ABSTRACT
The elements hydrogen, carbon, nitrogen, oxygen and sulphur possess two or more naturally occurring isotopes that are not involved in any form of radioactive decay process (Table 9.1). These elements are key constituents of rock-forming minerals as well as the hydrosphere, biosphere and atmosphere and their stable isotope ratios are powerful tracers of geological processes, especially those involving fluids. For light elements, the relative mass differences among their isotopes are large enough so that the physical and chemical properties of molecules containing one or other isotope are slightly different. For example, molecules of water containing the heavier isotopes of hydrogen, 2H (deuterium), or oxygen, 18O, have a slightly lower vapour pressure than water composed of the common light isotopes 1H and 16O. During a physical process such as evaporation, water molecules substituted with the lighter isotope pass into the vapour phase more rapidly than water molecules substituted by heavier isotopes, such that rainwater eventually becomes slighdy more enriched in 1H and 6O than sea water. Systematic isotopic variations in 2H/1H and 18O/16O ratios develop within the water cycle and different forms of natural waters possess characteristic stable isotopic signatures. Similarly distinctive isotopie fractionations are associated with the geochemical cycles of C, N and S and stable isotopes provide unique information on the source, temperature and mass balance of chemical fluxes between the surface and interior of the Earth. Natural abundances of stable isotopes and reference standards used in delta notation. Reference standards: V-SMOW – standard mean ocean water prepared by the International Atomic Energy Agency Vienna (<sup>2</sup>H/<sup>1</sup>H = 0.00015595; <sup>18</sup>O/<sup>16</sup>O = 0.0020052); PDB – belemnite from Pee Dee formation (<sup>13</sup>C/<sup>12</sup>C = 0.00112372); Air – atmospheric nitrogen (<sup>15</sup>N/<sup>14</sup>N = 0.0036765); CDT – Canyon Diablo Troilite (<sup>34</sup>S/<sup>32</sup>S = 0.0450451). Oxygen isotopes can be reported relative to PDB (carbonates) or V-SMOW (water and silicates)
|
Element |
Analysed as |
Isotope |
Natural abundance (%) |
Reference standard |
Terrestrial range (%) |
|---|---|---|---|---|---|
|
H |
H2 |
1H |
99.984 |
V-SMOW |
δD = −450 to + 50 |
|
2H |
0.01557 |
||||
|
C |
CO2 |
12C |
98.888 |
PDB |
δ 13C = −120 to + 15 |
|
13c |
1.1112 |
||||
|
N |
N2 |
14N |
99.634 |
Air |
δ 15N = −20 to + 30 |
|
15N |
0.366 |
||||
|
O |
CO2(O2) |
16O |
99.759 |
V-SMOW |
S18O = −50 to + 40 |
|
17o |
0.037 |
or PDB |
|||
|
18O |
0.204 |
||||
|
S |
SO2(SF6) |
32S |
95.0 |
CDT |
δ 34S = −65 to + 90 |
|
33S |
0.76 |
||||
|
34S |
4.22 |
||||
|
36S |
0.014 |
