Earth and Planetary Science Letters 291 (2010) 172–181
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Earth and Planetary Science Letters
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
Old Sm–Nd ages for cumulate eucrites and redetermination of the solar system initial
146
Sm/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
CNRS, UMR 6524, LMV, F-63038 Clermont-Ferrand, France
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
b
c
a r t i c l e
i n f o
Article history:
Received 29 May 2009
Received in revised form 5 January 2010
Accepted 6 January 2010
Available online 27 January 2010
Editor: T. Spohn
Keywords:
146
Sm–142Nd systematics
eucrites
early planetary differentiation
isotope distribution through solar system
a b s t r a c t
Short-lived 146Sm–142Nd and long-lived 147Sm–143Nd chronometers have been measured in three cumulate
eucrites (Binda, Moore County and Moama). The two major mineral phases (plagioclase and pyroxene)
present in these achondrites are characterized by a wide range of Sm/Nd ratios that allows well-resolved
Sm–Nd isochrons. This group of meteorites thus is suitable to better constrain the initial 146Sm/144Sm ratio of
the solar system. Binda and Moore County give concordant ages of 4544 ± 88 and 4542 ± 85 Ma,
respectively, with initial 143Nd/144Nd 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
system initial 146Sm/144Sm ratio is obtained using the five-point regression line determined for Binda. The
146
Sm/144Sm ratio of 0.00728 ± 57 obtained for this sample translates to a 146Sm/144Sm ratio at 4568 Ga of
0.0085 considering the age of isotopic closure obtained from 147Sm–143Nd systematics. When 146Sm–142Nd
data from the literature are examined in detail, four eucrites have concordant 147Sm–143Nd and 146Sm–142Nd
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
The early history of solar system evolution from planetesimal
formation to initial differentiation can be studied using short-lived
radioisotope systems to provide high temporal resolution. However,
extinct radionuclides must be used in combination with long-lived
chronometers in order to convert the relative time intervals provided by the extinct radioisotopes into absolute ages. Pb–Pb ages are
generally used to anchor the extinct nuclide relative timescale
because they provide absolute ages determined with the highest
precision. Sm–Nd systematics are composed of two decay schemes:
the extinct 146Sm–142Nd (T1/2 = 103 Ma) and the long-lived 147Sm–
143
Nd chronometers (T1/2 = 106 Ga). Because absolute ages can be
obtained from the latter, no additional anchor point is required for
Sm–Nd studies. In the case of a protracted cooling history for the
sample studied, coupled 146Sm–142Nd, 147Sm–143Nd systematics
⁎ Corresponding author.
E-mail address: M.Boyet@opgc.univ-bpclermont.fr (M. Boyet).
0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2010.01.010
present the advantage that both chronometers have the same isotopic
closure temperature. Like 26Al–26Mg (T1/2=0.73 Ma), 146Sm–142Nd
involves refractory lithophile elements, but the longer lifetime of
146
Sm is better suited for constraining the silicate evolution of planetary bodies produced during the first hundred million years of the
solar system history.
Eucrites are achondrites composed of both basaltic rocks and
gabbroic cumulates. Grouped with howardites and diogenites, these
meteorites are assumed to come from a small parent body of a few
hundred km diameter called 4-Vesta (McCord et al., 1970; Consolmagno
and Drake, 1977). The recent detection of correlated isotopic and
parent/daughter ratio variation for 26Al–26Mg, 60Fe–60Ni, 53Mn–53Cr
and 182Hf–182W in eucrites suggests that mantle–crust differentiation on
the HED (howardite–eucrite–diogenite) parent body occurred during
the first 5 Ma or less of solar system history (Shukolyukov and Lugmair,
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
Sm–143Nd and Pb–Pb isochron ages 100–150 Ma younger than
the circa 4.56 Ga ages obtained for basaltic eucrites (Lugmair et al.,
M. Boyet et al. / Earth and Planetary Science Letters 291 (2010) 172–181
1977; Hamet et al., 1978; Jacobsen and Wasserburg, 1984; Galer and
Lugmair, 1996; Tera et al., 1997; Blichert-Toft et al., 2002). Exceptions
include the old Sm–Nd ages determined for EET87520 (Lugmair et al.,
1991) and Y980318/433 (Nyquist et al., 2004, 2008). One proposed
explanation for the younger ages of cumulate eucrites is a longer
thermal history in the deep crust (Ghosh and McSween, 1998; Bogard
and Garrison, 2003), but this difference could also be due to late thermal
disturbance during later meteorite bombardment event(s).
We focused on the study of Sm–Nd systematics in three cumulate
eucrites, Binda, Moama and Moore County. Whole rock measurements
obtained on these samples were published in a previous paper (Boyet
and Carlson, 2005), as well as whole rock data for three basaltic
eucrites (Béréba, Nuevo Laredo and Pasamonte). Whole rock 146Sm–
142
Nd measurements of eucrites provide a slope corresponding to an
initial 146Sm/144Sm ratio = 0.0074 ± 0.0015 (Fig. 1), which is within
uncertainty of several estimates of the initial ratio of the solar system
(e.g. Jacobsen and Wasserburg, 1984; Lugmair and Galer, 1992;
Nyquist et al. 1994). Although the slope of this isochron is largely
defined by the data for the cumulate eucrite Moama due to its
relatively high Sm/Nd ratio, this result suggests that cumulate eucrites
could have formed early in solar system history, as did the basaltic
eucrites, a possibility also indicated by the old age obtained for the
cumulate eucrites EET87520 (Lugmair et al., 1991) and Y980318/433
(Nyquist et al., 2004, 2008). The main mineral phases of cumulate
eucrites (cpx and plagioclase) are characterized by a wide range of
Sm/Nd ratios that vary by a factor of 4. New, high precision Sm–Nd
measurements of these phases should allow a more precise estimate of
the 146Sm/144Sm ratio at the time of meteorite formation. These results
will be compared to previous estimates of this ratio calculated from
internal isochrons of different achondrite groups in order to i) better
constrain the initial solar system 146Sm/144Sm ratio, ii) discuss the
initial Sm–Nd isotopic homogeneity in the solar system at the time of
accretion and, iii) better understand the chronology of the HED parent
body differentiation and in particular, its crustal evolution.
2. Samples and analytical techniques
2.1. Sample description
173
or terrestrial contamination should affect the chemical composition of
found meteorites more strongly, such as Binda and Moama. However,
neither of these samples resided in extreme conditions such as the
meteorites found in the Sahara or in Antarctica, where strong
weathering is frequently observed (Crozaz et al., 2003). Almost all
basaltic eucrites are brecciated and composed of minerals and lithic
fragments set in a fine-grained matrix. The main portion of Binda is
brecciated but also contains unbrecciated clasts of coarse-grained
equigranular gabbro. Binda is the most magnesian of the cumulate
eucrites (Duke and Silver, 1967). This feature explains the fact that
Binda was previously classified as a howardite, the intermediate rock
type composed of a mixture of eucrite and diogenite. Cumulate
eucrites are mainly composed of augite and plagioclase, with minor
phases that include chromite, ilmenite, troilite, and rare metal. This
work was undertaken on pieces of meteorites (Moore County and
Moama) from which Pb isotope compositions were determined previously (Tera et al., 1997). In the same publication, 147Sm–143Nd measurements on Moore County are also reported.
