios (Blichert-Toft et al., 2003)). Next, Lu (+ Yb), U, and Hf are sequentially eluted from the resin. In general, the precision and accuracy of 176Lu/175Lu measurements

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is improved if some Yb is removed from the Lu cuts, resulting in smaller interference corrections for 176Yb on 176Lu. Mu¨nker et al. (2001) found that the Lu–Yb separation on Ln Spec resin improves as the HCl molarity of loading acid is increased from 2 to ~3.5 M, but at the expense of Lu yield in the subsequent 6 M HCl HREE cut. Zircons have close to chondritic Yb/Lu (i.e., ~6) but often contain b 50 ppm Lu, such that smaller detrital grains (5 Ag) generally have less than 250 pg of Lu. To obtain enough Lu for a precise ID measurement, we chose to maximize the Lu yield rather than attempt to lower the Yb/Lu in the Lu cuts. This is accomplished by eluting Lu with 8 ml of 3 M HCl, which typically results in column yields of N95%. As discussed below, the effects of elevated Yb/Lu in the Lu cuts on calculated initial eHf values of zircons are insignificant. The distribution coefficient for U on EichromR Ln Spec resin decreases with increasing HCl molarity up to at least 6 M HCl. Uranium is therefore eluted with 6 ml of 6 M HCl, at a column yield of N95%. Following Mu¨nker et al. (2001), Zr is subsequently eluted in 12 ml of a 6 M HCl–0.06 M HF mixture in which Hf remains adsorbed to the resin. Removal of Zr is not necessarily required for Hf isotope measurements by MC-ICPMS (BlichertToft et al., 1997; Mu¨nker et al., 2001) but, because the Mu¨nster IsoProbe is also used for Zr measurements, any unnecessary contamination of the instrument with excessive amounts of Zr is avoided. By removing Zr first, the Zr/Hf in the Hf cut is typically decreased from ~50 to b3. Hafnium is subsequently eluted in 2 M HF at a ~91% column yield. The complete procedure and elution scheme are summarized in Table 1 and Fig. 1. The elution Table 1 Single-column separation scheme for combined separation of Pb, Lu, U, and Hf using ca. 100 Al of Eichrom Ln Spec resin Step

Resin bed volume

Acid

Equilibrate Load sample, Pb Pb Lu U Rinse Zr

20 5 10 80 60 120

1 M HCl 1 M HCl 1 M HCl 3 M HCl 6 M HCl 6 M HCl–0.06 M HF 2 M HF

Hf

20

The resin bed is ~45 mm long and ~1.7 mm in diameter.

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Fig.1. Elution profile for the single-column separation procedure based on 1-ml cuts through a ~100-Al resin bed. A synthetic zircon matrix containing 50 Ag Zr, 10 Ag Hf, 1 Ag Lu, 100 ng Pb and U and 750 ng Yb was used for the column calibration.

scheme was calibrated with a synthetic solution that approximated a zircon matrix.

5. Hf and Lu isotope measurements by MCICPMS Hafnium and Lu isotope measurements were performed using the Micromass IsoProbe MC-ICPMS at Mu¨nster. The samples were dissolved in 0.56 M HNO3–0.24 M HF for Hf and in 0.1 M HNO3 for Lu measurements and introduced via a CetacR MCN6000 nebulizer. Due to the low amounts of Hf and Lu in detrital zircons, only 300 Al of acid were added to the samples, which led to intensities similar to the 24 ppb in-house Hf standard (~1 V for 177Hf). For washout between the samples, we used 2% HNO3 for Lu and 1 M HNO3–0.5 M HF for Hf. The collector configuration used for Hf isotope ratio measurements is shown in Table 2. Hafnium isotope ratios were corrected for interferences and mass bias as described by Mu¨nker et al. (2001). During the course of this study, the mean 176 Hf/177Hf of our 24 and 80 ppb in-house standard

