PIXE ANALYSES OF OLIVINE GRAINS IN SEMARKONA - MICRODISTRIBUTION AND CORRELATION OF TRACE-ELEMENTS

Proton-induced X-ray emission (PIXE) has been used to determine the concentrations of Ca, Ti, V, Cr. Mn, Fe, Ni, and Zn in isolated olivine grains and olivines within chondrules of the Semarkona meteorite (LL3.0). Measurements were performed on a section approximately 7 mum thick with a lateral resolution of 2 mum x 3 mum. Almost all grains show higher concentrations of Ca, Ti, V, Cr, Mn, Fe, and Ni at the grain boundaries relative to the grain cores. Large (greater-than-or-equal-to 1 mm) isolated grains have flat concentration profiles compared to olivine grains from chondrules; however, maxima in the concentration profiles of mobile elements near the boundaries were characteristic for isolated grains and were not observed in olivines within chondrules. We propose that the distribution of mobile elements in isolated grains has been modified by a short heating event. Model calculations for zoned and uniform initial profiles are presented. A good qualitative agreement between calculated and observed profiles enables an estimation of 1100-1800 K for the maximal temperature and a timescale of 1-1000 years for the diffusion process. The shapes of the initial distributions remain an open question.