Re-Os isotopic systematics and platinum group element composition of the Tagish Lake carbonaceous chondrite

doi:10.1016/j.gca.2004.10.005 Geochimica et Cosmochimica Acta, Vol. 69, No. 6, pp. 1619 1631, 2005 Copyright 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00.00 Re-Os isotopic systematics and platinum group element composition of the Tagish Lake carbonaceous chondrite ALAN D. BRANDON, 1, *MUNIR HUMAYUN, 2, IGOR S. PUCHTEL, 2, and MICHAEL E. ZOLENSKY 3 1 Mail Code SR, NASA Johnson Space Center, Houston, TX 77058, USA 2 Department of the Geophysical Sciences, University of Chicago, 5734 S Ellis Avenue, Chicago, IL 60637, USA 3 Mail Code ST, NASA Johnson Space Center, Houston, TX 77058, USA (Received May 11, 2004; accepted in revised form October 13, 2004) Abstract The Tagish Lake meteorite is a primitive C2 chondrite that has undergone aqueous alteration shortly after formation of its parent body. Previous work indicates that if this type of material was part of a late veneer during terrestrial planetary accretion, it could provide a link between atmophile elements such as H, C, N and noble gases, and highly siderophile element replenishment in the bulk silicate portions of terrestrial planets following core formation. The systematic Re-Os isotope and highly siderophile element measurements performed here on five separate fractions indicate that while Tagish Lake has amongst the highest Ru/Ir (1.63 0.08), Pd/Ir (1.19 0.06) and 187 Os/ 188 Os (0.12564 0.12802) of all carbonaceous chondrites, these characteristics still fall short of those necessary to explain the observed siderophile element systematics of the primitive upper mantles of Earth and Mars. Hence, a direct link between atmophile and highly siderophile elements remains elusive, and other sources for replenishment are required, unless an as yet poorly constrained process fractionated Re/Os, Ru/Ir, and Pd/Ir following late accretion on both the Earth and Mars mantles. The unique elevated Ru/Ir combined with elevated 187 Os/ 188 Os of Tagish Lake may be attributed to Ru and Re mobility during aqueous alteration very early in its parent body history. The Os, Ir, Pt, and Pd abundances of Tagish Lake are similar to CI chondrites. The elevated Ru/Ir and the higher Re/Os and consequent 187 Os/ 188 Os in Tagish Lake, are balanced by a lower Ru/Ir and lower Re/Os and 187 Os/ 188 Os in CMchondrites, relative to CI chondrites. A model that links Tagish Lake with CI and CM chondrites in the same parent body may explain the observed systematics. In this scenario, CM chondrite material comprises the exterior, grading downward to Tagish Lake material, which grades to CI material in the interior of the parent body. Aqueous alteration intensifies towards the interior with increasing temperature. Ruthenium and Re are mobilized from the CM layer into the Tagish Lake layer. This model may thus provide a potential direct parent body relationship between three separate groups of carbonaceous chondrites. Copyright 2005 Elsevier Ltd 1. INTRODUCTION The Tagish Lake meteorite represents some of the most primitive solar system material obtained to date (Brown et al., 2000). Because it is a low-density (between 1.11 1.67 g/cm 3 ), high-porosity (between 37% 58%), carbon-rich (3.5 6 wt%), matrix-dominated rock (Brown et al., 2000, 2002a; Grady et al., 2002; Zolensky et al., 2002), it is a very weak (friable) meteorite and, under normal circumstances, would have likely broken down rapidly after falling on the Earth s surface. The Tagish Lake carbonaceous (C) chondrite fragments were preserved by a set of fortunate circumstances. The meteoroid fell in winter. Portions of it landed on a frozen lake on January 18, 2000, and samples were recovered within one week while still frozen (Brown et al., 2000). Several months later, additional samples were collected, some of which were already breaking down and weathering even though they were recovered on and in the lake ice. As such, this meteorite provides the unique opportunity for the study of primitive solar system materials that are distinct from those in our present collection. From the * Author to whom correspondence should be addressed (alan.d.brandon1@jsc.nasa.gov). Present address: National High Magnetic Field Laboratory and Department of Geological Sciences, Florida State University, 1800 E Paul Dirac Drive, Tallahassee, FL 32310, USA. Present address: Isotope Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, MD 20742, USA. 1619 perspective of highly siderophile elements (HSE), consisting of Re, Au and platinum group elements (PGE - Os, Ir, Ru, Rh, Pt, and Pd) the Tagish Lake meteorite may help constrain compositional variability of these elements in the solar nebula resulting from processes operating in the earliest solar system. The Tagish Lake meteorite came from a parent body that underwent extensive aqueous alteration (Brown et al., 2000; Zolensky et al., 2002). Hence, HSE mobility during such processes can be investigated. In addition, Tagish Lake may potentially provide clues to the types of materials terrestrial planets were accreted from, and in particular, what types of materials were added after early differentiation within terrestrial planets. The Tagish Lake C-chondrite is volatile-rich and may be material that could make significant contributions to both atmophile element, such as H, C, N and noble gases (Ne, Ar, Kr, and Xe), and HSE budgets of Earth and Mars during late accretion (Anders and Owen, 1977; Drake and Righter, 2002). Brown et al. (2000) measured concentrations of Re, Os, Ir, Ru and Pd for one aliquot of Tagish Lake. The calculated Re/Os, Re/Ir, Ru/Ir, and Pd/Ir of this aliquot are 0.122 0.013, 0.102 0.009, 1.97 0.19, and 1.79 0.18, respectively. Friedrich et al. (2002) obtained concentrations of Re, Ir, Ru and Pd for 3 different aliquots and found Re/Ir, Ru/Ir, and Pd/Ir to be 0.0960 006, 2.65 0.04, and 1.083 0.14, respectively, although Friedrich et al. (2002) expressed caution on the Ru values owing to their distinct elevation relative to CI values. If these ratios from the 4 aliquots combined are indicative of the time-

