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Cyrkony nie są wieczne. Badania izotopowe O-U-Pb cyrkonów ze skał alkalicznych

Krzemińska Ewa, Poprawa Paweł

Przegląd Geologiczny

2024

Vol. 72, nr 12-1

730--–734

Despite the common belief that zircons are nearly indestructible i.e. zircons last forever, regardless of the geological evolution experienced by their host rocks, there is one hostile environment in which decomposition of this incredibly resistant mineral may occur. An alteration of the natural zircon by coupled dissolution-reprecipitation or by ion-exchange with an aqueous fluid are common for alkaline rocks. In those zircons, the abundance of non-formula elements increase and textural changes are frequently observed. These symptoms are accompanied by the disturbance of primarily isotopic signatures. The extent of these processes can be well inferred from the oxygen (δ18O) isotopic composition of zircon. A large contrast of the δ18O between values of the normal mantle/magmatic zircons (>5.3 ±0.6%o) and the results obtained from the porous zircons from the Elk syenite massif have been detected by SHRIMP IIe/MC analyses using a~20 fim Cs+ beam. Most of the grains had reduced δ 1SO, up to prominently negative δ18O values of-7.46 ±0.27%. These δ 18O-depleted signatures resulted from high-temperature alkaline fluids - zircon interaction after foid syenite emplacement. Then this would imply that porous textures, as illustrated in Fig. 1, could be induced by alkaline fluids and thus these grains could be used to date solely the post-emplacement, i.e. hydrothermal, stage of evolution.