2.2. Mineral separation
Pieces of Moama and Moore County were sampled from the
interior of the meteorites. For Binda, the fusion crust was thoroughly
removed and the part selected for separation was located more than
2 mm beneath the fusion crust. For Moama and Moore County,
mineral separates were obtained by handpicking under a microscope
in a clean environment. Care was taken during picking to eliminate
crystals where inclusions were detected. For Binda, plagioclase and
pyroxene separates also were obtained by handpicking, but two other
fractions, called light and heavy fractions (LH and HF, respectively)
were separated by density using undiluted methylene iodide.
Minerals were washed in Milli-Q deionized water and then leached
using 1 M HCl. Once dried, fractions were crushed in a new agate
mortar exclusively used for meteoritic material. Whole rocks were
crushed in the same mortar. Around 500 mg of each fraction for
Moama was separated and then dissolved for analysis (Table 1). The
higher Nd contents in Binda and Moore County allow the use of
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
Fig. 1. 146Sm–142Nd evolution diagram plots as ε142Nd (expressed relative to terrestrial
standard) vs 144Sm/144Nd for whole rock eucrites. The regression including only data
from Boyet and Carlson (2005) yields 146Sm/144Nd = 0.0074 ± 0.0015, which corre146
34
sponds to a differentiation event of 21+
Sm/144Sm solar system
− 27 assuming an initial
ratio of 0.0085.
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-methyllactic) 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
measurements of unspiked fractions (less than 10 and 40 pg, respectively measured on 3 blanks). For Sm and Nd content determinations
using isotope dilution, the total procedural blanks were less than 2 and
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
notation ([(142Nd/144Nd)sample / (142Nd/144Nd)std − 1] × 104) and are reported relative to the mean value obtained for the La Jolla standard (1.141853 ± 0.000006, ± 2σ). 143Nd 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.
Sample
Weight
(g)
Sm
(ppm)
Nd
(ppm)
147
Binda WR
Binda HF
Binda Cpx
Binda LF
Binda Plagio.
Moama WR
Moama Cpx
Moama Plagio.
Moore County WR
Moore County Cpx
Moore County Plagio.
0.20491
0.38170
0.24865
0.09284
0.08238
0.49083
0.54711
0.36797
0.22988
0.13976
0.08786
0.3566
0.2743
0.2229
0.5569
0.1399
0.1575
0.1788
0.0742
0.7071
0.8153
0.3079
0.9857
0.6628
0.4281
1.7503
0.8190
0.3772
0.3290
0.3507
2.082
1.5612
1.8386
0.2187
0.2502
0.3149
0.1924
0.1033
0.2523
0.3286
0.1279
0.2053
0.3157
0.1012
2.4. Isotope measurements
Isotopic measurements were performed by thermal ionization
using the DTM ThermoFisher Triton. Sm and Nd samples were loaded
onto zone-refined rhenium filaments and analyzed in static mode. Nd
isotope measurements for 142Nd/144Nd and 143Nd/144Nd determination were made using double Re filaments and the Nd+ ion. Analytical
runs consisted of 27 blocks of 20 ratios taken statically (8 s integration) using amplifier rotation (see detail in Boyet and Carlson,
2005). Ce and Sm interferences on masses 142 and 144 are monitored
by measuring 140 and 147 masses, respectively. Ce and Sm contributions on masses 142 and 144 were always lower than 2 ppm for
both standard and sample measurements. Nd isotope ratios were
corrected for mass fractionation to 146Nd/144Nd = 0.7219 using the
exponential law. Data are reported relative to a value of 143Nd/
144
Nd = 0.511860 for the La Jolla standard. The average measured
value for this standard during the course of these measurements was
0.511845 ± 0.000003 (±2σ). 142Nd/144Nd ratios are expressed in
epsilon notation and calculated relative to the mean 142Nd/144Nd
obtained for the terrestrial standards measured in the same barrel of
the samples: ([(142Nd/144Nd)sample / (142Nd/144Nd)std − 1] × 104).
During this work, 8 standard measurements produced an average
value for 142Nd/144Nd in the La Jolla standard of 1.141853 ± 0.000006
Sm/144Nd
ε142Nd ± 2σ
143
Nd/144Nd ± 2σ
0.08 ± 0.05
0.58 ± 0.09
1.28 ± 0.15
− 0.22±0.08
− 1.48 ± 0.22
0.57 ± 0.09
1.39 ± 0.08
− 0.72±0.34
0.05 ± 0.06
1.16 ± 0.07
− 1.38 ± 0.10
0.513319±2
0.514347 ± 4
0.516290 ± 7
0.512532±4
0.509920±10
0.514397 ± 4
0.516678 ± 3
0.510558±15
0.512996 ± 3
0.516271±3
0.509804 ± 4
(5 ppm of external reproducibility). The external reproducibility on
repeated standard analyses is slightly larger for all standards run over
a several month period. Sm isotope compositions of unspiked whole
rock samples were presented in Boyet and Carlson (2005). For spiked
fractions, Nd was measured as NdO+ emitted from a single Re
filament due to the small amount of Nd present. No difference outside
the analytical uncertainty was noted between the 143Nd/144Nd and
142
Nd/144Nd measured for the spike corrected aliquots compared to
the unspiked portions.
3. Results
Sm and Nd isotopic compositions and concentrations for Binda,
Moore County and Moama are presented in Table 1. Whole rock
sample measurements, already published in a previous paper (Boyet
and Carlson, 2005), also are reported in Table 1. For Binda, four
different mineral fractions were measured whereas only single plagioclase and the pyroxene separates were analyzed for Moore County
and Moama. The Sm isotope composition of unspiked fractions also
was measured for whole rock samples. These results, presented in our
previous paper (Boyet and Carlson, 2005), show that the Sm isotope
composition of these meteorites has not been disturbed by thermal
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
and the ratios measured in whole rock basaltic eucrites (Blichert-Toft
et al., 2002; Boyet and Carlson, 2005). Sm/Nd ratios between different
mineral phases show an even wider spread. Plagioclase is characterized by the lowest 147Sm/144Nd ratios (between 0.1012 and 0.1279)
and always have negative ε142Nd (−0.72 to −1.48 relative to the
terrestrial standard) and subchondritic 143Nd/144Nd ratios (<0.510).
In comparison, pyroxene is characterized by higher Sm/Nd ratios
(147Sm/144Nd > 0.31), large excesses in 142Nd (ε142Nd>1.2) and
radiogenic 143Nd/144Nd ratios (>0.516).
Results for Binda, Moore County and Moama are plotted in Figs. 2–4,
respectively, and are discussed individually below. For these meteorites,
line-fitting is done using the Isoplot program (Ludwig, 1991) with
Model 3 fits because the probability of fit is generally too low for using
Model 1 fits. Errors reported for the ages and 146Sm/144Sm ratios are 2σ
of the observed scatter of the data about the best-fit line. The 95%
confidence limits reported by Isoplot are not used here because they
give unrealistically large uncertainties, particularly when the number of
points on the line is small. Insets to Figs. 2a and 3a show error envelopes
for 2σ of the scatter and suggest that these errors better reflect the true
uncertainty of the data. Initial ε143Nd and their uncertainty are calculated using the procedure described by Fletcher and Rosman (1982).
Previous Sm–Nd studies reporting data for both 146Sm–142Nd and
147
Sm–143Nd chronometers measured on the same fractions have
been compiled and are presented in Table 2. Estimates of solar system initial 146Sm/144Sm derive not only from internal isochrons on
eucrites, but also from silicate clasts from mesosiderites, angrites, and
one study of the meteorite Acapulco. For that reason, results of these
studies also are provided in Table 2. In this table, data have been
normalized to 146Nd/144Nd = 0.7219. Isochron ages and uncertainties
for 147Sm–143Nd and 146Sm–142Nd have been re-calculated using
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).
