The determination of metallographic cooling rates of iron meteorites are presently based on measurements and modeling of disequilibrium profiles of Ni in taenite lamellae. Key elements of such models are the FeNiP phase diagram (in case of IABs also the FeNiC phase diagram) and the diffusivities of Ni in both kamacite and taenite. Until 1995 the data have been interpreted in terms of a linear cooling history yielding a constant cooling rate in the Widmanstatten pattern formation interval (-800 degrees - 300 degrees C). Rasmussen et al. provide the first attempt to estimate a two-stage cooling history. Thus the obtained cooling rates only apply in a limited temperature interval. We have initiated a search for alternatives to Ni as a cooling rate indicator that could yield cooling rates of iron meteorites in different temperature intervals. The idea is that elements with a strong partition between taenite and kamacite and a diffusivity significantly different from that of Ni will become immobilized at different temperatures. Disequilibrium profiles of these elements may therefore provide cooling rate information for other temperature intervals. We have applied, micro-PIXE analysis in the search for concentration profiles of Ge, Ga, and Cu in Toluca and Odessa (IAB), and Cape York (IIIAB).
Lunar and Planetary Science Conference
- Pub Date:
- March 1996
- COOLING RATES;
- METEORITES: IRON;