Experiments on the consolidation of chondrites and the formation of dense rims around chondrules
Introduction
Chondritic meteorites are the largest fraction of all meteorites making up more than 85% of the meteorites in our collections (Bischoff and Geiger, 1995). Age determination done by Amelin et al. (2002) has shown that the mm-sized spherical constituents, the chondrules, are only ∼2.5 Myr younger than the Ca- and Al-rich inclusions (CAIs), which are the first condensates of the Solar System. Because of their ubiquity and their formation time, the chondrites seem to be tightly connected to the coagulation phase of planetesimals whose formation conditions remain relatively unclear. The chondrules are making up between 0 and 80 vol.% of the chondrites and, thus, constitute one of their major fractions (Hewins et al., 2005). Other important constituents of chondrites are the matrix (0–100 vol.%), the opaque phases (0–70 vol.%), and the CAIs (0–3 vol.%) (Brearley and Jones, 1998, Zanda, 2004, Hezel et al., 2008, McSween, 1977).
Beitz et al. (2012b) have shown in laboratory experiments that chondrules covered with a thin dust rim tend to form large clusters very rapidly. Such a fast and/or local accretion process is in good agreement with the material complementarity of the chondrules, the dust rims, and the matrix, as reported in Palme et al. (1993). Beitz et al. (2012a) showed that a parent body formed by coagulation of chondrules and matrix material, while freely floating in the solar nebula, possesses a porosity which depends on its matrix fraction. However, the expected volume filling factors ϕ (defined as 1-porosity) are not consistent with values found in carbonaceous chondrites, which cover a range between ϕ = 0.58 and ϕ = 1.0 (Macke et al., 2011). Thus, Beitz et al. (2012a) concluded that additional compaction of the chondrite parent bodies is required.
Such a compaction process was suggested earlier by Trigo-Rodriguez et al. (2006) to explain the formation of high-density dust rims around chondrules that are found in several types of carbonaceous chondrites. The CM chondrites are famous for these fine-grained rims and all larger meteoritic constituents are surrounded by dust rims. Metzler et al. (1992) investigated the typical relation between the rim thickness and the chondrule diameter and found a linear relation with typical rim thickness of about 20% of the chondrule diameter. These fine-grained rims are typically denser than the surrounding matrix. This was investigated by Beitz et al. (2012a) for a fine-grained rim surrounding a chondrule in the CM2 chondrite Murchison using computer-aided tomography. They found a volume filling factor of the rim higher by Δϕ = 0.1 compared to the overall value of the Murchison chondrite (ϕ = 0.78, measured by Macke et al. (2011)). This enhanced (packing) density around the chondrule is in good agreement with the rim densities measured by Wilson et al. (1999) and also in the range of the rim volume filling factors of ϕ = 0.90–0.94 measured by Wasson (1995).
The aim of this new study is to investigate the degree of compaction and maximally achievable volume filling factor of different dust and chondrule-analog mixtures in a dynamic compaction process and to prove or disprove the formation of fine-grained overdense rims around the chondrules. High-velocity collisions occurred regularly during the planetesimal-formation process so that their impact on the porosity evolution of the growing planetesimals is interesting to study. Furthermore, we intend to derive the dynamic-pressure range in which such chondrule and dust mixtures were compacted and consolidated in the solar nebula to obtain the typical collisions velocities and constrain the processes responsible for the formation of planetesimals.
Section snippets
Experimental technique
We performed impact experiments with the basic idea to dynamically compress a cylindrical chondrule-dust sample of 2 cm diameter and up to 10 cm length by use of a fast aluminum projectile. The samples consisted of a mixture of micrometer-sized dust particles (as matrix analogs) and millimeter-sized solid beads (as chondrule analogs) and were aimed to represent a pre-chondrite body, i.e. a growing planetesimal. The cylindrical aluminum projectiles were accelerated in a powder gun to velocities of
Image analysis and data reduction
In this section, we describe the calculation of the volume filling factor from the reconstruction of the XRT images taken. In particular, we will describe the image processing and calibration procedure necessary to derive the density structure of the samples.
For the density reconstruction of the XRT slices, we first corrected for beam hardening. This was done by determining a radial intensity curve for a homogeneous sample, which only consisted of dust and for which no radial density
Results
In total, 46 impact experiments were performed in this study and 25 of these were successful and could be analyzed by the XRT method described above. We considered an experiment successful if the projectile hit the sample centrally and if the full intermixture of beads and dust was compressed to a solid body. In all other (unsuccessful) cases, either the projectile hit the inner wall of the nylon tube so that the pressure could not be derived (see below), or the dust-beads intermixture was
Discussion
The porosity of meteorites is an important parameter when studying the formation history of one of the oldest materials of the Solar System. Our measured porosities of the impact–compacted analog samples shall now be compared with the porosities of real chondrites. For this comparison, ordinary chondrites and carbonaceous chondrites were chosen. The ordinary chondrites were chosen because they make up most of the chondrites in our collections and therefore provide us with a huge amount of
Conclusions
Summarizing the above results, we found that the pressure, under which porous pre-chondritic bodies need to be compacted to reach their typical densities, ranges between ∼0.05 GPa and ∼2 GPa. This range is in a good agreement with the shock stage of chondrites and the fraction of intact chondrules. The maximum impact pressure, which we mainly used in this study, is independent of the projectile and target sizes and masses and, thus, only depends on the collision velocity and the initial
Acknowledgments
E.B. and J.B. thank the Deutsche Forschungsgemeinschaft (DFG) for support under Grant Bl 298/13-2 as part of the SPP 1385 “The first 10 Millon Years of the Solar System”. C.G. is grateful to the Japan Society for the Promotion of Science (JSPS) for the funding. We thank Akira Shimada from the Department of Earth and Planetary Sciences (Kyoto University) and Stephan Olliges from the Institut für Partikeltechnik (TU Braunschweig) for providing us with the X-ray tomography measurements. We also
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