Dating | geochronology | n3ws.info
Radioactive dating is a method of dating rocks and minerals using radioactive isotopes. This method is useful for igneous and metamorphic rocks, which including a small proportion of the radioactive isotope 14C (formed. Geologists use radiometric dating to estimate how long ago rocks formed, and to When molten rock cools, forming what are called igneous rocks, radioactive. Is radiometric dating possible on metamorphic rocks? timing of the last metamorphic reset plus the date the mineral grains originally formed.
The same margin of error applies for younger fossiliferous rocks, making absolute dating comparable in precision to that attained using fossils. To achieve this precision, geochronologists have had to develop the ability to isolate certain high-quality minerals that can be shown to have remained closed to migration of the radioactive parent atoms they contain and the daughter atoms formed by radioactive decay over billions of years of geologic time.
In addition, they have had to develop special techniques with which to dissolve these highly refractory minerals without contaminating the small amount about one-billionth of a gram of contained lead and uranium on which the age must be calculated. Since parent uranium atoms change into daughter atoms with time at a known rate, their relative abundance leads directly to the absolute age of the host mineral. In fact, even in younger rocks, absolute dating is the only way that the fossil record can be calibrated.
Without absolute ages, investigators could only determine which fossil organisms lived at the same time and the relative order of their appearance in the correlated sedimentary rock record.
Unlike ages derived from fossils, which occur only in sedimentary rocks, absolute ages are obtained from minerals that grow as liquid rock bodies cool at or below the surface. When rocks are subjected to high temperatures and pressures in mountain roots formed where continents collide, certain datable minerals grow and even regrow to record the timing of such geologic events. When these regions are later exposed in uptilted portions of ancient continents, a history of terrestrial rock-forming events can be deduced.
Episodes of global volcanic activityrifting of continents, folding, and metamorphism are defined by absolute ages. The results suggest that the present-day global tectonic scheme was operative in the distant past as well. Continents move, carried on huge slabs, or plates, of dense rock about km 62 miles thick over a low-friction, partially melted zone the asthenosphere below.
In the oceansnew seafloor, created at the globe-circling oceanic ridgesmoves away, cools, and sinks back into the mantle in what are known as subduction zones i. Where this occurs at the edge of a continent, as along the west coast of North and South America, large mountain chains develop with abundant volcanoes and their subvolcanic equivalents. These units, called igneous rockor magma in their molten form, constitute major crustal additions.
By contrast, crustal destruction occurs at the margins of two colliding continents, as, for example, where the subcontinent of India is moving north over Asia. Great uplift, accompanied by rapid erosion, is taking place and large sediment fans are being deposited in the Indian Ocean to the south.
Rocks of this kind in the ancient record may very well have resulted from rapid uplift and continent collision. When continental plates collide, the edge of one plate is thrust onto that of the other.
The rocks in the lower slab undergo changes in their mineral content in response to heat and pressure and will probably become exposed at the surface again some time later. Rocks converted to new mineral assemblages because of changing temperatures and pressures are called metamorphic.
Virtually any rock now seen forming at the surface can be found in exposed deep crustal sections in a form that reveals through its mineral content the temperature and pressure of burial. Such regions of the crust may even undergo melting and subsequent extrusion of melt magma, which may appear at the surface as volcanic rocks or may solidify as it rises to form granites at high crustal levels.
Magmas produced in this way are regarded as recycled crust, whereas others extracted by partial melting of the mantle below are considered primary. Even the oceans and atmosphere are involved in this great cycle because minerals formed at high temperatures are unstable at surface conditions and eventually break down or weather, in many cases taking up water and carbon dioxide to make new minerals.
If such minerals were deposited on a downgoing i.
These components would then rise and be fixed in the upper crust or perhaps reemerge at the surface. Such hot circulating fluids can dissolve metals and eventually deposit them as economic mineral deposits on their way to the surface.
