Old Sm–Nd Ages for Cumulate Eucrites and Redetermination of the Solar System Initial 146Sm/144Sm Ratio

Earth and Planetary Science Letters 291 (2010) 172–181

Contents lists available at ScienceDirect

Earth and Planetary Science Letters

journal homepage: www.elsevier.com/locate/epsl

Old Sm–Nd ages for cumulate eucrites and redetermination of the solar system initial 146Sm/144Sm ratio

Maud Boyet a,b,c,⁎, Richard W. Carlson d, Mary Horan d a Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France b CNRS, UMR 6524, LMV, F-63038 Clermont-Ferrand, France c IRD, R 163, LMV, F-63038 Clermont-Ferrand, France d Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd. NW. Washington DC, 20015, USA article info abstract

Article history: Short-lived 146Sm–142Nd and long-lived 147Sm–143Nd chronometers have been measured in three cumulate Received 29 May 2009 eucrites (Binda, Moore County and Moama). The two major mineral phases (plagioclase and pyroxene) Received in revised form 5 January 2010 present in these achondrites are characterized by a wide range of Sm/Nd ratios that allows well-resolved Accepted 6 January 2010 Sm–Nd isochrons. This group of meteorites thus is suitable to better constrain the initial 146Sm/144Sm ratio of Available online 27 January 2010 the solar system. Binda and Moore County give concordant ages of 4544±88 and 4542±85 Ma, 143 144 Editor: T. Spohn respectively, with initial Nd/ Nd ratios slightly higher, to within error, of chondritic. These ages are in agreement with most of the radiometric ages determined on basaltic eucrites. A best estimate for the solar 146 144 ﬁ Keywords: system initial Sm/ Sm ratio is obtained using the ve-point regression line determined for Binda. The 146 144 146 144 146Sm–142Nd systematics Sm/ Sm ratio of 0.00728±57 obtained for this sample translates to a Sm/ Sm ratio at 4568 Ga of eucrites 0.0085 considering the age of isotopic closure obtained from 147Sm–143Nd systematics. When 146Sm–142Nd early planetary differentiation data from the literature are examined in detail, four eucrites have concordant 147Sm–143Nd and 146Sm–142Nd isotope distribution through solar system systematics. Their weighted average 147Sm–143Nd age is equal to 4546±8 Ma. An initial 146Sm/144Sm ratio at 4568 Ma calculated from these samples is 0.0084±0.0005. A similar ratio of 0.0085±0.0007 is calculated if data from different groups of achondrites (angrite and mesosiderite) are included in the calculation. No difference in the 146Sm/144Sm ratios or initial 142Nd/144Nd ratios is observed among different groups of achondrites relative to ordinary chondrites. This work suggests that 146Sm was homogeneously distributed and that both Sm and Nd were isotopically uniform at the planetary scale in the solar system, at least in the region around where these planetary bodies formed. © 2010 Elsevier B.V. All rights reserved.

1. Introduction present the advantage that both chronometers have the same isotopic 26 26 146 142 closure temperature. Like Al– Mg (T1/2=0.73 Ma), Sm– Nd The early history of solar system evolution from planetesimal involves refractory lithophile elements, but the longer lifetime of formation to initial differentiation can be studied using short-lived 146Sm is better suited for constraining the silicate evolution of pla- radioisotope systems to provide high temporal resolution. However, netary bodies produced during the ﬁrst hundred million years of the extinct radionuclides must be used in combination with long-lived solar system history. chronometers in order to convert the relative time intervals pro- Eucrites are achondrites composed of both basaltic rocks and vided by the extinct radioisotopes into absolute ages. Pb–Pb ages are gabbroic cumulates. Grouped with howardites and diogenites, these generally used to anchor the extinct nuclide relative timescale meteorites are assumed to come from a small parent body of a few because they provide absolute ages determined with the highest hundred km diameter called 4-Vesta (McCord et al., 1970; Consolmagno precision. Sm–Nd systematics are composed of two decay schemes: and Drake, 1977). The recent detection of correlated isotopic and 146 142 147 26 26 60 60 53 53 the extinct Sm– Nd (T1/2 =103 Ma) and the long-lived Sm– parent/daughter ratio variation for Al– Mg, Fe– Ni, Mn– Cr 143 182 182 Nd chronometers (T1/2 =106 Ga). Because absolute ages can be and Hf– W in eucrites suggests that mantle–crust differentiation on obtained from the latter, no additional anchor point is required for the HED (howardite–eucrite–diogenite) parent body occurred during Sm–Nd studies. In the case of a protracted cooling history for the the ﬁrst 5 Ma or less of solar system history (Shukolyukov and Lugmair, sample studied, coupled 146Sm–142Nd, 147Sm–143Nd systematics 1993; Lugmair and Shukolyukov, 1998; Nyquist et al., 2003; Kleine et al., 2004; Bizzarro et al., 2005; Srinivasan, et al., 2007). Compared to these old ages for HED differentiation, most cumulate eucrites provide internal 147 –143 – – ⁎ Corresponding author. Sm Nd and Pb Pb isochron ages 100 150 Ma younger than E-mail address: [email protected] (M. Boyet). the circa 4.56 Ga ages obtained for basaltic eucrites (Lugmair et al.,

1977; Hamet et al., 1978; Jacobsen and Wasserburg, 1984; Galer and or terrestrial contamination should affect the chemical composition of Lugmair, 1996; Tera et al., 1997; Blichert-Toft et al., 2002). Exceptions found meteorites more strongly, such as Binda and Moama. However, include the old Sm–Nd ages determined for EET87520 (Lugmair et al., neither of these samples resided in extreme conditions such as the 1991) and Y980318/433 (Nyquist et al., 2004, 2008). One proposed meteorites found in the Sahara or in Antarctica, where strong explanation for the younger ages of cumulate eucrites is a longer weathering is frequently observed (Crozaz et al., 2003). Almost all thermal history in the deep crust (Ghosh and McSween, 1998; Bogard basaltic eucrites are brecciated and composed of minerals and lithic and Garrison, 2003), but this difference could also be due to late thermal fragments set in a fine-grained matrix. The main portion of Binda is disturbance during later meteorite bombardment event(s). brecciated but also contains unbrecciated clasts of coarse-grained We focused on the study of Sm–Nd systematics in three cumulate equigranular gabbro. Binda is the most magnesian of the cumulate eucrites, Binda, Moama and Moore County. Whole rock measurements eucrites (Duke and Silver, 1967). This feature explains the fact that obtained on these samples were published in a previous paper (Boyet Binda was previously classified as a howardite, the intermediate rock and Carlson, 2005), as well as whole rock data for three basaltic type composed of a mixture of eucrite and diogenite. Cumulate eucrites (Béréba, Nuevo Laredo and Pasamonte). Whole rock 146Sm– eucrites are mainly composed of augite and plagioclase, with minor 142Nd measurements of eucrites provide a slope corresponding to an phases that include chromite, ilmenite, troilite, and rare metal. This initial 146Sm/144Sm ratio=0.0074±0.0015 (Fig. 1), which is within work was undertaken on pieces of meteorites (Moore County and uncertainty of several estimates of the initial ratio of the solar system Moama) from which Pb isotope compositions were determined pre- (e.g. Jacobsen and Wasserburg, 1984; Lugmair and Galer, 1992; viously (Tera et al., 1997). In the same publication, 147Sm–143Nd mea- Nyquist et al. 1994). Although the slope of this isochron is largely surements on Moore County are also reported. defined by the data for the cumulate eucrite Moama due to its relatively high Sm/Nd ratio, this result suggests that cumulate eucrites 2.2. Mineral separation could have formed early in solar system history, as did the basaltic eucrites, a possibility also indicated by the old age obtained for the Pieces of Moama and Moore County were sampled from the cumulate eucrites EET87520 (Lugmair et al., 1991) and Y980318/433 interior of the meteorites. For Binda, the fusion crust was thoroughly (Nyquist et al., 2004, 2008). The main mineral phases of cumulate removed and the part selected for separation was located more than eucrites (cpx and plagioclase) are characterized by a wide range of 2 mm beneath the fusion crust. For Moama and Moore County, Sm/Nd ratios that vary by a factor of 4. New, high precision Sm–Nd mineral separates were obtained by handpicking under a microscope measurements of these phases should allow a more precise estimate of in a clean environment. Care was taken during picking to eliminate the 146Sm/144Sm ratio at the time of meteorite formation. These results crystals where inclusions were detected. For Binda, plagioclase and will be compared to previous estimates of this ratio calculated from pyroxene separates also were obtained by handpicking, but two other internal isochrons of different achondrite groups in order to i) better fractions, called light and heavy fractions (LH and HF, respectively) constrain the initial solar system 146Sm/144Sm ratio, ii) discuss the were separated by density using undiluted methylene iodide. initial Sm–Nd isotopic homogeneity in the solar system at the time of Minerals were washed in Milli-Q deionized water and then leached accretion and, iii) better understand the chronology of the HED parent using 1 M HCl. Once dried, fractions were crushed in a new agate body differentiation and in particular, its crustal evolution. mortar exclusively used for meteoritic material. Whole rocks were crushed in the same mortar. Around 500 mg of each fraction for 2. Samples and analytical techniques Moama was separated and then dissolved for analysis (Table 1). The higher Nd contents in Binda and Moore County allow the use of 2.1. Sample description smaller sample sizes (Table 1). Among the three meteorites analyzed in this study, only Moore County is an observed fall. Secondary perturbations due to weathering 2.3. Chemistry