runs was 0.282149 F 13 (2r) (i.e., an external reproducibility of 46 ppm). The intensities of these standards during measurements (~0.9–3 V of 177Hf) bracket the range of intensities at which Hf fractions of detrital zircons are typically measured. All Hf results are reported relative to a 176Hf/177Hf value of 0.282160 for our in-house standard (AMES Hf), which is isotopically indistinguishable from the JMC-475 Hf standard (Mu¨nker et al., 2001). For Lu isotope dilution (ID) measurements, online mass bias corrections were made by doping the samples with natural Re, then normalizing 176Lu/175Lu to the measured 187Re/185Re (Scherer et al., 1999) using the exponential law and a btrueQ 187Re/185Re of 1.6738 (de Bie`vre and Taylor, 1993). Following a protocol previously used by the Lyon group, the exponential law was also applied when correcting 176Lu for interference from 176Yb. However, we used Re instead of Yb itself to calculate the mass bias factor. The 187Re/185Re-normalized 176Yb/173Yb of our mixed Yb–Re standard averages ~0.7939. During Lu ID analysis, this btrueQ 176 Yb/173Yb and the Re mass bias factor are used to calculate a bmass-biasedQ 176Yb/173Yb, which is multiplied by the raw 173Yb/175Lu and subtracted from the

Table 2 Cup configurations for Hf and Lu isotope measurements using the Micromass IsoProbe at Mu¨nster (after Mu¨nker et al., 2001) Configuration

L3

L2

Ax

H1

H2

H3

H4

H5

H6

Hf Lu

173 Yb 173 Yb

175 Lu 175 Lu

176 Hf, Lu, Yb 176 Lu, Hf, Yb

177 Hf 177 Hf

178 Hf 178 Hf

179 Hf 179 Hf

180 Hf, Ta, W 180 Hf

181 Ta 185 Re

182 W 187 Re

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measured 176(Lu + Yb)/175Lu to obtain an Yb-corrected 176 Lu/175Lu. This ratio is then corrected for mass bias using the Re mass bias factor. Our Lu ID measurement method assumes that the exponential law accurately describes and relates the mass bias behaviors of elements having similar masses (i.e., Re, Lu, and Yb). Although in detail this assumption has been shown to be inaccurate (e.g., Mare´chal et al., 1999; Wombacher and Rehka¨mper, 2003; Albare`de et al., 2004), the exponential law does approximate the mass bias behaviors of these elements well enough to achieve ~1% analytical uncertainty on the measured Lu/Hf of whole rocks (e.g., Blichert-Toft et al., 1997; Mu¨nker et al., 2001). Adding a aHIBA column step (Gruau et al., 1988) to remove almost all Yb from the Lu cut Scherer et al. (1999, 2001) achieved an external reproducibility of 0.2% for Lu ID measurements using this normalization technique. However, to maximize the Lu yield for zircon analyses, no attempt was made to remove Yb from the Lu cut in the present study. The correction for the isobaric interference of 176Yb on 176Lu is therefore more significant, but it is not completely effective because the exponential law does not perfectly describe the mass bias. This, combined with a low amount of Lu available in a single zircon analysis, results in an analytical uncertainty of 1–2% on the measured Lu contents and Lu/Hf of zircons. We note, however, that the Lu/Hf of zircon is so low that even 10% uncertainties on their measured Lu/Hf ratios have little effect on calculated initial Hf isotope compositions (e.g., b0.1 e-unit for the 91500 fraction having the highest Lu/Hf ratio.) The simple Lu measurement procedure described here is more than adequate for zircons, but where high-precision Lu/Hf values are required (e.g., whole rocks or samples with high Lu/Hf), methods that remove Yb from the Lu cut or employ a more robust Yb interference correction (Blichert-Toft et al., 1997; Scherer et al., 1999; Mu¨nker et al., 2001; Blichert-Toft et al., 2002; Barfod et al., 2003; Vervoort et al., 2004) are recommended.

6. U and Pb measurement by TIMS The U and Pb measurements were performed on a VG Sector 54 at Mu¨nster. The samples were loaded on single Re filaments with the H3PO4-silica gel method

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(Cameron et al., 1967) for both elements. The loading blanks for Pb were b 1 pg. Lead was typically analyzed in static mode, using Faraday cups for all masses except 204Pb, which was measured with a Daly ion counter. Uranium was measured as UO2+; the 238UO2/233UO2 was analyzed either statically or by peak-hopping into the axial Faraday cup or the Daly ion counter. The typical run temperatures were 1150–1250 8C for Pb and 1200–1350 8C for U. Mass fractionations, estimated from repeated runs of standards (NBS-982 for Pb and U-500 for U), were 0.10 F 0.05% per amu for Pb and 0.04 F 0.02% per amu for U.

