Rhenium–osmium dating is a form of

radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...

based on the

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

of the

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numb ...

¹⁸⁷ Re to ¹⁸⁷ Os. This normally occurs with a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...

of 41.6 × 10⁹ y, but studies using fully ionised ¹⁸⁷ Re atoms have found that this can decrease to only 33 y. Both rhenium and osmium are strongly siderophilic (iron loving), while Re is also chalcophilic (sulfur loving) making it 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 Thermal ionization mass spectrometry (TIMS) is also known as surface ionization and is a highly sensitive isotope mass spectrometry characterization technique. The isotopic ratios of radionuclides are used to get an accurate measurement for the ele ...

Rhenium–osmium isochron

Rhenium–osmium dating is carried out by the isochron dating 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: :

\left(\frac\right)_ = \left(\frac\right)_ + \left(\frac\right) \cdot (e^-1),

where: : ''t'' is the age of the sample, : λ is the decay constant of ¹⁸⁷Re, : (''e''^λ''t''-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 is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...

deposit in the

Witwatersrand The Witwatersrand () (locally the Rand or, less commonly, the Reef) is a , north-facing scarp in South Africa. It consists of a hard, erosion-resistant quartzite metamorphic rock, over which several north-flowing rivers form waterfalls, which ...

mining Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the economic ...

camp,

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...

Rhenium-osmium isotopic evolution

Rhenium and osmium were strongly refractory and siderophile during the initial accretion of the Earth which caused both elements to preferentially enter the Earth's core. Thus the two elements should be depleted in the silicate Earth yet the ¹⁸⁷ Os / ¹⁸⁸ Os ratio of mantle is

chondritic A chondrite is a stony (non-metallic) meteorite that has not been modified, by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form pri ...

. The reason for this apparent contradiction is owed to the change in behavior between Re and Os in partial melt events. Re tends to enter the melt phase (incompatible) while Os remains in the solid residue (compatible). This causes high ratios of Re/Os in oceanic crust (which is derived from partial melting of mantle) and low ratios of Re/Os in the lower mantle. In this regard, the Re–Os system to study the geochemical evolution of mantle rocks and in defining the chronology of mantle differentiation is extremely helpful. Peridotite

xenolith A xenolith ("foreign rock") is a rock fragment ( country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in ig ...

s which are thought to sample the

upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appr ...

sometimes contain supra-chondritic Os-isotopic ratios. This is thought to evidence of subducted ancient high Re/Os

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

ic crust that is being recycled back into the mantle. This combination of radiogenic (¹⁸⁷ Os that was created by decay of ¹⁸⁷ Re) and nonradiogenic melts helps to support the theory of at least two Os-isotopic reservoirs in the mantle. The volume of both these reservoirs is thought to be around 5-10% of the whole mantle. The first reservoir is characterized by depletion in Re and proxies for melt fertility (such as concentrations of elements like Ca and Al). The second reservoir is chondritic in composition. Direct measurement of the age of continental crust through Re-Os dating is difficult. Infiltration of xenoliths by their commonly Re-rich magma alters the true elemental Re/Os ratios. Instead, determining model ages can be done in two ways: "Re depletion" model ages or the "melting age" model. The former finds the minimum age of the extraction event assuming the elemental Re/Os ratio equals 0 (komatiite residues have Re/Os of 0, so this is assuming a xenolith was extracted from a near-komatiite melt). The latter gives the age of the melting event inferred from the point when a melt proxy like Al₂O₃ is equal to 0 (ancient subcontinental lithosphere has weight percentages of CaO and Al₂O₃ ranging from 0 to 2%).

Pt–Re–Os systematics

The radioactive decay of ¹⁹⁰ Pt to ¹⁸⁶Os has a half-life of 6.5(3)×10¹¹ years (which is longer than the age of the universe, so it is essentially stable). However, in-situ ¹⁸⁷ Os / ¹⁸⁸ Os and ¹⁸⁶ Os / ¹⁸⁸ Os of modern plume related magmas show simultaneous enrichment which implies a source that is supra-chondritic in Pt/Os and Re/Os. Since both parental isotopes have extremely long half-lives, the Os-isotope rich reservoir must be very old to allow enough time for the daughter isotopes to form. These observations are interpreted to support the theory Archean subducted crust contributed Os-isotope rich melts back into the mantle.

References

{{DEFAULTSORT:Rhenium-osmium dating Radiometric dating Rhenium Osmium