Samples were dissolved in sealed PFA Savillex beakers using a

mixture of concentrated ultra-pure acid (HF–HNO3 in proportion 3:1) heated for two days at 130 °C. All samples produced a clear solution

in 6 N HCl after repeated treatments with concentrated HNO3.When complete dissolution was achieved, about ten percent of the solution was removed and spiked with 149Sm–150Nd in order to measure Sm and Nd concentrations by isotope dilution. This spike was calibrated against new standard solutions prepared from AMES Nd and Sm metal (see supplemental material of Boyet and Carlson, 2005). Rare earth elements were ﬁrst separated as a group using cation exchange columns (AGW50-X8 resin) in an HCl medium. Neodymium and samarium were then separated using 0.2 M alpha-hydroxyisobutyric (2-methyl- lactic) acid with pH adjusted to 4.7. This column was repeated three times for unspiked fractions in order to obtain negligible levels of Ce (interference on mass 142) and Sm (interferences on masses 144, 148, and 150) in the Nd fraction. We used a similar method for Sm and Nd separation of spiked fractions, but smaller columns were used for the ﬁrst step separation and Sm and Nd fractions were obtained from only one column pass with 2-methylactic acid. Total blanks for Sm and Nd are negligible relative to the amount of Sm and Nd analyzed for 146 142 142 Fig. 1. Sm– Nd evolution diagram plots as ε Nd (expressed relative to terrestrial measurements of unspiked fractions (less than 10 and 40 pg, respec- standard) vs 144Sm/144Nd for whole rock eucrites. The regression including only data tively measured on 3 blanks). For Sm and Nd content determinations from Boyet and Carlson (2005) yields 146Sm/144Nd=0.0074±0.0015, which corre- +34 146 144 using isotope dilution, the total procedural blanks were less than 2 and sponds to a differentiation event of 21− 27 assuming an initial Sm/ Sm solar system ratio of 0.0085. 10 pg for Sm and Nd, respectively, for this procedure. 174 M. Boyet et al. / Earth and Planetary Science Letters 291 (2010) 172–181

Table 1 Sm–Nd isotope measurements. Whole rock eucrite measurements are published in a previous study (Boyet and Carlson, 2005). Sm and Nd concentrations were determined by isotope dilution on a spiked aliquot taken after dissolution. Ratios were corrected for mass fractionation to 146Nd/144Nd=0.7219. 142Nd/144Nd ratios are expressed in epsilon 142 144 142 144 4 143 notation ([( Nd/ Nd)sample /( Nd/ Nd)std −1]×10 ) and are reported relative to the mean value obtained for the La Jolla standard (1.141853±0.000006, ±2σ). Nd data are reported relative to a value of 143Nd/144Nd=0.511860 for the La Jolla standard. The average measured value for this standard during the period when the eucrite analyses were performed was 0.511845 ±0.000003.

147 144 143 144 Sample Weight Sm Nd Sm/ Nd ε142Nd ±2σ Nd/ Nd±2σ (g) (ppm) (ppm)

Binda WR 0.20491 0.3566 0.9857 0.2187 0.08±0.05 0.513319±2 Binda HF 0.38170 0.2743 0.6628 0.2502 0.58±0.09 0.514347±4 Binda Cpx 0.24865 0.2229 0.4281 0.3149 1.28±0.15 0.516290±7 Binda LF 0.09284 0.5569 1.7503 0.1924 −0.22±0.08 0.512532±4 Binda Plagio. 0.08238 0.1399 0.8190 0.1033 −1.48±0.22 0.509920±10 Moama WR 0.49083 0.1575 0.3772 0.2523 0.57±0.09 0.514397±4 Moama Cpx 0.54711 0.1788 0.3290 0.3286 1.39±0.08 0.516678±3 Moama Plagio. 0.36797 0.0742 0.3507 0.1279 −0.72±0.34 0.510558±15 Moore County WR 0.22988 0.7071 2.082 0.2053 0.05±0.06 0.512996±3 Moore County Cpx 0.13976 0.8153 1.5612 0.3157 1.16±0.07 0.516271±3 Moore County Plagio. 0.08786 0.3079 1.8386 0.1012 −1.38±0.10 0.509804±4

2.4. Isotope measurements (5 ppm of external reproducibility). The external reproducibility on repeated standard analyses is slightly larger for all standards run over Isotopic measurements were performed by thermal ionization a several month period. Sm isotope compositions of unspiked whole using the DTM ThermoFisher Triton. Sm and Nd samples were loaded rock samples were presented in Boyet and Carlson (2005). For spiked onto zone-refined rhenium filaments and analyzed in static mode. Nd fractions, Nd was measured as NdO+ emitted from a single Re isotope measurements for 142Nd/144Nd and 143Nd/144Nd determina- filament due to the small amount of Nd present. No difference outside tion were made using double Re filaments and the Nd+ ion. Analytical the analytical uncertainty was noted between the 143Nd/144Nd and runs consisted of 27 blocks of 20 ratios taken statically (8 s inte- 142Nd/144Nd measured for the spike corrected aliquots compared to gration) using amplifier rotation (see detail in Boyet and Carlson, the unspiked portions. 2005). Ce and Sm interferences on masses 142 and 144 are monitored by measuring 140 and 147 masses, respectively. Ce and Sm con- 3. Results tributions on masses 142 and 144 were always lower than 2 ppm for both standard and sample measurements. Nd isotope ratios were Sm and Nd isotopic compositions and concentrations for Binda, corrected for mass fractionation to 146Nd/144Nd=0.7219 using the Moore County and Moama are presented in Table 1. Whole rock exponential law. Data are reported relative to a value of 143Nd/ sample measurements, already published in a previous paper (Boyet 144Nd=0.511860 for the La Jolla standard. The average measured and Carlson, 2005), also are reported in Table 1. For Binda, four value for this standard during the course of these measurements was different mineral fractions were measured whereas only single pla- 0.511845±0.000003 (±2σ). 142Nd/144Nd ratios are expressed in gioclase and the pyroxene separates were analyzed for Moore County epsilon notation and calculated relative to the mean 142Nd/144Nd and Moama. The Sm isotope composition of unspiked fractions also obtained for the terrestrial standards measured in the same barrel of was measured for whole rock samples. These results, presented in our the samples: ([(142Nd/144Nd)sample/(142Nd/144Nd)std− 1]× 104). previous paper (Boyet and Carlson, 2005), show that the Sm isotope During this work, 8 standard measurements produced an average composition of these meteorites has not been disturbed by thermal value for 142Nd/144Nd in the La Jolla standard of 1.141853±0.000006 neutron capture due to exposure to cosmic rays. The 147Sm/144Nd

Fig. 2. Sm–Nd systematics measured for Binda. (a) 147Sm–143Nd isochron diagram. (b) 146Sm–142Nd evolution diagram plotted as ε142Nd vs 144Sm/144Nd. The inset to 2a shows the 2σ error envelope for the regression in parts per 10,000. M. Boyet et al. / Earth and Planetary Science Letters 291 (2010) 172–181 175