Geochronological studies have provided documentary evidence that these rock-forming and rock-re-forming processes were active in the past. Seafloor spreading has been traced, by dating minerals found in a unique grouping of rock units thought to have been formed at the oceanic ridges, to million years ago, with rare occurrences as early as 2 billion years ago.
Other ancient volcanic units document various cycles of mountain building. The source of ancient sediment packages like those presently forming off India can be identified by dating single detrital grains of zircon found in sandstone.
Magmas produced by the melting of older crust can be identified because their zircons commonly contain inherited older cores. Episodes of continental collision can be dated by isolating new zircons formed as the buried rocks underwent local melting. Periods of deformation associated with major collisions cannot be directly dated if no new minerals have formed.
The time of deformation can be bracketed, however, if datable units, which both predate and postdate it, can be identified. The timing of cycles involving the expulsion of fluids from deep within the crust can be ascertained by dating new minerals formed at high pressures in exposed deep crustal sections. In some cases, it is possible to prove that gold deposits may have come from specific fluids if the deposition time of the deposits can be determined and the time of fluid expulsion is known.
Where the crust is under tension, as in Iceland, great fissures develop. These fissures serve as conduits that allow black lavacalled basaltto reach the surface. The portion that remains in a fissure below the surface usually forms a vertical black tubular body known as a dike or dyke.
Precise dating of such dikes can reveal times of crustal rifting in the past. Dikes and lava, now exposed on either side of Baffin Bayhave been dated to determine the time when Greenland separated from North America—namely, about 60 million years ago. Combining knowledge of Earth processes observed today with absolute ages of ancient geologic analogues seems to indicate that the oceans and atmosphere were present by at least 4 billion years ago and that they were probably released by early heating of the planet.
The continents were produced over time; the oldest preserved portions were formed approximately 4 billion years ago, but this process had begun about by 4. Absolute dating allows rock units formed at the same time to be identified and reassembled into ancient mountain belts, which in many cases have been disassociated by subsequent tectonic processes.
The most obvious of these is the Appalachian chain that occupies the east coast of North America and extends to parts of Newfoundland as well as parts of Ireland, England, and Norway. Relic oceanic crustformed between million and million years ago, was identified on both sides of the Atlantic in this chain, as were numerous correlative volcanic and sedimentary units.
Evidence based on geologic description, fossil content, and absolute and relative ages leave no doubt that these rocks were all part of a single mountain belt before the Atlantic Ocean opened in stages from about million years ago. Determination of sequence Relative geologic ages can be deduced in rock sequences consisting of sedimentary, metamorphic, or igneous rock units. In fact, they constitute an essential part in any precise isotopic, or absolute, dating program.
Such is the case because most rocks simply cannot be isotopically dated. Therefore, a geologist must first determine relative ages and then locate the most favourable units for absolute dating. It is also important to note that relative ages are inherently more precise, since two or more units deposited minutes or years apart would have identical absolute ages but precisely defined relative ages. While absolute ages require expensive, complex analytical equipment, relative ages can be deduced from simple visual observations.
Steno's four laws of stratigraphy. Most methods for determining relative geologic ages are well illustrated in sedimentary rocks.
These rocks cover roughly 75 percent of the surface area of the continents, and unconsolidated sediments blanket most of the ocean floor. They provide evidence of former surface conditions and the life-forms that existed under those conditions. The sequence of a layered sedimentary series is easily defined because deposition always proceeds from the bottom to the top.
This principle would seem self-evident, but its first enunciation more than years ago by Nicolaus Steno represented an enormous advance in understanding. Known as the principle of superpositionit holds that in a series of sedimentary layers or superposed lava flows the oldest layer is at the bottom, and layers from there upward become progressively younger.
On occasion, however, deformation may have caused the rocks of the crust to tilt, perhaps to the point of overturning them. The decay series of most interest to geologists are those with half-lives of tens, hundreds or thousands of millions of years.
If the proportions of parent and daughter isotopes of these decay series can be measured, periods of geological time in millions to thousands of millions of years can be calculated. 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: