Accurate Hf isotope determinations of complex zircons using the laser ablation split stream method

Article Volume 15, Number 1 23 January 214 doi: ISSN: 1525-227 Accurate Hf isotope determinations of complex zircons using the laser ablation split stream method Christopher M. Fisher and Jeffery D. Vervoort School of the Environment, Washington State University, Pullman, Washington 99163, USA (chris.fisher@wsu.edu) S. Andrew DuFrane Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada [1] The laser ablation split stream (LASS) technique is a powerful tool for mineral-scale isotope analyses and in particular, for concurrent determination of age and Hf isotope composition of zircon. Because LASS utilizes two independent mass spectrometers, a large range of masses can be measured during a single ablation, and thus, the same sample volume can be analyzed for multiple geochemical systems. This paper describes a simple analytical setup using a laser ablation system coupled to a singlecollector (for U-Pb age determination) and a multicollector (for Hf isotope analyses) inductively coupled plasma mass spectrometer (MC-ICPMS). The ability of the LASS for concurrent Hf 1 age technique to extract meaningful Hf isotope compositions in isotopically zoned zircon is demonstrated using zircons from two Proterozoic gneisses from northern Idaho, USA. These samples illustrate the potential problems associated with inadvertently sampling multiple age and Hf components in zircons, as well as the potential of LASS to recover meaningful Hf isotope compositions. We suggest that such inadvertent sampling of differing age and Hf components can be a significant cause of excess scatter in Hf isotope analyses and demonstrate that the LASS approach offers a robust solution to these issues. The veracity of the approach is demonstrated by accurate analyses of 1 reference zircons with well-characterized age and Hf isotopic composition, using laser spot diameters of 3 and 4 mm. In order to expand the database of high-precision Lu-Hf isotope analyses of reference zircons, we present 27 new isotope dilution-mc-icpms Lu-Hf isotope measurements of five U-Pb zircon standards: FC1, Temora, R33, QGNG, and 915. Components: 1,49 words, 11 figures, 2 tables. Keywords: laser ablation split stream; zircon; U-Pb; Hf isotopes; ICPMS; dating. Index Terms: 1115 Radioisotope geochronology: Geochronology; 14 Radiogenic isotope geochemistry: Geochemistry. Received 29 July 213; Revised 3 December 213; Accepted 4 December 213; Published 23 January 214. Fisher C. M., J. D. Vervoort, and S. A. DuFrane (214), Accurate Hf isotope determinations of complex zircons using the laser ablation split stream method, Geochem. Geophys. Geosyst., 15, 121 139, doi:. 1. Introduction [2] The increasing availability of in situ techniques like laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and secondary ion mass spectrometry (SIMS) has resulted in a proliferation of analytical techniques being applied to single mineral grains [e.g., Iizuka and 213. American Geophysical Union. All Rights Reserved. 121

Hirata, 25; Kemp et al., 25; Hiess et al., 29; Valley et al., 21]. For example, pioneering work by Hawkesworth and Kemp [24] determined U-Pb age, Lu-Hf isotopic composition, and O isotopic composition in microdomains of individual zircon grains in order to address mechanisms of crustal growth and recycling. Approaches such as these have the ability to provide improved resolution of geochemical evolution and geologic processes. However, each type of analysis is typically conducted separately and therefore consumes a different sample volume. This approach can produce robust results in relatively simple samples, but in complexly zoned minerals, analyzing different volumes for different geochemical systems can result in spurious data. This is especially true when there are drastic differences in the sampling volumes for different measurements. [3] The two most common approaches for measuring age and Hf isotopic composition in zircon are to (1) determine the age with ion microprobe (e.g., sensitive high-resolution ion microprobe (SHRIMP)) followed by Hf isotopic analysis by LA-MC (multicollector)-icpms on top of the small SIMS pit [e.g., Kemp et al., 25; Wooden et al., 213] or (2) determine the age with LA- ICPMS followed by Hf isotopic analysis by LA- MC-ICPMS in an adjacent part of the crystal but within the same cathodoluminesencse-defined zone [e.g., Iizuka et al., 29; Liu et al., 21]. In both cases, two very different sample volumes are consumed during the analyses (U-Pb versus Hf isotopes), and depending on the zircon growthzone geometry at depth, an incorrect age can potentially be assigned to the measured Hf isotope ratio. Incorrect age assignment will result in an inaccurate initial Hf isotope composition, especially when expressed in epsilon notation due to the strong age dependency of the CHUR (chondritic uniform reservoir) reference frame. In the case of U-Pb age measurement by ion microprobe, the volume of zircon needed for U-Pb analysis is typically 1% of what is required for an accurate and precise Hf isotopic measurement [Valley et al., 26]. In the case of U-Pb age measurement by LA-ICPMS, the relatively large volume of the laser pit can often preclude placing the Hf laser analysis on top of the U-Pb pit, and thus, in most instances it is necessary to place the Hf analyses in an adjacent portion of the zircon grain, or in a part of the crystal that appears to represent the same growth zone based on Cathodoluminescence (CL) images [e.g., Andersen et al., 29]. [4] A striking example of such sampling problems concerns the Hf isotope measurements of ancient detrital zircons from the Jack Hill metasedimentary belt. An initial study by Harrison et al. [25] reported results from zircons dated by SIMS and then analyzed for Hf isotopic composition using LA-MC-ICPMS and relatively large spot sizes of 6 8 mm. These data showed extreme heterogeneity in initial e Hf, with both positive and negative values, leading the authors to argue for rapid development of enriched continental crust and a complimentary depleted mantle. Valley et al. [26] challenged this conclusion on the basis of sampling problems associated with the very different analytical volumes used for the U-Pb age and Hf isotope measurements. Follow-up studies by Harrison et al. [28] and Kemp et al. [29], where Hf isotopes were determined concurrently with Pb/ 26 Pb ages, revealed a much more simple Lu-Hf evolution of the source materials of the Jack Hills zircons, underscoring the importance of concurrent age and Hf isotope composition measurement. 1.1. Simultaneous Determination of Age and Hf Isotopes [5] The first demonstration of simultaneous acquisition of age and Hf isotope composition was by Woodhead et al. [24] who utilized the ion optical capability of the Nu Plasma MC-ICPMS to measure Lu-Hf and Pb isotopes in alternating cycles (i.e., dynamic analysis) during a single ablation. A very similar method was applied to detrital zircons from the Jack Hills metasediments by Harrison et al. [28] and Kemp et al. [29], using the zoomoptic capabilities of the ThermoScientific Neptune. [6] While this method has the advantage of analyzing the same volume in an alternating measurement, there are a number of limitations to this approach that should be considered. First, much of the utility of the U-Pb system lies in the availability of three independent ages based on 235 Uand decay ( 26 Pb/,, and Pb/ 26 Pb) which allows the ability to assess the concordance of measured ages and thus for the identification of both ancient and modern Pb loss as well as physical mixing of cores and rims of different ages during analysis. Second, accurate and precise (2% precision or better) Pb/ 26 Pb ages are generally not possible in Phanerozoic age zircon, thus limiting the utility of simultaneous Pb/Pb age and Hf isotopic composition to zircons Precambrian or older. Third, this approach is relatively insensitive to detecting inadvertent mixing of young overgrowths when attempting to 122

