A PIXE, EPMA and SIMS study of the Chainpur meteorite: small grains of lead found in a chondrule

Abstract

Chondrules in the Chainpur meteorite were studied to determine trace-elemental sitings by means of particle induced X-ray emission (PIXE). One of those, a porphyritic pyroxene-type chondrule, revealed to have an extremely high concentration of lead. The lead concentration tends to be very localized around a spot of about 70 μm with a maximum concentration of 5 wt%. Further studies of the same sample revealed other spots of this kind in another type of chondrule. The porphyritic chondrule was also studied with an electron probe to obtain a better picture of this strange inclusion. This measurement showed that the lead is concentrated in even smaller grains of about 2–4 μm in size, incorporated in a glassy groundmass together with enstatite grains. Lead concentrations in these grains are about 50 wt%. Secondary ion mass spectrometry (SIMS) and additional PIXE measurements were done to exclude terrestrial 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, respectively. The isotopic ratios of the lead isotopes are significantly different from those of common lead. Very convincing is the fact that the isotopic ratios fall in the same ball park as the ratios found in other meteorites of the same kind. This very strongly suggests that contamination can be ruled out. Assuming the single-stage model to be valid for this kind of meteorite, the ratio μ = 204Pb 238 U is determined from the 207Pb 204Pb versus 206Pb 204Pb 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 body of the meteorite, roughly 500–650 Myr ago.