Variable Tl, Pb, and Cd concentrations and isotope compositions of enstatite and ordinary chondrites—Evidence for volatile element mobilization and decay of extinct 205Pb

New Tl, Pb, and Cd concentration and Tl, Pb isotope data are presented for enstatite as well as L‐ and LL‐type ordinary chondrites, with additional Cd stable isotope results for the former. All three chondrite suites have Tl and Cd contents that vary by more than 1–2 orders of magnitude but Pb concentrations are more uniform, as a result of terrestrial Pb contamination. Model calculations based on Pb isotope compositions indicate that for more than half of the samples, more than 50% of the measured Pb contents are due to addition of modern terrestrial Pb. In part, this is responsible for the relatively young and imprecise Pb‐Pb ages determined for EH, L, and LL chondrites, which are hence only of limited chronological utility. In contrast, four particularly pristine EL chondrites define a precise Pb‐Pb cooling age of 4559 ± 6 Ma. The enstatite chondrites (ECs) have highly variable ε114/110Cd of between about +3 and +70 due to stable isotope fractionation from thermal and shock metamorphism. Furthermore, nearly all enstatite meteorites display ε205Tl values from −3.3 to +0.8, while a single anomalous sample is highly fractionated in both Tl and Cd isotopes. The majority of the ECs thereby define a correlation of ε205Tl with ε114/110Cd, which suggests that at least some of the Tl isotope variability reflects stable isotope fractionation rather than radiogenic ingrowth of 205Tl from 205Pb decay. Considering L chondrites, most ε205Tl values range between −4 and +1, while two outliers with ε205Tl ≤ −10 are indicative of stable isotope fractionation. Considering only those L chondrites which are least likely to feature Pb contamination or stable Tl isotope effects, the results are in accord with the former presence of live 205Pb on the parent body, with an initial 205Pb/204Pb = (1.5 ± 1.4) × 10−4, which suggests late equilibration of the Pb‐Tl system 26–113 Ma after carbonaceous chondrites (CCs). The LL chondrites display highly variable ε205Tl values from −12.5 to +14.9, also indicative of stable isotope effects. However, the data for three pristine LL3/LL4 chondrites display an excellent correlation between ε205Tl and 204Pb/203Tl. This defines an initial 205Pb/204Pb of (1.4 ± 0.3) × 10−4, equivalent to a 205Pb‐205Tl cooling age of 55 + 12/−24 Ma (31–67 Ma) after CCs.


INTRODUCTION
Extinct radionuclide systems are powerful cosmochemical tools that provide quantitative information on the presolar production sites of elements as well as the time scales of early solar system [Correction added on 23, after first online publication: One of the authors' names was changed from Carl Palk to Emeliana Palk after publication.] processes. The nuclide 205 Pb, which decays to 205 Tl with a half-life of 15.1 Ma (Pengra et al. 1978), has thereby received attention for many years. This stems from the suggestion that it is unique among extinct radionuclides, as it may be produced solely by the s-process of nucleosynthesis (e.g., Blake et al. 1973;Yokoi et al. 1985;Wasserburg et al. 1994Wasserburg et al. , 2006. However, it has also been posited that significant 205 Pb can be generated by "mini r-processing" in exploding massive stars (Heymann and Liffman 1986). Aside from its possible role as a unique nucleosynthetic tracer, the 205 Pb-205 Tl decay system has many potential uses in chronometry. Both Pb and Tl are volatile and moderately siderophile elements, so that the Pb/Tl ratios of solar system materials are affected by numerous processes, including partial evaporation and condensation, metal-silicate equilibration, and fractional crystallization of solid metal during the cooling of planetary cores. As a result, the 205 Pb-205 Tl decay system may, in principle, be useful for dating a range of early solar system processes, including volatilization during thermal processing, planetary differentiation, and core crystallization.
Astrophysical models, which predict that 205 Pb was sufficiently abundant in the early solar system to produce meteorite reservoirs with distinct Tl isotope compositions, prompted a number of early studies of the 205 Pb-205 Tl decay system (Anders and Stevens 1960;Ostic et al. 1969;Huey and Kohman 1972;Arden 1983;Chen and Wasserburg 1994). Definitive evidence for the former presence of live 205 Pb in the solar system remained elusive, however, until the investigation of Nielsen et al. (2006a). The latter study found a strong positive correlation between e 205 Tl and 204 Pb/ 203 Tl ratios for metal samples of IAB iron meteorites, and this was interpreted as an isochron from in situ decay of formerly live 205 Pb, following metal segregation and crystallization. While this isochron demonstrated that live 205 Pb was present in the solar nebula, it failed to link the 205 Pb-205 Tl decay system to an absolute time scale and hence the initial solar system abundance of 205 Pb remained uncertain.
Such a link, which is necessary to enable chronological application of the 205 Pb-205 Tl system, was first established by Baker et al. (2010). These workers analyzed a comprehensive suite of carbonaceous chondrites (CCs) that defined an isochron, which was interpreted to record volatile element fractionation in the solar nebula at close to 4567 Ma with an initial solar system 205 Pb abundance of ( 205 Pb/ 204 Pb) SS,0 = (1.0 AE 0.4) 9 10 À3 . Further evidence for radiogenic variations in Tl isotope compositions was obtained in a 205  Tl study of IIAB and IIIAB iron meteorites (Andreasen et al. 2012). Notably, Andreasen et al. (2012) also re-evaluated the CC isochron of Baker et al. (2010) in light of their iron meteorite results. In detail, they questioned the accuracy of the Baker et al. (2010) 205 Pb-205 Tl isochron for CCs because it included a strongly weathered desert find (NWA 801, CR2; Connolly et al. 2007) and a sample of Colony CO3 with significant Pb contamination. Following elimination of these meteorites, Andreasen et al. (2012) inferred a revised initial solar system value of ( 205 Pb/ 204 Pb) SS,0 = 2 AE 1 9 10 À3 and considered this result as more robust because it does not conflict with their 205 Pb-205 Tl data for IIAB irons. Given the uncertainties inherent in the 205  Tl isochron studies of both Baker et al. (2010) and Andreasen et al. (2012), an initial solar system 205 Pb abundance that combines both results to ( 205 Pb/ 204 Pb) SS,0 = (1.8 AE 1.2) 9 10 À3 is applied here in the following.
The present study follows on from the investigations of Nielsen et al. (2006a), Baker et al. (2010), and Andreasen et al. (2012) to explore whether bulk rock samples of enstatite chondrites (ECs) and ordinary chondrites (OCs) display Tl isotope variations from the decay of 205 Pb, which can be exploited for chronology. Such work, however, must cope with two distinct problems that beset the application of the 205 Pb-205 Tl decay system.
First, Tl shows significant stable isotope fractionation in various terrestrial environments and meteorites (Rehk€ amper et al. 2002;Nielsen et al. 2006aNielsen et al. , 2006bNielsen et al. , 2006cNielsen et al. , 2017Nielsen and Rehk€ amper 2012). While such fractionations were initially deemed surprising due to the high atomic weight of Tl (Rehk€ amper and Halliday 1999), it has now been shown that stable Tl isotope effects are quite common in nature and of both mass-dependent and non-mass-dependent origin, whereby the latter are related to differences in nuclear volume and charge distribution (Schauble 2007). Such fractionations are problematic as Tl has only two stable isotopes, so there is no means to robustly differentiate between isotopic variations that are caused by radiogenic ingrowth of 205 Tl and stable isotope effects. Furthermore, stable isotope fractionation of Tl during nebular and parent body processes is supported by the highly mobile and volatile nature of the element (with a 50% condensation temperature of T C50 = 532 K; Lodders 2003), and evidence for such effects has already been found in both CCs and iron meteorites (Nielsen et al. 2006a;Baker et al. 2010). To screen their results for possible stable Tl isotope effects in CCs, Baker et al. (2010) employed stable isotope analyses of Cd. This approach to employ Cd as a monitor element is based on previous investigations, which revealed that Cd isotope compositions are readily altered by partial evaporation and condensation (Wombacher et al. 2003(Wombacher et al. , 2004(Wombacher et al. , 2008. The assumption that stable isotope fractionations of Cd and Tl are often at least roughly coupled is not unreasonable, given that Cd has a condensation temperature (of T C50 = 652 K; Lodders 2003) similar to Tl and both elements are known to be readily volatilized and mobilized during heating and metamorphic processes (Matza and Lipschutz 1977;Ngo and Lipschutz 1980;Paul and Lipschutz 1989;Wombacher et al. 2008;Tonui et al. 2014). In addition, both Tl and Cd are thought to be concentrated in troilite during nebular condensation (Lodders 2003) and this similarity in distribution is highlighted by the excellent correlation seen in this study for the Tl and Cd concentrations of both ECs and OCs. The application of Cd stable isotopes as a monitor for Tl isotope fractionations is, furthermore, supported by the absence of radiogenic isotope effects for Cd and the availability of five major Cd isotopes ( 110 Cd to 114 Cd) with abundances of >10%, which enable precise and accurate stable isotope measurements with the double spike technique (Ripperger and Rehk€ amper 2007;Rehk€ amper et al. 2011). While a decoupled isotope fractionation of these two elements cannot be excluded, Cd isotope analyses presently provide the most robust means to monitor possible Tl stable isotope fractionation. Hence, such analyses were also applied here for the ECs, while previous work has already shown that Cd stable isotope fractionations appear to be fairly common for OCs, with large isotope effects (of >5& for 110 Cd/ 114 Cd) recorded in some samples (Wombacher et al. 2008).
Second, many stony and iron meteorites, including falls, are pervasively contaminated by modern terrestrial Pb (e.g., G€ opel et al. 1985;Tera and Carlson 1999). Such contamination has an impact on 205 Pb-205 Tl isochron studies because they require the original uncontaminated indigenous "meteoritic" 204 Pb/ 203 Tl ratios, but these can be difficult, if not impossible, to recover. For some meteorites (e.g., iron meteorite metals), Pb contamination can be readily detected and quantified via Pb isotope analyses but corrections for such effects are problematic if contamination is severe and/or the indigenous meteoritic Pb isotope composition is not precisely defined. In targeted Pb isotope investigations of meteorites, the effects of terrestrial Pb contamination are commonly mitigated or avoided by extensive physical cleaning and chemical leaching of samples (G€ opel et al. 1985;Amelin 2006). Such an approach was not employed here because leaching has the potential to generate significant stable isotope effects for Tl (e.g., Nielsen et al. 2013). Hence, Pb isotope measurements were carried out for all samples and these data in combination with Pb-Tl elemental ratios were employed to assess and, using model calculations, semiquantitatively determine the extent of terrestrial Pb contamination.