Fig. 3. Sm–Nd systematics measured for Moore County. (a) 147Sm–143Nd isochron diagram. Data from this study are reported by white symbols, and literature values are shown by gray diamonds (Tera et al., 1997; Blichert-Toft, et al., 2002). The inset shows the 2σ error envelope for the regression in parts per 10,000. (b) 146Sm–142Nd evolution diagram plotted as ε142Nd vs 144Sm/144Nd. ratios for the bulk samples are clearly higher than the chondritic value confidence limits reported by Isoplot are not used here because they and the ratios measured in whole rock basaltic eucrites (Blichert-Toft give unrealistically large uncertainties, particularly when the number of et al., 2002; Boyet and Carlson, 2005). Sm/Nd ratios between different points on the line is small. Insets to Figs. 2aand3a show error envelopes mineral phases show an even wider spread. Plagioclase is character- for 2σ of the scatter and suggest that these errors better reflect the true ized by the lowest 147Sm/144Nd ratios (between 0.1012 and 0.1279) uncertainty of the data. Initial ε143Nd and their uncertainty are cal- and always have negative ε142Nd (−0.72 to −1.48 relative to the culated using the procedure described by Fletcher and Rosman (1982). terrestrial standard) and subchondritic 143Nd/144Nd ratios (<0.510). Previous Sm–Nd studies reporting data for both 146Sm–142Nd and In comparison, pyroxene is characterized by higher Sm/Nd ratios 147Sm–143Nd chronometers measured on the same fractions have (147Sm/144Nd>0.31), large excesses in 142Nd (ε142Nd>1.2) and been compiled and are presented in Table 2. Estimates of solar sys- radiogenic 143Nd/144Nd ratios (>0.516). tem initial 146Sm/144Sm derive not only from internal isochrons on Results for Binda, Moore County and Moama are plotted in Figs. 2–4, eucrites, but also from silicate clasts from mesosiderites, angrites, and respectively, and are discussed individually below. For these meteorites, one study of the meteorite Acapulco. For that reason, results of these line-fitting is done using the Isoplot program (Ludwig, 1991)with studies also are provided in Table 2. In this table, data have been Model 3 fits because the probability of fit is generally too low for using normalized to 146Nd/144Nd=0.7219. Isochron ages and uncertainties Model 1 fits. Errors reported for the ages and 146Sm/144Sm ratios are 2σ for 147Sm–143Nd and 146Sm–142Nd have been re-calculated using of the observed scatter of the data about the best-fit line. The 95% Isoplot, assuming an uncertainty of 0.2% on measured Sm/Nd ratios.

Fig. 4. Sm–Nd systematics measured for Moama. (a) 147Sm–143Nd isochron diagram showing data measured in this study (white symbols) and data from the literature (gray diamonds (Hamet, et al., 1978 and Jacobsen and Wasserburg, 1984). Regression of whole rock and the pyroxene fraction measured in this study are shown as a solid black line. The black dashed line shows regression of whole rock, pyroxene and plagioclase fractions measured in this study. The solid gray line shows regression of all data, including the literature data, but excluding our plagioclase measurement. (b) 146Sm–142Nd evolution diagram plotted as ε142Nd vs 144Sm/144Nd. 176 M. Boyet et al. / Earth and Planetary Science Letters 291 (2010) 172–181

Table 2 Coupled 146Sm–142Nd, 147Sm–143Nd systematics obtained in published studies of achondrites and the results from this work for Binda, Moore County and Moama. All isochrons have been re-calculated using the Isoplot program, except for a few samples noted * when data were not reported in the publication. Some previous studies have not been included because of evident isotopic disturbances in the sample (Y79251, Nyquist et al., 1997a,b), or a largely under-constrained regression (Angra dos Reis, Nyquist et al., 1994). Samples considered as “non-disturbed” for coupled 146Sm–142Nd and 147Sm–143Nd systematics have their names underlined. These samples are those used in the calculation of the solar system initial 146Sm/144Sm ratio. Sm–Nd data on chondrites have been obtained by measuring phosphate fractions and chondrules from six ordinary chondrites and one carbonaceous chondrite (Amelin and Rotenberg, 2004).

147 143 143 b c 146 144 d 142 e c 146 144 f g Sample name T Sm– Nd ε NdT MSWD Sm/ SmT ε NdT MSWD Sm/ SmTo Reference (Ma)a

Eucrite Binda 4544±88 1.0±0.9 47 0.00728±0.00057 −2.74±0.23 1.6 0.0085 This study Caldera 4544±19 0.4±0.8 2 0.00735±0.00132 −3.00±0.55 0.77 0.0088 1 Chervony Kut* 4580±30 3.3±0.9 0.00690±0.00150 −2.65±0.66 0.0069 2 EET87520* 4547±9 0.00690±0.00450 −2.71±0.23 0.0079 3 EET90020* 4482±30 1.8±0.3 0.00480±0.00200 0.0085 4 Ibitira 4474±67 1.2±0.4 3.6 0.00905±0.00280 −2.83±0.46 2.6 0.0171 5 Ibitira 4570±90 0.00820±0.00080 0.0082 6 Moama 4594±79 0.4±0.9 19 0.00593±0.00069 −2.13±0.08 0.08 0.0155 This study Moama 4466±42 0.0±0.6 2 0.00411±0.00130 −1.62±0.59 2.0 0.0082 7 Moore County 4542±85 1.0±1.0 43 0.00660±0.00120 −2.48±0.49 21 0.0073 This study Y980318* 4567±24 1.5±1.6 0.00600±0.00090 −2.8±0.6 0.0081 8 Y980433* 4542±42 1.5±1.1 0.00570±0.00050 0.0068 9

Angrite Angra dos Reis 4512±85 1.6±0.6 18 0.00632±0.00170 −2.39±0.68 1.4 0.0109 10 Angra dos Reis 4560±94 −0.1±0.2 0.46 0.0135±0.0064 −4.67±2.1 6.4 0.0143 7 LEW 86010 4558±34 −0.7±0.2 1.5 0.00710±0.00170 −2.57±0.60 0.25 0.0076 11 LEW 86010 4536±69 1.2±0.9 6.5 0.00780±0.00200 −2.74±0.88 4.6 0.0097 12

Mesosiderite Morristow Frag. A 4453±95 2.5±1.1 5 0.00630±0.00700 −0.90±3.15 5.2 0.0137 5 Morristow Frag. B 4483±40 1.8±0.7 0 0.00620±0.00260 −1.95±0.58 0 0.0110 5 Mt. Padburry 4494±45 −0.4±0.5 9.1 0.00560±0.00078 −1.95±0.37 1.4 0.0092 13 Vaca Muerta P.5 4505±166 3.7±1.4 56 0.00450±0.00440 −1.60±1.58 4.6 0.0069 13 Vaca Muerta P.12 4401±119 18.9±6.2 18 0.00628±0.00080 −0.29±0.95 0.6 0.0193 13 Vaca Muerta P.16 4653±219 −1.2±2.6 145 0.00530±0.00340 −1.34±1.58 18 0.0030 13 Vaca Muerta 4512±86 0.0±0.7 9.2 0.00760±0.00280 −2.57±1.17 10 0.0111 14

Acapulcoite Acapulco 4601±62 −0.1±2.4 4.1 0.00702±0.00178 −2.22±0.68 4.1 0.0070 5

Chondrites 0.00750±0.00270 −2.62±0.93 1.2 15

a 147Sm–143Nd ages calculated from the slope of the internal isochrons. b Initial e143Nd values are calculated at age T147Sm–143Nd using λ147Sm=6.54×10− 12 yr− 1 and the modern average chondrite values deﬁned by Bouvier et al. (2008). c Values of the MSWD (mean square of weighted deviates). d 146Sm/144Sm ratios at the age of Sm–Nd isotope closure (slope of the fossil isochrons). e Initial 142Nd/144Nd ratios are expressed relative to the terrestrial standard value. f 146Sm/144Sm ratios re-calculated at the age of solar system formation (4568 Ma) considering the age of Sm–Nd isotope closure obtained from 147Sm–143Nd systematics. g References: 1 (Wadhwa and Lugmair, 1996), 2 (Wadhwa and Lugmair, 1995), 3 (Lugmair et al., 1991), 4 (Nyquist et al., 1997a,b), 5 (Prinzhofer et al., 1992), 6 (Nyquist et al., 1999), 7 (Jacobsen and Wasserburg, 1984), 8 (Nyquist et al., 2004 and Nyquist et al., 2008), 9 (Nyquist et al., 2008), 10 (Lugmair and Marti, 1977), 11 (Lugmair and Galer, 1992), 12 (Nyquist et al., 1994), 13 (Stewart et al., 1994), 14 (Sharma et al., 1995), 15 (Amelin and Rotenberg, 2004).