He New Wave 213nm Nd:YAG laser analyze old cores. This is a result of the low radiogenic Pb contents in the younger zones compared to older cores and is discussed in detail below. Finally, precision of both Pb/Pb and Hf isotopes appear to be compromised by the overall shorter counting times. For example, using the routine of Kemp et al. [29], 5% of the analysis is dedicated to Hf isotopic measurement, 25% is dedicated to Pb isotope measurement, and the remaining 25% is used for magnet settling. While it is now possible to measure Hf and U-Pb isotopes using a single mass spectrometer, due to the availability of instruments with increased mass dispersion, the precision of both measurements will still be compromised by shorter counting times [Woodhead et al., 24]. [7] More recently, an alternative approach (herein referred to as LASS (laser ablation split stream)) allows simultaneous determination of U-Pb age and Hf isotopic composition on two mass spectrometers during a single laser ablation analysis. This technique was first described by Yuan et al. [28] and Xie et al. [28] and has since been explored by others [e.g., Tollstrup et al., 212; Liu et al., 212; Kylander-Clark et al., 213]. The ablated particles are evacuated from the sample cell in a single piece of tubing, which is then split downstream into two separate paths using a Y connection. Each of these tubes is attached to individual mass spectrometers, for different types of analysis. In the present study, a single-collector High Resolution-Inductively Coupled Plasma Mass Spectrometer (HR-ICPMS) (ThermoFinnigan Element2) is used to measure the U-Pb isotopic composition (age), and a MC-ICPMS (ThermoFinnigan Neptune) is used to measure the Lu-Hf isotopic composition of zircon (Figure 1). This technique is an improvement over the alternating Pb-Pb and Lu-Hf isotope measurements [e.g., Woodhead et al., 24; Harrison et al., 28; Kemp et al., 29] because of the ability of measuring (and thus 235 U), allowing the concordance of individual zircons to be assessed, which is an extremely important criterion for working with ancient zircons. While concurrent Pb/ 26 Pb ages offer numerous advantages for interpreting ancient Hf isotopic data as demonstrated by Kemp et al. [29, 21], it does not allow assessment of concordance and identification of ancient Pb loss, both of which complicate determination of the true crystallization age. [8] Here we describe a simplified LASS setup, which is used to simultaneously determine both the U-Pb age and Lu-Hf isotopic composition of zircon. Our analytical setup consists of two Thermo- Scientific ICPMS mass spectrometers coupled to a New Wave 213nm Nd:YAG (neodymium: yttriumaluminum garnet) laser. Compared to previous studies, this setup utilizes a more simplified sample-flow system to transport the ablated material to the ICP source, with similar or better precision to previous LASS studies, and without the need for addition gas flow control, baffles, or mixing chambers. The precision and accuracy of the technique are demonstrated by analyses of a number of widely available and well-known reference zircons, which have been characterized for both age and Hf isotopic composition. In order to explore the resolution of the technique for analysis of complexly zoned zircon samples and demonstrate its utility, we present simple models for mixing of an isotopically zoned zircon (i.e., different ages and Hf isotopic compositions) during the laser sampling process. In addition, we present two case studies of zircon from Proterozoic gneisses, which illustrate the benefits of the LASS approach. Finally, we report new Lu-Hf isotope data determined by isotope dilution-mc- ICPMS for five of these reference zircons. These data agree with previously published values and present the results for the commonly used (high rare earth element (REE)/Hf) zircon standard R33, thus providing benchmark analyses for comparison with laser ablation generated data. 2. Methods Thermo Neptune 2.1. LASS Analytical Setup [9] The LASS setup at Washington State University (WSU) is shown schematically in Figure 1. N 2 Ar sample gas Thermo Element2 Ar sample gas torch U-Pb age Lu-Hf isotopes Figure 1. Schematic diagram of the laser ablation split stream (LASS) instrumental setup used in the present study. See text for more details. torch 123

Table 1. (a) Cup Configuration and (b) Instrument Operating Parameters for Laser Ablation Analyses (a) Cup Configuration and Interferences L4 L3 L2 L1 Axial H1 H2 H3 H4 171 Yb 173 Yb 175 Lu 176 Hf 176 Yb 176 Lu 178 Hf 179 Hf 18 Hf 18 W 182 W REE-oxides 155 Gd 16 O 157 Gd 16 O 159 Tb 16 O 16 Gd 16 O 161 Dy 16 O 162 Dy 16 O 162 Er 16 O 163 Dy 16 O 164 Dy 16 O 164 Er 16 O 166 Er 16 O (b) Instrument Operating Parameters MC-ICPMS Sector Field-Inductively Coupled Plasma Mass Spectrometer Model ThermoFinnigan Neptune ThermoFinnigan Element2 Forward power 12 W Mass resolution Low (4) Low Gas flows-laser ablation Cool/plasma (Ar) 16 L/min.85 L/min Auxiliary (Ar).85 L/min.85 L/min Sample/nebulizer (Ar).6 L/min 1. L/min Carrier gas (He) 1.2 L/min 1.2 L/min Nitrogen 5 ml/min 5 ml/min Laser ablation Type New wave 213 nm (Nd:YAG) Masses measured Repetition rate 1 Hz 24 Pb, 26 Pb, 28 Pb, 232 Th, 235 U, Laser fluence 9 1 J/cm 2 Points per peak 1 Spot size 3 4 lm Samples are ablated using a NewWave 213 nm Nd:YAG laser using laser spot diameters of either 3 or 4 mm. The laser was operated at 1 Hz with a fluence of 9 1 J/cm 3. The ablated material is transported out of the laser cell using helium carrier gas and is mixed with a small amount (5 ml/ min) of N 2 gas which is added using a plastic Y split approximately 3 cm after exiting the laser ablation system. The ablated sample 1 He 1 N 2 mixture is then split into two separate paths, again using a Y split, an additional 3 cm downstream from the point of N 2 gas addition. One path carries the ablated aerosol to a ThermoScientific Element2 high-resolution ICPMS for U-Pb isotopic measurement, and a second path carries the ablated aerosol to a ThermoScientific Neptune MC-ICPMS for Lu-Hf isotopic measurement. Ar sample gas is added to each path 2 cm from the torch. When measuring Lu-Hf isotopes by laser ablation, the Neptune is equipped with a standard nickel sample cone and a nickel X skimmer cone which has been shown to improve sensitivity by a factor of 1.4 [Hu et al., 212]. Unlike other published methods, no additional mass flow controllers [e.g., Xie et al., 28], mixing chambers [Yuan et al., 28; Tollstrup et al., 212], or baffles [Tollstrup et al., 212] are used. [1] Typical instrument operation parameters used during LASS analysis are summarized in Table 1. The Yb-Lu-Hf cup configuration and instrument operation parameters are also summarized in Table 1. Data were collected in low-resolution, static mode using 6, 1 s integrations. 2.2. Data Reduction 2.2.1. U-Pb Data Reduction [11] The U-Pb data reduction protocol used in this study was presented in detail by Chang et al. [26] and is only briefly discussed here. Each analysis consists of a 3 s gas blank followed by 65 s of ablation. For each analysis, the first 6sof data produced as the sample signal reaches maximum signal intensity are not considered. The next 3 s of data are used to calculate U-Pb ages which is done by measuring the intensities at masses 22 Hg, 24 Pb 1 Hg, 26 Pb, Pb, 28 Pb, 232 Th, 235 U, and during 3 sweeps through the mass range. The last 3 s of the ablation is not used to calculate U-Pb ages, owing to variable and nonlinear U-Pb fractionation after about 35 s of ablation. The problem of U-Pb fractionation during laser ablation is a well-known phenomenon, which increases with increasing ablation depth. In the case where there is a change in measured Hf isotope composition during the final 3 s of ablation, that portion of data is not included when calculating the final. [12] The mean count rate of each isotope during the 3 s gas blank (3 sweeps) is subtracted from 124

