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.

Lead-Lead Isotope Geochemistry

Lead has four stable isotopes - Pb²⁰⁴, Pb²⁰⁶,Pb²⁰⁷,Pb²⁰⁸ and one common radiogenic isotope Pb²⁰² 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 surce of sediments and even the origin of people via isotopic fingerprinting of their teeth, skin and bones.

Samarium-Neodymium

Sm-Nd 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 archaological finds (pots, ceramics).

Rhenium-Osmium

Potassium-Argon

Noble Gas Isotopes

Helium-3

Helium-3 was trapped in the planet when it was created. Some He-3 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, He-3 will be degassed from oceanic sediment during subduction, so cosmogenic He-3 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 He-3 and a He-4 ion. This requires significant lithium to adversely affect the He-3, He-4 ratio.

All degassed helium is lost to space eventually, due to the escape velocity of helium exceeding that of Earth. Thus, it is assumed the helium content and ratios of Earth's atmosphere have remained essentially stable.

It has been observed that He-3 is present in volcano emissions and oceanic ridge samples. How He-3 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, He-3 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 He-4 is produced in the crust than in the mantle.

The ratio (R) of He-3 to He-4 is often used to represent He-3 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

He-3/He-4 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.

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 allows estimation of the age of recent ground waters.

General online stable isotope references

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