Model 3 ﬁts were used and the errors reported for ages and 146Sm/ agreement with the more precise 244Pu–Xe age of 4529±34 Ma 144Sm ratios are 2σ of the observed scatter of the data about the best- determined for this sample (Miura et al., 1998). The 146Sm–142Nd ﬁt line. Table 2 also includes a few results from abstracts in which only internal isochron is shown Fig. 2b (MSWD=1.6). The equation of the a regression age and initial Nd isotope ratio were reported without line in a 144Sm/144Nd–142Nd/144Nd plot is given by: accompanying isotopic data. These samples are starred in the table, and those results are transcribed as written in the abstracts with no 142Nd= 144Nd = 146Sm= 144Sm × 144Sm= 144Nd + 142Nd= 144Nd mes T T re-calculation. ð1Þ

3.1. Binda where the (146Sm/144Sm) ratio at the age of the isotopic closure (T) corresponds to the slope of the isochron. The Binda isochron defines an The heavy (HF) and light (LF) density separates from Binda show initial 146Sm/144Sm ratio equal to 0.0073±0.0006 and an intercept of isotopic composition intermediate between WR and pyroxene (px) −2.74±0.23 (calculated relative to the modern terrestrial standard for HF and between WR and plagioclase (plag), for LF, respectively, value). For the Binda isochron, ε142Nd calculated for a chondritic Sm/Nd indicating that these separates may not have been as mineralogically ratio of 0.1960 is equal to −0.19±0.07, similar to the mean of pure as were the hand-picked separates. Different mineral fractions chondritic samples (Boyet and Carlson, 2005). and the whole rock show good alignment in a 147Sm/144Nd vs. 143Nd/ 144Nd plot (Fig. 2a). The slope of 0.03017±0.00060 (MSWD=47) 3.2. Moore County corresponds to an age of 4544±88 Ma (λ147Sm=6.54×10− 12 yr− 1) and the intercept is 0.50677±4 (ε143Nd=1.0±0.9 calculated using Both Sm–Nd isochrons are defined by 3 points (Fig. 3). The slope of the following average chondrite parameters: 147Sm/144Nd=0.1960 the 147Sm–143Nd line yields an age of 4542±85 Ma (MSWD=46) and 143Nd/144Nd=0.512630 (Bouvier, et al., 2008)). This age is in that is nearly identical to the age obtained for Binda. The calculated M. Boyet et al. / Earth and Planetary Science Letters 291 (2010) 172–181 177 intercept is 0.50677±5 (ε143Nd=1.0±1.0). This age overlaps within ting our data for Binda, Moama and Moore County simultaneously gives error the Sm–Nd age of 4456±25 Ma and Pb–Pb age of 4484±19 Ma an age of 4557±42 Ma, initial ε143Nd=+0.78±1.3 and 146Sm/ reported for Moore County by Tera et al. (1997).InFig. 3a, other Sm–Nd 144Sm=0.00672±0.00046. The accuracy of this age depends on measurements obtained on this sample (whole rock and mineral whether all three samples are exactly the same age and formed with separates from Tera et al. (1997) and Blichert-Toft et al. (2002) are the same initial isotopic compositions. For reasons to be discussed shown for comparison. The 147Sm–143Nd age obtained in this study is below, we do not believe this to be the case. similar to the 244Pu–Xe age of ∼4548 Ma (Shukolyukov and Begemann, The large age uncertainties for the 147Sm–143Nd ages reported here 1996). The 146Sm–142Nd alignment gives a slope of 0.0066±12 and an stem largely from the scatter of the points about any best-fit line. For intercept of −2.48±0.49 ε-unit (MSWD=21), which corresponds to example, for Binda, the 143Nd/144Nd data deviate from the best-fit line a present day value of −0.20±0.19 when calculated relative to by ε143Nd=−0.96 for the whole rock to +0.66 for the plagioclase. chondritic Sm/Nd ratio (Fig. 3b). Both 146Sm–142Nd and 147Sm–143Nd Alternatively, the whole rock and plagioclase data have 147Sm/144Nd systematics are in agreement to suggest that Binda and Moore County ratios displaced from the best-fit line by +0.75% and −1.1%, have the same formation age. respectively. Both deviations are well outside of analytical error, leading to the conclusion that the internal Sm–Nd systematics of 3.3. Moama the cumulate eucrites have been disturbed. For Moama, additional evidence for disturbance comes from the observation that the age 147Sm–143Nd measurements obtained on Moama have been plot- determined here is older, outside of uncertainty, compared to the age ted with literature values in Fig. 4a. Considering only data measured in determined by Jacobsen and Wasserburg (1984), which is close to this study, we obtain the steepest slope (MSWD=19) yielding an age the 4439±97 Ma Pb–Pb age found by Tera et al. (1997). Additional of 4594±79 Ma and an initial 143Nd/144Nd ratio of 0.50667±0.00005 Moama plagioclase (Hamet et al., 1978) and whole rock (Blichert-Toft (ε143Nd=0.4±0.9). This age compares with an age of 4466±42 Ma et al., 2002) measurements lie distinctly off the lines reported here and initial ε143Nd=0.0±0.6 from Jacobsen and Wasserburg (1984) and by Jacobsen and Wasserburg (1984), in the case of the Blichert- and an age of 4520±50 Ma with initial ε143Nd=+1.9±0.6 from Toft et al. (2002) whole rock measurement by −14 in ε143Nd. Hamet et al. (1978). For Binda, the Sm–Nd age reported here agrees well with its Pu–Xe The slope of the 142Nd/144Nd–144Sm/144Nd correlation using our age (4529±34 Ma; Miura et al., 1998). For Moore County, the Sm–Nd data gives a 146Sm/144Sm ratio of 0.0059±7 (MSWD=0.08) and age is older than, but within uncertainty, of the Sm–Nd age reported intercept of −2.13±0.08 corresponding to an initial 142Nd/144Nd previously (4456±25 Ma) and the more precise Pb–Pb age of 4484± isotope composition of ε142Nd=−0.03±0.03 (Fig. 4b). Using just the 19 Ma reported for Moore County by Tera et al. (1997). Moore County, pyroxene and whole rock data changes these values to 146Sm/ however, is heavily contaminated with terrestrial Pb (Tera et al., 144Nd=0.0060±0.0009 and present day ε142Nd (at chondritic Sm/Nd 1997), which complicates interpretation of its Pb–Pb age in spite of its ratio) of −0.04±0.16. Using the data reported by Jacobsen and high precision. Moore County also has been examined for its Mn–Cr Wasserburg (1984) we calculate an initial 146Sm/144Sm=0.0041± systematics, but no evidence was found for live 53Mn, leading to only 0.0013 for their Moama analyses. The Pb–Pb age determined for Moama an upper age estimate of <4549 Ma (Lugmair and Shukolyukov, (4426±97 Ma; Tera et al., 1997) supports a younger age for Moama, but 1998). This minimum age is important, however, because it shows overlaps all Sm–Nd age determinations for Moama within error. that Moore County cannot be as old as some basaltic eucrites that display evidence of live 26Al (Piplia Kalan — Srinivasan et al., 2007), 4. Discussion 53Mn (Chervony Kut, Juvinas, Ibitira, but not Caldera, EET87520, or Pomozdino — Lugmair and Shukolyukov, 1998), and 182Hf (Stannern — 4.1. Sm–Nd age of cumulate eucrites Kleine et al., 2005). Four cumulate eucrites (Binda, Caldera, EET87520, Moore County) Using the new data, absolute ages estimated from 147Sm–143Nd provide concordant 146Sm–142Nd, 147Sm–143Nd results (Fig. 5). We have systematics for Binda and Moore County are 4544±88 Ma and 4542± classified these samples as isotopically undisturbed (at least for Sm–Nd) 85Ma, respectively. In the data reported here, Moama gives an and they will be used to provide an estimate of the solar system initial older age (4594 ± 79 Ma), but one that overlaps within error. Fit- 146Sm/144Sm ratio in the next section. First, we note that the weighted

