The isotopic composition of zirconium in terrestrial and extraterrestrial samples: implications for extinct 92Nb

Since Nb and Zr are only little fractionated during magmatic processes, the 92Nb-92Zr relative chronometer has the potential of dating the formation of the planetary bodies through their differentiation. The isotopic composition of zirconium in lunar, meteoritic and old and recent terrestrial samples has been determined. No isotopic variation has been found. However, the 92Zr/90Zr ratio of 3.8Ga. old zircons from the Isua acid conglomerate is on the lower limit of the range of the standard measurements. If considered an anomaly, it would correspond to a +1.5x10-492Zr relative deficiency or to a 3.0x10-494Zr relative excess. The data constrain the 92Nb/93Nb isotopic ratio at the time of formation of the solar system to be less than 0.007, so that the maximum sensitivity of the 92Nb-92Zr relative chronometer for the formation of planetary bodies is around 107a. (author)