THErmal history Of reDuced chondrULEs in the early solar system – THEODULE

After the textural, mineralogical, and chemical characterization of our samples using numerous analytical techniques (CT-scan, EMP, SEM, and EBSD), two different methods will be used for the determination of cooling rates: LA-ICP-MS analyses of chondrule-decorating metal grains for one, and FIB sample preparation plus TEM for the other.

CT-scan analyses in Paris (CM), Renazzo (CR), and Isheyevo (CH/CB) indicate the presence of type I chondrule-rich areas surrounded by millimetre-sized metal lumps. CT-scan analyses of these areas are in progress.

Textural, mineralogical, and chemical characterization of our samples, measurement of zoning profiles in Cu and Ga using LA-ICP-MS, and FIB sections in Ca-rich pyroxenes for TEM examination.

Nothing at this stage.

Meteoritic chondrules are the record of widespread melting in the early solar system, either in the protoplanetary disk or associated with the formation of planets. Mechanisms proposed for chondrule formation include melting of silicate dust aggregates by gas shock waves, melting during passage of dust through bow shocks around planetary embryos, and melting during collisions of planetesimals. The thermal histories of such events are being modelled and we can compare them to measured chondrule cooling rates as a criterion for the chondrule formation mechanism. Most estimated chondrule cooling rates are based on oxidized (Type II) chondrules, because their ferroan olivine crystals are strongly zoned, and the zonation in Fe/Mg and minor elements can be simulated in crystallization experiments and diffusion calculations. These cooling rates, even though somewhat controversial, are generally assumed to apply to all chondrules, and are one of the main reasons for the support of the gas shock heating mechanism. However, Type II chondrules are abundant only in ordinary chondrites, which are associated with S-type asteroids and therefore probably formed in the inner solar system. The cooling history of reduced (Type I) chondrules, dominant in carbonaceous chondrites (related to C-type asteroids and comets) and therefore formed widely throughout the protoplanetary disk is thus most important, yet very poorly constrained, because their forsteritic olivine is unzoned. We propose to apply two new methods to the determination of the cooling rates of Type I chondrules, taking advantage of several state-of-the-art techniques.

We will study samples of the Paris CM chondrite and the Renazzo CR chondrite, both from the MNHN collection, together with the Isheyevo CH/CBb chondrite. Computer tomography (CT) scanning will be used to localize a number of Type I chondrules with metal grains on the surface. We will prepare serial polished sections to intersect features of interest, and characterize the chondrules by SDD-SEM, EMP, and EBSD. A representative number of chondrules containing Ca-rich pyroxenes and metal will be selected for further studies. Metal grains on the chondrule surfaces will be analyzed by LA-ICP-MS and the diffusion profiles of Cu and Ga will be used to determine cooling rates. From the same samples, we will prepare FIB sections of Ca-rich pyroxenes for TEM examination. The objective will be the study at the nanometer scale of diopside-pigeonite exsolutions that developed during cooling. The exsolution wavelength will be used to determine cooling rates. We will thus have two new and independent cooling rates for temperatures near 1200°C. Concordance of the rates would remove controversy about chondrule cooling history. The obtained cooling rates will be confronted to models, and thus allow to evaluate the heating mechanisms that prevailed throughout the disk in the early solar system.