- Isotope geochemistry
Isotope geochemistry is an aspect of
geologybased upon study of the relative and absolute concentrations of the elements and their isotopesin the Earth. Broadly, the field is divided into two branches: stable and radiogenicisotope geochemistry.
Lead-lead isotope geochemistry
Leadhas four stable isotopes- 204Pb, 206Pb, 207Pb, 208Pb and one common radioactive isotope 202Pb with a half-lifeof ~53,000 years.
Lead is created in the Earth via decay of
transuranicelements, primarily uraniumand thorium.
geochemistryis 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 sedimentsand 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 scienceto fingerprint bullets, because each batch of ammunition has its own peculiar 204Pb/206Pb vs 207Pb/208Pb ratio.
Samarium- neodymiumis 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
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.
Rheniumand osmiumare chalcophile elements which are present at very low abundances in the crust. Rhenium undergoes radioactive decayto 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: ThoriumRatios - 231Pa / 230Th Uraniumis 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 adsorptionon settling particles, but not at equal rates. 231Pa has a residence equivalent to the residence time of deep waterin the Atlanticbasin (around 1000 yrs) but 230Th is removed more rapidly (centuries). Thermohaline circulationeffectively 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 d13Cand 231Pa/230Th can therefore provide a more complete insight into past circulation changes.
Noble gas isotopes
Helium-3was 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 platesare younger than continental plates). However, 3He will be degassed from oceanic sediment during subduction, so cosmogenic 3He is not affecting the concentration or noble gasratios of the mantle.
Helium-3 is created by
cosmic raybombardment, and by lithiumspallation reactions which generally occur in the crust. Lithium spallationis the process by which a high-energy neutron bombards a lithiumatom, 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 velocityfor the Earth. Thus, it is assumed the helium content and ratios of Earth's atmospherehave remained essentially stable.
It has been observed that 3He is present in
volcanoemissions and oceanic ridgesamples. 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
heliumand carbonin magmachemistry, 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 crystallattice of minerals within fluid inclusions.
Helium-4 is created by
radiogenicproduction (by decay of uranium/ thorium-series elements). The continental crusthas 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 hydrothermalprocesses, igneous geologyand 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
Tritiumwas 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]
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]
Faure G., 1986. "Principles of Isotope Geology",
John Wiley & Sons. ISBN 0-471-86412-9
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.
Arne D., Bierlein F.P., Morgan J.W., & Stein H.J., 2001. "Re-Os dating of sulfides associated with
goldmineralisation 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.
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