Geochimicaet CosmochimicaActa, Vol. 60, No. 23, pp. 4889-4893, 1996
Copyright © 1996ElsevierScienceLtd
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Pergamon
PPI S0016-7037(96) 00331-6
LETTER
Age of the eucrite "Caldera" from convergence of long-lived
and short-lived chronometers
MEENAKSHI WADHWA* and GONTER W. LUGMAIR
Scripps Institution of Oceanography, University of Califomia, San Diego, La Jolla, CA 92093, USA
(Received June 4, 1996; accepted in revised form September 17, 1996)
Abstract--The long-lived 1475m-143Ndchronometer has been used in conjunction with the short-lived
146Sm-142Nd and 53Mn-53Crchronometers to constrain the time of formation for the eucrite Caldera. The
combination of these three chronometers has produced results that are consistent with one another and
converge on a precise, high resolution age of 4537 +__ 12 Ma (within 95% confidence limits). So far
this is the youngest crystallization age that has been confirmed for a noncumulate eucrite. Thus, this
basaltic meteorite formed relatively late as compared to other noncumulate eucrites on a differentiated
asteroidal parent body, most likely between 17 and 41 million years after the formation of the first
condensates in the early Solar System. From these age constraints it is evident that Caldera either formed
within a magma body on a large ( > 5 0 0 km) asteroid or in an extensive impact melt on the crust of its
parent body.
We report here the results of a study of 1475m-143Nd,
1465m-142Nd (146Sm-decays to laZNd with a half-life of - 1 0 3
Ma), and 53Mn-53Cr (S3Mn decays by electron capture to
S3Cr with a half-life of ~3.7 Ma) systematics in the eucrite
Caldera, that has yielded a precise absolute age for this meteorite. Caldera, a find from Chile, is an unusual basaltic meteorite in that it is one of only two known non-Antarctic unbrecciated noncumulate eucrites, the other being Ibitira. Previous Sm-Nd work on Ibitira (Prinzhofer et al., 1992) has
shown that although the 147Sm-143Nd systematics yielded a
precise age of 4.46 ___ 0.02 Ga, the 146Snl]ln4sm ratio was
determined to be 0.009 ___0.001. This was taken to indicate
that Ibitira was indeed an ancient object that formed close
to ~4.56 Ga ago, but that its Sm-Nd systematics had been
subsequently disturbed by partial isotopic equilibration. Similarly, the formation ages of other noncumulate (but brecciated) eucrites, such as Juvinas and Chervony Kut, have also
been estimated to lie close to ~4.56 Ga (Lugmair et al.,
1975, 1994a,b; Wadhwa and Lugmair, 1995). Texturally,
Caldera differs from Ibitira, and from other (brecciated)
noncumulate eucrites, in having a relatively coarse grain size
(typically a few millimeters) and a hypidiomorphic granular
texture, indicative of a slower cooling rate (Boctor et al.,
1994; N. Z. Boctor, pers. commun.).
1. INTRODUCTION
High precision, high resolution formation ages of igneous
meteorites are of considerable interest since they provide
insight into timescales involved in the accretion and subsequent differentiation of meteorite parent bodies in the early
Solar System. High resolution ages of eucritic meteorites
are particularly interesting since these can place important
constraints on the chronology and evolution of the basaltic
crust on their asteroidal parent body. It has been recognized
that relative chronometry from the distribution of extinct
radionuclides (Podosek and Swindle, 1988, and references
therein), such as 26A1, 1465m, 53Mn, and 6°Fe, could potentially be used in combination with absolute chronometers,
such as 147Sm-143Nd, 87Rb-S7Sr, and 2°7pb-z°6pb, to obtain
precise absolute age dates that could allow a resolution of a
few million years for early Solar System events. In practice,
however, this has been impeded largely due to the analytical
difficulties of accurately measuring the decay products of
many of these short-lived radionuclides and, therefore, the
lack of an extensive database of correlated precise absolute
and relative ages for various primitive meteorite types. Moreover, an important prerequisite for the use of extinct radionuclides as reliable relative chronometers is that they be homogeneously distributed at least in the region of the Solar System where these meteorites formed. Only recently tighter
constraints regarding the distributions of at least some of
these nuclides have begun to emerge (Lugmair and Galer,
1992; Lugmair and Maclsaac, 1995; MacPherson et al.,
1995; Lugmair et al., 1996).
