• Scientific Dates for Prehistoric Times
    Awake!—1986 | September 22

    • The Rubidium-Strontium Clock

      Another radioactive clock for minerals has been developed more recently. It is based on the decay of rubidium into strontium. Rubidium decays incredibly slowly. Its half-life is 50 billion years! So little of it has decayed in even the oldest rocks that meticulous measurements are necessary to distinguish the added strontium-87 from the primordial strontium. There may be a hundred times more strontium than rubidium in the mineral, and even in a billion years, only a little more than 1 percent of the rubidium decays. In spite of these difficulties, the minute amount of strontium produced by decay has been measured in a few cases. This clock is valuable for checking the ages found by other methods.

      An exciting example of the use of this method was on a meteorite that astronomers believe might be like the rocks that theoretically fell together to form the planets, a remnant of the primordial material from which the solar system was made. The indicated age, 4.6 billion years, was consistent with this view.

      An outstanding success of the rubidium-strontium clock was in dating the same moon rock described above. Five different minerals in the rock were tested, and they joined in indicating an age of 3.3 billion years, the same as the potassium-argon age.a

  • Scientific Dates for Prehistoric Times
    Awake!—1986 | September 22

    • a A word of caution about the rubidium clock: The decay of rubidium is so inordinately slow that its half-life cannot be measured with accuracy by counting the beta rays from its decay. The half-life has been determined by comparing it with other long-lived elements. So in this sense, it is not a completely independent method.