Isotope geochemistry is an aspect of
geology based upon the study of natural variations in the relative abundances of
isotopes of various
elements. Variations in
isotopic abundance
In physics, natural abundance (NA) refers to the abundance of isotopes of a chemical element as naturally found on a planet. The relative atomic mass (a weighted average, weighted by mole-fraction abundance figures) of these isotopes is the atomi ...
are measured by
isotope ratio mass spectrometry
Isotope-ratio mass spectrometry (IRMS) is a specialization of mass spectrometry, in which mass spectrometric methods are used to measure the relative abundance of isotopes in a given sample.
This technique has two different applications in the ea ...
, and can reveal information about the ages and origins of rock, air or water bodies, or processes of mixing between them.
Stable isotope
The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...
geochemistry is largely concerned with isotopic variations arising from mass-dependent
isotope fractionation, whereas
radiogenic isotope geochemistry is concerned with the products of natural
radioactivity.
Stable isotope geochemistry
For most stable isotopes, the magnitude of fractionation from
kinetic and
equilibrium fractionation
Equilibrium isotope fractionation is the partial separation of isotopes between two or more substances in chemical equilibrium. Equilibrium fractionation is strongest at low temperatures, and (along with kinetic isotope effects) forms the basis o ...
is very small; for this reason, enrichments are typically reported in "per mil" (‰, parts per thousand).
These enrichments (δ) represent the ratio of heavy isotope to light isotope in the sample over the ratio of a
standard Standard may refer to:
Symbols
* Colours, standards and guidons, kinds of military signs
* Standard (emblem), a type of a large symbol or emblem used for identification
Norms, conventions or requirements
* Standard (metrology), an object ...
. That is,
:
‰
Hydrogen
Carbon
Carbon has two
stable isotope
The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...
s,
12C and
13C, and one radioactive isotope,
14C.
The stable carbon isotope ratio,
''δ''13C, is measured against Vienna Pee Dee
Belemnite (VPDB).
The stable carbon isotopes are fractionated primarily by
photosynthesis (Faure, 2004). The
13C/
12C ratio is also an indicator of paleoclimate: a change in the ratio in the remains of plants indicates a change in the amount of photosynthetic activity, and thus in how favorable the environment was for the plants. During photosynthesis, organisms using the
C3 pathway show different enrichments compared to those using the
C4 pathway, allowing scientists not only to distinguish organic matter from abiotic carbon, but also what type of photosynthetic pathway the organic matter was using.
Occasional spikes in the global
13C/
12C ratio have also been useful as stratigraphic markers for
chemostratigraphy Chemostratigraphy, or chemical stratigraphy, is the study of the chemical variations within sedimentary sequences to determine stratigraphic relationships. The field is relatively young, having only come into common usage in the early 1980s, but th ...
, especially during the
Paleozoic.
The
14C ratio has been used to track ocean circulation, among other things.
Nitrogen
Nitrogen has two stable isotopes,
14N and
15N. The ratio between these is measured relative to nitrogen in
ambient air.
Nitrogen ratios are frequently linked to agricultural activities. Nitrogen isotope data has also been used to measure the amount of exchange of air between the
stratosphere
The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air h ...
and
troposphere using data from the greenhouse gas
N2O.
Oxygen
Oxygen has three stable isotopes,
16O,
17O, and
18O. Oxygen ratios are measured relative to
Vienna Standard Mean Ocean Water (VSMOW) or Vienna Pee Dee Belemnite (VPDB).
Variations in oxygen isotope ratios are used to track both water movement, paleoclimate,
and atmospheric gases such as
ozone and
carbon dioxide.
Typically, the VPDB oxygen reference is used for paleoclimate, while VSMOW is used for most other applications.
Oxygen isotopes appear in anomalous ratios in atmospheric ozone, resulting from
mass-independent fractionation Mass-independent isotope fractionation or Non-mass-dependent fractionation (NMD), refers to any chemical or physical process that acts to separate isotopes, where the amount of separation does not scale in proportion with the difference in the mass ...
.
Isotope ratios in fossilized
foraminifera have been used to deduce the temperature of ancient seas.
Sulfur
Sulfur has four stable isotopes, with the following abundances:
32S (0.9502),
33S (0.0075),
34S (0.0421) and
36S (0.0002). These abundances are compared to those found in
Cañon Diablo troilite.