2. EXPERIMENTAL
A 1.5 g bulk sample of Caldera was gently crushed with a boron
carbide mortar and then sieved to obtain various grain-size fractions.
There is extensive yellow-brownstaining on almost all grain surfaces
in Caldera. Because of the unknown terrestrial residence time of this
meteorite, and the possible terrestrial origin of this staining, it was
clear that it had to be removed before reliable isotopic measurements
on mineral separates could be attempted. To remove the staining the
* Present address: Department of Geology, The Field Museum,
Roosevelt Road at Lake Shore Drive, Chicago, IL 60605, USA.
4889
4890
M. Wadhwa and G. W. Lugmair
Table 1, Sm-Nd isotopic results for Caldera. Nd isotopic data are normalized to 148Nd/144Nd = 0.241572 (equivalent to
H6Nd/I44Nd = 0.7219). Uncertainties (20) on ratios, given in parentheses, are for the least significant figures.
Sample
CAL TRI
Sm (ppm)
Nd (ppm)
147Sm/144Nd
1.626
4.994
0.1968 (4)
143Nd/144Nd
£142"
144Sm/144Nd
0.512688(14)
.04034(8)
-0.60 (0.37)
CALTR2
1.526
4.408
0.2092 (4)
0.513057(14)
0.04289(9)
-0.12 (0.37)
CAL PI
0.346
2.337
0.08957 (18)
0.509438(14)
0.01837(4)
-1.80 (0.37)
CALPx
1.572
3.411
0.2783 (6)
0.515121(14)
0.05705(11)
+0.68(0.36)
* £142 = {[(142Nd/144Nd)measured/(142Nd/144Nd)staadard]- I } x 104. The standard 142Nd/144Nd value of 1.141834 + 11 used
here is the mean of tenestrial standards given in Lugmair and Galer (1992).
125-250/zm fraction was washed in an ultrasonic bath for up to
2 h in 1.8 N HC1. Thereafter, plagioclase (P1) and pyroxene (Px)
mineral separates were prepared using a Frantz Isodynamic magnetic
separator followed by handpicking. In order to optimize the purity
of the mineral fractions, the P1 and Px separates were again cleaned
in 1.8 N HC1 for 15 rain and a second round of handpicking followed.
Two bulk samples (TR1 and TR2), weighing 161 and 127 mg,
respectively, were used for Sm-Nd analyses. TR1 was an unetched
chip while TR2 was crushed and then ultrasonically cleaned for a
half hour in 1.8 N HC1 before dissolution.
Mineral separates (P1 and Px), mostly clean of the surface coating,
and TR1 and aliquots of the acid-cleaned bulk sample TR2 were
dissolved in HF/HNO3. For the Mn-Cr analyses the rest of TR2
(TR in Table 2) was dissolved in HF/HNO3 to obtain the silicate
"Sil" (solution) and " S p " (residue) fractions. The silicate portion,
Sil, was decanted, and the chromite residue, Sp, was then washed
in H 2 0 and dissolved in a Teflon bomb in HF/HNO3. All samples
were eventually brought into solution in HC1 for aliquoting. The
parent solutions for mineral separates and bulk samples were allquoted into three portions: ~90% for isotopic measurements; ~5%
for samarium and neodymium concentrations; ~5% reserved as a
"safety fraction." Samarium, neodymium, and chromium separations were made using cation exchange chromatography.
Manganese and chromium concentrations were determined in aliquots of the dissolved samples of bulk rock TR, Px, silicate fraction
(Sil), and chromite (Sp) with graphite furnace atomic absorption
spectrometry. The accuracy of the measured concentrations was typically better than _+5%, and a 2a uncertainty of _+ 5% is estimated
for the Mn/Cr ratios.
Samarium and neodymium concentrations were obtained using
the isotope dilution technique. Chromium and neodymium isotopic
measurements were made on the La Jolla VG 54E single-collector
mass spectrometer using the Faraday cup. The number of repeat
measurements of the isotopic composition (each consisting of 30
blocks of 10 ratios) on each sample ranged from 2 to 6 for neodymium and 12 to 18 for chromium. Neodymium isotopic analyses were
carded out as in Lugmair and Galer (1992). Details of chromium
isotopic measurements will be described elsewhere.
