[meteorite-list] 238U/235U Variations in Meteorites: Extant 247Cm and Implications for Pb-Pb Dating

From: Shawn Alan <photophlow_at_meteoritecentral.com>
Date: Sat, 20 Feb 2010 11:10:12 -0800 (PST)
Message-ID: <746999.36052.qm_at_web113609.mail.gq1.yahoo.com>

Hello Listers,
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I found an article today on the topic of Pb-Pb dating and how it might be flawed. Down below is the abstract and the article and also a link to the pdf file which also has graphs and the reference section.
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Shawn Alan
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http://www.geo.umass.edu/petrology/PetSem/Brennecka%20et%20al_Science_2010_UPb.pdf
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                                Abstract
The 238U/235U isotope ratio has long been considered invariant in meteoritic materials (equal to 137.88). This assumption is a cornerstone of the high-precision lead-lead dates that define the absolute age of the solar system. Calcium-aluminum?rich inclusions (CAIs) of the Allende meteorite display variable 238U/235U ratios, ranging between 137.409 ? 0.039 and 137.885 ? 0.009. This range implies substantial uncertainties in the ages that were previously determined by lead-lead dating of CAIs, which may be overestimated by several million years. The correlation of uranium isotope ratios with proxies for curium/uranium (that is, thorium/uranium and neodymium/uranium) provides strong evidence that the observed variations of 238U/235U in CAIs were produced by the decay of extant curium-247 to uranium-235 in the early solar system, with an initial 247Cm/235U ratio of approximately 1.1 ? 10?4 to 2.4 ? 10?4.
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Meteorites can provide a wealth of information
about the formation and evolution
of the solar system. In chondrite
meteorites, calcium-aluminum?rich inclusions
(CAIs) represent the first solids to condense from
the cooling protoplanetary disk during the birth
of the solar system (1); therefore, the ages of
CAIs are generally considered to date the solar
system?sorigin (2?4). High-precision Pb-Pb dating
studies, which rely on a known ratio of parent
U isotopes, assume that the 238U/235U ratio is
invariant in meteoritic material (equal to 137.88)
(5). Uranium isotope variations in meteorites may
be produced by many mechanisms, including
the decay of extant 247Cm to 235U, nucleosynthetic
anomalies in U isotopes, or fractionation of
U isotopes during chemical reactions, as recently
observed on Earth (6, 7). Any or all of these mechanisms
may play some role in 238U/235U variability
in early solar system materials; however, the existence
and effect of 247Cm on the 238U/235U ratio can
be studied using geochemical proxies for Cm.
247Cm is only created in certain types of supernovae
during r-process nucleosynthesis. It
decays to 235U with a half-life of 15.6 million years
(My) (8?13). If 247Cm was present during the
formation of the solar system, it would be detected
by variations of 238U/235U in ancient meteoritic
materials in which the original solar
system Cm/U ratio may have been substantially
fractionated by processes associated with the
formation of the meteoritic materials. The CAIs
in chondritic meteorites are likely to be such materials,
because many of them experienced ele

1School of Earth and Space Exploration, Arizona State University,
Tempe, AZ 85287 USA. 2Institut fur Geowissenschaften,
Goethe-Universit?t, Frankfurt, Germany. 3Senckenberg Forschungsinstitut
und Naturmuseum, Frankfurt, Germany. 4Department
of Chemistry and Biochemistry, Arizona State
University, Tempe, AZ 85287, USA.
*To whom correspondence should be addressed. E-mail:
brennecka at asu.edu
?Present address: Institut f?r Geology und Mineralogie, Universit?t
zu K?ln, Cologne, Germany.
mental fractionation during condensation and
evaporation processes that were involved in their
formation and because Cm is more refractory
than U (14).
Quantification of the abundance of extant
247Cm has the potential to provide new constraints
on the origin of short-lived radionuclides
in the early solar system. If the 247Cm in the early
solar system was predominantly inherited from
galactic chemical evolution (13), then it should
be possible for us to determine the time interval
of free decay (D) between the last r-process nucleosynthetic
event and the formation of the solar
system (5, 1 , 15, 16). Supposed claims of large
variations in the 238U/235U ratio that were caused
by the decay of 247Cm (8, 9) were refuted in subsequent
studies (5, 10, 1 , 17). Here we present
high-precision 238U/235U ratios obtained from
13 CAIs of the Allende meteorite to quantify the
amount of 247Cm present in the early solar sys-
Fig. 1. 238U/235Uisotope
values for the samples of
this study. The box represents
the measured value

and analytical precision
of replicate analyses of
20? to 100?parts per
billion solutions of the
SRM950a standard. Error
bars are calculated as 2
times the standard deviation
(2SD) of multiple
runs of each sample, when
possible. In samples with
extremely limited uranium,
for which fewer than three
runs were possible, the
reported errors are conservatively
represented by
the long-term reproducibilities
(2SD) based on
multiple runs of SRM950a
measured over the course
of this study at the same concentration as the sample.

