Geochronology
Geochronology ( from Ancient Greek: γῆ (ge) " earth" and χρὁνος ( chronos ) " time period " and -logy ) is the science discipline, the events of earth's history and secondarily the time of origin of rocks and sediments (see Chronostratigraphy ) dated absolutely - time. Among other things, created from the acquired data the geological time scale in the time interval identified, designated as time- dated geochronological units and are illustrated.
Frequently geochronological units correspond to the formation time chronostratigraphical units, ie physically existent body of rock. The geochronology is by its very nature, however, immaterial and is therefore not strictly speaking a stratigraphic ( gesteinsdatierende ) discipline. The relationship between concrete geochronological units are always expressed in an older / younger relationship.
The dating of rocks can be absolute or relative.
Methods
Sedimentation
A first formulation of the storage law carried out in 1669 by Nicolaus Steno. The storage law states that layers lying below were formerly deposited as above lying layers. This allows the establishment of a relative sequence of layers. Separated from each other outcropping at the surface layers can be correlated with each other by index fossils ( stratigraphy ).
Up to the early 20th century, there was no direct methods for absolute dating of rocks. Estimates were based on rates of erosion of the mountains and sedimentation in oceans, or on observations of lava during volcanic eruptions.
The stratification consequences of sediments can be compared over time and classified together based GLOBAL NETWORK events (precipitation of the carried material into the atmosphere of large volcanic eruptions or meteorite falls ). For example, the accumulation of the rare element iridium on Earth, which is derived from certain meteorite falls is known (see iridium anomaly ).
Isotope measurement
With the discovery of different radioactivity measurement methods have been developed based on the examination of the amount ratio of the natural radioisotopes. The isotope ratios change due to different decay times ( half-life) or natural radiation ( radioactivity of the Earth or extraterrestrial radiation).
The first, based on the uranium-lead decay series age determination was published in 1913 by Arthur Holmes and the time was very controversial. Friedrich Georg Houtermans published in 1953, based on conducted by Clair Cameron Patterson uranium-lead isotopic measurements of meteorites, which is now accepted age of the Earth of about 4.5 billion years ago. Today different radioactive isotopes and their decay products are used to determine the age of rocks. The age of a rock can be interpreted differently depending on the method of examination. In igneous rocks, both the age of crystallization ( the emplacement in the Earth's crust ) and, depending surveyed, mineral can also buy several cooling age can be determined. Similarly, the period of metamorphosis event can be found in metamorphic rocks. In some sediments formed during the deposition of certain minerals (eg, glauconite in many marine (green ) sandstones ), the origin of the age can be determined by measuring radioactive isotopes. This age is interpreted as sedimentation ages.
Rubidium - strontium method
Rubidium 87Rb decays with a half -life of 47 billion years in 87Sr. The Radiometric dating is suitable for very old rocks. Since, in addition 87Sr also the stable 86Sr occurs, is obtained via the Isochronenmethode fairly accurate data for example, feldspars, hornblende and mica in the order of 1000 million years, with an error of several 10 million years.
Uranium-lead method
The uranium-lead method uses two decay chains:
The age of uranium-bearing minerals may now about the relationship of daughter isotopes to the remaining portion of the parent isotope: are determined by knowledge of the half -life of the parent isotope ( here U). When applicable, the existing prior to the radioactive decay content must be taken into account at the lead isotope 207Pb and 206Pb; this is done by measuring the content of non entstandenem by radioactive decay, ie already existing 204Pb: The 207Pb/204Pb and 206Pb/204Pb ratios unchanged are known from the measurement of meteorite material, therefore, from the 204Pb content may also be the original content of 207Pb and 206Pb are calculated; this must be subtracted from the measured content - the rest is created by radioactive decay.
A major advantage of the uranium-lead method is that you can usually use both decay chains, thus securing its result. Because of the high half-lives of the method is best suited to determine age from 1 million years.
Potassium and argon argon argon method
The potassium - argon method uses the decay products of potassium. Potassium itself occurs in nature in the form of three isotopes ago: 39K ( 93.26 %), 40K ( 0.012 %), 41K (6.73 %).
The radioactive 40K decays with a half -life of 1.277 · 109 years to 40Ar and 40Ca. The rarely occurring 40Ar is used for age determination. 40Ca comes as isotope of calcium is very common, so that the emergence of additional 40Ca from the decay of potassium is hardly measurable and therefore not suitable for age determinations.
To determine the 40Ar content of a rock, the rock must be melted. In the case of gas exiting the noble gas 40Ar is determined. Although the 40K content of the rock is determined, can be calculated from the change in the ratio of 40K to 40Ar between the time of rock formation or - solidification and the date of the determination of the ratio in the laboratory calculate the age of the rock.
Due to the relatively long half-life of 1.28 · 109 years these methods for rocks that are older than about 100,000 years is.
The 40Ar/39Ar-Methode utilizes the emergence of 39Ar from 39K by neutron bombardment of a rock sample in a reactor. After bombardment, the ratio of exiting the following melting of a rock sample isotopes 40Ar and 39Ar is determined.
As with the potassium - argon method is the daughter isotope 40Ar. As the isotope ratios of C are known, 39Ar, which is caused by neutron bombardment in the disintegration of 39 K may be used as a substitute for the K mother isotope.
Thus, only the ratio of 40Ar to 39Ar to determine the exiting gas. Analyzes of other isotopes by further analysis is required.
Radiocarbon method
The most geologically to determine the age of organic material suitable material younger radiocarbon method uses the decay of the damage caused by cosmic radiation in the upper atmosphere 14C (half-life: 5730 years ). It is designed for geological purposes only appropriate when carbon-containing objects are to be dated, which are less than 50,000 years old. Thus, it is limited to the Quaternary.
The main field of application of the radiocarbon method is the archeology and the archaeological stratigraphy.
Aluminum -beryllium method
The age determination on the aluminum isotope 26Al and the beryllium isotope 10Be in the mineral quartz is based on the (known) ratio of 26Al and 10Be, ( muon capture neutron spallation ) arise both by cosmic radiation on the surface of rocks / minerals. The ratio is dependent inter alia on the altitude, the geomagnetic latitude, the radiation geometry and a possible attenuation of the radiation by shields ( shipment, cover ). The specific radiation conditions and thus the ratio of 26Al to 10Be must be determined or estimated before the age determination.
From the date on which the material in question was shielded from cosmic radiation (eg, by incorporating in a cave ), the proportion of the two radionuclides by radioactive decay decreases at different rates, so that for the ratio of these radionuclides time of the study and the assumed (known) equilibrium ratio under irradiation and knowledge of the respective half-lives (see also chart of nuclides ) can be estimated age.
This method was also used to determine the age of fossil hominid bones. However, the bones can not be studied directly, but covers the surrounding sediments containing quartz used.
Samarium- neodymium method
Samarium 146 is transformed via alpha decay in 142 to neodymium. The long half -life of samarium allows age determinations of geologic time. Before 2012 it was thought a half-life of samarium 103 million years. Presumably, it is significantly shorter at 68 million years. From the measured ratio for stable isotope Sm144 estate of 0.007 and an initial ratio of 0.0094 followed by an age for the Earth of 4.57 billion years ago.
Other methods
- Neodymium - strontium method
- Lutetium -hafnium method
- Rhenium - osmium method