AN EXAMPLE OF REVERSE DISCORDANCE DURING ION MICROPROBE ZIRCON DATING: AN ARTIFACT OF ENHANCED ION YIELDS FROM A RADIOGENIC LABILE PB

Abstract

Sample #74459 contains 204Pb-rich zircons which, when analyzed by ion microprobe, yield U ages that are exceptionally older than their 207Pb206Pb ages. The 64 ion microprobe measurements define an age of 2681 +/- 6 Ma (1σ) for the zircons, with individual analyses giving 238U206Pb ages reaching 7.2 Ga. In order to verify such lowU ratios, isotope dilution analyses were conducted on 12 single grains previously analyzed by SHRIMP: the IDTIMS measurements yielded radiogenic238U206Pb ratios between 2.0 and 8.5 whereas the 34 ion microprobe analyses on the same 12 grains gave systematically lower values between 0.6 and 2.5.The discrepancy between the SHRIMP and IDTIMS data has been traced to the labile Pb. By plotting the 204Pb vs. 206Pb concentrations observed by the ion microprobe, an unusually low 204Pb206Pb ratio of #0.02 has been defined for the labile component. Apparently the secondary ion emission yield of the labile Pb we significantly greater than that of in situ radiogenic Pb, thereby invalidating SHRIMP's interelement calibration and leading to inaccurate U ages.These data provide important insights into the geochemistry of labile Pb; in #74459 the labile Pb was locally derived and was well mixed at the kilogram scale prior to becoming housed within the zircon grains. High-resolution TEM data suggest that the labile Pb is contained within amorphous microdomains which are intimately associated with crystalline zircon. Another feature of the labile Pb is its ubiquitous presence in the zircons whereas it was not taken up by the coexisting feldspar. This labile component has an age of #0 Ma.