1620 A. D. Brandon, M. Humayum, I. S. Puchtel, and M. E. Zolensky integrated Re/Os, as well as the Ru/Ir and Pd/Ir of the Tagish Lake material, it would evolve to a higher 187 Os/ 188 Os and have higher Ru/Ir and Pd/Ir than other C-chondrites, which have Re/Os of 0.084 0.028 and Re/Ir of 0.084 0.001, Ru/Ir of 1.44 0.22, and Pd/Ir of 1.06 0.29 (Walker et al., 2002; Horan et al., 2003). Hence, if such materials with higher than normal Re/Os, Ru/Ir, and Pd/Ir for C-chondrites were part or all late accretion materials, then a match between a primitive chondrite and the Earth s HSE pattern could be found. Coupling with the atmophile element-rich nature of Tagish Lake would then provide important constraints on the Earth volatile inventory. This paper presents precise measurements of the Re-Os isotope systematics and PGE concentrations for several fractions of Tagish Lake by isotope dilution using TIMS and ICP-MS. For intercomparison with the results of Walker et al. (2002) and Horan et al. (2003), data on Allende are also presented. These data are compared to the HSE data for other primitive C-chondrites (Walker et al., 2002; Horan et al., 2003) including CI, CM, CK, CR, and CV chondrites. Since highly siderophile elements include some of the most refractory elements to condense from a gas of solar composition (Sylvester et al., 1990, 1993; Campbell et al., 2001, 2003), these data also help constrain the variability of HSE in different regions of the solar nebula from which C-chondrites formed. Further, these measurements will determine whether this meteorite does in fact represent C-chondrite material with time-integrated Re/Os that is elevated with respect to CI chondrites, coupled with high Ru/Ir and Pd/Ir with similarities to the HSE patterns of planetary mantles. 2. SAMPLES A homogenized powder of Allende (USNM 3529) was obtained from the Smithsonian Institution s National Museum of Natural History. This sample was used in this study for the purpose of direct comparison of Re-Os isotope and HSE concentration data previously obtained for this powder (Walker et al., 2002; Horan et al., 2003; Becker et al., 2004). Several different fractions of the Tagish Lake meteorite were chosen for this study (Zolensky et al., 2002). Samples were chosen that were as far from fusion crust as possible. The carbonate-poor (C-P) lithology studied here was taken from the pristine 0.5 kg frozen sample collected within one week of the fall and sent to the Johnson Space Center for curation. This dominant lithology is a phyllosilicate-rich, matrix-supported assemblage of fine- to coarse-grained, phyllosilicate-rich clasts, sparse chondrules, aggregates, sparse calcium-aluminum inclusions (CAIs) and a variety of isolated grains of olivine, magnetite, Fe-Ni sulfides, Cr-Ni phosphides and uncommon Ca- Mg-Fe carbonates. It is virtually metal-free; very rare nuggets of metal are present within primary silicates. The matrix consists mainly of saponite, serpentine and Fe-Ni sulfides. Matrixsupported clasts consist of fine-grained phyllosilicates with minor Fe-Ni sulfides and/or magnetite. Most clasts range from 50 to 500 m in average diameter, and are rounded. Large pores are very common, and many of these have walls covered by Ca-carbonates. The carbonate-rich (C-R) lithology is also from the pristine sample at the Johnson Space Center. It is similar to the carbonate-poor lithology in most respects, and actually grades into it. The major difference is a significantly greater abundance of Ca-Mg-Fe carbonates. Carbonates are so abundant that in places they are replacing olivine. Although the abundance of chondrules is about the same in both the carbonate-rich and -poor lithologies, fine-grained clasts and CAIs are almost absent from the former. In general, the C-R lithology is notably finer-grained than any other common lithology in C-chondrites, excepting the finest-grained CV3 dark inclusions. KN2 and MH11 are both samples that were recovered several months after the fall, but were recovered from the top of the frozen lake, and show no significant evidence of terrestrial alteration. MC01 was also recovered several months after the fall, from within frozen lake ice. Upon removal from the enclosing ice this sample has fallen into mixture of slightly weathered chips as well as fine, black powder. This behavior is typical of water-soaked Tagish Lake samples. We assume that these samples contain both the carbonate-rich and poor lithologies. 3. ANALYTICAL TECHNIQUES 3.1. Spike Calibration For this study, an HSE mixed spiked of 99 Ru, 110 Pd, 185 Re, 190 Os, 191 Ir and 198 Pt (UC#000601) was used to obtain precise concentrations of their respective elements by isotope dilution. The abundances in spike UC#000601 were optimized for carbonaceous chondrite relative abundances. The 185 Re concentration was calibrated using N-TIMS against a gravimetric standard produced by dissolving pure Re metal, provided by Harry Becker (University of Maryland). The 190 Os concentration was calibrated by N-TIMS using the NOSY 1 and 2 standards (Yin et al., 2001; provided by Cin-Ty Lee, Rice University). The Ir, Ru, Pt, and Pd spike concentrations were calibrated using ICP-MS at The University of Chicago against in-house gravimetric standards produced by dissolving pure metals. The suitability of using this digestion technique and mixed spike is presented below in the Results section. 