Improved U-Th-total Pb dating of zircons by electron microprobe using a simple new background modeling procedure and Ca as a chemical criterion of fluid-induced U-Th-Pb discordance in zircon

We present a simple new background modeling method particularly for the det ermination of trace concentrations of Pb, U and Th to date zircons chemical ly by electron microprobe. This method reduces the acquisition time conside rably, and at the same time increases the precision of a single Pb, U and T h analysis. Depending on the analytical conditions, theoretical detection l imits down to 45 ppm (99% confidence level) for U, Th and Pb could be obtai ned. The final precision on age for a single measurement was between +/-8 a nd +/-35% (1 sigma) at Pb concentrations between 100 and 200 ppm in Protero zoic zircons. Precise U, Th and Pb analyses, however, do not necessarily fa cilitate a straightforward chemical age calculation. U-Th-total Pb data of zircons from four granites could not be objectively interpreted because of a severe disturbance of the U-Th-Pb system. However, it is shown that chemi cal ages derived only from zircon areas containing less than similar to 0.2 wt.% CaO agree very well with published conventional radiometric ages of t hese rocks. We propose a model which explains the Ca gain and Pb loss of zi rcons which have not undergone any metamorphism after emplacement to be the result of a hydration of radiation damaged zircon domains through low temp erature aqueous solutions. It is suggested that the Ca content can be used independently from U, Th and Pb analyses as a chemical criterion to discrim inate between altered zircon domains, having suffered a low temperature flu id-induced Pb-loss, and non-altered zircon domains for which there is a hig h probability that they will give concordant U-Th-Pb ages. (C) 2000 Elsevie r Science B.V. All rights reserved.