SAMPLES
A total of nine ECs were chosen for this study, encompassing three EH and six EL meteorites, of which five are falls (Table 1). In addition, 20 OCs were analyzed. In this case, all samples are falls, encompassing 10 L chondrites and 10 LL chondrites of petrological grades 3-6 (Table 1). While the EL6 chondrite NWA 3134 was purchased from a private collector and the EL3 chondrites MAC 02839 and PCA 91020 were obtained from the Antarctic Meteorite Collection of the NASA Johnson Space Center, all other meteorites were provided by the Natural History Museum London. The samples, obtained in the form of small chips, were initially prepared for analysis by sanding down sawn surfaces with silicon carbide paper and grinding to a powder using an alumina mortar and pestle.

Laboratory and Reagents
The analytical work was carried out in the mass spectrometry and ISO 6 clean room laboratories of the Imperial College MAGIC Research Center. All critical sample handling was performed in ISO 4 laminar flow hoods, using Teflon PFA beakers, >18.2 MΩ cm water, and mineral acids that were purified in-house using quartz or Teflon stills. Saturated bromine water (abbreviated as Br 2 -H 2 O in the following) and 0.1 M HCl containing 5% dissolved SO 2 (0.1 M HCl-5% SO 2 ) were made up following published methods (Rehk€ amper and Halliday 1999). All mixtures between mineral acids and between mineral acids and Br 2 -H 2 O were thereby prepared on a v/v basis, while the HCl-SO 2 mixture was made up on a w/w basis. An overview of the full analytical procedure is provided in Fig. S1 in supporting information.

Sample Digestion and Aliquoting
Approximately 1.5-2 g of meteorite powder was initially digested with a mixture of 5 mL 15 M HNO 3 and 6-7 mL 28 M HF on a hotplate for several days, dried, refluxed in 5-10 mL of a 1 + 1 mixture of 15 M HNO 3 and 6 M HCl, and dried again. Following this, the samples were taken up in 5-10 mL 6 M HCl, refluxed, and then centrifuged in 15 mL PP tubes. Any residues, if present, were subjected to further acid digestion with the described procedure, but using reduced acid volumes, until perfect solutions were obtained. The final centrifuged sample solutions were then split into three aliquots (1) a split of~2.5% was used for Tl and Pb concentration measurements by isotope dilution (ID); (2) a similarly small aliquot was taken for an initial determination of Cd concentrations by ID; (3) the majority of the solutions were reserved for the determination of Tl, Pb, and (for the ECs) Cd isotope compositions (see Fig. S1).

