Isotope geochemistry

Source: Wikipedia, the free encyclopedia.

Isotope geochemistry is an aspect of

isotopic abundance are measured by isotope-ratio mass spectrometry
, and can reveal information about the ages and origins of rock, air or water bodies, or processes of mixing between them.

radioactivity
.

Stable isotope geochemistry

For most stable isotopes, the magnitude of fractionation from

standard
. That is,

‰

Hydrogen

Main article: Hydrogen isotope biogeochemistry

Carbon

Main article: δ13C

stable isotopes, 12C and 13C, and one radioactive isotope, 14C
.

The stable carbon isotope ratio,

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.[1] Occasional spikes in the global 13C/12C ratio have also been useful as stratigraphic markers for chemostratigraphy, especially during the Paleozoic.[3]

The

14C
ratio has been used to track ocean circulation, among other things.

Nitrogen

ambient air.[2] 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 and troposphere using data from the greenhouse gas N2O.[4]

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).[2] Variations in oxygen isotope ratios are used to track both water movement, paleoclimate,[1] and atmospheric gases such as ozone and carbon dioxide.[5] Typically, the VPDB oxygen reference is used for paleoclimate, while VSMOW is used for most other applications.[1] Oxygen isotopes appear in anomalous ratios in atmospheric ozone, resulting from mass-independent fractionation.[6] Isotope ratios in fossilized foraminifera have been used to deduce the temperature of ancient seas.[7]

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.[5] Variations in sulfur isotope ratios are used to study the origin of sulfur in an orebody and the temperature of formation of sulfur–bearing minerals as well as a biosignature that can reveal presence of sulfate reducing microbes.[8][9]

Radiogenic isotope geochemistry

Main article: Radiometric dating

Radiogenic isotopes provide powerful tracers for studying the ages and origins of Earth systems.[10] 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 and Earth's mantle through geological time.

Lead–lead isotope geochemistry

Main article: Lead–lead dating

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

isotopic fingerprinting
of their teeth, skin and bones.

It has been used to date ice cores from the Arctic shelf, and provides information on the source of atmospheric lead pollution.

Lead–lead isotopes has been successfully used in forensic science to fingerprint bullets, because each batch of ammunition has its own peculiar 204Pb/206Pb vs 207Pb/208Pb ratio.

Samarium–neodymium

Main article: Samarium–neodymium dating

isotopic fingerprints
of geological materials, and various other materials including archaeological finds (pots, ceramics).

147Sm decays to produce 143Nd with a half life of 1.06x1011 years.

Dating is achieved usually by trying to produce an isochron 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 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

Main article: Rhenium–osmium dating

siderophile elements 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 sulfides 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

tectonic plates are younger than continental plates). However, 3He will be degassed from oceanic sediment during subduction, so cosmogenic 3He is not affecting the concentration or noble gas ratios of the mantle
.

Helium-3 is created by

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

Earth's atmosphere
have remained essentially stable.

It has been observed that 3He is present in

oceanic ridge samples. How 3He is stored in the planet is under investigation, but it is associated with the mantle
and is used as a marker of material of deep origin.

Due to similarities in

fluid inclusions
.

Helium-4 is created by

radiogenic production (by decay of uranium/thorium-series elements). The continental crust
has become enriched with those elements relative to the mantle and thus more He4 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:

3He/4He isotope chemistry is being used to date

igneous geology and ore genesis
.

Isotopes in actinide decay chains

Isotopes in the decay chains 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/Thorium – 231Pa/230Th

Atlantic basin (around 1000 yrs) but 230Th is removed more rapidly (centuries). Thermohaline circulation 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

ground waters. A small amount of tritium is also produced naturally by cosmic ray spallation and spontaneous ternary fission
in natural uranium and thorium, but due to the relatively short half-life of tritium and the relatively small quantities (compared to those from anthropogenic sources) those sources of tritium usually play only a secondary role in the analysis of groundwater.

See also

Notes

  1. ^
    ISBN 978-0-13-272790-7
    .
  2. ^ a b c "USGS -- Isotope Tracers -- Resources -- Isotope Geochemistry". Retrieved 2009-01-18.
  3. doi:10.1016/s0031-0182(02)00510-2
    . Retrieved 7 Jan 2017.
  4. S2CID 140545969
    .
  5. ^
    PMID 14664646
    .
  6. doi:10.1029/GL014i001p00080
    .
  7. S2CID 4239689
    .
  8. ISBN 978-0-582-06701-1
  9. ISSN 2662-4435
    .
  10. ^ Dickin, A.P. (2005). Radiogenic Isotope Geology. Cambridge University Press. Archived from the original on 2014-03-27. Retrieved 2013-10-10.

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

3He/4He

Re–Os

External links
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