3.2. Sample Preparation and Chemical Purification Procedures The Tagish Lake samples were ground in a ceramic alumina mortar and pestle. The mortar and pestle were cleaned using Ottawa quartz sand in ethanol. The sand was ground to a very fine powder before and after processing of the meteorites. After cleaning with Ottawa sand, the mortar and pestle was repeatedly rinsed with MilliQ ultrapure water. In addition, between different meteorite samples, the mortar and pestle was soaked in hot 8NHNO 3 for 10 min, and then rinsed repeatedly with ultrapure water. The sample powders were then loaded in quartz Carius tubes with the HSE mixed spike and inverse aqua regia (3 parts HNO 3, 1 part HCl) and sealed (Shirey and Walker, 1995). The samples were digested in an oven at 270 C for 72 to 96 hours. After digestion, Os was extracted from the aqua regia using carbon tetrachloride solvent, back extracted into HBr (Cohen and Waters, 1996), followed by purification by microdistillation (Birck et al., 1997). Following Os extraction, the aqua regia solutions were divided in half. One half of each was dried down and redissolved in 0.2 HNO 3. From these fractions Re was separated and purified by anion exchange column chemistry. The other halves were converted into chloride form, redissolved in 0.15 N HCl, and loaded onto cation exchange columns for PGE separation and purification following the procedures of Puchtel and Humayun (2001). 3.3. Re-Os and HSE Analyses Rhenium and Os analyses were performed on a ThermoFinnigan Triton multicollector thermal ionization mass spectrometer, at the Johnson Space Center. All samples were measured using Faraday cups in static mode. Rhenium was measured as ReO 4 on Ni filaments using Ba(NO 3 ) 2 as an electron emitter. Osmium was measured as OsO 3 on Pt filaments using Ba(OH) 2. Oxygen corrections were made using the oxygen isotopic composition measured on 2 ng loads of ReO 4 on the

Highly siderophile elements in Tagish Lake 1621 Faraday cups. The O 2 pressures in the source were maintained in the range of 1 3 10 7 mbar for all runs. After oxygen corrections were performed on the raw data, instrumental mass fractionation was corrected using 192 Os/ 188 Os 3.083 and the exponential law. Statistics were then applied to eliminate data beyond 2 for each run. The measured Re isotopic ratios were not corrected for fractionation, which was estimated to be negligible based on the data for the in-house Re standard ( 187 Re/ 185 Re 1.6733 19, 2 ). The Johnson Matthey Os isotope standard gave 187 Os/ 188 Os 0.11380 1 during the analytical period, within uncertainty of its established value (e.g., 0.1138067 21, Brandon et al., 1999). The measurements of Ir, Ru, Pt, and Pd isotopic compositions were performed on a ThermoFinnigan Element single-collector, magnetic sector, high-resolution ICP-MS at The University of Chicago (Puchtel and Humayun, 2001; Puchtel et al., 2004a). The sample solutions were introduced into the ICP-MS torch via a CETAC MCN6000 desolvating nebulizer. Typical count rates were 10 5 10 6 cps for PGE. The internal precisions of individual runs were better than 0.5% relative (2 aver ). Long-term reproducibilities of a 0.5 ppb in-house standard solution containing Ir-Ru-Pd-Pt, which characterize the external precision of the analyses, were 1% 2% (2 ) on all isotope ratios. Mass fractionation for Ru, Pd, Ir, and Pt, was corrected using 99 Ru/ 102 Ru 0.4044, 110 Pd/ 106 Pd 0.4288, 191 Ir/ 193 Ir 0.5942, and 198 Pt/ 195 Pt 0.2130, relative to those measured in the standard solutions that were run alternately with samples. The total analytical blank was 3.7 pg Os, 0.5 pg Ir, 3 pg Ru, 31 pg Pt, 7 pg Pd, and 10 pg for Re. Blank corrections applied were 0.1% for all elements. Table 1 reports the new HSE data and Os isotopes obtained in this study. In addition, these data are listed in electronic annex EA-1. 4. RESULTS One of the primary concerns when analyzing chondrites for bulk HSE abundances is whether the dissolution technique was efficient in accessing these elements in each sample (Walker et al., 2002). Highly siderophile element alloys, typically found within refractory inclusions such as CAIs (Calcium-aluminum inclusions), are the most difficult mineral phases to dissolve. Chondrites with the highest proportion of CAIs could contain as much as 50% of the HSE budget of the bulk sample in CAIs (Becker et al., 2001; Walker et al., 2002). Tagish Lake could potentially have HSE alloys in the observed CAIs or within the matrix. Standard digestion techniques employing HF and HCl may not dissolve HSE alloys (Walker et al., 2002). Hence, for this study, the Carius tube digestion technique was employed as described above. This technique evidently accesses HSE in refractory inclusions and results in 98% recovery in bulk chondrites (Becker et al., 2001; Walker et al., 2002). For example, Allende is relatively rich in CAIs and Becker et al. (2001) demonstrated that 95% of the Re and Os present were accessed in Allende CAIs they measured. The 187 Os/ 188 Os ratios of the bulk CAI were within 0.3% of the bulk chondrite such that minimal or no bias in this ratio was created as a result of using the Carius tube digestion technique. Therefore, to test the suitability of the Carius tube method for HSE access for C-chondrite material, and also as a test for interlaboratory calibration, a homogenized sample of Allende, which was analyzed six times by Walker et al. (2002), Horan et al. (2003), and Becker et al. (2004), was analyzed here (Table 1). The aliquot of Allende measured at JSC has 187 Os/ 188 Os of 0.12596 0.00001, similar to the aliquots previously measured with 187 Os/ 188 Os from 0.12614 0.00004 to 0.12643 0.00004 (Walker et al., 2002; Becker et al., 2004). Of key importance, the value of the JSC aliquot is 0.1, where is the per mil combined deviation in 187 Re/ 188 Os and 187 Os/ 188 Os datum from the reference IIIAB iron meteorite isochron (Table 1; Smoliar et al., 1996; Walker et al., 2002). This is within the range calculated for the Allende fractions measured by Walker et al. (2002) and Becker et al. (2004), which range from 0.8 to 6.6. The HSE concentrations for the Allende JSC aliquot also agree well with those for the aliquots measured by Walker et al. (2002), Horan et al. (2003), and Becker et al. (2004) as shown in Table 1. Of note is that for the HSE ratios (Table 1), the Allende aliquot values in this study lie within the range of values obtained independently in these previous studies that were produced with the same Carius tube digestion-isotope dilution technique, using a different source of spike. Therefore, it can be concluded that the new data reported here for both Allende and Tagish Lake are directly comparable to the chondrite data presented in Walker et al. (2002) and Horan et al. (2003) and