COPPER, ZINC, GALLIUM AND GERMANIUM DISTRIBUTIONS IN TAENITE LAMELLAE OF IRON-METEORITES AND THEIR IMPORTANCE FOR COOLING RATE ESTIMATIONS

One method for estimating cooling rates of meteorite parent bodies is to model measured nickel distributions in taenite lamellae of meteorites. Goldstein and Ogilvie (Geochim. Cosmochim. Acta 29, 893, 1965) and Rasmussen (Icarus 45, 564, 1981) developed techniques based on this idea to examine the cooling history in the temperature range between similar to 700 and similar to 400 degrees C. As a result of Instrumental Neutron Activation Analysis (INAA) Rasmussen et al. (Meteoritics 23, 105, 1988) postulated that some trace elements would also be good cooling rate indicators. They argued that elements with distinct diffusion behavior are sensitive to different temperature ranges. The new Heidelberg proton microprobe uses the method of Proton Induced X-ray Emission (PIXE) for elemental analysis. This microprobe is an appropriate instrument to measure distributions of trace elements with a spatial resolution of 2 mu m. We demonstrated on the iron meteorites Cape York (Agpalilik), Toluca and Odessa that the elements copper, zinc, gallium and germanium imitate the profiles of nickel in taenite lamella. The interpretation of the Zn, Ga and Ge profiles leads to the conclusion that these elements undergo diffusion mechanisms comparable to those of Ni. The numerical simulation of Cu distributions with a simplified model points out that little new formation can be obtained about the cooling history of the meteorites by modelling Cu profiles. To simulate Zn,Ga or Ge distributions, the use of ternary phase diagrams is necessary.