FIELD OBSERVATIONS OF THE STABILITY OF FISSION TRACKS IN RADIATION-DAMAGED ZIRCON

Annealing of fission tracks in zircon is a function of temperature, time, and total accumulated radiation damage. At moderate temperatures, radiation-damaged grains become fully annealed while grains with little or no radiation damage can be virtually unaffected. Differential annealing results in variable resetting and multiple age populations. Low-retentive zircon (LRZ), which have a partly disordered crystalline structure, have significant radiation damage and a low temperature of annealing (~180-200°C). High-retentive zircon (HRZ), which are nearly crystalline, fully anneal at temperatures of ~250-300°C depending on rate of cooling. Partially reset samples are those where LRZ are reset, HRZ are not reset, and therefore the cooling age is not concordant, but the young population of grain ages records the youngest thermal events. Full resetting of both LRZ and HRZ results in cooling ages that are concordant, or nearly so. Here we highlight the geological significance of different populations of zircon grain ages and relate those to the thermal events, with examples from the Cordillera Huayhuash (Andes, Peru) and the Sredinny Range (Kamchatka, Russia). In both cases, we use zircon fission track (ZFT) and helium dating of zircon (ZHe) to constrain the timing of the lower-temperature events. In the Cordillera Huayhuash, Cretaceous quartzites have been heated during intrusion of Miocene plutons along the crest of the Andes. In the Sredinny Range of Kamchatka, a vertical transect in schists in the core of the range show pervasive post-orogenic cooling at 40-42 Ma. In the later case, almost all samples have a weaker and variable secondary ZFT population between 19 and 29 Ma, and significantly shortened track lengths of 6-8 um. This work highlights the dramatic difference in effective closure temperature between damaged and undamaged zircon, and it underscores the need to understand the role of radiation damage in zircon thermochronology.