Fig. 5. 147Sm–143Nd ages of eucrites defined by internal isochrons. References for literature data are given in Table 2. Moama# is the age obtained in this study. Samples within the gray field have very similar 147Sm–143Nd ages. The light yellow bar corresponds to the weighted average of independent results, which is equal to 4546±8 Ma. 178 M. Boyet et al. / Earth and Planetary Science Letters 291 (2010) 172–181 average of 147Sm–143Nd ages obtained for these samples is well-defined pure p-process isotope, a p-process deficiency will be manifest in at 4546±8 Ma (MSWD=0.04). This time of Sm–Nd isotope closure, 142Nd because of the deficiency in 146Sm (Andreasen and Sharma, moreover, is consistent with the lack of live 53Mn in Moore County and 2006). Plotting C- chondrites in a Sm/Nd–142Nd/144Nd plot, a slope Caldera. It also agrees very well with the ages of Hf–W mineral isochrons of 146Sm/144Sm ∼0.035 at the age of chondrite formation is obtained, on basaltic eucrites that are approximately 20 Ma younger than solar a value significantly higher than the value of 0.008 estimated for the system formation (Kleine et al., 2005). solar system. The most likely explanation for this is that the cor- Samarium–Nd studies of samples from the HED parent body using relation is not a 146Sm–142Nd isochron, but is instead a mixing line both the 146Sm–142Nd and 147Sm–143Nd chronometers consistently yield between material with “normal” solar system isotopic composition younger ages than those obtained from very short-lived chronometers and isotopically anomalous pre-solar material. Nucleosynthetic on whole rock eucrites (Lugmair and Shukolyukov, 1998; Bizzarro et al., anomalies in Sm and Nd have not been detected in ordinary and 2005). Whole rock eucrites define both Mn–Cr (Lugmair and Shukolyukov, enstatite chondrites, eucrites, angrites, or lunar samples, hence the 1998)andHf–W(Kleine et al., 2004) isochrons consistent with the observed 142Nd/144Nd variations are explained by Sm/Nd fraction- differentiation of the HED parent body within 5 to 7 Ma after solar ation produced during the lifetime of 146Sm (Boyet and Carlson, system formation. Several eucrites also have internal isochron system- 2005; Andreasen and Sharma, 2006; Carlson et al., 2007; Boyet and atics in the Al–Mg (Srinivasan et al., 2007), Mn–Cr (Lugmair and Carlson, 2007; Brandon et al., 2009). Shukolyukov, 1998), and Hf–W(Kleine et al., 2005) systems consistent Samarium–Nd analyses obtained on Binda provide the most pre- with their eruption onto the surface of the HED parent body before cise estimate for the solar system initial 146Sm/144Sm ratio from this 4560 Ma. Differentiation and melting of the eucrite parent body thus study. As discussed in the previous section, this sample has a 147Sm– occurred early, likely as a result of heating by the decay of 26Al. 143Nd age (4545±89 Ma) similar to the Hf–W age of 4547±2 Ma Are 147Sm–143Nd ages obtained for cumulate eucrites related to a defined by most of the basaltic eucrites (Kleine et al., 2005). The large-scale thermal event on the HED parent body? The brecciated 146Sm/144Sm ratio calculated for Binda from the five-point regression texture of most the samples, as well as exsolution textures described line is defined with high precision (0.0073±6). This ratio translates to in pyroxenes (Takeda, 1997) indicate that successive thermal events a ratio at 4568 Ma equal to 0.0084±6, if Sm–Nd and Hf–W isotope have affected the HED parent body. Thermal metamorphism resulting closure occurred at the same time. A difference of 2 Ma on the from impacts was interpreted to be responsible for re-equilibration absolute age has little effect on the calculation since the major uncer- of W isotopes in basaltic eucrites approximately 20 Ma after solar tainty is the error on the slope of the fossil isochron. For example, system formation (Kleine et al., 2005). The similarity of our weighted when the 147Sm–143Nd age is used, we find a 4568 Ga 146Sm/144Sm average of 147Sm–143Nd ages for cumulate eucrites with the W ages ratio equal to 0.0085 (Table 2). suggests that the thermal event affected both basaltic and cumulate Sm–Nd results obtained on Caldera, Moore County and EET87520 eucrites. Using our results for Binda for which 5 different mineral are fully consistent with this estimate of the solar system initial 146Sm/ fractions define a precise 142Nd/144Nd–Sm/Nd isochron, the age of 144Sm ratio, however errors on initial 146Sm/144Sm ratios are higher. this thermal event can be further constrained. As represented in Fig. 2, When 146Sm/144Sm ratios are calculated back to the age of solar the slope of the internal isochron for Binda means that the last Sm–Nd system formation, using absolute ages defined by the 147Sm–143Nd +12 146 144 isotope closure occurred 23− 11 Ma after solar system formation for an chronometer for these 3 samples, Sm/ Sm ratios of 0.0075±45 to initial solar system 146Sm/44Sm ratio of 0.0085 (see ratio estimate in 0.0088±13 are calculated. The weighted mean of the initial solar the next section). The very old Al–Mg, Mn–Cr and Hf–W ages obtained system 146Sm/144Sm ratio calculated from the 4 eucrites with on some basaltic eucrites shows clearly that some eucrites escaped undisturbed Sm–Nd (Binda, Moore County, Caldéra and EET 87520) age resetting during the event recorded by Binda. However, even is 0.00837±0.00048 (MSWD=0.92). younger ages have been recorded by other isotope systematics in The two paired eucrites Y980318 and Y980433 have lower 146Sm/ some eucrites, including the younger U–Pb, Rb–Sr and Ar–Ar ages 144Sm ratios at the time of Sm–Nd isotope closure (0.0060±0.0009 reported for many samples (Birck and Allègre, 1978; Bogard and and 0.0057±0.0005, respectively) although their 147Sm–143Nd ages Garrison, 1995; Tera et al., 1997) indicating a prolonged period of are similar to those of other eucrites (4567±24 Ma and 4542± presumably impact-related resetting of eucrite ages. 42 Ma, respectively). The reason why these samples give lower 146Sm/144Sm values is not clear. Further evaluation of the data for 4.2. Solar system initial 146Sm/144Sm ratio these samples is hindered by the fact that the measured isotope ratios are not reported in these abstracts (Nyquist et al., 2004, 2008). We The precise determination of the initial abundance of short-lived note however than the first 146Sm/144Sm ratio determined for Y90318 radioisotopes is critical for their use to investigate the chronology of was significantly higher and equal to 0.0077±0.0012 at an age of the early solar system. Internal isochrons for CAIs found in car- 4560±150 Ma (Nyquist et al., 2004). bonaceous chondrites are currently used as pinning points for Four eucrite samples reported in Table 2 (Chervony Kut, EET translating the relative time scale provided by several extinct radio- 90020, Ibitira and Moama) have been excluded from the determina- isotope systems into absolute ages because these objects represent tion of the solar system initial 146Sm/144Sm ratio. For Chervony Kut, the first solids condensing in the solar nebula, and they have been we note that none of the measured points for this sample lie on the dated precisely using Pb–Pb systematics (4567 to 4568 Ma (Amelin regression line that defines the 147Sm–143Nd age (Wadhwa and et al., 2002; Bouvier et al., 2007). CAIs are not a good reference point Lugmair, 1995). The 147Sm–143Nd regression for Chervony Kut also for 146Sm–142Nd systematics because they can contain large isotope indicates an unreasonably high initial ε143Nd of 3.3±0.9, which these anomalies unrelated to the decay of 146Sm (McCulloch and Wasserburg, authors attributed to disturbance of the internal Sm–Nd systematics 1978) and because the internal 147Sm–143Nd systematics of CAIs can of this eucrite. We note however that the 146Sm/144Sm ratio deter- give anomalously young ages indicative of later resetting of the Sm–Nd mined on this sample appears to be consistent (=0.0069±0.0015) system (Scheinin, 1977). More recently, isotopic anomalies in Sm with other eucrite data although Chervony Kut also presents evidence and Nd have been identified at the whole rock scale in carbona- for live 53Mn, and thus must be older than e.g. Moore County (Lugmair ceous chondrites (Andreasen and Sharma, 2006; Carlson et al., and Shukolyukov, 1998). 2007). P-process deficits have been identified by large negative 144Sm Ibitira is characterized by two quite different, but overlapping within anomalies (100% p-process isotope). This magnitude of p-process error, 147Sm–143Nd age determinations (4468±67 Ma, Prinzhofer et al., deficit would have only a minor direct effect on 142Nd abundances 1992; 4.57±0.09 Ga, Nyquist et al., 1999). The older age is consistent (formed by 4% p-process and 96% s-processes), but because 146Sm is a with the high 146Sm/144Sm ratio of 0.009±0.003 (Prinzhofer et al., M. Boyet et al. / Earth and Planetary Science Letters 291 (2010) 172–181 179

1992) to 0.0082±0.0008 (Nyquist et al., 1999) determined for Ibitira regression, the 146Sm/144Sm ratio for LEW86010 calculated back to and the evidence for live 53Mn that gives an age of 4557 Ma for Ibitira 4568 Ma is in the range 0.0082–0.0084, a value similar to the estimate (Lugmair and Shukolyukov, 1998). Using the Mn–Cr age, the lower from the “concordant” eucrites described above. 146Sm/144Sm initial ratio determined for Ibitira (Nyquist et al., 1999) Mesosiderites are composed of mixtures of silicate clasts and would translate to a solar system initial value of 0.0089, on the high side, metal. Sm–Nd data have been published for three mesosiderites but within error, of the value calculated here from the data for Binda. The (Morristown, Mt. Padburry and Vaca Muerta), and for four different evidence for isotopic disturbance in Ibitira has been noted and fully pebbles of Vaca Muerta, ranging from basaltic to gabbroic textures discussed by Prinzhofer et al. (1992) and Nyquist et al. (1999).They (Stewart et al., 1994; Sharma et al., 1995). Silicate clasts contained in proposed that the Sm–Nd system remained closed in pyroxenes Vaca Muerta have been affected by secondary isotopic disturbance whereas isotope disturbances have affected both plagioclase and (leached fractions fall off the 147Sm–144Nd correlation lines, the ages phosphate phases. Secondary exchange could have occurred at 4485 have large uncertainties, and the initial ε143Nd of two pebbles are ±15 Ma, an age obtained by Ar–Ar systematics (Bogard and Garrison, dramatically different from chondritic, see Table 2). The one Vaca 1995). Moreover Ibitira is the only eucrite falling off the δ18O′–δ17O′ Muerta pebble with lowest age uncertainty has a chondritic initial mass dependant fractionation line formed by samples coming from the ε143Nd and gives a somewhat high solar system initial 146Sm/ HED parent body (Wiechert et al., 2004), which clouds its connection to 144Sm=0.0111±0.0028, but the large uncertainty allows this value the other eucrites. to overlap the estimates derived from eucrites and angrites. EET 90020 has a younger 147Sm–143Nd age (4482±30 Ma) and 146Sm–142Nd and 147Sm–143Nd results obtained for two fragments radiogenic initial 143Nd/144Nd ratio (ε143Nd=1.8±0.3), a feature of Morristown give very similar results, but are characterized by young shared by other samples with disturbed Sm–Nd (e.g. Moama, Chervony ages (4.45 to 4.48Ga) and positive initial ε143Nd (1.8 to 2.6) Kut and Ibitira) (Nyquist et al., 1997a,b). (Prinzhofer et al., 1992). The 146Sm/144Sm estimates are high Moama provides a particularly good example of the need for an (0.0063±0.0070 and 0.0062±0.0026) relative to the young 147Sm– accurate absolute age in order to derive an accurate estimate of the 143Nd ages, but the errors on the ratio are large enough to overlap initial 146Sm/144Sm of the solar system. Using our data and the essentially all estimates of solar system initial 146Sm/144Sm ratio. By 4594 Ma age, the solar system initial 146Sm/144Sm determined from contrast, Sm–Nd systematics in the silicate clast from Mt. Padburry are the Moama data is 0.00498, lower than essentially any other estimate consistent and leached fractions fall along the line defined by of this value. Using the Pb–Pb age (4426 Ma) for Moama (Tera et al., separated phases (Stewart et al., 1994), which was not the case for 1997) instead gives 146Sm/144Sm=0.0155 at 4568 Ma, which is other mesosiderite samples. A 147Sm–143Nd age of 4494±45 Ma higher than any other estimate. In contrast, the data of Jacobsen and (MSWD=9.1) is obtained. The 146Sm–142Nd regression yields a well- Wasserburg (1984) for Moama provide a solar system initial 146Sm/ defined slope of 0.00560±0.0078 (MSWD=1.4), which corresponds 144Sm=0.0082, which is indistinguishable from the value we derive to a value of 0.0092 for the 146Sm/44Sm at 4.568 Ga, slightly higher, but above for the “concordant” eucrites. overlapping within error, the value derived here from the eucrite data. Only two angrites, Angra dos Reis and LEW 86010, have been The 147Sm–143Nd age obtained on Acapulco (a primitive achon- analyzed for both Sm–Nd systematics. Because these samples have drite classified in the Acapulcoite group) is equal to 4601±62 Ma Sm–Nd ages close to those reported here for Binda and Moore County, with a 146Sm/144Sm ratio of 0.0070±18 (Prinzhofer et al., 1992). This and precise U–Pb ages indicating that they crystallized early in solar 146Sm/144Sm value is similar to the value obtained on angrite LEW system history, unlike most eucrites, they provide a good comparison 86010, for which consistent 147Sm–143Nd (4558±42 Ma; Lugmair for age cross calibration. The 147Sm–143Nd ages for Angra dos Reis and Galer, 1992) and Pb–Pb (4558.55±0.14 Ma; Amelin, 2008) ages (4512±85 Ma (Lugmair and Marti, 1977); 4560±50 Ma (Jacobsen have been obtained. A very similar Pb–Pb age of 4556.5 Ma has been and Wasserburg, 1984)) and LEW86010 (4558±34 Ma (Lugmair and measured on phosphates separated from Acapulco (Amelin, 2005; Galer, 1992) and 4536±69 Ma (Nyquist et al., 2004)) are consistent Amelin et al., 2006), suggesting that the 147Sm–143Nd age determined with their Pb–Pb ages (4557.65±0.13 Ma and 4558.55±0.14 Ma for on this sample cannot be used as reference for calculating the initial Angra dos Reis and LEW8601, respectively (Amelin, 2008)). These two 146Sm/144Sm solar system ratio. If the Pb–Pb age is used instead, a samples are younger than other angrites like d'Orbigny (4564.44± value of 0.0076 is obtained for the initial 146Sm/144Sm ratio, which is 0.12 Ma; (Amelin, 2008) and SAH 99555 (4564.86±0.38 Ma (Amelin, lower, but within error, of the value derived from the eucrite data. 2008); 4564.58±0.14 Ma (Connelly et al., 2008)) showing that When different groups of achondrites are considered separately different age groups of angrites exist, as they apparently do for (after data have been selected as discussed above), consistent values eucrites. The earliest estimate of solar system initial 146Sm/144Sm was are obtained for the solar system initial 146Sm/144Sm ratio. The precise obtained on Angra dos Reis (Lugmair and Marti, 1977). Due to the estimate of 0.0084±0.0006 is obtained for Binda when we assume small Sm/Nd fractionation between pyroxene and plagioclase in this that both Sm–Nd and Hf–W isotope closure occurred at the same time. sample (147Sm/144Nd from 0.13 to 0.20) and since the analytical An initial solar system 146Sm/144Sm ratio of 0.0084±0.0005 is cal- precision on Nd isotope ratios were larger than 50 ppm at this time, the culated when the 4 other non-disturbed Sm–Nd eucrites are added to initial 146Sm/144Sm ratio calculated for this sample has large errors the Binda data. When the angrite LEW8601 and the mesosiderite Mt. (0.0063±0.0017 (Lugmair and Marti, 1977) and 0.0135±0.0064 Padburry are included in the average calculation, we find a solar (Jacobsen and Wasserburg, 1984)). A 146Sm/44Sm ratio of 0.0071± system initial 146Sm/144Sm ratio equal to 0.0085±0.0007. This esti- 0.0017 was defined for LEW86010 at the age of Sm–Nd isotopic mate is consistent with the 146Sm/144Sm ratio of 0.0075±0.0027 closure, which translates to an initial value of 0.0076 at 4568 Ma defined by chondrites (Amelin and Rotenberg, 2004). The correlation (Lugmair and Galer, 1992). The same sample has been measured by between 147Sm–143Nd age and initial 146Sm/144Sm is shown in Fig. 6a. Nyquist et al. (1994). We note a small difference between the results of The only samples that fall significantly off this correlation are our calculations and those published in the original paper. We obtain a Y980318/Y980433 for which the data are reported only in graphical 146Sm/44Sm ratio equal to 0.0078±0.0020 instead of 0.0076±0.0009 form in abstracts (Nyquist et al., 2004, 2008). Prinzhofer et al. (1992) for the results reported by Nyquist et al. (1994). The difference be- noted the lack of such a correlation for their samples and explained comes significantly higher when data measured on leachates are the disturbance of the Sm–Nd systematics by exchange between omitted from the regression. We calculate a 146Sm/44Sm ratio equal to phosphates and plagioclase. Their correlation was significantly im- 0.0089±0.0020 whereas the value of 0.0080±0.0009 was reported in proved when 146Sm/144Sm ratios were plotted against 147Sm–143Nd Nyquist et al. (1994).UsingthePb–Pb formation age of LEW86010 and pyroxene model ages (Prinzhofer et al., 1992). This approach is not the 146Sm/144Sm ratio defined when leachates are not considered in the justified in our study because the pyroxene model ages are similar 180 M. Boyet et al. / Earth and Planetary Science Letters 291 (2010) 172–181

146Sm was homogeneously distributed in the inner solar system across the region of the nebula from which the parent bodies of the achondrites formed. The exception is the nucleosynthetic anomalies preserved in carbonaceous chondrites as discussed previously (Andreasen and Sharma, 2006; Carlson et al., 2007). The 142Nd/ 144Nd ratios measured on bulk meteorite samples cannot be directly compared because the samples evolved since the time of Sm–Nd isotopic closure with different Sm/Nd ratios. This is a particularly important issue for the cumulate eucrites that are characterized by superchondritic 147Sm/144Nd ratios. Initial ε142Nd determined from internal isochrons of a variety of achondrites are shown relative to their 147Sm–143Nd ages in Fig. 6b. The initial ε142Nd of the achondrites evolved within the ﬁeld deﬁned by the 142Nd/144Nd isotopic evolution of ordinary chondrites through time, assuming that ordinary chondrites have a present day ε142Nd=−0.16±0.03 (n=11, (Carlson and Boyet, 2008) relative to the terrestrial standard.

