Project 1.5: The chondrule formation process
Project 1.5: The chondrule formation process
Project 1.5 is led by Klaus Mezger and is the PhD project of Jan Hoffmann. The analytical work is performed in the isotope laboratory of the Institute of Geological Sciences, University Bern.
There are different approaches to study and try to understand the formation of planetary systems starting from a cloud of dust and gas. Information can be obtained by observation of exoplanetary systems in the making, numerical modelling of condensation and accretion processes from dust to planets or the direct study of material that is representative of the different stages of planetary system formation. The latter is only possible for our system because it is the only planetary system from which we have material that can be directly studied and analyzed in the laboratory.
Figure 1: Meteorite sample found in the Omani desert during the Omani-Swiss Meteorite Search Expeditions, January 2019. Cold and, in this case, hot deserts are preferred sampling areas for extraterrestrial material because of the little vegetation, dry climate and therefore good preservation of meteorites and different geological concentration mechanisms. The complete sample, broken into several fragments due to the impact and erosion, weight ca. 80 kg (Photo: top-Hammad Khan bottom-Malgorzata Sliz)
Chondrites are the most primitive rock samples from the Solar System available for direct study in the laboratory. They consist of spherules that were once molten and are surrounded by a fine grained matrix. The processes that led to the formation of chondrules and chondrites are not understood, but they seem to represent a key step in the formation of planets from a solar nebula. A single chondrite sample contains chondrules that are chemically, mineralogically and petrographically distinct.
Figure 2 Fragment of chondrite. The sample consists of chondrules of different size, texture, bulk chemical composition and mineralogy and they are surrounded by a fine grained matric.
Despite this diversity, the bulk composition of all chondrites is very similar as far as the refractory elements are concerned. The chemical homogeneity of bulk samples contrasts with the isotopic heterogeneity among the different chondrite groups. The processes that led to the formation of chondrules and chondrite are not understood, but they seem to represent a key step in the formation of planets from a solar nebula. The chondrule forming process was of short duration (ca. 1 Myr), but not the first accretion event in our solar system: chondrules emerged ca. 1.7 Ma after the formation of the first solids (e.g. CAIs) and first planetesimals.
Figure 3 Time interval of chondrule formation in Carbonaceous and Ordinary Chondrites. The two groups show similar time intervals for chondrule melting as determined with the 26Al/26Al chronometer. CR chondrites formed distinctly later (Pape et al., 2019).
Physical models have been proposed that suggest chondrule formation is due to collision of primitive or partially differentiated planetesimals while in contrast models based on element and isotope abundances would be more consistent with a process involving flash-heating and melting of dust aggregates in the disk. All these models need to satisfy the chemical and mineralogical observations made on chondrites and their different components. Several elements in various chondrite groups show isotopic variabilities that cannot be explained as a result of geological and/or cosmochemical processing, but are rather due to different nucleosynthetic origins. This indicates that different regions in the early Solar System had distinct isotope compositions, but the reasons for these differences remain enigmatic. By studying the stable isotope compositions of elements with a range of geo- and cosmochemical behaviors (Ca, Ba, Mo) in separated matrix and adjacent (ideally) individual chondrules of various types of chondrites we will evaluate chemical differentiation and possible (un)mixing processes during chondrule formation and subsequent chondrite formation.
Figure 4 Examples for chondrules from Ordinary Chondrites (Pape 2018)
- For which element is there isotope variability between matrix and chondrules?
- Which proposed chondrule formation model can be reconciled with the isotope data?
- Is the chondrule forming process triggered by planetesimal formation?
- Is there evidence for a late injection of material from a nearby supernova?
