Coarse-grained chondrule rims in type 3 chondrites |
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| Authors: | Alan E Rubin |
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| Institution: | Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024 USA |
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| Abstract: | Relatively coarse-grained rims occur around all types of chondrules in type 3 carbonaceous and ordinary chondrites. Those in H-L-LL3 chondrites are composed primarily of olivine and low-Ca pyroxene; those in CV3 chondrites contain much less low-Ca pyroxene. Average grain sizes range from ～4 μm in H-L-LL3 chondrites to ～10 μm in CV3 chondrites. Such rims surround ～50%, ～10% and ≤ 1% of chondrules in CV3, H-L-LL3 and CO3 chondrites, respectively, but are rare (≤1%) around CV3 Ca,Al-rich inclusions. Rim thicknesses average ～150 μm in H-L-LL3 chondrites and ～400 μm in CV3 chondrites.The rims in H-L-LL3 chondrites are composed of material very similar to that which comprises darkzoned chondrules and recrysiallized matrix. Dark-zoned chondrules and coarse-grained rims probably formed in the solar nebula from clumps of opaque matrix material heated to sub-solidus to sub-liquidus temperatures during chondrule formation. Mechanisms capable of completely melting some material while only sintering other material require steep thermal gradients; suitable processes are lightning, reconnecting magnetic field lines and, possibly, aerodynamic drag heating.CV chondrites may have formed in a region where the chondrule formation mechanism was less efficient, probably at greater solar distances than the ordinary chondrites. The lesser efficiency of heating could be responsible for the greater abundance of coarse-grained rims around CV chondrules. Alternatively, CV chondrules may have suffered fewer particle collisions prior to agglomeration. |
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