Which mineral is commonly used to date the oldest rocks on Earth due to its long-lived isotopes?

• A. Quartz
• B. Zircons
• C. Feldspar
• D. Calcite

Answer: (B)

Dating the oldest rocks hinges on minerals that lock in radioactive decay without being reset by later heating or alteration. Zircon is ideal for this because it forms early and preferentially incorporates uranium while excluding lead when it crystallizes, so the uranium-lead system starts right at the moment of formation. The uranium isotopes present (U-238 and U-235) decay to stable lead isotopes (Pb-206 and Pb-207) on long, well-known timescales, providing a reliable clock that can reach back to around 4.4 billion years. Zircon also has a high closure temperature, meaning the U-Pb system remains closed to diffusion until rocks experience very high temperatures, so the recorded age reflects the time of crystallization rather than later thermal events. In addition, zircons are extremely durable and resistant to weathering and metamorphism, allowing ancient crystals to survive long enough to be incorporated into younger rocks or to persist as detrital grains; this is why some of the oldest measurements come from zircons found in ancient crustal regions. Other minerals either don’t host a robust uranium-lead system, or their isotopic clocks reset more easily during geological processes, making them less reliable for pinning down the ages of Earth’s oldest rocks.