Samples for ID Concentration Measurements of Tl, Pb, and Cd
Highly enriched tracer solutions of 203 Tl and 208 Pb were added to and equilibrated with the Tl-Pb ID sample aliquots. The solutions were then dried, Shock stages from Bennett andMcSween (1996), Friedrich et al. (2004), Gattacceca et al. (2005), Izawa et al. (2011), Rubin (1994, 2004, , and St€ offler et al. (1991). The uncertainties for the concentration data are AE2% for Tl and Pb, AE2% for Cd in ECs, and AE10% for Cd in OCs. The quoted uncertainties for the isotopic data were determined as follows: for e 205 Tl based on 2SD of multiple bracketing standard runs; for e 114/110 Cd based on internal 2SE precision of sample runs (typically similar to 2SD of multiple standard runs), using a minimum value of 0.5; for Pb isotopes based on 2SE precision of samples and/or bracketing standard runs. The calculated f met-Pb values denote the molar fraction of indigenous meteoritic Pb present in the analyzed samples (see supporting information). a MAC 02839 and MAC 02837 are paired (Righter and Satterwhite 2004). b Wiggenhauser et al. (2016). c Baker et al. (2009). d Prytulak et al. (2013). e Hettmann et al. (2014). f Selected "best" values from GeoReM database (Jochum et al. 2005 Table S1 in supporting information).
The Cd ID sample aliquots were spiked with a 113 Cd-111 Cd double spike (Xue et al. 2012), evaporated to dryness, and taken up again in~500 lL 3 M HCl. The ion exchange procedure that was then applied for the separation of Cd was modified from the methodology of Wombacher et al. (2003) (see Table S2  Samples for Determination of Tl, Pb, and Cd Isotope Compositions The employed chromatographic methods were modified from Baker et al. (2010) and enable the separation and purification of separate Tl, Pb, and Cd fractions from a single solution aliquot prior to isotopic analysis. To this end, an appropriate weight of 113 Cd-111 Cd double spike was first added to the major sample aliquots from the initial digestion. Following equilibration by refluxing, the solutions were dried and the residues were taken up in 1 M HCl-2% Br 2 -H 2 O, ready for the first stage of the purification procedure. In this first step, a combined Pb-Cd and a separate Tl fraction were isolated from the sample matrix, using a procedure modified from the initial chemistry protocol employed by Baker et al. (2010) (see Fig. S1; Table S3 in supporting information). Importantly, this new method achieves nearly quantitative yields for all three elements on a routine basis.
A further small resin column was thereafter employed for the purification of the Tl fractions, following methods modified from Rehk€ amper and Halliday (1999) (see Table S4 in supporting information). The combined Pb-Cd fractions were dried and processed through a second stage of anion exchange chromatography to separate Pb and Cd from each other and remaining matrix elements (see Table S5 of the supporting information). The Pb fractions obtained in this second stage of the purification procedure were already sufficiently clean for isotopic analysis, while additional cleanup steps were necessary for Cd. In detail, Cd purification was achieved in two stages using small resin columns following procedures modified from Wombacher et al. (2003). An initial Cd separation by anion exchange chromatography was thereby followed by a cleanup column with TRU resin to better isolate Cd from potentially problematic elements, such as Sn and Mo (see Table S6 in supporting information).

Mass Spectrometry
All isotopic measurements were carried out with a Nu Instruments Nu Plasma HR MC-ICP-MS, using either an Aridus, Aridus II, or a Nu Instruments DSN desolvating sample introduction system fitted with glass or PFA nebulizers that were operated at a flow rate of about 100-130 lL min À1 . Prior to the analyses, the purified elemental fractions were dissolved in an appropriate volume of either 0.1 M HNO 3 (for Pb and Cd) or 0.1 M HNO 3 -0.1% H 2 SO 4 (v/v; for Tl).

Isotope Dilution Concentration Measurements for Tl, Pb, and Cd
The purified Tl ID fractions were doped with NIST SRM 981 Pb and an in-house Pt standard solution, and both 208 Pb/ 206 Pb and 198 Pt/ 196 Pt were monitored during the 203 Tl/ 205 Tl ID measurements for correction of the instrumental mass bias. While the mass bias correction of the Tl data using Pb provides more precise results (due to the highly correlated mass bias behavior of both elements), the less precise Pt-normalized Tl isotope data are superior for samples that contain small residual amounts of sample-derived Pb. Such sample-derived Pb produces systematic errors in the mass bias correction and may be present in the Tl ID samples, as the Tl is not purified by a second step of column chemistry (Table S2) (Nielsen et al. 2004). The Pb ID fractions were doped with NIST SRM 997 Tl, allowing use of the 205 Tl/ 203 Tl ratios for the correction of the instrumental mass bias encountered by 208 Pb/ 206 Pb. For Cd, the 111 Cd/ 110 Cd ratios were employed for the ID concentration calculations to avoid isotopes that suffer from isobaric interferences of Sn, which was present in traces in the Cd ID fractions.

Isotope Composition Measurements for Tl, Pb, and Cd
The determination of Tl isotope compositions largely followed the procedures of Baker et al. (2009). The purified Tl fractions were thereby doped with NIST SRM 981 Pb and a combination of external normalization to 208 Pb/ 206 Pb and standard-sample bracketing was applied for mass bias correction. All results are reported in e notation, whereby The Pb isotope measurements were conducted using standard-sample bracketing and external normalization relative to the 205 Tl/ 203 Tl ratio of admixed NIST SRM 997 Tl for mass bias correction. The analytical methods were similar to those outlined in Rehk€ amper and Mezger (2000), and all results are reported relative to the NIST SRM 981 Pb data of Galer and Abouchami (1998).
The stable Cd isotope measurements utilized previously described data acquisition and reduction methods (Ripperger and Rehk€ amper 2007;Xue et al. 2012). The Cd isotope data collected during the mass spectrometric analyses of the sample-double spike mixtures-were processed to determine the "true" isotope compositions of the unspiked samples, which were corrected for both instrumental mass bias and isobaric interferences. All results are reported in e 114/110 Cd notation, calculated analogous to Equation 1 using the 114 Cd /110 Cd isotope ratio. The e 114/110 Cd data are thereby given relative to NIST SRM 3108 Cd (Abouchami et al. 2013).

Blanks and Geological Reference Materials
Full procedural blanks were processed through the digestion and chemical separation procedure with every batch of 6-12 samples. For Tl and Cd, the mean blanks were about 5 and 50 pg, respectively (all blanks given are for a 100% sample aliquot). For samples with the lowest Tl and Cd contents of about 0.1 and 0.5 ng g À1 , respectively (Table 1), this amounts to blank contributions to the total element contents of about 3% for Tl and 5% for Cd. The great majority of the samples, however, are more enriched in these elements and the blank contributions are therefore mostly <1%. Given these observations, the Tl and Cd concentrations were corrected for blank contributions, while no corrections were undertaken for the isotope compositions, because such corrections are smaller than the analytical uncertainty for most samples and are poorly constrained for all (as the isotope composition of the blank was not determined).
The total procedural Pb blanks were significantly higher at between 0.9 and 2.8 ng. These relatively high blanks are attributed to the use of modified household hotplates for initial sample digestions, while dedicated Teflon-coated hotplates were employed in subsequent sample preparation steps. The latter conclusion follows from tests with the USGS AGV-2 reference material; when this sample was processed using only Teflon-coated hotplates, the corresponding Pb blanks averaged~90 pg. Despite the relatively high Pb blanks, the blank contribution to the total Pb content of samples is only moderate to negligible. For the samples with the lowest Pb concentrations (~50 ng g À1 ; Table 1), the blank contributes <3% to the total Pb budgets, but 20 of 26 samples have Pb concentration exceeding 200 ng g À1 , where the blank additions amount to <1%. Consequently, the measured Pb concentrations were blank-corrected but no corrections were applied to the isotope compositions, as such corrections would be minor and highly uncertain, given the uncertainty of the blank isotope composition.
Analyses of the USGS geological reference material AGV-2 yielded Tl, Pb, and Cd isotope compositions and Cd concentrations in good agreement with recently published results, thereby demonstrating the robustness of our methods (Table 1).