.28235.28225 MUNZirc 1 and 4 ALL-.282142 ± 38 (n=135) -.282145 ± 31 (n=78) 3um-.282137 ± 45 (n=57).28215.2825 3um 176 Yb/ 1% Yb correction.28195..5.1.15.2.25.3.35 Figure 2. LASS results ( versus 176 Yb/ ) for MUNZirc 1 and MunZirc 4 during the course of this study. Note that some analyses of MUNZirc 4 have 176 Yb/ >.28, which represents more than a 1% interference correction for 176 Yb. each sweep through the mass range collected during ablation (3 sweeps). Following this background correction, 26 Pb/,, and Pb/ 26 Pb ratios are determined for each sweep for both samples and standards, and final ages are calculated using a standard-sample bracketing approach. In order to account for variable U-Pb fractionation, the Pb/U ratios for the analyses are calculated using the intercept method of Sylvester and Ghaderi [1997], while the measured Pb/ 26 Pb is determined using the mean of all measurements during the analysis. [13] Analyses of unknown zircons are interspersed with analyses of standards, typically with 12 unknowns bracketed by approximately two analyses of two different zircon standards (Plesovice and FC1). Plesovice (337 Ma) [Slama et al., 28] was used to calibrate the - 26 Pb ages, and FC1 (199Ma) [Paces and Miller, 1993] was used for Pb/ 26 Pb, owing to much higher count rates for Pb (2 4 times higher than Plesovice). Typical count rates on the WSU Element2 for are 2.5 3 1 6 counts/s for National Institute of Standards and Technology (NIST) 61 glass (46 ppm U) when using a 4 mm laser spot with the laser and instrument parameters as reported in Table 1. Weighted mean ages and concordia plots were produced using the Redux software program (version 2.1.21) (Bowring et al., 211). 2.2.2. Lu-Hf Data Reduction [14] The Lu-Hf data reduction protocol generally follows that of Fisher et al. [211], except that data reduction was done using the Iolite software program (University of Melbourne) customized for inhouse usage. Each Hf isotope analysis consists of 3 s of on-peak baseline (gas blank) followed by 65 s of ablation. The Iolite auto spline is fit to the mean baseline intensity determined for each isotope and is subtracted from signal intensities measured for each measurement cycle, or integration during ablation [Paton et al., 21]. The most important correction for in situ Hf isotopic analysis is the 176 Yb interference on 176 Hf. The Yb mass bias is determined for each integration of the zircon analysis by measuring the 173 Yb/ 171 Yb ratio relative to 173 Yb/ 171 Yb 5 1.132685 [Segal et al., 23] using the exponential law. The 176 Yb interference is then corrected using the measured voltage of 173 Yb, the Yb mass bias, an adjusted 176 Yb/ 173 Yb of.79639, with the Yb mass bias and interference corrections determined for each integration. The 176 Yb/ 173 Yb used at WSU was calibrated based on numerous measurements of synthetic zircons with a wide range of Yb/Hf (Figure 2). The mass bias of Hf is determined by the measurement of 179 Hf/, using the exponential law and a value for 179 Hf/ of.7325 [Patchett and Tatsumoto, 198]. [15] In order to facilitate data collection, the Lu- Hf isotopic data are collected in a continuous, single data file that encompasses 15 individual ablation analyses (each file consisting of up to 24 individual 1 s integrations). This approach allows the operator to interact primarily with the single-collector ICPMS and laser. [16] As has been true with a number of other studies [Iizuka et al., 29; Kemp et al., 29; Bell et al., 211], we observe an offset between the measured of reference zircons and their accepted value, determined using purified Hf solutions of the same zircons analyzed by solution-mc-icpms and normalized to the 125

Table 2. Detailed Table of Solution Work for This Study Zircon Age (Ma) 2SE 176 Lu/ 2SE (i) e Hf() e Hf(i) FC21 199.282179 6.5.17 6.2.282158 221.4 2.5 FC21 rr1.282178 6.5.282157 221.5 3.2 FC21 rr2.282176 6.5.282155 221.5 3.1 FC123.282183 6.6.175 6.2.28216 221.3 2.6 FC121.282192 6.6.943 6.2.282172 221. 3. Mean.282183.19 221.3 2.9 2SD.12.132.4.6 R33 x 419.282762 6.6.24 6.4.282746 2.8 8. R33 y.282757 6.7.2491 6.5.282738 21. 7.7 R33 z.282763 6.7.239 6.5.282745 2.8 7.9 R3321.282764 6.6.1432 6.3.282753 2.7 8.2 R3322.282763 6.7.173 6.3.28275 2.8 8.1 R3323.282778 6.6.1515 6.3.282766 2.2 8.7 R33 a.282759 6.4.247 6.5.282759 2.9 7.7 Mean.282764.1989 2.8 8. 2SD.14.869.5.7 QGNG (A) 185.28161 6.7.786 6.2.281582 241.6 2.8 QGNG (A rr).28168 6.5.28158 241.6.1 QGNG (B).2816 6.4.699 6.1.281575 241.9 21. QGNG_k.28161 6.9.567 6.1.281581 241.9 2.8 QGNG b.28169 6.7.673 6.1.281586 241.6 2.7 QGNG a.281627 6.5.99 6.2.281595 241. 2.3 QGNG21.281623 6.6.838 6.2.281623 241.1 2.4 Mean.281611.745 241.5 2.6 2SD.2.247.7.8 Temora (A) 417.28271 6.9.1118 6.2.282692 23. 6. Temora_k.282699 6.11.1165 6.2.28269 23.1 5.9 Temora_j.28277 6.11.1398 6.3.282696 22.8 6.1 Temora_i.282695 6.9.1256 6.3.282686 23.2 5.8 Mean.28271.1234 23. 6. 2SD.1.246.3.3 915 163.28231 6.6.34 6.1.28234 216.8 6.8 915_e.28236 6.7.283 6.1.28231 216.9 6.7 915_d.282319 6.9.339 6.1.282312 216.5 7.1 915_c.282318 6.8.331 6.1.282311 216.5 7.1 Mean.282313.314 216.7 6.9 2SD.12.52.4.4 Epsilon Hf values were calculated using the CHUR values of Bouvier et al. (28) and (for initial values) the 176Lu decay constant of Söderlund et al. (24) and Scherer et al. (21). JMC475 Hf standard. As shown below, the offset cannot be related to an incorrect interference correction as all standards show the same deviation despite having a large range in Yb/Hf. Two approaches have been used to correct this residual bias, calibration to either a reference zircon [e.g., Iizuka et al., 29; Bell et al., 211] or to solution MC-ICPMS measurements of the Hf reference material JMC475 [e.g., Kemp et al., 29]. We have adapted the former approach as it more accurately reflects instrumental conditions (including mass bias) during the laser ablation analyses of unknowns and uses the U-Pb standard (Plesovice) and the reported value of.282482 6 13 to normalize all other zircons analyzed in this study. The typical correction factor is 1.15 (Figure 5a). [17] As discussed by Hu et al. [212], high-quality Hf isotope analyses by LA-MC-ICPMS require 18 Hf ion beams of 4 V. Typical sensitivity for the data reported here is 3.5 V of 18 Hf for zircon 915. This is nearly identical to that reported by Tollstrup et al. [212] using a laser fluence of 5 J/cm 2, and spot sizes and repetition rates identical to this study. The higher laser fluence used in the present study (1 J/cm 2 ) can be attributed to the higher sensitivity of the Neptune Plus equipped with X skimmer and Jet sample cones (14 V/ppm) compared to the standard Neptune used in this study (4 V/ppm). 2.3. Isotope-Dilution, Solution-MC- ICPMS Lu-Hf Analysis of Zircons [18] Twenty-six isotope dilution analyses of the Lu-Hf isotope composition of reference zircons of known age were conducted in order to better establish the Hf isotope composition of these zircons (FC1, Temora, R33, QGNG, and 915 (Table 2)). The origin and U-Pb age of these zircons are summarized along with the LASS results 126

