Variations in isotopic abundance are measured by isotope-ratio mass spectrometry, and can reveal information about the ages and origins of rock, air or water bodies, or processes of mixing between them.
The stable carbon isotope ratio, δ13C, is measured against Vienna Pee Dee Belemnite (VPDB)[clarification needed].
[2] Variations in oxygen isotope ratios are used to track both water movement, paleoclimate,[1] and atmospheric gases such as ozone and carbon dioxide.
[1] Oxygen isotopes appear in anomalous ratios in atmospheric ozone, resulting from mass-independent fractionation.
[8][9] Radiogenic isotopes provide powerful tracers for studying the ages and origins of Earth systems.
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.
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.
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.
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.
All degassed helium is lost to space eventually, as it is less dense than the atmosphere and thus steadily rises until subject to charge exchange escape.
Thus, it is assumed the helium content and ratios of Earth's atmosphere have remained essentially stable.
However, 3He is transported to the surface primarily trapped in the crystal lattice of minerals within fluid inclusions.
Comparing the ratio of tritium to helium-3 (3H/3He) allows estimation of the age of recent ground waters.