Mineralogy, K-Ar and ⁴⁰Ar-³⁹Ar dating studies of biotites from dacitic volcanics in the Iwami Silve area, Shimane Pref, Japan

Base on the methods of microscope, X-ray diffractometer and electron-probe microanalyzer, biotites from dacitic rocks in the Iwami silver mining area, Shimane prefecture, Japan have been studied in detail. Part of these biotites subjected different level of hydrothermal alteration. The dating results from those fresh or with weakly altered biotites indicated that they were formed during Quaternary. However, the ⁴⁰Ar-³⁹Ar dating results obtained from biotites with intensively hydrothermal alteration suggested that the K-Ar isotope equilibrium in biotite crystal was broken by the later higher temperature hydrothermal activity. It revealed that the results obtained from altered biotites were older than that obtained from fresh biotites, and may cause a big age error. So, caution should be paid to the samples from the area with intensively hydrothermal alterations. The critical temperature for heating to break the original K-Ar isotope system was estimated to be about 200 °C.