http://www.answersingenesis.org/articles/am/v7/n4/helium-in-rocks
The helium leakage rate has been determined in several experiments.2 All measurements are in agreement. Helium diffuses so rapidly that all the helium in these zircon crystals should have leaked out in less than 100,000 years. The fact that so much helium is still there means they cannot be 1.5 billion years old, as uranium-lead dating suggests. Indeed, using the measured rate of helium diffusion, these pre-Flood rocks have an average “diffusion age” of only 6,000 (± 2,000) years.
The helium leakage rate has been determined in several experiments.2 All measurements are in agreement. Helium diffuses so rapidly that all the helium in these zircon crystals should have leaked out in less than 100,000 years. The fact that so much helium is still there means they cannot be 1.5 billion years old, as uranium-lead dating suggests. Indeed, using the measured rate of helium diffusion, these pre-Flood rocks have an average “diffusion age” of only 6,000 (± 2,000) years.
