G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Technical Brief Volume 9, Number 3 20 March 2008 Q03017, doi: /2007gc ISSN: Click Here for Full Article Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb ages by laser ablation multicollector inductively coupled plasma mass spectrometry George E. Gehrels, Victor A. Valencia, and Joaquin Ruiz Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA (ggehrels@ .arizona.edu) [1] U-Th-Pb geochronology by laser ablation multicollector inductively coupled plasma mass spectrometry initiated during the mid to late 1990s as a reconnaissance tool, capable of generating ages of only moderate precision from relatively large volumes of zircon. New developments in instrumentation and experimental methodology, as described herein and by other researchers, now make it possible it to correct for common Pb accurately (using measured 204 Pb), to acquire geochronologic information rapidly (30 40 unknowns/h), to generate U-Pb ages with an accuracy of better than 1% for most zircon standards, and to conduct analyses on much smaller (e.g., 10 mmby6mm) volumes of material. These capabilities are driving important advances in many aspects of Earth science research. Components: 6453 words, 13 figures. Keywords: geochronology; LA-ICPMS. Index Terms: 1115 Geochronology: Radioisotope geochronology; 1194 Geochronology: Instruments and techniques; 1040 Geochemistry: Radiogenic isotope geochemistry. Received 31 August 2007; Revised 29 November 2007; Accepted 22 December 2007; Published 20 March Gehrels, G. E., V. A. Valencia, and J. Ruiz (2008), Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb ages by laser ablation multicollector inductively coupled plasma mass spectrometry, Geochem. Geophys. Geosyst., 9, Q03017, doi: /2007gc Introduction [2] U-Th-Pb geochronology is becoming an increasingly important tool in many aspects of Earth science research because technical developments have provided opportunities for improved precision and accuracy, enhanced spatial resolution, and more efficient data acquisition. Some of the most exciting advances in geochronology are being driven by laser ablation-inductively coupled plasma-mass spectrometers [Günther et al., 1997; Günther and Heinrich, 1999; Horn et al., 2000; Jackson et al., 2001; Horstwood et al., 2003; Košler and Sylvester, 2003; Woodhead et al., 2004; Simonetti et al., 2005, 2006; Chang et al., 2006; Gehrels et al., 2006; Horn and von Blanckenburg, 2007]. [3] The Arizona LaserChron Center (ALC) conducts U-Th-Pb geochronology with a multicollector inductively coupled plasma-mass spectrometer (GVI Isoprobe) that is coupled to a 193 nm Excimer laser ablation system (New Wave Instruments and Lambda Physik). These instruments have been particularly successful because they (1) can determine U-Th-Pb ages very efficiently, (2) generate ages with a precision and accuracy that is appropri- Copyright 2008 by the American Geophysical Union 1 of 13

2 ate for most geochronologic problems in Earth science, (3) provide geochronologic information with a horizontal resolution of 10 mm and a depth resolution of 1 mm, (4) provide opportunities for development of new geochronological techniques and applications, (5) are highly amenable to multiuser operation, and (6) provide an excellent tool for training researchers in the theory and practice of U-Th-Pb geochronology. This article describes the analytical methods that are used for U-Th-Pb geochronologic research at the ALC, with emphasis on technical developments that provide enhanced precision, accuracy, efficiency, and spatial resolution of U-Th-Pb ages and on the types of scientific advances that result from these developments. 