Radiogenic nuclide

Radiogenic nuclides (more commonly referred to as radiogenic isotopes) form some of the most important tools in geology. They are used in two principal ways:

1) In comparison with the quantity of the radioactive 'parent isotope' in a system, the quantity of the radiogenic 'daughter product' is used as a radiometric dating tool (e.g. uranium-lead geochronology).

2) In comparison with the quantity of a non-radiogenic isotope of the same element, the quantity of the radiogenic isotope is used as an isotopic tracer (e.g. 206Pb/204Pb). This technique is discussed in more detail under the heading isotope geochemistry.

Examples

For radiogenic isotopes that decay slowly enough, a primordial fraction is always present, since all sufficiently long-lived isotopes do in fact naturally occur primordially. An additional fraction of some of these isotopes may also occur radiogenically.

Lead is perhaps the best example of a partly radiogenic substance, as all four of its stable isotopes (Pb-204, Pb-206, Pb-207, and Pb-208) are present primordially, in known and fixed ratios. However, Pb-204 is only present primordially, while the other three isotopes may also occur as radiogenic decay products of uranium and thorium. Specifically, Pb-206 is formed from U-238, Pb-207 from U-235, and Pb-208 from Th-232. In rocks that contain uranium and thorium, the excess amounts of the three heavier lead isotopes allows the rocks to be dated.

Other substances considered partly radiogenic are argon-40, formed from radioactive potassium, and nitrogen-14, which is formed by the decay of carbon-14.

Other important examples of radiogenic elements are radon and helium, both of which form during the decay of heavier elements in bedrock. Radon is entirely radiogenic, since it has too short a half life to occur primordially. Helium, however, occurs in the crust of the Earth primordially, since both helium-3 and helium-4 are stable, and small amounts were trapped in the crust of the Earth as it formed. Helium-3 is entirely primordial. However, the global supply of helium (which occurs in gas wells and well as the atmosphere) is almost entirely (about 99%) radiogenic, as shown by its factor of 100 enrichment in radiogenic helium-4 relative to the primordial ratio of helium-4 to helium-3. The latter ratio is known from extraterrestrial sources.

Radiogenic nuclides used in geology

{| class="wikitable" !Parent nuclide !Product nuclide !Decay constant (yr⁻¹) !Half-life |- |¹⁹⁰Pt |¹⁸⁶Os |1.477 ×10⁻¹² |469.3 Byr |- |¹⁴⁷Sm |¹⁴³Nd |6.54 ×10⁻¹² |106 Byr |- |⁸⁷Rb |⁸⁷Sr |1.402 ×10⁻¹¹ |49.44 Byr |- |¹⁸⁷Re |¹⁸⁷Os |1.666 ×10⁻¹¹ |41.6 Byr |- |¹⁷⁶Lu |¹⁷⁶Hf |1.867 ×10⁻¹¹ |37.1 Byr |- |²³²Th |²⁰⁸Pb** |4.9475 ×10⁻¹¹ |14.01 Byr |- |⁴⁰K |⁴⁰Ar |5.81 ×10⁻¹¹ |11.93 Byr |- |²³⁸U |²⁰⁶Pb** |1.55125 ×10⁻¹⁰ |4.468 Byr |- |⁴⁰K |⁴⁰Ca |4.962 ×10⁻¹⁰ |1.397 Byr |- |²³⁵U |²⁰⁷Pb** |9.8485 ×10⁻¹⁰ |0.7038 Byr |- |¹²⁹I |¹²⁹Xe |4.3 ×10⁻⁸ |16 Myr |- |¹⁰Be |¹⁰B |4.6 ×10⁻⁷ |1.5 Myr |- |²⁶Al |²⁶Mg |9.9 ×10⁻⁷ |0.7 Myr |- |³⁶Cl |³⁶Ar/S |2.24 ×10⁻⁶ |310 kyr |- |²³⁴U |²³⁰Th |2.826 ×10⁻⁶ |245.25 kyr |- |²³⁰Th |²²⁶Ra |9.1577 ×10⁻⁶ |75.69 kyr |- |²³¹Pa |²²⁷Ac |2.116 ×10⁻⁵ |32.76 kyr |- |¹⁴C |¹⁴N |1.2097 ×10⁻⁴ |5730 yr |- |²²⁶Ra |²²²Rn |4.33 ×10⁻⁴ |1600 yr |}

References

Category:Radioactivity