7. Standard zircon 91500 7.1. Results of previous studies Many laboratories have analyzed the 91500 zircon for U–Pb, Lu–Hf, and other trace elements, and a partial compilation of the data is shown in Table 3. The first results for the 91500 standard were compiled by Wiedenbeck et al. (1995). In that study, three different laboratories (Ottawa, Toronto, and Zurich) were each supplied with approximately 10 mg of the standard, from which 3–4 independent TIMS U–Pb analyses were made by each group. The average 207Pb/206Pb age for the 11 analyses is 1065.4 F 0.6 Ma and the average 206Pb/238U age is 1062.4 F 0.8 Ma, corresponding to a 206Pb/238U of 0.17917 F 16 (2 s.d.). In addition, the Toronto laboratory also took 2% aliquots of the U–Pb solutions for Lu–Hf ID analyses by TIMS, with the remaining 98% aliquot being used for U–Pb and the Hf IC measurements. Their mean 176Hf/177Hf value was 0.282284 F 6 (2 s.d.), reported relative to their assumed true value of 0.282142 for JMC-475 (Wiedenbeck et al., 1995). To compare this result with more recent Hf data, it is reported here relative to the presently accepted value of 0.282160 for JMC-475, resulting in a value of 0.282302 F 6 (2 s.d.). Paquette and Pin (2001) demonstrated their newly developed extraction scheme for U–Pb analyses by analyzing six fragments (0.2–0.8 mg) of standard zircon 91500. The resulting 207Pb/206Pb age was 1066.5 F 1.1 Ma and the 206Pb/238U age was 1064.0 F 1.5 Ma. Their reported 206Pb/238U is 0.1795 F 3 (2 s.d.) and there-

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Author

Method

U, ppm

Pb, ppm

207

207

206

207 Pb/206Pb age [Ma]

206 Pb/238U age [Ma]

Hf, ppm

Lu, ppm

176

Wiedenbeck et al., 1995

TIMS

81.2

14.8

0.07488 F 2

1.8502 F 16

0.17917 F 16

1065.4 F 0.6

1062.4 F 0.8

5895

12

Nesbitt et al., 1997 Horn et al., 2000b Amelin et al., 2000 Griffin et al., 2000 Paquette and Pin, 2001 Lopez et al., 2001 Machado and Simonetti, 2001 Chen et al., 2002 Amelin and Zaitsev, 2002 Goolaerts et al., 2004 Nebel-Jacobsen et al., this study

LA-ICPMS LA-ICPMS MC-ICPMS LA-ICPMS TIMS TIMS LA-ICPMS TIMS TIMS MC-ICPMS TIMS and MC-ICPMS

130 – – – 76.5 80 – 79 77 – 82.83

– – – – 14 15 – 14 – – 15.15

– 0.07523 F 32 – – 0.07492 F 4 0.07494 F 7 – 0.0750 F 1 0.07488 F 7 – 0.07473 F 16

– 1.844 F 15 – – 1.8537 F 39 1.8585 F 75 – 1.846 F 3 1.8428 F 131 – 1.8539 F 155

–

–

–

5960 – 6247 – – – – – – – 6525

– – 13.3 13.9 – – – – – – 15.1

0.282284 F 6; 0.282302 F 6a – – 0.282320 F 28 0.282297 F 44 – – 0.282270 F 123 – – 0.282302 F 8 0.282305 F 12

Uncertainties are 2 s.d. in the last significant digits. a This value has been adjusted to the presently accepted b Weighted mean values. c This error is based on in-run statistics. d No error reported.

176

Pb/206Pb

Pb/235U

Pb/238U

0.179 F 1

1074 F 5

– – 0.1795 F 3 0.17990 F 62 – 0.1787 F 17c 0.17849 F 128 – 0.17991 F118

– – 1066.5 F 1.1 1066.6 F 1.4 – 1067 F 1.0 1065.3 F 2.0 – 1061.3 F 4.3

Hf/177 Hf value of 0.282160 (see text).