Sample name
T147Sm–143Nd
(Ma)a
ε143NdTb
Eucrite
Binda
Caldera
Chervony Kut*
EET87520*
EET90020*
Ibitira
Ibitira
Moama
Moama
Moore County
Y980318*
Y980433*
4544 ± 88
4544 ± 19
4580 ± 30
4547 ± 9
4482 ± 30
4474 ± 67
4570 ± 90
4594 ± 79
4466 ± 42
4542 ± 85
4567 ± 24
4542 ± 42
1.0 ± 0.9
0.4 ± 0.8
3.3 ± 0.9
Angrite
Angra dos Reis
Angra dos Reis
LEW 86010
LEW 86010
4512 ± 85
4560 ± 94
4558 ± 34
4536 ± 69
1.6 ± 0.6
− 0.1 ± 0.2
− 0.7 ± 0.2
1.2 ± 0.9
Mesosiderite
Morristow Frag. A
Morristow Frag. B
Mt. Padburry
Vaca Muerta P.5
Vaca Muerta P.12
Vaca Muerta P.16
Vaca Muerta
4453 ± 95
4483 ± 40
4494 ± 45
4505 ± 166
4401 ± 119
4653 ± 219
4512 ± 86
2.5 ± 1.1
1.8 ± 0.7
− 0.4 ± 0.5
3.7 ± 1.4
18.9 ± 6.2
− 1.2 ± 2.6
0.0 ± 0.7
Acapulcoite
Acapulco
4601 ± 62
− 0.1 ± 2.4
Chondrites
1.8 ± 0.3
1.2 ± 0.4
0.4 ± 0.9
0.0 ± 0.6
1.0 ± 1.0
1.5 ± 1.6
1.5 ± 1.1
MSWDc
146
Sm/144SmTd
ε142NdTe
MSWDc
0.00728 ± 0.00057
0.00735 ± 0.00132
0.00690 ± 0.00150
0.00690 ± 0.00450
0.00480 ± 0.00200
0.00905 ± 0.00280
0.00820 ± 0.00080
0.00593 ± 0.00069
0.00411 ± 0.00130
0.00660 ± 0.00120
0.00600 ± 0.00090
0.00570 ± 0.00050
− 2.74 ± 0.23
− 3.00 ± 0.55
− 2.65 ± 0.66
− 2.71 ± 0.23
1.6
0.77
− 2.83 ± 0.46
2.6
− 2.13 ± 0.08
− 1.62 ± 0.59
− 2.48 ± 0.49
− 2.8 ± 0.6
0.08
2.0
21
0.00632 ± 0.00170
0.0135 ± 0.0064
0.00710 ± 0.00170
0.00780 ± 0.00200
− 2.39 ± 0.68
− 4.67 ± 2.1
− 2.57 ± 0.60
− 2.74 ± 0.88
1.4
6.4
0.25
4.6
5
0
9.1
56
18
145
9.2
0.00630 ± 0.00700
0.00620 ± 0.00260
0.00560 ± 0.00078
0.00450 ± 0.00440
0.00628 ± 0.00080
0.00530 ± 0.00340
0.00760 ± 0.00280
− 0.90 ± 3.15
− 1.95 ± 0.58
− 1.95 ± 0.37
− 1.60 ± 1.58
− 0.29 ± 0.95
− 1.34 ± 1.58
− 2.57 ± 1.17
4.1
0.00702 ± 0.00178
0.00750 ± 0.00270
47
2
3.6
19
2
43
18
0.46
1.5
6.5
146
Sm/144SmTof
Referenceg
0.0085
0.0088
0.0069
0.0079
0.0085
0.0171
0.0082
0.0155
0.0082
0.0073
0.0081
0.0068
This study
1
2
3
4
5
6
This study
7
This study
8
9
0.0109
0.0143
0.0076
0.0097
10
7
11
12
5.2
0
1.4
4.6
0.6
18
10
0.0137
0.0110
0.0092
0.0069
0.0193
0.0030
0.0111
5
5
13
13
13
13
14
− 2.22 ± 0.68
4.1
0.0070
5
− 2.62 ± 0.93
1.2
15
a 147
Sm–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 146
Sm/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 146
Sm/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/
144
Sm ratios are 2σ of the observed scatter of the data about the bestfit line. Table 2 also includes a few results from abstracts in which only
a regression age and initial Nd isotope ratio were reported without
accompanying isotopic data. These samples are starred in the table,
and those results are transcribed as written in the abstracts with no
re-calculation.
agreement with the more precise 244Pu–Xe age of 4529 ± 34 Ma
determined for this sample (Miura et al., 1998). The 146Sm–142Nd
internal isochron is shown Fig. 2b (MSWD = 1.6). The equation of the
line in a 144Sm/144Nd–142Nd/144Nd plot is given by:
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
initial 146Sm/144Sm ratio equal to 0.0073 ± 0.0006 and an intercept of
−2.74 ± 0.23 (calculated relative to the modern terrestrial standard
value). For the Binda isochron, ε142Nd calculated for a chondritic Sm/Nd
ratio of 0.1960 is equal to −0.19 ± 0.07, similar to the mean of
chondritic samples (Boyet and Carlson, 2005).
The heavy (HF) and light (LF) density separates from Binda show
isotopic composition intermediate between WR and pyroxene (px)
for HF and between WR and plagioclase (plag), for LF, respectively,
indicating that these separates may not have been as mineralogically
pure as were the hand-picked separates. Different mineral fractions
and the whole rock show good alignment in a 147Sm/144Nd vs. 143Nd/
144
Nd plot (Fig. 2a). The slope of 0.03017 ± 0.00060 (MSWD = 47)
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
the following average chondrite parameters: 147Sm/144Nd = 0.1960
and 143Nd/144Nd = 0.512630 (Bouvier, et al., 2008)). This age is in
142
Nd =
144
Nd
mes
=
146
Sm=
144
144
144
142
144
Sm ×
Sm= Nd +
Nd = Nd
T
T
ð1Þ
3.2. Moore County
Both Sm–Nd isochrons are defined by 3 points (Fig. 3). The slope of
the 147Sm–143Nd line yields an age of 4542 ± 85 Ma (MSWD = 46)
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
intercept is 0.50677 ± 5 (ε143Nd = 1.0 ± 1.0). This age overlaps within
error the Sm–Nd age of 4456 ± 25 Ma and Pb–Pb age of 4484 ± 19 Ma
reported for Moore County by Tera et al. (1997). In Fig. 3a, other Sm–Nd
measurements obtained on this sample (whole rock and mineral
separates from Tera et al. (1997) and Blichert-Toft et al. (2002) are
shown for comparison. The 147Sm–143Nd age obtained in this study is
similar to the 244Pu–Xe age of ∼4548 Ma (Shukolyukov and Begemann,
1996). The 146Sm–142Nd alignment gives a slope of 0.0066 ± 12 and an
intercept of −2.48 ± 0.49 ε-unit (MSWD= 21), which corresponds to
a present day value of −0.20 ± 0.19 when calculated relative to
chondritic Sm/Nd ratio (Fig. 3b). Both 146Sm–142Nd and 147Sm–143Nd
systematics are in agreement to suggest that Binda and Moore County
have the same formation age.
