Rhenium-osmium dating
Encyclopedia
Rhenium-Osmium dating is a form of radiometric dating
based on the beta decay
of the isotope
187Re
to 187Os
. This normally occurs with a half-life
of 41.6 × 109 y, but studies using fully ionised 187Re
atoms have found that this can decrease to only 33 y. Both rhenium and osmium are strongly siderophilic (iron loving) and chalcophilic (sulfur loving) making them useful in dating sulfide ores
such as gold and Cu-Ni deposits.
This dating method is based on an isochron calculated based on isotopic ratios measured using N-TIMS (Negative – Thermal Ionization Mass Spectrometry).
method. Isochrons are created by analysing several samples believed to have formed at the same time from a common source. The Re-Os isochron plots the ratio of radiogenic 187Os to non-radiogenic 188Os against the ratio of the parent isotope 187Re to the non-radiogenic isotope 188Os. The stable and relatively abundant osmium isotope 188Os is used to normalize the radiogenic isotope in the isochron.
The Re-Os isochron is defined by the following equation:
where:
A good example of an application of the Re-Os isochron method is a study on the dating of a gold
deposit in the Witwatersrand
mining
camp, South Africa
.
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
based on the beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
of the isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
187Re
Rhenium
Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion , rhenium is one of the rarest elements in the Earth's crust. The free element has...
to 187Os
Osmium
Osmium is a chemical element with the symbol Os and atomic number 76. Osmium is a hard, brittle, blue-gray or blue-blacktransition metal in the platinum family, and is the densest natural element. Osmium is twice as dense as lead. The density of osmium is , slightly greater than that of iridium,...
. This normally occurs with a half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 41.6 × 109 y, but studies using fully ionised 187Re
Rhenium
Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion , rhenium is one of the rarest elements in the Earth's crust. The free element has...
atoms have found that this can decrease to only 33 y. Both rhenium and osmium are strongly siderophilic (iron loving) and chalcophilic (sulfur loving) making them useful in dating sulfide ores
Sulfide mineral
The sulfide minerals are a class of minerals containing sulfide as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, the sulfarsenides and the sulfosalts...
such as gold and Cu-Ni deposits.
This dating method is based on an isochron calculated based on isotopic ratios measured using N-TIMS (Negative – Thermal Ionization Mass Spectrometry).
Rhenium-Osmium isochron
Rhenium-Osmium dating is carried out by the isochron datingIsochron dating
Isochron dating is a common technique of radiometric dating and is applied to date certain events, such as crystallization, metamorphism, shock events, and differentiation of precursor melts, in the history of rocks...
method. Isochrons are created by analysing several samples believed to have formed at the same time from a common source. The Re-Os isochron plots the ratio of radiogenic 187Os to non-radiogenic 188Os against the ratio of the parent isotope 187Re to the non-radiogenic isotope 188Os. The stable and relatively abundant osmium isotope 188Os is used to normalize the radiogenic isotope in the isochron.
The Re-Os isochron is defined by the following equation:
where:
- t is the age of the sample,
- λ is the decay constant of 187Re,
- (eλt-1) is the slope of the isochron which defines the age of the system.
A good example of an application of the Re-Os isochron method is a study on the dating of a gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
deposit in the Witwatersrand
Witwatersrand
The Witwatersrand is a low, sedimentary range of hills, at an elevation of 1700–1800 metres above sea-level, which runs in an east-west direction through Gauteng in South Africa. The word in Afrikaans means "the ridge of white waters". Geologically it is complex, but the principal formations...
mining
Mining
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...
camp, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
.