A rock is found with the mineral zircon within it. Within the zircon grains, there is observed to be both 235U (a type of uranium) and 207Pb (a type of lead). Interestingly, there is the same number of atoms of 235U as there is of 207Pb in each zircon. If we know that the half-life of 235U is 713 million years, the mineral grain must be blank1 - Word Answer million years old?
