(U-Th)/He 연령측정법의 원리와 응용

Abstract

The (U-Th)/He dating utilizes the production of alpha particles (He atoms) during natural radioactive decays of U, U and Th. (U-Th)/He age can be determined from the abundances of the parent nuclides U, U and Th and the radiogenic He. Because helium is one of the noble gases (non-reactive) with a relatively small radius, it diffuses rapidly in many geological materials, even at low temperatures. Therefore, ingrowth of He during radioactive decay competes with diffusive loss at elevated temperatures during the geologic time scale, determining the amount of He existing today in natural samples. For example, He diffusion in apatite is known to be very rapid compared to that in most other minerals, causing a significant diffusive loss at ~80C or higher. At ~40C, He diffusion in apatite becomes slow enough to preserve most He in the sample. Thus, an apatite’s (U-Th)/He age represents the timing when the sample passed through the temperature range of 80-40C. The crustal depth corresponding to this temperature range is called a “partial retention zone.” Normal closure temperatures for a typical grain size and cooling rate are ~60-70C for apatite and ~200C for zircon and titanite. Because the apatite He closure temperature is lower than that of most other thermochronometers, it can provide critical constraints on relatively recent or shallow-crustal exhumation histories.