provide a meaningful bulk representation of their parent meteorites. The Tagish Lake aliquots measured in this study display a range in 187 Os/ 188 Os from 0.12564 to 0.12802. The most radiogenic Tagish Lake aliquot (C-P) has 187 Os/ 188 Os that is higher than that of all bulk C-chondrites measured to date, and falls outside of the range measured by Walker et al. (2002) of up to 0.12731 for Lancé. The 187 Os/ 188 Os for the C-P and other lithologies reflect time-integrated 187 Re/ 188 Os of 0.4145 (Re/Os 0.0860) to 187 Re/ 188 Os of 0.3854 (Re/Os 0.0800) and similar to their measured ratios, respectively, assuming no disturbance since 4.558 Ga (i.e., conforming to the IIIAB isochron, Table 1, Fig. 1). This long term Re/Os is significantly lower than the Re/Os (and Re/Ir) obtained by Brown et al. (2000) and Friedrich et al. (2002). This is shown in an HSEnormalized plot in Figure 2. While the Os and Ir abundances previously measured on different aliquots of Tagish Lake are similar to those obtained here, the Re abundances of all 4 previously measured aliquots were higher than the new measurements reported here. The fact that the values for the aliquots in the present study (Table 1) are consistent with conforming to the IIIAB isochron, means that the Re abundances for these fractions are consistent with their Os concentrations and isotopic compositions. A comparison with previous HSE analyses of Tagish Lake can be made. Brown et al. (2000) reported Ru, Pd, Os, Ir, and Pt abundances by fire assay-inaa, and Re abundances by aqua regia digestion-icp-ms, on samples of Tagish Lake. Their abundances for Ru (35%), Pd (60%), Re (30%) and Pt (25%) are higher than any value obtained here. The analytical technique used in that study for HSE abundances may not provide accurate analyses for these elements. Friedrich et al. (2002) used microwave digestion ICP-MS to analyze Pd, Ru, Re, Ir and Pt abundances in three samples of Tagish Lake (Fig. 2). They do not provide any specifics of their digestion methods. However, their mean Pd abundance (545 41) is similar to that obtained here (577 48), their mean Pt abundance (850 92) is 10% 15% lower than the mean of five Pt determinations here (960 40), but higher than the Pt determination for MH-11 of 752 ppb (Table 1). Iridium abundances agree between all three studies, being 485 36 (UC), 504 50 (Purdue) and 547 10 (Brown et al., 2000), with a maximum difference of 13%, excluding MH-11 which is 30% lower than the mean of all other samples measured here. The Ru abundances are quite variable, averaging 794 86 (this study), 1300 1400 (Friedrich et al., 2002) and 1080 90 (Brown et al., 2000). The values provided by

Table 1. Re-Os isotope systematics and platinum group element concentrations for the Tagish Lake and Allende meteorites (concentrations parts per billion). wt. (gr) Re Os 187 Os/ 188 Os 2 187 Re/ 188 Os a Ir Ru Pt Pd Os/Ir Pt/Os Pd/Os Pd/Ir Pt/Ir Ru/Ir Allende (USNM 3529) Walker and Horan b 0.202 716.7 1016 1364 678.3 0.950 1.903 1.418 Walker and Horan b 0.219 63.230 773.92 0.12620 0.39355 0.8 Walker and Horan b 0.238 62.580 770.06 0.12643 0.39149 3.1 Walker and Horan b 0.117 60.078 748.73 0.12615 0.38657 4.2 704.4 1336 661.9 1.063 1.784 0.884 0.940 1.897 Walker and Horan b 0.110 61.798 767.18 0.12614 0.38807 2.9 716.3 1345 681.8 1.071 1.753 0.889 0.952 1.878 Becker c 61.2 763.00 0.12638 0.38641 6.6 700.0 1140 1379 786.0 1.090 1.807 1.030 1.123 1.970 1.629 This Study 0.0523 63.476 784.87 0.12596 0.00001 0.38961 0.1 720.4 1118 1421 681.7 1.090 1.811 0.869 0.946 1.973 1.552 Tagish Lake Carbonate-Poor Fraction 1 0.0180 471.78 0.12790 0.00002 Fraction 2 0.0613 39.711 468.51 0.12802 0.00002 0.40833 5.7 461.2 758.3 973.3 563.4 1.016 2.078 1.202 1.221 2.110 1.644 Tagish Lake Carbonate-Rich Fraction 1 0.0167 496.25 0.12713 0.00004 Fraction 2 0.0459 41.498 499.78 0.12719 0.00002 0.40001 4.0 480.2 776.7 927.2 544.3 1.041 1.855 1.089 1.134 1.931 1.618 Tagish Lake bulk KN2 Fraction 1 0.0427 42.024 514.67 0.12641 0.00002 0.39336 1.5 506.8 846.5 968.1 600.9 1.016 1.881 1.168 1.186 1.910 1.670 KN2 Fraction 2 0.0421 42.102 508.61 0.12684 0.00002 0.39879 1.5 497.2 833.5 960.4 598.2 1.023 1.888 1.176 1.203 1.932 1.677 MC-01 0.0516 39.483 468.99 0.12688 0.00001 0.40557 3.5 477.7 755.1 976.4 578.9 0.982 2.082 1.234 1.212 2.044 1.581 MH-11 0.0505 31.124 380.69 0.12564 0.00001 0.39386 6.6 370.3 593.8 751.7 443.6 1.028 1.975 1.165 1.198 2.030 1.603 CI values d 37.83 448.73 430.5 636.0 855.5 551.3 1.042 1.907 1.229 1.280 1.987 1.477 a 10 4 ( 187 Os/ 188 Os chondrite (0.09524 (0.07887 187 Re/ 188 Os chondrite )). Following Walker et al. (2002). b Walker and Horan Allende data are from Walker et al. (2002) and Horan et al. (2003). c Becker Allende data are from Becker et al. (2004) and are listed here with his permission. d The CI values are the averages of Ivuna and Orgueil from Walker et al. (2002) and Horan et al. (2003). 1622 A. D. Brandon, M. Humayum, I. S. Puchtel, and M. E. Zolensky

Highly siderophile elements in Tagish Lake 1623 In addition, the Pd abundances are not elevated in the aliquots measured here, consistent with the Pd data of Friedrich et al. (2002), and the Ru abundances are significantly lower than those of Friedrich et al. (2002) and Brown et al. (2000). Hence, we do not find PGE abundances with as elevated Re/Os, Re/Ir, Ru/Ir, or Pd/Os as reported by Brown et al. (2000) or Friedrich et al. (2002). This might reflect chondrite heterogeneity, contamination in the previous samples, or instrumental/analytical bias. The observation that the new data presented here, where aliquots from four different Tagish Lake rock fragments were measured and show uniform HSE patterns is inconsistent with these differences being chondrite heterogeneity. The latter two possibilities cannot be separately resolved with the present study here. In the proceeding sections of this paper, only Tagish Lake HSE data measured for this investigation will be considered (Table 1). 5. DISCUSSION 5.1. Classification of Tagish Lake from HSE Fig. 1. 