.28221.2822 a).282785.282775 b).28219.282765.28218.282755.28217.28216 FC-1.282183 ± 12.282745.282735 R33.282764 ± 14.28273.28272.28271.2827.28269.28268 c) Temora.28271 ± 1.28269.28268.28267.28266.28265.28264 d) QGNG.281611 ± 2.282335.282325 e).282315.28235.282295.282285 915.282313 ± 12 Figure 3. Results of isotope dilution-solution MC-ICPMS analyses of reference zircons of known age. Error bars are 2SE of the mean, and reported values are the mean of all analyses with uncertainty given as two standard deviations. (a) FC1, (b) R33, (c) Temora, (d) QGNG, and (e) 915. below. With the exception of two duplicate analyses and one triplicate analysis, each zircon represents a separate aliquot and thus is spiked and put through chemistry separately. The details of the dissolution, spiking, column chemistry, and Lu-Hf mass spectrometry are discussed in detail in Goodge and Vervoort [26]. 3. Results 3.1. Solution-MC-ICPMS Analysis of the Hf Isotopic Composition of Reference Zircons [19] In an effort to improve the database of highprecision solution-mc-icpms Lu-Hf isotope data for reference zircons of known age, we present new isotope dilution analyses of reference zircons including FC1, Temora, R33, QGNG, and 915 (Table 2). The origin and U-Pb age of these zircons are summarized along with the LASS results below. 3.1.1. FC1 [2] Solution Hf isotopic analyses of three individual crystals of FC1, including triplicate Hf isotope analyses for one of the crystals, yield a mean of.282183 6 12 (2SD) and moderately high 176 Lu/ from.94 to.11 (Figure 3a). These results are in good agreement with those of Woodhead and Hergt [25], who report a of.282184 6 16 (2SD) and 176 Lu/ of.126 for five replicate runs of a single dissolution of FC1. 3.1.2. R33 [21] Seven individual crystals of R33 yield a mean of.282764 6 14 (2SD) with high and variable 176 Lu/ ranging from.143 to.249 (Figure 3b). To our knowledge, R33 has the highest 176 Lu/, and thus 176 Yb/, of any zircon of known age commonly analyzed as a reference mineral. Because of its homogenous and high ( 176 Lu 1 176 Yb)/, R33 is an excellent standard for monitoring 176 Yb and 176 Lu isobaric interferences on 176 Hf. 3.1.3. QGNG [22] Six individual crystals of QGNG yield a mean of.281611 6 2 (2SD) with relatively low 176 Lu/ ranging from.567 to.99 (Figure 3c). These results are in close agreement with Woodhead and Hergt [25], who 127

report a of.281612 6 4 (2SD) and 176 Lu/ of.731 for five replicate runs of a single dissolution of QGNG. 3.1.4. Temora [23] Four individual crystals of Temora yield a mean of.28271 6 1 (2SD) and moderately high 176 Lu/ from.1118 to.1398 (Figure 3d). These results are within analytical uncertainty of the results of Woodhead et al. [24] who report a mean of.282686 6 8 (2SD) for three separate aliquots of the Temora 2 zircon standard. Woodhead et al. [24] report a 176 Lu/ from.19 for one of the aliquots. It should be noted that our analyses were from the original collection of Temora (Temora 1) [Black et al., 23], which may have a different isotopic composition from Temora 2 [Wormald et al., 212]. 3.1.5. 915 [24] Four crystal fragments of the wellcharacterized zircon standard 915 yield a mean of.282313 6 12 (2SD) and 176 Lu/ of.283.339 (Figure 3e). This result is in close agreement with the compiled value of.28238 6 6 (2SD) [Blichert-Toft, 28]. 3.2. LASS Analysis of Reference Zircons [25] In order to test the precision and accuracy of the LASS technique presented here, multiple analyses of a number of reference zircons were undertaken using both 3 and 4 mm laser spot sizes. The results of these analyses are reported in supporting information Table S1 1 (Plesovice, FC1, MUNZirc 1 and 4) and in supporting information Table S2 (Mud Tank, GJ1, R33, QGNG, Temora, and 915). 3.2.1. Plesovice [26] In this study we used the Plesovice zircon as both the primary U-Pb isotopic standard as well as the Hf isotopic standard for final correction of. Because Plesovice was used as the U- Pb standard to normalize the other (unknown) analyses, it is not possible to assess the accuracy of its LASS age determinations. For Hf, we utilized Plesovice s extremely low (Lu 1 Yb)/Hf ratios to assess the measurements. The measurements using the 4 mm laser spot were extremely consistent and yielded a mean value of.282436 6 28 (2SD, n 5 289; Figure 4a). This value is in good agreement with the analyses conducted using a 3 mm laser spot, which yielded a 1 Additional supporting information may be found in the online version of this article. mean of.282432 6 33, 2SD; n 5 5). These values are lower than the accepted solution- MC-ICPMS of.282482 [Slama et al., 28]. Such biases have also been noted in other studies [Iizuka et al., 29; Kemp et al., 29; Bell et al., 211]. Similar biases are observed in all other reference zircons, and therefore we used the Plesovice zircon standard to determine the bias in measured and correct all other standards and unknowns using this bias. 3.2.2. FC1 [27] Zircon standard FC1 was extracted from the Anorthositic Series of the Duluth complex, Minnesota. FC1 has an age of 199 [Paces and Miller, 1993] and is used as the Pb/ 26 Pb standard in this study, owing to its high Pb contents compared to Plesovice. LASS analyses of FC1 yield a mean of.282182 6 47 (2SD, n 5 149; Figure 4b); using a 4 mm laser spot yielded a mean of.282183 6 41 (2SD, n 5 122). This value is in good agreement with the analyses conducted using a 3 mm laser spot, which yielded a mean of.282175 6 68 (2SD, n 5 27). All LASS data are in close agreement with solution-mc-icpms Hf presented here and by Woodhead and Hergt [25]. 3.2.3. Mud Tank [28] The Mud Tank zircon standard is derived from the Mud Tank carbonatite, Australia. These zircons are large (up to 1 cm) and readily available [Woodhead and Hergt, 25]. The currently accepted age for the carbonatite is 732 Ma [Black and Gulson, 1978]. Similar to other carbonatite zircon, Mud Tank has extremely low U contents (1 ppm) and thus low radiogenic Pb contents. The low U and Pb contents are reflected in the uncertainty of the LASS U-Pb data. Fifty-one analyses of Mud Tank zircon using both 3 and 4 mm laser spots give a weighted mean 26 Pb/ age of 731.9 6 4.9 Mean square weighted deviation (MSWD 5 1.3), in good agreement with the published age (Figure 5a) of 732 Ma. A number of the analyses are discordant, but this is likely not due to common Pb as they do not have anomalously high 24 Pb signals. Thus, we attribute the spread in ages to the relatively low and noisy Pb signal intensities for Mud Tank (typically 7 cps). [29] Woodhead and Hergt [25] observed heterogeneous for colored crystals of the Mud Tank zircon. In contrast, the colorless crystals displayed homogeneous Hf isotopic composition, with exceptionally low REE/Hf. Solution Hf isotopic analysis of colorless Mud Tank yields a of.28257 6 3 (2SD, n 5 5) 128

