A PIXE, EPMA AND SIMS STUDY OF THE CHAINPUR METEORITE - SMALL GRAINS OF LEAD FOUND IN A CHONDRULE

Chondrules in the Chainpur meteorite were studied to determine trace-e lemental sitings by means of particle induced X-ray emission (PIXE). O ne of those, a porphyritic pyroxene-type chondrule, revealed to have a n extremely high concentration of lead. The lead concentration tends t o be very localized around a spot of about 70 mu m with a maximum conc entration of 5 wt%. Further studies of the same sample revealed other spots of this kind in another type of chondrule.The porphyritic chondr ule was also studied with an electron probe to obtain a better picture of this strange inclusion. This measurement showed that the lead is c oncentrated in even smaller grains of about 2-4 mu m in size, incorpor ated in a glassy groundmass together with enstatite grains. Lead conce ntrations in these grains are about 50 wt%. Secondary ion mass spectro metry (SIMS) and additional PIXE measurements were done to exclude ter restrial origin of the lead compound. Industrial lead contains up to 6 wt% antimony, but none was found here with PIXE. Uranium and thorium were not found either above the detection limits of 5 and 18 ppm, resp ectively. The isotopic ratios of the lead isotopes are significantly d ifferent from those of common lead. Very convincing is the fact that t he isotopic ratios fall in the same ball park as the ratios found in o ther meteorites of the same kind. This very strongly suggests that con tamination can be ruled out. Assuming the single-stage model to be val id for this kind of meteorite, the ratio mu = Pb-204/U-238 is determin ed from the Pb-207/Pb-204 versus Pb-206/Pb-204 diagram and found to be around 6.5. Because no uranium was found in concentrations suggested by this ratio, it is likely that a recent metamorphic event separated the lead from its source region. A probable source region could be the troilite-rich rim which surrounds the chondrule. A candidate for the recent metamorphic event is the collisional break-up of the parent bod y of the meteorite, roughly 500-650 Myr ago.