Chemical Geology. Pb Pb dating of chondrules from CV chondrites by progressive dissolution. J.N. Connelly a,b,, M. Bizzarro a

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1 Chemical Geology 259 (2009) Contents lists available at ScienceDirect Chemical Geology journal homepage: Pb Pb dating of chondrules from CV chondrites by progressive dissolution J.N. Connelly a,b,, M. Bizzarro a a Geological Museum, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen, Denmark b The Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, 78712, USA article info abstract Article history: Received 15 April 2008 Received in revised form 25 October 2008 Accepted 1 November 2008 Editor: D. Rickard Keywords: Chondrules CV chondrite Pb-Pb Geochronology Progressive dissolution To refine absolute Pb Pb ages of chondrules, we have experimented with progressive dissolution procedures of chondrules in the CV chondrite Allende in an attempt to effectively remove terrestrial contamination and generate Pb fractions of sufficient size and spread in 207 Pb 206 Pb space to adequately define meaningful isochrons. Samples are extricated from the matrix, ultrasonicated, abraded and repeatedly pre-cleaned by ultrasonicating in distilled ethanol, water and acetone. Subsequent steps of weak HCl and HBr acids combined with ultrasonic agitation and modest heating effectively removes terrestrial contamination in most samples we analyzed. Subsequent exposure to warm 4 M HNO 3 typically isolates the least-radiogenic component of the stepwise dissolution series. Continuing the dissolution steps with HCl of increasing molarity with longer and higher temperature heating steps returns more radiogenic Pb fractions. The most radiogenic Pb fraction typically corresponds to the first warm 1 M HF step with subsequent steps in stronger HF yielding progressively less radiogenic signatures. Importantly, the Pb components extracted in the dissolution of the final residues with 28 M HF+14 M HNO 3 typically lie slightly below the isochron defined by previous steps, corresponding to a slightly younger 207 Pb /206 Pb model age than the inferred real age. The wide range of total Pb contents (20 N2000 pg) in different fractions returned by this method requires analyses by TIMS using a high-efficiency Pb emitter, 202 Pb/ 205 Pb double spike and diligent monitoring of laboratory Pb blank. As expected, U and Pb are highly and variably fractionated during the procedure such that no U/Pb age information is recovered. A multi-chondrule fraction from the CV chondrite Allende yields an age of ±0.81 Ma, which is statistically younger than calcium aluminum inclusions from the CV chondrite Efremovka Elsevier B.V. All rights reserved. 1. Introduction As the earliest formed solids in the solar system, calcium aluminum inclusions (CAIs) and chondrules contain petrographic, geochemical and isotopic records that constrain the evolution of the first few million years of the solar nebula. Available petrographic, U Pb and Al Mg isotopic data indicate that CAIs formed first and in a brief interval of perhaps less than 100,000 yr at ±0.15 Ma (Amelin et al., 2002; Bizzarro et al., 2004; Thrane et al., 2006; Amelin et al., 2006). Conversely, textures in chondrules indicate that they have a more complicated history that may include extensive and, in some cases, repeated reworking (Scott and Krot, 2005). As expected, this greater degree of complexity is also expressed by the isotopic data that infer a range of ages that span the first 3 Myr of the Solar System (MacPherson et al., 1995; Russell et al., 1996; Kita et al., 2000; Huss et al., 2001; Kita et al., 2005). Bizzarro et al. (2004) presented the first high-precision internal Al Mg isochron for a single chondrule in the CV chondrite Allende that corresponds to an age of 0.25 Myr after CAI formation. They concluded that at least a subset of Corresponding author. Geological Museum, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen, Denmark. address: connelly@mail.utexas.edu (J.N. Connelly). chondrules formed very early, approximately synchronous with CAI formation. Bizzarro et al. (2004) and Thrane et al. (2006) present compelling evidence to support the homogeneous distribution of 26 Al in at least the CAI formation region and the accretion region of the terrestrial planets. However, there exists an alternative proposal that 26 Al and other extinct nuclides formed heterogeneously throughout the disk by solar irradiation such that initial 26 Al contents may reflect different production rates rather than different ages (Shu et al., 1997; Lee et al., 1998; Shu et al., 2001; Gounelle et al., 2001). Unconstrained by any chronological certainty, this general model allows for CAIs and chondrules to have formed coevally and for isotopic and compositional differences to reflect different regions of a heterogeneous disk. With CAIs from the CV chondrite Efremovka precisely dated by Amelin et al. (2002, 2006), resolution of this debate requires a robust absolute age for chondrules from this same meteorite group. This task has been complicated by ubiquitous terrestrial Pb contamination in meteorites and the model dependency of Pb Pb ages determined from highly-radiogenic Pb separates resulting from aggressive leaching. In these cases, a lack of spread in 204 Pb/ 206 Pb vs 207 Pb/ 206 Pb space requires that the initial Pb isotopic composition be modeled or approximated by some proxy measurement. Whereas only a bonafide mineral (i.e /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.chemgeo