2. Sample Preparation and Laser Ablation [4] Zircon is commonly used for U-Th-Pb geochronology because it is present in many crustal rocks, contains moderate concentrations of U and Th (typically tens to thousands of ppm) but very little Pb (ppb-ppt) when it crystallizes and is resistant to alteration and disturbance of the U-Th- Pb isotopic system [Harley and Kelly, 2007]. For most applications in our lab, zircon crystals are extracted from a rock sample by standard mineral separation techniques and mounted in a 1-inchdiameter epoxy plug, the surface of which is sanded down to expose the interior portions of most grains. Along with the unknowns are fragments of a standard zircon crystal that has been dated by isotope dilution thermal ionization (ID-TIMS). The standard crystals are used to constrain Pb/(U-Th) fractionation and U and Th concentrations. [5] It is also possible to analyze grains in situ in a thin section (e.g., in cases where the petrographic context is critical) or loose grains attached to tape on a glass slide (e.g., when specific grains will be removed and reanalyzed for higher precision by ID-TIMS or for fission track or (U-Th)/He thermochronologic analyses). In the case of thin sections, standards are inserted into holes drilled adjacent to the unknowns, and there is little additional variation in Pb/(U-Th) fractionation. For loose grains, there is considerably (2X) greater uncertainty in Pb/(U-Th), presumably due to nonlaminar flow of carrier gas across the sample surface and the complexity of interactions between the laser beam and an irregular crystal surface. [6] Cathodoluminescence (CL) images are acquired for most samples because they enable placement of laser pits in specific portions of crystals, and because variations in CL texture aid in interpreting the origin (e.g., igneous, metamorphic, or hydrothermal) of the zircons [Hanchar and Miller, 1993; Nasdala et al., 2003; Corfu et al., 2003]. Such images need to be used with caution in detrital zircon analyses, however, because selection/rejection of grains according to CL characteristics can yield a biased age spectra. [7] Laser ablation (LA) takes place with a beam diameter of either 35 or 25 mm for most applications, or with a beam diameter of 15 or 10 mm if finer spatial resolution is needed. With a 35 or 25 mm beam, the laser is set at a repetition rate of 8 Hz and fluence of 4 J/cm 2, which ablates at a rate of 1 mm/s and yields an average pit depth of 12 mm. This generates a signal of 100,000 cps per ppm for U in zircon. For smaller beam sizes, the ablation rate is reduced to 0.5 mm/s by reducing the laser fluence and repetition rate, and average pit depth is 6 mm. In both cases the ablated material is removed from the ablation chamber in He carrier gas (following Eggins et al. [1998] and Günther and Heinrich [1999]), mixed with Ar, and passed through the plasma of the inductively coupled plasma mass spectrometry (ICP-MS). 3. Isotopic Analysis [8] Isotopic analysis is performed with a multicollector inductively coupled plasma-mass spectrometer (GVI Isoprobe) equipped with an S option interface (Figure 1). The instrument is equipped with a collision cell operated with an argon flow rate of 0.24 ml/min to create a uniform energy distribution, and the accelerating voltage is 6 kv. Collectors include nine Faraday detectors and four low-side channeltron multipliers, all of which are moveable, as well as an axial Daly photomultiplier Collector Configurations [9] Two different collector configurations are used to accommodate the wide range of signal intensities that result from variations in U concentration, age, and rate of ablation (Figure 1). For samples analyzed with a 35 or 25 mm beam, Pb isotope measurement is challenging because crystals that are old and/or of high U concentration commonly generate 206 Pb intensities that are >1,000,000 cps, which is too high for continuous measurement with a channeltron. Conversely, crystals that are young and/or of low U concentration commonly generate 2of13

3 Geosystems G 3 gehrels et al.: technical brief /2007GC Figure 1. Schematic diagram of the GV Isoprobe used for isotope ratio measurements at the ALC. Also shown are the two collector configurations used for zircon analyses. 207 Pb intensities that are <50,000 cps, which is too low for reliable measurement with a Faraday collector equipped with a ohm resistor. We have accordingly developed a configuration in which 238 U, 232 Th, 208 Pb, and 206 Pb are measured in Faraday collectors equipped with ohm resistors, 207 Pb is measured with a Faraday collector equipped with a ohm resistor, and 204 Pb is measured with a channeltron multiplier adjusted to have a gain of 1.0 relative to the Faraday collectors ( large zircon configuration of Figure 1). [10] The improvement in our ability to measure 206 Pb/ 207 Pb ages is shown in Figure 2a, which compares the precision of 206 Pb/ 207 Pb and 207 Pb/ 235 U ages resulting from measurement of 207 Pb with ohm