Radioactive elements decay at a certain constant rate and this is the basis of radiometric dating. But, the decay elements need to be set, much like you would re-set a stop watch for a runner, to ensure an accurate measurement. When minerals get subducted into the Earth and come back as volcanic magmas or ash, this essential re-sets the radiometric clock back to zero and therefore a reliable age date is possible. Sedimentary rocks may have radioactive elements in them, but they have been re-worked from other rocks, so essentially, there radiometric clock has not been re-set back to zero. However, sedimentary rocks can be age dated if a volcanic ash horizon or a diabase sill or dyke can be found within the sequence. For example, if you find a dinosaur bone in a sedimentary sequence and you find an ash layer 10 meter above the bone and another ash layer 20 meters below it, you can determine the age of the two ash layers. You can then infer that the dino must have lived some time between these two age dates.
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.
Igneous rocks and high-grade metamorphic rocks are the most likely to be entirely formed of minerals that crystallized when the rocks formed. As most fossils are found in clastic sedimentary rocks, which are made of weathered and eroded minerals and bits of rock of various ages, it is unlikely to be able to make an radiometric age determination. Because sedimentary rocks contain fragments of many rocks that could be different ages, radiometric dating is less useful for dating sedimentary . Radiometric dating on metamorphic rocks. Is radiometric dating possible on metamorphic rocks? Wouldn't the parent/ daughter ratio "reset" after the rock partially melts? 12 comments. share. save hide report. 76Upvoted. This thread is archived. New comments cannot be posted and votes cannot be cast.
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 : this ratio is then used to calculate the age. 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: this will be constant for a particular magma body but will vary between different bodies. Second, the amount of 87 Sr present will vary according to the amount produced by the decay of 87 Rb: this depends on the amount of rubidium present in the rock and the age.
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: U to Pb with a half-life of 4. 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.
Trace amounts of uranium are to be found in minerals such as zircon, monazite, sphene and apatite: these occur as accessory minerals in igneous rocks and as heavy minerals in sediments.
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.
Mar 23, Sedimentary rocks may have radioactive elements in them, but they have been re-worked from other rocks, so essentially, there radiometric clock has not been re-set back to zero. However, sedimentary rocks can be age dated if a volcanic ash horizon or a diabase sill or dyke can be found within the sequence. Sedimentary rocks cannot be dated directly using radiometric dating, which is based on the idea that when rocks are in liquid form, their radiometric clock resets. This technique is generally used to date igneous and metamorphic rock, which are rocks that were once melted due to extreme heat and pressure.
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: different provenance areas, for example continental cratons of different ages, can be distinguished by analysis of mud and mudstones.
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.
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: glauconite contains potassium and may be dated by K-Ar or Ar-Ar methods, but the mineral is readily altered and limited in occurrence. The formation of igneous rocks usually can be dated successfully provided that they have not been severely altered or metamorphosed.
Intrusive bodies, including dykes and sills, and the products of volcanic activity lavas and tuff may be dated and these dates used to constrain the ages of the rocks around them by the laws of stratigraphic relationships.
Earth Science: Crash Course History of Science #20
Dates from metamorphic rocks may provide the age of metamorphism, although complications can arise if the degree of metamorphism has not been high enough to reset the radiometric 'clock', or if there have been multiple phases of metamorphism. General stratigraphic relations and isotopic ages are the principal means of correlating intrusive igneous bodies.
Radiometric dating. Geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. Radioactive elements decay The universe is full of naturally occurring radioactive elements. 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. Thermal ionization mass spectrometer used in radiometric dating. Radiometric dating calculates an age in years for geologic materials by measuring the presence of a short-life radioactive element, e.g., carbon, or a long-life radioactive element plus its decay product, e.g., potassium/argon
Geographically separate units of igneous rock can be shown to be part of the same igneous suite or complex by determining the isotopic ages of the rocks at each locality. Radiometric dating can also be very useful for demonstrating correspondence between extrusive igneous bodies.
The main drawbacks of correlation by this method are the limited range of lithologies that can be dated and problems of precision of the results, particularly with older rocks. For example, if two lava beds were formed only a million years apart and there is a margin of error in the dating methods of one million years, correlation of a lava bed of unknown affinity to one or the other cannot be certain.
Email This BlogThis! Share to Twitter Share to Facebook. This of course presupposes that radiometric dating works consistently as a dating technique in the first place. The assumptions which underlie radiometric dating are covered in our radiometric dating article published here.
Sedimentary rocks radiometric dating
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