.28265.28255 a) Plesovice ALL-.282436 ± 28 (n=289) -.282437 ± 27 (n=239) 3um-.282432 ± 33 (n=5).28245.28235 note bias in Plesovice used to corrected all other zircons 3um.28225 5 1 15 2 25 3.2824 b).2823.2822.2821.282 FC1 ALL-.282182 ± 47 (n=149) 3um -.282183 ±41 (n=122) 3um-.282175 ±68 (n=27).2819..5.1.15.2.25.3 176 Lu/ Figure 4. LASS results for the Plesovice and FC1 zircon standards during the course of this study. (a) determinations of for Plesovice, and (b) versus 176 Lu/ for FC1. [Woodhead and Hergt, 25]. Twelve LASS analyses using a 3 mm spot yield a mean of.282512 6 34 (2SD); 4 mm analyses yielded a mean of.28252 6 28 (n 5 42, 2SD). The mean for all analyses is.282518 6 29 (2SD, n 5 53; Figure 5b). 3.2.4. GJ1 [3] GJ1 is single, gem-quality zircon crystal, acquired from a Sydney gem dealer [Jackson et al., 24]. Eight aliquots were measured for U-Pb age by Isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and yielded a Pb/ 26 Pb age of 68.5 6.4 Ma and a younger 26 Pb/ of 599.8 6 4.5 Ma, presumably due to a small degree of Pb loss [Jackson et al., 24]. Thirty-one analyses of the GJ1 zircon, using both 3 and 4 mmlaser spot, give a weighted mean 26 Pb/ age of 599.7 6 2.7 (MSWD 5.99), in excellent agreement with the published 26 Pb/ age (Figure 5c). [31] The Hf isotopic composition of GJ1 was determined by solution MC-ICPMS and yielded a mean of.282 6 5(2SD)[Morel et al., 28]. Twelve LASS analyses of GJ1 using a 3 mm spot yield a mean of.2829 6 43 (2SD); 4 mm spot analyses yield a mean of.282219 6 47 (2SD, n 5 19). The mean for all analyses is.28215 6 46 (n 5 19; Figure 5d) well within analytical uncertainty of the solution-mc-icpms analyses of GJ1. 3.2.5. R33 [32] Zircon R33 comes from a quartz diorite from the Braintree Complex, Vermont, and is currently in use as a SHRIMP U-Pb standard and as a standard for interlaboratory comparison by the EARTH- TIME initiative. Its 26 Pb/ age is well constrained at 418.9 6.4 Ma [Black et al., 24]. Forty-two LASS analyses yielded a weighted mean 26 Pb/ age of 42.2 6 1.6 Ma (MSWD 5 5.9; Figure 6a). This result is 4% lower than the reported ID-TIMS age. [33] The R33 zircon is unique among reference zircons with well-constrained ages, in that it has 129

26 Pb/ a) 6 Mud Tank Weighted Mean 26 Pb/ age 731.9 ± 4.9 MSWD = 1.3 (n=51) 65 7 75 8 85 9 95 3um 26 Pb/ 52 c) 54 GJ1 Weighted Mean 26 Pb/ age 599.7 ± 2.7 MSWD =.99 (n=31) 56 58 6 62 64 66 68 b) d).2827.2826.2825 Mud Tank ALL-.282518 ± 29 (n=53) 3um-.282512 ± 34 (n=12) -.28252 ± 27 (n=41).2824 3um.2823 1 2 3 4 5.2822.2821.282 GJ1 ALL-.28215 ± 46 (n=31) 3um-.2829 ± 43 (n=12) -.28219 ± 47 (n=19).2819 3um.2818 5 1 15 2 25 3 Figure 5. (a) Concordia diagram with weighted mean 26 Pb/ age and (b) for the Mud Tank zircon standard. (c) Concordia diagram with weighted mean 26 Pb/ age and (d) for the GJ1 zircon standard. Red ellipses on concordia diagrams denote analyses conducted with 4 mm laser spot; blue ellipses denote 3 mm laser spots. Grey diamonds on plots denote analyses conducted with 4 mm laser spot, and black squares denote analyses done using 3 mm laser spot. Error ellipses on concordia diagrams are expressed as 2r. Error bars on analyses are given as 2SE. The reported mean of Hf isotope analyses is given as 2 standard deviations (2SD). much higher 176 Lu/ and 176 Yb/ than most other zircon standards and thus makes an excellent material for monitoring isobaric interferences during in situ Hf isotopic analysis. LASS analyses conducted using a 3 mm laser spot size yield a mean of.282749 6 65 (2SD, n 5 36); 4 mm spot analyses yield a mean of.282728 6 45 (2SD; n 5 8). The mean of all analyses is.282749 6 46 (n 5 44; Figure 6b). These results are within analytical error of the solution-mc-icpms results presented here (.282764 6 14). It should be noted that analyses conducted using a 4 mm laser spot appear to be offset toward lower. 3.2.6. QGNG [34] The QGNG zircon standard is derived from a quartz gabbro-norite gneiss from the Donnington Suite of the Gawler Craton, Australia [Black et al., 23; Woodhead and Hergt, 25]. However, as documented by Black et al. [23], the U-Pb systematics are complicated by a small degree of Pb loss (.5%) which results in spread of 26 Pb/ ages. Based on calibration against the Temora standard, Black et al. [23] suggest a 26 Pb/ age of 1842. 6 3.1. Black et al. [23] report two Pb/ 26 Pb ages determined at two independent laboratories: eight nearly concordant analyses from the Royal Ontario Museum yield a Pb/ 26 Pb age of 1851.6 6.6, while three nearly concordant analyses from the Berkeley Geochronology Center yield a Pb/ 26 Pb age of 1854.4 6 2.1. More recently, Schoene et al. [26] reported a Pb/ 26 Pb age of 1851.5 6 5.8 (MSWD 5 1.2) and a 26 Pb/ age of 1848.7 6 2.7 Ma (MSWD 5 2.1) for seven chemically abraded grains. Eighteen analyses of QGNG using both 3 and 4 mm laser spot diameters by LASS yield a weighted mean Pb/ 26 Pb age of 184.8 6 5.3 (MSWD 5.56; Figure 6c). Although this result is slightly outside of analytical error with published ID-TIMS, the age offset is only.5%. [35] Eight analyses by LASS using a 3 mm spot yield a mean of.28162 6 39 (2SD); 4 mm spot analyses yield a mean of.28167 6 41 (2SD, n 5 8). The mean of all analyses is.282614 6 41 13