versus ohm resistors. Analyses are of standard zircons ranging in age from 91 to 1065 Ma, using the same operating conditions for both sets of measurements (Data Set S1 in the auxiliary material 1 ). The 207 Pb intensity (in counts per second) is shown for each sample. It is clear that the ohm resistor yields much better precision for count rates below 50,000 cps. [11] For applications where beam size and pit depth are reduced to improve spatial resolution, channeltrons are used for all Pb isotopes and Faraday collectors are used for 232 Th and 238 U ( small zircon configuration of Figure 1). By 1 Auxiliary materials are available at ftp://ftp.agu.org/apend/gc/ 2007gc reducing the laser beam diameter to 15 or 10 mm and the excavation rate to 0.5 mm/s, the intensities of the Pb peaks rarely approach 500,000 cps. As described below, this configuration yields ages that are similar in precision and accuracy to ages measured with the large zircon configuration, even though much less zircon (as low as 1 2 ng) is excavated. This configuration is used only when the improved spatial resolution is necessary, however, in an effort to prolong the lifetime of the channeltrons ( total counts). [12] Channeltron linearity over the range of 10,000 to 600,000 cps has been evaluated by measurement of 206 Pb (channeltron) / 238 U (faraday) (in solution) as a function of 206 Pb (channeltron) intensity (Figure 2b), and by comparison of the known age of zircons with the 206 Pb/ 238 U ages of zircons that have been analyzed with 206 Pb in a channeltron and 238 Uina Faraday detector (Figure 2c, data in Data Set S2). These plots show that measured ratios are accurate to within 1.5% over this intensity range, and that there is a correlation between intensity and offset. Experiments with channeltron corrections (e.g., dead time) to account for this nonlinear response are in progress Wet Versus Dry Plasma [13] As described by Günther and Heinrich [1999] and O Connor et al. [2006], signal intensity is significantly enhanced (due to more efficient energy transfer to the ablated ions), and the plasma is less affected by the arrival of ablated material 3of13

4 Figure 2. (a) Comparison of analyses conducted with measurement of 207 Pb with Faraday collectors using ohm (blue ellipses) versus ohm (red ellipses) resistors. All other aspects of acquisition were similar for the two data sets. Signal intensity (in counts per second) of 207 Pb during analysis with ohm resistors is indicated. Data are reported in Data Set S1. (b) Comparison of signal intensity versus 206 Pb/ 238 U in a solution, using a channeltron for measurement of 206 Pb and a Faraday collector for measurement of 238 U. Zero percent value corresponds to the signal intensity (254,000 cps) generated by the calibration standard during a typical analysis. (c) Comparison of known age of zircon standards (zero line) with measured 206 Pb/ 238 U ages using a channeltron for 206 Pb and a Faraday detector for 238 U (data reported in Data Set S2). Each symbol represents the age shift (expressed in %) of the weighted mean of 10 analyses of a sample. The 206 Pb/ 238 U ages are calibrated relative to a Sri Lanka zircon, as described in the text, which yielded an average 206 Pb intensity of 254,000 cps. 4of13

5 Figure 3. Ion intensities generated by laser ablation of a 564 Ma zircon with 518 ppm U and a 206 Pb/ 204 Pb of 16,000. The laser was fired for 12 s (starting at 0 s). Data from the first 3 s are ignored due to the rapidly changing signal intensities, the large spike in 204 Pb, and the delayed response of the 207 Pb collector (due to the longer time constant of the ohm resistor). Ages are calculated from data for seconds (because water dominates the plasma loading), if water is aspirated during laser ablation analysis. The impact for our system is a 2X increase in sensitivity, and elimination of the tendency for 204 Pb signal intensities to drop when ablated material is injected into the plasma. Optimal gas flow rates for this configuration are 0.36 L/min for He carrier gas, 0.20 L/min for Ar make-up carrier gas (mixed with He 60 cm upstream from the torch), 1.0 L/min Ar for intermediate gas, and 14 L/min Ar for coolant gas. Aspiration takes place with a microconcentric nebulizer with an uptake rate of 50 ml/min and an Ar flow rate of 0.34 L/min Data Acquisition [14] Data acquisition involves (1) a single 12-s integration on peaks with no laser firing to measure on-peak background intensities, (2) 12 s of laser ablation during which intensities are integrated once per second, and (3) 30 s with no laser firing to allow all sample material to purge through the system and to prepare for the next analysis. This yields a throughput of unknown analyses per hour. [15] Ion intensities achieved during laser ablation of a typical zircon (564 Ma, 518 ppm U, 206/204 = 16,000) are shown on Figure 3. Important values and patterns are as follows: [16] 1. The 204 intensity has a large spike during the first 0.2 s due to the presence of common Pb on the surface of the sample mount. [17] 2. Background 204 intensity is 310 cps. Most of this 204 is Hg, as indicated by a background 202/204 ratio that is indistinguishable from natural Hg and by a low 206/204 ratio. Reducing this background 204 Hg is one of our constant challenges. Useful strategies include using research grade (99.999% purity) He carrier gas, replacement of the Au hexapole rods with Al rods, using Al (rather than Ni) cones, avoiding analysis of mounts that have been coated with gold, and insertion of an Hg trap (made from gold-coated quartz beads, available from Brooks-Rand Corporation, into the He carrier gas line. [18] 3. Peak 204 intensity is 620 cps, which is typical for an average zircon crystal. Given that 202 Hg does not increase in intensity during ablation, this 204 must be Pb. [19] 4. The 207 Pb intensity has a slower response than the other signals. [20] 5. U, Th, and Pb decrease in intensity during most of the analysis but at different rates. These trends result from increasing degrees of interaction between the ablated material and the sample surface within the pit as pit depth increases [Günther and Hattendorf, 2001; Košler and Sylvester, 2003]. [21] 6. All intensities return to approximately background values within several seconds after the laser ceases firing. 4. Data Processing [22] All aspects of data reduction are conducted off-line with an Excel spreadsheet ( agecalc ) equipped with VBA macros. This system is fully automated to import data from Isoprobe files, perform all necessary corrections, and calculate ages, uncertainties, and error correlations. Following extraction from a set of Isoprobe files, only three corrections are applied prior to age calculation Depth-Related Fractionation [23] Because the first few seconds of acquisition have rapid changes in intensity, delayed response 5of13

6 Figure 4. Measured 206 Pb/ 204 Pb from analysis by LA-ICPMS at the ALC (during five different sessions, utilizing all three different collector configurations) and from analysis by ID-TIMS. Data and explanations are provided in Data Set S3. The general correspondence of values indicates that all three of our collector configurations yield robust 206 Pb/ 204 Pb measurements. in the 207 Pb signal, and a large spike in 204 Pb, the first 3 s of data are not used in calculating ages. The remaining 9 s of data are extracted from Isoprobe files as nine 1 s integrations, and isotope ratios are calculated from these integrated intensities. Because little time-dependent fractionation is apparent in 206 Pb/ 207 Pb, 206 Pb/ 204 Pb, and 208 Pb/ 204 Pb, the values returned for these ratios are simple averages and standard deviations. Depth-dependent changes in 206 Pb/ 238 U and 208 Pb/ 232 Th are accounted for by least squares projection back to the initial ratio (fourth second of acquisition), and the uncertainty of this value is calculated as the standard deviation of this initial intercept Common Pb Correction [24] The analytical procedures outlined above have been developed in order to generate reliable 204 Pb measurements because accurate common Pb correction is essential for robust (U-Th)/Pb geochronology [Mattinson, 1987]. For example, if a 206 Pb/ 238 U age is calculated without a common Pb correction, the age will be off by 0.2% if the true 206 Pb/ 204 Pb is 10,000, 0.4% for a 206 Pb/ 204 Pb of 5000, and as much as 1.2% for a 206 Pb/ 204 Pb of The accuracy of our measurements is shown on Figure 4, which plots the measured 206 Pb/ 204 Pb from our laboratory against the 206 Pb/ 204 Pb determined by ID-TIMS on zircons (and SRM 610 glass) from the same samples. The data for these analyses are presented in Data Set S3. [25] Because the composition of common Pb in a zircon crystal is commonly unknown, e.g., for detrital minerals, the common Pb composition is interpreted from Stacey and Kramers [1975] and conservative uncertainties of 1.0 for 206 Pb/ 204 Pb, 0.3 for 207 Pb/ 204 Pb, and 2.0 for 208 Pb/ 204 Pb (2-sigma) Figure 5. ID-TIMS data for the Sri Lanka zircon crystal that is used to correct for elemental and isotopic fractionation at the ALC. All uncertainties are at 2-sigma. Analytical techniques are described by Gehrels [2000], and the data are reported in Table S1. 6of13

