In this study, we test laser ablation zircon dating using the new Agilent 8900 ICP-MS at Union College. The advantage of this “triple-quad” ICP is the ability to introduce NH3 gas into the system after an initial mass filter, which can remove mercury interference from the signal. Mercury and lead both have isotopes with a mass of 204, however, the introduction of ammonia gas into the ICP drives the amount of mercury down, allowing us to assume that the 204 mass being counted is purely from lead, allowing for accurate U-Pb dating of zircons. Tests were done to determine what set of parameters in the ICP and laser ablation system created the most accurate counts of 204Pb, 206Pb, 207Pb, and 238U. Parameters tested included the flow rate of ammonia gas and helium gas in the ICP and LA system as well as the laser spot size, power, and burst duration, as well as raising the sample height in the ablation chamber. The practice of using “polishing shots”, a few low power large spot size laser shots to clean off the grain before fully ablating it, was also tested. It was determined that tuning the ICP in a liquid solution of 1 ppb Hg, Pb, Th and U first and then “hot-swapping”, connecting the LA system to the ICP while the plasma is still lit and tuned to the liquid parameters. It was found that a spot size of 50 mm, 80% laser energy with 300 shots and a lowered sample height gave the best results.
Adirondack zircons were chosen for their large size, relative abundance, U content and proximity to Union. The many orogenic events that created the mountains also provided an excellent, and well known, test subject for the ability of the new ICP system to accurately date differing ages for multiple orogenic events. Zircons were analyzed from multiple areas of the Adirondacks including the Marcy Anorthosite Massif, The Carthage-Colton Shear Zone, and the metasedimentary belt near Fort Ann. Previous dating studies by Seleck et al. (2005) and Wong et al. (2012) were used to compare and correlate ages found in the outcrops.
The zircons were first studied on the SEM for their backscatter electron and cathodoluminesence textures to help determine the morphology and allow for a better understanding of their histories. It was found that the zircons exhibited textures such as magmatic zoning, the presence of cores and rims as well the presence of unfeatured zircons for even dating. Grains ranged from rounded to fully euhedral and ranged from 75-300 mm.