Chemical Fractionation in Chondrites by Aerodynamic Sorting of Chondritic Materials

Details Chemical Fractionation in Chondrites by Aerodynamic Sorting of Chondritic Materials Chemical Fractionation in Chondrites by Aerodynamic Sorting of Chondritic Materials

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.434s&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 434

Computer Science

3

Cais, Chondrites, Chondrules, Nebula

Scientific paper

Aerodynamic sorting in the nebula has been invoked directly or indirectly to account for the size variations of chondrules in different groups [1], associated size variations of chondrules and metal spherules in a CR chondrite [2], and variations in the oxygen isotopic compositions of H-L-LL chondrules and whole rocks [3]. We suggest that aerodynamic sorting processes affected the relative abundances of all chondritic ingredients and were therefore a major source of chemical differences between asteroids and perhaps planets [4]. For chondrites that were derived from the same batch of chondritic ingredients, e.g., ordinary chondrites, aerodynamic sorting may account for all chemical differences [5]. Matrix Material: Matrix material accretes into planetesimals largely in the form of rims on all particles rather than as individual dust grains [6,7]. Aerodynamic sorting of particles does not cause significant chemical variation in bulk matrix abundance or composition because rim composition is not correlated with particle composition [6,7], and rim thickness apparently correlates with particle radius [7]. Metal-Troilite Spherules: There are at least two metal-troilite components: poorly characterized spherules that are probably ejected during chondrule formation and fine-grained material associated with matrix rims. Skinner and Leenhouts [2] suggest that aerodynamic sorting of the spherules was a potent metal-silicate fractionation process. Our preliminary data for metal-troilite spherules in Lance (CO3) support their model. Spherules and chondrules are closer in size than in the CR chondrite they studied, but this may result from the very much higher troilite abundance in CO chondrites, which produced a smaller density difference between chondrules and spherules. But we cannot exclude the possibility that the size distribution of metallic spherules was controlled by that of the chondrules from which they were ejected and not by aerodynamic sorting of spherules. Matrix rims on spherules preclude the possibility that the spherules were released from chondrules during parent body impacts. Refractory Inclusions: The similarity in the order of chondrule and CAI sizes suggests a common size sorting process. Chondrules decrease in mean size in the order CV (1.0 mm) > CM (0.3 mm) > CO (0.15 mm) > ALH85085 (0.02 mm) [8]. Refractory inclusions decrease in size in the order CV (commonly >1 cm) > CO > CM (<1 mm) > ALH85085 (0.03 mm) [8,9]. Although many chondrules and CAIs in ALH85085 are broken, mean sizes were not much bigger originally as the grain sizes of fragments are comparable to those of complete objects [6]. Size sorting must postdate CAI formation as these size relationships hold for all kinds of CAI that formed in very different ways. Density differences cannot account for the apparently larger mean size of CAIs relative to chondrules in CV3 chondrites. Model: We suggest that a common aerodynamic sorting process may have affected all ingredients during their fall toward the proto-Sun through nebula gas. This sorting probably occurred after chondrule formation and rim acquisition in a turbulent environment [7] and ended when planetesimal accretion halted inward motion. Intermittent turbulence may have cleared the midplane of chondritic materials to allow episodic accretion of distinctly different batches of material. This model and runaway accretion of planetesimals accounts for the correlation of chondrule size with metamorphic type in CO3 chondrites [8] and provides a mechanism for understanding chemical and isotopic variations within asteroids such as the ureilite parent body. References: [1] Rubin A. E. and Keil K. (1984) Meteoritics, 19, 135-143. [2] Skinner W. R. and Leenhouts J. M. (1993) LPSC XXIV, 1315-1316. [3] Clayton R. N. et al. (1991) GCA, 55, 2317-2337. [4] Scott E. R. D. and Newsom H. E. (1989) Z. Naturforsch., 44a, 924-934. [5] Haack H. and Scott E. R. D. (1993), this volume. [6] Scott E. R. D. et al. (1984) GCA, 48, 1741-1757. [7] Metzler K. et al. (1992) GCA, 56, 2873-2987. [8] Meteorites and The Early Solar System (J. F. Kerridge and M. S. Matthews, eds.), chapters 9.1 and 10.3. [9] Grossman J. N. et al. EPSL, 91, 33-54.

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