187 Re/ 188 Os- 187 Os/ 188 Os systematics for carbonaceous (C- Chond), enstatite (E-Chond), and ordinary (O-Chond) chondrites. Data only from Walker et al. (2002) and the present study (Allende, Tagish Lake) are plotted. The IIIAB isochron, is from Smoliar et al. (1996), and is used here for reference to Re-Os systems that have been closed since 4.556 Ga (Walker et al., 2002). Friedrich et al. (2002) were conservatively described as information values only. The value obtained by Brown et al. (2000) was 28% higher than the highest value obtained here, while the range in five analyses was only 12%, excluding MH-11. Fig. 2. The HSE concentration patterns for Tagish Lake aliquots normalized to the CI values of Horan et al. (2003). Note that Friedrich et al. (2002) considers the Ru values they measured to be of information value only owing the distinct difference relative to CI values of Anders and Grevesse (1989). Note the smaller suprachondritic Ru in the present data, compared with previous data. Also, note the linear scale on the ordinate. On the basis of amino acid contents, Tagish Lake is distinct relative to other C-chondrite groups, but closest in composition to CI chondrites (Brown et al., 2000, 2002b; Kminek et al., 2002). The oxygen isotopic compositions are intermediate between CI and CM chondrites (Brown et al., 2000). Simon and Grossman (2003) found that the anhydrous inclusions in Tagish Lake were most similar to those found in CM chondrites, although the abundances were lower (13%, compared with 39% 56% in CM chondrites). In contrast, Grady et al. (2002) suggested that Tagish Lake was most similar to CI chondrites in terms of carbon and nitrogen isotope compositions and carbon contents, but could be intermediate between CI and CM in classification. Mittlefehldt (2002) noted that the refractory elements (Al, Ca) and the moderately volatile lithophile element abundances in Tagish Lake were more similar to CM than CI, but that abundances of highly volatile elements (Zn, Se) were intermediate between CM and CI. Friedrich et al. (2002) found that trace element contents, for example, on a plot of Zn/Mn vs. Sc/Mn atomic ratios, that Tagish Lake, while generally being intermediate in these compositions to CM and CI chondrites, did not lie on mixing lines between these compositions. Hiroi et al. (2001) determined that the reflectance spectrum for Tagish Lake is distinct from other C-chondrites and consistent with its origin from D-type asteroids. The D- type asteroids lie beyond the main asteroid belt. All of these independent investigations indicate that Tagish Lake may be a distinct C-chondrite that does not belong to any group, but had a nebular history at least in part similar to CM and CI chondrites, and may have a genetic relationship to these two groups. In Tagish Lake, the ratios of key refractory lithophile elements, Al/Mg and Ca/Mg, reported by Mittlefehldt (2002) are identical to CM chondrite values and distinct from CI values (Kallemeyn and Wasson, 1981). The CM chondrites have systematically higher abundances of refractory elements relative to CI, for example by 36% 41% for Al, Ca and Sc, and 27% 35% for Ru, Os and Ir (Kallemeyn and Wasson, 1981). The refractory lithophile element abundances in Tagish Lake (Mittlefehldt, 2002) are higher than CI values (Kallemeyn and Wasson, 1981) by 12, 5 and 41% for Al, Ca and Sc, respectively, and the refractory siderophile element abundances in

1624 A. D. Brandon, M. Humayum, I. S. Puchtel, and M. E. Zolensky Fig. 3. Highly siderophile element normalized patterns for carbonaceous chondrites (from Horan et al., 2003, and this study). The CI1 normalizing values are from Horan et al. (2003). The values for Tagish Lake are the averages for concentrations in this study and are plotted in each diagram for comparison. The values for Allende are the averages for concentrations from Walker et al. (2002); Horan et al. (2003); Becker et al. (2004); and this study (Table 1). Tagish Lake (this study) are higher than CI values (Kallemeyn and Wasson, 1981) by 15, 4 and 9% for Ru, Os and Ir, respectively. Thus, both refractory lithophile and refractory siderophile elements are enriched by similar amounts (excluding Sc), but the factors are smaller than those observed in CM chondrites. This could be due to higher abundances of volatile constituents (e.g., C, Grady et al., 2002) in Tagish Lake compared to CM chondrites. Because of this difference in absolute abundances, Tagish Lake has a CI-like pattern (Figs. 2 and 3), with the exception of Ru. The Tagish Lake C-chondrite has HSE characteristics that are intermediate between CI and CM groups. However, the data for 187 Os/ 188 Os of CI and CM are both systematically lower than Tagish Lake, averaging 0.12645 1(n 4, 2 ), 0.12564 95 (n 5), and 0.1270 14 (n 8), respectively (Table 1; Walker et al., 2002). These systematics are consistent with previous work that suggests that Tagish Lake be classified within a distinct group (C2), but possibly closely related to CI and CM groups. 5.2. Origin of HSE Variation in Carbonaceous Chondrites and Parent Body Models Carbonaceous chondrites from all of the groups with the exception of CI1, have elevated HSE concentrations relative to Tagish Lake (Fig. 3). In Figure 4, HSE/Ir ratios, normalized to CI chondrite abundances (Horan et al., 2003), are plotted for C-chondrites. For (Pd/Ir) N vs. Ir (Fig. 4A), the various chondrite groups appear to plot along a simple mixing trend (2 points for CR2 being the exception). Because of the presence of refractory Ir in CAIs, the trend is a reflection of addition of refractory material (Fig. 5, Takahashi et al., 1978; Wasson and Fig. 4. Highly siderophile element concentration and ratios plotted vs. Pd/Ir, normalized to CI values (N) of Horan et al. (2003). Also shown are CI averages for Anders and Grevesse (1989) AG; and Palme and Beer (1993) PB. (A) vs. Ir, (B) vs. Pt/Ir, (C) vs. Ru/Ir. Values plotted are individual analyses for each sample using the data from Horan et al. (2003) and this study (Table 1) and include replicates. (B, C) Condensation curves are shown from 1000 K to 1350 K in 10 K increments, with P tot 10 Pa (from Campbell et al., 2001, 2003).