c) 26 Pb/ a).28295.28285.28275 32 R33 Weighted Mean 26 Pb/ age 42.2 ± 1.6 MSWD = 5.9 (n=42) 34 36 38 4 42 R33 ALL-.282749 ± 46 (n=44) 3um-.282754 ± 47 (n=36) -.282728 ± 43 (n=8) 46 48 5 3um.28265 3um 176 Yb/.28255..2.4.6.8.1.12.14 d) b) 26 Pb/ 16.2818.2817.2816 165 QGNG Weighted Mean 26 Pb/ 238 Pb age 184.8 ± 5.3 MSWD =.56 (n=18) 17 175 18 185 19 195 QGNG ALL-.282614 ± 41 (n=18) -.28267 ± 41 (n=8) 3um-.28262 ± 39 (n=1) 2.2815 3um.2814 2 4 6 8 1 12 14 16 18 2 Figure 6. (a) Concordia diagram with weighted mean 26 Pb/ age and (b) versus 176 Yb/ for the R33 zircon standard. (c) Concordia diagram with weighted mean Pb/ 26 Pb age and (d) for the QGNG zircon standard. Red ellipses on concordia diagrams denote analyses conducted with 4 mm laser spot; blue ellipses denoted 3 mm laser spots. Grey diamonds on plots denote analyses conducted with 4 mm laser spot, and black squares denote analyses done using 3 mm laser spot. Error ellipses on concordia diagram are expressed as 2r. Error bars on analyses are expressed as 2SE. The reported mean of Hf isotope analyses is given as 2 standard deviations (2SD). (Figure 6c). These results are in close agreement with the solution-mc-icpms analyses of.281611 6 2 and.281612 6 4 (this study and Woodhead and Hergt [25], respectively). 3.2.7. Temora [36] The Temora zircon standard is from Middledale Gabbroic Diorite within the Lachlan Orogen of eastern Australia and is a commonly used SHRIMP standard. Similar to R33, Temora is also currently in use as an interlaboratory standard by the EARTHTIME initiative and has a well-constrained published 26 Pb/ ID-TIMS age of 416.75 6.24 (MSWD 5 1.14) [Black et al., 23]. Twenty-five LASS analyses using both 3 and 4 mm spot sizes give a weighted mean 26 Pb/ age of 49.2 6 2.2 (MSWD 5 3.9), approximately 2% lower than the ID-TIMS age (Figure 7a). [37] Eleven LASS analyses using a 3 mm spot yield a mean of.2827 6 58; 14 analyses using 4 mm spots yield a mean of.282687 6 45. The mean using both spot sizes is.282693 6 52 (Figure 7b). These results are in close agreement with our solution-mc-icpms results and those reported by Woodhead et al. [24], albeit with larger external error than other reference zircon presented here. This may represent age and Hf isotopic heterogeneity within this standard as noted by Wormald et al. [212]. 3.2.8. 915 [38] The 915 zircon standard has been well characterized for U-Pb age by ID-TIMS, most recently by Schone et al. [26]. In that study, seven fragments of the 915 crystal yield a weighted mean 26 Pb/ ID-TIMS age of 163.4 6 1.4 (MSWD5.7). Twenty LASS analyses of 915 using a 4 mm laser spot yielded 26 Pb/ age of 165.4 6 5.8 (MSWD 5 1.4), in close agreement with the published ID-TIMS age (Figure 7c). [39] All analyses of 915 were conducted using a laser spot size of 4 mm and yielded a of.28232 6 42 (Figure 7d). These results are in excellent agreement with the compiled value of.28238 6 6[Blichert-Toft, 28]. 3.2.9. MUNZirc 1 and MUNZirc 4 [4] MUNZirc 1 and 4 are synthetic zircons designed as reference materials for Hf isotope 131

26 Pb/ a) 34 36 Temora Weighted Mean 26 Pb/ age 49.2 ± 2.2 MSWD = 3.9 (n=25) 38 4 42 44 4 46 48 5 54 52 3um 26 Pb/ b) 9 915 Weighted Mean 26 Pb/ age 165.4 ± 5.8 MSWD = 1.4 (n=2) 95 1 15 11 1 115 12 125 c).2829.2828.2827 Temora ALL-.282693 ± 52 (n=25) 3um-.2827 ± 58 (n=11) -.282687 ± 45 (n=14) d).2825.2824.2823 915 -.28232 ± 42 (n=2).2826 3um.2825 5 1 15 2 25.2822.2821 5 1 15 2 Figure 7. (a) Concordia diagram with weighted mean 26 Pb/ age and (b) for the Temora zircon standard. (c) Concordia diagram with weighted mean 26 Pb/ age and (d) for the 915 zircon standard. Red ellipses on concordia diagrams denote analyses conducted with 4 mm laser spot; blue ellipses denoted 3 mm laser spots. Grey diamonds on plots denote analyses conducted with 4 mm laser spot, and black squares denote analyses done using 3 mm laser spot. Error ellipses on concordia diagram are expressed as 2r. Error bars on analyses are expressed at 2SE. The reported mean of Hf isotope analyses is given as 2SD. analyses [Fisher et al., 211] with a homogeneous Hf isotope composition and a wide range of REE/ Hf. Although the MUNZirc series zircons were analyzed using the normal LASS setup, they do not contain U or Pb and thus provide no age information. MUNZirc 1 was doped to achieve a 176 Yb/ similar to typical granitic zircons, while MUNZirc 4 was doped to achieve 176 Yb/ much greater than is observed in natural zircons, thus ensuring that an accurate can be determined for a wide range of 176 Yb/. LASS analyses of MUNZirc 1 and 4 were conducted using both 3 and 4 mm diameter laser spots. These results demonstrate no correlation in 176 Yb/ and (Figure 2) and that accurate interference corrections can be made despite some analyses containing equal amounts of 176 Yb and 176 Hf (equivalent to an interference correction of 1, e Hf units). The mean, when using 3 mm spots, was.282137 6 45 (2SD, n 5 57), similar to the mean of.282145 6 31 (2SD, n 5 78) when a 4 mm spot was used. The combined results of both 3 and 4 mm spot sizes give a mean of.282142 6 38 (2SD, n 5 135; Figure 2). These results are within analytical uncertainty of the solution-mc-icpms value of.282135 6 31 (2SD) for MUNZirc 1 and 4 [Fisher et al., 211]. 3.2.1. Summary of LASS U-Pb and Hf Isotope Analyses of Zircon Standards [41] The results of LASS analyses of zircon standards of known age and Hf isotope composition are shown in Figures 5 7, and in almost all cases both U-Pb and Hf isotope compositions agree within analytical uncertainty with benchmark ID-TIMS U-Pb ages and ID-solution-MC-ICPMS Lu-Hf isotope ratios. Most samples have Hf isotope ratios with internal and external reproducibility better than 1.5 e Hf units, very similar to Hf analyses conducted without concurrent age measurement. The two most prominent exceptions are the larger external reproducibly for both Temora and R33, and the lower determined for R33 using 4 mm spot sizes. Similarly, the U-Pb ages determined for Temora and R33 are systematically lower than the ID-TIMS ages by 2% and 4%, respectively. As discussed before, the larger 132

