Computer Science
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29..470h&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 470-471
Computer Science
3
Cooling, Histories, Metallography, Meteoritic Composition, Temperature Profiles, Depletion, Diffusion Coefficient, Nonlinearity, Nucleation, Phase Diagrams, Widmanstatten Structure
Scientific paper
The interior of most differentiated meteorite parent bodies cooled through the temperature range where the Widmanstatten pattern forms at a fairly constraint rate can be used to calculate cooling rates representative of the temperature range where the Widmanstatten pattern forms. For some meteorite parent bodies there is evidence that the thermal evolution was significantly nonlinear. The thermal histories between 1000 and 600 K are deduced based on the Widmanstatten pattern. The alpha bandwidth can be used to determine cooling rates if alpha bands unaffected by impingement can be located. Alpha bandwidth cooling rates, where applicable, define the cooling rate at temperatures above 800 K. Metallographic cooling rates are, in most cases, based on a suite of measurements of the total width (TW) and the mid-profile Ni concentration (MPC) of taenite lamellae. Data should plot along isocooling rate curves in MPC-TW diagrams. The MPC of wide taenite lamellae cannot be changed at low temperatures and are primarily sensitive to the cooling rate at high temperature. In contrast, narrow lamellae equilibrated at low temperature are sensitive to cooling rates at low temperatures. In either case the data points will not plot along isocooling curves in MPC-TW diagrams, and deviations from the isocooling rate curves can be used to infer the cooling rate as a function of temperature. Ni depletion of kamacite close to the kamacite-taenite phase boundary can also be used to estimate cooling rates. The shape and width of the depletion profile is a function of the cooling rate, forming at temperatures below 770 K. It was suggested that the size of the island phase in the cloudy zone and the Mossbauer parameters are low-temperature cooling-rate indicators. Cooling rates based on schreibersite growth were also used to determine cooling rates down to around 520 K depending on the nucleation temperature of the individual schreibersites. These methods are based on the growth of different phases making it difficult to infer relatively small decreases in cooling rate.
Haack Henning
Rasmussen Lars K.
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