Compositional trends and cooling rates of group IVB iron meteorites |
| |
| Authors: | Kaare L Rasmussen Daniel J Malvin Vagn F Buchwald John T Wasson |
| |
| Institution: | 1. Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024, USA;2. Department of Metallurgy, Technical University of Denmark, 2800 Lyngby, Denmark |
| |
| Abstract: | Based on new neutron activation data for group IVB we find that log-element — log-Ni trends are best understood in terms of core formation and fractional crystallization. The limited compositional range found in group IVB seems to reflect the fact that, because of the low concentrations of S, P and C and the high concentration of Ni, values are nearer unity than are those in other magmatic groups. Mean volatile abundances in group IVB are much lower than those found in any group of chondritic meteorites, suggesting that these low abundances were not entirely the result of nebular processes, but that planetary outgassing was also involved.We calculated cooling rates on the basis of a computer simulation of the growth of kamacite crystals; these calculations are particularly straightforward for the high-Ni irons since no local bulk Ni enrichment is involved. We estimate a mean IVB cooling rate of 170–230 K/Ma, the lower values based on 20 K undercooling, the higher on no undercooling. There is no dependence of cooling rate on chemical composition. The mean cooling rate of the low-volatile groups IVB and IVA are both much higher than those typical of iron-meteorite groups. This indicates small parent bodies, and reinforces the above suggestion that the low volatile contents resulted from planetary outgassing.There is a small compositional hiatus in group IVB, but since the sets on both sides of the hiatus form continuous trends on log-element — log-Ni diagrams and have the same cooling rates, it appears that both sets originated in a single oxidized, refractory-rich parent body. This sampling hiatus corresponds to 26% of the original core, a value shown to be typical for a random sequence sampled 11 times. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
