Academic Affiliation: Union College
Raised in St. Thomas, U.S.V.I. Brandt spent his childhood exploring and admiring the tropical environment around him. It was these island experiences that sparked his curiosity and admiration for the Earth and its natural processes. Brandt currently attends Union College in New York, where he is completing his Bachelor of Science in Geology with interests in pursuing a Master’s degree in petroleum geology. This summer Brandt worked on measuring apatite chemistry and its potential implications for (U-Th)/He dating.
Measuring apatite chemistry: Potential implications for apatite (U-Th)/He geochronology
Apatite (U-Th)/He dating is a well-accepted geochronology tool used to determine the cooling history of a region. To optimize this dating technique, factors such as radiation damage and annealing must be identified and taken into account when interpreting data. Previous (U-Th)/He and fission track research suggests that apatite chemistry (specifically Cl) influences radiation damage annealing temperatures, and thus makes apatite chemistry another factor to consider. Testing this hypothesis is required to ensure the robustness of apatite (U-Th)/He data interpretation. This study examines the influence of apatite chemistry by comparing (U-Th)/He dates of suspected chlorapatites to previously published dates from corresponding locations. Two regionally adjacent samples from the Canadian Shield and one from Mt. Powell in the Gore Range, Colorado were screened for high-grade apatites. Apatite chemistry was determined using both an electron microprobe and a LA-ICP-MS. (U-Th)/He dates were acquired using a quadrupole mass spectrometer to measure He content, and an ICP-MS to measure U and Th concentrations in dissolved apatite solutions. Dates obtained from Mt. Powell were bimodal, suggesting multiple populations of apatites; however, chemical data from Mt. Powell exhibited minimal amounts of Cl. Despite being regionally adjacent, the Ontario samples differed in both dates and chlorine content. The rrm0, an annealibility proxy that accounts for multiple chemical variables, of both Ontario and Mt. Powell samples yielded values below the average apatite. This indicates that both groups of samples are more resistant to radiation damage annealing that more typical fluorapatites. Evidence suggests that chlorine is not the only element affecting annealing rates. Perhaps other lattice substitutions play a crucial role in determining the annealibility of each individual grain. Future studies could benefit by comparing rrm0 values to (U-Th)/He dates of the same grain.