3.3. Moama
147
Sm–143Nd measurements obtained on Moama have been plotted with literature values in Fig. 4a. Considering only data measured in
this study, we obtain the steepest slope (MSWD = 19) yielding an age
of 4594 ± 79 Ma and an initial 143Nd/144Nd ratio of 0.50667 ± 0.00005
(ε143Nd = 0.4 ± 0.9). This age compares with an age of 4466 ± 42 Ma
and initial ε143Nd = 0.0 ± 0.6 from Jacobsen and Wasserburg (1984)
and an age of 4520 ± 50 Ma with initial ε143Nd = + 1.9 ± 0.6 from
Hamet et al. (1978).
The slope of the 142Nd/144Nd–144Sm/144Nd correlation using our
data gives a 146Sm/144Sm ratio of 0.0059 ± 7 (MSWD= 0.08) and
intercept of −2.13 ± 0.08 corresponding to an initial 142Nd/144Nd
isotope composition of ε142Nd = −0.03 ± 0.03 (Fig. 4b). Using just the
pyroxene and whole rock data changes these values to 146Sm/
144
Nd= 0.0060 ± 0.0009 and present day ε142Nd (at chondritic Sm/Nd
ratio) of −0.04 ± 0.16. Using the data reported by Jacobsen and
Wasserburg (1984) we calculate an initial 146Sm/144Sm = 0.0041 ±
0.0013 for their Moama analyses. The Pb–Pb age determined for Moama
(4426 ± 97 Ma; Tera et al., 1997) supports a younger age for Moama, but
overlaps all Sm–Nd age determinations for Moama within error.
4. Discussion
4.1. Sm–Nd age of cumulate eucrites
Using the new data, absolute ages estimated from 147Sm–143Nd
systematics for Binda and Moore County are 4544 ± 88 Ma and 4542 ±
85 Ma, respectively. In the data reported here, Moama gives an
older age (4594 ± 79 Ma), but one that overlaps within error. Fit-
177
ting our data for Binda, Moama and Moore County simultaneously gives
an age of 4557 ± 42 Ma, initial ε143Nd = +0.78 ± 1.3 and 146Sm/
144
Sm = 0.00672 ± 0.00046. The accuracy of this age depends on
whether all three samples are exactly the same age and formed with
the same initial isotopic compositions. For reasons to be discussed
below, we do not believe this to be the case.
The large age uncertainties for the 147Sm–143Nd ages reported here
stem largely from the scatter of the points about any best-fit line. For
example, for Binda, the 143Nd/144Nd data deviate from the best-fit line
by ε143Nd = − 0.96 for the whole rock to + 0.66 for the plagioclase.
Alternatively, the whole rock and plagioclase data have 147Sm/144Nd
ratios displaced from the best-fit line by + 0.75% and − 1.1%,
respectively. Both deviations are well outside of analytical error,
leading to the conclusion that the internal Sm–Nd systematics of
the cumulate eucrites have been disturbed. For Moama, additional
evidence for disturbance comes from the observation that the age
determined here is older, outside of uncertainty, compared to the age
determined by Jacobsen and Wasserburg (1984), which is close to
the 4439 ± 97 Ma Pb–Pb age found by Tera et al. (1997). Additional
Moama plagioclase (Hamet et al., 1978) and whole rock (Blichert-Toft
et al., 2002) measurements lie distinctly off the lines reported here
and by Jacobsen and Wasserburg (1984), in the case of the BlichertToft et al. (2002) whole rock measurement by − 14 in ε143Nd.
For Binda, the Sm–Nd age reported here agrees well with its Pu–Xe
age (4529 ± 34 Ma; Miura et al., 1998). For Moore County, the Sm–Nd
age is older than, but within uncertainty, of the Sm–Nd age reported
previously (4456 ± 25 Ma) and the more precise Pb–Pb age of 4484 ±
19 Ma reported for Moore County by Tera et al. (1997). Moore County,
however, is heavily contaminated with terrestrial Pb (Tera et al.,
1997), which complicates interpretation of its Pb–Pb age in spite of its
high precision. Moore County also has been examined for its Mn–Cr
systematics, but no evidence was found for live 53Mn, leading to only
an upper age estimate of <4549 Ma (Lugmair and Shukolyukov,
1998). This minimum age is important, however, because it shows
that Moore County cannot be as old as some basaltic eucrites that
display evidence of live 26Al (Piplia Kalan — Srinivasan et al., 2007),
53
Mn (Chervony Kut, Juvinas, Ibitira, but not Caldera, EET87520, or
Pomozdino — Lugmair and Shukolyukov, 1998), and 182Hf (Stannern —
Kleine et al., 2005).
Four cumulate eucrites (Binda, Caldera, EET87520, Moore County)
provide concordant 146Sm–142Nd, 147Sm–143Nd results (Fig. 5). We have
classified these samples as isotopically undisturbed (at least for Sm–Nd)
and they will be used to provide an estimate of the solar system initial
146
Sm/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
at 4546 ± 8 Ma (MSWD = 0.04). This time of Sm–Nd isotope closure,
moreover, is consistent with the lack of live 53Mn in Moore County and
Caldera. It also agrees very well with the ages of Hf–W mineral isochrons
on basaltic eucrites that are approximately 20 Ma younger than solar
system formation (Kleine et al., 2005).
Samarium–Nd studies of samples from the HED parent body using
both the 146Sm–142Nd and 147Sm–143Nd chronometers consistently yield
younger ages than those obtained from very short-lived chronometers
on whole rock eucrites (Lugmair and Shukolyukov, 1998; Bizzarro et al.,
2005). Whole rock eucrites define both Mn–Cr (Lugmair and Shukolyukov,
1998) and Hf–W (Kleine et al., 2004) isochrons consistent with the
differentiation of the HED parent body within 5 to 7 Ma after solar
system formation. Several eucrites also have internal isochron systematics in the Al–Mg (Srinivasan et al., 2007), Mn–Cr (Lugmair and
Shukolyukov, 1998), and Hf–W (Kleine et al., 2005) systems consistent
with their eruption onto the surface of the HED parent body before
4560 Ma. Differentiation and melting of the eucrite parent body thus
occurred early, likely as a result of heating by the decay of 26Al.
Are 147Sm–143Nd ages obtained for cumulate eucrites related to a
large-scale thermal event on the HED parent body? The brecciated
texture of most the samples, as well as exsolution textures described
in pyroxenes (Takeda, 1997) indicate that successive thermal events
have affected the HED parent body. Thermal metamorphism resulting
from impacts was interpreted to be responsible for re-equilibration
of W isotopes in basaltic eucrites approximately 20 Ma after solar
system formation (Kleine et al., 2005). The similarity of our weighted
average of 147Sm–143Nd ages for cumulate eucrites with the W ages
suggests that the thermal event affected both basaltic and cumulate
eucrites. Using our results for Binda for which 5 different mineral
fractions define a precise 142Nd/144Nd–Sm/Nd isochron, the age of
this thermal event can be further constrained. As represented in Fig. 2,
the slope of the internal isochron for Binda means that the last Sm–Nd
12
isotope closure occurred 23+
− 11 Ma after solar system formation for an
initial solar system 146Sm/44Sm ratio of 0.0085 (see ratio estimate in
the next section). The very old Al–Mg, Mn–Cr and Hf–W ages obtained
on some basaltic eucrites shows clearly that some eucrites escaped
age resetting during the event recorded by Binda. However, even
younger ages have been recorded by other isotope systematics in
some eucrites, including the younger U–Pb, Rb–Sr and Ar–Ar ages
reported for many samples (Birck and Allègre, 1978; Bogard and
Garrison, 1995; Tera et al., 1997) indicating a prolonged period of
presumably impact-related resetting of eucrite ages.
