Premium
A TEM study of exsolution in Ca‐rich pyroxenes from the Paris and Renazzo chondrites: Determination of type I chondrule cooling rates
Author(s) -
Cuvillier Priscille,
Chaumard Noël,
Leroux Hugues,
Zanda Brigitte,
Hewins Roger H.,
Jacob Damien,
Devouard Bertrand
Publication year - 2018
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.13032
Subject(s) - chondrule , augite , pyroxene , microstructure , chondrite , geology , mineralogy , crystallization , plagioclase , texture (cosmology) , materials science , olivine , analytical chemistry (journal) , meteorite , quartz , chemistry , astrobiology , composite material , thermodynamics , physics , paleontology , image (mathematics) , chromatography , artificial intelligence , computer science
Abstract 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 Wo 40 , a modulated texture on (100) and (001) is the dominant microstructure, while areas with compositions higher than Wo 40 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.