Question

What exactly are the methods used for dating fossils and/or meteorites believed to be in excess of 1 billion years old? Are errors common?

A method called radioactive dating is used to determine the ages of meteorites as well as fossilized biological material. This method requires a ratio comparison between the amounts of one element to its respective radioisotope. For example, in dating human remains, we may compare the amount of normal carbon (C12) to its radioisotope (C14). For any given radioisotope, (whether it is carbon-14, uranium-235, or potassium-40), there is a specific time period, called its half-life, during which half of the sample will have decayed into another element in that time. Likewise, half of that remaining will have decayed in two half-lives, leaving only one quarter of the original sample. Thus, the radioisotopes provide accurate nuclear clocks. Depending on the material being dated, we choose to compare specific elements depending on their radioisotopic half-lives. For example, since the half-life of carbon-14 is only 5,700 years, it is ideal for dating relatively recent biological material here on earth, such as human bones. However, when dating objects that may be much older, like rocks and meteorites, we must choose elements with longer half-lives such as potassium-40, which has a half-life of 1.3 billion years. Typically, more than one element is used to date a meteorite, and the process is carried out for several different mineral grains in the same rock to eliminate uncertainties. The final age, checked by the agreement of the different methods, is usually accurate to within a few percentage points--that is to a few tens of millions of years in a meteorite several billion years old.
March 11, 2002