Variations in sulfur isotope ratios are used to study the origin of sulfur in an
ore
Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April ...
body and the temperature of formation of sulfur–bearing minerals as well as a biosignature that can reveal presence of sulfate reducing microbes.
[Rollinson, H.R. (1993). ''Using Geochemical Data: Evaluation, Presentation, Interpretation'' Longman Scientific & Technical. ]
Radiogenic isotope geochemistry
Radiogenic isotopes provide powerful tracers for studying the ages and origins of Earth systems.
They are particularly useful to understand mixing processes between different components, because (heavy) radiogenic isotope ratios are not usually fractionated by chemical processes.
Radiogenic isotope tracers are most powerful when used together with other tracers: The more tracers used, the more control on mixing processes. An example of this application is to the evolution of the
Earth's crust
Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...
and
Earth's mantle through geological time.
Lead–lead isotope geochemistry
Lead has four stable
isotopes:
204Pb,
206Pb,
207Pb, and
208Pb.
Lead is created in the Earth via decay of
actinide elements, primarily
uranium and
thorium.
Lead isotope
geochemistry
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
is useful for providing
isotopic dates on a variety of materials. Because the lead isotopes are created by decay of different transuranic elements, the ratios of the four lead isotopes to one another can be very useful in tracking the source of melts in
igneous rocks, the source of
sediments and even the origin of people via
isotopic fingerprinting of their teeth, skin and bones.
It has been used to date
ice core
An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier. Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper ones, and an ice core contains i ...
s from the Arctic shelf, and provides information on the source of atmospheric lead
pollution.
Lead–lead isotopes has been successfully used in
forensic science
Forensic science, also known as criminalistics, is the application of science to criminal and civil laws, mainly—on the criminal side—during criminal investigation, as governed by the legal standards of admissible evidence and criminal ...
to fingerprint bullets, because each batch of ammunition has its own peculiar
204Pb/
206Pb vs
207Pb/
208Pb ratio.
Samarium–neodymium
Samarium–
neodymium is an isotope system which can be utilised to provide a date as well as
isotopic fingerprints of geological materials, and various other materials including archaeological finds (pots, ceramics).
147Sm decays to produce
143Nd with a half life of 1.06x10
11 years.
Dating is achieved usually by trying to produce an
isochron
In the mathematical theory of dynamical systems, an isochron is a set of initial conditions for the system that all lead to the same long-term behaviour.
Mathematical isochron An introductory example
Consider the ordinary differential equation ...
of several minerals within a rock specimen. The initial
143Nd/
144Nd ratio is determined.
This initial ratio is modelled relative to CHUR - the Chondritic Uniform Reservoir - which is an approximation of the chondritic material which formed the solar system. CHUR was determined by analysing
chondrite and
achondrite
An achondrite is a stony meteorite that does not contain chondrules. It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystalliz ...
meteorites.
The difference in the ratio of the sample relative to CHUR can give information on a model age of extraction from the mantle (for which an assumed evolution has been calculated relative to CHUR) and to whether this was extracted from a granitic source (depleted in radiogenic Nd), the mantle, or an enriched source.
Rhenium–osmium
Rhenium
Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one ...
and
osmium are
siderophile element
The Goldschmidt classification,
developed by Victor Goldschmidt (1888–1947), is a geochemical classification which groups the chemical elements within the Earth according to their preferred host phases into lithophile ( rock-loving), siderophil ...
s which are present at very low abundances in the crust. Rhenium undergoes
radioactive decay to produce osmium. The ratio of non-radiogenic osmium to radiogenic osmium throughout time varies.
Rhenium prefers to enter
sulfide
Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...
s more readily than osmium. Hence, during melting of the mantle, rhenium is stripped out, and prevents the osmium–osmium ratio from changing appreciably. This ''locks in'' an initial osmium ratio of the sample at the time of the melting event. Osmium–osmium initial ratios are used to determine the source characteristic and age of mantle melting events.
Noble gas isotopes
Natural isotopic variations amongst the noble gases result from both radiogenic and nucleogenic production processes. Because of their unique properties, it is useful to distinguish them from the conventional radiogenic isotope systems described above.
