True (3D) chondrule size distributions: Basic data for the reconstruction of chondrule formation and asteroid accretion

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The asteroid belt between Mars and Jupiter is dominated by chondritic asteroids which mainly consist of chondrules, i.e. mm-sized beads of crystallized silicate melt. Components of such chondrules have also been found in comet Wild 2. This indicates that they were not only an important building block of planetary objects in the inner solar system, but also may have considerably contributed to the buildup of outer solar system bodies. They formed around 2±2 million years after the formation of the first solids in the protoplanetary disk in the course of unknown transient heating events. Obviously, chondrule formation was a very characteristic, nonetheless still enigmatic, process which transiently transformed vast amounts of material into melt droplets.High precision data on the mean chondrule size and chondrule size distributions are needed by astrophysicists in order to develop models on chondrule formation, chondrule size sorting, and asteroid accretion. However, a recently published literature survey reveals that large knowledge gaps concerning these parameters exist. Most available chondrule data were obtained from two-dimensional (2D) cut faces and do not reflect the true 3D chondrule sizes. Unfortunately, there is no reliable correction procedure at hand which is able to transform 2D into 3D data. Hence, more than 200 years after the discovery of chondrules we do not have a comprehensive and consistent database at hand for these important parameters.The measurement of these parameters is one central aspect of this research proposal and it is planned to establish a comprehensive data base for all major chondrite groups (H, L, LL, R, EH, EL, CV, CK, CR, CO, CM). These data shall be obtained by using high resolution X-ray computed tomography (µ-CT) techniques, available at the Technical University of Dortmund.In cooperation with astrophysicist from the University of Lund (Sweden) it is planned to use the newly obtained data to refine a model of asteroid formation by chondrule accretion on surfaces of preexisting planetesimals. Furthermore, these data shall be used to search for correlations between the individual chondrule size and other petrologic, chemical, and isotopic parameters, namely chondrule type, oxygen isotopic composition, and thickness of accretionary dust mantles.