Single Zircon Evaporation Thermal Ionisation Mass Spectrometry: Method and Procedures† U. S. Kl�otzli Laboratory for Geochronology, Department of Geology, University of Vienna, Geozentrum, Althanstrasse-14, A-1090 Vienna, Austria Zircon evaporation thermal ionisation mass spectrometry (TIMS) is used in geochronology to determine absolute 207Pb*/206Pb* ages and Th/U ratios of single zircon crystals. The process involves the breakdown of zircon (ZrSiO4) to porous baddeleyite (ZrO2) along a reaction front which progresses into the interior of the crystal.Evaporation of high quality zircons thus allows us to distinguish between crystal rim (overgrowth) and core, providing precise information about the time of magmatic crystal growth, partial dissolution, and/or metamorphic overgrowth. Derived Th/U ratios complement age data interpretation and provide valuable petrogenetic implications. A double Re-filament ion source is used. The zircon is encased in the evaporation filament and heated step-wise to 1200–1300 °C to strip off unsupported common and radiogenic Pb components.After cleaning, evaporation proceeds in temperature steps of ~ 20 °C. The evaporate (SiO2, Pb, REEs, and U from the zircon and Re from the evaporation filament) of each step is deposited for 45 min on the cold ionisation filament and subsequently analysed. Lead isotopic composition is determined using a dynamic secondary electron multiplier ion counter or static Faraday cup data acquisition schemes.Lead ratios are corrected for fractionation using correction factors derived from standard measurements of a 1 ng NBS SRM 982 sample. The precision on 207Pb/206Pb ratios is < 1%. Only high temperature steps ( > 1300 °C) with 206Pb/204Pb > 5000 are used for age calculations. The ages reported (single temperature step, multi-temperature step means) are weighted means calculated from at least 20 measured 207Pb*/206Pb* ratios with 2 standard errors of the mean.Precision of ages is strongly dependent on age range and varies between 0.1 and 10%. Keywords: Thermal ionisation mass spectrometry; zircon evaporation; geochronology; lead–lead dating; absolute dating The uranium–thorium–lead and lead–lead dating techniques for zircon and other U-bearing mineral phases (i.e., monazite, xenotime, allanite, sphene) are standard geochronological methods used in laboratories throughout the world. In particular, zircon dating has proven to be a very powerful tool, due to the large resistance of zircon against thermal and mechanical alteration, in establishing absolute geochronological information about magma generation and intrusion events and/or periods of high temperature metamorphic overprinting.Other commonly used geochronometers normally do not allow the geoanalyst to directly date such high temperature events. The U–Th–Pb and Pb–Pb dating techniques are based on the radioactive decay of 238U to 206Pb*, 235U to 207Pb*, and 232Th to 208Pb*, respectively (for fundamentals see Hunziker and J�ager,1 Faure,2 and Geyh and Schleicher3). Conventional U–Th–Pb dating principally comprises the chemical dissolution of the mineral under investigation, the purification and determination of the concentrations of U, Th, Pb (using isotope dilution techniques), and the determination of the isotopic composition of Pb.Therefore, conventional U–Th–Pb dating requires an ultra-clean laboratory environment, sophisticated isotope dilution thermal ionisation mass spectrometry (TIMS), complicated fractionation corrections, and error evaluation.Additionally, the different age information present in different domains of one zircon crystal is often lost due to the ‘integration’ effect of crystal dissolution, thus providing less age information than actually present. These restrictions can be overcome by applying either even more sophisticated analytical methods [i.e., partial dissolution experiments (Mattinson4), mechanical separation of rim and core (Steiger et al.5), vapour digestion combined with cathodoluminescence investigations (Wendt and Todt,6 Poller et al.7) or spot analysis using ion microprobes (Compston et al.,8 Wiedenbeck and Goswami9) and laser ablation mass spectrometric techniques (e.g., Jackson et al.,10 Hirata and Nesbitt11)].This paper presents an alternative zircon dating method: single zircon evaporation TIMS comprises the direct determination of 207Pb*/206Pb* ages (so called lead–lead ages).This method circumvents the need for the determination of elemental concentrations of the conventional U–Th–Pb dating techniques. Thus, the laborious and complicated work involved in the ultraclean laboratory and the isotope dilution TIMS can be completely avoided. Direct evaporation TIMS of finely ground zircon for Pb isotope analysis was first suggested by Kosztolanyi12 as early as 1965 and was later applied by Sunin and Malyshev13 to zircons from various rocks.But these early applications were severely hampered by analytical difficulties (for discussion see Kober.14) Gentry et al.15 and Kober14 suggested a mounting procedure for unground zircon crystals in a single-filament ion-source. To further enhance beam stability and duration Kober14 used a double-filament ion-source arrangement and a stepwise heating procedure resulting in a major improvement in the precision of the resulting evaporation 207Pb*/206Pb* ages.The method described in this contribution is partly based on the method of Kober,14,16 but advances analytical precision, accuracy and the possibilities of age data interpretation. Geochronological Background A fundamental prerequisite of absolute age determination is the assumption of a closed system behaviour of mother and daughter elements and the absence of any internal isotope fractionation since the time of closure with respect to diffusion of the elements under consideration.Thus, if one assumes that no spatial fractionation between U, Th, and Pb has taken place in the crystal lattice since the time of closure the 207Pb*/206Pb* ratio can directly be recalculated to an apparent lead–lead age: † Presented at Geoanalysis 97: 3rd International Conference on the Analysis of Geological and Environmental Materials, Vail, CO, USA, June 1–5, 1997. Analyst, November 1997, Vol. 122 (1239–1248) 1239206 235 207 206 235 238 1 1 1 1 235 238 Pb = U (e and Pb U ( e Pb Pb U e U e 238 207 238 235 * ) * ) * * ( ) ( ) l l l l t t t t - = - ® = - - Assuming that the U isotopic composition is constant (238U/ 235U = 137.88) and that the decay constants of 238U and 235U are accurately and precisely known (Steiger and J�ager17) the transcendental equation can iteratively be solved for t, providing an age-estimate for the isotopic system since its closure.Additionally the 208Pb*/206Pb* ratio is directly proportional to the Th/U ratio, an important mineralo- and petrogenetic indicator which is helpful for the interpretation of single zircon age spectra (Kl�otzli18).Process of Zircon Evaporation The process of evaporation involves the breakdown of the accessory silicate zircon (ZrSiO4, normally < 200 mm in size) to the porous oxide baddeleyite (ZrO2) and the associated loss of mainly Si, SiO2, Pb, REEs, U and Th along a reaction front which progresses into the interior of the grain (Ansdell and Kyser,19 Roddick and Chapman,20 Roddick21 and Kl�otzli18). Continuous evaporation or evaporation with stepwise increasing evaporation temperatures ideally results in a ‘depth’ profile through the zircon crystal from the outermost rim to the innermost core (Kober,14 Kl�otzli,18 Kl�otzli and Parrish22).The stepwise evaporation on a ‘cold’ ionisation filament, as described below, makes use of the silica-gel effect (routinely used in conventional Pb TIMS, Cameron et al.23) and the accumulation of Pb on the ionisation filament resulting in better ionisation efficiency and ion beam stability (Kober14; Kl�otzli18).Mass Spectrometer Parameters and Maintenance A double Re-filament arrangement in a conventional extendedgeometry thermal ionisation mass spectrometer (Finnigan MAT 262, Bremen, Germany) equipped with an 120 MHz ion-counting secondary electron multiplier (SEM-IC) is used.Faraday System For Pb isotopic analysis the Faraday system is used in a conventional way and is not described in detail. SEM-IC System The SEM-IC system is heavily used in zircon evaporation TIMS (90% of all analyses) and therefore its operation and performance is described in more detail. Additionally, the use of dynamic data acquisition and the chronic instability of SEM systems requires precise monitoring of the SEM-IC system parameters. The peak shape and the deflection voltages of the ion beam in the SEM are controlled routinely before and after a zircon evaporation session using a ~ 8 mV Pb+ or Re+ ion beam.The optimum HV for the counting efficiency of the SEM is adjusted once a week using a Finnigan-MAT automatic calibration routine. It is set ~100 V higher than the beginning of the SEM saturation plateau. Since July 1991 the optimum HV has increased from 1950 to 2950 V (at present), demonstrating the effect of ageing of the SEM through heavy usage.Count gains are in the range of 90–95% relative to the axial Faraday cup. The precise count gain relative to the Faraday system is not determined because no mixed acquisition schemes are used in zircon evaporation TIMS. This is due to nonsufficient intensity overlap between the Faraday and SEM which leads to inconsistent and temporarily strongly varying count gains. Proper focusing has to be checked more frequently than when using the Faraday system. The focus point of the very small evaporate deposit (compared with conventionally loaded samples) tends to move during evaporation due to thermal relaxation and/or stress release of the evaporation filament.Additionally, the gain across the conversion dynode of the SEM is not constant leading to fluctuations in the ion beam intensity if the ion beam moves around. Dead time correction for the counter is accomplished on-line using Finnigan MAT software routines. The applied correction is correct up to ca. 500 000 counts s21, about half of the maximum allowable beam intensity. At higher beam intensities counting problems arise with the more abundant Pb masses 206 and 208 leading to noticeably to high 207Pb/206Pb and to low 208Pb/206Pb. Because dead time effects are mass and intensity dependent, an appropriate dead time correction for higher ion beam intensities would be rather complicated to accomplish and would incorporate additional uncertainty to the analysis. To completely exclude any dead time effects of the SEM-IC system ion beam intensities above 500 000 counts s21 are avoided, both for standard and zircon evaporation analyses.Experimental Procedures Zircon Preparation The zircons used for evaporation analyses are washed in warm 3 m HNO3 for half an hour, rinsed with distilled water and dried. Criteria for choosing zircon crystals for analysis are: absence of cracks, micro-fissures and inclusions, no visible turbidity, suitable size (30 to 250 mm along c-axis, elongation maximum 1 : 10), and colour.In special cases (e.g., direct analysis of an inherited core) abrasion techniques are employed (Krogh;24 Kl�otzli25). Filament Preparation Rhenium filaments used for encasing zircons are preheated and cleaned at 4.5 A for 4–8 h. This long outgassing duration softens the filaments substantially, thus making it easier to bend the filament and to encase the zircon. Softened filaments are first pre-bent using a modified jig of Finnigan MAT and then formed by tweezers for zircon encasing. Ionisation filaments used for the analysis of the evaporated deposit are outgassed at 4.5 A for 45 min.Zircon Evaporation Zircon cleaning procedures Zircon evaporation analyses follows modified procedures originally described by Kober14 and modified by Kl�otzli.25,26 Fig. 1 gives a schematic representation of the evaporation procedures. Encased zircons are stepwise heated to 1200–1300 °C in order to strip off unsupported common and radiogenic Pb components with low activation energies.Such Pb is weakly bound to metamict zircon domains, cracks, micro-cleavages, and to Pb bearing inclusions within the crystal. Especially in metamict crystal domains activation energies for Pb are very low (0.1–0.4 eV, Tilton27) and Pb is thus readily mobilised. Non-metamict domains are much more retentive with Pb activation energies between 2.2–2.5 eV (Shestakov28). Pure common Pb is characterised by the presence of 204Pb and low 206Pb/204Pb (see below). 