Helium-3
Helium-3
Helium-3 (3He see also helion) is a light, stable isotope of helium with two protons and one neutron (the most common isotope, helium-4, having two protons and two neutrons in contrast). Other than protium (ordinary hydrogen), helium-3 is the ...
was trapped in the planet when it formed. Some
3He is being added by meteoric dust, primarily collecting on the bottom of oceans (although due to
subduction
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
, all oceanic
tectonic plates
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
are younger than continental plates). However,
3He will be degassed from oceanic sediment during
subduction
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
, so cosmogenic
3He is not affecting the concentration or
noble gas ratios of the
mantle
A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that.
Mantle may refer to:
*Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear
**Mantle (vesture), an Eastern Orthodox ve ...
.
Helium-3 is created by
cosmic ray bombardment, and by
lithium spallation reactions which generally occur in the crust. Lithium
spallation is the process by which a
high-energy neutron bombards a
lithium atom, creating a
3He and a
4He ion. This requires significant lithium to adversely affect the
3He/
4He ratio.
All degassed helium is lost to space eventually, due to the average speed of helium exceeding the
escape velocity
In celestial mechanics, escape velocity or escape speed is the minimum speed needed for a free, non- propelled object to escape from the gravitational influence of a primary body, thus reaching an infinite distance from it. It is typically s ...
for the Earth. Thus, it is assumed the helium content and ratios of
Earth's atmosphere have remained essentially stable.
It has been observed that
3He is present in
volcano emissions and
oceanic ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diver ...
samples. How
3He is stored in the planet is under investigation, but it is associated with the
mantle
A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that.
Mantle may refer to:
*Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear
**Mantle (vesture), an Eastern Orthodox ve ...
and is used as a marker of material of deep origin.
Due to similarities in
helium and
carbon in
magma chemistry, outgassing of helium requires the loss of
volatile components (
water,
carbon dioxide) from the mantle, which happens at depths of less than 60 km. However,
3He is transported to the surface primarily trapped in the
crystal lattice of minerals within
fluid inclusions image:Inclumed.gif, 250px, Trapped in a time capsule the same size as the diameter of a human hair, the ore-forming liquid in this inclusion was so hot and contained so much dissolved solids that when it cooled, crystals of halite, sylvite, gypsum, ...
.
Helium-4 is created by
radiogenic production (by decay of
uranium/
thorium-series
elements). The
continental crust
Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called ''sial'' bec ...
has become enriched with those elements relative to the mantle and thus more He
4 is produced in the crust than in the mantle.
The ratio (R) of
3He to
4He is often used to represent
3He content. R usually is given as a multiple of the present atmospheric ratio (Ra).
Common values for R/Ra:
* Old continental crust: less than 1
*
mid-ocean ridge 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% of a ...
(MORB): 7 to 9
* Spreading ridge rocks: 9.1 plus or minus 3.6
*
Hotspot
Hotspot, Hot Spot or Hot spot may refer to:
Places
* Hot Spot, Kentucky, a community in the United States
Arts, entertainment, and media Fictional entities
* Hot Spot (comics), a name for the DC Comics character Isaiah Crockett
* Hot Spot (Tr ...
rocks: 5 to 42
* Ocean and terrestrial water: 1
* Sedimentary formation water: less than 1
* Thermal spring water: 3 to 11
3He/
4He isotope chemistry is being used to date
groundwaters, estimate groundwater flow rates, track water pollution, and provide insights into
hydrothermal
Hydrothermal circulation in its most general sense is the circulation of hot water ( Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with t ...
processes,
igneous geology and
ore genesis
Various theories of ore genesis explain how the various types of mineral deposits form within Earth's crust. Ore-genesis theories vary depending on the mineral or commodity examined.
Ore-genesis theories generally involve three components: sou ...
.
(U-Th)/He dating of apatite as a thermal history tool
Isotopes in actinide decay chains
Isotopes in the
decay chain
In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay directl ...
s of actinides are unique amongst radiogenic isotopes because they are both radiogenic and radioactive. Because their abundances are normally quoted as activity ratios rather than atomic ratios, they are best considered separately from the other radiogenic isotope systems.
Protactinium
Protactinium (formerly protoactinium) is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds ...
/Thorium –
231Pa /
230Th
Uranium is well mixed in the ocean, and its decay produces
231Pa and
230Th at a constant activity ratio (0.093). The decay products are rapidly removed by
adsorption
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
on settling particles, but not at equal rates.
231Pa has a residence equivalent to the residence time of
deep water in the
Atlantic basin (around 1000 yrs) but
230Th is removed more rapidly (centuries).