7 Figure 6. Fractionation factors for 206 Pb/ 238 U and 206 Pb/ 207 Pb using the Sri Lanka zircon standard described above. See text for explanation. are assigned [Mattinson, 1987]. These uncertainties are propagated through all age calculations Fractionation Correction [26] Because fractionation of U, Th, and Pb occurs during laser ablation, as summarized by Günther and Hattendorf [2001] and Košler and Sylvester [2003], measured isotopic ratios for unknowns are corrected by comparison with matrix-matched standards that are analyzed once between every three to five unknowns. The primary standard used for zircon analyses is a Sri Lanka zircon crystal that yields an ID-TIMS age of ± 3.2 Ma (2-sigma). The ID-TIMS analyses are shown in Figure 5 and reported in Table S1. [27] Figure 6 is a plot showing 206 Pb/ 238 U and 206 Pb/ 207 Pb fractionation factors for a session involving analysis of 200 unknowns in which standards were analyzed once between every four unknowns. Each blue diamond is a standard (plotted as the known value divided by the measured value), the thick red line is the sliding window average of the closest 8 standards, the thin red lines show the standard error for this set of standards, and the vertical gray lines show the magnitude of a ±2% error about the average. Each unknown is accordingly adjusted for the closest 8 unknowns using a sliding window average. The total fractionation, transient variation in fractionation during a session, and scatter shown on these plots is typical for a zircon analysis with a beam diameter of 35 mm using mixed and ohm resistors U and Th Concentrations [28] U and Th concentrations are determined as a means of understanding discordance patterns (e.g., high U zircons are more susceptible to Pb loss) and because U/Th is a useful indicator of whether metamorphic fluids were present during zircon crystallization [Williams, 2001; Rubatto, 2002; Rubatto et al., 2001; Hoskin and Schaltegger, 2003; Harley et al., 2007]. The concentration of U and Th in unknowns is determined by comparison with the Sri Lanka zircon standard, which has an average U concentration of 518 ppm and Th concentration of 68 ppm (Table S1). U and Th concentration is determined by calculating the average intensity/concentration of 238 U and 232 Th for the standard analyses in a session, and then adjusting unknowns by this factor according to their measured 238 U and 232 Th intensities. U and Th concentrations are also calculated by comparison with chips of SRM 610 trace element glass, which are included on most mounts. In this case, the measured intensity of U and Th in the glass is compared with the known concentrations of 461 and 457 ppm (respectively), and this factor is then applied to the unknowns. In most cases, the two methods yield similar U and Th concentrations. The accuracy of our determinations of U concentration and U/Th is better than 20% based on analyses of zircon standards that have been analyzed in our laboratory and by ID-TIMS (Figure 7 and Data Set S4). 5. Calculation of Ages and Uncertainties [29] Ages are calculated from the isotope ratios following correction for collector gains, on-peak backgrounds, depth-related fractionation, common Pb, and elemental/isotopic fractionation. Uncertainties are propagated as either measurement errors or systematic errors. 7of13