4.2. Solar system initial
146
Sm/144Sm ratio
The precise determination of the initial abundance of short-lived
radioisotopes is critical for their use to investigate the chronology of
the early solar system. Internal isochrons for CAIs found in carbonaceous chondrites are currently used as pinning points for
translating the relative time scale provided by several extinct radioisotope systems into absolute ages because these objects represent
the first solids condensing in the solar nebula, and they have been
dated precisely using Pb–Pb systematics (4567 to 4568 Ma (Amelin
et al., 2002; Bouvier et al., 2007). CAIs are not a good reference point
for 146Sm–142Nd systematics because they can contain large isotope
anomalies unrelated to the decay of 146Sm (McCulloch and Wasserburg,
1978) and because the internal 147Sm–143Nd systematics of CAIs can
give anomalously young ages indicative of later resetting of the Sm–Nd
system (Scheinin, 1977). More recently, isotopic anomalies in Sm
and Nd have been identified at the whole rock scale in carbonaceous chondrites (Andreasen and Sharma, 2006; Carlson et al.,
2007). P-process deficits have been identified by large negative 144Sm
anomalies (100% p-process isotope). This magnitude of p-process
deficit would have only a minor direct effect on 142Nd abundances
(formed by 4% p-process and 96% s-processes), but because 146Sm is a
pure p-process isotope, a p-process deficiency will be manifest in
Nd because of the deficiency in 146Sm (Andreasen and Sharma,
2006). Plotting C- chondrites in a Sm/Nd–142Nd/144Nd plot, a slope
of 146Sm/144Sm ∼ 0.035 at the age of chondrite formation is obtained,
a value significantly higher than the value of 0.008 estimated for the
solar system. The most likely explanation for this is that the correlation is not a 146Sm–142Nd isochron, but is instead a mixing line
between material with “normal” solar system isotopic composition
and isotopically anomalous pre-solar material. Nucleosynthetic
anomalies in Sm and Nd have not been detected in ordinary and
enstatite chondrites, eucrites, angrites, or lunar samples, hence the
observed 142Nd/144Nd variations are explained by Sm/Nd fractionation produced during the lifetime of 146Sm (Boyet and Carlson,
2005; Andreasen and Sharma, 2006; Carlson et al., 2007; Boyet and
Carlson, 2007; Brandon et al., 2009).
Samarium–Nd analyses obtained on Binda provide the most precise estimate for the solar system initial 146Sm/144Sm ratio from this
study. As discussed in the previous section, this sample has a 147Sm–
143
Nd age (4545 ± 89 Ma) similar to the Hf–W age of 4547 ± 2 Ma
defined by most of the basaltic eucrites (Kleine et al., 2005). The
146
Sm/144Sm ratio calculated for Binda from the five-point regression
line is defined with high precision (0.0073 ± 6). This ratio translates to
a ratio at 4568 Ma equal to 0.0084 ± 6, if Sm–Nd and Hf–W isotope
closure occurred at the same time. A difference of 2 Ma on the
absolute age has little effect on the calculation since the major uncertainty is the error on the slope of the fossil isochron. For example,
when the 147Sm–143Nd age is used, we find a 4568 Ga 146Sm/144Sm
ratio equal to 0.0085 (Table 2).
Sm–Nd results obtained on Caldera, Moore County and EET87520
are fully consistent with this estimate of the solar system initial 146Sm/
144
Sm ratio, however errors on initial 146Sm/144Sm ratios are higher.
When 146Sm/144Sm ratios are calculated back to the age of solar
system formation, using absolute ages defined by the 147Sm–143Nd
chronometer for these 3 samples, 146Sm/144Sm ratios of 0.0075 ± 45 to
0.0088 ± 13 are calculated. The weighted mean of the initial solar
system 146Sm/144Sm ratio calculated from the 4 eucrites with
undisturbed Sm–Nd (Binda, Moore County, Caldéra and EET 87520)
is 0.00837 ± 0.00048 (MSWD = 0.92).
The two paired eucrites Y980318 and Y980433 have lower 146Sm/
144
Sm ratios at the time of Sm–Nd isotope closure (0.0060 ± 0.0009
and 0.0057 ± 0.0005, respectively) although their 147Sm–143Nd ages
are similar to those of other eucrites (4567 ± 24 Ma and 4542 ±
42 Ma, respectively). The reason why these samples give lower
146
Sm/144Sm values is not clear. Further evaluation of the data for
these samples is hindered by the fact that the measured isotope ratios
are not reported in these abstracts (Nyquist et al., 2004, 2008). We
note however than the first 146Sm/144Sm ratio determined for Y90318
was significantly higher and equal to 0.0077 ± 0.0012 at an age of
4560 ± 150 Ma (Nyquist et al., 2004).
Four eucrite samples reported in Table 2 (Chervony Kut, EET
90020, Ibitira and Moama) have been excluded from the determination of the solar system initial 146Sm/144Sm ratio. For Chervony Kut,
we note that none of the measured points for this sample lie on the
regression line that defines the 147Sm–143Nd age (Wadhwa and
Lugmair, 1995). The 147Sm–143Nd regression for Chervony Kut also
indicates an unreasonably high initial ε143Nd of 3.3 ± 0.9, which these
authors attributed to disturbance of the internal Sm–Nd systematics
of this eucrite. We note however that the 146Sm/144Sm ratio determined on this sample appears to be consistent (=0.0069 ± 0.0015)
with other eucrite data although Chervony Kut also presents evidence
for live 53Mn, and thus must be older than e.g. Moore County (Lugmair
and Shukolyukov, 1998).
Ibitira is characterized by two quite different, but overlapping within
error, 147Sm–143Nd age determinations (4468 ± 67 Ma, Prinzhofer et al.,
1992; 4.57 ± 0.09 Ga, Nyquist et al., 1999). The older age is consistent
with the high 146Sm/144Sm ratio of 0.009 ± 0.003 (Prinzhofer et al.,
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M. Boyet et al. / Earth and Planetary Science Letters 291 (2010) 172–181
1992) to 0.0082 ± 0.0008 (Nyquist et al., 1999) determined for Ibitira
and the evidence for live 53Mn that gives an age of 4557 Ma for Ibitira
(Lugmair and Shukolyukov, 1998). Using the Mn–Cr age, the lower
146
Sm/144Sm initial ratio determined for Ibitira (Nyquist et al., 1999)
would translate to a solar system initial value of 0.0089, on the high side,
but within error, of the value calculated here from the data for Binda. The
evidence for isotopic disturbance in Ibitira has been noted and fully
discussed by Prinzhofer et al. (1992) and Nyquist et al. (1999). They
proposed that the Sm–Nd system remained closed in pyroxenes
whereas isotope disturbances have affected both plagioclase and
phosphate phases. Secondary exchange could have occurred at 4485
± 15 Ma, an age obtained by Ar–Ar systematics (Bogard and Garrison,
1995). Moreover Ibitira is the only eucrite falling off the δ18O′–δ17O′
mass dependant fractionation line formed by samples coming from the
HED parent body (Wiechert et al., 2004), which clouds its connection to
the other eucrites.
EET 90020 has a younger 147Sm–143Nd age (4482 ± 30 Ma) and
radiogenic initial 143Nd/144Nd ratio (ε143Nd = 1.8± 0.3), a feature
shared by other samples with disturbed Sm–Nd (e.g. Moama, Chervony
Kut and Ibitira) (Nyquist et al., 1997a,b).
