Download Strontium isotopes in Earth/planetary science Stable isotopic

Stable
isotope
84
Sr
86
Sr
87
Sr
88
Sr
Relative
atomic mass
83.913 419
85.909 261
86.908 878
87.905 613
Mole
fraction
0.0056
0.0986
0.0700
0.8258
Strontium isotopes in Earth/planetary science
Stable isotopic fractionation of strontium is small because the relative differences between the
masses of stable strontium isotopes are small (mass numbers are 86, 87, and 88 for the most
abundant stable isotopes). Also, strontium is not subject to oxidation-reduction reactions in
normal terrestrial environments, which would cause isotopic fractionation to be more evident.
Nevertheless, current studies are exploring potential applications of stable strontium isotopic
fractionation; for example, it has been used as a proxy for temperature during coral growth and
for insights into the diets of ancient populations [294-296].
The relative abundance of natural radiogenic 87Sr in seawater is related to the relative
rates of processes that add or remove strontium in the ocean (seafloor spreading, mid-oceanridge hydrothermal activity, and continental weathering). Over geologic time, these processes
have fluctuated and the isotope-amount ratio n(87Sr)/n(86Sr) has changed systematically. By
measuring the n(87 Sr)/n(86 Sr) ratio in marine fossils of known age, it is possible to identify when
such environmental changes occurred. Conversely, it is possible to estimate the ages of marine
deposits by comparing measured n(87 Sr)/n(86 Sr) ratios with the global time chart; this process is
known as strontium isotope stratigraphy [296-298].
Strontium isotopes in forensic science and anthropology
The isotope-amount ratio n(87 Sr)/n(86 Sr) is highly variable in rocks, minerals, soils, and waters,
and it can be transmitted to plants (Figure 1), animals, and manufactured materials.
Measurements of n(87Sr)/n(86Sr) ratios can be used for forensic applications in food
authentication (determining where food came from), archaeology, crime-scene investigation, and
human migration [296-299].
Fig. 1: Variation in strontium isotope-amount ratios of twenty species of exotic wood from
thirteen countries (modified from [300]).
Strontium isotopes in geochronology
The 87 Rb-87Sr dating technique utilizes the fact that 87Sr is a product of radioactive 87Rb decay
(half-life = 4.96 × 1010 years) and is a useful tool for determining ages of rocks and minerals
spanning the age of the Earth (Figure 2) [296-299].
Fig. 2: Cross plot of n(87Sr)/n(86Sr) isotope-amount ratio and n(87Rb)/n(86Sr) mole ratio of
sphalerites (zinc sulfide mineral) from the Kipushi base metal deposit, Democratic Republic of
Congo (modified from [301]). 87 Sr is produced by decay of radioactive 87Rb. Rock containing
higher amounts of 87Rb will over time produce higher amounts of 87 Sr, for example sample
KI 1270/128 R. Rock containing lower amounts of 87Rb will over time produce smaller amounts
of 87 Sr, for example sample KI 1270/113 R. Assuming all the sphalerites in this figure were
formed at the same time, one can determine the age of formation of the sulfides from the slope of
the line through the data points (here 451.1 ± 6.0 million years), and this line is called an
isochron.
Glossary
atomic number (Z) – The number of protons in the nucleus of an atom.
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].
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 – 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.
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]
isotopic fractionation (stable-isotope fractionation) – preferential enrichment of one isotope
of an element over another, owing to slight variations in their physical, chemical, or biological
properties [706]. [return]
mass number (A) – total number of heavy particles (protons and neutrons, jointly called
nucleons) in the nucleus of an atom [703]. [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).
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.
proxy – a measured quantity that can be used to represent the value of another quantity in a
calculation. [return]
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.
radioactive isotope (radioisotope) – an atom for which radioactive decay has been
experimentally measured (also see half-life).
radiogenic – produced by the decay of a radioactive isotope, but which itself may or may not be
radioactive. [return]
reduction-oxidation (redox) - reduction – gain of an electron by an atom, oxidation – loss of an
electron by an atom. [return]
stable isotope – an atom for which no radioactive decay has ever been experimentally measured.
[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
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Anbar, A.D. Journal of Archaeological Science. 37 (9), 2352 (2010).
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297. B. L. Beard, and Johnson, C.M. Journal of Forensic Sciences. 45, 1049 (2000).
298. K. M. Frei, and Frei, R. Applied Geochemistry. 26 (3), 326 (2011).
299. G. Faure. Principles of Isotope Geology, 2nd Edition Wiley (1986).
300. K. Miller, Coplen, T.B., and Wieser, M. In Goldschmidt 22nd Conference, Montreal,
Canada (2012).
301. F. M. J. Schneider, and M. Brauns. Miner Deposita. 42, 791 (2007).
703. I. U. o. P. a. A. Chemistry. Compendium of Chemical Terminology, 2nd ed. (the "Gold
Book"). Blackwell Scientific Publications, Oxford (1997).
706. Coplen. Rapid Communications in Mass Spectrometry. 25 (2011).