1061 F 4 – – 1064 F 1.5 1066.3 F 3.1 – 1060d 1058.7 F 7.0 – 1066.5 F 6.4

Hf/177Hf

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Table 3 Published Lu–Hf and U–Pb data for standard zircon 91500

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fore within error of the results of Wiedenbeck et al. (1995). Lopez et al. (2001) used the zircon 91500 as a standard during their U–Pb measurements and presented a 206Pb/238U of 0.17990 F 62 (2 s.d.). This value is slightly higher than the previously reported ones, but they could reproduce the U/Pb within 0.3%. Also using the zircon 91500 as a standard, Amelin and Zaitsev (2002) reported a 206 Pb/238U of 0.17849 F 128 (2 s.d.), based on 8 measurements. Chen et al. (2002) studied both the behaviors of U and Pb and the isotope fractionation occurring in zircons during HF leaching of zircon 91500 and a Phalaborwa zircon. Two unleached fragments of zircon 91500 resulted in a mean 206 Pb/238U of 0.1787 F 17 (this error is based on in-run statistics) and a corresponding 206Pb/238U age of 1060 F 10 Ma. Nesbitt et al. (1997) presented the first laser ablation (LA) ICPMS results for trace elements in the 91500 zircon, measuring the concentrations of Hf and U, but not the isotopic compositions of Hf or Pb. Horn et al. (2000) determined Pb isotope compositions and U/Pb on 49 different spots (LA-ICPMS). They obtained a 207Pb/206Pb age of 1074 F 5 Ma (2 s.d.) and a 206Pb/238U age of 1061 F 4 Ma (2 s.d.). The first Hf isotope data acquired by the laser ablation method was published by Griffin et al. (2000). They determined an average 176Hf/177Hf ratio from 60 different spots and obtained a 176Hf/177Hf value of 0.282297 F 44 (2 s.d.). Machado and Simonetti (2001) also used LA-ICPMS and reported a mean 176 Hf/177Hf of 0.282270 F 123 (2 s.d.) for five ablation spots. The first solution ICPMS data for 91500 for Lu/Hf and Hf isotopic composition were presented by Amelin et al. (2000). Their six analyses yielded a mean of 0.282320 F 28 (2 s.d.) for 176Hf/177Hf. These workers also found a variation of 176Lu/177Hf between different fragments that exceed analytical errors. A recently published study of Goolaerts et al. (2004) reports a mean 176Hf/177Hf for 59 analyses of 4 fragments of 0.282302 F 8 (2 s.d.) using MC-ICPMS. All previously published data appear to indicate a variation in 176Hf/177Hf of 177 ppm and a total range in 206Pb/238U age of 7.6 million years, with all ages overlapping within errors. Element concentrations vary from 76.5 to 130 ppm for U and 5894 to 6247 ppm for Hf.

113

7.2. Results and discussion of standard zircon 91500 Two fragments (3 and 4) of the 91500 standard zircon were divided into fourteen pieces. The ten pieces of fragment 3 (A–J) were analyzed for U–Pb and all 14 pieces (fragments 3, A–J and 4, A–D) were used for Hf isotope measurements. All data are corrected for mass fractionation, blank, and interferences as described above. We corrected common Pb using the model of Stacey and Kramers (1975) and an assumed age of 1065 Ma. All results are listed in Tables 4 (U and Pb) and 5 (Lu and Hf). The ten analyses of fragment 3 (A–J) yield a mean 206 Pb/238U age of 1066.5 F 6.4 Ma, which is within error of previous results, and a mean 207Pb/206Pb age of 1061.3 F 4.3 Ma. The analyses range from concordant to slightly reversely discordant (Fig. 2). In our fragments, we did not observe the minor normal discordance reported by Wiedenbeck et al. (1995) who argued that slight Pb loss might have occurred along cracks in parts of the zircon crystal. All 10 analyses of fragment 3 lie on a single, distinct trend and have 207Pb/206Pb ages that are somewhat younger on average than published results. These low 207 Pb/206Pb ages cannot result from an inaccurate spike calibration or from incomplete sample-spike equilibration. Furthermore, shifting originally concordant points to the more reversely discordant positions of the observed data via fractionation would require that unreasonably high degrees of Pb or U fractionation consistently affected analyses of fragment 3, but not those of fragments 1 and 2, which are concordant and lie along the main trend of published results (Fig. 2). Fragments 1 and 2 were analyzed earlier by E. Scherer (unpublished data), using the same spike and TIMS procedure. We therefore consider the observed Pb-isotope characteristics of fragment 3 to be real, indicating heterogeneity with respect to fragments 1 and 2. Mean concentrations for all fragments are 83 F 21 ppm for U and 15.1 F 4 ppm for Pb. These results are in good agreement with all other published results in Table 3, except the preferred value for U given by Nesbitt et al. (1997). As the measured contents range from 99.1 to 65.9 ppm for U and from 18.1 to 11.7 ppm for Pb, the zircon appears to be heterogeneous with respect to these elements. The average 207 Pb/206Pb, 207Pb/235U, and 206Pb/238U ratios, however, all overlap with the results of previous studies,