1240 Analyst, November 1997, Vol. 122Evaporation-filament with zircon Ionisation-filament with evaporated deposit zircon evaporation and deposition on 'cold' ionisation-filament filament current/mA vs. temperature/°C Pb isotopic analysis and filament cleaning procedure 4500/1800 3000/1450 2200/1300 2000/1200 1500/950 1200/900 Ionisation-filament cleaning cycle Zircon cleaning cycle 1st evaporation/ analysis cycle 2nd evaporation/ analysis cycle 3rd evaporation/ analysis cycle 4th evaporation/ analysis cycle Check for zircon presence and Hf isotopic analysis time T4 T3 T2 T1 The progress of cleaning is always monitored with the EM-IC by mass scans from 203 to 209 (1 s integration time with 0.1 u steps).Additionally to common Pb, masses 204 (94Zr2 16O, 204Hg), 206 (94Zr2 18O), 207 (206Pb1H, organic material), 208 (207Pb1H, 96Zr2 16O, 176Hf16O2) can be occupied by isobaric molecules.The mass resolution of the MAT 262 is not sufficient to resolve these potential isobars. Therefore, before data acquisition can start, the absence of any isobars on the Pb isotope masses has to be checked. Masses 196 (90Zr2 16O), 202 (202Hg, 138BaP16O17O), 203 (203Tl), 205 (205Tl, 138BaP18O2, organic material) and 209 (208Pb1H, 209Bi, 177Hf16O2) are monitored to recognise the isobaric overlaps. In particular the isobar on mass 207 is critical because no constant ratio with any other non-Pb mass has been recognised which would allow an appropriate correction. Empirically the presence of 207 is always accompanied by 205 (with 207/205 < 15).Thus it is assumed that no isobaric 207 is present as soon as no 205 is present any more. Isobars of Zr2O and HfO2 normally do not pose any problems because the ionisation energy of the compound is far higher than the energies needed for Pb ionisation. Problems with isobars of BaPO2, Tl and Hg can arise when Clerici-solution (TlHCO2–Tl2C3H2O4) or Hg bearing heavy liquids (Thoulet-, Rohrbach-solution) were used for heavy mineral separation.PbH is sometimes present at very low temperatures ( < 850 °C) but disappears above 900 °C. Once no ‘low-temperature’ Pb or isobaric overlaps are present (0 min–2 h) the zircon temperature is raised by approximately 20 °C and, depending on ion beam intensities and experiment design, the isotopic composition of the evaporated Pb is either measured directly or the evaporate is deposited on the cold ionisation filament.Evaporation steps of 20 °C have been found to be a good compromise between spatial/temporal resolution (preferably small temperature steps) and sufficient ion beam intensity (preferably large temperature steps). The material evaporated from the evaporation filament consists mainly of a mixture of material from the zircon (70%) and Re from the filament (30%). From the zircon SiO2, most Pb, REEs, and about 50% of U and Th are quantitatively evaporated (Roddick and Chapman;20 Kl�otzli18).In particular, the amount of SiO2 evaporated depends strongly on crystal quality and to a somewhat lesser degree on evaporation temperature (Kl�otzli- Chowanetz et al.29). The amount of SiO2 deposited on the ionisation filament has a major influence on the quality of the subsequent Pb isotopic analysis. If only minor amounts of SiO2 are evaporated the silica-gel effect cannot function properly resulting in unstable ion beams and thus low analytical precision, although suitable amounts of Pb are present on the ionisation filament.This silica-gel effect sometimes jeopardises the analysis of highest quality zircons and leads to the situation that slightly metamics can be more easily analysed than completely non-metamict zircons, and that the absolute amount of Pb present in a crystal is not necessarily the dominant factor for the quality of a single zircon evaporation analysis. Total amounts of lead available are in the range of 1 pg to 1 ng for normal zircon crystals resulting in the analysis of sub-pg to subng amounts of Pb per evaporation step.The Pb+ ion yield is in the range of 1 3 1023 equivalent to the efficiency of the conventional silica-gel technique (Cameron et al.23). Evaporation procedures After a deposition step (15–45 min) Pb is analysed using either static Faraday or dynamic SEM-IC data acquisition procedures.During analyses, the evaporation filament is set to 1.2 A in order to prevent Pb being evaporated from the ionisation filament to be re-deposited on the evaporation filament (Ansdell and Kyser;30 Kl�otzli and Parrish22). After the Pb analyses the ionisation filament is raised to 4.5 A for some seconds, stripping off all remaining material deposited during the evaporation step. Then the next evaporation–analysis cycle can start. Evaporation temperatures are raised by approximately 20 °C from step to step until Pb evaporation from the zircon is complete.Depending on crystal quality, size, age and U and Pb content of the zircons, 2 to 8 evaporation–analysis cycles can be made. To check whether the Pb evaporation really is complete Fig. 1 Schematic representation of a single zircon evaporation analysis. T1–T4 symbolise increasing evaporation temperatures (+20 °C per evaporation step). See text for discussion. Analyst, November 1997, Vol. 122 1241or whether the zircon has just fallen off the evaporation filament, the evaporation filament is slowly raised to 3.5–3.8 A.The ionisation filament is set to 5 A. Presence of the zircon is controlled by monitoring the Zr+ ion beam on mass 90. Knowing whether Pb evaporation is complete or not is critical for the later interpretation of the evaporation data. High quality zircons can be heated to ~ 2000 °C. At these temperatures very stable Hf+ ion beams are attained thus making it possible to additionally determine the Hf isotopic composition of the zircon.The Hf isotopic composition gives important petrogenetic information, complementary to the Nd isotope system (e.g., crustal residence times, model ages, see Kl�otzli25 and references cited therein). Data Acquisition For compatibility reasons the acquisition schemes for the dynamic SEM-IC and static Faraday cup measurements are kept as similar as possible. In both modes and for normal acquisition ion beam intensities are measured in blocks of 10 scans with 4 s integration time and 2 s delay time each. Peak centering and intensity monitoring is done at the beginning of each block on mass 206.The background is measured on half-masses every 5 blocks with 15 s delay time and 32 s integration time. The background correction is made on-line during data acquisition. Acquisition schemes with masses analysed, integration times, and respective succession for the SEM-IC procedures are given in Table 1.For the SEM-IC analyses peaks are measured in succession of increasing mass in order to avoid problems with magnetic field stability (hysteresis). Data acquisition comprises 2 to 20 blocks, depending mostly on the durability of the ion beam. Acquisition is started or interrupted at a minimum ion beam intensity on mass 206 of 10 000 counts per s21 for the SEM-IC procedures and 5 mV for the Faraday cup procedures, respectively. Because of insufficient counting statistics, too low ion beam intensities can lead to unrealistic low 204Pb/206Pb ratios seemingly proving the absence of common Pb.Using the relatively high threshold intensities allows the recognition of 204Pb with appropriate precision. During the process of evaporation sometimes low to intermediate ion beam intensities ( < 5000 counts s21 on 206) normally exhibiting large intensity fluctuations can be registered. Special acquisition procedures with integration times of 16 s allows the analysis of these ion beams providing further information about the progress of evaporation.The same acquisition procedures are used to analyse low quality evaporation deposits from which only minor and unstable ion beams can be achieved. Age data derived from such procedures can then be compared to good age data found for other evaporation steps thus providing additional information for age data interpretation. Data Reduction and Fractionation Correction The subsequent data reduction is completely the same for both kinds of data acquisition schemes.Lead ratio calculation and statistical test with outlier elimination are performed using modified off-line software from Finnigan MAT. In Pb isotopic analysis no direct correction of machine discrimination and time dependent fractionation (internal normalisation) and other fractionation phenomena is possible. Natural lead is formed by a mixture of 4 different isotopes (204Pb, 206Pb, 207Pb, 208Pb), three of which are the stable daughter products of radioactive decay series (206Pb, 207Pb, 208Pb). So the amount of these 3 isotopes compared to 204Pb depends solely on the amount of radioactive mother isotope present and the time elapsed since the accumulation of the lead isotopes started.This then means that not one of all the possible ratios is constant and thus none can be used for the above mentioned corrections. This is the major drawback of the U–Pb and Pb–Pb dating techniques compared to other isotope systems (e.g., Rb/Sr, Sm/Nd), where the presence of at least two nonradiogenic isotopes resulting in one constant ratio allows the necessary corrections to be made.Therefore, 207Pb/206Pb and 208Pb/206Pb ratios are corrected using correction factors derived from NBS SRM 982 standard measurements with 1 ng of Pb loaded and using the conventional Si-gel technique (using the modified values given by Todt et al.31) These correction factors are determined individually for Faraday and SEM-IC procedures before and after the zircon analyses using the same data acquisition procedures as for zircon analyses. Two different sets of fractionation factors can be determined using both 208Pb/ 206Pb and 207Pb/206Pb values.The within error consistency of the two independently derived fractionation factors further proves the linearity and thus the correctness of the applied fractionation correction scheme.Compared to the overall analytical error of a single age the difference found in the 2 independently calculated fractionation factors is negligible. Because the 207Pb/206Pb ratio is the one of most interest, the correction factor derived from 207Pb/206Pb is used for the correction. Mass discrimination for Faraday procedures is about 50% of the mass discrimination of the SEM-IC procedures. 