8 Figure 7. Plots comparing the U concentration and U/ Th of zircon standards determined in our laboratory and by ID-TIMS. Gray shaded region shows an error of 20% from perfect correspondence. LA-ICPMS and available ID-TIMS data are presented in Data Set S4. [30] Measurement (or internal or random) errors arise from measurements that pertain to only a single analysis: these include 206 Pb/ 238 U and 206 Pb/ 204 Pb for 206 Pb/ 238 U ages, 206 Pb/ 207 Pb and 206 Pb/ 204 Pb for 206 Pb/ 207 Pb ages, and all three for 207 Pb/ 235 U ages. The 206 Pb/ 238 U- 207 Pb/ 235 U error correlation is calculated following Ludwig [1980, 2003]. [31] Systematic (or external) errors include four contributions, as follows: (1) uncertainties in decay constants for 238 U and 235 U, which are 0.16% and 0.21%, respectively [Jaffey et al., 1971] (including a factor of 1.5X to account for systematic errors in the original Jaffey et al. measurements [Mattinson, 1987]), (2) uncertainty in the age of the standard used for fractionation correction, (3) average uncertainty of the fractionation correction (sliding window standard error shown on plots of Figure 6), and (4) average uncertainty that arises from the composition of common Pb (described above). For most analyses, these systematic errors are 1% (2-sigma) for both 206 Pb/ 238 U and 206 Pb/ 207 Pb ages. [32] Ages are reported on the basis of 206 Pb/ 238 U for ages that are less than 1.2 Ga and on the basis of 206 Pb/ 207 Pb for ages that are older than 1.2 Ga. This is due primarily to the fact that 206 Pb/ 238 U ages are more precise for younger systems whereas 206 Pb/ 207 Pb ages or more precise for older systems (Figure 8). A second important factor is that 206 Pb/ 207 Pb ages are less sensitive to Pb loss, which is more common in older systems. Our strategy for determining which age to use, for example in a detrital study, is to determine a cutoff near 1.2 Ga that does not artificially divide a cluster of analyses. [33] For analyses of grains that are interpreted to be cogenetic (e.g., from an igneous rock), the weighted mean of a set of 206 Pb/ 238 Uor 206 Pb/ 207 Pb ages is calculated using Ludwig [2003]. For most samples Figure 8. Plot of Pb/ 238 U and 206 Pb/ 207 Pb ages selected at random from samples analyzed during spring Uncertainties are shown at 1-sigma in both Ma and percent and include only measurement (internal) errors. Solid blue line is a least squares regression of the 206 Pb/ 238 U ages. Solid red line is a power law fit of the 206 Pb/ 207 Pb ages. 8of13

9 Figure 9. Plot of 792 measurements of standard zircon conducted during four separate sessions in fall 2006 and spring 2007 (analyzed by D. Kimbrough and M. Grove, written communication, 2007). These analyses are used as a secondary standard to assess reproducibility and precision. [34] The precision of our U-Pb age determinations is shown on Figure 8. Figure 8 indicates that most 206 Pb/ 238 U ages and >1 Ga 206 Pb/ 207 Pb ages have an uncertainty of 0.5 2% (at 1-sigma level), whereas <1.0 Ga 206 Pb/ 207 Pb ages have considerably greater uncertainty. The greater uncertainty for young 206 Pb/ 207 Pb ages is due to the relative insensitivity of 206 Pb/ 207 Pb for young systems, as well as the difficulty of measuring small 207 Pb signals. [35] Secondary zircon standards are commonly analyzed in an effort to ensure accuracy and to evaluate reproducibility. As an example of the use of secondary standards, Figure 9 shows 792 analyses of standard zircon (136.6 Ma) that were conducted by M. Grove and D. Kimbrough (written communication, 2007) during four different sessions in fall 2006 and spring This analysis shows that the measurement techniques described above are reproducible within and between sessions. [36] The accuracy of our methods is determined by analyses of zircons that are well characterized by ID-TIMS (Figure 10 and Data Set S5). These standards have been analyzed during five separate sessions, with 10 analyses of each sample during each session, and no analyses discarded. Three sets of analyses were conducted utilizing Faraday collectors for 206 Pb, 207 Pb, and 208 Pb, one data set with mm pits and two sets with mm pits. Two sets of analyses were conducted with all Pb isotopes measured with channeltrons and 15 6 mm pits. Plotted are averages and standard deviations (at 2-sigma, including random and the MSWD of the weighted mean is 1.0 and the uncertainty ranges from 0.5% to 2% (2-sigma) depending primarily on age and U concentration. Systematic errors are not included in the uncertainty assigned to each analysis because uncertainties arising from decay constants, age of the standard, common Pb composition, and elemental/isotopic fractionation do not decrease as the number of analyses increases. Rather, systematic errors are propagated separately and added quadratically to the uncertainty of the weighted mean. Addition of the systematic errors yields a final age uncertainty of 1 2% (2-sigma) for most analyses. 