Isotope geochemistry is an aspect ofgeology based upon the study of natural variations in the relative abundances ofisotopes of variouselements. Variations inisotopic abundance are measured byisotope-ratio mass spectrometry, and can reveal information about the ages and origins of rock, air or water bodies, or processes of mixing between them.
Stable isotope geochemistry is largely concerned with isotopic variations arising from mass-dependentisotope fractionation, whereasradiogenic isotope geochemistry is concerned with the products of naturalradioactivity.
For most stable isotopes, the magnitude of fractionation fromkinetic andequilibrium fractionation is very small; for this reason, enrichments are typically reported in "per mil" (‰, parts per thousand).[1] These enrichments (δ) represent the ratio of heavy isotope to light isotope in the sample over the ratio of astandard. That is,
Carbon has twostable isotopes,12C and13C, and one radioactive isotope,14C.
The stable carbon isotope ratio,δ13C, is measured against Vienna Pee DeeBelemnite (VPDB)[clarification needed].[2] The stable carbon isotopes are fractionated primarily byphotosynthesis (Faure, 2004). The13C/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 theC3 pathway show different enrichments compared to those using theC4 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 global13C/12C ratio have also been useful as stratigraphic markers forchemostratigraphy, especially during thePaleozoic.[3]
The14C ratio has been used to track ocean circulation, among other things.
Nitrogen has two stable isotopes,14N and15N. The ratio between these is measured relative to nitrogen inambient 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 thestratosphere andtroposphere using data from the greenhouse gasN2O.[4]
Oxygen has three stable isotopes,16O,17O, and18O. Oxygen ratios are measured relative toVienna 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 asozone andcarbon 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 frommass-independent fractionation.[6] Isotope ratios in fossilizedforaminifera have been used to deduce the temperature of ancient seas.[7]
Sulfur has four stable isotopes, with the following abundances:32S (0.9502),33S (0.0075),34S (0.0421) and36S (0.0002). These abundances are compared to those found inCañon Diablo troilite.[5] Variations in sulfur isotope ratios are used to study the origin of sulfur in anorebody 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 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 theEarth's crust andEarth's mantle through geological time.
Lead has four stableisotopes:204Pb,206Pb,207Pb, and208Pb.
Lead is created in the Earth via decay ofactinide elements, primarilyuranium andthorium.
Lead isotopegeochemistry is useful for providingisotopic 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 inigneous rocks, the source ofsediments and even the origin of people viaisotopic fingerprinting of their teeth, skin and bones.
It has been used to dateice cores from the Arctic shelf, and provides information on the source of atmospheric leadpollution.
Lead–lead isotopes has been successfully used inforensic science to fingerprint bullets, because each batch of ammunition has its own peculiar204Pb/206Pb vs207Pb/208Pb ratio.
Samarium–neodymium is an isotope system which can be utilised to provide a date as well asisotopic fingerprints of geological materials, and various other materials including archaeological finds (pots, ceramics).
147Sm decays to produce143Nd with a half-life of 1.06x1011 years.
Dating is achieved usually by trying to produce anisochron of several minerals within a rock specimen. The initial143Nd/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 theSolar System. CHUR was determined by analysingchondrite andachondrite 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 andosmium aresiderophile elements which are present at very low abundances in the crust. Rhenium undergoesradioactive decay to produce osmium. The ratio of non-radiogenic osmium to radiogenic osmium throughout time varies.
Rhenium prefers to entersulfides more readily than osmium. Hence, during melting of the mantle, rhenium is stripped out, and prevents the osmium–osmium ratio from changing appreciably. Thislocks 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.
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 was trapped in the planet when it formed. Some3He is being added by meteoric dust, primarily collecting on the bottom of oceans (although due tosubduction, all oceanictectonic plates are younger than continental plates). However,3He will be degassed from oceanic sediment duringsubduction, so cosmogenic3He is not affecting the concentration ornoble gas ratios of themantle.
Helium-3 is created bycosmic ray bombardment, and bylithium spallation reactions which generally occur in the crust. Lithiumspallation is the process by which ahigh-energy neutron bombards alithium atom, creating a3He and a4He ion. This requires significant lithium to adversely affect the3He/4He ratio.
All degassed helium is lost to space eventually, as it is less dense than the atmosphere and thus steadily rises until subject tocharge exchange escape. Thus, it is assumed the helium content and ratios ofEarth's atmosphere have remained essentially stable.
It has been observed that3He is present involcano emissions andoceanic ridge samples. How3He is stored in the planet is under investigation, but it is associated with themantle and is used as a marker of material of deep origin.
Due to similarities inhelium andcarbon inmagma chemistry, outgassing of helium requires the loss ofvolatile 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 thecrystal lattice of minerals withinfluid inclusions.
Helium-4 is created byradiogenic production (by decay ofuranium/thorium-serieselements). Thecontinental 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) of3He to4He is often used to represent3He content.R usually is given as a multiple of the present atmospheric ratio (Ra).
Common values forR/Ra:
3He/4He isotope chemistry is being used to dategroundwaters, estimate groundwater flow rates, track water pollution, and provide insights intohydrothermal processes,igneousgeology andore genesis.
Isotopes in thedecay 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.
Uranium is well mixed in the ocean, and its decay produces231Pa and230Th at a constant activity ratio (0.093). The decay products are rapidly removed byadsorption on settling particles, but not at equal rates.231Pa has a residence equivalent to the residence time ofdeep water in theAtlantic basin (around 1000 yrs) but230Th is removed more rapidly (centuries).Thermohaline circulation effectively exports231Pa from the Atlantic into theSouthern Ocean, while most of the230Th remains in Atlantic sediments. As a result, there is a relationship between231Pa/230Th in Atlantic sediments and the rate of overturning: faster overturning produces lower sediment231Pa/230Th ratio, while slower overturning increases this ratio. The combination ofδ13C and231Pa/230Th can therefore provide a more complete insight into past circulation changes.
Tritium was released to the atmosphere during atmospheric testing of nuclear bombs. Radioactive decay of tritium produces the noble gashelium-3. Comparing the ratio of tritium to helium-3 (3H/3He) allows estimation of the age of recentground waters. A small amount of tritium is also produced naturally bycosmic ray spallation andspontaneousternary 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.
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