HR: 1340h
AN: V23B-1428    [Abstracts]
TI: ⁴He Implantation in Natural Diamond: Implications for Apatite (U-Th)/He Thermochronometry
AU: * Phillips, D
EM: dphillip@unimelb.edu.au
AF: The University of Melbourne, School of Earth Sciences Parkville, Melbourne, VIC 3010, Australia
AU: Kohn, B P
EM: b.kohn@unimelb.edu.au
AF: The University of Melbourne, School of Earth Sciences Parkville, Melbourne, VIC 3010, Australia
AU: Gleadow, A J
EM: gleadow@unimelb.edu.au
AF: The University of Melbourne, School of Earth Sciences Parkville, Melbourne, VIC 3010, Australia
AU: Harris, J W
EM: Jeff.Harris@ges.gla.ac.uk
AF: University of Glasgow, Department of Geographical and Earth Sciences, Glasgow, G128QQ, United Kingdom
AB: Current apatite (U-Th)/He thermochronometry protocols correct for ejection of α-particles from grain margins. However, the potential for implantation of ⁴He into apatite grains, from primary or secondary actinide minerals, has received more limited attention. Evidence for significant natural α-fluxes in the near- surface environment is provided by surface feature and He abundance studies on diamond. Intense α- damage induces a green colour centre in diamond, enabling visual assessment of natural α-implantation doses. Diamonds with transparent green coats and/or green spots occur in most primary and detrital diamond deposits worldwide, indicating that α-implantation rates into upper crustal minerals may be more significant than previously envisaged. Experiments on transparent green-coated natural diamonds reveal implanted αHe concentrations up to 0.015 cc/g, attributed to secondary uranium phases deposited by circulating groundwater (Shelkov et al., 1998). Implantation of similar α-dosages into apatite grains would increase (U-Th)/He ages by up to several hundred percent, dependent on α-dose rate, grain dimensions and actinide content. Investigation of actinide-rich granites in Australia has revealed the common juxtaposition of apatite and actinide phases such as monazite and zircon. In addition, secondary actinide-bearing phases (e.g. uraninite) are observed along joints, fractures, miarolitic cavities and weathering fronts, thus providing additional α-sources. These results demonstrate that (U-Th)/He thermochronometry analyses of apatite, particularly from actinide-rich, weathered granites and sediments, need to evaluate the potential for ⁴He implantation in the near-surface environment. Insight into the extent of this problem may be achievable through multiple analyses of single grains, in situ laser probe analyses, ⁴He/³He step-heating experiments, abrasion of grains and/or complementary apatite fission track analyses. Reference: Shelkov, D.A., Verchovsky, A.B., Milledge, H.J. and Pillinger, C.T., 1998, The radial distribution of implanted and trapped ⁴He in single diamond crystals and implications for the origin of carbonado: Chemical Geology, v. 149, p. 109-116.
DE: 1140 Thermochronology
DE: 8175 Tectonics and landscape evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting