Nuclear Tracks And Radiation Measurements最新文献

Thermochronology depends on isotopic systems for which the age is related to the time that a specified mineral cooled through its closure temperature. In tectonic studies, it is often of interest to examine regional variations in cooling age. When the variation in age within a data set exceeds the analytical errors on the age determinations, many options are available to model the spatial variation in age and to correlate it with other data or with the predictions of hypotheses. For example, trend surfaces [e.g. Davis J.C. (1986) Statistics and Data Analysis in Geology, 2nd edn. Wiley, New York] could be used to explain the variation that exceeds experimental error. In the case of interest in this study it may appear that the variations in age originate entirely from random analytical error. We show how geologically significant patterns that may be present in such apparently random data can be detected. Our analysis is based on characterizing how the extreme ages (oldest and youngest) are distributed among the sample localities. In particular, we explore whether the extremes are more dispersed or more clustered than could be expected from a random assignment of ages to the localities, as deduced from bootstrap simulations. This mode of analysis is non-parametric and requires no assumptions about the distributional form of the errors or the ages. The proposed analysis is applied to 34 zircon fission-track ages from the central Appalachian Piedmont, eastern U.S.A. Our results show that the older ages are concentrated near the center of the sample region and are surrounded by younger ages. This age pattern suggests that rocks now at the surface in the central part of the study area cooled first, followed by rocks located toward the periphery of the area.

The dating of volcanic hydrated glass shards presents considerable difficulties. However, within recent years the fission-track method based on the isothermal plateau technique has been successfully applied to such glasses. Age estimates obtained by this means compare very well with those based on co-existing mineral phases using the K-Ar, ⁴⁰Ar/³⁹Ar and thermoluminescence techniques. Annealing studies of nuclear tracks in the temperature range of 100–300°C have been made for various volcanic glass samples. The activation energy, E_a, for track annealing is determined in the frame of a model involving a unique value of E_a for a given glass sample. These annealing studies suggest that the thermal stability of nuclear tracks in volcanic glasses is mainly controlled by the SiO₂/Al₂O₃ ratio.

We dated, by fission tracks, 15 Macusanites from four sites located inside or in the vicinity of the Macusani ignimbritic field. Eight of these obsidian pebbles are from the new site of Chilcuno Grande, the remaining seven from the three sites of Caluyo Mayo, Chilcuno Chico and Samillia. The apparent fission-track ages of these obsidians range from about 4.6 to 3.2 Ma. As in all samples, fossil tracks show evidences of annealing; the apparent ages are only lower estimates of Macusanite formation time. Fission-track plateau ages could be determined in 12 samples. They range between 7.9 and 4.8 Ma. The distribution of available plateau ages suggests that Macusanites formed both during the main activity of the Macusani volcanic field (7 ± 1 MA ago) and late episodes (5.7−5.3 and 4.8−4.3 Ma ago).

Light-microscope examination of neutron-bombarded and chemically etched fluorapatite samples that have been annealed at 450°C for 5 h indicate that the etchable fission damage is zero. However, transmission electron microscopy (TEM) observations of material that has not been chemically etched confirm that unetchable crystalline damage remains prominent such that the track population is not eliminated. Using TEM to examine a suite of furnace-annealed samples, it is possible to reconstruct the physical process by which fission damage is repaired. Annealing is characterized by initial development of an irregular track boundary followed by segmentation of tre track length. As the extent of annealing progress,s, the spacing between segments increases and the segment's shape approaches a spherical morphology. Finally, the spheroidal segments may be annealed.