Constraining the Cooling Rates of Chondrules

Stockdale, Shannon; Franchi, Ian; Anand, Mahesh; Grady, Monica and Morgan, Dan (2017). Constraining the Cooling Rates of Chondrules. In: Workshop on Chondrules and the Protoplanetary Disk.



The cooling rate of chondrules is one of the important constraints on chondrule formation and can be used to distinguish between competing chondrule formation mechanisms. These mechanisms range from
shockwaves in the solar nebula to collisions between planetesimals. Dynamic crystallisation experiments are the most widely cited methods of determining chondrule cooling rates and have provided cooling rates from 1 to 3000 Kh-1. This is a very large range and in order to assess whether this is a true representation of chondrule cooling rates in the early solar system, it should be validated or constrained by using a more direct method.
Many type II, FeO-rich chondrules contain MgOrich relict olivine grains inherited from the precursor assemblage. Minor element concentrations of these grains bear a strong relationship to type I chondrules and therefore they likely originate from previous generations of these chondrules. These are important features as they can allow us to determine the cooling rate of their hosts.
During sub-solidus cooling, partial equilibration occurs between the relict grain and overgrowth crystal which creates diffusion profiles for a range of major, minor and trace elements. The amount of re-equilibration is dependent upon the cooling rate of the chondrule. Relatively broad diffusion profiles could indicate slower cooling rates as more equilibration has occurred. Narrow diffusion profiles should indicate faster cooling
rates as there was less time for equilibration.

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