Moama provides a particularly good example of the need for an
accurate absolute age in order to derive an accurate estimate of the
initial 146Sm/144Sm of the solar system. Using our data and the
4594 Ma age, the solar system initial 146Sm/144Sm determined from
the Moama data is 0.00498, lower than essentially any other estimate
of this value. Using the Pb–Pb age (4426 Ma) for Moama (Tera et al.,
1997) instead gives 146Sm/144Sm = 0.0155 at 4568 Ma, which is
higher than any other estimate. In contrast, the data of Jacobsen and
Wasserburg (1984) for Moama provide a solar system initial 146Sm/
144
Sm = 0.0082, which is indistinguishable from the value we derive
above for the “concordant” eucrites.
Only two angrites, Angra dos Reis and LEW 86010, have been
analyzed for both Sm–Nd systematics. Because these samples have
Sm–Nd ages close to those reported here for Binda and Moore County,
and precise U–Pb ages indicating that they crystallized early in solar
system history, unlike most eucrites, they provide a good comparison
for age cross calibration. The 147Sm–143Nd ages for Angra dos Reis
(4512 ± 85 Ma (Lugmair and Marti, 1977); 4560 ± 50 Ma (Jacobsen
and Wasserburg, 1984)) and LEW86010 (4558 ± 34 Ma (Lugmair and
Galer, 1992) and 4536 ± 69 Ma (Nyquist et al., 2004)) are consistent
with their Pb–Pb ages (4557.65 ± 0.13 Ma and 4558.55 ± 0.14 Ma for
Angra dos Reis and LEW8601, respectively (Amelin, 2008)). These two
samples are younger than other angrites like d'Orbigny (4564.44 ±
0.12 Ma; (Amelin, 2008) and SAH 99555 (4564.86 ± 0.38 Ma (Amelin,
2008); 4564.58 ± 0.14 Ma (Connelly et al., 2008)) showing that
different age groups of angrites exist, as they apparently do for
eucrites. The earliest estimate of solar system initial 146Sm/144Sm was
obtained on Angra dos Reis (Lugmair and Marti, 1977). Due to the
small Sm/Nd fractionation between pyroxene and plagioclase in this
sample (147Sm/144Nd from 0.13 to 0.20) and since the analytical
precision on Nd isotope ratios were larger than 50 ppm at this time, the
initial 146Sm/144Sm ratio calculated for this sample has large errors
(0.0063 ± 0.0017 (Lugmair and Marti, 1977) and 0.0135 ± 0.0064
(Jacobsen and Wasserburg, 1984)). A 146Sm/44Sm ratio of 0.0071 ±
0.0017 was defined for LEW86010 at the age of Sm–Nd isotopic
closure, which translates to an initial value of 0.0076 at 4568 Ma
(Lugmair and Galer, 1992). The same sample has been measured by
Nyquist et al. (1994). We note a small difference between the results of
our calculations and those published in the original paper. We obtain a
146
Sm/44Sm ratio equal to 0.0078 ± 0.0020 instead of 0.0076 ± 0.0009
for the results reported by Nyquist et al. (1994). The difference becomes significantly higher when data measured on leachates are
omitted from the regression. We calculate a 146Sm/44Sm ratio equal to
0.0089 ± 0.0020 whereas the value of 0.0080 ± 0.0009 was reported in
Nyquist et al. (1994). Using the Pb–Pb formation age of LEW86010 and
the 146Sm/144Sm ratio defined when leachates are not considered in the
179
regression, the 146Sm/144Sm ratio for LEW86010 calculated back to
4568 Ma is in the range 0.0082–0.0084, a value similar to the estimate
from the “concordant” eucrites described above.
Mesosiderites are composed of mixtures of silicate clasts and
metal. Sm–Nd data have been published for three mesosiderites
(Morristown, Mt. Padburry and Vaca Muerta), and for four different
pebbles of Vaca Muerta, ranging from basaltic to gabbroic textures
(Stewart et al., 1994; Sharma et al., 1995). Silicate clasts contained in
Vaca Muerta have been affected by secondary isotopic disturbance
(leached fractions fall off the 147Sm–144Nd correlation lines, the ages
have large uncertainties, and the initial ε143Nd of two pebbles are
dramatically different from chondritic, see Table 2). The one Vaca
Muerta pebble with lowest age uncertainty has a chondritic initial
ε143Nd and gives a somewhat high solar system initial 146Sm/
144
Sm = 0.0111 ± 0.0028, but the large uncertainty allows this value
to overlap the estimates derived from eucrites and angrites.
146
Sm–142Nd and 147Sm–143Nd results obtained for two fragments
of Morristown give very similar results, but are characterized by young
ages (4.45 to 4.48 Ga) and positive initial ε143Nd (1.8 to 2.6)
(Prinzhofer et al., 1992). The 146Sm/144Sm estimates are high
(0.0063 ± 0.0070 and 0.0062 ± 0.0026) relative to the young 147Sm–
143
Nd ages, but the errors on the ratio are large enough to overlap
essentially all estimates of solar system initial 146Sm/144Sm ratio. By
contrast, Sm–Nd systematics in the silicate clast from Mt. Padburry are
consistent and leached fractions fall along the line defined by
separated phases (Stewart et al., 1994), which was not the case for
other mesosiderite samples. A 147Sm–143Nd age of 4494 ± 45 Ma
(MSWD = 9.1) is obtained. The 146Sm–142Nd regression yields a welldefined slope of 0.00560 ± 0.0078 (MSWD = 1.4), which corresponds
to a value of 0.0092 for the 146Sm/44Sm at 4.568 Ga, slightly higher, but
overlapping within error, the value derived here from the eucrite data.
The 147Sm–143Nd age obtained on Acapulco (a primitive achondrite classified in the Acapulcoite group) is equal to 4601 ± 62 Ma
with a 146Sm/144Sm ratio of 0.0070 ± 18 (Prinzhofer et al., 1992). This
146
Sm/144Sm value is similar to the value obtained on angrite LEW
86010, for which consistent 147Sm–143Nd (4558 ± 42 Ma; Lugmair
and Galer, 1992) and Pb–Pb (4558.55 ± 0.14 Ma; Amelin, 2008) ages
have been obtained. A very similar Pb–Pb age of 4556.5 Ma has been
measured on phosphates separated from Acapulco (Amelin, 2005;
Amelin et al., 2006), suggesting that the 147Sm–143Nd age determined
on this sample cannot be used as reference for calculating the initial
146
Sm/144Sm solar system ratio. If the Pb–Pb age is used instead, a
value of 0.0076 is obtained for the initial 146Sm/144Sm ratio, which is
lower, but within error, of the value derived from the eucrite data.
When different groups of achondrites are considered separately
(after data have been selected as discussed above), consistent values
are obtained for the solar system initial 146Sm/144Sm ratio. The precise
estimate of 0.0084 ± 0.0006 is obtained for Binda when we assume
that both Sm–Nd and Hf–W isotope closure occurred at the same time.
An initial solar system 146Sm/144Sm ratio of 0.0084 ± 0.0005 is calculated when the 4 other non-disturbed Sm–Nd eucrites are added to
the Binda data. When the angrite LEW8601 and the mesosiderite Mt.
Padburry are included in the average calculation, we find a solar
system initial 146Sm/144Sm ratio equal to 0.0085 ± 0.0007. This estimate is consistent with the 146Sm/144Sm ratio of 0.0075 ± 0.0027
defined by chondrites (Amelin and Rotenberg, 2004). The correlation
between 147Sm–143Nd age and initial 146Sm/144Sm is shown in Fig. 6a.