Enstatite Chondrites
The Tl concentrations of the ECs vary by more than 2 orders of magnitude, from less than 1 ng g À1 to more than 100 ng g À1 (Fig. 1, Table 1). The Cd contents of these meteorites are similarly variable with concentrations from about 1.8 ng g À1 to nearly 860 ng g À1 . Clear systematics are apparent for both elements. First, the analyzed EH and the EL chondrites display similar abundance variations for both elements. Second, the less metamorphosed group of each clan (EH4 and EL3) generally has higher Tl, Cd concentrations compared to the more strongly metamorphosed meteorites (EH5 and EL6).
Comparable systematics are seen for Pb ( Fig. 1, Table 1). For this element, the concentrations in ECs range from <50 ng g À1 to more than 31,000 ng g À1 , with the less metamorphosed EH4/EL3 samples generally displaying higher Pb contents than the more equilibrated samples of groups EH5 and EL6. However, the EL6 Daniel's Kuil has a higher Pb content (of 8000 ng g À1 ) than the two analyzed EL3s (with <1700 ng g À1 Pb), while the EH4 Indarch has an unusually high Pb abundance of more than 31,000 ng g À1 . These latter observations most likely reflect significant contamination of both samples with terrestrial Pb and such contamination is expected to be more severe for Pb than for Tl, Cd, and other elements due to the ubiquity of anthropogenic Pb in the environment.
Notably, the combined EC data set reveals rather well-correlated Cd and Tl concentrations (R 2 = 0.83; Fig. 1a) with a mean Cd/Tl ratio of about 6. The plots of Pb abundance versus Tl and Cd (Figs. 1b and 1c) furthermore reveal that all but three of the samples are in accord with strongly correlated behavior between these elements. The same three samples, Indarch, Daniel's Kuil, and Atlanta, are thereby offset from both correlations (Figs. 1b and 1c) most likely due to recent terrestrial Pb contamination. The remaining meteorites display correlations with R 2 values of 0.99 and 0.88 for the Pb versus Tl and Cd trends, respectively, highlighting that all three elements may be controlled by similar processes and/or concentrated in the same phases in these meteorites.

L Chondrites
The Tl and Cd concentrations of the L chondrites vary by more than an order of magnitude, from about 0.1 to 26 ng g À1 and 0.4 to 7 ng g À1 , respectively ( Fig. 1; Table 1). For both elements, the less metamorphosed samples (of type L3/L4) tend to have higher concentrations than the more equilibrated L5/L6 chondrites. The Pb concentrations of the L chondrites are also highly variable at between 55 and 21,000 ng g À1 ( Fig. 1; Table 1) and there is no obvious correlation with petrographic type. Rather, it is apparent that Pb abundances are particularly variable for meteorites with relatively low Tl and Cd contents. Most likely, this reflects that samples with low indigenous volatile contents (as shown by Tl, Cd) are most susceptible to contamination by modern terrestrial Pb (Figs. 1b and 1c). Given these elemental systematics, it is unsurprising that Cd and Tl display somewhat correlated behavior in L chondrites, while there is no clear correlation of Pb with either Tl or Cd.

LL Chondrites
The Tl and Cd concentrations of LL chondrites vary by roughly 2 orders of magnitude, from about 0.3 to 19 ng g À1 and 0.8 to 290 ng g À1 , respectively ( Fig. 1; Table 1). In contrast to ECs and L chondrites, the correlation between Tl and Cd abundances is less well developed and there is no systematic variation of the concentrations with petrographic grade. In comparison, the Pb contents vary by less than a factor of 10, between about 270 and 1900 ng g À1 and no correlation is apparent between Pb and Tl or Cd concentrations (Figs. 1b and 1c). Rather, Pb abundances are fairly variable at both high and low Tl and Cd contents, most likely due to alteration of the pristine "meteoritic" Pb budgets by terrestrial alteration and contamination.

Overall Concentration Trends
Overall, the ECs and OCs display much more variable Pb, Tl, and Cd contents than the CCs of groups CI, CM, CO, CR, and CV that were analyzed by Baker et al. (2010) (Fig. 1). Baker et al. (2010) did not publish Cd concentrations for the CCs but results for six of their samples are now provided here (see Appendix). In detail, the EC and particularly the OC samples extend to significantly lower Pb, Tl, and Cd contents than the CCs, reflecting more severe volatile element depletion. In the following, the trends of the data are compared with the CI chondritic reference lines, defined by Cd/Tl = 4.76, Pb/Tl = 17.7, and Pb/Cd = 3.72 (Palme and Jones 2005), which are also shown in Fig. 1.
In a plot of Cd versus Tl contents, the EC, OC (and CC) data fall on a generally well-defined trend that essentially overlaps with the CI reference line (Fig. 1a). This suggests that the cosmochemical behavior of Cd and Tl is highly correlated during the volatile depletion processes that occurred in both the solar nebula and on meteorite parent bodies. However, a significant number  (Fig. 1a). Possibly, this may reflect preferential loss of Cd relative to Tl either during nebular processing or volatile remobilization. The lower condensation temperature of Tl in comparison to Cd (532 K versus 652 K; Lodders 2003) thereby argues against the former interpretation, while the latter is supported by the higher metamorphic mobility of Cd (relative to Tl), which has been inferred in previous work (e.g., Matza and Lipschutz 1977;Ngo and Lipschutz 1980;Paul and Lipschutz 1989;Tonui et al. 2014).
In the diagrams of Pb versus Tl and Cd, many EC and most OC samples plot above and to the left of the CI reference lines (Figs. 1b and 1c). These systematics most likely result from a combination of two processes.
First, Pb appears to be less impacted than Tl and Cd by the depletion processes that reduced the volatile element abundances of ECs and OCs relative to CIs. This may be due to the higher condensation temperature of Pb (T C50 = 727 K; Lodders 2003) or because it resides in more resistant phases (Fe alloys) than Tl and Cd, which are primarily hosted by troilite following nebular condensation (Lodders 2003). Second, given that the samples were not chemically leached prior to dissolution, it is likely that many of the meteorites are contaminated with substantial quantities of terrestrial Pb.

