Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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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.

       Potassium–argon systematics

Schematic illustration of rubidium–strontium systematics, showing evolution in the composition of four representative minerals (1–4) from initial composition (blue line) to current composition (red line) as 87Rb decays into 87Sr over time.

      However, because argon‐40 is a Noble element it generally occurs as a gas and therefore rarely occurs in minerals at the time they form. Therefore, argon‐40 that exists in minerals is likely the product of the radioactive decay of the potassium‐40. Assuming that there is no loss of this argon‐40 from the mineral and no addition from other sources, the ratio of40K/40Ar in the mineral should increase over time and yield reliable ages for the minerals and/or rocks in which it occurs. This is especially true for volcanic rocks because, at high temperatures, argon is a gas that escapes easily from the lava into the atmosphere (where it is the third most abundant gas, after nitrogen and oxygen). On the other hand, when the lava crystallizes to form potassium‐bearing minerals, argon‐40 produced by the decay of potassium‐40 tends to be trapped in the crystal lattice because its radius is larger than the spacing between atoms. Ideally, this sets the stage for using40K/40Ar to date such rocks, but as we shall see, many challenges remain. This ratio is most useful for dating samples that formed more than 100 Ka in which enough time has elapsed for accurately measurable argon‐40 to accumulate, although some dates as young as 25 Ka have been reported.

      Three isotopes of potassium exist and tend to occur in a known fixed ratio in mineral‐forming environments. The stable isotopes potassium‐39 and potassium‐41 constitute 93.25 815 and 6.73 025% of all potassium atoms. Radioactive potassium‐40 contributes only 0.0117% of all potassium atoms. The rarity of potassium‐40 means that its initial abundance in minerals or rocks must generally calculated from its known ratio to the other two isotopes that are much easier to measure accurately.

      In recent decades, many techniques have been developed for the purpose of producing more accurate and refined age dates. Of these, the most significant has been the evolution of40Ar/39Ar dating methods which compare the ratios of these two argon isotopes from a small portion of a sample to avoid the inaccuracies inherent in inhomogeneous samples such as whole rocks, parts of which may not be representative. In this method, the sample and a standard of known age containing potassium‐40 are bombarded with neutrons in a nuclear reactor to produce argon‐39 which does not exist naturally. The amount of argon‐39 produced under a standard set of conditions is a proxy for the amount of potassium‐40 in the sample. From this information, the40K/40Ar ratio can be calculated and the age of the sample determined. On the whole,40Ar/39Ar dating methods appear to be more accurate than conventional40K/40Ar40 methods and can date samples as young as 25 Ka, but they do require access