2 144 J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) internal) isochron can assure fulfillment of all prerequisite conditions for a reliable age determination, the fine-grained nature of chondrules makes mineral separation difficult. The current study was initiated to refine U Pb dating methods for chondrules to produce a robust model independent Pb Pb age for chondrules from Allende and, thus, to test the validity of the 26 Al 26 Mg chronometer. We evaluated methods to produce internal isochrons for chondrules by stepwise dissolution that exploits the range of solubilities of different chondrule phases. This method provides a means to evaluate whether terrestrial contamination is successfully removed and the projected initial Pb isotopic composition is reasonable as well as returning sufficient spread in Pb Pb space to reduce the final error on calculated ages The U Pb chronometer The U Pb isotopic system represents the only high-resolution absolute chronometer for dating early solar system materials but its utility is commonly compromised by complex systematics related to contamination by terrestrial Pb. Pb Pb ages may be calculated courtesy of the duality of the U Pb radiogenic system where 238 Udecaysinaseriesto 206 Pb and 235 U decays in a series to 207 Pb. Given the different decay rates of the two U isotopes, radiogenic Pb (Pb r ) will have a 207 Pb/ 206 Pb ratio that is uniquely related to the time before present that the sample formed or when Pb diffusion ceased in the sample. A sample comprising only Pb r would yield an accurate and precise age that corresponds to the formation age of that sample provided all Pb r wasformedinsituandthatthe 238 U/ 235 Uofthe sample is known. However, most, if not all, samples contain a component of initial Pb (Pb i ) such that it is not generally possible to isolate a pure Pb r component. Therefore, the 207 Pb/ 206 Pb ratio of Pb r must be determined by extrapolation of a line defined by analyses of fractions with different mixtures of Pb r and Pb i plotted in 204 Pb/ 206 Pb vs 207 Pb/ 206 Pb space to the y- axis where 204 Pb/ 206 Pb, and hence Pb i, are equal to zero. Identifying the appropriate 207 Pb/ 206 Pb ratio of Pb r with an acceptable error by this method requires that there are sufficiently radiogenic fractions with adequate spread to constrain the line and, therefore, the intercept. Ages determined from a highly-radiogenic component that is isolated by intense acid leaching exploits a short extrapolation to the y-axis to minimize error in the final age determination, but even modest values of 204 Pb/ 206 Pb will require an accurate estimate of the isotopic composition of Pb i to calculate an accurate age. Meteorites contain a third component of Pb, namely terrestrial contaminant Pb (Pb c ), which may comprise a number of different subcomponents with distinct isotopic compositions, including a ubiquitous laboratory blank. This last sub-component should represent a well characterized, minor constituent that may be accounted for in the final isotopic ratios whereas extraneous contamination (field, curation, etc) cannot be accurately subtracted. As such, determination of an accurate and precise age from Pb isotopic analyses generally requires that all Pb c is removed from the sample prior to dissolution such that only binary mixtures of Pb r and Pb i are represented in fractions used to define an isochron age. Assurances of complete removal of Pb c is best provided by linearity of data points and the extrapolation of lines to reasonable estimates of Pb i. It should be noted that pure mixtures of Pb r and Pb c will also define lines that project to the correct 207 Pb/ 206 Pb of Pb r but it is typically difficult to assess whether Pb i is completely absent. Recent work by Y. Amelin (e.g. Amelin et al., 2002; Krot et al., 2005; Amelin and Krot, 2007) using the Pb isotopic system has demonstrated that highly-radiogenic Pb can be isolated in residues of some aggressively acid leached meteorite samples. This permits precise ages to be calculated by either regressing multiple analyses of related materials or by determining or assuming the isotopic composition of Pb i for purposes of extrapolation through a single radiogenic point or cluster to a composition reflecting a pure Pb r. Any error in assigning the initial Pb i composition will result in an inaccurate age, despite the high precision resulting from the short extrapolation. This study tests whether stepwise dissolution of chondrules from Allende (CV chondrite) will effectively first remove Pb c and then parse the remaining Pb i and Pb r into separate fractions with sufficient spread in Pb Pb space to define an appropriate, well-constrained internal isochron that would better characterize the isotopic composition of Pb r and, therefore, the age of a sample. Linearity of points from this method would provide an internal test of whether a single growth evolution is likely for the sample analyzed and remove necessary assumptions about Pb i, thereby increasing confidence in the calculated compositions of Pb r. 2. Methods Chondrules (single or multiple) or chondrule fragments were extracted from a single, four centimetre diameter sample of Allende meteorite that was coarsely broken in a hydraulic press with the sample wrapped in paper. Chondrules from the resulting fragments were then excavated from matrix fragments and pre-cleaned using dental tools. Chondrules to be analyzed were