U-PB LASER ABLATION MC-ICP-MS ANALYSES OF APATITE AND ZIRCON FROM LEUCOGRANITES: CHALLENGES AND APPROACHES

Leucogranite dikes, sheets, and bulbous intrusions are ubiquitous in orogenic belts. They result from the partial melting of crustal rocks as a product of prograde metamorphism, decompression, and intrusion of higher temperature magmas. The presence of leucogranites in metamorphic environments and their cross-cutting relationships with metamorphic fabrics can be used to define deformational and thermal events by the absolute dating of their intrusions. However, the application of U-Pb laser ablation on either zircon or apatite has its challenges. Using U-Pb LA on zircons in leucogranites, we consistently run into problems such as: high common lead, a lack of abundance of zircon, high discordance, difficult morphology, and high uranium concentrations. However, this same method applied to apatite presents different problems such as: high common lead, limits on viewing morphology, high discordance, and low uranium concentrations; in addition to having a different closure temperature, making comparison with zircon data limited.

While the application of either of these methods is highly dependent on the amount of alteration and mineral abundance within a sample, a series of constraints can be applied to zircon and apatite, to mitigate variables, and deduce accurate and precise data. To constrain zircon data, we 1. Consider the relationship of age vs. discordance and only use grains that fall away from the trend lines. 2. Plot T-W diagrams for samples with high discordance to identify and constrain the 206Pb/204Pb ratio of common lead. 3. Select, if possible, grains that fall below the 204 cps from the FC1 standard value. 4. Calculate weighted mean ages using data above the generally accepted 3% discordance. 5. Consider the relationship of age vs. 204Pb and calculate an age intercept whenever there is a correlation, to compare with the weighted mean ages (Discordia). To accurately reduce apatite data, we 1. Apply a common lead correction (Stacey and Kramers, or Isochron). 2. Plot samples as either anchored or non-anchored Discordia diagrams. 3. Use 2 standards to check the validity of a Discordia line or common lead correction. 4. Use matrix matched standards, allowing for an increase in the power of the laser, and sensitivity of the collectors.