Pb Isotope Compositions and Evaluation of Terrestrial Pb Contamination
In the following, the measured Pb isotope compositions are employed to assess the extent to which the analyzed samples were affected by the addition of terrestrial Pb (Fig. 2). A plot of 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb for all ECs and OCs (Fig. 2a) demonstrates that the Pb isotope data fall into three broad groups.
(1) Samples with 206 Pb/ 204 Pb ≤ 13.5 have nearprimordial indigenous meteoritic Pb, similar to that seen in many CCs (e.g., Baker et al. 2010). This group encompasses four ECs, a single L chondrite, and three LL meteorites. (2) Four L chondrites and NWA 3134 EL6 have radiogenic Pb isotope compositions with 206 Pb/ 204 Pb ≥ 25. This radiogenic Pb is most likely also of indigenous origin, reflecting in situ decay of U and Th at high l ( 238 U/ 204 Pb) and variable j ( 232 Th/ 204 Pb) values (e.g., Unruh 1982;G€ opel et al. 1994 (G€ opel et al. 1985). In particular, many samples with such silicate Earth-like Pb isotopes also feature elevated Pb concentrations (Fig. 1) (Baker et al. 2010). The boxes approximate the compositions of the three Pb endmembers of (1) "near-primordial" indigenous meteoritic Pb; (2) "radiogenic Pb" also inferred to be of indigenous origin; and (3)  Pb present in the samples-primordial Pb, radiogenic Pb from in situ U, Th decay (which together comprise the indigenous meteoritic Pb of the meteorites), and extraneous terrestrial Pb from contamination of the meteorites (Fig. 2b). The plot thereby reveals two separate trends of data emanating from (near-) primordial meteoritic Pb characterized by 206 Pb/ 204 Pb ratios of between 9.5 and 10.5. (1) A very steep trend toward a composition characterized by 206 Pb/ 204 Pb > 40 and 204 Pb/ 203 Tl of between about 2 and 20. This is defined by samples with low Pb contents of ≤100 ng g À1 and hence reflects ingrowth of radiogenic Pb formed at high l, in accord with the results of previous studies (e.g., Unruh 1982;G€ opel et al. 1994).
(2) A shallower, curved trend toward 206 Pb/ 204 Pb % 18 and high 204 Pb/ 203 Tl values of >100 reflects the addition of terrestrial Pb. Combining the information from Pb concentrations (Fig. 1) and isotope compositions (Fig. 2) hence suggests that many, if not most, samples with 204 Pb/ 203 Tl ratios larger than about 10-20 are probably significantly affected by terrestrial Pb contamination, and this questions their use in isochron diagrams that attempt to exploit the extinct 205 Pb-205 Tl decay system for chronology. Assessments of data quality must be done with care, however, because natural deviations from the well-defined 204 Pb/ 203 Tl ratio of CCs (1.43 AE 0.14, 2SD; Baker et al. 2010) are expected for ECs and particularly OCs (e.g., Fig. 1). Parnallee, in particular, is characterized by near-primordial Pb with 206 Pb/ 204 Pb = 11.85 and a fairly high 204 Pb/ 203 Tl ratio of 28.1 (Table 1).
The recent 205 Pb-205 Tl isochron studies of CCs (Baker et al. 2010) and iron meteorites (Nielsen et al. 2006a;Andreasen et al. 2012) also encountered problems with terrestrial Pb contamination but were able to apply relatively simple mass balance calculations to quantify and correct for such effects. While such calculations are imprecise, particularly at high levels of contamination due to the unknown isotope composition of the extraneous Pb, they are relatively straightforward because CCs and iron meteorites are characterized by very low l and are hence expected to have primordial or near-primordial indigenous Pb isotope compositions. Due to the presence of radiogenic Pb in our ECs and OCs, a more sophisticated approach must be employed to estimate the isotope composition and mass fraction of indigenous meteoritic Pb in the samples. Such calculations were carried out here using an approach that is explained and discussed in the supporting information. Briefly, the U and Th contents of the samples were first estimated (as they were not measured in this study) and subsequently employed to infer in situ ingrowth of radiogenic Pb. The final estimates for the molar fraction of indigenous meteoritic Pb present in meteorites (f met-Pb ) were then obtained with a solver, which minimized the difference between measured Pb isotope composition of a sample and the calculated mixture (of primordial Pb + radiogenic Pb + terrestrial Pb), using the known (measured) total Pb concentration as an additional constraint (see supporting information for details). The results of these calculations (f met-Pb ; Table 1) take into account uncertainties in the estimated U, Th concentrations of the samples and the isotope composition of the terrestrial contaminant. The quoted uncertainties for f met-Pb (Table 1) are deemed to be reasonable for meteorites where U, Th contents are provided in the literature but they may be larger for samples where such data were not available and mean U, Th concentrations estimated for ECs and OCs were used instead. While all f met-Pb values of Table 1 are model-based estimates, the results for the latter samples must be assessed with particular care and they are thus given in parentheses.
Taking the calculated f met-Pb values at face value, the data reveal that the extent of Pb contamination is highly variable for the analyzed meteorites, ranging from nearly 0% to almost 100%, whereby modern terrestrial Pb provides more than 50% of the presentday Pb content for 15 of 29 samples (Table 1). The calculated extent of contamination is thereby in good agreement with qualitative assessments based on the 204 Pb/ 203 Tl ratios (Fig. 2b). In detail, the calculations confirm that the most contaminated samples are Indarch and Atlanta among the ECs (f met-Pb ≤ 0.2) and Ausson, New Concord, and Marion (Iowa) among L chondrites (f met-Pb < 0.15). For the LL chondrites, the f met-Pb estimates confirm that Benares, Khanpur, Appley Bridge, and Dhurmsala contain not only minor indigenous meteoritic Pb (at <30%) but also reveal similarly severe Pb contamination for other meteorites (Bishunpur, Tuxtuac, Mangwendi) with only Parnallee characterized by f met-Pb > 75% (Table 1).

Enstatite Chondrites
When all ECs are considered, a 207 Pb/ 206 Pb age of 4497 + 85 Ma is obtained, while exclusion of two clearly contaminated samples (Indarch, Atlanta) yields a much more precise result of 4553 AE 18 Ma ( Table 2). The latter age is thereby in good agreement with a Pb-Pb isochron age of 4577 AE 8 Ma obtained by  for the ECs Indarch EH4, Saint Saveur EH5, Saint Mark's EH5, and Hvittis EL6. However, although early investigations suggested a common parent body for EH and EL chondrites (e.g., Kaczaral et al. 1988;Kong et al. 1997), a number of petrographic, elemental, and isotopic studies now indicate that these two groups are derived from separate parent bodies (e.g., Sears et al. 1982;Keil 1989;Lin and Goresy 2002;Savage and Moynier 2013), and individual isochrons are hence discussed in the following.
Following elimination of strongly contaminated Indarch (Table 1), the remaining EH chondrites Abee and St Marks yield a two-point isochron with an age of 4527 Ma. This is clearly younger than the concordant Pb-Pb age of 4582 AE 6 Ma quoted for St Marks  and the 53 Mn-53 Cr isochron ages of~4565 Ma determined for the EH4 chondrites Abee and Indarch (Shukolyukov and Lugmair 2004), which are thought to reflect parent-daughter fractionation during condensation in the solar nebula. The relatively young age obtained here is thereby likely an artifact of the significant terrestrial Pb contamination recorded for our sample of St Marks (Table 1; f met-Pb % 24-58%).
For the EL chondrites, the samples remaining after exclusion of Atlanta (f met-Pb ≤ 19%; Table 1) have relatively pristine Pb isotope compositions, although a detailed evaluation of contamination was not possible for NWA 3134 (see supporting information). Notably, these samples define the most precise and meaningful Pb-Pb isochron of this study, equivalent to an age of 4559 AE 6 Ma. This result is identical, within error, to the less precise Pb-Pb age of 4588 AE 28 Ma for Hvittis EL6  and the~4560 Ma 53 Mn-53 Cr age for Khairpur EL6, which are thought to record metamorphic equilibration on the parent body (Shukolyukov and Lugmair 2004).

Ordinary Chondrites
Using the data for all L chondrites, the 207 Pb/ 206 Pb isochron age is 4533 AE 77 Ma. When three (Ausson, New Concord, and Marion) or five (plus Alfianello and Barwell) samples with the most obvious terrestrial Pb contamination are excluded, similar ages of 4543 AE 87 Ma and 4535 AE 37 Ma, respectively, are obtained (Table 2). Given the uncertainties, these results agree with published Pb-Pb ages of between 4529 and 4553 Ma for three L5/L6 chondrites , of~4550 Ma for L4/L5/L6 chondrites (Unruh 1982), and of about 4525-4555 Ma for silicates and phosphates of L5/L6 chondrites (G€ opel et al. 1994;Bouvier et al. 2007), which together are thought to reflect the protracted metamorphic history of the parent body.
For  G€ opel et al. 1994;Bouvier et al. 2007), which are considered to record metamorphic equilibration and cooling, the agreement is unsurprising given the large uncertainties of our LL chondrite ages.
In summary, the new Pb-Pb ages for OCs are only of limited utility due to the large associated errors. This reflects that precise Pb-Pb dating was not a key aim of this investigation such that no attempts were made to separate and analyze phases with particularly radiogenic Pb isotope compositions (e.g., phosphates and Regression calculated with Isoplot v3.41b using the "Model 2" fit, which assigns equal weights and zero error correlations to each point (Ludwig 2003). "Inverse" Pb-Pb ages derived using present-day 238 U/ 235 U = 137.88, and decay constants of k 235 U = 0.98571 Ga À1 (Mattinson 2010) and k 238 U = 0.155125 Ga À1 (Jaffey et al. 1971). a The low age uncertainty reflects that this is a two-point isochron.
leachates) or remove terrestrial Pb through initial leaching of the samples. As a consequence, even the most pristine OCs analyzed here feature a significant component of terrestrial Pb (Table 1), which overprints the metamorphic cooling history of the samples.

