X-Ray Fluorescence Microprobe Measurements of Trace Elements in Kamacite in Octahedrites

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Kamacite compositions have been determined using the X-ray microprobe (XRM) at the beamline X26A at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, USA. The XRM uses synchrotron radiation and has been used to study trace element chemistry of various materials (e.g., Sutton et al. 1987; Flynn and Sutton, 1992). The XRM has detection limits near 1 ppm in tens of micrometers^2 for many elements. The concentrations of Fe, Co, Ni, Cu, Ga, Ge, and As have been measured in different generations of kamacite lamellae in the iron meteorites Cape York (IIIAB), Buenaventura (IIIAB), Odessa (IAB), and Toluca (IAB). In the Cape York meteorite the differences in concentrations of Ni between earlier and later generations of kamacite lamellae (Rasmussen, 1989) are found to be less than 1%. It is conceivable that nucleation-controlled differences between different generations of kamacite lamellae will result in differences in trace element contents of the kamacites. An alternative explanation could, however, be that the diffusion-induced disequilibrium profiles are present in the kamacite lamellae, and that these are dominant. Presently we have too few data to distinguish between these two scenarios. Measurements of trace element partitioning between taenite and kamacite with the PIXE-probe have shown that other elements besides Ni might be used for cooling rate determination, provided their diffusion speeds and phase diagrams are investigated (Bajt and Rasmussen, 1991). Precise measurements of kamacite composition might reveal at what temperatures the kamacites nucleated, and help establish the high-temperature thermal history of octahedrites. References: Sutton S. R., Delaney J. S., Smith J. V., and Prinz M. (1987) Geochim. Cosmochim. Acta. 51. 2653-2662. Flynn G. J. and Sutton S. R. (1992) Proc. Lunar Planet. Sci. Vol. 22, pp. 171-184. Rasmussen K. L. (1989) Icarus. 80, 315-325. Bajt S. and Rasmussen K. L. (1991) Meteoritics. (abstract) 26, 314.

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