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Cosmochemistry

Isotopic Stopwatch

New high-precision analyses of the aluminum radioisotope 26 (26Al) reveal that it was distributed homogeneously across the nascent Solar System, giving researchers fundamental insight into its chaotic beginnings.
To come to this conclusion, scientists looked at the oldest solid materials in our Solar System, primitive meteorites known as chondrites. These are made up of round grain-like particles—called chondrules—and calcium-aluminum-rich inclusions (CAIs).
For the past 30 years, cosmochemists have dated meteorites using 26Al, a short-lived isotope with a half-life of just 730,000 years, which is believed to have been abundant during the formation of the Solar System, more than 4.56 billion years ago.

isotopic stopwatch

© CRPG/CNRS

The glassy matrix (green) embedding pyroxene crystals (purple) in this sample from the Semarkona meteorite is the oldest glass known in the solar system.



Though no longer in existence, this isotope radioactively decays to the magnesium isotope (26Mg), which can be effectively measured. This dating method, although extremely precise (on the order of 100,000 years), is still “relative,” since it can only date objects in relation to one another.
To make it reliable, and allow scientists to retrace a precise chronology of the formation of solids, one would have to prove that the aluminum isotope was distributed homogeneously throughout the early Solar System.
This is what Johan Villeneuve from CRPG1 and his colleagues set out to show.2
The team used the secondary ion mass spectrometer (SIMS), a high-precision tool that can measure isotope abundance in natural rock samples, to analyze the Semarkona meteorite. This chondrite, which fell in India in 1940, has remained relatively unaltered by thermal and chemical changes since its formation, more than four billion years ago. They found that although Semarkona's chondrules have very different formation ages, spanning a range of about three million years, their 26Al initial content is very homogeneous.
“Since the aluminum isotope's homogeneity is present in the oldest materials as well as in more recent constituents, this homogenisation must have occured very early on,” explains Villeneuve. This validates the use of 26Al as a chronometer to measure the timing of chondrule formation.
“It also helps us understand the Solar System's evolution models,” adds Villeneuve, “since such solids underwent the first condensation and accretion processes that finally led to the formation of planets.”
The next step is to establish possible correlations between the chondrules' formation times and their petrographic and chemical characteristics.

Marion Girault-Rime

Notes :

1. Centre de recherches pétrographiques et géochimiques (CNRS).
2. J. Villeneuve et al., “Homogeneous distribution of 26Al in the Solar System from the Mg isotopic composition of chondrules,” Science, 2009. 325: 985-8.

Contacts :

Johan Villeneuve,
CRPG, Nancy.
johanv@crpg.cnrs-nancy.fr


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