International Geologiical Congress - Oslo 2008

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PIP-04 From gas and dust to planets

 

Dating the formation of CAIs and chondrules - evidence from Hf-W chronometry

 

Thorsten Kleine, ETH Zurich (Switzerland)
Christoph Burkhardt, ETH Zurich (Switzerland)
Bernard Bourdon, ETH Zurich (Switzerland)
 

 

Ca-Al-rich inclusions (CAIs) and chondrules are the major components of chondritic meteorites and are among the oldest solids formed in the solar system. The relative timing of their formation has been constrained using Al-Mg and Pb-Pb chronometry and both chronometers indicate that chondrules formed ∼2-3 Ma later than CAIs. Although this is well established, two critical issues remain unsolved. First, there is a debate about the absolute age of CAIs and second, it is not known if chondrules from ordinary and carbonaceous chondrites formed at the same time. To address these issues we obtained internal Hf-W isochrons for CAIs, chondrites and angrites. From these data we obtain an absolute age for CAIs of 4568.6 ±0.7 Ma, which is ∼1.5 Ma older than their Pb-Pb age. The latter thus does not seem to date CAI formation. The Hf-W age for the formation of H chondrite chondrules is 1.7±0.7 Ma, consistent with ∼2 Ma Al-Mg ages for L and LL chondrules. In contrast, chondrules from most carbonaceous chondrites formed more than ∼0.5 Ma later, as constrained by differences between their Pb-Pb ages and the Hf-W age for CAIs.

The chondrule formation ages are inversely correlated with peak metamorphic temperatures reached inside the parent bodies of each chondrite group. Thermal modelling indicates that this correlation is consistent with temperature increases caused by heating from 26Al decay. The different thermal histories of ordinary and carbonaceous chondrites therefore most likely reflects their different initial 26Al abundance. 26Al was sufficiently abundant in ordinary chondrite parent bodies to cause intense thermal metamorphism, whereas the later formed parent bodies of carbonaceous chondrites contained too little 26Al to cause significant heating and metamorphism.

 

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