114 Table 4 U–Pb results for standard zircon 91500 and detrital zircons of the Takaka Terrane in New Zealand (sample NZ 518, Mt. Benson sandstone; NZMS co-ordinates E 2473368, N 6014423) Weight [mg]

U [ppm]

Pb [ppm]

Comm. Pb [pg]a

Raw 206Pb/ 204 Pbb

Atomic ratios

91500, fragment 3 A 1.11 B 1.24 C 2.14 D 1.27 E 1.20 F 1.10 G 1.26 H 1.67 I 1.16 J 1.47 Mean 2 s.d.

65.91 92.95 80.84 73.94 73.32 88.34 78.98 99.13 79.98 95.92 82.83 21.43

11.74 16.97 14.79 13.39 13.10 16.22 14.43 18.08 15.11 17.63 15.15 4.15

77 164 114 75 80 73 64 138 749 78 161

10,439 7,596 16,536 13,778 11,988 14,685 17,221 13,074 1,358 19,723 12,640

0.17906 F 42 0.17035 F 42 0.18052 F 21 0.17975 F 20 0.17920 F 21 0.18037 F 20 0.18042 F 21 0.17988 F 21 0.17916 F 35 0.18043 F 20 0.17991 0.00118

1.8438 F 46 1.8595 F 45 1.8602 F 23 1.8514 F 23 1.8425 F 23 1.8598 F 23 1.8613 F 23 1.8549 F 24 1.8445 F 39 1.8610 F 23 1.8539 0.0155

57 11 11 12

60.67 101.4 313.0 201.5

0.09802 F 227 0.09525 F 883 0.0959 F 435 0.09026 F 234

0.8259 F 37 0.7844 F 97 0.7943 F 48 0.7347 F 25

Sample

238

Pb/

U

Ages 207

235

Pb/

U

207

206

Pb/

Pb

0.07468 F 6 0.07478 F 4 0.07473 F 4 0.07470 F 4 0.07457 F 4 0.07478 F 4 0.07482 F 4 0.07479 F 4 0.07467 F 6 0.07481 F 4 0.07473 0.00016

0.0611 F 21 0.0597 F 50 0.0601 F 24 0.0590 F 13

206

Pb/238U

207

207

1061.8 F 2.3 1068.9 F 2.3 1069.8 F 1.2 1065.6 F 1.1 1062.6 F 1.2 1069.0 F 1.1 1069.3 F 1.2 1066.3 F 1.2 1062.4 F 2.0 1069.3 F 1.1 1066.5 6.4

1061.2 F 1.6 1066.8 F 1.6 1067.0 F 0.8 1063.9 F 0.8 1060.7 F 0.8 1066.9 F 0.8 1067.4 F 0.8 1065.2 F 0.9 1061.5 F 1.4 1067.4 F 0.8 1064.8 5.5

1059.9 F 1.6 1062.6 F 1.1 1061.4 F 1.1 1060.5 F 1.1 1056.9 F 1.1 1062.6 F 1.1 1063.7 F 1.1 1062.8 F 1.1 1059.6 F 1.6 1063.4 F 1.1 1061.3 4.3

602.8 F 16.3 586.5 F 52.0 590.3 F 25.6 557.1 F13.8

Pb/235U

611.3 F 20.6 588.0 F 55.4 593.6 F 27.1 559.3 F 14.7

Pb/206Pb

643.1 F 73.1 593.6 F 181.1 606.0 F 84.9 568.2 F 47.5

Uncertainties are 2 s.d. in the last significant digits. The data were processed with PBDAT (Ludwig, 1993). Uranium decay constants used for ages are k 238 U = 1.55125 F 0.00166  10 10 year 1 and k 235 U = 9.8485 F 0.0134  10 10 year 1 (Jaffey et al., 1971; 95% confidence limits). Natural 238 U/235 U is assumed to be 137.88. a Common Pb also includes blank Pb. b The 206 Pb/204 Pb is not corrected for blank, spike, or mass bias.

