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- Radiometric dating on metamorphic rocks : geology!
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.
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. The principle of solving simultaneous equations can be used to resolve these two unknowns. An alternative method is whole-rock dating, in which samples from different parts of an igneous body are taken, which, if they have crystallised at different times, will contain different amounts of rubidium and strontium present.
This is more straightforward than dating individual minerals as it does not require the separation of these minerals. Isotopes of uranium are all unstable and decay to daughter elements that include thorium, radon and lead. Two decays are important in radiometric dating: By measuring the proportions of the parent and daughter isotopes in the two decay series it is possible to determine the amount of lead in a mineral produced by radioactive decay and hence calculate the age of the mineral.
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Trace amounts of uranium are to be found in minerals such as zircon, monazite, sphene and apatite: Dating of zircon grains using uranium—lead dating provides information about provenance of the sediment. Dating of zircons has been used to establish the age of the oldest rocks in the world. Other parts of the uranium decay series are used in dating in the Quaternary.
These two rare earth elements in this decay series are normally only present in parts per million in rocks. The parent isotope is Sm and this decays by alpha particle emission to Nd with a half-life of billion years. The slow generation of Nd means that this technique is best suited to older rocks as the effects of analytical errors are less significant. The advantage of using this decay series is that the two elements behave almost identically in geochemical reactions and any alteration of the rock is likely to affect the two isotopes to equal degrees.
This eliminates some of the problems encountered with Rb—Sr caused by the different reactivity and mobility of the two elements in the decay series. This dating technique has been used on sediments to provide information about the age of the rocks that the sediment was derived from: Rhenium occurs in low concentrations in most rocks, but its most abundant naturally occurring isotope Re undergoes beta decay to an isotope of osmium Os with a half-life of 42 Ga.
This dating technique has been used mainly on sulphide ore bodies and basalts, but there have also been some successful attempts to date the depositional age of mudrocks with a high organic content. Osmium isotopes in seawater have also been shown to have varied through time. Radiometric dating is the only technique that can provide absolute ages of rocks through the stratigraphic record, but it is limited in application by the types of rocks which can be dated.
Introduction to Physical Geology Syllabus
The age of formation of minerals is determined by this method, so if orthoclase feldspar grains in a sandstone are dated radiometrically, the date obtained would be that of the granite the grains were eroded from. It is therefore not possible to date the formation of rocks made up from detrital grains and this excludes most sandstones, mudrocks and conglomerates.
Limestones are formed largely from the remains of organisms with calcium carbonate hard parts, and the minerals aragonite and calcite cannot be dated radiometrically on a geological time scale. Hence almost all sedimentary rocks are excluded from this method of dating and correlation.
An exception to this is the mineral glauconite, an authigenic mineral that forms in shallow marine environments: Living, well dead, organism can also be radiometrically dated. In this processes scientist compare the ratio of Carbon 12, a stable isotope of carbon, to that of Carbon 14, the radioactive isotope.
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Both Carbon 12 and Carbon 14 occur naturally within our environment at a constant ratio although carbon 14 is constantly decaying new carbon 14 is produced in the upper atmosphere as Nitrogen 14 is bombarded by cosmic radiation. Because Carbon 12 and Carbon 14 exists in a constant ratio within our environment it also exist in the exact same ratio within living organism. This is possible due to the fact that living organism are constantly replenishing carbon 14 levels through respiration.
However, when an organism dies respiration ceases and carbon 14 levels are no longer replenished.
What can be dated? Principles of Relative 4. Earth's Creation and the Concept of Deep Time.