Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.312z&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 312
Computer Science
1
Scientific paper
The calcium content in lherzolitic olivines is known to be pressure and temperature (Kohler and Brey 1990) dependent. Application of Ca-zoning for determining cooling rates of meteorites is facilitated by the absence of pressure effects (Kohler et al. 1991). Since kamacite-taenite exsolution lamellae and Ca-zoning in olivine are formed within the same temperature range (650-400 degrees C), both methods can be compared. Extrapolation of high temperature Ca-diffusion coefficients to these low temperatures yields higher cooling rates (2 to 3 orders of magnitude) than metallographic cooling rates. We estimated low temperature Ca-diffusion coefficients from a comparison of Ca- zoning in Landes olivine and corresponding metallographic cooling rates, derived from metal grains in Landes silicate inclusions (30 degrees C+-10/10^6 y) (Herpfer and Larimer 1992). Preliminary data showed Ca-zoning in olivines of silicate inclusions in Caddo County (IAB) (Palme et al. 1991). Further Ca-profiles in Caddo County olivines were measured by SIMS (Caltech) to obtain a better understanding of Ca-zoning. All analyzed olivine grains are zoned from ~120 ppm Ca in the core to 60-50 ppm at the rim. Small grains have lower core contents. Corresponding to the highest Ca-cores the maximum temperature recorded in Caddo County olivine is 630 degrees C, about 250 degrees C lower than the 2- px-temperature. Several Ca-profiles were determined on a single olivine grain (ol 2) with known crystallographic orientation. The following conclusions can be drawn from our study: 1. Identical Ca-zoning patterns for all profiles in ol 2, reflecting similar low temperature Ca-diffusion coefficients for different crystallographic directions. This allows to calculate cooling rates from zonation profiles of randomly oriented olivine grains. 2. Ca-zoning is independent of the adjacent minerals (see Fig. 1). Ca-profiles at the ol-cpx, ol-ol, ol-plag and ol- metal contact are identical. Grain boundary diffusion is apparently so fast that ol-cpx equilibration can be maintained even if no cpx is in contact to olivine. 3. Calculated diffusion profiles can be fitted to the measured profiles (3) Confirming that Ca-zonation is the result of a diffusion controlled process. With the adapted diffusion coefficient a cooling rate between 10 and 50 degrees C/10^6 y is obtained for Caddo County silicate inclusions. A slight decrease of fayalite content towards the rim was observed in ol 2. This zoning is less pronounced than Ca- zoning and appears to be restricted to the ol-metal border, presumably reflecting a reduction process. In summary, we have demonstrated the feasibility for using Ca-zoning in olivine as a cooling rate indicator in IAB-silicate inclusions. The resolution appears to be somewhat lower than that of metallographic cooling rates. Low temperature Ca-diffusion data in olivine are required to establish Ca-diffusion as an independent method for cooling rate determination at low temperature in metal bearing as well as metal free systems. References Kohler T.P. and Brey G.P. (1990) Geochim. Cosmochim. Acta. 54. 2375-2388. Kohler T., Palme H., and Brey G. (1991) N. Jb. Miner. Mh. 9. 423-431. Herpfer M.A. and Larimer J.W. (1992) submitted to GCA. Palme H., Hutcheon I.D., Kennedy A.K., Sheng Y.J. and Spettel B. (1991) Lunar Planet. Sci. (abstract) 22, 1015-1016. Figure 1, which in the hard copy appears here, shows Ca zoning in olivine (SIMS) for Caddo County.
Hutcheon Ian D.
Kennedy Allen K.
Palme Herbert
Zipfel Jutta
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