Peculiarities of oxygen isotope features in precious opals
The results of comparative investigation of  <sup>18</sup>O in precious opals of Primorye (the Raduzhnoye deposit), Australia, Ethiopia, and in modern opals from thermal springs of Mendeleev volcano on Kunashir Island (the Kuril Islands) are presented. It has been determined that oxygen isotope ratio in opals could serve as a criterion for assessment of the opals formation temperature. Low-temperature sedimentary opals are characterized by relatively heavy isotopic composition of oxygen, no matter sedimentary or volcanogenic rocks they were formed in. A-type opals from Australia and Slovakia are a good example. As for hydrothermal opals, they have light oxygen isotopic composition, which value is governed by the solution temperature at sedimentation. The higher solution temperature, the lighter isotopic composition of oxygen in precipitated opals and the closer it is to the oxygen isotopic composition of the hydrothermal solution.