Explain how isotopic dating using zircon crystals constrains the age of metamorphic rocks.

Explain how isotopic dating using zircon crystals constrains the age of metamorphic rocks.

• A. Zircon crystals incorporate uranium and exclude lead; U-Pb dating provides crystallization ages; metamorphism may reset or partially reset the clock
• B. Zircon crystals contain carbon-14 which dates metamorphism
• C. Zircon crystals always preserve the exact age regardless of metamorphism
• D. Zircon crystals are dissolved during metamorphism and provide no age information

Answer: (A)

Isotopic dating with zircon hinges on the uranium–lead decay system. Zircon crystals preferentially incorporate uranium when they form and exclude lead, so the ratio of uranium to lead in a zircon crystal records the time since crystallization. In rocks that later experience metamorphism, the clock can be reset if metamorphic conditions cause lead to diffuse or if new zircon grows during metamorphism. Zircon is highly resistant and often preserves the original age, but metamorphism can produce a younger overprint or create new zircon growth, so measured ages may reflect the original crystallization or a metamorphic event. By analyzing zircon from a metamorphic rock—checking features like core versus rim ages and concordance—geologists can constrain when the zircon formed and, therefore, the timing of metamorphism within that context.

Isotopic dating with zircon hinges on the uranium–lead decay system. Zircon crystals preferentially incorporate uranium when they form and exclude lead, so the ratio of uranium to lead in a zircon crystal records the time since crystallization. In rocks that later experience metamorphism, the clock can be reset if metamorphic conditions cause lead to diffuse or if new zircon grows during metamorphism. Zircon is highly resistant and often preserves the original age, but metamorphism can produce a younger overprint or create new zircon growth, so measured ages may reflect the original crystallization or a metamorphic event. By analyzing zircon from a metamorphic rock—checking features like core versus rim ages and concordance—geologists can constrain when the zircon formed and, therefore, the timing of metamorphism within that context.