Phosphorus -- A potential low-temperature cooling-rate indicator

Details Phosphorus -- A potential low-temperature cooling-rate indicator Phosphorus -- A potential low-temperature cooling-rate indicator

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.542u&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 542

Other

Cooling, Low Temperature, Metallography, Meteoritic Composition, Phosphorus, Diffusion, Kamacite, Lamella (Metallurgy), Schreibersite, Temperature Effects

Scientific paper

Most metallographical cooling rate determinations are based on Ni-diffusion profiles in taenite and will typically reflect the cooling rate in the temperature range 1000-700 K. One alternative method is based on the Ni-depleted Agrell zone in kamacite adjacent to taenite lamellae. Another method relies on the size and Ni content of exsolved schreibersite, rhabdite, which reflects the cooling rate at approximately 600 K in very slowly cooled meteorites. The distribution coefficient of Ni between schreibersite and kamacite depends strongly on temperature varying from 1 at 1200 K to more than 8 below 600 K. Partial redistribution of Ni with decreasing temperature leads to Ni-depletion profiles in kamacite adjacent to schrebersite. This method may provide a useful cooling-rate indicator in Ni-poor meteorites where taenite is scarce or absent, such as the hexahedrites or the coarsest octahedrites. The diffusivity of P in kamacite is much higher than that of Ni. Under similar circumstances, P diffusion profiles will record the cooling rate at lower temperatures than Ni profiles. Cooling rates based on P diffusion may thus provide an important supplement to Ni-based cooling rates. Low-temperature cooling rates may reveal whether the meteorite parent body cooled undisturbed to low temperature or rapidly after disruption at some low temperature. Santa Luzia, a P-rich member of the Ni-poor group IIAB was investigated. Our data on Santa Luzia allow us to compare cooling rates estimated by the established methods with the new methods based on Ni- and P-diffusion profiles around schreibersite.

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