Cd and Tl Isotope Compositions-Evaluation of Stable Isotope Fractionation
Application of the 205 Pb-205 Tl decay system for chronometry is rendered difficult if radiogenic variations in Tl isotope compositions are overlain by stable isotope effects. Such fractionations are evaluated in the following, in particular based on the stable Cd isotope data that were acquired here for ECs (Fig. 3) and are published in the literature for OCs (Wombacher et al. 2003(Wombacher et al. , 2008. For comparison, the Cd isotope compositions of the CCs analyzed by Baker et al. (2010) are also shown in Fig. 3. Notably, most of the latter chondrites (seven of nine samples) show a restricted range of e 114/110 Cd between +1.1 and +5.3. The higher e 114/110 Cd values of the two anomalous samples, Kainsaz CO3 (+15.6) and Leoville CV3 (+32.7), were previously interpreted to reflect stable isotope fractionation that occurred during elemental mobilization associated with heating and metamorphism on the parent body (Wombacher et al. 2008;Baker et al. 2010).

Enstatite Chondrites
The e 114/110 Cd data for the ECs are highly variable with most meteorites (six of eight samples) showing results between +2.9 and +10.4 and two samples (Khairpur EL6; PCA 91020 EL3) with extremely fractionated values of +65.7 and +72.9 (Fig. 3). Overall, the ECs thus display more variable Cd isotope compositions than CCs, which indicate that many samples record significant stable isotope fractionation, most likely as a result of parent body processes. The impact of such processes on Tl isotope compositions is also apparent. In detail, seven of the eight ECs for which isotopic data are available (excluding Khairpur EL6) define a strong linear correlation between e 205 Tl and e 114/110 Cd but the near-perfect correlation (R 2 = 0.99) is largely governed by a single sample, PCA 91020 (Fig. 3a). However, the correlation is still significant (with R 2 = 0.60) if only the six samples with the least fractionated Cd isotope compositions are considered (Fig. 3b). In addition, the elemental data reveal an excellent correlation between Cd and Tl abundances for the ECs (Fig. 1a), a trend that is also broadly linked to petrological type, whereby higher concentrations are generally observed in less equilibrated meteorites.
Together, the observations suggest that the abundances and isotope compositions of Cd and Tl are significantly controlled by the degree to which they were remobilized on the parent bodies. Additional constraints emerge when the data are scrutinized in more detail. In particular, PCA 91020 (EL3) and Abee (EH4) are more enriched in Cd but more depleted in Tl than MAC 02839 (EL3) and Indarch (EH4), respectively (Table 1). It is conceivable that these systematics reflect the distinct behavior of Cd and Tl during thermal metamorphism and mobilization, as has been observed in thermally altered CCs and heating experiments with carbonaceous material (e.g., Matza and Lipschutz 1977;Ngo and Lipschutz 1980;Paul and Lipschutz 1989;Wombacher et al. 2008;Tonui et al. 2014). It may also indicate that processes other than thermal metamorphism affected the Cd and Tl contents of these meteorites. Notably, Abee, with shock grades S2 to S4, shows brecciation from multiple impact events . Elemental mobilization at the high temperatures associated with shock processing may have altered the volatile budgets of this meteorite, although it displays nearly unfractionated isotope compositions (e 114/110 Cd = +3.0; e 205 Tl = À3.3; Table 1). Such isotope fractionation, however, is evident for the most heavily shocked EC analyzed in this study, PCA 91020 with a shock stage of S5 . The finding that this meteorite has the heaviest Tl and Cd isotope compositions recorded in this study (e 205 Tl = +22.2; e 114/110 Cd = +72.9) suggests that stable isotope fractionation is likely associated with the heating and elemental volatilization that occurs during impacts. Similarly, shock effects may also be present in other meteorites, such as Khairpur (EL6, S2; e 114/110 Cd = +65.7) and Atlanta (EL6, S2; e 114/110 Cd = +10.4), even though they nominally record lower shock stages. In this case, the meteorites were possibly highly shocked prior to metamorphism but the petrological shock features were subsequently erased by thermal annealing (Rubin 1983;. Given these uncertainties, it is problematic to unambiguously assign the mobilization and stable isotope fractionation observed for Cd and Tl in ECs to either thermal or shock metamorphism, with both processes likely contributing to the volatile element signatures of most samples.

Ordinary Chondrites
While no stable Cd isotope analyses were carried out for OCs, abundant data for such meteorites are available in the literature (Wombacher et al. 2003(Wombacher et al. , 2008. Notably, the studies of Wombacher and coworkers demonstrate highly variable Cd isotope compositions for OCs, with e 114/110 Cd values ranging from about À110 to +150. A similar isotopic variability was observed for H, L, and LL chondrites and particularly large isotope effects were seen for type 3 OCs. These findings were interpreted in the context of an "onion layer" parent body model for the OCs, as reflecting widespread redistribution and associated isotope fractionation of Cd during numerous episodes of partial evaporation and condensation during thermal metamorphism. However, it was also suggested that elemental redistribution and isotope fractionation was possibly enhanced by impact-induced breakup and reassembly of meteorite parent bodies (Friedrich et al. 2004;Wombacher et al. 2008).
These observations are in accord with the data of the present study. Notably, our samples show highly variable Tl isotope compositions that are unlikely to solely reflect differences in radiogenic ingrowth of 205 Tl at variable Pb/Tl ratios ( Fig. 4; see Discussion section).
Furthermore, most equilibrated L6/LL6 chondrites record a very narrow range of e 205 Tl values from À3.9 to +0.3 and, with only few exceptions (particularly Fig. 4. Plots of e 205 Tl versus measured 204 Pb/ 203 Tl ratios for the enstatite and ordinary chondrites of this study as well as the carbonaceous chondrites analyzed by Baker et al. (2010). Variable scaling is used in panels (a) to (c) to best reveal the structure and systematics of the data. The blue and green hashed lines in panel (b) highlight correlations for particularly pristine samples that are discussed in the text as possible 205 Pb-205 Tl isochrons for L and LL chondrites, respectively. The hashed black lines in panels (b) and (c) denote the 205 Pb-205 Tl isochron of carbonaceous chondrites determined by Baker et al. (2010). Symbols are as defined in the legend of among LL chondrites), the OCs are in accord with higher Tl and Cd concentrations for lower petrographic grades. Together, these results attest to significant stable isotope fractionation of both Cd and Tl during the elemental remobilization that accompanies parent body metamorphism. Additional isotope fractionation during shock metamorphism is also possible but is likely to have played a secondary role. This interpretation follows from the observation that most OCs analyzed in this study record only minor shock (S2, S3), even though there are a few notable exceptions, such as New Concord (S4) and Alfianello (S5) (Table 1). Furthermore, the data set reveals no clear correlations between Tl concentrations or isotope composition and shock grade, in accord with the Cd isotope findings of Wombacher et al. (2008) for OCs.

