Element that makes up 20.95 vol. % of the Earth’s atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth’s crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only ~1/1500 that of H. Atmospheric oxygen normally occurs in molecular form, O2. Ozone, O3, and monatomic oxygen, O2-, predominate in the upper atmosphere, where ozone shields the Earth from the Sun’s UV radiation. Oxygen combines with most other elements and, when this reaction occurs rapidly (with release of heat and light), it is described as combustion, or burning (exothermic). Oxygen usually occurs bonded with another element (e.g., CO, CO2), being too reactive to persist very long as O2. All O2 in Earth’s atmosphere is constantly renewed by photosynthetic activity. Oxygen is very common as ionic crystalline solids bonded with various metal ions to yield “oxides” and bonded with Si to form silicates.

Oxygen has three stable isotopes: 16O (99.757%), 17O (0.038 %), and 18O (0.205 %). Variations in the 18O/16O and 17O/16O ratios are reported in terms of their deviations (δ18O and δ17O) in parts per thousand (‰) from a standard. The usual reference is standard mean ocean water (SMOW), which plots at (0,0). Pure 16O plots at -1000 ‰ on both axes.





The oxygen atoms in terrestrial (and lunar) rocks do not have identical proportions of the three isotopes, but the ratios of these isotopes follow a simple relationship controlled by their masses. Rocks with the same 18O/16O ratio will have the same 17O/16O ratios. But if their 18O/16O ratios differ by say 0.2 %, their 17O/16O ratios will differ by half this amount, 0.1 %. This behavior, called “mass-dependent fractionation,” yields the “terrestrial mass-fractionation line” (slope = 0.52) on the isotope plot. All terrestrial rocks plot along this line, as do materials from the Moon. This coincidence of Earth and Moon isotopes is taken as evidence that the two bodies share a common origin. The mass-fractionation lines for other planets (Mars, 4 Vesta) are parallel, indicating slightly different ratios of the three isotopes, but still mass-dependent fractionation. In contrast, meteorites show evidence of formation in many isotopically distinct environments, permitting classification based on oxygen isotopic compositions.



Some or all content above used with permission from J. H. Wittke.