ultrasonicated in distilled water until matrix was removed from the surface of the chondrule, as determined by inspection using a binocular microscope at 25 magnification. Samples were subsequently abraded in a microabrader lined with corundum-coated mylar until the fragment or chondrule exterior was smooth (24 56 h). Fractions comprising single chondrules, multiple chondrules or chondrule fragments (Table 1) were taken to the clean laboratory for a pre-cleaning treatment that comprised repeated ultrasonicating and rinsing in ultraclean water. This pre-cleaning procedure was repeated 3 times beyond the point when the water was clear after 5 min of ultrasonicating. None of these washes were processed for isotopic analyses. Some samples were coarsely crushed using an agate mortar and pestle whereas other samples were processed as whole chondrules or fragments (Table 1). Samples that were crushed were pre-cleaned again by repeated ultrasonicating and rinsing in ultraclean water. The samples were transferred to a 7 ml Savillex vial and were treated to a second precleaning step in water, acetone, ethanol and very dilute HCl while gently heated (75 C) on a hotplate and/or ultrasonicated. This was repeated for a minimum of 5 cycles of acetone/ethanol/water. After a final rinse 3 times in water, each fraction was subjected to a series of different acid steps as defined in Table 1. The acid was carefully removed from the residue and transferred into a clean 7 ml Savillex capsule using a pre-cleaned Eppendorf pipette after which the residue was rinsed and ultrasonicated twice in water only the first water rinse was added to the preceding acid step. To ensure dissolution of fine, suspended material that was inadvertently transferred to the Savillex capsule, approximately 0.1 ml of 28 M HF was added to fractions that lacked this acid after it was isolated from the residue. Centrifuging may be a better method of removing the suspended material but this was not evaluated for fear of increasing the Pb blank during handling. The fractions were dried down, re-dissolved in concentrated HCl HNO 3 mixtures to bring them fully in solution at which time a mixed 202 Pb 205 Pb 235 U spike ( 205 Pb/ 202 Pb= ; 205 Pb/ 206 Pb=1208; 205 Pb/ 207 Pb=1620; 205 Pb/ 208 Pb=496; 205 Pb/ 204 Pb = 11410; 205 Pb/ 235 U=119.66; 235 U/ 238 U=1405; spike prepared by F. Corfu, Oslo University and calibrated against NBS-982) was added to each fraction to determine Pb isotopic mass fractionation during analyses (Amelin and Davis, 2006) and elemental abundances. This mixture was dried down and finally re-dissolved in 1 M HBr, the appropriate acid for the first chemical separation step. Pb was separated from the matrix in two passes through ml anion columns following Lugmair and Galer (1992) except that the samples were loaded in the first pass onto the columns with 1 M HBr. Uranium was separated from the wash solution from the Pb chemistry using U-TEVA resin chemistry after Davis et al. (1997). Pb from different chondrite fractions was analysed on two mass spectrometers in two laboratories following the same protocol and using a Pb emitter made from silicic acid after Gerstenberger and Haase (1997). All beam intensities larger than 2 mv were analysed by faraday detectors in static mode. Samples with a 204 Pb signal intensity too small for a faraday detector but with large enough intensities of the other Pb masses (including 202 Pb

3 J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) Table 1 Isotopic data for Allende chondrules

4 146 J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) and 205 Pb) were analysed by faraday detectors in static mode but with 204 Pb in an axial secondary electron multiplier ion counting system (SEM-IC; Finnigan MAT 262, Oslo) or Daly photomultiplier (VG-354, Copenhagen). To avoid the necessity of an accurate SEM-IC vs. faraday calibration, these samples were also analyzed dynamically at lower filament temperatures with all masses analyzed sequentially measured in an axial SEM-IC or Daly. Samples with all beams too small to be effectively measured by faraday detectors were dynamically analysed with all masses sequentially measured in an axial SEM-IC or Daly detector. Analyses using the Daly detector in Copenhagen are less accurate than those from Oslo as the former instrument has not been well calibrated for small Pb samples. As such, the data from Copenhagen from fractions C1, C2, C4 and C7 were used only to evaluate and refine the stepwise dissolution procedures. Using information gained in these early tests, Allende fraction C12 was subjected to our optimized step dissolution sequence and were analysed at the University of Oslo to determine an accurate age. The mass spectrometer protocols used for each fraction are listed in Table 1. Final ratios, errors and error correlations (Table 1) were calculated using an in-house program and PbDat (v. 1.24, Ludwig, 1993). Ages were calculated using Isoplot Ex v. 3 (Ludwig, 2003), decay constants of Jaffey et al. (1971) and a natural 238 U/ 235 U ratio of Chemical analyses of fractions of Allende chondrule C12 were produced by an Elan 6100 quadrupole ICP-MS at the Geological Survey of Denmark and Greenland. 