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

[Shawn Alan]

Hello Listers,
 
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. 
 
Shawn Alan
 
http://www.geo.umass.edu/petrology/PetSem/Brennecka%20et%20al_Science_2010_UPb.pdf
 
 
 
                                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. 
 
 
 
 
 
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-Universität, 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 für Geology und Mineralogie, Universität 
zu Köln, 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 
¼¼
206Pb* 238Uel238 t - 1 137:88 el238 t - 1 
ð1Þ 
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.