Other Stable Isotopes

Stable isotopes of nitrogen, oxygen and helium.

Element	Stable isotopes	Terrestrial abundance (%)	Common standards
Nitrogen	¹⁴N and ¹⁵N	99.63 and 0.37	Atmospheric nitrogen
Oxygen	¹⁶O and ¹⁸O	99.763 and 0.200	Standard Mean Oceanic Water (V-SMOW) and PDB (for low temperature geothermometry)
Helium	⁴He and ⁴He		Atmospheric helium (R_A)
CAUTION: Reported isotopic values for a sample will differ depending on the standard used

Oxygen

Oxygen isotopic data derived from combustion of petroleum compounds or kerogen reflects the isotopic composition of the oxidising gas and has no geological significance. The oxygen isotopic composition of silicate and carbonate minerals can be useful when determining the history of cementation (and hence porosity) in a reservoir (Cox et al., 2010). Another use is the estimation of sea surface temperature indirectly from planktonic foraminifera.

Nitrogen

Nitrogen in gas samples may often represent atmospheric contamination, and so isotopic ratios are rarely useful. However, the isotopic ratios of nitrogen in sedimentary organic matter are beginning to be used as source and diagenetic process indicators. For further information see .

Helium

Helium isotope ratios can be used to determine the presence of mantle-derived gases, most notably carbon dioxide. The ³He/⁴He ratio of a sample (R_M) is usually expressed relative to that of air (R_A) by the ratio R_M/R_A, which can be corrected for atmospheric contamination using the formula (Giggenbach et al., 1993):

R/R_A = [(R_M/R_A)(He/Ne)_M - (He/Ne)_A] / [(He/Ne)_M - (He/Ne)_A]

The correction requires the concentration of neon in the gas sample to be measured.

For further information on all of the above isotope groups see 'Isotope interpretation'.

References

Cox, P.A., Wood, R.A., Dickson, J.A.D, Al Rougha, H.B., Shedel, H. & Corbett, P.W.M. (2010). Dynamics of cementation in response to oil charge: Evidence from a Cretaceous carbonate field, U.A.E. In: Sedimentary Geology vol. 228 pp.246-254.

Giggenbach, W.F., Sano, Y. & Wakita, H. (1993). Isotopic composition of helium, and CO2 and CH4 contents in gases produced along the New Zealand part of a convergent plate boundary. In: Geochimica et Cosmochimica Acta vol. 57 pp. 3427-3455.

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