Download Stable isotope Relative atomic mass Mole fraction Os 183.952 489

†
Stable
Relative
Mole
isotope
atomic mass
fraction
184
Os†
183.952 489
0.0002
186
†
Os
185.953 84
0.0159
187
Os
186.955 75
0.0196
188
Os
187.955 84
0.1324
189
Os
188.958 14
0.1615
190
Os
189.958 44
0.2626
192
191.961 48
0.4078
Os
Radioactive isotope having a relatively
long half-life and a characteristic
terrestrial isotopic composition that
contributes significantly and reproducibly
to the determination of the standard
atomic weight of the element in normal
materials. The half-lives of 184Os and
186
Os are 1.1 × 1013 years and
2 × 1015 years, respectively.
Osmium isotopes in Earth/planetary science
The isotope-amount ratio n(187Os)/n(186Os) in rocks can be transferred to fluids, such as
magmas and groundwaters (Figure 4.76.1). Variations in the inherited n(187 Os)/n(186 Os) ratios
can provide a useful tracer for fluid sources and migration paths [299, 511, 524]. Meteorites
and meteorite dust impacting the Earth have different osmium isotopic compositions than
terrestrial rocks and sediments. As a result, n(187Os)/n(186Os)-ratio studies provide evidence of
continuing extraterrestrial additions to the Earth over geologic time, as well as providing a
method for prospecting in the sedimentary record for large meteorite impact events that may
have affected life on Earth [299, 525].
Fig. 1: The isotope-amount ratio n(187Os)/n(186Os) in rocks can be transferred to fluids, such as
magmas. (Image Source: U.S. Geological Survey Hawaiian Volcano Observatory, Kilauea)
[168].
Osmium isotopes in geochronology
Some 187Os is radiogenic as a result of being formed by beta decay of radioactive 187Re, which
has a half-life of 4.16 × 1010 years. Variations in the isotope-amount ratio n( 187Os)/n(186Os) and
mole ratio n(187 Re)/n(186Os) are used for geochronology; for example, variations in these ratios
have been used to determine the ages of the Earth, moon, and meteorites [299, 511, 524, 525].
Kirk et al. [526] measured rhenium-osmium isotopic abundances in gold and pyrites from
conglomerates of the Central Rand Group of South Africa (Figure 2), which have produced over
48,000 metric tons of gold and have accounted for 40 percent of the world’s total historic
production [527]. The gold and rounded pyrites from the conglomerates yield an age of
~3.0 × 109 years. Kirk et al. find that this age is much older than that of conglomerate, and they
conclude that the gold is detrital (material wearing away by weathering or erosion) and was not
deposited by later hydrothermal fluids.
Fig. 2: Cross plot of n(187Os)/n(188Os) isotope-amount ratio and n(187Re)/n(188Os) mole ratio of
gold and pyrite from South Africa’s Witwatersrand Supergoup gold deposits (modified from
[526]). The turquoise diamonds are gold-bearing samples from Vaal Reef, and they form an
isochron with an age of ~3.0 × 109 years. The purple filled circles are euhedral (crystals having a
flat surface and sharp angles) pyrites from the Venterdorp Contact Reef, and they have a much
younger age of 672 ± 510 million years.
Osmium isotopes used as a source of radioactive isotope(s)
Os can be used for the production of the medical radioisotope 195mPt via the 192Os (α, n) 195mPt
reaction.
192
Glossary
atomic number (Z) – The number of protons in the nucleus of an atom.
beta decay (β-decay) – radioactive decay process resulting in emission of a beta particle of
either positive or negative charge (an electron or positron). [return]
electron – elementary particle of matter with a negative electric charge and a rest mass of about
9.109 × 10–31 kg.
element (chemical element) – a species of atoms; all atoms with the same number of protons in
the atomic nucleus. A pure chemical substance composed of atoms with the same number of
protons in the atomic nucleus [703]. [return]
gamma rays (gamma radiation) – a stream of high-energy electromagnetic radiation given off
by an atomic nucleus undergoing radioactive decay. The energies of gamma rays are higher
than those of X-rays; thus, gamma rays have greater penetrating power.