Finally, elemental X-ray mapping was performed on three thin
sections of Caldera to identify any possible phases rich in rare earth
elements (REEs). These analyses were made with the Cameca SX50 electron microprobe at the University of Chicago.
3.1. Sm-Nd
mapping of thin sections of Caldera showed that a calcium
phosphate mineral (containing percent-level LREEs) was
present in very low abundances ( < 0 . 0 1 modal % ) .
There is a large spread in the S m / N d ratios between plagioctase and pyroxene in Caldera (Table 1 ). G o o d isochrons
for both the ~47Sm-]43Nd and the 146Sm-~4ZNd systems were
obtained (Figs. 1 and 2). Note that even the data from the
unetched bulk sample, TR1, fall on the isochrons, which
indicates that if the surficial staining on mineral grains is
indeed of terrestrial origin, only insignificant amounts of
REEs could have been introduced. The ~47Sm- ]4aNd system
yields an age of 4544 _+ 19 M a with an initial ~43Nd/]"Nd
ratio of 0.506741 _+ 24. The short-lived 146Sm-~4ZNd system
results in a ~46Sm/]~Sm ratio o f 0.0073 _+ l l and an initial
e~42 of - 3 . 0 _+ 0.5. These S m - N d results, within errors, are
indistinguishable from results previously obtained on other
achondrites such as the angrites, Angra dos Reis, and
L E W 8 6 0 1 0 ( L u g m a i r and Marti, 1977; Lugmair and Galer,
1992; Nyquist et al., 1994), and the n o n c u m u l a t e eucrites,
Juvinas, Ibitira, and C h e r v o n y Kut ( L u g m a i r et al., 1975;
Prinzhofer et al., 1992; W a d h w a and Lugmair, 1995). This
indicates that both S m - N d systems in all these meteorites
closed contemporaneously within the uncertainties afforded
by the data, i.e., 2 0 - 3 0 M a for J47Sm-143Nd and > 1 5 M a
0.520
0.517
i
•
I ,= 0. 74t
/
II
"o
~: 0.514
TRI~
/
TR2
.
,
z
~ 0.511
3. RESULTS AND DISCUSSION
0.508
Systematics
°.5 5 c 0 3
Results of Sm-Nd analyses are given in Table 1. Note that
mass balance constraints from concentrations of samarium
and n e o d y m i u m require the presence of a trace phase rich in
REEs. Previous petrographic studies (Boctor et al., 1994) had
failed to identify any phosphates in Caldera. However, X-ray
CALDERA
Age = 4.544 + 0.019 Ga
,
0.100
o.~,5o
~
0.200
147Sm/144Nd
o.2s--~- o.3so
0.300
0.400
FIG. 1. 147Sm-143Nd evolution diagram for Caldera. The age is
calculated using k = 6.54 × 10 -12 a -~. The deviations of the data
points from the best-tit line in e-units (parts per 104 ) and the individual 2~7 errors are shown in the inset figure.
Age of the eucrite meteorite Caldera
Table 2. Mn-Cr isotopic results for Caldera. Cr isotopic data are aocmalized to
4
2
,~
50Cr#2Cr= 0.051859. 20 m~rt in eomt,matiem are t,/i~:aly ~ ~lt.
CALDERA
j
14eSm/1~Sm " 0.0073 4" 11 •
. 1(142)--3.0+0.5~
I
3
1
4891
I
oi
Staple
CALTR
i TR2
TR1
/
i
0400
PI
i
0.02
SSlVlaP~
eX531"
4326
2095
2.33
1.12 + 0,09
CALSp
9402
6.29 x 105
0.02
1.12+0.08
CAL Sil
4457
1000
5.03
1.11 4"0.11
CAl. Px
7428
1072
7.82
i.15 4" 0.12
"t(53) = {[(53Cr#2Cr)measmed/(J3Cr/J2Cr)standardl-l} x I04. The stlmdard vahm of
i
i
0.04
~,~Sm/-~Nd
0.06
i
0.08
FIG. 2. '46Sm-142Ndevolution diagram for Caldera, plotted as ~142
vs. 144Sm/~44Nd.The dotted lines show the range of uncertainty (2a)
in the standard value of -0.1 e-units.
for t46Sm-i42Nd (although in the case of Ibitira, the Sm-Nd
systematics were subsequently disturbed by partial isotopic
equilibration; Prinzhofer et al., 1992).
