Abstract

This study provides details of the textural and compositional changes in zircon as a result of heating from approximately 1400 to 1650°C under vacuum in a mass spectrometer during analysis by Pb-evaporation from a single zircon grain. Electron microprobe analyses of partially analyzed zircon grains show that the zircon reacts preferentially around the margins, along cracks, and around inclusions at low temperatures, whereas the more crystalline internal portions of the zircon react at higher temperatures. The reaction involves the breakdown of zircon to form a porous zirconium oxide with concomitant release of silica and other trace elements, including Pb, which are collected on a filament and subsequently ionized to measure the isotopic composition of the Pb. The reaction front migrates inwards from the margin of the zircon sample with increasing temperature, such that Pb from the most retentive sites in the crystal is released at the highest temperatures. These domains are the least likely to have undergone Pb loss, and thus the 207Pb/206Pb ratios obtained at these remperarures are considered to represent best the age of the zircon. The Hf isotopic composition of zircon cannot be obtained using the evaporation technique because of uncorrectable mass interferences with 176Hf. The use of this technique to determine the 207Pb/206Pb age of other U-bearing minerals such as titanite, baddeleyite, and apatite is complicated by different breakdown reactions within these minerals, especially the lack of silica, which is needed for successful analysis of the isotopic composition of the Pb.

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