6. Reproducibility, Precision, and Accuracy Figure 10. Comparison of LA-ICPMS ages with ID- TIMS ages for well-characterized zircons that range in age from 28 to 1434 Ma (data in Data Set S5). Each square is the weighted mean of a set of 10 LA-ICPMS measurements, and error bars show the standard deviation (expressed at 2-sigma) of the weighted mean. No analyses were rejected from any of the sessions. All ages shown are 206 Pb/ 238 U ages. Analyses were conducted during five different sessions between November 2005 and March The average age offset for all analyses is 0.15% and all means are within 2% of the ID-TIMS ages. 9of13

10 Figure 11. Relative age probability plot of detrital zircon grains from Lesser Himalayan strata, Greater Himalayan strata, and Tethyan strata in the Nepal Himalaya [from Gehrels et al., 2003]. The Greater Himalayan strata are structurally juxtaposed over rocks of the Lesser Himalaya along the Main Central Thrust. Differences in detrital zircon age spectra of Lesser Himalayan strata and Greater Himalayan/Tethyan strata suggest that the Main Central Thrust is a fundamental crustal boundary, separating Lesser Himalayan strata that accumulated on the Indian craton from a Greater Himalayan/Tethyan terrane that originated in the paleo-tethys ocean basin. systematic errors) of 206 Pb/ 238 U ages for 10 analyses from each sample. The average precision of all analyses is 2.1% (1-sigma). [37] This analysis shows that a set of 10 analyses yields an average age that is within 2% of the known (ID-TIMS) age for all standards, that most samples are within 1%, and that the average ages for R33 and Temora (perhaps the best behaved of the various standards) are accurate to within 0.3%. The average offset of all analyses is 0.15%, which indicates that there is no significant bias in our analytical methods. [38] Of interest are the systematic shifts of the LA- ICPMS ages of some samples relative to the ID- TIMS ages (Figure 10). Compositional analyses indicate that these shifts may be related to trace element concentrations (especially Nd), as suggested by Black et al. [2004], although physical aspects of the zircons (e.g., density of inclusions or fractures) may also be important. More detailed analyses of the chemical and physical nature of zircons, and perhaps treatment by chemical abrasion and/or high-temperature annealing, may yield improvements in the precision and accuracy of U-Pb ages by LA-ICPMS. 7. Spatial Resolution [39] The spatial resolution of laser ablation (beam size down to 8 mm with our system) enables U-Pb ages to be determined with a horizontal resolution of 10 mm. The vertical resolution is on the order of 4 6 mm for an entire analysis (when analyses are conducted with a reduced ablation rate and Pb isotopes are measured with channeltrons), although each 1-s integration within an analysis yields age information with a spatial resolution of 0.5 to 1 mm. This is still considerably larger than analysis by SIMS, where a vertical resolution of less than 0.1 mm is readily achievable [e.g., Breeding et al., 2004]. 8. Applications 8.1. Detrital Zircon Provenance Studies [40] Most of the geochronologic analyses conducted at the ALC are on detrital zircon grains, as this application takes maximum advantage of the high efficiency of laser ablation-icpms techniques. As described by Gehrels et al. [2006], 100 unknowns are analyzed per sample in an effort to recognize all of the major age components present, in approximately their original proportions. Ages are portrayed on a relative age probability plot, and important age peaks are recognized as containing at least three overlapping analyses. Programs for plotting, analyzing, and statistically comparing age spectra are available at the ALC web site ( [41] U-Th-Pb geochronologic analyses conducted at the ALC are contributing to the rapid advances in detrital zircon provenance research given that 40,000 analyses of detrital zircon grains are conducted each year, with samples gathered from many different regions of the world. An example of a detrital zircon data set that has important tectonic implications is shown in Figure 11. These data demonstrate that Greater Himalayan and Tethyan 10 of 13

