Radiometric dating of rocks

Posted on: August 20, 2023 in Explaining Geology

Radiometric dating of rocks is the key tool used by geologists to put an absolute age on a rock. Stratigaphy and fossil correlation can only give relative dates, and that only if the beds cover a wide area and contain fossils, and fossils of species that are relatively rapidly evovling and short lived [as species not as individuals]. these methods are useless for metamorphic rocks and for most igneous rocks.

Although geologists knew the sequence of geological events, it was not until the radioactive dating of rocks was developed in 1905 that they could put absolute ages in millions of years on these events.

Fortunately there are several radioative elements within the earth.

Underlying principles of radiometric dating of rocks

• Radioactive elements occur in most igneous and metamorphic rocks.
• One parent radioactive atoms decays to for one daughter atom.
• Half the number of radioative atoms decay in a fixed period of time
• The time it takes for half the parent atoms to decay to daughter atoms is called the half-life.
• If the half-life for a particular element is known and if the number of parent and daughter atoms in
  a mineral or rock is known, then the age of that mineral or rock can be calculated.
• This assumes that none of the daughter was present at the start, that none of either element has been added or lost.
• The half life of every radioactive substance is different.
• Uranium – lead dating , (U-Pb), is one of the oldest and most refined of the radioactive dating
  methods. It can be used to date rocks that formed and crystallised from about 1 million years to over
  4.5 billion years. There are two uranium-lead series ,the uranium series from ²³⁸U to ²⁰⁶Pb, with a half-life of 4.47 billion years and the actinium series from ²³⁵U to ²⁰⁷Pb, with a half-life of 710 million years.
• Other elements that are used in radioactive dating by geologists are potassium to argon, rubidium to
  strontium.
• Potassium-Argon dating has the advantage that the argon is an inert gas that does not react chemically and would not be expected to be included in the solidification of a rock, so any found inside a rock is very likely the result of radioactive decay of potassium. Since the argon will escape if the rock is melted, the dates obtained are to the last molten time for the rock. The half life is 5.81 x 10^-11yr^-1.
• The rubidium-strontium pair is often used for dating and has a non-radiogenic isotope, strontium-86, which can be used as a check on original concentrations of the isotopes. ⁸⁷Rb decays into the ground state of ⁸⁷Sr with a half-life of 4.88 x 10¹⁰ years. This process is often used along with potassium-argon dating on the same rocks. The ratios of rubidium-87 and strontium-87 to the strontium-86 found in different parts of a rock sample can be plotted against each other in a graph called an isochron which should be a straight line. The slope of the line gives the measured age. The oldest ages obtained from the Rb/Sr method can be taken as one indicator of the age of the earth.
• If the element has a short half life, it decays relatively quickly so can be used only for dating young objects, e.g. carbon-14 has a half life of 5,730 years and can only be used to date objects up to 50,000 years old – ideal for Quaternary studies and archaeology.