Y. Nebel-Jacobsen et al. / Chemical Geology 220 (2005) 105–120

Takaka Terrane 1-01 2-04 2-08 2-09

206

Y. Nebel-Jacobsen et al. / Chemical Geology 220 (2005) 105–120

115

Table 5 Lu–Hf results for standard zircon 91500 and detrital zircons of the Takaka Terrane in New Zealand (sample NZ 518) Lu [ppm]

Hf [ppm]

176

176

176

177

177

177

91500, fragment 3 A 1.11

10.6

6150

0.000243

B C

1.24 2.14

16.6 15.5

6247 6253

D E F G H

1.27 1.20 1.10 1.26 1.67

15.0 13.2 24.1 49.8 13.5

6260 6296 6452 6608 8232

I J

1.16 1.47

11.1 29.2

6807 6212

0.000378 0.000351 0.000352 0.000339 0.000297 0.000531 0.00107 0.000233 0.000230 0.000232 0.000668

0.282307 F 7 0.282307 F 7 0.282302 F 5 0.282297 F 5 0.282297 F 5 0.282302 F 6 0.282300 F 6 0.282310 F 10 0.282306 F 8 0.282315 F 6 0.282315 F 6 0.282301 F 5 0.282306 F 7

0.282302 0.282301 0.282294 0.282290 0.282297 0.282295 0.282294 0.282300 0.282284 0.282311 0.282310 0.282296 0.282293

16.5 16.5 16.6 16.8 16.8 16.6 16.7 16.3 16.5 16.2 16.2 16.7 16.5

6.9 6.8 6.7 6.5 6.5 6.7 6.5 6.9 6.3 7.2 7.2 6.6 6.6

1.13 1.02 1.04 1.14

11.9 11.6 13.3 10.9 15.1b 10.7

29748 29657 5991 6788 6525c 1136

0.0000567 0.0000555 0.000316 0.000227

0.282314 F 7 0.282251 F 22 0.282295 F 6 0.282311 F 6 0.282305d 0.000012

0.282313 0.282250 0.282288 0.282306 0.282297d 0.000016

16.2 18.4 16.9 16.3 16.5d 0.4

7.3 5.1 6.5 7.1 6.7d 0.6

0.002030 0.0006536 0.0001855 0.0004079

0.282422 F 23 0.282485 F 21 0.282247 F 11 0.282396 F 8

0.282452 0.282313 0.282256 0.282399

12.4 10.2 18.6 13.3

0.3 3.2 6.3 0.2

Sample

Fragment 4 A B C D Meana 2 s.d.

Weight [mg]

Takaka Terrane 1-01 2-04 2-08 2-09

Lu/ Hf

Hf/ Hf

Hf/ Hfinitial

eHf

eHfinitial

Uncertainties are 2 s.d. in the last significant digits. For calculation of initial eHf the chondritic values of 176 Lu/177 Hf = 0.0332 and Hf/177 Hf = 0.282772 from Blichert-Toft and Albare`de (1997) and the k 176 Lu of 1.867  10 11 from Scherer et al. (2001), Scherer at al. (2003) and So¨derlund et al. (2004) were used. a Means exclude same-solution replicates. Some outlier values were excluded from the means using a 2j filter, b excluding 3G, c excluding 4A and 4B, and d excluding 4B. 176

except for the 207Pb/206Pb of Horn et al. (2000) and Chen et al. (2002), which are slightly higher. Excluding one outlier (4B, Table 5 and Fig. 3), the 176 Hf/177Hf values range from 0.282295 to 0.282315, corresponding to a present-day eHf of 16.9 to 16.2 and initial eHf of 6.3–7.3 (using the chondritic values of 176Lu/177Hf = 0.0332 and 176Hf/177Hf = 0.282772 from Blichert-Toft and Albare`de (1997) and the k 176 Lu of 1.867  10 11 from Scherer et al. (2001), Scherer at al. (2003), and So¨derlund et al. (2004)). The measured 176Lu/177Hf values vary by a factor of 19 from 0.0000555 to 0.00107, a greater range than previously reported for the 91500 zircon. Amelin et al. (2000) reported a variation of ~10% in 176Lu/177Hf.

