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The Absolute Chronology and
Thermal Processing of Solids
in the Solar Protoplanetary Disk
James N. Connelly,1* Martin Bizzarro,1* Alexander N. Krot,1,2 Åke Nordlund,3
Daniel Wielandt,1 Marina A. Ivanova4
Transient heating events that formed calcium-aluminum–rich inclusions (CAIs) and chondrules
are fundamental processes in the evolution of the solar protoplanetary disk, but their chronology
is not understood. Using U-corrected Pb-Pb dating, we determined absolute ages of individual
CAIs and chondrules from primitive meteorites. CAIs define a brief formation interval corresponding
to an age of 4567.30 T 0.16 million years (My), whereas chondrule ages range from 4567.32 T 0.42
to 4564.71 T 0.30 My. These data refute the long-held view of an age gap between CAIs and
chondrules and, instead, indicate that chondrule formation started contemporaneously with
CAIs and lasted ~3 My. This time scale is similar to disk lifetimes inferred from astronomical
observations, suggesting that the formation of CAIs and chondrules reflects a process intrinsically
linked to the secular evolution of accretionary disks.
The only record of our solar system’s form- ative stages comes from the earliest sol- ids preserved from the protoplanetary disk
that now reside as millimeter- to centimeter-sized objects—calcium-aluminum–rich inclusions
(CAIs) and chondrules—in chondrite meteorites.
These complex objects have been the subject
of intense study in an attempt to decipher their
origins and, in turn, use them as records of the
dynamics of the protoplanetary disk that led
to the formation of the solar system (1–8). Most
CAIs formed as fine-grained condensates from a
gas of approximately solar composition in a high-temperature environment (>1300 K) at total
pressure ≤10–4 bar, with a subset experiencing
re-melting to form distinct coarser igneous tex-
1Centre for Star and Planet Formation and Natural History
Museum of Denmark, University of Copenhagen, DK-1350
Copenhagen, Denmark. 2Hawai‘i Institute of Geophysics and
Planetology, University of Hawai‘i at Manoa, HI 96822, USA.
3Centre for Star and Planet Formation and Niels Bohr Institute,
University of Copenhagen, DK-2100 Copenhagen, Denmark.
4Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow 119991, Russia.
*To whom correspondence should be addressed. E-mail:
firstname.lastname@example.org (J.N.C.); email@example.com (M.B.)
tures (9). In contrast, most chondrules represent
coalesced dust aggregates that were subsequently rapidly melted and cooled in lower-temperature
regions (<1000 K) and higher ambient vapor
pressures (≥10−3 bar) than CAIs, resulting in igneous porphyritic textures (10). Despite their formation by different mechanisms (condensation
versus dust accretion) in distinct environments
(11), these objects share common histories of exposure to brief, high-temperature events at least
once in their respective evolutions.
The current perception of the relative timing of CAI and chondrule formation is based on
the short-lived 26Al-26Mg chronometer [26Al decays to 26Mg with a half-life of 0.73 million
years (My)], which has led to a growing consensus that chondrules formed 1 to 2 My after
CAIs (12). This age difference has long been
used as a central observation in defining models of chondrule formation and, in addition, implies that the melting of CAIs and chondrules
was produced by different mechanisms and/or
heat sources. However, the 26Al-26Mg dating method critically depends on the disputed assumption of homogeneous distribution of 26Al in space
and time within the protoplanetary disk (13).
33. A. Annunziata, C. Galli, M. Marinelli, T. Pau, Eur. J. Org. Chem.
2001, 1323 (2001).
Acknowledgments: This work was supported by the National
Science Foundation (graduate research fellowships for S.E.C.
and K.J.L.) and by the Gordon and Betty Moore Foundation.
Metrical parameters for the structure of copper complex 1
are available free of charge from the Cambridge Crystallographic
Data Centre under reference CCDC-896019.
22 June 2012; accepted 10 September 2012
Materials and Methods
Figs. S1 to S41
Tables S1 to S3
In contrast, chronologies based on long-lived
radioisotope systems rely on the knowledge of
the present-day abundances of the parent and
daughter isotopes in a sample and therefore are
free from assumptions of parent nuclide homogeneity. Of the various long-lived radioisotope
systems, the Pb-Pb dating method is the most
powerful tool to establish a high-resolution chronology of the first 10 My of the solar system.
This chronometer is based on two isotopes of
U, 238U and 235U, that decay in a chain to stable
Pb isotopes, 206Pb and 207Pb, respectively, resulting in 207PbR/206PbR (where R = radiogenic) ratios that correspond to the amount of time passed
since the system closed, by Eq. 1
; ; ðel1t − 1Þ
ðel2t − 1Þ
where l1 and l2 reflect the decay constants for 235U
and 238U, respectively; t, time. The 207PbR/206PbR
ratio of an inclusion is calculated by extrapolating from an array of measured Pb isotopic values
that represent varying mixtures of radiogenic
Pb and its initial Pb isotopic composition, which
should approximate that of the solar system defined by the Nantan iron meteorite (14). However,
attempts to date individual CAIs and chondrules
by this approach have historically been frustrated
by the difficulties in analyzing the small amounts
of Pb in these inclusions. In addition, the 238U/235U
ratio required for Eq. 1, which has traditionally
been assumed to be 137.88 in all solar system
materials, was demonstrated to vary in CAIs by
35 e units (deviations in parts per 104), corresponding to offsets in calculated Pb-Pb ages of up to
5 My (15). The observation of U isotope variability, attributed to the decay of the short-lived
247Cm nuclide (247Cm decays to 235U with a half-life of 15.6 My) voided all published Pb-Pb ages
for solar system materials that were based on an
assumed 238U/235U ratio and made clear the need to
have measurements of the U isotopic compositions for all materials dated by the Pb-Pb method.
To establish an assumption-free absolute chro-
nology of CAI and chondrule formation, we have
developed improved methods for the precise anal-
ysis of small amounts of Pb and U by thermal