Highly siderophile elements in Tagish Lake 1625 Fig. 6. 187 Re/ 188 Os- 187 Os/ 188 Os (A) and (Ru/Ir) N - 187 Os/ 188 Os (B) systematics for the Tagish Lake, CI1, and CM2 chondrites. Data as in Figure 4. Fig. 5. Pd/Ir vs. the average volume (vol) % of refractory inclusions for each C-chondrite group (compiled by Brearley and Jones, 1998). Data as in Figure 4. Kallemeyn, 1988; Jochum, 1996; Horan et al., 2003). The Tagish Lake replicates plot close to the CI composition, intermediate between CI and CM. The (Pt/Ir) N in Tagish Lake samples is identical to that of other C-chondrites, which plot mostly within 5% of the CI value, with the exception of Murray and Ornans (Fig. 4B). The compositional trajectory of condensates from a gas of solar composition at 10 Pa total nebular pressure (from Campbell et al., 2001) is shown for comparison. The 30% spread in (Pd/Ir) N at constant (Pt/Ir) N is consistent with volatility-based fractionations among chondrite groups, including mixing of refractory with CI-like material. The apparently correlated variations of (Pd/Ir) N and (Pt/Ir) N in Ornans and Murray are not consistent with condensation trends and, if real, must imply other factors that vary Ir independently of Pt and Pd. Several different mechanisms could have resulted in the different HSE variations observed in the C-chondrites. As noted above, the primary bulk HSE concentrations in carbonaceous chondrites are dependent on the amount of HSE-rich refractory material present and on conditions during condensation (Figs. 4 and 5). However, secondary variations that deviate from those expected during condensation require additional mechanisms, likely from parent body processes. In the case of Tagish Lake and most of the C-chondrites shown in Figures 3 and 4, the values for the Re-Os isotopic systematics indicate virtually no Re/Os fractionation since the time of IIIAB iron meteorite closure at 4.556 Ga, at least on the scale of the aliquots of samples measured ( 42 mg, this study; Walker et al., 2002; Horan et al., 2003). Hence, these relationships are consistent with mechanisms that fractionate HSE very early on the parent bodies. The exceptions to this for the samples considered here are Vigarano and Lancé, which have values of 20 and 12.2, respectively, for some fractions and fall far off the IIIAB isochron (Fig. 1; Walker et al., 2002). Walker et al. (2002) interpreted the discrepant values for these C- chondrites, as well as some ordinary and enstatite chondrites that are not on or very close to the IIIAB isochron (Fig. 1), to result from very recent processes such as shock mobility during impact expulsion from the parent body, or Re, and possibly Os redistribution during alteration on Earth. One appealing possibility is that some or all of the HSE variation could result from aqueous alteration. Aqueous alteration occurred within the first 5 to 20 Ma of solar system history in C-chondrites as evidenced by I-Xe isotope systematics (Swindle, 1998). Tagish Lake has HSE contents that are intermediate to CM2 and CI1 rocks (Figs. 3 and 4). The degree of alteration of Tagish Lake is also intermediate between CM2 and CI1, such that it is possible that HSE may have been in part removed and/or redistributed during aqueous alteration where CM2 Tagish Lake CI1. Tagish Lake displays elevated (Ru/ Ir) N and plots in a field that lies above and with only minimal to no overlap with the other C-chondrites (Fig. 4C, also observed in the HSE normalized pattern, Fig. 3). The Ru anomaly ( 7% 14% enrichment in Ru/Ir) sets Tagish Lake apart from other C-chondrite types. Particularly, the CM chondrites, Murchison, Murray and Mighei, have (Ru/Ir) N lower than CI chondrites, while CV chondrites are slightly higher than CI. However, the (Ru/Ir) N for mean CM, CO and CV chondrites in Kallemeyn and Wasson (1981) are within 1% of the Horan et al. (2003) CI-chondrite mean, although their CI chondrite value is 5% higher. The Ornans analysis of Horan et al. (2003) is an exception. Also, shown in Figure 4C is the compositional trajectory of condensates from a solar composition gas at a total nebular pressure of 10 Pa. It should be noted that both Ru and Ir are significantly more refractory than Pd, so that Ru/Ir fractionation on the basis of differential volatility is not expected. The C-chondrite data are broadly consistent with this, with the notable excess of Ru in Tagish Lake, and the possible deficiency of Ru in CM chondrites. Tagish Lake is not compositionally intermediate between CM and CI chondrites in (Ru/Ir) N (Fig. 4C). Further, this is not ascribable to high-t nebular processes. In addition, Tagish Lake is not intermediate between CM and CI with respect to Re-Os isotope systematics. Tagish Lake has elevated 187 Os/ 188 Os relative to both groups, as a consequence of long-term elevated Re/Os (Fig. 6A). Whatever the process is that resulted in elevated Re/Os in Tagish Lake compared to CM and CI chondrites, may have also resulted in elevated (Ru/Ir) N (Fig. 6B). Rhenium is more mobile than other HSE (including Ru) during oxidation at low oxygen fugacities (Palme et al., 1998). It may, therefore, be useful to consider the possibility of low-t parent-body processes affecting the siderophile element abundances, since

1626 A. D. Brandon, M. Humayum, I. S. Puchtel, and M. E. Zolensky Fig. 7. A single parent body model for CM, Tagish Lake, and CI chondrites. Ruthenium and rhenium mobility from the CM-layer into the Tagish Lake layer may explain the elevated Ru/Ir and more radiogenic 187 Os/ 188 Os (i.e., long-term elevated Re/Os) in this layer. The Tagish Lake-layer is a mixture of CM-CI materials consistent with its composition. Heating of ice-rich material in the interior CI layer results in aqueous alteration in a gradient decreasing towards the exterior of the parent body. See text for additional explanation. Tagish Lake, CI and CM chondrites have all experienced aqueous alteration. Campbell et al. (2003) measured the composition of a secondary sulfide vein cross-cutting an Allende CAI in TS68 by laser ablation ICP-MS, and noted the exceptionally high Ru content (1137 ppm). They also reported Mo, Rh, W and Os (2.1 ppm), but found other siderophile elements, including Re, to be below detection limits ( 100 ppb). Mixing between a CI composition chondrite and 40 80 ppm by weight of such sulfide material could be sufficient to increase the Ru abundance in the bulk rock by 7% 14% relative to CI chondrites, without affecting the abundances of the other elements measured here. It should be noted here that the results of Campbell et al. (2003) simply demonstrated Ru mobility on small spatial scales. The Ru excesses observed in the present study, if a result of secondary sulfide addition, could imply that similar Ru mobility may occur on a larger spatial scale. The HSE ratio plots show that Tagish Lake plots both in a distinct field relative to the other groups of C-chondrites, and on mixing lines between CM and CI chondrites (Fig. 4). Thus, Tagish Lake cannot be considered as a simple mixture between any two groups. These relationships show that Tagish Lake has either undergone a distinct nebular history or distinct parent body history (or both) relative to other C-chondrites. One possible scenario that may explain the compositional relationships between CI, CM and Tagish Lake is to consider that these chondrites are derived from a single parent-body (Fig. 7). The center