Tl Isotope Compositions and 204 Pb/ 203 Tl-Evidence for Live 205 Pb
In comparison to the CCs, the ECs and particularly the OCs display significantly larger variations in Tl isotope compositions and 204 Pb/ 203 Tl ratios (Fig. 4). Despite this large variability, evidence for the former existence of live 205 Pb is not straightforward to find, as both Tl isotope fractionation and Pb contamination obscure the effects of 205 Tl ingrowth from the decay of 205 Pb.

Enstatite Chondrites
For ECs, the Tl isotope compositions of all but one sample show variability similar to that seen in the CCs, with e 205 Tl values between À3.0 and +0.8, while the anomalous sample PCA 91020 extends this range to e 205 Tl = +22.2 (Fig. 4a). For 204 Pb/ 203 Tl, most samples fall within a narrow range of between about 1.9 and 2.7, but one from each group (Indarch EH4 and Atlanta EL6) has 204 Pb/ 203 Tl % 14 (Fig. 4b). In addition, Daniel's Kuil is characterized by 204 Pb/ 203 Tl % 42 but this sample is not plotted in Figs. 3 and 4 because isotopic data are not available.
The high 204 Pb/ 203 Tl ratios of Indarch (EH4) and Atlanta (EL6) are almost certainly a product of terrestrial Pb contamination (with f met-Pb <20%; Table 1) and the data are hence unsuitable for isochron calculations. Similarly, the high and hence possibly radiogenic e 205 Tl = +22.2 for PCA 91020 is accompanied by an extremely fractionated Cd isotope composition, suggestive of stable isotope fractionation (Figs. 3a and  4). The remaining five ECs display a far more restricted range of Tl isotope compositions and 204 Pb/ 203 Tl ratios, which might be interpreted to define a correlation that is offset from, but nearly subparallel to, the data array of the CCs (Fig. 4c). Interpretation of this correlation as an isochron is unlikely to be robust, however, given the significant Cd isotope fractionations and the correlated Cd and Tl isotope compositions observed for the ECs (Fig. 3).
In summary, these findings indicate that the e 205 Tl values of ECs may well show some variability that reflects in situ decay of 205 Pb but this cannot be convincingly separated from the stable isotope effects, which are likely to have also impacted Tl isotope compositions. A detailed evaluation of possible radiogenic variations of e 205 Tl is furthermore complicated by the terrestrial Pb contamination, which significantly perturbs the 204 Pb/ 203 Tl ratios of many ECs (Table 1).
Mez€ o-Madaras resembles a CC, both in terms of 204 Pb/ 203 Tl ratio and e 205 Tl (Fig. 4c), in agreement with its highly volatile-rich composition and unequilibrated petrology, which indicates only weak thermal processing. In contrast, the most anomalous Tl isotope compositions are seen for Tennasilm L4 (e 205 Tl % À21) and New Concord L6 (e 205 Tl % À10), even though these meteorites have very different Tl concentrations of about 4.8 ng g À1 and 0.35 ng g À1 , respectively. Given that large Cd stable isotope fractionations were documented for L chondrites (Wombacher et al. 2008), it is likely that the Tl isotope compositions of Tennasilm and New Concord were also altered by stable isotope effects and this cannot be ruled out for other L chondrites.
When considering only those samples which are least likely to be perturbed by terrestrial Pb contamination (with f met-Pb > 0.4) and Tl stable isotope fractionation (e 205 Tl ≥ À10), the L chondrites show limited evidence of live 205 Pb. Focusing on the two most weakly metamorphosed and pristine meteorites, Mez€ o-Madaras L3.7 and Saratov L4, these define a two-point isochron that provides an initial 205 Pb/ 204 Pb = (9.1 AE 4.1) 9 10 À5 (Fig. 4b, blue dashed line), equivalent to a Pb-Tl age of 65 + 17/À25 Ma (or 39-82 Ma) for L3/L4 chondrites relative to CCs. Given the evidence for significant addition of terrestrial Pb to these and other L chondrites (Table 1), it is more useful to investigate possible isochron relationships using 204 Pb/ 203 Tl data that are corrected for such contamination (Table 3; see supporting information). In this case, the two-point isochron of Mez€ o-Madaras and Saratov defines an initial 205 Pb/ 204 Pb of (1.7 AE 1.9) 9 10 À4 . This suggests equilibration of the Pb-Tl system more than 22 Ma after CCs, but is also in accord with the absence of any live 205 Pb on the parent body (Fig. 5, blue dashed line). A similar but slightly more precise result emerges when Mocs 54647 is also considered in the isochron calculation. This equilibrated L6 chondrite displays radiogenic Pb ( 206 Pb/ 204 Pb % 39; Table 1), which is indicative of only limited Pb contamination (f met-Pb = 43-92%) and possibly a very low primordial Pb content (Table 3). An L chondrite isochron defined by Mez€ o-Madaras, Saratov, and Mocs 54647 also yields a highly uncertain initial 205 Pb/ 204 Pb = (1.5 AE 1.4) 9 10 À4 , which is in accord with late equilibration of the Pb-Tl system at 54 + 60/À28 Ma, equivalent to 26-113 Ma after CCs (Fig. 5, solid blue line).

LL Chondrites
Among the chondrites studied to date, the LL group displays the largest range of Tl isotope compositions, with e 205 Tl values of between À12.5 and +14.9. The 204 Pb/ 203 Tl ratios are also highly variable, with results ranging from 0.66 to 152 (Fig. 4) Further results for the two samples with the most extreme Tl isotope compositions, Tuxtuac (e 205 Tl % À12) and Olivenza (e 205 Tl % +15), also give rise to suspicion. Notably, both meteorites are heavily metamorphosed LL5 chondrites. Tuxtuac has a low Pb concentration and a 204 Pb/ 203 Tl ratio that is lower than found in any CC (Figs. 1b and 4b). As it also has a terrestrial Pb isotope composition, the Pb content of this meteorite most likely reflects exchange with terrestrial Pb and overall Pb loss. In contrast, Olivenza features a much higher Pb concentration coupled with relatively high Tl, which produces a 204 Pb/ 203 Tl similar to that of CCs. In addition, it has a relatively low 206 Pb/ 204 Pb % 13.2, indicative of a fairly uncontaminated Pb budget with f met-Pb = 50-68% (Table 1). However, given the comparatively low Pb/Tl ratio and e 205 Tl = +14.9, Olivenza plots far above the 205 Pb-205 Tl isochron of CCs, regardless of the exact slope of the latter (Fig. 4b). For this positive e 205 Tl to be of radiogenic origin, Olivenza would need to be derived from a parent body that was originally characterized by very high Pb/Tl, followed by significant Pb loss, either on the parent asteroid or by the meteoroid. While possible, such an evolution is considered less likely than a scenario whereby the high e 205 Tl is due to stable isotope fractionation. Consideration of only the least metamorphosed (LL3 and LL4) and least contaminated (Table 1) samples, Bishunpur LL3.1, Parnallee LL3.6, and Soko-Banja LL4, hints at a correlation between e 205 Tl and 204 Pb/ 203 Tl (Fig. 4b, dashed green line). If interpreted as an isochron, this correlation defines an initial 205 Pb/ 204 Pb of (1.26 AE 0.17) 9 10 À4 , equivalent to an age of 58 + 11/À24 Ma (34-69 Ma) relative to CCs. Given that the Pb budgets of these meteorites are subject to significant terrestrial contamination (Table 1), it is again more useful to evaluate possible isochronous relationships using 204 Pb/ 203 Tl data corrected for terrestrial Pb contamination. Such a correction yields lower 204 Pb/ 203 Tl ratios for all meteorites but the change is particularly prominent for Soko-Banja, with 204 Pb/ 203 Tl decreasing from~25 to~18 (Table 3). Notably, the contamination-corrected 204 Pb/ 203 Tl data also provide a precisely defined correlation (Fig. 5, green line). This is unlikely to be a fortuitous outcome, given the clear evidence for terrestrial Pb contamination of the samples, but suggests that the correction procedure produces robust results.
Evaluation of the revised isochron yields an initial 205 Pb/ 204 Pb ratio of (1.4 AE 0.3) 9 10 À4 , equivalent to an age of 55 + 12/À24 Ma (31-67 Ma) for LL3/LL4 chondrites after CCs. Notably, this result is remarkably similar to the isochron age obtained using the uncorrected 204 Pb/ 203 Tl ratios. Overall, the data thus provide credible evidence for the former presence of live 205 Pb on the LL chondrite parent body during cooling. The youngest possible 205 Pb-205 Tl age of 31 Ma after CCs is thereby still younger than the Pb-Pb ages of about 4545-4557 Ma, which were determined for silicates and phosphates of LL5/LL6 chondrites G€ opel et al. 1994;Bouvier et al. 2007), and that are thought to record the cooling history of the parent body. This observation suggests a low closure temperature for the 205 Pb-205 Tl system relative to U-Pb closure in silicates (at~1000-1100 K; Cherniak 2001) and phosphates (~800 K; Cherniak et al. 1991), in accord with the highly mobile character of Tl even during moderate thermal overprinting (Matza and Lipschutz 1977;Ngo and Lipschutz 1980).

