Electron Microprobe Zircon Fission-Track Dating: A New Take on An Old Thermochronometer
We present a novel method of fission-track geo/thermochronology, which utilizes an electron microprobe to both image fission-track etch pits and determine U concentration in zircon. Utilizing this paired set of information, cooling ages of single crystal are determined. Typically zircon fission-track (ZFT) dating has been used as a thermochronometer, which records the time that a mineral passes through the ~220-260°C thermal window; above which ion tracks in the crystal lattice formed from the spontaneous fission decay of 238U are kinetically unstable and therefore anneal and disappear.
Electron microprobe fission-track (EM-FT) dating has advantages over previous techniques in that it does not involve thermal neutron irradiation,which increases time required for analyses. The standard "external detector method" is susceptible to alignment and contact problems in the detector mineral pairing and also requires counting a second set of induced fission-tracks. In EM-FT only one set of tracks is counted and no external detector is used because uranium concentration is determined directly on the crystal of interest.
Wavelength dispersive spectroscopy (WDS) allows simultaneous acquisition of complementary elements such as Th and Hf. Finally imaging etch pits with an electron beam permits counting higher track densities (>107 tracks cm-2) than can be counted using optical microscopy. The ability to count higher track densities and effectively unlimited U concentrations using WDS opens the possibility of dating of zircon grains which are both older (Precambrian) and higher U than can be typically dated.
Zircons that are both older and higher in U accumulate more self-induced radiation damage over time. Zircons with higher quantities of radiation damage anneal at much lower temperatures - perhaps as low as 180°C opening up the possibility that EP-FT analysis may be able to date the time at which rocks cool through the upper portion of the gas window.
Here we test the applicability of the method on seven well known ash and hypabyssal formations including the Fish Canyon Tuff (28.305 ± 0.036 Ma), Peach Springs Tuff (18.51 ± 0.1 Ma), Tardree Rhyolite (61.32 ± 0.09 Ma), Nisatai Dacite, (21.0 ± 0.3 Ma), Bishop Tuff (0.734 ± 0.024 Ma), Buluk Tuff (16.4 ± 0.2 Ma), and Mt. Dromedary Intrusive Complex (98.8 ± 0.6 Ma). We also present the result of model simulations and correction equations developed to choose proper coating mediums and eliminate X-ray peak interferences.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California