204Pb/206Pb ratios are not corrected. Table 2 gives the appropriate NBS SRM 982 data. Recommended standard values used are from Todt et al.31 Fig. 2 shows plots of the NBS SRM 982 data for the SEM-IC and the Faraday procedures. Table 3 gives a compilation of the derived correction factors, while Table 4 shows the influence of the applied correction on final 207Pb*/206Pb* ages using the SEM-IC Table 1 Peak succession and position used for zircon evaporation analysis and accompanying NBS SRM 982 standard measurements for both the SEM-IC and the Faraday collector systems Integration Detector time/delay system Mass succession/position time* Ziron cleaning SEM-IC 203–209 mass scan 1/0.1 u Common lead and isobars SEM-IC 206, 207, 208, 209, 202, 4/2 present 203, 204, 205 Common lead present SEM-IC 206, 207, 208, 204 4/2 (206Pb204Pb < 50 000) Faradays 204 = 7, 206 = 5, 207 = 4, 208 = 3 4 Radiogenic lead only SEM-IC 206, 207, 208 4/2 (206Pb204Pb > 50 000) Faradays 206 = 5, 207 = 4, 208 = 3 4 Radiogenic lead with very SEM-IC 206, 207, 208 16/2 low intensity, unstable * Measured in seconds. 1242 Analyst, November 1997, Vol. 1220.4650 0.9965 0.9970 0.9975 0.9980 0.9985 0.9990 0.9995 1.0000 1.0005 0.4655 0.4660 0.4665 0.4670 0.4675 0.4680 1 ng NBS 982 ICm mean 1 ng NBS 982 ICm NBS 982 Standard val207Pb/206Pb 207Pb/206Pb 208Pb/206Pb 208Pb/206Pb 0.460 0.462 0.464 0.466 0.468 0.470 0.472 0.474 0.476 0.478 0.480 1.015 1.013 1.011 1.009 1.007 1.005 1.003 1.001 0.999 0.997 0.995 ( a) ( b) 1 ng NBS 982 Far mean 1 ng NBS 982 Far NBS 982 Standard values system fractionation correction.In most cases, the applied correction is well within the analytical error of the raw data. Relative corrections range from ~ 10% for Tertiary age to < 0.1% for Archean ages. The absolute age shift is between 25 Ma and 23 Ma, respectively (1 Ma = 106 y). For the Faraday system corrections are about half the size and in the opposite direction. Time dependent fractionation (Rayleigh-type fractionation) cannot be accounted for using this simple correction calculation.But regarding the very small relative mass differences involved, the effect is assumed to be negligibly small and must thus not be accounted for. The calculated correction factors thus account for all biases of the SEM-IC and Faraday systems together with mass discrimination of the mass spectrometer and other fractionation effects, but not for time dependent fractionation effects. Bias characteristics of the SEM-IC compared to the Faraday cups are not independently incorporated in the data reduction because no mixed acquisition schemes (SEM-IC and Faraday cups used at the same time) are used in zircon evaporation analysis.Fractionation effects due to variation of heating procedures and ionisation temperatures can only be overcome by keeping the procedures and ionisation temperatures as similar and as Table 2 NBS SRM 982 standard values for the SEM-IC and Faraday collector systems (period 28.11.96 to 4.03.97) NBS 982 SEM-IC NBS 982 Faraday No.of analyses 25 2SE* % 25 2SE* % 208/206 measured 1.0052 ± 0.0030 0.30 0.9979 ± 0.0003 0.03 207/206 measured 0.4679 ± 0.0024 0.52 0.4665 ± 0.0002 0.04 208/204 measured 35.76 ± 0.70 1.96 36.72 ± 0.33 0.91 207/204 measured 17.62 ± 0.46 2.60 17.14 ± 0.18 1.07 206/204 measured 36.01 ± 0.70 1.95 36.51 ± 0.35 0.95 * Errors are 2 standard errors of the mean. Fig. 2 NBS SRM 982 standard measurements. Standard amount loaded for all measurements is 1 ng.(a) Plot of 25 dynamic SEM-IC collector measurements (period 02.12.96 to 04.03.97). (b) Plot of 19 static Faraday collector measurements (period 28.11.96 to 14.02.97). All error bars are 2sm. Recommended standard values of Todt et al.31 are given for comparison. Errors for recommended standard values are smaller than plotting symbol. Analyst, November 1997, Vol. 122 1243constant as possible for both standard measurements and for zircon evaporation analyses. For the applied acquisition schemes ionisation temperatures are constant within ±20 °C.After fractionation correction all relevant information from the acquired data is written as ASCII files for further evaluation. Pb Blank Assessment As no chemical treatment is used no severe problems with external Pb blank contributions are encountered in zircon evaporation analysis. But problems with Pb blank contributions from former zircon evaporation analyses could arise by the reactivation of deposits in the ion source coming especially from the first shielding plate.In order to avoid sample to sample cross contamination the shielding plate is changed and cleaned on a regular basis. Additionally an ion source blank is measured before each zircon analysis using a blank Re-single filament which is heated step-wise up to 5.5 A. The ion source can effectively be cleaned by using a defocused Re+ ion beam of 1–2 V intensity for 5 min duration. The process of cleaning can be monitored with mass 39K.Mass scans from 203 to 209 with 1 s integration time and 0.1 u steps are employed for monitoring the Pb blank. After cleaning, ion source Pb blank levels are in the range of < 10 counts s21 at 2200 °C and < 0.1 counts s21 at 1400 °C. The Pb blank of the used Re-filament material is negligibly small and thus poses no problems. Common Pb Correction The addition of common Pb to the radiogenic Pb poses one of the most significant problems in U–Th–Pb and Pb–Pb geochronology.Different methods are used to establish appropriate corrections needed to achieve geologically meaningful age Table 3 Lead mass fractionation values derived from NBS SRM 982 standard measurements (Table 2). Mass fractionation is independently calculated for 207Pb/206Pb and 208Pb/206Pb for both the SEM-IC and the Faraday collector systems. See text for discussion Mass Fraction* discrimim/ t 2SE† % nation/u 2SE† % 208/206 SEM-IC (25) 1.0050 ± 0.0030 0.301 20.00251 ± 0.0000076 0.301 207/206 SEM-IC (25) 1.0020 ± 0.0052 0.523 20.00200 ± 0.0000104 0.523 208/206 Far (19) 0.9978 ± 0.0003 0.031 0.00112 ± 0.0000004 0.031 207/206 Far (19) 0.9989 ± 0.0004 0.037 0.00107 ± 0.0000004 0.037 * m/t = measured value/true value.† Errors are 2 standard errors of the mean. Table 4 Influence of Pb isotope fractionation correction on final 207Pb*/206Pb* ages for the SEM-IC system. For comparison 3 different age ranges are shown. See text for discussion Age/ 207Pb/206Pb 2SE* % Ma 2SE* %† Tertiary age— Measured 0.04700 ± 0.00047 1.00 49.2 ± 23.9 48.5 Corrected 0.04691 ± 0.00053 1.13 44.5 ± 27.0 60.7 Age difference 24.8 10.7 Palaeozoic age— Measured 0.05800 ± 0.00058 1.00 530 ± 22 4.14 Corrected 0.05788 ± 0.00065 1.13 525 ± 25 4.71 Age difference 24.4 0.84 Archean age— Measured 0.45000 ± 0.00450 1.00 4085 ± 15 0.36 Corrected 0.44910 ± 0.00507 1.13 4082 ± 17 0.41 Age difference 23.0 0.07 Fractionation factor: 207/206 SEM-IC: 1.00200 ± 0.00524 (0.52%) * Errors are 2 standard errors of the mean.† Relative shift in corrected age. Table 5 Pb isotopic and age data for the zircon evaporation analysis of sample 2E92-B Weinsberg granite (cf. lower plot of Fig. 5). For discussion see text Evaporation 207/206 Block temperature*/ 2SE‡ age§/ 2SE†/ 2SE† No. °C 207/206† 2SE‡ % Ma Ma (%) 208/206† 2SE† Th/U¶ 2SE† Sample 2E92-B 2E92BC01 10 1398 0.05360 0.00022 0.4 354 9 2.6 0.0198 0.0049 0.060 0.015 2E92BC02 10 1443 0.05350 0.00019 0.4 350 8 2.3 0.0181 0.0047 0.055 0.014 2E92BC03 10 1463 0.05816 0.00025 0.4 536 10 1.8 0.0808 0.0051 0.243 0.015 2E92BC04 10 1482 0.05820 0.00018 0.3 537 7 1.3 0.0790 0.0053 0.238 0.016 2E92BC05 10 1504 0.05808 0.00028 0.5 533 11 2.0 0.0779 0.0054 0.235 0.016 Mean C01-C02, rim 0.05355 0.00006 0.1 352 3 0.7 Mean C03-C05, core 0.05815 0.00021 0.4 535 8 1.5 * Error on evaporation temperature is estimated to be ±10 °C.† Mean from individual scan ratios.‡ All errors reported are 2 standard errors of the mean. § Mean ages derived from individual scan ratios and not from individual scan ages. ¶ Th/U at apparent 207Pb/206Pb age. 1244 Analyst, November 1997, Vol. 122information. Fig. 3 shows the influence of the addition of common Pb to the radiogenic Pb for different age ranges. It is evident that common Pb can very dramatically change the apparent 207Pb/206Pb age. Any reasonable correction procedures must thus rely on precise knowledge of the common Pb isotopic composition, a fact very difficult to establish.Using wrong or non-precise common Pb compositions will unavoidably lead to wrong 207Pb*/206Pb* ages. If for the age calculation correct error assessment procedures are used the precision of the common Pb isotopic will have a large and negative impact on the achievable final age precision (see below). Thus the only reliable method to gain sound age data is to avoid common Pb contamination completely.In this respect, zircon evaporation analysis is superior to conventional U–Th– Pb dating. The sites of unsupported common Pb within a zircon crystal are metamict crystal domains, Pb bearing inclusion, fissures, and cracks. The common Pb can very effectively be removed from the crystal as described above. In order to minimise additionally the contribution of common Pb only high temperature steps ( > 1300 °C) with 204Pb/206Pb < 0.0002 (206Pb/204Pb > 5000) are used for age calculations.Therefore, the influence of common Pb (at least for zircons older than 500 Ma) is negligible and no common Pb correction has to be applied at all. During routine analysis 204Pb/ 206Pb is normally < 0.00001. As the 207Pb/206Pb ‘age’ of common Pb tends to be higher than the purely radiogenic 207Pb/206Pb age admixture of common Pb can be recognised by decreasing 207Pb/206Pb ages with increasing evaporation temperatures at low temperature steps.Rarely, evaporation data from zircons exhibiting a large common Pb contribution to the total Pb can be corrected for this common Pb component (Kl�otzli et al.33). Fig. 3 Influence of common Pb correction on measured 207Pb/206Pb ratios with varying 206Pb/204Pb and different age ranges. (a) Plot of the apparent 207Pb*/ 206Pb* age after common Pb correction. (b) Plot of the relative age difference between the apparent 207Pb/206Pb age and the 207Pb*/206Pb* age after common Pb correction. Dashed vertical lines designates 206Pb/204Pb = 5000.Common Pb composition for both plots are 206Pb/204Pb = 18.700 and 207Pb/ 204Pb = 15.628, respectively (mean crustal Pb, Stacey and Kramers.32) For discussion see text. Analyst, November 1997, Vol. 122 1245Age Calculation Age calculation and statistics are made using Microsoft Excel spreadsheets closely following evaluation routines given by York,34 Ludwig35 and Roddick et al.36 and with ISOPLOT of Ludwig.37 Decay constants used are from Steiger and J�ager.17 Reported ages are weighted-mean ages calculated from at least 20 measured 207Pb*/206Pb* ratios.Weighting factors for the individual ratios are derived from counting statistics. Errors reported are either 2 standard deviations (2s) or 2 standard errors of the mean (2sm). Correlation between 206Pb and 207Pb during data acquisition is assumed to be 0, so the correlation coefficient equals 0 for error calculation on 207Pb*/206Pb* ratios and ages.Bootstrap analysis and Monte Carlo simulations of measured 207Pb*/206Pb* ratio spectra are used to check whether or not obtained mean ratios and errors are statistically meaningful. Chi squared tests are used to check for proper Gaussian distribution of the 207Pb*/206Pb* ratios. It is assumed, that the variation of the 207Pb*/206Pb* of a single evaporation step of an analysis of mono-aged Pb should follow a Gaussian normal distribution (variation derived from counting statistics alone).Data sets not following a Gaussian distribution probably exhibit a mixture of different Pb components, which then precludes any significant age information. Variations in the 208Pb*/206Pb* cannot be checked in this respect because they primarily reflect changing Th/U of the evaporated zircon domain (Kl�otzli.18) Basic statistics have to be made using the raw data because of the non-linear age transformation obscuring any relevant non-Gaussian distributions.Reports are in the form of plots giving the most important parameters: evaporation temperature, 207Pb*/206Pb* ratios and ages, number of ages, mean values, 208Pb*/206Pb* and Th/U ratios (see examples). Error Assessment Proper error assessment is one of the major problems in analytical geochronology and is often not rigorously done (York,34 Ludwig,35,37 Mattinson,4 Roddick et al.,36 Kl�otzli25). Very often ages reported from single zircon evaporation analysis include errors which are simply derived from the internal analytical scatter of individual ages excluding any external error sources.Such a simple approach to error assessment is neither justified nor correct and should be avoided completely in as much as the involved mathematics for the correct error calculation are rather simple. In the present report all errors (except errors on the decay constants of the U isotopes) are propagated into the final mean ages. Error propagation is done using the standard Gaussian error propagation formula.Errors incorporated in calculations are: errors on individual ratios from counting statistics, errors derived from fractionation correction factors, errors from standard measurements and from the recommended standard values, and the weighted errors from individual temperature steps. If a common Pb correction is applied to the age data the precision of the common Pb isotopic composition is incorporated as well.Precision and Accuracy The accuracy of the method is demonstrated by a number of studies comparing single zircon evaporation data with conventional U/Pb data or with ion probe data (i.e., Ansdell and Kyser,30 Kl�otzli,26 Kl�otzli and Parrish,22 Kober,14 Kr�oner and Seng�or,38 Kr�oner et al.,39 M�uller et al.,40 Peindl and H�ock,41 Kl�otzli-Chowanetz et al.,29 Kl�otzli et al.33). The internal precision of the method is defined above, but is not of significant interest in a geological context.The more important external precision of the method can only be assessed by a complete error propagation scheme as shown above and comparison with the external reproducibility of individual zircons with the same age. At present no internationally recommended standard zircon is available for zircon evaporation analysis. In house reproducibilities of 27 analyses from a zircon population from an Ordovician alkali gneiss are in the range of 487.2 ± 9.7 Ma (2%).It is thus assumed that under normal circumstances the external precision of the method is in the range of 1–5%, depending mostly on the age range investigated (Kl�otzli,25 Bernhard et al.42). Zircon evaporation age data is interpreted to be significant if at least 3 crystals of a sample population exhibit (each within at least 2 temperature steps) within error concordance. It is then assumed that such age data reflects true concordant 207Pb*/206Pb* ages which can then be interpreted as geologically meaningful.Examples Age data derived from zircon evaporation analysis is often reported in the form of histograms showing age range versus number of ages or mass scans or blocks of the highest temperature steps from a number of zircons. The amount of information provided with such diagrams is rather scarce, sometimes even misleading. For instance, no individual errors are incorporated into a simple histogram and the reported mean age does not necessarily correspond to the most frequent 207Pb*/ 206Pb* ratio or age.All age information from lower temperature steps and from 208Pb*/206Pb* ratios is lost or omitted. If such a compilation is shown it should be done in the form of probability density plots of ages and not as histograms with arbitrary class widths. The examples presented here show all major aspects of single zircon evaporation dating, their possible representation and interpretation. Example 1 (Fig. 4) presents two plots of evaporation temperature versus 207Pb*/206Pb* age for two zircons from the paragneiss–migmatite boundary of the Winnebach migmatite in the Upper Austroalpine � Otztal–Stubai nappe of the Eastern Alps, Austria (Kl�otzli-Chowanetz et al.29).Width of plotted boxes is the estimated error on an evaporation temperature of ±10 °C. The height of the boxes is calculated as 2s of the mean age of individual temperature steps. Assigned errors of mean ages reported are all 2sm.The examples clearly demonstrate the complementary ‘behaviour’ of zircons during evaporation analysis. Evaporation of crystal 8830-E (Fig. 4) resulted in a perfect age plateau of 484 ± 6 Ma over 8 evaporation steps ranging from 1350 to 1490 °C. Conversely, evaporation of zircon 8830-C (Fig. 4) resulted in a staircase of increasing ages with increasing evaporation temperature. Both zircons were evaporated to completeness. The age plateau of 8830-E is interpreted to represent the crystallisation age of a core-free zircon (at least in respect to Pb isotopic systematics).Based on zircon typology and additional conventional zircon dating, the crystallisation event is attributed to the migmatite formation during the Ordovician. Zircon 8830-C exhibits (within ion steps) the same age for the migmatite formation event at 480 ± 6 Ma. The single step ages of 561 and 632 Ma possibly represent older metamorphic events. Similar but better defined ages were found in other zircons from the same locality, further supporting this interpretation.The higher temperature staircase ( > 1460 °C) is interpreted as representing a mixture of an old, possibly Archean Pb component with Pb of Cambrian or Proterozoic age. If no additional information for the two older metamorphic events would exist, the age staircase must be interpreted as representing a mixture between the Archean and the Ordovician Pb component.The age of 2355 ± 85 Ma is interpreted as representing a minimum age estimate for the formation or recrystallisation of an inherited zircon core. The evaporation temperature profile of zircon 8830-C directly proves within one zircon crystal the existence of differently old 1246 Analyst, November 1997, Vol. 122Pb components with individual levels of activation energy which can very effectively be separated by evaporation analysis (Kl�otzli18). Example 2 (Fig. 5) presents two plots of block number versus 207Pb*/206Pb* and 208Pb*/206Pb* for two zircons from granitoids of the South Bohemian Pluton, Austria (Kl�otzli and Parrish,22 Kl�otzli et al.33) The block number gives the progress of evaporation with steps of increasing temperature as indicated by labelled evaporation temperatures (in °C).Each block represents the mean of 10 mass scans. For easier reading errors on individual blocks are not shown. They are in the range of the symbol size.Assigned errors of mean ages reported are 2sm. Both zircons demonstrate the presence of an inherited core and a later overgrowth. Inherited cores and overgrowth could be analysed by at least two evaporation steps thus providing plateau ages which can be interpreted as being geologically meaningful. For zircon 4690-A both ages (336 ± 4 Ma and 635 ± 12 Ma, respectively) are interpreted to represent magmatic growth events. This interpretation is further supported by the large intra- and inter-evaporation step variation in 208Pb*/206Pb* at constant 207Pb*/206Pb* indicative for magmatic Th/U variation in the evaporated zircon domain.The analysis of the deposit of the third evaporation step of 4690-A (at 1515 °C) shows the typical pattern of a reversed deposit (Kl�otzli18), direct evidence for the mixing of differently old zircon domains during evaporation. The 208Pb*/ 206Pb* spectrum obtained for the inherited core of 2E92-B (535 ± 8 Ma) does not show significant variation.This reflects constant Th/U ratios throughout the inherited core. This is interpreted as reflecting crystal homogenisation during a high temperature metamorphic overprint leading to the formation of charnockitic rocks (Kl�otzli et al.32). The exact meaning of the Variscan overgrowth (352 ± 3 Ma) is still a matter of debate. Outlook To further enhance the precision of 207Pb*/206Pb* ages from zircon evaporation analysis static SEM-IC data acquisition using multi-collector SEM-IC systems has to be established.It should be possible to achieve the same precision for Pb isotopic analysis as is routinely found for multi-collector Faraday systems (i.e., 10 times better in precision as at present). One possible way of upgrading is by substituting the Faraday cups of a MAT 262 with ion-counting channeltrons as demonstrated by Fig. 4 Plots of evaporation temperature versus 207Pb*/206Pb* age for two zircons from the paragneiss–migmatite boundary of the Winnebach migmatite in the Upper Austroalpine � Otztal-Stubai nappe of the Eastern Alps, Austria (Kl�otzli-Chowanetz et al.29).Width of plotted boxes is estimated error on evaporation temperature of ±10 °C. Height of boxes is calculated as 2s of the mean age of individual temperature steps. For discussion see text. Fig. 5 Plots of block number versus 207Pb*/206Pb* and 208Pb*/206Pb* for two zircons from granitoids of the South Bohemian Pluton, Austria (Kl�otzli and Parrish,22 Kl�otzli et al.33) Block number gives progress of evaporation with steps of increasing temperature as indicated by labelled evaporation temperatures (in °C).Each block represents the mean of 10 mass scans. For easier reading errors on individual blocks are not shown. They are in the range of the symbol size. Assigned errors of mean ages reported are 2sm. For discussion see text. Analyst, November 1997, Vol. 122 1247Richter et al.43 or by using a newly designed Wien-filter TIMS (Laue et al.44).Additionally, the zircon mounting and encasing procedures can substantially be improved by using preformed Re filaments and micro-manipulators. The author thanks the following for spending tedious hours at the MS, for discussion, criticism, advice, a steady hand during zircon mounting, and critical reviews of an earlier version of this paper: F. Bernhard, E. Chowanetz, W. Frank, V. H�ock, G. Hoinkes, M. Jelenc, S. Meli, B. M�uller.Financial support by the Austrian Science Foundation is also acknowledged. References 1 Lectures in Isotope Geology, ed. Hunziker, J. C., and J�ager, E., Springer Verlag, Berlin, 1979. 2 Faure, G., Principles of Isotope Geology, 2nd edn., Wiley, New York, 1986. 3 Geyh, M., and Schleicher, H., Absolute Age Determination: Physical and Chemical Dating Methods and Their Application, Springer Verlag, Berlin, 1990. 4 Mattinson, J. M., Chem. Geol., 1987, 66, 151. 5 Steiger, R., Bickel, R.A., and Meier, M., Terra Abstr., 1993, 1–5, 395. 6 Wendt, J. J., and Todt, W., Terra Abstr., 1991, 3, 507. 7 Poller, U., Liebetrau, V., and Todt, W., in Proceedings of the V. M. Goldschmidt Conference, Heidelberg, 1996, volume 1, p. 119. 8 Compston, W., Williams, I. S., and Clement, S. W., in Proceedings of the 30th American Society of Mass Spectrometry Conference, 1982, p. 593. 9 Wiedenbeck, M., and Goswami, J. N., Geochim. Cosmochim Acta, 1994, 58, 2135. 10 Jackson, S.E., Longerich, H. P., Horn, I., and Dunning, G. R., in Proceedings of the V. M. Goldschmidt Conference, Heidelberg, 1996, vol. 1, p. 283. 11 Hirata, T., and Nesbitt, R. W., Geochim. Cosmochim. Acta, 1995, 59, 2491. 12 Kosztolanyi, C., Compt. Rend. Acad Sci., 1965, 261, 5849. 13 Sunin, L. V., and Malyshev, V. I., Geochem. Int., 1983, 20, 34. 14 Kober, B., Contrib. Mineral. Petrol., 1997, 93, 482. 15 Gentry, R. V., Sworski, T. J., McKown, H. S., Eby, R. E., and Christie, W.H., Science, 1982, 216, 296. 16 Kober, B., Contrib. Mineral. Petrol, 1996, 96, 63. 17 Steiger, R. H., and J�ager, E., Earth Planet. Sci. Lett., 1977, 36, 359. 18 Kl�otzli, U. S., Chem. Geol., 1997, submitted for publication. 19 Ansdell, K. M., and Kyser, T. K., Am. Min., 1993, 78, 36. 