A TEM study of exsolution in Ca-rich pyroxenes from the Paris and Renazzo chondrites: Determination of type I chondrule cooling rates

1Priscille Cuvillier,2,3Noël Chaumard,1Hugues Leroux,2,4,5Brigitte Zanda,2,4Roger H. Hewins1Damien Jacob,6Bertrand Devouard
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13032]
1Unité Matériaux et Transformations, Université Lille 1 and CNRS, Villeneuve d’Ascq, France
2Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Université, Muséum National d’Histoire Naturelle, UPMC Université Paris 06, IRD & CNRS, Paris, France
3WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin, USA
4Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
5Institut de Mécanique Céleste et de Calcul des Ephémérides, Observatoire de Paris, Paris Cedex, France
6Aix-Marseille Université, CNRS, IRD, CEREGE UM34, Aix en Provence, France
Published by arrangemenr with John Wiley & Sons

We conducted a transmission electron microscope study of the exsolution microstructures of Ca-rich pyroxenes in type I chondrules from the Paris CM and Renazzo CR carbonaceous chondrites in order to provide better constraints on the cooling history of type I chondrules. Our study shows a high variability of composition in the augite grains at a submicrometer scale, reflecting nonequilibrium crystallization. The microstructure is closely related to the local composition and is thus variable inside augite grains. For compositions inside the pyroxene miscibility gap, with a wollastonite (Wo) content typically below 40 mole%, the augite grains contain abundant exsolution lamellae on (001). For grain areas with composition close to Wo40, a modulated texture on (100) and (001) is the dominant microstructure, while areas with compositions higher than Wo40 do not show any exsolution microstructure development. To estimate the cooling rate, we used the spacing of the exsolution lamellae on (001), for which the growth is diffusion controlled and thus sensitive to the cooling rate. Despite the relatively homogeneous microstructures of augite grains with Wo < 35 mole%, our study of four chondrules suggests a range of cooling rates from ~10 to ~1000 °C h−1, within the temperature interval 1200–1350 °C. These cooling rates are comparable to those of type II chondrules, i.e., 1–1000 °C h−1. We conclude that the formation of type I and II chondrules in the proto-solar nebula was the result of a common mechanism.

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