1. RADIOMETRIC METHODS ROCK DATING

2. The various isotopes of the same element = same atomic number but differ in terms of atomic mass They differ in the number of neutrons in their nuclei but have the same number of protons.

3. Half-lives of these isotopes and the parent-to-daughter ratio in a given rock sample can be measured, then a calculation based on the total number of elapsed half lives gives the absolute (radiometric) date at which the parent began to decay, i.e., the age of the rock. Some rock-forming minerals contain naturally occurring radioactive isotopes with very long half-lives.

4. Of the three basic rock types, igneous rocks are most suited for radiometric dating. These rocks have not changed since they solidified from cooling magma containing the parent isotope. Metamorphic rocks may also be radiometrically dated. However, radiometric dating generally gives the age of metamorphism, not the original rock. Most ancient sedimentary rocks cannot be dated radiometrically, so other methods will need to be used to give an estimation of the absolute age.,.

5. Radiometric Methods: Uranium–lead (U–Pb) Detects an age range of 1 mill y to 4.5 bill y, precision 0.1–1%. The method relies on two separate decay chains where U-238 decays to Pb-206, with a half-life of 4.47 billion years and U-235 decays to Pb-207, with a half-life of 704 million years. A commonly used method as it can detect the absolute age of very old and quite young rocks.

6. Radiometric Methods: Uranium–lead (U–Pb) Detects an age range of 1 mill yr to 4.5 bill yr, precision 0.1–1%. The method relies on two separate decay chains where U-238 decays to Pb-206, with a half-life of 4.47 billion years and U-235 decays to Pb-207, with a half-life of 704 million years. A commonly used method as it can detect the absolute age of very old and quite young rocks.

7. Potassium-argon (K–Ar) This involves decay of radioactive Potassium-40 to Argon-40. Potassium-40 has a half-life of 1.3 billion years, and so this method is used to date the oldest rocks. Argon is an inert gas: Potassium is always tightly locked up in minerals eg potassium feldspar. Argon (an inert gas) is not part of any minerals, it is simply trapped in the minerals. In the lab the mineral is ground up and heated in a laser furnace. The Argon-40 is measured in a mass spectrometer.

8. Radiocarbon dating Carbon-14 is a radioactive isotope of carbon. Half-life of 5,730 years. Carbon- 14 is continuously created in the upper atmosphere and ends up as a trace component in atmospheric carbon dioxide. An organism acquires carbon during its lifetime. Plants through photosynthesis and animals through consumption.

9. When an organism dies, it stops acquiring new carbon-14 and the existing isotope decays. The proportion of carbon-14 left when the remains of the organism are examined provides the time elapsed since its death. The carbon-14 dating limit is around 58,000 to 62,000 years. Useful for dating sedimentary rocks with organic material trapped inside.

10. Argon–argon (Ar–Ar) dating This newer method converts a stable form of potassium (39K) into 39Ar when irradiated with neutrons in a nuclear reactor. Method is useful for aging volcanic and igneous rocks, since the age is determined from when the liquid rock cools into solid rock.

11. USES Ar-Ar and K- Ar dating is useful for older rocks (billions of years) Radio Carbon dating is useful for recent rock up to 60,000 years. Ref: https://en.wikipedia.org/wiki/Argon–argon_dating https://en.wikipedia.org/wiki/K–Ar_dating https://en.wikipedia.org/wiki/Radiocarbon_dating