HIGH-PRECISION PB-207 PB-206 ZIRCON GEOCHRONOLOGY USING A SMALL ION MICROPROBE

A small geometry ion microprobe (Cameca ims 4f) has been successfully used for the routine measurement of Pb-207/Pb-206 ratios in individual zircons with a spatial resolution of less-than-or-equal-to 20 mum. In spite of the lower sensitivity of the small ion microprobe, resulting from operating at high mass resolution (M/DELTAM > 4600) in order to resolve isobaric molecular interferences, we can discern Archean compo nents whose ages differ by greater-than-or-equal-to 50 Ma. A single me asurement requires 90 min of data acquisition; thus, a comprehensive s tudy of a zircon population can be accomplished in a few days. Three s ets of Precambrian zircons, analyzed earlier by other techniques, yiel ded ages which are within +/-2 sigma of their ''true'' values. As our first ''unknown'' we analyzed zircons from a sample of orthogneiss col lected from the Aravalli Craton of northwestern India. For this sample , we obtained a minimum age of 3281 +/- 3 Ma (1 sigma), which we belie ve closely approximates the time of crystallization of the gneiss's ig neous protolith.