The only samples that fall significantly off this correlation are
Y980318/Y980433 for which the data are reported only in graphical
form in abstracts (Nyquist et al., 2004, 2008). Prinzhofer et al. (1992)
noted the lack of such a correlation for their samples and explained
the disturbance of the Sm–Nd systematics by exchange between
phosphates and plagioclase. Their correlation was significantly improved when 146Sm/144Sm ratios were plotted against 147Sm–143Nd
pyroxene model ages (Prinzhofer et al., 1992). This approach is not
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
146
Sm 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/
144
Nd 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
Fig. 6. (a) 146Sm/144Sm decay through time using an initial 146Sm/144Sm solar system
ratio of 0.0085 ± 0.0007, calculated from the samples with undisturbed Sm–Nd
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,
1996), and EET87520 (Lugmair, et al., 1991). Data for eucrites Y980318/433 (Nyquist
et al., 2008) have lower 146Sm/144Sm ratios than other eucrites and are shown with
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
reference. The silicate clast comes from the mesosiderite Mt Padburry (Stewart, et al.,
1994). The uncertainty on the146Sm/144Sm value is represented by the gray field.
(b) Initial ε142Nd determined from internal isochrons of the same samples are compared
to the evolution of 142Nd/144Nd in ordinary chondrites. The shaded field shows the
142
Nd evolution of ordinary chondrites with time, given their present day ε142Nd =
− 0.16 ± 0.03.
within error to absolute Sm–Nd ages defined by internal isochrons.
The 146Sm/144Sm decay curve using this value as well as “nondisturbed” samples are represented in Fig. 6. The new estimate of the
146
Sm/144Sm ratio is much more precise than, but consistent with, the
estimate of 0.0076 ± 0.0017 suggested by Lugmair and Galer (1992).
The precision of 0.0007 obtained on the initial 146Sm/144Sm solar
system ratio translates to an uncertainty of ∼12 Ma on time intervals
defined from the slope of fossil isochrons.
This work brings additional constraints on the Sm and Nd isotopic
distribution within the solar system. The overlapping 146Sm/144Sm
ratios measured from different groups of achondrites suggest that
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 shortlived 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 summarized 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 becomes 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
isotopically uniform at the planetary scale in the solar system, at least in
the region where the achondrites and terrestrial planets formed.
Acknowledgements
We thank Fouad Tera for giving us these samples, Tim Mock for his
assistance in the mass-spectrometry room, and Nabil Boctor for helpful discussion on the petrology of these samples. Insightful reviews
by Thorsten Kleine, Klaus Mezger, Steve Galer and an anonymous
reviewer helped improve this manuscript. This work was supported
by the Carnegie Institution of Washington and NASA Cosmochemistry grant NNX08AH65G. The DTM Triton was purchased with the
aid of a grant from the National Science Foundation (EAR-0320589).
The research leading to these results has received funding from the
European Research Council under the European Community's
Seventh Framework Programme (FP7/2007-2013 Grant Agreement
no. 209035).
References
Amelin, Y., 2005. Meteorite phosphates show constant 176Lu decay rate since 4557 million
years ago. Science 310, 839–841.
Amelin, Y., 2008. U–Pb ages of angrites. Geochim. Cosmochim. Acta 72, 221–232.
Amelin, Y., Rotenberg, E., 2004. Sm–Nd systematics of chondrites. Earth Planet. Sci. Lett.
223, 267–282.
M. Boyet et al. / Earth and Planetary Science Letters 291 (2010) 172–181
Amelin, Y., Krot, A.N., Hutcheon, I.D., Ulyanov, A.A., 2002. Lead isotopic ages of chondrules
and calcium–aluminium-rich inclusions. Science 297, 1678–1683.
Amelin, Y., Wadhwa, M., Lugmair, G.W., 2006. Pb isotopic dating of meteorites using
202
Pb–205Pb double spike: comparison with other high-resolution chronometers.
Lunar Planet Sci. XXXVII, 1970.
Andreasen, R., Sharma, M., 2006. Solar Nebula heterogeneity in p-process samarium
and neodynium isotopes. Science 314, 806–809.
Birck, J.-L., Allègre, C.J., 1978. Chronology and chemical history of the parent body of
basaltic achondrites studied by the 87Rb–87Sr method. Earth Planet. Sci. Lett. 39,
37–51.
Bizzarro, M., Baker, J.A., Haack, H., Kasper, L.L., 2005. Rapid timescales for accretion and
melting of differentiated planetesimals inferred from 26Al–26Mg chronometry.
Astrophys. J. 632, L41–L44.
Blichert-Toft, J., Boyet, M., Télouk, P., Albarède, F., 2002. 147Sm–143Nd and 176Lu–176Hf in
eucrites and the differentiation of the HED parent body. Earth Planet. Sci. Lett. 204,
167–181.
Bogard, D.D., Garrison, D.H., 1995. 39Ar–40Ar age of the Ibitira eucrite and constraints on
the time of pyroxene equilibration. Geochim. Cosmochim. Acta 59, 4317–4322.
Bogard, D.D., Garrison, D.H., 2003. 39Ar–40Ar ages of eucrites and thermal history of
asteroid 4 Vesta. Meteor. Planet. Sci. 38, 669–710.
Bouvier, A., Blichert-Toft, J., Moynier, F., Vervoort, J.D., Albarède, F., 2007. Pb–Pb dating
constraints on the accretion and cooling history of chondrites. Geochim.
Cosmochim. Acta 71, 1583–1604.
Bouvier, A., Vervoort, J.D., Patchett, P.J., 2008. The Lu–Hf and Sm–Nd isotopic composition of CHUR: constraints from unequilbrated chondrites and implications for
the bulk composition of terrestrial planets. Earth Planet. Sci. Lett. 273, 48–57.
Boyet, M., Carlson, R.W., 2005. 142Nd evidence for early (> 4.53 Ga) global differentiation of the silicate earth. Science 309, 576–581.
Boyet, M., Carlson, R.W., 2007. Early lunar differentiation and a non magma ocean origin
for the lunar crust. Earth Planet. Sci. Lett. 262, 505–516.
Brandon, A.D., Lapen, T.J., Debaille, V., Beard, B.L., Rankenburg, K., Neal, C., 2009. Reevaluating 142Nd/144Nd in lunar mare basalts with implications for the early
evolution and bulk Sm/Nd of the Moon. Geochim. Cosmochim. Acta 73, 6421–6445.
Carlson, R.W., Boyet, M., 2008. Composition of the Earth's interior: the importance of
early events. Phil. Trans. R. Soc. A 366, 4077–4103.
Carlson, R.W., Boyet, M., Horan, M.F., 2007. Chondrite barium, neodymium, and samarium
isotopic heterogeneity and early Earth differentiation. Science 316, 1175–1178.
Connelly, J.N., Bizzarro, M., Thrane, K., Baker, J.A., 2008. The Pb–Pb age of angrite
SAH99555 revisited. Geochim. Cosmochim. Acta 72, 4813–4824.
Consolmagno, G.J., Drake, M.J., 1977. Composition and evolution of the eucrite parent
body: evidence from rare earth elements. Geochim. Cosmochim. Acta 41, 1271–1282.
Crozaz, G., Floss, C., Wadhwa, M., 2003. Chemical alteration and REE mobilization in
meteorites from hot and cold deserts. Geochim. Cosmochim. Acta 67, 4727–4741.
Duke, M.B., Silver, L.T., 1967. Petrology of eucrites, howardites and mesosiderites.
Geochim. Cosmochim. Acta 31, 1637–1665.