3. Results 3.1. Allende chondrules C2, C4 and C7 To evaluate the effects of different acids and acid strengths in stepwise dissolution of chondrules, three samples comprising abraded, pre-cleaned, uncrushed, single or multiple chondrules were treated by slightly different methodsasdescribed intable 1. The results are shown graphically in Fig. 1A C. Chondrule fractions C2, C4 and C7 share the common attribute that the early steps are dominated by Pb i with lesser but varying degrees of Pb c and Pb r.thefirst acid steps of C4 and C7 chondrules plot on a line between primordial Pb (PAT, Tatsumoto et al., 1973) and a subset of the remaining fractions indicating that most of the Pb c was effectively removed in the pre-cleaning steps (ethanol acetone water weak HCl on a hotplate and in an ultrasonicator). In general, the isotopic composition of the Pb extracted in progressively stronger HCl becomes more radiogenic falling on or close to a line defined by the majority of the fractions in each sample. Consistent departures from the general pattern of pre-hf steps becoming more radiogenic include the HBr step in C4 and C7, which contains a more radiogenic signature than subsequent 6 M HCl steps (Fig. 1A C). This implies that HBr removed a radiogenic phase and, as demonstrated in C7, may extract a phase containing Pb c that was not affected by earlier steps of 2 M HCl and 4 M HNO 3. It is also clear that HBr does not liberate large quantities Pb of either Pb r or Pb i when applied after 2MHCland4MHNO 3. 6 M HCl extracted Pb in roughly equal proportions from Pb i and Pb r reservoirs of C4 and C7 but had apparently nearly exhausted the Pb i componentinc2asl7hadonly6pgofpbandl9and L10 were very radiogenic. The first application of HF, regardless of acid strength, consistently liberates the most radiogenic Pb. Sample C2 was attacked with 28 M HF 14 M HNO 3 as the first HF step after 6 M HCl, producing a fraction with highly radiogenic Pb consistent with the Pb i reservoir having been nearly exhausted by this point. HF steps of chondrules C4 and C7 show the interesting and important property that the first HF step extracts the most radiogenic Pb and that increased HF strength becomes slightly more enriched in Pb i, indicating that this component had not been as effectively removed in earlier steps of C4 and C7 relative to that in C2. Also important to note is that the dissolution of the final residual fractions in C4 and C7 with 28 M HF 14 M HNO 3 in a bomb at 200 C falls below the line defined by the majority of points indicating that they still contained a component of Pb c that had not been previously digested or that there was an undocumented additional component of Pb blank in the last step. Presuming the line defined by the majority of earlier steps corresponds to the real age of these chondrules, it is clear that including the component represented in the final dissolution of the residue of C4 and C7 in an isochron would have resulted in a slightly younger age. In summary, the approach of stepwise dissolution of samples with increasingly stronger acids returns the desired result of producing progressively more radiogenic fractions that define a line corresponding to an isochron. A modest pre-cleaning with ethanol, acetone, water and very weak HCl on a hotplate and in an ultrasonic is effective in removing most of the terrestrial contamination of these chondrules. The most obvious difference between these three procedures lies in the degree to which the final residues are radiogenic. Whereas this may represent compositional variations between chondrules, it may also reflect the aggressiveness of acid dissolution/leaching prior to the application of HF. The major difference between C2 vs C4/C7 stepwise dissolution is the more aggressive use of 4 M HNO 3 early in the procedure of C2, which apparently removes phases containing Pb i more effectively than HCl Allende chondrule C1 After obtaining the results from the dissolution of fractions C2, C4 and C7, a large (39 mg) chondrule was broken into two pieces that were 6.4 mg and 33.0 mg (labeled C1a and C1b, respectively). Chondrule C1a, processed first to characterize this sample, was subjected to a full stepwise dissolution procedure but only the final dissolution step was analyzed as an unspiked sample. Correction for blank Pb was based on signal intensity and experience with other fractions of comparable size and dissolution sequences. The full dissolution of the final residue represents the only HF step in this sequence and produced a radiogenic signature that exceeded all subsequent analyses of C1b (Fig. 1D). The remaining larger piece of chondrule (C1b) was processed differently than C1a, with the former lacking an early HNO 3 step and the dissolution of silicates was facilitated by a series of progressively stronger HF steps. This chondrule responded differently than C2, C4 and C7 in that it lacked a systematic shift from a composition dominated by Pb i to more radiogenic compositions and the early steps, most notably L1-5 in 2 M HCl, are much more radiogenic than comparable steps from the other chondrules. Conversely, none of the steps produced a radiogenic signature as high as the final residue of C1a. The erratic jumps in relative proportions of Pb i and Pb r in the Notes to Table 1 Hp =hotplate. US=ultrasonicator. =not included in age regression. a -Mode of mass spectrometer. b -Mass spectrometer facility. SF=static mode with all masses measured in faraday collectors. DD=all masses measured in dynamic mode using a daly - photomultiplier ion counting system. SF-DD6/4=all masses measured in static mode using faraday collectors except the 206Pb/204Pb ratio which was collected in dynamic mode using a daly - photomultiplier ion counting system. =unspiked fractions.