3.2. Mn-Cr
Cr (ppm)
(+5~t)
---_---_--_----,-~~----_--_-
~-1
Me (ppm)
P x ~
Systematics
The considerably finer resolution of the 53Mn- 53Cr system,
compared to both Sm-Nd systems, paints an interestingly
different picture which is, however, consistent with the SmNd systematics. Mn-Cr systematics are summarized in Table
2 and illustrated in Fig. 3. Minerals in Caldera have a wider
range of 55Mn/52Cr ratios, from ~ 0 (chromite) to ~ 8 (pyroxene), as compared to other noncumulate eucrites (Lugmair et al., 1994a, b). Inspite of this wide range in 55Mn/
52Cr ratios, the 53Cr/52Cr ratios in all the samples measured
are the same within uncertainties. Although typical errors
are 8 - 1 2 ppm, the total range in 53Cr/S2Cr ratios is only
about 4 ppm, i.e., 1.11-1.15 e-units, with an average of 1.12
e. It should be noted that since Caldera has a short cosmic
ray exposure age of ~ 14 Ma (Shukolyukov and Begemarm,
1996), spallogenic isotope effects in chromium are negligible. Since the S3Cr/52Cr ratios in all samples are the same,
an upper limit for the 53Mn/55Mn ratio of 1.2 x 10 -7 can
be calculated from the uncertainties. This means that the
short-lived radionuclide 53Mn, which was present in the incipient solar nebula, was no longer extant by the time the
Mn-Cr system closed in Caldera.
53Cr/57-Cr is taken as 0.1134507.
T h e errors on f.(53) given here are not
would be smaller, but are bascd on estimates f r o m the i~roducibility of the repe.~
eneasurementsof the sampte and standards.
indicates that formation of Caldera must have occurred ~ 19
Ma after Chervony Kut. Although a precise crystallization
age for Chervony Kut is not known, it is obvious that the
upper limit on the age of this differentiated meteorite cannot
exceed that of the first condensates in the early Solar System,
which is anchored at 4566 __. 2 Ma (G6pel et al., 19911.
Therefore, within 95% confidence limits, the upper limit on
the age of Caldera cannot exceed 4549 Ma.
It should be noted that if the upper limit of the ~3Mn/~Mn
ratio in Caldera is compared with a 53Mn/55Mn ratio of (4.4
_ 1.0) x 10 -2, as determined for calcium aluminum inclusions (CAIs) by Birck and All~gre (1985), an age difference
of at least 30 Ma is obtained for these objects, thereby leading to an upper limit of 4536 Ma for the age of Caldera.
However, there appear to be clear discrepancies in assuming
that (1) the initial value of the 53Mn/55Mn ratio for the Solar
System was (4.4 _+ 1.1) × 10 -5, as represented by the
average value of this ratio in CAIs and/or (2) the 53Mn/
55Mn ratio was homogeneous in the early Solar System. This
can be seen by the fact that comparison of the 53Mn/55Mn
ratio in CAIs (Birck and All~gre, 1985) with that determined
in the LEW86010 angrite (Lugmair et al., 1992) indicates
that CAIs are older than the angrites by - 2 0 Ma, whereas
Pb-Pb systematics (G6pel et al, 1991; Lugmair and Galer,
2.50
[~Mn/UMn
•
._
<1.2E-7 [
H^vo(.co.l.12,
,
t.~
/ ~F
[ ~o
,
."
~ ~
,
i
3.3. A g e C o n s t r a i n t s f o r C a l d e r a
-r
6) 1 . 0 0
From the long-lived ~47Sm-t43Nd chronometer, the age of
Caidera is established as 4544 + 19 Ma, allowing an age
range of 4525-4563 Ma for its formation. However, further
constraints for this age interval can be obtained from the
s h o r t - l i v e d 5 3 M n - 5 3 C r and 146Sin-142Nd chronometers. It was
shown for the eucrite Chervony Kut that S3Mn was still present at the time of its solidification and that the 53Mn/55Mn
ratio at that time was (4.3 ___ 0.4) x 10 -6 (Lugmair et al.,
1994a and unpubl, data). If both eucrites originate from the
same parent body with a common 53Mn/55Mn reservoir (see
below) the upper limit of the 53Mn/55Mn ratio in Caldera
2anmm, which
~"
R
0,501
Earth & Moon
0'00 1
0.0
,
,
2.0
.
