LASER ABLATION U-PB ZIRCON DATING BY TRIPLE QUAD ICP-MS; CAN MERCURY INTERFERENCE REMOVAL LEAD TO MORE PRECISE AGES: AN EXAMPLE FROM THE ADIRONDACK HIGHLANDS, NY

The Adirondack Mountains in upstate New York are polymetamorphic rocks that formed during the stages of the Grenville Orogeny where major orogenetic events are separated by 50-100 Ma, so a minimum precision of 2-3 % on absolute dates of 1 Ga is required to resolve the difference between these events. For this study Adirondack zircons were collected from previously dated outcrops (Seleck et al. 2005, Wong et al. 2012) due to their size, abundance and U content, and to allow comparison with SHRIMP ages. Greater precision would increase our ability to make meaningful interpretations about the duration of metamorphic events. One potential way to increase precision on quadrupole instruments with low mass resolution is to avoid the Hg interference on ²⁰⁴Pb by reacting it away with ammonia gas in a collision cell.

In this study we test Hg interference removal using "triple-quad" ICP-MS. Having a quadrupole mass analyzer before and after the reaction cell limits the masses entering the cell to only 204, and those exiting the cell, again to only 204, completely avoiding the measurement of molecular reaction products. All work was performed using the Agilent 8900 coupled with the solid state NdYAG Cetac 213 at Union College. Tests were initially performed in liquid mode in a solution of 1 ppb Hg, Pb, Th and U. NH3 gas flow was adjusted to reduce the ²⁰²Hg, while still maintaining a large Pb signal. When optimized (at 8% flow rate) ²⁰²Hg counts were present at 0.3 % of their no gas levels, while Pb recovery on masses 206 and 207 were 100%. Corrected ²⁰⁴Pb values also show 100% of the Pb makes it through the reaction cell, once the counts for the ²⁰⁴Hg have been subtracted. Uranium does decrease in sensitivity, showing counts which are 40% of those found when running in no gas mode. For ablated zircons, total ²³⁸U counts are reduced to 25% of their no gas values but, except for very low U grains, the standard deviation on individual ablations is the same with or without the cell gas. Counts at mass 204 drop by 50% when run in ammonia mode, and 206/204 ratios increase by a factor of 2, suggesting that the ammonia is making a real difference eliminating the Hg interference on the ²⁰⁴Pb. For laser ablation dating, precision is the main factor limiting better resolution of age differences in the Grenville and other polymetamorphic terranes.