Место издания:Санкт Петербургский госуниверситет г. Санкт-петебург
Аннотация:One of the most important parts of any paleomagnetic research is to estimate the time of acquisition of the components of natural remanent magnetization. In addition to classical paleomagnetic tests, low-temperature thermochronology and scanning electron microscopy (SEM) are widely used to solve this task. In our report, we briefly describe the basics of these methods, including the real examples of their use on the equipment of the Resource Sharing Center at the Sсhmidt Institute of Physics of the Earth RAS. Among the methods of low-temperature geochronology, the apatite fission-track analysis (AFT dating) occupies one of the key positions. The density of tracks of spontaneous fission of uranium-238 makes it possible to estimate the time elapsed since the last cooling of rocks below 120°C; the distribution of track lengths reflects the cooling rate to ~40°C. Thus, the fission-track analysis of apatite, in combination with the results of Ar/Ar isotope dating of feldspar and mica, allow estimation of the thermal evolution of rocks from 350°C and below and gives very useful information for estimating the age of the magnetization, especially, intrusive magmatic rocks. Scanning electron microscopy together with X-ray phase analysis has long been successfully used to estimate the time of acquisition and nature of magnetization of rocks by studying the composition and structure of magnetic minerals. The use of ferrocolloid makes it possible to reveal the details of the internal structure of magnetic minerals using an optical microscope. An analysis of the diffraction patterns of the magnetic fraction provides information on the crystallographic parameters of all mineral phases in the sample and their ratios. These methods require a great deal of research experience, which has been accumulated over decades of work in this direction at the IPE RAS. We present the "Atlas of Microstructures", which is designed as a guide for the diagnosis of magnetic minerals using optical and electron microscopy taking together with X-ray phase analysis data. It should be noted that the above methods have been successfully implemented at the Resource Sharing Center of the IPE RAS: optical microscopy and fission-track analysis (dating) of apatite are carried out using Olympus BX53 and BX51M microscopes with a total magnification of up to x1000; for scanning electron microscopy we use Tescan MIRA LMS and VEGA II microscopes, and for X-ray phase analysis – Stoe Stadi MP diffractometer.
