LASER CUTTING OF ZIRCON FOR CA-TIMS GEOCHRONOLOGY: ADDING SPATIAL RESOLUTION TO HIGH-PRECISION DATES

U-Pb dates from zircon analyzed by chemical abrasion thermal ionization mass spectrometry (CA-TIMS) are highly precise, yet most lack spatial resolution because zircon is generally dissolved as whole grains (ideally polished and CL imaged) or less commonly as fragments from crudely broken grains. Poor spatial resolution results in inaccurate ages of geological events when dating grains with resolvable age heterogeneity, such as those with older antecrystic or xenocrystic cores or younger metamorphic rims, due to mixing of age domains. To add spatial resolution to CA-TIMS, I developed a sampling technique that extracts small, well-characterized domains from zircon wafers (30-40 μm thick) created by polishing opposite sides of grains. Both sides are imaged by CL and analyzed by LA-ICPMS for U-Pb dates and trace element compositions in numerous spots, allowing domains to be identified and projected through the wafer. Domains are isolated by cutting with a 6 μm wide laser beam and age homogeneity is assessed by dating several fragments per domain by CA-TIMS. The fragments are small, many are <0.5 μg and some <0.1 μg, which is similar to the mass of zircon ablated in a typical LA-ICPMS spot. Obtaining high-precision CA-TIMS dates from these fragments depends on several factors that are more critical than when dating whole grains, including low Pb and U blanks (<0.2 pg and 0.01 pg, respectively) and high ionization on the mass spectrometer. Use of this technique on three samples shows that accurate and precise ages of growth events can be obtained from zircon with complex core-rim relationships: (1) Hadean zircon from the Jack Hills, Australia; (2) Phanerozoic zircon from the Poplar Mountain Gneiss, Pelham Dome, Massachusetts; and (3) Jurassic zircon from the Carmacks Copper deposit, Yukon. This technique should be considered for zircon that requires high age and spatial resolution.