CONCLUSIONS
Thallium, Pb, and Cd concentration and Tl, Pb isotope analyses were carried out for enstatite as well as L-and LL-type ordinary chondrites, with additional Cd isotope measurements for the former. The Tl and Cd contents of all three suites typically vary by 1-2 orders of magnitude and for the enstatite and L chondrites, the concentrations are well correlated with petrographic grade, whereby less metamorphosed samples have the highest abundances of both elements. This feature was most likely established during parent body metamorphism and associated mobilization of volatile elements. The Pb contents reveal less variability and Pb isotope compositions suggest that this reflects pervasive contamination of many meteorites with terrestrial Pb. Model calculations indicate that more than 50% of the total measured Pb concentrations for 15 of 29 samples are due to addition of modern Pb. In part, this accounts for the relatively young and imprecise Pb-Pb ages determined for EH, L, and LL chondrites, which are of little chronological utility. In contrast, four relatively pristine EL chondrites with f met-Pb > 80% define a precise Pb-Pb age of 4559 AE 6 Ma, in accord with the metamorphic cooling ages determined for equilibrated EL chondrites in previous studies Shukolyukov and Lugmair 2004).
The Cd isotope compositions of the ECs are variable due to stable isotope fractionation, whereby two samples have e 114/110 Cd % +70 and the remainder display e 114/110 Cd values of about +3 to +10. All but one of these meteorites is characterized by e 205 Tl of between À3.3 and +0.8, while PCA 910020 features e 205 Tl % +22, presumably due to coupled fractionation of both Cd and Tl stable isotopes. Excluding PCA 910020 and three meteorites with 204 Pb/ 203 Tl >14 (due to Pb contamination), the remaining five ECs might be interpreted to define a 205 Pb-205 Tl isochron. This interpretation is questionable, however, as a correlation of e 205 Tl with e 114/110 Cd is supported by all but one of these samples. This covariation suggests that at least some of the observed Tl isotope variability reflects stable isotope fractionation rather than radiogenic ingrowth from decay of 205 Pb. The available evidence, including the shock history of the meteorites, thereby suggests that the observed Cd and Tl stable isotope effects are a consequence of elemental mobilization during both thermal and shock metamorphism on the parent body.
The e 205 Tl data for most L chondrites range between À4 and +1 but there are two outliers with e 205 Tl ≤ À10, which probably reflect stable isotope effects associated with elemental remobilization during metamorphism. Evaluation of the samples that show the least evidence for Pb contamination or stable Tl isotope fractionation provides an initial 205 Pb/ 204 Pb of (1.5 AE 1.4) 9 10 À4 , in accord with late equilibration of the 205 Pb-205 Tl system at 26-113 Ma after CCs.
The LL chondrites display highly variable 204 Pb/ 203 Tl ratios and the most variable Tl isotope compositions seen in this study, with e 205 Tl between À12.5 and +14.9. Data evaluation suggests that the highest 204 Pb/ 203 Tl ratios are due to Pb contamination, while the most extreme Tl isotope compositions reflect stable isotope fractionation. Persuasive evidence for in situ decay of 205 Pb is, however, provided by three particularly pristine samples, all LL3/LL4 chondrites, which display an excellent correlation between e 205 Tl and 204 Pb/ 203 Tl. Interpretation of this correlation as an isochron yields an initial 205 Pb/ 204 Pb = (1.4 AE 0.3) 9 10 À4 , equivalent to a metamorphic cooling age of 31-67 Ma for LL3/LL4 chondrites relative to CCs.
In summary, our analyses reveal highly variable indigenous Tl and Cd contents in ECs and OCs, while the Pb concentrations are strongly affected by terrestrial contamination. In addition, the Tl isotope compositions of some but possibly many meteorites were altered by stable isotope fractionation. Together, these issues hinder the search for evidence of live 205 Pb in both ECs and OCs, and restrict the chronological application of the 205 Pb-205 Tl decay system. However, with more basic understanding, the sensitivity with which Tl and Cd concentrations and isotope compositions react to secondary mobilization and alteration processes might be exploited to improve understanding of both nebular and parent body processes that affect volatile element budgets and distributions. Cd concentrations are only available for samples to which the Cd double spike was added before the separation chemistry.

SUPPORTING INFORMATION
Additional supporting information may be found in the online version of this article: Fig. S1. Flowchart of the full analytical procedure for the determination of Cd-Tl-Pb concentrations and isotope compositions. Table S1. Anion exchange chemistry for the separation of Pb and Tl from spiked sample solution aliquots for subsequent concentration measurements by isotope dilution MC-ICP-MS. Table S2. Anion exchange chemistry for the separation of Cd from spiked sample solution aliquots, for subsequent Cd concentration measurements by isotope dilution MC-ICP-MS. Table S3. Anion exchange chemistry for the separation of a combined Pb-Cd and a Tl fraction from sample solutions for subsequent measurements of isotope compositions. Table S4. Anion exchange chemistry for purification of Tl fractions obtained from the first-stage chemistry of Table S3. Table S5. Anion exchange chemistry for the separation and purification of Pb and Cd from the combined Pb-Cd fractions obtained from the first-stage chemistry of Table S3. Table S6. Chromatographic procedure for purification of Cd from the Cd fractions obtained from the secondstage chemistry of Table S5. Table S7. Published U-Th concentration data determined by mass spectrometry for samples relevant to this study. Table S8. Estimated mass fractions of meteoritic Pb in the samples (f met-Pb ) and key parameters that constrain the estimates. Presentation and discussion of modeling to estimate terrestrial Pb contamination of meteorites.