20 Roddick, J. C., and Chapman, H. J., EOS, Trans. Am. Geophys. Union, 1991, 72, 531. 21 Roddick, J. C., USGS Circular, 1994, 1107, 269. 22 Kl�otzli, U. S., and Parrish, R. R., Mineral. Petrol., 1996, 58, 197. 23 Cameron, A. E., Smith, D. E., and Walker, R. L., Anal. Chem., 1969, 41, 525. 24 Krogh, T. E., Geochim. Cosmochim. Acta, 1982, 46, 637. 25 Kl�otzli, U. S., Mitt. � Osterr. Miner. Ges., 1994, submitted for publication. 26 Kl�otzli, U. S., Mitt. � Osterr. Miner. Ges., 1993, 138, 123. 27 Tilton, G. R., J. Geophys. Res., 1960, 65, 2933. 28 Shestakov, G. I., Geochim. Int., 1972, 9, 801. 29 Kl�otzli-Chowanetz, E., Kl�otzli, U. S., and Koller, F., Schw. Miner. Petrol. Mitt., 1997, in the press. 30 Ansdell, K.M., and Kyser, T. K., Geology, 1991, 19, 518. 31 Todt, W., Cliff, R. A., Hanser A., and Hofmann, A. W., Geophys. Monograph, 1996, 95, 429. 32 Stacey, J. S., and Kramers, J. D., Earth Planet. Sci. Lett., 1975, 26, 207. 33 Kl�otzli, U. S, Koller, F., Scharbert, S., and H�ock, V., Chem. Geol., 1997, submitted for publication. 34 York, D., Earth Planet. Sci. Lett., 1969, 5, 320. 35 Ludwig, K. R., Earth Planet. Sci. Lett., 1980, 46, 212. 36 Roddick, J.C., Loveridge, W. D., and Parrish, R. R., Chem. Geol., 1987, 66, 111. 37 Ludwig, K. R., USGS Open-file Report, 1992, 91-445. 38 Kr�oner, A., and Seng�or, A. M. C., Geology, 1990, 18, 1186. 39 Kr�oner, A., Todt, W., Humbrian Res., 1992, 59, 15. 40 M�uller, B., Kl�otzli, U. S., and Flisch, M., Geol. Rundschau, 1995, 84, 457. 41 Peindl, P., and H�ock, V., Terra Abstr., 1993, 1/5, 392. 42 Bernhard, F., Kl�otzli, U. S., Hoinkes, G., and Th�oni, M., Mineral.Petrol., 1996, 58, 171. 43 Richter, S., Ott, U., and Begemann, F., Int. J. Mass Spec. Ion. Proc., 1994, 136, 91. 44 Laue, H. J., Tegtmeyer, A., and Wegener, M., Spectromat Inf. Sheet, 1995, 1. Paper 7/04114D Received June 12, 1997 Accepted September 1, 1997 1248 Analyst, November 1997, Vol. 122 Single Zircon Evaporation Thermal Ionisation Mass Spectrometry: Method and Procedures† U. S. Kl�otzli Laboratory for Geochronology, Department of Geology, University of Vienna, Geozentrum, Althanstrasse-14, A-1090 Vienna, Austria Zircon evaporation thermal ionisation mass spectrometry (TIMS) is used in geochronology to determine absolute 207Pb*/206Pb* ages and Th/U ratios of single zircon crystals.The process involves the breakdown of zircon (ZrSiO4) to porous baddeleyite (ZrO2) along a reaction front which progresses into the interior of the crystal. Evaporation of high quality zircons thus allows us to distinguish between crystal rim (overgrowth) and core, providing precise information about the time of magmatic crystal growth, partial dissolution, and/or metamorphic overgrowth.Derived Th/U ratios complement age data interpretation and provide valuable petrogenetic implications. A double Re-filament ion source is used. The zircon is encased in the evaporation filament and heated step-wise to 1200–1300 °C to strip off unsupported common and radiogenic Pb components. After cleaning, evaporation proceeds in temperature steps of ~ 20 °C.The evaporate (SiO2, Pb, REEs, and U from the zircon and Re from the evaporation filament) of each step is deposited for 45 min on the cold ionisation filament and subsequently analysed. Lead isotopic composition is determined using a dynamic secondary electron multiplier ion counter or static Faraday cup data acquisition schemes. Lead ratios are corrected for fractionation using correction factors derived from standard measurements of a 1 ng NBS SRM 982 sample.The precision on 207Pb/206Pb ratios is < 1%. Only high temperature steps ( > 1300 °C) with 206Pb/204Pb > 5000 are used for age calculations. The ages reported (single temperature step, multi-temperature step means) are weighted means calculated from at least 20 measured 207Pb*/206Pb* ratios with 2 standard errors of the mean. Precision of ages is strongly dependent on age range and varies between 0.1 and 10%. Keywords: Thermal ionisation mass spectrometry; zircon evaporation; geochronology; lead–lead dating; absolute dating The uranium–thorium–lead and lead–lead dating techniques for zircon and other U-bearing mineral phases (i.e., monazite, xenotime, allanite, sphene) are standard geochronological methods used in laboratories throughout the world.In particular, zircon dating has proven to be a very powerful tool, due to the large resistance of zircon against thermal and mechanical alteration, in establishing absolute geochronological information about magma generation and intrusion events and/or periods of high temperature metamorphic overprinting. Other commonly used geochronometers normally do not allow the geoanalyst to directly date such high temperature events.The U–Th–Pb and Pb–Pb dating techniques are based on the radioactive decay of 238U to 206Pb*, 235U to 207Pb*, and 232Th to 208Pb*, respectively (for fundamentals see Hunziker and J�ager,1 Faure,2 and Geyh and Schleicher3).Conventional U–Th–Pb dating principally comprises the chemical dissolution of the mineral under investigation, the purification and determination of the concentrations of U, Th, Pb (using isotope dilution techniques), and the determination of the isotopic composition of Pb. Therefore, conventional U–Th–Pb dating requires an ultra-clean laboratory environment, sophisticated isotope dilution thermal ionisation mass spectrometry (TIMS), complicated fractionation corrections, and error evaluation.Additionally, the different age information present in different domains of one zircon crystal is often lost due to the ‘integration’ effect of crystal dissolution, thus providing less age information than actually present. These restrictions can be overcome by applying either even more sophisticated analytical methods [i.e., partial dissolution experiments (Mattinson4), mechanical separation of rim and core (Steiger et al.5), vapour digestion combined with cathodoluminescence investigations (Wendt and Todt,6 Poller et al.7) or spot analysis using ion microprobes (Compston et al.,8 Wiedenbeck and Goswami9) and laser ablation mass spectrometric techniques (e.g., Jackson et al.,10 Hirata and Nesbitt11)].This paper presents an alternative zircon dating method: single zircon evaporation TIMS comprises the direct determination of 207Pb*/206Pb* ages (so called lead–lead ages). This method circumvents the need for the determination of elemental concentrations of the conventional U–Th–Pb dating techniques.Thus, the laborious and complicated work involved in the ultraclean laboratory and the isotope dilution TIMS can be completely avoided. Direct evaporation TIMS of finely ground zircon for Pb isotope analysis was first suggested by Kosztolanyi12 as early as 1965 and was later applied by Sunin and Malyshev13 to zircons from various rocks. But these early applications were severely hampered by analytical difficulties (for discussion see Kober.14) Gentry et al.15 and Kober14 suggested a mounting procedure for unground zircon crystals in a single-filament ion-source. To further enhance beam stability and duration Kober14 used a double-filament ion-source arrangement and a stepwise heating procedure resulting in a major improvement in the precision of the resulting evaporation 207Pb*/206Pb* ages.The method described in this contribution is partly based on the method of Kober,14,16 but advances analytical precision, accuracy and the possibilities of age data interpretation.Geochronological Background A fundamental prerequisite of absolute age determination is the assumption of a closed system behaviour of mother and daughter elements and the absence of any internal isotope fractionation since the time of closure with respect to diffusion of the elements under consideration. Thus, if one assumes that no spatial fractionation between U, Th, and Pb has taken place in the crystal lattice since the time of closure the 207Pb*/206Pb* ratio can directly be recalculated to an apparent lead–lead age: † Presented at Geoanalysis 97: 3rd International Conference on the Analysis of Geological and Environmental Materials, Vail, CO, USA, June 1–5, 1997.Analyst, November 1997, Vol. 122 (1239–1248) 1239206 235 207 206 235 238 1 1 1 1 235 238 Pb = U (e and Pb U ( e Pb Pb U e U e 238 207 238 235 * ) * ) * * ( ) ( ) l l l l t t t t - = - ® = - - Assuming that the U isotopic composition is constant (238U/ 235U = 137.88) and that the decay constants of 238U and 235U are accurately and precisely known (Steiger and J�ager17) the transcendental equation can iteratively be solved for t, providing an age-estimate for the isotopic system since its closure.Additionally the 208Pb*/206Pb* ratio is directly proportional to the Th/U ratio, an important mineralo- and petrogenetic indicator which is helpful for the interpretation of single zircon age spectra (Kl�otzli18).Process of Zircon Evaporation The process of evaporation involves the breakdown of the accessory silicate zircon (ZrSiO4, normally < 200 mm in size) to the porous oxide baddeleyite (ZrO2) and the associated loss of mainly Si, SiO2, Pb, REEs, U and Th along a reaction front which progresses into the interior of the grain (Ansdell and Kyser,19 Roddick and Chapman,20 Roddick21 and Kl�otzli18). Continuous evaporation or evaporation with stepwise increasing evaporation temperatures ideally results in a ‘depth’ profile through the zircon crystal from thhe innermost core (Kober,14 Kl�otzli,18 Kl�otzli and Parrish22).The stepwise evaporation on a ‘cold’ ionisation filament, as described below, makes use of the silica-gel effect (routinely used in conventional Pb TIMS, Cameron et al.23) and the accumulation of Pb on the ionisation filament resulting in better ionisation efficiency and ion beam stability (Kober14; Kl�otzli18).Mass Spectrometer Parameters and Maintenance A double Re-filament arrangement in a conventional extendedgeometry thermal ionisation multi-collector mass spectrometer (Finnigan MAT 262, Bremen, Germany) equipped with an 120 MHz ion-counting secondary electron multiplier (SEM-IC) is used. Faraday System For Pb isotopic analysis the Faraday system is used in a conventional way and is not described in detail. SEM-IC System The SEM-IC system is heavily used in zircon evaporation TIMS (90% of all analyses) and therefore its operation and performance is described in more detail. Additionally, the use of dynamic data acquisition and the chronic instability of SEM systems requires precise monitoring of the SEM-IC system parameters. The peak shape and the deflection voltages of the ion beam in the SEM are controlled routinely before and after a zircon evaporation session using a ~ 8 mV Pb+ or Re+ ion beam.The optimum HV for the counting efficiency of the SEM is adjusted once a week using a Finnigan-MAT automatic calibration routine. It is set ~100 V higher than the beginning of the SEM saturation plateau. Since July 1991 the optimum HV has increased from 1950 to 2950 V (at present), demonstrating the effect of ageing of the SEM through heavy usage. Count gains are in the range of 90–95% relative to the axial Faraday cup. The precise count gain relative to the Faraday system is not determined because no mixed acquisition schemes are used in zircon evaporation TIMS.This is due to nonsufficient intensity overlap between the Faraday and SEM which leads to inconsistent and temporarily strongly varying count gains. Proper focusing has to be checked more frequently than when using the Faraday system. The focus point of the very small evaporate deposit (compared with conventionally loaded samples) tends to move during evaporation due to thermal relaxation and/or stress release of the evaporation filament.Additionally, the