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| Paper: |
The Chondrite Types and Their Origins |
| Volume: |
341, Chondrites and the Protoplanetary Disk |
| Page: |
953 |
| Authors: |
Wood, J.A. |
| Abstract: |
Recent advances in the dating of ferromagnesian chondrules are summarized, and the conclusion seems unavoidable (the author's earlier convictions notwithstanding) that chondrules comprising a given chondrite are the product of nebular activity extending over a million years or more. Continuing chondrule-forming activity, probably successive shock events, can explain the non-solar major element chemical compositions of the ordinary chondrites (OC): they are the cumulative effect of repeated small changes in the local system composition that accompanied each chondrule- forming event. In particular Fe,Ni metal was increasingly lost from the system with time, presumably by incorporation in planetesimals (most of them unsampled) at the nebular midplane. Fe/Si in the system progressed smoothly with time through values appropriate to the H, L, and LL OC groups. The parent bodies of each of these groups were accreted in a relatively short time, during which Fe/Si was essentially constant, then accretion effectively ceased, probably because the parent body was perturbed into an inclined orbit and no longer spent much time in the chondrule-rich nebular midplane. The perturbations are probably ascribable to the same density waves (gravitational irregularities) that created chondrule-forming shocks. Implementation of this concept is also explored for carbonaceous and enstatite chondrites. Maintenance of the differences in composition of the various chondrite types, which formed at differing radial distances, requires that turbulent diffusion was not effective in the zone of chondrite formation. Instead periodic disturbances by the nebular density waves were probably responsible for remixing solids from the midplane zone into the body of the nebula. |
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