of a small asteroid is considered to represent accreted presolar dust and ice that has never experienced high temperature nebular processes (i.e., CI precursors devoid of CAIs and chondrules). Inward migration of this body resulted in continued accretion of material that bears CAIs and chondrules, in addition to presolar dust (i.e., CM-precursors). Thus, some refractory-enriched material is present in the outer layer. Heating of the interior of the body resulted in aqueous alteration of the entire body with temperatures around 373 K in the interior, and 273 K near the exterior, based on oxygen isotope thermometry (Clayton and Mayeda, 1984). Transport of Ru (S, Zn, etc.) from the CM-layer and deposited as Ru-rich sulfides, and transport of Re in oxidizing aqueous fluids, into mixed CM-CI material could account for both the similarities and the differences between Tagish Lake and CM-CI mixtures. The source of this excess Ru and Re is the altered CAI material in the CMs. Thus, CMs could be slightly depleted in Ru and Re, while Tagish Lake is enriched, relative to Ir and CI abundances (Figs. 4 6). This scenario would require that as aqueous alteration intensified downward into the CI layer, conditions changed resulting in a halt of the progression of Ru and Re into that layer, leaving only the Tagish Lake layer enriched in these elements relative to the other HSE. The issue of what were the variable conditions during condensation and early parent body processes that caused the variations in HSE between the different C-chondrite groups is at present poorly constrained. The model presented above that could link CM, CI, and Tagish Lake chondrites through Re and Ru mobility during aqueous alteration, will not explain the differences in HSE patterns for the other groups. Several observations can be made with the present data set. First, refractory inclusions (i.e., fremdlinge) themselves show the variation in HSE from different C-chondrite groups (Palme and Wlotzka, 1976; Fegley and Palme, 1985; Sylvester et al., 1990, 1993; Campbell et al., 2003). This observation, combined with the scatter and unique characteristics of HSE normalized patterns between groups (Figs. 3 and 4), indicates that the HSE variation is not a simple case of mixing between refractory grains and matrix within the solar nebula. Second, if Tagish Lake (and thus in our model, CI and CM chondrites) comes from a D-type asteroid lying beyond the main asteroid belt and some of the other C-chondrite groups arise from asteroids closer to the sun, the implication is that the HSE variation within the C-chondrites must be related to differing locations within the solar nebula and the unique characteristics thereof. Differing locations within the solar nebula, with greater and lesser amounts of dust, temperature, ices, etc., also led to larger or lesser amounts of refractory inclusions. Each location for each C-chondrite group could have resulted in early-formed fremdlinge with HSE concentrations dependent on these variables and by early parent-body processes that may give rise to redistribution of HSE such as proposed for Tagish Lake above. In summary, the HSE concentrations in Tagish Lake are similar to CI chondrites with the exception of Re/Os and Ru. One possibility that may explain the HSE variations of this meteorite is nebular condensation followed by Re and Ru mobility resulting from aqueous alteration during early processing on a parent body. If this model holds, it may be possible to link Tagish Lake to CI and CM chondrites in a single parent body (Fig. 7). A single parent body model for these chondrites, which was formed by gradational accretion of material with CI to Tagish Lake to CM outwards, is broadly consistent with other compositional parameters discussed above, such as car-

Highly siderophile elements in Tagish Lake 1627 bon, nitrogen, and oxygen isotopes, where Tagish Lake compositions are intermediate to CI and CM chondrites. Alternatively, Tagish Lake may have formed in a distinct parent body and its distinctive Re and Ru enrichments may result from volatility-dominated processes unique to its parent body and its formation location within the solar nebula. The differences between Tagish Lake, CI, CM and the other C-chondrites is at present unresolved, but the aqueous alteration model presented above may provide a basis for further work on this issue. 5.3. Implications for Late Accretion on Terrestrial Planets It is now well accepted that terrestrial planets were largely accreted (95% 99%) within the first 50 m.y of solar system history and grew by the continuous addition of small planetesimals that themselves had formed earlier from even smaller objects (Wetherill, 1986; Chambers and Wetherill, 1998). The prevailing models for terrestrial planet accretion and differentiation call upon early forming magma oceans generally concurrent with iron-rich core formation. Tungsten isotope measurements of terrestrial and Martian materials are consistent with core formation occurring within 50 m.y. or less after the onset of condensation of the solar system (Lee and Halliday, 1997; Kleine et al., 2002; Schoenberg et al., 2002; Yin et al., 2002; Halliday, 2004). In such models, extraction of the core likely leaves the silicate portions of the Earth and Mars strongly depleted in the highly siderophile elements (Chou, 1978; Morgan et al., 1981, 2001; Morgan, 1986). Earth and Mars-sized objects were formed at least in part from planetesimals that were already differentiated at the time of accretion (Taylor and Norman, 1990; Carlson and Lugmair, 2000). These planetesimals were mostly stripped of their volatiles and differentiated into ironrich cores and silicate mantles, and possibly juvenile crust (Carlson and Lugmair, 2000). Magma ocean development during early differentiation of terrestrial planets would also result in strong outgassing, which would leave early interiors of Earth and Mars relatively volatile depleted in constituents that subsequently formed the atmospheres and oceans (Porcelli and Pepin, 2000). Impact driven accretion would remove any protoatmosphere during the late stages of accretion (Halliday, 2004). In addition, Xe loss could have resulted by removal of a large portion of the Earth s atmosphere at 50 to 80 m.y. after solar system formation from the giant impact that formed the Moon (Porcelli et al., 2001). A result of such processes is that late accretion of materials, usually taken to be the last 1% of material accreted to terrestrial planets, subsequent to core formation, magma ocean development and atmospheric removal is necessary to refertilize the silicate portions of Earth and Mars with volatile elements and possibly HSE to their estimated or present-day measured concentrations (Kimura et al., 1974; Anders and Owen, 1977; Chou, 1978; Morgan et al., 1981, 2001; Morgan, 1986; Porcelli and Pepin, 2000; Righter et al., 2000; Holzheid et al., 2000; Drake and Righter, 2002). A crucial question for constraining early Earth and Mars volatile inventories is whether acquisition of these elements and HSE were coupled during late accretionary processes (Anders and Owen, 1977). If the abundances of these elemental groups were coupled, then the volatile inventories of Earth and Mars might be inferred from the abundances of highly siderophile elements in their silicate mantles. For this to be possible, volatile-rich primitive materials must be found that