Fission Track Etching Method And Radiation Induced Annealing Mechanism In Zircon

An important hypothesis of thermal history reconstruction by fission track is that“heat”is the only energy source of the shortening of fission tracks induced by the spontaneous events of ²³⁸U.Therefore,the thermal history of rocks can be reconstructed by measuring the length of the etched fission tracks.However,there is an also evidence that the?-decay of U and Th causes the shortening of fission by?-irradiation,which would impact on the reliability of thermochronological applications of thermal history reconstructions relying solely on thermal annealing.As zircon generally has a high concentration in U and Th,it more likely has the radiation induced annealing effect by?-decay than apatite does.One should expect this radiation annealing effect at lower temperatures.However,the studies of fission tracks in zircon are relatively limited.The etching conditions of the apatite are friendly(room temperature,etching time 20 s),whereas,the etching conditions of the zircon fission track are very harsh.A special container is required for melting alkali as the etchant,and a prolonged etching time up to hundreds of hours is needed.Because of the different degrees of radiation damage,the standards for fill etching are different for different zircons.This is further complicated by the track orientation in the crystal(or the orientation effects).All these account for the limited studies for fission track etching and radiation induced fission tracks annealing in zircon.Therefore,it is important to understanding the mechanisms of track etching and radiation induced fission track annealing in zircon.As the etchant advances along the track,the gradual increase in track length and track width provides important experimental data for the improvement of the fission track annealing model.In this thesis,we have used Pakistan zircon to conduct the three etching experiments as follow.First,we have conducted a step etching experiment,by which the track length as a function of etching time is obtained from individual tracks.This provides direct evidence of how the etchant advances along the track,and how the track length and width is increased during the entire track etching process.Second,we have modified an etch-anneal-etch method by intentionally shortening the first etching step to a very short time(e.g.,only 30 s),annealing all the unetched tracks,and then etching for another 58 h to-enlarge the initially etched track to 1?m above the optical observation threshold.We have found that the etchant enters into the tracks in 30 s approaching to a length of 6.29?m.This is significantly different from previous speculation that track length increases gradually as the etchant advances along the track.Third,by studying four different zircons with similar origins,we have found that the length of confined fission tracks in zircon has negative correlations with the density of surface tracks,as well as the?-irradiation dose.The two negative correlations indicate that a higher U concentration or a higher track density in zircon accounts for a shorter length of confined tracks.When a fission track is collided by alpha decay recoils,the track width is narrowed,which hinders the etchant advancing along the track and results in the shortening of etched tracks in zircon.The above results are of great importance to the understanding of track etching and radiation induced-track annealing in zircon.They have important implications for fission track annealing model and the thermochronological applications,particularly in low temperatures.