DETERMINING THE ZIRCON FISSION-TRACK CLOSURE TEMPERATURE

Low-temperature thermochronometry is widely used to determine cooling histories of crustal rock, and to constrain timing and rate of exhumation processes such as normal faulting and erosion. The concept of an effective closure temperature is widely used to relate cooling ages to temperature. While effective closure temperatures for most low-temperature thermochronometers are well constrained, there are serious discrepancies, however, for the closure temperature of the zircon fission-track (ZFT) system. Field-based studies yield ZFT closure temperatures of 210-240ºC depending on cooling rate. Laboratory studies proposed much higher ZFT closure temperatures of up to 360ºC. A new field-based estimate of the zircon fission-track closure temperature is proposed from 13 new zircon fission-track cooling ages form the Gold Butte block, SE Nevada. The Gold Butte block has become a reference site for natural calibration of low-temperature chronometers. The block was rapidly exhumed by Miocene normal faulting, and exposes a section of the crust extending to an original depth of ~18km. Numerous thermochronometry studies have been published for the block: apatite FT, muscovite Ar-Ar, titanite, zircon and apatite (U-Th)/He thermochronometers, and apatite, titanite and zircon U-Pb chronometers. Thus the paleothermal structure and exhumation history of the block are well resolved. Here, the thermal history of the Gold Butte block is re-evaluated, resulting in a new geothermal gradient estimate of 17.0 C/km and a very slow cooling rate of 0.46 ± 0.02 C/m.y before onset of rapid Miocene cooling. For these thermal conditions the closure temperature of the zircon fission-track system is determined at 194 ± 18 C, which is in good agreement with previous field-based estimates. Due to the problem of variation in zircon fission-track annealing with radiation damage, field studies are probably the best way to calibrate the closure temperature of the zircon fission-track system. The slowly cooled part of the Gold Butte block reflects slow erosional exhumation at 0.0273 km/m.y. for about 130 m.y., while the rapidly cooled part provides a slip rate of 10.5 km/m.y. of the hanging wall that caused the fast Miocene exhumation of this crustal section.