.
.
410
610
SSMnl,~Cr
.
810
10.0
FIG. 3. 53Mn-53Crevolution diagram for Caldera, plotted as (c 53)
vs. 5SMn/52Cr. For reference, the solid horizontal line through (0,0)
represents the terrestrial standard; dashed fines are 53Mn-53Cr isocitrons for the eucrites Chervony Kut (CK) (Lugmalr et al., 1994a,
and unpubl, data) and Juvinas (JUV) (Lugmalr et al., 1994h).
4892
M. Wadhwa and G. W. Lugmair
1992) indicate an age difference of only ~ 8 Ma. This raises
the possibility that the inferred 53Mn/55Mn ratio in CAIs
could be "artificially" elevated as a result of the presence
of either nucleosynthetic anomalies (Papanastassiou, 1986)
or 53Cr excesses from the decay of 53Mn in the interstellar
medium. Additionally, it has recently been shown (Lugmair
and Maclsaac, 1995; Lugmair et al., 1996) that the initial
53Mn/55Mn ratio in the parent bodies of different meteorite
groups may have been different. Therefore, we are certain
that the direct comparison of 53Mn/55Mn ratios between meteorites may give meaningful age differences only when the
meteorites are believed to have either originated on the same
parent body or if it can be shown by Mn-Cr systematics that
meteorite families are closely related to one another (see
Lugmair et al., 1996). In the case of the noncumulate eucrites Caldera, Chervony Kut, and also Juvinas, it is evident
from Fig. 2 and from Lugmair et al. (1996) that their MnCr isochrons all intercept, within error, very close to their
respective bulk rock Mn/Cr ratios of ~ 1.7 to 2 at a common
53Cr excess of ~1.1 e-units. This suggests that all three
meteorites originated from a reservoir with the same initial
53Mn/55Mn ratio (which is much higher than measured so
far for any other meteorite type). Thus, Mn-Cr relative chronometry appears feasible between these objects.
A lower bound on the age of Caldera is estimated from
Sm-Nd systematics. Lugmair and Galer (1992) evaluated all
existing ~46Sm/144Smdata for consistency and interlaboratory
bias. Their best estimate for the ~46Sm/l~Sm ratio at 4558
Ma is 0.0070 _+ 10. Subsequent determinations of the 1465n~
~44Sm ratio for angrites as well as other meteorites that
formed close to ~4.56 Ga (Prinzhofer et al., 1992; Nyquist
et al., 1994; Stewart et al., 1994; Sharma et al., 1995) confirm that the above value for this ratio at 4558 Ma is indeed
reasonable. The 1465m/~44Smratio obtained by us for Caldera
is 0.0073 _+ 11, which is in agreement, within error, with
the above value. To determine a lower limit for the age of
Caldera, we compare the maximum value for the best estimate of the J46Srn]l~Sm ratio at 4558 Ma (i.e., 0.0080) with
the minimum allowable value for Caldera (i.e., 0.0062).
This indicates that the maximum time difference between
the age of the angrite LEW86010 (4558 Ma) and that of
Caldera cannot be more than ~38 Ma, suggesting that the
minimum age of Caldera cannot be less than - 4 5 2 0 Ma.
(Note: If we were to use the upper limit of the measured
~46Sm/144Sm value for LEW86010 of 0.0088, as determined
by Lugmair and Galer (1992), then the minimum age of
Caldera would be 4506 Ma.) However, this age is younger
than the minimum age (from 2a uncertainties) obtained from
the J47Sm-14aNd chronometer. Therefore, the minimum age
allowed by this long-lived chronometer (i.e., 4525 Ma) is the
strict lower bound on the formation age of Caldera. Finally, it
should be noted that if the nominal best estimate value of
0.0070 is taken for the ~46Sm/1445mratio in the Solar System
at 4558 Ma before present (Lugmair and Galer, 1992), the
lower limit on the age of Caldera (for a minimum ~46Sm/
~44Sm ratio of 0.0062, within 2a errors) becomes 4540 Ma.
