Cooling rates of type I chondrules from Renazzo: Implications for chondrule formation

1,2Noël Chaumard,3Munir Humayun,1Brigitte Zanda,1,4Roger H. Hewins
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13040]
1Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Sorbonne Universités, Muséum national d’histoire naturelle, UPMC Université Paris 06, UMR CNRS 7590, IRD, UMR 206, Paris Cedex 05, France
2Department of Geoscience, WiscSIMS, University of Wisconsin-Madison, Madison, Wisconsin, USA
3Department of Earth, Ocean & Atmospheric Science, and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida, USA
4Department of Earth & Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
Published by arrangement with John Wiley & Sons

Cooling rates are one of the few fundamental constraints on models of chondrule formation. In this study, we used Cu and Ga diffusion profiles in metal grains to determine the cooling rates of type I chondrules in the Renazzo CR2 chondrite. To improve previous estimations of cooling rates obtained using this method, we used CT scanning and serial polishing of our sections to analyze equatorial sections of large metal grains. Through the cores of these metal grains situated at the surface of chondrules, the cooling rates calculated range from 21 to 86 K h−1 for a peak temperature Tp ~ 1623–1673 K. A metal grain embedded in the core of a chondrule exhibits a cooling rate of 1.2 K h−1 at a Tp ~ 1573 K. We also measured Cu-Ga diffusion profiles from nonequatorial sections of metal grains and calculated a lower range of cooling rates of 15–69 K h−1 for Tp ~ 1473–1603 K compared to our results from equatorial sections. The high cooling rates inferred from the lightning model (several thousand K h−1) are clearly at odds with the values obtained in this work. The X-wind model predicts cooling rates (~6–10 K h−1) lower than most of our results. The cooling rates calculated here are in close agreement with those inferred from shock wave models, in particular for temperatures at which olivine crystallizes (from ~10 to several hundreds K h−1 between 1900 and 1500 K). However, the chemical compositions of metal grains in Renazzo are consistent with the splashing model, in which a spray of metal droplets originated from a partially molten planetesimal. Volatile siderophile element depletion is explained by evaporation before metal was engulfed within silicate droplets. Liquid metal isolated from the liquid silicate crystallized during cooling, reacted with the ambient gas, and then re-accreted within partially molten chondrules.

Discuss

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s