Isotope geochemistry


Isotope geochemistry

Isotope geochemistry is an aspect of geology based upon study of the relative and absolute concentrations of the elements and their isotopes in the Earth. Broadly, the field is divided into two branches: stable and radiogenic isotope geochemistry.

Lead-lead isotope geochemistry

Lead has four stable isotopes - 204Pb, 206Pb, 207Pb, 208Pb and one common radioactive isotope 202Pb with a half-life of ~53,000 years.

Lead is created in the Earth via decay of transuranic elements, primarily uranium and thorium.

Lead isotope geochemistry 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 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.

amarium-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 1.06x1011 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

Rhenium and osmium are chalcophile 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.

Protactinium:Thorium Ratios - 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 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 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 d13C and 231Pa/230Th can therefore provide a more complete insight into past circulation changes.

Noble gas isotopes

Helium-3

Helium-3 was trapped in the planet when it was created. Some 3He is being added by meteoric dust, primarily collecting on the bottom of oceans (although due to subduction, all oceanic 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 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 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 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 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.

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:
* Old continental crust: less than 1
* mid-ocean ridge basalt (MORB): 7 to 9
* Spreading ridge rocks: 9.1 plus or minus 3.6
* Hotspot 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 processes, igneous geology and ore genesis.
* [http://www.geotrack.com.au/uthhe/u-th-he-techinfo.htm (U-Th)/He dating of apatite as a thermal history tool]
* [http://lvo.wr.usgs.gov/helium.html USGS: Helium Discharge at Mammoth Mountain Fumarole (MMF)]

Ground water 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. Comparing the ratio of tritium to helium-3 (3H/3He) allows estimation of the age of recent ground waters.
* [http://water.usgs.gov/lab/3h3he/background/ USGS Tritium/Helium-3 Dating]
* [http://wwwrcamnl.wr.usgs.gov/isoig/period/he_iig.html Hydrologic Isotope Tracers - Helium]

ee also

* Cosmogenic isotopes
* Environmental isotopes
* Geochemistry
* Isotopic signature
* Radiometric dating

General online stable isotope references

* [http://pubs.usgs.gov/info/seal2/ USGS: Stable Isotopes and Mineral Resource Investigations in the United States]
* [http://wwwrcamnl.wr.usgs.gov/isoig/res/funda.html USGS: Fundamentals of Stable Isotope Geochemistry]
* [http://www.science.uottawa.ca/~eih/ch1/ch1.htm Environmental Isotopes]
* [http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html Fundamentals of Isotope Geochemistry]

References

Faure G., 1986. "Principles of Isotope Geology", John Wiley & Sons. ISBN 0-471-86412-9

3He/4He
Burnard P.G., Farley K.A., & Turner G., 1998. "Multiple fluid pulses in a Samoan harzburgite", "Chemical Geology", 147: 99-114.

Kirstein L. & Timmerman M., 2000. "Evidence of the proto-Iceland lume in northwestern Ireland at 42Ma from helium isotopes", "Journal of the Geological Society, London", 157: 923-927.

Porcelli D. & Halliday A.N., 2001. "The core as a possible source of mantle helium", "Earth and Planetary Science Letters", 192: 45-56.

Re-Os
Arne D., Bierlein F.P., Morgan J.W., & Stein H.J., 2001. "Re-Os dating of sulfides associated with gold mineralisation in central Victoria, Australia", "Economic Geology", 96: 1455-1459.

Martin C., 1991. "Osmium isotopic characteristics of mantle-derived rocks", "Geochimica et Cosmochimica Acta", 55: 1421-1434.


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Isotope-ratio mass spectrometry — Isotope ratio mass spectrometer with gas bench in foreground Acronym IRMS Classification mass spectrometry Other techniques …   Wikipedia

  • Geochemistry — The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, and the cycles of matter and energy that transport the Earth s… …   Wikipedia

  • Stable isotope — Graph of isotopes/nuclides by type of decay. Orange and blue nuclides are unstable, with the black squares between these regions representing stable nuclides. The unbroken line passing below many of the nuclides represents the theoretical… …   Wikipedia

  • Compatibility (geochemistry) — In geochemistry, compatibility is a measure of how readily a particular trace element substitutes for a major element within a mineral. Compatibility of an ion is controlled by two things: its valence and its ionic radius. Both must approximate… …   Wikipedia

  • Geology and Geochemistry — ▪ 2008 Introduction Diamond inclusions in ancient terrestrial rock provided clues about the early history of Earth s crust. Scientists studied slow earthquakes and the crystalline structure of Earth s inner core. International scientific studies… …   Universalium

  • Petroleum geochemistry — is the application of chemical principles to the study of the origin, migration, accumulation, and alteration of petroleum...(John M. Hunt, 1979). [Hunt, J.M., 1979. Petroleum Geochemistry and Geology . Freeman, San Francisco ISBN 0716710056]… …   Wikipedia

  • Earth Sciences — ▪ 2009 Introduction Geology and Geochemistry       The theme of the 33rd International Geological Congress, which was held in Norway in August 2008, was “Earth System Science: Foundation for Sustainable Development.” It was attended by nearly… …   Universalium

  • Donald J. DePaolo — is an American professor of geochemistry in the Department of Earth and Planetary Science at the University of California, Berkeley and Associate Laboratory Director for Energy and Environmental Sciences at the Lawrence Berkeley National… …   Wikipedia

  • Radiometric dating — (often called radioactive dating) is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.[1]… …   Wikipedia

  • δ18O — Foraminifera samples …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.