5 J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) Fig. 1. A E. 204 Pb/ 206 Pb vs 207 Pb/ 206 Pb diagrams for Allende fractions with each fraction labeled with sequence number, acid used and total Pb (after blank correction) in parentheses. Dark square symbols represent those fractions that define a line whereas the light square symbols represent those fractions that do not fall on a line. MT = average modern terrestrial Pb (Stacey and Kramers, 1975), PAT = primordial Pb defined by Tatsumoto et al., 1973). progressive steps indicates that there is a reservoir or reservoirs of Pb i that was not effectively removed early in the stepwise dissolution procedure and is, instead, intermittently sourced by different steps. Most obvious is the fact that L11, which used 28 M HF and the first use of HNO 3 in this sequence, contains a less radiogenic Pb than earlier steps in HCl. The dissolution array for C1a and C1b collectively highlights the importance of using HNO 3 early in the procedure to effectively remove apb i hosting phase that is not effectively removed by HCl Allende multi-chondrule fraction C12 Integrating the results from chondrules C1 C7, a stepwise dissolution procedure was devised for the multi-chondrule fraction C12 to obtain an accurate and precise age by: 1) optimizing the removal of Pb c early and with minimum loss of either Pb i or Pb r, and 2) maximizing the dispersion in Pb Pb space of subsequent dissolution steps by preferentially isolating Pb i early in the dissolution process with 4 M HNO 3.This

6 148 J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) Chemical data for chondrule C12 Fig Pb/ 206 Pb vs 207 Pb/ 206 Pb diagrams for Allende chondrule fractions from C12 that define an isochron. Fractions labeled with sequence number, acid used and total Pb (after blank correction) in parentheses. Error ellipses represent 2-sigma errors. Errors and error correlations were calculated using PbDat (Ludwig,1993) whereas ages, errors on ages and the mean square of weighted deviates (MSWD) of isochron utilized Isoplot Ex v.3 (Ludwig, 2003). fraction also tested whether the most radiogenic component is returned in lowest molarity HF of Pb c and whether the final residue consistently contains remnant Pb c. The results have been presented in Connelly et al. (2008a,b) and are shown in Fig. 1E. Recognizing that 2 M HBr has minimum effect on at least those phases hosting Pb i, this acid was selected to start the dissolution procedure. Steps L1 (2 M HBr) and L2 (2 M HCl) fall below the line defined by the remaining steps (except L10) and are inferred to contain a significant component of Pb c (approximately 33% and 27%, respectively). These early steps successfully remove Pb c as the next 6 steps define a statistically acceptable line that passes through PAT and is inferred to represent an isochron corresponding to an age of ±0.81 Ma (mean square of weighted diviates [MSWD]=0.59; Fig. 2). The first step in this progression (4 M HNO 3 that was heated to 110 C for 20 min) yielded 992 pg of Pb that was dominated by Pb i. A subsequent step of water heated overnight contained no measurable Pb. The next three steps in HCl (L5 L7) contain a total of 277 pg of Pb with similar isotopic compositions reflecting subequal amounts of Pb i and Pb r. The HF steps began with cold 1 M HF that yielded the most radiogenic component with a 204 Pb/ 206 Pb ratio of Subsequent HF steps liberated a component of Pb from a Pb i source that had not been exhausted by earlier steps. The full dissolution of the final residue plots below the isochron (Fig. 1E), indicating that a phase with a minor amount of Pb c must have persisted through all previous steps. A total of 2.42 ng of Pb was extracted from this multi-chondrule fraction for an average Pb concentration of 0.11 ppm. To better understand the sequence in which phases were breaking down during the stepwise dissolution procedure, the washes of each step of C12 from the anion exchange chemistry were analysed for major elements plus Co, Ni and P (Table 2). The results do not provide a unique solution but rather a general guide as to which major silicate phases broke down at different steps in the procedure. Using the fingerprint of low Mg/Na and Mg/Al ratios of plagioclase, it is clear that the first two steps L1 and L2 are dominated by this phase. The high Mg/Al, Mg/Na, Mg/Ti, Mg/Ca in the subsequent steps L3 and L4 indicate that olivine began to break down during these steps. Given the high olivine/ plagioclase modal abundances of these chondrules, it is possible that plagioclase continued to breakdown through these steps but its chemical signature was overwhelmed by the olivine signal. The decrease in all these ratios in L5 but with a Mg/Na ratio higher than L1 and L2 indicates a contribution by pyroxene by this step. The remainder of the steps contain different proportions of olivine and pyroxene except for the last step, which appears to be dominated by pyroxene as indicated by the high Mg/Fe and Mg/Na ratios and intermediate Mg/Al and Mg/Ca ratios. These data do not account for sulfides or oxide phases that may have been present. Sulphides and phosphates are expected to have low and high 238 U/ 204 Pb (μ) values, respectively, and breakdown early in the procedure. Oxides likely have very little effect on the U Pb budget but would break down late in the procedure. Assuming the μ values of the three major silicates increase in the order of plagioclase olivine pyroxene, the silicate breakdown modeled after geochemical data is broadly consistent with the Pb Pb data, except for the last HF steps where Pb r /Pb i decreases. The Pb budget of the early steps is likely strongly influenced by sulfides present in the chondrules with the predominance of Pb i in the early steps indicating the effective breakdown of sulfides with HCl and, especially, HNO Evaluation of blank Pb correction Blank subtraction is critical to the final age, error and fit ofthe isochron derived from the blank corrected data. The amount of Pb c contributed by chemistry and mass spectrometry has been determined over the course of this study by direct measurement of blank samples processed through chemistry this has yielded an average blank estimate of 1.8±0.5 pg (n=10) that includes the mass spectrometer loading blank of approximately 0.4 pg. A full procedural blank is estimated by mimicking all steps, including transfer of acids and cleaning solutions by Eppendorf pipettes, of a full dissolution series to yield one blank estimate for every fraction analyzed. This has yielded an average full procedural blank of 3.0±1.0 pg (n=15). Diligent care in sample preparation and careful monitoring of Pb blanks through the full procedure assures that Pb blanks are consistent and, therefore, correctable to within the assigned error on the blank. In turn, this error is propagated through the calculation such that the blank Table 2 Chemical analyses of washes after Pb separation for dissolution steps of Allende chondrule fraction C12 Al Ca Fe Mg Mn K Na Ti Co Ni P C12-L1 2M HBr C12-L2 2M HCl C12-L3 4M HNO C12-L4 H 2 O C12-L5 2M HCl C12-L6 6M HCl C12-L7 6M HCl C12-L8 1M HF C12-L9 1M HF C12-L10 28M HF NOTE: Concentration in ppm of solution run by an Elan 6100 quadrupole ICP-MS at the Geological Survey of Denmark and Greenland relative to 50 and 250 ppb standard solutions. Aliquots of the washes from the Pb chemistry were dried down and redissolved in 10 ml of 1M HCl for analyses.