Our measured absolute concentrations of Lu and Hf also show large variations. Lutetium contents range from 10.9 to 29.2 ppm, except for one piece (3G) that contains 50 ppm Lu. Hafnium ranges from 5991 to 8232 ppm, with two outliers that have ~29,700 ppm Hf (4A and 4B). Possible explanations for the high Lu and Hf outliers include the presence of inclusions, incomplete spike-sample equilibration, or actual heterogeneities in the zircon itself. As mentioned earlier, the initial eHf of zircon is affected only slightly by even large errors in Lu/Hf, so that the initial eHf values cannot be used to screen for incomplete spike-sample equilibration among zircon digestions. However, that the two ~29 000 ppm Hf samples are pieces of the

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Fig. 2. U–Pb concordia diagram for standard zircon 91500. Mu¨nster data (black ellipses), including the ten individual runs of Mu¨nster fragment 3, A–J, from the present study, and two earlier analyses of Mu¨nster fragments 1 and 2 by E. Scherer (unpublished data). For comparison, TIMS data from other laboratories are shown (gray ellipses; Wiedenbeck et al., 1995, Paquette and Pin, 2001; Lopez et al., 2001; Chen et al., 2002, and Amelin and Zaitsev, 2002). The lines marked Pb/Pb and U/Pb are parallel to shifts that would be caused by 1) Pb fractionation and 2) U/Pb inaccuracy (e.g., due to uncertainty in spike calibration, incomplete sample-spike equilibration, or unusual degree of U fractionation). The error ellipses already account for the estimated magnitudes of these effects. Data were plotted with Isoplot 2.49 by Ludwig (2001).

Fig. 3. 176Hf/177Hf of standard zircon 91500 reported here compared to previously reported values. Values of Wiedenbeck et al. (1995) were originally reported relative to an assumed true 176Hf/177Hf of 0.282142 for JMC-475 but have been adjusted here to reflect the currently accepted JMC-475 value of 0.282160. For LA-ICPMS studies, only means are given (Griffin et al., 2000, n = 60; Machado and Simonetti, 2001, n = 5). The horizontal line and grey bar show the mean 176Hf/177Hf and 2 s.d. reproducibility from this study; brackets indicate replicate analyses of the same single solution.

Y. Nebel-Jacobsen et al. / Chemical Geology 220 (2005) 105–120

same fragment and that they have very similar Lu/Hf (within 2.1%) suggests that the elevated Hf contents are indeed a real feature and not an artifact of poor spike-sample equilibration. We do not know the reason for the anomalously low eHf of piece 4B, but incomplete spike-sample equilibration cannot be the cause. The elevated Hf contents of pieces 4A and B may indicate the presence of distinct Hf-rich zones in the standard zircon. CL imaging has shown that while much of the 91500 is relatively unzoned, some fragments show extreme CL banding (Wiedenbeck et al., 2004). However, no anomalous Hf contents have been reported for the standard zircon 91500 prior to this study.

8. Detrital zircons of the New Zealand Takaka Terrane The chemical separation for U, Pb, Lu, and Hf presented here was developed to analyze detrital zircons, which are typically smaller than 150 Am in diameter. To demonstrate the applicability of the method to detrital zircons, four grains of the Mount Benson Sandstone, New Zealand with sizes ranging from 60 to 100 Am in diameter were analyzed. The Mount Benson Sandstone belongs to Middle Cambrian sediments of the Haupiri Group, which is part of