Fletcher, I.R., Rosman, K.J.R., 1982. Precise determination of initial εNd from Sm–Nd
isochron data. Geochim. Cosmochim. Acta 46, 1983–1987.
Galer, S.J.G., Lugmair, G.W., 1996. Lead isotope systematics of noncumulate eucrites.
Meteor. Planet. Sci. 31, A47.
Ghosh, A., McSween Jr, H.Y., 1998. A thermal model for the differentiation of asteroid 4
Vesta based on radiogenic heating. Icarus 134, 187–206.
Hamet, J., Nakamura, N., Unruh, D., Tatsumoto, M., 1978. Origin and history of the
cumulate eucrite Moama as inferred from REE abundances, Sm–Nd and U–Pb
systematics. Proc. Lunar Planet. Sci. Conf. 9th, pp. 1115–1136.
Jacobsen, S.B., Wasserburg, G.J., 1984. Sm–Nd isotopic evolution of chondrites and
achondrites, II. Earth Planet. Sci. Lett. 67, 137–150.
Kleine, T., Mezger, K., Münker, C., Palme, H., Bischoff, A., 2004. 182Hf–182W isotope systematics of chondrites, eucrites and martian meteorites: chronology of core formation
and early mantle differentiation in Vesta and Mars. Geochim. Cosmochim. Acta 68,
2935–2946.
Kleine, T., Mezger, K., Palme, H., Scherer, E., Münker, C., 2005. The W isotope composition of eucrite metals: constraints on the timing and cause of the thermal
metamorphism of basaltic eucrites. Earth Planet. Sci. Lett. 231, 41–52.
Ludwig, K.R., 1991. ISOPLOT: a plotting and regression program for radiogenic isotope
data. USGS Open File Report, pp. 91–445.
181
Lugmair, G.W., Galer, S.J.G., 1992. Age and isotopic relationships among the angrites
Lewis Cliff 86010 and Angra dos Reis. Geochim. Cosmochim. Acta 56, 1673–1694.
Lugmair, G.W., Marti, K., 1977. Sm–Nd–Pu timepieces in the Angra Dos Reis meteorite.
Earth Planet. Sci. Lett. 35, 273–284.
Lugmair, G.W., Shukolyukov, A., 1998. Early solar system time-scales according to
53
Mn–53Cr systematics. Geochim. Cosmochim. Acta 62, 2863–2886.
Lugmair, G.W., Scheinin, N.B., Carlson, R.W., 1977. Sm–Nd systematics of the Serra de
Magé eucrite. Meteoritics 12, 300–301.
Lugmair, G.W., Galer, S.J.G., Carlson, R.W., 1991. Isotope systematics of cumulate eucrite
EET-87520. Meteor. Planet. Sci. 26, 368.
McCord, T.B., Adams, J.B., Johnson, T.V., 1970. Asteroid Vesta: spectral reflectivity and
compositional implications. Science 168, 1445–1447.
McCulloch, M.T., Wasserburg, G.J., 1978. Barium and neodymium isotopic anomalies in
the Allende meteorite. Astrophys. J. 220, L15–L19.
Miura, Y.N., Nagao, K., Sugiura, N., Fujitani, T., Warren, P.H., 1998. Noble gases, 81Kr–Kr
exposure ages and 244Pu–Xe ages of six eucrites, Béréba, Binda, Camel Donga,
Juvinas, Millbillillie, and Stannern. Geochim. Cosmochim. Acta 62, 2369–2387.
Nyquist, L.E., Bansal, B., Wiesmann, H., Shih, C.-Y., 1994. Neodymium, strontium and
chromium isotopic studies of the LEW86010 and Angra dos Reis meteorites and the
chronology of the angrite parent body. Meteoritics 29, 872–885.
Nyquist, L.E., Bogard, D., Takeda, H., Bansal, B., Wiesmann, H., Shih, C.-Y., 1997a. Crystallization, recrystallization, and impact-metamorphic ages of eucrites Y792510
and Y791186. Geochim. Cosmochim. Acta 61, 2119–2138.
Nyquist, L.E., Wiesmann, H., Reese, Y., Shih, C.-Y., Borg, L.E., 1997b. Samarium–neodymium
age and manganese–chromium systematics of eucrite Elephant Moraine 90020.
Meteor. Planet. Sci. 32, A101–A102.
Nyquist, L.E., Reese, Y.D., Wiesmann, H., Shih, C.-Y., 1999. Two ages for Ibitira: a record
of crystallization and recrystallization. Meteor. Planet. Sci. 34, A87–A88.
Nyquist, L.E., Reese, Y., Wiesmann, H., Shih, C.-Y., Takeda, H., 2003. Fossil 26Al and 53Mn
in the Asuka 881394 eucrite: evidence of the earliest crut on the asteroid 4 Vesta.
Earth Planet. Sci. Lett. 214, 11–25.
Nyquist, L.E., Takeda, H., Shih, C.-Y., Wiesman, H., 2004. Sm–Nd age and initial 87Sr/86Sr for
Yamato 980318: an old cumulate eucrite. Lunar Planet Sci. Conf. XXXV, 1330–1331.
Nyquist, L.E., Shih, C.-Y., Reese, Y.D., 2008. Sm–Nd for norite 78236 and eucrite
Y980318/433: implications for planetary and solar system processes. Lunar Planet
Sci. XXXIX, 1437–1438.
Prinzhofer, A., Papanastassiou, D.A., Wasserburg, G.J., 1992. Samarium–neodymium
evolution of meteorites. Geochim. Cosmochim. Acta 56, 797–815.
Scheinin, N.B., Sm–Nd systematics of extraterrestrial objects and questions of excess
142
Nd, PhD thesis, University of California, 1977.
Sharma, M., Papanastassiou, D.A., Wasserburg, G.J., 1995. Sm–Nd systematics of a large
eucrite clast in the Vaca Muerta mesosiderite. Lunar Planet Sci. XXVI, 1271–1272.
Shukolyukov, A., Begemann, F., 1996. Pu–Xe dating of eucrites. Geochim. Cosmochim.
Acta 60, 2453–2471.
Shukolyukov, A., Lugmair, G.W., 1993. 60Fe in eucrites. Earth Planet. Sci. Lett. 119,
159–166.
Srinivasan, G., Whitehouse, M.J., Weber, I., Yamaguchi, A., 2007. The crystallization age
of eucrite zircon. Science 317, 345–347.
Stewart, B.W., Papanastassiou, D.A., Wasserburg, G.J., 1994. Sm–Nd chronology and
petrogenesis of mesosiderites. Geochim. Cosmochim. Acta 58, 3487–3509.
Takeda, H., 1997. Mineralogical records of the early planetary processes on the
howardite, eucrite, diogenite parent body with reference to Vesta. Meteor. Planet.
Sci. 32, 841–853.
Tera, F., Carlson, R.W., Boctor, N.Z., 1997. Radiometric ages of basaltic achondrites and
their relation to the early history of the Solar System. Geochim. Cosmochim. Acta
61, 1713–1731.
Wadhwa, M., Lugmair, G.W., 1995. Sm–Nd systematics of the eucrite Chervony Kut.
LPSC XXVI, 1453–1454.
Wadhwa, M., Lugmair, G.W., 1996. Age of the eucrite “Caldera” from convergence of longlived and short-lived chronometers. Geochim. Cosmochim. Acta 60, 4889–4893.
Wiechert, U.H., Halliday, A.N., Palme, H., Rumble, D., 2004. Oxygen isotope evidence for
rapid mixing of the HED meteorite parent body. Earth Planet. Sci. Lett. 221,
373–382.