T130 Monazite: The Ultimate Geologic Record

Monday, 10 October 2011
1:30 PM - 5:30 PM
Room 102BC

FISSION TRACK DATING OF MONAZITE: ETCHING EFFICIENCIES AS A FUNCTION OF U CONTENT

Paper number: T130-14
Program: Technical Sessions
Time: 5:00 PM – 5:15 PM

Authors

Abstract

Monazite sand grains from a phosphate placer deposit derived from Paleozoic-age granite along with grains from recently (Eocene to Miocene) exhumed terrains were analyzed to test the feasibility of this mineral as a fission-track thermochronometer. Chemical compositions, including U concentrations, were determined by electron microprobe analysis for grain mounts of monazite sand grains (average [U] = 3578 ± 156 (1 s.d.) ppm) supplied by the University of Minnesota Department of Earth Sciences, monazite grains from the Valhalla (average [U] = 1667 ± 124 ppm) and Okanogan (average [U] = 1128 ± 116 ppm) migmatite cored gneiss domes of the Omineca crystalline belt, and monazite grains from Catnip Canyon (average [U] = 423 ± 6 ppm) in the Catalina core complex, southeastern Arizona. Polished grain mounts were etched in boiling 37% HCl for 45 minutes. Latent fission tracks were revealed in the Valhalla and Okanogan samples, however, few to no tracks were visible in the placer monazite sand grains or the Catnip Canyon sample after etching.

Lack of visible tracks could result from syn- to post-exhumation latent track annealing due to the predicted low annealing temperature for fission tracks in monazite. While this may explain the absence of tracks in the Catnip Canyon sample, questions still remain regarding lack of visible tracks in the Paleozoic-age sand grains for which latent tracks have been successfully etched in previous experiments. Alternatively, we considered whether chemical and/or crystallographic variations, along with differential radiation damage could produce differences in etching efficiency. To address this scenario, we re-polished and etched the Catalina grain mount for a total of 80 minutes and found that the longer etching time revealed fission tracks in the Catnip Canyon monazite grains.

Results suggest the that differences in latent fission track etching efficiency in monazite may be a function of U concentration, where crystals containing low U contents require longer etching times. These results suggest that fission tracks in monazite do not completely anneal at surface temperatures over geologic timescales and that monazite can potentially be exploited as a new ultra-low temperature thermochronometer.