Читать книгу Earth Materials онлайн

110 страница из 155

Several isotopes of the relatively rare element rubidium (Rb) exist; some 27% of these are radioactive⁸⁷Rb. Most Rb⁺¹ is concentrated in continental crust, especially in substitution for potassium ion (K⁺¹) in potassium‐bearing minerals such as potassium feldspar, muscovite, biotite, sodic plagioclase, and amphibole. Radioactive⁸⁷Rb is slowly (half‐life = 48.8 Ga) transformed by beta decay into strontium‐87 (⁸⁷Sr). Unfortunately, the much smaller strontium ion (Sr⁺²) does not easily substitute for potassium ion (K⁺¹) and therefore tends to migrate into minerals in the rock that contain calcium ion (Ca⁺²) for which strontium easily substitutes. This makes using⁸⁷Rb/⁸⁶Sr for age dating a much less accurate method than the U/Pb methods explained previously.

One basic concept behind rubidium–strontium dating is that the original rock has some initial amount of⁸⁷Rb, some initial ratio of⁸⁷Sr/⁸⁶Sr and some initial ratio of⁸⁷Rb/⁸⁶Sr. These ratios evolve through time in a predictable manner. Over time, the amount of⁸⁷Rb decreases and the amount of⁸⁷Sr increases by radioactive decay so that the⁸⁷Sr/⁸⁶Sr ratio increases. At the same time, the⁸⁷Rb/⁸⁶Sr ratio decreases by an amount proportional to sample age. A second basic concept is that the initial amount of⁸⁷Rb varies from mineral to mineral, being highest in potassium‐rich minerals. As a result, the rate at which the⁸⁷Sr/⁸⁶Sr ratio increases depends on the individual mineral. For example, in a potassium‐rich (rubidium‐rich) mineral, the⁸⁷Sr/⁸⁶Sr ratio will increase rapidly, whereas for a potassium‐poor mineral it will increase slowly. For a mineral with no⁸⁷Rb substituting for K, the⁸⁷Sr/⁸⁶Sr ratio will not change; it will remain the initial⁸⁷Sr/⁸⁶Sr ratio. The⁸⁷Rb/⁸⁶Sr ratio of the whole rock however decreases at a constant rate that depends on the decay constant.