Many different types of so-called dating methods exist: some provide relative ages while others provide absolute ages.
The most commonly used absolute dating methods are based on the radioactive decay of chemical elements. Parent nuclides will decay and become daughter products, and the ratio between the two tells us how much time has elapsed since a certain geological event, for example a volcanic eruption, the formation of an ore body, or the break-up of a continent.
These methods are commonly referred to as geochronological methods. Thermochronological methods are related to this, and provide information on temperature histories. Temperature increases with depth in the earth, and knowing the temperature history of a rock sample can thus provide information on how much erosion has taken place, how deep it was burried, and, for example, also what the potential for oil and gas in a particular geological horizon is.
We use different dating techniques depending on what geological process or event is being studied. Below we describe the dating techniques that are in use at NGU and for which NGU researcher have specialized expertise.
A noble gas mass-spectrometer can determine the isotopic composition of argon gas extracted from geological samples by heating them in an oven or by hitting the sample with a laser. The photograph shows the argon laboratory at NGU.40Ar/39Ar geochronology allows measurement of ages for common rock-forming minerals and whole rocks that contain K. The method is based on the natural, radioactive decay of 40K to 40Ar; the half-life of this decay process allows measurement with high precision of the Ar isotope content of minerals and rocks ranging in age from that of the Moon to several hundred thousand years old .
The NGU 40Ar/39Ar geochronology laboratory analyzes bulk mineral separates, single grains and whole-rock samples including white mica, biotite, phlogopite, amphibole, K-feldspar, plagioclase, glauconite, glass and volcanic whole rocks. Applications include establishing sedimentary provenance, timing of faulting, volcanic events, crustal cooling (after high-grade metamorphism or igneous intrusion), and crustal unroofing.
The U-Pb method is the geologist¿s favorite method to date high-temperature geologic events, like crystallization of magmatic rocks and high-temperature metamorphism. U-Pb geochronology is based on two independent decay series, the decay of 238U to 206Pb with a half life of 4.47x109 y, and the decay of 235U to 207Pb with a half life of 0.704x109 y.
The U-Pb system allows the dating of U-rich (and initially Pb-poor) minerals like zircon (ZrSiO4), baddeleyite (ZrO2), monazite (CePO4), and titanite (CaTiSiO5). Analyses of these minerals for U and Pb isotopes provide an estimate of the time elapsed since the crystal started to behave as a closed system. Zircon, monazite and baddeleyite are robust and so U-Pb analyses provide the age of crystallization of these minerals. Titanite is less robust and is behaving as a closed system only below 650-600 °C. Consequently, titanite analyses may record cooling of the sample through 650-600 °C.
One single crystal of zircon may record different geological events, and analytical methods with micrometer-scale spatial resolution are needed to unravel such complex histories. Click for a larger image.Zircon is the most used mineral in U-Pb geochronology, as it is an ubiquitous heavy accessory mineral in silica-rich crustal rocks. It gives the age of a crystallization of a wide range of plutonic and volcanic rocks, like granite and rhyolite. Zircon is also a high-temperature metamorphic mineral and therefore provides the age of metamorphism in various gneisses. In metamorphic rocks, zircon has commonly a core-rim structure: the core crystallized during the magmatic intrusion of the rock before it was deformed while the rim crystallized during metamorphism and deformation.
One single crystal of zircon thus may record different geological events and analytical methods with micrometer-scale spatial resolution are necessary to unravel such complex histories. Baddeleyite is rare, but it is a very much sought-after mineral to date silica-poor mafic magmatic rocks, like gabbro. Monazite and titanite are useful to date metamorphic events and regional unroofing.
Zircon is also a detrital mineral present in clastic sediments. A very fast developing application of U-Pb geochronology is the evaluation of the provenance of sediments. The age distribution of individual zircon grains in the sediment can be compared with the age distribution of rocks in possible catchment areas of the basin.
Shiny silver-colored molybdenite from Vatterfjorden i Lofoten, Northern Norway.One very powerful and relatively new chronometer is obtained from two relatively uncommon elements, rhenium (Re) and osmium (Os). One isotope of Re (187Re) decays to an isotope of Os (187Os). The decay rate is defined by the half-life, the time that it takes for half of the parent 187Re to transform to daughter 187Os. The long half-life for the Re-Os chronometer, about ten times the age of the Earth, means that Re-Os can be used to determine events early in Earth's Geochronologists sample fresh shale for Re-Os dating from the Biri Formation, Southern Norway. history, and to date meteorites.
In addition, minerals and rocks with very high levels of Re, such as sulphide minerals like molybdenite and organic-rich black shales, permit accurate dating of much younger events in Earth's history. Thus, the Re-Os chronometer accesses an enormously wide range of geological time.
The elements Re and Os are unique relative to elements forming all other geologic clocks in that they are highly Sample and spike are sealed together in a glass tube in an acid solution. concentrated in minerals that are mined for metals such as copper, nickel, gold, and molybdenum. This means that the accumulation of metals in the Earth's crust can be directly dated and understood relative to their hosting geologic terrains and the larger tectonic picture. This provide useful information to mineral exploration today.
How do we measure these two modest elements, Re and Os, which occur at the parts per million- to parts per Isotopes of Re and Os are counted as negative ions on a mass spectrometer.trillion level in the Earth's crust? Re and Os are separated from a rock or mineral under pressure and elevated temperature in an acid solution that contains a spike. A spike contains very precisely known quantities of isotopes of Re and Os. Thus, the balance of the Re and Os in the solution belongs to the mineral. After chemical purification to isolate the Re and the Os from the solution, a mass spectrometer is used to count the charged molecules known as ions containing the Re and Os. The stream of ions is separated by a sensitive magnetic field in a vacuum so that different isotopes of Re and Os are individually and precisely counted.
Thermochronology, Fission track dating
Ages obtained from isotopic dating methods are based on the ratio of parent and daughter isotopes. In the case of the fission track method, the daughter product is not another isotope, but a trail of physical damage to the crystal lattice resulting from spontaneous fission of the parent nucleus.
When the rate at which spontaneous fission occurs is known, the accumulation of such trails, known as fission tracks, can be used as a dating tool. Analogous to diffusional loss of daughter isotopes, the damage trails in the crystal lattice disappear above a threshold temperature by the fission track annealing process. Although the physics behind the annealing process are poorly understood, the outcome is empirically well known. Annealing initially causes the length of fission tracks to decrease and may eventually completely repair the damage to the crystal lattice. The latter is known as total annealing. The rate at which annealing takes place is a function of both mineral properties and temperature history.
Fission tracks in geological samples have been well-studied in mica, volcanic glass, titanite and zircon. However, by far the most research has been done on fission tracks in apatite, a widely disseminated accessory mineral in all classes of rocks. Retention of fission tracks in natural minerals takes place only at temperatures well below that of their crystallisation temperature.
The amount of fission tracks per volume and their length will be a sensitive function of the annealing process and of the cooling history of the sample being studied. A cooling history can be constrained by thermal history modelling of fission track data (fission track age and fission track length distribution). Fission track analysis and thermal history modelling of apatite fission track data provide powerful tools with which to assess regional cooling and denudation histories.