scatter seen in Hf isotopes Temora may represent heterogeneity within this standard. The larger degree of scatter observed in the measured Hf isotope composition for R33 may be due to the higher (Lu 1 Yb)/Hf in this natural zircon sample. 4. Discussion 4.1. Mixing Scenarios [42] One of the most powerful applications of the LASS approach for zircon is the ability to detect mixed analyses on the basis of both U-Pb age and Hf isotope composition. For example, Hf isotope data for magmatic zircons can be filtered by removing analyses with concurrently measured U- Pb ages outside of the population of interest, present as either older cores or younger overgrowths. [43] Since precise Hf isotope analyses require longer ablation times (typically 6 s) and relatively large laser spot sizes, the analytical volume consumed during Hf analysis is significantly larger than that required for U-Pb analyses, especially by SIMS. Ultimately, this leads to a very different sample volume being used to measure U-Pb and Hf isotope composition. Because of the necessity for larger sample volumes for Hf isotope measurements, inadvertent sampling of different zones becomes more likely and is potentially undetectable unless age and Hf isotope data are collected simultaneously. 4.2. Mixing During Ablation [44] We present three mixing scenarios that can occur during LASS zircon analysis in order to describe how this mixing might be manifest in the data. The model incorporates a 2. Ga zircon with a of.2813 and a 75 Ma overgrowth with of.2825. Both age domains have the same Hf concentrations but different uranium (and thus radiogenic Pb) concentrations: (1) a core with 1 times more U than the rim; (2) equal U concentrations in both core and rim; and (3) the core with one-tenth the U as the rim. The results of this simple model are shown in Figures 8 and 9. [45] A number of important points regarding the ability of LASS to detect mixing are illustrated in this plot. In scenario A, where the core has 1 times more U than the rim (Figures 2a and 2b), a relatively large proportion of rim material is needed to contribute to the analysis to detect mixed - 26 Pb or Pb/ 26 Pb ages. For example, nearly 9% of the rim material incorporated with only 1% core material is needed to detect a mixed Pb/ 26 Pb age, and nearly 5% of the rim material needs to be included to detect a mixed 26 Pb/ age. This is due to the differential leveraging by the different U concentrations. Both mixing scenarios, however, result in relatively large deviations in Hf isotope composition from the targeted core because Hf concentrations do not vary in zircon and are approximately the same in both core and rim. [46] In scenario B, where both core and rim have equal U concentrations, mixing can be detected in the in the - 26 Pb ages with smaller amounts of rim material, but large amounts of rim material are still required to detect such mixing in Pb/ 26 Pb ages. Finally, in scenario C where the core has 1 times less U than the rim, mixing in both - 26 Pb and Pb/ 26 Pb ages can be detected with smaller amounts of rim material. [47] These simple modeling results demonstrate that the relative amount of U (and thus radiogenic Pb) is potentially the most limiting factor for detecting mixing of Hf isotopes on the basis of age. In the case where the target is an older core, detectingmixingonbasisof Pb/ 26 Pb is ineffective unless the rim material has much higher U than the core. The presence of mixed components, however, is easier to detect using - 26 Pb ages provided the cores do not have substantially more U than the rims. The situation is much improved when the target is younger material around older cores. In this case it will be possible to detect mixing of U-Pb age components provided the younger material does not have substantially higher U than the older cores, and that any mixing is greater than 5 1%. However, it is clear from Figure 3 that Pb/ 26 Pb is especially sensitive for detecting inclusion of older core material, regardless of differences in the U content between younger and older materials. [48] It should be noted that this is a simplistic model and that natural systems are often more complicated. Some additional factors which may preclude more complicated modeling include (1) samples where there is a range of ages and Hf isotope compositions in inherited cores; (2) samples which undergo partial Pb loss prior to new growth of magmatic zircon; and (3) samples which include more than two age-hf isotope components. 4.3. Case Studies From the Clearwater Metamorphic Core Complex, Northern Idaho [49] The samples used in the following case studies were collected from the Clearwater 133

.2826.2824.2822.28218.28216.28214.28212 a) Rim has 1X less U than core Pb loss 5% mix 5 1 15 2 25 26 Pb/ age.45.4.35.3.25.2.15.1.5 26 Pb/ b) Rim has 1X less U than core 5% mix 1 2 3 4 5 6 7 8.2826.2824.2822.28218.28216.28214.28212 c) Pb loss 5 1 15 2 25 26 Pb/ age Rim and core have equal U contents 5% mix.45.4.35.3.25.2.15.1.5 26 Pb/ d) 5% mix Rim and core have equal U contents 1 2 3 4 5 6 7 8.2826.2824.2822.28218.28216.28214.28212 e) 5% mix Rim has 1X more U than core Pb loss 5 1 15 2 25 26 Pb/ age.45.4.35.3.25.2.15.1.5 26 Pb/ f) Rim has 1X more U than core 5% mix 1 2 3 4 5 6 7 8 Figure 8. Mixing diagrams for measured U-Pb and for a hypothetical zircon with core and rim of different ages. Tick marks are made at 1% mixing increments. See text for more details on end-member zircon age and Hf isotope composition. (a, b) versus 26 Pb/ and concordia plot for the case when the zircon rim has 1 times less U than core. (c, d) versus 26 Pb/ and concordia plot for the case when the zircon rim and core have equal U contents. (e, f) versus 26 Pb/ and concordia plot for the case when the zircon rim has 1 times more U than core. metamorphic core complex, northern Idaho. The Clearwater metamorphic complex consists of a mixture of Archean age gneisses (2.6 Ga), which contain no known components of inheritance in their zircon populations. Also present within the complex are relatively voluminous amounts of Proterozoic gneiss of tonalitic composition, which have a small range of magmatic ages of 1.86 Ga. Zircon U-Pb dating of these Proterozoic gneisses has revealed that many of these units contain 1.86 Ga zircons which possess 2.6 Ga cores, identical in age to the Archean rocks found proximally. Zircon in the Proterozoic age samples also contain Cretaceous and younger overgrowths. The complexity of the Proterozoic gneiss, which can contain multiple distinct age components in a single zircon, provides an excellent opportunity to test the utility of LASS on complex zircons samples. 4.3.1. Case Study 1: Clearwater Tonalite Gneiss [5] In order to explore the effects of mixing core and rim analyses during Lu-Hf LA-MC-ICPMS 134