7 J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) after leaching of chondrules by Amelin and Krot (2007).This may reflect true variation between chondrules or that Amelin and Krot (2007) employed a leaching procedure that more effectively attacked the phases containing Pb i prior to final dissolution of the residue. An alternative possibility is that the more protracted dissolution procedure using a wider range of acids and acid strengths may have compromised the eventual integrity of the high μ phases. If correct, this may require parallel processing of two samples, one by a protracted dissolution procedure intended to provide a spread of Pb compositions and the other following the procedures of Amelin and Krot (2007) intended to provide highlyradiogenic values. In some cases, mineral separates of high μ phases may be necessary to insure highly-radiogenic phases Defining initial Pb (Pb i ) Fig. 3. Plot shows variation of MSWD (mean square of weighted deviates y axis) for the data uniformly corrected for different amounts of Pb blank (x axis). The minimumvalue for the MSWD between 1.5 and 3.0 pg supports the assigned Pb blank of 3.0±1.0 pg of Pb. error is reflected in error on the reported age. Since the sample to blank ratio varies from fraction to fraction, a second check of whether blank is appropriately subtracted comes from assessment of the quality of fit of the regression of the data array as a function of the blank assignment. An inappropriate blank assignment would tend to randomize a data array that would otherwise define an acceptable isochron. A plot of the linearity of the data array (as estimated by the calculated MSWD) as a function of different amounts of blank assigned uniformly to all fractions (Fig. 3) shows a minimum at approximately 2.5 pg. Blank assignments less than 1.5 pg and greater than 3.5 pg results in an array that does not define an acceptable line in Pb Pb inverse isochron space. This is inferred to indicate that the appropriate blank assignment lies within this range, an assessment consistent with the independent estimate of 3.0±1.0 pg from multiple blank analyses processed in tandem with samples. 4. Discussion Aggressive acid cleaning of meteorite samples reported in previous studies (e.g. Amelin et al., 2002; Krot et al., 2005) prior to full digestion of the residue has the demonstrated advantage of removing Pb c and Pb i, thereby producing highly-radiogenic analyses that results in a short or no extrapolation in Pb Pb space to an inferred pure Pb r composition. However, even minor amounts of residual Pb i require an assumption or definition of Pb i to calculate an accurate age. Stepwise dissolution procedures defined here appear to effectively remove Pb c in the first steps while preserving Pb i and Pb r in the residue that can be partitioned in subsequent steps allowing for the definition of true internal isochrons in Pb Pb space. More work is involved in processing multiple steps from a single sample but linearity of different fractions by this method tests whether: 1) Pb c has been successfully removed early in the procedure, 2) an appropriate Pb i is inferred from the isochron, and 3) any initial Pb is present with a highly-radiogenic signature resulting from an initial in growth due to a high U/Pb (so-called aborted high µ Pb ).FailuretoremovePb c early in the procedure will result in Pb in subsequent steps being derived from a three-component system, which will cause scatter of points in Pb Pb space. Furthermore, nothing is theoretically lost in this approach as samples are processed such that the final residue should represent a radiogenic fraction comparable to what would have been obtained by analyzing a single final residue after aggressive leaching/cleaning procedures. However, we note that, except for fractions C2-L9 and C2-R, the most radiogenic fractions analysed in this study are less radiogenic than the analyses of residues Pb Pb dating requires either direct measurement or robust estimate of the 207 Pb/ 206 Pb of Pb r. As such, it is a common goal of most Pb Pb chronological studies to select and process samples to derive a highly-radiogenic Pb fraction that minimizes the extrapolation to the intercept, which corresponds to the best estimate of the pure Pb r component. However, even a sample with a highlyradiogenic composition corresponding to a 204 Pb/ 206 Pb ratio of will return an offset of ca. 1 Myr for a model age depending on whether the initial composition is inferred to be that of averaged modern terrestrial Pb (MT, Stacey and Kramers, 1975) or that of primordial Pb (PAT, Tatsumoto et al., 1973). MT is never preferentially assigned instead of PAT as an estimate of Pb i in a meteorite when calculating model ages but some data arrays defined by multiple analyses project through MT rather than PAT (Baker et al., 2005; Amelin and Krot, 2007). This infers that Pb i in that sample was either more primitive than PAT (i.e. pre-pat; see Tera and Carlson, 1999) or that MT was not fully removed from the fractions processed. The prospect of Pb isotopic heterogeneity in the early Solar System allows for the possibility of a pre-pat Pb