COMPARISON OF NEW LA-MC-ICPMS (U-PB) APATITE AGES TO SHRIMP-RG (U-PB) ZIRCON AGES FROM PLUTONIC ROCKS OF THE BENDELEBEN MOUNTAINS, SEWARD PENINSULA, ALASKA

New U-Pb apatite ages for pegmatite and gabbro samples from the Bendeleben Mountains have been collected and are compared to already documented zircon ages from the same rocks to provide more evidence of the use of apatites as a reliable age dating method using the U-Pb system. Though apatites have most commonly been used for low-temperature thermochronology studies using the (U-Th)/He system or fission-track analysis, the inherent difficulties, such as low U concentrations, high common lead, and lack of reliable standards are gradually being overcome for the U-Pb system, allowing for dating to a closure temperature of 450-550° C. Zircon dating was performed in 2009 using the Stanford University/USGS Sensitive High Resolution Ion Microprobe –Reverse Geometry (SHRIMP-RG) instrument. Apatite dating was performed in 2014 at the University of Arizona Laserchron Center using a Laser-Ablation Multicollector Inductively Coupled Plasma Mass Spectrometer (LA-MC-ICPMS). Zircons yielded concordia ages of 86.1 +/- 0.3 Ma for a pegmatite sample (09BEN20) and 82.8 +/- 0.5 Ma for a gabbro sample (09BEN61) using the SHRIMP-RG. Apatites dated on the LA-MC-ICPMS yielded concordia ages of 85.4 +/- 1.0 Ma and 81.4 +/-1.6 Ma respectively for the same rocks. Weighted mean average ages of zircons from the same rocks were 85.7 +/- 0.9 Ma for the pegmatite and 82.7 +/- 1.4 Ma for the gabbro. Weighted mean average ages of apatites from the same rocks were 85.1 +/- 0.8 Ma and 81.4 +/- 1.6 Ma respectively. When compared, all concordia ages and all weighted mean average ages were within error of each other despite the differences in mineralogy and analysis method. Importance of apatite mount preparation procedure was also exemplified in this study as each sample was run twice under different preparation procedures. In the first run, all apatites were cleaned with 2% HNO₃ and 1% HCl similar to procedures used for zircon mount preparation. Though this is customary for zircons, acid-etching of the softer apatites resulted in abnormal ages and noticeably higher error during analysis. Both were run twice as a result, once using acid-etched samples and once again using non-acid etched grains of the same samples. The ages reported above were from non-acid etched grains and were within error of zircon ages from the same samples collected on the SHRIMP-RG.