.2826.2824.2822.28218.28216.28214 Rim and core have equal U contents Rim has 1X more U than core Rim has 1X less U than core.28212 5 1 15 2 Pb/ 26 Pb age 25 Figure 9. Mixing diagrams for measured Pb/ 26 Pb age and for a hypothetical zircon with core and rim of different ages. Tick marks are made at 1% mixing increments. See text for more details on end-member zircon age and Hf isotope composition. analyses of zircon, we analyzed a Proterozoic tonalite gneiss (7RAB4), which was known to contain younger rims (8 Ma) which overgrow 1.87 Ga cores [Brewer et al., 28]. Because the younger rims are only present as thin overgrowths, a full rim analysis was not possible (Figure 1a). However, in order to test the relationship between measured age and Hf isotope composition, the core and rim were intentionally mixed on number grains. The results of these analyses are shown in Figures 1b 1d, and results are given in supporting information Table S3. [51] The intentional mixing of core and rim domains, as expected, produces a wide range of Hf isotopic compositions and ages. As seen in the Wetherill concordia diagram, most of these analyses fall on a discordia line with widely varying 26 Pb/ ages (55 to 1852 Ma) but relatively consistent Pb/ 26 Pb ages (1.87 to 1.71 Ga). The Hf isotopic compositions, on the other hand, are highly heterogeneous. The initial, calculated at 1.87 Ga, ranges from.281597 to.281991 or in terms of initial epsilon Hf, is a range of e Hf.2 14.2, a very large range of 14 e Hf units. When these data are viewed in isolation, one interpretation from the U/Pb data would be that these zircons exhibit recent Pb loss with the zircons produced in one magmatic event at 1.87 Ga. In this case the Hf isotopic compositions in the zircons could be interpreted as mixing of mantle and crustal components, all mixed in a single magma at 1.87 Ga. The Pb/ 26 Pb ages in this case are, unfortunately, not very helpful as they seem to indicate a single population of zircons that formed at 1.87 Ga and experienced severe recent Pb loss. With the combination of age and Hf isotopic composition from the LASS technique for this sample, in conjunction with the CL images, we can see that the 26 Pb/ and Hf isotopic heterogeneity is due to the mixing of domains of different ages and Hf isotopic compositions and not due to Hf isotopic heterogeneity in a single magmatic population. [52] It is important to note that the relatively uniform Pb/ 26 Pb ages would preclude identification of anomalous ages in this sample using Pb/Pb ages alone. As such, it is crucial that U-Pb data are collected concurrently with the Hf data. 4.3.2. Case Study 2: Granodiorite Gneiss [53] Sample CW11 94 is a medium grained, foliated biotite-granodiorite from a small pluton within the Clearwater metamorphic core complex, northern Idaho [Guevara, 212]. This sample was dated by LA-ICPMS at Boise State University. A total of 23 laser spots from 22 grains, representing both cores and rims, yielded a weighted mean Pb/ 26 Pb age of 187 6 9 Ma. In addition, two xenocrysts were also analyzed and yielded ages of 249 6 25 and 2336 6 69 Ma. Following the LA-ICPMS U-Pb analyses, the same grains were analyzed for Lu-Hf isotopes at the University of Florida, using the method described in Foster et al. [212]. The Hf isotope data yield a large range in e Hf varying from 25.2 to 5.5, with a majority of the analyses 2 to22. While such a large range in initial Hf isotopic is not expected in a typical igneous rock hand sample, it is potentially possible to generate a large range of initial Hf isotopic composition for a single age of zircon. Possible mechanisms include incomplete magma mixing of melts with differing isotopic compositions [Shaw and Flood, 29] and inefficient homogenization of magma from which entrained zircons dissolved [Villaros et al., 212]. [54] LASS analyses of sample CW11 94 were done using the same grain mount as the previous experiment. Twenty-six analyses were done on 25 135

.2821.282.2819 a) b) 1753 Ma 1631 Ma 1412 Ma Rim has 1X less U than core.2821.282.2819.2818.2818.2817 Rim has 1X more.2817 U than core.2816 Pb loss.2816 RAB4 RAB4.2815.2815 2 4 6 8 1 12 14 16 18 2 2 4 6 8 1 12 14 16 18 2 26 Pb/ age 1693 Ma c) d) Rim and core have equal U contents 26 Pb/ 15 14 13 12 11 1 9 8 7 6 Rim has 1X more U than core Rim has 1X less U than core Pb/ 26 Pb age 16 17 18 Rim and core have equal U contents Figure 1. Results of LASS analyses of sample RAB4. (a) Representative CL images of RAB4, (b) concordia diagram, (c) versus 26 Pb/ age, and (d) versus Pb/ 26 Pb age. zircon grains. A majority of the analyses (21) yield Pb/ 26 Pb ages from 19 to 1775 Ma (Figure 11a; results are given in supporting information Table S4). Excluding four analyses yields a Pb/ 26 Pb age of 1859 6 6 (MSWD 5 1.1), within error of the age determined using LA- ICPMS U-Pb only analyses. These same 21 grains yield a homogeneous initial isotopic composition.281674 6 39 (2SD), an initial e Hf of 2.1 6 1.4 (2SD; Figure 11b). These results are consistent with crystallization of these zircons from a homogeneous magma. In addition, four analyses yield markedly older Pb/ 26 Pb ages ranging from 196 to 2626 Ma and are interpreted to represent inherited cores. Three of these analyses yield much less radiogenic Hf isotope compositions, which are increasingly less radiogenic with increasing Pb/ 26 Pb ages. This sample clearly demonstrates the ability of the LASS technique to screen for Hf isotope and age anomalies, thus allowing isolation of analyses of the zircon age population of interest. 4.4. Heterogeneous Populations or Analytical Artifacts? [55] Much has been made of heterogeneous Hf isotopic compositions in zircon populations, both in magmatic rocks [e.g., Shaw and Flood, 29; Villaros et al., 212] and in detrital zircon populations. In the case of detrital zircon populations, heterogeneity in the Hf isotopic compositions is expected, especially when diverse sources are contributing to the zircon populations. However, it is important to ensure that additional heterogeneity is not added to the population s Hf isotopic signature by analytical artifacts. In the case of magmatic zircon populations, heterogeneity between individual magmas is not common, excluding the obvious case of inherited xenocrysts or overgrowths. If heterogeneity does exist in a zircon population it has important implications for the generation and evolution of magmas (i.e., magma mixing, incorporation of crustal components), but it is important to demonstrate this heterogeneity is real and not an artifact of the analysis. [56] These artifacts have two sources, which can be monitored in two different ways. One source relates to the accuracy and precision of the Hf analysis as discussed in section 2.2.2. This is most likely due to difficulty in assessing the accuracy of the interference corrections for 176 Yb and 176 Lu on 176 Hf if only low Lu1Yb reference zircons are used to judge data quality. Heterogeneity in magmatic samples can be best assessed by analyzing 136