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.

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Lead-lead isotope geochemistry

Lead has four stable isotopes - ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb and one common radiogenic 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.

Samarium-neodymium

Main article: Samarium-neodymium dating

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.

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.

• (U-Th)/He dating of apatite as a thermal history tool
• 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.

• USGS Tritium/Helium-3 Dating
• Hydrologic Isotope Tracers - Helium

See also

• Radiometric dating
• Isotopic signature
• Cosmogenic isotopes
• Geochemistry

General online stable isotope references

• USGS: Stable Isotopes and Mineral Resource Investigations in the United States
• USGS: Fundamentals of Stable Isotope Geochemistry
• Environmental Isotopes
• Fundamentals of Isotope Geochemistry

References

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

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

Porcelli D. & Halliday A.N., 2001. The core as a possible source of mantle helium. Earth and Planetary Science Letters, 192, pp. 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, pp. 1455-1459.

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