composition, but there is no data to directly support such a scenario. Attempts to measure the Pb isotopic composition of iron meteorites as proxies for the Pb isotopic composition in the early Solar System have all concluded that the values of Canyon Diablo measured by Tatsumoto et al. (1973) are representative (Oversby, 1973; Göpel et al., 1985; Connelly et al., 2008a,b). A more primitive Pb isotopic composition than that of PAT might be confirmed by Pb isotopic analyses of CAIs, chondrules and/or differentiated meteorites that are both more and less radiogenic than MT (to ensure that the line is not a mixture of highly radiogenic Pb and MT) and that projects back to a pre-pat isotopic composition. That no such analyses are known infers that a pre-pat reservoir did not exist and that data arrays defined by radiogenic fractions that extrapolate through MT are likely caused by Pb r mixed with variable amounts of Pb c rather than Pb i. Except in rare cases in which a sample contains no Pb i, the inclusion of MT in analyses used to constrain Pb i will return an age older than the true age. A linear array of points in 204 Pb/ 206 Pb vs 207 Pb/ 206 Pb space that projects through or near PAT rather than MT provides assurance that MT has been successfully removed and that a permissive Pb i component is inferred by the isochron. The step dissolution method defined here provides sufficient spread in 204 Pb/ 206 Pb vs 207 Pb/ 206 Pb space to evaluate whether MT has been successfully removed in pre-cleaning and early steps and that the inferred isotopic composition of Pb i is reasonable with respect to our estimate of PAT. The issue of defining and evaluating Pb i is seemingly overcome in cases where aggressive leaching returns residual fractions that, within error, have a 204 Pb/ 206 Pb ratio of zero or near zero. However, since this scenario does not allow for evaluation of whether an internal isochron exists in Pb Pb space, it allows for the possibility that a highlyradiogenic Pb i component from the precursor or source reservoir is present. Undetected pre-crystallization radiogenic Pb i incorporated in any meteorite will make the final model age older than the true age of

8 150 J.N. Connelly, M. Bizzarro / Chemical Geology 259 (2009) crystallization. In such a scenario, stepwise leaching is anticipated to generate a spread of points along the y-axis of a Pb Pb plot Matrix Pb (Pb m ) The matrix in Allende has Pb concentrations of ca. 1 ppm, approximately 10 times that of chondrules, and has varying Pb isotopic compositions that fall on or near the line defined by the preferred fractions of chondrule C12 (Tatsumoto et al., 1976 and Chen and Tilton, 1976). Given this concentration contrast, it is vital that all matrix is removed prior to analyses to avoid the non-radiogenic component being dominated by Pb from the matrix, thus voiding the basic assumption for an accurate isochron age calculation that the non-radiogenic Pb component must be initial Pb. Since matrix Pb will be approximately collinear with analyses of materials formed in the first few million years of the solar system, alternative means must be used to ensure matrix Pb was not included in the analyses. First, we are confident that the ultrasonicating and abrasion treatments prior to the stepwise dissolution was effective in removing matrix material from the chondrule. Inspection at 25 magnification using a binocular microscope confirmed that no dark matrix was visible against the contrast of the lighter chondrule at this resolution after these treatments. Secondly, chondrule C12 contained 1.6 ng of non-radiogenic Pb, representing approximately 67% of the total amount of Pb. With concentrations of Pb in the matrix at 1 ppm (Tatsumoto et al., 1976 and Chen and Tilton, 1976), approximately 1.6 mg of matrix (or 8% of our sample) would have been inadvertently included in the analyses if all non-radiogenic Pb was derived from matrix. The lack of any observable matrix at 25 magnification precludes this possibility. As such, we conclude that the non-radiogenic Pb in C12 represents initial Pb that was incorporated into the chondrule when it formed such that this basic assumption for calculating an accurate isochron is fulfilled Age of Allende chondrules We report an age of ±0.81 Ma for a multi-chondrule fraction from the CV chondrite Allende. Amelin and Krot (2007) report a preferred Pb Pb age for multigrain fractions of Allende chondrules of ±1.0 Ma based on the average age of seven wash residue sets (2 washes, 1 residue). From the same data set, the regression of the eight residues yields an age of ±2.4 Ma (MSWD=0.76) whereas the average of eight residue model ages (using PAT as Pb i ) is ±0.8 Ma (MSWD=0.85). Their preferred age of ±1.0 Ma and the age reported here for a multigrain fraction of chondrules overlap within error but the age cited by Amelin and Krot (2007) is skewed to a slightly older value and overlaps withtheacceptedageofcaiformation(amelin et al., 2002). Reasons for this offset may include: 1) a real difference in ages of the chondrules in the two studies, 2) a systematic bias in mass spectrometry results of one or both of