117

the Paleozoic Takaka Terrane in the South Island of New Zealand. The Mount Benson Sandstone mainly consists of coarse- to medium-grained sandstones with minor siltstones and tuff bands (Mu¨nker and Cooper, 1999). Cathodoluminescence images were taken at the University of Go¨ttingen and are shown in Fig. 4. Most of the zircons from this sample show different episodes of zircon growth and inherited cores. Only grains with the simplest zonation and growth history were chosen for analyses. All results are listed in Table 4 (U–Pb) and Table 5 (Lu–Hf). Due to the relatively lower concentrations of U and Pb in detrital zircons, the uncertainties of measured ratios and ages are higher than for the standard zircon 91500. The 206Pb/238U ages range from 554.7 to 600.6 Ma and the zircons therefore belong to a characteristic age-population commonly referred to as bRoss– DelamerianQ in sediments of the SE-margin of Gondwana (e.g., Berry et al., 2001; Ireland et al., 1998). The present day 176Hf/177Hf of these grains varies between 0.282247 and 0.282485, the corresponding eHf values from 18.6 to 10.2. Initial eHf values, which are indicators of the amount of recycled crust in the zircon’s original rock, show a total variation from 6.3 (grain 2-08) up to + 3.2 (grain 2-04). We interpret grain 2-08 to have been derived from a rock that contained some recycled crust, whereas grain 2-04

Fig. 4. CL images of the detrital zircons of the Takaka Terrane analyzed in this study. The images were made at the University of Go¨ttingen.

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was derived from a relatively juvenile crustal domain. These results demonstrate the potential of the method because the zircons appear to originate from crustal sources that have different crustal residence ages. Previous provenance studies based on Nd isotopes (e.g., Wombacher and Mu¨nker, 2000) and U–Pb ages alone could not reveal this diverse pattern because Mt. Benson sandstone sediments appear to have relatively uniform initial eNd ( 3.1; Wombacher and Mu¨nker, 2000) and the U–Pb ages of the measured zircons are all similar. A detailed geological interpretation of this sample will be discussed in a separate publication.

9. Conclusions The new technique presented here allows the efficient acquisition of both U–Pb age and initial Hf isotope composition from a single zircon, thus providing complementary information that is highly useful for provenance and crustal growth studies. After examination of the zircon by cathodoluminescence and electron microscopy and digestion of selected grains, the separation of all four elements (U, Pb, Lu, and Hf) from individual grains can be performed in a single, rapid ion exchange column step without splitting the dissolved zircon solution into aliquots. Given the high yields, almost all of each element in the zircon grain is available for analysis. The blank levels are all sufficiently low (less than 10 pg) to permit accurate isotope analyses. Analyses of less than 100 pg of Pb can be performed by TIMS in static mode with Faraday cups (only 204 Pb in an ion-counter). Using this technique, precise measurements of 176 Hf/177Hf, Lu/Hf, and U/Pb by isotope dilution using TIMS and MC-ICPMS are possible for grain sizes as small as 50 Am in diameter, as demonstrated by analyses of four detrital zircon grains from a Cambrian sediment from New Zealand. The New Zealand data also highlight the advantage of determining the Hf isotope composition in addition to the U–Pb age of single grains for provenance studies. In the case of the Mount Benson Sandstone, the four zircons had similar crystallization ages and may have been considered to originate from a single population. However, the high variability of 176Hf/177Hf

among the grains hints that they may have been derived from different crustal domains that contained different amounts of reworked crust but originate from the same thermal overprint, as indicated by similar U–Pb ages. Analyses of several fragments of the standard zircon 91500 agree with previous studies with respect to the U–Pb data but also suggest an inhomogeneous character of this standard with respect to U and Pb concentrations. With only one exception out of 14 analyses, the measured present-day Hf isotope ratios agree within errors and are also consistent with previously published data. Anomalously high Hf contents in two analyzed splits suggest the presence of Hf-rich domains in the standard zircon. Furthermore the large range of measured Lu/Hf points to an even more heterogeneous distribution of Lu and Hf in the zircon than already suggested by Amelin et al. (2000). Zircon 91500 seems to be a suitable standard for Hf isotopic measurements by laser ablation because of its relatively homogenous 176Hf/177Hf (~0.7 e-unit range, excluding one outlier), but it may not be suitable for Lu/Hf.

Acknowledgements We are grateful to Michael Wiedenbeck for providing a few fragments of the 91500 zircon standard and to Heidi Baier for assistance with the chemistry and TIMS analyses. We also thank Oliver Nebel, Stephan Schuth, and Arno Rohrbach for helpful comments and suggestions on an early version of the manuscript. We thank Janne Blichert-Toft and an anonymous reviewer for thoughtful and constructive reviews. A. Kronz from the University of Go¨ttingen is thanked for his support in collecting the CL images. This work was supported by the Deutsche Forschungsgemeinschaft grant Mu 1406/4-1. [RR]

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