half-life (radioactive) – the time interval that it takes for the total number of atoms of any
radioactive isotope to decay and leave only one-half of the original number of atoms. [return]
isochron – a line indicating age of formation of a suite of rock or mineral samples on a cross plot
of amount ratios of isotopes of the same element and mole ratios of isotopes of different
elements (one of which is radioactive and decays to an isotope of the other element). The time
formed can indicate time since metamorphism, crystallization, shock events, differentiation of
precursor melts, etc. For examples, see neodymium Figure 1, samarium Figure 1, rhenium Figure
1, and osmium Figure 2. [return]
isotope-amount ratio (r) – amount (symbol n) of an isotope divided by the amount of another
isotope of the chemical element in the same system [706]. [return]
isotope – one of two or more species of atoms of a given element (having the same number of
protons in the nucleus) with different atomic masses (different number of neutrons in the
nucleus). The atom can either be a stable isotope or a radioactive isotope.
isotopic composition – number and abundance of the isotopes of a chemical element that are
naturally occurring [706]. [return]
meteorite – a meteoroid that has survived atmospheric passage and fallen to the Earth’s surface
in one or more recoverable fragments. See also chondrites [705]. [return]
neutron – an elementary particle with no net charge and a rest mass of about 1.675 × 10–27 kg,
slightly more than that of the proton. All atoms contain neutrons in their nucleus except for
protium (1H).
normal material – a reasonably possible source for an element or its compounds in commerce,
for industry or science; the material is not itself studied for some extraordinary anomaly and its
mole fractions (isotopic abundances) have not been modified significantly in a geologically brief
period [4]. [return]
positron – the antimatter counterpart of the electron, with a mass identical to that of the electron
and an equal but opposite (positive) charge.
proton – an elementary particle having a rest mass of about 1.673 × 10–27 kg, slightly less than
that of a neutron, and a positive electric charge equal and opposite to that of the electron. The
number of protons in the nucleus of an atom is the atomic number.
radioactive decay – the process by which unstable (or radioactive) isotopes lose energy by
emitting alpha particles (helium nuclei), beta particles (positive or negative electrons), gamma
radiation, neutrons or protons to reach a final stable energy state.
radioisotope (radioactive isotope) – an atom for which radioactive decay has been
experimentally measured (also see half-life). [return]
radiogenic – produced by the decay of a radioactive isotope, but which itself may or may not be
radioactive. [return]
stable isotope – an atom for which no radioactive decay has ever been experimentally measured.
standard atomic weight – an evaluated quantity assigned by the IUPAC Commission on
Isotopic Abundances and Atomic Weights (CIAAW) to encompass the range of possible atomic
weights of a chemical element that might be encountered in all samples of normal terrestrial
materials. It is comprised of either an interval (currently for 12 elements) or a value and an
uncertainty (a standard Atomic-weight uncertainty), and currently there are 72. A standard
atomic weight is determined from an evaluation of peer-reviewed scientific publications. [return]
tracer - substance used for tracking purposes. [return]
X-rays – electromagnetic radiation with a wavelength ranging from 0.01 to 10 nanometers—
shorter than those of UV rays and typically longer than those of gamma rays.
References
4.
C. Kendall, and Coplen, T.B. Hydrological Process. 15, 1363 (2011). 10.1002/hyp.217
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Geological Survey Hawaiian Volcano Observatory (HVO). 2014 Feb. 26.
http://hvo.wr.usgs.gov/kilauea/update/archive/2003/May/main.html
299. G. Faure. Principles of Isotope Geology, 2nd Edition Wiley (1986).
511. A. Schersten. Re-Os, Pt-Os and Hf-W isotopes and tracing the core in mantle melts.
MantlePlumes.org. 2014 Feb. 25. http://www.mantleplumes.org/Os-W.html
524. A. D. Brandon, and Walker, R.J. Earth and Planetary Science Letters. 3-4 (232), 211
(2005).
525. F. S. Paquay, Ravizza, G.E., Dalai, T.K., and Peucker-Ehrenbrink, B. Science. 320
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526. J. R. J. Kirk, J. Chesley, J. Walshe, and G. England. Science. 297, 1856 (2002).
527. a. W. E. L. M. H. E. Frimmel. Society of Economic Geologists Special Publication. 9, 17
(2002).
703. I. U. o. P. a. A. Chemistry. Compendium of Chemical Terminology, 2nd ed. (the "Gold
Book"). Blackwell Scientific Publications, Oxford (1997).
705. American Geological Institute Glossary of Geology. American Geosciences Institute,
Alexandria, Virginia (2011).
706. Coplen. Rapid Communications in Mass Spectrometry. 25 (2011).