Isotope dating of rocks
In cases where particular minerals are to be dated, these are separated from the other minerals by using heavy liquids (liquids with densities similar to that of the minerals) in which some minerals will float and others sink, or magnetic separation using the different magnetic properties of minerals.The mineral concentrate may then be dissolved for isotopic or elemental analysis, except for argon isotope analysis, in which case the mineral grains are heated in a vacuum and the composition of the argon gas driven off is measured directly.Radiometric dating uses the decay of isotopes of elements present in minerals as a measure of the age of the rock: to do this, the rate of decay must be known, the proportion of different isotopes present when the mineral formed has to be assumed, and the proportions of different isotopes present today must be measured.This dating method is principally used for determining the age of formation of igneous rocks, including volcanic units that occur within sedimentary strata.Detectors at the end of the tube record the number of charged particles of a particular atomic mass and provide a ratio of the isotopes present in a sample.This is the most widely used system for radiometric dating of sedimentary strata, because it can be used to date the potassium-rich authigenic mineral glauconite and volcanic rocks (lavas and tuffs) that contain potassium in minerals such as some feldspars and micas.The radiometric decay series commonly used in radiometric dating of rocks are detailed in the following sections.
However, the proportion of potassium present as 40 K is very small at only 0.012%, and most of this decays to 40 Ca, with only 11% forming 40 Ar.Part-way along the tube a magnetic field induced by an electromagnet deflects the charged particles.The amount of deflection will depend upon the atomic mass of the particles so different isotopes are separated by their different masses.The discovery of radioactivity and the radiogenic decay of isotopes in the early part of the 20th century opened the way for dating rocks by an absolute, rather than relative, method.Up to this time estimates of the age of the Earth had been based on assumptions about rates of evolution, rates of deposition, the thermal behaviour of the Earth and the Sun or interpretation of religious scriptures.
Search for isotope dating of rocks:
This may not always be the case because addition or loss of isotopes can occur during weathering, diagenesis and metamorphism and this will lead to errors in the calculation of the age.