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Zircon age dating

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Do zircons—crystals of zirconium silicate—contain clocks you can trust?

Crystals of zirconium silicate can be found inside many sorts of rocks in the earth’s crust.

From it they suggest information the secular world finds surprising about hospitable conditions on the early earth.

When we examine the assumptions underlying their claims, however, we find their conclusions are built on a wobbly house of cards.

Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time.

If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept.

Despite debate about the accuracy of the uranium “clocks” they contain, scientists led by University of Wisconsin’s John Valley say they’ve found one grain with a confirmed age of 4.4 billion years.

Magmatic zircon in high-grade metamorphic rocks is often characterized by complex textures as revealed by cathodoluminenscence (CL) that result from multiple episodes of recrystallization, overgrowth, Pb-loss and modifications through fluid-induced disturbances of the crystal structure and the original U-Th-Pb isotopic systematics.

Many of these features can be recognized in 2-dimensional CL images, and isotopic analysis of such domains using a high resolution ion-microprobe with only shallow penetration of the zircon surface may be able to reconstruct much of the magmatic and complex post-magmatic history of such grains.

Oxygen isotopes and atoms of radioactive uranium trapped in the crystals are, many believe, frozen in time.

Confined as they are in these crystals, many scientists believe that the ratio of parent radioactive uranium to daughter lead atoms in zircons can be used to calculate the ages of the crystals.