11 Figure 12. Plot of U/Pb age and U/Th from zircons extracted from granitic bodies of the Coast Mountains batholith in coastal British Columbia (from G. E. Gehrels, unpublished data, 2007). This plot shows the power of assembling a large database to reconstruct the magmatic history of a region and the utility of using U/Th from zircons to recognize periods of metamorphism. strata in the Himalayan mountain system bear little resemblance to strata of the Lesser Himalaya, which requires large-scale tectonic transport of the Greater Himalaya over the Lesser Himalaya along the Main Central Thrust [Gehrels et al., 2003] Igneous History [42] Igneous samples are first imaged with CL to determine whether there is evidence for inherited cores and/or younger overgrowths. A beam size is then selected that allows for analysis of homogeneous domains. Analyses are conducted until there are at least 20 measurements for each domain and weighted mean plots are prepared for each. The petrogenesis and age significance of each domain is then determined from the CL images and from examination of plots of age versus U concentration (for evidence of Pb loss) and age versus U/Th (for evidence of metamorphic fluids during zircon growth). [43] U-Th-Pb geochronologic research at the ALC is also contributing to understanding the history and tectonic significance of magmatism in orogenic belts around the world through analysis of 10,000 igneous zircon grains per year. An example of an igneous data set is shown in Figure 12, which presents U-Th-Pb zircon analyses from 63 different granitic bodies in the Coast Mountains batholith of British Columbia [Gehrels et al., 2007]. The ages help define the main phases of magmatism in this segment of the batholith, and U-Th values indicate that two of these phases were associated with largescale generation of metamorphic fluids. Figure 13. Age and U/Th map of a zoned zircon grain from the Coast Mountains batholith in coastal British Columbia (G. E. Gehrels, unpublished data, 2007). Each of the 84 analyses was conducted with a beam diameter of 10 mm and a pit depth of 4 mm. Such maps, together with CL images, provide a powerful tool for understanding the petrogenesis of zircons that have experienced multiple phases of growth. 11 of 13

12 8.3. Age Mapping [44] The spatial resolution and sample-throughput efficiency of LA-ICPMS make it possible to investigate complex zircon crystals by generating age and U/Th maps. Figure 13 shows a zircon crystal in which two distinct phases of zircon growth are clearly visible in a CL image (from G. E. Gehrels, unpublished data, 2007). Eighty-four analyses were conducted on this crystal with a laser beam diameter of 10 mm and a pit depth of 4 mm. The ages clearly demonstrate that there are two phases of zircon growth, one at 92.6 ± 1.3 Ma and a younger phase at 58.3 ± 1.1 Ma. The high U/Th values (average 20.1) in outer portions demonstrate that zircon growth at 58.3 Ma was accompanied by metamorphism. 9. Conclusions [45] Analysis of zircons by LA-MC-ICPMS at the ALC yields individual ages with a precision and reproducibility of 1 2% and sets of ages that in most cases are accurate to better than 1%. Given the high efficiency of the described methodology, with a throughput of analyses per hour, combined with the fine spatial resolution of laser ablation, U-Pb geochronology by LA-MC-ICPMS is poised to have a major impact on the generation and application of U-Pb geochronology in the Earth sciences. [46] It is also apparent from the recent development of new instrumentation and new measurement strategies, as described herein and by Horn et al. [2000], Jackson et al. [2001], Horstwood et al. [2003], Košler and Sylvester [2003], Woodhead et al. [2004], Simonetti et al. [2005, 2006], Chang et al. [2006], Gehrels et al. [2006], and Horn and von Blanckenburg [2007] that there are many opportunities to improve the precision, accuracy, efficiency, and spatial resolution of laser ablation-icpms geochronology. Acknowledgments [47] The ALC is supported with funds from the National Science Foundation for acquisition of our LA-ICPMS (EAR ) and for facility support (EAR ). 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