the analytical data sets, 3) an inappropriate correction for instrumental mass fractionation in one or both of the data sets, 4) inappropriate laboratory blank correction and/or 5) an inappropriate assumption that the chondrule washes accurately represent the initial Pb in the chondrules analysed by Amelin and Krot (2007). Given that both studies analysed multiple chondrule fractions of the same meteorite, it seems unlikely that there is a systematic age difference in the average of these two data sets. We presume that both studies report accurate raw ratios within their respective stated external reproducibilities. A significantly inappropriate blank correctionwould cause a bias in the data but both studies report low and reproducible blanks such that this seems unlikely to generate a systematic bias. The definition of a statistically valid line in this study for chondrule fraction C12 also suggests that an appropriate amount of blank Pb has been subtracted. Consequently, the offset in ages between the two studies most likely reflects either an inappropriate correction for instrumental mass fractionation or an inappropriate assumption that the washes accurately reflect initial Pb in the chondrules. Given that a double spike was not yet available to Amelin and Krot (2007), the accuracy of the mass fractionation correction applied to their samples cannot be assessed. Use of the washes to constrain the isochron requires that the washes reflect a simple binary mix of initial and radiogenic Pb, a condition that necessitates that all terrestrial contamination was removed in earlier cleaning/leaching steps. Assuming that the isotopic composition of terrestrial contaminant would lie beneath a true isochron, a residual component of terrestrial contamination would result in an older estimated 207 Pb/ 206 Pb age. The HNO 3 washes appear to form a field rather than a line with an upper bounding surface in 204 Pb/ 206 Pb vs 207 Pb/ 206 Pb space corresponding to the line defined by the accepted C12 fractions of this study, which approximately intersects primordial Pb. As such, it appears that some of the washes of Amelin and Krot (2007) contain a minor component of terrestrial contamination that skews their wash-residue ages to ages that are slightly older than the age determined here. Recognizing that the data of Amelin and Krot (2007) is more radiogenic than any points defined in this study but lacks sufficient spread to provide an evaluation of Pb i, Connelly et al. (2008a) combined the two data sets to define an isochron corresponding to an age of ±0.45 Ma (MSWD=1.02). The isochron projects through PAT, indicating that Pb c was successfully removed and these chondrules evolved as a closed system since ±0.45 Ma. This age is 1.66±0.48 Myr younger than CAI formation in CV chondrites (Amelin et al., 2002), requiring that these chondrules formed or were reprocessed after CAIs. As discussed in Connelly et al. (2008a), this age offset is consistent with that predicted by the short-lived 26 Al 26 Mg chronometric system and, thus, validates the underlying assumptions of homogeneous distribution of 26 Al in the early Solar System (see Kita et al., 2005 for review). 5. Conclusions The method of stepwise dissolution of chondrules outlined in this paper provides a means to remove terrestrial contamination and create sufficient spread in Pb Pb space to define an internal isochron and the radiogenic 207 Pb/ 206 Pb composition. Acceptable linearity of a sufficient number of data points provides assurance that terrestrial Pb contamination was successfully removed and that Pb evolved in a single-stage and as a closed system. This method is made viable by the recent capacity to accurately and precisely analyze small amounts of Pb by TIMS (sub 50 pg) using a 202 Pb 205 Pb double spike and high-efficiency emitter while maintaining a consistently low blank. This method was successfully applied to a multi-chondrule fraction from CV chondrite Allende. Experiments with single and multiple chondrule fractions from Allende served to define the appropriate acid sequence that was ultimately used to date chondrule fraction C12 at ±0.81 Ma. Combining this data with the more radiogenic analyses of Amelin and Krot (2007), Connelly et al. (2008a) report an age of ±0.45 Ma for Allende chondrules that supports 26 Al/ 26 Mg relative age offsets between chondrules and CAIs in CV chondrites (Kita et al., 2005). Lastly, this method appears to be effective for Pb Pb geochronology on other matrixes including a differentiated meteorite from the angrite group (SAH99555; Connelly et al., 2008b). Acknowledgements Financial support for this project was provided by the Danish National Science Foundation, the Carlsberg Foundation and the Jackson School of Geosciences at The University of Texas at Austin. Fernando Corfu is gratefully acknowledged for generously providing 202 Pb 205 Pb double spike and access to his mass spectrometry facilities at the University of Oslo. References Amelin, Y., Davis, W.J., Isotopic analysis of lead in sub-nanogram quantitites by TIMS using a 202 Pb 205 Pb spike. 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