gain across the conversion dynode of the SEM is not constant leading to fluctuations in the ion beam intensity if the ion beam moves around. Dead time correction for the counter is accomplished on-line using Finnigan MAT software routines. The applied correction is correct up to ca. 500 000 counts s21, about half of the maximum allowable beam intensity. At higher beam intensities counting problems arise with the more abundant Pb masses 206 and 208 leading to noticeably to high 207Pb/206Pb and to low 208Pb/206Pb. Because dead time effects are mass and intensity dependent, an appropriate dead time correction for higher ion beam intensities would be rather complicated to accomplish and would incorporate additional uncertainty to the analysis.To completely exclude any dead time effects of the SEM-IC system ion beam intensities above 500 000 counts s21 are avoided, both for standard and zircon evaporation analyses.Experimental Procedures Zircon Preparation The zircons used for evaporation analyses are washed in warm 3 m HNO3 for half an hour, rinsed with distilled water and dried. Criteria for choosing zircon crystals for analysis are: absence of cracks, micro-fissures and inclusions, no visible turbidity, suitable size (30 to 250 mm along c-axis, elongation maximum 1 : 10), and colour. In special cases (e.g., direct analysis of an inherited core) abrasion techniques are employed (Krogh;24 Kl�otzli25).Filament Preparation Rhenium filaments used for encasing zircons are preheated and cleaned at 4.5 A for 4–8 h. This long outgassing duration softens the filaments substantially, thus making it easier to bend the filament and to encase the zircon. Softened filaments are first pre-bent using a modified jig of Finnigan MAT and then formed by tweezers for zircon encasing. Ionisation filaments used for the analysis of the evaporated deposit are outgassed at 4.5 A for 45 min.Zircon Evaporation Zircon cleaning procedures Zircon evaporation analyses follows modified procedures originally described by Kober14 and modified by Kl�otzli.25,26 Fig. 1 gives a schematic representation of the evaporation procedures. Encased zircons are stepwise heated to 1200–1300 °C in order to strip off unsupported common and radiogenic Pb components with low activation energies. Such Pb is weakly bound to metamict zircon domains, cracks, micro-cleavages, and to Pb bearing inclusions within the crystal.Especially in metamict crystal domains activation energies for Pb are very low (0.1–0.4 eV, Tilton27) and Pb is thus readily mobilised. Non-metamict domains are much more retentive with Pb activation energies between 2.2–2.5 eV (Shestakov28). Pure common Pb is characterised by the presence of 204Pb and low 206Pb/204Pb (see below). 1240 Analyst, November 1997, Vol. 122Evaporation-filament with zircon Ionisation-filament with evaporated deposit zircon evaporation and deposition on 'cold' ionisation-filament filament current/mA vs.temperature/°C Pb isotopic analysis and filament cleaning procedure 4500/1800 3000/1450 2200/1300 2000/1200 1500/950 1200/900 Ionisation-filament cleaning cycle Zircon cleaning cycle 1st evaporation/ analysis cycle 2nd evaporation/ analysis cycle 3rd evaporation/ analysis cycle 4th evaporation/ analysis cycle Check for zircon presence and Hf isotopic analysis time T4 T3 T2 T1 The progress of cleaning is always monitored with the EM-IC by mass scans from 203 to 209 (1 s integration time with 0.1 u steps).Additionally to common Pb, masses 204 (94Zr2 16O, 204Hg), 206 (94Zr2 18O), 207 (206Pb1H, organic material), 208 (207Pb1H, 96Zr2 16O, 176Hf16O2) can be occupied by isobaric molecules. The mass resolution of the MAT 262 is not sufficient to resolve these potential isobars. Therefore, before data acquisition can start, the absence of any isobars on the Pb isotope masses has to be checked.Masses 196 (90Zr2 16O), 202 (202Hg, 138BaP16O17O), 203 (203Tl), 205 (205Tl, 138BaP18O2, organic material) and 209 (208Pb1H, 209Bi, 177Hf16O2) are monitored to recognise the isobaric overlaps. In particular the isobar on mass 207 is critical because no constant ratio with any other non-Pb mass has been recognised which would allow an appropriate correction. Empirically the presence of 207 is always accompanied by 205 (with 207/205 < 15).Thus it is assumed that no isobaric 207 is present as soon as no 205 is present any more. Isobars of Zr2O and HfO2 normally do not pose any problems because the ionisation energy of the compound is far higher than the energies needed for Pb ionisation. Problems with isobars of BaPO2, Tl and Hg can arise when Clerici-solution (TlHCO2–Tl2C3H2O4) or Hg bearing heavy liquids (Thoulet-, Rohrbach-solution) were used for heavy mineral separation.PbH is sometimes present at very low temperatures ( < 850 °C) but disappears above 900 °C. Once no ‘low-temperature’ Pb or isobaric overlaps are present (0 min–2 h) the zircon temperature is raised by approximately 20 °C and, depending on ion beam intensities and experiment design, the isotopic composition of the evaporated Pb is either measured directly or the evaporate is deposited on the cold ionisation filament. Evaporation steps of 20 °C have been found to be a good compromise between spatial/temporal resolution (preferably small temperature steps) and sufficient ion beam intensity (preferably large temperature steps).The material evaporated from the evaporation filament consists mainly of a mixture of material from the zircon (70%) and Re from the filament (30%). From the zircon SiO2, most Pb, REEs, and about 50% of U and Th are quantitatively evaporated (Roddick and Chapman;20 Kl�otzli18). In particular, the amount of SiO2 evaporated depends strongly on crystal quality and to a somewhat lesser degree on evaporation (Kl�otzli- Chowanetz et al.29).The amount of SiO2 deposited on the ionisation filament has a major influence on the quality of the subsequent Pb isotopic analysis. If only minor amounts of SiO2 are evaporated the silica-gel effect cannot function properly resulting in unstable ion beams and thus low analytical precision, although suitable amounts of Pb are present on the ionisation filament.This silica-gel effect sometimes jeopardises the analysis of highest quality zircons and leads to the situation that slightly metamict zircons can be more easily analysed than completely non-metamict zircons, and that the absolute amount of Pb present in a crystal is not necessarily the dominant factor for the quality of a single zircon evaporation analysis. Total amounts of lead available are in the range of 1 pg to 1 ng for normal zircon crystals resulting in the analysis of sub-pg to subng amounts of Pb per evaporation step.The Pb+ ion yield is in the range of 1 3 1023 equivalent to the efficiency of the conventional silica-gel technique (Cameron et al.23). Evaporation procedures After a deposition step (15–45 min) Pb is analysed using either static Faraday or dynamic SEM-IC data acquisition procedures. During analyses, the evaporation filament is set to 1.2 A in order to prevent Pb being evaporated from the ionisation filament to be re-deposited on the evaporation filament (Ansdell and Kyser;30 Kl�otzli and Parrish22).After the Pb analyses the ionisation filament is raised to 4.5 A for some seconds, stripping off all remaining material deposited during the evaporation step. Then the next evaporation–analysis cycle can start. Evaporation temperatures are raised by approximately 20 °C from step to step until Pb evaporation from the zircon is complete. Depending on crystal quality, size, age and U and Pb content of the zircons, 2 to 8 evaporation–analysis cycles can be made.To check whether the Pb evaporation really is complete Fig. 1 Schematic representation of a single zircon evaporation analysis. T1–T4 symbolise increasing evaporation temperatures (+20 °C per evaporation step). See text for discussion. Analyst, November 1997, Vol. 122 1241or whether the zircon has just fallen off the evaporation filament, the evaporation filament is slowly raised to 3.5–3.8 A.The ionisation filament is set to 5 A. Presence of the zircon is controlled by monitoring the Zr+ ion beam on mass 90. Knowing whether Pb evaporation is complete or not is critical for the later interpretation of the evaporation data. High quality zircons can be heated to ~ 2000 °C. At these temperatures very stable Hf+ ion beams are attained thus making it possible to additionally determine the Hf isotopic composition of the zircon. The Hf isotopic composition gives important petrogenetic information, complementary to the Nd isotope system (e.g., crustal residence times, model ages, see Kl�otzli25 and references cited therein).Data Acquisition For compatibility reasons the acquisition schemes for the dynamic SEM-IC and static Faraday cup measurements are kept as similar as possible. In both modes and for normal acquisition ion beam intensities are measured in blocks of 10 scans with 4 s integration time and 2 s delay time each. Peak centering and intensity monitoring is done at the beginning of each block on mass 206.The background is measured on half-masses every 5 blocks with 15 s delay time and 32 s integration time. The background correction is made on-line during data acquisition. Acquisition schemes with masses analysed, integration times, and respective succession for the SEM-IC procedures are given in Table 1. For the SEM-IC analyses peaks are measured in succession of increasing mass in order to avoid problems with magnetic field stability (hysteresis).Data acquisition comprises 2 to 20 blocks, depending mostly on the durability of the ion beam. Acquisition is started or interrupted at a minimum ion beam intensity on mass 206 of 10 000 counts per s21 for the SEM-IC procedures and 5 mV for the Faraday cup procedures, respectively. Because of insufficient counting statistics, too low ion beam intensities can lead to unrealistic low 204Pb/206Pb ratios seemingly proving the absence of common Pb.Using the relatively high threshold intensities allows the recognition of 204Pb with appropriate precision. During the process of evaporation sometimes low to intermediate ion beam intensities ( < 5000 counts s21 on 206) normally exhibiting large intensity fluctuations can be registered. Special acquisition procedures with integration times of 16 s allows the analysis of these ion beams providing further information about the progress of evaporation. The same acquisition procedures are used to analyse low quality evaporation deposits from which only minor and unstable ion beams can be achieved.Age data derived from such procedures can then be compared to good age data found for other evaporation steps thus providing additional information for age data interpretation. Data Reduction and Fractionation Correction The subsequent data reduction is completely the same for both kinds of data acquisition schemes. Lead ratio calculation and statistical test with outlier elimination are performed using modified off-line software from Finnigan MAT.In Pb isotopic analysis no direct correction of machine discrimination and time dependent fractionation (internal normalisation) and other fractionation phenomena is possible. Natural lead is formed by a mixture of 4 different isotopes (204Pb, 206Pb, 207Pb, 208Pb), three of which are the stable daughter products of radioactive decay series (206Pb, 207Pb, 208Pb).So the amount of these 3 isotopes compared to 204Pb depends solely on the amount of radioactive mother isotope present and the time elapsed since the accumulation of the lead isotopes started. This then means that not one of all the possible ratios is constant and thus none can be used for the above mentioned corrections. This is the major drawback of the U–Pb and Pb–Pb dating techniques compared to other isotope systems (e.g., Rb/Sr, Sm/Nd), where the presence of at least two nonradiogenic isotopes resulting in one constant ratio allows the necessary corrections to be made.Therefore, 207Pb/206Pb and 208Pb/206Pb ratios are corrected using correction factors derived from NBS SRM 982 standard measurements with 1 ng of Pb loaded and using the conventional Si-gel technique (using the modified values given by Todt et al.31) These correction factors are determined individually for Faraday and SEM-IC procedures before and after the zircon analyses using the same data acquisition procedures as for zircon analyses.Two different sets of fractionation factors can be determined using both 208Pb/ 206Pb and 207Pb/206Pb values. The within error consistency of the two independently derived fractionation factors further proves the linearity and thus the correctness of the applied fractionation correction scheme. Compared to the overall analytical error of a single age the difference found in the 2 independently calculated fractionation factors is negligible.Because the 207Pb/206Pb ratio is the one of most interest, the correction factor derived from 207Pb/206Pb is used for the correction. Mass discrimination for Faraday procedures is about 50% of the mass discrimination of the SEM-IC procedures. 