Thermohaline circulation
Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to tempe ...
effectively exports
231Pa from the Atlantic into the
Southern Ocean, while most of the
230Th remains in Atlantic sediments. As a result, there is a relationship between
231Pa/
230Th in Atlantic sediments and the rate of overturning: faster overturning produces lower sediment
231Pa/
230Th ratio, while slower overturning increases this ratio. The combination of
δ13C and
231Pa/
230Th can therefore provide a more complete insight into past circulation changes.
Anthropogenic isotopes
Tritium/helium-3
Tritium was released to the atmosphere during atmospheric testing of nuclear bombs. Radioactive decay of tritium produces the noble gas
helium-3
Helium-3 (3He see also helion) is a light, stable isotope of helium with two protons and one neutron (the most common isotope, helium-4, having two protons and two neutrons in contrast). Other than protium (ordinary hydrogen), helium-3 is the ...
. Comparing the ratio of tritium to helium-3 (
3H/
3He) allows estimation of the age of recent
ground water
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
s. A small amount of Tritium is also produced naturally by
cosmic ray spallation
Cosmic ray spallation, also known as the x-process, is a set of naturally occurring nuclear reactions causing nucleosynthesis; it refers to the formation of chemical elements from the impact of cosmic rays on an object. Cosmic rays are highly ener ...
and
spontaneous
Spontaneous may refer to:
* Spontaneous abortion
* Spontaneous bacterial peritonitis
* Spontaneous combustion
* Spontaneous declaration
* Spontaneous emission
* Spontaneous fission
* Spontaneous generation
* Spontaneous human combustion
* Spontan ...
ternary fission
Ternary fission is a comparatively rare (0.2 to 0.4% of events) type of nuclear fission in which three charged products are produced rather than two. As in other nuclear fission processes, other uncharged particles such as multiple neutrons and g ...
in natural uranium and thorium, but due to the relatively short half-life of Tritium and the relatively small quantities (compared to those from humandmade sources) those sources of Tritium usually play only a secondary role in the analysis of groundwater.
USGS Tritium/Helium-3 Dating
See also
*
Cosmogenic isotope
Cosmogenic nuclides (or cosmogenic isotopes) are rare nuclides (isotopes) created when a high-energy cosmic ray interacts with the nucleus of an ''in situ'' Solar System atom, causing nucleons (protons and neutrons) to be expelled from the atom ...
s
*
Environmental isotopes The environmental isotopes are a subset of isotopes, both stable and radioactive, which are the object of isotope geochemistry. They are primarily used as tracers to see how things move around within the ocean-atmosphere system, within terrestrial ...
*
Geochemistry
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
*
Isotopic signature
*
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 ...
*
Isotope-ratio mass spectrometry
Isotope-ratio mass spectrometry (IRMS) is a specialization of mass spectrometry, in which mass spectrometric methods are used to measure the relative abundance of isotopes in a given sample.
This technique has two different applications in the ea ...
*
Sulfur isotope biogeochemistry Sulfur isotope biogeochemistry is the study of the distribution of isotopes of sulfur, sulfur isotopes in biological and geological materials. In addition to its common isotope, 32S, sulfur has three rare stable isotopes: 34S, 36S, and 33S. The dist ...
*
Urey-Bigeleisen-Mayer equation
Notes
References
General
*
Allègre C.J., 2008. ''Isotope Geology'' (Cambridge University Press).
*Dickin A.P., 2005. ''Radiogenic Isotope Geology'' (Cambridge University Press).
*
Faure G., Mensing T.M. (2004), ''Isotopes: Principles and Applications'' (John Wiley & Sons).
*Hoefs J., 2004. ''Stable Isotope Geochemistry'' (Springer Verlag).
*Sharp Z., 2006. ''Principles of Stable Isotope Geochemistry'' (Prentice Hall).
Stable isotopes
Environmental Isotopes (University of Ottawa)
(C. Kendall & E.A. Caldwell, chap.2 in ''Isotope Tracers in Catchment Hydrology''
dited by C. Kendall & J.J. McDonnell 1998)
Stable Isotopes and Mineral Resource Investigations in the United States (
USGS)
3He/4He
*
*
*
Re–Os
*
*
External links
National Isotope Development CenterReference information on isotopes, and coordination and management of isotope production, availability, and distribution
Isotope Development & Production for Research and Applications (IDPRA)U.S. Department of Energy program for isotope production and production research and development
{{Authority control
Geochemistry
Geophysics
Geochronological dating methods