5. Conclusion

Coupled 146Sm–142Nd and 147Sm–143Nd measurements on whole rocks and mineral separates of two cumulate eucrites (Binda and Moore County) give concordant results and suggest that the last Sm–Nd isotopic closure occurred at ∼4547 Ga. This event cannot be related to the HED planetesimal melting that apparently occurred during the first 5 Ma of solar system history as identified from studies of very short- lived chronometers in eucrites, but is fully consistent with the ages obtained from Hf–W internal isochron studies on eucrites. Moama has a more complex history and probably a multi-stage evolution as illustrated by initial radiogenic Nd isotope composition and range of Sm–Nd ages. The 146Sm/144Sm ratios defined for Binda and Moore County are consistent with previous estimates made from angrite, eucrite and mesosiderite internal isochrons. All the 146Sm–142Nd literature data obtained on achondrite internal isochrons have been sum- marized and carefully examined in order to provide the best estimate of the solar system 146Sm/44Sm ratio at 4568 Ma. A ratio equal to 0.0084± 0.0005 is obtained from considering eucrite data only; this value be- comes 0.0085±0.0007 when data obtained for one angrite (LEW8601) and for the mesosiderite Mt Padburry are also included. No difference in the initial 146Sm/144Sm and 142Nd/144Nd ratios are observed among different groups of achondrites compared to ordinary chondrites. This work suggests that Sm and Nd were homogeneously distributed and Fig. 6. (a) 146Sm/144Sm decay through time using an initial 146Sm/144Sm solar system isotopically uniform at the planetary scale in the solar system, at least in ratio of 0.0085±0.0007, calculated from the samples with undisturbed Sm–Nd the region where the achondrites and terrestrial planets formed. systematics, as described in the text. Ages and initial 146Sm/144Sm ratios obtained from internal isochrons for individual samples are from Table 2. Eucrites shown with a gray circle are Binda and Moore County (this study), Caldera (Wadhwa and Lugmair, Acknowledgements 1996), and EET87520 (Lugmair, et al., 1991). Data for eucrites Y980318/433 (Nyquist 146 144 et al., 2008) have lower Sm/ Sm ratios than other eucrites and are shown with We thank Fouad Tera for giving us these samples, Tim Mock for his open symbols. Two measurements of the angrite LEW86010 are represented (Lugmair and Galer, 1992; Nyquist et al., 1994) and its Pb–Pb age (Amelin, 2008) is given for assistance in the mass-spectrometry room, and Nabil Boctor for help- reference. The silicate clast comes from the mesosiderite Mt Padburry (Stewart, et al., ful discussion on the petrology of these samples. Insightful reviews 1994). The uncertainty on the146Sm/144Sm value is represented by the gray field. by Thorsten Kleine, Klaus Mezger, Steve Galer and an anonymous ε (b) Initial 142Nd determined from internal isochrons of the same samples are compared reviewer helped improve this manuscript. This work was supported to the evolution of 142Nd/144Nd in ordinary chondrites. The shaded field shows the 142 by the Carnegie Institution of Washington and NASA Cosmochem- Nd evolution of ordinary chondrites with time, given their present day ε142Nd = −0.16±0.03. istry grant NNX08AH65G. The DTM Triton was purchased with the aid of a grant from the National Science Foundation (EAR-0320589). within error to absolute Sm–Nd ages defined by internal isochrons. The research leading to these results has received funding from the The 146Sm/144Sm decay curve using this value as well as “non- European Research Council under the European Community's disturbed” samples are represented in Fig. 6. The new estimate of the Seventh Framework Programme (FP7/2007-2013 Grant Agreement 146Sm/144Sm ratio is much more precise than, but consistent with, the no. 209035). estimate of 0.0076±0.0017 suggested by Lugmair and Galer (1992). The precision of 0.0007 obtained on the initial 146Sm/144Sm solar References system ratio translates to an uncertainty of ∼12 Ma on time intervals defined from the slope of fossil isochrons. Amelin, Y., 2005. Meteorite phosphates show constant 176Lu decay rate since 4557 million This work brings additional constraints on the Sm and Nd isotopic years ago. Science 310, 839–841. – – 146 144 Amelin, Y., 2008. U Pb ages of angrites. Geochim. Cosmochim. Acta 72, 221 232. distribution within the solar system. The overlapping Sm/ Sm Amelin, Y., Rotenberg, E., 2004. Sm–Nd systematics of chondrites. Earth Planet. Sci. Lett. ratios measured from different groups of achondrites suggest that 223, 267–282. M. Boyet et al. / Earth and Planetary Science Letters 291 (2010) 172–181 181