204Pb/206Pb ratios are not corrected. Table 2 gives the appropriate NBS SRM 982 data. Recommended standard values used are from Todt et al.31 Fig. 2 shows plots of the NBS SRM 982 data for the SEM-IC and the Faraday procedures.Table 3 gives a compilation of the derived correction factors, while Table 4 shows the influence of the applied correction on final 207Pb*/206Pb* ages using the SEM-IC Table 1 Peak succession and position used for zircon evaporation analysis and accompanying NBS SRM 982 standard measurements for both the SEM-IC and the Faraday collector systems Integration Detector time/delay system Mass succession/position time* Ziron cleaning SEM-IC 203–209 mass scan 1/0.1 u Common lead and isobars SEM-IC 206, 207, 208, 209, 202, 4/2 present 203, 204, 205 lead present SEM-IC 206, 207, 208, 204 4/2 (206Pb204Pb < 50 000) Faradays 204 = 7, 206 = 5, 207 = 4, 208 = 3 4 Radiogenic lead only SEM-IC 206, 207, 208 4/2 (206Pb204Pb > 50 000) Faradays 206 = 5, 207 = 4, 208 = 3 4 Radiogenic lead with very SEM-IC 206, 207, 208 16/2 low intensity, unstable * Measured in seconds. 1242 Analyst, November 1997, Vol. 1220.4650 0.9965 0.9970 0.9975 0.9980 0.9985 0.9990 0.9995 1.0000 1.0005 0.4655 0.4660 0.4665 0.4670 0.4675 0.4680 1 ng NBS 982 ICm mean 1 ng NBS 982 ICm NBS 982 Standard values 207Pb/206Pb 207Pb/206Pb 208Pb/206Pb 208Pb/206Pb 0.460 0.462 0.464 0.466 0.468 0.470 0.472 0.474 0.476 0.478 0.480 1.015 1.013 1.011 1.009 1.007 1.005 1.003 1.001 0.999 0.997 0.995 ( a) ( b) 1 ng NBS 982 Far mean 1 ng NBS 982 Far NBS 982 Standard values system fractionation correction.In most cases, the applied correction is well within the analytical error of the raw data.Relative corrections range from ~ 10% for Tertiary age to < 0.1% for Archean ages. The absolute age shift is between 25 Ma and 23 Ma, respectively (1 Ma = 106 y). For the Faraday system corrections are about half the size and in the opposite direction. Time dependent fractionation (Rayleigh-type fractionation) cannot be accounted for using this simple correction calculation. But regarding the very small relative mass differences involved, the effect is assumed to be negligibly small and must thus not be accounted for.The calculated correction factors thus account for all biases of the SEM-IC and Faraday systems together with mass discrimination of the mass spectrometer and other fractionation effects, but not for time dependent fractionation effects. Bias characteristics of the SEM-IC compared to the Faraday cups are not independently incorporated in the data reduction because no mixed acquisition schemes (SEM-IC and Faraday cups used at the same time) are used in zircon evaporation analysis. Fractionation effects due to variation of heating procedures and ionisation temperatures can only be overcome by keeping the procedures and ionisation temperatures as similar and as Table 2 NBS SRM 982 standard values for the SEM-IC and Faraday collector systems (period 28.11.96 to 4.03.97) NBS 982 SEM-IC NBS 982 Faraday No.of analyses 25 2SE* % 25 2SE* % 208/206 measured 1.0052 ± 0.0030 0.30 0.9979 ± 0.0003 0.03 207/206 measured 0.4679 ± 0.0024 0.52 0.4665 ± 0.0002 0.04 208/204 measured 35.76 ± 0.70 1.96 36.72 ± 0.33 0.91 207/204 measured 17.62 ± 0.46 2.60 17.14 ± 0.18 1.07 206/204 measured 36.01 ± 0.70 1.95 36.51 ± 0.35 0.95 * Errors are 2 standard errors of the mean.Fig. 2 NBS SRM 982 standard measurements. Standard amount loaded for all measurements is 1 ng. (a) Plot of 25 dynamic SEM-IC collector measurements (period 02.12.96 to 04.03.97). (b) Plot of 19 static Faraday collector measurements (period 28.11.96 to 14.02.97).All error bars are 2sm. Recommended standard values of Todt et al.31 are given for comparison. Errors for recommended standard values are smaller than plotting symbol. Analyst, November 1997, Vol. 122 1243constant as possible for both standard measurements and for zircon evaporation analyses. For the applied acquisition schemes ionisation temperatures are constant within ±20 °C. After fractionation correction all relevant information from the acquired data is written as ASCII files for further evaluation.Pb Blank Assessment As no chemical treatment is used no severe problems with external Pb blank contributions are encountered in zircon evaporation analysis. But problems with Pb blank contributions from former zircon evaporation analyses could arise by the reactivation of deposits in the ion source coming especially from the first shielding plate. In order to avoid sample to sample cross contamination the shielding plate is changed and cleaned on a regular basis.Additionally an ion source blank is measured before each zircon analysis using a blank Re-single filament which is heated step-wise up to 5.5 A. The ion source can effectively be cleaned by using a defocused Re+ ion beam of 1–2 V intensity for 5 min duration. The process of cleaning can be monitored with mass 39K. Mass scans from 203 to 209 with 1 s integration time and 0.1 u steps are employed for monitoring the Pb blank.After cleaning, ion source Pb blank levels are in the range of < 10 counts s21 at 2200 °C and < 0.1 counts s21 at 1400 °C. The Pb blank of the used Re-filament material is negligibly small and thus poses no problems. Common Pb Correction The addition of common Pb to the radiogenic Pb poses one of the most significant problems in U–Th–Pb and Pb–Pb geochronology. Different methods are used to establish appropriate corrections needed to achieve geologically meaningful age Table 3 Lead mass fractionation values derived from NBS SRM 982 standard measurements (Table 2).Mass fractionation is independently calculated for 207Pb/206Pb and 208Pb/206Pb for both the SEM-IC and the Faraday collector systems. See text for discussion Mass Fraction* discrimim/ t 2SE† % nation/u 2SE† % 208/206 SEM-IC (25) 1.0050 ± 0.0030 0.301 20.00251 ± 0.0000076 0.301 207/206 SEM-IC (25) 1.0020 ± 0.0052 0.523 20.00200 ± 0.0000104 0.523 208/206 Far (19) 0.9978 ± 0.0003 0.031 0.00112 ± 0.0000004 0.031 207/206 Far (19) 0.9989 ± 0.0004 0.037 0.00107 ± 0.0000004 0.037 * m/t = measured value/true value.† Errors are 2 standard errors of the mean. Table 4 Influence of Pb isotope fractionation correction on final 207Pb*/206Pb* ages for the SEM-IC system. For comparison 3 different age ranges are shown. See text for discussion Age/ 207Pb/206Pb 2SE* % Ma 2SE* %† Tertiary age— Measured 0.04700 ± 0.00047 1.00 49.2 ± 23.9 48.5 Corrected 0.04691 ± 0.00053 1.13 44.5 ± 27.0 60.7 Age difference 24.8 10.7 Palaeozoic age— Measured 0.05800 ± 0.00058 1.00 530 ± 22 4.14 Corrected 0.05788 ± 0.00065 1.13 525 ± 25 4.71 Age difference 24.4 0.84 Archean age— Measured 0.45000 ± 0.00450 1.00 4085 ± 15 0.36 Corrected 0.44910 ± 0.00507 1.13 4082 ± 17 0.41 Age difference 23.0 0.07 Fractionation factor: 207/206 SEM-IC: 1.00200 ± 0.00524 (0.52%) * Errors are 2 standard errors of the mean.† Relative shift in corrected age.Table 5 Pb isotopic and age data for the zircon evaporation analysis of sample 2E92-B Weinsberg granite (cf. lower plot of Fig. 5). For discussion see text Evaporation 207/206 Block temperature*/ 2SE‡ age§/ 2SE†/ 2SE† No. °C 207/206† 2SE‡ % Ma Ma (%) 208/206† 2SE† Th/U¶ 2SE† Sample 2E92-B 2E92BC01 10 1398 0.05360 0.00022 0.4 354 9 2.6 0.0198 0.0049 0.060 0.015 2E92BC02 10 1443 0.05350 0.00019 0.4 350 8 2.3 0.0181 0.0047 0.055 0.014 2E92BC03 10 1463 0.05816 0.00025 0.4 536 10 1.8 0.0808 0.0051 0.243 0.015 2E92BC04 10 1482 0.05820 0.00018 0.3 537 7 1.3 0.0790 0.0053 0.238 0.016 2E92BC05 10 1504 0.05808 0.00028 0.5 533 11 2.0 0.0779 0.0054 0.235 0.016 Mean C01-C02, rim 0.05355 0.00006 0.1 352 3 0.7 Mean C03-C05, core 0.05815 0.00021 0.4 535 8 1.5 * Error on evaporation temperature is estimated to be ±10 °C.† Mean from individual scan ratios. ‡ All errors reported are 2 standard errors of the mean. § Mean ages derived from individual scan ratios and not from individual scan ages.¶ Th/U at apparent 207Pb/206Pb age. 1244 Analyst, November 1997, Vol. 122information. Fig. 3 shows the influence of the addition of common Pb to the radiogenic Pb for different age ranges. It is evident that common Pb can very dramatically change the apparent 207Pb/206Pb age. Any reasonable correction procedures must thus rely on precise knowledge of the common Pb isotopic composition, a fact very difficult to establish. Using wrong or non-precise common Pb compositions will unavoidably lead to wrong 207Pb*/206Pb* ages.If for the age calculation correct error assessment procedures are used the precision of the common Pb isotopic will have a large and negative impact on the achievable final age precision (see below). Thus the only reliable method to gain sound age data is to avoid common Pb contamination completely. In this respect, zircon evaporation analysis is superior to conventional U–Th– Pb dating.The sites of unsupported common Pb within a zircon crystal are metamict crystal domains, Pb bearing inclusion, fissures, and cracks. The common Pb can very effectively be removed from the crystal as described above. In order to minimise additionally the contribution of common Pb only high temperature steps ( > 1300 °C) with 204Pb/206Pb < 0.0002 (206Pb/204Pb > 5000) are used for age calculations. Therefore, the influence of common Pb (at least for zircons older than 500 Ma) is negligible and no common Pb correction has to be applied at all.During routine analysis 204Pb/ 206Pb is normally < 0.00001. As the 207Pb/206Pb ‘age’ of common Pb tends to be higher than the purely radiogenic 207Pb/206Pb age admixture of common Pb can be recognised by decreasing 207Pb/206Pb ages with increasing evaporation temperatures at low temperature steps. Rarely, evaporation data from zircons exhibiting a large common Pb contribution to the total Pb can be corrected for this common Pb component (Kl�otzli et al.33).Fig. 3 Influence of common Pb correction on measured 207Pb/206Pb ratios with varying 206Pb/204Pb and different age ranges. (a) Plot of the apparent 207Pb*/ 206Pb* age after common Pb correction. (b) Plot of the relative age difference between the apparent 207Pb/206Pb age and the 207Pb*/206Pb* age after common Pb correction. Dashed vertical lines designates 206Pb/204Pb = 5000. Common Pb composition for both plots