Radiometric Dating Of Metamorphic Rocks ;

Radiometric Dating Of Metamorphic Rocks

radiometric dating of metamorphic rocks

Key concepts Absolute Dating The problem: By the mid 19th century it was obvious that Radiometric dating of metamorphic rocks was much older than years, but how old? This problem attracted the attention of capable scholars but ultimately depended on serendipitous discoveries. Early attempts: Initially, three lines of evidence were pursued:

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To calculate the age of a rock it is necessary to know the half-life of the radioactive decay series, the amount of the parent and daughter isotopes present in the rock when it formed, and the present proportions of these isotopes. It must also be assumed that all the daughter isotope measured in the rock today formed as a result of decay of the parent.

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.

It is therefore important to try to ensure that decay has taken place in a 'closed system', with no loss or addition of isotopes, by using only unweathered and unaltered material in analyses.

The radiometric decay series commonly used in radiometric dating of rocks are detailed in the following sections. The choice of method of determination of the age of the rock is governed by its age and the abundance of the appropriate elements in minerals. Practical radiometric dating The samples of rock collected for radiometric dating are generally quite large several kilograms to eliminate inhomogeneities in the rock. The samples are crushed to sand and granule size, thoroughly mixed to homogenise the material and a smaller subsample selected.

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.

Measurement of the concentrations of different isotopes is carried out with a mass spectrometer. In these instruments a small amount micrograms of the sample is heated in a vacuum to ionise the isotopes and these charged particles are then accelerated along a tube in a vacuum by a potential difference.

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. 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. Potassium—argon and argon—argon dating 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.

One of the isotopes of potassium, 40 K, decays partly by electron capture a proton becomes a neutron to an isotope of the gaseous element argon, 40 Ar, the other product being an isotope of calcium, 40 Ca. The half-life of this decay is However, the proportion of potassium present as 40 K is very small at only 0.

Argon is an inert rare gas and the isotopes of very small quantities of argon can be measured by a mass spectrometer by driving the gas out of the minerals. K—Ar dating has therefore been widely used in dating rocks but there is a significant problem with the method, which is that the daughter isotope can escape from the rock by diffusion because it is a gas.

The amount of argon measured is therefore commonly less than the total amount produced by the radioactive decay of potassium. This results in an underestimate of the age of the rock. The problems of argon loss can be overcome by using the argon—argon method. The first step in this technique is the irradiation of the sample by neutron bombardment to form 39 Ar from 39 K occurring in the rock.

The ratio of 39 K to 40 K is a known constant so if the amount of 39 Ar produced from 39 K can be measured, this provides an indirect method of calculating the 40 K present in the rock. Measurement of the 39 Ar produced by bombardment is made by mass spectrometer at the same time as measuring the amount of 40 Ar present. Before an age can be calculated from the proportions of 39 Ar and 40 Ar present it is necessary to find out the proportion of 39 K that has been converted to 39 Ar by the neutron bombardment.

This can be achieved by bombarding a sample of known age a 'standard' along with the samples to be measured and comparing the results of the isotope analysis.

The principle of the Ar—Ar method is therefore the use of 39 Ar as a proxy for 40 K. Although a more difficult and expensive method, Ar—Ar is now preferred to K—Ar. The effects of alteration can be eliminated by step-heating the sample during determination of the amounts of 39 Ar and 40 Ar present by mass spectrometer. Alteration and hence 40 Ar loss occurs at lower temperatures than the original crystallisation so the isotope ratios measured at different temperatures will be different.

The sample is heated until there is no change in ratio with increase in temperature a 'plateau' is reached: If no 'plateau' is achieved and the ratio changes with each temperature step the sample is known to be too altered to provide a reliable date. Other radiometric dating systems Rubidium—strontium dating This is a widely used method for dating igneous rocks because the parent element, rubidium, is common as a trace element in many silicate minerals.

The isotope 87 Rb decays by shedding an electron beta decay to 87 Sr with a half-life of 48 billion years. The proportions of two of the isotopes of strontium, 86 Sr and 87 Sr, are measured and the ratio of 86 Sr to 87 Sr will depend on two factors. First, this ratio will depend on the proportions in the original magma: Second, the amount of 87 Sr present will vary according to the amount produced by the decay of 87 Rb: The rubidium and strontium concentrations in the rock can be measured by geochemical analytical techniques such as XRF X-ray fluorescence.

Two unknowns remain: Group of conformable layers lying between unconformities. Unconformities are so common that today that sequence stratigraphy - the mapping and correlation of conformable sequences - is a major field in Geology. With unconformities factored in, the age of the Earth would have to be much greater than 36 million years. Similar attempts yielded results that varied widely between 3 million and 1.

Evolution stokes the fire: By the s century, the controversy surrounding evolution prompted new attention. After all, if the Earth were too young for there to have been time for evolution, the evolution debate would be over. Ocean salinity: In John Joly , acting on suggestion of Edmund Halley , attempted estimate based on the salinity of the ocean.

He calculated the amount of salt being transported into the oceans by rivers and compared this to the salinity of sea water, obtaining an age of 90 million years.

Sir William Thomson, Lord Kelvin , during the late 19th century, assumed that the Earth had originally been molten then, using averge melting point of rocks and the laws of thermodynamics, determined that the Earth would completely solidify within 20 million years. Both uniformitarians and evolutionists were uncomfortable, since their notions required a much older Earth, but the quantitative rigor of Thomson's approach made his the most prestigeous estimate of his day.

As it developed, both Joly and Tomson were leaving vital but unknown information out of their equations. Joly missed that salt is removed from the oceans by various processes. Kelvin could not have know that new heat is generated inside the Earth by radioactive decay nuclear fission , because the process had not been discovered.

The discovery of radioactivity: Ironically, radioactive decay, which frustrated Kelvin's purpose, ended up providing the true key to the absolute dating of rocks. Antoine Becquerel Discovered natural radioactivity In the following years, a large number of radioactive isotopes and their daughter products became known.

Pierre and Marie Curie: Discovered that the radioactive element radium continuously releases newly generated heat - radiogenic heat. With this discovery, it became clear that the decay of radioactive substances provided a continuous source of new heat that Thomson hadn't accounted for. The Earth might, indeed, be much older than his calculations indicated. But how old? Radiometric dating: At the beginning of the 20th century, Ernest Rutherford and Frederick Soddy developed the concept of the half-life - For any radioactive substance, there is a specific period of time in which half of a sample will decay to a daughter substance.

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