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

Sample #74459 contains Pb-204-rich Zircons which, when analyzed by ion microprobe, yield U/Pb ages that are exceptionally older than their P b-207/Pb-206 ages. The 64 ion microprobe measurements define an age of 2681 +/- 6 Ma (1 sigma) for the zircons, with individual analyses giv ing U-238/Pb-206 ages reaching 7.2 Ga. In order to verify such low U/P b ratios, isotope dilution analyses were conducted on 12 single,orains previously analyzed by SHRIMP: the IDTIMS measurements yielded radiog enic U-238/Pb-206 ratios between 2.0 and 8.5 whereas the 34 ion microp robe analyses on the same 12 grains gave systematically lower values b etween 0.6 and 2.5. The discrepancy between the SHRIMP and IDTIMS data has been traced to the labile Pb. By plotting the Pb-204 vs. Pb-206 c oncentrations observed by the ion microprobe, an unusually low Pb-204/ Pb-206 ratio of similar to 0.02 has been defined for the labile compon ent Apparently the secondary ion emission yield of the labile Pb was s ignificantly greater than that of in situ radiogenic Pb, thereby inval idating SHRIMP's interelement calibration and leading to inaccurate U/ Pb ages. These data provide important insights into the geochemistry o f 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 contai ned within amorphous microdomains which are intimately associated with crystalline zircon. Another feature of the labile Pb is its ubiquitou s presence in the zircons whereas it was not taken up by the coexistin g feldspar. This labile component has an age of similar to 0 Ma.