have HSE abundances and Os isotopic compositions that match, either individually or in combination with other meteorite types, the HSE compositions of the silicate portions of Earth and Mars. Alternatively, materials that are strongly HSE-depleted but volatile-rich, and vice versa, could have both contributed, or the mechanisms of volatile and HSE replenishment were decoupled. A third possible scenario is that mantles of the terrestrial planets did not acquire their present HSE budgets through late accretion, but instead through early differentiation processes, possibly during magma ocean re-equilibration (Righter and Drake, 1997; Frost et al., 2004). Each of these scenarios has important consequences to the earliest history of terrestrial planetary differentiation with possible implications for the types of materials present in the inner solar system during those events. The compositions of C-chondrites are such that 1% (necessary for HSE replenishment) of a C-chondrite veneer would not supply enough of many moderately and highly volatile elements (e.g., Na, K, Pb, Bi, Cs, In) to replenish the interior of Earth to their calculated abundances (Palme and O Neill, 2003). However, atmophile elements such as H, C, and N, and noble gas concentrations for the Earth and Mars are consistent with a significant proportion of a late veneer of C-chondrite composition (Anders and Owen, 1977; Robert, 2001; Drake and Righter, 2002). The D/H ratios of C-chondrites are identical to Earth surface water providing a compelling reason to consider water delivery to Earth from such a source. The D/H ratios for Mars rocks also overlap those of C-chondrites (Robert, 2001). Depending on the conditions for late veneer accretion and water delivery, C-chondrites could supply enough water by mass corresponding to one Earth ocean (Drake and Righter, 2002). So the question then becomes not whether all volatile elements can be replenished by the same material that replenish HSE, but whether atmophile element and HSE are replenished by C-chondrite late accretion. The Earth and Mars primitive upper mantles have been estimated to have HSE concentrations within a similar range, 0.002 to 0.007 times relative to CI chondrites (Chou, 1978; Morgan et al., 1981, 2001; Morgan, 1986; Newsom, 1990; Warren et al., 1999). 0.5 to 1% of chondritic late accretion that is mixed into the Earth s mantle could account for these HSE characteristics (Morgan et al., 2001). Several alternatives to a late veneer model for explaining HSE concentrations in the Earth and Martian mantles have been proposed. These include, core separation at the base of a deep magma ocean (800 1000 km in Earth, Murthy, 1991; Righter and Drake, 1997), incomplete core extraction (Jones and Drake, 1986), and core reflux into terrestrial mantles either early after initial differentiation, or by gradual addition over time (Snow and Schmidt, 1998). The first two models are dependent on having metal/silicate partition coefficients (D metal/silicate ) that are within a few percent of each other for the HSE to explain the abundance patterns and Os isotopic compositions of mantle peridotites (Meisel et al., 2001; Becker et al., 2004). The D metal/silicate for different HSE have orders of magnitude difference and do not support these models (e.g., O Neill et al., 1995; Holzheid et al., 2000). In the latter hypothesis, the 186 Os- 187 Os systematics of mantle-derived materials suggest that only very small portions of recent to present-day mantle plumes potentially carry a core

1628 A. D. Brandon, M. Humayum, I. S. Puchtel, and M. E. Zolensky Fig. 8. The calculated 187 Re/ 188 Os vs. measured 187 Os/ 188 Os for chondrites (Walker et al., 2002; this study). The 187 Re/ 188 Os values are those calculated assuming that the 187 Os/ 188 Os for each sample aliquot lies on the IIIAB isochron (Smoliar et al., 1996). The proposed values (Meisel et al., 2001; Brandon et al., 2000b) for the present-day Earth primitive mantle (PUM), and the Mars primitive mantle, are plotted. signature (Walker et al., 1997; Brandon et al., 1998, 1999, 2003). Abyssal peridotites and Os-rich alloys from ophiolites, do not show such a signature (Walker et al., 1997; Brandon et al., 1998, 2000a). Hence, these systematics are inconsistent with the addition of core material back into the mantle over time in the Earth as a mechanism that raises the HSE concentrations observed to their present levels. Thus, while all three models require additional consideration and testing, within the present understanding of HSE behavior, they do not appear to be as plausible as a late veneer addition to explain both the Os isotopic compositions and the HSE concentrations of terrestrial mantles. However, there are drawbacks to having late accretion materials with elevated HSE that are atmophile element-rich. The 187 Os/ 188 Os for Earth s primitive upper mantle (EPUM) is estimated to be 0.1296 0.0008, reflecting a time-integrated evolution with 187 Re/ 188 Os of 0.4346 (Meisel et al., 2001). This 187 Os/ 188 Os is more radiogenic than those for C-chondrites including Tagish Lake, but falls within the upper range of both enstatite and ordinary chondrites (Walker et al., 2002). Therefore, the Os isotope data at present are consistent with a late accretion on Earth of materials with time-integrated Re/Os ratios similar to relatively dry enstatite and ordinary chondrites rather than C-chondrites (Fig. 8). If these characteristics of the Earth s mantle are representative of the late accretion of materials added subsequent to core formation, then HSE and atmophile element enrichment must be decoupled from each other during this process. The present-day 187 Os/ 188 Os of the Martian primitive upper mantle (MPUM) has been estimated at 0.132 0.001 (Fig. 8) based on the isotopic composition of the SNC lherzolites (LEW 88516, ALHA 77005, Y793605), which have chondritic 182 W/ 184 W and 142 Nd/ 144 Nd isotopic compositions (Brandon et al., 2000b). This estimate of the present-day 187 Os/ 188 Os of Fig. 9. The 187 Os/ 188 Os vs. Ru/Ir (A) and Pd/Ir (B) systematics for chondrites (Walker et al., 2002; Horan et al., 2003, this study). All data plotted are averaged values with the exception of Tagish Lake from Table 1. Also plotted are the estimated values for the Earth s primitive upper mantle (PUM, Meisel et al., 2001; Becker et al., 2004). MPUM reflects the time-integrated evolution of a reservoir with a 187 Re/ 188 Os of 0.4651. If these values represent the HSE characteristics of late accretion material on Mars, then no bulk chondritic material analyzed to date has the necessary characteristics. The maximum 187 Os/ 188 Os measured on a bulk chondrite to date is 0.13045 on Avanhandava H5 (Walker et al., 2002). The HSE ratios of Ru/Ir and Pt/Ir for Earth s upper mantle peridotites (Schmidt et al., 2000; Becker et al., 2004) and mantle sources of komatiites (Puchtel et al., 2004a,b) are also too high to be explained by a late veneer of any known chondrite (Fig. 9). Although portions of Tagish Lake have the highest 187 Os/ 188 Os of any C-chondrite measured to date, its Ru/Ir and Pd/Ir, coupled with 187 Os/ 188 Os, are all too low to explain the composition estimated for the Earth s mantle. Some ordinary and enstatite chondrites have the requisite 187 Os/ 188 Os and Pd/Ir ratios, but no chondrites measured to-date have Ru/Ir and Pd/Ir ratios elevated enough to be a potential late accretion