摘要
用锆石U－Pb同位素体系测定锆石的生成年龄是目前应用最广泛的地质测年法之一。这些年龄的解释一直被锆石中放射成因铅的部分丢失所困扰。一般来说Pb通过蜕晶质锆石中的扩散，晶质锆石中的扩散，蜕晶质中的淋滤和蜕晶质的重结晶四个途径丢失。笔者认为，这四种丢失方式受温度的影响。温度低于600℃～650℃时，锆石晶体发生蜕晶质化，在蜕晶质化的锆石中Pb容易扩散或淋滤丢失。如果岩石的温度一直保持在这个温度之上，锆石不发生蜕晶质化，Pb很少丢失。只有温度超过1000℃，Pb才有可能通过扩散丢失。在地壳环境中，Pb的扩散不重要。一般来说锆石熔融再结晶，才能导致钻石U－Pb体系的重设。另外，由于蜕晶质化引起的放射成因铅丢失使锆石不一致线的下交点年龄不一定有实际地质意义，解释时要和地质情况相结合。
It is the most widely used technique to date the high precision mineral growth ages in igneous and metamorphic rocks with the U - Pb method. The interpretation of these ages,perticularly in metamorphic settings, is hampered by an incomlete understanding of the common phenomnon of partial Pb - loss in zircon. In principle, this Pb - loss may occur in four very diffrent ways: diffusion in metamict zircon, diffusion in pristine zircon, leaching from metamict zircon and recrystallization of metamict zircon. The four Pb - loss process are controled by temperature If temperature below c. 600 ℃ zircon becorne metamict and may lose Pb by chemical alteration, leaching and possibly diffusion in the metamict material; Heating of even partially metamict zircons to about 600 - 650℃ or higher causes Pb - loss as a result ofrecrystallization of the damaged lattice; Form about 650 ℃ to at least 1000 ℃there is noappreciable Pb - loss because Pb - diffusion in the pristine lattice is extremly slow; Ahae at least 1000℃ zircon may lose Pb slowly as a result of diffusion in the pristine lattice.It seems that theonly way for zircon to became completely reset under crustal conditions is through dissolusion and subsequent reprecipitation. Under all other conditions in the continental crust zircons ae foreer.
出处
《前寒武纪研究进展》
1998年第3期51-56,共6页
Progress in Precambrian Research