are 206Pb/204Pb = 18.700 and 207Pb/ 204Pb = 15.628, respectively (mean crustal Pb, Stacey and Kramers.32) For discussion see text.Analyst, November 1997, Vol. 122 1245Age Calculation Age calculation and statistics are made using Microsoft Excel spreadsheets closely following evaluation routines given by York,34 Ludwig35 and Roddick et al.36 and with ISOPLOT of Ludwig.37 Decay constants used are from Steiger and J�ager.17 Reported ages are weighted-mean ages calculated from at least 20 measured 207Pb*/206Pb* ratios.Weighting factors for the individual ratios are derived from counting statistics. Errors reported are either 2 standard deviations (2s) or 2 standard errors of the mean (2sm). Correlation between 206Pb and 207Pb during data acquisition is assumed to be 0, so the correlation coefficient equals 0 for error calculation on 207Pb*/206Pb* ratios and ages. Bootstrap analysis and Monte Carlo simulations of measured 207Pb*/206Pb* ratio spectra are used to check whether or not obtained mean ratios and errors are statistically meaningful.Chi squared tests are used to check for proper Gaussian distribution of the 207Pb*/206Pb* ratios. It is assumed, that the variation of the 207Pb*/206Pb* of a single evaporation step of an analysis of mono-aged Pb should follow a Gaussian normal distribution (variation derived from counting statistics alone). Data sets not following a Gaussian distribution probably exhibit a mixture of different Pb components, which then precludes any significant age information.Variations in the 208Pb*/206Pb* cannot be checked in this respect because they primarily reflect changing Th/U of the evaporated zircon domain (Kl�otzli.18) Basic statistics have to be made using the raw data because of the non-linear age transformation obscuring any relevant non-Gaussian distributions. Reports are in the form of plots giving the most important parameters: evaporation temperature, 207Pb*/206Pb* ratios and ages, number of ages, mean values, 208Pb*/206Pb* and Th/U ratios (see examples).Error Assessment Proper error assessment is one of the major problems in analytical geochronology and is often not rigorously done (York,34 Ludwig,35,37 Mattinson,4 Roddick et al.,36 Kl�otzli25). Very often ages reported from single zircon evaporation analysis include errors which are simply derived from the internal analytical scatter of individual ages excluding any external error sources.Such a simple approach to error assessment is neither justified nor correct and should be avoided completely in as much as the involved mathematics for the correct error calculation are rather simple. In the present report all errors (except errors on the decay constants of the U isotopes) are propagated into the final mean ages. Error propagation is done using the standard Gaussian error propagation formula. Errors incorporated in calculations are: errors on individual ratios from counting statistics, errors derived from fractionation correction factors, errors from standard measurements and from the recommended standard values, and the weighted errors from individual temperature steps.If a common Pb correction is applied to the age data the precision of the common Pb isotopic composition is incorporated as well. Precision and Accuracy The accuracy of the method is demonstrated by a number of studies comparing single zircon evaporation data with conventional U/Pb data or with ion probe data (i.e., Ansdell and Kyser,30 Kl�otzli,26 Kl�otzli and Parrish,22 Kober,14 Kr�oner and Seng�or,38 Kr�oner et al.,39 M�uller et al.,40 Peindl and H�ock,41 Kl�otzli-Chowanetz et al.,29 Kl�otzli et al.33).The internal precision of the method is defined above, but is not of significant interest in a geological context. The more important external precision of the method can only be assessed by a complete error propagation scheme as shown above and comparison with the external reproducibility of individual zircons with the same age.At present no internationally recommended standard zircon is available for zircon evaporation analysis. In house reproducibilities of 27 analyses from a zircon population from an Ordovician alkali gneiss are in the range of 487.2 ± 9.7 Ma (2%). It is thus assumed that under normal circumstances the external precision of the method is in the range of 1–5%, depending mostly on the age range investigated (Kl�otzli,25 Bernhard et al.42).Zircon evaporation age data is interpreted to be significant if at least 3 crystals of a sample population exhibit (each within at least 2 temperature steps) within error concordance. It is then assumed that such age data reflects true concordant 207Pb*/206Pb* ages which can then be interpreted as geologically meaningful. Examples Age data derived from zircon evaporation analysis is often reported in the form of histograms showing age range versus number of ages or mass scans or blocks of the highest temperature steps from a number of zircons.The amount of information provided with such diagrams is rather scarce, sometimes even misleading. For instance, no individual errors are incorporated into a simple histogram and the reported mean age does not necessarily correspond to the most frequent 207Pb*/ 206Pb* ratio or age. All age information from lower temperature steps and from 208Pb*/206Pb* ratios is lost or omitted. If such a compilation is shown it should be done in the form of probability density plots of ages and not as histograms with arbitrary class widths.The examples presented here show all major aspects of single zircon evaporation dating, their possible representation and interpretation. Example 1 (Fig. 4) presents two plots of evaporation temperature versus 207Pb*/206Pb* age for two zircons from the paragneiss–migmatite boundary of the Winnebach migmatite in the Upper Austroalpine � Otztal–Stubai nappe of the Eastern Alps, Austria (Kl�otzli-Chowanetz et al.29).Width of plotted boxes is the estimated error on an evaporation temperature of ±10 °C. The height of the boxes is calculated as 2s of the mean age of individual temperature steps. Assigned errors of mean ages reported are all 2sm. The examples clearly demonstrate the complementary ‘behaviour’ of zircons during evaporation analysis.Evaporation of crystal 8830-E (Fig. 4) resulted in a perfect age plateau of 484 ± 6 Ma over 8 evaporation steps ranging from 1350 toation of zircon 8830-C (Fig. 4) resulted in a staircase of increasing ages with increasing evaporation temperature. Both zircons were evaporated to completeness. The age plateau of 8830-E is interpreted to represent the crystallisation age of a core-free zircon (at least in respect to Pb isotopic systematics). Based on zircon typology and additional conventional zircon dating, the crystallisation event is attributed to the migmatite formation during the Ordovician.Zircon 8830-C exhibits (within error for the two first evaporation steps) the same age for the migmatite formation event at 480 ± 6 Ma. The single step ages of 561 and 632 Ma possibly represent older metamorphic events. Similar but better defined ages were found in other zircons from the same locality, further supporting this interpretation. The higher temperature staircase ( > 1460 °C) is interpreted as representing a mixture of an old, possibly Archean Pb component with Pb of Cambrian or Proterozoic age.If no additional information for the two older metamorphic events would exist, the age staircase must be interpreted as representing a mixture between the Archean and the Ordovician Pb component. The age of 2355 ± 85 Ma is interpreted as representing a minimum age estimate for the formation or recrystallisation of an inherited zircon core.The evaporation temperature profile of zircon 8830-C directly proves within one zircon crystal the existence of differently old 1246 Analyst, November 1997, Vol. 122Pb components with individual levels of activation energy which can very effectively be separated by evaporation analysis (Kl�otzli18). Example 2 (Fig. 5) presents two plots of block number versus 207Pb*/206Pb* and 208Pb*/206Pb* for two zircons from granitoids of the South Bohemian Pluton, Austria (Kl�otzli and Parrish,22 Kl�otzli et al.33) The block number gives the progress of evaporation with steps of increasing temperature as indicated by labelled evaporation temperatures (in °C).Each block represents the mean of 10 mass scans. For easier reading errors on individual blocks are not shown. They are in the range of the symbol size. Assigned errors of mean ages reported are 2sm. Both zircons demonstrate the presence of an inherited core and a later overgrowth.Inherited cores and overgrowth could be analysed by at least two evaporation steps thus providing plateau ages which can be interpreted as being geologically meaningful. For zircon 4690-A both ages (336 ± 4 Ma and 635 ± 12 Ma, respectively) are interpreted to represent magmatic growth events. This interpretation is further supported by the large intra- and inter-evaporation step variation in 208Pb*/206Pb* at constant 207Pb*/206Pb* indicative for magmatic Th/U variation in the evaporated zircon domain.The analysis of the deposit of the third evaporation step of 4690-A (at 1515 °C) shows the typical pattern of a reversed deposit (Kl�otzli18), direct evidence for the mixing of differently old zircon domains during evaporation. The 208Pb*/ 206Pb* spectrum obtained for the inherited core of 2E92-B (535 ± 8 Ma) does not show significant variation. This reflects constant Th/U ratios throughout the inherited core. This is interpreted as reflecting crystal homogenisation during a high temperature metamorphic overprint leading to the formation of charnockitic rocks (Kl�otzli et al.32).The exact meaning of the Variscan overgrowth (352 ± 3 Ma) is still a matter of debate. Outlook To further enhance the precision of 207Pb*/206Pb* ages from zircon evaporation analysis static SEM-IC data acquisition using multi-collector SEM-IC systems has to be established. It should be possible to achieve the same precision for Pb isotopic analysis as is routinely found for multi-collector Faraday systems (i.e., 10 times better in precision as at present).One possible way of upgrading is by substituting the Faraday cups of a MAT 262 with ion-counting channeltrons as demonstrated by Fig. 4 Plots of evaporation temperature versus 207Pb*/206Pb* age for two zircons from the paragneiss–migmatite boundary of the Winnebach migmatite in the Upper Austroalpine � Otztal-Stubai nappe of the Eastern Alps, Austria (Kl�otzli-Chowanetz et al.29).Width of plotted boxes is estimated error on evaporation temperature of ±10 °C. Height of boxes is calculated as 2s of the mean age of individual temperature steps. For discussion see text. Fig. 5 Plots of block number versus 207Pb*/206Pb* and 208Pb*/206Pb* for two zircons from granitoids of the South Bohemian Pluton, Austria (Kl�otzli and Parrish,22 Kl�otzli et al.33) Block number gives progress of evaporation with steps of increasing temperature as indicated by labelled evaporation temperatures (in °C). Each block represents the mean of 10 mass scans. For easier reading errors on individual blocks are not shown. They are in the range of the symbol size. Assigned errors of mean ages reported are 2sm. For discussion see text. Analyst, November 1997, Vol. 122 1247Richter et al.43 or by using a newly designed Wien-filter TIMS (Laue et al.44). Additionally, the zircon mounting and encasing procedures can substantially be improved by using preformed Re filaments and micro-manipulators. 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Paper 7/04114D Received June 12, 1997 Accepted September 1, 1997 1248 Analyst, Nov