From the above, it is clear that combining the constraints
from the ~47Sm-143Nd and the Mn-Cr systematics restricts
the age range for the formation of Caldera between 4525
and 4549 Ma. Additionally, if one uses the nominal best
estimate value of 0.0070 for the ~46Sm/144Sm ratio in the
Solar System at 4558 Ma (Lugmair and Galer, 1992), the
age range for Caldera becomes 4540-4549 Ma. Conservatively, however, the maximum allowable age range for Caldera, within 95% confidence limits, is the former, which can
be translated to an absolute age of formation of 4537 _+ 12
Ma. It should be noted that the Pb-Pb age for Caldera was
recently reported as 4516 _+ 3 Ma (Galer and Lugmair,
1996). This is lower than our estimated age range for Caldera and, as noted by these authors, could be a result of the
protracted cooling history of this meteorite, such that the
times and temperatures of isotopic closure for the Sm-Nd
and Mn-Cr systems were different from those of the Pb-Pb
system.
3.4. Implications for Basalt Formation on the Eucrite
Parent Body
Compared to the age of formation of the refractory CAIs
in chondrites (GOpel et al., 1991), it is evident that the
eucrite Caldera formed between 17 and 41 million years
after the condensation of the first solid objects in the early
Solar System. In contrast, Sm-Nd systematics in another
unbrecciated noncumulate eucrite, Ibitira, are consistent with
an old age close to ~4.56 Ga (Prinzhofer et al., 1992). Two
of the brecciated noncumulate eucrites, Chervony Kut and
Juvinas, show clear evidence of the presence of live 53Mn
and are believed to have formed within a few (i.e., <10)
million years of the formation of CAIs (Lugmair et al.,
1994a,b). Therefore, the formation age of 4537 _+ 12 Ma for
Caldera is thus far the youngest age that has been precisely
established for any noncumulate eucrite. At that time, the
short-lived radionuclide, 26A1,very likely a major heat source
for differentiation in the early Solar System, was extinct
(Lee et al., 1977; MacPherson et al., 1995, and references
therein). Thus, the source magma for Caldera was either
generated at depth in a relatively large asteroidal body
( >500 km in diameter, which could perhaps retain heat for
millions of years after the complete decay of a6AI), or was
formed in an impact melt on the surface of the eucrite parent
body (EPB), when the chromium isotopes were completely
reequilibrated.
As stated above, 53Mn-53Cr data for some noncumulate
eucrites indicate that these objects have crystallized, most
likely as basaltic flows on the EPB, < 10 million years after
the formation of CAIs. If Caldera indeed crystallized in a
magma body, it implies that endogenous basalt formation
on the EPB may have lasted millions of years longer than
has previously been assumed. However, certain petrographic
characteristics of Caldera, such as the presence of minute
"relict" pyroxene blebs within plagioclase grains, appear
consistent with its formation in an impact melt (N. Z. Boctor,
pers. commun.). In this case, the coarse-grained crystalline
texture of Caldera would require that the impact produced
basaltic melt pool be extensive enough to allow slow cooling.
Therefore, an impact origin for Caldera would imply that
the crust of the EPB was extensively remelted by energetic
impacts that produced relatively deep pools of basaltic melt
Age of the eucrite meteorite Caldera
on its surface. More detailed petrographic and geochemical
studies on this rock are needed to evaluate this possibility
further.
Finally, the combination of long-lived and short-lived
chronometers, such as in the case of Caldera, will yield high
resolution absolute age dates for many other meteorite types
in the near future as increasing amounts of self-consistent
datasets for long-lived and short-lived radionuclides become
available. This will eventually contribute towards a much
higher resolution picture of the sequence of condensation,
accretion and differentiation events in the early Solar
System.
Acknowledgments--We would like to thank Chris Maclsaac for his
invaluable assistance in the laboratory and Deniz Oezen for diligently handpicking the plagioclase and pyroxene mineral separates
used in this study. We are grateful to Ian Steele for helping with
the x-ray mapping of sections of Caldera. We thank Dimitri Papanastassiou, Doug Macdougall, and an anonymous reviewer for very
helpful comments. Support for this work was provided by NASA
grant NAGW 3285.
Editorial handling: J. D. Macdougall
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