Downloaded from www.sciencemag.org on January 26, 2010

www.sciencemag.org SCIENCE VOL 327 22 JANUARY 2010 449

REPORTS
Fig. 2. (A) 232Th/238Uand
(B) 144Nd/238U ratios plotted
versus 235U/238Uratios,the
reciprocal values of our
measured 238U/235Uratios.
The gray dashed lines represent
the 2SD errors on
the best-fit line (solid
black). Errors on the y-axis
data are T2SD; x-axis error
bars are T5% of the de

termined value of the elemental
ratio.
group II CAIs, suggests a complex condensation
history involving fractional condensation
(21, 22). The four CAIs of this study that have
the highest Nd/U and Th/U ratios (as well as
the lowest 238U/235U ratios) are all classified as
group II CAIs by their REE patterns (Fig. 3).
Because of the lower condensation temperature
of U relative to Nd and Th (23), the fractional
condensation history that resulted in the characteristic
group II REE pattern in these objects
is likely to have produced the relatively high
Nd/U and Th/U ratios.
The correlation of both Th/U and Nd/U with
U isotope ratios in the CAIs indicates that the
238U/235U variations do not arise from nucleosynthetic
anomalies or U isotope fractionation,
neither of which easily give rise to such a trend,
and instead provide evidence for the presence
of extant 247Cm in the early solar system. Under
this interpretation, deviations from the best-fit
lines in Fig. 2 could be caused by heterogeneity
of 238U/235U in the solar nebula, Th and Nd
acting as imperfect proxies for Cm, or 238U/235U
fractionation following Allende CAI formation,
possibly from variable redox during secondary
alteration processes (7).
In contrast to our findings, a recent study did
not detect deviations in the 238U/235U ratio among
a variety of bulk meteorite samples, including
Allende and Murchison (1 ). Given the reported
precision of the study?s U isotope analysis, the
144Nd/238U ratios should have been sufficient to
reveal detectable variations in 238U/235U from
247Cm decay. Although the 238U/235U value of bulk
Murchison samples agrees within error with our
observed values, those for bulk Allende differ
well outside of reported errors. The reason for
this disagreement is unclear at this time.
The initial 247Cm/235U ratio in the early solar
system can be estimated by using the slopes of
the best-fit lines in Fig. 2 (1 ). UsingThandNd
as proxies for Cm, we estimate the initial solar
system 247Cm/235U ratio to be 2.4 ? 10-4 T 0.6 ?
10-4 and 1.1 ? 10-4 T 0.2 ? 10-4, respectively.
The difference between the estimates may be
due to slight differences in the geochemical behavior
of Th and Nd or possibly because of uncertainties
in the assumed solar system Nd/U or
Th/U ratios. Nevertheless, these values are, on
average, higher than the upper limit derived previously
using analyses of the U isotope com-

Fig. 3. REE patterns of four group II CAIs analyzed in this study, normalized to CI chondrites. All other
CAI samples studied here (except 3531-D, for which the REE abundances were not measured) display
flat REE patterns, indicating chondritic relative abundances of these elements (light gray lines).
Fig.4.Ageadjustmentrequiredforsamplesfoundnottohavea238U/235Uvalueof137.88,asassumedinthePb-Pbageequation(Eq.1).TheshadedregionrepresentstherangeofUisotopecompositionsreportedinthisstudy,andtheasterisksrepresentthespecific238U/235Uratiosmeasuredinthesesamples.
Downloaded from www.sciencemag.org on January 26, 2010

positions of bulk chondritic meteorites (1 ). Our
estimates are, however, in agreement with the
upper limit of ~4 ? 10-3 that was determined previously
based on analyses of CAIs (12). If 247Cm
is inherited from galactic chemical evolution,
the range of initial solar system 247Cm/235U
ratios estimated here translates to D ~ 110 to
140 My. This value is similar to, but more
precise than, previous estimates of D based on
the inferred initial solar system abundances of
other r-process?only radionuclides such as 244Pu
and 129I, but does not match the significantly
shorter estimate of D (~30 My) derived from the
initial abundance of 182Hf (16). However, because
182Hf was overabundant in the early solar
system compared with its expected abundance
from galactic chemical evolution, it may have
been injected into the presolar molecular cloud or
the solar nebula by a nearby supernova event [for
example, (13)].
450 22 JANUARY 2010 VOL 327 SCIENCE www.sciencemag.org

REPORTS

Our findings also have implications for precise
dating of early events in the history of the
solar system. The Pb-Pb age equation (Eq. 1)
has been used for decades to calculate the absolute
ages of both meteoritic and terrestrial materials
(24). This equation assumes that 238U/235U
is invariant at any given time, and that the present-
day value is 137.88.
206Pb* 235Uel235 t - 11 el235 t - 1
??
206Pb* 238Uel238 t - 1 137:88 el238 t - 1
?1?
Here, l is the decay constant for the specific
isotope and t is the age. Any deviation from this
assumed 238U/235U would cause miscalculation
in the determined Pb-Pb age of a sample. A
difference of up to 3.5 per mil (?) implies that a
correction of up to ?5 My would be required if
the Pb-Pb ages of these CAIs were obtained
using the previously assumed 238U/235Uvalue
(Fig. 4).
Because 238U/235U variations in solar system
materials are not restricted to CAIs, this requirement
may extend to high-precision Pb-Pb dating
of other materials as well. It is possible, however,
that the 238U/235U values of bulk chondrites
are controlled to a substantial degree by CAIs,
which may be heterogeneously distributed at the
scale at which these analyses were made.
The Pb-Pb dating technique is the only absolute
dating technique able to resolve age differences
of <1 My in materials formed in the
early solar system. Whereas the full range of
238U/235U ratios reported here would result in an
overestimation of the ages of these CAIs by up
to 5 My, the largest excesses (>3.5?)in 235U
occur in the group II CAIs that appear to have
experienced the largest Cm/U fractionation.
For non?group II CAIs, the age overestimation
is =1 My. The apparent discrepancies between
absolute Pb-Pb ages and relative (for example,
26Al-26Mg, 53Mn-53Cr, and 182Hf-182W) ages
(2, 4, 25, 26) may therefore place limits on the
uncertainty of the age of the solar system.
Received on Sat 20 Feb 2010 02:10:12 PM PST