Amelin, Y., Krot, A.N., Hutcheon, I.D., Ulyanov, A.A., 2002. Lead isotopic ages of chondrules Lugmair, G.W., Galer, S.J.G., 1992. Age and isotopic relationships among the angrites and calcium–aluminium-rich inclusions. Science 297, 1678–1683. Lewis Cliff 86010 and Angra dos Reis. Geochim. Cosmochim. Acta 56, 1673–1694. Amelin, Y., Wadhwa, M., Lugmair, G.W., 2006. Pb isotopic dating of meteorites using Lugmair, G.W., Marti, K., 1977. Sm–Nd–Pu timepieces in the Angra Dos Reis meteorite. 202Pb–205Pb double spike: comparison with other high-resolution chronometers. Earth Planet. Sci. Lett. 35, 273–284. Lunar Planet Sci. XXXVII, 1970. Lugmair, G.W., Shukolyukov, A., 1998. Early solar system time-scales according to Andreasen, R., Sharma, M., 2006. Solar Nebula heterogeneity in p-process samarium 53Mn–53Cr systematics. Geochim. Cosmochim. Acta 62, 2863–2886. and neodynium isotopes. Science 314, 806–809. Lugmair, G.W., Scheinin, N.B., Carlson, R.W., 1977. Sm–Nd systematics of the Serra de Birck, J.-L., Allègre, C.J., 1978. Chronology and chemical history of the parent body of Magé eucrite. Meteoritics 12, 300–301. basaltic achondrites studied by the 87Rb–87Sr method. Earth Planet. Sci. Lett. 39, Lugmair, G.W., Galer, S.J.G., Carlson, R.W., 1991. Isotope systematics of cumulate eucrite 37–51. EET-87520. Meteor. Planet. Sci. 26, 368. Bizzarro, M., Baker, J.A., Haack, H., Kasper, L.L., 2005. Rapid timescales for accretion and McCord, T.B., Adams, J.B., Johnson, T.V., 1970. Asteroid Vesta: spectral reﬂectivity and melting of differentiated planetesimals inferred from 26Al–26Mg chronometry. compositional implications. Science 168, 1445–1447. Astrophys. J. 632, L41–L44. McCulloch, M.T., Wasserburg, G.J., 1978. Barium and neodymium isotopic anomalies in Blichert-Toft, J., Boyet, M., Télouk, P., Albarède, F., 2002. 147Sm–143Nd and 176Lu–176Hf in the Allende meteorite. Astrophys. J. 220, L15–L19. eucrites and the differentiation of the HED parent body. Earth Planet. Sci. Lett. 204, Miura, Y.N., Nagao, K., Sugiura, N., Fujitani, T., Warren, P.H., 1998. Noble gases, 81Kr–Kr 167–181. exposure ages and 244Pu–Xe ages of six eucrites, Béréba, Binda, Camel Donga, Bogard, D.D., Garrison, D.H., 1995. 39Ar–40Ar age of the Ibitira eucrite and constraints on Juvinas, Millbillillie, and Stannern. Geochim. Cosmochim. Acta 62, 2369–2387. the time of pyroxene equilibration. Geochim. Cosmochim. Acta 59, 4317–4322. Nyquist, L.E., Bansal, B., Wiesmann, H., Shih, C.-Y., 1994. Neodymium, strontium and Bogard, D.D., Garrison, D.H., 2003. 39Ar–40Ar ages of eucrites and thermal history of chromium isotopic studies of the LEW86010 and Angra dos Reis meteorites and the asteroid 4 Vesta. Meteor. Planet. Sci. 38, 669–710. chronology of the angrite parent body. Meteoritics 29, 872–885. Bouvier, A., Blichert-Toft, J., Moynier, F., Vervoort, J.D., Albarède, F., 2007. Pb–Pb dating Nyquist, L.E., Bogard, D., Takeda, H., Bansal, B., Wiesmann, H., Shih, C.-Y., 1997a. Crys- constraints on the accretion and cooling history of chondrites. Geochim. tallization, recrystallization, and impact-metamorphic ages of eucrites Y792510 Cosmochim. Acta 71, 1583–1604. and Y791186. Geochim. Cosmochim. Acta 61, 2119–2138. Bouvier, A., Vervoort, J.D., Patchett, P.J., 2008. The Lu–Hf and Sm–Nd isotopic com- Nyquist, L.E., Wiesmann, H., Reese, Y., Shih, C.-Y., Borg, L.E., 1997b. Samarium–neodymium position of CHUR: constraints from unequilbrated chondrites and implications for age and manganese–chromium systematics of eucrite Elephant Moraine 90020. the bulk composition of terrestrial planets. Earth Planet. Sci. Lett. 273, 48–57. Meteor. Planet. Sci. 32, A101–A102. Boyet, M., Carlson, R.W., 2005. 142Nd evidence for early (>4.53 Ga) global differenti- Nyquist, L.E., Reese, Y.D., Wiesmann, H., Shih, C.-Y., 1999. Two ages for Ibitira: a record ation of the silicate earth. Science 309, 576–581. of crystallization and recrystallization. Meteor. Planet. Sci. 34, A87–A88. Boyet, M., Carlson, R.W., 2007. Early lunar differentiation and a non magma ocean origin Nyquist, L.E., Reese, Y., Wiesmann, H., Shih, C.-Y., Takeda, H., 2003. Fossil 26Al and 53Mn for the lunar crust. Earth Planet. Sci. Lett. 262, 505–516. in the Asuka 881394 eucrite: evidence of the earliest crut on the asteroid 4 Vesta. Brandon, A.D., Lapen, T.J., Debaille, V., Beard, B.L., Rankenburg, K., Neal, C., 2009. Re- Earth Planet. Sci. Lett. 214, 11–25. evaluating 142Nd/144Nd in lunar mare basalts with implications for the early Nyquist, L.E., Takeda, H., Shih, C.-Y., Wiesman, H., 2004. Sm–Nd age and initial 87Sr/86Sr for evolution and bulk Sm/Nd of the Moon. Geochim. Cosmochim. Acta 73, 6421–6445. Yamato 980318: an old cumulate eucrite. Lunar Planet Sci. Conf. XXXV, 1330–1331. Carlson, R.W., Boyet, M., 2008. Composition of the Earth's interior: the importance of Nyquist, L.E., Shih, C.-Y., Reese, Y.D., 2008. Sm–Nd for norite 78236 and eucrite early events. Phil. Trans. R. Soc. A 366, 4077–4103. Y980318/433: implications for planetary and solar system processes. Lunar Planet Carlson, R.W., Boyet, M., Horan, M.F., 2007. Chondrite barium, neodymium, and samarium Sci. XXXIX, 1437–1438. isotopic heterogeneity and early Earth differentiation. Science 316, 1175–1178. Prinzhofer, A., Papanastassiou, D.A., Wasserburg, G.J., 1992. Samarium–neodymium Connelly, J.N., Bizzarro, M., Thrane, K., Baker, J.A., 2008. The Pb–Pb age of angrite evolution of meteorites. Geochim. Cosmochim. Acta 56, 797–815. SAH99555 revisited. Geochim. Cosmochim. Acta 72, 4813–4824. Scheinin, N.B., Sm–Nd systematics of extraterrestrial objects and questions of excess Consolmagno, G.J., Drake, M.J., 1977. Composition and evolution of the eucrite parent 142Nd, PhD thesis, University of California, 1977. body: evidence from rare earth elements. Geochim. Cosmochim. Acta 41, 1271–1282. Sharma, M., Papanastassiou, D.A., Wasserburg, G.J., 1995. Sm–Nd systematics of a large Crozaz, G., Floss, C., Wadhwa, M., 2003. Chemical alteration and REE mobilization in eucrite clast in the Vaca Muerta mesosiderite. Lunar Planet Sci. XXVI, 1271–1272. meteorites from hot and cold deserts. Geochim. Cosmochim. Acta 67, 4727–4741. Shukolyukov, A., Begemann, F., 1996. Pu–Xe dating of eucrites. Geochim. Cosmochim. Duke, M.B., Silver, L.T., 1967. Petrology of eucrites, howardites and mesosiderites. Acta 60, 2453–2471. Geochim. Cosmochim. Acta 31, 1637–1665. Shukolyukov, A., Lugmair, G.W., 1993. 60Fe in eucrites. Earth Planet. Sci. Lett. 119, Fletcher, I.R., Rosman, K.J.R., 1982. Precise determination of initial εNd from Sm–Nd 159–166. isochron data. Geochim. Cosmochim. Acta 46, 1983–1987. Srinivasan, G., Whitehouse, M.J., Weber, I., Yamaguchi, A., 2007. The crystallization age Galer, S.J.G., Lugmair, G.W., 1996. Lead isotope systematics of noncumulate eucrites. of eucrite zircon. Science 317, 345–347. Meteor. Planet. Sci. 31, A47. Stewart, B.W., Papanastassiou, D.A., Wasserburg, G.J., 1994. Sm–Nd chronology and Ghosh, A., McSween Jr, H.Y., 1998. A thermal model for the differentiation of asteroid 4 petrogenesis of mesosiderites. Geochim. Cosmochim. Acta 58, 3487–3509. Vesta based on radiogenic heating. Icarus 134, 187–206. Takeda, H., 1997. Mineralogical records of the early planetary processes on the Hamet, J., Nakamura, N., Unruh, D., Tatsumoto, M., 1978. Origin and history of the howardite, eucrite, diogenite parent body with reference to Vesta. Meteor. Planet. cumulate eucrite Moama as inferred from REE abundances, Sm–Nd and U–Pb Sci. 32, 841–853. systematics. Proc. Lunar Planet. Sci. Conf. 9th, pp. 1115–1136. Tera, F., Carlson, R.W., Boctor, N.Z., 1997. Radiometric ages of basaltic achondrites and Jacobsen, S.B., Wasserburg, G.J., 1984. Sm–Nd isotopic evolution of chondrites and their relation to the early history of the Solar System. Geochim. Cosmochim. Acta achondrites, II. Earth Planet. Sci. Lett. 67, 137–150. 61, 1713–1731. Kleine, T., Mezger, K., Münker, C., Palme, H., Bischoff, A., 2004. 182Hf–182Wisotopesys- Wadhwa, M., Lugmair, G.W., 1995. Sm–Nd systematics of the eucrite Chervony Kut. tematics of chondrites, eucrites and martian meteorites: chronology of core formation LPSC XXVI, 1453–1454. and early mantle differentiation in Vesta and Mars. Geochim. Cosmochim. Acta 68, Wadhwa, M., Lugmair, G.W., 1996. Age of the eucrite “Caldera” from convergence of long- 2935–2946. lived and short-lived chronometers. Geochim. Cosmochim. Acta 60, 4889–4893. Kleine, T., Mezger, K., Palme, H., Scherer, E., Münker, C., 2005. The W isotope com- Wiechert, U.H., Halliday, A.N., Palme, H., Rumble, D., 2004. Oxygen isotope evidence for position of eucrite metals: constraints on the timing and cause of the thermal rapid mixing of the HED meteorite parent body. Earth Planet. Sci. Lett. 221, metamorphism of basaltic eucrites. Earth Planet. Sci. Lett. 231, 41–52. 373–382. Ludwig, K.R., 1991. ISOPLOT: a plotting and regression program for radiogenic isotope data. USGS Open File Report, pp. 91–445.